There are many applications, for example in the aerospace industry, in which sealants, gaskets, O-rings, solid propellant binders, etc., must have low temperature flexibility, high temperature stability, solvent resistance, and thermal and oxidative stability. Although recent prior art materials have been described which exhibit some of the above properties, there have not heretofore been provided polymers which satisfy the above requirements and additionally have satisfactory low temperature flexibility.
Polyurethanes have been prepared in the past by reacting hydroxyl-containing hydrocarbon polymers, such as poly(oxypropylene) triols, with aliphatic or aromatic diisocyanates. Such prior art polyurethanes, though widely useful for many applications, do not have the low temperature flexibility, tensile and elongation, and the hydrolytic, thermal, and oxidative stability required for many other applications (such as solid propellant binders and other uses in the aerospace industry). Recently, fluorine-containing polyurethanes have been disclosed in the art which do have some thermal and oxidative stability, but they and their methods of preparation suffer from a number of disadvantages or limitations, as discussed below.
In Vysokomolekulyarnye Soedineiya Vol. (A) 9, No. 11, p. 2482 (1967) and Jour. of Polymer Sci. Part A-1, Vol. 5, p. 2757 (1967), non-rubbery fluorine-containing polyurethanes are disclosed as being prepared by either the reaction of chloroformate derivatives of hydroxy compounds with fluorine-containing diamines (which reaction evolves corrosive, bubble-forming hydrogen chloride) or the reaction of fluorine-containing hydroxy compounds (rather than prepolymers) with aliphatic diisocyanates. These polyurethanes have a high ratio of urethane groups to the total weight of the polymer, and consequently a low fluorine content. NASA Publication No. SP-5901(01), p. 14 (1968), published by NASA's Office of Technology Utilization, discloses fluorine-containing polyurethanes, having pendant --CF.sub.3 groups in the backbone, prepared by reaction of an excess of aliphatic diisocyanate with hydroxyl- and fluorine-containing prepolymers having hydroxyl functionalities typically less than two, using undesirably high reaction temperatures. Though these prior art fluorine-containing polyurethanes do have some thermal and oxidative stability, they do not have very low temperature flexibility -- a property which is highly desirable where such products are used, for example, as low temperature adhesives and propellant binders.