| As a natural polymer material,wood and bamboo are widely used to prepare new composite materials.Improving the mechanical strength of wood bamboo composite is an important way to promote product upgrading and industrial development.In this study,the effects of wood and bamboo bonding interphase induced by high voltage electric field(HVEF)have been systematically investigated.The mechanism of HVEF induction on the interphase of composite materials have been systematically revealed.These results have important theoretical value and practical significance for improving the quality and efficiency of wood bamboo composite materials.In this study,the advanced equipments such as ESR(electron spin resonance),dynamic contact angle device,X-ray photoelectron spectrometer,rotational rheometer,fluorescence microscope and nano-indentation device were selected in order to investigate the effects of HVEF on the physicochemical properties of wood and bamboo,the effects of HVEF on the chemical structure and rheological properties of adhesives under a series of HVEF parameters.The aggregation effect of adhesive at bonding interface induced by HVEF has also been revealed and the micro-mechanical prediction model is established.The main conclusions of this study are as follows:1.After HVEF treatment,the surface activity of wood and bamboo increased significantly.Moreover,with the increase of voltage/time,the surface free radicals,O/C ratio and the number of oxygen groups increased significantly while the contact angle decreased.Under the condition of 60kV,the surface activity highly increased.The increment of free radicals was 26%,the decrease of initial contact angle was 22%,the decrease of equilibrium contact angle was 23%,the increment of free energy component was 43%～75%,the increment of O/C ratio was 34%,the increment of oxygen-containing groups were 39%(C-OH),149%(C-O or C=O)and 97%(O-C=O),respectively.Varied surface characteristics of different species and different sections were obtained under HVEF treatment.The change ratio was based on the surface characteristic parameters:cross section<tangential section<radial section;ayous<poplar<masson pine<Chinese fir.Under HVEF,the triggered silver particles reacted with wood functional groups with different oxidation forms of Ag(0),Ag(I)and Ag(III).The highest concentration of silver particles in wood samples was 10.71%.Nano-scale silver particles were obtained with a certain crystal morphology and the average particle size was less than 50 nm.For bamboo,the surface activity of treated bamboo skin was significantly higher than that of bamboo pith.Under the condition of 60kV,the free radical,O/C ratio and oxygen-containing groups of bamboo skin increased by 35%,32%,33%(C-OH),136%(C-O or C=O)and 71%(O-C=O)respectively.Therefore,under HVEF treatment,the physical and chemical properties of wood and bamboo can be significantly improved,which is conducive to improving the interphase properties of composite materials.2.With the increase of voltage/time,significantly improved inter-molecular reactions of urea formaldehyde resin and phenol formaldehyde resin were obtained.After 60kV/8 min treatment,significant increment of the characteristic peaks of C-O groups were obtained.Under HVEF treatment,the temperature/frequency dependence of the rheological behaviors of the two resins changed significantly.In the temperature spectrum test,the viscosity,storage modulus and loss modulus of urea formaldehyde resin and phenol formaldehyde resin increased significantly after treatment.There was no significant difference in the increment of rheological property parameters between the two resins under various treatment conditions,but the reaction characteristics of the two adhesives were different with the water factor.In the frequency spectrum test,with the increase of temperature constant,the increment of viscosity,storage modulus and loss modulus of urea formaldehyde resin and phenol formaldehyde resin decreased.Therefore,the degree of inter-molecular polymerization of phenol formaldehyde and urea formaldehyde resin can be significantly improved and the viscoelasticity of the resin can be improved under HVEF treatment.3.After HVEF treatment,the distribution of adhesive at the bonding interphase was continuous and uniform.The penetration depth was significantly reduced.The density and bonding strength at the bonding interphase were significantly increased,and the delamination rate was reduced.After treatment,the maximal density at interphase is 1081 kg/m~3,which was32%higher than the control.The bonding strength increased from 0.66MPa to 1.25MPa and the wood breaking rate increased to 85%,and the delamination rate decreased to 5.97%.Varied effects of HVEF treatment on different sections(C,cross section,T,tangential section and R,radial section)and different species were achieved.According to the change ratio of the interface characteristics,the sequence was:C-C<T-T<T-T_⊥<R-T<R-R;ayous<poplar<masson pine<Chinese fir.The results showed that the improvement of the interphase properties of softwood induced by HVEF was higher than that of hardwood.According to the correlation analysis,the increase of lignin content significantly increased with the increment of free radicals,the decrease of penetration depth and the increment of bonding strength.However,there was a negative correlation between the content of extract,the diameter of cell cavity and the decrease of penetration depth and the increment of bonding strength.For bamboo material,the bonding strength was significantly improved after HVEF treatment.The bonding strength of bamboo skin and bamboo skin was 9.51MPa,and the bamboo failure ratio was 60%.In the combination of bamboo pith and bamboo pith,the maximum bamboo failure ratio was 85%,which was increased by 70%.Therefore,under HVEF treatment,the continuous and uniform distribution of bonding interphase adhesives can be obtained,which can significantly improve the bonding performance of wood bamboo composite,and is conducive to the efficient utilization of wood bamboo composite.4.According to the vertical density profile at the bonding interface,the laminated stiffness and stress distribution model of the bonding interface has been established.The results showed that the relative error was less than±15%.Based on the distribution model,the macroscopic mechanical properties of composite are predicted with the combination of composite mechanics and laminated plate theory,including elastic modulus,bending strength,shear modulus and shear strength.The results showed that the prediction error of mechanical properties is less than 30%.With the stiffness and strength distribution model,the effect of HVEF treatment can be quantitatively characterized and the mechanical properties of HVEF treated composites can be predicted.As a result,strengthening mechanism of bonding interphase can be revealed with the the stiffness and strength distribution model. |
