Polyvinyl chloride (PVC)

Several 19th and 20th chemists succeeded in producing the plastic polyvinyl chloride (PVC) but production on an industrial scale didn't commence until the 1920s in the USA. Even then, use of the new material languished until the Second World War accelerated demand for a synthetic replacement to costly natural rubber; especially to replace the the insulating material for wiring on military ships.

Today, PVC is the third largest-selling commodity plastic in the world after polyethylene and polypropylene. PVC's low cost, excellent durability and processability, make it the material of choice for dozens of industries including, of course, the manufacture of vinyl gramophone (phonograph) records.

Chemical properties

PVC (C2H3Cl) has an amorphous structure with polar chlorine atoms in the molecular structure. In this, PVC is different from other plastics which have only carbon and hydrogen atoms in their molecular structure (like Polyethylene). The factor most strongly influencing the durability of a material is resistance to oxidation by air. PVC, having the molecular structure where the chlorine atom is bound to every other carbon chain, is highly resistant to oxidative reactions. Other general purpose plastics with structures made up only of carbon and hydrogen are more susceptible to deterioration by oxidation.

Mechanical properties

When vinyl is mentioned, most people think of shower-curtains and car seat-covers. But these are highly plasticised vinyl resins containing anywhere from 20% to 40% of a liquid plasticiser.

Pure PVC is an amorphous, mechanically stable material which is basically rigid and transparent at normal temperature. The rigidity of pure PVC is due to the short distances between the molecules and the strong intermolecular forces between them. When heated, the energies of molecular motions become greater than the intermolecular forces which results in the softening of the resin. If plasticisers are added to PVC when molten, the plasticiser molecules make their way between the PVC molecules and prevent the PVC polymer molecules from coming closer with each other. If enough plasticiser is added, the polymer molecules are kept apart even at normal temperature and softness is maintained. The vinyls used for records are copolymers where the primary polymeric material is PVC combined in the ratio of approximately 85 parts vinyl chloride to 10 parts vinyl acetate. The remaining 5% is made up from heat-stabilisers and colourants.

Processability

PVC is a thermoplastic material which means it may be softened by the application of heat without undergoing appreciable chemical change. While hot, it may be formed into any desired shape and then cooled to a solid state. Usually this cycle of heating, forming, and cooling may be repeated as often as desired.

The processability of a thermoplastic material depends largely on its melt viscosity. PVC has a relatively low softening temperature compared with other plastics (92°C compared with 152°C for polypropylene) but it is not suitable for injection moulding because its melt viscosity is comparatively high. (That's to say, it's "gooey".) On the other hand, since PVC is an amorphous plastic with no phase transition, PVC products may be moulded with a high dimensional accuracy which they retain when cold. PVC is therefore ideally suited to the dimensionally accurate compression moulding process of making records.

The future?

Now that new vinyl record pressing machines are being produced for the first time in 30 years, there's some renewed interest in the selection of materials for records. One idea is to use thermosetting polyimides; materials which are used for space-suits and for high-performance electrical insulation. Although expensive, polymides possess better thermal stability than PVC and have excellent mechanical properties which could reproduce the groove better and offer a more frictionless surface.

The idea of using thermosetting resins for records is not new. But thermosetting resins (as opposed to thermoplastic resins) are characterised by the property of undergoing a permanent physical and chemical change during the molding operation. In the past, the record industry was built upon the policy of regrinding and reusing reject records; even records returned from dealers and distributors. So that, in the heyday of LP records (especially in America), as much as 20% to 30% of the material used in record manufacture was derived from re-cycling. The reuse of thermosetting materials in this way is not possible and this prevented thermosetting materials being developed for use by the record industry.



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