Bonding of Vulcanised Rubbers

In the lightly cross-linked polymers (e.g. the vulcanised rubbers) the main purpose of cross-linking is to prevent the material deforming indefinitely under load. The chains can no longer slide past each other, and flow, in the usual sense of the word, is not possible without rupture of covalent bonds. Between the crosslinks, however, the molecular segments remain flexible. Thus under appropriate conditions of temperature the polymer mass may be rubbery or it may be rigid. It may also be capable of ciystallisation in both the unstressed and the stressed state. 

The proximity of the methyl group to the double bond in natural rubber results in the polymer being more reactive at both the double bond and at the a-methylenic position than polybutadiene, SBR and, particularly, polychlor-oprene. Consequently natural rubber is more subject to oxidation, and as in this case (c.f. polybutadiene and SBR) this leads to chain scission the rubber becomes softer and weaker. As already stated the oxidation reaction is considerably affected by the type of vulcanisation as well as by the use of antioxidants. 

Modern bonding systems usually consist of a primer coat, often with a secondary tie coat, plus a tacky solution to assist in the application of the rubber. The bonding systems currently in use are usually suitable both for autoclave vulcanisation and vulcanisation at 100°C with atmospheric pressure steam or hot water. Ambient vulcanisation bonding systems have to be chemically active at the lower temperatures and are therefore specialist in nature. 

Ultrasonic vulcanisation also tends to change the interfacial property of the rubber and the reinforcing materials to improve bonding. Improved wetting and flow characteristics produced by ultrasonic vulcanisation have the potential to increase the interfacial bond strength between the rubber and the reinforcing materials currently used. 

Vulcanisation of rubber Natural rubber becomes soft at high temperature (>335 K) and brittle at low temperatures (<283 K) and shows high water absorption capacity, it Is soluble in non-polar solvents and Is non-resistant to attack by oxidising agents. To improve upon these physical properties, a process of vulcanisation is carried out. This process consists of heating a mixture of raw rubber with sulphur and an appropriate additive at a temperature range between 373 K to 415 K. On vulcanisation, sulphur forms cross links at the reactive sites of double bonds and thus the rubber gets stiffened. 

In thermoplasts the parts of different chains are bound to each other by weak secondary bonds, which are broken at higher temperatures. In thermosets and vulcanised rubbers the chains are permanently linked by primary chemical bonds, which, also at higher T, do not allow flow. 

Rubber has been used by the mining industry, both in the form of cured rubber and in its uncured state, for bonding and vulcanising to metal surfaces of tanks and vessels for over half a century. It has been used to protect such items of the plant and equipment from the deleterious effects of abrasive wear, caused by elements such as coal dust, ore particles in slurry and solid form and dusty fumes. 

These rubbers are based on atoms of silicon chains rather than carbon atoms. Their unique structure is responsible for their extreme temperature properties. The most common types of silicone rubbers are specfically polysilaxanes. The Si-O-Si bonds can rotate much more freely than the C-C bond or the C-O bond. So the silicone chain is much more flexible and less affected by temperature. Silicone rubber is vulcanised by the action of peroxides which crosslink the chains by abstracting hydrogen atoms from the methyl side groups, allowing the resulting free radicals to couple into a crosslink. Some varieties of polysiloxanes contain some vinyl methyl siloxane units, which permit sulfur vulcanisation at the double bonds. 

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