Interface Gluability And Its Affecting Factors Of Bamboo/Poplar Composite

As the contradiction of wood demand and supply in China, a series of policies for wood saving and substitution have been released to ensure the sustainable development of wood industry. Developing bamboo/wood composites is one way to make full use of both advantages of bamboo and plantation wood.Lots of researches have been doing on bamboo/wood composites most of which focus on the product development, production process and so on. But there are few researches on the interface of bamboo/wood Composite. Therefore this paper focused on the relationship between mechanical properties of interface of bamboo/wood and bonding strength of bamboo/wood composite with different processing parameters, humidity condition and thermal condition. Three kinds of element materials which were bleached bamboo, carbonized bamboo and poplar veneer and two types of adhesives which were emulsion polymer isocynate (EPI) and urea formaldehyde (UF) were used in the experiment. Research methods and results were as follows:1) Key processing parameters for bamboo/poplar veneer composite processing had been studied, and the relationship of surface properties of elements and glue penetration in composite interface and the bonding strength of composite had been studied.Results showed that the optimum glue spreading and pressure for composites glued by EPI were 250g/m2 and 1.6MPa, respectively; the optimum glue spreading, pressure and filler content for composites glued by UF were 300g/m2, 1.2MPa and 0%, respectively. Bonding strength of composites glued by UF was little high than that of EPI. Bonding strength of composites composed by different elements was as follows: bleached bamboo/bleached bamboo > carbonized bamboo/carbonized bamboo > bleached bamboo/poplar veneer > carbonized bamboo/poplar veneer > poplar veneer/poplar veneer.Wettability, surface activity and surface chemical constituents were studied by contact angle meters, x-ray photoelectron spectroscopy (XPS) and fourier transform infrared (FTIR), respectively. And penetrability of adhesive on the surface of bleached bamboo, carbonized bamboo and poplar veneer was studied by fluorescence microscope. Result showed that surface wettability and penetration of adhesives on the surface of elements played some roles on the improvement of bonding strength of composite, and the effect of penetration, especially the penetration of EPI on the surface of poplar veneer and UF on the surface of bamboos, was bigger than the wettability of elements.2) Strain distribution in the interface of composites studied by digital speckle correlation method (DSCM) and finite element method (FEM).Results showed that the maximum shear strain was at the end of glueline. Composite glue by UF had lower shear strain and peel strain, but higher shear stress and peel stress, compared with composite glued with EPI. Strains and stresses in the interface were symmetric and the maximum stresses and strains were both at the end corner of glueline contacting with substrates, when composite was composed by substrates with same MOE, otherwise strains and stresses in the interface were asymmetric and the maximum shear stress and peel stress were at the end corner of glueline contacting with poplar veneer and bamboo, respectively; strains close to poplar veneer were higher than that to bamboo. The maximum stresses and strains of composites were as follows: poplar veneer/poplar veneer > bleached bamboo/poplar veneer > carbonized bamboo/carbonized bamboo > bleached bamboo/bleached bamboo, which means composite composed by substrates with higher MOE had lower maximum strains and stresses.3) The dimensional change and bonding strength of bamboo/poplar veneer under humidity cycle was studied.The humidity cycled between 40% and 90% under 80℃. Results showed that the dimensional change of bleached bamboo was closed to that of poplar veneer, and lower than that of carbonized bamboo. The dimensional changes of bamboo/poplar veneer were close to those of bamboo elements. Composite glued with EPI had larger dimensional change than that of glued with UF, which related to the MOE of adhesives.Bleached bamboo/poplar veneer had higher bonding strength than carbonized bamboo/poplar veneer, which because the big differential dimensional change between carbonized bamboo and poplar veneer in the humidity cycle. There was a decrease in bonding strength of composite glued with UF, while even a few of increase in that of composite glue with EPI as the increase of humidity cycle. 4) The dimensional change and bonding strength of bamboo/poplar veneer under thermal cycle was studied.The thermal was cycled between 80℃and room temperature. Results showed that thermal cycle had the biggest effect on the elemental dimension in radical direction, followed by tangential direction and longitudinal direction. Poplar veneer had the largest dimensional change, followed by carbonized bamboo and bleached bamboo. As the increase of thermal cycle, the composite dimension in longitudinal direction and radical direction was decreased; while decreased at the first two cycles and after that it changed little in tangential direction. The dimensional change of composite glued with EPI was lower than that of glued with UF in longitudinal direction, and higher than that of glue with UF in radical and tangential direction. 5) Temperature, strain and stress distribution in carbonized bamboo/poplar veneer under nonisothermal condition was studied with finite element method.Results showed that temperature was gradually decreased from heating side to non-heating side. And it increased in nonlinear form at first and then turn to linear form as a function of time. The maximum stresses were located at the end of the glueline. Peel strain and stress was higher than shear strain and stress in the composite. As the increase of temperature, the maximum stress and strain in the composite were oscillated around a certain value.Composite warped up under the nonisothermal condition. Carbonized bamboo/carbonized bamboo had larger deformation than carbonized bamboo/poplar veneer, and also higher strain and stress in the interface.