As with other forms of radiation energy, gamma radiation can be used to polymerize or co-polymerize certain gaseous, liquid, or solid monomers. Gamma-radiation-induced reactions are similar in nature and mechanism to those induced by uv radiation. Consequently, the same classes of polymers used for uv curing are amenable to gamma curing. These include the general class of ethylenic compounds such as vinyls, acrylics, allylics, and polyesters. Epoxy resins, however, are difficult to polymerize by gamma irradiation. High dosages of 4 × 108rads and greater are required and, even so, only a small percentage increase in molecular weight occurs.[82] However, indirect techniques may be used to radiation cure epoxies. The epoxy resin may be modified to incorporate vinyl or ethylenic groups that are radiation sensitive. Gamma irradiation can then induce polymerization through these functional groups.
As with plasma, another use of gamma radiation consists of grafting gaseous or liquid monomers onto the surfaces of solid plastics or coatings, thus altering the surfaces to improve their wetting, antistatic, adhesion, or surface resistivity properties, depending on the group that is grafted. As an example, acrylic groups have been grafted onto Teflon film to give it antistatic properties.[83] Other specific applications in which the moisture resistance, chemical resistance, and radiation stability of polymers have been improved by gamma irradiation grafting have been described by
DasGupta.[84]
Commercial gamma-radiation equipment utilizes one of three gamma sources: cobalt 60, cesium 137, or iridium 192. Dose rates may range from 5 × 104 rads/hour to 6.4 × 104 rads/hour.
Although extensive experimental work has been performed on using gamma radiation to polymerize monomers, very little is reported on the practical applications of this technique for electronics. However, gamma-radiation curing, as well as other radiation-curing methods, hold much promise for future electronic applications. Among some potential uses are the curing of insulating coatings or capacitor dielectrics, the fabrication of detailed microinsulation patterns, and the development of materials having precise bulk or surface electrical properties.