Polyvinyl Chloride
Michael W. Allsopp, Giovanni Vianello*
[Also see: PVC: A Health Hazard From Production through Disposal Paul Goettlich 25oct01]
1. Introduction
Poly(vinyl chloride), PVC, a polymer prepared from vinyl chloride monomer (VCM),
where n = 700 1500, holds a unique position amongst all the polymers produced today. It is relatively inexpensive and is used in such a wide range of applications that its versatility is almost unlimited. However, if it were discovered today it would probably be shelved as a somewhat intractable and thermally unstable material. How can this apparent contradiction be explained ?
The uniqueness of PVC can be considered under three headings: morphology, versatility, and molecular structure.
1.1. Morphology
As made, PVC is particulate in nature and comes in two main sizes depending on the process used. Suspension and mass polymerizations give grains (particles) of 100 180 ΅m in diameter, whereas the emulsion process affords a latex of particle size 0.1 3.0 ΅m. The latter is dried to yield friable grain-like structures of 5 50 ΅m.
Because of this unique particulate structure the most frequently used word in the vocabulary of the PVC technologist is morphology. In no other polymer is it as important as it is in PVC.
In its polymerization, a growing PVC chain becomes insoluble in VCM above a chain length of about 10 units [6] so PVC is essentially insoluble in its monomer and the process is thus classified as a precipitation suspension polymerization. However, PVC is heavily swollen and partly solvated by the monomer to the extent of 27 wt % [7] and this has a major influence on the polymerization itself (see Section 4.1. Suspension Polymerization of Vinyl Chloride) as well as the final properties and end uses of PVC. Hence, the way the PVC separates from the monomer, its future growth mechanism, and the swelling of the polymer by the monomer are critically important in its formation, handling, and subsequent processing.
1.2. Versatility
Poly(vinyl chloride) is a generic name. Each producer makes a range of PVC polymers which vary in morphology and in molecular mass, depending on the intended end use. In industry, the K-value and viscosity number are used to represent molecular mass, and producers often reflect these parameters in the grade codes used to define different products (e.g., S 68/173 refers to a suspension type material with a K-value of 68, and VY 110/57 to a resin with a viscosity number of 110). The calculation of the K-value is given in [8] and the relationship between these parameters and molecular mass in [9].
PVC with K = 66 68 can be processed in rigid formulations to give pipes, conduit, sheet, and window profiles; K = 65 71 in flexible formulations for flexible sheet, flooring, wallpapers, cable coverings, hoses, tubing, and medical products, and PVC with low K-values (55 60) in formulations for injection molding of pipe and conduit fittings, integral electrical plugs, and blow molding of bottles and other containers.
In contrast, poly(methyl methacrylate) of even very high molecular mass is still soluble in its monomer and produces only glasslike solid beads at the end of polymerization which have no internal morphology and as such are limited to a narrow range of end uses (e.g., moldings for optical applications such as covers for car lights and illuminated signs.
1.3. Molecular Structure
Amongst the range of polymeric materials produced today PVC is unique because the bulky chlorine atom imparts a strongly polar nature to the PVC polymer chain, and the essentially syndiotactic conformation of the repeat unit in the chain leads to a limited level of crystallinity [10]. This results in good mechanical properties, particularly stiffness at low wall thickness, high melt viscosity at relatively low molecular mass, and the ability to maintain good mechanical properties even when highly plasticized. This enables a wide range of softness and flexibility to be achieved and hence leads to an even wider variety of end uses.
2. Physical and Chemical Properties
2.1. Vinyl Chloride Monomer (VCM)
Vinyl chloride monomer (VCM), bp 13.4 °C, is a gas at room temperature and pressure. Therefore, it is handled as a compressed volatile liquid in all polymerization operations. Its vapor pressure over the typical polymerization temperature range of 50° to 70 °C is 800 1250 kPa. As a result PVC polymerization reactors are thick-walled jacketed steel vessels with a pressure rating of 1725 kPa. VCM is slightly soluble in water (0.11 wt % at 20 °C). Whilst this has some influence on the suspension polymerization process it is critically important to the success of the emulsion polymerization process described in Section 4.3. Emulsion Polymerization of Vinyl Chloride. The polymerization of VCM is strongly exothermic, and its specific heat and heat of evaporation of 1.352 kJ kg1 K1 and 20.6 kJ/mol, respectively, allow the use of a condenser to remove the heat of reaction as well as by the more conventional jacketed vessel systems. Its explosive limits in air are 4 22 vol % and plant design, particularly when handling unreacted VCM in the recovery system, must be designed and operated accordingly.
2.2. Poly(Vinyl Chloride) (PVC)
PVC is never used alone. It is always mixed with heat stabilizers, lubricants, plasticizers, fillers, and other additives to make processing possible, all of which can influence its physical and mechanical properties. Table (1) lists properties of rigid (unplasticized) PVC with a total additives content of <10 %. Table (2) list properties of flexible (plasticized) PVC where the range of physical properties varies widely, depending on the plasticizer content. This can vary between 20 and 100 phr so as a guide properties typical of a plasticizer content of 50 phr are given.
In addition the K-value (molecular mass) of the PVC can also influence properties significantly (see Section 4.1.4.3. K-Value (Molecular Mass)).
PVC has extremely good chemical resistance to all but low molecular mass chlorinated solvents. Therefore, it is widely used in the construction and lining of chemical plants.
3. Resources and Raw Materials
VCM is produced industrially by two main reactions:
1) Hydrochlorination of acetylene
2) Thermal cracking of 1,2-dichloroethane produced by direct chlorination or oxychlorination of ethylene in the balanced process. Presently, more than 90 % of the VCM produced is based on this route, full details of which are given in Chlorinated Hydrocarbons - 3.1. Vinyl Chloride (VCM).
In an ideal situation a PVC plant is fully integrated, beginning with ethylene and chlorine (salt), but various levels of integration are employed worldwide. Stand-alone PVC plants, supplied with VCM by sea, road, or rail, are well known as are ones making the bulk of their VCM requirements from scratch on site, with supplementary supplies of 1,2-dichloroethane for cracking to satisfy peak demand. Finally, there are fully integrated plants which have the benefit of uninterrupted supplies of base raw materials (ethylene and salt) and where the monomer is supplied by pipeline at a significant cost benefit. It can cost up to 120 DM/t to transport VCM within continental Western Europe depending on distance and type of freight used, i.e., road, rail or ship.
4. Production
There are three main processes used for the commercial production of PVC: suspension (providing 80 % of world production), emulsion (12 %) and mass, also called bulk (8 %).
4.1. Suspension Polymerization of Vinyl Chloride
4.1.1. Introduction
The layout of a typical suspension PVC plant is shown in Figure (1).
The suspension polymerization process is essentially a bulk polymerization process carried out in millions of small "reactors" (droplets). Liquid vinyl chloride under its autogeneous vapor pressure is dispersed in water by vigorous stirring in a reactor (autoclave) of 25 150 m3 capacity, fitted with a jacket and/or condenser for heat removal and baffles for optimum agitation. This results in the formation of droplets of average diameter 30 40 m which are stabilized against coalescence by one or more protective colloids (granulating agent). The other essential ingredient is a monomer-soluble free radical initiator.
The formulations charged to the reactors (otherwise known as autoclaves, kettles, or polymerizers) are normally referred to as recipes. A basic recipe for suspension processes can be simply water, VCM, initiator, and suspension agent, for example:
VCM 100 parts
Water 90 130 parts
Protective colloid 0.05 0.15 parts
Initiator 0.03 0.08 parts
These quantities vary depending on the PVC grade, reactor size, plant type, etc.
Whilst this will produce a particulate PVC it is unlikely to have the optimum morphology. This will only be achieved if other additives are employed, such as oxygen, buffers, secondary or tertiary granulating agents, chain-transfer or chain-extending agents, comonomers, antioxidants, together with the right level of agitation and homogenization and the correct charging procedure and timing for each additive addition. A combination of just the right number and degree of the above parameters to achieve the optimum morphology usually determines the quality of a PVC grade.
After charging, the reactor contents are heated to the reaction temperature of 45°75 °C. Reactors are 80 95 % full at this stage. Heat causes some of the initiator to decompose into free radicals, and the monomer in the droplets begins to polymerize. The strongly exothermic reaction (1540 kJ/kg) is controlled by removing heat via the jacket and/or by boil-off into a condenser from which the condensed monomer is returned to the reactor.
Progress of the reaction in a jacketed reactor can be followed by continuously monitoring the water temperature in the jacket since the flow rate is constant and a constant batch temperature is maintained by progressively reducing the cooling water temperature (see Fig. (2)).
Although PVC is insoluble in its monomer (see Section 1.1. Morphology) the polymer is swollen by ca. 27 wt % VCM to form a coherent gel [7]. Therefore, as the conversion increases towards 70 % and beyond, the pressure in the reactor starts to fall; slowly at first, but then much more rapidly as the last of the free liquid monomer is consumed. Polymerization continues in the gel phase, very rapidly at first since chain termination is hindered by lack of mobility of the growing chain, but, as conversion increases beyond 80 85 %, monomer starvation rapidly reduces the rate.
The reaction is terminated at a predetermined pressure by either adding a chain terminator and/or venting off the unreacted monomer to the recovery plant. After venting, the resin in the aqueous slurry can still contain 2 3 % unreacted monomer, which is removed by stripping, either batchwise (as in older plants) or continuously in a column. The batch is discharged from the reactor to a feed vessel and then fed continuously through a stripper column (Fig. (3)). The unreacted monomer is recovered, liquified, and reused in later polymerizations. After passing through a heat exchanger the slurry is fed to a continuous centrifuge to give a wet- cake with 20 30 % moisture content. The remaining water is then removed by conventional flash and/or fluid-bed drying to give a dry, free-flowing powder with a residual VCM content below 1 ppm.
4.1.2. Additives
Water.
The water used in the process should be demineralized (i.e. of low
conductivity) since ionic species, especially sodium ions, can affect the
performance of other additives such as protective colloids and influence final
polymer properties such as volume resistivity.
Protective Colloids (Granulating Agents).
The protective colloids used in the
suspension polymerization process are of two types: primary and secondary. The
main function of the primary granulating agents is to control grain size, but
they also affect porosity and other morphological properties. The demand for
polymers with reduced residual VCM levels and better plasticizer adsorption led
to the need for higher and more uniform porosity within each grain and this is
achieved by using a secondary stabilizer in addition to the primary.
Although a wide variety of surface active materials have been or are still used, the substances most commonly used today are cellulose ether derivatives and partially hydrolyzed poly(vinyl acetates) or poly(vinyl alcohols) (PVAs). The latter are usually preferred and are used alone or in combination with the cellulose type, which they are gradually replacing. Typical PVA protective colloids are listed in Table (3).
As PVA primary granulating agents are prepared by alkaline hydrolysis of poly(vinyl acetates) they are complex mixtures containing fractions with different degrees of hydrolysis and molecular mass and having a block distribution of the OH groups [11]. Because of this complexity suspension processes need fine-tuning around the chosen agent since decreasing the average degree of hydrolysis of the PVA increases the porosity of the PVC grains and affects the grain size, as shown in Figure (4). This behavior is used in conjunction with agitation to achieve the optimum size and porosity. Below 70 % hydrolysis the PVAs are insoluble in water and lose their ability to stabilize the VCM droplets adequately so the normal range of PVA granulating agents used are in the range 70 to 85 %.
Many surfactants of low molecular mass have been claimed as useful secondary stabilizers but in practice few can be used without adverse side effects. One exception is the nonionic sorbitan monolaurate. Again, PVAs are much more widely used. However, this family is prepared by acid hydrolysis of low molecular mass poly(vinyl acetates) and has a random distribution of acetate groups [11]. Initially these materials, which are insoluble in water, were prepared as methanolic solutions (e.g., Polivic S 202) but more recently aqueous-based, low-hydrolysis PVAs (e.g., Alcotex A 55, and Polivic S 404 W), which are safer to handle and are environmentally more acceptable, have been developed and are now very widely used in the industry. Typical secondary protective colloids are listed in Table (3).
The use of secondary granulating agents affects the mean grain size and grain size distribution so the concentration of the primary must be adjusted to compensate. Since each producer uses autoclaves of different size and shape, equipped with different agitation and baffle systems, with or without condensers, and makes many different types of resin, no absolute formula is available which determines the magnitude of this change in primary concentration; this must be developed in-house by the suspension polymerization technologist.
Initiators.
A range of initiators have been used, but currently a relative
few are widely employed. At one time benzoyl peroxide and azo initiators were
typically used, but diacetyl peroxides, peroxydicarbonates (PDC), and alkyl
peroxyesters are now preferred. Typical initiators are listed in Table (4).
The half life t1/2 at a particular temperature is the main factor which determines the choice of an initiator for a given PVC grade. The initiator should have a half-life of ca. 2 h at reaction temperature to optimize reaction time. Since the best plant output is obtained when the reaction profile is as square as possible (see Section 4.1.3. Kinetics) two initiators of different activity are often chosen to try to achieve this ideal. The more reactive initiator is used to start the polymerization but then as this exhausts itself the slower one takes over to completion. When preparing K 70 80 resins at temperatures at or below 52 °C, a peroxydicarbonate (e.g., 6, 7,8 or 9) is used as the primary initiator, with an active initiator such as 1 helping to bolster the slow initial rate which is obtained at these low temperatures. In the intermediate range of K 63 68 a single peroxydicarbonate (PDC) is usually sufficient, but at or below K 60 the PDC needs the extra radical flux of a slow initiator to prevent exhaustion of the reaction, so a less active diacyl peroxide such as 4 is used as a secondary.
Three other factors are also important: water solubility, storage and handling, and reactor fouling.
The initiators' water solubility can affect their efficiency by making them more susceptible to hydrolysis and can affect their distribution between VCM droplets; the higher the solubility the more homogeneous the distribution.
The ease of storage and handling has a major influence on the acceptability of an initiator to a PVC producer. In general the shorter the half-life of the initiator, the more unstable it is, and the lower its self-decomposition temperature. The stability also depends on the radical energy and physical form. Solid crystalline initiators such as 8 (BCHPC) are more stable than their liquid analogues, e.g., 5 (EHPC). Many of the more active initiators must to be stored under refrigeration which tends to restrict their acceptability. Liquid initiators have the advantage that they can be automatically metered into reactors, which is essential in larger computer-controlled plants. However, the current trend is to use initiators as finely divided aqueous dispersions. This has the advantage of improving their homogeneity of distribution throughout the VCM droplets, allows them to be added at any time or temperature during the charging procedure, and renders them very safe to handle since their heat of decomposition is less than the heat of evaporation of the carrier. Dispersion of 25 40 % are typical, and systems must be carefully designed to ensure complete homogenization of the dispersion is maintained and accurate metering achieved.
4.1.3. Kinetics
The kinetic profile seen in Figure (2) is typical of many VCM suspension polymerizations conducted at the lower end of the temperature range and exhibits the classical acceleration with conversion as shown by the gradually decreasing jacket temperature. At higher temperatures the reaction rate is more constant and a square-wave profile is obtained. Three different phases in the reaction can be identified:
Low Conversion (< 5 %).
Polymerization occurs largely in the monomer phase
because the quantity of polymer produced is so small. It is only this early
stage that follows classical kinetics where the rate of polymerization is
dependent on the square root of the initiator concentration:
where
[M] = monomer concentration
[ I ] = initiator concentration
kp = propagation rate constant
kd = initiator decomposition rate constant
kt = termination rate constant
As the amount of polymer phase increases with conversion the rate deviates more and more from this equation due to a decrease in kt . A full description of the models used to ex-plain the deviations from ideality is given in [12] , [13].
Medium Conversion (5 65 %).
Polymerization continues in both the free
monomer and polymer monomer gel phases. This region shows the classical
acceleration with increasing conversion [14]. Since termination occurs primarily
by the diffusion together and collision of two bulky macroradicals, it is
considerably slower in the viscous polymer phase. As a result the termination
rate constant is dramatically reduced and the rate of polymerization in this
phase is much faster. As the amount of polymer phase increases the rate of
polymerization accelerates.
High Conversion (> 65 %).
No free monomer is left to maintain the
autogeneous pressure, which falls, and polymerization occurs in the polymer gel
phase of rapidly increasing viscosity as the remaining VCM is consumed. Hence,
the rate of polymerization increases still further to reach a maximum just after
pressure drop but then decreases progressively due to increasing monomer
starvation. When the instantaneous polymerization rate falls below the overall
rate of reaction the batch is terminated, either by venting off unreacted VCM to
the recovery plant or by adding a chain terminator and then venting. Depending
on the properties required, a conversion of 70 95 % is chosen. It is
uneconomic to continue to very high conversion, and as properties such as
plasticizer absorption, dry up rate (i.e., the time taken to achieve a
free-flowing PVC plasticizer dry blend), and initial color heat
stability deteriorate rapidly above 85 90 % conversion, most flexible resins
are stopped at around 85 % and rigid resins at around 90 % conversion.
4.1.4. Morphology
Control of the morphology of each PVC grain is the main parameter controlling the quality of a PVC resin. This subject covers mean grain size, grain size distribution, and, most important of all, porosity (cold plasticizer absorption) and bulk density, which are closely interrelated.
4.1.4.1. Control of Grain Size
At the beginning of the batch the bulk vinyl chloride phase is broken down into droplets of 30 40 ΅m mean size under the influence of agitation and by the presence of the water-soluble protective colloid, which is adsorbed at the monomer water interface. As polymerization proceeds, graft copolymerization of PVC onto the interfacial layer of protective colloid reduces its mobility and protective ability, and droplet coalescence begins to occur at around 4 5 % conversion. As the conversion increases, changes in the surface layer of the droplet due to further polymerization suppress the droplet coalescence step, which ceases at 20 % conversion, and the grain size remains largely constant for the remainder of the polymerization. Altering the stirrer speed after this point has no further effect on the mean grain size (MGS) [15] of the final product, which is controlled to between 100 and 180 ΅m, depending on resin type, by a combination of initial agitator speed and protective colloid(s) concentration and type.
Polymerizations with high concentrations of colloid and less agitation give spherical grains of low porosity ( < 10 %) and very high bulk density ( > 700 g/L) and are of little interest, generally due to problems of VCM removal and poor gelation. Flexible resins are made with a low MGS (100 130 ΅m), rigid polymers higher (150 180 ΅m), and low K resins with intermediate size (130 160 ΅m).
In extreme cases it is possible to stabilize the droplets to such a degree that no coalescence occurs. Specialized VCM vinyl acetate copolymers for gramophone record production fall into this category as do blending resins (30 ΅m MGS) for use as viscosity depressants in plastisols (see Section 4.3. Emulsion Polymerization of Vinyl Chloride).
As well as achieving the desired MGS it is equally important to control the size distribution. A Gaussean distribution is normally produced and it is important to make this as narrow as possible since a coarse fraction (> 250 ΅m) can give rise to fisheyes (dispersion faults), and make VCM removal difficult as well as resulting in loss of yield due to screen rejects, while fines (< 60 ΅m) can be lost in aqueous and gaseous effluent or cause powder flow problems.
4.1.4.2. Control of Porosity and Bulk Density
Porosity in PVC grains is created by a complex series of interrelated steps in the formation and growth of the submicroscopic structure within each droplet and also depends on the unique nature of the VCM PVC system.
PVC is insoluble in its monomer, and the growing oligomers start to precipitate when the chain length exceeds 10 monomer limits [6]. This initial precipitation results in the formation of microdomains about 15 20 nm in diameter [16]. This occurs at as low as 0.001 % conversion [17]. The microdomains rapidly become unstable and aggregate to form domains (primary particle nuclei) of about 0.1 ΅m diameter. After this stage no further domains are formed and the number of primary particles is fixed. All new PVC radicals that are produced in the monomer phase collide with and are precipitated onto the vast existing surface area of primary particles before a new domain can be formed. Since the existing PVC phase is swollen with monomer and contains initiator, initiator radicals, and growing chains undergoing chain transfer, primary particles grow rapidly by accretion of new micro radicals and microdomains and by gel-phase polymerization. They in turn become unstable and a final agglomeration step to form 1 2 ΅m primary particle aggregates takes place. By the end of the polymerization the primary particles have grown to 1 m within the aggregates, which themselves have grown to 2 5 ΅m. The whole mechanism is shown in diagrammatic form in Figure (5) and discussed in detail in [18].
The arrangement of the aggregates of primary particles within each monomer droplet has a major influence on the morphology of the final PVC grain. The conversion of VCM of density 0.85 g/cm3 at 52 °C to PVC of 1.4 g/cm3 produces a volume contraction of 39 %. If the polymerizing droplets contract totally the final grain has no porosity. If, however, the contraction is prevented totally the final grain porosity can be as high as 39 %. Therefore, the better the three-dimensional spatial arrangement of the aggregates relative to one another the higher the porosity that is achieved.
The main aim of the polymer technologist is to create the best possible spatial network of aggregates relative to one another so that a network is formed that is strong enough to resist droplet contraction and hence maximize porosity. The sooner this is achieved in the polymerization the better the porosity attained. In practice 34 35 % is easily achievable. At ca. 65 70 % conversion all of the free monomer is consumed, and the only VCM remaining is that swelling the polymer gel (27 %) and in the vapor space and dissolved in the water. At this stage the voids in the grain (the porosity) are filled with batch water, so that the quantity of water in interstitial spaces between the grains is reduced and a significant increase in slurry viscosity is observed since less mobilizing water is available to separate the grains.
The control of bulk density or apparent density (AD) is more important than even porosity to the processor. The latter is concerned with charge size and cycle times in high speed mixers, output rates, motor torque and degree of gelation in extruders, and mechanical properties of the article being produced. These parameters are very dependent on AD. The reason for introducing the control of porosity before control of AD is that the latter is very heavily influenced by the former. Apparent density depends on porosity, grain shape, and grain size distribution in order of priority and is best described by examining the packing fraction (PF) concept, i.e., the ability of the somewhat irregular PVC grains to fill a space to the maximum efficiency.
CPA = cold plasticizer absorption (porosity)
AD = apparent density
Most extruders used in processing PVC are of the twin screw type and these operate like a simple pump: increasing the feed rate by 10 % (10 % higher AD) increases the discharge or output rate by 8 9 % (see Chap. 8. Processing and End Uses). In practise, controlling porosity is relatively easy since a number of effective secondary stabilizers are available. Maximizing the AD is much more difficult since control of grain shape comes from a complex interaction of both agitation and choice of primary and secondary granulating agents together with in-house expertise of charging techniques and use of other additives. A range of grain shapes which can be produced is shown in Figure (6).
In practice typical property targets for porosity and AD are:
1) For flexible resins (K 70): porosity of > 30 % and AD of > 500 g/L
2) For rigid extrusion resins (K 66 68): porosity 20 % and AD 580 g/L
3) For bottle resins (K 57 60): porosity 18 20 % and AD 560 g/L
4.1.4.3. K-Value (Molecular Mass)
PVC is again rather unique in its relationship between reaction temperature and K-value. With PVC the rate of polymer formation by chain chain termination is small due to the influence of gel phase polymerization described above. Instead, since chain transfer to monomer is a predominant step in the polymerization of VCM, the length of the polymer chain (degree of polymerization, DP) is determined by the ratio of the rate of chain propagation to chain transfer:
[M]= monomer concentration
[R·n] = radical concentration
kp= propagation rate constant
ktr= chain-transfer rate constant
As kp and ktr depend only on temperature the molecular mass of PVC is controlled by the reaction temperature (see Figure (7)).
If a particularly low molecular mass polymer is required or if a reactor's pressure rating is too low to operate at very high temperatures an additional chain-transfer agent can be employed. This reduces the molecular mass by increasing the chain-transfer rate constant above that for monomer alone in the above equation. As a result DP decreases, depending on the quantity of chain transfer agent used.
The general K-value range of commercial resins is K 55 to K 75, with a few producers providing speciality resins outside this range to K 50 and K 80 and with just one or two K 90 100 polymers also being available. Despite the relatively narrow K-value/molecular mass range of most commercial resins (see Table (5)) the polar nature of the PVC chain gives rise to a considerable melt viscosity span. Mechanical properties increase slightly with increasing K-value so wherever possible the highest K-value polymer is used. However, the power and mechanical robustness of equipment and the complicated melt flow pattern requirements in molds often restrict the K-value which can be used. For example, most bottles are made from K 57 polymers, some from K 60, but this is the limit for the application because bottles are blow molded so restricting the melt viscosity which can be handled.
4.1.4.4. Problem Areas
There are relatively few problems in the operation of the suspension polymerization process. However, to obtain the best possible quality and consistency some points need special attention.
In the past reactor fouling was bad and mechanical cleaning between each batch essential. With the advent of clean-wall technology, high-pressure rinse systems, and improved recipes it is now possible to operate in a closed manner, thereby reducing problems of VCM exposure (see Chap. 5. Environmental Protection). Reactor opening for cleaning is reduced to once in several hundred batches. It is essential to ensure that the reactor is rendered completely free of polymer between batches since this is the most common cause of fisheyes. Equally important is to ensure there is no chance of cross-grade contamination in the downstream operations of slurry handling, stripping, drying, and storage. Contamination of the product by foreign substances is eliminated by good housekeeping and plant management together with the right choice of metals of construction and design. Stainless steel is widely used in many sections of PVC plants.
4.2. Bulk or Mass Polymerization of Vinyl Chloride
The mass polymerization process is virtually identical to that taking place within each monomer droplet in the suspension process. As such the mechanism and kinetics are very similar to those described in Section 4.1.3. Kinetics. The major difference is in the mechanical operation of the process.
About 10 % of world PVC production is provided by the mass process. The process was developed by Pechiney St. Gobain (PSG) who are now part of the French ATO Company. All mass producers are licensees of the PSG technology. The process is carried out in two stages:
1) In the first stage monomer and initiator are charged and vigorously agitated in a vertical, stainless-steel autoclave (prepolymerizer) of 8 25 m3 capacity, fitted with a water- cooled jacket and condenser. Rapid polymerization takes place at 62 75 °C to give ca. 100 ΅m aggregated spherical flocs composed of 0.1 ΅m primary particles, which form the basis or seeds of the final PVC grain. Conversion is taken to 7 12 % in ca. 30 min, by which time the initiator used is exhausted.
2) The slurry from the prepolymerizer is discharged into the second-stage reactor, which has a volume of 12 50 m3, with fresh initiator and more VCM. Up to five second-stages can be fed from one prepolymerizer. In the second stage the 0.1 ΅m primary particles making up each seed grow in size and fuse together to give the final grain of PVC of 130 160 ΅m diameter. As the conversion increases the physical nature changes from a wet powder at ca. 20 % to a dry one at 40 % conversion. The heat of polymerization is removed from the growing grains by evaporation of the VCM and condensation on the cooled wall of the reactor or in a water- cooled reflex condenser(s). The second stage takes 3 9 h, depending on K-value.
Originally, the second stage reactor was a horizontal jacketed vessel with one or more condensers, agitated by slowly rotating (6 10 rpm) ribbon blenders with some blades set very close to the reactor wall to keep it clean. However, it proved difficult to discharge and clean the reactor to a sufficiently high standard to prevent the formation of fisheyes (dispersion faults) in PVC grades intended for plasticized applications [19]. The product can contain up to 10 % of coarse material which must be sieved out and recovered by grinding, milling, and reclassifying.
This led to the development in 1978 of a vertical second-stage reactor of up to 50 m3 capacity fitted with two independent agitators designed to overcome the above discharge problems. However, there has been little interest in this development and all recent investment in new PVC plants has been in the suspension process. In theory the mass process should be cheaper since it does not need a drying stage. However, it is a two-stage process, it is difficult to remove traces of unreacted monomer, and it gives a significant amount of oversized material, all of which offset its advantages.
The mass process is discussed in more detail in [19] , [20].
4.3. Emulsion Polymerization of Vinyl Chloride
4.3.1. Introduction
The emulsion polymerization process involves the polymerization of monomer in an aqueous medium containing surfactant and a water-soluble initiator, producing PVC latices. PVC latices are colloidal dispersions of spherical particles, ranging in size between 0.1 and 3.0 ΅m. Most PVC latices are spray dried and then milled to obtain fine powders, made up of agglomerates of latex particles. When mixed with plasticizers they disperse readily to form stable suspensions. During mixing most of the agglomerates are broken down into the original latex particles. Such dispersion of fine particles in plasticizers are known as plastisols or pastes, and the powder is called dispersion or paste polymer. The surfactant layer around the particle surface prevents their adsorbing the plasticizer at room temperature so they can be used as liquids and may then be spread on to fabric or other substrates, poured on molds, or deposited on formers to produce flooring, wall covering, artificial leather, balls, toys, or protective gloves. There are other grades of PVC polymers, produced by emulsion polymerization, that do not form plastisols and that are used as blends with suspension PVC grades for extrusion application or in the manufacture of battery separator plates. These so-called emulsion polymers are of only minor economic interest. Sales in latex form are very limited; latices are used in water-based paints, printing inks, and impregnated fabrics. The total production capacity of emulsion PVC is given in Table (6).
4.3.2. Emulsion Polymerization Processes
The ways to produce PVC latices are:
1) The classic emulsion polymerization, in which the particle size and particle size distribution are controlled by a water-soluble initiator and a surfactant [21][22][23][24][25]. They are monodisperse with a particle size typically less than 0.1 ΅m.
2) Seeded emulsion polymerization, in which the particle size distribution of the final latex depends on the amount and the size of the latex used as seed and on the amount of surfactant and the way in which it is added [26] , [27]. A typical material has 0.2 and 1.2 ΅m particles (see Fig. (8)).
3) Microsuspension polymerization, in which a monomer-soluble initiator is used. Polymerization takes place within the fine droplets in which the monomer is dispersed by passing a coarse emulsion of monomer, initiator, and surfactant through a mechanical homogenizer [28][29][30]. The resulting particle size distribution is Gaussian between 0.2 and 1.2 ΅m.
4) Polymerization of fine emulsions achieved by using a combination of a typical surfactant such as sodium dodecyl sulfate and a long-chain fatty alcohol. This gives rise to spontaneous emulsification of the monomer into very fine droplets. In this case the polymerization can be carried out by using a water-soluble or a monomer-soluble initiator [31]. The latex particle size distribution is the same as for microsuspension polymerization.
The industrial polymerization techniques used for carrying out these processes are the following:
1) Batch polymerization where all the monomer is charged at the beginning
2) Semibatch or semicontinuous polymerization, with the monomer added in stages or continuously as reaction proceeds
3) Continuous polymerization where monomer and other components are fed continuously into the reactor and the latex is withdrawn from the bottom of the reactor
All these processes are in current use to achieve a wide range of latex characteristics, which are related to the rheological properties of the PVC plastisols [32].
4.3.3. Description of the Batch Process
The emulsion polymerization of PVC consists of the following stages:
1) Polymerization, 2) VCM removal, 3) latex storage, 4) drying, 5) milling, 6) packing and storage. Figure (9) shows a schematic representation of the stages required for a plant which produces all the emulsion, seeded, and microsuspension grades.
A recipe for a simple batch emulsion polymerization (in parts by weight) is as follows:
Demineralized water 110 140
Vinyl chloride 100
Emulsifier 0.1 1
Initiator 0.1 0.2
The polymerization takes place in an autoclave resistant to the VCM vapor pressure. Pressures of 6.4 10.0 bar correspond to polymerization temperatures of 40 60 °C, depending on the desired molecular mass. The polymerization reaction is strongly exothermic with a heat of polymerization of 1534 kJ/kg. Heat removal can be a problem as the size of the autoclave increases because the ratio between surface area and volume becomes less favorable. For this reason industrial plants commonly use water cooling, and external condensers. Modern plants use autoclaves with capacities ranging between 30 and 80 m3. Large autoclaves can present agitation problems because low shear rates are required to maintain mechanical stability of the latex and to avoid coagulation.
Initiators.
Although organic peroxides slightly soluble in water such as
methyl ethyl ketone peroxide and 1-hydroperoxy-1'-hydroxydicyclohexyl
peroxide can be used, the most widely used initiators are ammonium or potassium
peroxosulfate. Their decomposition in aqueous solution is first order with
respect to the peroxosulfate ion concentration [33]. The rate may be increased
significantly by using a reducing agent. Redox catalysis involves the use of
transition metal ions such as copper or iron [34]. The systems widely used in
emulsion polymerization of vinyl chloride are potassium peroxosulfate sodium
bisulfate copper or iron. Redox systems can also be used in microsuspension,
an example being dilauroyl peroxide copper ascorbic acid [35].
Emulsifiers are very important in the emulsion polymerization of vinly chloride because they determine not only the latex characteristics but also the properties of the final product [36][37][38]. Moreover, the nature and the concentration of the emulsifier, together with the agitation, are major parameters controlling the mechanical stability of the latex during polymerization and handling, and the major cause of reactor wall encrustation and lump formation. Since the latex is dried by spray drying, the emulsifier remains in the resin and can influence properties of the product such as the flowability of the powder, plastisol formation, the viscosity of the plastisol, and also the properties of the final article such as heat stability, water adsorption, and coat adhesion. For this reason a wide variety of emulsifiers are used. During polymerization anionic types are usually used, while nonionics are generally added to the latex after polymerization. Typical emulsifiers are the sodium salts of alkyl sulfates, alkyl sulfonates, alkylbenzenesulfonates, dialkyl sulfosuccinates, alkyl ethoxysulfates, fatty acid soaps, alkyl phenol ethoxylates, and fatty acid ethoxylates.
Particle Formation.
Emulsifiers are characterized by the critical
concentration at which their molecules aggregate to form micelles [39] , [40].
In Figure (10) a plot of surface tension versus emulsifier concentration is
shown; the point at which the curve exhibits a marked change is known as the
critical micelle concentration (CMC). Conventional emulsion polymerization is
carried out at a concentration above the CMC. In this way, particle formation is
controlled by the micelles, which solubilize the monomer by swelling and become
the loci of polymerization. The nucleation phase stops when the micelles
disappear, and the emulsifier is absorbed on to the surface of the particles.
Depending on the balance between the hydrophilic and hydrophobic part of the
molecule, each emulsifier plays a specific role in the emulsion polymerization,
as they have different CMC values, surface covering powers, and stabilizing
effects.
VCM Removal.
The polymerization is terminated at 90 % conversion and is
followed by the venting of unreacted monomer to a gas holder. After degassing,
the latex contains 3 % vinyl chloride and further reductions must be made to
fulfil legal requirements for environmental protection. The residual VCM is
removed by stripping under vacuum in a stripping vessel. Although VCM loss from
particles by diffusion is fast, VCM removal from an emulsion is much more
difficult than from suspension slurries as the surfactants lead to large amounts
of foam formation. Continuous processes using a thin-film evaporator or which
spray the latex into an evacuated chamber against a countercurrent stream of gas
reduce considerably the problems of foaming and coagulation [41][42][43][44].
The residual vinyl chloride monomer content of the latex after stripping ranges
between 200 and 2000 ppm, depending on the type of latex and the operating
conditions.
Drying and Milling.
Although there are several different techniques to
separate polymer from water, such as freeze drying, drum drying, or coagulation
followed by centrifugation, the most widely used is spray drying because this
confers a special structure to the dried particle that is important for the
production of plastisols. A fine spray of latex is introduced into a hot air
stream where evaporation takes place.
The spray is obtained by various types of atomizers, such as:
1) Spinning disks or wheels rotating at 15 000 20 000 rpm
2) Single pressure nozzles operating at 100 500 bar
3) Two-fluid pressure nozzles using compressed air up to 4 bar as atomizing fluid [45]
The drying air temperature at the inlet is 150 240 °C and at the outlet, 55 80 °C, depending on the quality of the required product and the type of latex. By varying the drying conditions it is possible to change the resin characteristics, and hence its use, even from a typical paste grade to a general-purpose emulsion resin, e.g., for rigid extrusion. The dry powder consists of spherical aggregates, with the size usually ranging from 1 to 100 m diameter with an average of 30 40 ΅m. The aggregates are called secondary particles, to distinguish them from those of the latex, which are known as primary particles. The way the aggregates form is complex and depends on many parameters such as primary particle size, type and amount of emulsifier, mean residence time in the drying chamber, droplet size of the spray, and temperature [46]. Figure (11) shows a scanning electron micrograph of spray-dried PVC particles formed from bimodal seeded latex.
Plastisol-grade resins, usually made from multimodal latex (i.e., with a Gaussian particle size distribution), are milled to reduce the size of the coarse fraction, which can cause problems in coating applications of the plastisol. Generally the viscosity of the plastisol increases when the polymer has been milled, so it is desirable to reduce milling as far as possible and to use air classification to remove the coarse fraction (> 65 ΅m) under control. In industrial production, air classifiers or mills or a combination of both are used. After milling, the average particle size of the resin is in the range of 5 15 ΅m.
Polymers for rigid extrusion and for manufacturing plate separators in batteries are sold in unmilled form. Resins are characterized by analysis of molecular mass, emulsifier and salt content, pH of aqueous extract, granulometry, and viscosity and rheological behavior of the pastes.
4.3.4. Plastisols
Plastisols, commonly called pastes, are dispersions of PVC powders in plasticizers. A wide range of plasticizers are used in plastisol formulations, but the most widely used are C4 C12 esters such as phthalates, adipates, azelates, sebacates, trimellitates, and phosphates. Chlorinated paraffins are used as secondary plasticizers. A good plasticizer should have low volatility, low color value, neutral reaction, resistance to hydrolysis, insolubility in water, flame resistance, and nontoxicity. As no plasticizer can satisfy all these properties several plasticizers are mixed to achieve the desired properties of the final product. Table (7) shows the influence of plasticizers on plastisol properties; Table (8), the influence of plasticizers on final product properties; and Table (9), the influence of resin characteristics on plastisol and final product properties.
The amount of plasticizer used in plastisol formulations plays a major role in determining the hardness and flexibility of the end product and can vary from 40 to 130 phr. In Figure (12) the influence of plasticizer content on plastisol viscosity is shown.
The other essential ingredients in plastisol formulations are:
1) Heat stabilizers, which generally are the same compounds used for suspension polymers.
2) Dyes and pigments suitable for coloring plastisols. The most widely used is titanium dioxide.
3) Fillers, generally inorganic materials such as calcium carbonate, barite, silicate, kaolin or china clay, are used primarily to lower costs and also to impart special properties such as hardness, abrasion resistance, and no sticking.
4) Blowing agents for foamed PVC production such as azodicarbonamide.
5) UV adsorbers to prevent the decomposition action of sunlight.
6) Esters of poly(ethylene glycol), phosphate esters, and fatty acid amines and amides are usually employed as antistatic agents.
7) Viscosity depressants, such as white spirit, poly(ethylene glycol) monolaurate, alkylphenols, low molecular mass paraffins.
8) Extender resin or filler polymer a PVC grade of spherical form, made by the suspension process and therefore cheaper than dispersion resin, which is used to reduce cost and viscosity of the plastisol [47] , [48].
In Table (10) the influence of plastisol ingredients on the final product characteristics is shown.
4.3.5. Polymer Plasticizer Interaction
Figure (13) illustrates the types of dependence of viscosity on shear rate;
1) Newtonian (viscosity is independent of shear rate)
2) Pseudoplastic (viscosity decreases as shear rate increases)
3) Dilatant (viscosity increases with increasing shear rate)
Over a wide range of shear rate a plastisol may exhibit all three types of behavior since the viscosity of plastisols rarely displays true Newtonian behavior. At low shear rates they are generally pseudoplastic, at intermediate values they show dilatancy, and may sometimes become pseudoplastic again at higher shear rates, as shown in Figure (14). There is no certainty on the exact mechanism as the rheology of PVC plastisols is complex and affected by many factors such as:
1) Polymer/plasticizer mixing ratio
2) Type of plasticizer and its interaction with polymer
3) Particle size and their distribution of primary particles
4) Structure, shape, porosity, and surface of the secondary particles
5) Surfactants, added during polymerization or in the paste
6) Molecular mass and molecular mass distribution
7) Paste-making conditions such as mixing temperature, time, and intensity [49].
4.3.6. Gelation and Fusion
Gelation is defined as the change from liquid to semisolid state, and fusion, when an homogeneous phase at molecular level between polymer and plasticizer is reached. These processes have been examined by determining viscosity changes of the liquid system, the tensile strength, and changes in complex viscosity and viscoelastic properties over the temperature range 25 200 °C.
At the beginning of heating, plasticizer viscosity decreases, as does the plastisol viscosity. At the same time, polymer particles adsorb plasticizer and swell, reducing the fraction of liquid plasticizer, and the viscosity then increases rapidly. When the liquid phase is completely absorbed by the particles, the system becomes dry. As the process continues the interparticle boundaries disappear and the polymer chains become increasingly entangled and development of the physical properties begins. At this point the gelation stage is complete and the fusion process starts, as illustrated in Figure (15). Further heating of plastisol makes the polymer flow into a continuous mass. The melt viscosity, after having reached a maximum, starts to decrease and the process passes through the fusion stage. Morphology of the particles, particle size, particle size distribution and molecular mass of the resin affect the viscosity behavior. Low molecular mass resin starts the gelation phase earlier and completes the gelation process at lower temperature. The same result is obtained by using plasticizers with higher solvating action, while larger particle size increases gelation temperature or time because it slows the penetration of the plasticizer into the resin [50].
4.3.7. Applications
Vinyl plastisols can be applied by spread coating, knife coating, roll coating, molding, dipping and spraying; the most widely used application is coating.
Coating.
Vinyl plastisol can be used to coat paper, fabrics, metals, felt,
and glass fibers to produce wallpapers, floor coverings, vinyl leather, conveyor
belts, and tarpaulins. Each application needs its own formulation, in which the
appropriate resin grade and plasticizer must be used to give the required
rheological characteristics. In the case of fabric coating where direct
spreading is employed, it is important to avoid penetration of plastisol into
the fabric; therefore, the viscosity behavior of the paste must be strongly
pseudoplastic at low shear rate and have moderate viscosity at high shear rate.
In wallpapers, where thin coatings are applied, a low-viscosity resin with
Newtonian behavior is required. In production of floor coverings, three or even
four coatings are applied: an impregnation layer to saturate the fibers, one or
two foam coats, and a wear layer with high mechanical strength. Each coat, which
needs different grade resin to achieve the required properties, is gelled at 150
°C before application of the next layer. When the last coat is applied the
entire coating is fused in an oven at 200 °C. The foam layer can be chemically
or mechanically foamed [51]. Usually mechanical embossing is used to create
surface textures and apply colored patterns.
Dipping.
The product, mainly work, household, and surgical gloves, is
obtained by dipping metal or ceramic molds or an article to be coated into the
plastisol and then withdrawing it, draining it, and then curing it in an oven.
Molding
Molding is used for producing hollow articles such as balls, dolls, and toys.
The process requires a plastisol with low viscosity at low shear rate that has a
short gelation time and is easy to deaerate. In rotational molding the plastisol
is poured into a cold mold that rotates around two perpendicular axes whilst
entering an oven where it is heated with air to 200 250 °C. After gelation
and fusion, the mold is cooled in a waterbath.
Further information is given in [32] , [39] , [40] , [45] , [48] , [52][53][54].
4.4. Chlorinated Poly(Vinyl Chloride), CPVC
The softening point (heat distortion temperature or Tg ) of PVC can be increased by chlorination of the polymer, either in a solvent or as a suspension. The normal chlorine content of 56.7 % is raised to 63 68 %, depending on type, and this increases the softening point from ca. 78 83 °C (depending on formulation) to at least 120 °C.
The older method involves chlorination of the PVC dissolved in a chlorinated solvent, but the process is expensive because of the need for an extensive solvent recovery system. However, it has the benefit of yielding a homogeneous material. The product is difficult to handle physically because of its very low bulk density.
The most common commercial process in use today disperses the PVC grains in water and swells them with a chlorinated hydrocarbon. The reactor is degassed to remove oxygen and heated to reaction temperature (50 60 °C); chlorine is added continuously and the system irradiated with UV light to produce chlorine radicals. Hydrogen chloride is produced as a byproduct and is removed at the end by washing with an inorganic base, after which the product is dried [55]. The degree of chlorination is variable, with the surface layers of the grains being preferentially treated.
CPVC is less thermally stable than conventional PVC and requires higher temperatures to develop good mechanical properties. Hence, it is difficult to process and quite expensive, but blends with suspension PVC can be used to achieve a specific softening point. CPVC products have not been very successful commercially but have found a specific niche in the market. They are used for hot-water systems in mobile homes in the United States and for the manufacture of fibers, for use in thermal underwear, in France.
5. Environmental Protection
Potential problem areas associated with the production, handling, and use of PVC are small if a few simple measures to avoid environmental contact and pollution are followed.
Losses of VCM from polymerization plants have been reduced to extremely low levels following conversion to closed-lid operation as a result of the toxicological problems discussed in Chapter 10. Toxicology and Occupational Health. Although modern column strippers reduce VCM levels in PVC to less than 10 ppm, a small quantity of VCM is lost from the drier section. The older batch strippers present more of a problem since they cannot achieve such low residual VCM contents.
PVC powder can be lost in centrifuge and scrubber effluent and by overfilling stock tanks and from pipeline leaks. It is important that all plant drains which are likely to contain PVC are directed to settling pits where flocculation treatment can be used so that the final discharge is free of PVC. All wet waste is collected and reclaimed as second- or third-grade material for use in noncritical applications such as one-trip horticultural products and fencing.
Powder losses from drier stacks can be minimized by ensuring that drier cyclones are adequately sized to the air conveying rates used. Some modern plants use bag filter systems to meet current regulations that restrict the amount of PVC dust loss to 50 mg/m3 air [56].
Compared to other polymers and typical metals PVC requires less energy per unit weight for its production [57]. Whilst plastics in general are blamed for long-lasting litter problems, it is significant that only 12 % of PVC is used for consumables, while 64 % is incorporated into products with an expected useful life of 15 years or more, probably much more. Disposal is therefore less of an immediate problem than is often thought.
As landfill sites are used up more municipal waste is being incinerated. Though inherently nonflammable, PVC will burn in an existing fire to release hydrogen chloride gas, which can be scrubbed out satisfactorily. Fears about the increased release of dioxins during incineration due to the presence of PVC in waste have proved to be unfounded [58].
6. Quality Specifications and Analysis
All PVC producers test a number of parameters on every batch produced as part of routine quality control operation. These include Kvalue, mean grain size and grain size distribution, apparent density, porosity, contamination, dispersion, color and heat stability, and residual VCM content. Most plants use statistical process control techniques to ensure that product consistency from batch to batch meets the stringent requirements of today's customers. It is difficult to give a typical testing regime since every plant makes batches of different size, has slurry blending facilities of differing degrees of sophistication, and has hoppers of different capacity on their drying plants. For a modern plant operating large autoclaves there is considerable testing of each polymerization batch (wet stage) together with additional testing of the dried and blended material as a cross check.
In-house tests have been collected and correlated by national and international standards organizations to make them universally applicable. ISO has defined tests and standards set up by other organisations such as the ASTM and DIN so that ISO methods are now widely accepted worldwide. ASTM D 1755 defines a system for classifying general purpose (mass and suspension) and dispersion (paste) resins [59].
7. Storage and Transportation
Most plants handle more than 80 % of their production in bulk, and storage and supply in 25 kg bags is diminishing all the time as customers convert. After drying, the material is conveyed by blow-eggs (vessels in which the powder is pressurized and fluidized) of 2 15 t capacity via pipeline to the silo farm where it is stored in silos of capacity 100 500 t. PVC is discharged either directly or via a smaller loading hopper into 20 t road tankers, the most popular form of PVC transportation. Sometimes big-bags made of woven polypropylene are used for 1 t lots of PVC, and 20 t ISO metal containers are used for long distance deliveries. PVC is not a difficult material to handle and the logistics side of the operation is straightforward. It is essential to prevent cross-contamination of the PVC by other products and thorough inter-trip cleaning isessential, since many operators use their tankers for back carriage of a wide range of products, many nonpolymeric.
8. Processing and End Uses
Due to its unique combination of properties PVC is never handled on its own. Instead a complex formulation incorporating several additives is used. A typical base formulation contains: PVC resin, heat stabilizer(s), internal lubricant(s), external lubricant(s), processing aid, and additionally, impact modifier, filler(s), pigment, UV stabilizer, as well as primary and secondary plasticizers (for flexible applications; see Plasticizers).
PVC is intrinsically unstable because of molecular defects in some of the polymer chains [60] , [61] and when subjected to heat they initiate a self-accelerating dehydrochlorination reaction. Stabilizers neutralize the HCl produced and introduce nucleophilic substitution reactions that prevent further degradation [60] , [61] (see also Plastics, Additives - 4. Heat Stabilizers for Poly(Vinyl Chloride)). Table (11) gives a summary of the heat stabilizers in regular use.
The polar, highly viscous PVC melt sticks easily to metal walls of extruder barrels, calenders, mills, etc. so an external lubricant is employed to assist the smooth passage of the melt. Internal lubricants help to reduce melt viscosity and prevent overheating and so help to ensure good color of the final product. Processing aids improve the surface appearance of extruded sections and reduce melt defects such as screw memory, where the helical nature of the screw can be seen as regular ripples in the pipe surface. Many other additives can be used, e.g., impact modifiers for bottles and UV stabilizers and fillers (e.g., TiO2) for house sidings and window frames to ensure the best possible in-service performance or longevity.
In all extrusion and some other conversion processes the PVC grain is not broken down to its constituent primary particles [62] , unlike emulsion PVC processing (see Section 4.3.6. Gelation and Fusion). Instead the suspension grains (150 ΅m) gradually lose their original form by fusion and elongation under the influence of heat, pressure, and shear so that in badly processed PVC grain memory can be detected by optical microscopy of cross sections of extrudates [62]. In practise converters take great care to ensure that the degree of gelation (i.e., lack of grain memory) in the final PVC article is very high. Degree of gelation can be measured by determining the degree of attack by a poor solvent such as acetone or methylene chloride or by measurement of flow pressure to assess the strength and elasticity of partially fused material directly [63].
The enormous subject of processing, including different aspects such as formulations, types of processing equipment, gelation, rheology and mechanical properties is covered more widely in [62] , [64][65][66][67][68].
The versatility of PVC can be gauged by the very broad summary of typical end uses given in Table (12).
9. Economic Aspects
After many years when plant capacity exceeded demand the world economic recession of the late 1970s to early 1980s produced a massive shake-out in the world PVC industry, particularly in Western Europe and North America. A number of producers withdrew, especially those using old, small reactor plants, and several amalgamated so that the number of U.S. producers dropped from 21 in 1980 to 14 in 1990 (Table (13)).
In Western Europe 31 producers dwindled to just 17 by 1990 (Table (14)) and such was the success of the rationalization that the Western European market was essentially in balance in 1988. Several other world zones achieved similar success, notably Japan, Taiwan, and the Soviet Union.
The U.S. market did not balance and together with massive investment of ca. 1.3 Χ 106 t of new capacity over the period 1988 to 1991 has generated well over 106 t of overcapacity in the region and created a free-fall in prices in 1990 1991.
This has changed the scene from 1988 when the main importers of PVC were South-East Asia, the Indian subcontient, Oceania, the Middle East and Africa and the exporters Eastern Europe, North, South and Central America, and Western Europe. In 1991, the major change is that Western Europe has become a major importer, the United States increased exports by nearly 500 %, and imports into the South East Asia region have doubled since 1988. Figures for world PVC production by region are given in Table (15).
Further rationalization is already forthcoming with one-time leading producer in the United States, BF Goodrich, announcing closure of three older plants in 1992/93, which represents 25 % of its current capacity. A number of forecasters suggest PVC demand will continue to grow at anything between 1.5 and 5 % and this optimism is well supported by a number of further investments in new plants, particularly in China, South East Asia, the Indian subcontinent and, to a lesser extent, in Western Europe.
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[66] : N. L. Perry in[61] vol. 2, p. 601, 1st ed., 1976; vol. 2, p. 1, 2nd ed., 1988.[61]: D. Braun, E. Bazdadea in L. I. Nass, C. A. Heiberger (eds.): Encyclopaedia of PVC, 2nd ed., vol. 1, Marcel Dekker, New York 1986.
[67] : W. Henschel, P. Franz in W. V. Titow (ed.): PVC Technology, 4th ed., Elsevier Applied Science Publishers, Barking 1984.
[68] : E. D. Owen: Degradation and Stabilisation of PVC, Elsevier Applied Science Publishers, Barking 1984.
* Ullmann's Encyclopedia of Industrial Chemistry Wiley-VCH Verlag GmbH, Weinheim, Germany
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