CONTROLLING THE REFRACTIVE INDEX OF EPOXY ADHESIVES WITH ACCEPTABLE YELLOWING AFTER AGING
CHRISTOPHER C. AUGERSON, & JOHN M. MESSINGER
In repairing a broken glass object, factors that will make the repair less visible are usually considered. One such factor, important in the repair of colorless glass, is the approximate match between the index of refraction of the adhesive and that of the glass (Ogden 1975; Tennent and Townsend 1984a; Messinger and Lansbury 1989). Tennent and Townsend (1984b) describe a glass repair where the difference between the index of refraction of the glass and that of the adhesive is 0.04; this repair is clearly visible. However, in another repair of the same glass, the difference of refractive indexes is approximately 0.01, and the repair is not easily visible under normal lighting conditions.
Recently, Messinger and Lansbury (1989) developed a method of controlling the index of refraction of epoxy adhesives to a small degree. This control was achieved by adding a mixture of 1,2-epoxy-3-[2,4,6-tribromophenoxyl] propane and 1,2-epoxy-3-[2,4,6-triiodophenoxyl] propane to either of two commercially available epoxies. Ablebond mixed with such additives produced a range of refractive index from 1.565 to 1.59. The resulting adhesives were found to have yellowing characteristics within acceptable standards when aged at room temperature in the dark as defined by Down (1984). HXTAL NYL-1 mixed with such additives produced a range of refractive indexes from 1.515 to 1.545. However, the halogenated additives caused the adhesives made with HXTAL NYL-1 to yellow beyond the acceptable standards.
This study was conducted to determine whether mixtures of Ablebond resin and HXTAL NYL-1 resin could be used to produce epoxies with a range of refractive index from 1.515 to 1.565, which also pass the standards of Down (1984) for yellowing.