Fast-curing epoxy systems

The present invention relates to a composition comprising a) at least one epoxy resin, b) at least one amine having at least two secondary amino groups which are both part of an organic ring system, and c) at least one salt of a very strong Brønsted acid with a counterion selected from metal ions, metal-containing ions, phosphonium ions and ammonium ions, and to processes for production thereof and use thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. § 119 patent application which claims the benefit of European Application No. 18172954.2 filed May 17, 2018, which is incorporated herein by reference in its entirety.

FIELD

Epoxy resins, especially those that are prepared from bisphenol A and epichlorohydrin, are known raw materials for the production of high-quality casting resins, coating compositions, composites and adhesives. Aromatic epoxy resins cured with polyamines have not only good chemical and solvent resistance but also good bond strength on many substrates.

BACKGROUND

EP 0 969 030 A1 discloses epoxy/amine coating systems wherein the amine component is an aliphatic amine. The compositions may include a catalyst. In the examples, the hardener used is additionally aminoethylpiperazine.

U.S. Pat. No. 4,775,734 A discloses the curing of epoxy resins with aminoethylpiperazine, bis(aminopropyl)piperazine or piperidine inter alia, using catalytic amounts of tetrafluoroborate or hexafluorophosphate salts of various amines.

EP 3 255 078 A1 discloses epoxy resin compositions comprising at least one epoxy compound, 2-(2,2,6,6-tetramethylpiperidin-4-yl)propane-1,3-diamine, and optionally a catalyst which may be an inorganic salt inter alia.

US 2012/0010330 A1 discloses the curing of an epoxy resin with, inter alia, a cyclic diamine selected from piperazine and homopiperazine in the presence of at least 10% of an alkylphenol. However, disadvantages of the use of phenolic additives are their allergenicity and their toxicity. The curing of the compositions disclosed can be effected in the presence of a catalyst, which may be calcium nitrate.

WO 2017/074810 A1 discloses compositions comprising an epoxy resin, a polyetheramine and a further amine hardener which may be an imidazole or imidazoline inter alia, and a metal triflate catalyst.

Another common factor in the prior art compositions is that they cure too slowly or only at high temperatures.

For many applications, however, it is important that surface curing or complete through-curing proceeds particularly rapidly or else at low temperatures. Firstly, productivity can be increased or else the next processing step can be accelerated. The latter is important particularly in cold regions in order, for example, to provide ships or pipelines with a corrosion protection coating, such that continued operation is possible there in winter as well.

SUMMARY

The problem addressed in the present context is thus that of providing epoxy systems that react much more quickly under the same conditions than the formulations known to date. Another problem addressed was that of providing epoxy formulations that partly or fully cure more quickly than is possible to date at temperatures well below room temperature. It would be further desirable if they were very substantially free of toxic and allergenic additives.

DETAILED DESCRIPTION

These present problems are solved by the compositions according to the invention comprisinga) at least one epoxy resin,b) at least one cyclic amine having at least two secondary amino groups which are both part of an organic ring system, andc) at least one salt of a very strong Brønsted acid with a counterion selected from metal ions, metal-containing ions, phosphonium ions and ammonium ions.

Preferably, the compositions according to the invention are additionally phenol-free, meaning that they do not contain any phenols or substituted phenols. Substituted phenols are understood here to mean compounds derived structurally from phenol. More particularly, the compositions according to the invention preferably do not contain any alkylated phenols. Corresponding phenol-free compositions have the advantage of having low toxicity and allergenicity.

Component a) is at least one epoxy resin. All epoxy compounds are suitable in principle for this purpose.

Suitable epoxy compounds are described, for example, in EP 675 185 A2. Useful compounds are a multitude of the known compounds containing more than one epoxy group, preferably two epoxy groups, per molecule. These epoxy compounds may be either saturated or unsaturated and aliphatic, cycloaliphatic, aromatic or heterocyclic and may also have hydroxyl groups. They may additionally contain such substituents that do not cause any troublesome side reactions under the mixing or reaction conditions, for example alkyl or aryl substituents or ether moieties.

Preferred epoxy compounds here are glycidyl ethers which derive from polyhydric phenols, especially bisphenols and novolaks, and which have molar masses based on the number of epoxy groups ME (“epoxy equivalent weights”, “EV value”) between 100 and 1500 g/eq, but especially between 150 and 250 g/eq.

Very particular preference is given to using liquid diglycidyl ethers based on bisphenol A and bisphenol F having an epoxy equivalent weight of 150 to 200 g/eq.

Further useful components A preferably include: poly(N-glycidyl) compounds obtainable by dehydrohalogenation of the reaction products of epichlorohydrin and amines such as aniline, n-butylamine, bis(4-aminophenyl)methane, m-xylylenediamine or bis(4-methylaminophenyl)methane. However, the poly(N-glycidyl) compounds also include triglycidyl isocyanurate, triglycidylurazole and oligomers thereof, N,N′-diglycidyl derivatives of cycloalkyleneureas and diglycidyl derivatives of hydantoins.

In addition, it is also possible with preference to use polyglycidyl esters of polycarboxylic acids which are obtained by the reaction of epichlorohydrin or similar epoxy compounds with an aliphatic, cycloaliphatic or aromatic polycarboxylic acid such as oxalic acid, succinic acid, adipic acid, glutaric acid, phthalic acid, terephthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, naphthalene-2,6-dicarboxylic acid and higher diglycidyl dicarboxylates, for example dimerized or trimerized linolenic acid. Examples are diglycidyl adipate, diglycidyl phthalate and diglycidyl hexahydrophthalate.

Also preferred are glycidyl esters of unsaturated carboxylic acids and epoxidized esters of unsaturated alcohols or unsaturated carboxylic acids. In addition to the polyglycidyl ethers, it is possible to use small amounts of monoepoxides, for example methyl glycidyl ether, butyl glycidyl ether, allyl glycidyl ether, ethylhexyl glycidyl ether, long-chain aliphatic glycidyl ethers, for example cetyl glycidyl ether and stearyl glycidyl ether, monoglycidyl ethers of a higher isomeric alcohol mixture, glycidyl ethers of a mixture of C12 to C13 alcohols, phenyl glycidyl ether, cresyl glycidyl ether, p-tert-butylphenyl glycidyl ether, p-octylphenyl glycidyl ether, p-phenylphenyl glycidyl ether, glycidyl ethers of an alkoxylated lauryl alcohol, and also monoepoxides such as epoxidized monounsaturated hydrocarbons (butylene oxide, cyclohexene oxide, styrene oxide), in proportions by mass of up to 30% by weight, preferably 10% to 20% by weight, based on the mass of the polyglycidyl ethers.

A detailed enumeration of the suitable epoxy compounds can be found in the handbook “Epoxidverbindungen and Epoxidharze” [Epoxy Compounds and Epoxy Resins] by A. M. Paquin, Springer Verlag, Berlin 1958, Chapter IV, and in Lee Neville “Handbook of Epoxy Resins”, 1967, Chapter 2.

It is also possible to use mixtures of the epoxy compounds mentioned.

Preference is given to using mixtures of the aforementioned epoxy compounds as component A).

Useful amines b) having at least two secondary amine groups that are both part of an organic ring system in principle include all compounds having at least two secondary cyclic amino groups. Furthermore, the amines b), as NH carrier, may also have further primary or acyclic secondary amino groups. Preferably, however, the amines used in the compositions according to the invention do not have any further primary or acyclic secondary amino groups, meaning that they have exclusively at least two secondary amino groups that are part of an organic ring system. In addition, however, the amines used in accordance with the invention may also have tertiary amino groups that are unreactive.

An “organic ring system” is understood to mean a heterocyclic system which may have one, two or at least three nitrogen-containing heterocycles (optionally fused to non-nitrogen-containing heterocycles or non-heterocycles). The organic ring system may also have heteroatoms other than nitrogen in the nitrogen-containing cycles, especially oxygen, sulfur and/or phosphorus. “A nitrogen-containing cycle” in the present context is understood to mean a simple nitrogen-containing cycle or a simple nitrogen-containing cycle that shares a ring bond with a non-nitrogen-containing heterocycle, a cycloalkane, a cycloalkene or an aromatic hydrocarbon (i.e. a nitrogen-containing cycle to which a non-nitrogen-containing heterocycle, a cycloalkane, a cycloalkene or an aromatic hydrocarbon is optionally fused).

The amines having at least two secondary amino groups may also be corresponding reaction products of compounds having secondary cyclic amino groups and optionally further primary or acyclic secondary amino groups with groups reactive toward NH groups, especially mono- and polyepoxides (adducts), polyisocyanates, polycarbodiimides, phenols (Mannich reactions with aldehydes and ketones), and substituted phenols (exchange of a Mannich base), polyesters, polycarboxylic acids, polyacrylates and polymethacrylates.

In a preferred embodiment, the amines b) are amines having at least two secondary cyclic amino groups, of which at least two belong to different cycles. More preferably, the amines b) are amines having two to four secondary cyclic amino groups on two to four different cycles. Corresponding amines having at least two different cycles are preferably also understood to mean fused heterocycles composed of at least two simple heterocycles.

Most preferably, the at least two secondary cyclic amino groups belong to the same cycle. Even further preferred amines b) have the formula (I)

in whichR1to R4is H or an organic radical, and
X=—(Y1)m-(A1)n-(Y2)o-(A2)p-(Y3)q-(A3)r-(Y4)s(II)where, independently of one another,m, n, o, p, q, r and s=0 or 1,A1, A2, A3=alkylene or alkenylene radical andY1, Y2, Y3, Y4=NR5, PR5, O or S, with R5independently=H or organic radical,where any two organic radicals selected from R1to R5and any radicals present in the alkylene and/or alkenylene radicals A1, A2, A3may also form one or more further rings,with the proviso that at least one of the radicals selected from Y1, Y2, Y3, Y4present is NR5where R5═H.

The cyclic amines are saturated or partly unsaturated in relation to the ring shown in formula (I). Corresponding amines react more quickly than aromatic amines and imidazolines not covered by the formula. Further rings optionally present in the cyclic amine may be not only saturated or unsaturated, but also aromatic. Preferably, the further rings optionally present in the cyclic system are nonaromatic, i.e. saturated or unsaturated.

Preferred cyclic amines are saturated in relation to the ring shown in formula (I).

Preferably, at least one of the carbon atoms adjacent to the ring-attached amino group=H, i.e. at least one of the R1, R2,R3and R4radicals=H. Further preferably at least two of the R1, R2, R3and R4radicals=H. Preferably at least three, more preferably all, of the R1, R2, R3and R4radicals=H. Correspondingly unsubstituted amines react more rapidly than other amines.

In the X radical, if present, A1, A2and A3are independently alkylene or alkenylene radicals. These may in turn bear organic radicals. If the alkylene and/or alkenylene radicals themselves have one or more organic radicals, any two organic radicals are selected from the organic radicals in the alkylene and/or alkenylene radicals may also form one or more further rings with one another or with any organic R1to R5radicals present.

Radicals selected from R1to R5present and any radicals present in the alkylene and/or alkenylene radicals A1, A2, A3may be substituted by at least one —NHR6or —NH2group, where R6=organic radical. This means that, even in formula (I), as well as the at least two cyclic amino groups, there may also be further primary or acyclic secondary amino groups.

The indices x, y and z, in accordance with their pertinence to the A1, A2and A3radical, may preferably also be referred to as indices x1, y1and z1, as x2, y2and z2and as x3, y3and z3.

Preferably, X has a chain length of 2 to 15 atoms. Preferably, the sum total of all x, ½.y and z and of m, o, q and s has a value from 2 to 15, further preferably a value from 2 to 8, even further preferably a value from 2 to 5 and most preferably a value from 2 to 4. In other words, preferably, 2≤m+o+q+s+x1+½.y1+z1+x2+½.y2+z2+x3+½.y3+z3≤15. Further preferably, 2≤m+o+q+s+x1+½.y1+z1+x2+½.y2+z2+x3+½.y3+z3≤8, further preferably, 2≤m+o+q+s+x1+½.y1+z1+x2+½.y2+z2+x3+½.y3+z3≤5, most preferably, 2≤m+o+q+s+x1+½.y1+z1+x2+½.y2+z2+x3+½.y3+z3≤4.

The amines of the formula (I) are (optionally bi/poly)cyclic compounds since both carbon atoms adjacent to the amino group are attached on either side to the substituted (hetero)alkylene radical of the formula (II).

Further preferably, formula (I), as well as the nitrogen atom of the ring-attached NH group, has not more than one further heteroatom in the cycle, i.e. X preferably has the formula (IIa)
X═—(CR7R8)x—(Y)o—(CR11R12)z—  (IIa)

Correspondingly, in formula (I), any two organic radicals selected from R1to R8, R11and R12may also form one or more further rings.

The R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11and R12and radicals may be organic radicals. Preferred organic radicals are alkyl, alkenyl, alkynyl, aryl or aralkyl radicals. These may be heteroatom-substituted within their chain and/or their substituents. Preferred organic radicals are alkyl, alkenyl, alkynyl, aryl or aralkyl radicals which may bear one or more ether, carboxyl, amino, amido, urea, carbamate, carbonate, amidino or guanidino groups within the chain. In addition, preferred organic radicals may one or more substituents selected from organic radicals optionally having ether, carboxyl, amino, amido, urea, carbamate, carbonate, amidino or guanidino groups that may optionally also be substituted by hydroxyl, nitrile or halogen radicals or may themselves be hydroxyl, nitrile or halogen radicals.

The compounds of the formula (I) may also be used in a mixture.

The compositions according to the invention further comprise at least one salt of a very strong Brønsted acid with a counterion selected from metal ions, metal-containing ions, phosphonium ions and ammonium ions. Corresponding salts serve as catalyst. Such a very strong acid in the present context is understood to mean a salt having a pKa (or in the case of polyprotic acids having a pKa of the first protolysis stage) of not more than −9.01. Preferably, the pKa of the very strong acid is between −9.5 and −25, more preferably between −9.9 and −21.

The pKa is defined as the negative decadic logarithm of the equilibrium constant Ka and is considered to be a measure of the strength of an acid. The smaller a pKa, the stronger the acid. The pKAis determined as disclosed in F. G. Bordwell, “Equilibrium Acidities in Dimethylsulfoxide Solution”,Acc. Chem. Res.1988, 21, 456-463.

Preferred salts are the corresponding salts of the very strong acids collated in the first column in Table 1 below:

The counterions of these very strong acids are selected from metal ions, metal-containing ions, phosphonium ions and ammonium ions. Preferred metal ions are alkaline earth metal cations, especially beryllium, magnesium, calcium, strontium and barium ions, and lithium, aluminium, bismuth, zinc and europium ions.

Metal-containing ions are ions that contain at least one metal but are not pure metal ions. They are preferably metal compound ions, for example tetraphenylstibonium ions.

The counterions may also be ammonium ions (NH4+) or optionally alkylated and/or arylated phosphonium ions. Very particular preference is given to ammonium ions.

Preferred salts of very strong acids are calcium triflate, europium triflate, barium triflate, aluminium triflate, bismuth triflate, lithium triflate, lithium perchlorate, barium perchlorate, lithium hexafluorophosphate and zinc perchlorate. Very particular preference is given to calcium triflate, europium triflate, lithium hexafluorophosphate and lithium perchlorate.

Preference is given to salts having good solubility in organic media (e.g. benzyl alcohol), i.e. salts having a solubility of at least 10 g/l in benzyl alcohol at 20° C.

Preferably, the compositions according to the invention may further include d) amines that do not have at least two secondary amino groups as part of an organic ring system, and/or e) further auxiliaries or additives.

Such amines d) are preferably di- or polyamines. These may be monomeric, oligomeric and/or polymeric compounds. Preferred monomeric and oligomeric compounds are selected from the group consisting of diamines, triamines and tetramines. The amine group of the di- or polyamines d) may be attached to a primary, secondary or tertiary carbon atom, preferably to a primary or secondary carbon atom. It is also possible to use mixtures of di- and/or polyamines as component d).

Components d) used may be the following amines, alone or in mixtures:aliphatic amines, especially the polyalkylenepolyamines, preferably selected from ethylene-1,2-diamine, propylene-1,2-diamine, propylene-1,3-diamine, butylene-1,2-diamine, butylene-1,3-diamine, butylene-1,4-diamine, 2-(ethylamino)ethylamine, 3-(methylamino)propylamine, diethylenetriamine, triethylenetetramine, pentaethylenehexamine, trimethylhexamethylenediamine, 2,2,4-trimethylhexamethylenediamine, 2,4,4-trimethylhexamethylenediamine, 2-methylpentanediamine, hexamethylenediamine, N-(2-aminoethyl)ethane-1,2-diamine, N-(3-aminopropyl)propane-1,3-diamine, N,N″-1,2-ethanediylbis(1,3-propanediamine), dipropylenetriamine, adipic dihydrazide and hydrazine;oxyalkylenepolyamines, preferably selected from polyoxypropylenediamine and polyoxypropylenetriamine (e.g. Jeffamine® D-230, Jeffamine® D-400, Jeffamine® T-403, Jeffamine® T-5000), 1,13-diamino-4,7,10-trioxatridecane, 4,7-dioxadecane-1,10-diamine;cycloaliphatic amines, preferably selected from isophoronediamine (3,5,5-trimethyl-3-aminomethylcyclohexylamine), 4,4′-diaminodicyclohexylmethane, 2,4′-diaminodicyclohexylmethane and 2,2′-diaminodicyclohexylmethane, alone or in mixtures of the isomers, 3,3′-dimethyl-4,4′-diaminodicyclohexylmethane, N-cyclohexyl-1,3-propanediamine, 1,2-diaminocyclohexane, 3-(cyclohexylamino)propylamine, TCD diamine (3(4),8(9)-bis(aminomethyl)tricyclo[5.2.1.02.6]decane), 4-methylcyclohexane-1,3-diamine,amines having one cyclic amino group and at least one non-cyclic primary or secondary amino group, preferably 1-(3-aminoethyl)piperazine (AEP), 1-(3-aminopropyl)piperazine;araliphatic amines, preferably xylylenediamines;aromatic amines, preferably phenylenediamines, especially phenylene-1,3-diamine and phenylene-1,4-diamine, and 4,4′-diaminodiphenylmethane, 2,4′-diaminodiphenylmethane, 2,2′-diaminodiphenylmethane, optionally alone or in mixtures of the isomers;adduct hardeners, especially reaction products of epoxy compounds, especially glycidyl ethers of bisphenol A and F, with excess amine;polyamidoamine hardeners, especially polyamidoamine hardeners obtained by condensation of mono- and polycarboxylic acids with polyamines, very especially by condensation of dimer fatty acids with polyalkylenepolyamines;Mannich base hardeners, especially by reaction of mono- or polyhydric phenols with aldehydes, especially formaldehyde, and polyamines, Mannich base hardeners; andMannich bases, especially based on phenol and/or resorcinol, formaldehyde and m-xylylenediamine, and also N-aminoethylpiperazine and blends of N-aminoethylpiperazine with nonylphenol and/or benzyl alcohol, phenalkamines which are obtained in a Mannich reaction from cardanols, aldehydes and amines.

It is also possible to use mixtures of the aforementioned di- or polyamines as component d).

Preferred further auxiliaries and admixtures e) may be the compounds customary in epoxy chemistry. Preferred auxiliaries and admixtures e) are pigments, solvents, levelling agents, modifiers, for example benzyl alcohol or nonylphenol, degassing agents, flatting agents, reactive diluents, and conventional catalysts such as salicylic acid.

In a preferred embodiment, no amines d) are used. In a preferred embodiment, no further conventional catalysts are used, meaning that the possible auxiliaries and admixtures are limited to pigments, solvents, levelling agents, modifiers, degassing agents, flatting agents and reactive diluents.

The composition of components a) to e) in the composition according to the invention is preferably the following proportions by weight, based on the total mass of the composition:a) epoxy resin 30-95%b) cyclic amine 1-50%c) salt of the very strong Brønsted acid 0.001-5%, preferably 0.1-3%,d) further amine 0-48% ande) further auxiliaries or additives 0-48%.

Preferably, in addition, the proportion by weight of the amines b) based on the total mass of the amines b) and d) is at least 10% by weight, more preferably at least 30% by weight, even more preferably at least 50% by weight and more preferably still at least 70% by weight.

The invention further provides a process for producing a composition according to the invention, in which at least one epoxy resin a), at least one cyclic amine b) and at least one salt of a very strong acid c) are mixed with one another.

The invention further provides for the use of the compositions according to the invention as a casting resin, coating composition, composite or adhesive or as a constituent thereof.

To demonstrate the particular reactivity of the catalysts claimed, model experiments are first compared with one another. For this purpose, 0.025 mol (3.75 g) of 1,2-epoxy-3-phenoxypropane is added to a mixture of 22.68 g of toluene (solvent) and 2.08 g of tetradecane (internal standard). To this are added 0.025 mol (2.13 g) of piperidine, and 0.06 g of the particular catalyst. Immediately after the mixing, a GC is taken and the content of 1,2-epoxy-3-phenoxypropane is compared with the content of tetradecane. After 4 h at room temperature, by means of GC analysis, the residual content of 1,2-epoxy-3-phenoxypropane (EP) is ascertained (calibrated by the internal tetradecane standard). This gives the following results:

Comparison of the Catalysts

To demonstrate the particular reactivity of the amines claimed, model experiments are first compared with one another. For this purpose, 0.025 mol (3.75 g) of 1,2-epoxy-3-phenoxypropane is added to a mixture of 22.68 g of toluene (solvent) and 2.08 g of tetradecane (internal standard). To this are added 0.025 NH equivalent of an amine and 0.06 g of calcium triflate. Immediately after the mixing, a GC is taken and the content of 1,2-epoxy-3-phenoxypropane is compared with the content of tetradecane. After 4 h at room temperature, by means of GC analysis, the residual content of 1,2-epoxy-3-phenoxypropane (EP) is ascertained (calibrated by the internal tetradecane standard). This gives the following results:

It is clearly apparent from the model experiments that the compositions claimed are much more reactive than conventional systems.

100 parts homopiperazine (Aldrich) are intimately mixed with 66 parts benzyl alcohol and 380 parts Epikote 828 (epoxy equivalent 190, Hexion) and a DSC is taken immediately thereafter.

0.5 part calcium triflate (Aldrich) is dissolved in 100 parts homopiperazine (Aldrich) and 66 parts benzyl alcohol and this mixture is intimately mixed with 380 parts Epikote 828 (epoxy equivalent 190, Hexion) and a DSC is taken immediately thereafter.

0.5 part calcium nitrate (Aldrich) is dissolved in 100 parts isophoronediamine (Evonik Industries) and 66 parts benzyl alcohol and this mixture is intimately mixed with 445 parts Epikote 828 (epoxy equivalent 190, Hexion) and a DSC is taken immediately thereafter.

In the inventive experiment, the exothermic peak is lower, as is the onset of this exothermic peak. The reactive composition according to the invention is thus more reactive than without catalyst or without amines having two cyclic NH groups.