| Wood thermoplastic composites (WPCs) are manufactured by dispersing wood fibers or wood flour (WF) into molten plastics to form composite materials by processing techniques such as extrusion, thermoforming, and compression or injection molding. The most commonly used thermoplastics in manufacturing WPCs are polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC). Among the many polymers that have been used as matrices for preparation of WPCs, PVC is certainly one of the most popular one. In terms of stiffness, creep behaviour, exterior appearance, abrasion and scratch resistance, PVC based composites are superior to mainly used polyolefin based matrices. Thus, WF-PVC composites are now being widely used in the building construction applications like doors or windows, interior decoration, furniture and many the other residential products with growing markets. In these areas, fire safety is an important issue. Despite good flame retardancy of PVC, WF-PVC composite lights easliy for the easy flammability of the high content of wood flour and other low-molecular-mass additives. Additionally, PVC can generate high levels of black smoke and toxic gases (i.e., chlorine compounds, benzene, and other aromatic compounds) when it is forced to burn. Therefore, it is necessary to improve the flame retardancy and smoke suppression of WF-PVC composites in order to expand its application.In this dissertation, thermal and burning properties of WF-PVC composite was investigated, flame retarded WF-PVC composites with a variety of metal compounds were prepared. A number of pioneering work on flame retardancy and smoke supperssion properties of the composites was carrried out, while a certain amount of original results were achieved.Part I:Thermal and burning properties of WF-PVC composite.The present study deals with the effects of wood flour on thermal and burning properties of wood flour-poly(vinyl chloride) composites (WF-PVC) using thermogravimetric (TG), cone calorimetry (CONE) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS).TG tests indicated strong interaction occurred between wood flour and PVC during the thermal degradation of WF-PVC composites. Wood flour decreased the temperature of onset of decomposition of PVC. However, the char formation could be increased by adding wood flour to PVC. Moreover, HCl evolution from PVC may function as an acid-catalyst of WF at high temperature. CONE test indicates that wood flour had positive effects on heat release and smoke emission of PVC. Comparing with PVC, WF-PVC reduced average heat release rate (av-HRR) and the peak HRR (pk-HRR) by about14%and28%, respectively; smoke production rate (SPR) was also decreased. The degradation mechanism was studied by Py-GC/MS. The results showed that the volatile pyrolysis products of WF-PVC were very different from PVC. The yields of HCl and aromatic compounds decreased dramaticly, and the aliphatic compounds increased by the incorpration of WF. Part â…¡:Transition metal oxides on the thermal behavior and flame retardancy of WF-PVC composites.Some transition metal oxides have showed good flame ratardancy and smoke inhibition on PVC materials, but there are no certain ways to follow. Since the flame retardancy ability of the transition metal oxides on PVC is related with their capacity of binding with HCl. Meanwhile, the majority of the oxides exhibit catalytic activity, their catalytic selectivity to different materials may be important. In this section, investigations of the thermal decomposition, the flame retardancy and the smoke emission behaviour of WF-PVC formulations containing the first-row transition metal oxides, TiO2, Fe2O3, NiO, CuO and ZnO, were carried out using thermogravimetry and cone calorimetry.According to TGA results, the first-row transition metal oxides can be divided into two kinds based on their effects in WF-PVC composites. The first is represented by TiO2; The second includes the oxides Fe2O3, NiO, CuO and ZnO; The presence of TiO2increased the intial decompositon temperature and the reaction activation energy of the composites. It demonstrated that TiO2played some positive effects on the initial thermal stability of WF-PVC comosites. Fe2O3, NiO, CuO and ZnO showed an excellent pyrolysis catalysis to the composites. The addtion of these oxdies decreased the initial decomposition temperature, meanwhile, the peak mass loss rate increased and the mass loss decreased during the first thermal decomposition period; In the later stages, the temperature at maximum peak of the thermal degradation was shifted to a higher temperature, while the mass loss rate was decreased. In addition, the amount of residual char increased obviously; TiO2hardly affected the fire performance characteristics of the composites, whereas Fe2O3, NiO, CuO and ZnO are effective flame and smoke retarders for WF-PVC composites. The time to ignition was remarkbly prolonged, and the heat and smoke release was also reduced obviously. From the char residue, these metal oxides not only promoted char yeild but also improved char morphology; Among of these metal oxides, ZnO was doing the best. In contrast with untreated composites, the incorpration of ZnO lowed total heat release by36.4%and the total somke prodcution by55.0%, while raised char residue by56.4%. Moreover, the time to ignition of the composites was significantly prolonged from23s to141s, which suggested a fire accident can be greatly reduced.Part â…¢:Zinc compounds as flame retardants and smoke suppressants for WF-PVC composites.Transition metal oxides, especially ZnO has proved to be an effective flame and smoke retarders for WF-PVC composites. Recently, the effects of some divalent metal salts on PVC have attracted much attention, they can also be used as flame retardants in PVC because of their reduced toxity. Here zinc stannate, zinc borate and zinc molydbate were selected and incorporated into WF-PVC composites, the activity on flame retardancy and smoke suppression of the metal and the environment surrounding the metal ion resulted form the compounds was studied.It was found that, in the presence of zinc stannate and zinc borate, the onset decomposition temperature of the composites was shifted to a lower temperature, which means these kinds of flame retardants did some negative effect on the thermal stability of WF-PVC composites. Howerver, they promoted the formation of char. zinc molydbate had little effects on thermal decomposition of the composites; The mechanism of thermal decomposition of the composite was analyzed by Fourier transform infrared (FTIR) spectroscopy using the attenuated total reflectance (ATR) method. The decrease of C-H bonds and increase of C=C bonds indicated WF and ZB promote the crosslinking and charring reactions of the composites.According to the cone calorimetry (CONE) tests, zinc stannate and zinc borate are effective flame and smoke retarders for WF-PVC composites. The time to ignition was greatly increased, for example, the addtion of zinc stannate expanded the time to ignition from23s to214s. Heat and smoke release of the composites during burning was also greatly decreased. The total smoke production of per init mass WF-PVC composite was463.4m2/Kg, while the presence of zinc stannate and zinc borate decreased the smoke production to230.8and200.5m2/Kg separately. Heat-reducing synergisms resulted when zinc stannate was combined with zinc borate. The total heat release was reduced from36.4to17.57kw m-2for the incorporation of zinc stannate and zinc borate systems; Mechanical results indicated that ZB had some negative effects on the strength properties of WF-PVC, however the modulus increased compared with the untreated composite.Part â…£:preparation, characterization and application of zinc borophosphate in WF-PVC compositesBorophosphate, as a new branch of inorganic materials, has shown fascinating structural diversities and potential applications in optical, magnetic, catalytic and ion-exchanged aspects. Recently metal borophosphates attracted much attention in the flame retardant field due to their potential applications as flame retardants. In this section, two kinds of zinc borophosphates were synthesized by using boric acid flux method combined with hydrothermal synthesis method. The results of thermogravimetric and cone calorimetry tests showed that zinc borophosphate improved thermal decomposition behaviour of the composites, and endowed WF-PVC composites with certain flame retardance and smoke suppression capability. |
