C.F. Cullis

The significance of thermoanalytical measurements in the assessment of polymer flammability

Abstract Thermoanalytical techniques have been widely used for the study of polymer decomposition processes, but they cannot accurately represent the very complex process of polymer combustion. The present work discusses some of the advantages and limitations involved in the use of thermoanalytical measurements to elucidate selected aspects of the constituent processes involved during the combustion of an organic polymer. There is no correlation between the flammability of different organic polymers and their thermal stability, even if the latter property is represented by an improved parameter such as the temperature at which 1% of the polymer has decomposed. An attempt has been made to correlate the inverse of the flammability (as measured by the limiting oxygen index) with the ratio of the heats of combustion and of gasification; but the results are not entirely satisfactory. Thermal analysis can, however, be very useful for the study of the effects of additives on a polymeric substrate. The lower heating rates involved, compared with those associated with a fire, allow determinations to be made not only of the ease of decomposition of an organic polymer but also of the thermochemistry of its reactions, of the effects of different atmospheres and of the extent of volatilization of certain species. In particular, thermoanalytical measurements carried out on a polymer-additive system can aid in the prediction of the effectiveness and mode of action of additives as flame retardants.

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.

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.

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 metal chelates on the oxidation of polyolefins at high temperatures

Abstract The oxidation of carboxy-terminated polybutadiene (CTPB) and polypropylene (PP) has been investigated at temperatures above 150°C. Studies have also been made of the effects on the oxidation of these polymers of a number of transition metal chelates, namely, the bis(3,5-diisopropyl salicylato) (DIPS) derivatives of Co(II), Ni(II) and Cu(II), bis(3,5-diisopropyl salicylato) hydroxyaquo Cr(III), the bis( N -n-butyl salicylaldimino) (NBS) compounds of Cu(II) and Ni(II), and bis(stilbene dithiolato)nickel (II) (NiSDT). At the temperatures concerned, the antioxidant activity of some of the chelates, in particular the DIPS compounds, was limited by their thermal stability. NiSDT was found to be a uniquely effective antioxidant at temperatures up to ∼290°C. Mechanisms have been proposed to account for the dual role, as both catalysts and inhibitors of thermal oxidation, of the DIPS and NBS chelates.

The effect of red phosphorus on the flammability and smoke-producing tendency of poly(vinyl chloride) and polystyrene

Abstract Studies have been made of the ways in which red phosphorus, both on its own and in conjunction with a bromine and a nitrogen compound, affects the combustion behaviour of two thermoplastic polymers. With plasticised poly(vinyl chloride) (PVC), red phosphorus by itself considerably increases both flammability and smoke formation; but in the additional presence of bromine and nitrogen, reasonably good flame retardance can be obtained with smoke levels not much higher than those from the base polymer. Thermal analysis of PVC-phosphorus mixtures shows that oxidation of the phosphorus is largely inhibited until all the chlorine has been lost. Microanalysis indicates that the major part of the phosphorus remains initially in the solid residue. Red phosphorus by itself has some flame-retardant action on polystyrene (PS) but greatly enhances smoke formation. Even with a bromine and a nitrogen compound present, it is difficult to obtain satisfactory flame retardance without an unacceptable increase in smoke levels. Thermoanalytical studies show that phosphorus incorporated into PS is readily oxidised; and microanalytical measurements indicate that the greater part of the phosphorus is volatilised. Thus, with PVC, where phosphorus acts principally in the condensed phase, the formation of reactive phosphoric and phosphonic acids is apparently inhibited; whereas with PS, the reduced species which cause flame quenching are largely converted to inactive phosphorus oxides.

The thermal degradation and oxidation of polybutadiene

Abstract Detailed studies have been made of the successive stages in the thermal degradation in air and nitrogen of carboxy-terminated polybutadiene (CTPB). During oxidation at high temperatures, a protective surface film is first formed; this film ruptures at temperatures where pyrolysis leads to the formation of volatile products in the bulk of the polymer. Thermogravimetric curves for the degradation of CTPB in nitrogen are complex in shape; it appears that the free-radical crosslinking and cyclisation reactions cause an increase in the thermal stability of the polymer during degradation.

Metal chelates as flame retardants and smoke suppressants for thermoplastic polymers

Abstract Metal chelates have been incorporated into polystyrene and polypropylene to study their effects on the combustion of these polymers, in the absence and presence of a chlorinated hydrocarbon. In particular, the effect of ferrocene on the flammability of the polypropylene-Cereclor 70 system has been studied comprehensively by a triangular diagram method. The additives have been shown to interact synergistically to increase char and thus produce a system of low flammability; the Cl/Fe ratio needed for optimum flame retardance is considerably higher than stoichiometric. It has been found that, if the presence of the chlorine compound decreases the amount of smoke produced from a polymer-metal chelate system, this is indicative of some degree of flame-retardant activity of the metal-halogen system.

Studies of the effects of phosphorus and its compounds on the combustion of cellulose

Abstract Cellulose cotton cloth has been treated with phosphorus, both by direct phosphorylation and by addition of various compounds containing up to 98.6% P. Studies have been made of the effects of these treatments on thermal decomposition and flammability of the cloth. It has been shown that, in terms of unit weight of phosphorus introduced, phosphorylation is a more efficient method of imparting flame retardance than incorporation of additives. When the various additives used were compared on this basis, tritolyl phosphate and triphenyl phosphine were the most effective, while red phosphorus was the least so. Flame retardance of cellulosic materials is generally related to the extent of increased charring. However no such generalization can be made with regard to the phosphorus-based additives studied in this work, since some function primarily in the gas phase and others in the condensed phase. There is thus no obvious correlation between degree of flame retardant effectiveness and mechanism of flame retardance. The predominant effect of nitrogen compounds, both on their own and when used in conjunction with phosphorus compounds, was to decrease flammability, but there appeared to be no simple connection between the magnitude of their effect and any particular mode of action.