Durability of isocyanate resin adhesives for wood V: Changes of color and chemical structure in photodegradation

Abstract

The effect of ultraviolet (UV) light irradiation on the color and chemical structure of water-cured polymeric diphenylmethane diisocyanate (PMDI) was investigated using a UV long-life fade meter. Control treatment was performed without UV light irradiation using a thermohygrostat for comparison. Two kinds of resin were used in this study: that to which only water had been added, and resin to which a small amount of polyol and water had been added. In addition, lauan (Shorea spp.) wood was used as a reference. The photodegradation of the resins over a period of up to 300 h was observed using a colorimeter and Fourier transform infrared (FT-IR) spectroscopy. When the resins were treated with UV light, the color difference (ΔE* ab) of the resins increased signifi cantly in a short time, and then reached a near-constant value. For lightness, L* decreased rapidly for a few hours and then decreased gradually. The color darkened compared with that of the wood used. When treatment was performed without UV light irradiation, ΔE* ab and ΔL* of the resins showed negligible change. Based on the results of FT-IR analysis, severe degradation such as cleavage of the main chemical bond was hardly observed under UV light irradiation irrespective of the type of resin. © 2008 The Japan Wood Research Society.