A. Richard Horrocks

Complex char formation in flame-retarded fibre-intumescent combinations—II. Thermal analytical studies

Abstract It is well known that flame retardant fibres, when heated, decompose differently when compared to untreated ones and this is especially so for char-promoting flame-retarding species such as Lewis-acid generators in cellulosic fibres. Certain intumescent systems when dispersed about flame retardant fibres, interact and develop a unique ‘char-bonded’ structure which has been shown to enhance flame and heat resistant properties. These interactive fibre-intumescent combinations require that both the flame retardant fibre and intumescent form chars by chemically and physically compatible mechanisms, usually by a semi-liquid intermediate phase. Examples analyzed to date are flame retarded viscose fibres (Visil, Kemira; Lenzing FR Viscose) which although comprising different flame-retarding species, generate polyacids on heating. Intumescents based on ammonium polyphosphate behave in a similar way thus enabling physicochemical interactions of both charring components to occur. Initial studies of char structure using scanning electron microscopy have already shown evidence for this kind of interaction. In this paper, the mode of thermal degradation of these systems has been studied by thermal analysis (DSC, TGA) and the nature of char determined by IR studies. The results of these investigations are used to more fully understand the mechanisms involved.

Flame retardant textile back-coatings. Part 2. Effectiveness of phosphorus-containing flame retardants in textile back-coating formulations

Selected phosphorus-containing flame retardants, including some intumescents, have been formulated with selected resins and applied as back-coatings to both cotton and cotton-polyester (35:65) blended fabrics. While all formulations raise the limiting oxygen index, only those based on ammonium polyphosphate and a cyclic phosphonate enable samples to pass a small-scale version of the simulated match test, BS5852: 1979, Source 1. As expected, the back-coatings containing intumescents promoted higher levels of char formation, but these did not reflect in their performance to the match test where most incurred failures. Thermogravimetric analysis suggests that the more effective flame retardants as exemplified by ammonium polyphosphate, are those which liquefy by melting and/or decomposition well below 300 °C. It is proposed that this enables wetting by these products of the back face of the fabric and their diffusion to the front face where, as the temperature rises towards 300 °C, char formation occurs before ignition of surface fibres can take place.

Complex char formation in flame retarded fibre-intumescent combinations: 1. Scanning electron microscopic studies

Abstract A recently disclosed development1–3 comprising a composite of flame retardant cellulose fibres and an intumescent has shown unusually high flame and heat resistance with a char structure that can withstand air exposure temperatures up to 1200°C. Char structures of selected flame retardant cellulose fibres when alone and blended with inert E-glass and aromatic char-forming novoloid fibres in the presence of an ammonium polyphoshate-based intumescent have been studied using scanning electron microscopy. Evidence suggests that a ‘char-bonded’ structure is formed in which flame retardant cellulose fibrous chars have interacted with intumescent char during its formation. Similar interactions are not seen with E-glass and novoloid fibre residues. The char-bonded structure is considered to have reduced air permeability and so has considerably enhanced surface oxidative resistance at high temperatures. An outline mechanism is proposed and is based on known pyrolysis mechanisms for the flame retarded cellulose fibre and intumescent components.

Burning behaviour of foam/cotton fabric combinations in the cone calorimeter

Abstract The burning behaviours of polyurethane foam/cotton fabric combinations were investigated using cone calorimetry. One non-flame retarded and two flame retarded polyurethane foams containing melamine and melamine plus a chlorinated phosphate respectively, were combined with four cotton fabrics, i.e. two types of commercial cotton, one without flame retardant (CN) and another flame retarded with Proban (CPR); and another two flame retardant cottons which were treated with Pyrovatex (CPY) and (NH4)2HPO4 (CDA) respectively in the laboratory.

Evidence of interaction in flame-retardant fibre-intumescent combinations by thermal analytical techniques

Abstract Flame retardants when applied to cellulosic fibres, change their decomposition in such a way that significant conversion to carbonaceous char occurs. Intumescents, when dispersed on these flame-retardant (FR) fibres, enhance this property. They not only produce an expanded and thermally protective char barrier, but also interact with FR fibres to form a so-called char-bonded structure which has been shown to possess unusually high resistance to air oxidation at temperatures in excess of 500°C. The effectiveness of the char-bonded structure as a flame and heat barrier is considered to be dependent on the efficiency of the interaction of fibre and intumescent char-forming chemistries. Previous studies have demonstrated the interaction between various flame-retarded viscose fibres and phosphate-based intumescents; current research extends the work to include flame-retardant cottons. In this paper, these interactive properties are studied by using the thermal analytical techniques, TMA, TGA and DSC, which enable the various pyrolysis transitions and associated volatilisation and intumescent char-formations to be studied. In this way a greater understanding of the mechanism of complex char-formation and its subsequent oxidation may be investigated.

Char formation in flame-retarded wool fibres. Part 1. Effect of intumescent on thermogravimetric behaviour

The char-forming characteristics of wool are well known but have been little studied during recent years, although flame-retardant activity may be associated with enhanced char formation in many cases. Recently, intumescent flame-retardants have been shown to react synergistically with flame-retarded cellulosic fibres and generate unexpectedly high levels of char. Extension of this concept to wool, a naturally intumescing fibre, shows that similar synergistic activity may be observed. This paper describes and discusses the initial work which has involved the application of a number of phosphate-based intumescents bound in a resin system to the surface of 100% wool fabrics in the absence and presence of zirconium-based flame-retardant (ZIRPRO®) and to unretarded wool/flame-retarded (FR) viscose blend fabrics. Thermal analytical studies (TGA) of treated fabrics and synthesised mixtures of respective powdered fibre/intumescent combinations demonstrate that enhanced char formation may occur and that char shows higher-than-expected oxidative resistance above 500°C. Scanning electron microscopy studies have demonstrated the physical nature of the interaction and show that, under the experimental heating conditions used to generate chars, the wool component appears to have passed through a semi-liquid state prior to or simultaneously with the onset of char formation. A qualitative model is proposed to explain the interactions taking place between fibre and intumescent char-forming reactions which compares well with earlier results observed for flame-retardant cellulosic fibre — intumescent interactions. Copyright

Correlation of physicochemical changes in UV-exposed low density polyethylene films containing various UV stabilisers

Melt blown biaxially orientated, unstabilised and stabilised low density polyethylene (LDPE) films with various thicknesses were exposed in two accelerated artificial weathering devices with xenon arc (Xenotest) and fluorescent tube UV-B (QUV) sources under controlled temperature and humidity. The structural changes during combined photo- and thermal degradation have been studied using tensile tensiometric, IR spectrophotometric, and thermal analytical (DSC) techniques. The effects of hindered amine light stabilisers (HALS) and film thickness on the time-dependent changes in elongation, carbonyl group concentration, crystallinity and onset temperature (T on ) of post-fusion DSC oxidation exotherm have been observed. Photo-oxidation is accompanied by increased crystallinity which maximises as mechanical properties start to deteriorate significantly and the rate of carbonyl group formation increases. While poor correlation exists between reduction in mechanical properties and increased carbonyl index values, the former correlates well with the DSC derived T on values for unstabilised and stabilised films. This suggests that thermal analysis may be useful in detecting physicochemical changes occurring in exposed films more effectively than other techniques such as IR.

Quantification of Zinc Hydroxystannate** and Stannate** Synergies in Halogen-containing Flame-retardant Polymeric Formulations

Zinc stannate and hydroxystannate are used as synergists in fire-retardant systems in conjunction with halogenated species where their behavior is generally considered to be similar to antimony-containing synergists while offering the additional properties of smoke suppression and relative nontoxicity. The literature most often compares relative synergistic behaviors qualitatively but this article determines such behavior quantitatively using Lewin’s synergistic effectiveness parameter, ES, calculated from sample limiting oxygen index (LOI) data. Flame-retardant formulations comprising zinc stannate (ZS), zinc hydroxystannate (ZHS), or antimony III oxide (ATO) in combination with selected and polymer-compatible bromine-containing flame retardants were formulated in commercial grades of poly(vinyl chloride) (PVC)), thermoset polyester resin, and polyamide 6. PVC formulations simulated both commercial cable and plastisol applications and comprised either a phthalate or aryl phosphate ester as plasticizer in combination with selected synergists. All formulations were subjected to flammability testing using LOI, UL94 (vertical sample mode), and cone calorimetric (including smoke analysis) methods. For all PVC/synergist combinations containing the phthalate plasticizer, synergy was evident with 2.0 > ES > 1.0 and a relative effectiveness order ATO > ZHS > ZS. Zinc borate present as a cosynergist also has a quantifiable, positive effect. In the presence of the phosphate ester plasticizer, the reverse order is observed with marginal levels of synergy being evident (1.2 > ES > 1.0). In polyester resin formulations, ATO and ZHS exhibit similar levels of synergy when present with dibromoneopentyl diglycol with the former being superior when decabromodiphenyl ether is the flame retardant. However, in all polyamide 6 formulations, the highest levels of synergistic effectiveness are observed (ES ≥ 5.0) with ZS. Maximum values of ES correspond to a molar ratio of Sn/Br<0.3 suggesting the formation of SnBr2 and SnBr4 as the effective flame-retarding species.

FTIR analysis of gases evolved from cotton and flame retarded cotton fabrics pyrolysed in air

The gaseous products produced by pyrolysing various samples of cotton and flame retarded cotton fabrics in air at different temperatures (300–1200 °C) were identified and quantified via FTIR. CO and CO2 evolutions were investigated in detail. The flame retardant samples showed enhanced CO and CO2 concentrations at lower pyrolysis temperatures (300–450 °C). These evolutions showed similar temperature dependence behaviour for all samples tested. At higher temperatures (450–800 °C), a second pyrolysis stage is identified where product evolution increased in concentration towards respective maximum values within this range. At higher temperatures still (800–1200 °C) pyrolysis product oxidation occurred which thus reduced the concentrations of all oxidisable products. The relative evolutions of both CO and CO2 are commensurate with the known vapour phase and condensed phase activities of bromine- and phosphorus-nitrogen-containing flame retardants, respectively.

The flammability of polyacrylonitrile and its copolymers

The burning behaviour and the influence of flame-retardant species on the flammability of fibre-forming homopolymer and copolymers of acrylonitrile are reported. A pressed powdered polymer sheet technique has been developed which allows a range of polymer compositions in the presence and absence of flame regardants to be assessed for limiting oxygen index, burning rate and char residue determinations. The method has been used to provide a rapid, reproducible and convenient means of screening possible flame-retardant systems. It has been shown that the mechanism of thermal degradation of acrylic polymers is dependent on the rate of heating. At low heating rates cyclisation is the main reaction pathway, whereas at high heating rates commensurate with those encountered in fires volatile-forming chain scission predominates. Furthermore, the effectiveness of a particular flame retardant is directly proportional to its char-forming tendency. The most effective of the flame retardants studied was ammonium polyphosphate, and a mechanism by which this operates is suggested.