Crash-Durable Adhesive with Enhanced Stress Durability

Abstract

A heat-curable structural adhesive includes a non-rubber-modified epoxy resin, a reaction product of a carboxyl- or amine-terminated butadiene polymer or copolymer and a bisphenol F-based epoxy resin, a elastomeric toughener containing capped isocyanate groups, one or more epoxy curing agents a moisture scavenger and a heat activatable catalyst comprising a mixture of a solid or liquid tertiary amine compound that has a boiling temperature of at least 130° C. and a novolac resin. The structural adhesive develops excellent bonding properties when cured at moderate temperatures, especially from 120 to 170° C., and in addition performs very well in environmental aging stress testing. The adhesive is particularly good for bonding aluminum to another metal, or bonding aluminum to aluminum.

What is claimed is: 
     
         1 . A heat-curable structural adhesive comprising
 A) at least one non-rubber-modified epoxy resin;   B) a reaction product of a carboxyl- or amine-terminated butadiene polymer or copolymer and a bisphenol F-based epoxy resin;   C) at least one elastomeric toughener containing capped isocyanate groups;   D) one or more epoxy curing agents;   E) from 0.5 to 10 weight percent of a moisture scavenger, based on the total weight of the heat-curable structural adhesive; and   F) a heat activatable catalyst comprising a mixture of a solid or liquid tertiary amine compound that has a boiling temperature of at least 130° C. and a novolac resin, and further wherein the elastomeric toughener and the rubber portion of the rubber-modified epoxy resin together constitute from 15 to 25% of the total weight of the heat-curable adhesive.   
     
     
         2 . The heat-curable structural adhesive of  claim 1  which
 a) exhibits an impact shear strength of at least 20 N/mm as measured according to the ISO 11343 wedge impact method on 2 mm-thick aluminum 6111 alloy substrates after curing at 160° C. for 10 minutes, 
 b) exhibits a storage modulus of at least 900 MPa at 50° C. as measured by dynamic mechanical analysis according to ASTM E2254-09; and/or 
 c) withstands at least 45 cycles of the environmental aging under stress test after curing for 10 minutes at 160° C. 
 
     
     
         3 . The heat-curable structural adhesive of  claim 2  wherein the storage modulus at 50° C. is up to 1400 MPa. 
     
     
         4 . The heat-curable structural adhesive of  claim 3  which when cured exhibits a water absorption of no greater than 2.2% by weight. 
     
     
         5 . The heat-curable structural adhesive of  claim 4  wherein component A) has an average epoxy equivalent weight of 170 to 600 and includes at least one diglycidyl ether of a polyhydric phenol compound. 
     
     
         6 . The heat-curable structural adhesive of  claim 5  in which the elastomeric toughener and the rubber portion of the rubber-modified epoxy resin together constitute from 16 to 22% of the total weight of the heat-curable adhesive. 
     
     
         7 . The heat-curable structural adhesive of  claim 6  wherein the elastomeric toughener constitutes from 5 to 16% of the total weight of the heat-curable adhesive. 
     
     
         8 . The heat-curable structural adhesive of  claim 7  wherein the novolac resin is a phenol-formaldehyde resin that softens at a temperature of 130 to 200° C. 
     
     
         9 . The heat-curable structural adhesive of  claim 8  wherein the moisture scavenger is calcium oxide, and is present in an amount of 2 to 5% of the total weight of the structural adhesive. 
     
     
         10 . The heat-curable structural adhesive of  claim 9  which contains at least 0.5% by weight of component F). 
     
     
         11 . The heat-curable structural adhesive of  claim 10  wherein the isocyanate groups of the elastomeric toughener are capped with a monophenolic compound, a primary or secondary aliphatic, cycloaliphatic or aromatic monoamine, a monothiol compounds or a benzyl amine. 
     
     
         12 . The heat-curable structural adhesive of  claim 11  wherein the isocyanate groups of the elastomeric toughener are capped with a polyphenol or an aminophenol. 
     
     
         13 . A method for bonding an aluminum member to a second metal member, comprising forming a layer of a heat-curable structural adhesive of  claim 1  between and in contact with the aluminum member and the second metal member to form an assembly and then heating the assembly including the structural adhesive at a temperature of at least 120° C. up to 170° C. to cure the structural adhesive and form an adhesive bond between the aluminum member and the second metal member. 
     
     
         14 . The method of  claim 13  wherein the second metal member is welded to the aluminum member after the layer of heat-curable structural adhesive is formed and before the structural adhesive is cured. 
     
     
         15 . The method of  claim 14  wherein a coating is applied to the assembly before the structural adhesive is cured. 
     
     
         16 . The method of  claim 15  wherein the structural adhesive and the coating are cured simultaneously. 
     
     
         17 . The method of  claim 16  wherein the second metal member is aluminum. 
     
     
         18 . The method of  claim 17  wherein the assembly is all or a portion of a vehicular frame.