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PVC--Polyvinylchloride What is PVC?
- By Skip Thacker
- Published 02/13/2006
- Commodity Plastics
-
Rating:
Skip Thacker
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. Currently retired, but available for PVC help.
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.
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
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.
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Article Series
This article is part 1 of a 4 part series. Other articles in this series are shown below:
-
PVC--Polyvinylchloride What is PVC?