Study On Regulating Mechanism Of The Photostability Of Wood Flour/polypropylene Composites

In order to improve the photostability of wood-plastic composites,in this study,wood flour/polypropylene(WF/PP)composites were investigated as the subject.Firstly,for figuring out the mechanisms,the main compoents in WF named lignin and cellulose were separated to make PP-based composites at different ratios.Their photodegradation behaviors and mechanisms during accelerated UV weathering were studied.Further,based on the mechanisms,organic antioxidants(Vitamin E and antioxidant 168),inorganic layered double hydroxide(LDH),and organin-inorganic anti-weathering compound additives(LDH-based additives and lignin-clay complexes)were systhsied and used in WF/PP composites.During 960 h UV weathering,the changes in surface color,surface gloss,flexural and impact properties,surface morphologies,and surface chemistry were studied,accompanied by characterizations using AFM,ATR-FTIR,DSC,SEM,TG,and XPS.The primary conclusions are shown below:(1)The photodegradation of PP was mainly due to its chain scission,which resulted in the decreased mechanical properties,with the impact strength retention ratio of only 2.7%.The cellulose-based composites exhibited better color stability but more significant deterioration in mechnical properties.After 960h weathering,their impact strength decreased to 29.3%,while their flexural MOR and MOE retention ratios were only 36.0%and 27.0%,respectively.The discoloration of composites was attributed to lignin,especially at high contents.However,composites containing lignin showed less loss of mechanical properites and less surface cracks on weathered surface than other groups,confirming its functions of stabilization and antioxidation.(2)The addition of Vitamin E(Ve)successfully alleviated the photodegradation process of WF/PP composites,especially at higher loading levels(0.4 wt%and 1.2 wt%).Composites containing Ve showed less discoloration,less surface cracks,and less loss of mechanical properites after weathering.Comparatively,the compound systems of Ve and antioxidant 168 were more effective on slowing down the photodegradation process.For composites containing 0.2 wt%Ve and 1.0 wt%antioxidant 168,their flexural properties rentation ratio was almost 100%after 960 h weathering.However,their color stability was not good enough especially at the later stage of weathering,due to the leaching of the additives.(3)A kind of layered double hydroxide,MgAl-CO3-LDH(LDH),was synthesized through coprecipitation method.Its effects on the photostability of WF/PP composites during weathering were investigated.The addition of 1 wt t%LDH improve the mechanical and thermal properties of composites.Their flexural strength and modulus retention ratios after weathering were as high as 98.0%and 80.0%,respectively.However,the anti-weathering efficiency of LDH was only maintained 480 h because of the leaching of LDH(4)Owning to the ready exchangeability of LDH interlayer anions,5-sulfosalicylic acid(SA)and sodium ligninsulfonate(SL)were intercalated into LDH layer to make two kinds of organic-inorganic compound additives,namely LDH-SA and LDH-SL,respectively.Compared to LDH,the thermal stability,UV absorbance,and anti-leaching properties of LDH-SA and LDH-SL were improved.Moreover,their efficiency was extened to longer than 720 h.Comparatively,composites containing LDH-SL showed better color stability,less surface cracks,and less loss of mechanical properties during weathering Moreover,it still maintained the surface color of light brown even after 960 h weathering.The inorganic-organic compound systems have both UV shielding and UV absorbance activities(5)Lignin(L)was used as an antioxidant and mixed with diatomite(DMT)and montmorillonite(MMT)were used to prepare two lignin-clay complexes,namely lignin-diatomite(L-DMT)and lignin-montmorillonite(L-MMT),respectively.Ligin was absorbed on the surface of clays,and some even penetrated into pores of DMT,improving their anti-leaching property.Comparatively,the complexes improved the impact properties and alleviated the discoloration,as well as loss of mechanical properties during weathering.L-MMT exbihited photocatalytic effect at the later stage of weathering,while L-DMT was more effective in this study.