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CE Refresher Polymerization of Vinyl Chloride URL 17981 Suspension, emulsion, bulk and solvent techniques convert vinyl chloride mono mer into polyvinyl chlorides. Initiating the reaction and controlling polymer size, length and molecular weight depend to a large extent on the chemistry and on the type of multiple-phase relations that occur during the course of polymerization. IYLE F. ALBRIGHT, Purdue University Polyvinyl chloride polymers (commonly called PVC polymers) are the second largest family of high poly mers in the U.S., based on the amount produced. In 1966, production was over 2 billion lb., or over 17% of all plastics. The term "PVC polymers" as used here will include materials produced by polymerizing pure vinyl chloride (VC), or mixtures of comonomers that are predominantly vinyl chloride. PVC polymers are by far the most important mem ber of the family of vinyl polymers,* which normally includes polyvinyl chloride, polyvinyl acetate, polyvinyl alcohol, polyvinyl acetals and polyvinylidene chloride. Production of PVC polymers since 1955 has grown almost 15%/yr. in the U.S.niT This rate is expected to remain at this level for the next live years. In many parts of the world, production has increased even more rapidly than in the U.S. In 1946, the U.S. produced 80% of all PVC, but it only makes about 25% cur rently. The amount of PVC produced in Common Mar ket countries now surpasses that in this country, and Japanese output is over half of that in the U.S. Part of the major increase for PVC production in the rest of the world is caused by delays in getting adequate production of other thermoplastic polymers, including polyethylene. When other polymers are pro duced abroad in larger quantities, they may displace PVC for certain uses. Nevertheless, the average annual growth rate for PVC polymers is estimated to be sub stantial in most countries, and to vary between 12 and 14% for the Common Market from 1965 to 1970. Increased production of PVC during the last few years has been caused in part by a steady and sig nificant decrease in the selling price. Present prices for To meet the author, see Chem. Eng., Feb. 13. 1967. p. 164. Some authors use the term "vinyl polymers" differently. For example. Btllmeyer' uses it to refer to all polymers in which the repeating unit of the polymer has the formula (--CH*--CHX-->. In this case. X Is any atom except hydrogen, or it is a group of atoms. the general-purpose grade PVC are 15^/lb., or much lower for quantity purchases. Projections based on recent process improvements (particularly those for vinyl chloride manufacture) indicate that PVC will sell for less than 10^/lb. (based on 1965 dollars) by 1970." At least 23 companies in the U.S. presently produce and sell PVC resins. The uses for PVC are many and varied, as indicated in Table I.38 Extremely wide variations in the physical properties are possible because of several major modi fications that include plasticized polymers or copoly mers produced primarily from vinyl chloride. PVC pro ducers are spending large amounts of money in an effort to find new uses and to develop improved prod ucts. They have high hopes for: 1. Developing clear PVC polymers that can be used End-uses for polyvinyl chloride polymers and copolymers during 1965 In the U.S.--Table I Flexible polyvinyl chlorides Coatings for cables..................................................................... 12% Films and sheets......................................................................... 18% Flooring.......................................................................................... 17% Coatings......................................................................................... 15% Miscellaneous extruded products......................................... 14% Miscellaneous uses.................................................................... 14% 90% Rigid polyvinyl chlorides Tubes and fittings...................................................................... 5% Bottles, records, etc................................................................... 3% Films and sheets.................................... 2% Data calculated from Olivier.0 10% Chemical Engineering--Moy t, 1967 151 CE REFRESHER . . . URL 17982 Th Chemistry of Vinyl Chloride Polymerization Vinyl chloride or mixtures of comonomers that are predominantly vinyl chloride are polymerized com mercially by free-radical chain reactions that are quite similar in the basic steps to those of high-pressure polymerization of ethylene.* The basic steps and kinetics for vinyl chloride polymerization follow. Initiation is normally accomplished with a compound that forms free radicals at relatively lew temperatures. Initiators in large-scale use for vinyl chloride poly merization are lauroyl peroxide, isopropyl percarbonate, and azo-bis-isobutyronitrile.* About 30'i of all peroxide initiators (for polymerization of various polymers) are used for vinyl polymers, including polyvinyl ace tate. Consumption of peroxide initiators totals about 13 to 14 million lb./yr. Chemical steps that occur during initiation are: I --2R* n. + CHj=CHCl -- n--CH,--CHCl (1) (2) where R* is a free radical obtained from the initiator I, and k* is the rate constant for Eq. (1). The initiator fragment R* becomes incorporated in the free radical formed in Eq. (2). The rate of decomposition of I is: -d(I)M - MI) (3) where the parentheses around I in Eq. (3) indicate concentration of I in the solution. [Parentheses will be used similarly in other kinetic equations in this article.] Since two free-radicals R* are formed from each decomposition of an initiator molecule, and since only a fraction f of R* reacts via Eq. (2): rf(M*)/rf/ - (4) where M* is the free radical formed after the addition of a vinyl chloride molecule to a free radical such as R. Propagation involves addition of vinyl chloride to free radical at the end of a growing chain. kv --R(--CH---CHCl),--CH?--CHCl 4- CHr=CHCl----- R(--CH,--CHCl),+--CH,--CHCl (5) where n varies from zero to a very large number. The reaction-rate constant kv for the propagation steps is considered to be independent of chain length, i.e., the value of n. Such an assumption has been verified for many types of free-radical polymerizations. The kinetic equation for Eq. (5) is then: -rffCHs--CHCl)/<ft - MM) (CH,--CHCl) (6) where M* is, in this case, the concentration of: R(--CH,--CHCl),--CH,--CHCl Termination steps, in which the concentration of free radicals decreases, often involve the reaction between two growing chains M. Coupling is essentially the com bination of (M),, and (M*) to form where n and m refer to the number of (--CH*-CHC1-- > groups in each chain. Disproportionation results in two polymer molecules, M* and M.; one of the molecules is saturated, and the other has a double bond at one end. The rate equation is: -rf(M)Aft - 2k, (M*)1 (7) where ki is the rate constant for termination by both coupling and disproportionation. The Integer, 2, is re These steps were described In an earlier article of this se ries, Chem. Eiit,., Dec. IS, IGGG, p. 114. quired, since two free-radicals are destroyed by each termination reaction. The steady-state approximation is often applicable to free-radical polymerizations, including those of vinyl chloride. This approximation says that d(M*)/d ap proaches and remains equal to zero for most of the polymerization. Therefore, the sum of Eqs. <4) and (7) is zero, and hence: (M-) - (I)*'* (8) Substituting Eq. (8) into Eq. (G) yields: - - (~>0 {CHl==sCHC1) (9) If f has a low value, it is then essentially directly proportional to (M*). In this case, the rate of vinyl chloride polymerization is proportional to (CH,=CHC1)*=. Since (I) and (CH,=CHC1) vary with time, Eq. (9) or its modifications has frequently been used only for determining initial rates of polymerization--in which case, the quantities (I) and (CH^CHd) refer to the initial concentrations. Eq. (9) has been found to rep resent the kinetic data for some polymerizations of vinyl chloride that occur in dilute solutions. This evi dence tends to substantiate the proposed model for the reaction. However, when concentrated solutions or un diluted vinyl chlorides are used, the equation does not lit the data. In suspension polymerization, which is of major industrial importance, vinyl chloride is present in an undiluted state. Chain-transfer steps are methods of terminating the growth of a free-radical chain; but, in the process, the radical is transferred to another molecule. The net result is that there is no decrease in the concentration of free radicals. Chain transfer can occur with vinyl chloride as follows: R(--CH*--CHCl)*--CH,--CHCl + CH,=CHC1----- R(--CH.--CHCl)*--CIIjCHCl, + CH,=CH Vinyl chloride can add to the radical CHi=CH to form another growing chain. However, some of these radicals act to terminate and destroy other free radi cals.1* For example: 2CH-*CH -- CH*=CH--CH=CH, (Butadiene) Butadiene acts as a chain terminator. When it adds to a growing chain, an unreactive resonance-stabilized allvl radical is formed. Chain transfer can also occur with the initiator or PVC polymer molecule. With the latter, a long-chain branch will form. Chain-transfer steps in which an other active radical is formed do not affect the over-all rate of polymerization. Usually, the molecular weight of the polymer is decreased when transfer is with vinyl chloride or with the initiator; but it is not de creased when transfer is with a polymer molecule. Operating conditions during polymerization affect the characteristics of PVC molecules. As a rule, in creased temperature and increased concentrations of initiator decrease the molecular weight of the PVC polymer. Chain branching becomes of increased im portance when the concentration of polymer builds up, i.e. at high degree of polymerization. Various additives such as those used in suspension and emulsion poly merization may result in chain-transfer steps. 152 May 8, 1967--Chemical Engineering for food packaging. At present, PVC film has found wide acceptance as a meat-wrap, and some believe that it will dominate this field by 1968. Suitable plasticizers that can get clearance from the Food and Drug Admin istration are not always available for other types of film, or blow-molded bottles. Reynolds Metals has pub licized some features of its process to form a heatshrinkable PVC film that is plasticized with an epoxidized soybean oil, or similar material.2 This latter film is proposed for irregularly shaped articles such as fruits, vegetables or frankfurters. Recently Air Reduc tion Co. has announced a propylene-modified PVC that is clear and has received FDA clearance.6 2. Increasing use of PVC polymers in buildings.3- An effort is being made to develop suitable polymers for siding, window frames, conduits, gutters and til ing. In many cases, it is necessary to have archaic building codes changed to allow use of these plastics for building components. Polymerization Fundamentals More than one phase exists during polymerizations of vinyl chloride. This is particularly true in suspen sion or emulsion polymerization--presently the two most common commercial methods. In both methods, at least two phases are always present, namely the discontinuous organic phase and the continuous water phase. However, as polymerization progresses, there is both a vinyl chloride phase and a PVC phase also present. Mass transfer is then of importance during the polymerization reaction. Suspension Polymerization Winslow and Matreyek21 have presented a schematic diagram of the states of dispersion of vinyl chloride for suspension polymerization. This is shown in Fig. I. The dispersed vinyl chloride droplets suspended throughout the water phase are subjected to shear by means of mechanical agitation. As a result, the larger unstabilized droplets are broken up into smaller ones that tend to coalesce and reform into larger droplets. A dynamic equilibrium between dispersion and coales cence then occurs. The degree and type of dispersioncoalescence is important in terms of the porosity and bulk density of the final polymer product. Protective colloids such as starch, proteinaceous ma terials or, frequently, polyvinyl alcohol (as indicated in Fig. 1) are added to stabilize the vinyl chloride droplets and help prevent agglomeration of the PVC droplets. These colloids are soluble in water but in soluble in vinyl chloride. One of their important prop erties is that they can increase the viscosity of the water layer, and hence delay the process of coales cence.23 Inorganic materials are sometimes added to help prevent or regulate coalescence. Materials such as kaolin, barium sulfate, magnesium carbonate, talcum, neutral phosphates, and bentonite clay have been found effective. These inorganic particles are quite insoluble in either phase, and concentrate at the interface be tween the water and organic phases. The initiator that starts polymerization is soluble in vinyl chloride. Viscosity of the organic phase in creases as polymerization occurs, and polymer mole cules form throughout the droplets, which become syrupy. Agglomeration of particles is a problem during this phase of polymerization. Since PVC is quite insoluble in vinyl chloride, a "PVC phase" is produced in. the dispersed organic droplets. When this phase occurs, auto-acceleration is noted by an increase in the rate of polymerization instead of the decrease that is expected as vinyl chloride concentration decreases. Auto-acceleration has been explained by a decrease in the rate of termination steps for the over-all reac tion. Evidence has been obtained to indicate that po lymerization occurs in both the vinyl chloride and PVC phases. Polymerization in the PVC phase occurs as vinyl chloride diffuses to the active sites of that phase.18 These active sites tend to be located close to the surface of the semisolid PVC phase. Due to their limited mobility in the PVC phase, the active sites cannot easily react with each other to cause coupling or disproportionation reactions that destroy free rad icals. When auto-acceleration occurs, the degree of polymerization also increases--thus supporting the URL 17983 SUSPENSION POLYMERIZATION oc curs with a suitable stabilizer in a series of steps, as illustrated schemat ically. The type and degree of coales cence is important relative to the bulk density and porosity of the final poly vinyl chloride particles.--Fig. 1 Vinyl chloride bulk monomer Adsorbed molecular film of polyvinyl alcohol Agitation (Shear) Coalescence " a* | Interfacial tension o Ooo oo o o 0 ;<$& ' I Stabilizer I such as I polyvinyl T al'coh' ol 1 -O O q O o o 0oOo00 O Ckewical EagiuHring--May 1,1967 153 URL 17984 CE REFRESHER . . . hypothesis that there is a decreased possibility of the emulsifier has been transferred to the polymer chain-terminating steps. phase, and as the polymer particles grow, a lesser and lesser amount of emulsifier is available for a given Emulsion Polymerization surface area. The amount of emulsifier affects the stability of the The theory of emulsion polymerization is not yet emulsion20; the number of micelles, and hence the completely understood. Although the general concept number of polymer particles produced; and, in some as outlined by Harkins14 is widely accepted, the mech cases, the rate of polymerization. The results of Peg- anism may not be entirely applicable to all types of gion1 differ from those of Harkins14 relative to the emulsion polymerization of vinyl chloride. For example, effect of some of the operating variables. Peggion sug Peggion and others19 report data for some polymeriza gests that in the early stages of polymerization some tions, which do not agTee with the earlier mechanism. vinyl chloride is polymerized while it is dissolved in At least four components are involved in emulsion the water. He points out that a rather appreciable polymerization--water, vinyl chloride, initiators, and amount (0.6% by weight) of vinyl chloride is soluble an emulsifying agent. Water is the continuous phase; in water at 50 C. After the polymer groups formed by vinyl chloride is the discontinuous phase. The initi initiation in the micelle grow, and after the micelles ators are water soluble, and the emulsifying agent disappear, polymer formed in solution coagulates and stabilizes the emulsion formed when the system is precipitates on other polymer particles. Peggion indi agitated. Emulsifiers, either anionic or cationic, behave cates that the mechanism for polymerizing vinyl chlo as essentially normal electrolytes at low concentra ride may be quite different than that for less-soluble tions. When the emulsifier concentration is increased, monomers. surface tension decreases between phases, and conduc Termination steps involving coupling or dispropor tivity of the mixture increases. Eventually a critical tionation are rare in emulsion polymerization because concentration is achieved.. Above this concentration, the number of PVC polymer chains for each particle surface tension and conductivity change less rapidly. is small. Hence, PVC polymers produced by emulsion The emulsifier that was previously distributed uni polymerization tend to have high molecular weights. formly begins to agglomerate into groups (called Occasionally, a second free radical enters the polymer micelles) that contain 20 to 30 molecules.14 particle and causes termination, or initiates another The initiator in the water phase forms a free radical polymer chain. that migrates to the micelle. Here the radical reacts There is still a need for considerably more investi with vinyl chloride to initiate the polymer chain, as gation of emulsion polymerization. Many operating shown in Eq. (2). variables are of importance, and each has complex Propagation reactions occur as additional vinyl chlo effects on the final reaction and on the character of the ride molecules combine with the growing free radical, emulsion. There is evidence that no sharp line of differ as shown in Eq. (5). A polymer particle starts to ence exists between emulsion and suspension polym form, and the emulsifier collects at the surface. Vinyl erization. chloride molecules then diffuse from the dispersed droplets of vinyl chloride through the water phase and Bulk Polymerization through the emulsifier to the growing chain. As polym erization progresses, and as the other polymer par The characteristics of bulk polymerization are essen ticles grow in size, more emulsifier is needed at the tially identical to those of suspension polymerization surface of the particles. In some cases when 12 to if the temperature can be adequately controlled. Meth 20% conversion is reached during emulsion polymeri ods of temperature control are quite different, and will zation, the emulsifier micelles have disappeared and be discussed in a later article that will describe a the emulsifier is all located at the surface of the par commercial bulk-polymerization process. ticles.14 At higher conversions (perhaps 60%), all of the monomer is in the polymer phase. Solvent Polymerization ft The number of polymer particles formed and hence their size seems to be controlled in the early stages The kinetics and mechanism of solvent polymeriza of polymerization. Initially, there is competition be tion have been thoroughly discussed by Mickley, Mi tween growing polymer particles and micelles for each chaels and Moore.18 A solid PVC phase often forms as vinyl chloride molecule that is transferred. If a grow polymerization progresses, and some solvents are bet ing polymer chain forms in most micelles, there will ter for solubilizing PVC than others. When a solid then be many particles, but each will be small because PVC phase occurs, auto-acceleration also occurs. The the molecules of vinyl chloride will be limited. In some reaction mechanism is in many respects similar to that cases, the number of polymer particles remains almost of suspension polymerization. constant after the micelles disappear.1 Evidently, poly merizations cannot easily be initiated in dispersed vinyl chloride droplets. Characteristics of PVC Molecules Auto-acceleration sometimes occurs in emulsion po Vinyl chloride monomers can combine in several lymerization,1 and apparently both monomer and poly ways during propagation. In most cases, vinyl chloride mer phases exist in the polymer particles. After all of molecules react to form a head-to-tail arrangement of 1S4 May 1,1967--Chemical Enfintariaf Effect of molecular weight on unplasticixed PVC and blends of PVC and ABS polymers--Table II Vygen 65* Molecular weight of PVC.............................. Solution viscosity.......................................... Tensile strength, psi...................................... Flexural strength, psi.................................... Flexural modulus, psi.................................... Notched Izod at 77 F., ft.-lb./in................ Heat distortion (264 psi.) for: 10-mil deflection, *C.................................. 60-mil deflection, *C.................................. 62,000 0.70 7,750 11,750 420,000 0.44 69 75 Product of General Tire & Rubber Co. t Blend contains 70 parts Vygen 85, and 30 parts ABS. $ Blend contains 70 parts Vygen 120, and 30 parts A8S. Vygen 85* 74,000 0.80 7,775 12,000 420.000 0.50 69 76 Vygen 120* 107,000 1.18 7.850 12.500 440,000 0.80 75 80 Vygen 85/ABSf Vygen 120/ABS* 74,000 107,000 6.150 9,225 330,000 14.0 6,250 9,600 360,000 18.0 66 72 74 78 URL 17985 Effect of molecular weight on plasticized PVC--Table III Vygen 85* Vygen 105* Vygen 110* Vygen 120* Molecular weight................................................................. Solution viscosity.................................................... ............ Tensile strength, psi............................................................ Ultimate elongation, %....................................................... Tensile strength at 100% elongation, psi.......... .......... 83,000 0.93 2,490 300 1,400 93.000 1.03 2.730 340 1,420 107,200 1.18 2,890 350 1,460 Product of General Tire & Rubber Co. The formulation for the plasticized PVC (Vygen) is: 100 parts resin, 50 parts plasticizer (dioctyl phthalate), and 2 parts of Ba-Cd stabilizer. the repeating units ( --CH2--CHCI--) in the chain12: tail XX (_CHr--CHCI--CHf-CHCl--) XX head Occasionally, a few tail-to-tail and head-to-head ar rangements occur: (--CHCI--C'Hj--CH*--CHCI--) (_CH,--CHCI--CHCI--CHi--) Crystallinity Factors The carbon atom to which the chlorine atom is at tached is asymmetric. As a result, PVC polymers can occur in various stereospecific arrangements*-- namely, atactic, syndiotactic and isotactic.1-24- 23 Com mercial PVC has been reported to be primarily syndio- The importance of these stereospecific arrangements of the repeating group of a polymer Is dramatically demonstrated by polypropylene. Atactic polypropylene is so mechanically weak that it is of no commercial Importance as a plastic. Isotactlc polypropylene has found large-scale uses both as a plastic and a synthetic fiber. The atactic form of the polymer because of the random d- and l- arrangement of the repeating units of the polymer does not pack as well as either the ail d- or all Iarrangement of the isotactic polymer. The syndiotactic form that has an alternating d- and l* arrangement Is presumably inter mediate in packing ability to the atactic and Isotactic forms. tactic in nature but is said to have considerable amounts of atactic regions.1-ss Some polymer experts believe that PVC is primarily atactic. In any case, commercial PVC is only slightly crystalline--perhaps being as high as 10 to 15%.24-28 Crystallinity varies with the method of polymerization. The polymer macro molecule is thought to be screw-shaped. Attempts have been made to produce isotactic PVC, and partial success seems to have been realized. The following points must be considered to determine whether isotactic PVC would have important commer cial interest: 1. The unplasticized product would presumably be quite crystalline, having increased density and tensile strength. These improved properties would be of defi nite interest. 2. The unplasticized product would presumably have a higher softening temperature.24 A problem with presently available PVC is a high softening tempera ture that is almost as high as its decomposition tem perature. This is especially true of high-molecularweight PVC. The decomposition problem with isotactic PVC would likely be accentuated, perhaps to an unrea sonable extent. If so, conventional methods of extru sion and injection molding might not be applicable with the unplasticized product. Further, modification of these molding methods would likely be expensive. Cbemicel Engineering -May 8, 1967 155 CE REFRESHER . . . URL 17986 3. If isotactic PVC were plasticized to destroy its improved packing and crystallinity, it is questionable whether the material would have any significantly im proved properties as compared with presently available plasticized materials. Molecular Weight of PVC Polymers Commercial PVC polymers have number-average molecular weights that vary from about 50,000 to 150,000.2,1- 25 Routine laboratory tests for PVC gener ally involve the measurement of solution viscosity of the polymer rather than the direct measurement of molecular weight. The relationship between solution viscosity 17 and molecular weight M for a specific poly mer is: i; = KM where K and a are constants for a given polymer-solvent combination. The higher-molecular-weight PVC resins are plas ticized and used for flexible tubing, welting, electrical components, garden hose and calendered film. These products are obtained by extrusion, and the resin is subjected to elevated process conditions for only a short period of time.* Intermediate-molecular-weight resins are used in film and sheet, coated fabrics and rigid products. Low-molecular-weight resins are used in fluidized-bed coatings, phonograph records and injec tion-molded parts. Tables II and III indicate the effect of molecular weight on the physical properties of unplasticized and plasticized PVC polymers.* Molecular weight has a sig nificant effect on the impact resistance of unplasticized polymers, and on the elongation and tensile strength at 100% elongation of plasticized products. Even better impact strength is obtained for rigid polymers (con taining little or no plasticizers) by blending nitrile rubber or ABS polymers* with PVC polymers. As in dicated in Table II, a blend of 70% PVC and 30% ABS produces impact strengths up to 20 times greater than pure PVC. Tensile and flexural strengths of the blends are somewhat lower. tions to occur on a given polymer chain. Evidence sup porting these facts is: 1. The rate of dehydrochlorination increases as molecular weight decreases, i.e. the rate is essentially proportional to the number of end-groups. 2. Chlorination of the double bonds decreases the rate of dehydrochlorination. Initiator fragments on the end of PVC polymer molecules may also be the starting point for decom position reactions. Chain branching, which occurs to only a relatively small extent, also decreases the sta bility of the polymer because tertiary carbon atoms exist with chlorine atoms attached. Such chlorine atoms are much easier to abstract than those attached to secondary carbon atoms. If any head-to-head ar rangement occurred, the polymer molecule would be relatively unstable at the point where the two chlorine atoms were adjacent. The exact mechanism for dehydrochlorination is not completely understood but depends on the presence or absence of oxygen. Inorganic stabilizers are added to most PVC resins. Quality Control Tests The following tests are generally made on batches of the finished polymer before shipment to the con sumer: (a) solution viscosity in order to determine average molecular weight, (b) bulk density, (c) plas ticizer takeup, (d) irreversible plasticizer takeup, (e) moisture content, (f) mill stability, (g) clarity, (h) "fish eyes'* or gel particles, (i) foreign particles such as dirt, (j) particle-size distribution, and (k) press stability. For electrical-grade PVC, the following addi tional tests are made: conductivity and pH. Plasticizer takeup is of interest since it indicates the rate at which the plasticizer and PVC resin mix, and hence the rate at which extrusion or injection molding can be done. Careful control of quality is therefore essen tial in order to produce suitable polymers. Chemical Factors Affecting Stability Compounding of PVC Polymers PVC polymers are relatively unstable with regard to temperature (especially 200 C., or higher) and light8 Hydrogen chloride is then evolved. Some of the double bonds thus formed are attacked by oxygen or enter into cross-linking reactions. Severe degradation of physical properties and appearance may result, de pending on the plasticizer. Although all factors affect ing stability are not known, several features of the polymer molecule contribute to this instability. (Hence, variations In the techniques of polymerization that minimize these features are important.) The end-groups of PVC molecules are often the weak point at which decomposition reactions begin. Such an end-group is, -- CHC1--CHs--CC1 = CH*. A free-radical mechanism can easily begin at the double bond. Addi tional double bonds are produced during dehydrochlorination, allowing additional decomposition reac " Terpolymen of acrylonitrile <A), butadiene (B), and sty rene (S). The polymers of vinyl chloride are often compounded (i.e., blended) with plasticizers and stabilizers. Lubri cants and pigments are sometimes added. Hard, hornlike PVC is converted to a softer and rather flexible material by compounding it with a plasticizer. Generally, plasticizers decrease the tensile strength of the polymer, decrease the processing time for extrusion or molding operations, increase the allow able elongation of the polymer, and increase the im pact strength and the low-temperature flexibility.10-18 The exact role of the plasticizer in PVC is not known, but it acts to partially solvate the polymer chains. As a result, separation of the chain is increased and, hence, the otherwise strong intermolecular forces be tween the chains are decreased. Any crystallinity originally present in PVC polymers is destroyed by the plasticizer. Table IV gives one example of the relationship among the various degrees of plasticization and im- 154 May I, 1967--Chemical EnginMriag portant physical properties. Desired properties for the plasticizer include: 1. Adequate compatibility with the PVC resin-- that is, the degree to which PVC resin is solvated by the plasticizer. With high compatibility, a more or less true solution is formed. With lower compatibility, there may be a phase separation. For example, at higher temperatures such as are used in extrusion or molding, a single phase may be present. As the mixture is cooled, a PVC-continuous phase forms and a plas ticizer-discontinuous phase is also present. The degree of compatibility has an important effect on the proper ties of the final product. In addition, and especially with low compatibility, the plasticizer may slowly dif fuse out of the final product. Hence the physical prop erties could change significantly with time. 2. Low volatility, in order to minimize loss of plas ticizer from the product and to minimize odor. 3. Good stability--particularly in regard to heat and light, or other forms of radiation. 4. Nonflammability. 5. Nontoxicity. 6. Satisfactory low-temperature properties. 7. Reasonable cost. No plasticizer meets all of these characteristics, and selection of the plasticizer or mixture of plasticizers involves a compromise. Several classifications have been given to plasti cizers. External plasticizers are those additives that are mixed physically with the PVC resin, whereas internal plasticizers react chemically and are incor porated into the polymer chain. The following discus sion pertains to external plasticizers that are divided into primary and secondary ones. Primary plasticizers are highly compatible with the resin, but secondary ones are only of intermediate compatibility. Plasticizers are commonly organic esters with a high molecular weight, about 300 to 1,500. In 1965, phthalic anhydride esters amounting to 679 million lb. were produced as plasticizers, of which di-(2-ethylhexyl) phthalate was the major one.7 Other phthalate esters include various C< to Cio alkyl phthalates. Esters of sebacic, adipic, azelaic and phosphoric acids are also good plasticizers. Alcohols for these esters are fre quently obtained by using the Oxo process. Straightchain alcohols such as produced by Continental Oil Co.'s Alfol process may have certain advantages as compared with branched-chain alcohols.50 Epoxidized oils and esters, polymeric esters having molecular weights ranging from about 2,000 to 5,000, chlorinated polyethylene, and other polymers such as ABS resins (see Table II) have been used commer cially as plasticizers or as special blending materials.* Stabilizers Certain stabilizers have been found useful in min imizing decomposition reactions caused by heat, light or ultraviolet radiation. Heat is always a factor in the extrusion or molding operations of the polymer. Most stabilizers are metal salts. Coprecipitated barium and cadmium laurates, sometimes with zinc laurate, are used in most PVC resins as stabilizers against heat oxidation.21 By making an opaque plastic, stabilization against ultraviolet or visible light is accomplished. With transparent PVC, several stabilizers have now been found to be quite successful. In general, the sta bilizers are used in relatively low concentrations, fre quently less than 1 to 2% in the final polymeric mate rial. A detailed analysis of the complicated problem of stabilization, and available stabilizers is given by Chevassus and deBroutelles.8 Other Additives Fillers, pigments or dyes, lubricants, and fungicides or pesticides are sometimes added, depending on the final use of the plastic. The fillers are generally cheap extenders such as clays used for certain electricalgrade PVC resins. The clays absorb free acid and other polar compounds. Asbestos fillers are used in certain floor-tile products. Copolymers Copolymers prepared from a mixture of comonomers containing 60'*, or more, of vinyl chloride, with the remainder being primarily vinyl acetate, are of com mercial importance.13 Copolymers account for perhaps 25% of the production capacity of vinyl chloride poly mers. Copolymers tend to improve two important phys ical properties of the homopolymer of vinyl chloride, namely flexibility, and limited solubility in solvents. External plasticizers may provide one method for in creasing flexibility, but such plasticizers are not always adequate for solubility requirements. URL 17987 Effect of plasticizing with dioctyl phthalate--Table IV Parts of dloctyl phthalate per 100 parts of Vyoen 120* 0 30 40 50 60 70 Tensile strength, psi....................................................................... Elongation, %.................................................................................... Shore A hardness. 10 sec............................................................. 7,750 5to25t 115 3,550 265 98 3,200 295 92 2,850 345 86 2,500 370 79 2,100 410 72 * Vy^an 120 (a product f General Tire & Rubber Co.) has an intrinsic viscosity of 1. IB and a molecular weight of 107.000. t Estimated from Plastics Properties Chart in "Modern Plastics Encyclopedia 1965,' McGraw-Hill, New York, 1964. Chenical Engineering-May t, 1967 157 CE REFRESHER . . URL 17988 Properties of rigid press-polished sheets of vinyl chloride/vinyl acetate copolymers--Table V Vinyl Acetate Content Property Mill roll temperature for softening, " F. 3% 340 15% 250 Hardness, Rockwell M............................... 0 50 Notched impact, ft.*lb./in......................... 0.65 0.20 Abrasion loss in 2,000 cycles, %............ 0.02 0.13 Heat distortion (66 psi.), C.................... Tensile strength, psi................................... 67 8,200 57 8,500 Yield stress in flexure, psi........................ 12,800 12,200 In one sense, copolymers are internal plasticizers. Certain comonomers such as vinyl stearate or longchain esters of maleic anhydride are sometimes co polymerized into the final product. In such a case, the plasticizer is chemically bonded into the polymer chain and hence is a true interna) plasticizer. Vinyl Chloride/Vinyl Acetate Copolymers The properties of copolymers of vinyl chloride and vinyl acetate are dependent on the relative ratio of the two comonomers that have reacted, assuming that the final polymer molecules have the same molec ular weights. Copolymers containing less than 10% vinyl acetate have physical properties quite similar to those of the homopolymer of vinyl chloride, except for the lower temperatures required for compound ing. Differences in mechanical properties tend to in crease rapidly as concentrations of vinyl acetate in crease above 10%. Table V gives the properties of two copolymers containing 3% and 15% vinyl acetate.13 Copolymers containing 13#, or more, of vinyl ace tate and having relatively low molecular weights are used for protective and decorative coatings, flexible film, floor tiles, and compression moldings where ex ceptionally good flow characteristics are required.13 Phonograph records are one example in which precise duplication, and hence excellent flow characteristics, are needed. A vinyl chloride/vinyl acetate ratio of 87/13 for the copolymer has been used. If the heat stability problem can be solved, relatively low molecu lar weight homopolymers can be used as an alterna tive solution.24 Copolymers containing less than 13% vinyl acetate and having relatively high molecular weights are used for rigid sheeting, extruded rods and calendered articles. When the copolymer is to be used for protective or decorative coatings, a small amount of maleic anhy dride or other polymerizable carboxyl hydrocarbon is sometimes copolymerized with the vinyl chloride and vinyl acetate.1* Carboxyl groups improve the adhesion properties of the coatings. In other cases, a portion of the acetate groups on the polymer chain is removed by hydrolysis to produce hydroxyl groups that also improve adhesion. Vinyl chloride/vinyl acetate copolymers are pro duced by batch polymerizations. Hence, the relative ratios of vinyl chloride and vinyl acetate that react tend to vary with the time of the run, unless vinyl chloride (the more reactive comonomer) is added in order to maintain a constant ratio of comonomers for the reaction.1* Vinyl chloride is also copolymerized commercially in significant amounts with acrylonitrile and vinyl- idene chloride.22 Other types of copolymers have been reported, but their commercial importance is small. A copolymer of vinyl chloride and propylene has recently been announced by Air Reduction Co. The properties announced for this copolymer are such that it will likely find important uses--especially since it has received clearance from the Food and Drug Ad ministration for "clear" food containers.* References 1. Blllmeyer, F. W., "Textbook of Polymer Science." Inter* science. New York. 1962. 2. Reynolds Expand# PVC Film Plant, Chrm. Eng. Jan. 1. ] 962. p. 46, 3. Rigid PVC I# Set for Sale# Growth, Chrm. Eng. Xnra, July 29. 1963, p. 34. 4. Competitive Growth for Peroxide Catalyst?. Cheni. Eng. New*, Feb. 24, 1964, p. 25. 5. Vinyl Siding Makers Gear for Battle, Chew. Eng. .Veins, Oct. 19. 1964. p. 26. 6. Propylene-Modified Bottle* Could Ease Problems of PVC Bottle? for Foods, Chrm. Eng. New*. Mar. 7. 1966, p. 31. 7. Plasticizer Sales Pass One Billion Pounds, Chrm. Bng. AVir a, Aug. 1. 1966, p. 20. 8. Chevassu*. F. and deBroutelles. R.. "The Stabilisation of Polyvinyl Chloride." translated by C. J. P. Elchkorn and E. E. Sarmiento. St. Martin's Press, New York. 1963. 9. Douglas. W. C.. Gyenge. .1. M.. Haeklm. G. and Hanley, A. J.. PVC Fabrication* and Applications, AIChE Seminar by General Tire ft Rubber Co.. Oct. 1964. 10. Fedor, W. S,, Plasticisers, Chrm. Eng. Vetca. Nov. 13, 1961, pp. 118-138. M. Fedor, W. S., Commodity Forecasting, Chem. Eng. Newt, Sept. 12. 1966. pp. 80-9'*. 12. Flory. P. J., "Principles of Polymer Chemistry," Cornell University Pres*. Ithaca. X. V., 197.8, 13. Ham. G. E.. ''Copolymerization," pp. S87-637, Interscience, New York, 1964. 14. Harkin*. W. D.. A General Theory of the Mechanism of Emulainn Polymerization. J. ACS., SB. 1428-1444 (1947). 1 Miekley. H. S., Michaels. A. S. and Moore. A. E., Kinetics of Precipitation Polymerization of Vinyl Chloride. J. Polymer Sot.. SO. 121 (1962). 16. Mllkovleh. R.. Theory of Polymerisation, AIChE Seminar by General Tire ft Rubber Co.. Oct. 1964. 17. Olivier. G.. What's the Future for PVC, Ilutlmcarbon Procca*.. Sept. 1966. p. 281. 18. Park. R. M., Plasticizer--Versatile, Nereasarv Resin Modi fier. ilsitlrocorbxn Procraa.. Mar. 1962. p. 120. 19. Pegglon. E., Tpata. F. and Talamln, O., A Kinetic Studv of the Emulsion Polymerization or Vinyl Chloride. Mokrom>ilekvlurr Chrmir, 71, 173 (1964). 2(1 Powers, J, R. (to B. F. Goodrich Cn>. "Polymerization of Vinyl Compounds," T\ S. Patent 2.320.9**9 (Sept. .'*. 197*0). 21. Richard. W. R.. Stabilizers--Key to Ageless Plastics, 11 yarncurbon Proccxa., Mar. 1962, p. 123, 22. Schlldknerhl. C. E., "Vinyl and Related Polymers." Wiley. New York, 197*2, 23. Schildknecht, C. E., "Polymer Processes," Interscience, New >ork, 1956. 24. Smith, TV. M., "Vinyl Reains." Reinhold. New York, 197*8. -j. Smith, w. M., ".Manufacture of Plastics," Vol. I, pp. 303343, Relnhold, New York, 1964. 26. Spencer, F. J., Progress in Polymers Today. Hydrocarbon Proccea.. July 1966, p. 83. ,27. Winslow. F. H. and Matreyek. \V,, Particle Size in Sus pension Polymerization. Ind. Eng. Chem., 43 1108 (1951). Key Concepts for This Article Active (*) Reviewing Passive (9) Input/Fcedsteck (1) Output/Frdart <S> Processes* Monomers* Plastics* Chemistry* Vinyl chloride* Polymer?* Polymeriza- Vinyl acetate* Copolymers* tion* Plasticizers* Polyvinylchloride* Polymers* Stabilizers* (Words In bold are role Indicators; numbers correspond to I^C-AIChE system except for Role 8 modification. Asterisks mark key concepts suggested for Indexing. Others are added to Ijppro'c reading ax an abstract. Indexing is deai-ri)>ed In f'licm. Eng.. Oct. 11. 1965. p. 187; or you may order Kev Concept reprint, 50f, using Reader Service Postcard.) 156 May 8, 1967--Chemical Engineering