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US-12624262-B2 - High modulus, toughened one-component epoxy structural adhesives with high aspect ratio fillers

US12624262B2US 12624262 B2US12624262 B2US 12624262B2US-12624262-B2

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.

Inventors

  • Felix KOCH
  • Sergio Grunder
  • Andreas Lutz

Assignees

  • DDP SPECIALTY ELECTRONIC MATERIALS US, LLC

Dates

Publication Date
20260512
Application Date
20231205

Claims (16)

  1. 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) 5 to 20 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 about 12 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 wherein at least 10 weight-% of component D) has an aspect ratio of at least 20 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. 2 . The one-component epoxy structural adhesive of claim 1 , wherein the aspect ratio of the mineral filler in the form of fibers is at least 9.
  3. 3 . The one-component epoxy structural adhesive of claim 1 , 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.
  4. 4 . The one-component epoxy structural adhesive of claim 1 , which contains 10 to 30 weight percent of the toughener.
  5. 5 . The one-component epoxy structural adhesive of claim 1 , wherein the isocyanate groups of the toughener are capped with a phenol, polyphenol or aminophenol.
  6. 6 . The one-component epoxy structural adhesive of claim 1 , 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.
  7. 7 . The one-component epoxy structural adhesive of claim 1 , wherein the curing agent includes one or more of dicyandiamide, methyl guanidine, dimethyl guanidine, trimethyl guanidine, tetramethyl guanidine, methylisobiguanidine, dimethylisobiguanidine, tetramethylisbiguandidine, heptamethylisobiguanidine, hexamethylisobiguanidine, acetoguanamine and benzoguanamine.
  8. 8 . The one-component epoxy structural adhesive of claim 1 , 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.
  9. 9 . The one-component epoxy structural adhesive of claim 1 , which further contains at least one low aspect ratio filler, wherein the aspect ratio of the low aspect ratio filler is 6 or less.
  10. 10 . The one-component epoxy structural adhesive of claim 1 , which is devoid of a rubber that does not contain capped isocyanate groups.
  11. 11 . The one-component epoxy structural adhesive of claim 1 , 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.
  12. 12 . The one-component epoxy structural adhesive of claim 1 , 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.
  13. 13 . The one-component epoxy structural adhesive of claim 1 , 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.
  14. 14 . The one-component epoxy structural adhesive of claim 1 , 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.
  15. 15 . The one-component epoxy structural adhesive of claim 1 , 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.
  16. 16 . The one-component epoxy structural adhesive of claim 1 wherein the mineral filler in the form of fibers has a diameter of about 12 μm.

Description

This application claims priority to U.S. patent application Ser. No. 17/035,996, filed on Sep. 29, 2020, which claimed priority to U.S. patent application Ser. No. 15/755,080, filed on Feb. 25, 2018 and now abandoned, which claimed priority to International Application No. PCT/US16/049604, filed on Aug. 31, 2016, which claimed priority to U.S. Patent Application Ser. No. 62/216,401, filed on Sep. 10, 2015. All parent applications are incorporated herein by reference in their entireties. This invention relates to one-component epoxy structural adhesives that contain a toughener. Toughened one-component epoxy structural adhesives are used extensively in the automotive and other industries for metal-metal bonding as well as bonding metals to other materials. Often, these structural adhesives must strongly resist failure during vehicle collision situations. Structural adhesives of this type are sometimes referred to as “crash durable adhesives”, or “CDAs”. This attribute is achieved through the presence of certain types of materials in the adhesive formulation. These materials are often referred to as “tougheners”. The tougheners have blocked functional groups that, under the conditions of the curing reaction, can become de-blocked and react with an epoxy resin. Tougheners of this type are described, for example, in U.S. Pat. Nos. 5,202,390, 5,278,257, WO 2005/118734, WO 2007/003650, WO2012/091842, U.S. Published Patent Application No. 2005/0070634, U.S. Published Patent Application No. 2005/0209401, U.S. Published Patent Application 2006/0276601, EP-A-0 308 664, EP 1 498 441A, EP-A 1 728 825, EP-A 1 896 517, EP-A 1 916 269, EP-A 1 916 270, EP-A 1 916 272 and EP-A-1 916 285. These tougheners are highly effective in imparting toughness, as indicated by the ISO 11343 dynamic impact peel strength test. However, the presence of the tougheners greatly reduces the elastic modulus of the cured adhesive. The elastic modulus refers to the slope of the stress-strain curve of the cured adhesive when subjected to tensile testing using methods such as DIN EN ISO 527-1. Because of the low elastic modulus of the cured adhesive, the bonded assembly of substrates and adhesive is more flexible (i.e., less stiff) than is wanted. It is common to incorporate a particulate filler into structural adhesives of this type, for rheological reasons and to decrease cost per unit weight. However, this has been found to impart only a modest increase in elastic modulus, and can result in a significant loss of dynamic impact peel strength. What is desired is a one-component, structural adhesive that when cured exhibits high dynamic impact peel strength and a high elastic modulus. Preferably, these benefits are obtained without significant increase in the viscosity of the uncured adhesive. This invention is 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, andE) 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. The presence of the fibrous filler having the specified dimensions has surprisingly been found to impart a substantial increase in the elastic modulus of the cured adhesive. Surprisingly, this benefit is achieved without significant loss of dynamic impact peel strength or deterioration of other important properties. The invention is also a method comprising forming a layer of the structural adhesive of the invention at a bondline between two substrates, and curing the layer to form an adhesive bond between the two substrates. At least one and preferably both of the substrates may be metals. Suitable epoxy resins include those described at column 2 line 66 to column 4 line 24 of U.S. Pat. No. 4,734,332, incorporated herein by reference. The epoxy resin should have an average of at least 1.8, more preferably at least 1.9 epoxide groups per molecule. Preferably, at least a portion of the epoxy resin(s) is not rubber-modified, meaning that, prior to curing, the epoxy resin is not chemically bonded to a rubber. Suitable epoxy resins include diglycidyl ethers of polyhydric phenol compounds such as resorcinol, catechol, hydroquinone, biphenol, bisphenol A, bisphenol AP (1,1-bis(4-hydroxylphenyl)-1-phenyl ethane), bisphenol F, bisphenol K and tetramethylbiphenol; diglycidyl ethers of aliphatic glycols and polyether glycols such as the diglycidyl ethers of C2-24 alkylene glycols and poly(ethylene oxide) or poly(propylene oxide) glycols; polyglycidyl ethers of phenol-formaldehyde novolac resins (epoxy novolac resins), alkyl substituted phe