Testing latent heat storage in heat protection clothing

In work environments exposed to heat, there is a risk of scalding or heat stroke for the worker. The aim of the HEATex research project is to enable workers to stay longer in heat-exposed environments. For this purpose, a special textile is being developed in which phase change materials (PCM) are incorporated. The PCM content of the textile can convert heat into enthalpy of fusion in heat-exposed environments. The textiles are investigated for their heat transfer under the influence of different types of heat sources, mechanical pressure and humidity levels.

The aim of the research project HEATex (Development of Heat Exposed Occupational Safety Textiles) at the Institute of Textile Technology of the RWTH Aachen University is to enable longer residence times for workers in heat exposed environments. In Germany, around 10 per cent of the workforce is exposed to high temperatures at their workplaces (steel production, fire brigade, etc.)[1]. This results in accidents at work due to external heat effects. In order to enable workers to stay longer in heat-exposed environments, a textile is being developed in which phase change materials (PCM) are incorporated.

The PCM content of the textile can convert heat into enthalpy of fusion in heat-exposed environments. The developed textiles also contain a layer of aluminium that reflects heat radiation. A honeycomb structure improves air circulation and thus improves moisture transport. The effectiveness of the layered elements of the textiles is investigated with the aid of two test benches. One test bench analyses the behaviour of the samples under the influence of heat radiation, the other under the influence of contact heat [2]. In the following, the performance of the tests and their most important results will be explained.

Test procedure and results

Six test specimens are examined. They differ in their layer structure and layer thicknesses. Samples 1 and 2 consist of a knitted fabric with a honeycomb structure which is between 4 mm (curvature) and 1.5 mm (valley) thick, a 2 mm thick PCM layer and a 3 mm (sample 1) or 5 mm (sample 2) thick spacer fabric. Samples three to six consist of the same knitted fabric with honeycomb structure and the same spacer fabric. In addition, the samples 3-6 contain a 0.1 mm thick reflective layer of aluminium foil and a 3 mm thick stiffened PCM layer.

During the tests, the surface temperature of the samples is recorded. This is measured with an electronic contact thermometer. The measuring probe of the thermometer is attached to the surface layer of the sample. The temperatures are plotted over time (see Fig. 2). The surface temperature approaches the final temperature asymptotically. This asymptotic behaviour of the curves is comparable to that of an ideal curve of the transient temperature curve at constant ambient conditions [3]. The slope of the surface temperature decreases in the range of approximately 30 degree Celsius for a short time. This sink results from the absorption of the thermal energy by the PCM. The use of PCM in textiles can therefore delay a rise in temperature and thus contribute to protection against the effects of heat.

During the tests, the so-called threshold time is determined. This is the time it takes for the textiles to warm up from 25 degree Celsius room temperature to a critical temperature of 41 degree Celsius.

In order to evaluate the textiles for use as heat protection underwear, a utility value analysis is carried out. To this end, the realistic nature of the individual tests is first evaluated with the aid of a pair-wise comparison. This weighting of the test setup is weighted with the previously determined threshold times of the respective samples and added up.

Since a strongly varying behaviour of the samples is observed when using contact and radiant heat, the evaluations are only carried out separately. In the results for the contact heat test bench, sample 4 scores best. Sample 6 then follows. Consequently, solid samples with stronger PCM and an aluminium layer are best suited for contact heat protection. Thin specimens with little PCM and no reflective layer perform very poorly here, whereas the test stand for radiant heat performs best with specimen 2, which consists of a soft base PCM. Then sample 6 follows again, but much closer to the average than before. Samples with less PCM and without a reflective layer perform best here. This is followed by solid samples with more PCM and a reflective aluminium layer. The examination of the test results of both test rigs leads to the conclusion that textile 6 has the protective properties with the greatest continuity in the Accordingly, heat protection textiles such as textile 6 should have a honeycomb structure, a spacer fabric, a reflective layer and a PCM layer.

The next steps are the development and testing of new textile samples or textile layer structures. These can be tested with the existing test benches.

References

• TÃœV 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
• Melliand: Test setup for investigating the heat transfer of heat protection textiles made of 3D spacer fabric | Lena Barth, Christoph Peiner, Lukas Lechthaler
• Uni Magdeburg: Instationary heat conduction | URL: http://www.uni-magdeburg.de/isut/TV/Download/Kapitel_7_Waerme-_und_Stoffuebertragung.pdf

The article is authored by Paul Grünefeld, Kevin Krause, Lena Barth, Lukas Lechthaler, Christoph Peiner, Thomas Gries, who are from the Institut für Textiltechnik of RWTH Aachen University.