Polyvinyl chloride (PVC) is a tough, strong thermoplastic material which has an excellent combination of physical and electrical properties. The products are usually characterized as plasticized or rigid types. Polyvinyl chloride (and copolymers) is the second most commonly used polyvinyl resins in one of the most versatile plastics.
The plasticized types, either soft copolymers or plasticized homopolymers, are somewhat elastic materials which are familiar in the form of shower curtains, floor coverings, raincoats, dishpans, dolls, bottle-top sealers, prosthetic forms, wire insulations and films, among others.
Rigid polyvinyl chloride products, which may consist of the homopolymer, copolymer, or polyblends, are used in the manufacture of phonograph records, pipe, chemically resistant liners for chemical-reaction vessels, and siding and window sashes.
The monomer is frequently prepare from chlorine, acetylene, and ethylene by a combination of processes which affords complete utilization of the chlorine.
The polymerization of vinyl chloride, chemical reaction, and its copolymerization with other vinyl monomers may be initiated by peroxide or azo compounds and carried out in bulk or in aqueous emulsion or suspension systems. The structure and properties of the product are quite dependent on polymerization temperature; the lower the temperature, the higher the softening point of the resin. Polymers prepared in bulk or suspension are used for many applications as molded, extruded, or calendered objects. Polymers prepared in emulsion are often molded by dipping or pouring techniques.
Because polyvinyl chloride products have a tendency to lose hydrogen chloride at high temperatures, a stabilizer, such as a tin or lead compound, is included in the final composition. Fillers are also commonly incorporated.
Copolymers with monomers, such as vinyl acetate or propylene can be processed at lower temperatures than the homopolymer. The acetate copolymer is especially useful in floor tiles, though homopolymers are used as well.
Blends or “alloys” of polyvinyl chloride with small amounts of rubbery materials, such as the interpolymer of acrylonitrile, butadiene, and styrene (ABS) have been produced for applications such as panels and pipe in which impact resistance, as well as hardness and strength, is desired.
Chlorination of polyvinyl chloride is sometimes effected to obtain a stiffer resin at the expense of processability.
PVC is the most common of all vinyl polymers. The polymer of vinyl chloride, PVC, is produced by three basic processes…
1. Mass Process
Vinyl chloride is polymerized in a carrying medium. The polymerization is stopped at a low conversion and the polymer is separated from the residual monomer which is recycled.
2. Emulsion Process
Vinyl chloride is emulsified with water by use of emulsifying agents. The polymerization is carried out to a high degree of conversion with a small amount of recovered monomer being recycled. The product is obtained as an emulsion or spray and dried to produce a very fine powder.
3. Suspension Process
Vinyl chloride is suspended as small droplets of monomer in water and then polymerization is carried to a high degree of conversion. A small amount of monomer is recovered and recycled. A granular product is obtained by centrifuging and drying. While a range of products can be made by the first two methods as above, they are generally limited to specialty products which are difficult to impossible to make by the suspension process. The largest proportion of PVC is produced, particularly in the USA, the by suspension process because it requires the least capital investment and has the lowers operation costs.
Advantages and Limitations of
PVC and Related Polymers
Mechanical Properties: Good combination of stiffness and impact strength (rigid formulation), toughness, extensibility, high ratio of strength to weight (flexible formulations). Acrylic-PVC alloy with high impact strength available. Limitations are recovery from bending or stretching damped compared to rubber. Comparatively low heat distortion and softening temperature (even rigid compositions – except CPVC).
Physical Properties: Good electrical insulation (enhanced by suitable formulation).
Environmental Resistance/Flammability: Non-flammability (reduced or enhanced by some plasticizer) limitations are the tendency to progressive degradation at elevated temperatures. Stiffening and embrittlement at low temperatures (rigid compositions) and flexible unless specially formulated.
Application to Food and Medicine: PVC and PvdC used in packaging applications. Blown bottles and containers also made from PVC (non-toxic grades).
Formulation and Processing: Formulation versatility giving materials ranging from elastomers to rigid engineering thermoplastics and processing possibilities from paste coatings to injection molding. Limitation is that adhesion to many substrates limited in the absence of a primer.
Miscellaneous: Dimensional stability at room temperature for rigid formulations. Almost unlimited range of colors – transparent to opaque according to formulation. Low cost. Limitation is the high specific gravity for polymeric material.
Comparison of Characteristics of
PVC and Related Polymers
PVC (Flexible): Wide range of flexibility possible – competes for rubber applications at lower cost. Ability to form pastes (plastisols) by addition of plasticizers for coating and casting. Limitations are that flexible grades are most susceptible to staining, chemical and microbiological attack due to presence of plasticizers. Creep properties are inferior to rigid PVC.
PVC (Rigid): Lower coefficient of friction and better abrasive wear resistance than flexible grades. Easier to process and cheaper than CPVC. Can be glass fiber-reinforced to give improved strength and stiffness and lower coefficient of thermal expansion. Limitations are that it is attacked by strong acids and alkalix, swollen hydrocarbons and it has a lower maximum service temperature than CPVC.
PVC Copolymners (Less Rigid than Rigid PVC): Processing characteristics and impact strength improved by blending with copolymers. Limitations are that creep properties are generally inferior to rigid PVC, and lower tensile strength.
CPVC (Rigid): Considerably higher upper service temperature than rigid PVC. Best creep properties of PVC types. Resistant to acids, alkalis, most organic solvents, oil and grease. Limitations are that it’s more difficult to process than PVC due to higher softening point and melt viscosity and it has higher cost.
PvdC (Flexible): Best resistance to all acids and most common alkalis. Unaffected by aromatic and aliphatic hydrocarbons, alcohols and esters. Best resistance to permeation by organic and aqueous vapors. Limitations are that it has higher cost and thermal processing is more difficult.
Typical Applications of PVC
Rigid Piping: Potable water conduits, water mains, drainage, soil pipe systems, rain water systems and gutters, gas conduits, venting, furniture and crafts.
Rigid Structural and Cladding Elements: Paneling and partitioning (interior), wall cladding (exterior), building panels, glazing, roofing, window frames and hollow bricks.
Flexible Misc. Products: Flexible “see through” doors, folding doors, roof underlayment, greenhouse glazing, suspended ceilings and floorings (continuous and tiles).
Flexible insulation and sheathing (wire and cable), batter separators. Rigid terminal boxed and conduit.
Rigid Foil (Thin Sheeting): may be fabricated into containers, blister and skin packs, nesting trays (for confectionary and cookies/crackers).
Flexible Film: used directly for wrapping food and other products and “shrink” or “cling” wrap.
Rigid Blown Bottles and Containers: used for oil, wine, beverages, shampoos, and certain cosmetics.
Coated Fabric and Paper Applications (Flexible)
Leather Cloth: used in upholstery, clothing, travel and fancy goods. Protective and foul weather clothing. Tarpaulins, life rafts, hover rafts, conveyer belts.
Rigid Foam: used as a core in sandwich structure (building and boat building), buoyancy blocks, fishing floats, insulation (thermal and acoustic) and shock absorbent materials.
Flexible Foam: upholstery (especially automotive), leather cloth and fancy goods, carpet backing, underlays, embossed wall papers, foam flooring, shoe soles and soft toys.
Some of these find application in adhesive and thin protective coatings (i.e. paints). Those vinyl chloride copolymers which are of direct interest to the product designer are used mainly where a rigid polymer with PVC-like service properties are required, but where easier thermal processing is necessary. This type of copolymer is also the material of vinyl sheeting used for thermoforming, which must combine rigidity with easy heat-softening under fabricating conditions. Vinyl chloride/acrylic ester copolymer has been used as the material of blow molded bottles and film, for improved impact strength and/or heat weldability.
CPVC: The principal use for this material in in hot water pipes where its combination of generally PVC-like properties with a much higher softening point makes it particularly suitable.
PvdC: The most important application of this material is as a coating or laminate component imparting good gas, moisture, vapor, scent and flavor barrier properties to plastic packaging films and containers. Heat-shrinkable and heat-sealable films have been produced from PvdC, as well as heat-sealable layers or coating on other polymer films. The solvent resistance of PvdC pipes has been utilized in special applications, as in chemicals installations and apparatus. Use of extruded PvdC products in this and other fields also includes gaskets, valve seats, tape for wrapping joints and conveyer belts. PvdC moldings are represented by spray-gun handles, acid dippers and components in equipment for rayon manufacture.
Processing of PVC and Related Polymers
Calendering: Sheet, film (flexible or rigid), flooring and floor tiles (flexible)
Extrusion: Sheet, film, pipe, profiles (all flexible or rigid), coated wire and cable (flexible coating)
Compression molding: Thin sheeting or laminates, cellular articles
Injection molding: Pipe fittings, electrical fittings, gaskets, footwear (including micro cellular soles)
Blow Molding: Bottles and other containers
Paste Processing (Coating by Dipping, Spreading or Transfer Techniques): Coated fabrics, vinyl wall-papers, PVC Backed carpets, coated work gloves, coated metal sheet and articles
Powder Processing (Coating by Dripping or Spraying): Coated metal sheet and wire trays
Powder Processing (Spreading on Temporary Support): Sintered porous sheets for battery separators
Thermoforming: Blister packs, nesting trays, containers and ceiling panels