Metal salts of phosphoric acid esters as cross linking catalysts

The present invention is directed to a catalyst for the epoxy reaction with carboxyl and or anhydride functional compounds for use in coating, sealant, adhesive and casting applications. More particularly, the present invention is directed to the use of zinc or tin (II) salts of a mono- or di-ester of phosphoric acid (also referred to as alkyl acid phosphate) having the structure:

(RO) n —P(═O)—(OH) m

wherein n+m═3 and n is between 2 to 1 preferably between 1.7 to 1.2 and the metal counter ion Zn or Sn (II) is in a molar equivalent ratio of 0.7 to 1.5 per mole of the alkyl acid phosphate. The use of Zn or Sn(II) alkyl acid phosphate as a catalyst in the epoxy-carboxylanhydride reaction improves the stability of the reactants at room temperature and avoids yellowing or blistering in the coating produced. Furthermore, the improved stability of the reactants in the presence of the catalyst enables a single packaged product for the epoxy-carboxy/ahydride mixture.

It has been long recognized that epoxy compounds react with carboxylic acids or with anhydrides. It is also known that this reaction can be catalyzed. Antoon and Koenig ( J. Polym. Sci ., Polym. Chem. Ed. (1981) 19(2):549-70) studied the mechanism of catalysis by tertiary amines of the reaction of anhydrides with epoxy resins, typically a glycidyl ether of bisphenol. They pointed out that it is the quaternary ammonium salt zwitterion that initiated the polymerization reaction. Matejka and Dusek studied the reaction of phenylglycidyl ether model compounds with caproic acid in the presence of a tertiary amine as the catalyst ( Polym. Bull . (1986) 15(3):215-21). Based on their experimental data, they suggested that this is an addition esterification process.

Metal salts and amines have been used as catalysts for the epoxycarboxyl/anhydride reaction. For example, 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), a strong basic amine and its salts are being promoted as catalysts for epoxy-carboxyl/anhydride polymer systems. It is known that the salts of amines usually improved the pot life of such polymer systems. Whittemore et. al. (U.S. Pat. No. 3,639,345) disclosed thermosetting resins using an epoxy functional bisphenol A and a trimellitic anhydride ester with an amine, an imidazole or an aminoalkyl phenol, as the catalyst.

Metal salts or Lewis acid catalysts are also promoted for epoxy resins. The metal salts has found applications as catalysts for epoxycarboxyl/anhydride coatings. The catalytic effect of metal salts was recognized by Connelly et. al. (ZA 6,907,152) who described the use of zinc acetate, chromium acetate, iron octoate, zinc naphthenate, cobalt naphthenate and manganese naphthenate as catalysts. Metal salts of Mg, Ca, Sr, Ba, Zn, Al, Sn and Sb have been disclosed by Lauterbach (U.S. Pat. No. 4,614,674) as catalysts in combination with waxes as matting agents for powder coatings. Wright et. al. disclose (U.S. Pat. No. 4,558,076) a fast curing coating formulation comprising a carboxyl functional polymer, a tertiary amine, a polyepoxide and an Al, Ti, or Zn alkoxide or complex as the catalyst.

A major problem with the known catalysts is the poor stability of the combination of the epoxy and carboxyl/anhydride reactants at ambient room temperature. The increase in viscosity requires the epoxy and the carboxyl/anhydride compounds to be formulated into two separate packages. A further problem is the yellowing tendency of amines during the bake or heating cycle. In addition, it is known that the use of amines result in films that are sensitive to humidity leading to blistering of the film. It would be desirable to have a catalyst that does not require the separate packaging of epoxy and carboxyl/anhydride reactants and does not cause yellowing or sensitivity to humidity leading to blistering

Metal salts such as zinc carboxylates have been shown to be effective catalysts in the above referenced patents. However, the problem with di and polyvalent metal salts is salt formation with the carboxyl groups of the reactant through ionic crosslinking leading to an instant increase in viscosity or gelation. Although covalent bonds are not formed in this process, this reaction can lead to very highly viscous formulations with poor flow quality resulting in poor film properties.

A class of metal alkyl acid phosphates which effectively catalyze the reaction of epoxy-carboxyl/anhydride have been developed. The use of these catalysts in the coating process not only reduce yellowing, but also provided excellent room temperature stability and excellent humidity resistance. The improved stability with the use of the metal alkyl acid phosphates of the invention provides for the formulation of a single packaged product.

The present invention provides a metal (M) alkyl acid phosphate catalyst wherein the alkyl acid phosphate has the formula:

(RO) n —(P═O)—(OH) m

and wherein:

a. each R is selected from the group consisting of:

a C 1 to C 18 alkly, cycloalkyl, or aryl;

a linear or branched C 6 to C 18 alkyl substituted with —(O—CH 2 —CH 2 —) o or —(O—CH—CH 3 —CH 2 —) p , wherein o or p is from 1 to 20; and

a β-hydroxyethyl compound, R′—X—CH 2 —CH—OH—CH 2 —, wherein R′ is a C 6 to C 18 alkyl or cycloalkyl or aryl, X is either —CH 2 —, —O— or —COO—;

b. n+m=3 and n is between 2 to 1; and

c. M is Zn or Sn (II) in a mole equivalent of 0.7 to 1.5 moles per mole of alkyl acid phosphate.

When R is a linear or branched alkyl substituted with —(O—CH 2 —CH 2 —) o or —(O—CH—CH 3 —CH 2 —) p , it may be the reaction product of a C 6 to C 18 alcohol with ethylene oxide or propylene oxide. When R is a β-hydroxyethyl compound, it may be the reaction product of an epoxide with phosphoric acid to provide R′—X—CH 2 —CH—OH—CH 2 —, wherein R′ is an alkyl or cycloaliphatic or aromatic radical with between C 6 to C 18 carbons, X is —CH 2 —, —O—, or —COO—. The length of the R or R′ chain controls the solubility of the compound in solvents and the compatibility of the compound with other resins.

The catalysts of the present invention are prepared by reacting a zinc or tin (II) alkoxide, oxide or carboxylate with an alkyl or aryl phosphoric acid. The metal alkyl or aryl acid phosphates are useful for catalyzing the reaction between an epoxy compound and a carboxyl compound selected from the group consisting of a carboxylic acid and an anhydride.

1. Antoon and Koenig, J. Polym. Sci ., Polym. Chem. Ed. (1981) 19(2):549-70.

2. Matejka and Dusek, Polym. Bull . (1986) 15(3):215-21.

3. Wright et. al, U.S. Pat. No. 4,558,076.

4. Whittemore et. al. U.S. Pat. No. 3,639,345.

5. Connelly et. al. ZA 6,907,152.

6. Lauterbach, U.S. Pat. No. 4,614,674.

The present invention provides a catalyst, a metal (M) salt of an alkyl acid phosphate with the formula:

(RO) n —(P═O)—(OH) m

wherein:

a. each R is selected from the group consisting of:

i) a C 1 to C 18 alkyl, cycloalkyl, or aryl;

ii) a linear or branched C 6 to C 18 alkyl substituted with —(O—CH 2 —CH 2 —) o or —(O—CH—CH 3 —CH 2 —) p , wherein o or p is from 1 to 20; and

iii) a β-hydroxyethyl compound, R′—X—CH 2 —CH—OH—CH 2 —, wherein R′ is a C 6 to C 18 alkyl or cycloalkyl or aryl, and X is either —CH 2 —, —O— or —COO—;

b. n+m=3 and n is between 2 to 1; and

c. M is Zn or Sn (II) in a mole equivalent of 0.7 to 1.5 moles per mole of alkyl acid phosphate.

When R is a linear or branched alkyl substituted with —(O—CH 2 —CH 2 —) o or —(O—CH—CH 3 —CH 2 —) p , it may be the reaction product of a C 6 to C 18 alcohol with ethylene oxide or propylene oxide. When R is a β-hydroxyethyl compound, it may be the reaction product of an epoxide with phosphoric acid to provide R′—X—CH 2 —CH—OH—CH 2 —, wherein R′ is an alkyl or cycloaliphatic or aromatic radical with between C 6 to C 18 carbons, X is —CH 2 —, —O— or —COO—. The length of the R or R′ chain controls the solubility of the compound in solvents and the compatibility of the compound with other resins.

When R′ is an alkyl, cycloalkyl or aryl, it is typically a methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, dodecyl, hexadecyl, phenyl, nonyl-phenyl, cyclohexyl, or dodecyl-ethenyl-oxy- with 5 to 15 repeating ethylene oxide units.

The RO— groups are typically derived from methanol, ethanol, propanol, isopropanol, b-butanol, isobutanol, dodecanol, hexadecanol, phenol, nonyl phenol, cyclohexanol, dodecanol ethylene oxide reaction products with 5 to 15 repeating ethylene oxide units.

The alkyl phosphorous acids for producing the compounds of the present invention are obtained by reacting any of the above alcohols or phenols with phosphorous pentoxide, either in an solvent or in an excess of the alcohol. This reaction produces a mixture of mono- or di- alkyl or mono- or di- aryl esters of phosphoric acid. Depending on the reaction conditions and whether an excess of alcohol is used the ratio of di- to mono- ester can vary from 4 to 1 (80/20) to 1:4 (20:80). When the ratio of the diester to the monoester is 4:1, n is 1.7. When the ratio of the diester to the monoester is 1:4, n is 1.2. Under practical reaction conditions the ratio is usually 3:2 (60/40). If desired a diester of phosphate can be converted to a mono ester by trans-esterification.

Other suitable reactants with phosphorous pentoxide are compounds with RO—CH 2 —CH—OH—CH 2 —O—, RCOO—CH 2 —CH—OH—CH 2 —O, or R—CH 2 —CH—OH—CH 2 —O—moieties. These moieties are obtained by reacting an epoxide such as a α-olefin epoxide, a mono glycidyl ether or glycidyl ester with phosphoric acid either in an inert solvent or in bulk.

The catalysts of the present invention are prepared by reacting a zinc or tin (II) alkoxide, oxide or carboxylate with the alkyl and/or aryl acid phosphate obtained as described above. The reaction can be carried out in a solution or also in bulk. The metal alkyl/aryl acid phosphates produced are useful for catalyzing the reaction between an epoxy compound and a carboxyl compound selected from the group consisting of a carboxylic acid and an anhydride.

The epoxy compounds useful in our invention are the polylglycidyl ether of bisphenol A or F or NOVOLAk® phenol formaldehyde resins with a molecular weight of between about 350 to 10000, preferably between 380 and 4000. These resins may be used as solids or viscous liquids. The diglycidyl esters of di and polycarboxylic acids are also useful for the present invention. Other glycidyl functional polymers that are useful include the polymers of the glycidyl ester of methacrylic acid, epoxidized oil, cycloaliphatic epoxies and triglycidyl isocyanurate. Cycloaliphatic epoxy compounds useful for the invention include: 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate, spiro[1,3-dioxane-5,3′-[7]oxabicyclo[4.1.0]heptane], 2-(7-oxabicyclo[4.1.0]hept-3-yl), 3,4-epoxycyclohexyl) methyl 3,4-epoxycyclohexylcarboxylate, 1,2-epoxy-4-(epoxyethyl)cyclohexane, 7-Oxabicyclo[4.1.0]heptane-3,4-dicarboxylic acid, bis(oxiranylmethyl) ester, 1,3,5-triglycidyl isocyanurate (TGIC), epoxidized soybean oil, epoxidized linseed oil.

Compounds with carboxyl or anhydride functional groups suitable in the present invention are the mono- di- or poly-carboxyllic acids or anhydrides. Examples of acids and anhydrides suitable for the present invention are: adipic acid; glutaric acid; glutaric anhydride; sebasic acid; 1,10 decanedioic acid; fumaric acid; maleic acid and maleic anhydride; succinic acid; phthalic acid and phthalic anhydride; 8,9,10-trinorborn-5-ene-2,3-dicarboxylic acid and 8,9,10-trinorborn-5-ene-2,3-dicarboxylic anhydride; cyclohexene-1,2-dicarboxylic acid; diphenyl-2,2′-dicarboxylic acid; methylnorbornene-2,3-dicarboxylic anhydride; cyclohexene-1,2-dicarboxylic acid; tetrahydrophthalic anhydride; 5-methyltetrahydrophthalic anhydride; octahydro-4,7-methano-1H-indene-5,-dicarboxylic acid; 1,2-cyclohexanedicarboxylic acid; dimeric fatty acids; alkenyl succinic acids and anhydrides; dicarboxylic acid anhydrides such as: succinic or glutaric anhydride, alkenylsuccinates with an alkenyl group from C 6 to C 18 ; aromatic anhydrides such as: o-phthalic anhydride, trimellitic acid anhydride or linear anhydrides of diacids.

Also suitable in this invention are carboxyl containing acrylic resins obtained by polymerizing a carboxyl functional monomer such as acrylic, methacrylic, maleic, fumaric, itaconic or the half ester of maleic or fumaric with acrylic or styrene or acrylonitrile monomer. Additionally acrylic polymers with anhydride groups such as the copolymers of acrylic monomers with maleic or itaconic anhydride. Examples for tri carboxylic acids/anhydrides are 1-propene-1,2,3-tricarboxylic acid; 1,2,4-benzenetricarboxylic acid; an adduct of abietic acid with fumaric acid or maleic anhydride; trimellitic anhydride; and citric acid. Examples for monoacids are the C 12 to C 18 fatty acids saturated and unsaturated.

Other compounds suitable for the invention as crosslinkers include mono, di or poly glycidyl esters, the reaction products of mono, di and polycarboxylic acids with epichlorohydrine; glycidyl ethers of aliphatic ethers of diols, triols and polyols, such as 1,2,3-propanetriol glycidyl ether; alkyl (C 10 -C 16 ) glycidyl ether; lauryl glycidyl ether; glycerin 1,3-diglycidyl ether; ethylene diglycidyl ether; polyethylene glycol bis(glycidyl ether); 1,4-butanediol diglycidyl ether; 1,6-hexanediglycidyl ether; bis(2,3-epoxypropyl) ether; homo and copolymers of allyl glycidyl ether; ethoxylated alcohol(C 12 -C 14 ) glycidyl ether.

Other than the glycidyl ether of bisphenol A and F and of phenol formaldehyde polymers, phenyl glycidyl ether, p-t-butylphenol glycidyl ether, hydroquinone diglycidyl ether, glycidyl p-glycidyloxybenzoate, p-nonylphenol glycidyl ether, glycidyl ether reaction product of 2-methyl phenol and formaldehyde polymer are also useful in the present invention.

It has to be understood that the use of monofunctional compounds and diluents can reduce the crosslinking density and therefore adversely affect the film properties. Therefore the use of monofunctional compounds has to be balanced with the use of higher functional crosslinkers.

The ratio of the epoxy compound to the carboxyl or anhydride in the formulation can be 0.5 to 1 to 5 to 1 depending on the crosslinking density desired. Normally the optimum crosslinking density is achieved when the ratio of functional epoxy groups and carboxyl groups is one to one under ideal conditions. However, with most epoxy formulations some self-condensation of the epoxy groups takes place. For example, it is necessary to use an excess of epoxy groups to react all the carboxyl or anhydride groups so that a film with no free carboxyl groups are present, if excellent detergent or alkali resistance in a film is desired. However, if better adhesion and flexibility is desired, then the ratio can be adjusted so that some of the unreacted carboxyl groups remain.

The ratio of epoxy to carboxy functional groups is important for primer applications where corrosion resistance is an important requirement. In such a formulation the level of epoxy resin can be reduced. The ratio of epoxy to carboxyl groups is also dependent on the functional groups in the reactant system. For example, if one reacts a carboxyl functional acrylic resin with a difunctional epoxy resin, it might be desirable to use an access of carboxy groups. If an acrylic resin which has a high molecular weight is used, it usually contains many carboxyl groups, a typical acrylic resin might have an acid number of 56 and a molecular weight of 20,000. In such a resin the average chain contains 20 carboxyl groups. To achieve crosslinking in such a system, theoretically three carboxy groups have to be reacted to form an effective network. The epoxy in such a formulation might be a diglycidyl ether of bisphenol A, a difunctional crosslinker. A person with skill in the coating art would therefore use an excess of carboxyl groups and a deficiency of epoxy groups to achieve a good network. Most crosslinking reactions do not go to completion. If the crosslinkers have reacted to an average to 75%, it indicates that some molecules of the crosslinking agents have completely reacted, with some molecules having reacted only at one end and some molecules having not reacted at all. By having an excess of carboxy groups on the acrylic, one could assure a higher conversion of all the epoxy groups. This problem is typical in can coatings, where it is important to eliminate any unreacted epoxy resin to prevent any leaching of epoxy resin into the food.

Typical cure temperatures for the formulations of the present invention are between 100 to 300° C. for a time period from several seconds to hours. Preferred are cure temperatures from 120-250° C. for 30 seconds to 30 minutes.

The formulation of the present invention is useful for producing coatings, adhesive films, or in casting or molding. Typical applications include use as corrosion resistant primers for automotive applications, or can or coil coatings, or automotive clear coats. The coatings can be applied as a high solids or a powder coating.

The following examples illustrates the invention and are not to be used to limit the scope of the invention.