Structures Of Reacting Products Of Isocyanate With Cellulose And Polyurethane Adhesion Mechanism For Woods

Polyurethane (PU) is a high-performance adhesive which can be applied to bond a variety of materials and different substrate materials. The application of PU in woodwork industry to replace three traditional aldehyde adhesives archives remarkable economic and social benefits. However, its application to furniture industry is restricted in our country by its high cost. It is considerably valuable for researching the adhesion mechanism of PU for wood and developing advanced and low cost PU. The primary difficulty lying in research of the adhesion mechanism of PU to wood are as the follows:first, none of adhesion reactions can be separated because the woods are porous and complex in component, and PU has the reactive functional group-NCO; second, there are no accurate test technology for adhesion strength. In this paper, a novel method of modeling wood and PU by decomposing and substituting the wood by components which have pre-defined similar characteristics was developed and hence different adhesion reactions were decoupled. Inverse gas chromatography (IGC) was used to research the mechanism of interfacial adsorption in originality and scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), combined with the test of the mechanics performance were applied in the research of the adhesion mechanism of mechanical anchoring and interfacial diffusion in depth, respectively. Products structures of phenly isocyanate reacting with cellulose were also characterized by nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and the mechanism of chemical bonding were investigated.The chemical mechanism of PU boning wood was studied by modeling wood by cellulose, which is the main composition in wood and has a thorough defined chemical structure, and by modeling PU using phenyl isocyanate (PI), which is a compound with mono functional group, so that PI cannot cure crosslinking and the -NCO reactivity is unique. The product structures of 98% 15N-labelled PI reacting with microcrystalline cellulose with different moisture contents were characterized by applying 15N and 13C CP/MAS NMR and FTIR. The effects of moisture content of woods, reaction temperature and time etc. on reaction were studied in detail. It was found that PI reacts with anhydrous cellulose and produces (bi)allophanate, and urethane, however, PI reacts with water predominantly as using hydrous cellulose and results in urea and biuret. At lower temperature (<130℃) and short time (<4 h) PI reacts mostly with water, while the urea and urethane are main products at high temperature and long time. The chemical bonding exists between wood and PU, but the chemical bonds is less for the PU bonding wood with the moisture content mostly within 8%-12%.The adsorption mechanism of PU bonding woods was studied by means of IGC in order to determine the surface dispersive free energy, surface acidity and basicity of the fraxinus manshurica, birch, poplar and cured phenol formaldehyde adhesives (PF) and cured PU at various-NCO content. The results indicate that these three kinds of woods have high surface energy and are acidity dominant amphiprotic materials, whereas cured PU and PF were basicity dominant amphiprotic materials, and the surface dispersive free energy of PU varied largely due to a increasing content of -NCO. The surface free energy of the fraxinus manshurica, poplar and cured PU was determined, and it was found that the dispersive component was larger than the acidic-basic components. Test of the interface adhesive works of fraxinus manshurica/PU and poplar/PU revealed the dispersive component of their adhesive works was 5-6 times larger than the acidic-basic components. The results demonstrated the existence of adsorptive interactions of PU boning wood and dispersive reactions of adsorptive were far more than acidic-basic interactions.The mechanical anchoring mechanism of adhesion were studied by observing the cross section morphology of plywood using SEM, and by combing with the test of adhesion strength of PU bonding PE boards made by polyethene (PE) boards, which were prepared to surface glazed, surface rough, inner air chamber or porosity connected. PE was selected to simulate wood due to its not any chemical reactions and acidic-basic interactions with PU. The adhesive in wood vessels, pits and scalariform perforations at the bonding cross section of birch plyboard, and the bonding nails at adhesive side was observed by SEM, which reveals the forming of semi-interpenetrating polymer networks of adhesive in wood within a definite thickness from the wood bonding side. The shear strength of plywood increases with the increasing of the surface roughness, the aperture and the porosity. The result demonstrates the mechanical anchoring mechanism of adhesion existing in bonding of adhesives to wood. The summation of dispersive and diffusion interactions contribute to bonding strength more than the mechanical anchoring reactions in existing not any the chemical interactions and the acidic-basic interactions.The diffusion mechanism of PU bonding wood was investigated in detail using the model of PU/cellulose film, which eliminates the interface mechanical anchoring reactions. The interface morphology of cured PU/cellulose film was observed by SEM. A blurring interface of the test piece cured at 140℃and a clear-cut interface of it cured at 100℃were observed. Whereas an interfacial crack of a PF/cellulose film was found, suggest the PU possessing a grater diffusion interactions to cellulose film than that of PF. A diffusion thickness of 300 nm of PU in PU/cellulose film cured at 140℃was determined by an electron spectrometer with SEM. The crystallinity index of cellulose film was very low caculated by XRD testing, that shows the PU diffusion by amorphous fraction of wood. It was tested by XPS that the surface N content of cellulose film cured conglutinating PU without-NCO, and then immerged solvent and then eluviated the surface adhesive layer, which revealed an increase with the increasing of cured temperature and time even exceed the N content of the PU bulk. All of these proved the diffusion of PU molecule chain segments and phases separation accrued gradually with curing of PU/cellulose film. The analysis of the data of strength test and the breach mode of cellulose film bonding test specimen showed that elevation of the cured temperature was propitious to the diffusion of molecule segments, and high-NCO content lead to an excessive crosslinking against the phases separation of PU, thus reduced the bonding strength. The curing temperature and the ratio of polyethylene glycol in PU are important factors for diffusion. The results show that the interface diffusion is the main interactions effecting adhesion strength.These researches, in a detailed and systematical way, shows that the chemical bonding, adsorption, mechanical anchoring and diffusion mechanism exist in the adhesion of PU to wood; and the contribution of these various interactions to adhesion are ranked as following: diffusion interactions> adsorption (dispersion> acidic-basic) interactions> mechanical anchoring interactions> chemical bonding interactions.