Tecnología GPRS

The thermal protection and comfort of the thermal liner face cloth has been deter- mined through analyzing the porosity, temperature profiles and DSC thermograph.

Figure 4. Observation of the form stability of composite powder by heating at 120"C and comparing the melting behavior with pure PCM.

In a complete FFPC, the face cloth of thermal liner stayed behind the moisture barrier which is wind and water proof. The face cloth S1 is expected to be imper- meable to air as it was coated on both sides with aerogel and PCM that created binder films on both surfaces. The porosity measured by using jPOR macro in ImageJ software identified that the uncoated fabric had 14% porous area which reduced to 3.9% on composite powder coated surface and 4% on aerogel coated surface. The SEM images in Figure 5 show that both the coated surfaces have been significantly covered by binder film, leaving very few pores exposed. The actual understanding of the porosity of coated fabric can be determined comparing air permeability of coated fabric with its uncoated version. In current case, the air permeability testing results supported the expectation by showing that the coated fabric is impermeable to air. The normal uncoated fabric showed air permeability of 98.53 mL/cm2/s at 10 Pa while none of the coated fabric samples (S1 and S2) showed any indication of air flow at 10 Pa or even 100 Pa. However, when the pressure drop was increased to 500 Pa, coated samples showed air permeability of 4.5 and 1.5 mL/cm2/s respectively for S1 and S2. This analysis indicates the imper- meable nature of coated fabric as anticipated which is beneficial from protection perspective as it will enhance the resistance to convective heat transfer.

To test the thermal resistance, temperature changes behind each layer of FFPC assembly were analyzed. The time–temperature graph in Figure 6 shows that the face cloth S1 had superior thermal protection, in terms of temperature raise behind thermal liner, against incoming heat fluxes from an external heat source. The fire- fighter clothing assembly was exposed to a 250"C heat source at 10 cm distance. The temperature behind the outer shell fabric (sensor P4) immediately raised above 70"C within the first few minutes and then gradually became stable around 80"C. Temperature between moisture barrier (sensor P3) and thermal liners (sensor P1 and P2) rose comparatively at a slower rate. The temperature behind the face cloth of thermal liner increased very slowly and after 21 min 42 s it reached around 55"_C

for S1 and around 65"C for S2. Second degree burn takes place generally at 55"C. Hence, the test was stopped when the temperature behind the inner most layer

Figure 5. SEM images of coated and uncoated fabric surface showing the presence of binder film which covers the pores in fabric.

reached to 55"C. Now two vital points, the time to reach pain threshold and time to receive burn, can be identified from Figure 6. Face cloth S2 took 4 min 42 s before reaching a point when the wearer would start to feel pain and took 9 min 45 s before the wearer would theoretically receive burn. This time frame was 10 min 30 s and 21 min 42 s respectively for the face cloth S1. Hence, it can be said that the face cloth S1 delayed the time to ‘‘pain threshold’’ for 5 min and would be able to provide extra 11 min to the wearer before receiving burn in the current experimen- tal situation. In general, it may be argued that if the developed face cloth, i.e. S1 could actually provide extra 5 min to the firefighter before feeling thermal pain. The standard deviation of time to pain threshold was found only 0.3 min. Hence, it is certain that the face cloth S1 is capable of extending time to pain threshold and pain alarm time for firefighters. As a result, the temperature between skin-clothing microclimate will remain in comfort zone for longer duration and also the possi- bility of second degree burn injury will be lowered. This extended time frame or time delay to discomfort is the evidence of enhancing comfort and protection by the aerogel/PCM embedded face cloth.

Furthermore, the phase change behavior of embedded PCM and the resultant extra time allowance in the comfort zone of wearer can be explained through the rate of temperature change. The temperature difference curve between S2 and S1 shows the rate of temperature change. The changes can be noted in three stages. In the first stage, higher rate of temperature change can be seen from a sharp upward

Figure 6. Time–temperature profiles.

trend in the temperature difference graph. The sharp upward trend indicates the slowest temperature increase of S1 in the melting zone of eicosane. Hence it can be assumed that the embedded PCM went through a phase change process at this stage, slowing down the temperature increase of S1 in comparison to S2. In the second stage, a relatively stable temperature difference (i.e., nearly zero rate of temperature change) between S2 and S1 was observed which is most likely due to the ‘‘near to completion’’ status of phase change process of the embedded PCM. Finally at the third stage when the phase changes completed, the temperature difference between S2 and S1 was minimizing, showed a down trend in the curve and rate of temperature change initiated with a reverse trend. As the average human body temperature fall in the range of 36.1–37.2"_{C [44,45] and the maximum}

phase transition of the embedded PCM on skin-side occurs at 37–39"_{C, the body}

temperature will start to rise beyond this phase transition. However, the body temperature will not increase as long as the outbound heat fluxes will find any way to be mitigated. The embedded phase change material will act as a mitigation media by absorbing the outbound body heat. The PCM embodiment in the skin- side of face cloth offers the best opportunity to absorb outbound metabolic heat than its embodiment in any other layer. Hence, it is more likely that the embedded PCM will go for phase transition by metabolic heat fluxes and will slow down the body temperature raise.

A firefighter can produce around 300–500 W during their 15 min work [31] and Pause estimated that roughly around 40 to 60 kJ energy is required to be stored by PCM in order to provide cooling effect and to avoid heat stress over a desired period [46]. The DSC graphs in Figure 7 shows that each gram of composite powder is capable of taking 177 J of heat energy, which indicates that roughly 338 g of composite powder coating uptake will be required to provide effective cooling effect for the amount of time as described above.

Figure 7. DSC thermograph of the composite powder.

The amount of metabolic heat absorbed by the embedded PCM will mainly depend on the actual amount of PCM present to go through phase transition and on its melting enthalpy (!Hm). The SEM images in Figure 5 give an idea of

the amount of PCM composite powder on the fabric surface in current investiga- tion. From the three stage observations of temperature changes in Figure 6, it is obvious that if the amount of PCM increased in S1, then the first stage will be elongated. As a consequence, the treated liner will be capable to offer more extra time to the firefighter before they will feel pain or receive burn.

Conclusion

The thermal protection and comfort of proposed thermal liner’s face cloth is inves- tigated by applying nanoporous aerogel particles on the ambient side of the face cloth and composite of phase change material on the next to skin side. The aim of this study was to enhance protection against incoming external heat flux by the addition of aerogel and to overcome the body heat blockage problem of aerogel embedment through the incorporation of PCM. The study identified that the sim- ultaneous use of PCM and aerogel can offer superior thermal protection and com- fort in terms of pain threshold and pain alarm time. The PCM/aerogel composite powder was also found to be stable at elevated temperature which indicates its practicality and safe use in skin side of the face cloth. It is certain that the final performance of the proposed combination will depend on the amount of PCM for phase transition and its melting enthalpy. The enhancement of protection and comfort due to proposed combination is inevitable and proven by the current study.

Acknowledgement

The authors are grateful to Australian government for enabling this study through endeav- our scholarship. They also like to thank Mr Martin Gregory for his outstanding support with laboratory instruments. Thanks to fellow doctoral researchers who extended their supporting hand to conduct tests. Special thanks to Mr Mac Furgusson and Mrs Siti Hana Nasir for their cooperation in coating and thermal imaging.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, author- ship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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