No abstract

No abstract

A wood surface, which is exposed to a high temperature condition, can experience inactivation. Surface inactivation reflects physical and chemical modifications of the wood surface. Consequently, these changes result in reduced ability of an adhesive to properly wet, flow, penetrate, and cure. Thus, an inactivated wood surface does not bond well with adhesives.The changes in surface chemistry, wettability, and adhesion of inactivated wood surfaces, including heartwood of yellow-poplar (Liriodendron tulipifera) and southern pine (Pinus taeda), were studied. Wood samples were dried from the green moisture content condition in a convection oven at five different temperature levels ranging from 50 to 200°C. The comparative characterization of the surface was done by X-ray photoelectron spectroscopy (XPS), sessile drop wettability, and fracture testing of adhesive bonds. Additionally, several chemical treatments were utilized to improve wettability and adhesion of inactivated wood surfaces.The comparative analysis helped elucidate clear relationships between surface chemistry, wettability, and bond performance in regard to surface inactivation. XPS results showed that wood drying caused modification in wood surface chemistry. The oxygen to carbon ratio (O/C) decreased and the C1/C2 ratio increased with drying temperature. The C1 component is related to carbon-carbon or carbon-hydrogen bonds, and the C2 component represents single carbonoxygen bond. A low O/C ratio and a high C1/C2 ratio reflected a high concentration of non-polar wood components (extractives/VOCs) on the wood surface, which modified the wood surface from hydrophilic to more hydrophobic. A hydrophobic wood surface repelled water and wettability of this surface was low (i.e., a high contact angle). Wettability was directly related to the O/C ratio and inversely related to the C1/C2 ratio.iii Contact angle decreased with time and increased with the temperature of exposure. A dependence of wood species was evident. Southern pine had a lower wettability than yellowpoplar, which was due to a greater concentration of non-polar hydrocarbon-type extractives and heat-generated volatiles on the surface. Solvent extraction prior to drying did not improved wettability, whereas, extraction after drying improved wettability. A contribution of extractives migration and pyrolysis products deposition played a significant role in the heat-induced inactivation process of southern pine.The maximum strain energy release rate (G max ) obtained by fracture testing showed that surface inactivation was insignificant for yellow-poplar when exposed to drying temperatures < 187°C. The southern pine was most susceptible to inactivation particularly when bonded with phenol-formaldehyde (PF) adhesive. A typical surface inactivation for southern pine occurred at drying temperatures > 156°C.Chemical treatments improved the wettability of inactivated wood surfaces, but an improvement in adhesion was not evident for specimens bonded with polyvinyl-acetate (PVA) adhesive. Of the chemica...

No abstract

Bonding of untreated, intermediate (hydro-thermolysed) and heat treated wood with melamine-urea-formaldehyde (MUF), phenol-resorcinol-formaldehyde (PRF) and polyurethane (PUR) adhesives was studied. An industrial heat treatment process (Plato(R)) was used, which included two separate heat treatment stages and a drying stage in-between. Laminated beams having four lamellas were prepared from untreated and treated timber for mechanical testing of the bond lines. The results of the tests showed that heat treatment affected the shear strength and the delamination of the laminated wood depending on the adhesive system used for bonding. The PUR and MUF adhesives performed in a rather similar way, and better than the PRF adhesive. The shear strength of laminated wood bonded with the waterborne MUF and PRF adhesives decreased for the specimens made of hydro-thermolysed timber and decreased further for the specimens made of fully heat treated timber. The difference in adhesive bond shear strength between untreated, intermediate and fully treated wood was less obvious in the case of the PUR adhesive. Delamination of the PRF bond line decreased drastically for all the specimens made of heat treated timber

No abstract

No abstract

No abstract