Aminoalkoxypentaerythritols

The present invention relates to aminoalkoxypentaerythritol compounds, and to methods for curing epoxy resins wherein aminoalkoxypentaerythritol compounds are employed as a curing agent. The present invention also relates to epoxy resin compositions containing aminoalkoxypentaerythritol compounds as epoxy curing agents.

Epoxy resins were first introduced commercially in the United States in about 1950 and since then their use has grown rapidly. Epoxy resins may be broadly defined as resinous intermediate materials which are characterized by the presence of the epoxy group, ##STR3## In general, epoxy resins are not used by themselves but rather they require the addition of a curing agent or hardener to convert them into a thermoset material. Epoxy resins have gained wide acceptance in structural applications and in protective coatings because of their generally excellent toughness, adhesion, chemical resistance, and electrical properties. The combination of these properties is generally not found in any other single plastic material.

A relatively large number of chemical reagents are available or known to have utility as curing agents or hardeners which may be added to epoxy resins to convert them to thermoset materials. It is also known that in the curing process both the epoxy and the hydroxyl groups of the resin may be involved. Curing agents are available whereby curing may be accomplished at room temperature or upon heating. Curing may take place in general either by a coupling or addition process, or by catalytic polymerization.

The known curing agents or hardeners for epoxy resins fall into three categories: (1) the acidic type, e.g., acid anhydrides; (2) aldehyde condensation products, e.g., phenol-, urea-, and melamine-formaldehyde resins; and (3) amine type, e.g., aliphatic and aromatic amines, polyamides, tertiary amines, and amine adducts. The novel aminoalkoxypentaerythritol compounds of the present invention may be employed as the third type, namely the amine type, of epoxy curing agent.

The aminoalkoxypentaerythritol compounds of the present invention may be represented by the following structural formula: ##STR4## Where R.sub.1, R.sub.2, and R.sub.3 are independently selected from H and ##STR5## where R.sub.4 is selected from H and a lower alkyl group having from 1 to about 4 carbon atoms. Preferably in the above structural formula, all of the available hydroxyl groups are reacted so that R.sub.1, R.sub.2 and R.sub.3 are all ##STR6## and R.sub.4 has the value mentioned above. Even more preferably such reaction is accomplished by means of acrylonitrile which is subsequently reduced so that R.sub.4 is H.

The present invention also relates to the use of such compounds as epoxy curing agents and to epoxy resin compositions which contain 100 parts by weight of at least one epoxy resin and from about 15 parts to about 50 parts, preferably about 20 parts to about 30 parts by weight of the aminoalkoxypentaerythritol compounds of the invention. Such cured products have good flexibility, high heat distortion temperatures and excellent solvent resistance.

The aminoalkoxypentaerythritols of the present invention may be prepared by a two step process. In a first step, a suitable ethylenically unsaturated nitrile, e.g., acrylonitrile, crotonitrile, or methacrylonitrile may be reacted with pentaerythritol under conventional reaction conditions in the presence of a conventional cyanoethylation catalyst such as sodium hydroxide, potassium hydroxide or lithium hydroxide. In this first step one, several or all of the free hydroxyl groups may be cyanoalkylated to provide a cyanoalkylated intermediate. The preferred unsaturated nitrile for use in the cyanoalkylation is acrylonitrile, and preferably all of the available hydroxyl groups of the pentaerythritol are cyanoalkylated. According to such preferred embodiment, the intermediate compound which is provided is tetra(-2-cyanoethoxy)pentaerythritol. For convenience the details of the invention will be described with particular reference to acrylonitrile as the cyanoalkylation agent and to tetra(-2-cyanoethoxy)pentaerythritol as the cyanoalkylated intermediate.

The second step in the preparation of the novel compounds of the invention is the hydrogenation of the cyanoalkyl groups of the cyanoalkylated intermediate, e.g., tetra(-2-cyanoethoxy)pentaerythritol. Such reduction may be accomplished with hydrogen and ammonia either in the presence or absence of a solvent and a metal catalyst. The corresponding tetraamine may be produced in high yields as shown in the equation below: ##STR7##

The temperature at which the above described reduction of tetranitrile may be carried out can vary widely. However, generally the temperature may be within a range of from about 30.degree.-150.degree. C. and preferably in the range of 80.degree.-125.degree. C. Likewise, the period of time required for the reaction to go to substantial completion can vary widely, such being dependent on the hydrogen pressure and the particular catalyst employed as well as the temperature at which such reaction is carried out. Generally, however, the reaction proceeds to completion when the reactants are contacted at the required temperature for a period of time from about 0.5 to about 4 hours. Pressures in the range of about 1000 to about 5000 psi can be used to accomplish the reduction. While pressure in the higher range (2100-5000psi) can be used and may be advantageous, the preferable range of pressure is 1000-2000 psi. The use of ammonia in this reduction is a critical factor in the process to maintain high yields of the corresponding tetramine. The presence of ammonia serves to inhibit formation of secondary amines. The amount of ammonia used can vary from 5 moles of ammonia per mole of tetranitrile to 30 moles per mole of tetranitrile. Preferably 12 to about 20 moles of ammonia are used per mole of tetranitrile.

The hydrogenation of the tetranitrile can be carried out in the presence or absence of a solvent. When solvent is employed, any suitable solvent which will not interfere with the desired hydrogenation can be employed, such as cycloaliphatic ethers, e.g., dioxane, tetrahydrofuran, and the like and higher boiling hydrocarbons, e.g., hexane, cyclohexane, heptane, decane, toluene, xylenes, and the like, and alcohols, e.g, methyl alcohol, ethyl alcohol, isopropyl alcohol, isobutyl alcohol and the like.

In carrying out the reduction of the tetranitrile any suitable reduction catalyst can be employed. Typical of such reduction catalysts are raney nickel, cobalt, palladium, platinum, ruthenium, rhodium, osmium, iridium, iron, including salts and oxides thereof and the like. Further, such catalysts can be in their free metal state or extended on a support such as charcoal, aluminum, kieselguhr and the like.

The amount of catalyst employed in the reduction can vary widely. However, generally the amount of catalyst will vary from about 1 to 30 weight percent, preferably from about 5 to 10 weight percent.

The process of reducing the tetranitrile has been described as a batch operation carried out in a high pressure stirred autoclave. However, other reducing modes will give equally good results. A continuous flow reactor may be used with suspended or fixed bed solid catalyst operating at the proper temperature, pressure, and flow rate to give the desired reduction. Alternatively, the desired reduction can be accomplished by incremental addition of the tetranitrile to a batch type reactor at the proper described operating conditions.

One particularly important use for the novel compounds of the present invention is their use as epoxy curing agents for polyepoxides. The polyepoxides which can be cured at elevated temperatures using the amino compounds as herein described are those polyepoxides possessing at least two ##STR8## groups. These groups may be terminal, i.e., ##STR9## groups, or they may be in an internal position. However, especially desirable results can be obtained when the epoxy groups are terminal. The polyepoxides may be saturated or unsaturated, aliphatic, cycloaliphatic, aromatic or heterocyclic, and may be substituted such as with hydroxy groups, ether radicals and the like. Further, the polyepoxides can be monomeric or polymeric. Such polyepoxides, and their preparation, are well known in the art.

The curing of the polyepoxides with the above-described amino compound curing agents of the present invention may be accomplished by simply mixing the two components together. While the reaction between the two components may occur slowly at room temperature, improved results can be obtained if the mixture is heated to a temperature of from about 50.degree. C. to about 280.degree. C. for a period of time of from about 1 to about 12 hours and thereafter post-curing the reaction product for an additional period of time from about 1 to about 8 hours at a temperature of from about 140.degree. C. to about 225.degree. C. With a small casting, curing of the reaction mixture can be obtained by heating the reaction mixture for about 2 hours at a temperature of from about 80.degree. C. to about 100.degree. C. and thereafter post-curing the reaction product at a temperature of from about 140.degree. C. to about 225.degree. C. for an additional 2 hours or so.

In curing polyepoxides it is generally desirable that the polyepoxide be in a mobile condition when the curing agent is added to ensure uniform mixing. If the polyepoxide is extremely viscous or solid at room or casting temperature, the polyepoxide may be heated to reduce the viscosity or a volatile liquid solvent which can escape from the polyepoxide composition containing the novel amino compound curing agent by evaporation before and/or during the curing of such polyepoxide composition can be added to the polyepoxide to reduce its viscosity. Typical of such volatile liquid solvents are ketones, such as acetone, methyl ethyl ketone and the like, ethers, such as ethyl acetate, butyl acetate and the like, ether alcohols, such as methyl, ethyl or butyl ethers of ethylene glycol and chlorinated hydrocarbons, such as chloroform.

In addition to the use of the amino compounds of the present invention as epoxy curing agents, many other uses can readily be envisioned by those skilled in the art. Thus, not only do the compounds of the present invention find utility as epoxy curing agents but such compositions can be employed as oil and fuel adductive intermediates. Further, the polyamines may be employed for the formation of diisocyanate compositions for the incorporation into polyurethane compositions, and the compound may be further reacted to form novel and useful polyamides.

In order to more fully describe the preparation and use of the novel compounds of the present invention the following examples are given. However, such examples are presented for illustration only and are not to be construed as unduly limiting the scope of the present invention. Unless otherwise indicated, all parts and/or percentages given in these examples are by weight.

In a 5000 cc. three necked flask equipped with an overhead stirrer, reflux condenser, nitrogen purge, dropping funnel, and thermometer was charged 144 ml. of water, 9.0 gm. of anhydrous lithium hydroxide, 240 ml. THF, and 816.9 gm. (6.0 moles) of pentaerythritol. The overhead stirrer was then adjusted to high speed and the reaction contents were preheated to .about.40.degree. C. with a water bath. Acrylonitrile was then added through the dropping funnel. Over the course of 3.3 hours a total of 1820 ml. of acrylonitrile was added and the reaction temperature reached a maximum of 60.degree. C. while the water bath was maintained above 40.degree. C. if necessary. The reaction flask was then post-heated at 55.degree.-60.degree. C. for an additional 6 hours. The reaction contents were cooled to room temperature and neutralized with glacial acetic acid. The reaction contents were then filtered. The crude filtered reaction product was stripped of all excess volatiles such as THF, water and acrylonitrile under a vacuum (15-30mmHg) and at a temperature not exceeding 60.degree. C. to give tetra(-2-cyanoethoxy)pentaerythritol. This non-viscous liquid tetranitrile was considered to be suitable for use without further purification.

In a two liter autoclave was charged 252 gm. of tetra(-2-cyanoethoxy)pentaerythritol, 556 gm. of toluene and 20 gm. of 5 percent ruthenium on alumina. The autoclave was pressure checked to 2000 psi with hydrogen for leaks. Afterwards, 160 gm. of liquid ammonia was charged. The pressure was then adjusted to 400 psi with hydrogen gas and the autoclave was heated to 125.degree.-130.degree. C. where a pressure of 1600-1850 psi developed. Samples were pulled from the autoclave and analyzed until the reaction was near completion. After three hours at 125.degree.-130.degree. C., the reaction was post-heated at 145.degree.-150.degree. C. for an additional 45 minutes. The autoclave was then cooled and the contents were emptied. The excess ammonia, toluene and any n-propylamine were removed from the product under vacuum (15-30 mmHg) to give a colorless liquid. This colorless liquid was stripped further under higher vacuum (0.5 to 2.0 mmHg) to remove any reduced dicyanoethylated water impurity present. An IR spectrum of the final stripped tetra(-3-aminopropoxy)pentaerythritol indicated that all the nitrile groups had been reduced. A potentiometric titration of the product (a composite of Examples 2-6) with 1N HCl resulted in a neutralization equivalent of 10.57 milliequivalents of HCl per 1 gm. of product. The theoretical value for tetra(3-aminopropoxy)pentaerythritol (MWT=364.15 g/mole) was calculated to be 10.98milliequivalents of HCl per gm. of tetramine which is in good agreement with the observed experimental value.

In a two liter autoclave was charged 288 gm. of tetra(-2-cyanoethoxy)pentaerythritol, 552 gm. of toluene and 20 gm. of 5percent ruthenium on alumina (the autoclave contained 20 gm. of 5 percent ruthenium on alumina from Example 1). The autoclave was pressure checked to 2000psi with hydrogen for leaks. Afterwards, 152 gm. of liquid ammonia was charged. The pressure was then adjusted to 400 psi with hydrogen gas and the autoclave was heated to 125.degree.-130.degree. C. where a pressure of 1600-1800 psi developed. Samples were pulled from the autoclave and analyzed until the reaction was near completion. After three hours at 125.degree.-130.degree. C., the reaction was post-heated at 145.degree.-150.degree. C. for an additional four hours. The autoclave was then cooled and the contents were emptied. The excess ammonia, toluene and any n-propylamine were removed from the product under vacuum (15-30 mmHg) to give a colorless liquid. This colorless liquid was stripped further under higher vacuum (0.5 to 2.0 mmHg) to remove any reduced dicyanoethylated water impurity present. An IR spectrum of the final stripped tetra(-3-aminopropoxy)pentaerythritol indicated that all the nitrile groups had been reduced.

In a two liter autoclave was charged 25.0 gm. of tetra(-2-cyanoethoxy)pentaerythritol, 552 gm. of toluene and 20 gm. of 5 percent ruthenium on alumina. The autoclave was pressure checked to 2000 psi with hydrogen for leaks. Afterwards 149 gm. of liquid ammonia was charged. The pressure was then adjusted to 400 psi with hydrogen gas and the autoclave was heated to 125.degree.-130.degree. C. where a pressure of 1650-1700 psi developed. Samples were pulled from the autoclave and analyzed until the reaction was near completion. After three hours at 125.degree.-130.degree. C., the reaction was post-heated at 145.degree.-150.degree. C. for an additional 30 minutes. The autoclave was then cooled and the contents were emptied. The excess ammonia, toluene and any n-propylamine were removed from the product under vacuum (15-30 mmHg) to give a colorless liquid. This colorless liquid was stripped further under higher vacuum (0.5 to 2.0 mmHg) to remove any reduced dicyanoethylated water impurity present. An IR spectrum of the final stripped tetra(-3-aminopropoxy)pentaerythritol indicated that all the nitrile groups had been reduced.

In a two liter autoclave was charged 260 gm. of tetra(-2-cyanoethoxy)pentaerythritol, 560 gm. of toluene and 20 gm. of 5 percent ruthenium on alumina (the autoclave contained 20 gm. of 5 percent ruthenium on alumina from Example 4). The autoclave was pressure checked to 2000 psi with hydrogen for leaks. Afterwards 148 gm. of liquid ammonia was charged. The pressure was then adjusted to 400 psi with hydrogen gas and the autoclave was heated to 125.degree.-130.degree. C. where a pressure of 1500-1550 psi developed. Samples were pulled from the autoclave and analyzed until the reaction was near completion. After three hours at 125.degree.-130.degree. C., the reaction was post-heated at 145.degree.-150.degree. C. for an additional 1 hour. The autoclave was then cooled and the contents were emptied. The excess ammonia, toluene and any n-propylamine were removed from the product under vacuum (15-30 mmHg) to give a colorless liquid. This colorless liquid was stripped further under higher vacuum (0.5 to 2.0 mmHg) to remove any reduced dicyanoethylated water impurity present. An IR spectrum of the final stripped tetra(-3-aminopropoxy)pentaerythritol indicated that all the nitrile groups had been reduced.

In a two liter autoclave was charged 250 gm. of tetra(-2-cyanoethoxy)pentaerythritol, 550 gm. of toluene and 20 gm. of 5 percent ruthenium on alumina. The autoclave was pressure checked to 2000 psi with hydrogen for leaks. Afterwards 150 gm. of liquid ammonia was charged. The pressure was then adjusted to 400 psi with hydrogen gas and the autoclave was heated to 125.degree.-130.degree. C. where a pressure of 1800-1900 psi developed. Samples were pulled from the autoclave and analyzed until the reaction was near completion. After three hours at 125.degree.-130.degree. C., the reaction was post-heated at 145.degree.-150.degree. C. for an additional 1 hour. The autoclave was then cooled and the contents were emptied. The excess ammonia, toluene and any n-propylamine were removed from the product under vacuum (15-30 mmHg) to give a colorless liquid. This colorless liquid was stripped further under higher vacuum (0.5 to 2.0 mmHg) to remove any reduced dicyanoethylated water impurity present. An IR spectrum of the final stripped tetra(-3-aminopropoxy)pentaerythritol indicated that all the nitrile groups had been reduced.

To five beakers each containing 100 parts of epoxy resin based on diglycidyl ether of bisphenol A (n=0.2, WPE=185-195) were added the corresponding ##STR10## parts of tetra(-3-aminopropoxy)pentaerythritol prepared in Examples 2-6: 24.6 parts, 25.1 parts, 25.6 parts, 26.1 parts, and 26.6 parts. After mixing each beaker thoroughly for 2 minutes and centrifuging at a speed of 3000 rpm, these resin mixtures were placed in an aluminum mold and were cured for 2 hours at 80.degree. C. and for another 2 hours at 150.degree. C. The crosslinked products had glass transition temperatures measured with a differential scanning calorimeter (Perkin Elmer Model DSC-2) as are summarized in the table below.

The above examples clearly indicate the preparation of the novel compounds of the present invention. Further, Example 7 illustrates the use of tetra(-3-aminopropoxy)pentaerythritol as a curing agent.