“Study on the mechanism of wood/bamboo/adhesive activation and bonding enhancement induced by high voltage electric field”

Thesis supervisors: Nicolas BROSSE (LERMAB), Xiaoning LU (Nanjing Forestry University)

Ecole doctorale : SIMPPÉ

jury :

In this study, the advanced equipments 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. 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. 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/m3, which was 32% 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%. 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.