effect of stabilizers on properties of PVC (polymer)1

"Effect of stabilizers on properties of PVC.”

INTRODUCTION Poly (vinyl chloride) (PVC) is one of the most fascinating, interesting, universal and oldest thermoplastics of all. On one hand it is the polymer which comes into contact with a baby’s skin only a few minutes after its birth, in many countries the bracelet on which the name and the birthday of the newborn are noted is made of PVC. A lot of other medical items such as oxygen tents and blood bags are also based on PVC because of the chemical and physical properties of PVC not offered by other materials. On the other hand many PVC products, for example toys as children or more durable items as u-PVC windows, cladding, electrical insulation on wires and cables, resilient flooring, pipes for land drainage, sewage and drinking water are with us throughout our lives. Even the coffins that we are finally buried in are probably covered in PVC foil or veneer and lined with flexible PVC film. Polyvinyl chloride, known as PVC, is one of the world’s leading synthetic polymers. It has many uses, ranging from long-term construction applications, such as pipes used in the transportation of potable water, to short-term uses, such as food packaging. Because of the presence of chlorine, it is a highly polar polymer, which allows a wide range of additives to be incorporated within it. This variety of additives permits a broad range of physical property characteristics; hence there is not one PVC composition but many. This also explains the great diversity of applications in which PVC has been used from the time of its original commercial development in the early 1930s, when a few hundred tonnes were produced, to its annual global consumption rate of nearly 25 million tones in the year 2000.

College Of Engg.& Tech. Akola

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Worldwide demand for plastic pipe is forecast to increase more than four percent per year through 2007, continuing to outpace gains for overall pipe demand. China- which is already one of the largest national markets will register some of the strongest increases, with plastic pipe demand growing over eight percent annually through 2007. A number of other countries? Including India, Russia, and Turkey? Will also exhibit strong sales gains, fueled by acceleration in infrastructure construction activity and industrialization. In this project an attempt has been made to improve the thermal and physical property of the PVC pipe by optimizing the additives such as stabilizer for reducing the cost of PVC pipe.


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MARKET DATA Polyvinyl chloride is the leading plastic pipe resin in global use, accounting for over two-thirds of plastic pipe demand by weight. PVC pipe is popular because of its low cost, durability, strength and ease of extrusion, allowing it to make inroads against non-plastic pipe materials. Demand for high density polyethylene pipe will benefit from use as small-diameter pipe in natural gas transmission, as conduit for electrical and telecommunications applications, and as corrugated pipe for drains and sewers.

Table:


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Table:

Table:


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Literature Survey Polyvinyl chloride, better known as PVC or vinyl, is an in expensive plastic so versatile it has become completely pervasive in modern society. The list of products made from polyvinyl chloride is exhaustive, ranging from phonograph records to drainage and potable piping, water bottles, cling film, credit cards and toys. More uses include window frames, rain gutters, wall paneling, doors, wallpapers, flooring, garden furniture, binders and even pens. Even imitation leather is a product of polyvinyl chloride. In fact, it's hard to turn anywhere without seeing some form of this plastic. During the fifty years following the end of World War II, PVC has become the second most widely used plastic in the world. Today more than 20 million tonnes are Manufactured each year. In Europe the total consumption of PVC products was estimated at approximately 7.4 million tonnes in 1998, which corresponds to ca. 5.5 million tonnes of PVC polymer (ECVM). About one third of the total production of PVC polymer is used for the production of flexible PVC products, the remaining two thirds fall into the category of rigid PVC products. Virgin PVC is thermally and photochemically unstable and depending on requirements and desired characteristics of the application, a number of additives are applied to reduce these problems prior to manufacture of PVC products. The quantitatively important classes of additives are heat stabilizers, plasticisers, and inert fillers, the latter generally added to reduce cost and get more volume for a given amount of polymer Other classes of additives for specific applications include pigments, impact modifiers,


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lubricants, fillers, UV stabilizers, biocides (to prevent fungal growth on flexible PVC) and antistatic agents. In the following sections an overview is provided of the main application areas of PVC and the most important classes of additives. The average quantities of these components used in PVC products are given wherever possible. It has to be borne in mind that a wide range of applications and substances are added to different PVC products, accordingly, a more detailed and quantified inventory of components applied to PVC is hardly possible within the scope of this study. PVC products with long and extra long service life are predominately used in the building and automotive sector, while PVC products for packaging purposes have a short service life. Except for cables, flexible PVC has a rather short to medium-term service life and the majority of rigid PVC products is in use for a long time period. PVC products with very long lifespan are presently still retained in the use phase. Thus, the present PVC waste stream is not directly correlated with the current production, in particular PVC products with a comparatively long life-span used in large quantities in the building sector are just beginning to appear in the waste stream and will certainly effect future PVC waste stream composition. All PVC products contain at the minimum a PVC polymer, a stabiliser and a lubricant. Various other components are incorporated in PVC, sometimes in large quantities with regard to the polymer. Generally, the percentage of additives varies between 10 and 25 % in weight of the resin, in the case of rigid products. Flexible PVC can contain plasticisers up to 60 % of the weight, the average content is about 30%. Although the composition of PVC products is broadly known, the actual formulation of a product differs depending on the year of production and on compounding specifics of College Of Engg.& Tech. Akola

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different PVC converters, thus even in the same application (e.g. window profiles, pipes, films) the composition of the PVC material varies. In the following sections the most commonly used additives are described in brief. The effect of additives on the properties and characteristics of the product is summarised in the following overview:

Table:

•

Stabilizers

are ingredients that are generally added to the PVC

polymer in order to prevent thermal degradation and hydrogen chloride evolution during processing and to give the finished article optimum properties (heat and UV stability). Approximately 1-8 % may be added to PVC formulation depending on other components


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and the final application. The most important group of stabilisers are metal salts (i.e. calcium and zinc stearates, basic lead sulphate and lead phosphite) • organo metals (i.e. mono- and diorganotin, tin thioglycolate) • organo phosphites (i.e. trialkyl-phosphites) •

epoxy compounds (i.e. epoxidised soya bean oil, sunflower oil and linseed oil)

• antioxidants, polyols (i.e. BHT, pentaerythritol) •

Lubricants are used in amounts of 1 – 4 weight %, they are added to prevent the plastic from adhering to the metal walls of the moulding machines, and to modify the properties of mixtures. Examples of such lubricants are waxes, fatty acids (stearic acid etc), fatty alcohols, etc.

•

Fillers are added to improve certain properties, i.e. mechanical and electrical resistance, and may comprise 50% of a PVC formulation. In rigid PVC for building applications, no more than 5 % fillers are added. The main fillers are of mineral origin: calcium carbonate (limestone), talc, chalks etc.

•

These are mainly organic pigments and colouring agents and mineral

pigments

(titanium oxide, iron oxide, chromium oxide, cadmium

oxide, etc). Typical amounts are 0,1 weight%, but up to 15% may be used in extraordinary cases. (see para. 11 for the use of cadmium as pigment). In many applications, the presence of chlorine in PVC provides satisfactory fire performance, but the addition of flame retardants is used in particular in some flexible PVC building materials in relatively high concentrations – up to 10 – 20 weight %. Flexible PVC has a lower College Of Engg.& Tech. Akola

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chlorine-content than rigid PVC and is therefore more flammable. Chlorinated paraffins are commonly used as •

flame-retardants:-

the chlorinated paraffins used are almost

exclusively of medium chain length or longer Chlorinated paraffins also have plasticising properties and therefore are also used as secondary plasticiser for PVC products such as floorings, cable insulation, garden hoses, coatings, and shoes. •

Other additives:-

are added for specific purposes, such as

biocides or fungicides. Impact modifiers are used in concentrations of up to 15 weight % to improve the impact resistance of rigid PVC.


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The Economic and Technical Importance of PVC :Stabilizers Polyvinyl chloride (PVC) was one of the first thermoplastics developed. It has become worldwide a very important bulk plastic over its almost 70 year history. PVC consumption in different geographic areas and expecteddemand through 2000 are shown in Fig. 3.1.PVC consumption (Mio.t).


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PVC – including the various copolymers of vinyl chloride and chlorinated PVC – is expected to remain important among thermoplastics because of its compatibility with a large number of other products (e. g., plasticizers, impact modifiers), in contrast to other plastics. Because PVC’s mechanical properties can be adjusted over a wide range, yielding everything from rigid to flexible end products, there are many different processing methods and applications for PVC. The toxicological problems which at one time were major obstacles in the manufacture and processing of PVC were solved satisfactorily many years ago. When PVC was first developed, flexible PVC was dominant, but rigid PVC production has increased continually and is now approximately twothirds of total consumption in many countries. The low thermal stability of PVC is well known. Despite this fact, processing at elevated temperatures is possible by adding specific heat stabilizers that stop the damage. This is one of the main reasons PVC has become a major bulk plastic. The development and production of suitable heat stabilizers followed the production of PVC from the beginning, and remains a precondition for processing and application in the future. Consumption of heat stabilizers in Western Europe was approximately 150,000 tons in 1995 and is estimated to be 170,000 tons by the year 2000. The consumption of thermal stabilizers for PVC worldwide is estimated to be 450,000 tons.

Thermal Degradation and Stabilization of PVC:Mechanism of PVC Degradation:When PVC is processed at high temperatures, it is degraded by dehydrochlorination, chain scission, and crosslinking of macromolecules.


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Free hydrogen chloride (HCl) evolves and discoloration of the resin occurs along with important changes in physical and chemical properties. The evolution of HCl takes place by elimination from the polymer backbone; discoloration results from the formation of conjugated polyene sequences of 5 to 30 double bonds (primary reactions). Subsequent reactions of highly reactive conjugated polyenes crosslink or cleave the polymer chain, and form benzene and condensed and/or alkylated benzenes in trace amounts depending on temperature and available oxygen (secondary reactions). Dehydrochlorination of PVC in the Absence of Air (Primary Degradation) Any mechanism of degradation has to explain a series of experimental facts. Structural irregularities, such as tertiary or allylic chlorine atoms, increase the degradation rates measurably at the beginning of the process by a rapid dehydrochlorination that starts the degradation process (Scheme 3.1). Initial rates of degradation are proportional to the content of these irregularities. However, PVC degrades even if these irregularities are eliminated by special polymerization conditions or treatments because of the dehydrochlorination of normal monomer units (random elimination) (Scheme 3.1). It is estimated that after allowing for the differences in concentrations and reaction rates, the rate of random degradation in commercial PVC because of normal chain secondary chlorine atoms has the same order of magnitude as does degradation that results from structural irregularities. Cis-ketoallylic structures, although very reactive in dehydrochlorination (Scheme 3.1), are not present in commercial PVC but can be generated by thermal oxidative processes. After the reactive irregularities initially present are exhausted, degradation continues because of the elimination initiated from normal monomer units. These findings indicate that thermal degradation in PVC is an intrinsic property of this polymer and that changes in synthesis conditions College Of Engg.& Tech. Akola

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or specialtreatments that eliminate structural irregularities improve the stability of PVC, but can not completely eliminate its degradation. Stabilizers must be used.

Not all allylic chlorine atoms preexisting and/or formed in the degradation process accelerate degradation. Single double bonds can be identified in degraded PVC by NMR spectroscopy. Double bond sequences, once formed, do not increase by continuation of degradation. There are allylic chlorides with some forms of alkenic double bonds that are stable under degradation conditions . The conjugated polyene sequences are generated in apparently parallel processes from the first moment of degradation. For relatively low


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conversions, their concentrations increase linearly with time. Zero order rate constants calculated as slopes of these lines decrease exponentially with the increase of the number of double bonds in the sequence. In the thermal degradation of solid PVC, an induction period is observed, and then for higher conversions, the degradation rate increases with time, indicating an autocatalytic process. Hydrogen chloride formed in the degradation increases both the degradation rate and the mean number of double bonds in the polyene sequence, and consequently plays an essential catalytic role in PVC degradation . Some local configurations and conformations of the polymer chain of PVC, such as the conformation GTTG (G for Gauche T for Trans) at the end of certain isotactic sequences, favor degradation. These conformations exhibit a high local mobility relative to the remaining structures in PVC and possess some chlorine atoms with very high degrees of freedom. Both features make possible the adoption of the conformation enabling the elimination reaction. It follows that dehydrochlorination is possible only for specific local conformations. Along the same line, PVC molecules at the surface of primary particles in the solid state have a much higher conformational mobility than molecules in the interior. PVC degradation consequently is expected to take place predominantly at the surface of primary particles. It is well known that dehydrochlorination of PVC proceeds violently in the presence of Lewis acids such as FeCl3, ZnCl2, ,AlCl3, SiCl4, GeCl4, SnCl4, BCl3, and GaCl3.

This process is

responsible for the very fast discoloration of PVC in the presence of Zn or Sn carboxylates that act as stabilizers till the corresponding halides are formed and fast dehydrochlorination starts. The reaction mechanism of a complex chemical process such as PVC degradation defines the sequence of elementary reactions leading from reactants to products and describes each College Of Engg.& Tech. Akola

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of these reactions. The mechanism of PVC degradation should explain the above fundamental observations and should also agree with the observations related to PVC stabilization that are discussed later in this chapter. The dehydrochlorination of PVC is a very specific chemical process because of the existence of a long series of alternating CHCl and CH2 groups in the polymer backbone that makes possible a chain of multiple consecutive eliminations. However, the parallel formation of conjugated polyene sequences containing 1 to 30 double bonds cannot be explained by a simple consecutive elimination. The chain reaction model from Scheme 3.2 can explain this apparent contradiction .

The first elimination from a monomer residue from the chain (-CH2CHCl-) or a structural irregularity such as a tertiary chlorine atom (-CH2CCl<) forms an active intermediate or a stable monoalkene. This active


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intermediate partitions between a stable sequence of two double bonds and a new intermediate . The fate of the second intermediate is analogous to that of the first one and the process continues in this way, generating all the double bond sequences. The concentration of each intermediate is lower than the concentration of the previous one. A simple steady state approximation shows that all polyene sequences in the distribution are formed

simultaneously

and

the

apparent

rate

constants

decrease

exponentially in agreement with experiment . There is a general consensus that the intermediates in the degradation process are allylic sequences with progressively increased numbers of conjugated double bonds However, the mechanism of initiation, propagation, and termination steps is controversial. An early mechanism hypothesized that the intermediates were allylic radicals (Scheme 3.3).

The major problem with this mechanism is that the chlorine atom is known to be so reactive as to be non-selective. Data on model compounds showed that the allylic hydrogen atom, (>C=CH-CH2-CCl<), has only slightly


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higher reactivity toward abstraction by a chlorine atom that is free to diffuse throughout the polymer matrix than does a hydrogen atom from a secondary carbon (-CH2-CCl<). The above mechanism consequently generates primarily isolated double bonds and not the observed sequences of conjugated polyenes, owing to the much higher concentration of hydrogen on secondary carbon atoms . This radical mechanism also fails to explain the very important catalytic role of HCl. In addition, there are no reliable proofs that radicals are intermediates in PVC degradation in the absence of initiators and/or oxygen . An ion pair mechanism (Scheme 3.4) was considered for the initiation step by ionization of chlorine followed by rapid elimination of a proton. A much faster ionization of the activated allylic chlorine formed was considered responsible for the chain reactions . However, this mechanism does not explain the previously presented experimental facts. Moreover, it does not postulate any interruption reactions of the degradation chain. The only product of degradation should be a polyene resulting from elimination of all chlorine atoms from the PVC molecule. Consequently, the ion pair mechanism cannot explain the real distribution of polyene sequences as a function of the number of double bonds. This mechanism also does not explain the catalytic role of HCl. Formation enthalpy of Cl2Hcomplexes of 3 to 4 kcal/mol as an intermediate for the catalyzed process in this mechanism does not compensate for the high activation enthalpy of C-Cl ionization (140–180 kcal/ mol). A concerted elimination mechanism postulated by A. R. Amer and J. S. Shapiro, modified by M. Fisch and R. Bacaloglu , and based on experimental data and molecular orbital calculations

and

additional experimental data from W. H. Starnes and coworkers

best

explains the experimental facts (Scheme 3.5). The first step is slow College Of Engg.& Tech. Akola

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formation of a double bond randomly along the polymer chain via a 1,2unimolecular elimination of HCl through a four-center transition state (Scheme 3.5) or a

six-center transition state in the catalytic presence of HCl or metal chloride dike ZnCl2 (Scheme 3.6). Structural irregularities such as allylic or tertiary chlorine atoms eliminate much faster than do normal secondary chlorines in the chain.


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The second and third steps of the processes constitute the chain elimination, regardless of the initiation site. In the second step, an HCl molecule is eliminated from a cis _-alkyl-allyl chlorine through a six-center transition state, generating a conjugated diene or polyene. Next is an HCl-catalyzed, 1,3 chlorine rearrangement, generating a new cis _-alkyl-allyl chlorine from the conjugated polyene. The second and third processes may continue as long as HCl is still present in the system. Elimination of HCl stops the 1,3-rearrangement of chlorine and consequently, the reaction chain. This explains the very important role of HCl in PVC degradation. All the metallic chlorides that are Lewis acids and can form complexes with chloroalkanes may have a similar role. (Scheme 3.7). The analogy with chloroalkanes and chloroalkenes provides important support for the above mechanism, as it was shown that activation parameters for the initiation of PVC degradation correlate with the activation parameters of gas phase elimination from secondary chloroalkanes and fall on the same College Of Engg.& Tech. Akola

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straight line. This suggests that both processes may have the same mechanism: a 1,2-elimination of hydrogen chloride from a synperiplanar conformation involving backbone chlorine through a transition state of four centers. In the same way, it was shown that the chain reactions may have the same mechanism as the elimination from cis _-alkyl-allylic chlorides (chloroalkenes): a 1,4-elimination of hydrogen chloride through a transition state of six atoms from a cis allylic structure. Only the cis-allylic system with one double bond that has a relatively low activation enthalpy of dehydrochlorination is reactive in chain propagation in the degradation process. Trans conjugated polyenes are much more stable in the absence of HCl. To dehydrochlorinate, they require a 1,2-elimination at the one end that has a much higher activation enthalpy. This process is similar to a random initiation and is much more likely statistically to occur at a different place in the polymer molecule or on a different polymer molecule entirely. In this way, a chain reaction, once interrupted, does not continue, and the sequences of conjugated polyenes remain as such in the system. Trans conjugated polyenic systems are known to be very stable relative to their cis isomers because of their favorable conformation that allows polymer packing to occur.


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Thermal Oxidative Degradation of PVC:During processing, in addition to thermal dehydrochlorination, the polymer is exposed to thermo-oxidative degradation resulting from oxygen; in addition, mechanical stress may cause chain scission. The main feature in thermo-oxidative degradation is dehydrochlorination as in thermal degradation. The presence of oxygen causes the dehydrochlorination process to accelerate, but the discoloration is not as severe as during thermal degradation. The polyene sequences are shorter as a result of the reaction between them and oxygen. The overall activation energy of dehydrochlorination is practically the same

for

thermal

and

thermo-oxidative

processes

.

The

initial

dehydrochlorination proceeds by the same mechanism. The most significant damage during the commercial processing of PVC occurs as a result of mechano-chemical reactions in the presence of entrapped oxygen. The shear forces cause chain scission, generating radicals. Thermally-initiated HCl loss is followed by radical oxidation of polyenes to form peroxy radicals and hydroperoxides. Hydroperoxides decompose to generate alkoxy and hydroxy radicals that accelerate the oxidation process and form ketones and acid chlorides . Ketoallylic chlorides initiate the thermal dehydrochlorination process, as described earlier.


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Although the radical process is much more complex than shown in Scheme 3.8, it is clear that thermo-oxidative degradation does not differ in any essential way from thermal degradation. Dehydrochlorination, the most important process, has the same mechanism in both types of degradations.


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Secondary Processes in PVC Degradation:In PVC degradation at low dehydrochlorination levels, polyene concentrations increase linearly and in parallel with HCl evolution. At higher dehydrochlorination levels, the increase in polyene concentration levels off. The plateau value is lower when degradation temperatures and oxygen

pressures

are

higher.

When

the

plateau

is

reached,

dehydrochlorination level for all double bond sequences show that no consecutive reactions to longer polyenes take place . In the absence of oxygen during the thermal degradation of solid samples, a measurable increase in molecular weight occurs and the molecular weight distribution becomes wider and shifts toward higher values. At some point during degradation, the melt viscosity increases considerably, as can be observed by increasing torque in a Brabender Plasticorder experiments [33]. The crosslinking process is catalyzed by HCl . The most important crosslinking reaction is the Diels-Alder condensation of cisoid trans-trans dienes with other polyenes (Scheme 3.9). Benzene is formed in very small amounts even at temperatures as low as 160 to 170 °C by an intramolecular process (Scheme 3.9). At higher temperatures, substituted benzenes and condensed aromatic hydrocarbons are formed by radical scission of Diels- Alder condensation products and radical cyclization of polyenes. In the presence of oxygen, the same reactions take place, but they are more complex because of the processes described earlier. Oxidative scission of the chain predominates and, in general, the molecular weight of the polymer decreases .


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Heat Stabilization of PVC:The degradation of PVC at elevated temperatures required in thermoplastic processing is an intrinsic characteristic of the polymer and consists of dehydrochlorination, auto-oxidation, mechano-chemical chain scission, crosslinking, and condensation reactions. This degradation must be controlled by the addition of stabilizers. The heat stabilizer must prevent the dehydrochlorination reaction that is the primary process in degradation. There are two ways the stabilizer can act: • By reacting with allylic chlorides, the intermediates in the zipper degradation chain. This process should be faster than the chain propagation itself, requiring a very active nucleophile. However. the reactivity of the nucleophile should not be so high as to react with the secondary chlorine of the PVC chain, a process that rapidly exhausts the stabilizer. To be effective, the stabilizer must be associated by complex formation with polymer chlorine atoms, which means it should have a Lewis acid character. This association should take place in regions where the polymer molecules have maximum mobility; in other words, where the conformation of the polymer can favor the degradation processes. • Once the degradation starts, it is very fast and can be stopped only if the stabilizer is already associated with the chlorine atom that becomes allylic. These regions are the surfaces of the primary particles of PVC, where the stabilizer molecules are associated with the chlorine atoms. The exceptional effectiveness of such stabilizers at very low concentrations is explained by their entropically favorable position for stopping degradation. In general, these stabilizers, because of their effectiveness, prevent the formation of


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polyenes longer than four to five double bonds and maintain very good early color in the polymer. These stabilizers are called primary stabilizers. • Scavenging the hydrogen chloride generated by degradation is another way to stop the process as the HCl is a catalyst for the chain propagation reaction and the initiation step. However, the diffusion of HCl is quite slow because HCl is associated with the double bond where it was generated. When HCl diffuses away from the reaction center, the zipper degradation reaction stops. The stabilizer should scavenge HCl with high effectiveness to avoid its catalytic

effect

in

chain

initiation

that

starts

another

zipper

dehydrochlorination chain. Because this type of stabilizer cannot prevent the dehydrochlorination in its early stages, polyenes longer than four to five double bonds are formed. PVC discolors and the initial color is not maintained. However, by scavenging HCl, this type of stabilizer avoids the autocatalytic degradation and consequently, overall degradation is much slower. These stabilizers provide very good long term stability and are usually referred to secondary stabilizers. To have good stabilization of PVC with good early color and long term stability, the two types of stabilizer should be combined appropriately for each particular PVC formulation. Stabilization is complicated by the fact that primary stabilizers become strong Lewis acids by reacting with the HCl that catalyzes the initiation and propagation of PVC degradation. To avoid this, secondary stabilizers should react efficiently with HCl to protect the primary stabilizers. Another possibility is to include compounds called costabilizers in the system. Costabilizers form relatively stable complexes with the chloro derivatives of primary stabilizers (the Lewis acids) and suppress their degradative effect.


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The most important classes of stabilizers and how they act in PVC stabilization is briefly discussed below.

Alkyltin Stabilizers:The most commercially important alkyltin derivatives are the mono and dimethyl-, butyland octyltin alkyl thioglycolates, mercaptopropionates and alkyl maleates. All these compounds react with HCl to form the corresponding tin chlorides (Scheme 3.10). However, their stabilization effect does not correlate with the amount of HCl reacted nor with the rate of this reaction. It has been established that in the stabilization of PVC with alkyltin alkyl thioglycolates, alkyl thioglycolates are released by reaction with HCl . These alkyltin compounds consequently function as secondary stabilizers, but this is not the main mechanism of their action.

During PVC stabilization with alkyltin alkyl thioglycolates, thioglycolate groups are incorporated into the polymer chain as was determined by 113Sn and 14C tagging, Alkyltin thioglycolates exchange thioglycolate groups with chlorine atoms in reactions with model allylic chlorides and the reactivity in this process parallels the PVC stabilization effect . The main College Of Engg.& Tech. Akola

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stabilization mechanism of these compounds is consequently substitution of allylic chlorine and they are primary stabilizers (Scheme 3.11). In alkyltin thioglycolates, one thioglycolate group bonds to tin to form a complex and is not active in stabilization. Alkyltin mercaptopropionate groups do not form such complex structures; all mercaptopropionate groups are active in stabilization and their activity is higher compared to the corresponding thioglycolates on a molar basis. In general, monoalkyltins are more reactive than dialkyltin derivatives; however, as a result of the very fast exchange of

thioglycolate groups, compositions comprised of at least 40 to 50% mono content exhibit activity equal to that of the monoalkyltin derivatives themselves. Consequently, pure monalkyltin derivatives are not required to obtain maximum stabilization. Based on their high compatibility with PVC and difficulty of extracting them from PVC blends, it has been postulated that tin stabilizers associate with chlorine atoms at the surface of PVC primary particles which explains their high efficiency in PVC stabilization (Scheme 3.12).


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Mercapto compounds generated by the reaction of alkyltin mercapto derivative with HCl add to double bond sequences and by this process, retard PVC’s discoloration . Dialkyltin di(alkyl maleates) are able to add in a Diels Alder reaction to polyene sequences and reduce the discoloration of degraded PVC . In both cases, the average polyene sequence length is shortened, thereby shifting the absorption maximum toward the ultraviolet and away from the visible wavelengths. 3.2.2.2 Mixed Metal Stabilizers Metal carboxylates stabilize PVC by either mechanism, depending on the metal. Strongly basic carboxylates derived from metals such as K, Ca, or Ba, which have little or no Lewis acidity are mostly HCl scavengers. Metals such as Zn and Cd, which are stronger Lewis acids and form covalent carboxylates, not only scavenge HCl, but also substitute carboxylate for the allylic chlorine atoms (Scheme 3.13). It has been shown that when the concentration of the metal carboxylates is decreased, the ester group introduced into the backbone by direct substitution can be eliminated by


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reaction with HCl or by thermal degradation at higher temperatures (reversible blocking mechanism) (Scheme 3.13). IR spectroscopy has shown that Zn carboxylates associate with PVC molecules at the surface of primary particles and are, consequently, very effective in the substitution of allylic chlorine. The synergism between Zn or Cd carboxylates and Ba or Ca carboxylates is attributed to fast exchange reactions between zinc or cadmium chlorides and barium or calcium carboxylates. These reactions regenerate the active zinc or cadmium carboxylates and also avoid the catalytic effect of zinc or cadmium chlorides in PVC degradation (Scheme However, it has been shown that the synergistic effect is increased by preheating zinc and calcium stearates together. In this way, a complex zinc stearate is formed that is more active in allylic chlorine substitution (Scheme 3.14).


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The damaging effect of Zn or Cd chlorides in PVC degradation can be considerably reduced by using costabilizers that form metal complexes with them. The most common costabilizers used with solid Cd and Zn carboxylates are polyols .

Alkyl Phosphites Stabilizers:Dialkyl phosphites have no effect on PVC degradation. Trialkyl phosphites scavenge HCl by an Arbuzov reaction and form dialkyl phosphites. They react also with allylic chlorides, but this process plays a secondary role (Scheme 3.15). When used alone, phosphites are secondary stabilizers, giving good long term stability but poor early color. However, in the presence of zinc di(dialkyl phosphites) (formed from zinc salts and trialkyl phosphites as stabilization proceeds), allylic substitution is considerably


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increased and becomes the dominant process in PVC stabilization. The early color is very considerably improved.

Diketones Stabilizers:Diketones and similar compounds with active methylenes react in the presence of Zn carboxylates as catalysts with allylic chlorides generated by PVC degradation by a Calkylation process . The stabilization effect increases with the CH acidity of these compounds .

Epoxidized Fatty Acid Esters Stabilizers:Epoxides are HCl scavengers and are also reported to be effective in allylic chlorine replacement in the catalytic presence of Zn and Cd salts (Scheme 3.16) . College Of Engg.& Tech. Akola

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Hydrotalcites Stabilizers:Hydrotalcite, a natural mineral, is the hydroxycarbonate of Mg and Al with the exact formula: Mg6Al2 (OH)16CO3.4H2O. It is constituted from infinite sheets of octahedra of Mg2+ six-fold coordinated to OH-, sharing edges (brucite-like sheets), where Al3+ substitutes for some of the Mg2+ ions. A positive charge is generated in the hydroxyl sheet that is compensated for by CO3 2- anions, which lie in the interlayer regions between two sheets. In the free space of these interlayers, there is water of crystallization, associated by hydrogen bonds with both OH- and CO3 2anions. Hydrotalcite-like clays with anions of weak acids react with strong acids such as HCl and exchange the anions with Cl-. This reaction allows hydrotalcite-like clays to be used as HCl scavengers in PVC stabilization . Product Groups and Their Specific Chemical and Application Characteristics Despite the great variety of thermostabilizers known, only a few have gained industrial importance. According to their chemical composition, they are usually divided into four groups: tin stabilizers, mixed metal carboxylates, College Of Engg.& Tech. Akola

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lead stabilizers, and metal free stabilizers. Besides the thermostabilizers, there is the important group of costabilizers, which are products with no significant efficiency alone, but which are used together with stabilizers to provide strongly enhanced effects.

Tin Stabilizers:As early as 1936, Yngve recommended not only tetraalkyltin but also alkyltin carboxylates as PVC stabilizers . In 1950, Firestone filed patent applications for organotin mercaptides, which became extremely important in further developments in PVC technology . Organotin compounds with at least one tin-sulfur bond are generally called organotin mercaptides, sulfur-containing tin stabilizers, or thiotins. Organotin salts of carboxylic acids – mainly maleic acid or half esters of maleic acid – are usually known as organotin carboxylates, and the corresponding stabilizers are sometimes called sulfur-free tin stabilizers.

Organotin Mercaptides and Organotin Sulfides:Sulfur-containing organotin compounds are among the most efficient and most widely used heat stabilizers. They can be described by the following structures (Scheme 3.17):


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Among these, the liquid thioglycolates are the predominant group on the market. Mono- and diorganotin mercaptides are often used in combination, because these mixtures improve initial color as well as the long-term heat stability of PVC (synergistically) . This is shown in Fig. 3.2.

Fig 3.2 Synergism of mono- and dioctyltin isooctyl thioglycolates exemplified by yellowness Index (YI) as a function of milling time (t) in the College Of Engg.& Tech. Akola

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dynamic heat stability test on a two-roll mill at 200 °C a: dioctyltinbis(isooctylthioglycolate),b:monooctyltintris(isooctylthioglycolate),c: dioctyltin-bis

(isooctylthioglycolate)

and

20%

80%

monooctyltin-tris

(isooctylthioglycolate) The term “dialkyltin” is also used for mixtures of dialkyltin with smaller amounts ofmonoalkyltin compounds in order to exploit the synergistic effect of the combination of mono- and dialkyltin. Very efficient, solid stabilizers of a type not mentioned above are derived from ß-mercaptopropionic acid (Scheme 3.18): Sulfides of mono- and diorganotin are used in liquid mixtures with certain tin stabilizers, mainly together with thioglycolates and reverse esters. The heat stabilizing effect of these organotin stabilizers depends on the type of mercapto group they contain. These groups are directly involved in the stabilizing reaction; they can either replace the labile chlorine directly according to Scheme 3.11, or add onto polyene sequences after intermediate formation of the mercaptide HSR . Organotin mercaptides are able to react with HCl, to annihilate initiating sites by substitutionand also help impede auto-oxidation. The combination of

these

functions

gives

the

organotin

mercaptides

exceptional

thermostabilizing properties not found in any other class of stabilizer.Details about the mechanism of organotin stabilization can be found in Section 3.2 . The organotin-sulfur stabilizers, especially as mixtures of mono- and dialkyl-tin i-octyl thioglycolates, can be used in all PVC applications where high thermostability is required. They can stabilize all homopolymers, emulsion, suspension, and bulk PVC (E-, S-, MPVC), as well as copolymers of vinyl chloride, graft polymers, polyblends, and postchlorinated PVC (CPVC). One of the most appreciated properties of the whole organotin stabilizer group is the absolute College Of Engg.& Tech. Akola

35


crystal clarity of finished articles, an advantage in the manufacture of rigid PVC packaging and transparent film, bottles, and containers. Organotin thioglycolates are used also in the manufacture of plasticized PVC hoses, profiles, sheet, and transparent top coats or layers. Sulfur-containing organotin stabilizers are not, in general, self-lubricating. Therefore, the high processing temperatures necessary for optimum transparency may cause the hot melt to adhere to the metal surfaces of processing equipment, unless suitable lubricants are added. Adding highpolymeric processing aids based on PMMA to organotin-stabilized PVC imparts better flow properties and improves the surface quality of finished articles, e.g., in calendering films, extruded profiles and sheet, blown bottles, and injection molded fittings. Organotin mercaptides should not be used with cadmium- or leadcontaining stabilizers or pigments because the resulting formation of cadmium or lead sulfide can discolor the PVC (“sulfur staining”). Organotin stabilizers migrate from rigid PVC only very slightly. This fact, together with favorable toxicological properties, is the basis for the worldwide approval of certain types of methyl- and octyltin isooctylthioglycolates for use in food packaging and potable water pipe . As described in the previous Section (3.2.2.), organotin stabilizers are transformed during processing into the corresponding organotin chlorides. Methyltin chlorides have considerably higher vapor pressure than the analogous butyl- or octyltin compounds. Because of their volatility during processing, the maximum allowed concentration (MAC) for tin (0.1 mg/m3) must be monitored and enforced strictly, especially in open systems such as calendering. Butyltin mercaptides are widely used as stabilizers in the College Of Engg.& Tech. Akola

36


production of films, sheet, injection moldings, floor tiles, and wall coverings. In the US, they are also used for pipe extrusion, and siding with high titanium dioxide content is manufactured almost exclusively with organotin mercaptides. Transparent and translucent articles for outdoor use can be stabilized with sulfur-containing organotin stabilizers only if suitable UV absorbers are also present. A special application for sulfur-containing organotin stabilizers is the production of foamed, rigid PVC profiles and sheet. Reverse esters are mercaptides of mono- or di-methyl or butyltin, based on mercaptoethanol esters of long chain fatty acids. They are especially effective in the manufacture of PVC pipe and siding. Approvals for water pipe exist for certain organotin reverse esters. The liquid estertin iso-octyl thioglycolates (alkyl-O-CO-CH2CH2)2Sn(SCH2COO-i-octyl)2 are also efficient nontoxic stabilizers, but they have not developed significant market share.

Organotin Carboxylates:Only carboxylates carboxylate with the following structures are of practical interest (Scheme 3.19):

As already mentioned, organotin derivatives of maleic acid may have an additional stabilizer function, i.e., the Diels-Alder reaction . Their performance is good in all types of suspension, emulsion, and bulk PVC.


37


Optimum results are obtained when they are combined with small amounts of phenolic antioxidants, particularly in plasticized PVC, impact-modified PVC, and PVC copolymers. Because stabilizers containing maleic acid occasionally lead to eye and mucous membrane irritations, there have been many attempts to replace them with other systems. For many

years,

organotin stabilizers free of maleic acid have been on the market. These consist of a combination of organotin carboxylates, e.g., laurates, and a small amount of an organotin mercaptide. Just as with sulfur-free organotin stabilizers, when used in a suitable formulations, this combination gives rigid PVC high transparency and excellent weathering stability. In the melt, PVC stabilized with alkyltin maleates tends to stick to hot contact areas of the processing equipment. However, this problem can be prevented by suitable lubricants. Organotin carboxylates work especially well in the manufacture of rigid or plasticized PVC articles for outdoor use, as for example, transparent and translucent double-walled panels for greenhouses, siding, and window profiles, particularly when pigmented.


38


Recommended Formulations Sr.no 1 2 3 4 5 6 7 8 9

Ingridient PVC Calcium Carbonate Stearic Acid Tio2 Paraffin wax Tribasic Lead Sulfate Dibasic lead sterate Calcium sterate Lead sterate

% 100 16-12 .0.6 0.4. 0.6 1.6 0.8 0.8 0.4

Experimental Work In this project we have processed the PVC pressure pipe of different compounding minimizing the percentage of stabilizer and we have taken the


39


thermal stability test as well as other standard test for determining the properties of PVC pipes. We have made various batches of different composition which is given below.

BATCH –A Sr.no 1 2 3 4 5 6 7 8 9

Ingredients PVC Calcium Carbonate Steraic Acid Tio2 Paraffin wax Tribasic Lead Sulfate Dibasic lead sterate Calcium sterate Lead sterate

% 100 16-12 0.6 0.4. 0.6 0.7 0.5 0.5 0.2

BATCH –B Sr.no 1 2 3 4 5 6 7 8

Ingredients PVC Calcium Carbonate Stearic Acid Tio2 Paraffin wax Tribasic Lead Sulfate Dibasic lead sterate Calcium stearte


% 100 16-12 .0.6 0.4. 0.6 0.8 0.55 0.55 40


9

Lead stearte

0.25

BATCH –C Sr.no 1 2 3 4 5 6 7 8 9

Ingredients PVC Calcium Carbonate Stearic Acid Tio2 Paraffin wax Tribasic Lead Sulfate Dibasic lead sterate Calcium sterate Lead sterate

% 100 16-12 .0.6 0.4. 0.6 0.9 0.6 0.6 0.3

BATCH –D Sr.no 1 2 3 4 5 6 7 8

Ingredients PVC Calcium Carbonate Stearic Acid Tio2 Paraffin wax Tribasic Lead Sulfate Dibasic lead sterate Calcium sterate


% 100 16-12 .0.6 0.4. 0.6 1. 0.65 0.65 41


9

Lead sterate

0.35

BATCH –E Sr.no 1 2 3 4 5 6 7 8 9

Ingredients PVC Calcium Carbonate Stearic Acid Tio2 Paraffin wax Tribasic Lead Sulfate Dibasic lead sterate Calcium sterate Lead sterate

% 100 16-12 .0.6 0.4. 0.6 1.1 0.7 0.7 0.4

The processing parameters are as below.

Processing Parameters Single screw exruderwiht belt drive barrel temperature: zone 1 125 °C zone 2 150 °C zone 3 160 °C Die temperature 165°C m/c RPM 12 vacuum pressure 250mmof Hg output 2m/min sizing unit length 3feet cooling tube length 5feet Traction unit RPM 12 Traction unit length 3feet Compounding Compounding temperature Compounding time

110 -120°C 15 min

Pipe Dimension diameter of pipe

25mm


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length of pipe thickness of pipe weight of pipe

3m 1.5mm 450 gm

Test for compounds. Static Thermal stability of PVC:Theory:Resin sample is heated and emitted vapour are passed through a tube containing canes red paper . The liberation of HCL is indicated by the colour change.

Principle :During the degradation of PVC ,HCL is liberated which changes the colour of canes red paper to blue. Apparatus:i.

Red indicator paper : 6 mm X50 mm.

ii.

Stop-watch with accuracy of 0.2 sec.

iii.

Oill bath : Capable of maintaining the temperature to the accuracy of 1°C in the range of 120 to 210°C ( Chetan Electrical Worksmake ).

iv. v.

Stirrer with variable speed (Remi make). Test tube : 18 + 1 mm diameter ,150 + 2 mm length.

vi.

Rubber stopper : With a hole to accommodate 2+3 mm diameter tube.

vii.

Silicon oil : Viscosity of 50 + 5cp

Safety:Carry out the experiment in the fuming chamber with glass door shut. Wear safety glasses during test.


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Procedure :1) Heat the oil bath to test temperature (180°C) and maintain in it

0+2°C. 2) Fill the test tube with sample PVC resin up to 50+2 mm depth ( do

not top the tube to form a compact mass) . 3) Place small glass tube in hole of stopper .Roll one end of moist cargo

red paper / blue litmus and insert it in tube so that 30 + 2 mm of paper strip extends from the glass tube. 4) Stopper the test tube with the stopper and position the glass tube in

the stopper so that end of paper is 25 + 2 mm above sample surface. 5) Immerse the in oil bath to the level of upper surface of sample and simultaneously start stop watch. 6) Measure the time till the indicator paper shown first signs of colour change.

Observation:Sample Batch A compound Batch B compound Batch C compound Batch D compound Batch E compound


Temp °C 180 180 180 180 180

Time for degradation(min) 102 114 110 117 115

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Test for PVC pipes: Following test are generally performed for assessing suitability of PVC pipes Dimensions of the pipes: Pipes are checked for outside diameter and thickness with the help of ball ended micrometer tolerance on outside diameter is +0.3 mm for 16-90 mm O.D pipes + 0.4 mm for 110 mm pipes and 0.5 mm for 140 X160 mm O.D. pipes. Wall thickness for a working pressure of 4kg/cm2/90mm O.D. pipes is min .2.1 and max.2.6mm for 110 mm OD / 4kg/cm2 other size and working pressure ratings, dimensions are available in. Visual appearance: The pipes shall be reasonably round. The internal and external surface of pipe shall be smooth and clean, reasonably free from grooving and other defects. The end shall be clean cut and square with axis of the pipe. Short Term Hydraulic Test: A sample of pipe equal to 10 times the nominal size of the pipe(not less than 25 cms or more than 75 cms) shall withstand a circumferential stresses of 360kg/Cm2 for at least 1 hour. At 27±1°C without any sign of leaking or weeping. Internal Presser to be applied is P= 2δ S/D-S. Where P = Pressure to be applied in Kgf/cm2 δ

=

Circumferential stress in Kgf/cm2

S

= Minimum wall thickness in mm.

D

= Mean OD in mm


45


4kgf/cm2 pressure is applied for 1hr Pass Pass Pass Pass Pass

Batch A B C D E

Reversion Test: A 20 cm peace of pipe on which two marks are made 100 mm apart, and is placed in oven at 150 °C for 1 hour After the stipulated time is over the reduction in length of 100 mm mark is measured. The Shrinkage is less than 5% than the sample was passed the reversion test was accepted for the ISI test. Batch A B C D E

Shrinkage 2% 3% 4% 3% 3%

Water absorption:The 4.5 cm length of pipe is cut and weight. The beaker of 500 ml is taken then the distilled water of 250ml is taken into this beaker. And the piece of pipe is immersed for 24 hr. and this pipe is weighted after


46


immersion the difference in the weight of pipe gives us the percentage of water absorption. Batch A B C D E

% of water absorption for 24 hr 1.38% 0.81% 1.72% 1.46% 0.98%

Sulphuric Acid Test for PVC pipe: In this test the pipe sample of measured length and weight was immersed in H2SO4 bath for 14 days. After the period of pipe was dried with the cloth and then its weight was measured the weight is increased or decreased by 0.010-0.013 Grams the sample was accepted other wise not. Batch A B C D E

Difference in weight in gm 0.06 0.01 0.08 0.10 0.017

Impact strength test: 30cm pipe pieces are cut and conditioned at 0°± 1° for 1hr specimen shall be tested on following the weight machine within 10 sec of removal from the bath. Mass of striker and height of free fall shall be under: O.D.(mm) 20 25-40 50-63

Total mass of kg 0.25 0.25 0.25

Height (meter) 0.5 1.0 2.0

80-100

0.50

2.0


47


125

1.0

2.0

Each specimen shall be drawn with longitudinal parallel line not less then 50 mm. the weighted striker shall be allowed to fall freely on the mark line. If the specimen does not fail by cracking or splitting, the specimen shall be rotated to the next longitudinal line and so on. The process shall be repeated until all the marked lines have been tested or until a failure is recorded. If 14 strikes are made without failure pipes shall be deemed to have passed the test. Batch A B C D E

striker of 0.25kg weight is fall at height of 1 meter Pass Pass Pass Pass Pass

Result & Discussion Comparison table:Test Batch Thermal

A

B

C

D

E

102

114

110

117

115

stability (in min) College Of Engg.& Tech. Akola

48


Water

1.38%

0.81%

1.72%

1.46%

0.98%

absop. Reversion

2%

3%

4%

3%

3%

Test I. H. S. P Impact

PASS PASS

PASS PASS

PASS PASS

PASS PASS

PASS FAIL

0.06

0.01

0.08

0.10

0.017

Strength H2SO4 Test (in gm)

It is clear from the above table of various batches of different proportion of stabilizer. The batch B is satisfying criteria of ASTM. For thermal stability, Batch B obtain good thermal stability with optimum percentage of stabilizers and from this it considered as the formulation of making low cost PVC pipe with good properties.


49


Conclusion Worldwide demand for plastic pipe is forecast to increase more than four percent per year through 2007, continuing to outpace gains for overall pipe demand. This continues increase in production would not been possible without the improvement in processing of PVC pipe with changes in additives proportion during compounding. This makes us to thing that why not we make the pipe which gives good individual properties in PVC pipe system with economical aspect. After thermal stability test and other test for various properties we have concluded that % of stabilizer used in batch B improved the properties such as thermal stability which reduces the percentage of stabilizer to about 30 percent. Thus the batch B is giving similar properties like standard formulation so it reduces the cost of processing to two percent.


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References BOOKS: 1) PVC Technology by A.S.ATHALYE & Dr.PRAKASH TRIVEDI first edition 1994 page no: (197-199), (168-169),(92-120). 2) PVC Stabilization by JERRY WYE PITCH 4th edition page no (12-

30),(198- 220), 3) PVC Technology by W.Y.Titow Elsevier applied science publication, London and new York page no 4-23,37-49,59-65 4) Plastic material 5th edition (1995), J.A. Brydson butter worth

London page no. 293-345. 5) Polymer science V.R. Gowerikar new age international Pvt Ltd Pub new Delhi, page no. 236-238


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WEBSITES: 1) www.cmie.com 2) www.specialchem4.com 3) www.plasticnews.com 4) www.ril.com 5) www.finolex.com


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effect of stabilizers on properties of PVC (polymer)1

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