User:2beeps
From Wikipedia, the free encyclopedia
PVC 101 PRIMER
PVC, the plastic most people are scared to death to run! Perhaps the most versatile of all resins, PVC is one of the major plastics in use today, yet it continues to be an enigma to anyone not fully familiar with the parameters of proper handling and processing. Fears of degradation and HCl evolution, the whinings of Greenpeace and other environmentalists with their anti-chlorine agenda, and the phthalate plasticizer issue all combine to fuel the unease of those considering use of PVC in their products. Perhaps the following little “course” on PVC will help ease some of these concerns.
“PVC--Poly(vinylchloride)-101”
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 U.S 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.
Major Markets for PVC
Building & Construction
Pipe & Fittings ( Potable water, sewer, irrigation, teleduct, fibrotic cable
duct, drain/waste/vent, spiral wound large diameter culvert,
chemical and food processing piping, fire sprinkler piping)
Siding, Gutters, Downspouts
Window Profiles
Fencing
Decking/Docking
Flooring & Cove Base
Wall Covering
Wire & Cable
Single Ply Roofing Membrane
Landfill Liners
Transportation
Instrument Panels, Dashboards
Auto, Mass Transit &Aircraft Interiors & Seating
Under Hood Wiring
Under Car Abrasion Coating
Floor Mats
Arm Rests
Foamed Gaskets
Window Trim
Body Side Molding
Convertible rear windows (Obsolete! but big years ago!)
Electronic/Appliances
Keyboards
Component Housings
Electrical Cord Jacketing
Fiber Optic Sheathing
Floppy Disk Jackets
Various Components in Phone Systems, Power Tools, Refrigerators,
Washers, Air Conditioners, Computers.
Medical
Blood Bags, and Tubing
Catheters
Surgical Gloves, and Sheeting
Prosthetics
Single Dose Medication Packaging
Packaging
Meat and Produce Film
Jar Lid Gasketing(Foamed Plastisol)
Clear Blister Pack Sheet (Thermoformed)
Clear & Opaque Bottles
Home & Leisure
Garden Hose
Toys, Dolls, Inflatables
Shoe Soles
Fishing Lures
Vinyl Coated Metal Racks & Shelving
Boat & Dock Pads
Tarpaulins
Credit Cards (copolymer)
Patio Furniture, Fabric, Strapping
Shower Curtains
Swimming Pool Liners
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 Back 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 entire 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. VCM is no longer an issue.
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. Presence of toxic heavy metals is no longer an issue.
However, the following PVC issues remain “active”, as Greenpeace and others continue to attack ALL uses of Chlorine and especially PVC--the largest user of Chlorine. For a more complete and accurate, up-to-date accounting of all these issues, please log onto the “Chlorophiles’” web site: www.ping.be/~ping5859/index.html
A series of fires,( Beverly Hills Nightclub in Newport KY, Air Canada crash in Cincinnati, MGM Hotel in Las Vegas, and Dusseldorf Airport in Germany) thrusted PVC and evolved HCl onstage, mainly involving flexible electrical wire & cable insulation and rigid conduit in plenum areas. Several court trials and hearings in each case determined that PVC itself did not cause any of the fires--and in fact will not support combustion on its own. The emitted HCl fumes were shown to be in very small proportion to the total smoke and fire volatiles evolved. HCl is considered toxic, but not highly toxic, and a very corrosive gas that attacks mucous membranes of eyes, nose, throat , and lungs if the concentration is high enough. However it was shown that most fire fatalities were from suffocation--lack of oxygen--a condition of all fire situations.
In spite of a 50 year history of safe usage--including medical applications-- the widely used plasticizer, DOP (or more accurately Di-2 ethyl hexyl phthalate) continues to come under attack , first cited as a potential carcinogen (DiBUTYL phthalate fed in massive doses to rats caused some tumor formation) by extrapolated association to phthalates NOT used in PVC, and more recently cited as a potential endocrine mimic and disrupter. Many toy producers and medical device producers succumbed to environmental political pressures and phased out DOP plasticized PVC products.
A highly toxic group of organic chlorine-containing compounds, Dioxins, can be formed during inefficient (under 700-800F) combustion of ANY mixture containing chlorine, hydrocarbons, oxygen, and moisture. Burning solid wastes--especially hospital wastes with saline solutions present-- including paper can produce some dioxins. Wood fires can also produce dioxins. Incinerated PVC can also produce dioxins. However the PVC content of solid consumer wastes and hospital wastes is not very large, and Dioxin analyses of PVC-containing vs. PVC-free waste incineration showed virtually identical, small, Dioxin production! Further, if incineration is done at sufficiently high temperatures (over 800f), Dioxin and other toxic organics are destroyed! In spite of this, Greenpeace and the uninformed continue to claim that PVC is the major source of Dioxin!
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.)
PVC Information Resources
The Vinyl Institute www.vinylinfo.org/
The Chlorophiles www.ping.be/~ping5859/index.html
By George (Skip) Thacker, PVC Technical Services
P.O. Box 1377
Silver City, NM 88062
E-MAIL: 2beeps@silvercity-nm.com
BIOGRAPHIC BACKGROUND
EDUCATION: B. A. in Chemistry, Wabash College 1956 MBA Program, Xavier University, 1960-61
TECHNICAL ORGANIZATIONS: Society of Plastic Engineers, Fellow Emeritus Grade
Member; Vinyl Division
The Chlorophiles.
PROFESSIONAL EXPERIENCES: Over 35 years in research, development, technical services, sales and marketing activities centered primarily on polymer additives to improve processing and end-use properties of plastics.
Company affiliations included Olin Mathieson Chemical Company (now Olin Corporation), Cincinnati Milacron,(both the old Advance Division, now Akcros, and Milacron Chemicals, now Rohm & Haas),and Anderson & Company, which represents a number of major additive and lab test equipment suppliers. Currently retired, but also available as”PVCTechnical Services”, providing “hands-on” help in PVC science and technology.
PUBLICATIONS:
“Photodegradation of Rigid Polyvinyl Chloride” L. B. Weisfeld, G. A. Thacker, L. I. Nass; SPE Journal, Vol. 21, No. 7, July, 1965, pp 649-658 (originally presented at SPE Antec, Boston, 1964)
“Stabilization of Polyvinyl Chloride” Optimum Stabilizer Selection; publication of Advance Division, Carlisle Chemical Company, 1967
“Effect of Stabilizers on the Melt Rheology of Poly(Vinyl Chloride)”; L. B. Weisfeld, G. A. Thacker, L. Giamundo, Advances in Chemistry Series, No. 85
“Stabilization of Polymers and Stabilizer Processes” 1968, pp 38-44, American Chemical Society
“Stabilizing Compounds” Modern Plastics Encyclopedia 1969-70
“Stabilizers” Modern Plastics Encyclopedia 1969-70
“U. V. Absorbers and Light Stabilizers” Modern Plastics Encyclopedia, 1971-72
“Antioxidants” Modern Plastics Encyclopedia, 1971-72
“Twin Screw Extrusion of PVC” Paper presented at SPE Antec, 1972
“Heat Stabilizers” Modern Plastics Encyclopedia, 1973-74
“Lubricants” Modern Plastics Encyclopedia, 1974-75
“Rigid PVC - Prime Stimulant for Stabilizer Development” Paper presented at 2nd Annual National Compounding Conference, New York University, Sept. 18, 1973
“Rigid PVC Extrusion I” Publication of Cincinnati Milacron Chemicals, 1976 (Concerns Single vs Twin Screw Processing)
“Rigid PVC Extrusion II - Problems and Remedies” Publication of Cincinnati Milacron Chemicals, 1978 (Concerns Twin Screw Processing Hints)
BOOKS
“Engineering With Rigid PVC” I. Luis Gomez, author, Marcel Dekker, Inc., N. Y., 1984, G. A. Thacker reviewed all chapters and contributed the forward.
“Encyclopedia of PVC”, edited by L.I. Nass, Marcel Dekker, NYC, publisher. Second Edition, Volume 4, Chapter 8, “Compounding and Processing PVC: General Principles of Plant Operation for Optimum Profitability”, authored by G.A. Thacker and R.F. Grossman .
