The thermal protective performance (TPP) rating of a fabric is the amount of energy in cal cm-2 which must be applied to the fabric until it is estimated that seconddegree burns of the skin behind the fabric would occur, using the Stoll criterion. As the heat flux of 2 cal cm-2 s-1 (84 kW m-2) is used, the TPP rating is simply found by multiplying the time to exceed the Stoll second-degree burn criterion by two. The larger this number, the greater the protection factor of the fabric system [4, 7, 10].
Thermal protective performance test methods use data from the work of Stoll& Chianta to estimate the time it takes for second-degree burn damage to begin for a given exposure. This data is converted to the total amounts of energy that must be absorbed by the skin, and to skin temperature rises, to cause second-degree burns for a given amount of exposure to heat and length of exposure. The temperature rise recorded can hence be compared with this criterion to estimate the time required to produce a seconddegree burn.
The test apparatus constructed for this study is based on the NFPA test method 1971 (Figure 1) [7]. This equipment combines two burners and a bank of nine quartz tubes calibrated to provide a 50% radiative and 50% convective heat flux at a total flux of 84±5 kW m-2 (2±0.1 cal cm-2 s-1). A shutter protects the fabric sample from high heat flux before and after the test run. The specimens measure 150 mm by 150 mm and are placed horizontally on the heat sources,between an upper and lower mounting plate. The heat sensor is mounted in direct contact with the back surface of the fabric that would normally rest against the skin. In standard NFPA 1971, there is a spacer between the heat sensor and the back surface of the fabric in the case of single-layer fabrics. However, we did not use this, because it was found to increase all the values recorded and to distort the results with some materials more than others [8]. The heat sensor consists of a copper calorimeter and an insulating board in which the calorimeter is mounted. The copper calorimeter is blackened and has a 40 mm diameter with a thickness of 1.6 mm; three thermocouples are secured in the disk, positioned at 120o intervals.
The TPP tests for all these specimens were repeated three times. The fabric lay ers were sewn together so that one just touched another. Before beginning the tests, the sensor was cooled to approximately body temperature, below 38 oC. After the beginning of the exposure, the temperature was recorded every three seconds. The temperature rise was calculated by subtracting the starting temperature from the recorded temperatures, and temperature rise versus time curves were plotted. The curve recorded in terms of the rises in temperature was compared with Stoll’s second-degree burn criteria,in order to calculate the protection time.The TPP ratings were determined by multiplying the protection time by the heat flux density of 2 cal cm-2 s-1.
We sent samples of four types of outerlayer fabrics (1, 2, 3 and 4) and one type of third-layer assembly (4 + 6) to the Du Pont Engineering Fibres Laboratory in Meyrin, Switzerland to be tested there. Then we compared the results of the Du Pont tests with the results from our apparatus. A spacer of 6 mm was used in the Du Pont tests for single-layer fabrics.The effects of laundering on TPP rat ing, area mass, air permeability and tear strength were examined. Before washing, and after 1 cycle, 3, 5, and 10 cycles,these properties of each specimen were measured.