Rubber Compounding

01 Sep 2020 | Posted by Andrew Onions

Rubber Compounding

MacLellan Rubber provides customized and standardized rubber compounds, producing elastomer sheeting’s and polymer blends for a variety of applications. There is a common misconception, promoted by some suppliers that a rubber compound is one ingredient, namely the rubber polymer, and that this is the same for every polymer type. This is wholly inaccurate.

A rubber compound could be a combination of 3 to 30 different ingredients with thousands of different compositions from a multitude of chemical suppliers.

Through formulation long-chain molecules are chemically linked together, forming networks and transforming the material from a viscous liquid to elastic solid, this is what happens during vulcanisation.

A typical rubber formation will have six core elements:

Polymer – Filler – Antioxidant – Antiozonants – Oil – Curative

Some of these will be individual chemicals, others will be a mixture of multiple chemicals depending upon the performance requirements of the finished product.

Polymers:

This is the most important ingredient in a rubber compound, polymers give the basis for chemical, physical and processing properties. Typically polymers are distinguished by chemical performance and viscosity rating. Generally, the higher percentage of Polymer in a compound the better performance.  For example a Nitrile (NBR) compound will have varying amounts of Acrylonitrile (ACN) - the higher the ACN, the more oil resistance. For lower grade, cheaper compounds, some polymers are mixed typically with SBR or regrind.

Fillers:

Carbon Black is one of the most common fillers. This acts as a reinforcing agent giving the rubber compound physical strength and its black color. Reinforcing agents of the rubber compound act as stress arrestor and are required to have high specific area. This means that the particle must be smaller than 1 μm in size. A primary particle size as small as 0.1 μm can be obtained which can give a specific area of a few hundred square meters per gram of filler. Changing the particle size can increase physical properties while also increasing compression set.

White clays and Mineral fillers are used for colored compounds. This helps makes the compound white to add the desired color requested. These types of fillers are typically less reinforcing than carbon black. Blending of these fillers with carbon black helps reduce cost of the overall compound.

Antioxidant:

Antioxidants help protect the compound from high temperature while in use and while the compound is being mixed. These ingredients can absorb free radicals that can break the polymers bonds and reduce service life of the compound

Antiozonants:

The most common antiozonant is wax, used to bloom out on the surface after molding to protect the rubber from ozone attack. A common example can be seen when purchasing new tires. The surface of a new tire will have a wax feel or show a slight haze.

Oil (processing aids):

Various oils are used to help incorporate all the dry ingredients used in the rubber compounding. They also help to reduce the viscosity of the overall compound to help with processing especially when moulding. Adding more oil can also lower the hardness of the rubber. Thus, lower Shore A compounds will typically have more oil in the formulation.

Curatives:

Sulfur is the most common curative for rubber compounds. Rubber polymers are just entangled hydrocarbon chains than won’t hold shape. In 1839, Goodyear discovered that rubber chains can be bonded together by heating the rubber with sulfur, called vulcanization. In this way bonds of carbon – sulfur – carbon are formed.

Another common vulcanizing agent, peroxide creates direct bonds to the carbon chains forming carbon – carbon bonds. These bonds take more energy to break, thus typically giving higher service temperatures and low compression sets. Some polymers can only be cross-linked with peroxides.

Developing the Compound you Need

There are many other ingredients that can be used to give different properties. Because of all the combinations, not all compounds are the same. Rubber compounding is a science to assure required properties by varying the ratios of ingredients and compensating for the interactions. This is why most compounders consider their rubber formulations proprietary. It can take hours of testing to perfect the mechanical properties while maintaining good processing.

For advice on the right material for your application contact your account manager, fill in the online contact form, or email us at salea@maclellanrubber.com