ENR – Epoxyprenes
Epoxidized Natural Rubber
Natural rubber (NR) is one of the most important commercial elastomers produced from the sap of the Hevea Brasiliensis tree. It is used for a large number of applications including tires, hoses, gloves, rubber bands, and adhesives. However, as a commodity elastomer, its applications are limited by low resistance to heat, oxygen, UV, etc. and by the high solubility in hydrocarbons and other hydrophobic solvents and oils. These limitations result from the chemical structure of cis-1,4-polyisoprene which contains double bonds that are prone to thermal and oxidative degradation. To improve its stability, natural rubber is sometimes chemically modified by introducing hydrophilic groups along the isoprene backbone, for example, by epoxidation which yields epoxidized natural rubber. This reaction is typically carried out with a peracid formed by the reaction of acetic or formic acid with hydrogen peroxide. It produces rubber with randomly dispersed epoxide groups along the polymer backbone.
Epoxidized natural rubber (sometimes called Epoxyprene and abbreviated to ENR) has much improved heat and chemical resistance because the oxirane ring increases the hydrophilicity and reduces the number of double bonds in the backbone. Epoxidation also improves the miscibility with polar polymers such as PVC, NBR, ECO etc.
The three most common levels of epoxidation are 50, 25 and 10 mole percent, referred to as ENR-50, ENR-25 and ENR-10 respectively. ENR-50 undergoes strain crystallization like NR, but with oil resistance comparable to a medium acrylonitrile NBR and gas permeability similar to butyl rubbers.1 Silica-reinforced ENR rubbers or blends with natural rubber have lower rolling resistance than NR and better wet traction than oil-extended styrene-butadiene rubber (OESBR) and have been considered as tire tread materials.2
COMMERCIAL ENR Elastomers
The two major manufacturers of epoxyprene rubber are San-Thap International and Sanyo Corporation.
ENR-25 and ENR-50 can be used in many applications where natural rubber is used but where improved heat and oil resistance and/or lower gas permeability is required. Applications include anti-vibration mounts, footwear components, tires, bearings, and adhesives.
1C.S.L. Baker, I.R. Gelling, R. Newell, Rubber Chem. & Technol., Vol. 58, No. 1, 67-85 (1995)
2K. Sengloyluan, K. Sahakaro, W.K. Dierkes, J.W.M. Noordermeer, European Poly. J., Vol. 51, p.p. 69-79 (2014)
3I.R. Gelling, J. nat. Rubb. Res. 6(3) 184-205 (1991)