TG/DTG/DTA evaluation of flame retarded cotton fabrics and comparison to cone calorimeter data

Abstract

Unbleached cotton fabrics (UCF) with 12.5% polypropylene scrim treated with two phosphate–urea based fire-retardant (FR) formulations were evaluated for FR properties using thermogravimetry/differential thermogravimetry/differential thermal analysis (TG/DTG/DTA) method. In addition to testing the two FR-treated unbleached cotton fabrics (CF-FR1 and CF-FR2), bleached cotton fabric (BCF) treated with the two FR formulations (BCF-FR1 and BCF-FR2) was evaluated. Both formulations were washable with add-on of FR chemicals at 18.7% (FR1) or 17.4% (FR2) for UCF and 22.5% (FR1) or 24.9% (FR2) for BCF. The decreasing order of sums at maximal rates of samples degradation in air environment according to DTG method was: BCF (21.40%/min) > UCF (12.91%/min) > BCF-FR2 (12.83%/min) > BCF-FR1 (11.68%/min) > CF-FR2 (10.20%/min) > CF-FR1 (9.73%/min). It indicates that both formulations cause the decrease of thermooxidation of the products at slower rates than the starting material. Several endo- and exothermic peaks observed by DTA in inert and oxidative environment gives additional information about the degradation process. The order of decreasing thermal responses of the studied samples based on sums of DTA peak values of endothermic and exothermic peaks in air environment is: UCF (0.597 °C/mg) > BCF (0.120 °C/mg) > CF-FR1 (0.089 °C/mg) > BCF-FR1 (0.077 °C/mg) > CF-FR2 (0.062 °C/mg) > BCF-FR2 (0.053 °C/mg). This is in agreement with the cone calorimeter results according to which the flammability properties are improving with the decreasing heat release rates or ignition time prolongation in order: UCF > CF-FR1 > CF-FR2. The advantage of TG/DTG/DTA method is slower linear heating rate, which allows the more detailed evaluation of the light and flammable cotton fabric.

Introduction

At the present time, it is more and more evident that flammable materials like cotton fabrics need to be tested by several independent methods to properly evaluate effectiveness of the flame retardancy formulations (Alongi et al., 2012b, Gao et al., 2009, Hagen et al., 2009, Mostashari and Baie, 2010). Cone calorimeter testing is less sensitive when flammable and low density materials are tested in small quantities and high heat fluxes. In such cases, the heat release rate values are not giving the complete picture. Thermogravimetry is a predominant and important analytical method to evaluate the thermal processes of cotton. Also the use of limited oxygen index (LOI) method brings a difference picture due to atmosphere manipulation, but could be misleading at high FR add-on. It is also informative to know the temperature of the studied sample during the course of degradation, which is dependent upon the architecture of the burning chamber. This can be better achieved on thermobalance than with cone calorimeter. The effectiveness of flame retardant could then be evaluated on the basis of volatiles formed with increasing temperature, which is related to amount of residue produced. Also the effect of thermooxidation during the process could be observed with TG/DTG/DTA when comparing the degradation process in inert and oxidative environment (Šimkovic, Antal, Balog, Košík, & Plaček, 1985). A more effective flame retarded material exhibits smaller differences during the course of degradation for inert in comparison to oxidative conditions (Chang et al., 2011, Tian et al., 1999). The problem of residue behavior is especially important when large amounts of flame retardants are added (Chang et al., 2007, Parikh et al., 2003). When evaluating the residue behavior of fabric samples with initial mass of 2 g, the cone calorimeter might provide unreliable data due to the low sensitivity of a balance with a 0–500 g range (White, Nam, & Parikh, 2012). For such applications TG/DTG/DTA should be used for comparison.

The future of flame retardants is in a more environmental approach, and especially for cotton, less plastic containing and more polysaccharide containing composites need to be studied (Horrocks, 2011, Šimkovic, 2008). Examples of the use of the cone calorimeter for light composite sample evaluation are known (Šimkovic et al., 2005, Šimkovic et al., 2007, Šimkovic et al., 2012, Šimkovic, 2012). It indicates that at lower heating levels the shape of heat release rate (HRR) curve is broadening due to slower increasing of temperature in the heating chamber. Under these circumstances the heating conditions of cone calorimeter and TG/DTG/DTA are closer and might be complementary for the study of flame retarding conditions. In this way it could be learned if the samples could be tested properly with cone calorimeter due to several times smaller thicknesses used than for UCF sample. This series of TG/DTG/DTA tests is a follow-up to an evaluation of the FR fabrics in a cone calorimeter (White, Nam, & Parikh, 2012).

The goal of the present work was to use a TG/DTG/DTA system to understand the behavior and to evaluate the effectiveness of two phosphate–urea flame retardants in improving the fire performance of UCF and compare the results with cone calorimeter tests on the same materials. Additionally BCF samples modified with the same formulations were evaluated with TG/DTG/DTA. These formulations were developed for mattresses applications and tested on different settings (Nam et al., 2010, Parikh et al., 2003, Uppal et al., 2010). Similar concepts were studied by other authors with the help of thermogravimetry (Liodakis, Fetsis, & Agiovlastis, 2009). This method was already used before (Šimkovic, Antal, Balog, Košík & Plaček, 1985) when DTG and DTA are used for evaluation of cellulose thermooxidation. Problems related to textile or decreasing the sample size when tested by cone calorimeter are also known (Limdholm et al., 2012, Schartel and Hull, 2007).