Heretofore, when high curing temperatures of about 150.degree. C. or higher were employed to effect rapid curing, and often gain better mechanical properties and solvent-resistance of aqueous dispersion anionic polyurethanes, the product would become discolored. When discoloration posed a problem, high temperature curing was avoided.
It was known to employ chain extenders in forming water-dispersible polyurethanes such as is disclosed in U.S. Pat. No. 3,432,456 issued to Oertel et al. on Mar. 11, 1969 and U.S. Pat. No. 3,475,377 issued to Rosendahl et al. on Oct. 28, 1969. It was not known, however, that aqueous dispersion of anionic polyurethanes could be cured at these high temperatures of over 150.degree. C.
Other related art includes U.S. Pat. No. 3,149,998 issued to Roland. J. Thurmaler on Sept. 22, 1964, discloses the use of hydrazides, semicarbazides and their analogs for the color-stabilization of polyurethanes at ordinary temperatures significantly below 150.degree. C., and not with the present anionic polyurethanes; U.S. Pat. No. 4,147,679 issued to Scriven et al. on Apr. 3, 1979, discloses the use of hydrazines and hydrazides as chain-extenders at low temperatures to improve polyurethane characteristics such as flexibility, hardness, drying, and coating properties; U.S. Pat. No. 3,415,768 issued to Dieterich et al. on Dec. 10, 1968, relates to the low temperature cure of polyisocyanates in combination with diamines such as ethylene diamine and/or alternatively hydrazine as chain-extenders so as to increase mechanical strength and resistance to solvents; and Reiff et al. in U.S. Pat. No. 3,920,598 issued on Nov. 18, 1975, discloses nonionic polyurethanes which are not necessarily suseptable to discoloration at high temperatures. All the foregoing patents are incorporated herein by reference.
It is a principle object of the present invention to provide water based polyurethanes which have high thermal oxidative color stability and yet which also have good physical and mechanical properties.
It is another object of the present invention to provide highly useful water-dispersible anionic polyurethanes which are cured more rapidly by employing high temperatures, particularly when cross linking agents are employed.
It is still a further object of the present invention to provide a process for preparing the aforesaid polyurethanes which is readily and inexpensively performed and practical in commercial utilization.