Baljinder K. Kandola

Flame-Retardant Treatments of Cellulose and Their Influence on the Mechanism of Cellulose Pyrolysis

Abstract Cellulose, either as a major component in wood or as the prime textile fiber cotton, is most frequently implicated in fire, causing injuries and fatalities [1]. When ignited, cellulose undergoes thermal degradation, form-ing combustible volatile compounds which become involved in the propaga-tion of fire. Fortunately cellulose has a chemical composition which makes it easily amenable to interactive flame-retardant treatments. Because flam-mability is a relative rather than an absolute concept, there are no truly flame-retardant fabrics, and the best that can be attained is some given level of flame resistance. Barker and Drews [2] proposed that with cellulose, the problem of fire can be described as two distinct phenomena, glowing and flaming, which present different potential hazards and should be ap-proached in different ways. Glowing is a direct oxidation of solid cellulose or its degradation products. It is generally a slow combustion and is of great concern for only specific items, such as c...

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.

Simultaneous reduction and surface functionalization of graphene oxide with POSS for reducing fire hazards in epoxy composites

Simultaneous reduction and surface functionalization of graphene oxide (GO) was realized by simple refluxing of GO with octa-aminophenyl polyhedral oligomeric silsesquioxanes (OapPOSS) without the use of any reducing agents. The presence of OapPOSS made the hydrophilic GO hydrophobic, as evidenced by the good dispersion of the OapPOSS-reduced GO (OapPOSS-rGO) in tetrahydrofuran solvent. The structure of OapPOSS-rGO was confirmed by XPS, FTIR and TEM. A morphological study showed that, due to the good interfacial interaction between the functionalized graphene and epoxy, OapPOSS-rGO was dispersed well in the matrix. With the incorporation of 2.0 wt% of OapPOSS-rGO, the onset thermal degradation temperature of the epoxy composite was significantly increased by 43 °C. Moreover, the peak heat release rate, total heat release and CO production rate values of OapPOSS-rGO/EP were significantly reduced by 49%, 37% and 58%, respectively, compared to those of neat epoxy. This dramatically reduced fire hazards was mainly attributed to the synergestic effect of OapPOSS-rGO: the adsorption and barrier effect of reduced graphene oxide inhibited the heat and gas release and promoted the formation of graphitized carbons, while OapPOSS improved the thermal oxidative resistance of the char layer.

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.

Smoke, CO, and CO2 Measurements and Evaluation using Different Fire Testing Techniques for Flame Retardant Unsaturated Polyester Resin Formulations

Smoke is considered to be the main fire hazard but its production depends on major variables, principally the chemical character and the burning rate of the polymer plus the availability of oxygen and hence ventilation. The main aim of this work is to study the effect of smoke suppressants on flammability and smoke production of flame retarded unsaturated polyester resin-nanocomposites using four different testing regimes representing different fire scenarios. Samples containing zinc borate, zinc stannates, ammonium polyphosphate with and without nanoclay are analyzed for smoke generation using cone calorimetry (well-ventilated fire), a tube furnace (fully developed fire), and a smoke density chamber (under-ventilated fire). Carbon monoxide (CO) and carbon dioxide (CO2) measurements using thermogravimetry-evolved gas analysis (TG-EGA), cone calorimetry, and tube furnace have also been analyzed and compared. Results have confirmed that the production of smoke, CO, and CO2 depend upon smoke suppressants and fire conditions used during testing samples. From this study it is evident that tin additives have very little influence on flammability of unsaturated polyester resin but they reduce smoke formation. The slight flame retardant action of the Res/APP/ZB sample is due to enhanced cross-linking of APP in the presence of zinc borate, whereas zinc stannates do not promote cross-linking of APP and hence show no improvement in flame retardancy. Finally, the presence of nanoclay in flame retarded resin shows significant reduction in smoke formations in both well-ventilated and under-ventilated fire condition. However, in the presence of smoke suppressants used in this study, the nanoclay is not instrumental in further suppressing smoke formation.

The Potential for Volatile Phosphorus-containing Flame Retardants in Textile Back-coatings

A number of volatile phosphorus-containing flame retardant species has been identified as possible replacements for bromine-containing formulations used in textile back-coatings because of the need for vapor-phase activity. The selected retardants include tributyl phosphate (TBP), a monomeric cyclic phosphonate Antiblaze CU (Rhodia Specialites) and the oligomeric phosphate-phosphonate Fyrol 51 (Akzo). When combined with an intumescent char-forming pentaerythritol derivative (NH1197, Chemtura) and applied as a back-coating on to cotton and polypropylene substrates, significant improvements in overall flame retardancy are observed. One sample applied to cotton and comprising both TBP and intumescent passed the simulated match-ignition test, BS5852:1979:Part1 after a water soak at 40°C for 30 min. Determination of residual phosphorus within chars shows that there is significant volatile activity present in these formulations. Addition of volatile nitrogen as melamine also demonstrated improved flame retardancy in similar formulations.

Thermo-mechanical Responses of Fiber-reinforced Epoxy Composites Exposed to High Temperature Environments. Part I: Experimental Data Acquisition

This first part of a series of papers on the thermo-mechanical responses of fiber-reinforced composites at elevated temperatures reports the experimental results required as input data in order to validate the kinetic, heat transfer, and thermo-mechanical models being developed and to be discussed in subsequent papers. Here the experimental techniques used for the determination of physical, thermal, and mechanical properties and their significance for particular models are discussed. The fire retardant system used to improve the fire performance of glass fiber-reinforced epoxy composites is a combination of a cellulosic charring agent and an interactive intumescent, melamine phosphate. Thermogravimetry is used to obtain kinetic parameters and to evaluate the temperature-dependent physical properties such as density, thermal conductivity, and specific heat capacity, determined using other techniques. During flammability evaluation under a cone calorimeter at 50 kW/m2 heat flux, thermocouples are used to measure temperatures through the thicknesses of samples. To investigate their thermo-mechanical behavior, the composites are exposed to different heating environments and their residual flexural modulus after cooling to ambient temperatures determined. At a low heating rate of 10°C/min and convective conditions, there was a minimal effect of fire retardant additives on mechanical property retention, indicating that fire retardants have no effect on the glass transition temperature of the resin. On the other hand, the fire-retarded coupons exposed to a radiant heat from cone calorimeter, where the heating rate is about 200°C/min, showed 60% retention of flexural modulus after a 40-s exposure, compared to 20% retention observed for the control sample after cooling specimens to ambient temperatures.

Towards the design of sandwich panel composites with enhanced mechanical and thermal properties by variation of the in-plane Poisson's ratios

The finite element (FE) method has been used to study the mechanical and thermal properties of both conventional and re-entrant (i.e. negative Poissons ratio) honeycombs, which may be used as the cores of sandwich panel composites. Failure of the honeycomb structures was simulated using a crack propagation method developed in-house. The cell-wall stress build up in the conventional honeycomb was calculated to be significantly reduced relative to the re-entrant honeycomb under (2D) hydrostatic loading, implying that the conventional core will undergo significantly less internal damage than the re-entrant core. Conversely, the re-entrant honeycomb performs better than the conventional honeycomb under thermal loading conditions. The size and pathway of the crack formed during the simulation is dependent on the failure stress distribution used in the crack propagation routine.

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.

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.