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¢ Necessity of Warp Sizing During the weaving process, the yarns are subjected to three basic physical stresses. These are stretch, strain and abrasion. Although these forces exist in varying proportions depending upon the type of loom and the fabric styling, all three are forces that must be considered in all cases. Therefore, the ideal sizing material would produce a smooth, tough, elastic film which would adhere to the yarn. It should smooth to friction and abrasion. It should be tough to endure the load or strain and it should be elastic to allow flexibility and sufficient stretch. Advantage of using Polyvinyl Alcohol in Warp Sizing PVA is an excellent film provider. Its tough film is easily removed (desized) with hot water. It leads to the following benefits: -Superior Abrasion resistance - Adhesion to synthetic fibres -flexibility/ elongation -strength - user friendly slashing performance. Yarn sized with PVA can run at lower add-on because of the adhesion and strength advantage PVA provides over natural binders. It requires in quantity only 1/3 rd of the starch. The excellent abrasion resistance means less shedding on the slasher and in the weave room. The inherent flexibility of films of PVA resins eliminates the need for high Relative humidity in the weave room. A RH of 65-75% are recommended PVA solutions are thermally stable and can be maintained for lower periods of time at high temperature. PVA are widely reclaimed and reused for sizing, thus reducing effluent levels from the finishing plant. PVA and wax together will provide the optimal size performance. Wax is needed to reduce dryer can sticking, weaker film for easier split, minimize clinging on looms and improved lubrication for the size coating. However, excessive use of waxes can lead to poor adhesion, brittleness, roughness and decreased abrasion resistance. Also waxes can be difficult to remove at desizing stage which can lead to quality problems in desized fabrics. It is important to select a wax that contains an effective emulsifier. It will act to prevent wax from redepositing back on the fabric during desizing. Common wax is tallow. Starch is primarily used as an extender for PVA to reduce formulation cost. However antistats
are needed with starch containing formulations to minimize static on warp yarns. Generally they are not needed with 100% PVA sizes. Antistats funciton as humectants, helping to retain moisture in the film while simulatneously plasticising the film. It includes urea, ethylene glycol and glycerol. Recommended level is 3-7%. Defomers: Size solution can exhibit foam. For this we need defoamers in the levels of 0.25 to 1.00% based on the weight. Binders- They are used for synthetic fibres- two major types are polyester or polyacrylic solution. To avoid lappets warp density: For ring Spun 100% cotton - Spacing between adjecent ends should not be less than the diameter of the yarn. For Ring spun P/C blend spacing should not be less than 1.5 times the yarn diameter. For open end yarns, number of yarns per inch should be 10% less than the ring spun yarns of comparable count. Viscosity: A properly sized warp will have completely encapsulating (360o) the yarn surface to hold down loose fibres. Internal penetration must be sufficient (15-25%) to anchor the size film to the surface of the yarn. Too low a size viscosity allows liquid to penetrate too deeply into the yarn. Too high a viscosity will not allow sufficient penetration to anchor the size. If ends are tightly packed in the size box, viscosity should be lowered to improve penetration. Temperature of the size box is important for the right viscosity. High temperature may cause PVA to form skin- causing hard size formation when the slasher is stopped. Recommended temperature is 160-185 o F. Drying can temperature should be set at the minimum to dry the yarn to hte desired moisture content of 5-8%. Yarn stretch in cotton should be 1-6%, PC should be 1-1.5%, and Rayon/ Acrylic should be 35%. Desizing agents: Can be water for PVA, NaoH for oils/waxes, HCL for starch, Enzyme for starch, Solvents for oils/waxes or peroxide for PVA. p
c Sizing The process consists of laying warp yarn parallel and sizing the yarn with a mixture to strengthen it to withstand the rigours of weaving. Warp yarn is withdrawn in sheet from from warp beams which are placed at the back of the sizing machine. The yarn is then passed through sow box. Size solution is applied by immersion. After removing surplus solution that occurs at this state, the yarn is dried and arranged on a loom
beam. The objective of sizing is RESISTANCE to ABRASION This objective is achieved by applying on the yarn a unifrom and smooth protective film of suitable sizing material. We also want that in the sized yarn there should be - Some increase in tensile strength in the yarn - Minimum loss of extensibility in yarn ( about 4.4-4.6% elongation at break is required for cotton) -Required moisture content ( 8-10% of cotton) - Good quality of sized beam (neither too soft nor too firm + free from yarn defects) - Good productivity and efficiency - Reduced Cost Process Control in Sizing The process control program in sizing should, therefore, comprise of the following aspects: 1. Selecting the correct size receipe and size pick -up level 2. Ensuring correct preparatio of size paste 3. Control of - size pick up - stretch -moisture content -quality of beam -machine speed -machine efficiency -a method to calculate the expected level of productivity p p
c Control of size pickup For control on variation of size pick up , the two steps required are: a. determination of average size pick up on the beam b. Suitable adjustment in sizing conditions a. Determination of average size pick-up
Example: The weight of sized yarn on a beam was found to be 82.5 lbs. The beam contains 1050 yards of warp, whose count before sizing was 50s cotton.If the number of ends in the warp is 3000, calculate the following: a. The weight of size on the yarn b. The % of size put on the yarn c. The count of sized yarn a. Weight of size on warp= weight of sized warp- weight of same length of unsized warp Now weight of unsized warp= (No of ends x length of warp in yds)/(count of unsized yarn x 840)= (3000x1050)/(50x840) = 75 lbs Therefore weight of size on warp = 82.5 -75 = 7.5 lbs b. Percentage of size on warp = (wt of size x 100)/ wt of unsized warp = (7.5x1000)/75 = 10% c. Count of sized yarn = (No of ends of the warp x length of warp in yds)/(wt of lbs of warp x 840) = (3000x1050)/(82.5x840)= 45.5s cotton b. Control on sizing condition 1. Viscosity of size paste in size box: Any variation in the concentration or temperature alters the viscosity of the paste which in turn affects both the level of size pick up and extent of penetration. Initially as the viscosity increases, the size pick-up also increases. But as the viscosity increases beyond a point, the size pick up is reduced. 2. Sqeezing pressure and condition of squeezing nip: The squeesing pressure determines the extent of penetration of the size paste between the fibres of the yarn and also of the removal of excess size paste and hence the level of the size pick up. 3. Speed of the sizing machine: Other sizing conditions remaining unchanged, the size pick up increases with increasing sizing speed and vice versa. This is because the time available to squeeze the surplus size from the yarn is less at high speeds. 4. Depth of immersion roller in size paste: the depth of immersion roller in the paste determines the duration for which the yarn remains immersed in the paste. this duration in turn influences both the level of size pick up and the extent of size penetration. 5. Level of size paste in the size box: Variation in the level of size paste is an important source of size pick-up variations both within and between beams. 6. Density of ends: When the density of ends is high, difficulties are encountered in obtaining adequate and uniform size penetration. Therefore size pick up may vary at these fabrics. p
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Control of Yarn Stretch During sizing, the yarns are under tension, this results in a slight permanent stretch in the yarn. It leads to a decrease in extensibility or elongation at break of the sized yarn, which leads to more breakage at the loom shed. The various zones of stretch control on modern sizing machines are: 1. Creel zone: start- last warper beam, end-Dry nip 2. Wet Zone; Start- dry nip, end- first drying cylinder 3. Drying Zone: start- first drying cylinder, end- last drying cylinder 5. Splitting Zone: start-last drying cylinder, end- drag roll 6. Winding Zone: start- drag roll, end- loom beam Control of Stretch in Creel Zone The creel stretch on the existing type of sizing machines has to be controlled manually. The yarn tension in the creel zone increases gradually with reduction in diameter of the warper's beam. To counter this, the tightening of the beam is required to be adjusted suitably as the sizing progresses. Control of Stretch in the Wet Zone The control of stretch in this zone can be done with the help of 'positive dry nip'. On multicylinder sizing machines, stretch can be controlled by synchronising the PIV gear during the cylinders with that driving he finishing squeeze roller in the wet zone. Control of Moisture in sized yarns A moisture control of 8-10% should be maintained in the sized cotton yarns. With excessive drying, the size film becomes brittle and harsh. Very high moisture content is also undesirable because it makes the size films soft and the yarn sticky. Quality of Sized Beams A satisfactory weaver's beam should unwind well on the loom. These are some of the important package faults: 1. Density of sized beams: A loosly packed weaver's beam does not work well. The density is mainly influenced by two factors:
a. effectiveness of the friction clutch or the DC drive b. effectiveness of the bream pressing motion. 2. Broken ends, missing ends, crossed ends, sticky ends The major sources of all these faults are a. lappers b. invisible breaks during sizing A lapper is an accumulaiton of layers of yarn on the warper's beam. Those end breaks during sizing that do not form lappers are called invisible breaks. Both lappers and invisible breaks result in missing and broken ends in the sized beams. Crossed Ends- these are formed during weaving whenever the leading end is not available in the appropriate place on the beam,and, therefore, the weaver has to knot the trailing end to an end that is far away. This happens because in some cases the leading end of an invisible break migrates to a distant place. Sticky ends - These are caused when broken ends from the warper's beam migrate to the yarn of another warper's beam. In order to control these faults, it is necessary to control the incidence of lappers and invisible breaks. Factors affecting lappers: - End breakage rate at warping - Efficiency of warp stop motion at warping - condition of beam flanges (warper's) Factors affecting invisible breaks - High stretch at sizing - weak places in the yarn Defective selvedges There are two types of defective selvedges that cause more difficulty in unwinding during weaving than the ends of the beam of thebeam. These are: 1. Sunken selvedges 2. Bulging selvedges These defects can be controlled by a. correctly setting the expandable comb at the headstock b. Using the correct size of beam pressing roller so that it reaches both the beam flanges. c. Ensuring that beam flanges are true.
Formation of ridges on the Beam Ridges on the beam are formed when the ends that are taken in one dent of the comb do not spread out. To minimise the falut the eccentric dancing rollers at the headstock should be adjusted properly. p p
Cost of Sizing The cost of sizing ( excluding cost of sizing chemicals) is generally expressed as cost per unit length. Hence contribution of sizing cost can be calculated as: cost of sizing= cost per unit length x tape length cost of sizing chemcials is expressed as cost per unit length of size mixture. The cost per piece of fabric can be calculated as: Cost of sizing chemicals per piece length= cost/kg x (warp weight per piece length x size add on %) p p
h Efficiency of Sizing Department The major causes of loss of machine efficiency are: 1. Creeling, which includes removing the exhausted beams, mounting new beams, denting, leasing etc. 2. Replacement of weaver's beam on consumption Efficiency in sizing Let yarn count= 34 Set length on warper's beam= 18000 m length of yarn on weavers beam= 1500m Total number of ends= 4000 Full running speed= 50 m/min No of lappers per 3000 ends per 1000 m = 3 Time to cut a lapper = 90 sec
Time to doff a beam and put new lease rods = 600 seconds time to change a set = 6300 sec Time loss/1000 m due to miscellaneous causes= 60 sec Calculations Sizing time of one beam with no stops : R = (1500x60)/50 = 1800 seconds Lappers per beam of 1500 m per 4000 ends = 6 time lost in cuttin glappers per beam = 540 seconds Number of weavers beam per set= 12 time lost per beam in a set change = 6300/12=525 seconds time loss in beam doffing and leasing per beam = 600 seconds Miscellaneous losses per beam of 1500 m = 90 sec total time to size one beam = 3555 seconds Machine efficiency = (1800 x 100)/3555 = 50.6% p p
Choice of Size Receipe The suitability of size receipe for the type of yarn should take into account, fabric construction, and end use as follows: 1. Cotton Yarn- Starch Based Polyester etc.- Polyvinyl Alcohol (PVA) based 2. Fine and Superfine cotton- thin boiling Starches Coarse Count- ordinary starch 3. Heavy Fabrics- Modified starches Light and Medium- Ordinary starches 4. conventional sizing Machines- Low viscosity starch High speed sizing machines- High viscosity starches 5. Unlbleached calendered sort of fabrics- More weighting agents, antiseptic softners etc. are used. Bleached sorts- ordinary starches 6. Finer Counts and Heavier Constructions- More size pickups Coarse counts- Less size pickups 7. Plied yarns- low size pick up or nil even
Ingredients for water Based Sizes 1. Adhesives- Potato starch, starch from cereals ( corn, wheat, rice etc0, carboxy methyl cellulose (CMC), Polyviny alcohol (PVA), Polyviny Chloride (PVC) 2. Lubricants- Mineral Waxes, vegetable waxes, animal fats, mineral oils, vegetable oils 3. Additives- Salicylic Acid, Zinc Chloride, Chloride emulsifiers. Suggested Size Receipe Ordinary starch=100 Gum= 2 Mutton tallow= 7.5 Preparation of Size Receipe 1. Ingredients Water= 18.5" Modified Starch = 50 kg CMC= 5 kg PVA= 10 kg Gum = 1.5 kg Softner= 1.2 kg Antistat= 0.8 kg Mixing: Took 17" water. Added Starch, Gum, CMC and PVA slowly and one by one as the slurry is stirred. Softner and antistat added in storage kettle. Cooking 1. cold stirring at 50 rpm = 12 min 2. Transfer into pressure cooker through sieve=== 45 deg C 3. Stirrer rpm in cooker== 55 4. Close lid, open steam inlet, open air vent 5. Close air vent valve when steam is seen escaping through it, temperature = 96 deg C, Pressure = 10 PSI 6. time between point 3 and 4 == 15 min 7. Cooking starts, steam continued till temperature = 130 deg c, Pressure = 40 PSI 8. Time for getting 130 deg C after closing air vent valve = 25 min 9. Stem in-let closed 10. 10 minutes after closing steam, Temp= 124 deg C, Pressure = 32 PSI 11. Total cooking time after getting 130 deg C= 35 min 12. Flow time of paste taken through sample valve= 24 seconds
13. Transferred to storage beck 14. Final volume = 18.5 " 15. Stirrer RPM in storage beck = 20 16. Temperature after 10 min. of transfer = 100 deg C 17. Refrectometer reading= 16 Lab Data 1. Size add on = 18 % 2. Increase in strength % = 5.2 3. Elongation at break % os sized yarn = 6.8% p
