| Nowadays,aluminium alloy and polyvinyl chloride(PVC)are still the most commonly used materials for door and window frames,occupying more than 80%market due to their advantages in performances.However,with the increased environmental protection consciousness,people come to realize the high energy consumption and severe pollution during the production and the issue when dealing with the wastes.Wood-made doors and windows are the most traditional and environmentally friendly;however,the high cost of the woody products caused by the resources crisis limits the sales.On the other hand,wood polyolefin composites(WPCs)have dominated the market of the outdoor building decoration materials due to their excellent properties,especially the environment friendly characteristics.However,the lack in the development of high-value WPC products restrict the development of the industry.Based on the above-mentioned development trend of the door and window frames and the WPC products,we proposed to prepare door and window frames using WPC,and focused on the investigation of the key techniques during production.The main contents and innovative results can be summarized as follows:(1)Air-flow drying of wood fibers.In order to satisfy the required stiffness and toughness of the door and window frames,we used the air-flow drying method to pretreat the wood fibers.Compared with the common wood flour/high density polyethylene(HDPE)composites,the products using the air-flow dried wood flour had improved tensile strength,flexure strength and impact strength.For instance,at the wood content of 60%,the increase in the strength was 12.7%,25.5%and 65.3%,respectively.The impact strength of our composites reached up to 13.3kJ/m2.This can be explained by the decreased extractive content of the air-flow dried wood flour at high temperatures-the extractive content was decreased by 32%at 240?.These results confirmed the feasibility and superiority of our air-flow drying method in drying wood fibers for the preparation of advanced WPC products.(2)Exploration of extrusion parameters and characterization of WPC.The basic optimized formula of WPC was studied and obtained via the orthogonal experiment and variance analysis.For example,the optimum formula of WPC based on PP was:wood fiber 60 parts,PP 36 parts,MAPP 4 parts,lubricant 3 parts and talcum powder 5 parts.The results showed that the extruded velocity during preparation and the mechanical properties of the resultant composites reached high levels in the lab when using this optimum formula.The effect of the die temperature on the molding and the mechanical properties of the WPC composites was also studied.It was found that the effect of the die temperature on the molding of WPC based on HDPE was minor and the suitable extrusion temperature was 170±5?;however,the temperature had great effect on the molding of WPC based on PP,i.e.,the molding temperature range of WPC based on PP was narrow,and the suitable extrusion temperature was 170±:5?.In addition,the temperature significantly influenced the mechanical properties of the products due to the crystalline change of the polymers used.According to the usage requirements of windows,the changes of WPC in the sizes and mechanical performances with temperature were also researched.The results demonstrated that the machining stress of WPC was discharged after thermal treatment at 80?,and the size of WPC was reduced by 0.05%compared to the original value when the temperature cooled to room temperature.The XRD results showed that the effect of temperature in the range of-10-80? on the crystallization behavior of WPC based on PP was slight,whereas.the effect for WPC based on HDPE was obvious.(3)Design of WPC window frames and optimization of extrusion process.We selected the building external windows with the strictest usage requirements as the research objects.The section of the window frames was designed based on the specific service environment,and the heat-insulating property and wind resistance performance of the windows were studied and calculated to satisfy the required environmental demands.The extrusion die was designed and prepared,and the experiment was conducted on the pilot equipment.There was a difference between the designed size and actual size after extrusion.Specifically,the dimensional deviations of transmission lock slots and sealing strip grooves had great effects on subsequent installation.Besides,the impact strength of the window frames was weak.Therefore,the cross-section shape of the window frames was improved by adding reinforcing ribs and adjusting the sizes of the transmission lock slots and sealing strip grooves.Additionally,the aluminium alloy gusset plate was added on the outside to improve the ageing resistance of the WCP window frame.(4)Assembly and tests of the WPC windows.Extrusion experiments were performed using the modified die.The extrusion speed for window sashes whose cross section is the most complicated could reach 1.1 m/min and hold steady.The WPC windows were assembled with the above profiles and characterized.The result showed that the air impermeability,water proofness,wind load resistance,heat-shielding performance,and sound insulation was scale 8,3,6,5 and 5,respectively,which meet the relevant requirements for building windows.(5)Recyclable properties of WPC for door and window frames.The wood fiber/HDPE composite and wood fiber/PP composite prepared based on the formula of door and window frames were reprocessed totally 7 times.The mechanical property,micro structure,crystallinity,rheology property,thermal property and water resistance of the composites were characterized,and the effect of antioxidant 1010 on the properties of the composites was also studied.The results showed that with the increasing recycle number,the tensile and flexure strength of WPC based on HDPE firstly increased and then decreased.The tensile and flexure strength of WPC after reprocessed seven times decreased by 24%and 20%,respectively.With the reprocessing time increasing,the impact strength decreased by 30%.The antioxidant 1010 did not prevent the properties of WPC based on HDPE.The water resistance of WPC based on HDPE increased with the recycle time.SEM showed that the size of the wood flour decreased and the distribution of the wood flour in the matrix tended to be uniform.However,the mechanical properties of the WPC based on PP decreased with the increase of the recycle number,and the protecting effect of the antioxidant was more obvious.The thermal property tests showed that the thermal resistance performance reduced after reprocessing for both WPCs. |
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