Controllable synthesis of three kinds of zinc borates and flame retardant properties in polyurethane foam

Depending on the molar ratios of ZnO:B2O3:H2O, zinc borates can be classified into many kinds of categories. In this paper, we successfully synthesized three kinds of zinc borates (2ZnO·3B2O3·7H2O, 2ZnO·3B2O3·3.5H2O and 3ZnO·3B2O3·5H2O) by controlling the reaction conditions, using Na2B4O7·10H2O, H3BO3 and Zn(NO3)2·6H2O as the reactants. These three zinc borates transform with each other if the reaction conditions were appropriately controlled. The morphologies of three products are peculiar. The synthesized 2ZnO·3B2O3·3.5H2O is anomalous spheric-like or flaky, 3ZnO·3B2O3·5H2O is flower-like, and 2ZnO·3B2O3·7H2O is club-shaped. According to the thermodynamic performance of three kinds of zinc borates, we selected 2ZnO·3B2O3·3.5H2O which had the best flame retardant properties, then doped it into the polyurethane to synthesize polyurethane/zinc borate materials. The maximum decomposition temperature of the polyurethane foam doped with zinc borate was increased by 54 °C before 300 °C. Furthermore, the maximum decomposition temperature was increased by 104 °C after 400 °C.

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► A new method for preparing 2ZnO·3B2O3·3.5H2O has been proposed.
► The morphologies of three types of zinc borates are peculiar.
► Zinc borates transform with each other by appropriately controlled.
► Polyurethane/zinc borate materials have excellent retardant properties.