Study of UV Curable Rubber-Toughened Epoxy Systems

Rubber-toughened epoxy resins are used extensively in various structural applications. Current thermal curing processes limit the possible structural designs and require higher inputs in terms of pressure, temperature and time. UV and electron beam curing offers almost instantaneous curing of even complex structural shapes using minimal inputs. Various formulations were quenched from single phase at high temperatures to various temperatures below the corresponding cloud point temperatures, before being allowed to cool to room temperature. These quenched formulations were then cured using UV and thermal energy. The cured material toughness increases with decreasing particle size by over 150%. The rate of cooling affects the particle size achieved during phase separation. Particle size decreases with faster cooling rates. Particle size of rapidly cooled samples changes with time at room temperature. This is thought to be due to Ostwald ripening in which densely populated small-sized particles coalesce together forming small number of large-sized particles. This phenomenon is predicted to reduce the intended shelf life of the B-stage resin.

Triaryl sulphonium hexafluoroantimonate (TASHFA) cured two phase and three component samples were partly uncured even after curing under UV radiation for 15 minutes. Additional thermal curing step in a conventional oven resulted in complete cure. FT-IR studies of the samples confirmed the presence of unreacted epoxide rings in the UV cured samples. Additional short-time thermal curing step, without any additional peroxide catalyst, was found to be sufficient for complete cure. DMA results showed that some amount of rubber remains dissolved in the epoxy matrix. This dissolved rubber contributes to the observed shift of 30^oC in the epoxy tan δ peak for the fastest cooled sample. The amount of the particulate rubber, dispersed in the matrix, reduced with an increase in the cooling rate.