High modulus, toughened one-component epoxy structural adhesives with high aspect ratio fillers

Abstract

One-component toughened epoxy structural adhesives contain fibrous mineral fillers that have an aspect ratio of ≥6. The presence of the fibrous filler leads to a significant increase in elastic modulus while having little if any adverse effect on other properties such as dynamic impact peel resistance.

1 . A one-component epoxy structural adhesive comprising in admixture:
 A) at least one epoxy resin;   B) a reactive toughener containing urethane and/or urea groups and capped isocyanate groups;   C) one or more epoxy curing agents;   D) 1 to 40 weight percent, based on the total weight of the epoxy structural adhesive, of a mineral filler in the form of fibers having a diameter of 3 μm to 25 μm and an aspect ratio of at least 6 , and   E) one or more epoxy curing catalysts,   
       wherein the curing agent(s) and epoxy curing catalyst(s) are selected together such that the structural adhesive exhibits a curing temperature of at least 60° C. 
     
     
         2 . The one-component epoxy structural adhesive of  claim 1 , which contains 5 to 20 weight percent of component D). 
     
     
         3 . The one-component epoxy structural adhesive of  claim 1  or  2 , wherein the mineral filler in the form of fibers is wollastonite. 
     
     
         4 . The one-component epoxy structural adhesive of  claim 2 , wherein the aspect ratio of the mineral filler in the form of fibers has an aspect ratio of at least 9. 
     
     
         5 . The one-component epoxy structural adhesive of  claim 2 , which contains 40 to 70% by weight of component A), and component A) includes one or more bisphenol diglycidyl ethers having an epoxy equivalent weight of 170 to 400. 
     
     
         6 . The one-component epoxy structural adhesive of  claim 2 , which contains 10 to 30 weight percent of the toughener. 
     
     
         7 . The one-component epoxy structural adhesive of  claim 2 , wherein the isocyanate groups of the toughener are capped with a phenol, polyphenol or aminophenol. 
     
     
         8 . The one-component epoxy structural adhesive of  claim 2  wherein the toughener is made in a process that includes the steps of forming an isocyanate-terminated prepolymer by reacting an excess of a polyisocyanate with a 300 to 3000 equivalent weight polyol to form a prepolymer, optionally chain-extending the prepolymer and then capping the isocyanate groups of the prepolymer or chain-extended prepolymer. 
     
     
         9 . The one-component epoxy structural adhesive of  claim 2  wherein the curing agent includes one or more dicyandiamide, methyl guanidine, dimethyl guanidine, trimethyl guanidine, tetramethyl guanidine, methylisobiguanidine, dimethylisobiguanidine, tetramethylisbiguandidine, heptamethylisobiguanidine, hexamethylisobiguanidine, acetoguanamine and benzoguanamine. 
     
     
         10 . The one-component epoxy structural adhesive of  claim 2 , wherein the catalyst includes one or more of p-chlorophenyl-N,N-dimethylurea, 3-phenyl-1,1-dimethylurea, 3,4-dichlorophenyl-N,N-dimethylurea, N-(3-chloro-4 -methylphenyl)-N′,N′-dimethylurea, benzyldimethylamine, 2,4,6-tris(dimethylaminomethyl)phenol, piperidine 2-ethyl-2-methylimidazol, N-butylimidazol, 6-caprolactam and 2,4,6-tris(dimethylaminomethyl)phenol. 
     
     
         11 . The one-component epoxy structural adhesive of  claim 2  which further contains at least one low aspect ratio filler. 
     
     
         12 . The one-component epoxy structural adhesive of  claim 2  which is devoid of a rubber that does not contain capped isoycanate groups. 
     
     
         13 . The one-component epoxy structural adhesive of  claim 2 , which exhibits an elastic modulus of at least 2200 MPa when cured for 30 minutes at 180° C. and tested in accordance with DIN EN ISO 527-1. 
     
     
         14 . The one-component epoxy structural adhesive of  claim 2 , which exhibits an elastic modulus of at least 2500 MPa when cured for 30 minutes at 180° C. and tested in accordance with DIN EN ISO 527-1. 
     
     
         15 . The one-component epoxy structural adhesive of  claim 2  which exhibits a dynamic impact peel strength of at least 20 N/mm when applied between oily 1.0 mm thick HC420LAD+Z100 steel coupons, cured at 180° C. for 30 minutes and then tested at 23C in accordance with the ISO 11343 wedge impact method at 23° C. 
     
     
         16 . The one-component epoxy structural adhesive of  claim 2  which exhibits a dynamic impact peel strength of at least 25 N/mm when applied between oily 1.0 mm thick HC420LAD+Z100 steel coupons, cured at 180° C. for 30 minutes and then tested at 23C in accordance with the ISO 11343 wedge impact method at 23° C. 
     
     
         17 . A method comprising forming a layer of the structural adhesive of  claim 1  at a bondline between two substrates, and curing the layer to form an adhesive bond between the two substrates. 
     
     
         18 . The method of  claim 17  wherein at least one of the substrates is a metal. 
     
     
         19 . The one-component epoxy structural adhesive of  claim 2 , wherein at least 10 weight-% of component D) has an aspect ratio of at least 20. 
     
     
         20 . The one-component epoxy structural adhesive of  claim 19  which exhibits a dynamic impact peel strength of at least 30 N/mm when applied between oily 1.0 mm thick HC420LAD+Z100 steel coupons, cured at 180° C. for 30 minutes and then tested at 23C in accordance with the ISO 11343 wedge impact method at 23° C.