1. U.S. Pat. 2 721 189 (1954/1955) to Du Pont de Nemours and Co. (A.W. Anderson and N.G. Merckling).
2. U.S. Pat. 3 074 918 (1957) to Du Pont de Nemours and Co. (H.S. Eleuterio).
3. U.S. Pat. 2 963 447 (1958/1960) to Standard Oil of Indiana (E.F. Peters and B.L. Evering).