Polyurethane systems used in the manufacture of finished polyurethane resin bodies or articles can be classified in many ways, according to both chemical derivation and to type of final fabrication process. For example, a given polyurethane system may be classified according to the type of backbone (e.g., polyether or polyester), type of isocyanate employed (aromatic or aliphatic), type of chain extender (e.g., polyol, polyamine, water, etc.) and according to many other reactive components, additives, solvents, etc., that may be employed at one point or another, if not in the final processing step. Moreover, under each principal category there may be, and generally are, two or more subclassifications which are of commercial importance. For example, two important subclasses of polyether urethanes are the polyoxypropylene/ethylene class and the polyoxytetramethylene class.
With respect to type of final fabrication process, polyurethane resin-forming systems may be classified as belonging to one of two principal groups: (1) the group in which a chemical reaction of the polyurethane resin-forming system occurs in the final processing step to provide a polyurethane resin with mature physical properties, as is usually the case in polyurethane foam systems and castable elastomer-forming systems, for example; and, (2) the group in which there is essentially no chemical reaction in the final processing step, as in the case of polyurethane thermoplastic resins which are employed in thermoforming operations and in certain adhesive and coating applications.
In systems of group (1), that is, polyurethane resinforming systems in which a high molecular weight polymer resin with mature physical properties is first obtained in the forming step in which the end-use article is produced, it is generally desirable that the reactive polyurethane resin-forming system have a pot life sufficiently long to enable convenient handling, such as the filling of a mold with the reactive system in a liquid state, together with rapid curing characteristics which require a minimum of heat input to supplement the natural heat of reaction of the system. Such curing characteristics are desirable in the interest of energy conservation and minimum and demold time and/or curing time, which factors relate to the ultimate manufactured cost of the finished polyurethane resin bodies.
In recent years, as energy costs and labor and production overhead costs have increased sharply, great attention has been given new techniques for reduction of curing energy requirements and the combined production time and investment factors as they relate to the costs of finished articles. For example, new manufacturing techniques such as reaction injection molding (RIM) have been developed for the manufacture of molded polyurethane resin bodies as well as for molded bodies derived from other types of reactive polymer systems. Among the advantages of the RIM molding technique is the facile employment of highly reactive systems with short pot lives. Not only can these systems be demolded more rapidly than systems used in traditional molding operations but, as a result of the greater reactivity, post-curing time and energy requirements often are sharply reduced as well.
Also, especially in the manufacture of polyurethane foams, there is a trend toward utilization of systems which offer improved curing characteristics even though the purchased costs of the chemicals are greater than the costs of alternate systems which provide adequate final physical properties. In many cases the "premium" systems provide the lower final manufactured costs by means of increased production rates and/or reduced time/temperature post-cure requirements. Moreover, the polymer resin physical properties provided by the premium systems generally are superior to those of standard systems and, in the end-use application, an additional cost/performance advantage may be realized which complements the original manufactured cost advantage of the finished polyurethane article.
The advantages of RIM and other advanced polyurethane resin fabrication techniques notwithstanding, there is a continuing search for polyurethanes resin-forming systems of group (1) with improved pot life/curing requirement characteristics. If pot life can be increased without increasing demold time and/or post-curing time/temperature requirements, it is generally the case in a given system that catalysis can be employed so as to further reduce post-cure requirements while keeping the pot life above the minimum acceptable level. Therefore new systems which, without catalysis, provide improved combinations of pot life and curing characteristics, are of great general interest. As a great variety of catalysts for urethane systems is commercially available, and in view of the fact that catalysis in this field has been extensively studied (see, for example, J. H. Saunders and K. C. Frisch, "Polyurethanes: Chemistry and Technology," Part I, Interscience, New York-London (1962) pp. 129-211), it is then a comparatively straightforward matter to catalyze any such inherently improved polyurethane resin-forming system so as to optimize its pot-life/curing characteristics for a given application.
If a given polyurethane resin-forming system is considered as a reference point with respect to the combination of pot life and curing characteristics which it provides, there are a number of independent variables which can generally be manipulated for the purpose of improving that combination of properties. That is, for the purpose of extending the pot life without increasing the cure requirements of time and/or temperature; for the purpose of reducing the cure requirements without shortening the pot life; or, hopefully, for the purpose of simultaneously extending the pot life while reducing the cure requirements. Such independent variables include, among others, the starting temperature of the reactants when they are first mixed to provide the reactive polyurethane resin-forming system; external heat input to the reaction mixture; the choice of catalyst and concentration thereof; and, the choice of reactants and their relative proportions.
Although variation of all of the above independent variables is necessary in general in the process of optimizing a polyurethane resin-forming system for a given application, the choice of reactants provides the greatest opportunity for adjustment of the characteristics of the reacting polyurethane resin-forming system as well as the physical properties which it will exhibit in its final cured state. Even though at the present time there are dozens of polyisocyanate products and hundreds of polyols and other reactive hydrogen compounds commercially available and economically feasible for use in formulation of new polyurethane resin-forming systems, there is a continuing effort to develop new reactants which impart improved combinations of pot life and curing characteristics and/or final physical properties. These development efforts are directed toward all three principal types of reactive intermediates employed in polyurethane resin-forming systems: the polyisocyanates, the polyol resins, and the low-molecular-weight intermediates commonly termed "chain extenders". For example, in the case of polyol resins, there has been a major effort in recent years to develop new types of "capped" polyether polyols; these are starting polyoxypropylene polyols which have been further reacted with ethylene oxide, principally for the purpose of providing a higher ratio of primary to secondary hydroxyl groups in the finished polyether polyol. Primary hydroxyl groups are substantially more reactive with isocyanates than are secondary hydroxyl groups, and the "capped" polyether polyols therefore offer, among other features, improved curing characteristics in polyurethane resin-forming systems. Such polyols are particularly useful in polyurethane foam-forming systems.
Since, on a weight basis, the polyol resin component is typically the principal component of polyurethane resin-forming systems, often accounting for more than half the total weight of reactive components, it is particularly desirable to improve individual reactivity characteristics of the polyol component, if at all possible, in any effort to improve the overall reactivity and curing characteristics of the polyurethane resin-forming system. However, with the notable exception of the aforementioned capped polyether polyols, it is generally difficult to make such improvements without bringing into play accompanying performance and/or economic disadvantages. For example, if the hydroxyl functionality of the polyol is increased so as to shorten demold time, many physical properties of the finished polyurethane resin product will be affected and the overall result generally is undesirable; the optimum polyol resin functionality usually is predetermined on the basis of final physical property considerations rather than pot life and/or curing characteristic considerations. Similarly, if some or all of the hydroxyl groups of the polyol resin are replaced by other reactive hydrogen groups (leaving aside the question of whether such modification is economically and/or technically feasible) the pot life and curing characteristics can indeed be significantly changed; but, in general where there is an improvement in one characteristic, the other will be adversely affected. If some or all of the hydroxyl groups of a polyol resin are replaced by primary amino groups, for example, the curing time and curing energy requirements of a given derived polyurethane resin-forming system will be sharply reduced. However, the pot life of the system also will be sharply reduced, and there will be other behavioral changes in both the reacting system and the final, cured product, which other changes will generally not be acceptable.
In polyurethane resin-forming systems, particularly in those where the polyol resin component is di-functional (the functionality being predetermined for purposes of achieving certain final polyurethane physical properties), curing to a demoldable or handleable state often involves some degree of secondary, network-forming reactions of residual isocyanate such as the reaction of residual isocyanate with carbamate N--H groups formed in an earlier stage of the polymerization process. Other reactive hydrogen-containing groups, when present, also are involved in like reactions with residual isocyanate. The carboxamide group, --[NHC.dbd.O]--, for example, has been incorporated in polyol resins as a reactivity and final physical property modifier, but not in the proportions and distribution, or for the purposes contemplated by this invention.
For example, polyester polyol resins generally are manufactured by means of condensation polymerization reactions in which it is not possible to control the distribution of carboxamide groups among individual polyester molecules. In such reactions, an average of one or two carboxamide groups per polyester polyol molecule can be built in by inclusion of the appropriate amounts of, for example, monoethanolamine or hexamethylenediamine, respectively, but the actual polyester polyol product will be comprised of a significant proportion of molecules with no carboxamide groups, some molecules with the desired number (one or two) carboxamide groups, and another significant proportion of molecules with a surplus of carboxamide groups. While such mixtures are capable of offering some improvement in pot life/curing characteristics without undue sacrifice of other desirable behavioral characteristics, the overall improvements by these means are not as great as might be desired. While some polyester polyols containing a molecularly-uniform distribution of the carboxamide group have been disclosed in the prior art in connection with various polymer applications, including polyurethane resins (see, for example, U.S. Pat. Nos. 2,933,477; 2,933,478; 2,990,379; 3,169,945; and, 3,186,971), there has heretofore been no recognition of the surprising and unexpected behavior obtained in accordance with the invention by the incorporation of such special carboxamide-modified polyols in a polyurethane resin-forming system.