PVC is never handled on its own. Instead a complex formulation incorporating several additives is used. A typical base formulation contains PVC resin, heat stabilizers, internal lubricants, external lubricants, processing aid, and additionally, impact modifier, fillers, pigment, UV stabilizer, as well as primary and secondary plasticizers. The typical end uses of PVC are given in following Table
The emulsion polymerization process involves the polymerization of monomer in an aqueous medium containing surfactant and a watersoluble initiator, producing PVC latices. PVC latices are colloidal dispersions of spherical particles, ranging in size between 0 1 and 3.0 ~lm most PVC latices are spray dried and then milled to obtain fine powders. When mixed with plasticizers they disperse readily to form
stable suspensions. During mixing most of the agglomerates are broken down into original latex particles. Such dispersions 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 plasticizers at room temperature so they can be used as liquids and may then be spread on to fabric or other substrate, poured on molds or deposited to produce flooring, wall covering, artificial leather, balls, toys or gloves.
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
A recipe for a simple batch emulsion polymerization is as follows:
Demineralized water 110 – 140 parts
Vinyl Chloride 100
Emulsifier 0.1-1.0
Initiator 0.1-0.2
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This process was developed Pechiney St. Gobain (PSG) which is now part of the French ATO Company. This process is carried out in two stages:
1)In the first stage monomer and initiator are charged and vigorously agitated. In a vertical, stainless prepolymerizer of 825m3 capacity, fitted with a water cooled jacket and condenser. Rapid polymerization takes place at 62-75°C to give 100 flm aggregated spherical flocs composed of 0.1 flm primary particles. Conversion of 7-12 % in about 30 min is obtained. By this time initiator is exhausted
2)The slurry from prepolymerizer is discharged into the secondstage reactor with volume 12-50 m3, with fresh initiator and more VCM In the second stage the primary particles grow in size and fuse together to give grain of PVC 130-160 flm diameter As the conversion increases the physical nature changes from a wet powder at 20% to a dry one at 40% conversion. The heat of polymerization is removed by evaporation of VCM and condensation on the cooled wall of the reactor or in a water-cooled reflux condenser. The second stage takes 3-9 hr, depending on K-value.
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Suspension polymerization is essentially a bulk polymerization process carried out in millions of small reactors (droplets). Liquid vinyl chloride under pressure is dispersed in water by vigorous stirring in a reactor of 25-150 m3 capacity, fitted with a jacket and/or condenser for heat removal and baffles for optimum mixing. This results in the formation of droplets of average diameter 30-40 ~lm which are stabilized against coalescence by protective colloids. The other essential ingredient is a monomer-soluble free radical initiator.
Suspension PVC plant
a) Reactor; b) Blowdown vessel; c) VCM recovery plant; d) Stripping column; e) Heat excbanger; f) Centrifuge; g) Driers
The formulations charged to the reactor 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 0.05-0.08 part
Initiator 0.03-0.08 parts
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These quantities vary depending on the PVC grade, reactor Size, plant type, etc.
After charging, the reactor contents are heated to the reaction temperature of 45°C-70°C. Reactors are 80-90% 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 is controlled by removing heat via jacket and/or by boil-off into a condenser .
Progress of the reaction can be followed by 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 temperature. As the conversion increases towards 70% and beyond, the pressure in the reactor starts to fall. The reaction is terminated at a predetermined pressure by either addition chain terminator and/or venting off the unreacted monomer to the recovery.
The batch is discharged from the reactor to a feed vessel and then fed continuously through a stripper column. The unreacted monomer is recovered. 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 or fluid-bed drying to give a dry, free flowing powder with a residual VCM content below 1 ppm.
Water used in the process should be demineralized since IOniC species, especially sodium ions, can affect the performance of other additives and influence final polymer properties. Protective colloids used today are cellulose ether derivatives and partially hydrolyzed polyvinyl acetates or polyvinyl alcohols. Diacetyl peroxides, peroxydicarbonates, and alkyl peroxyesters are the preferred initiators.
PVC is unique in its relationship between reaction temperature and Kvalue. With PVC the length of the polymer chain (degree of polymerization, DP) is determined by the ratio of the rate of chain propagation to chain transfer Both the rate constants kp and ktr depend only on tempertature therefore the molecular mass of PVC is controlled by the reaction temperature. The general K-value range of commercial resins is K55 to K75. K-value is determined from the solution viscosity measurement in specific solvent at 25°C.
Polyvinyl chloride, PVC, is a polymer prepared from vinyl chloride monomer (VCM).
where n = 700 – 1500
It is relatively inexpensive and is used in wide range of applications. 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 um in diameter, whereas the emulsion process gives latex of particle size 0.1-30 um. The later is dried to yield friable grain-like structures of 5-50 um.
In its polymerization, a growing PVC chain becomes insoluble in VCM above a chain length of 10 units so PVC is essentially insoluble in its monomer and thus classified as a precipitation suspension polymerization.
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 the producers often reflect these parameters in the grade codes used to define different products (e.g.S68/173 refers to a suspension type material with K-value of 68, and VY 110/57 to a resin with viscosity number 110).
PVC with K=66-68 can be processed formulations to give pipes, conduits, 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 (5560) in formulations for injection molding of pipe and conduit fittings, electrical plugs, and blow molding of bottles and other containers.
Molecular Structure
Amongst the range of polymeric material produced today PVC is unique because the bulky chlorine atom imparts a strongly polar nature to the PVC chain, and essentially syndiotactic conformation of the repeat unit in the chain leads to a limited level of crystallinity. This results in good mechanical properties, particularly stiffness at low wall thickness, high melt viscosity at relatively low molecular mass, and 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 even wider variety of end uses.
Vinvl Chloride Monomer
Vinyl chloride monomer, boiling point 13.4C 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°C to 70°C is 8.0 to 12.5 barg. As a result PVC polymerization reactors are thick-walled jacketed steel vessels. VCM is slightly soluble in water. The polymerization of VCM is strongly exothermic. VCM has a pleasant smell and narcotic effect but can cause death by inhalation Exposure to VCM causes acroosteolysis (AOL), which affects mainly hands and feet. VCM also causes a very rare liver cancer, angiosarcoma. The knowledge that VCM is a human carcinogen has led to the introduction of very stringent controls to limit the exposure of workers and the general public to the monomer. During a shift an employee’s exposure must not exceed 5 ppm over a period of 15 min or less. The annual average in the plant’s airspace should be less than 3 ppm.
VCM is produced industrially by two main reactions:
PVC is never used alone. It is always mixed with heat stabilizers, lubricants, plasticizers, fillers, and other additives to make processing possible. Un plasticized PVC is known as rigid PVC and has total additives content less than 10%. Plasticized PVC known as flexible PVC has plasticizer content 20 to 100 phr (parts per hundred). The physical properties vary widely between the two types.
In addition the K-value (molecular mass) of the PVC can also influence properties. PVC has extremely good chemical resistance to all but the chlorinated solvents.
There are three main processes used for the commercial production of PVC