Preferable are thermal initiators that are rated with a ten-hour half-life at high temperatures, for example above 120??? C., including organic peroxides such as t-butyl cumyl peroxide, di(t-amyl) peroxide, 2,5-di(t-butylperoxy)-2,5-dimethyl-3-hexyne, cumene hydroperoxide, t-butyl hydroperoxide, and the like.