Property of rubber
The term "rubber" refers generically to different products, natural and artificial, which are used for the production of the most various articles. The raw material is represented by caucciù which is a natural product from plantations, but which can also be obtained synthetically in the chemical industry. The natural rubber and artificial are mixed with various additives to obtain the so-called "compound" that is subjected to curing at high temperatures in order to make them buy an elastic behavior.
The additives in a rubber compound may vary from 2-3% (in the case of a rubber band) to over 60% by weight and will include some or all of the following:
Curatives - Active chemicals which bring about the cross-linking of the long chain rubber polymer. Sulphur was the first to be discovered and is still commonly used.
Reinforcing Fillers - Materials which increase the strength of the material. Carbon black and silica are the most commonly used.
Fillers - Relatively inert chemicals, such as clays, which increase the bulk of the compound (excess use of inert fillers can cheapen materials but often has an adverse effect on performance).
Pigments - Added to produce specified colors. They can only be used with compounds which do not contain carbon black.
Plasticizers - Added to aid processability or to produce specified properties.
Anti-Oxidants/Anti-Ozonants - Chemicals which are added to help the compound resist surface attack, especially by ozone.
Process Aids - Resins, soaps, polyethylene.
The constituents are weighed out and combined by a mixing process which must blend the ingredients thoroughly.
This is achieved either by an internal mixer, where the compound is mixed by two meshing rotors in an enclosed case; or by open mill mixing, adding the ingredients carefully into the “nip” between two steel rollers.
The result of either process is a batch of uncured rubber compound ready to molded in a desired shape.
Rubbers have found many applications in industrial and consumer goods because they are the only group of materials able to provide elastic properties across a wide range of temperatures (well above and below the temperature ranges of plastics and other elastic materials).
The rubber family includes a diverse range of materials and the main properties are summarized below with examples of actual applications:
It can be used over a temperature range from -80°C to +300°C
It is available in a wide range of colors
It can be electrically insulating, conductive or anti static
It can withstand extremes of weather and outdoor environments indefinitely
It can withstand exposure to fuels, oils and chemicals while retaining its properties
It can be made flame retardant and self extinguishing
It can maintain tension and compression forces indefinitely
It is conformable, adaptable and can accommodate movement, shock, thermal changes
It can absorb vibration and noise and act as an insulator
It can be gas tight and used as a fluid seal or separator
It has low thermal conductivity and can be used to reduce heat transfer
It has friction properties similar to human skin
It can have a clean, smooth surface which is non-stick and suitable for hygienic applications
It is compatible with other engineering materials (e.g. metals, plastics and ceramics) and can be combined with them in many different ways, including bonding.
Many of these properties can be combined by suitable compounding. Some properties are only available in one type of rubber.