1. U.S. Pat. 3,206,403 (1965); 3,475,393 (1969).
2. U.S. Pat. 2,998,414 (1961); 2,316,940 (1965).
3. Sekiyu Gakkai Shi,9, No. 6, 432?440 (1966) [Russian translation No. 7343318, VINITI (1968)].
4. U.S. Pat. 3,052,648 (1962).
5. U.S. Pat. 3,216,940 (1975).