Photodegradation of adhesives used in wood composite materials

The weathering of wood composites is caused by a complex combination of chemical and mechanical effects. Wood composites such as glulam beams are increasingly being used outdoors where their service life depends to some extent on the durability of the adhesive used in the composite. Increases in the durability of adhesives used in such composite materials would prolong their service life and enable them to compete more effectively with other structural materials such as concrete and steel. This study attempted to improve our understanding of the photodegradation of adhesives and the relationship between wood and adhesive photodegradation. The effectiveness of a UV light absorber and hindered amine light stabilizer (UVA and HALS) at protecting adhesives from photodegradation was also investigated. First, the effect of adhesive type (melamine formaldehyde, epoxide, and emulsion polymer isocyanate), stabilizer and adhesive stabilizer interaction on tensile strength, weight loss and discoloration of adhesive dog-bone samples exposed in two different weatherometer devices (QUV and Xenon-arc) was examined. Structural and chemical changes of the adhesive specimens were examined using Fourier Transform Infrared (FTIR) spectroscopy and Scanning Electron Microscopy (SEM). Secondly, the effects of adhesive type (melamine formaldehyde, epoxide), stabilizer and adhesive stabilizer interaction on surface roughness and discoloration of wood-adhesive-dowel samples exposed to solar radiation was examined. Profileometry and SEM was used to examine the surface of dowels in the region where they were exposed to both wood and sunlight. An epoxide adhesive (butyl glycidyl ether of bisphenol-A with polyamide) used in the aircraft industry showed outstanding resistance to weathering. The other adhesives were not as resistant to weathering, but the addition of a UVA/HALS photostabilizer to the adhesives generally increased their photostability (particularly color changes of the epoxy adhesives and weight loss of the MF adhesive). Greater degradation of adhesive samples occurred when they were exposed in a QUV weatherometer than in a Xenon-arc weatherometer. The synergistic effect of moisture and UV radiation on the degradation of adhesives may account for this observation. Adhesive dowels embedded in wood did not show greater degradation (erosion) in the region where they were exposed to both wood and sunlight. Therefore the hypotheses that wood photosensitizes adhesives could not be supported by experimental findings. Further refinement of the experimental methodology developed in this thesis would be desirable to retest this hypothesis. All of the four adhesives that were tested possessed some interesting characteristics that might make them suitable for use in glulam exposed outdoors, but out of the four the two epoxy adhesives appeared to have the greatest potential.Forestry, Faculty ofGraduat