Suitable azo compounds include, but are not limited to, non-water-soluble azo compounds such as 1-1???-azobiscyclohexanecarbonitrile, 2-2???-azobisisobutyronitrile, 2-2???-azobis(2-methylbutyronitrile), 2-2???azobis(propionitrile), 2-2???-azobis(2,4-dimethylvaleronitrile), 2-2???azobis(valeronitrile), 2-(carbamoylazo)-isobutyronitrile and mixtures thereof; and water-soluble azo compounds such as a