By Liz White, UT staffBoulder City, Nevada-Amerityre Corp. says independent tests show that in heat aging and ozone-resistance, its polyurethane tyre compounds, "have outperformed comparable rubber compounds." Such tests offer an intitial prediction of a tyre's potential long-term durability, the firm claimed. The developer of polyurethane tyres said that the US National Highway Traffic Safety Administration (NHTSA) has been studying the effects of ozone exposure and tyre heat build up and aging on tyre performance. According to Amerityre, NHTSA intends to propose an oven-aging test for FMVSS 139, the safety standard relating to highway-use tyres, to take effect July 2007.Amerityre says these results "are significant in that they demonstrate that its polyurethane compounds show minimal changes in physical properties as a result of extended exposure to heat and ozone."The 70 Shore A PU material had a hardness increase of 0-2 percent after 70h at 100°C, while the hardness of comparable rubber compounds increased from 12-20 percent. In 50 ppm ozone, the PU had no cracking after 144h, while the rubber compounds showed initial cracking after 30h. The tests show that use of Amerityre's polyurethane materials, "can be expected to produce a tyre with better and more consistent performance," claimed Richard Steinke, Amerityre's president and ceo."