NR – Natural Rubber
IR – Polyisoprene
Hevea rubber (NR) is the only non-synthetic rubber and has been in commercial use since the beginning of the 20th century. It is a natural occurring elastomer extracted from the sap of the Hevea Brasiliensis tree grown on plantations. It is fully biodegradable and consists mainly of cis-1,4-polyisoprene:
The synthetic equivalent is polyisoprene (IR) which can be produced by polymerization of 2-methyl-1,3-butadiene with Ziegler-Natta catalyst. Both NR and IR rubbers are often mixed with other ingredients such as oils, fillers, crosslinking agents, and antioxidants, which allows for a versatile variation of the elastomer properties.
Natural and synthetic polyisoprene are inexpensive elastomers with good fatigue resistance and are, therefore, an excellent choice for dynamic applications at low and ambient temperatures. The tensile strength, elongation, and abrasion resistance of natural rubber is excellent over its working temperature range of approximately -50°C to +100°C. The chemically similar synthetic polyisoprene has lower strength than natural rubber but better low-temperature properties. In applications where consistency and good processability is required, synthetic polyisoprene is often the better choice. Since the polymerization of isoprene is carried out under narrowly controlled conditions, there is very little variation of composition, structure, and resulting properties, whereas natural rubber contains naturally occuring impurities and the molecular weight varies with the growth conditions of the rubber tree. Both has a noticeable impact on the physical properties and processeability of the rubber. In general, synthetic rubber has superior resistance to aging and weathering and is easier to process due to its lower viscosity. It is also often more compatible with other rubbers such as SBR and EPDM.
Natural and sythetic rubber differ in the microstructure; natural rubber consist almost entirely of the cis-1,4 polymer whereas synthetic isoprene is a blend of cis-1,4, trans-1,4 and 3,4 polymer. The amount of cis-1,4 is typically in the range of 90 to 98 %. An increase in cis-1,4 usually lowers the glass transition temperature, increases the crystallinity, and improves the mechanical strength. Thus the tensile strength and tear resistance of synthetic polyisoprene are usually somewhat lower than those of natural rubber.
Although natural rubber (NR) and poyisoprene (IR) can be used with water, polar organics (organic acids, alcohols, ketones) and some dilute acids and alkalis, other elastomers such as EPDM are preferable for these applications. Both natural and synthetic isoprene rubber compounds are attacked by non-polar solvents, fuels and petroleum based oils. They also have poor heat resistance and are susceptible to attack by ozone due to the presence of double bonds in the polymer backbone that are prone to thermal and oxidative degradation. The degradation generally occurs through chain scission and causes a drop in the mechanical properties.
COMMERCIAL IR/NR Elastomers
Major manufacturers of polyisoprene rubber are Firestone, Eni, Cray Valley, LG Chem, GoodYear, and Lanxess.
Due to its outstanding strength and excellent compression set, isoprene rubber is the preferred material for many engineering applications. Typical uses include anti-vibration mounts, drive couplings, tires, springs, bearings, rubber bands, and adhesives.
The typical working temperature range is -50°C to +100°C.