Thermally thin materials are defined as materials for which there is no significant heat gradient throughout their thickness. In other words, the whole volume is heated at the same temperature at all times. On the contrary, the heat gradient is significant in thermally thick materials. The flammability of thermally thin materials, such as textiles, is of great concern. Indeed, they are present in buildings such as in curtains and upholstered furniture and can greatly contribute to residential fires. Moreover, some textiles are subject to specific fire issues (nightwear, business uniform). Therefore, many studies have been carried out in the past to better understand the flammability of thermally thin materials and to provide efficient flame retardant solutions.

A cone calorimeter is probably the apparatus the most used by the scientific community working on the flammability of materials. It provides quantitative data on various properties, including ignitability, heat release, gas, and smoke release. A comprehensive review can be found to make the best use of this device. It has been highlighted that the choice of setup conditions influences significantly the fire performances and these conditions must be chosen carefully. This is especially the case for thermally thin materials, such as textiles or films. Indeed, such materials are semi-transparent and may not absorb the entire applied heat flux. Moreover, they can distort during burning and, therefore, the exposed surface area is not constant anymore, making the calculation of the heat release rate (HRR) false. Some researchers have already investigated the effect of setup (substrate, edge effects, grid) on fire performances of textiles in order to limit these phenomena. However, despite these disturbances, the cone calorimeter still remains a useful tool even for such materials and has often been used to characterize the fire performances of various textiles. Nevertheless, the role of the textile structure on flammability properties is not clear. It was shown that flammability of polypropylene (PP) materials in the cone calorimeter is mainly driven by their area density and not by the textile structure. However, PP melts at 160 °C, which is a temperature much lower than its decomposition temperature and, therefore, the textile structure disappears prior to the decomposition and, thus, has no influence on fire behavior.

Time-to-ignition (TTI) has been extensively studied and some models have been proposed for its prediction under various experimental conditions, such as flat or cylindrical geometries, thermally thick or thin materials, auto-ignition or piloted ignition, constant or variable heat flux. Equations (1) and (2) are quite often used to predict the time-to-ignition for, respectively, thermally thick and thermally thin materials under constant heat flux:

With k being the heat conductivity, ρ the density, c the specific heat, l the sample thickness, Tig the temperature at ignition, T0 the room temperature,

the heat flux, and CHF the critical heat flux.

Models have also been proposed to account for steady burning in the case of thermally thick materials. Of course, the steady state is not always reached even for thermally thick materials. Indeed, some phenomena, such as char accumulation, may reduce the heat transfer leading to a continuously decreasing heat release rate (HRR). It has proposed a model to predict the burning rate of charring polymers.

Thermally thin materials are “characterized by a sharp peak in HRR, since the whole sample is pyrolyzed at the same time. In this case, the pHRR (peak of heat release rate) becomes dependent on their total fire load”. It means that these materials do not show any steady burning rate and pHRR is only dependent on the sample weight at fixed conditions. Indeed, fire load is the product of the fuel mass and the effective heat of combustion. In a cone calorimeter, which is a well-ventilated test, effective heat of combustion is usually close to the heat of complete combustion. The latter can easily be calculated using Huggett’s relation [23] if the material burns completely or can be measured using pyrolysis combustion flow calorimeter. Nevertheless, to the best of our knowledge, there has been no attempt to check if pHRR of thermally thin materials may be easily calculated only from their fire load. It was attempted to predict the pHRR of upholstered furniture containing a fabric using a response surface analysis and using several parameters, including thermal inertia, heat of combustion, and heat flux, but they considered a much more limited set of materials and test conditions.

The behavior you're observing, where the Heat Release Rate (HRR) decreases but the Peak Heat Release Rate (pHRR) increases, can be attributed to a number of factors in the material's combustion and thermal behavior. Here are some potential causes for this:

In summary, this behavior suggests that the material might undergo a complex combustion process with initial heat absorption or slow combustion followed by a rapid release of energy. It would be helpful to examine the composition and structure of the material to pinpoint the exact mechanisms.

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