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.
