CONTROLLING THE REFRACTIVE INDEX OF EPOXY ADHESIVES WITH ACCEPTABLE YELLOWING AFTER AGING
CHRISTOPHER C. AUGERSON, & JOHN M. MESSINGER
HXTAL NYL-1 resin and Ablebond resin were mixed in a variety of weight percent (100:0, 87.5:12.5, 75.0:25.0, 62.5:37.5, 50.0:50.0, 37.5:62.5, 25.0:75.0, 12.5:87.5, 0:100). Two separate sets of mixtures were prepared, from two separate resin batches.
Each resin mixture was prepared in a clean, glass test tube and stirred thoroughly with a glass rod. The HXTAL NYL-1 catalyst was added to each total resin mixture, in a proportion of 4:1 (resin:catalyst) by weight. Again, each resin mixture was stirred thoroughly with a glass rod. Preliminary tests, not reported here, indicated that if either of the two stirring procedures (blending the resins and adding the catalyst) were not thorough, then the resulting cast epoxies did not have uniform hardening or yellowing characteristics.
All epoxy mixtures were cast into small blocks measuring 3 � 12 � 20 mm. This size is required to fit the sample window on the Abbe refractometer. These mixtures were cured at 59–60 �C for 6 hours. After cooling overnight, the indexes of refraction were measured with an Abbe refractometer, according to the method ofMessinger and Lansbury (1989). Refractive indexes were similarly measured after one year of dark aging. The UV-visible absorbance of the adhesive blocks was also measured after one year of dark aging, according to the method of Messinger and Lansbury (1989). As in their study, the degree of yellowing was represented by the value of At, calculated according to the method of Down (1984).1