Two classic approaches to flame retardancy of rubber are the incorporation of a halogen-antimony system and of a water-generating heat-sink additive such as ATH or MDH. Often, these two systems are used together. A “standard package” may be 15–20 phr of chloroparaffin (70% Cl, a solid chloroparaffin). An improved package to pass a more stringent test might use up to 30 phr of a chloroparaffin, 8–12 phr antimony trioxide, and 50–75 phr ATH. A liquid product with 60% Cl content was said to allow better processing.
As discussed earlier, the most widely used zinc borate is 4ZnO , which is often used synergistically with ATH or MDH, with which it sinters and forms a barrier. It also releases water with heat absorption at 325°C. In a typical rubber, 150 phr of ATH and 10 phr of zinc borate can be as effective as 190 phr ATH. Zinc borate also performs well in halogen-antimony systems, as it allows replacement of some or most of the antimony trioxide, and can form a glassy inorganic char with antimony oxide. Zinc metaborate, 2ZnO∙2B2O3∙3H2O (Marshall Additive Technologies’ ZB200) is also used and is sometimes lower in cost.