M.M. Hirschler

Thermal analysis and flammability of polymers Effect of halogen-metal additive systems

Abstract Thermal analyses have been carried out on systems containing a thermoplastic polymer (ABS or HDPE), a halogenated hydrocarbon (decabromobiphenyl or a chlorinated wax) and one or two metal-containing compounds. The results were compared with flammability measurements on the same systems, as determined by limiting oxygen index. Thermal analyses of the additives on their own has shown the extent to which the metal compounds are volatilised as their metal halides, thus indicating their possible similarity to the mechanism of flame-retardant action of antimony oxide. Such results are shown to be misleading in terms of providing an indication of flame-retardant efficiency. However it is possible to obtain meaningful correlations between the information obtained from thermal analysis of complete systems, i.e. including the polymeric substrate, and comprehensive studies (e.g. by triangular diagrams) of the flammability.

Char formation from polyolefins. Correlations with low-temperature oxygen uptake and with flammability in the presence of metal halogen systems

Abstract The oxidative degradation and combustion of polypropylene were investigated by simultaneous thermal analysis (STA) and by flammability measurements. STA shows that, while halogen compounds and metal oxides invariably increase char formation from the polymeric substrate, metal chelates can act both as inhibitors and as promoters of carbonisation. In general, however, there appears to be a direct correlation between the effect of an additive on oxygen uptake and that on char formation. Pro-oxidants which catalyse char formation are effective condensed-phase flame retardants, as a result of their producing fewer volatile species from the polymer.

Comprehensive study of the effect of composition on the flame-retardant activity of antimony oxide and halogenated hydrocarbons in thermoplastic polymers

Abstract Flame retardance has been studied in the systems (a) antimony (III) oxide and decabromobiphenyl in acrylonitrile-butadiene-styrene terpolymer (ABS) and (b) antimony (III) oxide and chlorinated paraffin in high-density polyethylene (HDPE). Values of limiting oxygen index for over 70 compositions in each system have been measured and fitted by fourth-order polynomials. The triangular plots of these polynomials show complexities of the systems which cannot be revealed by traditional methods. Optimum atomic ratios for this synergistic system were found to be higher than the commonly accepted value of 3: Br/Sb = 9 and Cl/Sb = 6–7. ABS was found to be more amenable to flame retardance than HDPE; possible reasons are discussed.

Studies of the effects of phosphorus-nitrogen-bromine systems on the combustion of some thermoplastic polymers

Abstract A study has been made of red phosphorus (P) (or phosphates) and nitrogen compounds as flame retardants/smoke suppressants for thermoplastic polymers [plasticized poly(vinyl chloride), polystyrene, high density polyethylene and poly(methyl methacrylate)], both on their own and in combination with a brominated flame retardant (decabromobiphenyl). The techniques used were limiting oxygen index, the Michigan smoke chamber, thermoanalysis and i.r. spectrometric analysis. The nitrogen compounds used serve as mild flame retardants but perhaps their most important effect is to decrease smoke formation from the polymers and in particular to nullify the large increase in smoke levels caused by the incorporation of P. In many systems, there is flame retardance and smoke suppression synergism, since the combination of P and nitrogen compounds decreases flammability and specific smoke formation to a greater extent than the additive effects of the individual materials. The nitrogen compounds appear to act by extending considerably the temperature range over which the system decomposes, and thus decreasing the average rate of production of volatiles. They also catalyse oxidation of P, which is the flame retardant mechanism for this material. The nitrogen compounds also break down to yield inert gases, which cause dilution of the gas mixture to outside the flammable range. All nitrogen compounds used, except benzotriazole, give large quantities of ammonia on decomposition; benzotriazole is the only nitrogen compound used which does not reduce flammability. In conclusion, it is considered that the disadvantages of P as a flame retardant outweigh its advantages for the systems studied.

Effect of oxygen on the thermal decomposition of poly(vinylidene fluoride)

Abstract Thermogravimetric studies have been made of successive stages of the thermal decomposition of poly(vinylidene fluoride) during programmed heating, at low and high heating rates, in nitrogen, air and oxygen. A single-stage (probably chain-stripping) mechanism operates in nitrogen; it is second order in polymer and has a high activation energy (approx. 180 kJ mol −1 ). The presence of oxygen in the atmosphere drastically changes the behaviour of the reaction which becomes zero order with respect to the polymer. TG and DTG charts of reaction in nitrogen and in air differ mainly in the appearance of the carbon burn-off reaction. However the kinetic activation parameters of the reaction in air are very similar to those of the much more complex three-stage reaction in oxygen. The effects of the high heating rates are noticeable predominantly in that the separate stages become much less easy to distinguish and the temperature of initial breakdown is lowered, although the temperature at which 1% of the mass has been lost remains essentially unchanged.

The effect of combinations of aluminium(III) oxides and decabromobiphenyl on the flammability of and smoke production from acrylonitrile-butadiene-styrene terpolymer

Studies have been made of the flammability [expressed in terms of the limiting oxygen index (LOI)] and the smoke-forming tendency [expressed in terms of the maximum smoke density (Ds)] of systems containing widely varying proportions of an acrylonitrile-butadiene-styrene terpolymer (ABS), decabromobiphenyl (DBB) and (a) anhydrous aluminium(III) oxide, (b) aluminium(III) oxide monohydrate and (c) aluminium(III) oxide trihydrate. The values of LOI and Ds have been plotted by polynomials up to the fourth order and plotted in triangular diagrams. The optimum aluminium/halogen atomic ratios for flame retardance are different for the three oxides. Flame-retardant synergism has been observed, however, between all three oxides and DBB and is particularly marked with anhydrous alumina and the halogen compound. All the systems also give a significant degree of smoke-suppression. Simultaneous thermal analyses strongly suggest that the flame-retardant action of the aluminium compounds is not purely physical and is largely confined to the condensed phase.

Red phosphorus as a flame retardant for a thermoplastic nitrogen-containing polymer

Studies were made of the flame-retardant and smoke-suppressant activity of red phosphorus, in the presence of a brominated (decabromobiphenyl) and/or a nitrogenated (melamine) compound, on acrylonitrile-butadiene-styrene terpolymer. These studies were complemented by simultaneous thermal analyses of the polymeric systems. Red phosphorus is oxidised on heating and the thermal decomposition of any system containing this additive includes a substantial weight gain stage at ca 700 K. Melamine is volatilised very early on and thereafter acts only as a smoke suppressant in the gas phase. Decabromobiphenyl and red phosphorus interact positively in decreasing the flammability of the substrate, particularly at atomic ratios not lower than stoichiometric. The acrylonitrile content in ABS is very important, suggesting that the additives act mainly in the gas phase but that they also form some sort of charry layer on the polymer surface.

Binary mixtures of metal compounds as flame retardants for organic polymers

Abstract Studies have been made of the effect on the flammability of thermoplastic polymers of the partial or total replacement of one metal compound by another in the presence also of a suitable halogen compound; particular attention has been paid to systems where the primary flame retardant is antimony(III) oxide. With each binary metal compound system investigated, ten different compositions have been chosen so as to provide a symmetrical arrangement of points within a triangular design; resulting calculated values of the limiting oxygen index for each polymer-flame retardant system for a given polymer are shown as a graphical contour analysis. Comprehensive studies of several systems show that both iron(III) oxide and aluminium oxide monohydrate can significantly enhance the flame-retardant action of antimony(III) oxide but that several other metal compounds, although not as effective as Sb 2 O 3 , may nevertheless be used as adequate partial replacements for it. The Fe 2 O 3 -SnO 2 -H 2 O system can also act as an effective flame retardant under certain conditions. The SnOue5f8ZnO system perhaps best illustrates the importance of the polymer substrate and of the total additive loading as factors controlling the flame-retardant effectiveness. For all the systems studied, however, ABS is a much better substrate than HDPE. The results of a reasonably detailed study of the flame retardance conferred by several different compositions of a binary metal compound mixture give a much more reliable indication of the effects on polymer flammability of the constituent metal compounds than are obtained simply by replacement of a given concentration of one compound by another.

The combined action of aluminium oxides and halogen compounds as flame retardants

Abstract Studies have been made of the effects of mixtures of aluminium oxides and organic halogen compounds on the flammability of polyethylene and an acrylonitrile-butadiene-styrene copolymer (ABS). There is considerable synergism in the flame-retardant action, particularly when anhydrous alumina and a bromine compound are introduced into ABS. The addition of relatively small amounts of alumina prevents the saturation effects otherwise observed when increasing quantities of halogen compounds on their own are incorporated into the polymers. Chemical interaction between aluminium oxides and the halogens occurs in the condensed phase.

Effects of magnesium oxide/hydroxide on flammability and smoke production tendency of polystyrene

Abstract Investigations have been carried out on the effects of magnesium oxide and magnesium hydroxide on the thermal degradation of polystyrene. The results have been correlated with the effects of the additives on the flammability of and smoke production from the polymer. Both metal compounds are extremely efficient smoke suppressants but neither is of much interest as a flame retardant on its own. The flammability of the system can be decreased by the use of brominated additives, but at the expense of a very heavy increase in smoke production. Ternary additive systems, containing also a different metal oxide, can be used to obtain a significant degree of flame retardance while retaining a high measure of smoke suppression. Interpretations are given of the action of the additives on the degradation of the polymer.