Effect of moisture content on thermal protective performance of fabric assemblies by a stored energy approach under flash exposure

Abstract

Firefighters wearing protective clothing perspire profusely in the process of performing their duties, and sweat increases moisture in the inner layers of multilayer protective clothing. Also, the outer shell fabrics inevitably become wet. In this study, two kinds of outer shell fabrics (aramid IIIA fabric and aramid 1313 and flame-retardant viscose-blended fabric) and three kinds of thermal liner fabrics with different thicknesses were selected. Two wetness conditions were investigated to simulate the sweating in thermal liner fabric with or without the wet outer shell fabric. A modified thermal protective performance (TPP) tester was employed to explore the effects of moisture and its distribution on stored thermal energy developed in six fabric systems and on TPP under flash exposure. Pearson correlations were established to analyze the relationships of the fabric systems’ thickness and second-degree burn time, and of absorbed energy and second-degree burn time in different configurations. The statistical analysis from these obtained data indicated that the thickness of fabric systems had no significant correlation for second-degree burn time (p > 0.05), but the absorbed energy exhibited a strong relation (the lowest R2 value could reach 0.8070 and p-values were all much less than 0.05). Performance results for the wet thermal liner indicated that the negative impact on thermal protection reached the greatest degree in 15% wetness, but in some extreme situations (100% wetness), the performance was improved (the maximum increase can achieve 116.2% over performance in dry condition). However, the existing moisture in the outer shell showed a positive effect. These findings will enable the engineering of textile materials that achieve high performance protection from thermal hazards and give some guidance to firefighters during operations.

Quantification of the energy storage caused dual performance of thermal protective clothing containing with moisture exposed to hot steam

“…Results from the protection time and absorbed energy all demonstrated that the thermal protective performance during steam exposure was increased with the external moisture content. This result was consistent with those found in the flame or high radiant heat exposures, showing that external moisture enhanced thermal protective performance.…”

“…The heat flux increased immediately when steam exposure started and then reached a peak value in 1.4‐2 seconds. A previous study by Zhang has shown that under flame exposure, the heat flux of the sensor behind a three‐layer fabric system with moisture preconditioned, arrived at the maximum value in approximately 10‐20 seconds. The steam exposure greatly accelerated the occurrence of the peak heat flux, indicating that the steam imposed a much quicker heat transfer at the beginning of exposure comparing to the flame exposure.…”

“…Moisture absorption strongly affects heat transfer as it changes thermal properties, that is, thermal conductivity and heat capacity of the fabric. The effects of internal and external moisture exposed to flame or radiant heat have been investigated by previous studies . Lawson et al found that under flame exposure, external moisture decreased heat transfer through the fabric whereas internal moisture did the opposite, increasing heat transfer.…”

“…Barker et al observed that approximately 15% moisture content reduced protective performance most severely under a radiant heat exposure. It was reported that under a flame exposure, 15% internal moisture content approached the minimum thermal protection but saturation moisture improved thermal protection . Taken together, the presence of moisture within fabrics has a complex effect on heat transmission and its effect is highly influenced by the type and intensity of thermal exposure, moisture amount, and moisture location.…”

Quantitative assessment of the thermal stored energy in protective clothing under low-level radiant heat exposure

“…Some researchers used this test method to compare the results obtained by the stored energy approach and the conventional approach that ignores the energy discharge. [16][17][18] Based on these studies, a 'stored energy test' (SET) apparatus is introduced in the newly developed ASTM F2731-11 standard. 19 Using the SET apparatus, Barker et al 10 and Su et al 20 studied the heat transmission and thermal energy storage in fabrics exposed to low-level radiant heat.…”