ONE-COMPONENT TOUGHENED EPOXY ADHESIVES

Abstract

One-component epoxy adhesives containing a phosphorus-modified epoxy resin, a toughener and an epoxy resin that is neither rubber-modified nor phosphorus-modified. These adhesives are structural adhesives useful in automotive applications. They exhibit particularly good corrosion resistance.

Claims

1. A one-component epoxy adhesive comprising in admixture A) a non-rubber-modified, non-phosphorous-modified epoxy resin or mixture thereof, the non-rubber-modified, non-phosphorous-modified epoxy resin or mixture thereof being a liquid at 23 C., B) one or more reactive urethane group- and/or urea group-containing polymers having a number average molecular weight of up to 35,000, at least one polyether and/or diene rubber segment having a weight of at least 1000 atomic mass units, and capped isocyanate groups, C) at least one epoxy curing catalyst, D) a curing agent and E) 3.5 to 50 weight-%, based on the weight of the adhesive, of an epoxy-containing adduct of an epoxy resin and a phosphorus acid, said one-component toughened epoxy adhesive containing no more than 2 parts by weight of a plasticizer per part by weight of component B) and containing no more than 7 weight percent of core-shell rubber particles, and wherein the adhesive exhibits a curing temperature of at least 60 C.

2. The one-component epoxy adhesive of claim 1, wherein component E is a reaction product of at least one epoxy resin and a phosphorus acid at a ratio of 0.05 to 0.4 equivalents of POH and/or PO.sup.M.sup.+ moieties per equivalent of epoxy groups provided by the at least one epoxy resin.

3. The one-component epoxy adhesive of claim 1, wherein Component E is a reaction product of a diglycidyl ether of a polyphenol having an epoxy equivalent weight of 150-225 and the phosphorus acid.

4. The one-component epoxy adhesive of claim 1, which contains 3.5 to 15 weight-% of component E, based on the weight of the adhesive.

5. The one-component epoxy adhesive of claim 1, wherein component A includes at least one diglycidyl ether of a bisphenol.

6. The one-component epoxy adhesive of claim 1, wherein component A includes a first diglycidyl ether of a bisphenol that has an epoxy equivalent weight of up to 225 and a second diglycidyl ether of a bisphenol that has an epoxy equivalent weight of greater than 225 to 750

7. The one-component epoxy adhesive of claim 1, wherein component B wherein the capped isocyanate groups are capped with a monophenol or polyphenol.

8. The one-component epoxy adhesive of claim 1, wherein component D includes dicyandiamide.

9. The one-component epoxy adhesive of claim 1, wherein component C includes a urea compound.

10. The one-component epoxy adhesive of claim 1, which contains 0 to 0.75 weight-% glass microspheres, based on the weight of the one-component epoxy adhesive.

11. A method for bonding two substrates, comprising forming a layer of the adhesive of claim 1 at a bondline between two substrates to form an assembly, and then curing adhesive layer at the bondline by heating to a temperature of at least 130 C. to form a cured adhesive bonded to the two substrates at the bondline.

12. A method for forming a bonded and coated assembly, comprising 1) forming a layer of claim 1 at a bondline between a first and a second substrate to form an assembly that includes the first and second substrates each in contact with the adhesive composition at the bondline; then 2) immersing the assembly into a coating bath to form a layer of an uncured coating on at least a portion of an exposed surface of the assembly; and then 3) heating the assembly to a temperature of at least 130 C. to cure the adhesive to form a cured adhesive bonded to the substrates at the bondline and simultaneously cure the coating layer.

Description

EXAMPLES 1-3 AND COMPARATIVE SAMPLES A, B AND C

[0074] One-component adhesive Examples 1-3 and Comparative Samples A-C are prepared by mixing ingredients as indicated in Table 1. All have curing temperatures of at least 100 C.

[0075] The Component B material is prepared by mixing 58.8 parts of a 2000 number average molecular weight polytetrahydrofuran diol (PolyTHF 2000 from BASF) with 14.4 parts of a 2800 number average molecular weight hydroxyl-terminated poly(butadiene) (Poly BD R45 HTLO from Cray Valley). This mixture is dried under vacuum at 120 C. and cooled to 60 C. 11.65 parts of hexamethylene diisocyanate are added, followed by 0.06 parts of a tin catalyst, and the ingredients are allowed to react to produce an isocyanate-terminated prepolymer. The prepolymer is then reacted sequentially at 100-105 C. with o,o-diallylbisphenol A and cardanol to cap the isocyanate groups. The resulting Component B material has a number average molecular weight of 6200 g/mol and a polydispersity of 2.8, as measured by gel permeation chromatography in tetrahydrofuran using universal calibration.

TABLE-US-00001 TABLE 1 Parts by Weight Sample Designation Ingredient A* B* 1 C* 2 3 Component A.sup.1 42.85 39.85 37 34 37.68 38.3 Component B 20 20 20 20 20 20 Component C.sup.2 0.8 0.8 0.8 0.8 0.8 0.8 Component D.sup.3 3.65 3.65 3.50 3.65 3.60 3.70 Component E.sup.4 0 3.0 6.0 0 6.0 6.0 Silane Coupling Agent.sup.5 1.2 1.2 1.2 0.8 1.2 1.2 Fillers.sup.6 28 28 28 28 28 28 Glass Microspheres 1.5 1.5 1.5 0.75 0.75 0 Epoxy-functional diluent.sup.7 2.0 2.0 2.0 2.0 2.0 2.0 .sup.1Liquid mixture of a diglycidyl ether of bisphenol A having an epoxy equivalent weight of about 186 and a diglycidyl ether of bisphenol A resin having an epoxy equivalent weight of about 520. The amount of the latter is 9.5 parts in all cases, with the former constituting the remainder of the indicated weight. .sup.250/50 by weight mixture of an aliphatic bis urea sold by Emerald Materials as OmicureU-35M and a 4,4-methylene bis (phenyldimethylurea) sold by Emerald Materials as Omicure U-52M. .sup.3Dicyandiamide sold as Amicure CG 120 G by Air Products. .sup.4Phosphoric acid-modified diglycidyl ether of bisphenol A, sold by Asahi Denka as EP49-10P2 epoxy resin. .sup.5Dynasylan GLYEO from Evonik Industries. .sup.6Mixture of fumed silica, calcium oxide, wollastonite, calcium carbonate and colorant. .sup.7Monoglycidyl ether of p-tertiarybutyl phenol.

[0076] Corrosion resistance is evaluated on each of Comparative Samples A-C and Examples 1-3 by measuring lap shear strength (per DIN EN 1465) on samples that have and have not undergone environmental aging.

[0077] Lap shear specimens are prepared using 1.2 mm-thick HC420LAD+Z100 galvanized steel test strips. The test strips are degreased and then re-greased by dip coating them into a solution of 90% heptane and Anticorit PL3802-395 corrosion prevention oil (Fuchs Lubricants UK). The adhesive in each case is applied to one of the strips, and 0.3 mm glass beads are sprinkled on top of the adhesive before overlaying the second strip. The bond area is 1025 mm with a thickness of 0.3 mm as determined by the glass beads. The assembled test specimens are held together with metal clips and baked at 150 C. for 45 minutes. The specimens are then dipped into an E-coat bath and cured for 30 minutes at 180 C.

[0078] In all cases, lap shear specimens are prepared in multiples. Fresh samples are tested for lap shear strength after equilibrating to 23 C. Aged samples are tested after undergoing 90 cycles of the Volkswagen PV 1210 protocol and being equilibrated to 23 C.

[0079] Impact peel strength is evaluated according to ISO 11343 on samples that have not undergone environmental aging. The adhesives are used to bond DX 56+Z (EN 10 346) low carbon galvanized steel to DC 04+ZE (EN 10 152) electrogalvanized steel. The substrates are degreased and re-greased prior to assembling the test specimens as per the lap shear strips. The adhesive composition is applied to one metal strip, with spacers to adjust the adhesive layer thickness to 0.2 mm. The second metal strip is then applied to the adhesive layer. Bond area is 2030 mm. The test specimens are held together with metal clips and cured for 30 minutes at 180 C. The impact load is 90 J at drop weight speed of 2 m/s. Impact peel strength is reported as average impact load at plateau using a Zwick-Roell impact tester.

[0080] The elastic modulus, tensile strength and elongation of the cured adhesives are measured according to DIN ISO 527-1, after curing the adhesive for 30 min at 180 C. in a hot press between metal plates.

[0081] Plastic viscosity at 45 C. is measured on a Bohlin CS-50 Rheometer, under conditions C/P 20 and up/down 0.1-20s.sup.1, and is calculated using the Casson model.

[0082] Results of the foregoing testing are as indicated in Table 2.

TABLE-US-00002 TABLE 2 Sample Designation Test A* B* 1 C* 2 3 % Component E 0 3 6 0 6 6 Lap Shear Strength (MPa) Initial 29.1 29.1 28.9 35.3 32.2 32.2 After corrosion aging 14.4 12.3 17.3 14.2 20.6 20.9 Loss after aging (%) 50 57 40 60 36 35 Impact Peel Resistance 34.0 34.1 33.6 35.0 35.4 35.7 (N/mm) Elastic Modulus (MPa) 1940 ND 1837 2032 1800 ND Tensile Strength (MPa) 28.7 ND 28.1 32.9 28.3 ND Elongation at break (%) 6.0 ND 7.2 7.5 7.5 ND Plastic Viscosity, 74 75 89 77 67 66 45 C. (Pa)

[0083] Comparative Sample A loses 50% of its lap shear strength after corrosive aging. Comparative Sample B, which contains 3% of the phosphorus-modified epoxy resin, performs even more poorly, losing 57% of its lap shear strength. The presence of 3% of the phosphorus-modified epoxy resin surprisingly does not increase initial lap shear strength, as seen by comparing the values for Comparative Samples A and B.

[0084] Example 1 exhibits essentially the same initial lap shear strength and impact peel resistance as do Comparative Samples A and B, again confirming that the presence of the phosphorus-modified epoxy resin at these levels has negligible effect on adhesive strength. However, the corrosion-aged sample has a lap shear strength that is 20% greater than Comparative Sample A and 40% greater than Comparative Sample B. These results demonstrate a large improvement in resistance to corrosive aging.

[0085] The results for Comparative Sample C and Examples 2 and 3 show the same pattern. Comparative Sample C loses 60% of its initial lap shear strength after corrosive aging, compared to only about 35% for each of Examples 2 and 3. The corrosion-aged Examples 2 and 3 have lap shear strengths about 45% greater than Comparative Sample C. This is achieved even though, as before, the presence of the phosphorus-modified epoxy resin surprisingly has no significant positive impact on the initial lap shear results.

[0086] Examples 2 and 3 also are notable because of their content (or absence in the case of Example 3) of hollow glass microspheres. Hollow glass microspheres are known to impart corrosion resistance to structural adhesives. This is reflected in the greater loss of lap shear strength after corrosion testing of Comparative Sample C vs. Comparative Sample A. In those cases, removing half of the hollow glass microspheres (Comparative Sample C) results in a loss of 60% of initial strength vs. only a 50% loss for Comparative Sample A). Example 2 demonstrates much less loss of lap shear strength compared to both Comparative Samples A and C. The presence of 6% of the phosphorus-modified epoxy resin overcomes the negative effect of reducing the amount of microspheres. Example 3 demonstrates that this advantage is obtained even in the absence of hollow glass microspheres.