RUBBER and VULCANIZATION
Post 345 ⇒ by Gautam Shah →
Vulcanization is a cross or inter linking process between individual polymer chains. Raw rubbers have little elasticity, stability, and are deformable but with interlinking the flexible polymer molecules get entangled. The interlinked entanglement can be temporary as such molecules can again disentangle and flow under stress.
The interlinking is carried out by addition of sulfur, or other equivalent curatives or accelerators. The term vulcanization also refers to other process where cross linkage is encouraged, as for example cellulose treated in a zinc chloride solution to cross-link the cellulose fibers.
Some form curing of rubber was carried out since prehistoric times. Ancient Mesoamericans, extracted latex from a rubber tree Castilla elastica, and mixed with juice of a local vine, Ipomoea alba, to create processed rubber as early as 1600 BC. Modern process of ‘curing’ the rubber, the vulcanization, was named after Vulcan, the Roman god of fire, metalworking and forge, in the 19th C.
First rubber products remained stable during winter period, but with summer heat became liquid gum like mass. The process of vulcanization corrected that defect while offering many stable products such as tyres, shoes, soles, hoses, and conveyor belts. Uncured rubber is used for adhesives, insulating and friction tapes, coatings, etc.
Rubber has good resistance to abrasion and so used for tyres, industrial belts and conveyor belts. It is flexible and elastic so used for gaskets, bendable pipes, foundation packing as vibration absorber, stretchable threads, bands and stripes. Rubber is relatively impermeable to water and gases, and so useful for hoses, balls, rain-wear, diving gears, and balloons, cushions. Rubber is used for inner lining of chemical storage vessels. It is a poor conductor, so used for insulation as well as for electrical insulation. Vulcanized rubber is used to make articles such as clarinet and saxophone mouth pieces, bowling balls and hockey pucks.
Vulcanization affects properties of rubber on several fronts: such as better strength, stiffness, hardness, fatigue and abrasion resistance, chemical stability, reduced deformations superior behaviour in temperature variations. Vulcanization is made agents like sulfur, sulfur donors, peroxide, thiazole and sulfonamide accelerators, metal oxides, polyvalent metal ions. Curing is heat treatment, usually after a product is formed. The product is heated and maintained at the required temperature under steam pressure. Many articles are vulcanized in moulds that are compressed by hydraulic presses.
The optimum amount of sulfur to be added to the rubber is about 10% by weight. Adding an excess of sulfur produces a very brittle and inelastic substance called Ebonite. Man-made or synthetic rubber can also be vulcanized, and the process is similar. Vulcanized rubber is about 10x stronger than natural rubber and is also about 10x more rigid. Polymers that are elastic are sometimes called elastomers.
“It is often difficult to distinguish a covalently interlinked Elastomer from one that is merely tangled or one that is held together by strong intermolecular associations. One means of distinguishing is to test whether the polymer dissolves in a compatible solvent or merely swells without dissolving. Covalently interlinked molecules do not dissolve. Interlinking is therefore necessary for good solvent resistance or for use at high temperatures.”