Polymer Science & Engineering Course

Poly(vinyl chloride) 1. Introduction Poly(vinyl chloride) (PVC) has a chemistry and a physical structure that makes it broadly unique in the polymer world. PVC (often referred to vinyls or vinyl resins) is made commercially at several molecular weights, depending on the intended applications: from Mw = 39000 g/mol, to Mw = 168000 g/mol. PVC chemistry follows:

(I)

where n, i.e. degree of polymerization, ranges commercially from 625 to 2700. PVC has grown to be one of the major plastics of the world. It was the largest group of thermoplastic materials; however, the vinyl resins have been suppressed in volume by the olefin polymers. PVC is second in volume to polypropylene among plastic materials, Figure 1. The volume of each individual categories of polyethylene is smaller than PVC’s.

1.1 Raw material The chemical process for making PVC involves three steps: first, production of the monomer, vinyl chloride; then the linking of these monomer units in a polymerization process; and finally the blending of the polymer with additives, Figure 2. In this section, the production of raw materials will be considered. Ethylene comes from oil or natural gas which is refined and 'cracked' by heating ethane, propane or butane or naptha from oil. Ethylene reacts with chlorine to form ethylene dichloride (EDC), which is finally cracked to the monomer. Chlorine is produced from electrolysis of salt (NaCl). In this process, the dissolved salt is chemically decomposed by passing an electric current through it. This produces chlorine and sodium. Sodium reacts with water to form caustic soda (sodium hydroxide) and hydrogen gas. The manufacture of PVC accounts for 35% of the chlorine produced industrially and 0.3% of the worlds supply of gas and oil, Figure 3. Most other polymers are largely

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Polymer Science & Engineering Course hydrocarbons and thus are highly dependent on supplies of oil and gas; whereas PVC is much less dependent on the supply of oil and gas.

Figure 1. Annual worldwide use of plastics (year 2000).

Figure 2. PVC production.

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Figure 3. the raw materials for PVC.

2.2 Chlorine content Vinyl compounds often contain nearly 50% chlorine. Because of this, in a fire, vinyl provides only about half the fuel compared to other polymers. Halogens in flameretardants, including chlorine in PVC, additionally provide condensed phase and gas phase combustion resistance by a radical- trapping, flame-poisoning mechanism. This is a reaction of the neutral halogen with hydrogen on the fuel to remove hydrogen as a fuel element and to form hydrogen halide. Thus, vinyl is unique in its resistance to combustion because it has lower intrinsic fuel content than other polymers, it can form hydrogen chloride in the condensed phase to reduce available fuel, and it can inactivate hydrogen from fuel in the gas phase. This chemistry is as follows:

R−X

+

Flame retardant

H ~~~~~~~~ → R • Hydrogen on fuel

+

Flame retardant fragment

H − Cl

+

• ~~~~~~~~

(1)

Fuel, less hydrogen

Additionally, the hazards in a fire are mainly carbon monoxide and heat; these are especially a problem with materials other than vinyl that readily burn. When forced to burn, vinyl produces carbon monoxide, carbon dioxide, and hydrogen chloride. Of these, the most hazardous is carbon monoxide. Hydrogen chloride is an irritant gas that can be lethal at extremely high levels, which are never reached or even approached in real fires.

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3. Polymerization Vinyl chloride monomer (VCM) is a clear, colorless liquid that boils at -13 ºC. VCM is polymerized via free-radical methods, Scheme 1. VCM has a liquid density at normal polymerization temperature between 0.85-0.9 g/cm3. The polymer has a density of 1.4 g/cm3 which is a sign of the large shrinkage during polymerization.

Scheme 1. VCM polymerization.

3.1 VCM vs. oxygen Oxygen is an effective but undesirable shortstop for the polymerization reaction. An alternating copolymer of oxygen (peroxide, R-O-O-R') and VCM (called poly(vinyl chloride peroxide)) will form until oxygen is consumed and the polymerization reaction starts. Thus, oxygen must be excluded from the polymerization.

3.2 VCM vs. nitrogen Nitrogen is quite soluble in VCM, so it is not good practice to use nitrogen pressure to move the monomer about the plant or the laboratory. Different alternatives are available:

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Polymer Science & Engineering Course o Pumps o Slight heating of the storage tank to generate enough pressure for monomer transfer o Gravity transfer

Nitrogen does not interfere with the polymerization; however, because nitrogen is soluble in the monomer and not soluble in the polymer, significant pressure can build up in the polymerization vessel during polymerizations containing nitrogen, especially if there is not a lot of headspace in the vessel.

3.3 Polymerization Because of the low boiling point of the monomer and the limited stability of the polymer, polymerization of vinyl chloride must be carried out under conditions giving rapid reactions at low temperatures (polymerization temperature is 50 ºC). Bulk polymerization cannot be carried to high conversions because of local overheating leading to deterioration of the polymer. Solvents are often used to prevent this effect. PVC is mostly produced by suspension polymerization since the reaction can be controlled better. PVC solubility in its monomer

PVC is highly insoluble in its own liquid monomer. PVC precipitates as tiny particles during polymerization and the agglomeration of these particles produces a porous internal structure in the resin particles. VCM, however, is fairly soluble in PVC, and thus the particles of PVC precipitated during the polymerization are highly softened by the monomer. The rate of polymerization of the VCM inside the swollen particle is substantially faster than the rate of polymerization inside the liquid phase, because the gel prevents mobility of the growing polymer radicals and therefore results in a substantial lowering of the radical termination rate in the gel; notice that polymerization termination takes place by either combination or disproportionation. Thus, PVC polymerizations are auto-accelerating. As the conversion increases, more of the fast polymerizing (swollen) phase present. With a long half-life initiator, this results in a reaction rate that is proportional to the conversion. Long-life initiators are generally not used, so the acceleration tends to drop off as the active initiator is consumed. Polymerization kinetics

A typical reaction rate curve of heat versus conversion is shown in Figure 4. This curve is generated by a computer model simulation, but similar curves result from real

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Polymer Science & Engineering Course measurements of the heat release from actual polymerization reactions. The solid line is the predicted reaction curve and the dotted line is the actual heat that must be removed from polymerization reactor. Note that the dotted line is slightly lower than the solid line, because of the effect of water injection. Starting at about 60% conversion, a rate peak is observed. The rate peak starts when all of the liquid monomer has been consumed and pressure-drop starts. Further polymerization produces a change in the composition of the remaining swollen polymer phase. As the composition changes, the local mobility of the growing polymer chain is reduced relative to the mobility of the low molecular weight monomer. The radicals are not able to find each other easily. The termination rate is therefore reduced, causing the concentration of free radicals to rise and the rate of polymerization rises accordingly. At approx. 75% conversion, the composition has changed enough that now the monomer has reduced mobility and concentration and the polymerization rate starts to fall owing to a reduced propagation rate. At just over 90% conversion, the rate essentially approaches zero, because the polymer VCM phase passes through the glass transition temperature and essentially all mobility stops. Commercially, reacting beyond approximately 80-85% conversion is not very efficient. In order to keep this reaction under control during the rate peak, the polymerization reactor used for this reaction would need to be able to remove 2250 kilowatts of energy, allowing a slightly excess heat removal capability for margin of error. Note that the polymerization depicted in Figure 4 has a built-in safety feature. The reaction rate accelerates at 60% conversion as the pressure is dropping. A run away reaction at this conversion is not likely to produce a reactor over-pressurization situation.

Figure 4. Plot of heat load versus conversion predicted by a computer model for an isothermal polymerization.

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3.4 Molecular weight The molecular weight of PVC is controlled by altering the polymerization temperature, chain transfer to monomer controls the molecular weight. A typical radical will chain transfer about ten times before it ultimately terminates. The higher the polymerization temperature, the lower the molecular weight. As the temperature increases, the rate of chain transfer to monomer increases faster than the chain propagation rate. The reaction temperature can be isothermal for narrow molecular weight distribution or temperature ramped to give wider distribution. Normal commercial polymerization temperatures range from 50°C to approximately 70°C. Below 50 °C, the reactions become too slow and the molecular weight is generally too high for most processing needs. Above 70°C, the reactor pressure becomes too high. If extremely fast polymerizations are run, however, e.g., with a total polymerization time faster than approximately three hours, the concentration of free radicals can get high enough that the termination reaction can start to play a role in molecular weight control. Normal polymerization times run from 3 to 6 hours, but reactor design can greatly affect this. It should be pointed out that for PVC, the term chain transfer to monomer takes on a special meaning. Mechanistically, it is not a classical chain transfer mechanism. The chain transfer process in PVC actually starts with a monomer unit adding in a head-tohead fashion. The resulting growing radical structure does not continue to polymerize fast enough and generally, before another monomer can add, the structure rearranges, inserts a double bond at the end of the chain, and another new radical is formed, which in turn initiates a new growing chain. The process looks like chain transfer to monomer, but this mechanism is somewhat unique to PVC.

3.4.1 Molecular weight extension There are occasions, when a molecular weight is desired higher or lower than that produced at polymerization temperatures between 50 and 70°C. Polymerization temperatures lower that 50°C can be used without over-pressurization risk, but as the temperature is lowered, the reaction temperature approaches the temperature of the cooling water and the ability to remove heat is reduced or lost entirely. Higher molecular weight products can be made at normal polymerization temperatures with the addition of a multifunctional monomer such as di-allyl phthalate. This produces a branched polymer of higher molecular weight but not higher crystallinity. The amounts of multifunctional ingredients that can be used are limited, however, because eventually a cross-linked

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Polymer Science & Engineering Course product results. It is often good practice to meter the multifunctional monomer to keep its concentration low. Making lower molecular weight polymers is a bit more challenging. Going up in polymerization temperature is easy from a heat transfer perspective but usually the polymerization vessel cannot withstand the VCM vapor pressure. In order to make lower molecular weight PVC it is often necessary to employ chain transfer agents. Unlike polyolefins, however, there are no effective chain transfer agents yet. All of them have some problem. Some produce PVC with poor heat stability. Some retard the polymerization rate too much. Some require such a high level that they must be recovered with the VCM (with the resulting contamination of the recovered monomer). The most popular chain transfer agent in use today is 2-mercapto-ethanol (2-ME), which is not a real chain transfer agent, but rather it is a chain terminator. Additional initiator is needed to overcome the rate reduction. Another problem associated with making low molecular weight polymer is that high temperature and low molecular weight produces particularly soft and sticky primary particles and porosity is lost. Often, one's ability to make a very low molecular weight PVC is related to having a secondary dispersant that can produce enough porosity to allow effective VCM recovery.

3.5 Structure PVC has a linear structure formed by either head to tail (most common) or head to hear addition of monomer molecules to the growing polymer chain, Figure 5. Thus, the molecular structure is primarily –CH2- units alternating with –CHCl- units. (A)

(B)

Figure 5. Head-to-tail (A) and head-to head (B) addition of VCM to the growing PVC chain, X ≡ Cl.

PVC shows a tendency to add via syndiotactic placement; it is found that PVC is approximately 56 % syndiotactic at 50 ºC polymerization temperature. This irregularity of structure accounts for its low crystallinity. However, PVC achieves strength because of the bulky polymer chains as a consequence of the large Cl groups on every other carbon.

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3.6 Stability PVC is almost certainly the least naturally stable polymer in commercial use. During processing, storage and utilization, PVC degrades as it is exposed to high temperatures, high mechanical stresses or ultraviolet light, all in the presence of oxygen. Degradation of the polymer occurs by successive elimination of hydrogen chloride (HCl), which is called dehydrochlorination, yielding long polyenes, Scheme 2, which are consequently causing discoloration, deterioration of the mechanical properties and a lowering of the chemical resistance.

Scheme 2 Dehydrochlorination

Therefore, PVC requires stabilization for practically any technical application. Stabilization mainly proceeds by the addition of compounds, such as metal oxides, carbonates, fatty acid salts as well as HCl acceptors as ethylene oxide compounds. These additives stabilize PVC by slowing down the dehydroclorination reaction and absorption of the evolved hydrogen chloride.

3.6.1 The causes for the low thermal stability of PVC It is known that the quality, or thermal stability, of PVC decreases when monomer conversion increases. VCM is polymerized in a batchwise process, which means that the monomer supply gets more and more exhausted with increasing monomer conversion. As a consequence side-reactions by the macroradicals will increasingly occur, resulting in

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Polymer Science & Engineering Course the formation of a lot of different types of structural irregularities. Some of these defects are shown to have a dramatic influence on the thermal stability. The most occurring structural defects in PVC are a wide range of branches, which are formed by various routes. Some of them seem to affect the thermal stability while others are completely harmless. The frequently occurring branches and the most important types of branches concerning the thermal stability of PVC are described below. Besides head-to-head units within the polymer backbone, head-to-head emplacement of VCM to a growing polymer chain can also result in other types of irregularities within the chain. Chloromethyl (MB) and 1,2-dichloroethyl branches (EB) result from one or two successive 1,2-Cl shifts respectively, followed by regular chain growth as is shown in Scheme 3. The MB and EB structures are expected to have minor, if any, influence on the initiation of dehydrochlorination of PVC. This insignificant influence of MB and EB is probably due to the absence of tertiary chlorine at the branchpoint carbons, as it has been proven that an increase in the amount of tertiary chlorine in the polymer chain increases the thermal degradation. Other types of branching do contain tertiary chlorine at the branchpoint carbon such as the 2,4-dichloron-butyl branch and various types of long chain branching.

Scheme 3. Chemical consequences of head-to-head addition during the polymerization of VCM

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Polymer Science & Engineering Course The 2,4-dichloro-n-butyl branch (BB) is formed via a 1,5-backbiting mechanism (Scheme 4). The growing macroradical abstracts a hydrogen atom from the CH2 group at the fifth position in the chain leaving a radical at that point, after which propagation continues from there and a polymer chain bearing a butyl branch is created. Long chain branching (LCB) results from hydrogen abstraction, from a chloromethylene or a methylene unit of a polymer chain, by a growing macroradical or possibly a chlorine atom (Scheme 5). The newly formed macroradical will propagate further, generating long chain branches. About 66% of all LCB formed do contain tertiary chlorine at the branchpoint carbon, meaning that hydrogen abstraction from the chloromethylene unit occurs more often.

Scheme 4. 1.5-backbiting mechanism generating a 2,4-dichloro-n-butyl branch

Scheme 5. Formation of long chain branching after hydrogen abstraction from chloromethylene and methylene by macroradicals or chlorine atoms

Both BB and LCB are mainly formed when the monomer supply is almost exhausted. Due to monomer starvation the growing macroradicals start reacting with themselves or with neighboring polymer chains as described. Chapter 3: PVC

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Polymer Science & Engineering Course Diethyl branches (DEB) seem also to be present in PVC fractions produced at very high VCM conversions and therefore when monomer supply is almost exhausted. This type of branching is believed to contribute significantly to the thermal degradation of the polymer, due to the presence of two tertiary chlorines at the branchpoint carbons, Scheme 6. Internal unsaturation, especially internal allylic chlorines, IA in Scheme 5, proved to be one of the main structural defects influencing the thermal stability. Just like the tertiary chlorines the amount of internal allylic chlorines increases dramatically when monomer supply becomes exhausted. When the amount of allylic and tertiary chlorines in commercial PVC as well as their reactivity towards thermal degradation is taken into account, it seems that tertiary chlorine has a somewhat higher reactivity towards dehydrochlorination. The effect of head-to-head units, Scheme 3, in PVC on the thermal stability is still not conclusively proven, but it is known that the amount of these units is very small, presumably 0-0.2 per 1000 monomeric units. So if these structures have any influence on the thermal stability, it will be minor compared to other structural irregularities. Note that chloride groups in any of the defect structures are not stable and hence are “labile”. The order of chloride lability is: internal allylic chlorides ≈ tertiary chlorides > terminal allylic chlorides > ordinary secondary chlorides. Labile chlorides account for less than 0.5 % of all chlorides in PVC, but their effect on heat stability is dramatic. Because of their presence, PVC begins to noticeably degrade at temperatures as low as 100 ºC and degrades rapidly in its processing range of 170-220 ºC.

Scheme 6. Diethyl branches

3.7 Plasticization PVC is a very tough and rigid material with extensive applications. Its range of utilization is significantly expanded by plasticization, which converts rigid PVC, to flexible PVC. Plasticization involves blending PVC with plasticizers (e.g., diisooctyl

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Polymer Science & Engineering Course phthalate, tritolyl phosphate, epoxidized oils). The plasticizer imparts flexibility by acting effectively as an ‘‘internal lubricant’’ between PVC chains.

3.7.1 Mechanisms of plasticization For a plasticizer to be effective, it must be thoroughly mixed and incorporated into the PVC polymer matrix. This is typically obtained by heating and mixing until either the resin dissolves in the plasticizer or the plasticizer dissolves in the resin. The plasticized material is then molded or shaped into the useful product and cooled. Different plasticizers will exhibit different characteristics in both the ease with which they form the plasticized material and in the resulting mechanical and physical properties of the flexible product. Several theories have been developed to account for the observed characteristics of the plasticization process. Two of these theories will be described in this course. According to the Lubricating Theory of plasticization, as the system is heated, the plasticizer molecules diffuse into the polymer and weaken the polymer-polymer interactions (van der Waals' forces). Here, the plasticizer molecules act as shields to reduce polymer-polymer interactive forces and prevent the formation of a rigid network. This lowers the PVC Tg and allows the polymer chains to move rapidly, resulting in increased flexibility, softness, and elongation. The Gel Theory considers the plasticized polymer to be neither solid nor liquid but an intermediate state, loosely held together by a three-dimensional network of weak secondary bonding forces. These bonding forces acting between plasticizer and polymer are easily overcome by applied external stresses allowing the plasticized polymer to flex, elongate, or compress.

3.8 Copolymers VCM does not co-polymerize well. It has an unfavorable reactivity ratio with just about every other monomer except vinyl acetate (VAC), 5-40 % vinyl acetate, Structure II. Because of the unfavorable reactivity ratios, making other copolymers usually involves long reactions with slow metering of one of the monomers (the one that reacts fastest). This is required to produce what is known as a uniform co-polymer, where the composition at the beginning of the polymerization is similar to that made at high conversion. There is another general problem associated with copolymers. PVC is insoluble in the monomer and this produces the porous internal structure that facilitates monomer recovery. One of the reasons for this insolubility is that the polymer has a certain level of crystallinity. Random copolymerization tends to break up the crystallinity. Also, the fact that a second monomer is present changes polymer solubilities Chapter 3: PVC

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Polymer Science & Engineering Course and makes the polymer become monomer soluble. This produces a non-porous particle and alters the reaction kinetics dramatically. Today, most copolymers are made using microsuspension or emulsion methods, because these processes produce small particles that can be stripped of residual monomer. Copolymers entangle and fuse at lower temperatures and flow more easily at lower melt temperatures, because they have low or no crystallinity. Note that this copolymer has a Tg of 28 ºC. Polymers containing around 10 % vinylidene chloride, Structure III, have better tensile properties than PVC. Acrylic esters have also been used to improve copolymer solubility and workability.

(II)

(III)

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