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.

What is claimed is: 
     
         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, wherein the mineral filler in the form of fibers is wollastonite, and the diameter is D50 as measured by microscopy, 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., wherein the curing temperature is the lowest temperature at which the structural adhesive achieves at least 30% of its lap shear strength according to DIN ISO 1465 at full cure within 2 hours.   
     
     
         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 2 , wherein the aspect ratio of the mineral filler in the form of fibers has an aspect ratio of at least 9. 
     
     
         4 . 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. 
     
     
         5 . The one-component epoxy structural adhesive of  claim 2 , which contains 10 to 30 weight percent of the toughener. 
     
     
         6 . The one-component epoxy structural adhesive of  claim 2 , wherein the isocyanate groups of the toughener are capped with a phenol, polyphenol or aminophenol. 
     
     
         7 . 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. 
     
     
         8 . 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. 
     
     
         9 . 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. 
     
     
         10 . The one-component epoxy structural adhesive of  claim 2  which further contains at least one low aspect ratio filler, wherein the aspect ratio of the low aspect ratio filler is 6 or less. 
     
     
         11 . The one-component epoxy structural adhesive of  claim 2  which is devoid of a rubber that does not contain capped isocyanate groups. 
     
     
         12 . 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. 
     
     
         13 . 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. 
     
     
         14 . 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 23 C in accordance with the ISO 11343 wedge impact method at 23° C. 
     
     
         15 . 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 23 C in accordance with the ISO 11343 wedge impact method at 23° C. 
     
     
         16 . 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. 
     
     
         17 . 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 23 C in accordance with the ISO 11343 wedge impact method at 23° C.