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2. Christ, R.E., and Hanford, W.E., U.S. Patent 2,333,639 (1940, to DuPont).
3. British Patent 580 524 (1941, to ICI Ltd.): British Patent 574 134 (1942, to ICI Ltd.).
4. Martin, T.G., and Seeger, N.V., U.S. Patent 2,625,535 (1953, to Goodyear Tire & Rubber Co.)