Structural adhesives having improved wash-off resistance and method for dispensing same

Abstract

An uncured adhesive contains 2 to 10 weight percent of particles of a semi-crystalline organic material, preferably a polyester having a number-average molecular weight of 2000 to 10,000, a hydroxyl number of 10 to 60, and a melting temperature of 50 to 125 C. The adhesive is applied by heating it just prior to application, to melt the particles. After application, the adhesive is cooled to below the melting temperature of the semi-crystalline organic material, and then cured. The process allows the adhesive to be stored and pumped at ambient temperatures, due to the moderate viscosity of the material. Upon melting and re-cooling the semi-crystalline organic material, the adhesive assumes a high yield stress that imparts very good wash-off resistance. In preferred embodiments, the adhesive composition includes an epoxy resin and an epoxy curing agent.

Claims

1. A method for applying an adhesive composition, comprising the steps of: A. introducing an uncured adhesive composition into an automated dispensing system, wherein the uncured adhesive composition contains 2 to 10% by weight of a solid, particulate semi-crystalline organic material dispersed in a phase that is liquid at 20 C., wherein at least 90 weight-% of the particles of the semi-crystalline organic material have a size of 250 nm to 500 m, the semi-crystalline organic material has a crystalline melting temperature of 50 to 140 C., wherein the uncured adhesive composition has a curing temperature greater than the crystalline melting temperature of the semi-crystalline organic material and wherein the uncured adhesive composition has a viscosity of 1000 Pas or less at 20 C. at a shear rate of 3 s.sup.1, a viscosity of 400 Pas or less at 30 C. and a shear rate of 3 s.sup., and a Casson yield stress of 125 Pa or less; B. melting the semi-crystalline organic material by heating the uncured adhesive composition in the automated dispensing system to a temperature at least equal to the crystalline melting temperature of the semi-crystalline organic material but below the curing temperature of the uncured adhesive composition; C. applying the heated uncured adhesive composition from the automated dispensing system to either or both of two substrates and contacting the substrates such that the uncured adhesive composition is located between and in contact with the substrates to be bonded together, forming a bondline and then; D. prior to curing the uncured adhesive composition, cooling the uncured adhesive composition on the substrate to below the melting temperature of the semi-crystalline organic material, wherein the Casson yield stress of the uncured adhesive composition after cooling is 200 to 1500 Pa and then E. curing the adhesive composition on the substrate.

2. The method of claim 1, wherein the automated dispensing equipment includes storage apparatus for storing the uncured adhesive composition, a header system through which the uncured adhesive composition is transported to one or more dosers which meter the uncured adhesive composition, and one or more conduits through which the metered adhesive composition is transferred from each doser to a corresponding application station, wherein step B is performed by applying heat and/or mechanical energy at the doser(s), the one or more conduits and/or the application station(s), and the storage and header system are maintained at a temperature below the melting temperature of the semi-crystalline organic material.

3. The method of claim 1, wherein the storage and header system are maintained at a temperature of no greater than 35 C.

4. The method of claim 1, wherein the semi-crystalline organic material is a polyester having a number average molecular weight of 2000 to 10,000, a hydroxyl number of 10 to 60, and a melting temperature of 50 to 125 C.

5. The method of claim 4 wherein the polyester has a number average molecular weight of 2500 to 8500 and a melting temperature of 50 to 80 C.

6. The method of claim 1, wherein the adhesive composition includes an epoxy resin and an epoxy curing agent.

7. The method of claim 6, wherein the adhesive composition further includes at least one elastomeric component selected from one or more of (a) a reactive toughener that has isocyanate groups that are blocked or capped with a phenolic compound, an aminophenolic compound, a primary or secondary aliphatic or cycloaliphatic amine, a benzyl alcohol, an aromatic amine, a benzyl amine or a thiol, (b) rubber-modified epoxy resin and (c) a core-shell rubber.

8. The method of claim 1, wherein at least one of the two substrates is a vehicular frame member.

9. The method of any claim 8, wherein the two substrates and applied adhesive composition are coated by immersion into a coating, and the coating and adhesive composition are cured at the same time.

10. The method of claim 1, wherein at least one of the two substrates is a vehicular frame member.

11. The method of claim 1 wherein at least 95 weight percent of the particles have a size of 500 nm to 50 m.

Description

EXAMPLES 1-3 AND COMPARATIVE EXAMPLES C-1 AND C-2

(1) Examples 1-3 and Comparative Samples C-1 and C-2 are made from the following formulations:

(2) TABLE-US-00001 TABLE 1 Parts By Weight Ingredient Comp. C-1 Comp. C-2 Ex. 1 Ex. 2 Ex. 3 Epoxy Resin 40.7 20.7 39.9 40.7 40.7 Rubber- 20.2 20.2 20.2 20.2 20.2 Modified Epoxy Toughener 15.5 15.5 15.5 15.5 15.5 Cardura N10 1.0 1.0 1.0 1.0 1.0 Epoxy Silane 0.6 0.6 0.6 0.6 0.6 Pigment 0.1 0.1 0.1 0.1 0.1 Dicyanamide 4.8 4.8 4.8 4.8 4.8 Calcium Oxide 2.0 2.0 2.0 2.0 2.0 Calcium 5.5 5.5 5.5 5.5 5.5 Carbonate Talc 0.3 0.3 0.3 0.3 0.3 Fumed Silica 8.5 3.5 3.5 3.5 3.5 TDAMP 0.8 0.8 0 0.8 0.8 Accelerine 0 0 0.8 0 0 2191 Catalyst Dynacoll/Epoxy 0 25 0 0 0 Resin Premix Dynacoll 0 0 5.8 0 0 Powder Rheocin 0 0 0 5 0 Rheotix 240 0 0 0 0 5

(3) To produce Comparative Samples C-1 and C-2, the ingredients are mixed on a planetary mixer for about 5 minutes. Waste heat raises the temperature of the mixture to about 50 C. A scrape-down is performed, and the components are mixed further for 30 minutes under vacuum. In Comparative Sample C-2, the Dynacoll 7330 is not present in particulate form.

(4) To produce Example 1, the ingredients are mixed, taking care to maintain the temperature at 30 C. or below to avoid any softening or melting of the Dynacoll 7330 particles. As a result, the product is a dispersion in which the Dynacoll 7330 is present in the form of fine particles. The different catalyst used in this example is due to the melting temperature of the Dynacoll 7330. The TDAMP catalyst used in the other formulations becomes activated near the melting temperature of Dynacoll 7330; to avoid premature curing, this catalyst is replaced in Example 1 with Accelerine 2191, which becomes activated at a higher temperature. This allows the particle melting step to be performed without curing the adhesive.

(5) Examples 2 and 3 are prepared in the same general manner as Example 1, maintaining the temperature below the melting temperature of the semi-crystalline organic material in each case. Because the semi-crystalline organic materials used here have melting temperatures below about 50 C., the TDAMP catalyst can be used in these formulations without causing the adhesive to cure during the particle melting step.

(6) The viscosity of each of Examples 1-3 and Comparative Samples C-1 and C-2 is measured at 20 C. and 30 C., in each case at a shear rate of 3 s.sup.1. Results are as reported in Table 2 below.

(7) The Casson yield stress for each of the samples is measured, with results as indicated in Table 2.

(8) Each of the samples is then heated for five minutes at the temperature indicated in Table 2. In the case of Examples 1-3, this heating regimen is sufficient to melt the particles of the semi-crystalline organic material without curing the adhesive. The material is then cooled, and the viscosity at 20 C. (3 s.sup.1 shear rate) and Casson yield stress are again measured. Results are as indicated in Table 2.

(9) Test specimens for testing lap shear strength, 23 C. impact peel strength and 40 impact peel strength are prepared from each of the samples. For lap shear strength testing, test specimens are prepared and tested in accordance with DIN EN 1465, using a bonding area of 2510 mm, an adhesive layer thickness of 0.2 mm and a test speed of 10 mm/min. The substrate is 0.8 mm cold rolled steel that is solvent cleaned to remove any oil. For impact peel strength testing, specimens are prepared and tested in accordance with ISO 11343, using a bonding area of 3020 mm, an adhesive layer thickness of 0.2 mm and a test speed of 2 m/s. The samples are brought back to about 23 C. for testing unless otherwise indicated.

(10) TABLE-US-00002 TABLE 2 Test Condition/Property Comp. C-1 Comp. C-2 Ex. 1 Ex. 2 Ex. 3 Initial Viscosity, 1625 1246 754 803 735 20 C., Pa .Math. s Initial Viscosity, 853 551 294 332 N.D. 30 C., Pa .Math. s Initial Casson Yield 621 177 71 105 85 Stress, Pa Heating 70 70 83 60 60 Temperature, C. Post-heating 1718 1353 1790 1147 1065 Viscosity, 20 C., Pa .Math. s Post-heating Casson 550 318 724 230 204 Yield Stress, Pa % Increase Yield 11.4 80 1020 383 340 Stress Lap Shear Strength, 18.4 18.5 18.4 18.6 Not MPa Determined Impact Peel Strength, 30.9 39.6 30.2 37.2 Not N/mm Determined

(11) Example 1 demonstrates the advantages of this invention. Prior to performing the melting step, this sample has a viscosity very much lower than that of the Comparative Samples. The viscosity of Example 1 is lower at 20 C. than that of the Comparative Samples at 30 C. This lower viscosity removes the need to heat the adhesive so it can be pumped easily through the header system of automatic processing equipment. The Casson yield stress of the adhesive prior to the melting step is very much lower than that of the Comparatives, which is consistent with the particulate form of the Dynacoll 7330 material. This presents no difficulty, as a high Casson yield stress in not needed until the adhesive is applied to the substrate. In Example 1, Casson yield stress is sacrificed at a point in time when it is not needed, in exchange for a large benefit in viscosity.

(12) Once Example 1 undergoes the heating step, its viscosity increases to become similar to that of the Comparative Samples. This is consistent with the melting of the Dynacoll 7330 particles. Yield stress also increases to actually become greater than those of the Comparative Samples. Thus, this invention provides a material and process which allows for a low viscosity material to be handled without applied heat until just prior to application, where heat is applied to melt the dispersed particles of semi-crystalline organic material, thereby increasing the yield stress and wash-off resistance.

(13) Examples 2 and 3 show similar effects, although the effects in these cases are not as pronounced as with Example 1. This may be due to the lower crystalline temperature of the semi-crystalline organic material, compared to that used in Example 1. Providing the semi-crystalline organic material in the form of solid particles reduces viscosity until the heating step is performed, at which point the yield stress and viscosity increase on demand to produce an adhesive that is highly resistant to wash-off.