Mechanism Of Low Temperature Plasma Modified Wood Veneer To Achieve High Adhesive Efficiency

Wood-based composite is a kind of biomass composite material with the highest yield and the most widely used varieties.At present,the adhesive applying method widely used in wood composite materials leads to excessive consumption of adhesive,which will increase the production cost and the formaldehyde emission of the final product.It greatly limits the green and sustainable development of wood-based composites.In this thesis,atmospheric low temperature plasma treatment was utilized as the surface pretreatment of wood materials and afterwards,air spray atomization was applied as the method of adhesive applying for manufacturing wood-based composites with reduced adhesive consumption and fine mechanical properties.The adhesive consumption during producing plywood can be reduced to 100-60 g·m^-2 in this way.Comparative investigation between proposed method and traditional method are conducted,and the potential of industrial application of this method is also evaluated.On that basis,the permeability of adhesive and the mechanical properties in the adhesion interface were studied by means of a variety of techniques.The mechanism of plasma treatment on the surface of modified wood materials and the microscopic deformation mechanism of modified adhesion interface were further revealed.The research results in this paper are helpful to fundementally understand the mechanism of enhanced adhesive consumption in plasma-modified adhesion interfaces.Besides improving mechanical performance of plywood,the plywood preparation method proposed in this thesis can greatly reduce adhesive consumption in plywood production,thus achieving low-cost and friendly environmental performance of industrial application of wood composites.It can also provide theoretical guidance and practical basis to popularize and optimize plasma treatment technology.The main research results of this paper are as follows:(1)Combining low-temperature plasma and air spray atomization can reduce adhesive consumption to 57.44 g·m^-2 while guarantee the mechanical performance of plywood meet the requirement of Chinese National Standard.As compared to traditional adhesive applying method(roller coating,adhesive consumption:120 g·m^-2),the corresponding adhesive consumption of plywood can be successfully reduced by 52.13%.Plasma treatment leaded to oxidation of veneer surface and an active surface layer of veneer with nano-sized etching can be accordingly formed.On that basis,the capillarity between veneer surface and adhesive can be enhanced,leading to improved wettability and spreading of adhesive on veneer.With an adhesive consumption of 60 g·m^-2,the coverage of atomized adhesive on plasma-treated veneer is 96.31%.After curing,atomized adhesive can form into a continuous and defect-free adhesive layer,which improves the stress transferring efficiency of adhesion interface.Also, the concentration of shearing stress can be reduced due to the lacking of defects in adhesive layer after plasma treatment.Plasma treatment is proved to be effective for activating veneers from multiple species and thus improving the wettability of adhesive with different molecular weight and solid content.Plasma treatment combined with air spray atomization exhibits good adaptation to raw materials with different properties,indicating a promising potential for industrial application.(2)After plasma treatment,atomized adhesive formed into annular-filling in the vessels of interface with an adhesive consumption of 57.44 g·m^-2.Corresponding filled area is 136.31?m².The EP and AP of adhesive are 278.63?m and 11.06?m,respectively.Plasma treatment oxidized and etched the veneer surface,thus elevating the capillarity between veneer and adhesive.The penetration of adhesive can be accordingly enhanced.Plasma treatment combined with air spray atomization exhibits a good adaption to raw materials.Plasma treatment can also enhance the penetration of adhesive into cell wall,which is beneficial for promoting the diffusion of adhesive and then the formation of network structure of adhesive and cell wall.The mechanical interlock between adhesive and wood can be accordingly improved.(3)Particles in plasma exhibit high energy and they can cleave the chemical bonds.Cellulose and lignin undergo obvious oxidation in plasma.Plasma treatment leads to cleavage of inter-molecule hydrogen bond in cellulose,thus reducing the crystallinity of cellulose to38.81%.On that basis,the penetration channel of adhesive into cell wall can be enlarged.Also,the free hydroxyl in cellulose can act as the potential reactive sites for adhesive reacting with cell wall,the?-glucosidic bond in cellulose exhibits lowest BDE(251.87 k J·mol^-1)and a active chemical nature.Thus,the cleavage of?-glucosidic bond is the first-step reaction during oxidation of cellulose.The free radical reaction initialed by cleavage of?-glucosidic bond leads to cleavage of C4-C5 covalent bond in the pyranose ring.Oxygen-containing groups(hydroxy and carbonyl)can be grafted.Carboxyl can be generated by further oxidation of carbonyl.Plasma treatment can also lead to the oxidation of lignin macromolecules.Plasma-induced oxidation degradation mainly occurred in the aliphatic region of lignin.The oxidative reaction of lignin first occurred in the C?-O covalent bond(BDE=50.51 kcal·mol^-1).Through the free radical reaction,the C?-C?covalent bond in?-O-4 substructure cleaved and the oxygen-containing groups can be accordingly grafted.The new-grafted oxygen-containing groups can provide reaction sites for adhesive and cell wall,thus promoting the diffusion of adhesive,The mechanical interlock between adhesive and wood can be enhanced by formation stable chemical bonds between adhesive and cell wall.(4)Plasma treatment effectively improves the mechanics of cell wall in adhesion interface.The Er and H of cell wall in the plasma-treated interface with an adhesive consumption of 60 g·m^-2are 16.28 GPa and 588.41 MPa,respectively.Plasma treatment enhances the diffusion and penetration of adhesive into cell wall,thus the cell wall can be strengthened after curing of adhesive.The stress transferring efficiency of adhesion interface can be improved with enhanced cell wall mechanics.Plasma treatment exhibits good adaptation to raw materials for improving cell wall mechanics.The diffusion of adhesive into cell wall can be improved by plasma treatment and stable chemical bonds between adhesive and cell wall can be formed.On that basis,the adhesion energy between adhesive and cell wall improved by 45.56%.Plasma treatment promotes the formation of multi-layer adhesion interface with fine mechanics,which improving the stress transferring efficiency and strain distribution.The whole adhesion interface is thus imparted with better load-bearing capacity.(5)Investigation into the failure mechanism of plasma-treated adhesion interface reveals that plasma treatment improves the penetration gradient of adhesive in adhesion interface.The enlarged adhesion interface is beneficial for stress and strain distribution.Plasma treatment limits the initial and prolongation of cracks in CML and cell wall.FEA analysis also proves that plasma treatment improves the stress and strain distribution in adhesion interface.The total stress and stain of plasma-treated adhesion interface are below then the corresponding value of untreated one,which suggests that plasma treatment imparted adhesion interface with better rigidity.(6)The mechanism underlying the plasma-induced high adhesion efficiency between wood and adhesive can be concluded as follows:(i)Plasma treatment oxidized and etched the veneer surface.The capillarity between veneer and adhesive can be thus improved,which promotes the wettability and spreading of adhesive.The continuous and defect-free adhesive layer can be formed,which reduced the concentration of stress.(ii)Plasma treatment also enlarged the penetration channels of adhesive,thus improving the mechanical interlock between veneer and adhesive.The diffusion of adhesive into cell wall can be also improved by plasma treatment and stable chemical bond between cell wall and adhesive can be formed,thus improving mechanical interlock.(iii)Plasma-enhanced diffusion of adhesive into cell wall also leads to strengthen of cell wall,thus improving the mechanics of adhesion interface.Plasma treatment limits the initial and prolongation of cracks in adhesion interface.On that basis,the rigidity of adhesion interface improves,which promotes the uniform distribution of stress and strain.