Longer lasting heat-resistant rubber developed for US Army Tank Pad (top) is being modified for application in longer lasting windshield wiper (bottom).

Research involving both the University of Maryland and DAMILIC Corporation scientists has resulted in the invention of a new formulation for the manufacture of Styrene-Butadiene Rubber (SBR) products. The new rubber has improved wear resistance, hot tear strength, ozone resistance, crack initiation, and crack growth characteristics. DAMILIC Corporation has secured rights to this technology through a patent and formation of the subsidiary DAMILIC Rubber Co., owned jointly with the University of Maryland.

With the University of Maryland's improved formulation, the sulfur is reduced or eliminated, and an additional crosslinking agent is added. Curing is accomplished in a two step process, with the second step consisting of a radiation cure.

The resulting product exhibits an improvement in ozone resistance - with more than 12,000 times the resistance of conventionally cured formulations. This special radiation curable formula survived for two months in an ozone chamber (at elevated ozone concentrations) at the National Institute of Standards and Technology. Standard sulfur/heat cured SBR samples failed within 3 hours under the same conditions. A summary of the results of laboratory tests are shown below. This ozone resistance translates into improved aging characteristics.

Mechanical Characteristics of E-Beam v. Sulfur Cured Rubber

Mechanical Properties	Sulfur Cured SBR	EB Cured SBR
Tensile Strength	3300 psi	3390 psi
200% Modulus	650 psi	655 psi
Hot Tear	130 lb/in	182 lb/in
Elongation at Break	515%	610%
Crack Initiation	20,000 cycles	>200,000 cycles
Crack Growth	25 mil/min	9.4 mil/min

Testing results in general have demonstrated that radiation curing of rubber offers a new technology to the rubber industry for the manufacture of materials with properties unattainable by traditional chemical curing techniques.