Storage-stable heat-activated tertiary amine catalysts for epoxy resins

Abstract

Epoxy adhesive compositions contain a heat-activatable catalyst. The heat-activatable catalyst includes a tertiary amine catalyst and a novolac resin that has a weight average molecular weight of at least 3000. One-component epoxy adhesive formulations that contain the heat-activatable catalyst have unexpectedly good storage stability.

Claims

1. A one-component epoxy adhesive, comprising A) one or more epoxy resins; B) at least one epoxy hardener having a melting temperature of at least 50 C.; and C) a particulate heat-activatable catalyst composition having a volume average particle size of 100 nanometers to 10 microns comprising a mixture of a liquid aminophenol compound selected from the group consisting of 2,6-bis(dimethylaminomethyl)phenol and 2,4-bis(dimethylaminomethyl)phenol, or a solid aminophenol compound that has a boiling temperature of at least 130 C. and a melting temperature of no greater than 60 C.; and a phenol novolac resin that has a weight average molecular weight of 10,000 to 25,000 and a softening temperature of 130 to 200 C.; wherein the weight ratio of the aminophenol compound and the phenol novolac resin is from 40:60 to 70:30.

2. The one-component epoxy adhesive of claim 1 wherein the heat-activatable catalyst composition is made in a process that includes the steps of: a) mixing the aminophenol compound with a liquid solution of the phenol novolac resin in ethanol or methanol, at a temperature above the melting temperature of the aminophenol compound but not greater than 80 C. and not greater than the boiling temperature of the ethanol or methanol and at proportions such that from 0.1 to 10 equivalents of phenol groups are provided by the phenol novolac resin per equivalent of tertiary amine groups provided by the aminophenol compound, whereby the reaction mixture separates into a liquid phase containing mainly the ethanol or methanol and a precipitate of the aminophenol compound and the phenol novolac resin; b) separating the precipitate from the liquid phase while maintaining the temperature of the precipitate at no greater than 80 C. and c) drying the separated precipitate at a temperature of no greater than 80 C. to remove residual ethanol or methanol from the precipitate.

Description

EXAMPLES 1-2

(1) 44 parts of a 10,000 molecular weight phenol-formaldehyde novolac resin having a softening temperature of 130 C. (Resicure 3057, from SI Group) are melted at 140 C. 56 parts of 2,4,6-tris(dimethylaminomethyl)phenol are added to the molten novolac resin, and the resulting mixture is stirred at 140 C. for 30 minutes. A clear solution is obtained, which is cooled to room temperature and ground to a powder. The resulting heat-activatable catalyst is designated Example 1.

(2) Example 2 is prepared by dissolving 44 parts of a 24,000 molecular weight phenol-formaldehyde novolac resin (Resicure 5200, from SI Group) having a softening temperature of 180 into methanol. The resulting solution is combined with 56 parts of 2,4,6-tris(dimethylaminomethyl)phenol and brought to reflux for two hours to form a clear solution. The mixture is cooled to room temperature and the methanol is removed by evaporation. The resulting solid is dried and ground to a powder.

EXAMPLE 3

(3) A simple epoxy adhesive formulation is prepared to evaluate the curing activity of heat-activatable catalyst Examples 1 and 2. The formulation contains DER 331 epoxy resin, dicyandiamide and various amounts of heat-activatable catalyst Example 1. DER 331 is a liquid diglycidyl ether of bisphenol A, available from The Dow Chemical Company. It has an epoxy equivalent weight of approximately 187. Three samples, designated Examples 3-A through 3-E, are prepared. The ratios of these ingredients are as given in Table 1.

(4) Curing characteristics are evaluated by dynamic scanning calorimetry on a Q2000 instrument from TA Instruments. 5-15 mg of sample are tested under dry nitrogen. The samples are heated from 20 C. to 250 C. at 10 C./minute, held at 250 C. for 30 minutes, then cooled to room temperature at 10 C./minute and then reheated to 250 C. at 10 C./minute. The cure on-set temperature, peak exotherm temperature, T.sub.g of the cured resin and enthalpy are all determined. Results are as indicated in Table 1.

(5) TABLE-US-00001 TABLE 1 Parts by Weight Ex. 3-A Ex. 3-B Ex. 3-C Ex. 3-D Ex. 3-E Ingredients Epoxy resin 93.49 92.31 89.94 93.7 91.8 Dicyandiamide 5.26 5.19 5.06 5.3 5.2 Catalyst Example 1 1.25 2.50 5.00 0 0 Catalyst Example 2 0 0 0 1 3 Test Results On-set 146 136 126 146 132 temperature, C. Peak temperature, 169 160 148 164 151 C. T.sub.g, C. 123 123 123 134 125 Enthalpy, J/g 255 282 338 289 310

EXAMPLE 4 AND COMPARATIVE SAMPLE A

(6) A toughened crash-durable epoxy adhesive is prepared by mixing 54.65 parts of a mixture of liquid diglycidyl ethers of bisphenol-A, 13.22 parts of an adduct of a butadiene-acrylonitrile rubber and an epoxy resin, available commercially from Schill and Seilacher as Struktol 3914, 14 parts of a polyurethane toughener having capped terminal isocyanate groups, 1.49 parts of a wetting agent, 0.72 parts of an epoxy silane, 4.33 parts of dicyandiamide, 5.16 parts of fumed silica, 1 part of a heat-activatable catalyst and 5.43 parts of fillers.

(7) For Example 4, the heat-activatable catalyst is the Example 1 catalyst. For Comparative Sample A, the heat-activatable catalyst is 2,4,6-tris(dimethylaminomethyl)phenol in a poly(vinylphenol matrix). Active catalyst levels are the same in Example 4 and Comparative Sample A.

(8) The storage stabilities of Example 4 and Comparative Sample A are evaluated at various temperatures as indicated in Table 2 below, by making viscosity measurements at the start of testing and after storing at the specified temperatures for the indicated periods of time. Testing is performed on a Bohlin CS-50 rheometer and a 4/20 mm plate/cone system. The samples are conditioned at 45 C. for five minutes. While holding the sample at this temperature, the shear rate is increased from 0.1/second to 20/second over five minutes, and then decreased back to 0.1/second at the same rate. Viscosity at 10/second is measured. Results are as indicated in Table 2.

(9) TABLE-US-00002 TABLE 2 Storage Initial Conditions Viscosity, Final Ratio, (Temperature, Pa .Math. s Viscosity, Final/Initial % time) (10/sec) cP Viscosity Increase** 23 C., 24 weeks Ex. 4 130 152 1.17 17% Comp. Sample A* 126 174 1.38 38% 30 C., 24 weeks Ex. 4 130 176 1.35 35% Comp. Sample A 126 235 1.86 86% 40 C., 24 weeks Ex. 4 130 402 3.1 210% Comp. Sample A 126 561 4.4 340% *Not an example of the invention. **Calculated as 100% [(final viscosity initial viscosity)/initial viscosity].

(10) In all cases, significantly better storage stability is seen with the catalyst of the invention.

EXAMPLES 5-6

(11) A 10,000 molecular weight phenol-formaldehyde novolac resin having a softening temperature of 130 C. (Resicure 3057, from SI Group) is ground until it passes through a 100 US mesh screen. 2,4,6-tris(dimethylaminomethyl)phenol (Ancamine K54, from Air Products and Chemicals) is added to an equal weight of the ground novolac resin, and the resulting mixture is stirred at room temperature for 30 minutes.

(12) A 5-15 mg portion of this mixture is then evaluated by DSC on a Model Q2000 modulated analyzer from TA Instruments. The sample is heated from 20 C. to 200 C. at a rate of 10 C./minute. An exotherm is seen commencing at a sample temperature of about 64-67 C. and reaching a peak at about 83-84 C. This exotherm is interpreted as hydrogen bound formation or other complexation between the phenol groups of the novolac resin and the amine groups of the catalyst.

(13) Another portion of the mixture is heated from 23 to 130 C. while measuring its viscosity. The viscosity of the mixture is below 5000 cP until the temperature reaches about 70 C. As the temperature increases from 70 C. to about 90 C., the viscosity of the mixture increases rapidly to over 250,000 cP. The viscosity then decreases to about 14,500 cp at 110 C. and to about 4,000 cp at 130 C.

(14) A third portion of the mixture is heated to 130 C., cooled to room temperature and ground to a powder that passes through a 100 US Mesh screen. The resulting heat-activatable catalyst is designated Example 5, and is formulated into Epoxy Adhesive Example 5 as described below.

(15) Example 6 is prepared and tested in the same manner, except that the ratio of 2,4,6-tris(dimethylaminomethyl)phenol to novolac resin is 44:56 by weight. When evaluated by DSC in the manner described with respect to Example 5, an exotherm is seen commencing at a sample temperature of about 63-68 C. and reaching a peak at about 82-84 C. When the viscosity is measured as described with respect to Example 5, it is seen to increase rapidly from less than 10,000 cP at 65 C. to almost 600,000 cP at about 85 C., after which the viscosity decreases to about 52,000 cP at 110 C. and to about 18,000 cP at 130 C.

(16) A third portion of the mixture is heated to 130 C., cooled to room temperature and ground to a powder. The resulting heat-activatable catalyst is designated Example 6, and is formulated into Epoxy Adhesive Example 6 as described below.

(17) Epoxy Adhesive Examples 5 and 6, which contain heat-activatable catalysts 5 and 6, respectively, are prepared by mixing the ingredients indicated in Table 3:

(18) TABLE-US-00003 TABLE 3 Parts by Weight Ingredient Adhesive Ex. 5 Adhesive Ex. 6 Epoxy Resin A.sup.1 49 49 Carboxyl-terminated butadiene- 4 4 acrylonitrile rubber Capped polyurethane toughener.sup.2 25 25 Epoxy Resin B.sup.3 2 2 Epoxy Silane.sup.4 0.2 0.2 Dicyandiamide.sup.5 5 5 Fumed silica 4 4 Fillers 10.8 10.8 Catalyst Example 1 0.9 0 Catalyst Example 2 0 0.9 .sup.1Epoxy Resin A is a liquid diglycidyl ether of bisphenol A having an epoxy equivalent weight of approximately 192. It is available from The Dow Chemical Company as DER 331. .sup.2An isocyanate-terminated polyurethane prepolymer prepared from a polyether polyol and an aliphatic diisocyanate, in which the isocyanate groups are capped with o,o-diallyl bisphenol A, and is made as described in Example 13 of EP 308 664. Toughener A has an M.sub.n of 6,900 and an M.sub.w of 13,200. .sup.3Epoxy resin B is a diglycidyl ether of a poly(propylene glycol), which has an epoxy equivalent weight of about 320. It is available from The Dow Chemical Company as DER 732. Toughener B is made according to processes described in US 2005/0070634 A1. .sup.4Dynasilan A187, available from Evonik Industries AG. .sup.5NC700, from Cardolite. .sup.5Amicure CG-1200, from Air Products and Chemicals.

(19) Impact peel testing is performed on each of Adhesive Examples 5 and 6 at 23 C. in accordance with ISO 11343 wedge impact method. The adhesives are cured for 20 minutes at 165 C. on cold rolled steel substrates. Testing is performed at an operating speed of 2 m/sec. Results are as indicated in Table 4 below.

(20) The storage stabilities of Adhesive Examples 5 and 6 are evaluated storing the adhesive at 40 C. in a sealed container for one week. Viscosity measurements are made at the start of testing and after storing. Testing is performed at 38 C. on an ARES-G2 shear rheometer and a 25 mm parallel plate system. The samples are conditioned at 38 C. for one minute. While holding the sample at this temperature, the shear rate is brought to a shear rate of 30/second for four minutes. Viscosity at a shear rate of 30/second is measured. Results are indicated in Table 4.

(21) TABLE-US-00004 TABLE 4 Impact Peel Initial Final % Viscosity Strength, Example No. Viscosity, cP Viscosity, cP Increase N/mm 5 115 139 20 30 6 113 128 14 31

(22) The data in Table 4 demonstrates that a one-part epoxy adhesive in accordance with the invention exhibits good storage stability, and cures well to form a strong adhesive bond.

EXAMPLES 7-8 AND COMPARATIVE SAMPLE B

(23) 56 parts by weight of a 10,000 molecular weight phenol-formaldehyde novolac resin having a softening temperature of 130 C. (Resicure 3057, from SI Group) is mixed with 350 parts of methanol and dissolved by heating to 45 C. The resulting solution is stirred and 44 parts by weight of room temperature 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30, from The Dow Chemical Company) is slowly added. A mixture of the novolac resin and 2,4,6-tris(dimethylaminomethyl)phenol) precipitates from the solution as the catalyst is added, forming a solid phase and a liquid phase that contains mainly methanol. The methanolic liquid phase is poured off. The solid phase is then pulverized and dried at 50 C. for two hours. The resulting material is designated as Catalyst Example 7.

(24) A one-part epoxy adhesive (Adhesive Example 7) identical to Adhesive Examples 5 and 6 is prepared, except that the Catalyst Example 7 is substituted for the catalysts present in Adhesive Examples 5 and 6.

(25) The impact peel strength of Adhesive Example 7 is measured in the manner indicated with respect to Adhesive Examples 5 and 6. The average impact peel strength over three samples is about 31 N/mm, and cohesive failure is seen in all cases.

(26) Adhesive Example 8 is made in the same manner as Adhesive Example 7, except that the catalyst is a 44/56 by weight mixture of the Resicure 3057 novolac resin and 2,4,6-tris(dimethylaminomethyl)phenol, made by melting the resin, adding the aminophenol catalyst to the molten novolac resin, stirring the resulting mixture at 140 C. for 30 minutes and the cooling the resulting mixture to room temperature and grinding it to a powder.

(27) Comparative Example B is also made in the same manner as Adhesive Example 7, except that the catalyst is a commercially available 2,4,6-tris(dimethylaminomethyl)phenol/poly(vinyl phenol) mixture (EP 796 from Huntsman Chemicals).

(28) Storage stability of Adhesive Examples 7 and 8 and Comparative Sample B are determined as described with respect to Adhesive Examples 5 and 6. Results are indicated in Table 5.

(29) TABLE-US-00005 TABLE 5 Viscosity after 1 Initial week at 40 C. Sample Viscosity, cP storage, cP % increase Adhesive Example 7 113 119 5 Adhesive Example 8 115 166 44 Comparative Sample B 109 290 151

(30) As can be seen from the data in Table 5, the adhesives containing the heat-activated catalyst of the invention are substantially more storage stable at 40 C. than one (Comparative Sample B) that instead contains the commercially available catalyst in which the tertiary amine is in a poly(vinyl phenol) matrix. Surprisingly, Adhesive Example 7 is also significantly more storage stable than Adhesive Example 8, which indicates that the manner in which the heat-activated catalyst is prepared can be important to its storage stability.