The epoxy resin in timber composites is also exposed to attack by incident sunlight. Aromatic epoxy adhesives are more easily degraded by UV light. All epoxy resins containing aromatic groups can absorb sufficient UV radiation to cause bond dissociation [1,185-187]185-187.] The UV photons absorbed by polymers result in photo-oxidativephoto-oxidation reactions thatwhich alter the chemical structure, resulting in material deterioration. The chemical reactions usually cause molecular chain scission or chain crosslinkingcross-linking as are shown in Figure 2.11. Chain scission of epoxies decreases the molecular weight, giving rise to reduced strength and heat resistance. Chain cross-linking causecauses excessive brittleness and can result in microcrackingmicro-cracking. Some epoxies exhibit a colourcolor change when exposed to UV radiation. In addition to inducing chain scission and increasing crosslinkcross-link density, photo-oxidativephoto-oxidation reactions can also result in the production of chromophoric chemical species. The photodegradationphoto-degradation of epoxy resins occurs when they exposeare exposed to the cracking of the surface with a loss in the stiffness and strength. The initial step in the photodegradationphoto-degradation of adhesives involves photoabsorptionphoto-absorption at a particular wavelength, which depends on the structure of the resin and the presence of impurities. Polymers or epoxy resins have molecules that contain C-H, C-C, C-N, C-O and C-Cl bonds and; hence they are not expected to absorb wavelengths longer than 190 nmmm. Short wavelength photons absorbed by central structural components can cause severe degradation, such as bond scission. Free radicals, however, are formed after adhesives are irradiated with UV light of wavelengths less than 300 nmmm. Epoxies can be degraded via a photo-oxidativephoto-oxidation pathway or photolytically. In the latter, epoxy resins contain chromophores thatwhich absorb UV light directly, without first undergoing oxidation reactions. For example, aromatic polymers absorb only a small portion of the radiation that causes photochemicalphoto-chemical changes.