1. M.E.B. Jones, British Patent 1,016,245 (1962)
2. U.S. Patent 4, 008, 203 (1977).
3. H.A. Vogel, British Patent 1,060,546 (1963)
4. H.A. Vogel, J. Polym. Sci., Part A-1, 8, 2035 (1970).
5. A.G. Farnham and R.N. Johnson, British Patent 1, 078, 234 (1963)