Plastics, Polymers & Rubber

Together they form the activator system for sulphur vulcanisation. Zinc oxide supplies zinc, and stearic acid reacts with it to form zinc stearate, which dissolves in the rubber. This zinc complex works with the accelerator to make sulphur cross-linking faster and more efficient, so the compound reaches a good state of cure in less time. Recipes commonly use a few parts of each per hundred of rubber, set by the compound design.

Internal lubricants are polar and compatible with PVC: they reduce friction between polymer chains, lower melt viscosity and promote fusion. External lubricants, such as paraffin, polyethylene and Fischer-Tropsch waxes, are mostly non-polar, move to the melt surface and reduce sticking to hot barrels, screws and dies, which delays fusion. Rigid PVC needs the two in balance: too much external lubricant gives poor fusion and plate-out, too little causes sticking and burning.

Both migrate, or bloom, to the film surface after extrusion. Slip agents, usually fatty amides such as erucamide or oleamide, form a thin lubricating layer that lowers the coefficient of friction so film runs smoothly on bag-making and packing lines; erucamide blooms more slowly than oleamide. Antistatic agents such as glycerol monostearate or ethoxylated amines draw a thin layer of moisture to the surface that dissipates static, reducing cling and dust pick-up.

Plastics grades are designed for hot, dry polymer melts rather than water or solvent. They carry lighter surface treatments, often with an organic coating, so they disperse well in polyolefins and PVC and release little moisture at processing temperatures. Moisture from heavily treated grades can cause lacing, small holes in thin film. Paint grades carry heavier treatments for dispersion and durability in a liquid coating. Choose by polymer, processing temperature and outdoor exposure.