Study On The Process And Performance Of PAN-Based Carbon Fiber Reinforced Structural Laminated Veneer Lumber

In order to build a community with a shared future for mankind,China has incorporated“carbon peak and carbon neutrality”into the overall economic and social development.The timber industry is a major component of the national economy.The production and use of timber and its products can reduce carbon emissions.The development of timber structures can reduce energy consumption and reduce environmental pollution.At present,the production and application of laminated veneer lumber in China are mainly concentrated in non-structural application fields such as indoor furniture and decoration.With the improvement of the mechanical properties of materials,ordinary laminated veneer lumber is facing the problems of insufficient market competitiveness and limited application fields,which restricts the application and development of laminated veneer lumber in the field of engineering structure in China to a certain extent.Therefore,in order to speed up the technological innovation of the wood-based panel industry,promote the green cycle low-carbon development model,and improve the green low-carbon processing technology innovation system,it is necessary to optimize and adjust the industrial structure of the wood-based panel.Eucalyptus is an important artificial timber tree species in southern China,but its low density,poor material and many defects seriously restrict its efficient utilization.In order to solve the problem that the current eucalyptus is difficult to meet the needs of structural materials due to its poor wood properties,improve the high-value utilization of eucalyptus resources,and develop high-strength structural wood composite materials,this paper combined eucalyptus veneer with PAN-based carbon fiber to prepare structural laminated veneer lumber（SLVL）,and studied and analyzed the characteristics of raw materials and their bonding mechanism,main process parameters,structural forms,and laying rates.The main research contents and conclusions are as follows:（1）The analysis of glue distribution in the bonding interface showed that epoxy resin adhesive and phenolic resin adhesive effectively penetrated between carbon fiber and eucalyptus veneer.The gaps between the carbon fibers were filled with cured epoxy resin adhesive,and the cell cavity,tracheids and pits of eucalyptus parenchyma were filled with phenolic resin adhesive,showing densification characteristics.After the surface modification of PAN-based carbon fiber by silane coupling agent KH550,the formed coating can increase the roughness of the fiber surface and improve the interfacial wettability and compatibility of different polar materials.Nanoparticles Si O₂can toughen epoxy resin.The bonding between eucalyptus veneer,PAN-based carbon fiber and adhesive in PAN-based carbon fiber reinforced SLVL is relatively close,reflecting good interfacial bonding performance.The composite SLVL still maintains some crystal structure characteristics of the original carbon fiber and eucalyptus.The crystallite size of carbon fiber increases and the degree of graphitization increases,indicating that carbon fiber can play its reinforcing role in eucalyptus SLVL composites.（2）The number of PAN-based carbon fiber reinforced layers and the laying structure have a significant effect on the physical and mechanical properties of the structural laminated veneer lumber.Different carbon fiber layout structures will make the enhancement effect different:When the PAN-based carbon fiber is arranged near the weak position of the specimen subjected to large tension and compression,the enhancement effect of the overall mechanical properties of the plate is more significant.The hot pressing pressure and the addition amount of nano-Si O₂toughening particles have a great influence on the impregnation stripping performance and horizontal shear strength of the sheet.When the hot pressing pressure is 2.0 MPa and the addition amount of nano-Si O₂is2.5%,the impregnation stripping performance,horizontal shear strength,elastic modulus and static bending strength of the sheet are the best.The influence trend and optimum level of each process factor on elastic modulus and static bending strength under different loading modes are basically the same,and the influence of each factor change is not significant.（3）The optimum process parameters were obtained in the test range:carbon fiber layer 4,nano-Si O₂2.5%,hot pressing time 1.25min/mm,hot pressing pressure 2.0 MPa.On the basis of ensuring that the basic mechanical properties of SLVL can be met,the material utilization rate and cost savings are considered to avoid waste caused by excess performance.The process parameters of PAN-based carbon fiber reinforced SLVL are optimized as follows:carbon fiber layer 2,nano-Si O₂2.5%,hot pressing time 1.25 min/mm,hot pressing pressure2.0 MPa.In addition,the test results show that the physical and chemical properties meet the index requirements.Among them,the elastic modulus and static bending strength can reach up to 180E level,and the horizontal shear strength can reach up to 65V-55H level.（4）In the process of productive test,epoxy resin and polyamide resin cross-linked to form a three-dimensional network structure,which was easy to increase the viscosity of the adhesive and reduce the fluidity,which was not conducive to the development of the coating process.The veneer has the anisotropic characteristics of dry shrinkage and wet expansion.When the moisture content is high and the process parameters are not properly controlled in the pressure relief stage in the later stage of hot pressing,too much water vapor in the core layer of the plate is likely to cause rapid volume expansion and poor exhaust,resulting in delamination and bubbling.In order to avoid the occurrence of plate defects caused by improper process,15%ethanol solution can be added to reduce the viscosity of the adhesive,increase the fluidity and improve the operability.The moisture content of the plate was controlled by the drying process,and the hot pressing process parameters were reasonably set to reduce the internal and external pressure difference of the plate.（5）Based on the influence of the structure of PAN-based carbon fiber reinforced SLVL on its dimensional stability,the size of the plate will shrink or swell with the change of ambient temperature and humidity.The vertical core composite structure is conducive to improving the dimensional stability and water absorption and expansion resistance of the product.Veneer and carbon fiber play a role in controlling deformation and reinforcement of the whole plate.The total size change rate decreased by 41.18%and 48.53%respectively,the 24h water absorption rate decreased by 33.14%and 39.11%respectively,and the water absorption thickness expansion rate decreased by 31.82%and39.39%respectively.In addition,the arrangement position of carbon fiber has a certain influence on the anti-deformation ability of the product.When the carbon fiber is added during the secondary pressure,it can work together with the veneer to block the water from entering the adhesive layer and reduce the combination of water molecules and free hydroxyl groups.The enhancement effect on dimensional stability is more significant.（6）The form of the vertical core composite structure and loading mode have a great influence on the mechanical properties.When the laying position of the carbon fiber is perpendicular to the loading direction,the enhancement effect on the overall mechanical properties is more significant,and the horizontal shear strength is increased by 30.51%and 39.51%,respectively.The static bending strength and elastic modulus were increased by 24.54%and 14.74%,44.86%and 30.16%,respectively.The high strength and high elastic modulus of carbon fiber reinforced materials were fully utilized,which was beneficial to improve the overall mechanical properties of SLVL.（7）The concept of PAN-based carbon fiber laying rate is introduced.The different laying rates of carbon fiber have a significant effect on the mechanical properties of the product.With the increase of the laying rate,the horizontal shear strength,static bending strength and elastic modulus of different loading methods show an increasing trend,but the strength of the unit carbon fiber laying area increases.The increase of horizontal shear strengthτ_Z/A_Z,static bending strengthσ_Z/A_Zand elastic modulus E_Z/A_Zincreased first and then decreased with the increase of laying rate.When the laying rate was 30%～50%,the increase of mechanical properties reached the maximum.The increase of elastic modulus and static bending strength was significant,and the increase of elastic modulus could reach 51.873 MPa/cm².When the laying rate is 70%,the increase of mechanical properties is weakened.Choosing the appropriate carbon fiber laying rate can improve the overall mechanical properties of LVL,improve the material utilization rate and avoid waste caused by material and performance surplus.