BR – Polybutadiene
Polybutadiene rubber (BR, PBD) is one of the cheapest and largest-volume synthetic general-purpose elastomers which is sometimes used as a substitute for natural rubber (NR). It is produced either by anionic polymerization or by coordination polymerization of 1,3-butadiene typically in a non-polar solvent. The latter is often the method of choice because it allows for both tight control of the molecular weight (MW) and excellent stereoregularity.1 The 1,3-butadiene can enter the growing polymer chain in three different ways yielding the three structural isomers cis-1,4, trans-1,4 and vinyl-1,2, as shown in the figure below.
Both cis-1,4- and trans-1,4-polybutadiene have a similar low glass transition temperature (Tg) of approximately -107°C whereas 1,2-vinyl has a much higher Tg of about 0°C and thus is not suitable for most elastomer applications. Dispite similar Tg's, cis and trans PBD have very different mechanical properties and uses. For example, cis-1,4 rich PBD has very low crystallinity, and the lowest melting point, whereas polybutadiene with a high trans-1,4 content has much higher crystallinity and is much harder and less flexible.
Butadiene rubbers are typically cured by sulfur systems and are often compounded with other polymers and additives such as oils, fillers, crosslinking agents, and antioxidants, which allows for a versatile variation of the elastomeric properties.
Butadiene rubbers are not resistant to oil, gasoline and hydrocarbon solvents. They also have poor heat resistance and are susceptible to attack by ozone due to the presence of double bonds in the polymer backbone which are prone to thermal-oxidative degradation. The degradation generally occurs through oxidative crosslinking and causes embrittlement.
COMMERCIAL IR/NR Elastomers
Some major manufacturers of poybutadiene rubbers are Firestone, Cray Valley, AsahiKasei, GoodYear, and Lanxess. Three types of polybutadiene are available: high-cis, medium-cis, and low-cis. Among these, medium-cis rubber is the most popular grade.
The major use of cis polybutadiene rubber is in tires. It goes into side walls and treads. To optimize performance such as traction, rolling and abrasion resistance, it is typically compounded with other elastomers such as natural rubber and SBR. Other applications are golf ball cores, inner tubes of hoses for sandblasting, and covers for pneumatic and water hoses. Polybutadiene is also used as a toughening agent in plastics, in particular in high-impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS).
High vinyl and high trans polybutadiene are produced on a much smaller scale and are used on a much smaller scale. For example, high-vinly PBD is sometimes used in high quality tires to improve rebound resilience whereas high-trans PBD is used in the outer lay of golf balls.