Low-melting sulphate glasses and glass-ceramics, and their utility as fire and smoke retarder additives for poly(vinyl chloride)

Abstract

Low-melting sulphate glasses and glass-ceramics, which are effective fire and smoke retarder additives for poly(vinyl chloride) (PVC), have been developed. They are based on low-melting, glass-forming compositions in the K₂SO₄-Na₂SO₄-ZnSO₄ ternary system. The preferred ternary sulphate glass melts at 420° C and has the composition K₂SO₄ · Na₂SO₄ · 2ZnSO₄, which also corresponds to the composition of a congruently melting ternary compound. In a simple, tin-stabilized PVC compound, 10 phr (parts per hundred of resin) resulted in a 54% reduction in smoke in the flaming mode of the NBS Smoke Chamber. 40 phr gave a smoke reduction of 83%. The preferred ternary sulphate glass also works in concert with low levels of other known smoke retarders. For example, 10 phr of the ternary glass and 5 phr of MoO₃ gave a smoke reduction of 78%, while 40 phr of the glass and 5 phr of MoO₃ gave a smoke reduction of 93%. The addition of transition metal sulphates to low-melting compositions in the K₂SO₄- Na₂SO₄-ZnSO₄ system led to the development of low-melting glasses and glass-ceramics containing Ni²⁺, Cu²⁺ and VO²⁺. These transition metal modified sulphates were also very effective smoke retarders in the simple PVC compound. All of the low-melting sulphate glasses and glass-ceramics promoted intumescence and char formation during combustion of the simple PVC compound. In this way they effectively formed thermal barriers to retard further combustion.