Study On Interface Layer Bonding Properties Of Heat-treated Glulam

It is an effective way to improve the dimension stability and durability of glulam in timber structure to produce glulam by using solid wood sawn timber after heating treatment.After high temperature heating treatment,the surface bonding quality of solid timber is the key technology of heating treatment glulam production.In this project,the interlaminar interface of heat-treated glulam of Pinus sylvestris var.mongolica was taken as the research object,and the material properties of glulam laminates before and after heat treatment,bonding properties and environmental durability of Glulam interlaminar interface,and shear properties of Glulam short beams were systematically studied,the main research results are as follows:1)The flexural property is an important index to evaluate the mechanical properties of heattreated glulam laminates.At high temperature,the cellulose and hemicellulose in glulam laminates degrade and the lignin content increases,leading to the change of their mechanical properties.The flexural strength of 180 ℃ heat-treated laminates is higher than that of untreated and 215 ℃ heat-treated laminates.The elastic modulus of flexural resistance increases with the increase of heat treatment temperature.Compared with the untreated glulam laminates,the elastic modulus of flexural resistance of the glulam laminates treated at 215 ℃ increases by 27%.2)Color stability can directly reflect the surface durability of glulam laminates.To high temperature heat treatment wood,heat treatment temperature is higher its color is deeper.The substances produced by hemicellulose degradation under high temperature conditions have antioxidant properties,which can delay the aging process of wood surface,and make the surface of heat-treated glulam laminates have better color stability,which is conducive to improving the surface durability of glulam laminates.The color difference of heat-treated laminates was lower than that of untreated laminates after being irradiated by UV lamp for 360 h.3)Wettability is an important technical index to evaluate the adhesion of wood surface.After high temperature heat treatment,the hydrophilic groups inside the wood decreased,and the surface wettability of heat-treated glulam laminates decreased.The higher the heat treatment temperature,the more obvious the decrease of wettability.The surface wettability of heat-treated laminated wood was improved by planing treatment,which made the surface of laminated wood more smooth and reduced the adverse effect of surface carbonization layer.The planing thickness increased,and the improvement of the surface wettability of heat-treated laminated wood was small.When the planing thickness was 0.6mm,the laminated wood could better meet the requirements of technological composite.4)The interlaminar failure characteristics of heat-treated glulam are wood failure,interlaminar failure and mixed failure.When the strength of wood itself is lower than the bonding strength of the interface,it is shown as wood failure,most of which occurs in untreated glulam.The interface bonding property of heat-treated glulam is poor,mainly due to interface failure,which mostly occurs in the 215 ℃ treated glulam.Mixed failure is the main failure characteristic of most heat-treated glulam when the strength of wood itself is close to the bonding strength of wood interface.According to the SEM characterization of heat-treated glulam,the untreated glulam interlaminar interface is continuous and has good bonding.There are large cracks in the interface joints of heat-treated glulam wood,and high temperature heat treatment is not conducive to interface bonding.5)The interlaminar shear properties of heat-treated glulam with different heat treatment temperatures,gearing degrees and types of adhesives were studied by block shear test.The high temperature heat treatment reduces the wettability of the glulam laminates.At the same time,the embrittlement layer formed on the surface of the glulam laminates impeded the flow of adhesives,which led to the increase of heat treatment temperature and the decrease of interlaminar shear properties.The surface of heat-treated glulam laminates can be planed to improve the interlaminar shear performance obviously.The type of adhesive has a great influence on the interlayer interface of heat-treated glulam,and the properties of one-component polyurethane adhesive are better than that of isocyanate adhesive.6)In high humidity environment,the interlaminar shear performance of heat-treated glulam is stable.The higher the heat treatment temperature is,the less the influence of environmental humidity on the interlaminar shear strength of heat-treated glulam is.In the natural aging environment,the interfacial shear strength of heat-treated glulam decreased by polynomial function with the increase of aging time.With the increase of aging time,the interlaminar shear properties of heat-treated glulam tend to a certain stable state,which indicates that hightemperature heat treatment can effectively improve the stability and durability of the glulam.7)The short beam shear test method with symmetrical tri-point loading can make up for the shortcoming that the block shear test method is susceptible to cut damage,and it can more truly reflect the composite quality of glulam interlaminar interface under engineering application.In the process of flexural loading,the strain distribution in mid-span of heat-treated glulam was asymmetry,and the strain value of each layer was significantly lower than that of untreated glulam under the same load condition,which indicated that the properties of layers of heat-treated glulam were greatly different,and the synergy between layers was not as good as that of untreated glulam.Heat treatment can easily make the glulam wood fiber become hard and brittle,and its initial flexural stiffness increases significantly with the increase of heat treatment temperature.When the shear span ratio is 2,the initial stiffness of 215 ℃ heat-treated glulam short beam is23.4% higher than that of untreated glulam.The smaller the shear span ratio,the higher the bearing capacity of the specimens.When the shear span ratio is 2,the shear strength of 180 ℃heat-treated glulam short beam is higher than that of untreated and 215 ℃ heat-treated glulam.