Curable resin compositions

Details Curable resin compositions Curable resin compositions

2000-06-09

2003-06-10

Dawson, Robert (Department: 1712)

Coating processes

With post-treatment of coating or coating material

Heating or drying

C528S087000, C528S088000, C528S118000, C528S122000, C528S408000, C528S117000, C544S358000, C546S184000, C548S400000

Reexamination Certificate

active

06576297

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to ambient temperature curable compositions, comprising an epoxy resin and an amine-functional curing agent and useful as coatings, adhesives or sealants, and to a process for curing the compositions.
BACKGROUND OF THE INVENTION
The use of heterocyclic secondary amines such as piperidine, pyrrolidine, aminoethylpiperazine or anabasine as curing agents is discussed in the textbook “Handbook of Epoxy Resins” by Lee and Neville published by McGraw-Hill, 1967. They are generally used in small amounts to promote mainly self-condensation of epoxide groups. Piperidine, for example, is customarily used at 5-7% by weight based on an epoxy resin such as the diglycidyl ether of bisphenol A (DGEBA). 2,3-Bipiperidine is described by Forostyan et al. in Plasticheskie Massy, 1965(1), 16-17 as providing cures with DGEBA in 2 hours at 80° C.
US-A-4581454 describes adducts of aminohydrocarbyl piperazines and urea which are used as curing agents for epoxy resins, particularly in reaction injection moulding.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, a process for forming a layer of cured epoxy resin on a substrate comprises coating the substrate with a composition comprising an epoxy resin and an amine-functional curing agent and allowing the coating thus applied to cure at ambient temperature, and it is characterised in that the curing agent comprises a material containing at least two heterocyclic secondary amine groups. By a heterocyclic secondary amine group we mean a secondary amine group in which the amine nitrogen atom forms part of a heterocyclic ring.
The invention also provides for the use of a compound containing at least two heterocyclic secondary amine groups as curing agent for an epoxy resin, characterised in that the epoxy resin and the compound containing at least two heterocyclic secondary amine groups are applied together as a coating to a substrate and allowed to cure on the substrate at ambient temperature.
DETAILED DESCRIPTION OF THE INVENTION
The ambient temperature at which the coating is cured is generally below 40° C. and frequently below 25° C., and it may be below 10° C. or even below 0° C., down to −20° C. for example. At these low temperatures the epoxy resin compositions of the invention cure more rapidly than known epoxy resin compositions such as those using a diprimary amine as curing agent. We believe that initial curing of the epoxy resin is essentially through epoxide/amine reaction rather than self-condensation of epoxide groups.
The epoxy resin of the composition can in general be any of the epoxy resins described in the above textbook by Lee and Neville, preferably a glycidyl-type epoxy resin containing glycidyl ether or ester groups. The epoxy resin can for example be a glycidyl ether of a bisphenol such as DGEBA or can be a condensed or extended glycidyl ether of a bisphenol. Such glycidyl ethers derived from a bisphenol generally have an epoxy functionality of 2 or slightly less, for example 1.5 to 2. The epoxy resin can alternatively be a glycidyl ether of a polyhydric phenol, for example an epoxy novolak resin, or an aliphatic or cycloaliphatic di- or poly-glycidyl ether. Examples of epoxy resins containing glycidyl ester groups are homopolymers or copolymers of a glycidyl ester of an ethylenically unsaturated carboxylic acid such as glycidyl methacrylate or glycidyl acrylate, or the diglycidyl ester of dimerised fatty acid.
In most cases the curing agent preferably contains more than two, for example at least three, heterocyclic secondary amine groups, although mixtures of a curing agent with at least three such groups and a curing agent with at least two but less than three such groups can be used. It is strongly preferred that either the epoxy resin contains an average of more than two epoxy groups per molecule or the curing agent contains an average of more than two heterocyclic secondary amine groups per molecule, for example at least 2.5 or 3 heterocyclic secondary amine groups per molecule. The heterocyclic secondary amine groups can for example be part of a heterocycle containing 3 to 12 atoms in the ring, for example a saturated heterocycle such as a piperidine, piperazine, pyrrolidine, azetidine, aziridine, imidazolidine, oxazolidine, thiazolidine or homopiperazine (1,4-diazacycloheptane) ring, an unsaturated heterocycle such as an imidazoline ring or even an aromatic ring having a secondary amine group such as pyrrole or imidazole. For rings containing two hetero-N-atoms such as piperazine or imidazolidine, it is usually preferred that only one N atom in an individual ring is present as a secondary amine group; the ring can be attached to the remainder of the curing agent molecule through the other N atom, for example the other N atom of a piperazine ring.
Thus, according to another aspect of the invention a coating, sealant or adhesive composition curable at ambient temperatures of 40° C. or below comprises an epoxy resin and an amine-functional curing agent and is characterised in that the curing agent comprises a material containing at least three heterocyclic secondary amine groups.
The curing agent can for example be the reaction product of a primary amino-substituted heterocyclic secondary amine with a compound containing two or more, preferably at least three, groups which are reactive with primary amine groups but substantially unreactive with heterocyclic secondary amine groups. The primary amino-substituted heterocyclic secondary amine can for example be N-(2-aminoethyl)piperazine, 2-(2-aminoethyl)imidazoline, N-(3-aminopropyl)piperazine, 4-(aminomethyl)piperidine, 2-(aminomethyl)piperidine, 3-(aminomethyl)piperidine or 3-aminopyrrolidine, or a substituted derivative of any of the above containing for example one or more alkyl or alkoxy substituents. The groups which are reactive with primary amine groups but not with heterocyclic secondary amine groups can for example be beta-dicarbonyl groups such as acetoacetate groups and similar beta-ketoester groups or beta-diketone groups, other aldehyde or ketone groups, for example the aldehyde groups of glutaraldehyde, terminal urea groups —NHCONH
2
or imide- forming groups such as cyclic anhydrides or half-esters of vic-dicarboxylic acid groups capable of forming cyclic imides. Acid groups, particularly carboxylic acid groups, and their lower alkyl esters will also react preferentially with primary amine groups rather is than heterocyclic secondary amine groups. The curing agent can alternatively be formed by reaction of a polyfunctional reagent with pyridine substituted by a reactive functional group, followed by hydrogenation of the pyridine ring to generate secondary amine groups.
A di- or poly-acetoacetate ester, for example, can be reacted with N-aminoethylpiperazine or another primary amino-substituted heterocyclic secondary amine to bond the heterocylic secondary amine to the acetoacetate ester through an imine or enamine linkage. The reaction is shown below for trimethylolpropane tris(acetoacetate).
trimethylolpropane tris(2-piperazinoethyl)aminocrotonate (may exist in ketimine or enamine form)
Trimethylolpropane tris(acetoacetate) can be prepared from trimethylolpropane and a lower alkyl acetoacetate such as t-butyl acetoacetate by heating to transesterify, with removal of the volatile alcohol such as t-butanol by distillation. Poly(acetoacetate)esters can similarly be formed from other polyols such as pentaerythritol, 1,6-hexanediol, trimethylolethane or sorbitol or hydroxy-functional polymers such as acrylic polymers having pendant hydroxyl groups, for example homo- and co-polymers of 2-hydroxyethyl acrylate or methacrylate and polyesters, including hyperbranched or dendritic polymers having surface hydroxyl groups and 2,4,6-tris(hydroxymethyl)phenol and other compounds and oligomers formed by novolak condensation of an optionally substituted phenol and formaldehyde.
Glutaraldehyde will react with a primary amine to form one imine linkage followed by aldol condensation of the other aldehyde

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