| Wood bonding is physical and chemical interacttion between the wood and adhesive, which indicate that energy was exsited in the surface or interface that determined the intrinsic quality of adhesive phenomenol. The effect of property of wood and adhesive on the bonding strength were remarkable, so it was important to investigate the surface and interface properties of wood and adhesive. The fast growing wood and phenonlic formaldhyde adhesive as the mainly materials to investigated:(1) measured the relative equilibrium contact angle by the sessile drop method using the contact angle intsructment (OCA20), and the surface free energy were calculated; the differences of four calculated methods used to calculated the surface free energy were compared.(2) The influence of different machined process on the wettability of wood were determined.(3) The surface properties of liquid and solid PF adhesives were investigated by using the OCA20, X-ray photoelectron spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR), nuclear magnetic resonance (13CCP/MAS NMR).(4) The relationships betweeen the surface free energy, wettability and bonding strength were studied, and the curing properties were also investigated during the hot process. And the conclusions were followed(1) Four different methods were used to obtain the relative equilibriun contact angle, and also the obtained contact angles were used to calculated the surface free energy, the result indicated that the penetration of fast-growing poplar wood was better than that of the other wood materials, and also the surface free energy of wood samples were higher than that of the wood material, the disperse compent of wood surface was higher than the polar component and base component was higher than that of acid component.(2) The surface free energy of wood plastic composites (WPCs) calculated by four methods were significant different. The surface free energy of WPCs were not only depends on the chemical components or physical structure but also on the different calculated methods, species of reference liquids and the amount of reference liquids.(3) The wettability of different processing the wood surface is different. The contact of water on the rotary wood samples were higher than that of planed and sawn wood samples, and the surface free energy of the rotary wood samples was lower. The effect of surface roughness on contact angle was insignificant compared with surface structure morphology. (4) The fresh wood surface was more easily wetted by water than was the aged wood sample. The contact angles increased as the surface roughness decreased. The disperse and polar components of fresh wood both increased as the grit number increased. For aged wood, the surface free energy decreased as the grit number increased, but the disperse component increased. The effect of roughness on the acid-base components, acid component, and base component (calculated using the vOCG method) were unremarkable. The surface free energy of wood samples (obtained using the liquid parameters given by Volpe and Siboni) can effectively balance the relationship between the acid and base components. All coefficient of determination R-squared values were over90%, and the value of the Modified model was99%. The models can be used to accurately describe the adhesive wetting process.(5) The molar ratio had a great impact on the surface properties of phenolic resin. When the molar ratio was above1.75, the change of PF liquid surface tension were small, the dispersion component took hold; when the molar ratio was2.5, the surface tension and the dispersive component maximized, polar component minimized, the smaller the surface tension of a phenolic resin, the better the wettability. Total surface free energy and polar component of PF resin showed a decreasing trend after the first increase with the decrease of the molar ratio, closely related to-OH in resin surface. The cured resin with the different molar ratio, the change of FTIR peak was not obvious, only a benzene ring skeleton-CH vibration absorption peak. With the lower molar ratio, the peak intensity increased gradually, mainly due to the increasing phenol content.(6) The effects of surface roughness on bonding strength of fresh wood and aged wood were different. According to the S-D equation, the bonding strength of fresh wood enhanced with increasing permeability; but for aged wood, the correlation was not obvious, the relationship when using the M-D model were basically the same as using S-D equation. The relationship between K value and the bonding strength with Santoni model to calculate was also not distinct.(7) Core temperature increased over time and when the hot pressing temperature(i.e, temperatures of upper and down pressing plates) rised, plywood core heating rate was gradually increased, the average bond strength enhanced over time, and eventually reached a relative equilibrium value. FTIR suggested that polymerization reaction hydroxymethyl in resin structure, reducing the relative quantity.13CCP/MAS NMR analysis showed that the ratio of methylene bridge bond carbon atoms and unreacted hydroxyl methyl carbon atoms increased with heating curing time, indicated the increase of cure degree, but also enhancement the bonding strength. |
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