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.

Claims

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 about 12 ?m to 25 ?m 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 is 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 of 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 16 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.

18. The one-component epoxy structural adhesive of claim 1 wherein the mineral filler in the form of fibers having a diameter of about 12 ?m.

Description

EXAMPLE 1 AND COMPARATIVE SAMPLES A-D

[0103] Structural Adhesive Example 1 and Comparative Samples A-D are prepared by blending ingredients as indicated in Table 1:

TABLE-US-00001 TABLE 1 Parts By Weight Comp. Comp. Comp. Comp. Component Samp. A Samp. B Samp. C Samp. D Ex. 1 Epoxy Resin A 67.46 60.64 60.64 60.64 60.64 Toughener A 19.0 19.0 19.0 19.0 19.0 GLYEO 0.6 0.6 0.6 0.6 0.6 Colorant 0.4 0.4 0.4 0.4 0.4 Dicyanamide 6.74 6.06 6.06 6.06 6.06 EP796 0.8 0.8 0.8 0.8 0.8 Hydrophobic 5.0 5.0 5.0 5.0 5.0 Fumed Silica Calcium Oxide 0 7.5 0 0 0 Wollastonite A 0 0 0 0 7.5 Wollastonite C 0 0 7.5 0 0 Wollastonite D 0 0 0 7.5 0

[0104] Test samples for tensile strength, elongation and elastic modulus measurements are made by curing a portion of each sample for 30 minutes at 180? C. Test specimens are cut from the cured samples and evaluated according to DIN EN ISO 527-1.

[0105] Impact peel testing is performed for each adhesive sample. The substrates are 1.0 mm-thick HC420LAD+Z100 steel coupons. The test coupons for the impact peel testing are 90 mm?20 mm with a bonded area of 30?20 mm. They are cleaned with heptane and then re-greased by dipcoating them into a 9:1 by volume solution of heptane and a corrosion prevention lubricant (Anticorit PL 3802-39S). The adhesive sample is then applied to the bond area of one coupon and squeezed onto the other coupon to prepare each test specimen, with spacers present to maintain an adhesive layer thickness of 0.2 mm. The assembled test specimens are cured at 180? C. for 30 minutes. The impact peel testing is performed in accordance with ISO 11343 wedge impact method. Testing is performed at an operating speed of 2 m/sec with samples at a temperature of 23? C.

[0106] Lap shear specimens are made using coupons of the same steel, except they are 1.2 mm thick. The specimens are made by sprinkling glass beads (0.2 mm diameter) onto one of the coupons, applying the adhesive sample, and then positioning the second coupon over top the adhesive. The bonded area in each case is 25?10 mm, and the adhesive layer thickness is controlled by the glass beads to 0.2 mm. The test specimens are cured for 30 minutes at 180? C. and evaluated for lap shear strength in accordance with DIN ISO 1465. Testing is performed at 23? C. and a test speed of 10 mm/minute.

[0107] Viscosity and yield stress are measured on a Bohlin CS-50 rheometer, C/P 20, up/down 0.1-20 s.sup.?1, with data evaluated according to the Casson model.

[0108] Results of this testing are as indicated in Table 2.

TABLE-US-00002 TABLE 2 Comp. Comp. Comp. Comp. Samp. A Samp. B Samp. C Samp. D Ex. 1 Filler None CaO Woll. C Woll. D Woll. A Filler Aspect N/A ~1 5 3 13 Ratio Property Elastic 2035 2067 2205 2162 2733 Modulus, MPa Tensile 47.6 44.2 46.5 46.5 43.6 Strength, MPa Elongation 9.5 7.6 7.3 8.2 6.1 at Break, % Impact Peel 32.0 31.9 31.1 31.0 32.2 Str., N/mm Lap Shear 40.6 37.8 39.5 40.3 38.5 Str., MPa Lap Shear 2.2 1.9 1.9 2.1 2.0 Elongation, % Yield stress, 63.6 78.8 86.5 85.5 56.1 45? C., Pa Viscosity, 18.0 23.9 22.9 24.2 26.8 45? C., Pa .Math. s CaCO.sub.3-calcium carbonate; CaO-calcium oxide; Woll. A-Wollastonite A; Woll. C-Wollastonite C; Woll. D-Wollastonite D.

[0109] Comparative Sample A is an unfilled control. In each of Comparative Samples B-D and in Example 1, the Comparative Sample A formulation is modified by adding 7.5 parts of a filler and removing an equal quantity of epoxy resin and dicyanamide. The amount of the toughener is kept constant across these samples.

[0110] Comparative Sample B shows the effect of adding a low aspect ratio calcium oxide filler. The addition of this filler has very little effect on any of the properties except for increasing yield stress and viscosity mildly.

[0111] The addition of low aspect ratio wollastonite as in Comparative Samples C and D results in a small (6-8.5%) increase in elastic modulus. Like the calcium oxide, these fillers increase yield stress and viscosity somewhat, but have little other effect on the properties of the cured adhesive.

[0112] Example 1 contains a high aspect ratio wollastonite filler. The elastic modulus is increased by 34% over Comparative Sample A, which is several times the increase seen with the fillers of Comparative Samples B. C and D. Other tensile properties, lap shear strength and, surprisingly, impact peel resistance are not significantly different than those of Comparative Samples A-D. Yield stress and viscosity are slightly higher than the filled comparative samples.

EXAMPLES 2-4 AND COMPARATIVE SAMPLES A, E, F AND G

[0113] Structural Adhesive Examples 2-4 and Comparative Samples E, F and G are prepared by blending ingredients as indicated in Table 3:

TABLE-US-00003 TABLE 3 Parts By Weight Component Comp. E Comp. F Comp. G Ex. 2 Ex. 3 Ex. 4 Epoxy Resin A 53.4 53.82 53.82 53.82 53.82 33.17 Epoxy Resin B 0 0 0 0 0 16.59 Toughener A 19.0 19.0 19.0 19.0 19.0 19.0 GLYEO 0.6 0.6 0.6 0.6 0.6 0.6 Colorant 0.4 0.4 0.4 0.4 0.4 0.4 Dicyanamide 4.80 5.38 5.38 5.38 5.38 4.58 EP796 0.8 0.8 0.8 0.8 0.8 0.8 Hydrophobic Fumed 5.0 5.0 5.0 5.0 5.0 5.0 Silica Calcium Carbonate 7.0 0 0 0 0 0 Calcium Oxide 6.5 15.0 0 0 0 6.0 Microspheres A 2.5 0 0 0 0 0 Microspheres B 0 0 0 0 0 3.86 Wollastonite A 0 0 0 15.0 0 10 Wollastonite B 0 0 0 0 15.0 0 Wollastonite D 0 0 15.0 0 0 0

[0114] These adhesives are tested in the same manner as described with respect to Example 1 and Comparative Samples A-D. Results are as indicated in Table 4. The results for Comparative Sample A are repeated for reference.

TABLE-US-00004 TABLE 4 Comp. A Comp. E Comp. F Comp. G Ex. 2 Ex. 3 Ex. 4 Filler Type, None Mix.sup.1, CaO, Woll. D, Woll. A, Woll. B, Woll. Mix.sup.2, Amount 16 wt. % 15 wt. % 15 wt. % 15 wt. % 15 wt. % 19.86 wt. % Properties Elastic 2035 2121 2247 2483 3418 3470 2899 Modulus, MPa Tensile 47.6 37.9 41.4 43.6 44.5 51.9 44.2 Strength, MPa Elongation 9.5 3.8 5.6 6.1 2.9 4.0 3.9 at Break, % Impact Peel 32.0 25.8 29.7 30.4 30.9 30.4 27.0 Str., N/mm Lap Shear 40.6 31.8 36.1 38.0 35.7 38.0 36.4 Str., MPa Lap Shear 2.2 1.5 1.8 1.9 1.7 1.9 1.8 Elongation, % Yield stress, 63.6 107.2 106.0 102.3 66.2 124.3 89.6 45? C., Pa Viscosity, 18.0 44.8 37.5 39.3 37.8 32.3 57.8 45? C., Pa .Math. s .sup.1Mixture of calcium carbonate, calcium oxide and hollow glass spheres. .sup.2Mixture of Wollastonite A, calcium carbonate and glass microspheres.

[0115] Comparative Sample A is again the unfilled baseline structural adhesive. In each of Comparative Samples E, F and G and Examples 2-4, the Comparative Sample A formulation is modified by adding filler and removing an equal quantity of epoxy resin and dicyanamide. The amount of the toughener is kept constant across these Samples. The epoxy resin component also is changed in Example 4.

[0116] Comparative Samples E, F and G shows the effect of adding low aspect ratio fillers. Comparative Example (1, which contains 15 weight-% a low aspect ratio wollastonite filler, exhibits an increase in elastic modulus of about 22%, compared to the baseline case. This increase in elastic modulus is far less than seen with Example 1, which contains only 7.5 weight-% of the high aspect ratio wollastonite filler. Comparative Examples E and F show only a small benefit in elastic modulus. All of Comparative Samples E, F and G show some loss in impact peel resistance and lap shear strength.

[0117] Examples 2 and 3 each exhibit increases in elastic modulus of about 70%, compared to the baseline case. Other properties are close to or better than Comparative Samples E, F and G. Examples 2 and 3, in comparison with Comparative Samples E, F and G, clearly demonstrate the large and unexpected effect of selecting a high aspect ratio filler. Examples 2 and 3 in comparison with Comparative Sample G shows that this effect is not due to the differences in the type of filler.

[0118] Example 4 is formulated somewhat differently than the other adhesives, but nonetheless exhibits a large improvement in elastic modulus compared to any of the Comparative Samples.

EXAMPLE 5 AND COMPARATIVE SAMPLE H

[0119] Structural Adhesive Example 5 and Comparative Sample H are prepared by blending ingredients as indicated in Table 5:

TABLE-US-00005 TABLE 5 Parts By Weight Component Comp. Sample H Ex. 5 Epoxy Resin A 14.0 Epoxy Resin B 30.0 Epoxy Resin C 3.0 Epoxy Resin D 2.0 Toughener B 19.5 GLYEO 0.55 Colorant 0.35 Dicyanamide 4.70 Curing Accelerants 1.5 Polyvinyl butyral terpolymer 0.65 Hydrophobic Fumed Silica 5.70 Calcium Carbonate 7.6 0 Calcium Oxide 6.5 6.5 Microspheres A 1.75 1.75 0.2 mm glass beads 2.0 2.0 Wollastonite A 0 7.6

[0120] These adhesives are tested in the same manner as described above, with results as indicated in Table 6. The substrates for the dynamic impact peel strength testing are 0.75 mm grade DX56 D+Z100MB hot dipped zinc coated steel and 0.75 mm DC04 ZE50/50 electrolytically tine coated steel, each from Thyssen Krupp.

TABLE-US-00006 TABLE 6 Comp. H Ex. 5 Filler Type, Low aspect High aspect ratio Amount ratio mixture. wollastonite, low aspect ratio calcium oxide, microbeads Properties Elastic Modulus, 1837 2508 MPa Tensile Strength, 31.0 35.0 MPa Elongation at 4.6 3.7 Break, % Impact Peel Str., 31.8 30.0 N/mm Yield stress, 249 408 45? C., Pa Viscosity, 65.0 70 45? C., Pa .Math. s .sup.1Mixture of calcium carbonate, calcium oxide and hollow glass spheres. .sup.2Mixture of Wollastonite A, calcium carbonate and glass microspheres.

[0121] Once again, the inclusion of a high aspect ratio mineral filler leads to a large improvement in elastic modulus while having only a small effect on other properties. In Example 5, the benefits of the high aspect filler are seen even when additional, low aspect ratio fillers are present.

EXAMPLES 6-15

[0122] Example 1 is repeated 10 times, in each instance replacing the wollastonite fibers with glass or carbon fibers as indicated in Table 7. Results of testing of the resulting cured adhesives are as indicated in Table 8.

TABLE-US-00007 TABLE 7 Parts by Weight Component Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Epoxy Resin A 53.82 53.82 47.00 40.18 53.82 60.64 53.82 47.00 58.37 53.82 Toughener A 19.00 19.00 19.00 19.00 19.00 19.00 19.00 19.00 19.00 19.00 GLYEO 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 Colorant 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 Dicyanamide 5.38 5.38 4.70 4.02 5.38 6.06 5.38 4.70 5.83 5.38 EP796 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 Hydrophobic 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 Fumed Silica Glass Fiber A 15 0 0 0 0 0 0 0 0 0 Glass Fiber B 0 15 22.50 30.00 0 0 0 0 0 0 Glass Fiber C 0 0 0 0 15 0 0 0 0 0 Carbon Fiber A 0 0 0 0 0 7.5 15 22.5 0 0 Carbon Fiber B 0 0 0 0 0 0 0 0 10 15

TABLE-US-00008 TABLE 8 Designation Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Fiber loading, % 15 15 22.5 30 15 7.5 15 22.5 10 15 Fiber Aspect Ratio, 11 15 15 15 29 11 11 11 21 21 diameter (?m) (14) (14) (14) (14) (16) (7) (7) (7) (7) (7) Properties Elastic 3083 3191 3217 3787 2786 3900 4775 5663 3779 4056 Modulus, MPa Tensile 49.7 52.6 50.4 50.1 51.6 60.8 68.3 63.8 58.9 58.0 Strength, MPa Elongation at 4.3 4.5 4.05 2.8 5.15 4.2 3.7 2.8 4.8 5.0 Break, % Impact Peel 32.6 36.0 31.2 29.3 35.8 34.2 32.9 23.2 33.6 32.5 Str., N/mm Lap Shear 35.0 37.7 32.2 27.7 41.4 39.3 36.0 28.8 40.1 37.1 Str., MPa Lap Shear 35.0 1.8 1.44 1.7 1.8 2.1 1.9 1.2 1.8 2.0 Elongation, % Yield stress, 122.4 108.4 153.6 127 145.1 71.9 75.6 ND 78.0 83.5 45? C., Pa Viscosity, 33.9 32.1 46.6 155 61.5 48.2 38.7 ND 28.9 37.1 45? C., Pa .Math. s

[0123] As with previous examples of the invention, Examples 6-15 demonstrate high elastic moduli, compared with Comparative Samples A, E, F and G, even at high fiber loadings. Impact peel strengths are generally comparable to or better than those of the Comparative Samples, at equivalent fiber loadings.