Structural epoxy resin adhesives containing elastomeric tougheners capped with ketoximes

Abstract

Structural adhesives are prepared from an elastomeric toughener that contains urethane and/or urea groups, and have terminal isocyanate groups that are capped with ketoxime compound. The adhesives have very good storage stability and cure to form cured adhesives that have good lap shear and impact peel strengths, even at −40° C.

1 . A structural adhesive, comprising:
 A) at least one epoxy resin;   B) one or more reactive elastomeric tougheners containing capped isocyanate groups; and   C) one or more epoxy curing agents;   
       wherein at least 90% of the capped isocyanate groups of the reactive tougheners in the structural adhesive are capped with a ketoxime compound. 
     
     
         2 . The structural adhesive of  claim 1 , wherein at least 95% of the capped isocyanate groups of all reactive tougheners in the structural adhesive are capped with a ketoxime compound and 1% or fewer of the capped isocyanate groups are capped with an epoxy-functional capping group. 
     
     
         3 . The structural adhesive of  claim 1 , wherein the reactive toughener(s) contain at least one segment of a polyether or a butadiene homopolymer or copolymer, the segment having a molecular weight of at least 800 daltons. 
     
     
         4 . The structural adhesive of  claim 1 , wherein the epoxy resin includes at least one diglycidyl ether of a polyhydric phenol, the adhesive further comprises a liquid rubber-modified epoxy resin, a core-shell rubber or both a liquid rubber-modified epoxy resin and a core-shell rubber; and the adhesive contains from about 8 to 30 weight percent of the reactive toughener. 
     
     
         5 . A structural adhesive of  claim 1 , which is a one-part adhesive that contains a curing agent and a catalyst, and which cures rapidly at a temperature of 140° C. or higher but slowly if at all at a temperature of 80° C. or lower. 
     
     
         6 . The structural adhesive of  claim 1 , which further contains at least one monomeric or oligomeric, addition polymerizable, ethylenically unsaturated material having a molecular weight of less than about 1500. 
     
     
         7 . The structural adhesive of  claim 1 , which contains at least one epoxide-terminated liquid rubber. 
     
     
         8 . A method comprising applying the structural adhesive of  claim 1  to the surfaces of two metal members, and curing the structural adhesive to form an adhesive bond between the two metal members.