| MadSci Network: Chemistry |
What most people think of when they see the term "rubber" is the black material that tires are made of. Or they think of "Natural Rubber" which comes from rubber-tree sap as a latex (identical to milk-weed sap). The rubber that most people are familiar with is actually a complex mixture of many components since "rubber" by itself is of little value. The rubber compound is a mixture of a polymer, reinforcing agents, vulcanizing agents such as sulfur, antioxidants, and oils. The black of a tire comes from carbon black which is actually condensed smoke like lamp black that you can see from a candle flame. Carbon black gives strength and abrasion resistance to the rubber polymer. Carbon black is primarily carbon from the incomplete burning of hydrocarbons (oils).
Polymers are formed from many monomer units like a string of pearls or a chain link. The individual links of the molecular chain are the monomers. There are many different polymers of many different properties. Plastics are also polymers. Rubber polymers are normally refered to as "elastomers" for elastic-polymers. Polymers are long molecular chains. Polymers vary according to the monomer units used to make them. Different properties in the final rubber compound result from these various polymers. The rubber inside a tire is different from the rubber in the tread that meets the road is different from the rubber on the side is different from the rubber in a seal or the rubber of a bouncing ball. Natural rubber is a naturally produced polymer of isoprene and has certain properties. Synthetic or man-made rubber are polymers of other monomers and have different sometimes superior properties to natural rubber that make them more useful for certain applications where those properties are more desirable.
Butyl is the term for a family of polymers that are a co-polymer of isobutylene and isoprene. Chemically it is poly(isobutylene-co-isoprene) or poly(2-methyl-1-propene-co-2methyl-1,3-butadiene) Grades of butyl rubber are distinguished by molecular weight and by isoprene ratio. Butyl rubber has very low unsaturation in the main chain (resulting from higher isobutylene content) which produces the unique set of desirable properties characteristic of butyl rubber. These properties include: low gas permeability, thermal stability, high hysteresis (damping, i.e. no bounce), ozone and weathering resistance, chemical and moisture resistance. Butyl rubber was first commercially produced in the US in 1942 in response to the critical need for synthetic rubber to replace the lost supplies in the war.
The unique properties of butyl can be used by engineers in areas where those properties are beneficial. Butyl makes a very poor ball; no bounce. But no bounce is good for an energy absorber such as a motor mount to dampen engine vibration. Low gas permeability is good for the innerliner of a tire or for inner-tubes which lets a tire hold air better. In the old-days before butyl it was neccessary to frequently put air in tires because the air would slowly leak out. It does not leak through butyl.
Thomas Satterfield
Project Chemist
Continental General Tire Co.