This application relates to a two-part polyurethane based adhesive useful in bonding porous and nonporous materials. The adhesive is especially useful in bonding glass into window frames, for example windshields into automobiles.
Polyurethane adhesive compositions typically comprise at least one urethane prepolymer. Adhesives useful for bonding to nonporous substrates, such as glass, to metal, are well known (see U.S. Pat. Nos. 4,374,237 and 4,687,533 incorporated herein by reference). When glass is installed in automobiles on a production line, a one-part moisture curable polyurethane adhesive is preferably used, as the equipment needed for the application of such an adhesive in an automobile assembly plant is less expensive than the equipment needed to apply a two-part adhesive. One part polyurethane adhesives are disclosed in U.S. Pat. Nos. 4,374,237 and 4,687,533.
In the automotive after market replacement industry, glass is often bound into automobiles through the use of two-part moisture curable polyurethane adhesives. Two-part polyurethane adhesives are used because they offer rapid initial cure allowing for rapid drive-away times. Two-part polyurethane adhesives are well known, see for example U.S. Pat. No. 4,835,012, incorporated herein by reference, and DE 4,210,277. Despite the relatively rapid drive-away times afforded by two-part polyurethane adhesives, the market demands faster curing adhesives which allow even faster drive away times, for instance about 60 minutes from application and more preferably about 30 minutes from application. If the adhesive cures too rapidly, then the window installer loses the necessary time to install and properly place the glass into the frame before the adhesive becomes too intractable to work with. Working time is defined as the period from application of adhesive until the adhesive becomes too intractable to work with, and is preferably about 10 to 12 minutes.
Future regulations in the U.S. will require dual airbags in vehicles. During crashes the airbags inflate and exert additional pressure on the windshield. Federal Transportation Agency regulations require that windshield remain in place at crashes of up to 30 mph (48 KPH). This requires adhesives which have enhanced strength at the drive away time. To meet such a standard, the lap shear strengths of the adhesive should be preferably 150 psi (1033 kPa) at the designated drive away time as determined according to ASTM D-3163.
Therefore, what is needed is a two-part polyurethane adhesive which facilitates faster drive away time, which meets the strength requirements defined above and which still provides for a reasonable working time to facilitate proper placement of glass in window frames.