What is PVC?
What is PVC? PVC is regarded as perhaps the most versatile thermoplastic resin, due to its ability to accept an extremely wide variety of additives. PVC has a unique degradation sequence and Inherent Properties.
What is PVC?
Inherent Properties of PVC:
Containing 56.5% chlorine and 43.5% ehylene from petroleum feedstrocks, PVC is much less dependent than most other thermosplastic 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 agent, 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) donimated the earlier years (40’s, 50’s, 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 stablizer 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 peged 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.