Firefighting clothing is one of the most widely used varieties of protective clothing, which can protect the human body from heat damage. The fire scene where firefighters work is not a dry environment, especially when firefighters put out an open fire, the human body will sweat a lot, the clothes inside will absorb a lot of sweat, and the firefighting water can also be transferred to the inner layer through the outer surface of the clothes. This water evaporates into water vapor at high temperature, which is transmitted to the surface of human skin, which can burn human skin. Therefore, it is of practical significance to study the thermal protection properties of moisture-containing fabrics.

Experimental part:

Test material:

The structure of fire suits is generally divided into four layers, which are composed of the outermost layer, the waterproof and breathable layer, the heat insulation layer and the comfort layer. From the various layers of fabrics suitable for making fire-fighting suits, one was selected as the experimental fabric to compare the changes in thermal protection performance of different layers of fabrics after moisture absorption.

testing method:

The TPP value reflects the thermal protection ability of the fabric against thermal radiation and thermal convection. The larger the value, the better the thermal protection performance of the thermal protective clothing; conversely, the worse. The test method is to place the sample horizontally on a specific heat source. Within a specified distance, a heat source occurs in two different forms of heat transfer - thermal convection and thermal radiation. A copper heat flow meter placed on the opposite side of the sample measures the temperature on the backside of the sample. The flame needs to be in direct contact with the sample to achieve a heat flow of 84kw/m2 to the fabric surface. The heating curve was measured with a copper heat flow meter on the back of the sample and compared with the Stoll standard curve to obtain the time t required for secondary burn, and multiplied by the exposure heat energy q to obtain the TPP value. The calculation type is TPP=2×q type: q is the specified radiant heat flow (84kw/m2); t: the time required to cause secondary burns. The TPP experiment was carried out using a TPP thermal protection performance tester according to the NFPAl976 standard.

The samples are divided into 5 groups, numbered from 1 to 5, each group of samples includes A.B.C.D4 fabrics, 3 samples are taken from each tissue, and each sample is placed in a sealed plastic bag. Add water, and spray 5.10.15.20ml of water evenly for the second to fifth groups of samples. The hygroscopic sample was placed in a constant temperature and humidity environment (temperature of 20 °C, relative humidity of 65%) for 24 hours to allow the sample to fully absorb water. The TPP value of the sample after moisture absorption was measured by the TPP experiment, and the average value of each tissue was 3 times the TPP value.

Experimental results:

①Influence of moisture content of single-layer fabric on thermal protection performance. Among the four kinds of fabrics, the moisture content of fabric B is the lowest, which is lower than 3%, and has basically no change compared with that before moisture absorption. Therefore, the secondary burn time and TPP value of fabric B after moisture absorption did not change much compared with those before moisture absorption. This is because the B fabric is a polytetrafluoroethylene film, which is a waterproof and moisture-permeable fabric with excellent performance. There is basically no condensed water on the surface, so the water content is very low. Because the comfort layer flame retardant cotton fabric D has good water absorption and the highest water content, followed by the outer layer fabric A, and the heat insulation layer insulation felt C has the middle moisture absorption. The hygroscopicity of these three-layer fabrics increased with the addition of water, and the secondary burn time and TPP value increased with the increase of water content. The moisture content of the single-layer fabric has a very high positive correlation with the secondary burn time and TPP value. The greater the moisture content, the greater the TPP value, and the longer the corresponding skin reaches the second burn time. Therefore, the influence trend of the moisture content on the TPP value is basically the same as that on the second burn time.

②The linear regression model of TPP value and secondary burn time took TPP value and secondary burn as variables, and moisture content as the regression independent variable, and SPSS software was used to establish a single-variable linear regression model.

The univariate linear regression model of TPP value and water content is TPP value=10.732+0.072×water content, and the univariate linear regression model of secondary burn time and water content is secondary burn time=5.164+0.062×water content.

Experimental results:

The moisture content of the single-layer fabric was significantly correlated with the time to secondary burn and TPP value, and the time to secondary burn. There is an obvious linear regression relationship between IPP value and water content. When the water content increased by 1%, the second burn time was prolonged by 0.062s, and the TPP value was increased by 0.072kJ/cm2. The second burn time and TPP value increased with the increase of water content. Under the combined heat transfer condition of strong radiation and convection (82.21kw/m2), water helps to improve the thermal protection performance of the single-layer fabric. The higher the water content, the better the thermal protection of the single-layer fabric. In standard environments, the thermal protection performance of aqueous fabrics is superior to that of dry fabrics. In this case, the moisture in the fabric will evaporate rapidly, and before the radiative convection heat is transferred to the back of the fabric, part of the heat will be taken away by the steam, reducing the heat reaching the human body, thereby improving the thermal protection performance of the single-layer fabric. The greater the water content, the more water evaporated, the more heat taken away, the faster the heat dissipation, and the further improvement of the thermal protection performance of the fabric. This conclusion applies to moisture content less than 50%.