CONTROLLING THE REFRACTIVE INDEX OF EPOXY ADHESIVES
John M. Messinger, & Peter T. Lansbury
ONE CAN INDEED RAISE THE REFRACTIVE INDEX of two commercially available epoxy resins with additives 2 and 3, albeit to a limited degree. The refractive index of Ablebond¯ can be raised from about 1.565 to about 1.59. That of HXTAL¯ NYL-1 can be raised from about 1.515 to about 1.545. While the addition of these additives does cause yellowing in the cured HXTAL¯ resin, the amount of yellowing imparted to Ablebond¯ is well within acceptable limits. Decreasing the proportion of hardener present in an epoxy mixture does not seem to affect the tensile strength of Ablebond¯ until the proportion of hardener is below 22 parts hardener per 100 parts resin.∗ (32 to 100 is recommended for Ablebond¯.) Decreasing the proportion of hardener present in an epoxy mixture does not affect the tensile strength of HXTAL¯ until the proportion of hardener is below 17 parts hardener per 100 parts resin. (25 to 100 is recommended for HXTAL¯.) With low concentrations of hardener, both of these mixtures may be readily softened with methylene iodide and bromonaphthalene.
∗ Tensile strength was considered normal if manual flexing did not crack the epoxy blocks and/or a fingernail could not dent the blocks.