Study On Thermal Degradation And Multilayered Distribution Fire Retardant Performance Of Wood Plastic Composites

Wood plastic composites (WPCs) have become one of the most important part on wood materials science, polymer materials science and bio-composites area in recent years. However, the wood fiber materials and plastic matrices are highly flammable, which greatly limit their commercial utilization. Therefore, it is important and necessary to research and develop fire retardant treated WPCs to expand their commercial utilization. The most effective and easiest way to improve the fire retardant property of WPC is additive of fire retardants during the preparing process. The consequence is that the mechanical properties of the composites dramatically decreased and the costs increased.In this dissertation, three of the most common fire retardants (magnesium hydroxide, expandable graphite, and ammonium polyphosphate) for wood and polyolefin were used on wood fiber and polypropylene composites (WPPC). Single and multi-layer structured WPPC filled with magnesium hydroxide (MH), expandable graphite (EG), or ammonium polyphosphate (APP) were prepared. Thermal degradation, fire retardancy and smoke suppression properties of composites were determined, while the main results are as follows:(1) Modulated thermogravimetric analysis (MTGA) and thermogravimetric analysis (TGA) were used to investigate thermal degradation behavior of wood fiber (WF), polypropylene (PP) and its composites. The activation energy, pre-exponential factor and probable degradation mechanisms of each decomposition process were also determined by MTGA and mathematical model.The results of MTGA and TGA indicated that MH did not influence on the thermal degradation behavior of WPPC in the first decomposition stage (WF decomposition stage), but increased the decomposition temperature of WPPC in the second decomposition stage (PP decomposition stage). Both EG and APP reduce the decomposition peak temperature of WPPC in the first decomposition stage and increase decomposition peak temperature in the second decomposition stage, which promoted thermal degradation of wood flour and improved the thermal stability of PP. The decomposition process of pure PP did not change due to the too little maleic anhydride grafted polypropylene and lubricant in proportion of the sample. A general activation energy range of 161-178 kJ mol-1 at WF degradation stage, and 234-305 kJ mol"1 at PP degradation stage were proposed for WPPC with and without fire retardants. Moreover, the degradation mechanism of WF occurred by diffusion in three dimension processes (Ginstling-Brounshtein equation) when the conversion value was below 0.8. PP and fire retardants had no direct influence on the degradation mechanism of WF. In the PP decomposition stage, the degradation of WPPC without fire retardant was followed phase boundary controlled reaction mechanism. However, WPPC incorporation of MHC or APPC followed nucleation and growth mechanisms (Avrami equation 2), whereas WPPC with EG can be described by nucleation and growth mechanism (Avrami equation1).(2) According to the results of preceding chapter, the influence of interaction between fire retardants on thermal degradation behavior of WPPC was investigated.The decomposition rate of wood fiber and the decomposition temperature of MH were both reduced after adding MH and APP in nitrogen, but the thermal stability of PP was not further improved. However, the compound of EG and APP could further promoted thermal degradation of wood fiber and improved the thermal stability of PP, which displayed good synergistic action. In addition, the activation energy of WPPC in the beginning of decomposition stage was reduced, and the activation energy of WPPC was increased during the whole thermal degradation process. The general activation energy values were 163-253 kJ mol-1 at WF degradation stage, and were 234-305 kJ mol-1 at PP degradation stage.In air atmosphere, the decomposition peak temperature of WPPC in the first decomposition stage and the beginning of decomposition temperature of the second decomposition stage were both increased by compound with MH and APP. It indicated that fire retardants promoted char formation, more char layer can endure the higher temperatures to protect PP from decomposing. However, the initial decomposition temperature of WPPC with EG and APP was reduced, and the decomposition peak of WPPC shifted to higher temperature. It showed good synergistic effects in flame inhibition on both wood fiber and polypropylene. Moreover, the general activation energy of WPPC with EG and APP was higher than the control sample in the first decomposition stage, but the values in second decomposition stage fall in between. The general activation energy values were 120-280 kJ mol-1 at WF degradation stage, and were 205-258 kJ mol-1 at PP degradation stage.(3) The fire retardancy properties and mechanism of fire retardant WPPC were analyzed by CONE and SEM. The flexural property and water absorbency were also tested by mechanical testing machine.The heat release was reduced with the increase of the amount of fire retardants, but the smoke production was increased with the increase of the amount of APP. EG showed the best performance on flame retardancy and smoke suppression, and the worst property on flexural strength and modulus. The water absorption (WA) and thickness swelling rate of water absorption (TS) were both increased after adding MH or APP, but decreased after adding EG. It can be observed by SEM that the char residue of WPPC with MH was a coralloid network structure with plenty of holes, the char residue of WPPC with APP was a continuous and blocky with less holes in the surface, and wormlike char for the WPPC with EG. The fire retardant mechanism of three fire retardants can be summarized as below:(a) MH, MH was decomposed as the result of the endothermic reaction, in which heat energy was consumed. Meanwhile, a dense plume of water vapor obtained as a by-product of decomposing created a nonflammable protective layer on the surface of the composites to dilute oxygen, and the magnesium oxide decomposed from MH attached on the surface of composites to improved thermal stability of the composites. (b) APP, firstly, the phosphates decomposed from APP was able to cross-link with wood fiber, which conducive to dehydration of cellulose and char formation. Secondly, a plenty of nitrogen and ammonia nonflammable gases decompose from APP can also dilute the concentration of oxygen and flammable gases decompose from PP. (c) EG, acidic EG promoted thermal degradation of WF and mass loss at lower temperatures. EG could form a thick and porous carbon layer when heated, which was able to hinder the heat and oxygen transfer to matrix. Thus delay and terminate the decomposition of composites. In addition, none toxic and corrosive gas produced from EG, and the porous char can absorb a large amount of flue gas, thus greatly reduce smoke emission.(4) Multilayer structured fire retardant WPPC was prepared. Heat release, smoke production and mass loss were determined by CONE. The morphology, microstructure and micro area element distribution of carbon residue were characterized by SEM. The difference and mechanism of action between uniform distribution and multilayered distribution of a fire retardant were investigated to obtain a suitable fire retardant for multilayered distribution on WPPC.Compared with the single layer fire retardant WPPC, both the heat and smoke release of multilayer WPPC with APP were increased. The total heat and smoke release and the peak of heat release rate of three layer WPPC with MH were decreased, and the flexural strength and modulus were improved. The heat and smoke release of double and three layer WPPC with EG were both reduced. Especially for three layer WPPC with EG, the total heat and smoke release were decreased by 18% and 31%, respectively, and the flexural strength increased by 31%. The char residues of core layer of multilayer WPPC with EG were constituted by a thick, continuous and compact char structure through the observation of SEM. In order to further research the property of multilayered distribution, the uniform distribution and multilayered distribution of EG and APP were investigated. The CONE results indicated that multilayered distribution (EG in the cap layer and APP in the base layer) showed the better performance on reduce the heat and smoke release. SEM results showed that almost all of the particles combined with char layer to form a more firm and compact physical structure char. Moreover, the relative content of N/C, and P/C were increased. The fire retardant mechanism of multilayered distribution of EG and APP can be explained that the cap layer with EG formed an intumescent structure char layer could effectively protect the base layer from degradation and absorb the volatiles from the base layer. Thus greatly improved the thermal stability and reduced the smoke production of the whole composites.