The Influence of Elevated Temperatures on Wood-Adhesive Joints by Fourier Infrared Spectrum Analysis
YUE Kong1, CHENG Xiu-cai2, JIA Chong3, LIU Wei-qing1, LU Wei-dong1
1. College of Civil Engineering, Nanjing Tech University, Nanjing 211800, China
2. Nanjing Institute of Product Quality Inspection, Nanjing 210019, China
3. College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China
Abstract：The wood-adhesive joints played an important role in transferring stress, which was an important parameter for bearing capacity of wood members. So, the bonding performance in high temperature determined the fire resistance of wood members. Larch wood, structural resorcinol-phenol-formaldehyde (PRF) and melamine-urea-formaldehyde adhesive (MUF) were selected as objects, and wood moisture content, density, parallel-to-grain tangential shear strength of solid wood, and bonding properties of joints with different adhesives of a total of 216 specimens exposed to elevated temperature ranging from 20 to 280 ℃ were tested. Fourier transform infrared spectroscopy (FTIR) was used to reveal the influences of high temperature on wood-adhesive joints. The results showed that physical reactions occurred to larch wood, and wood color change was not obvious, because there was only density reduction caused by water release at a temperatures ranging from 20 to 150 ℃. When the temperature increased until 200 ℃, thermal degradation of larch wood started, the density decreased slowly and the color gradually deepened. When temperature continued to increase, the wood specimens sharply darkened, and thermal degradation was intensified. So the density loss increased. At 280 ℃, larch wood was charred, and its color was completely converted to black. The density was 72.49% of that at room temperature. The relationship between parallel-to-grain tangential shear strength of larch wood and high temperature was negatively correlated. The shear strength of larch wood was 9.654 MPa at 20 ℃. The shear strength decreased with the increase of high temperature ramped from 20 to 110 ℃. At 150 ℃, wood shear strength decreased to 60.68% of that at room temperature. The shear strength of larch wood decreased obviously when exposed to elevated temperature ranging from 150 to 280 ℃. At 280 ℃, wood shear strength decreased to 1.054 MPa. The bonding properties of joints exposed to high temperature attributed to the thermal stability of adhesives. At room temperature, larch wood has good bonding properties with PRF and MUF. The shear strengths of joints of larch wood with PRF and MUF were 9.071 and 9.619 MPa, respectively, and the wood failure percentage was above 80% at room temperature. With the increase of high temperature, the shear strength of wood-adhesive joints decreased obviously, and the joints with PRF exhibited better than MUF when exposed to a higher temperature. The shear strengths of joints with PRF and MUF both decreased at 20～150 ℃, and was similar to that of larch wood. At 150 ℃, shear strength of joints with PRF and MUF were 60.61% and 60.92% of that at room temperature, respectively, and the wood failure percentage was more than 70%. The shear strength of wood-PRF joints decreased rapidly at the temperatures ranging from 150 to 280 ℃, which was similar to that of larch wood, and was 0.774 MPa at 280 ℃. The bonding properties of wood-MUF joints were more affected by high temperature. Wood failure percentages of joints with MUF were 10% at 220 ℃, and the shear strength was 0 MPa at 280 ℃. FTIR analysis showed that there was no obvious change in the chemical structure of PRF at 20～150 ℃. The further chemical crosslinking and the breaking of ether bond and methylene bridge occurred to PRF as the temperature was higher than 150 ℃. A slight pyrolysis occurred to PRF, but the chemical structure still remained entire. There was no obvious change in chemical structure of MUF exposed to high temperature ranging from 20 to 150 ℃ as same as that of PRF. When the temperature was higher than 200 ℃, the characteristic peak of hydroxymethylwas weakened, the isocyanate group appeared, and the thermal degradation grew severe. So, PRF had a higher heatresistance than MUF. The study can provide data support for the selection of raw materials, and provide a basis for improving the theory and method of fire resistance design for timber structure.
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