The Invalidation Behaviour Of Fire Protective And Heat Insulating Flexible Composite Fabrics

In order to keep focused on their job and avoid getting burn injury when exposed to flashover environment with a high temperature, firefighters and other special working staff should wear functional clothing to ensure them to be safe and convenient to move and protect from thermal strike. The non-static use of textile materials in the fire scene with a high temperature and heat fluxes, would inevitably result in insufficiency of the thermal protective performance and the aging decomposition of the material. This will lead to not only the changes of the morphological structure, mechanical, physical and chemical properties, but also the abnormal behavior and the invalidation of the thermal protective performance. Recently firefighting protective clothing is weak in the flame temperature resistant and the exposure time (less than10min at800℃and higher) when exposed to a fire scenario, the essence of which is the inadequate of high temperature resistance and heat insulating property. To address these issue and meet the requirement of light-weight, flexible and non-mobility obstacles, and achieve high efficient, intelligent, reliable fire protective performance, the research focuses on the developing of fire protective and thermal insulating flexible composite fabrics, which are mainly used to protect human from high temperature field, based on the analysis of the fire environment with a temperature of800℃and high heat fluxes.Considering the coupled heat transfer of thermal conduction, radiation and convection of the fire environment, the existing thermal reflective materials, high-temperature resistant fiber materials and heat storage phase change materials (PCMs) were compared and evaluated. To obtain high efficient and long term thermal protective performance, the overall composite fabric was designed with three functional layers:the thermal reflective layer, the heat storage composite layer and the thermal comfortable liner. The tentative processing and preparation of the composite fabrics were also implemented practically.Take into account the high heat fluxes of flashover, the basalt fiber and glass fiber were used as heat insulation materials of the composite fabric after comparing the thermal properties of the existing high temperature resistant fiber materials. The aluminium foil was selected as thermal radiation reflective material, due to its high reflectance and high cost performance. The outer thermal reflective layer, which is composed of aluminum foil and the woven substrate fabrics made of basalt fiber or glass fiber, could reflect most of the radiant heat. Pentaerythritol was used as phase change material based on the principle of high fusion heat, safe in use and easy access. In view of the application and the relatively fixed location of PCM, the punched felt, made of thermal resistant fiber, was used as the matrix of heat storage composite layer. The thermal comfortable liner made of flame retardant cotton fiber, was used to insulate the residual heat and make firefighters feel comfortable. To make the composite fabric flexible and the PCM fixed in the fabric, the sewing process was adopted to the forming of the composite fabric.In order to characterize the fire protective performance and the thermal invalidation behaviour of the fabric when subjected to fire exposure, a fire protective performance testing apparatus entitled "FPPAS-1"(Flaming Protective Performance Analysis System-1) with self-owned intellectual property right was designed and developed. The apparatus could simulate the fire environment with high temperature and intensive heat fluxes. During the fire exposure experiment, the fabrics were exposed to the fire scenario simulated for5min, and then natural cooling was performed for10min. The real time temperature of the right and back surfaces of the fabrics and the temperature curves versus time, were measured by the distributed thermal sensors. Four kinds of8indexes of fire protective fabrics, especially the fire protective and thermal insulating efficiency E, the critical temperature safe factor TcS’ the endurable time safe factor EtS and the fire protective safe index, were proposed and established to rate fire insulating performance and evaluate security of the fabrics. In addition, the repeated high temperature exposure and fire assaults experiments, and the calibration of measurement precision, were conducted on the apparatus.Flexible composite fabrics prepared presented excellent fire protective performance, due to the effective function bearing of the functional structure layer. It was revealed that the composite fabric made of basalt fiber material and PCM, the weight per unit area of which is832.8g/m2, exhibited the best thermal protective performance by comparing the fire protective and thermal insulating indexes. The long term thermal insulating temperature of the fabric under fire exposure is509℃, which is83℃higher than the composite fabric without PCM. The result illustrated that the PCM has effectively play a role of heat absorbing and thennal insulating during the fire exposure.The retention rate of the thermal insulating temperature of the composite fabric made of basalt fiber and PCM, was81.2%after5times of fire exposure experiment. Apart from slight charring and getting yellow, no brittle damage occurred on the surface of the fabric. The temperature differential energy was calculated between the composite fabric with PCM and without PCM, which revealed that the efficient of PCM is limited to16.7%～30.7%, which were attibuted to the weight and heat loss of PCM. The mechanical properties of the composite fabrics showed that both the strength and the bending stiffness had decreased during the repeated fire exposure experiment, but the retention rate remained at more that65%and75%, respectively. All the results indicated that the composite fabric maintained thermal protective performance as well as mechanical properties after5times of repeated fire exposure experiment.Based on the previous theories, a heat transfer model of the fire protective testing apparatus has been established. The heat transfer model, comprehensively considering the factor of fire scene, fabric, air layer and human skin, is a coupled multilayer heat transfer model of heat conduction, convection and radiation. Matlab software was used to simulate and solve the heat transfer model on account of the boundary conditions of the fire exposure experiment. The temperature distribution of the composite fabric and the single functional layers were numerically calculated. By comparing the numerical results with the experimental results of the composite fabrics and the single functional layers, it is revealed that the heat transfer model coincide with the experimental to great extent. What is delighted is the coincidence degree is close to100%during the steady heat transfer stage. So the proposed numerical heat transfer model is precise and reliable.Great progress and breakthrough has been made in developing the fire protective and thermal insulating composite fabrics and the heat transfer mechanism, by numerical analysis of heat transfer model and the evaluation of the fire protective performance of the composite fabrics. But thermal protective clothing with more efficient and longer thermal protection, better PCMs and the mechanism of PCMs still need further investigated and developed.