Study On The Effect Of Hydrothermal Treatment Of The Reconstituted Glue Properties Of Waste Wood-based Panels

With the continuous improvement of people’s living standards in China,industries such as construction,furniture and interior decoration consume a large number of woodbased panels every year,and a large number of wood-based panels and their products are eliminated every year.Urea-formaldehyde(UF)resin has become the most widely used adhesive in the wood-based panel industry because of its excellent processing performance and low production cost.If traditional landfills and incineration are used to dispose of waste wood-based panels,the residual UF resin will cause environmental pollution problems.Using waste wood-based panels to produce recycled wood-based panels can alleviate the problem of raw material supply and demand in my country’s wood-based panel industry to a certain extent and is more conducive to the sustainable development of the wood industry.However,in preparing recycled wood-based panels from waste wood-based panels by re-gluing,the urea-formaldehyde resin remaining inside the panels will harm their re-gluing.As a green,environmentally friendly,and low-energy-consumption biomass material pretreatment technology,hydrothermal treatment is widely used to degrade organic polymers.This study selected waste particleboard,fiberboard and plywood containing UF resin residues as the research objects.First,three hydrothermal treatment methods of hydrochloric acid,sodium hydroxide and hydrogen peroxide were used to remove the residual UF resin in waste wood-based panels.Then,the chemical properties of the basic unit of waste wood-based panels and their gluing and recombination characteristics after hydrothermal treatment were further explored,and the feasibility of recycling waste wood-based panels after hydrothermal treatment was verified.The main research contents and findings are as follows:(1)The analysis of total nitrogen of waste wood-based panels before and after hydrothermal treatment showed that the total nitrogen content of waste particleboard,fiberboard and plywood decreased gradually with the increase in hydrothermal temperature and time.The effect of hydrothermal temperature on removing UF resin from waste wood-based panels is more significant.Different hydrothermal treatment environments are suitable for different kinds of waste wood-based panels.The results showed that after hydrothermal treatment at 120℃ for 60 min,the total nitrogen content of acid-treated waste fiberboard decreased by 78.81%,and the mass loss was 54.51%.After alkali treatment of waste particleboard,the total nitrogen content decreased by 72.56%,and the mass loss was 30.44%.After hydrothermal oxidation treatment of waste plywood,the total nitrogen content decreased by 80.76%,and the mass loss was 16.03%.The removal of urea-formaldehyde resin from waste wood-based panels will help to improve the mechanical properties of recycled wood-based panels when they are recombined and used.(2)Fourier transform infrared spectroscopy(FTIR),X-ray diffraction(XRD),scanning electron microscopy(SEM)and X-ray photoelectron spectroscopy(XPS)were used to explore the effects of different hydrothermal treatment methods on the chemical properties of waste wood-based panels.FTIR analysis showed that the strength of the amide group was weakened,indicating that the hydrothermal treatment destroyed the urea-formaldehyde resin and the adhesive interface between the UF resin and the wood components.XRD analysis showed that hydrothermal oxidation had minor damage to the crystallization zone in waste plywood,which reduced the crystallization index of waste plywood by 8.11%.However,acid and alkali treatment destroyed the amorphous area of cellulose and increased the crystallization index of waste wood-based panels.After acid and alkali treatment,the crystallization index of waste fiberboard and particleboard increased by 4.82%and 10.81%,respectively.SEM analysis showed that the surface structure of the waste wood-based panels was destroyed after hydrothermal treatment.The high specific surface area of waste woodbased panels helps the adhesive penetrate more quickly into the surface of the wood material.XPS analysis showed that the nitrogen content on the surface of the waste wood-based panels decreased and the active groups increased significantly after hydrothermal treatment.This result helps that when the waste wood-based panels are re-glued and reconstituted to make new wood-based panels,the adhesive reacts with the chemical components in the waste wood-based panels.All of these are beneficial to forming a more stable glued interface,thereby improving the performance of recycled wood-based panels.(3)The effect of hydrothermal treatment on the properties of recycled wood-based panels and their reorganization mechanism was explored.The internal bond strength(IB),static bending strength(MOR),elastic modulus(MOE)and 24 h water absorption thickness swelling ratio(TS)of the recycled wood-based panels were measured.After the shavings and fibers obtained by mechanical separation of waste wood-based panels are re-glued after hydrothermal treatment,the mechanical properties of the waste woodbased panels after hydrothermal treatment can meet the performance requirements of P1-type particleboard in the GB/T 4897 2015 standard.Regenerated fiberboard can also meet the performance requirements of ordinary MDF in the dry state in GB/T 117182009.Acid treatment can destroy the UF resin on the surface of more fiber units,which significantly improves the mechanical properties of the recycled fiberboard.Alkaline treatment can substantially improve the water-resistance of the board,and the recycled particleboard is reduced from 10.23%to 6.76%.The improvement of the mechanical properties of the plywood after hydrothermal oxidation treatment mainly comes from the removal of the adhesive and the destruction of the cellulose crystallization area,which are all conducive to the penetration of the adhesive during the sizing process and form a more vital bonding interface.The three hydrothermal treatment methods have laid a theoretical foundation for the recycling and recombination of waste wood-based panels.They have essential research significance for realizing the clean and efficient utilization of waste woodbased panels.