Curing agents for epoxy resins

This application is a continuation of International Patent Application No. PCT/US2009/064929 filed Nov. 18, 2009, which claims the benefit of U.S. Provisional Patent Application No. 61/116,328 filed Nov. 20, 2008, the contents of both of which are incorporated herein by reference.

Epoxy resins are used as adhesives, coatings, and molding compounds in the manufacture of articles that undergo thermal cycling either during fabrication or operation.

Two common curing agents for epoxy resins are aromatic amines or phenols. Aromatic amine curing agents give higher cross-link density, and consequently higher Tg values than phenol curing agents. It would be desirable to have a curing agent for epoxy resins that give a balance of properties to the cured epoxy.

This invention is a curable composition comprising an epoxy resin and a curing agent for the epoxy resin that has two or more different functionalities, both reactive with epoxy. The functionalities are those that can either react and form covalent bonds with the epoxy or accelerate the epoxy curing reaction. The presence of the two or more different functionalities on the curing agent will cause the formation of mixed epoxy polymer networks, which can enhance the physical and mechanical properties of the cured epoxy.

The curing agent will be a compound having one or more five- or six-membered aromatic rings, and substituted on the one or more five- or six-membered rings at least two functionalities selected from the group consisting of hydroxyl, amine, imidazole, azine, hydrazide, anhydride, and Lewis acid groups. The five- or six-membered aromatic ring optionally may contain heteroatoms (nitrogen or sulfur); optionally may be fused to one or more other aromatic or aliphatic cyclic rings; and optionally may contain an electron-withdrawing or electron-donating group.

Exemplary structures of these curing agents include

in which f 1 and f 2 independently are selected from the group consisting of hydroxyl, amine, azine, hydrazide, anhydride, imidazole or Lewis acid, provided that f 1 and f 2 are different; (the repeating aryl rings in the first structure, non-fused, connected by R, can be connected at the ortho, meta, or para positions on the benzene ring);

n and m independently are numerals from 0 to 4, provided that when n is 0, x is at least 1;

x is a numeral from 0 to 100, preferably 0 to 20, provided that when x is 0, n is at least 1 and R 1 is f 2 ;

R is a monovalent or polyvalent radical, which includes hydrocarbyl, heteroatom-containing hydrocarbyl, substituted heteroatom-containing hydrocarbyl, hydrocarbylene, substituted hydrocarbylene, heteroatom-containing hydrocarbylene, substituted heteroatom-containing hydrocarbylene, polysiloxanes, polysiloxane-polyurethane block copolymers, and combinations of any of the above linked with one or more linking groups selected from the group consisting of a covalent bond, —O—, —S—, —NR 3 —, —O—C(O)—, —O—C(O)—O—, —O—C(O)—NR 3 —, —NR 3 —C(O)—, —NR 3 —C(O)—NR 3 —, —N═N—, —S—C(O)—,

in which R 3 is independently for each position selected from the group consisting of hydrogen, alkyl, alkylene, alkoxy, acrylate, aryl, and maleimide;

R 1 and R 2 independently are selected from the group consisting of —R, —H, —CH 3 , —OCH 3 , —O(CH 2 ) n CH 3 , —(CH 2 ) n f 3 in which n is a numeral from 1 to 20 and f 3 is a hydrocarbyl group containing at least one carbon to carbon unsaturated bond (e.g. a vinyl group, and allyl group, an aryl group);

in which R 3 independently for each position is selected from the group consisting of hydrogen, alkyl, alkylene, alkoxy, acrylate, aryl, and maleimide; provided that when x is 0, R 1 is f 1 .

The functionalities are chosen so that one will provide a densely cross-linked and high Tg polymer network, and the other a less densely cross-linked and lower Tg polymer network. This combination will provide both strength and toughness (flexibility) in the cured composition.

The introduction of bulky side chains, R 1 and R 2 , on the exemplary structures can increase steric hindrance and thus reduce reactivity and improve the pot life of the formulation. One example of this is depicted in the following reaction scheme:

In another embodiment, to further reduce reactivity, a bulky side chain with a carbonyl group can be introduced to stabilize the transition state of the reaction intermediate. This is depicted in the following reaction scheme:

In one embodiment, the curing agents have both amine and phenol functionality. The amine and phenol functionalities work simultaneously to increase cure speed and give balanced performance to the cured adhesive. The amine-cured epoxy component gives a more densely cross-linked, and consequently higher Tg, polymer network than the phenol-cured epoxy component. The phenol-cured epoxy component provides a more linear polymer structure, which contributes to stress reduction.

Particular curing agents are selected from the group consisting of

in which R 1 and R 2 independently are selected from the group consisting of —H; —CH 3 ; —OCH 3 ; —O(CH 2 ) n CH 3 ;

and —(CH 2 ) n f 3 , in which n is a numeral from 1 to 20, preferably 1 to 10, and f 3 is a hydrocarbyl group containing at least one vinyl functionality;

and

in which R 3 independently for each position is selected from the group consisting of hydrogen, alkyl, alkylene, alkoxy, acrylate, aryl, and maleimide;

and in which R 4 is selected from the group consisting of

and —(CH 2 ) n —, in which n is a numeral from 1 to 6.

Suitable epoxy resins for formulating the curable compositions include bisphenol, naphthalene, and aliphatic type epoxies. Commercially available materials include bisphenol type epoxy resins (Epiclon 830LVP, 830CRP, 835LV, 850CRP) available from Dainippon Ink & Chemicals, Inc.; naphthalene type epoxy resins (Epiclon HP4032) available from Dainippon Ink & Chemicals, Inc.; aliphatic epoxy resins (Araldite CY179, 184, 192, 175, 179) available from Ciba Specialty Chemicals, (Epoxy 1234, 249, 206) available from Dow, and (EHPE-3150) available from Daicel Chemical Industries, Ltd. Other suitable epoxy resins include cycloaliphatic epoxy resins, bisphenol-A type epoxy resins, bisphenol-F type epoxy resins, epoxy novolac resins, biphenyl type epoxy resins, naphthalene type epoxy resins, and dicyclopentadienephenol type epoxy resins.

The stoichiometric ratio of amine curing agent to epoxy resin should be as close to 1:2 as possible to form a highly cross-linked, dense polymer network. The stoichiometric ratio of phenol curing agent to epoxy should be as close to 1:1 as possible to form a low cross-linked, non-dense polymer network.

In some compositions it may be desirable to include a filler. Suitable nonconductive fillers include silica, calcium carbonate, mica, titania, sand, glass, and polymer fillers. When included, fillers will be present in amounts of 10˜90% by weight of the formulation.

Sample formulations include those disclosed in the following table.

The compounds used in the Examples were purchased from Aldrich, with the exception that 1-[(2-methyl-1H-imidazole-1yl)methyl]-2-naphthalenol was purchased from Huntsman. The compounds have the following structures: