Abstract
This work aims to elucidate whether the hypothesis of zero oxygen at the mixture layer when flame takes place is assumable for every kind of material. For that purpose, we investigated the oxygen concentration there by cone calorimeter tests. A modified holder was developed in order to collect oxygen in this mixture layer. In addition, thermogravimetric tests were carried out so as to relate the possible effects of the presence of oxygen in the atmosphere where the pyrolysis process takes place, since the cone calorimeter does not allow to control the oxygen level of the atmosphere during the experiment. The reaction rates and per cent of residue in the cone calorimetric tests were measured and compared with the results from thermogravimetric tests. Six products were analysed which can be classified in three main groups: lignocellulosic, thermoplastic polymers and thermoset polymers. Cone calorimetric results showed that for some of the materials analysed (PET, Nylon and PUR foam) the oxygen level at mixture layer decreased until values close to zero. The comparison of reaction rates between cone calorimetric and thermogravimetric tests revealed the char layer created in cone calorimetric tests over the exposed face for brushed fir, Nylon and PET established an important heat barrier that modifies the thermal behaviour of these materials.
Highlights
Oxygen concentration has a strong effect on fire behavior of materials, i.e. they decompose in different ways, and mass loss rate become faster [1]
Thermal analysis techniques have been used for years [2] to evaluate experimental properties of materials, especially those related to thermal decomposition and oxidation parameters, such as, specific heat, conductivity and few reaction ones
Thermogravimetry –TG- and differential scanning calorimetry –DSC [3] are the most common. Those techniques and bench scale calorimetric analysis, i.e. cone calorimeter [4] have been used to obtain a set of parameters which tunes pyrolysis models appropriately
Summary
Oxygen concentration has a strong effect on fire behavior of materials, i.e. they decompose in different ways, and mass loss rate become faster [1]. In cone calorimetric test, according to the procedures of ASTM E 1354 04a [4], smoke measuring system is located in the tube of the exhaust system of the cone calorimetric and obviously that position of the gas sampler is not suitable to measure the oxygen concentration in the mixture layer since combustion effluents are highly dissolved in the surrounded air. We assumed that between sample surface and flame leading edge there is a vessel, mixing layer, in which oxygen concentration depends on consumption from the flame and displacement due to mass flow from the sample (refer to Fig. 4 a). We check the flow of the released gases from the sample in the cone to verify if the gas sampling tube layout was collecting air from the surrounding atmosphere and it would affect the results. As the contribution of the oxygen concentration from surrounding atmosphere was demonstrated to be insignificant, we can consider that samples are collecting oxygen concentration in the mixture layer adequately
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