Thermal Stabilization Mechanisms Of Boric Acid-Modified Wood Fiber And Its Application In Engineering Plastics-based Wood-plastic Composites

Wood-plastic composites(WPCs)have excellent economic,social and ecological benefits,having been widely used in transportation,building decoration,outdoor planks,automobiles,and other industry fields.However,compared to quality wood,mechanical properties and anti-creep properties of WPCs are lower since the nature of polymers and the incompatibility between wood fibers and non-polar polyolefin.Engineering plastic molecular chains have polar functional groups,which are conducive to the interfacial compatibility between polymers and wood fibers.Also,compared with polyolefin,engineering plastics have excellent physical and mechanical properties.Hence,they have great potential for preparing high-performance WPCs.However,the melting point temperature of most engineering plastics is higher than the wood degradation temperature.The mismatched temperatures limited the manufacture of WPCs based on engineering plastics.Polyamide(PA)and polycarbonate(PC)are currently the two most widely used engineering plastics in the world.At the same time,the processing temperature of PA and PC is lower than other types of engineering plastics.Therefore,PA 6 and PC as the representative of engineering plastics were chosen to explore the relationship between the preparation,structural and properties of engineering plastic-based WPCs.In this paper,the thermal stability of wood fibers will be improved by boric acid modification.Then,the modified wood fibers were filled into engineering plastics to prepare high-performance WPCs.The key technical issues will be studied.The preparation of WPCs based on engineering plastics is of great significance to improve the performance of WPCs,expend their application fields,promote the use of WPCs in electronic products,mechanical equipment and building structures,and increase the added value of products.The main contents and results are as follows:Effect of boric acid on wood fiber thermal properties and its mechanism.A boric acid solution was used to treat wood fiber,holocellulose,microcrystalline cellulose(MCC),lignin and small molecular substitution compounds(glycerin,mannose and glucose).Then,these treated mixtures were characterized using FTIR,XRD,XPS,thermogravimetry(TGA)and mass spectrometry.Results showed that boric acid treatment can complex or esterify with the hydroxy groups of wood fibers,thereby changing the chemical structure of wood fibers.boric acid changed the crystalline behaviors of wood fiber and its crystallinitydecreased by 22.4%.Boric acid treatruents improved the thermal stability of wood fiber,especially the initial degradation temperature.The initial degradation temperature of wood fiber increased from 200.8? to 280.0? after boric acid modification.The mechanism of boric acid treatment to improve the thermal stability of wood fiber is the combination of physical protection,increased degree of chemical cross-linking and introducing high-energy chemical bonds.The change of the chemistry structure of wood fiber altered their thermal degradation chemical pathway,increasing their char residues.Preparation and properties of borated wood fiber/polyamide 6(PA 6)composites..Untreated and borated wood fibers were compounded with polyamide 6 to prepare WPCs,respectively.The effects of wood fiber content and boric acid on the color,torque rheology,thermal stability and mechanical properties of WPCs were compared and analyzed.The results showed that boric acid slowed down the thermal degradation of lignocellulosic fibers,thereby slowing the darkening of the resulting composites.The flexural and tensile properties of the resulting composites increased with the increase in wood fiber content.Their flexural strength and modulus increased by 174.5%and 293.8%respectively.Compared with the untreated wood fiber/PA 6 composites,the strength of borated wood fiber/PA 6 slightly reduced,especially in the case of low wood fiber content,indicating that boric acid has a negative effect on mechanical strengths.However,when the wood fiber content was more than 40%,boric acid treatment reduced the decrease in mechanical strength which resulting from wood fiber degradation.Boric acid influenced thermal degradation behavior of wood fiber and PA 6.Torque rheological results showed that the balancing torque of the resulting composites decreased after boric acid modifications,which indicates that boric acid is beneficial to the processing of the resulting composites.Preparation and properties of borated wood fiber/polycarbonate(PC)composites.Polycarbonate-based WPCs were prepared using injection molding via improving the thermal stability of wood fibers with boric acid and their structure and properties were analyzed.FTIR,XRD and micromorphological analysis revealed the resulting composite has a typical structure of WPCs.Torque rheological results showed that the addition of wood fibers reduced the balancing torque of the resulting composite.The flexural modulus of the resulting composites increased by a maximum of 147.2%after adding borated wood fibers while flexural strength barely changed.The little change in mechanical strength of the resulting composites was because boric acid reduced the strength of wood fiber which only played a role in filling.TGA results showed that adding wood fibers reduced the thermal stability of the resulting composite.As expected,the creep resistance and coating properties of the composites gradually increased with increasing wood fiber content.The water contact angle of the composites gradually decreased as the wood fiber content increased.Preparation and properties of wood fiber/PC-high-density polyethylene(HDPE)composites.Blending different types of polymers can result in new polymer systems that have excellent properties and high cost performance.In the case of increasing the thermal degradation temperature of wood fibers with boric acid modification,wood fiber/PC/HDPE composites were prepared by injection at PC processing temperature,and their surface chemical structure,thermal properties,creep properties,mechanical properties and rheological properties were characterized.The results showed that the functional group content of PC molecular chains on the surface of the resulting composite gradually increased with the increase of PC content,indicating PC was uniformly distributed in the composites.Also,there was no characteristic absorption of the typical groups of wood fibers or other groups,which indicates that the polymer is enriched on the surface of wood-plastic,and the coating effect on the internal wood fibers is sufficient.Despite the addition of compatibilizers,PC and HDPE were incompatible.Boron compounds precipitated on the surface of wood fiber,resulting in a weak interface between wood fibers and polymers.Furthermore,the interface between PC and HDPE was obvious,and PC was dispersed in the HDPE matrix in the form of micro or nanoparticles.The crystallization temperature of the resulting composites increased after the addition of PC,indicating that the PC nanoparticles played a role in nucleating.However,the crystallinity of the resulting composites barely changed.The addition of PC improved the thermal stability of the resulting composites.The tensile and flexural strength of the resulting composites increased 22.7%and 27.6%respectively with the increase in PC content while the impact strength of the resulting composites decreased.Although adding PC increased the storage energy,loss modulus,and complex viscosity of the resulting composites,their frequency dependence did not change.Boric acid modification increases the thermal degradation temperature of wood fibers,providing the possibility to prepare polar engineering plastic-based WPCs with high performance.It provides a new idea for using engineering plastics to prepare high-performance wood-plastic composite materials.