Epoxy adhesive resistant to open bead humidity exposure

Abstract

Epoxy adhesives are made using core-shell rubbers and small amounts if any of other elastomeric materials. The epoxy adhesives contain a mixture of latent curing agents and/or high levels of calcium oxide. The adhesives exhibit excellent bonding properties even when an open or closed bead is exposed to humid air for prolonged periods before the epoxy adhesive is cured.

Claims

1. A one-component, heat-curable epoxy adhesive comprising in admixture: A) 45 to 75 weight percent, based on the weight of the epoxy adhesive, of a mixture of non-rubber-modified epoxy resins containing at least 80% by weight of a polyphenol polyglycidyl ether or a mixture of two or more polyphenol polyglycidyl ethers, wherein the mixture of non-rubber-modified epoxy resins is a liquid at 23° C. and has an epoxy equivalent weight of 175 to 250; B) 12 to 25 weight percent, based on the weight of the epoxy adhesive, of core-shell rubber particles; C) 5 to 25 weight percent, based on the weight of the epoxy adhesive, of one or more particulate inorganic fillers that include calcium oxide, wherein the calcium oxide constitutes at least 5 weight percent of the weight of the epoxy adhesive; and D) two or more latent epoxy curing agents including a first latent epoxy curing agent that becomes activated when heated to a temperature within the range of 60 to 120° C. and a second epoxy curing agent that becomes activated when heated to a temperature of at least 140° C., the two or more epoxy curing agents together being present in an amount sufficient to cure the epoxy adhesive, wherein the epoxy adhesive contains no more than 2 weight percent, based on the weight of the mixture of non-rubber-modified epoxy resins, of elastomeric compounds terminated in epoxy or epoxy-reactive groups and/or which contain urethane and/or urea groups.

2. The one-component, heat curable epoxy adhesive of claim 1, further comprising a latent catalyst.

3. The one-component, heat-curable epoxy adhesive of claim 2 wherein the second latent epoxy curing agent that becomes activated when heated to a temperature of at least 140° C. includes one or more of dicyandiamide, methyl guanidine, dimethyl guanidine, trimethyl guanidine, tetramethyl guanidine, methylisobiguanidine, dimethylisobiguanidine, tetramethylisbiguandidine, heptamethylisobiguanidine, hexamethylisobiguanidine, acetoguanamine and benzoguanamine.

4. The one-component, heat-curable epoxy adhesive of claim 3 wherein the first latent epoxy curing agent that becomes activated when heated to a temperature within the range of 60 to 120° C. includes at least one of ethylene diamine, diethylene triamine, triethylene tetraamine and tetraethylene pentaamine blocked with a blocking group selected from one or more of phenol, an alkyl-substituted phenol, benzyl alcohol, an alkyl-substituted benzyl alcohol, or an aliphatic carboxylic acid having 2 to 20 carbon atoms.

5. The one-component, heat curable epoxy adhesive of claim 2 wherein the mixture of non-rubber-modified epoxy resins includes at least one liquid diglycidyl ether of bisphenol A, at least one solid diglycidyl ether of bisphenol A and at least one liquid or solid diglycidyl ether of bisphenol F, and up to 5% monohydrolyzed species that are present as impurities in one or more of the non-rubber-modified epoxy resins.

6. The one-component, heat curable epoxy adhesive of claim 2 wherein component (C) includes fumed silica.

7. The one-component, heat curable epoxy adhesive of claim 2 wherein the mixture of non-rubber-modified epoxy resins, the core-shell rubber particles, the particulate inorganic fillers, the latent curing agents and latent curing catalyst together constitute at least 93% by weight of the epoxy adhesive.

8. The one-component, heat curable epoxy adhesive of claim 1 wherein the mixture of non-rubber-modified epoxy resins, the core-shell rubber particles, the particulate inorganic fillers and the two or more latent curing agents together constitute at least 93% by weight of the epoxy adhesive.

9. The one-component, heat curable epoxy adhesive of claim 1 containing no more than 1 weight percent, based on the weight of the epoxy adhesive, of elastomeric compounds terminated in epoxy or epoxy-reactive groups and/or which contain urethane and/or urea groups.

10. The one-component, heat curable epoxy adhesive of claim 1 containing no more than 0.25 weight percent, based on the weight of the epoxy adhesive, of elastomeric compounds terminated in epoxy or epoxy-reactive groups and/or which contain urethane and/or urea groups.

11. A method comprising forming a layer of an epoxy adhesive of claim 1 at a bondline between two substrates, and curing the layer to form an adhesive bond between the two substrates.

12. The method of claim 11 including the steps of applying the epoxy adhesive onto one of the substrates in the form of a bead having a cross-sectional area of 20 mm.sup.2 or less, exposing the bead to air containing at least 5 g of water vapor per cubic meter for a period of at least 48 hours, then applying the second substrate to the adhesive bead to form an adhesive layer at a bondline and curing the adhesive layer at the bondline to form the adhesive bond.

Description

EXAMPLES 1-3 AND COMPARATIVE SAMPLES A-D

(1) One-component, heat-curable epoxy adhesives of the first aspect of the invention (Examples 1-3) and Comparative Samples A-D are prepared by blending ingredients as indicated in Table 1:

(2) TABLE-US-00001 TABLE 1 Sample Designation A* B* C* D* 1 2 3 Ingredient Parts by Weight Epoxy Resin A 40.7 40.7 40.7 40.7 0 0 0 Epoxy Resin B 0 0 0 0 19.4 19.4 19.4 Epoxy Resin C 0 0 0 0 6 6 6 CSR Dispersion 0 0 0 0 52.0 51.1 51.1 Rubber-Modified 23.8 23.8 23.8 23.8 0 0 0 Epoxy Resin Elastomeric Toughener 12.0 12.0 12.0 12.0 0 0 0 CaO 6.0 10.0 14.0 18.0 6.0 8.3 6.0 CaCO.sub.3 0 0 0 0 0 0 0 Pigment 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Calcium Metasilicate 6.0 2.0 0 0 3.2 2.4 1.4 Fumed Silica 5.1 5.1 3.1 3.1 2.0 2.0 2.0 Glass Balloons 0 0.3 0.3 0.3 0 0 0 DICY 5.1 5.1 5.1 5.1 5.1 5.1 5.1 Ancamine 2014AS 0 0 0 0 5.0 5.0 8.3 Catalyst A 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Catalyst B 0.6 0.6 0.6 0.6 0 0 0 A-187 0.3 0.3 0.3 0.3 0.3 0.3 0.3 % Epoxy Resins ~55% ~55% ~55% ~55% 56.6 56.1 56.1 % Core-Shell Rubber 0 0 0 0 20.8 20.4 20.4 % CaO 6 10 14 18 6 8.3 6 % Particulate 17.4 17.7 17.3 20.9 11.5 13.0 9.7 Inorganic Fillers *Not an example of this invention.

(3) A bead of Comparative Adhesive A, 5 mm high and 8 mm wide with a circular top, is applied to a 2 mm-thick sheet of 5754 aluminum alloy coated with an Alcoa 951 conversion coating and a silicone lubricant. The bead is exposed to 35° C./80% relative humidity conditions (corresponding to 31.7 g of water vapor/cubic meter) for 72 hours. A second aluminum sheet is then applied on top of the adhesive bead, to form an ASTM D1876 test panel having a 0.25 mm adhesive bond line thickness and 76 mm overlap. Excess adhesive is removed. The assembly is cured at 205° C. for 30 minutes, cooled, and T-peel strength is tested at a pull rate of 127 mm/minute. The bond strength is 9.7 N/mm. The failure mode is cohesive failure.

(4) When this experiment is repeated with an adhesive bead only 3 mm high and 4 mm wide, the T-peel strength falls to 7 N/mm. The cured adhesive is seen to be highly porous. These experiments demonstrate the sensitivity of Comparative Adhesive A to aging. The smaller bead leads rapidly to poor results.

(5) The foregoing experiment is repeated using each of Comparative Samples A-D and Examples 1-3, except the bead size in each case is 2.5 mm high and 6 mm wide. Results are as indicated in Table 2.

(6) TABLE-US-00002 TABLE 2 Sample Designation A* B* C* D* 1 2 3 T-Peel Strength, N/mm 7.2 6.4 5.8 6.7 9.7 10.0 8.7 Porosity Rating.sup.1 1 1 1 1 2 2 2 *Not an example 0of this invention. .sup.11-porous through adhesive layer. 2. Porosity only at the edges of the adhesive layer.

(7) As shown by the data in Table 1, Examples 1-3 exhibit markedly better T-peel strength and much less porosity than Comparative Samples A-D. Notably, increasing the amount of calcium oxide in the Comparative Samples does not improve porosity and in addition causes T-peel strength to deteriorate.

EXAMPLES 4-6 AND COMPARATIVE SAMPLE E

(8) Examples 4-6 (of the second aspect of the invention) and Comparative Sample E are made and tested in in the previous examples. The formulations for these epoxy adhesives are given in Table 3. Results are as in Table 4.

(9) TABLE-US-00003 TABLE 3 Sample Designation E* 4 5 6 Ingredient Parts by Weight Epoxy Resin B 19.4 19.4 19.4 19.4 Epoxy Resin C 6.0 6.0 6.0 6.0 CSR Dispersion 51.1 51.1 51.1 51.1 CaO 6.0 10.0 14.0 18.0 Pigment 0.3 0.3 0.3 0.3 Calcium Metasilicate 8.0 4.0 0 0 Fumed Silica 2.0 2.0 2.0 2.0 Glass Balloons 1.0 1.0 1.0 1.0 DICY 5.1 5.1 5.1 5.1 Catalyst A 0.2 0.2 0.2 0.2 Catalyst B 0.6 0.6 0.6 0.6 A-187 0.3 0.3 0.3 0.3 % Epoxy Resins ~56.1% ~56.1% ~56.1% 53.9 % Core-Shell Rubber 20.4 20.4 20.4 19.6 % CaO 6 10 14 17.3 % Particulate Inorganic 17.3 17.3 17.3 16.6 Fillers *Not an example of this invention.

(10) TABLE-US-00004 TABLE 4 Sample Designation E* 4 5 6 T-Peel Strength, N/mm 8.8 11.5 12.3 12.0 Porosity Rating.sup.1 1 2 3 3 *Not an example of this invention. .sup.11-porous through adhesive layer. 2. Porosity only at the edges of the adhesive layer. 3. No visible porosity.

(11) As shown by the data in Table 4, increasing the amount of calcium oxide from 6 to 10 weight percent increases T-peel strength significantly and reduces porosity. Further increasing the amount of calcium oxide further increases the T-peel strength and reduces porosity substantially. Note that the effect of increasing the amount of calcium oxide is different in this set of samples than is seen in Comparative Samples A-D above. In the presence of a urethane toughener and rubber-modified epoxy resin, increasing calcium oxide has a deleterious effect. When those materials are replaced with a core-shell rubber, increasing calcium oxide leads to very significant improvements.

EXAMPLES 7-11 AND COMPARATIVE SAMPLE F

(12) Examples 7-11 (of the second aspect of the invention) and Comparative Sample F are made and tested in in the previous examples, except the bead size is only about 2.1 mm high and 5 mm wide. The formulations for these epoxy adhesives are given in Table 5. Results are as in Table 6.

(13) TABLE-US-00005 TABLE 5 Sample Designation 7 8 9 10 11 F* Ingredient Parts by Weight Epoxy Resin A 0 0 0 23.4 23.4 0 Epoxy Resin B 19.4 0 0 0 0 0 Epoxy Resin C 6.0 25.4 17.0 0 0 0 Epoxy Resin D 0 0 0 0 0 25.4 CSNL 0 0 0 2.0 2.7 0 CSR Dispersion 51.1 51.1 59.4 51.1 51.1 51.1 CaO 18.0 18.0 18.0 18.0 18.0 18.0 Pigment 0.4 0.4 0.4 0.4 0.4 0.4 Fumed Silica 2.0 2.0 2.0 2.0 3.0 2.0 Glass Balloons 2.0 2.0 2.0 2.0 2.0 2.0 DICY 5.1 5.1 5.1 5.1 5.1 5.1 Catalyst A 0.2 0.2 0.2 0.2 0.2 0.2 Catalyst B 0.6 0.6 0.6 0.6 0.6 0.6 A-187 0.3 0.3 0.3 0.3 0.3 0.3 % Epoxy Resins 53.9% 53.9 50.2% 52.6 50.2 53.9 % Core-Shell Rubber 19.6 19.6 22.8 19.6 19.6 19.6 % CaO 17.3 17.3 17.3 17.3 17.2 17.3 % Particulate Inorganic 21.5 21.5 21.5 21.5 21.3 21.5 Fillers *Not an example of this invention.

(14) TABLE-US-00006 TABLE 6 Sample Designation 7 8 9 10 11 F* T-Peel Strength, N/mm 9.0 9.0 10.4 9.2 10.6 7.2 Porosity Rating.sup.1 2 2 2 2 2 1 *Not an example of this invention. .sup.11. Porous through adhesive layer. 2. Porosity only at the edges of the adhesive layer.

(15) The data in Table 6 shows the effect of varying the epoxy resin. Good results are obtained when the epoxy resin mixture contains 90% by weight or more polyphenol polyglycidyl ethers, even when some diluent is present (Example 11). Conversely, including a large amount (about 40% of the epoxy resin mixture) of another epoxy resin (Comparative Sample F) leads to a large increase in porosity and large loss in T-peel strength.