MIE determination and thermal degradation study of PA12 polymer powder used for laser sintering

摘要

Pulverized materials such as metallic or polymer powders play a considerable role in many industrial processes. Their use requires the introduction of preventive safeguards to control the plant's safety. PA12 polymer powder processing by laser sintering is characteristic of this tendency. The present work concerns PA12 powder (bimodal particle size distribution: 10 μm and 55 μm) and relates to explosion sensitivity and the thermal degradation of this powder, which can occur during laser sintering. Minimum Ignition Energy is determined using a modified Hartmann tube combined with the Langlie method developed in the PRISME Laboratory. This study shows the influence of parameters such as distance between the electrodes, powder concentration and arc power on MIE values. Theses parameters vary in the range of 3-6 A for the current intensity of the spark and the electrode gap in the range of 2.5-4 mm. The MIE is obtained for a spark gap of 3 mm and current intensity of the 4 A spark in our device. It shows that the MIE is less than 40 mJ for concentrations approaching 1000 g/m~3. At lower concentrations (under 150 g/m~3) the MIE increases but discrepancies in measurements appear, probably because of the static electricity that creates strong irregularities in dust dispersion. The second part of this study concerns the thermal degradation of the PA12 which is performed by thermogravimetric experiments coupled with mass spectrometric (MS) analysis for gas investigation. The mass loss measurement combined with the gas analysis allows the principal stages of degradation to be determined so as to calculate the kinetics parameter PA12. Experiments have been performed for different heating rates between 1 and 30 K min~(-1) and the reproducibility of experiments has been verified. The activation energy is determined using two methods: Freidman and KAS. For a reaction rate of between 0.2 and 0.6, the activation energy is nearly constant. The KAS method gives a value of E_a = 250 kJ mol~(-1) and the Friedman method gives E_a = 300 kJ mol~(-1). The gas analysis by MS shows that oxidation begins at over 350℃ and finishes at under 650℃ with the formation of CO_2 and H_2O. Other major peaks with an m/z ratio of 29, 28 and 30 are noticed in this range of temperature. They show the presence of intermediate species such as C_2H_6, NO or CH_2O. The presence of HCN is also detected (m/z ratio of 27).