Investigations on the flame-retardation of cellulosic fibrous materials

Abstract

The flame-retardation of Pyrovatex CP on cellulose substrate has been studied. Since levoglycosan, responsible for the flammability of cellulose, is formed in the main decomposition process by dehydration, the amounts of water vapour released from untreated and treated cellulose during this process were measured. The water contents of the decomposition products obtained in thermal measurements were absorbed in dioxan and determined dielectrometrically. The results show that the water content of the decomposition products of untreated cellulose is much higher than that of treated cellulose. A reaction mechanism is suggested. The mechanism of action of phosphorus-containing flame-retardants is still an open question, the solution of which is difficult, among others because these compounds are marketed with different compositions. Inorganic phosphates were first applied as fire-retardants, but organic phosphates have recently become widespread, mostly in the form of phosphorus amides and halogenated phosphorus esters. It has been found [1] that there is a synergetic effect if phosphorus and nitrogen or phosphorus and halogen are applied in combination. Among flame-retardants containing phosphorus alone, a unique example is triphenyl phosphonate, investigated by Bostic et al. [2]. This compound has been found to exert its effect in the gas phase only, through a free-radical mechanism similar to that of hydrogen bromide. For triphenyl phosphite, it was assumed by Arney and Kuryla [3] that nonvolatile additives exert their effects by inhibiting the formation of levoglycosan, i.e. suppressing the dehydration step. This takes place through a phosphorylation reaction at the most reactive carbon atom of cellulose, Ca. Phosphorylation may proceed more easily with aryl derivatives than with the corresponding alkyl compounds, since the latter are more liable to elimination scission of the C-O bond, and the resulting oxyphosphoric acid has a catalytic effect on dehydration, i.e. on the formation of levoglycosan. The effects of phosphorus and nitrogen are superimposed when THPOH is applied in combination with ammonia, urea or guanidine, or when Pyrovatex CP (dimethyl phosphopropionic amide) is used. In their investigations of P and N-containing flame-retardants, Garn and Denson [4-6] subjected a mixture of cellulose and flame-retardant to pyrolysis, and identified the pyrolytic products