This invention relates to diisocyanate-modified polyesters and more specifically to their use as hot melt adhesives and coatings.
There is a need for polymeric substances useful as hot melt adhesives which can form structural metal-to-metal bonds. An exceptional hot melt adhesive would form bonds having T-Peel values (ASTM D 1876-69) about 50 lbs./inch-width and a Lap Shear values (ASTM D 1002-64) above 2000 psi. An especially preferred adhesive would not require pretreatment of the bonded surfaces. Further, there is a need for polymeric substances which can adhere to glass, such as bottles, to form a coating which will inhibit shattering on impact. An especially useful coating should withstand multiple washing cycles without appreciable loss of properties.
The prior art discloses the use of diisocyanates to increase the molecular weight of polymers. The prior art also shows adhesives formed from diisocyanates and polyesters where the ratio of isocyanate group to hydroxyl and carboxyl group is about unity. Relatively low-temperature reactions between diisocyanates and polyesters which leave free isocyanate groups for bonding are well known. However, it is well documented in the literature that isocyanate functionalities and urethanes decompose at temperatures above about 470.degree. F. (243.degree. C.). Thickwalled articles produced from a diisocyanate-modified polyester are described in U.S. Pat. No. 3,553,157. Buck et al., U.S. Pat. No. 2,876,725 discloses a method of sealing a metal seam using a copolyester heated to 300.degree. to 550.degree. F. In one example, Buck et al. include a triisocyanate within their copolyester adhesive which was heat activated in a seam at 425.degree. F. The copolyesters of Buck et al. are produced by reacting a diol with a mixture of aromatic and aliphatic diacids. Other relevant prior art include U.S. Pat. Nos. 3,115,479; 3,373,143; 3,097,961; 2,982,754; and British Pat. Nos. 610,140 and 684,400.