Note: Apple Rubber is a custom molder, not a material supplier.
Sometimes, there is a misconception that a rubber compound is one ingredient and is the same for every different polymer type. This could not be further from the truth. A rubber compound could be a combination of 3 to 15 different ingredients of thousands of different compositions and vendors.
A typical rubber formation based on parts per hundred is:
Polymer 100 phr
Filler 30 – 60 phr
Antioxidant 1 – 3 phr
Antiozonants 1 – 3 phr
Oil 5 – 30 phr
Cure 5 – 10 phr
Polymers:
The most important ingredient in a rubber compound, polymers give the bases for chemical, physical and molding properties. Typical polymers are distinguished by chemical performance and viscosity rating. An example for a Nitrile (Buna-N) compound is the amount of Acrylonitrile (ACN). The higher the ACN, the more oil resistance. These polymers are then sold with varying ACN content. The Mooney viscosity can vary to give better processing performance.
Filler:
Carbon Black is one of the most common fillers. This gives the rubber compound physical strength and its black color. There are various particle sizes and surface activities. 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 molding. Adding more oil can also lower the hardness of the rubber. Thus, lower Shore A compounds will typically have more oil in the formulation.
Cures:
Sulfur is the most common curative for rubber compounds. Rubber polymers are just entangled hydrocarbon chains than won’t hold shape. Back in 1839, Goodyear discovered that rubber chains can be bonded together by heating the rubber with sulfur, called vulcanization. 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.
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 hundreds of variations and hours of testing to perfect the mechanical properties while maintaining good processing.