Effects of Heat on Structures of Cotton, Polyester, and Wool Fibers in a Triblended Fabric With and Without Flame Retardant

Abstract

Fabrics marketed as flame resistant are often blends of more than one fiber. The effect of flame-resistant finishes and of heat on different fibers is not the same. A previous study compared differences in morphology of cotton and polyester fibers, and of cotton and wool fibers in two blended fabrics before and after heating. This report shows, using scanning electron microscopy (SEM), the progressive changes that occur during stages of heat stress in fibers in a triblended fabric. Samples of a cotton, polyester, and wool (60/25/15) triblend, unfinished and finished with bis[tetrakis(hydroxymethyl)phosphonium] sulfate (THPS), urea, and trimethylolmelamine, were studied before and after burning. The SEM examination compares responses of each fiber to heat and shows that of the treated fabrics, polyester responds first by melting, wool bubbles and flows, and the external structure of cotton is relatively unchanged. Energy dispersive x-ray analyses show the location of the flame retardant agent in cotton and wool fibers. Thermal analysis data are correlated with these structural changes.When blended fabrics are chemically finished, dif ferent fiber types respond differently to the finish and thus affect the final performance of the fabric. We reported the effects offlame-retardant finishing and heat on fabrics blended of cotton with polyester or wool in our earlier work [3]. This paper reports results from the study of a triblended fabric of cotton, polyester, and wool finished for Dame resistance.Scanning electron microscopy was used to show chemical coating on fiber surfaces and, in conjunction with energy dispersive x-ray analysis (EDX), also distribution of chemicals within individual fibers. Fabrics were studied after burning to obtain information on mechanisms of burning and on responses of the different types of fibers to heating. Chemical and morphological structures of the fibers affect melting and decomposition points as well as the nature of the deformations that occur during heat stress.Experimental The fabric studied was an 8 oz/sq yd (280 g/M2 60/ 25/15 cotton/polyester/wool twill that had been finished by a pad/dry/cure process in a formulation containing bis[tetrakis(hydroxymethyl)phosphonium] sulfate (THPS), urea, and trimethylolmelamine (TMM) as described by Beninate et al. [1]. Samples for the study were taken from three areas of a 45 degree ignition strip such as that shown in Figure 1: (a) is an unburned area, (b) is the completely charred area, and (c) the area where the effecrts of the heat were first found. Preparation of samples for microscopical examination was as given in our previous paper [3]. FIGURE 1. Fabric edge ignition strip showing the unburned aret (a), the completely charred area (b), and the intermediate zone (c).

Blends of Wool with High Performance Fibers as Heat and Fire Resistant Fabrics

“…This molten char can coat the p-aramide fibers and then, slow down their subsequent thermal oxidative degradation. This kind of mechanism has been already described in the case of blend wool/cotton [15,16]. The authors reported that the wool melt stabilized the material when the wool fuses and flows onto cotton fibers.…”

Scanning electron microscopic studies of wool/intumescent char formation

“…It is noteworthy that the swollen ‘pod‐like’ structures do not occur to the same extent as in the flame retardant treated samples, suggesting that the production and durability of these structures is somehow assisted by the presence of the flame retardant present either from the Visil fibres or from the intumescent systems. Examination of SEM evidence in earlier work17 referred to the above suggests that the presence of the flame retardant (THPS) generated more well defined ‘pod‐like’ features, while pure wool chars exhibited only general swelling or ‘ballooning’.…”

“…At 300 °C, while the axial splitting cuticular rupture still predominates, it is also noticed however, that the pyrolysing wool fibres have started to swell and now resemble ‘pod‐like’ structures with regular narrower or nodal points along the fibre. These ‘pod‐like’ structures have been noted but not commented upon by Goynes and Trask17 who studied char formation in wool fibres treated with tetrakis(hydroxyphospohonium) sulphate (THPS). In these earlier studies axial splitting of the cuticle was not observed however.…”