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Tech | September 2019

Investigating heat transfer of heat protection textiles

In a heat-exposed working environment there is a risk of scalding and heat stroke for the employee. The heat protection textile developed as part of the HEATex research project is used here as underwear. In order to realistically validate the effect of the various layer structures with phase change materials, two test benches for contact and radiant heat are designed and constructed. In this simulated working environment, the heat protection textiles are also exposed to pressure and moisture in order to reproduce real conditions of use, explain Lena Barth, Lukas Lechthaler, Christoph Peiner, Thomas Gries and Markus Tutsch.

As a part of the ZIM project HEATex (development of heat-exposed occupational safety textiles, funding code: ZF4018754CJ6), a heat protection textile with 3D spacer fabric and phase change material is being developed. The textile is used as underwear for employees in heat-exposed environments. In Germany, around 10 per cent of employees are exposed to high temperatures at their workplaces (metal, glass, ceramics and steel production as well as forges, foundries, fire brigades, ...).[1]

During work there are regular scalds and, in the worst case, heatstroke at a body temperature of over 40°C. The structure of the heat protection textile aims to prevent direct contact between external protective clothing and skin as well as to improve the absorption and transport of the body’s own moisture. A solid phase change material is used in the heat protection textiles, which absorbs large quantities of energy during the transition to liquid phase through a phase change. In addition, an aluminium layer is used as a radiation reflector. In order to validate the heat protection effect of the textiles, test benches are being developed. This enables an abstract simulation of real working conditions in heat-exposed environments.

Approaches

In the beginning, the conditions under the employee’s outer protective clothing are considered. The protective clothing causes a reduction of the incoming contact and radiant heat. Due to the fact that there is hardly any air movement below the tight-fitting protective clothing, the influence of convection can be classified as negligible. Finally, a test bench for contact and radiant heat is developed and put into operation, on which the textile layer structures can be examined under the influence of pressure and humidity.

The pressure is used to simulate the dead weight of the upper protective clothing and possible further stress caused by equipment. By compressing the textile, its volume is reduced and the air present in the textile layer is minimised. This has a negative effect on the insulation properties.

When adding moisture, the changed textile properties are investigated by the welding process. The thermal conductivity of the textile can be increased by that. Since the exchange of air with the environment is minimal and the clothing fits tightly, the small amount of air underneath the protective clothing can only absorb a small amount of moisture. Possible cooling effects due to evaporation enthalpy are therefore negligible below the outer protective clothing.

The test bench for contact heat is based on DIN EN ISO 12712-1.[2] A heating plate with a temperature of 130°C is used as the heat source. Optionally, a pressure of 60 Pa with a weight of 1070 g is applied to the sample. With a pressure atomizer, the textile is moistened with 16 g/m2 water during corresponding tests. The sample is clamped in a frame. During the test, the frame is positioned above the heat source, with the textile resting on the heating plate to ensure energy input through heat conduction.

A thermocouple is mounted in the top layer with honeycomb structure of the textile. It is positioned centrally in the honeycomb at a flat angle. Digital measurements are used to record temperature curves and detect possible temperature fluctuations at an early stage. In order to avoid these fluctuations, a lateral shielding against convection is installed around the test bench. In addition, a temperature measurement is carried out by means of an infrared camera. The temperature distribution on the surface layer in particular is measured and clearly visualized (Figure 1).

The development of the test bench for radiant heat is based on DIN EN ISO 6942 B.[3] In comparison to the previous test bench, the components are arranged vertically. In order to prevent heat radiation from entering before the measurement, the frame is positioned horizontally when the sample is clamped. A quartz heater with a power of 1500 W is used as heat source. The distance between source and sample is 50 cm. Possible pressure effects are reproducibly simulated by a stamp applied on one side. If necessary, the sample is moistened with a pressure atomizer, similar to the previous test stand. The temperature is also measured with a digital thermometer and an infrared camera (Figure 2).

Setup

Despite the simple measuring setup, the heat protection performance of the samples can be compared even with a small number of tests. The use of both test benches proves to be an essential component for the evaluation of the heat protection performance. With the different textile layer structures, a variance in the measurement results for contact and radiant heat can be seen.

When using the test benches, the reduction of possible air movement, which can lead to falsification of the measurement results, must be particularly taken into account. Temperature fluctuations of the heat sources and the environment must also be reduced. It is recommended that the test stand be set up in a standard textile climate at a temperature of 20°C and a relative humidity of 65 per cent. At the test benches just described, tests on heat protection textiles from STS Textiles GmbH, Grünbach, are carried out at the Institute for Textile Technology of RWTH Aachen University. The performance and evaluation of the tests are described in detail in the article “Performance of tests and results for latent heat storage in heat protection textiles”.

References:

TÜV Rheinland: Hitze-Check am Arbeitsplatz H.E.A.T Analyse – Objective Klimamessung nach DIN EN 27243 und Beurteilung der Hitzebelastung am Arbeitsplatz. URL: http://www.presseportal.de/pm/31385/2279470, Zugriff am 24.11.2016

DIN 12127-1 12.2015: Schutzkleidung gegen Hitze und Flammen – Bestimmung des Kontaktwärmedurchgangs durch Schutzkleidung oder Materialien

DIN EN ISO 6942 B 09.2002: Schutzkleidung - Schutz gegen Hitze und Feuer

  • Lena Barth, Lukas Lechthaler, Christoph Peiner and Thomas Gries are with Institut für Textiltechnik of RWTH Aachen University, Germany.
  • Markus Tutsch is with STS Textiles GmbH & Co. KG, Grünbach.
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