Mercerization Mercerization is a finishing treatment of cotton with a strong caustic alkaline solution in order to improve the luster, hard and other properties, was names after its discoverer, John Mercer, and has been in use for sometime. It has been seeing an increase in application recently.
Effect of Mercerization: 1.
Improve Luster.
2.
Increase ability to absorb dye.
3.
Improve reaction with a verity of chemicals.
4.
Improve stability of form.
5.
Improve strength/elongation.
6.
Improve smoothness.
7.
It has been shown that the increase in the luster occurs because of an effect.
8.
The cotton fiber do convoluted.
9.
The cross-sectional shape changes.
Factors of Mercerizing: In mercerizing followings are important: 1.
Twaddle
2.
Temperature
3.
Tension
4.
Time
(1) Twaddle (Concentration of NaOH): If the concentration of NaOH is increased above 56oTw improvement in luster will be attained but if it is decrease below 48o Tw. The quality of luster will begin to be adversely affected.
(2)
Temperature:
High degree of luster is attained at temperature 18-20oC. As the temperature is
increased the quality of luster is adversely affected but on lowering the temperature no improvement in the luster is obtained.
(3) Tension: For acquiring better luster the material must be stretch to its original dimension (both in warp and weft direction during mercerization). If the material is allowed to shrink during mercerizing then quality of luster will be impaired on the other hand if the material is stretched more no improvement in luster is achieved.
(4) Time: The optimum time for mercerizing is 30-60 seconds by increasing the duration of time no applicable improvement in the quality of mercerization can be achieved but if the time limit is less than 30 seconds in the quality of mercerization will be improved. Read more: http://textilelearner.blogspot.com/2012/04/mercerizing-finishing-process-effect-of.html#ixzz3dOVUurln\
A treatment of cotton yarn or fabric to increase its luster and affinity for dyes. The material is immersed under tension in a cold sodium hydroxide (caustic soda) solution in warp or skein form or in the piece, and is later neutralized in acid. The process causes a permanent swelling of the fiber and thus increases its luster.It is the process of treatment of cellulosic material with cold or hot caustic conditions under specific conditions to improve its appearance and physical as well as chemical properties.
Purpose 1.To 2.To 3.
To
What
of improve
is
improve improve the
the
dye
mercerizing uptake
the the and
mercerizing
moisture
lusture strength regain.
process
The mercerizing involves these three subsequent steps, a. Impregnation of the material in in relaxed state,cold caustic solution of required strength and wettability.. b. Stretching while the material is still impregnated in the caustic solution. c. Washing off the caustic soda from the material while keeping the material still in the stretch state.
Fig.The material flow chart in yarn mercerizing
Theory
of
mercerizing
Neales theory in Broad The hydroxyl groups on long cellulose chains attract the water molecules when cellulose takes up water, the structure expands transversily as some of the mutual secondary valency linkages are replaced by water hydroxyl linkages.Now in alkali solution ,some of the hydroxyl hydrogen atoms are replaced by sodium atoms and a system of high ionic concentration is established.Owing to osmosis ,water tends to enter this system and more secondary linkages are broken and are replaced by linkages with alkali in water.When the alkali cellulaose is washed the sodium ions and the hydroxyl ions are diffused away, and the osmotic pressure falls, the cellulose gel contracts by virtue of its elasticity. During this contraction , hydroxyl –hydroxyl linkages are reform but not in such a great number as before and the orientation of the micells is more random.The greater proportions of free hydroxyl groups accounts for the increase in the reactivity and absorptive capacity.The decrease in this high absorptive capacity on drying,particularly at high temperature ,is due to the formation of new secondary linkages on account of greater amplitude of thermal vibrations
of
hydroxyl
groups
Changes
as
during
A. 1.Swelling 2.Cross sectional 3.Shrinkage
suggested
by
mercerizing
Urqhart.
process
Fiber morphology along
changes
from with
level beam
shape longitudinal
to
round
shape. direction.
B. Molecular level 1.Hydrogen bond readjustment 2.Orientation (parallelization) of molecular chains in amorphous region along the direction of fiber length. 3.Orientation of the crystallinity in the direction of the fiber length. 4.Increased crystallinity C. 1.Increased 2.Liberation 3.Increase 4.Increase
of
Q.Why 1.Due Swelling
rate heat
of during in in
there
the
is
Chemical reaction on hydrolysis caustic treatment.(heat of sorption the alkali the absorption
a
swelling
and
in
and heat
of
Changes oxidation of reaction) absorption. iodine.
cellulose?
to is
due
to
molecular
attraction
with
associated
hydration
in
this
Swelling instance.
Since the alkali cellulose is more hydrated than native cellulose ,maximum swelling concentration is result of attraction of alkali cellulose in solution on one hand and free alkali on the other. Hydration of the cellulose increases with increased fixation of alkali in solution of rinsing concentration upto a certain limit, after which the free alkali exerts a dehydrating effect on alkali cellulose to a greater extent. Dissociation of alkali ions from the alkali cellulose compound corresponds to an absorption of OH- ions ,and in this manner a negative charge results.The cellulose molecules repel each other and absorb water ,this absorption being greater, the greater is the charge. If however the dissociation of alkali cellulose salt is forced back ,then there is reduction in the charge. Further if the concentration of the electrolyte is high in swelling liquor ,then charge of cellulose particles is shielded by free ions and force of repulsion is diminished. 2.Due to osmotic phenomenon. The fiber is surrounded by a hardened euticle which acts as a dialyzing membrane to induce osmotic action ,which is better explained with the help of Neales theory Q.Why there is a rise in temperature of caustic solution during mercerization? Ans ;-Because when cellulose is treated with a moderately concentrated caustic solution ,the heat is evolved due to heat of sorption and heat of reaction. Q.Why there is increase in luster of yarn after mercerization? Ans:-The cotton hair swells in strong caustic soda solution ,which change its cross section from squashed circular pipe shape to an oval shaped . if the fibers are placed under a tension or stretched position in the swollen state and then washed to reduce the caustic concentration below a particular limit, then there is an increase in the luster of the fiber.
Q.What should be caustic concentration for yarn mercerization? Ans:-Although mercerization can be done with the caustic concentration ranging between 150Be to 380Be, As for as the economy of the process and optimum mercerizing effect in yarn , the concentration of the caustic used is 250Be(225 gpl) to 32 0Be(325 gpl) at room temperature. Q. What shall be the ideal temperature for mercerization? Ans:- The relation between the shrinkage of cotton fiber, the concentration of alkali and the processing temperature was studied by H. Flecken, and the result is shown in this Figure.
He treated cotton yarn at 0°C to 40°C, using 30°/38° Bé caustic soda and measured the shrinkage by mercerization, and found that the least shrinkage variation is obtained at 10°C and 30° Bé. Therefore, robust conditions insensitive to the concentration of alkali and temperature must be selected for stable mercerization.
Q.Whether the recovered caustic from mercerizing process is reusable? Ans:-The caustic soda from the washing process is recoverable and can be reusable ,but it has to be free from the contaminants and must be purified to remove the undesirable impurities. Q.What is the mercerizing cycle for yarn mercerizing?
A typical mercerizing cycle in yarn mercerizing is having following steps, 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Loading
of
material Lowering Caustic
on
the
rollers of tray
Pre Free Lye Washing First
tray wash Second Third Final
and
in
rest
position. rollers movement tensioning shrinkage tensioning Squeezing movement tensioning wash wash wash Squeezing
14. 15.
Lifting Unloading
Of the
Rollers hanks.
What is the significance of different steps in mercerizing cycle? 1.Pre tensioning This is the first step in yarn mercerizing cycle ,which runs to uniformly distribute the greig yarn hanks on the rollers in combination with reversal of direction of motion of rollers, without any entanglement. The material should become completely wet in this step. 2.Shrinking This is the actual caustic treatment step or the mercerizing step in which the yarn is allowed to shrink freely , the yarn runs for sufficient time in relaxed state to react completely with the caustic , higher the shrinkage achieved better will be the mercerizing. 3. The
lye
tensioning
is
done
Lye to stretch
the
yarn
back
to
tensioning original length.
4.Squeezing To remove the unbound caustic solution from the material, so that the material can be washed effectively, and quickly to reduce the caustic content . This step also ensure the to minimize the wastage of excess caustic liquor during washing. 5. Washing and tensioning The stretch applied to get the maximum luster , material is washed along with stretching beyond its original length produce better luster. The amount of stretch applied depends upon the luster required and quality of yarn. The washing temperature is kept near boil to make washing efficient and short. Time or sequence of washing is so adjusted to reach a residual caustic content below 10% within a shortest possible time. 6.Final caustic content The final caustic content should be below 10% ,because if it is more it will be sufficient to carry out further mercerizing effect and the material will shrink back. During the storage of mercerized material , the water from the exposed areas will evaporate and got concentrated locally causing mercerizing effect ,which is called local mercerizing and will lead to patchy dyeing. The level of residual caustic in the yarn is achieved less than 3% , if it is on the higher side ,then the washing is not effective.
Read more: http://textilelearner.blogspot.com/2012/02/mercerizing-object-of-mercerizing.html#ixzz3dOWasbDc
Barium Activity Number: The ability of mercerized cotton fibre to absorb more iodine, dyes, moisture and alkalis, and to undergo oxidation or hydrolysis at increased rates compared to unmercerized cotton, has been used to develop various methods of determining the degree of mercerization. The most successful method consists of measuring preferential absorption of barium hydroxide from its dilute solution by mercerized cotton. The extent of this absorption is more than that by unmercerized cotton under the same conditions. Thus ratio multiplied by 100, is known as barium activity number, barium number or baryta number. Barium activity number was measured by AATCC test method 89-1998. One gram of mercerized and unmercerized cotton yarns were each cut into small pieces, dried over phosphorous pentoxide for 5 hours, then conditioned at 65% relative humidity at 311Cbefore testing and consequently treated with 30 ml of 0.25 N barium hydroxide solution in 100 ml conical flasks. After2 h, 10 ml of the solution is titrated against 0.1 N hydrochloric acid. A blank is also run without any yarn sample. For b, s and u being the titration readings of the blank, mercerized and unmercerized samples, respectively, the barium activity number may be given as in following equation.
Barium
activity
number
=
{(b-s)/(b-u)}
×
100
Appropriately large numbers of conditioned samples are taken, taking into consideration the moisture regain of the samples. Thus, if the moisture regains 8%, 1.08g of the conditioned cotton is weighted to get 1g of the bone dry cotton. The barium activity number of good mercerized cotton may be around 150-160. Read more: http://textilelearner.blogspot.com/2013/01/barium-activity-number-ban.html#ixzz3dOWqFjDu
1.
Alkaline
Treatments
Mercerization, the treatment of cotton with a strong caustic alkaline solution in order to improve the luster, hand and other properties, was named after its discoverer, John Mercer, and has been in use for some time. It has been seeing an increase in application recently. Recently, there has been wide use of so-called alkaline reduction processing, which treats polyester with a strong caustic alkaline solution to dissolve and remove the surface film in order to improve the hand. The methods and effects involved in the processing of cotton and polyester are different, but, both involve treatment with a strong alkaline solution before dyeing to improve the properties of the fiber, and so both can be considered together to be alkaline treatments. Furthermore, in the handling of blended and union weaves of polyester and cotton, both fibers can be treated effectively with alkalis at the same time, and so it is important that the two treatments be given equal consideration in such a case.
1.1
Mercerization
Processing
If cotton is dipped into a strong alkaline solution such as lithium hydroxide, caustic soda, or potassium hydroxide, the fibers will swell and shrink. If the fibers are placed under tension while in this swollen state and then rinsed with water, the alkali will be removed and a permanent silk-like luster will result. Alternatively, after swelling, if the alkali is rinsed off when the fiber is in its shrunk state, an increase in luster may not be discernable, but the fibers will fix in that shrunk state, thus giving good elasticity to external stress. The former is known as tension mercerization and is often simply called mercerization, while the latter is referred to as slack mercerization. Due to considerations of cost and efficacy, only caustic soda is used as the alkali in industry.
The effects of mercerization
Improved luster
Increased ability to absorb dye
Improved reactions with a variety of chemicals
Improved stability of form
Improved strength/elongation
Improved smoothness
Improved hand
Appearance is improved through increased luster, a deepening of the color and the production of a transparent look, the feel of the fabric is improved through a resulting soft hand and improved smoothness, and strength and elongation are also improved, along with the addition of good stretching ability. The treatment and handling can be adjusted to fit different requirements, thus allowing for the best application of the results of different processing. In presenting here basic information regarding mercerization, the work of J.I. Marsh (Mercerising, Chapman and Hall Ltd, 1951) and Matsui (Senshoku kogyo, Vol. 21, No. 11, pp.10-27) were used as references. A few points that are considered important for dyeing in the future are discussed here.
1.1. a The Effect of Caustic Alkalis on Cotton In the dyeing of cotton, it is well known that if too much caustic soda is used in vat dyes and other dyes which use caustic soda, the dye's ability to be absorbed will decline, this tendency being especially strong in weak alkaline vat dyes. This is thought to be the result of competition for absorption between the dye and the caustic soda. Caustic soda has an affinity for cellulose fibers, and through routine dyeing experience, it is well known that the removal of caustic soda through rinsing is very difficult when compared with the removal of acid. However, within the range of concentrations of caustic soda generally used in dyeing, the properties and form of cotton does not incur any particular effects, but if the alkaline concentration is gradually increased, they will be affected. Due to the different effects on different yarns, which are a collection of single fibers, or on different knits and wovens (and, in fact, its effect on yarn or knits and wovens is that which is desired) a variety of factors have complex cumulative effects, and the basic behavior of cotton exposed to certain alkalis is difficult to ascertain accurately, but
clarification
has
come
through
using
cotton
hairs
(single
cotton
fibers).
Qualitative observations of the behavior of cotton when exposed to caustic soda solutions of different concentrations were first conducted by Pop and Hubner (J.S.C.I. 23, p.404, 1904). Subsequently, researchers have repeatedly conducted experiments which included quantitative measurements, but the results have lacked consistency. While the reasons for this may be related to experimental procedure or certain errors, essentially, factors involved in the type and the maturation process of natural macro molecules like those in cotton can result in considerable differences in the resulting properties, structure and configuration. In addition to the variations in the responses to alkalis which result from these factors, a precise experimental procedure is difficult to determine, and this can also be considered a factor contributing to the difficulties. In the results observed to date, the behavior of cotton hairs exposed to different concentrations of caustic alkaline solutions can be summarized as follows. If a cotton hair is dipped in different alkaline solutions, no change in appearance will be visible up to 10°Bé, but above 11°Bé, the hair slowly loses its natural twist (this usually being in the order of 150-300 times per inch). Above 13°Bé, untwisting and shrinkage in the longitudinal direction now gradually starts to increase, and as the concentration nears 16°Bé, untwisting and shrinkage advance rapidly. Between 18 and 22°Bé, shrinkage of the length reaches its maximium and untwisting for the most part ceases. However, in the untwisting and shrinkage which have occurred to this point, while both are related to the swelling of the fiber, the untwisting usually occurs first, and is followed subsequently by the swelling. Nearing 24°Bé, swelling and untwisting occur at the same time, and between 33 and 44°Bé, swelling occurs before untwisting, and the rate of shrinkage that occurs with increases in the concentration of the alkaline solution decreases. After the point at which the greatest rate of shrinkage is reached, the rate of increase of untwisting slows down, but increases more or less linearly with increases in the concentration of the alkali. In the data reported by different researchers, the greatest discrepancies exist in the data related to the rates of shrinkage and swelling of the hairs. The concentrations which showed the greatest rate of shrinkage straddled the area between 18 and 22°Bé, and the concentrations for the greatest rate of swelling were distributed from 18 to 26°Bé. In these very widely distributed results, at concentrations displaying the greatest rate of contraction, these being in the range from approximately 18-22°Bé to around 33°Bé, not only did the actual alkaline concentrations displaying the greatest rates of shrinkage and swelling differ, but also the rate of shrinkage itself also varied greatly, sometimes
displaying an undulating decline, and sometimes displaying a smooth and gradual decline. In many cases, however, the rate of shrinkage started to decrease rapidly above 33°Bé. As can be seen, in mercerization, the results observed for the behavior of hairs over a range of alkaline concentrations, while important, show great disparities, and many points are still awaiting clarification. While many conjectures can be made regarding these problems, the essence of mercerizing cotton is that in the swelling of cellulose fibers due to exposure to alkalis, the natural crystalline structure of the cellulose relaxes and under an appropriate tension, the dimensions can be set by the conditions, and rinsing with water while these conditions are maintained removes the alkali and converts the cellulose to a new crystalline structure, fixing the dimensions. These being the basic principles, the degree of swelling of the cellulose is the most important factor and it is related to the alkaline concentration. However, because the swelling of the cellulose hair in the alkaline solution accompanies a change in the form of the hair itself, accurate measurements are extremely difficult to attain, and the results to date for the alkaline concentrations which display the greatest degree of swelling are, as previously noted, spread over a wide range of between 18 and 22°Bé. Other methods of measuring the degree of mercerization of cellulose include the X-ray diffraction method in which the degree of mercerization can be determined by comparing cellulose I, which has the crystalline structure of natural cellulose, with cellulose II, which has the crystalline structure of mercerized cellulose. The results concerning the degree of mercerization of a cellulose hair in different concentrations of caustic alkaline solutions obtained with the X-ray diffraction method showed that in 17°Bé NaOH 10% mercerized, in 18-19°Bé 40-45%, in 19-20°Bé 70%, in 20-21°Bé 80%, 22°Bé 90-95%, 23-24°Bé 95-100%, and in concentrations above 2425°Bé 100% mercerized. According to these results, the concentration of the caustic alkaline solution at normal temperatures needs to be above approximately 24°Bé to ensure the complete mercerization of cellulose fibers (cotton hairs) in their free state. This gives consistency to the effects of practical mercerization, and, at this concentration, the swelling and the untwisting of the cotton hair start more or less simultaneously. At alkaline concentrations in which the swelling occurs subsequently to the untwisting, the crystalline structure of the cellulose fiber shows signs that it has not undergone complete mercerization.
1.1. b Behavior
in changing alkaline solutions
In the previous section, the behavior of cellulose hairs dipped directly into an alkaline solution of fixed concentration was discussed. Here, the behavior of hair undergoing treatments involving gradual changes of concentration, being first dipped in strong alkaline solution and then into progressively weaker alkaline solutions, and conversely, first dipped in a weak alkaline solution and then into progressively stronger alkaline
solutions, is considered. The first to conduct quantitative measurements of the changes of a cotton hair undergoing such alkaline treatments were Willows, Barratt and Parker (J.T.I., 13, p.29, 1922). Their results are shown in Figs. 1 and 2.
Fig.1Changes in the length of a cotton hair when dipped in decreasing concentrations of alkaline solution (NaOH)
Fig.2 Changes in the length of a cotton hair when dipped in increasing concentrations of alkaline solution (NaOH) From these results, in comparison with direct treatment with a single concentration, when the hair is treated with increasing concentrations of alkaline that start from a weak solution and get progressively stronger, the concentration that displays the highest rate of shrinkage shifts much higher and the rate of the increase in shrinkage becomes extremely low. In contrast to this, if the opposite treatment is carried out, the alkaline
concentration that displays the greatest rate of shrinkage shifts conversely lower. These trends are visible in Fig. 3. This behavior is conjectured to be due to the fact that the diffusion of the caustic soda inside the cellulose fiber in its swollen state is extremely slow.
Fig.3 Changes in the length of a cotton hair when treated with a caustic soda solution [Collins and Williams (J.T.I., 14, p.287, 1923)] However, there is no data that has actually measured the diffusion of the caustic soda inside the alkali-swollen cellulose. The mercerization of cellulose that is exposed to increasing concentrations of alkaline solution can be considered to display behavior that is similar to that of the wet-on-wet method, that is, the wet mercerization method. The cloth not having dried before mercerization, wet mercerization involves application of a strong uniform squeezing agent and exposure in that state to a strong caustic alkaline for mercerization. Because the drying of the cloth before mercerization is omitted, this is very effective as a measure for saving energy, and is used widely in industry. Because the water content of the cloth before mercerization (usually around 50%) steadily dilutes the caustic soda, in order to ensure the practical effectiveness of wet mercerization, the concentration of the alkaline solution must be preserved through the steady addition of a correspondingly high-concentration alkaline solution. Furthermore, to avoid a rise in temperature due to the dilution heat of the alkali and the heat generation that accompanies the cellulose fiber's absorption of the alkali, the alkaline solution must be cooled, thus allowing the preservation of a constant temperature. Measures to preserve the uniformity of treatment have already been implemented and many factories over many years have made continual efforts to ensure the industrial success of wet mercerization.
However, many factories have now, for effectiveness, abandoned wet mercerization and have returned to the previously used dry mercerization. Of course, there are many reasons for this, including those relating to equipment costs and management, but one major reason is in regards to quality, because in wet mercerization problems concerning efficacy and uniformity can occur easily. One possible reason for wet mercerization not being as stable as dry mercerization is that the behavior of cellulose fibers in alkaline solutions is considered uniform, and the measures to prevent the diffusion of the caustic alkali inside the swollen cellulose are insufficient. In contrast to this, in dry mercerization, the alkaline solution for the first dipping must have a concentration sufficient for mercerization, and if it sufficiently penetrates the inside of the fiber with only the usual degrees of temperature and tension control, no major considerations are required with regard to the change in concentration of the alkaline solution that acts on the cellulose's structure, and management of the mercerization is extremely simple.
1.1.
c
Absorption
of
the
alkali
and
swelling
The cotton hair swells in a strong caustic soda solution, and on viewing the changes in the cross-section that occur during the mercerization process (see Fig. 4), the cross section, originally shaped like a squashed circular pipe, clearly becomes oval-shaped, thus enhancing the luster. The large differences in the swelling that occur due to the concentration of the alkaline solution are relative to the longitudinal shrinkage of the hair.
Fig.4 Changes in the cross-section of a cotton hair during mercerization The effect of the concentration of the alkaline solution on the shrinkage of the length and the dilation of the diameter, along with the increase in the volume of the hair, are shown in Fig. 5. The transformation of the cross sectional diameter in relation to changes in the hair's length are shown in Fig. 6. Fig.5 Changes in the dimensions of a cotton hair treated with caustic soda
Fig.6 Relationship between the changes in length and cross-sectional diameter of a cotton hair As can be seen from Fig. 5, the maximum increase in the volume of the cotton hair occurred for a 16% solution of NaOH, that is, a concentration nearing 22°Bé. However, after repetitions of the experiment, the concentration of alkaline solution that was determined to display the greatest rate of swelling and the greatest rate of longitudinal shrinkage for the hair was different each time, the results being distributed between 18 and 22°Bé. In all cases, however, if this alkaline concentration was exceeded, any subsequent increases in concentration resulted in a reduction in the degree of swelling. Many researchers have, in addition, investigated the changes in the amounts of alkaline and water absorbed by the hair for different concentrations of alkaline solution, and representative results are shown in Fig. 7. It can be seen that the concentration of alkaline solution which displayed the greatest rate of swelling for the hair also displayed the greatest amount of absorbed water, and in solutions above this concentration, while the rate of absorption of the alkali increased, the rate of absorption of the water decreased.
Fig.7 Alkaline concentration versus the cotton's absorption of water and NaOH
There are many possible reasons for this, but Neal's explanation using Donnan Membrane Equilibrium (J.T.I., 20, p. 373, 1929) enjoys much support. However, this cannot be considered sufficient explanation for all the behavior exhibited by the cellulose fiber in the alkaline solution. The visible changes in the cotton hair in various concentrations of alkaline solution have been discussed above, but this is still a weak foundation for a theoretical explanation for just the externally visible changes. Due to this, examination of the transformations that occur in the crystalline structure of the molecules of the cellulose is necessary. Analysis of the transformations of the crystalline structure of the cellulose hair in the alkaline solution with the X-ray diffraction method has been conducted by Katz and Mark (Z. Electrochem., 31, 105, 157, 1925), Katz and Viewg (ibid., 157), Suich and Walff (Z. phys. chem., 8, 221, 1930) and Hers and Trogus (ibid., B12, 381, 1931). According to these results, the cellulose hair undergoes no visible changes in concentrations up to around 8% NaOH (approx. 12°Bé), but at 12.5% NaOH (approx. 18°Bé), the generation of alkali cellulose becomes apparent. According to the above mentioned observations, above 11°Bé, the untwisting of the hair starts but is incomplete, and after that, as the concentration increases, the untwisting and the shrinkage of the hair gradually increase, and as the concentration nears 16°Bé, these two increase rapidly, and around 18°Bé (while results differ, in the range of 1822°Bé), the rate of shrinkage reaches its maximum and more than half of the untwisting is completed. This point, according to X- ray diffraction method, is also the point at which the cellulose's structural transformation commences. So, up to 18°Bé, the question is why does this kind of swelling and shrinkage occur even though no reaction occurs between the alkali and the cellulose. However, at concentrations below 18°Bé, no structural changes in the cellulose are noticeable with X-ray diffraction. In contrast to the molecules of the cellulose that are structural units, the alkali cellulose I examined with X-ray diffraction is a compound with 1 mol of NaOH appended, that is C6H10O5•NaOH. For many of those who have examined the amount of alkali absorbed by the cellulose hair, the point at which there was a change in the degree of absorption was sometimes at concentrations of 8% (approx. 12°Bé), and sometimes at concentrations nearing 13.0% (approx. 18-19°Bé), and so due to the generation of an inflection point at which the molecules of cellulose that were structural units were observed to absorb 0.5 mol NaOH, the generation of a compound of C12H20O10•NaOH has been suggested. Reconciliation of the differing views is problematic, but clarification of the differences could start from consideration of the fact that the cotton hair has a complex structure, and so is not a simple singular thing. In a cotton hair, which consists of natural cellulose, around 75% of the cellulose has a crystalline structure, the rest being of an amorphous structure or having constituents of
low molecular weight which, even if reacting with the alkali, would not be noticeable through X-ray diffraction, and nor would the reactant produced through reactions between the alkali and the outer skin of the micelle. The outer skin of the micelle, the non-crystallized cellulose and the constituents of low molecular weight are considered to constitute over 50% of the total cellulose. Because the reaction of these constituents of cellulose with alkalis cannot be observed with Xrays, the existence of compounds like C12H20O10•NaOH is not verifiable, but due to the absorption of the alkali, observers believe that over 50% of the constituents of the cellulose absorb around 0.5 mol in alkaline solutions of low concentrations, thus generating the C12H20O10•NaOH compound. However, if only amorphous structured cellulose, other cellulose constituents of low molecular weight and the outer layer of micelles react with the alkalis, when the whole structure of natural cellulose has not reacted with the alkali, the verification of the generation of a reactant with 0.5 mol of NaOH attached in the molecules of the cellulose that are structural units is not possible, and the existence of this type of reactant is adamantly denied by some. That is, it is considered that there is simply a phenomenon having such an external appearance that is produced during the process of the generation of 1 mol of molecules of cellulose that are structural units and 1 mol of attached NaOH. According to X-ray observation, the production of alkali cellulose I does not change up to concentrations of 18% (approx. 24 °Bé), but if the concentration goes above this, the production of alkali cellulose II can be observed, and at a concentration of 22% (approx. 28-29 °Bé) alkali cellulose I disappears completely. If alkali cellulose I is dried, there is a change in the X-ray interference pattern, and so the result is called alkali cellulose III. In contrast, if alkali cellulose II is dried, no change is visible through X-ray observations, meaning no structural changes occur due to drying. While alkali cellulose I can take water into its structure, alkali cellulose II appears to be unable to do so, and due to this difference, in the treatment of cellulose hairs in alkaline solution, after the maximum rate of swelling is reached in highly concentrated alkaline solutions, the amount of alkali absorbed increases, but the amount of water absorbed decreases, and this is consistent with a decrease in the production of alkali cellulose I and an increase in the production off alkali cellulose II with any increase in the concentration of the alkaline solution. Alkali cellulose I and II become hydrated cellulose, or mercerized cellulose, after rinsing with water. However, according to results of analysis to determine at what concentration of alkaline solution the original structure can be preserved if it is treated with gradually decreasing concentrations of low-concentrate alkaline solution during the rinsing process, while concentrations above 18°Bé are necessary for the generation of alkali cellulose I, it can only exist at concentrations of up to 10% (approx. 15°Bé), and alkali cellulose II is produced at concentrations above 18% (approx. 24°Bé), but at alkaline
concentrations below 6.6% (approx. 10°Bé), the original structure will undergo only very slight degradation, and thus can be considered extremely stable. This is because alkali cellulose has little ability to structurally coexist with water, and as mentioned previously (Figs. 1 and 3), this is evidence of the behavior during mercerization that includes rinsing accompanied by a gradual decrease in the alkaline concentration. In conclusion, due to X-ray diffraction observations of the reaction between the cellulose hair and the alkaline solution, it is believed that in an alkaline solution of low concentration, the alkali cannot combine with the cellulose molecules inside the micelle, and so in this state only the outer skin of the micelles and the cellulose that is not a part of a crystalline structure, that is, the material with low molecular weight, react with the alkali. As the concentration increases, the crystalline structure of the cellulose swells and relaxes, and when it reaches it most swollen state, the alkali penetrates the inside of the micelle, and undergoes a complete reaction with the cellulose.
1.1.d Effects of Constraint of the Hair on Swelling The results of many researchers determined the alkaline concentrations for which the swelling of the cellulose was greatest as being in the range 18-22°Bé. This concentration range does not represent the completion of the production of alkali cellulose I through the mercerization reaction, but rather the beginning, the end being nearer the higher alkali concentration of 24°Bé as mentioned previously. After swelling reaches its greatest point, NaOH thoroughly penetrates the interior of the micelle, and a reaction between the alkali and the micelle occurs, completing the generation of alkali cellulose I. From 18-22°Bé, the range displaying the greatest degree of swelling, to around 24°Bé, the cotton hair first contracts momentarily, and then swells again, and at 24°Bé or above a second swelling peak was observed. These observations cannot be disregarded as baseless occurrences. Fig.8 The longitudinal changes of a single scoured cotton hair in a single caustic soda solution (tensile force: 50mg) (Williams, Barratt and Parker)
Fig. 9 The change in the cotton hair's volume after mercerization For a natural cellulose like the cotton hair, the structure consists of a complex assembly, and furthermore it is of course not unlikely that the balance between the generation of alkali cellulose I and alkali cellulose II is derived from the characteristics of hydration. In measuring the absorption of alkali and the swelling of a cellulose fiber in an alkaline solution, measurements and calculations can be based on the changes in the length and the diameter, or alternatively, the water can be removed from the swollen fiber by compression or with a centrifuge, after which the composition of the liquid and attached alkali absorbed by the fiber can be analyzed, thus determining the degree of swelling and the amount of water and alkali absorbed. The degree of alkali absorption can also be determined by measuring the concentration of the alkali solution before and after the treatment of the cellulose, but getting reliable results is difficult due to the fine conditions required in the operations and other considerations, and in the course of an experiment, discrepancies among the determined values are very significant. However, rigorous measurement with a single experimental method can provide useful information regarding trends based on relative changes. For example, according to the behavior observed for woven fabric and fibers of raw cotton exposed to alkali, woven fabric only displays around half the degree of swelling displayed by fibers if both are treated with the same concentration of alkaline solution, but no such difference exists between their respective degrees of alkali absorption (see Figs. 10 and 11). Fig. 10 The absorption of alkali solution by fibers & fabrics
Fig. 11 The amount of NaOH absorbed by raw cotton Furthermore, in the results of these experiments, there was little visible increase in the degree of swelling of a fiber accompanying increases in the concentration of the alkaline, but for the woven fabric there were, and this is in apparent opposition to the results for cotton hairs. As another example, aside from the hair, the concentration at which yarn showed the greatest degree of swelling was determined to be 20% NaOH (26.5°Bé), and this far
surpasses the range of 18-22°Bé as determined for the cotton hair, and so cannot be dismissed as being simply due to experimental error. Inferring from these results, it is believed that when cotton hair under physical restraint, that is, made into a yarn or a woven fabric that restrains the hair's freedom due to twisting and to crossing of twisted yarn, is treated with an alkali, it displays different behavior because the shrinking and swelling displayed originally cannot occur due to the constraining forces. As an illustration of this, the report by H. Flecken (Textil Praxis, Juni, 365, 1970) shows that even for the same type of yarn, the behavior of single, double and triple yarns display differences, as does the shrinkage of woven fabric and hairs (see Fig. 12).
Fig. 12 The concentration of NaOH versus the shrinkage of cotton hairs and cotton fabrics Due to the degree of constraint on the swelling and shrinkage of single fibers, ie. hairs, changes will of course occur in the numerical values related to the swelling of cellulose fibers, but in general, increases in the degree of constraint moves the peaks of these values towards higher alkaline concentrations. While the diffusion and penetration of the alkali solution from the exterior to the interior of the hair occurs freely for hairs under no constraint, for hairs under constraint, not only will the swelling of the exterior of the hair narrow the gaps in the micelles, thus delaying the interior diffusion and penetration of the alkaline solution, but also the concentration gradient, which is the driving force for the internal diffusion of the alkali, will be raised, thus lowering the swelling rate itself. Thus, the result is that when a single fiber is under great constraining force, the concentration displaying the greatest rate of swelling will increasingly move towards alkalis of higher concentration.
1.1.e Heat generation during mercerization and the effect of temperature In mercerization, aside from the heat production due only to the behavior of the cellulose fiber and the alkaline solution, as in the reaction between the cellulose and the alkali, the swelling, and the hydration, there is also dilution heat which is generated through the dilution of a strong alkali either with the water content of the cellulose which has been absorbed from the air, or the water present during mercerization in a wet state, as in wet mercerization.
1.1.e.1 The reaction heat of the cellulose fiber and the alkali Heat is generated by the cellulose, which acts as a very weak acid, bonding with the alkali. Heat is also believed to be produced by the deformation of the molecular structure caused by the hydration and swelling of the alkali cellulose. This kind of heat generation, due to the reaction of the cellulose fiber with the alkaline solution, has been measured for some time, but the values relating to it determined by different researchers vary significantly. However, all the results show that heat generation due to the reaction between the cellulose and the alkali, like heat generation in other typical chemical reactions, is not simple, and varies according to the concentration of the alkali and other factors. The results of Neal (J.T.I. 20, 1929, p.373), which can be considered to be relatively reliable, are displayed in Fig. 13. According to these results, from a concentration of around 120g/L NaOH, or about 15-16°Bé, the reaction heat increases sharply, and from around 240g/L, or 26°Bé, the reaction heat increases in more or less constant proportion with increases in concentration.
Fig. 13 The reaction heat of cellulose and NaOH These results accord well with the results for the untwisting and swelling of the cotton hair and those measured for the relationship between the formation of alkali cellulose I and the concentration of the alkali as observed with X-rays. There is even a very slight generation of heat when cellulose fibers swell in water. While different researchers recorded different results for this heat generation, it is in the range of 2.6-4 Cal/g of cotton. In contrast to this, with an increase in the concentration of the alkali solution of 1M in the range from 15-16°Bé to 19-20°Bé for which the swelling was greatest, the heat generation was 8.93 Cal/g, and from a concentration of around 22°Bé, at which the production of alkali cellulose I was more or less complete, the rate of increase of generated heat was 3.40 Cal/g cotton/M NaOH. Above 42°Bé, this became 3.0 Cal/g cotton/M NaOH. While the time required for the completion of this heat generation for concentrations below about 34-35°Bé was less than five minutes, above 42°Bé it was ten minutes, and above 22M NaOH, 100 minutes was required, the reaction needing a very long period of time to complete. For concentrations that are used in practical mercerization, heat generation per kilogram of cotton is 20Kcal for 22°Bé, 22 Kcal for 24°Bé, 24Kcal for 26°Bé, 27 Kcal for 28°Bé, and 29Kcal for 30°Bé. Increases in the temperature of the alkali solution or of the yarn or the fabric will influence the effectiveness of the mercerization, so mercerization is normally carried out under a cooling process. At concentrations below 22°Bé, significant differences in behavior with regards to heat generation are displayed between fibers that have been mercerized once before and
fibers that have not been mercerized at all. Items that have been mercerized once before display the absorption of alkali and generation of heat even in alkali solutions of low concentration. (See Fig. 14)
Fig.14 The heat generated by mercerization (Okamura, Naturwiss, 21, 393, 1933)
1.1.e.2 Heat generation through dilution of the alkali solution It is well known that if a strong caustic soda solution is diluted with water, a large amount of heat will be generated, but in the mercerization of cellulose fibers, heat generation also occurs due to the dilution of the alkali solution by water that has been absorbed by the cellulose from the air. With regards to this effect, Tschilikin (Textilber, 14, 1933, p.404) reported that the addition of 15cc of water to a 491g/L solution of NaOH to produce 500cc of solution yields 788 Cal/g water, and similarly, in a 604g/L solution of NaOH yields 1312 Cal/g water. Thus in practical mercerization under these conditions, the mercerization of 300g of cotton yarn having an absorbed water content of 5% with 485cc of alkaline solution will result in an increase in the temperature of the solution of approximately 23°C in the former case and by approximately 40°C in the latter due to only the dilution heat of the absorbed water. This would appear to be excessive, but calculation of the dilution heat of the alkali solution gives 1.99Kcal/mol for the former and 2.69Kcal/mol for the latter, neither of which are excessive values. However, if the concentration of the alkali solution is significantly greater than that used in practical mercerization, at around 40°Bé or 45°Bé, then this cannot necessarily be used as practical data. Because there is no accurately measured data available to determine the extent to
which the dilution heat influences the mercerizing process, the following results were determined through calculations with the aid of a chemistry handbook. The infinite dilution of approximately 36°Bé NaOH and 23°Bé NaOH (data for concentrations between these two values is unavailable) yields 0.9 Kcal/mol and 0.06 Kcal/mol respectively, and so in mercerization under the above conditions, even with the factors mentioned above, the increase in temperature of a 36°Bé solution would only be 913°C, and only 0.3-0.9°C for a 23°Bé solution. So in practical mercerization at a concentration of around 30°Bé, the increase in temperature of the solution due to the water absorbed in the yarn can be estimated to correspond to only around 1-2°C under the above conditions. However, in wet mercerization, the alkaline solution used for feeding must be a highly concentrated solution of 49-50°Bé, and the yarn or the fabric has a high water content, so the resulting amount of dilution heat is large, and the resulting temperature increase cannot be ignored. While the concentration of the alkaline solution used for feeding in wet mercerization is presently 49-50°Bé, if the concentration of the caustic soda solution is reduced to 2829°Bé the dilution heat becomes 2.13 Kcal/mol, and 2.89Kcal/mol at 24-25°Bé, and 3.1 Kcal/mol at 22°Bé, thus allowing the problem to be ignored.
1.1.e.3 The effect of temperature during mercerization The reaction between the cellulose fiber and the alkaline solution is an exothermic reaction, and any increase in the treatment temperature reduces the absorption of the alkali, thus reducing the effectiveness of the mercerization. Furthermore, increasing the concentration of the alkaline solution to counteract the reduced absorption in order to achieve the same effects from the mercerization will not necessarily be successful. Sisson analyzed a cotton yarn treated with a wide range of alkaline concentrations and processing temperatures with X-ray diffraction, divided the results into the three divisions of complete mercerization, partial mercerization and un-mercerized and created a chart showing the relationship between the alkaline concentration and the temperature. The result is shown in Fig. 15.
Fig. 15 The temperature and concentration for the mercerization of cotton According to these results, regardless of the increase in the concentration of the alkali, above about 60°C, complete mercerization does not take place. The concentration of around 30°Bé in mercerization at room temperature, that is, around 20°C, is in the middle of the chart for complete mercerization, and this, interestingly, is fairly consistent with stable conditions determined by experience in practical mercerization and with the results of all researchers. For example, these results are consistent with the results of Beltzer (R.G.M.C., 1902, 6, 25, 34; see Fig. 16) who investigated the relationship between shrinkage and the effects of temperature and alkaline concentration in the mercerization of cotton yarn, and the results show that the concentration of 20°Bé represents a line beyond which behavior during mercerization changes.
Fig. 16 The shrinkage of yarn in caustic soda solutions of different temperatures and concentrations Similarly, the results of Birtwell, Chblenens et al (J.T.I., 21, 1930, p.85; see Fig. 17) show that temperature has its greatest influence on the shrinkage of cotton yarn at concentrations of 3N NaOH, (approx. 16°Bé), and above 4N (approx. 20°Bé), that influence diminishes.
Fig. 17 The shrinkage of cotton yarn in NaOH H. Flecken (Tex. Praxis., Juni, 365, 1970; see Fig. 18) measured the shrinkage of cotton yarn at concentrations of 30°Bé and 38°Bé for temperatures from 0°C to 40°C, and reported very small variations for temperatures in the range of 10-30°C at a concentration of 30°Bé, and this result can also be found in the center of Sisson's mercerization chart.
Fig. 18 Temperature versus shrinkage rate in the mercerization of cotton yarn From the above, it can be seen that in order to conduct stable mercerization, appropriate conditions are those at which the influence of the alkali concentration and the temperature are minimal, that is, those conditions as displayed in the center of Sisson's mercerization chart. While luster, hand, and dimensional stability are the three results most commonly demanded of mercerization, the extents to which these three are achieved are not only determined by a combination of the changes relating to the absorption of alkali, the shrinkage and the swelling, and neither are these three characteristics all affected by different conditions in the same way.
In any case, in industrial processing, economy and operability are also major concerns. Due to this, processing is not necessarily best implemented by treating the relationship between alkaline concentration and processing temperature as a function according to the aims of mercerization, and sometimes all the factors involved should be treated independently. In regards to this, the effect of temperature on the hand of the fabric is especially large, as is that of the tension exerted during the processing. A typical method, called cold mercerizing, involves processing with an alkaline solution at temperatures below 5°C, commonly in the range of minus 10-15°C. In this processing, which gives the yarn or fabric a feeling of transparency and a harshness like that of linen, due to which this processing is also called imitation linen finishing, the alkaline solution is in the range of 15-30°Bé. However, when the concentration is below 15°Bé and the temperature is as low as minus 10°C, the degree of shrinkage decreases and the swelling and the relaxing of the cellulose structure due to the absorption of alkali is insufficient to gain these results. While one reason why alkali processing at low temperatures such as these produces a harder hand is that it is not able to produce the same degree of swelling as processing at room temperature or higher, another important factor is considered to relate to the fact that by lowering the temperature, the freedom of the cellulose's molecular structure is fixed in a restrained state. In contrast to cold mercerization, processing at high temperatures is good for producing a soft hand, but at temperatures above 60°C, processing produces partial mercerization, complete mercerization not taking place. Thus, in mercerization to produce a soft hand in addition to a good luster and stability of form, adjustments cannot be limited to the temperature conditions of the alkali dip. Consideration of the entire process involved in the completion of mercerization is necessary. The same can also be said of cold mercerization.
1.1.f
Tension
During
Mercerization
In mercerization there are two types of tension, one produced by the constraining force in opposition to the swelling caused by the twisting of the hair or the structural density of the fabric when the single fibers which constitute the yarn or the fabric, ie the hairs, absorb alkaline solution and swell, the other being intentionally exerted on the yarn or fabric during mercerization. The former type of tension occurs due to the relationship between the force of the swelling and the constraining force in opposition to it, and although not certain, as it is due to the force of the swelling, it can be expected to increase with increases in the concentration of the alkaline solution or decreases in the processing temperature. The resulting negative tension can be derived by measuring the load required to keep the yarn or the fabric at the same length as that before processing, but because this tension cannot be adjusted during mercerization itself, if any adjustments are required, measures must be taken during the design stage of the yarn or the fabric.
The latter tension, being a tension intentionally exerted during mercerization, can be considered in three stages, these being during the penetration of the alkaline solution and the swelling, during the fixing of the dimensions and the enhancing of the luster, and during the removal of the alkali. Along with the concentration of the alkaline solution and the temperature during the treatment, control of the tension during the different stages is important in the supervision of the mercerizing process.
1.1.f.1
Tension
and
penetration
of
the
alkali
In the initial penetration of the alkaline solution and swelling of the fibers, the surface tension of caustic soda solution increases with increases in concentration, and for temperatures of around 18°C, at a concentration of 24°Bé, a concentration commonly used in mercerization, it is approximately 84dyncm-1, and at 30°Bé it is 89dyncm-1. The surface tension of water at the same temperature is 73.05dyncm-1, and for acids, and especially organic acids, the surface tension decreases with increases in concentration, becoming significantly lower than that of water, although for inorganic acids in the range of practical application, there is very little difference from water. From this, it is clear that the wetting of the yarn or the fabric with the alkaline solution during mercerization is not easy to achieve, and after the cellulose fibers come into contact with the alkaline solution, the surface of the yarn or the fabric will swell, and because the spaces between individual fibers contract, the air inside the yarn or the fabric will be less likely to escape to the outside, thus making the penetration of the alkaline solution more difficult, easily producing a state called wetted surface. The wet ability of a material or a yarn can be substantially reduced by insufficient preprocessing or excessive drying, so sufficient pretreatment along with treatment when the amount of residual water is close to that naturally absorbed by cotton, is desirable because these factors raise the degree of wetting and the internal penetration of the alkali. In practical mercerization, the dipping time for yarn and material that has been processed adequately is commonly set at around 40-50 seconds, and results of investigations into the effects of tension during this time show that when there is no tension the shrinkage nears equilibrium after 60 seconds, but under high tension the same processing time will only produce about half the amount of shrinkage, and around 120 seconds, or twice as much time, is necessary to produce the same amount of shrinkage as when there is no tension. These results were determined by the point at which the degree of shrinkage more or less reaches a value of equilibrium, but from the original results concerning sufficient penetration of the alkali into the inside of the cellulose fibers’ micelle and the subsequent reaction with the cellulose, the time required was determined to be two minutes for hairs and around five minutes for yarn or fabric. Thus in industrial mercerization, an alkaline dip of less than one minute will result in partial mercerization of only around 70% for yarn and 40-60% for fabric. In order to conduct adequate mercerization, in addition to sufficient pre-processing of the yarn or fabric to enhance its wettability, measures must be taken to ensure that no
tension is in force during dipping in alkali despite any occurrence of shrinkage, while a sufficient amount of time is also allowed. This kind of tension control is easy in the case of yarn done as a batch, but in the continuous treatment of fabric it is difficult to achieve, there being many cases of large deficiencies. Penetration agents for use in mercerization enhance the wettabilty during the alkali processing by accelerating the penetration of the alkaline solution into the structure of the yarn or fabric, but the excessive time required for the diffusion of the alkaline inside the hair remains a problem. High temperature mercerization is a method for allowing the rapid penetration and diffusion of the alkaline inside the hair to allow complete mercerization as far as the inner layer of the micelle. In addition to lowering the surface tension of the alkaline solution by raising its temperature, the internal diffusion of the alkali itself is greatly accelerated because the swelling decreases due to a drop in the cellulose’s absorption of alkali.Thereafter, the temperature is lowered in order to increase the absorption of alkali and thus increase the rate of the mercerization reaction. Due to considerations of cost, however, this method is little used.
1.1.f.2 Tension after the swelling due to the alkali The tension introduced after the swelling of the yarn or the fabric is a mechanical tension exerted to resist the force of the shrinkage of the shrunk yarn or fabric and, by stretching, it fixes the dimensions as required. At this time, the problem in the processing is not the degree of tension required but the amount of stretching. For the generation of a good luster, stretching to the original length before the processing is usually the norm, but stretching beyond the original length will enhance the luster even more. However, stretching is not often beyond the original length of the yarn or the fabric due to the mechanical difficulties and the negative effects on hand and strength. In so-called tensionless mercerization, which is totally without stretching, the improvement to luster is extremely slight. The results of investigations into the relationship between the degree of stretching and the luster of a cotton hair (J.T. Marsh) are shown in Table 1. Table 1 The relationship between the degree of stretching of a cotton hair during mercerization and the resulting luster.(J.T. Marsh)
The increase in luster is due to the cellulose hair, swollen with the alkali, becoming more circular (it becomes, in fact, elliptical), which stretching enhances, and the surface of the hair becoming smoother. The luster of cotton fibers is decided by the ratio between the long and short axes of the cross section of the cellulose air, and it improves as the cross section becomes more circular. (See Fig. 19)
Fig. 19 The axial ratio of the cross-section of single cotton fibers and their luster While mercerization can greatly improve luster, it cannot make up for deficiencies in the luster of the raw cotton itself, and in order to produce products of superior luster, primary considerations relate to the choice of raw cotton, the twisting and manufacture of the cotton yarn and the structure of the fabric. While changes in the mercerization process do influence the improvement of luster to a certain extent, any effect that surpasses the more basic variations cannot be expected. As mentioned previously, after fibers have swollen sufficiently in mercerization under conditions free from externally-applied tension, applying tension to yarn in batches is easy, but in the continuous treatment of fabric, because control is difficult to exert, supervision of the tension and the dimensions of the fabric are incapable of allowing conditions ideal for free shrinkage during the penetration of the alkali and the swelling. The result is close to that of mercerization at fixed length (a method which involves treatment while preserving a certain length, not allowing the shrinkage of the fabric which arises from the swelling and shrinkage accompanying the penetration of the alkali), the penetration of the alkali being less than ideal, and the effects of mercerization being reduced. Thus, this is one reason why the mercerization of yarn is valued in the production of high-grade products. In addition, the fixing of the dimensions by tension during the process, along with the removal of the alkali through the washing which follows, plays an important role in enhancing the shrink-resistance of a product. Cellulose with relaxed bonding due to swelling is fixed in the new dimensions at this
time, and the tension is maintained as the alkali is removed, because crystallization due to the bonding of cellulose molecules in their new positions must be allowed to occur. The fiber in its swollen state, in addition to having swelling sufficient to cause distortion to the cellulose’s molecular chains or structure due to the stretching, must also hold enough alkaline solution or water to prevent the intermolecular bonding of the cellulose. In processing for which the conditions are similar to those of mercerization at fixed length, the alkaline solution required for swelling does not enter the yarn or fabric in sufficient quantities before the time allotted for swelling is up due to the tension or the wringing of the liquid. During that time, free alkaline solution will be absorbed by the fibers, and a certain degree of swelling will occur, but the fibers are stretched when there is still an insufficient amount of alkali or water between the molecules in the cellulose’s structure, or in the air gaps in the micelle. The result is that the yarn or fabric will break because it cannot withstand the tension. When alkali cellulose fibers in a relaxed state are distorted, as can occur easily in weak fine count yarn or fabric, if the water which fills the spaces between the molecules is insufficient, the hydrogen bonds will break and the fiber will snap, but if the amount of water is sufficient, the alkali cellulose is considered to be able to respond to the distortion by deforming. The amount of water content required at this point is decided by the amount of absorbed alkali, and in cotton fabric it will be at least above 100%, while in the case of yarn, more is necessary. In tensionless mercerization in which no tension is exerted after the swelling which follows the absorption of the alkali, if total mercerization does not take place in the inside of the cellulose structure, the desired degree of stretching will of course not be attained. If the alkali-swollen fiber is stretched and the dimensions set as required, rinsing with water in that state will remove the alkali, but if the tension is relaxed when the alkali has still not been sufficiently removed, the fiber will swell and shrink again, and the effectiveness of the setting of the dimensions will be reduced due to the remaining alkali cellulose. Any Alkali cellulose I and II generated can exist for a short time at around 15°Bé or below 10°Bé respectively, as has been shown through X-ray observations. In measurements of the shrinkage of the cellulose, when hair that is treated with high concentrations of alkali is treated with alkali solutions of decreasing concentration (see Fig. 3), even at concentrations of 10°Tw, if the degree of shrinkage is still close to its maximum, treatment under a fixed tension until the alkali is more or less totally removed was found to be ideal. In industry, practical mercerization involving the hydrolytic cleavage of the alkali cellulose inside the cellulose and the preservation of the tension until the absorbed alkali is completely removed is nearly impossible, and so in practical production, the tension is released when the alkaline concentration of the cleaning liquid reaches an appropriate point, and the rinsing continues until neutralization.
When the alkaline concentration of the cleaning liquid goes below 5°Tw, or 3.5°Bé, the setting of the dimensions is considered to be sufficient, but the concentration cannot be adequately supervised and is basically decided not by the alkaline concentration of the cleaning liquid but by the amount of alkali that remains in the fiber. One of the most effective methods for removing alkali solution is the use of hot water, but in this too the sufficient preservation of the tension of the swollen fiber is very important, and in order to prevent the fibers from becoming brittle, the temperature must be kept below 80°C. Additionally, vacuum dehydration allows the deformation of the swollen fiber due to mangle nip to be avoided while effective cleaning is still carried out.
1.1.g
Drying
After
Mercerization
Fibers in their wet state at the conclusion of mercerization have a very high degree of swelling, and have large internal air gaps. But if the fibers are dried, these gaps will contract as the water evaporates. The final fixed sizes of the gaps inside the fibers are altered by different temperature and tension conditions during the drying. An idea of these changes can be determined by considering the changes in dye absorption rates and in moisture absorption rates. H. Flecker (see above), using Benzopurpurin B4, investigated the changes in the degree of absorption as relating to the alkaline concentration (Fig. 20), the degree of tension (Fig. 21), and the type of drying after mercerization. Fig. 20 Alkaline concentration versus the absorption of dyestuff
Fig. 21 Tension during mercerization versus the absorption of dyestuff
Fig. 22 Drying conditions after mercerization versus the absorption of dyestuff According to his investigations, a relationship does of course exist between variations in the cellulose micelle's degree of swelling and the amount of dyestuff absorbed as determined by alkaline concentration, and the air gaps in the fiber can vary in size up to 20% according to the level of tension that is applied. That is, in comparison to untreated fiber, absorption of dyestuff is twice as high after mercerization, and 2.4 times as high after tensionless mercerization. Furthermore, the absorption of dyestuff is reduced by one third after natural drying in air, and by nearly one half after drying at 110°C in comparison with non-dried fiber, which suggests the contraction of the air gaps inside the fiber. The repeated hydration and dehydration of the cellulose fibers results in the contraction of the micelles' air gaps, and while the reduction of the hygroscopicity and the amount of dyestuff absorbed are well known, the influence on cellulose fiber and cellophane is especially severe immediately after mercerization when the internal structure has a low level of crystallization. According to L.E. Hessler (Tex. Res. J., 24, p.822, 1954), the level of crystallization of cotton before mercerization was 89%, while after mercerization it was 64%, and that of viscose rayon was 45%. The results were particularly severe in cellulose having a low level of crystallization such as cellophane (see Fig. 23).
Fig. 23 The effect of preprocessing on cellophane's moisture absorption In addition, the results of Coward and Spencer (J.T.I., 14. p.32, 1923), who measured changes in the amount of absorbed water using a centrifuge, are shown in Table 2. Table 2: Changes in the amount of water absorbed due to conditions of washing and drying following mercerization
From the above, it can be seen that the size of the air gaps in the cellulose's micelles, that is, the fiber's internal volumetric capacity, is altered by the tension and temperature conditions during the rinsing and drying at the conclusion of the mercerization process which, in addition to altering the hygroscopic characteristics and producing variations in the absorption and reactivity of the dye, also significantly influence the hand of the fabric. Despite these factors, items lacking mercerization are of course inferior to mercerized items, and the drying which follows mercerization is best done at a low temperature under tensionless conditions, drying methods involving an ironing effect as with a cylinder drying machine being best avoided. These points in particular should be kept in
mind
during
the
1.1.h
dyeing
and
finishing
Types
of
of
mercerized
products.
Mercerization
Mercerization is widely used, and in the mercerization of different kinds of cellulose products, including blended products, the machine used and the treatment conditions must be selected in accordance with the type of fiber, the form that it is in and its properties, and also in accordance with the aims and the timing of the mercerization. The wide range of treatment methods can approximately be broken down into the following divisions. Parentheses denote established terminology. 1.
Classification
according
to
the
form
of
a) Yarn
the
product
mercerization
Batch :
Hank mercerization
Cheese mercerization
Continuous :
Single end mercerization
Tow mercerization
Warp mercerization
b) Knit Mercerization
Open mercerization
Closed mercerization (Round mercerization, tubular knit mercerization)
c) Cloth mercerization
2.
Chainless mercerization (Roller mercerization)
Chain mercerization (Stenter mercerization)
Batch-up mercerization Classification
a) Water content
according
to
the
marcerizing
conditions
Dry mercerization
Wet mercerization
b) Tension
Fixed-length mercerization
Tension mercerization
Tensionless mercerization
c) Alkaline concentration
Low-concentration alkaline mercerization
High-concentration alkaline mercerization
Two-step mercerization
d) Temperature
Ambient-temperature mercerization
High-temperature mercerization
Low-temperature mercerization
3. Classification according to timing
Gray mercerization
Pre-dyeing mercerization
Post-dyeing mercerization
4. Classification according to the number of treatments
Single mercerization
Double mercerization
5. Classification according to the type of alkali used
Caustic soda mercerization
Ammonia mercerization
6. Other
Alkali pad-dry method
Alkali pad-steam method
While other variations are also used, mercerization in industry is generally implemented according to a combination of the basic factors as listed above.
1.
Hank
mercerization
This is currently the most commonly used method of mercerization, and generally entails rolling a 54-inch long (the length of one loop) hank weighing about 500g a number of times between two adjustable rollers. The yarn is moved by the turning of the rollers, with penetration of the alkali, application of tension and rinsing occurring automatically. In this, one cycle takes about three to five minutes, and four to eight kilograms can be treated at one time. In the latest machines, all operations are automated, including control of the alkaline solution's concentration and temperature and the addition and recovery of the alkali, along with application of tension on the yarn and rinsing. The only manual operation is the paying in and out of the yarn, meaning that the procedure can be implemented with a high degree of effectiveness. If, in order to increase the level of efficiency, the length of the hank is increased, handling becomes difficult and if the weight of the hank is increased, the changes in length during the shrinkage and extension of the internal and external sections of the hank can differ, and variations in the length of the yarn in each loop can increase due to disarrangement of the yarn, which can all result in uneven mercerization. An advantage of hank mercerization is that during the yarn's absorption of the alkali, treatment can be carried out without tension, and so the alkali solution is able to penetrate the inside of the yarn sufficiently, and after the fibers have swelled sufficiently, any level of tension can be applied and the yarn rinsed for removal of the alkali. This allows the production of goods with satisfactory mercerization effects. However, if the winding or the handling of the hank is inappropriate, disarrangement of the lengths of yarn in one loop can result in different tensions, leading to uneven mercerization, which can often result in patchy dyeing. Uneven mercerizing due to uneven tension is an unavoidable problem in current methods of mercerization. While in theory reduction of the amount of yarn in one hank increases the evenness of the mercerization, this not only reduces productivity, but also results in many yarn-piecing defects during production of the weave or knit due to inadequate yarn length. For these reasons, using yarn that has been hank mercerized and then dyed for
finishing into solid-color fabrics can result in a barre effect, preventing the fabric's use in a product, and so hank mercerization of pre-dyed yarn is mainly used for products with narrow stripes or a checkered design when barre is not noticeable. Another problem in hank mercerization is that conventional rinsing after mercerization is insufficient, and without neutralization through separate rinsing with hot water, the remaining alkali can cause problems. While in theory there are no reasons preventing sufficient neutralization through removal of the alkali in this method, complete treatment of batches in hank form has a low efficiency, and so is not used due to the cost and the level of productivity. Thus, if treatments must be conducted separately, the effectiveness of simple conventional rinsing during the alkali treatment is negated. Furthermore, handling in hank form is necessary in the scouring and bleaching which follow, as well as in the dyeing, and there is a tendency for the quality of the yarn to deteriorate due to disarrangement of the yarn. Finally, after drying, winding from the hank to a cone or cheese is necessary, and the effort and labor hours needed for this are a major disadvantage.
2.
Cheese
mercerization
Carrying out mercerization, scouring, bleaching, and, in some cases, dyeing, along with oiling or sizing, with the yarn in cheese form results in a major rationalization, which can raise productivity and reduce costs. However, mercerization in cheese form can only be expected to achieve halfmercerization, and not the same degree of evenness as hank mercerization or other types of mercerization. This prevents the method from being used beyond a limited number of possible applications. However, this method is considered very valuable in those applications in which it provides a satisfactory degree of quality. One problem is how to limit the difference in shrinkage between the inside and the outside of the cheese. Important factors in this are the adjustment of the twisting and the density of the winding of the yarn, the size of the take-up tube, the thickness of the layers during the winding, and the alkali concentration and temperature during the treatment.
3.
Single-end
mercerization
This method, also called, cone-to-cone, or cheese-to-cheese, involves taking up yarn into a cheese or cone shape, and, with one machine per cone, conducting alkali penetration, rinsing (with hot and cold water), neutralization, rinsing again, and drying in consecutive order with the correct scheduling, and then taking up into a cheese or cone form. The yarn speed in this being approximately 450m/min, the productivity per machine is low, and the equipment costs are high, but because the sequence is automated, it only requires a very small number of staff.
The mechanism for conducting mercerization with these machines involves three revolving rollers: two squeezing rollers which are pressed tightly together and a third roller placed, at a certain distance, more or less parallel to these two. Yarn is lined up in parallel from one end to the other of the third roller, which is removed from the nip space of the two squeezing rollers, and moved in a spiral perpendicular to the roller, during which time the alkali penetrates, tension is applied and rinsing (both with hot and cold water) and neutralization occur. This is basically a form of fixed-length mercerization, and while the tension is not freely adjustable, in addition to adding finely engraved grooves to the surface of the third, slightly separated, roller, the diameter of the center and the two ends of the rollers can be adjusted in response to the shrinkage of the yarn due to the absorption of alkali and can apply tension after the absorption and swelling. However, these factors are also determined by the rollers, and the conditions cannot be changed according to the yarn and the desired effects as in hank-mercerizing, so the quality of the yarn after treatment is limited to a certain range. A problem in this form of mercerization is the relative difficulty of controlling the tension on the yarn as it is introduced, and differences in the level of tension between machines and between cheeses or cones can occur easily and lead to patchy dyeing. Due to this, in addition to giving special attention to tension control, it is important to adopt centralized supervision in order to ensure the same conditions for each machine, including those of alkali concentration and temperature. Other problems relate to yarn breakage, yarn overlap, and yarn skewing. While mechanical supervision is important, the yarn count and quality of the yarn undergoing treatment also has a large influence, and so choice of chop number and supervision of quality are very important. In general, this method of treatment requires two-fold yarn with a yarn count less than 60, and it is unsuitable for the treatment of yarn with fine yarn counts higher than this.
4.
Tow
mercerization
Normally, 400 or so yarns are wrapped around a beam or a ball with a warper and 8 to 10 of these beams or balls are set in a stand. Yarn is unreeled from the balls or the beams at the same time and lined up in ropes made with light twisting, which are mercerized continuously in a manner similar to that of roller mercerization of fabric. A large number of threads are gathered together and lightly twisted into a rope-like form in order to prevent the problems that arise as threads break and become entwined on the rollers. However, if too many threads are twisted together, the mercerization may be uneven, but if there are too few, problems can occur when threads break, and so it is important that the number twisted together be appropriate. The equipment used in this method looks like a row of soapers, and each treatment bath is driven separately, tension on the yarn is controlled, and the shrinkage due to
swelling during absorption of the alkali and the level of strain after this can be adjusted freely. This type of equipment can produce a large amount of yarn of consistent quality and so this method is suitable for the production of mercerized yarn for use in knits, and the treatment of fine yarn that is two-fold yarn with a yarn count of around 100-110 is also possible. However, when treating several thousand threads of yarn at once at a rate of around 20m/min, it is difficult to ensure that each yarn is sufficiently mercerized in comparison to the single-end method, and the resulting swelling of the yarn can easily be somewhat greater than that of the single-end method. However, in addition to producing a soft hand in the final product, this type of mercerization is very even, and so it is the best method for attaining level dyeing. One problem concerning the type of equipment used is the separation of the yarns in the rope after it is dried at the conclusion of the mercerization process, and the way in which the yarns are unwound is very important for ensuring the smoothness of the operation.
5.
Warp
mercerization
While tow mercerization involves the treatment of a lot of yarn lined up in rope-form, in warp mercerization yarn is wound onto a beam and fed into a machine with the same system as in a slasher-sizing machine. Mercerization takes place with sheets of separate threads, and the machinery used can be exactly the same as that in tow mercerization. Thus, in the warp-beam method and the tow method, only the handling is different, and aside from measures for achieving penetration, there are no major differences between the two methods. Machines for these methods have been produced for quite some time, and, due to considerations concerning yarn breakage, these machines have been used with the tow method with dozens of yarns at a time. If mercerization can be carried out with the warp-beam method, it can be more rational than the tow method because yarn that has been mercerized from beam to beam can be extracted in beam-form. A problem, however, is that during treatment the breakage of a single thread can lead to major difficulties, and so if the yarn is not of very good even quality, industrial implementation of this method is difficult. A representative example of continuous treatment with a number of gathered threads is the continuous dyeing of indigo denim, but the important factors in this can be learned from treatment in rope-form used in order to avoid problems associated with yarn breakage even when the yarn to be dyed has a thick yarn count of around 10.
1.1.h.2
Knit
Mercerization
1).
Open
Mercerization
This type of mercerization involves treatment of circular knits after they have been opened, and fabric that has had its selvage gummed as required is treated like a weave. This method is used in the mercerization of products with strict shrinkage restrictions in both the vertical and horizontal directions and fabric with motifs in which skewing often occurs. Recently, demand for this kind of mercerization has been increasing, especially in reply to improvements in knitted products. Originally, horizontal stitches in single knits became spiral shaped, and because of this fabrics would naturally have a tendency to twist, the selvage curling when open-cut, thus making fabric difficult to handle. Thus, to prevent these problems from occurring and to conduct treatment continuously in an open state, all types of mechanical means are necessary, and while preventing excessive shrinkage in the horizontal direction and curling of the selvage, immersion in alkali and swelling of the yarn must be carried out. Machine manufacturers have tried various measures to achieve this, including adhesive transfer from one roller to another, use of a roller with a large diameter, meshed engagement with an irregular roller, grooves on the surface of the roll and the use of a screw roll, but mainstream methods involve, in the last phase, attaining the required vertical and horizontal dimensions on a pin stenter, and, while preventing distortion due to sagging, removing the alkali and setting the dimensions. Dimension setting for cotton knits is precise, and important factors in the quality of the product include a residual shrinkage kept below 2-3%, a satisfactory level of elongation and tensile recovery strength, no deformation of the stitches, three dimensional swelling, a soft hand and good luster. Because skewing and uneven stitch density must be avoided, a high degree of technique and supervision are required in mercerization, and even now this technology is not perfect, and different companies continue to conduct committed research.
2).
Closed
Mercerization
(Tubular
Mercerization)
This method involves not opening round knits but mercerizing them in their tubular state. Knits are usually treated as two flat pieces of material which have been laid together. For this, strong tension or pressure applied longitudinally to the folds of the two edges of the long sides of the fabric causes differences in yarn density on both faces of one of the knits, which results in a line of broken stitches on the outside of the fabric and the formation of a concave broken line called an edge mark on the inside of the fabric. To prevent the production of this edge mark, between the swelling of the yarn with alkali and the rinsing to remove the alkali, the control of the tension and adjustment of the nip are of course important, but other methods that prevent the formation of edge marks include introducing air to the inside of the tube at strategic spots due to which the knit expands to a cylindrical shape, setting several round plastic blades or rings with
alterable circumferences at intervals inside the tubular knit, or inserting a bar-shaped frame inside, thus preventing the fixing of the edges during the treatment, and also expanding the width of the fabric. However, when solid material is inserted into the tube in order to increase the width of the fabric, the frictional resistance increases with increases in the width of the fabric, and because a great deal of force is required to move the fabric, there is a limit to the degree to which a fabric can be expanded in the horizontal direction, and even when air is introduced inside, preservation of air pressure above the level of resistance to the expansion of the fabric is limited because of the escape of the air. For these reasons, in closed mercerization, setting the dimensions of the fabric in its entirety is basically very difficult, and there are of course certain constraints in regards to this. However, the result obtained is very good in comparison with products which have not been mercerized in terms of appearance and quality, and with regards to the setting of the dimensions, and so aside from products of especially high quality, if the design of the yarn or of the knitting and the settings of the mercerizing machine are appropriate, the production of goods which satisfactorily meet the requirements of the market is possible even with round mercerization. Among the machines for this type of treatment, machinery which blows air into the knit is considered superior because tension control is simple, and so machinery of this sort has been constructed. Furthermore, even when air is not blown in, air contained in the structure of circular knits themselves collects inside the tube during the moisture extraction process, resulting in a cylindrical shape forming naturally, and so some machines make use of this quality in conducting effective treatment.
1.1.h.3 1)
Cloth
Mercerization
Chainless
mercerization
This method of mercerization running fabric through a number of rollers without the use of a clip stenter is also called roller mercerization. The machine used has a number of stainless rollers, or stainless and rubber rollers, of a relatively-large diameter tiered zigzag in close contact to each other inside a long trough, with the lower tier designed to submerge in alkaline solution for mercerization. The absorption of alkaline solution and fabric swelling take place as fabric sequentially glides through the surface of these rollers, and, although this movement from roller to roller in close contact with them reduces the widthwise contraction to a minimum, the resulting fabric expansion remains within a limited range, thus displaying the mechanism of mercerization at fixed length. A similar device is used for the removal of most alkali following this initial stage of alkali penetration and fabric swelling, and an open-width soaping machine for further removal and neutralization.
Therefore, the machinery required is extremely concise and the cost is low, in comparison with the chain mercerization method described in the following chapter. However, this method is subject to a considerable number of constraints due to inflexible widthwise control over fabric depending on the kind and use. With all cotton and its blends with polyester, in machines of this type there is trouble in the dimension settings of 100% cotton and blends with low polyester content, while blends with high ratios of polyester, even those of a plain structure, there will be no problem since control by heat setting is possible, with only a limited widthwise shrinkage being expected from mercerization. Roller mercerization is not at all suitable, particularly for these sheer plain weaves. This type of machine is widely in use in Europe, in contrast to its scarce usage in Japan.
2)
Chain
mercerization
In order to make up for the shortcomings of the roller mercerizing machine, a clip stenter is used for post-mercerization treatment, in which a widthwise tension is applied then most alkali is showered off the fabric kept on the stenter, followed by thorough alkali removal and neutralization using an open-width washing machine. As for practical machinery, a heavy padding mangle is used for the application of alkaline solution in the 2 dip/2 nip method, with sufficient time allowed for penetration and swelling of the fabric in a timing cylinder, instead of undergoing an operation using so many rollers and so much solution as in roller mercerization, to ensure reduced use of the alkali. Since the chain mercerizing machine operates at an extremely high speed of 120200m/min, a clip stenter is commonly used after two consecutive treatments of alkali application/penetration. It is a device of considerable size, capable of holding, while maintaining a widthwise tension, 70-90m fabric at a speed of 120m/min., or 117-150m at 200/m, so that sufficient time is allowed, approximately 35 seconds for polyester/cotton blends, and 45 seconds for 100% cotton, between the initial application of alkali solution and the subsequent start of showering the alkali of the fabric. Furthermore, thorough removal of the alkali is ensured in this stenter stage, through repeated showering and vacuum treatment. The efficiency of the vacuum treatment will be most influential in the removal of alkali, especially in cases of using heavy cotton weaves, deficiencies in alkali removal makes the showering, even in an increased amount, an ineffective flow over the fabric surface and allows the fabric to be released from the stenter while still immature, resulting not only in incomplete setting of the widthwise dimension but also in fluctuations in the dyeing stages that follow. Moreover, in the case of sheer cotton weaves, sufficient application of alkali solution will be important, since the relationship between controls over tensile strength for the obviation of crease production during the timing cylinder stage, controls over the fabric width on the stenter and the amount of alkaline solution required is extremely delicate.
Considering the points mentioned above, the performance of chain mercerizing machines developed to date seems hardly satisfactory. The removal of the remaining alkali after the stenter stage barely comes into question in terms of the resulting mercerization effect, however, a crucial watershed will be whether the remaining alkali can be reduced to less than 3°Bé before the fabric leaves the stenter.
3)
Batch-up
mercerisation
In this method, an alkaline solution is padded onto fabric which is then rolled up, and when padding is completed the alkali is removed through continuous cold rinsing. Although the use of the method is not common in Japan, a certain degree of application, including in knits, can be found in Western Europe. Despite costs for facilities being remarkably low, it is not an interesting method except for some special cases, as quality management and productivity remain problematic. Still, for the growing cases of carrying out alkali reduction for the polyester side of cotton/polyester blends to achieve both the mercerization of cotton and the alkali reduction of polyester in a single treatment, the application of this cold batch method is particularly interesting as a device that can combine the two separate stages which would otherwise raise facility problems.
Classification Conditions
According
to
the
Mercerizing
a) Classification according to the water content of substrate during treatment There is dry mercerization and wet mercerization, the ‘dry’ and ‘wet’ denoting the state of the substrate, i.e. yarns, knits and weaves, during treatment. Dry mercerization is the method commonly implemented for its stable mercerizing effects and easy management, except for when the wet method is used in order to omit the drying stage required for the dry method after scouring and bleaching of a yarn or weave. This wet mercerization requires, as mentioned previously, not only a sizable cooling device as a measure against the generation of a considerable amount of heat due to the water content of the fabric and yarn used, an alkaline solution of a higher concentration than the dry method and a longer processing time to ensure thorough alkali penetration within the cellulose fiber as well as sufficient swelling of the fiber, but also a thorough control over the alkaline solution to achieve success, especially when dealing with a weave, as there are quite a few flaws in the methods of maintaining a constant concentration and even dispersion of alkali in the treatment bath. Despite the fact that carrying out complete scouring and bleaching primarily proves much better for cellulose fiber in terms of ease of mercerization and the resultant effect,
the norm has been that, because no sizing agent is present in a yarn or knit from which little trouble is expected, gray mercerization is carried out with the addition of an alkali penetrating agent, then scouring and bleaching follow. If the wet method were implemented as frequently as the dry method, scouring and bleaching would naturally be carried out before mercerization for a greater mercerizing effect, improved product quality and reduced cost. As for weaves, because their weaving stage generally includes warp sizing and waxing, carrying out gray mercerization will not produce a good result even with the use of a penetrating agent no matter how powerful it might be, which is why desizing, scouring and bleaching are carried out before mercerization. Even in this case, the fact that many manufacturers carry out mercerization after going through a drying stage indicates how difficult the management of product quality is with the wet method. However, only a limited effect of mercerization can be expected especially in gray yarns and knits, and in order to apply mercerization in producing quality goods or to produce superior mercerization results, smooth operation should be made possible in all cases with the wet method, holding a true, full command over the process.
b)
Classification
according
to
tension
The chief objective of mercerization is the improved luster, which, as described earlier, can be attained by applying tension to fiber while in a swollen and shrunken state, and there are two ways of attaining this state of tension; one is by letting the fiber swell and shrink with the use of alkali while preserving its original length, and the other by letting the fiber contract freely then applying tension before bringing it back to the original length. The former is classified for convenience as ‘fixed-length mercerization’ and the latter ‘tension mercerization’, however, conditions somewhere between the two are most often the case in practical mercerization. Mechanical or operational reasons are usually responsible for this, and it is rare that conscious efforts are made with regard to the resultant effect. Equally, the configuration for the length of time required for maintaining tension is dealt with from the standpoint of meeting the standards of finished products as part of customary commercial requirements, and the industrial case of configurations being set genuinely with regard to the mercerization effect is hardly seen. Because neither configurations for tension application nor dimensions are set in order to maximize the resultant effect, inadequacy is commonly found not only in the quality of the finished product but also in the fabric’s shrinkage and hand. As has been seen so far, tensionless mercerization is a process in which the fiber is freely allowed to swell and shrink due to alkali absorption without any tension being applied, and after the alkali is washed off, dimensions are set, and is a process from which a stretch cotton with around 20% elasticity can be attained, using a fiber which is designed to produce a minimal resistance to contraction in alkali in its yarn and woven/knit state.
Among cotton knits and weaves commercially available nowadays, many have semistretch, if not full-stretch, properties.
c) Classification according to alkaline concentration As mentioned earlier, an alkali concentration higher than 24-26°Bé is required in the treatment liquor under ambient temperature conditions in order to achieve the expected mercerization effect with a concentration around 30°Bé resulting in a higher stability, however, supposing that the amount of alkali absorption by the cellulose fiber is a criterion for the effect of mercerization, any discussion based only on alkaline concentration will be meaningless because the amount of alkali absorbed by the fiber is determined by both the alkaline concentration and the processing temperature. The concentration used in practical industrial mercerization is therefore the same as that of preliminary mercerization, i.e. in the range between 20°Bé and 30°Bé. Though it is natural, in the industrial context, that an alkaline concentration as low as possible is preferred out of concern for production costs, by lowering the processing temperature, an effect which is equivalent to that which results from a high-temp/high-concentration treatment can be attained even with a low alkaline concentration. While the lower limit of concentration has conventionally been 22-24°Bé, a lower concentration of around 20°Bé may be used depending on the kind of weave. All alkaline concentrations within this range can be regarded as the same in terms of the resultant practical mercerization effect and it is the standard range used in mercerization. Equally, mercerization using an alkali solution of an even lower concentration between 14°Bé and 18°Bé is called low-concentration mercerization, in which the usual mercerizing effect cannot be attained. This mercerization process with a low alkaline concentration mainly aims to improve dyestuff absorption in cotton and to produce a supplementary effect of scouring, and with little alteration in the fabric dimensions and improvement in luster being expected, it only requires a simple device, effective in the production of casual wear that takes advantage of cotton’s natural feel and pile superior in softness, and has traditionally been used in post-treatment for the improved hand and dimension-setting of preliminarily mercerized knits. Meanwhile, an alkali solution of a high concentration of 31-32°Bé is often used in mercerization which is carried out at a high temperature ranging from 50-60°C up to 80°C. There is a method of mercerization that takes place in two steps using two solutions of a high and low alkaline concentration, being referred to as BP116553 in Textils Manufactullr; June P-24, 1971. (sic) This two-step mercerization uses initially an alkaline solution of 30-32%, i.e. 36-38°Bé, followed by that of 10%, i.e. 15°Bé, aiming at the creation of a product superior in luster and strength by inducing a higher degree of swelling in the fiber than the standard (onestep) mercerization method, for which the fiber, while being free of tension, is completely transformed into alkali cellulose II in the first step, then stretched to the original length with tension being applied, and sustaining the same state, is allowed to
transform
d)
into
alkali
cellulose
Classification
I
in
a
low-concentration
according
to
alkali
solution.
timing
There are usually three options as to when mercerization can be implemented during the course of dyeing and finishing. There are three different stages at which the process is carried out. Gray mercerization is carried out in the very first stage of the whole process, i.e. normally after singeing while the substrate is still in the loom state, or in other words, a method that deals with gray goods, then there is pre-dyeing mercerization, which occurs before dyeing and after the pretreatment stages of desizing, scouring and bleaching, and lastly post-dyeing mercerization takes place after dyeing, during the finishing of the yarn, weave or knit. As has been stated earlier, gray mercerization is more frequently implemented with yarns and knits, while pre-dyeing mercerization is generally applied in solid-dyed weaves, and post-dyeing mercerization in yarn-dyed products, such as gingham, certain top-end knits and some special high-quality weaves. Post-dyeing mercerization is used for gingham not because it is counted as one of the quality goods that the method deals with, but rather because the use of pre-dyeing mercerization would raise problems in terms of productivity and costs as it is often seen with quality yarn-dyed weaves, which are made of yarns that are mercerized before being dyed and finished. In contrast to this, when post-dyeing mercerization is used for top-end knit products or a certain kind of weave, some particular effects of mercerization are expected from this. Among various reasons that support this application, the first thing that can be pointed out is that the dyeing and finishing process causes deterioration in luster, which is observed when a yarn or weave is mercerized prior to dyeing and finishing. Deterioration in luster due to dyeing and finishing can be attributed to distortion of the yarn, roughening and depredation of the fiber surface caused by dyestuff adhesion and the influence of chemicals used in the dyeing process. Post-dyeing mercerization is therefore the one which can produce a significant difference in the quality of the finished product, such as the luster, the depth and integrity of shade as well as the shape. However, carrying out complete post-dyeing mercerization requires the use of dyestuff with a high fastness to high-concentration alkaline treatments, or otherwise the treatment has to be kept within an extent which does not adversely affect the dyeing result. Naturally, the result of such incomplete mercerization will not be the same as that which is usually expected from post-dyeing mercerization and will be inferior to that of thorough pre-dyeing mercerization, and most gingham products fall into this category. Moreover, whether mercerization is carried out while in the form of a yarn, weave or knit will considerably affect the quality of the final product. That is to say, in the case of yarn mercerization, each single yarn as it is not woven or knit is free of physical restraints and can be mercerized without any change in the shape of its circular cross section as opposed to weave or knit mercerization, in which a yarn can easily become flattened and loose shape, thereby resulting in a product of
inferior quality, because of its state of being tied together and due to mechanical handling. Therefore, in the choice, carrying out yarn dyeing and mercerization prior to weaving or knitting is preferred for achieving a superior quality in the finished product.
e) Classification according to the frequency of treatments As mentioned in the previous chapter, the effect of mercerization on the finished product quality differs considerably according to when the process is carried out, however, a satisfactory quality cannot always be attained simply by timing the process well. There are cases when the fastness of the dyestuff used is inadequate for the process depending on the hue regardless of the fact that carrying out post-dyeing yarn mercerization naturally involves problems in the handling itself and in productivity, and for which nothing but pre-dyeing mercerization is available as an option. Furthermore, as thorough pre-dyeing mercerization inevitably results in the hardening of hand, it is not always the best method in terms of the resultant product quality and hand. In addition, there is another problem in that a product quality as good as that of postdyeing mercerization cannot be attained because, as mentioned earlier, the effect of mercerization deteriorates in the subsequent dyeing process. A method that can cover such shortcomings is double mercerization, in which thorough yarn mercerization is carried out first, followed by dyeing of the yarn then knitting or weaving before going through mercerization again, with the dyeing process sometimes being shifted after the second mercerization. In order to maximize benefits from the effect of the first mercerization and to eliminate the disadvantages involved in it, a lower alkali concentration should be used in the second mercerization under milder conditions than that of the first round. Top-end knit products should require such a handling much more than weaves because of their structural characteristics.
f) Classification according to the type of alkali used Among the kinds of alkali with which the same kind of effect as mercerization is observed through bonding itself with cellulose fiber and making it swell, apart from caustic soda, lithium hydroxide and caustic potash are known. What is notable is that, with lithium hydroxide, an extremely high level of swelling can be observed at a concentration one half that of caustic soda and, as for caustic potash, that the effect of mercerization can be attained without causing damage in rayon fiber, its action being more moderate than caustic soda. However, with the cost of both chemicals being particularly high compared to caustic soda, cases of their application are scarce in contrast to the almost exclusive use of caustic soda. Moreover, this tendency is expected to remain unchanged for a long time to come.
Other than inorganic alkalis, there are also organic amines, quaternary ammonium bases and ammonia with which the same effect of mercerization can be observed, resulting from their action that induces swelling in cellulosic fiber. Of these substances, the industrial application of liquid ammonia has begun, which is generally referred to as ammonia mercerization. Although no effect of mercerization is observed when ammonia is used in a solution or in its gas state, when it is allowed to act on cellulosic fiber while in a liquid form the swelling can be completed within an extremely short time span. Mercerization effects will result through applying tension to the swollen fiber and removing alkali from it. Because the surface tension of liquid ammonia at the ambient temperature is 25.95dyn cm-1, which is smaller than one third of around 90dyn cm-1 in caustic soda mercerization, its wetting and penetrating properties are so extremely good that complete penetration is possible with a half to two thirds of a second of soaking, and the swelling of the cellulose within a few to 15 seconds. This is again one third of the time required in caustic soda mercerization, showing the advantage of being a quick process. However, the boiling point of liquid ammonia being around -33°C, the swelling induced in the cellulosic fiber is quite unstable and the fiber soon returns to its original state due to the vaporization of ammonia even when the temperature is maintained below the boiling point. Here, as in caustic soda mercerization, tension should be applied while in the swollen state, and maintaining the same state, the ammonia has to be removed in order to attain the effect of mercerization. With regard to the environment in which tension is applied, Peabody’s patented method uses a 90% ammonia gas with a 80% ammonia content being maintained during the process and removes the ammonia at first in a saturated ammonia solution at a temperature between -7°C and -10°C, secondly after squeezing and during drying at 28°C and finally during drying at 100°C. Including the use of hot water (88°C) in the Prograde method, these conditions used for the removal of ammonia can considerably affect not only the cost of ammonia recovery but also the result of mercerization, however, details concerning the use of such conditions are not known. When ammonia-mercerized products are compared to their alkali-mercerized counterparts, the former is said to be considerably inferior to the latter in the degree of improvement in dyestuff absorption and insufficient in luster, but superior in the degree of improvement in strength and the durability of dimension stability, as well as in maintaining cotton’s soft hand
Others In addition to the conventional method of mercerization, which consists of soaking in a caustic soda solution, leaving the fabric to swell while tension is applied and rinsing off the alkali, various other methods have been released with modifications made in order
to
enhance
the
resultant
effect.
Typical among those are Babcock’s Mercevic (sic) method and Sandoz’s SM method (Spannrahmen Mercerisation; stenter mercerization), the latter being said to be suitable for the mercerization of knits. Though, as stated earlier, the use of stenters has begun in the mercerization of knits in order to properly align knit stitches, what is different about the SM method, consisting of the same process from soaking in alkali for free contraction to pinning on a stenter for stitch alignment as the conventional method, is that, after those stages, the alkali-soaked knit fabric is dried whilst still on the stenter. In contrast to conventional methods in which the alkali is removed through showering after those stages, in the SM method, the fabric is removed from the stenter after being dried on it, then the removal of alkali and neutralization takes place in the usual way before the fabric is dried again while its width is adjusted on the stenter. Although details are not known as to what this interim drying process after the addition of alkali signifies in the SM method, it can be assumed that the operation from soaking in alkali to pinning would become easier through the application of a relatively high pickup rate and a weak alkaline solution, as well as the aligning of knit stitches on the stenter, and since the alkaline solution is concentrated during the drying process, it would result in high-concentration mercerization. It is conceivable that interesting results can be obtained from this, given that appropriate conditions are set to minimize deterioration in cellulose. Meanwhile, there is another method that can be called the pad-steam method, in which the fabric is fed into a normal or high-pressure steamer after the padding of the alkaline solution, thereby simultaneously carrying out mercerization and scouring, or thereby preventing the hardening of fabric hand due to mercerization. This pad-steam method is the opposite of high-temperature mercerization so to speak. Whereas high-temp mercerization takes an approach of cooling the fabric after allowing some time for high-temp/high-concentration alkali soaking to improve the resultant effect, in this pad-steam method, quick steaming is carried out in saturated, normal or high-pressure steam of around 5kg/cm2 after the adding of a relatively strong alkaline solution of about the concentration of that used in high-temp mercerization under ambient temperatures. In this, high-pressure steaming is said to be particularly effective. Generally speaking, the higher the temperature of the alkaline treatment, the poorer and the less sufficient the effect of mercerization results, however, it is assumed that, from carrying out high-pressure steaming, no degradation is expected in luster because water itself induces swelling in cotton, as well as that products with a soft hand can be obtained. Of course, there is no question that scouring effects can also be obtained by going through this process. Unlike the conventional methods that simply control the fabric tension under a single set of conditions with the alkaline concentration and process
temperature maintained at a fixed level, these new processes are a manifestation of efforts towards attaining more rational, multi-purpose effects through breaking down the mechanism of every action in the process of mercerization and deliberately altering the alkaline concentration, temperature or the moisture content (which automatically leads to alteration in the alkali concentration) during processing, in order to realize optimum conditions and to produce optimum effects. At present, these methods cannot be said to have been perfected sufficiently, nor are they widespread generally. One reason for this is that such changes during processing would cost considerably in terms of facilities and energy use, moreover, another considerable factor is that no technology has yet been established to make possible these conditions, or for them to take effect. However, the use of different methods to those existing at present is expected to spread if more emphasis is given to mercerization, with more attention paid to it not as something from the past but as a technology for the future. Read more: http://textilelearner.blogspot.com/2013/06/mercerization-process-of-cotton-fabric.html#ixzz3dOXPKPk3
5. Mercerization
1844 - John mercer observed the effect. 1850 – Filed a patent for treatment of cotton with concen. NaOH solution with improvement in strength & dyeability. 1890 – Horace Lowe; found if shrinkage is controlled, the fabric becomes lustrous. Tension Mercerization (Lowe) & Slack Mercerization (Mercer)
Mercerization is carried out for the following purpose.
1.
Improving lustre (silky look, in tension mercerization only).
2.
Improving tensile strength.
3.
Improved dimensional stability.
4.
Increasing uniformity of dyeing & improving color yield (savings in dyestuff).
5.
Improving Elasticity or Stretch material (in slack mercerization only).
Mercerization is a process of impregnating the textile material with a concentrated solution of cold NaOH, keeping it in contact with this cold solution for a given time with or without tension, and subsequently rinsing it.
MECHANISMMercerization causes swelling in fibre Swelling causes Cross-section to become rounder, loss of convolutions & detwisting leading to more lustrous surface Opening of fibre structure Increase in amorphous content due to decrystallization Although mercerization is accompanied by swelling, it is more closely related to internal modification of fiber. Effect is observed at 18-24 % w/w NaOH solutions. Higher no of –OH groups available as compared to un-mercerized cotton Higher moisture regain, dye-uptake and reactivity The effect of mercerization is related to the size of the sodium hydroxide hydrates (NaOH. xH O). The concentration of NaOH solution determines the value of x or the size of the hydrate. A dilute solution has larger sized hydrates, which can cause swelling in cotton fibre by entering 2
the amorphous phase of the fibre. At higher concentrations, the hydrate volume is smaller. Such hydrates cause lower swelling but have the ability to penetrate the crystalline phase and change the crystal structure. Such changes are capable of inducing important structural modifications in fibre structure.
In the table given below, the composition of NaOH hydrate, corresponding NaOH concentration and the type of hydrate formation are shown:
Sodium Hydroxide Hydrates: Composition, Type and Concentration of Maximum Frequency Composition of hydrate
NaOH Conc. (% by Wt.)
Type of hydrate
NaOH.20H O
6-9
Hydrated ion pair
NaOH.12H O
13.5-15
Hydrated ion pair
NaOH.10H O
18
Solvated dipole hydrate
NaOH.H O
69
Dipole hydrate
NaOH.7H O
22.8-24.1
Solvated Dipole hydrate + D.H. (7.4 A°)
NaOH.5H O
30.2-30.9
Dipole hydrate
NaOH.4H O
34.8-35
Dipole hydrate
NaOH.3.5H O
38-38.8
Dipole hydrate
NaOH.3.1H O
41.8-42.6
Dipole hydrate (Dia 6 A°)
NaOH.2H O
52.6
Dipole hydrate
2
2
2
2
2
2
2
2
2
2
In the table given below, the variation in the hydrodynamic volume with changing hydrate type is given. Composition of hydrate
Hydrodynamic volume (dia Å)
Dipole Hydrate
5-8
Solvated dipole hydrate
8-10
Hydrated ion pair
10-15
Separated Hydrated ion pair 15-20 When a sodium hydroxide solution of a given concentration is used for treatment of a cellulosic fabric (cotton), it forms a ternary complex, the composition of which is governed by the concentration of NaOH solution. This complex is known as soda cellulose. These soda celluloses have well defined crystalline character which is verified from their x-ray diffraction
patterns.
Given below is the phase diagram of soda celluloses.
Phase diagram of soda celluloses
In the figure given above, it can be seen that soda celluloses of defined x-ray diffraction pattern are formed on treatment with NaOH solutions of different concentrations at different temperatures. Although it seems that there are well defined boundaries for formation of various soda celluloses, in reality it is not the case.
Na-Cell I forms when cotton or ramie is treated with 12-19% (w/w) NaOH solutions at 20°C. Decomposition of Na-Cell I leads to cellulose II formation. At higher concentrations, Na-Cell II is formed. Drying of Na-Cell I results in formation of Na-Cell III.
Washing of Na-Cell I and II results in formation of Na-Cell IV. In fact, Na-Cell IV is only a cellulose hydrate as washing removes the alkali and decomposes the ternary complex. In a way it is a swollen form of cellulose.
At low temperatures, Na-Cell V is formed. During washing of Na-Cell V, Na-Cell IV is formed first and then Cellulose II is formed.
In the table given below, composition of Na-Celluloses and corresponding hydrates is given.
Soda Celluloses: Composition and corresponding sodium Hydroxide hydrates Soda celulose type
Comoposition
Corresponding hydrate
C H O , NaOH, 3H O 6
10
5
2
NaOH. 5H O 2
NaOH. 7H O 2
Na-Cell I NaOH. 10H O 2
(C H O ) , NaOH. 2H O 6
10
5
2
2
NaOH. 12H O 2
(C H O ) , NaOH. 1.5H O 6
Na-Cell II
10
5
2
2
C H O , NaOH. H O 6
10
5
NaOH. 3-3.5H O
2
2
NaOH. 2H O 2
Na-Cell III
C H O , NaOH. 2H O 6
10
5
NaOH. 4H O
2
2
CH O , HO 6
10
5
2
C H O , 0-0.3NaOH. H O 6
10
5
2
Na-Cell IV (Q)
NaOH. 20H O 2
NaOH. 12H O 2
C H O , 1.3 or 1.5NaOH. 2H O 6
10
5
2
Na-Cell V C H O , NaOH. 4-5H O 6
10
5
2
NaOH. 5H O 2
NaOH. 7H O 2
What are Cellulose I and Cellulose II?
The native form of cellulose which occurs in cotton and other natural cellulosic fibres, is known as cellulose I. It has it unique crystal diffraction pattern (monoclinic). It is a thermodynamically less stable form of cellulose. When cellulose in converted to regenerated cellulosic fibres like viscose, dissolution of cellulose is an intermediate step. During regeneration, it gets converted to a different crystalline form, known as Cellulose II.
During mercerization also, some native cellulose gets converted to cellulose II. The extent of this conversion depends on process conditions like temperature, tension, NaOH concentration etc.
In the table given below, the dimensions of unit cell of cellulose I and II are given:
Crystal form
Dimension a (Å)
Dimension b (Å )
Dimension c (Å )
Β (degrees)
Cellulose I
8.35
10.30
7.9
84
Cellulose II
8.14
10.3
9.14
62
The figure below arrangement of cellulosic chains in crystalline register in cellulose I and II respectively. It is easy to see that the formation of intra and inter chain hydrogen bond formation in the two crystalline forms.
Swelling of cellulose fibres in Alkali solutions
It must be clear that mercerization involves disruption of crystalline part of cellulose. For this to happen, alkali solutions of sufficiently high concentrations which can form alkali hydrates of such sizes which can enter the crystalline phase must be employed. Swelling occurs during mercerization but it is not the sufficient condition for mercerization to take place.
This will be clear from the figure shown below which shows swelling of cotton fibres with different alkali concentrations at different temperatures.
Cellulose--model films and the fundamental approach. Eero Kontturi, Tekla Tammelin, Monika Osterberg Chemical Society Reviews , 35(12):1287-304: 2007;
Swelling of cotton fibres at different alkali concentration and temperature
In the figure, the first sharp peak due to swelling appears at around 10% NaOH concentration. This could be very high at low temperatures. In fact, if viscose was to be treated with 10% NaOH solution at 0 °C, it will dissolve!
The swelling occurring at this concentration is solely due to amorphous swelling and does not result in any of the effects associated with mercerization. It is temporary in nature, but can cause large shrinkage due to high degree of swelling involved.
At higher concentrations, the swelling goes down and increases again at around 30% concentration. This swelling is caused by disruption of crystalline phase of cellulose. Generally the mercerization is carried out at 18-24% NaOH concentration and the swelling caused is much less than that occurring at 9-10% NaOH concentration. Hence it can be concluded that swelling is necessary but not the sufficient condition for mercerization to occur.
The effect of mercerization on various physical and other properties of cotton is discussed in following sections.
Structural Modification
Effect of NaOH Concentration
It has been discussed that mercerization leads to conversion of cellulose I to cellulose II. Native cellulose is Cellulose I and regenerated cellulose (example – viscose) is cellulose II. However, during mercerization, conversion of cellulose I to cellulose II is only partial. Hence mercerized cotton is a combination of both crystalline forms. The extent of conversion is dependent on various process parameters like concentration of NaOH, time of treatment, temperature and tension employed in the process.
In the figure given below, the effect of NaOH concentration on crystallinity of cotton is given (time 60 sec, temp 20 °C, tension- to maintain length change=0%)
Effect of alkali concentration of crystallinity of cotton
From the figure it can be inferred that the formation of cellulose II is favoured at higher alkali concentrations. The overall crystallinity of cotton also goes down slightly on mercerization.
Effect of temperature
Temperature seems to have a similar effect on conversion of cellulose I to cellulose II. Overall crystallinity also goes down. This is shown in the figure below:
Effect of temperature on crystallinity of cotton (time 60 sec, temp 20 °C, tension- to maintain length change=0%)
Enhancement in lustre
Cotton owes its non lustrous appearance to its non uniform twisted, convoluted, bean shaped fire cross section. As there is continuous change in the fibre cross section along its length, it has poor specular reflection which is responsible for high lustre. On mercerization, the fibre swells gradually and the false twist and the convolutions gradually disappear. The cross section becomes almost circular and the fibre acquires the shape of a regular uniform cylinder. This process of gradual change in the fibre cross section
is depicted in figure below:
Gradual change in cross section of cotton fibre on mercerization
In the above figure, stage 1-5 show swelling of cross section of fibre. It is easy to see conversion of a bean shaped flat cross section to almost round shape and gradual disappearance of lumen. Stage 6 and 7 are associated with removal of NaOH from fibre on washing and drying. It is important to note that on loss of sodium hydroxide during washing and subsequent drying, the fibre shrinks in cross sectional area, but maintains its rounder shape. Overall, there is very little gain in fibre volume due to mercerization, and hence the density of the fibre remains unaffected.
The rounder, more uniform (convolution-less) and regular cross section results in more specular reflection which results in higher lustre. However, it must be noted that the process of removal of convolutions and other surface irregularities (wrinkles and creases) is greatly aided by tension. Hence tension mercerization always results in more lustrous product as compared to a slack mercerized one.
The presence of protruding fibre ends on the yarn surface suppresses lustre. Hence cotton yarns meant to provide high lustre must also be singed in order to maximize lustre.
Mechanical properties
Mercerization has been found to increase the tensile strength of cotton fibres even though a drop in overall crystallinity is recorded. The main reason for this phenomenon to occur is swelling of fibres. The presence of many convolutions and false twists in the fibres cause weak spots to be present. These weak spots act as zones of stress concentrations due to lower cross sectional area on application of a tensile stress. Swelling, which is an integral part of mercerization process, removes convolutions and false twists and the fibres acquire a more uniform cross section along its length. With weak spots gone, the fibres exhibit enhanced tensile strength as compared to a non-mercerized fibre.
Increase in tensile strength can be summarized as:
Removal of convolutions results in removal of weak spots at the point of reversal
Fibres have more uniform, circular and smoother cross section after mercerization
The fibrills are arranged parallel along the fibre axis
Fibre alignment along fibre axis is better in case of tension mercerization as compared to slack mercerization
Effect of tension
Mercerization has a significant effect on mechanical properties of cotton. In the figure given below, tenacity, breaking elongation and Young’s modulus of cotton mercerized with 300 gpl NaOH solution at 20 °C for 60 sec are plotted. The tension is varied to allow 10% shrinkage to 4% stretch. The elongation and modulus are indicated on secondary vertical axis. It is clear that the enhancement in both tenacity and modulus is higher at higher tension. It is easy to visualize this since it results in better alignment of molecular chains in fibre axis direction (higher orientation). However, in case of elongation, it is reverse. With increasing tension, the elongation at break decreases monotonically. It is to be expected as increasing orientation would lead to lower elongation. At negative tension though (shrinkage occurring) the breaking elongation is higher than that of unmercerized cotton.
Hence it can be concluded that if improving the tenacity of cotton yarn is the main objective, mercerization should be carried out under tension.
Effect of Alkali concentration
The effect of alkali concentration on tenacity, breaking elongation and Young’s modulus of cotton mercerized with NaOH solution of variable concentration at 20 °C for 60 sec at constant length are plotted in the figure given below. The alkali concentration is varied from 100 to 300 gpl. Again, the elongation and modulus are indicated on secondary vertical axis.
Effect of Tension on Mechanical Properties of Cotton Yarn during Mercerization
Effect of NaOH conc. on Mechanical Properties of Cotton Yarn Mercerized at Constant Length
Dyeing Properties
Since mercerization results in change in physical structure of cotton, as manifest in change in crystal structure, crystallinity and lustre, it affects the dyeing behavior of cotton. The important changes that cause this are:
Mercerization decreases the crystallinity (increases amorphous content) Modifies the texture Overall effect – greater moisture absorption, greater reactivity towards chemical reagents and dye molecules
Mercerized cotton has higher
Dye absorption ( dye pickup)
Rate of dyeing Visual colour yield as compared to unmercerized cotton Improvement is more marked in slack mercerization (lower tension)
Increase in colour yield can allow for a saving of 15-50% in dyestuffs. Indeed many times, this is the reason for subjecting cotton goods to mercerization.
Interesting - mercerized cotton appears darker than unmercerized cotton even when the dye absorption is same.
The effect of mercerization on dye absorption and apparent color yield is shown in the figure below. The concentration of sodium hydroxide is varied from 150 to 350 gpl.
Effect of Mercerization on Dyeing Properties of Cotton Yarn at constant length (dye- CI Direct blue 1)
The increased Visual color yield (darker shade) of mercerized cotton is due to increased dye absorption as well as the optical effect.
Kubelka Munk Equation
Defines the colour yield in terms of reflectance of dyed material
K – Light absorption factor, S – light scattering factor, C is concentration and a is a constant
R - reflectance
K changes linearly with concentration (within some limits)
Upon mercerization
1.
S decreases (less internal scattering from a rounder x-section)
2.
Non uniformity of the operation causes increase in colour yield (apparent)
Non-uniformity of the mercerization results in peripheral fibres absorbing most of the dye. This may cause the cotton goods to appear darker in shade.
If improving the dyeability is the only aim then 150-190 gpl NaOH treatment is ideal.
In general, the increased dye absorption and the optical effect may contribute roughly equally towards increased Visual colour yield after mercerization.
Wetting agents for mercerization:
Mercerisation liquor of concentrated NaOH solutions is generally applied at low temperatures (15-20 °C). Under these conditions, liquors possess high viscosity and swelling of fibers is high and rapid. This results in poor and non uniform penetration of cotton textiles by NaOH solutions.
Sometimes goods are mercerized in greige or non-scoured state. The presence of sizes or cotton waxes
further hinders penetration of NaOH in the cotton yarns/fabric. Therefore the use of wetting agents to increase the wetting of goods by mercerizing liquor is resorted to, which results in faster and more uniform treatment of cotton goods.
Since the liquor is highly alkaline, the prospective wetting agent must be:
Soluble and stable in alkali solutions of mercerizing strength
Should be efficient (effective at low concentration)
Should have low affinity to fiber
Should be low foaming
Normal wetting agents are not sufficiently soluble in moderately concentrated caustic soda solution. Products based on cresylic acid (a mixture of o-, m- and p-cresols) were used sometime back. However these are toxic and non-biodegradable in nature and its use in mercerization in on wane. Addition of solvent additives such as alcohols (e.g. cyclohexanol), ethers, ketones, lower fatty acid amides increases wetting power of mercerizing liquor.
The biodegradable wetting agents
Certain alkylarylsulphonates some organophosphonates (e.g. sodium methyloctylphosphonate) alkylated diphenyloxide sulphates (e.g. dodecyldiphenyloxide sulphate) especially sulphated aliphatic alcohols (e.g. 2-ethylhexyl sulphate). most effective have chains with 4-8 carbons Example: sulphated 2-ethylhexanol, activated with about 10% butanol and unsulphated 2ethylhexanol. Hot mercerization
Although mercerization is an exothermic process and is more efficient at lower temperatures (typically a temperature range of 15 to 20 °C is used), it has following drawbacks:
At lower temperatures, NaOH solution of mercerizing strength has high viscosity and surface tension
When a yarn/fabric is mercerized, swelling of fibres at yarn surface due to mercerization causes compaction of yarn structure
This results in poor penetration of NaOH solution in the core of yarn
The treatment becomes non-uniform
The basic problem of poor penetration leading to non uniform mercerization can be tackled by carrying out mercerization with hot (60 to 100 °C) NaOH solution of mercerizing strength. This is known as hot mercerization.
However, since mercerization won’t take place at high temperature, the temperature of the treated fabric has to be brought down.
Therefore, hot mercerization becomes a two stage process, in which the first stage is treatment of cotton fabric with hot NaOH solution of mercerization strength and the second stage is the reduction in temperature of the treated fabric to bring about the mercerization.
As the temperature of the fabric is brought down, swelling may start and the fabric may shrink and hence the dimensions may need to be maintained during cooling stage by desired applied tension.
Advantages:
Hot NaOH solution has low viscosity and higher penetration power Swelling of cotton fibres in hot NaOH solution is low, hence more uniform treatment Faster process, leading to: higher productivity and compact unit Period of contact between material and NaOH can be reduced by upto 50% The fabric can be stretched to greater degree as the fabric becomes more plastic More uniform coloration Better dye uptake (less colour yield) Can permit elimination of scouring process However, under conditions prevailing in hot mercerization, degradation of cellulose at high temperatures in
presence of air can take place.
The sequence
Saturation of cotton with NaOH under relaxed conditions (50 oC to boiling point) for 4-20 seconds Controlled hot stretching Cooling the stretched material to less than 25 °C temperature Tension controlled washing (till NaOH concentration in fabric falls below 60 gpl level) Final washing under relaxed conditions Since NaOH is also an agent for carrying out scouring of cotton fibre, yarn/fabrics at high temperatures, hot mercerization can also be suitably modified to carry out both mercerization and scouring in one operation.
Sequence for combined mercerization and scouring
Hot saturation of fabric with alkali solution Steaming of relaxed or stretched material for 10 min at atmospheric pressure 5 sec under pressure (130-140 °C) Cooling and mercerization Washing There is another variation of hot mercerization. The sequence is as follows:
Washing of fabric at 95 °C Hot squeezing (high speed steam injection) Impregnation in NaOH at 20 °C Impregnation in NaOH at 30 °C Stabilization under tension Washing The advantages of this variant are:
More uniform treatment faster process, hence process time can be reduced
More productivity or more compact unit Higher stretching is possible; this will result in higher mechanical strength/lustre Higher stretch is possible if stretching proceeds while cooling Uniform dyeability Dyeuptake is better but colour yield is lower
Mercerization of blends
Sometimes blends of cotton and viscose need to be mercerized to bring dyeuptake and lustre of cotton at par with that of viscose
Problems faced in mercerization of cotton/viscose blends
Solubility of viscose in alkaline solutions causes weight and strength loss
Dissolution occurs at 80-130 gpl concentration of NaOH
(Viscose becomes highly swollen)
This problem can be solved by using a modifying sequence of mercerization as described below:
Hot impregnation (saturation with hot mercerization liquor to minimize swelling of viscose) Addition of electrolyte in the mercerizing liquor Use of KOH or admixture of NaOH and KOH (KOH induces less swelling) HOT water rinse (at 90 °C); viscose solubility is less at higher temperature Addition of electrolyte (NaCl) in rinsing bath (electrolyte suppresses swelling of viscose) Machines for Mercerization:
Fabric Mercerization with Tension
1. Stenter type or chain mercerizing machine
2. Chainless mercerizing machine 3. Chainless and pad less mercerizing machine
Hank Mercerizing Machine
Yarn mercerization
Yarns are mostly mercerized in hank form. Yarn mercerizing machines generally consist of two rollers, on which the hank can be mounted. A device can be used to apply tension (to stretch) on the hanks. After the hanks are mounted, the rollers can be lowered into a tray which consists of caustic soda solutions of mercerizing strength. The hanks are allowed to saturate with alkali, the tension is generally applied later. After sufficient time is allowed for mercerization (1-2 min) the hanks are squeezed and rinsed. The tension is released once the residual alkali concentration drops below 60 gpl. For optimum gain in tensile strength and lusture, a tension causing stretch of 0.5-3% is applied, higher tension results in lower tensile strength.
Fabric mercerizing machines:
Fabric mercerizing machines are generally continuous operation machines with the ability to apply controlled lengthwise tension. There are mainly two types of machines:
1.
Those equipped with a chain to apply a controlled width wise tension (Pad chain mercerizing machine)
2.
Chainless machines, which apply the width wise tension by means of expander bars or bowed roller (Chainless mercerizing machine)
Pad chain mercerizing machine
In these machines fabric is saturated with alkali solution and padded. The saturated fabric passes over a set of rollers (airing rollers) which allows the swelling or mercerisation to take place. Fabric is again saturated and squeezed and passed into a stenter where length and width wise tension is applied. Width wise tension is applied using stenter clips on chain.
This is followed by sprinkling of fabric with water to remove alkali. The tension is maintained till the alkali concentration drops below 60 gpl. After this, fabric can be rinsed and neutralized in open width washing machine (Tension may be relaxed).
The tension applied by means of clips in weft direction can be higher which can impart higher lusture to the fabric. However the tension in width wise direction can cause the fabric to expand in a non uniform way, as shown in the figure below. Also there is a risk of fabric getting torn near the edges where it is held by clips.
Chainless mercerizing machine tackle above problem by using bowed rollers (figure below) to apply width wise tension. The clips of chain mercerization machine are replaced by an expander unit consisting of bowed rollers. The stretch or tension applied in weft direction is determined by warp wise tension in the fabric which in turn is controlled by oscillating rollers. Other features are more or less similar to chain mercerization units.
Determination of degree of mercerization:
Although there are many methods to do this, one quantitative test based on the ability of mercerized cotton to absorb barium hydroxide is widely used. Mercerized cotton can absorb more Barium hydroxide than un-mercerized cotton and this is the basis for this test. The test method is described below:
Mercerized and un-mercerized cotton samples (2 gm each) are treated with 30 ml of N/4 Barium hydroxide for two hours in conical flasks at room temperature. Preferential absorption of barium hydroxide by cotton samples reduces the strength of Barium Hydroxide solutions. The amount of Ba(OH) absorbed can be determined by taking 10 ml of Ba(OH) solution from each of the flasks and titrating it with N/10 HCl, 2
2
phenolphthalein being used as an indicator. Fresh Ba(OH) is also titrated (blank titration) to accurately assess the actual Ba(OH) in solution. 2
2
The Barium Activity Number is given as:
BAN = (B-M) / (B-C)*100
Where B = Volume of HCl (ml) required for blank titration
M = Volume of HCl (ml) required with mercerized cotton
C= Volume of HCl (ml) required with un-mercerized cotton
For completely mercerized cotton the value of BAN is around 155 and for semi mercerized cotton it varies in between 115 and 130.
Causticizing Quite similar to the mercerization process as introduced by Mercer, that is, without tension, does not cause appreciable causticizing collection of material fact in some states there is even a slight lengthening due to absorption of caustic bath .
The causticizing is a treatment of textile finishing slightly higher intensity for the purge yarns and fabrics, it is useful to clean the cotton fiber, and give a slight improvement in dimensional stability, is used to particular to improve the dyeing affinity. Normally using concentrations of 8 to 9 ° Bé where you get the higher elongation of the fiber. Up to a concentration of 9.5 ° Bé causticizing considering the caustic treatment carried out, as it does not change the physical state and the convolution of the fiber.
Causticization also known as semi-mercerization is the process of treatment of cotton with caustic soda concentration varying from 11%-16%. The causticization differs from mercerization in two aspects viz concentration of caustic soda used is lower than that of mercerization and no tension applied during causticization. The process is usually carried out after scouring or after bleaching for the limited purpose to improve the dye uptake of fabric. The conventional operation of causticization is a separate treatment of dried fabric after scouring or bleaching involving the additional operations like drying before causticization, washing for the removal of akali followed by drying before dyeing. This adds to cost and extended process time resulting in loss of productivity. In the present investigation, a process of
causticization of cotton in grey stage by padding or treatment on jigger with 14% (W/V) caustic soda followed by two stage combined desizing, scouring, and bleaching and dyeing without drying on jigger is suggested for reduction in cost and higher productivity. The results showed complete removal of starch, instantaneous water drop absorption and good whiteness. The causticization of grey fabric showed improvement in dye uptake of selected direct, reactive and vat dyes to an extent of 24% and 55% compared to uncausticized cotton fabric. There was no loss of tensile strength after the causticization and combined pre- treatment, however causticization gave increase in % extension. The work reported is of technical nature of industrial importance.
