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.

Claims

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.

Description

EXAMPLES 1-2 AND COMPARATIVE SAMPLES A-D

[0061] One-component heat-curable structural adhesive Examples 1-2 and Comparative Samples A-D are prepared by mixing the components listed in Table 1.

TABLE-US-00001 TABLE 1 Comp. Comp. Comp. Comp. A B C D Ex. 1 Ex. 2 Non-Rubber Modified 43.97 45.71 48.41 32.66 41.0 34.56 Epoxy Resin A.sup.1 Rubber-Modified Bisphenol A- 0 11.54 7.48 0 0 0 based Epoxy Resin A.sup.2 Rubber-Modified Bisphenol A- 0 0 7.48 0 0 0 based Epoxy Resin B.sup.3 Rubber-Modified Bisphenol F- 23.80 0 0 31.70 23.80 26.50 based Epoxy Resin.sup.4 Reactive Diluent 0 1.14 3.15 0 0 0 Toughener A.sup.5 9.00 0 0 12.00 0 0 Toughener B.sup.6 0 17.31 9.67 0 12 12 Surfactant 0.38 0.68 1.28 0.28 0.3 0.28 Color Pigment 0.3 0.2 0.6 0.3 0.3 0.16 Dicyanamide 5.38 5.08 5.61 5.38 5.1 4.0 Inorganic Fillers 16.72 9.98 11.84 16.72 12.7 4.0 Calcium Oxide 0 4.9 3.65 0 4.0 3.0 Flame Retardant package 0 0 0 0 0 14.8 Polyvinyl butryal 0 2.7 0 0 0 0 2,4,6- 0 0.78 0.82 0 0 0 tris(dimethylaminomethyl)phenol in poly(vinylphenol) matrix Blocked diethylene triamine 0.50 0 0 0.51 0.2 0.1 catalyst 2,4,5- 0 0 0 0 0.6 0.7 tris(dimethylaminomethyl)phenol in novolac resin matrix % Rubber 4.66 4.62 5.98 6.21 4.66 5.20 % Rubber + Toughener 13.8 21.91 15.7 18.2 16.8 17.2 .sup.1Non-Rubber-Modified Epoxy Resin A is a diglycidyl ether of bisphenol A sold by The Dow Chemical Company as D.E.R. 331. It has an epoxy equivalent weight of about 186. .sup.2Rubber-Modified Bisphenol A-Based Epoxy Resin A is a reaction product of 60% by weight of a mixture of a ~180 epoxy equivalent weight diglycidyl ether of bisphenol A with 40% by weight of a carboxyl-terminated butadiene/acrylonitrile rubber sold by Noveon as Hycar 1300X8. .sup.3Rubber-Modified Bisphenol A-Based Epoxy Resin B is a reaction product of 60% by weight of a mixture of a ~180 epoxy equivalent weight diglycidyl ether of bisphenol A with 40% by weight of a carboxyl-terminated butadiene/acrylonitrile rubber sold by Noveon as Hycar 1300X13. .sup.4Rubber-Modified Bisphenol F-Based Epoxy Resin A is a reaction product of 70.2% by weight of a mixture of a ~180 epoxy equivalent weight diglycidyl ether of bisphenol F with 19.6% by weight of a carboxyl-terminated butadiene/acrylonitrile rubber sold by Noveon as Hycar 1300X13, which is further diluted with 10.2% of a solid diglycidyl ether of bisphenol A having an epoxy equivalent weight of 1600-2000 sold by The Dow Chemical Company as D.E.R. 667. .sup.5Toughener A is an isocyanate-terminated polyurethane prepolymer prepared from a polyether polyol and an aliphatic diisocyanate, in which the isocyanate groups are capped with o,o-diallyl bisphenol A, and is made as described in Example 13 of EP 308 664. .sup.6Toughener B is the same as Toughener A, except the capping groups are phenol rather than o,o-diallylbisphenol A.

[0062] Duplicate test coupons are prepared and are evaluated for lap shear strength in accordance with DIN EN 1465, using 2 mm-thick 6111 aluminum alloy coated with DC290 lubricant. Testing is performed at a test speed of 10 mm/minute and at 23 C. Test samples are prepared using each adhesive. The bonded area in each case is 2510 mm. The adhesive layer is 0.2 mm thick. Duplicate test specimens are cured for 30 minutes at 180 C. Results are as indicated in Table 2.

[0063] Impact peel testing is performed in accordance with the ISO 11343 wedge impact method. Testing is performed using an Instron Dynatup 8250 device operated at 2 mm/sec. Test coupons are 100 mm20 mm with a bonded area of 2030 mm The substrate is 0.8 mm-thick cold rolled steel that has been cleaned with acetone before applying the adhesive. A 0.15 mm10 mm wide Teflon tape is applied to the coupons to define a 2030 mm bond area. Impact peel testing is performed on samples cured for various times and at various temperatures as indicated in Table 2. In some cases, impact peel testing is also performed on samples cured for 10 minutes at 160 C. Results are as indicated in Table 2.

[0064] Environmental aging under stress testing is performed as described above. Curing onset temperature is measured by differential scanning calorimetry.

TABLE-US-00002 TABLE 2 Comp. Comp. Comp. Comp. Property A B C D 1 2 Lap Shear (MPa), 25 33.2 33 26.5 35.9 32.9 28.7 min cure @ 180 C. Lap Shear (MPa), 10 23 ND ND ND 28 24.9 min cure @ 160 C. RT Impact Peel (N/mm) 25 minute cure @ 20.0 33 17 33.2 24.8 26.0 180 C. 10 minute cure @ 19.7 ND ND ND 30 25.8 171 C. 10 minute cure @ 11.1 ND ND ND 25.8 21.2 160 C. Environmental Aging Under Stress (No. of Cycles to failure) 25 minute cure @ 52 36 35 47 48 50 180 C. 10 minute cure @ 49 ND ND ND 59 58 160 C. Curing onset temp., 167 ND ND 167 158 154 C. 50 C. Storage 1110 830 830 920 930 1010 modulus, MPa Water uptake, % 1.75 2.2 2.4 2.0 2.1 1.8 Wet G onset temp., 64.9 55.2 50.5 50.5 63.5 58.1 C. NDNot determined

[0065] In Tables 1 and 2, Comparative Sample A represents a baseline case. Without a catalyst as required herein, poor impact peel values are seen at the lower temperature cures, and especially at the 160 C. cure temperature. In addition, environmental aging under stress results at the 160 C. for Comparative Sample A are poorer than at the higher temperature cure. The poorer impact peel and stress aging results correlate to the higher curing onset temperature for this sample, which is 167 C. The sample does not cure well at the lower curing temperature.

[0066] Comparative Samples B and C show the effect of replacing the rubber-modified bisphenol F-based epoxy resin with a rubber modified bisphenol A-based epoxy resin. The environmental aging under stress results fall off dramatically with this change in rubber-modified epoxy resin, even when calcium oxide is present and, as in Comparative Sample C, the amount of rubber is increased.

[0067] Comparative Sample D demonstrates the effect of increasing the amount of rubber and toughener. This increases impact peel strength at the 180 C. cure but not significantly at the 160 C. cure. The curing onset temperature is 167 C. (equal to Comparative Sample A) and inadequate curing is achieved at the 160 C. cure temperature.

[0068] In Examples 1 and 2, the selection of catalyst, type of rubber-modified epoxy resin (bisphenol F type instead of bisphenol A type as in Comparative Samples B and C) and presence of calcium oxide leads to an adhesive which has excellent lap shear strength, excellent impact peel strength even at a 160 C. cure temperature, and also exhibits excellent performance on the environmental aging under stress test even when cured at only 160 C. Quite surprisingly, the environmental aging under stress values after curing at 160 C. are even higher than those obtained with the 180 C. cure.