Study Of Microencapsulated Ammonium Polyphosphate And Its Synergistic Effect On The Property And Mechanism Of Flame-Retardant Wood-Plastic Composites

Ammonium polyphosphate(APP)is a phosphorus-nitrogen flame retardant,which is notoxic and relatively efficient,and has been widely used in wood-plastic composites(WPCs).However,because of its poor compatibility with polymer and poor water resistance in matrix,the incorporation of APP has negative effect on the properties of matrix,especially the mechanical properties.Moreover,the flame-retardant efficiency of APP is limited when it is used alone.In addition,the matrix modified by APP produces a large amount of smoke during combustion process.In order to over come these disadvantages,in this study,APP were modified and synergistic combined with other flame retardants,and then they were used in wood flour/polypropylene composites.The melamine-urea-formaldehyde(UMF)resin and crosslinked ?-cyclodextrin microencapsulated APP,coupling agent modified APP/pentaerythritol combined system,APP/zeolites combined system,and APP/aluminum hydroxide combined system were studied respectively.The effects of them on the thermal degradation behavior,flame retardancy,smoke suppression property,and physical-mechanical properties of wood flour/polypropylene composites were investigated.In addition,the morphology,degree of graphitization,chemical composition,chemical bonding behavior of the composites combustion residue were investigated to explore the flame-retardant mechanism and or synergistic flame-retardant mechanism.The main conclusions are summarized as follows:(1)UMF resin microencapsulated APP(UMF-APP)were prepared and used in flame-retardant modification of wood flour/polypropylene composites.UMF-APP had low water solubility and high thermal stability as compared with APP.The composites containing UMF-APP had better flame retardancy and physical-mechanical properties than the ones containing APP.When the UMF resin to APP ratio was 1:2,the composites containing UMF-APP showed the highest flame retardancy among all the tested composites.When the UMF resin to APP ratio was 1:7,the composites containing UMF-APP showed the best mechanical properties.(2)Crosslinked ?-cyclodextrin were used to microencapsulate APP to prepare CD-APP that containing both acid source and carbon.Then,the CD-APP were used in the flame-retardant modification of wood flour/polypropylene composites.After microencapsulated,the initial water contact angle of APP increased,and the water solubility of APP significantly decreased.The composites containing CD-APP(WPC/CD-APP)had high thermal stability and flame retardancy as compared to the ones containing APP(WPC/APP).The CD-APP provided the composites combustion residue with higher graphitization and thermal stability than APP,it displayed both gas-phase and solid-phase flame-retardant mechanism on the composites combustion process.Moreover,the WPC/CD-APP showed better mechanical properties than the WPC/APP.(3)KH-550 coupling agent were used to surface modify APP and pentaerythritol(PER)to prepare k-APP and k-PER.Then,the k-APP or k-APP/k-PER combined system(k-APP/k-PER=3:1)were used in the flame-retardant modification of wood flour/polypropylene composites.The k-APP/k-PER combined system provided the composites with better thermal stability,higher flame retardancy and mechanical properties than the APP/PER system did.(4)The combination of APP and 4A or 13X zeolites provided the wood flour/polypropylene composites with high flame retardancy,mechanical properties,and smoke suppression effect.When the addition amount of 4A zeolites was 2wt%,the composites showed the highest limiting oxygen index.When the addition amount of 4A and 13X zeolites was 6wt%and 2wt%,respectively,the composites showed the best mechanical properties.Moreover,the addition of zeolites to the composites containing APP decreased the smoke production and toxicity during the composites combustion.(5)The combination of APP and aluminum hydroxide(ATH)provided the wood flour/polypropylene composites with high flame retardancy and smoke suppression effect.When the addition amount of APP and ATH was 20wt%and 10wt%,respectively,the composites(WPC/APP-20/ATH-10)performed the best flame retardancy among all the tested composites.Moreover,the combustion residue of the WPC/APP-20/ATH-10 was the most compact one among all the residues.The aluminum metaphosphate produced by the pyrolysis reaction between APP and ATH had the ability to enhance the composites combustion residue.When the addition amount of ATH was 30wt%,the composites(WPC/ATH-30)showed the best smoke suppression effect,the lowest water uptake rates,and the highest impact strength among all the tested composites.