PVC - Polyvinylchloride - What is PVC?

PVC, Poly(vinyl chloride), or "Vinyl" is the second largest volume plastic resin produced and consumed worldwide. Volume estimates for year 2000 world PVC production are in the 44 Billion pound range (20 MM metric tons), with about 14 billion pounds (6.35 MM metric tons), 32%, from US producers.

PVC resin is a product of the polymerization of vinyl chloride monomer or VCM (CH2=CHCl), in a "head-to-tail" manner via free radical catalysts. The resultant (ideal) PVC is a hydrocarbon chain (like polyethylene) but with a chlorine atom on every other carbon. (~CH2-CHCl-CH2-CHCl-CH2-CHCl-CH2~) Being an imperfect world, there is some chain branching during polymerization, which are weak points subject to degradation. More on this later.

How is it made?

The main polymerization methods for VCM include Suspension, Emulsion, and Bulk or Mass methods. Solution polymerization, once used for coil-coating PVC's, is no longer employed.

In Suspension polymerization, VCM droplets (containing free radical catalyst) are agitated with suspending agents in water for a given time and temperature to achieve the desired molecular weight (or "K value"). This is the most common production method, and furnishes "popcorn-like", irregularly shaped resin grains that can absorb liquid plasticizers and additives to form dryblend powder compounds. Most flexible and rigid PVC calendering, molding, and extrusion (from powder or pellets) is done with Suspension PVC (S-PVC).

Emulsion polymerization consists of emulsifying very small VCM droplets in water, with a water soluble free radical catalyst . Depending on the type of "soap" or emulsifying agent, agitation, and temperature, Emulsion PVC of varying molecular weight is produced. These resin particles are much smaller than Suspension PVC, and are smooth surfaced, non absorbent to plasticizers at ambient temperatures. Emulsion PVC resins(E-PVC), also called "Dispersion resins" and "Paste resins, are used to make Plastisols and Organosols for molding, dipping and coating applications.

Bulk (or Mass) polymerization entails just the VCM monomer, containing catalyst, in a two stage reactor. The first stage reactor, with reflux condenser, agitates the VCM monomer to about a 10% conversion to polymer. This slurry is then transferred to a horizontal reactor with a ribbon blending type low RPM agitator, where polymerization is finished as a dry powder. This PVC (M-PVC) is similar in particle size and shape to S-PVC, and is used in the same (mostly rigid) processes as S-PVC. The main difference between M-PVC and S-PVC of the same molecular weight or K-Value is the higher bulk density of M-PVC.

After all the above reactions are complete, the PVC resin is "steam-stripped" and dried in order to remove any residual VCM monomer--down to a fraction of a PPM.

Up to now, we have only discussed PVC HOMOPOLYMERS. With both S-PVC and E-PVC methods, vinyl chloride monomer can-and is- COPOLYMERIZED with other co-monomers, mainly vinyl acetate, to form PVC/PVAc copolymers. For equivalent molecular weights, vinyl copolymers show lower melt viscosities, higher tolerance for additive fillers, higher burn sensitivity to Zinc-containing stabilizers, and better cold-draw properties than homopolymers. They have found some specialty application niches, to be discussed later.

Inherent Properties of PVC

Containing 56.5% Chlorine and 43.5% Ethylene from petroleum feedstocks, PVC is much less dependent than most other thermoplastic resins on the fluctuations of supply and demand of the petroleum industry. Its chlorine content is derived from table salt! PVC's chlorine content provides inherent flame & fire retardancy. Other additives (plasticizers, modifying resins) may burn, but PVC will not support combustion on its own.

PVC is regarded as perhaps the most versatile thermoplastic resin, due to its ability to accept an extremely wide variety of additives: Plasticizers, stabilizers, fillers, process aids, impact modifiers, lubricants, foaming agents, biocides, pigments, reinforcements. Indeed, PVC by itself CANNOT be processed! It must have at least a stabilizer, a lubricant, and if flexible, a plasticizer present.

PVC products can run the gamut from a wiggly fishing worm to a high impact computer housing, pipe, windows and fencing, and all in between. Clear or opaque, flexible PVC applications(flooring, automotive, wire & cable)) dominated the earlier years (40's, 50's, early 60's), but with the advent of reciprocating screw injection molding and twin screw extrusion in the 60's, rigid PVC began to flex its muscle in pipe& fittings, siding, electrical junction boxes, fencing, docking, to the point today where rigid PVC applications account for about 70% of all PVC processed!

Physical properties of course will vary widely depending on types and amounts of additives chosen. Based on cost/performance, many consider rigid PVC to be "the poor man's engineering resin"!

PVC has a unique degradation sequence. Unlike most other polymers that exhibit mainly oxidative degradation with peroxide formation and chain scission, protected by antioxidants, PVC (while ALSO undergoing oxidative degradation) has a nasty habit of releasing HCl under heat and shear of processing---an "unzippering effect" that rapidly progresses to catastrophic charred blackening if left unchecked. The art and science of stabilization --a whole industry sector-- has developed very effective protective stabilizer additives to retard this type of degradation. This HCl elimination is most likely to start at a "weak link" site---typically a chlorine on a carbon at a branching site in the chain. The result is a series of alternating (or conjugated) double bonds, and the onset of visible discoloration(yellowing) has been pegged at 7-8 conjugated double bonds. However a UV black light can see early degradation at 3-4 double bonds before it becomes visible to the eye.

PVC History

Vinyl chloride was initially prepared in the lab by a French chemist (Regnault) in the early 19th century, and polymerized as a curiosity in the 1870's. By itself a hard, horny, intractable material that degraded with heat to give off HCl, poly(vinylchloride) had no commercial importance until about 1933, when a young chemist at B.F. Goodrich in Akron Ohio, Waldo Semon, dissolved PVC resin in hot dibutyl phthalate and hot tricresyl phosphate---and found upon cooling that the mixture gelled to a rubbery, elastomeric state that could be remelted and cooled!

With this discovery of plasticization, PVC became a commercially viable product, especially during World War II, and the shortage of rubber. PVC insulated wire & cable, coated fabric, waterproof boots and shoes, self-sealing aircraft fuel tanks, and other flexible PVC products rapidly emerged as the war ended.

Over the ensuing 45-50 years, PVC's annual production grew from a few hundred million pounds to about 14 billion pounds (U.S., 2000) as new uses and markets were developed. As stated earlier, the largest volume applications for PVC in the earlier years were flexible, plasticized products such as vinyl asbestos floor tile (obsoleted), vinyl flooring, wire & cable insulation, calendered supported (Naugahyde) and unsupported film and sheeting, hose, footwear and the like. Quantum improvements in extrusion and injection molding machinery and extrusion die design, together with significant improvements in stabilizer and lubricant technology allowed for the rapid growth of rigid PVC applications, mainly for the building and construction industries.

Current Trends in PVC

A hot new area for research and development in rigid PVC (and HDPE as well) centers on the incorporation of high percentages of WOOD FLOUR as a filler into the resin during extrusion, resulting in wood-like profiles that can be sawed, nailed, and screwed just like natural wood, but without the negatives of splintering and decay. Many potential markets are ready to open up--some have already-- as this technology becomes more refined and more widely available.

Also recently available, complete modules of hollow rigid PVC profiles, fitted together with tongue-and groove and concrete filled, provide low cost very durable rot-proof, hurricane proof, and earthquake proof housing for third world areas.

Environmental Concerns

In 1973, some PVC production workers who had been exposed to massive doses of VCM in cleaning out polymerization reactors for a 15-17 year period were found to have a rare liver cancer; angiosarcoma. The US EPA , NIOSH, and the PVC industry took rapid and thorough steps to reduce-and monitor- worker VCM exposure down to single digit parts per million, and PVC resin production added steam-stripping prior to drying in order to ship virtually VCM-free PVC resin to customers.

Toxic heavy metals used in PVC stabilizers and pigments have been removed and are still being removed as viable alternatives are found. These include Lead, Cadmium, and Strontium. Calcium, Zinc, Barium, Phosphites, Epoxy Oils, Organo-tins, and All Organic stabilizers are the currently used stabilizers in PVC compounds.

However, PVC issues remain "active", as Greenpeace and others continue to attack ALL uses of Chlorine and especially PVC--the largest user of Chlorine.


Editors Note:
PVC continues to be a controversial material with extreme charges from both sides. Plastics.com feels that anyone interested in the environmental issues here should look deeply into the details. A group of PVC industry workers supplies a links page with links to both sides of the issue;
www.ping.be/chlorophiles

Future of PVC

PVC's properties, cost, and versatility point to a bright future of growth in many existing products and markets as well as new, yet unimagined products. This bright future will, however, depend on how well the industry deals--in a rational and scientific manner-- with the environmental misconceptions that continually arise. Here are but a few forces that could drive potential product development and growth (courtesy of the Vinyl Institute, Vinyl 2020 Report, 1996):

• Global warming and demands for innovations in irrigation, shoreline protection and water control. Deforestation and the rising cost of wood as a building material.
• Worldwide urbanization and its demands for low cost innovations in housing, water distribution, sanitation and infrastructure---especially in the "second and third worlds".
• The health care revolution and the growing demand for products--including body parts-- that utilize vinyl's properties.
• The communications revolution and the need to "wire" the growing information superhighway.
• The environmental revolution, in which PVC can be promoted as a "Green" alternative. (Lower dependence--46.5%-- on nonrenewable petroleum feedstocks, Lower energy costs to produce pipe, siding, etc. vs. cast iron, aluminum, Lower BTU loss than aluminum and wood windows (foam-filled PVC), Longer lasting non corrosion water and sewer piping than ductile iron, cement asbestos pipe, as a few examples.)