The analysis of polymer chemical decomposition is often highly dependent on the test conditions. In fact, thermal analysis tends to be far more sensitive to instrumental parameters than other branches of chemical analysis. Some of the boundary conditions in thermal analysis have been widely studied in the literature, such as the heating rate or the atmosphere. However, the influence of the sample mass, the gas flow or the use of lid have not been enough studied for thermoplastic polymers.The aim of this paper is to analyse how the experimental boundary conditions of the Simultaneous Thermal Analysis apparatus affect the thermal decomposition of thermoplastic polymers. To do so a set of 35 experimental tests have been performed including variation of the sample mass, gas flow rate, heating rate, atmosphere and the use of lid in the crucible. Results enable us to analyse the influence in the mass loss and in the energy release or absorbed in the thermal decomposition of thermoplastic polymers, showing the impact of each boundary condition over the thermal decomposition. A comprehensive analysis of the thermal decomposition behaviour of the PVC and LLDPE by considering the influence of all the boundary conditions of the STA is covered. It is especially remarkable the influence of gas flow in the oxidative reactions, and of heating rate in the chemical reactions that thermoplastics polymers undergo in their decomposition. Additionally, sample mass comparison shows only deviation in the oxidative reactions, and do not show deviation for the non-oxidative reactions. That seems to show a higher effect on the results because of the energy release in the decomposition reactions than because of the thermal lag due to heat transfer on the sample, as it is usually thought.
Understanding a material's fire behaviour implies to know the thermal decomposition processes. Thermal analysis techniques are widely employed to study thermal decomposition processes, especially to calculate the kinetic and thermal properties. Cardboard boxes are widely employed as rack-storage commodities in industrial buildings. Hence, the characterization of the cardboard is considered a key factor for fire safety engineering, because it enables the determination of its thermal behaviour at high temperatures. The employment of mathematical or computational models for modelling the thermal decomposition processes is commonly used in fire safety engineering (FSE). The fire dynamics simulator (FDS) software is one of the most commonly used computational fluid dynamics software in FSE to address thermal analysis. To properly set up FDS and obtain accurate results, the numerical values of the thermal and kinetic properties are needed as input data. Owing to the large number of variables to be determined, a preliminary study is bound to be helpful, which can well-assess the influence of each variable over the pyrolysis model, discarding or restricting their influence. This study, based on the Monte Carlo method, presents a sensitivity analysis for the variables utilized as input data by the FDS software. The results show the conversion factor α, i.e. the mass involved in each reaction, and the triplet kinetic has a major impact on the reproduction of the thermal decomposition process in fire computer modelling.
Thermal analysis has been proven to be an efficiently technique to analyse thermal decomposition reactions of different type of materials. This technique is widely used in different fields. Among them, fire science, where polymeric materials are very common, has a particular issue, being the combustion reactions recurrent on these analyses. Thermal analysis has different particularities depending on the studied material. For instance, polymeric materials could undergo different decomposition reactions that are highly dependent on definition of the thermal analysis boundary conditions. The International Confederation for Thermal Analysis and Calorimetry (ICTAC) (Vyazovkin et al. in Thermochim Acta 590:1–23, 2014) and standards (ISO 11358-1. Plastics—Thermogravimetry (TG) of polymers—Part 1: General principles. ISO. 2014; https://www.iso.org/standard/59710.html. Accessed 31 Jan 2022), (ISO 11357-1. Plastics — Differential scanning calorimetry (DSC) — Part 1: General principles. ISO. 2016; https://www.iso.org/standard/70024.html. Accessed 31 Jan 2022) stablish how to set-up these boundary conditions in the thermogravimetric (TG) and differential scanning calorimetry (DSC) standards. As far as initial amount of sample mass is concern, some discrepancies can be found between the standards. For instance, the standards suggest a sample mass between 10 and 100 mg for TG and between 2 and 40 mg for DSC, whereas the ICTAC recommendations suggests that the sample mass times the heating rate should not exceed 100 mg K·min−1 in thermo-oxidative decomposition analysis, which is equivalent to samples masses lower than 10 mg for heating rates of 10 K·min−1, or lower than 5 mg for heating rates of 20 K·min−1. This discrepancy may lead to obtain different results from the tests. Additionally, according to the thermal and thermo-oxidative decomposition of polymers, the ICTAC remarks the influence on the results of the sample thicknesses, carrier gas and heating rates, but it does not analyse the influence of self-heating as it does for the hazardous materials. This work presents a study of the self-heating influence in the thermal decomposition processes of two widely used polymers as poly methyl methacrylate (PMMA) and linear low-density polyethylene (LLDPE). TG/DSC tests are used to evaluate the thermal decomposition processes. Boundary conditions of the tests definition as sample mass, atmospheres, and heating rate are considered to evaluate its influence on the polymers self-heating effect on the thermal decomposition. It also includes how to check if TG/DSC tests follows the theoretical principles of the thermal analysis, or if the results are affected by the self-heating. In the present study, a series of 32 experimental tests has been performed, analysing 16 boundary conditions. These experimental tests allow evaluating the influence of selected boundary conditions on the mass loss, the heat flux, and the materials decomposition reactions. Additionally, we analyse the effect of the boundary conditions on the temperature of the sample. Results show the impact of each different boundary conditions of the self-heating effect, and its influence in the final thermal decomposition measured and they represent an aid to define the suitable conditions to perform TG/DSC test on PMMA and LLDPE, or similar polymer materials. This is done by the evaluation of the influence of the self-heating in parameters as the sample temperature lags defined in [1], the reactions heat fluxes, and the difference between the sample and the programmed temperature. It is also analysed the influence of the auto-ignition temperature in the thermal analysis. It is remarkable the PMMA auto-ignition temperature effect on the TG/DSC results. Finally, some useful recommendations have been defined.
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 percent 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 Keywords Cone calorimeter, thermal oxidation of char, oxygen concentration, carbon dioxide concentration, pyrolysis behaviour, thermogravimetric analysis. Nomenclature ̇c one Incident flux from cone calorimeter/kW•m-2 ṁp eak Peak of the mass flow released by the sample and measured by the cone in the exhaust tube/kg•s-1 T ̅ Average temperature of the surface of the sample during the test after the ignition/ºC q̇ cone peak HRR peak during the test released by the sample in cone test without gas sampling tubes/kW•m-2 ṁs Sampling mass flow/kg•s-1 q̇ gas peak HRR peak during the test released by the sample in cone test with gas sampling tubes/kW•m-2 / ṁa vg Average mass flow released by the sample and measured by the cone in the exhaust tube/kg•s-1
Thermal analysis techniques play a crucial role to characterize solid phase thermal decomposition, since it provides information about how mass is lost (Thermal Gravimetric Analysis) and energy released (Differential Scanning Calorimetry). However, most of the INPUT thermal parameters and kinetic properties to be used in fire computer modelling cannot be obtained directly from those tests. Early works looked forward achieving those parameters employing indirect fitting methods, which enable the user to obtain a set of parameters capable to simulate accurately the mass loss curve (TG) or its derivative (DTG). This work aims to study the possibility of adding the energy released as a new target in the process, applying the analysis to Linear Low-Density Polyethylene (LLDPE). Results obtained in the present work reveal the major challenge of getting a set of parameters that can also fit DSC curve. The level of accuracy of the fitting to TG curve is higher than to DSC curve. This fact increases the value of the errors when both curves are used as targets to approach. As a result, this paper includes an alternative to consider the effects of the DSC curve.
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