One-component toughened epoxy adhesives containing a mixture of latent curing agents

Abstract

A one-component toughened epoxy-modified polyurethane and/or urea adhesive includes a mixture of dicyandiamide and a dihdyrazide as curing agents. The mixture of curing agents permits the adhesive to be cured at lower temperatures while developing good adhesive and mechanical properties in the cured adhesive.

Claims

1. A method for bonding two substrates, comprising forming a layer of an adhesive comprising in admixture A) at least one non-rubber-modified epoxy resin, B) at least one toughening agent, C) at least one epoxy curing catalyst and D) a curing agent mixture that includes dicyandiamide and one or more dihydrazide compounds, the dicyandiamide and one or more dihydrazide compounds being present at a weight ratio of 0.63 to 99:1, at a bondline between the two substrates to form an assembly and then curing the adhesive layer at the bondline by heating to a temperature of 130 to 165° C. to form a cured adhesive bonded to the two substrates at the bondline, wherein one of the substrates is a metal and the other substrate is a thermoplastic organic polymer, a thermoset organic polymer or a fiber composite.

2. The method of claim 1, wherein the two substrates have coefficients of linear thermal expansion that are different from each other by at least 5×10.sup.−6 m/m-K.

3. A method for forming a bonded and coated assembly, comprising 1) forming a layer of the adhesive comprising in admixture A) at least one non-rubber-modified epoxy resin, B) at least one toughening agent, C) at least one epoxy curing catalyst and D) a curing agent mixture that includes dicyandiamide and one or more dihydrazide compounds, the dicyandiamide and one or more dihydrazide compounds being present at a weight ratio of 1:99 to 99:1 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 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.

4. The method of claim 3 wherein the temperature is 130 to 175° C.

5. The method of claim 3 wherein the temperature is 133 to 165° C.

6. The method of claim 3, wherein the first and second substrates have coefficients of linear thermal expansion that are different from each other by at least 5×10.sup.−6 m/m-K.

7. The method of claim 3, wherein one of the substrates is a metal and the other substrate is a thermoplastic organic polymer, a thermoset organic polymer or a fiber composite.

Description

EXAMPLES 1-6 AND COMPARATIVE SAMPLES A AND B

(1) One-component adhesive Examples 1-6 and Comparative Samples A and B are prepared by mixing ingredients as indicated in Table 1. DSC analysis is performed at 10° C./min from 0° C. to 250° C. to determine curing onset temperature, peak exotherm temperature and enthalpy of curing for the adhesive composition. The chamber is cooled to 0° C. and a second scan is performed at 10° C./min up to 200° C. to determine the glass transition temperature of the cured adhesive.

(2) TABLE-US-00001 TABLE 1 Parts By Weight Comp. Comp. Sample Sample Ingredient A* Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 B* NRM Epoxy 52.90 52.59 52.29 51.99 51.69 51.10 50.81 48.32 Resin RM Epoxy 11.46 11.46 11.46 11.46 11.46 11.46 11.46 11.46 Resin Toughener 17.20 17.20 17.20 17.20 17.20 17.20 17.20 17.20 Dicyandiamide 5.10 4.76 4.42 4.08 3.75 3.09 2.77 0 Adipic Acid 0 0.65 1.30 1.93 2.57 3.81 4.42 9.68 Dihydrazide Adhesion 0.68 0.68 0.68 0.68 0.68 0.68 0.68 0.68 Promoter Glycidyl 1.13 1.13 1.13 1.13 1.13 1.13 1.13 1.13 neodecanoate Colorant 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 CNSL 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Fillers 10.18 10.18 10.18 10.18 10.18 10.18 10.18 10.18 Catalyst 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 DSC Results Curing Onset 166° C. 161° C. 156° C. 148° C. 145° C. 144° C. 143° C. 161° C. Temp. Peak Exotherm 178° C. 176° C. 174° C. 172° C. 171° C. 160° C. 158° C. 176° C. Temp. Glass 110° C. 101° C. 102° C. 103° C.  97° C.  97° C.  98° C. 112° C. Transition Temp. *Not an example of this invention.

(3) The combination of dicyandiamine and dihydrazide curing agents lead to a substantial lowering of curing onset temperature for each of Examples 1-6, compared to Comparative Sample A in all cases, and compared to Comparative Sample B in the cases of Examples 2 to 6. Peak exotherm temperature is also reduced significantly. The lower onset temperature means that curing commences earlier during the heating process, and accordingly proceeds for a longer time, even though the heating regimen in the same in all cases.

(4) Lap shear specimens are made using 1.6 mm cold rolled steel coupons. The specimens are made by cleaning the coupons, sprinkling glass beads (0.254 mm diameter) onto one of the coupons, applying the adhesive sample, and then positioning the second coupon on top of the adhesive. The bonded area in each case is 25×12.7 mm, and the adhesive layer thickness is controlled by the glass beads to 0.254 mm. The test specimens are cured for 30 minutes at 140° C. and evaluated for lap shear strength in accordance with DIN ISO 1465. Testing is performed at 23° C. and a test speed of 12.7 mm/minute. Results are as indicated in Table 2.

(5) TABLE-US-00002 TABLE 2 Sample Comp. Comp. Sample Sample A* Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 B* Lap Shear 9.3 32.9 28.2 27.2 29.2 27.9 25.0 15.2 Strength (MPa) *Not an example of this invention.

(6) These results show a dramatic increase in lap shear strength when the adhesives of the invention are cured at the somewhat low temperature of 140° C.

(7) A qualitative cure test is performed on each of Comp. Samples A and B and Examples 1-6. 6 mm beads are applied to a panel and placed into a 140° C. preheated oven. At five minute intervals, a spatula is dragged through the beads and the beads are visually examined. Incomplete cure is indicated by this test if the spatula cuts through the bead, smears it or otherwise deforms it. After 25 minutes, all of Examples 1-6 have cured, whereas neither of the Comparative Samples has done so.

(8) Storage stability is evaluated on Comparative Samples A and B and Example 1. The viscosity of each of the freshly prepared samples is measured at 38° C. using a 25 mm parallel plate viscometer at a shear rate of 3 s.sup.−1. The samples are each aged in closed containers at 50° C., with samples being withdrawn periodically for viscosity testing. Results are as in Table 3.

(9) TABLE-US-00003 TABLE 3 Aging 38° C. Viscosity (Pa .Math. s) Time Comp. Comp. (hr) Sample A* Ex. A Sample B* 0 107.0 105.9 138.3 51 108.6 105.6 146.5 119 141.4 145.1 309.4 219.5 275.5 331.7 540.5 310 383.5 407.1 5079 *Not an example of this invention.

(10) As can be seen from the data in Table 3, the adhesive containing the dihydrazide curing agent (Comparative Sample B) has poor thermal stability on this test. Comparative Sample A demonstrates the good thermal stability obtained with dicyandiamide. Surprisingly, Example 1 exhibits a thermal stability very close to that of Comparative Sample A, despite the presence of the dihydrazide curing agent.

(11) Impact peel testing is performed on Comparative Samples A and B and Example 4 as follows. The test coupons for the impact peel testing are 100 mm×20 mm with a bonded area of 30×20 mm. The adhesive sample is applied to the bond area of a 0.8 mm GMC-5E cold rolled steel coupon (ACT Laboratories) after cleaning the coupon with acetone. Another steel coupon is placed into contact with the adhesive and the assembly squeezed under a weight of about 10 kg to prepare each test specimen, with spacers present to maintain an adhesive layer thickness of 0.254 mm. The assembled test specimens are cured at 170° C. for 30 minutes. The impact peel testing is performed in accordance with the ISO 11343 wedge impact method. Testing is performed under a 90 Joule impact load at an operating speed of 2 m/sec with samples at a temperature of 23° C. or at −40° C., as indicated in Table 4.

(12) In addition, lap shear strength is evaluated on these samples as indicated before, after a 170° C./30 minute cure.

(13) Results of the testing are as indicated in Table 4.

(14) TABLE-US-00004 TABLE 4 Comp. Comp. Test Sample A* Ex. 4 Sample B* Impact Peel Strength, 23° C. (N/mm) 36.8 33.3 15.9 Impact Peel Strength, −40° C. 18.4 21.4 5.0 (N/mm) Lap Shear Strength (MPa) 35.3 35.3 30.3 *Not an example of this invention.

(15) The data in Table 4 shows that the adhesive of the invention exhibits good adhesion characteristics even at a higher temperature cure. The ability to cure the adhesive over a wide range of temperatures is a significant advantage in a manufacturing setting, where conditions may not always be closely controlled.

EXAMPLES 7-9 AND COMPARATIVE SAMPLES C AND D

(16) One-component adhesive Examples 7-9 and Comparative Samples C and D are prepared by mixing ingredients as indicated in Table 5. DSC analysis is performed to determine curing onset temperature, peak exotherm temperature, and enthalpy of curing for the adhesive composition, and to determine the glass transition temperature of the cured adhesive.

(17) TABLE-US-00005 TABLE 5 Parts By Weight Comp. Comp. Sample Sample Ingredient C* Ex. 7 Ex. 8 Ex. 9 D* NRM Epoxy Resin 33.70 33.00 32.32 31.67 31.05 Core-Shell Rubber 50 50 50 50 50 Dicyandiamide 2.96 2.17 1.42 0.70 0 Adipic Acid Dihydrazide 0 1.49 2.92 4.29 5.61 Adhesion Promoter 0.68 0.68 0.68 0.68 0.68 Glycidyl neodecanoate 1.13 1.13 1.13 1.13 1.13 Colorant 0.10 0.10 0.10 0.10 0.10 CNSL 0.50 0.50 0.50 0.50 0.50 Fillers 10.18 10.18 10.18 10.18 10.18 Catalyst 0.75 0.75 0.75 0.75 0.75 DSC Results Curing Onset Temp. 154° C. 145° C. 143° C. 139° C. 169° C. Peak Exotherm Temp. 164° C. 156° C. 156° C. 168° C. 178° C. Enthalpy of Cure, J/g 177 156 133 125 121 Glass Transition Temp. 107° C.  95° C.  83° C.  72° C.  70° C. *Not an example of this invention.

(18) These systems toughened with a core-shell rubber exhibit generally exhibit lower curing onset and peak exotherm temperatures than those of Examples 1-6. Nonetheless, the combination of dicyandiamide and dihydrazide even in these cases results in a significant depression of both values.