Recycling of Rubber
Practical Action
properties for specialist applications. The synthetic rubbers commonly used for tyre
manufacture are styrene-butadiene rubber and butadiene rubber (both members of the Buna
family). Butyl rubber, since it is gas-impermeable, is commonly used for inner tubes. Table
1 below shows typical applications of various types of rubber.
Type of rubber
Natural rubber
Styrene-butadiene rubber (SBR) and
Butadiene rubber (BR)
Butyl rubber (IIR)
Application
Commercial vehicles such as lorries, buses and
trailers.
Small lorries, private cars, motorbikes and
bicycles.
Inner tubes.
Table 1: Applications of different classes of rubber in the manufacture of vehicle tyres.
The raw materials that make up tyres are natural and synthetic rubbers, carbon, nylon or
polyester cord, sulphur, resins and oil. During the tyre making process, these are virtually
vulcanised into one compound that is not easily broken down.
Production of rubber products
The modern process of rubber manufacture involves a sophisticated series of processes such
as mastication, mixing, shaping, moulding and vulcanisation. Various additives are included
during the mixing process to give desired characteristics to the finished product. They
include:
Polymers
Activators
Fillers (carbon black)
Anti-degradants
Plasticisers
Vulcanisation accelerators
Vulcanisation agents
Fire retardants
Colorants or pigments
Softeners
Fillers are used to stiffen or strengthen rubber. Carbon black is an anti-abrasive and is
commonly used in tyre production. Pigments include zinc oxide, lithopone, and a number of
organic dyes. Softeners, which are necessary when the mix is too stiff for proper
incorporation of the various ingredients, usually consist of petroleum products, such as oils or
waxes; pine tar; or fatty acids. The moulding of the compound is carried out once the desired
mix has been achieved and vulcanisation is often carried out on the moulded product.
Vulcanisation
To understand the process of vulcanisation it is worth discussing, briefly, the molecular
structure of rubber. Crude latex is made up of a large number of very long, flexible, molecular
chains. If these chains are linked together to prevent the molecules moving apart, then the
rubber takes on its characteristic elastic quality. This linking process is carried out by
heating the latex with sulphur (other vulcanising agents such as selenium and tellurium are
occasionally used but sulphur is the most common). There are two common vulcanising
processes.
Pressure vulcanisation. This process involves heating the rubber with sulphur under
pressure at a temperature of 150oC. Many articles are vulcanised in moulds that are
compressed by a hydraulic press (see Figure 1 below).
Free vulcanisation. Used where pressure vulcanisation is not possible, such as with
continuous, extruded products, it is carried out by applying steam or hot air. Certain
types of garden hose, for example, are coated with lead, and are vulcanised by passing
high-pressure steam through the opening in the hose.
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