Polybutadiene

Polybutadiene is a synthetic rubber that is a polymer formed from the polymerization of the monomer 1,3-butadiene. It has a high resistance to wear and is used especially in the manufacture of tires. It has also been used to coat or encapsulate electronic assemblies, offering extremely high electrical resistivity. It exhibits a recovery of 80% after stress is applied, a value only exceeded by elastin and resilin.

Polymerization of butadiene

1,3-Butadiene is an organic compound which is a rather simple conjugated diene hydrocarbon; the chemical structure is shown as a reactant in the diagram below. A hydrocarbon diene molecule has two carbon-carbon double bonds (i. e. between two sets of carbon atoms). Polybutadiene can be formed from many 1,3-butadiene monomers undergoing free radical polymerization to make a much longer polymer chain molecule.

A chain propagating step in this chemical reaction involves a free radical near the end of a growing polymer chain forming a covalent bond with the #1 carbon in a 1,3-butadiene monomer molecule being added, resulting in a polymer chain intermediate with a substituted allyl free radical at the end of the chain. This allyl free radical, formed from the butadiene just added, can further bond to another monomer molecule at either the #2 or #4 carbons of the previous butadiene monomer. Most of the time, the new monomer bonds to the #4 or terminal carbon of the previous butadiene, resulting in a 1,4-addition of the previous butadiene unit. In a 1,4-addition, the two double bonds of the previous butadiene unit are turned into single bonds and a new double bond is formed between the #2 and #3 carbons. This new double bond may have either a "cis" or a "trans" configuration. A smaller fraction of the time (perhaps 20%), the new monomer bonds to the #2 carbon of the previous butadiene, resulting in a 1,2-addition of the previous butadiene unit. The double bond between the #1 and #2 carbons turns into a single bond in the previous butadiene unit, and the double bond between the #3 and #4 carbons remains intact in a short vinyl side group available for branching or cross-linking. "Cis" or "trans" configurations are not applicable in 1,2-additions of butadiene. See the following reaction diagram for examples of 1,2- and 1,4-addition in a polybutadiene chain.

The "trans" double bonds formed during polymerization allow the polymer chain to stay rather straight, allowing sections of polymer chains to line up against each other and effectively form microcrystalline regions in the material. The "cis" double bonds cause a bend in the polymer chain, preventing polymer chains from lining up and forming crystalline regions and resulting in larger regions of amorphous polymer. It has been found that a substantial percentage of "cis" double bond configurations in the polymer will result in a material with flexible elastomer (rubber-like) qualities. In free radical polymerization, both "cis" and "trans" double bonds will form in percentages which depend on temperature. There are different catalysts available which can result in polymerization either in the "cis" or the "trans" configurations.

Properties

Polybutadiene is a highly resilient synthetic rubber. Due to its outstanding resilience, it can be used for the manufacturing of golf balls. Heat buildup will be less in polybutadiene rubber based on vulcanization subjected to repeated flexing during service. This property leads to its use in the sidewall of truck tires. Good abrasion resistance of this rubber also leads to its use in the tread portion of truck tires; however, skidding may be a problem in passenger car tires due to low rolling resistance. For high temperature curing, polybutadiene may be blended with natural rubber and other rubbers, due to resistance in reversion of physical properties. Polybutadiene rubber can be used in water seals for dams due to its low water absorption properties. Rubber bullets and road binders can be also produced by polybutadiene rubber.

Copolymers

1,3-butadiene is normally copolymerized with other types of monomers such as styrene and acrylonitrile to form rubbers or plastics with various qualities. The most common form is styrene-butadiene copolymer, which is a commodity material for car tires. It is also used in block copolymers and tough thermoplastics such as ABS plastic. This way a copolymer material can be made with good stiffness, hardness, and toughness.Because the chains have a double bond in each and every repeat unit, the material is sensitive to ozone cracking.

Processing

Polybutadiene rubber is seldom used alone, but is instead mixed with other rubbers. Polybutadiene is difficult to band in a two roll mixing mill. Instead, a thin sheet of polybutadiene may be prepared and kept separate. Then, after proper mastication of natural rubber, the polybutadiene rubber may be added to the two roll mixing mill. A similar practice may be adopted, for example, if polybutadiene is to be mixed with styrene-butadiene.

In an internal mixer, natural rubber and/or styrene-butadiene may be placed first, followed by polybutadiene.

The plasticity of polybutadiene is not reduced by excessive mastication.

ee also

*Polymer degradation
*Ozone cracking

References