Published - 24th Jun 2019
Part 2 – Rubber Compounding / Mixing
Most Thermoplastic and Thermoset Elastomers need to be mixed with other ingredients to modify their properties and to make them easier to process and made into the material we recognise as rubber. The raw rubber that has ingredients mixed into it is called a Rubber Compound.
For Thermoset Rubbers the most important additives are those chemicals that ” vulcanise” or ”cross link” the polymer chains. Why do we need to do this? This process converts the very plastic easily deformed material that is useless for most applications into the Elastic, resilient material that we recognise as Rubber. This process is called vulcanisation or sometimes referred to as curing. Vulcanisation involves heating a rubber compound containing materials such as sulphur or organic peroxides. These chemicals can form chemical links or bonds between the polymer chains.
The polymer chains are arranged in a random matrix that can be relatively easily deformed,i.e it is very plastic and the formation of these chemical links between the main polymer chain binds it all together to form a 3 dimensional lattice that now has some memory and wants to return to its original position after being deformed- it has become elastic.
Other materials will be added to the raw polymer to change such physical propertires as Hardness, Tensile Strength, Compression Set, Weathering/Ageing. Materials will also be added to speed up the process of vulcanisation and improve the cross link density.
The list of ingredients a Rubber Technologist has at their dispoal is very large they include;
Fillers/Extenders such as Carbon Black, Clays, Calcium Carbonate and Silica. These will re-enforce and/or change the hardness ior extend (cheapen) the compound.
Oils/Process Aids. These can change the hardness, improve low temperature performance and make the compound process more easily.
Accelerators- These speed up the vulcanisation process
Acclerator activators- these assist in the vulcanisation process
Antioxidants and anti-ozonants to improve the long term ageing of the rubber
Pigments- These can be added to make coloured compounds
Blowing agents can also be added to the compound to make sponge rubber. Blowing agents are materials that, when heated, give off a gas- usually Nitrogen or Carbon Dioxide. As the gas is generated it causes the uncured rubber to expand forming cells. It then vulcanises to set it in shape, This is a specialised process for Industrial Rubber
The final compound is like a cake recipe and it will usuall be based on a 100 parts of raw polymer. A very basic Natural Rubber compound may be something like this:
Natural Rubber 100 parts Base Polymer
Carbon Black 50 parts for re enforcement
Clay 30 parts to extend- cheapen the mix
Oil 25 parts to reduce the hardness
Zinc Oxide 5.0 parts to activate the cure system
Stearic Acid 2.0 parts to activate the cure system
Sulphur 1.5 parts to form the cross links (vulcanise) in the polymer
Accelerators 1.5 parts to speed up vulcanisation
Antioxidant 1.0 part to improve ageing
Anti-ozanant wax 2.0 parts to improve ozone resistance (ozone will attack the polymer chain and break them this can often be seen as cracks when the material is strectched- in everydat terms the rubber has ”perished”