WO1996015172A1

WO1996015172A1 - Polyamine curing agents for epoxy resins exhibiting reduced carbonation

Info

Publication number: WO1996015172A1
Application number: PCT/US1995/014065
Authority: WO
Inventors: Dieter H. Klein; Christophe P. Marche
Original assignee: The Dow Chemical Company
Priority date: 1994-11-10
Filing date: 1995-10-31
Publication date: 1996-05-23
Also published as: GB9422729D0

Abstract

Novel amine-terminated epoxy adducts are made by reacting a core epoxy resin, which contains an adduct made by reacting lower molecular weight excess epoxy resin with an amine chain extender that contains on average 1.8 to 2.2 amine hydrogen atoms per molecule, with an excess of polyamine compound at a temperature of 50 °C to 140 °C. The adducts are useful as hardeners in aqueous and high solids curable coating formulations that contain epoxy resins.

Description

POLYAMINE CURING AGENTS FOR EPOXY RESINS EXHIBITING REDUCED CARBONATION

The present invention relates to the art of curing agents for epoxy resins, and more specifically to curing agents suitable for aqueous epoxy resin emulsions and high solids epoxy resin formulations.

It is well known to protect metal, concrete and other abricated materials by applying and curing a composition that contains epoxy resin and a curing agent. Particular resin and curing agents can be selected depending upon whether the coating is to be cured at elevated temperature (such as paint on a car body panel) or must be cured at ambient temperature (such as paint on a bridge or outdoor structure).

Polyamines are common curing agents used in curing such coating systems. However, compositions of polyamine and epoxy resin are susceptible to "carbonation" or "blushing", which is a discoloration caused by reaction of amine groups with carbon dioxide and humidity in the air. It is known that adducts of epoxy resins and polyamines are less susceptible to blushing than other forms of polyamine curing agents. See Tess, "Epoxy Resin Coatings", Epoxy Resins (2d Edition), at 743-744 (Marcel Dekker Inc. 1988). For example, the following patents describe adducts of polyamines and epoxy resins that are useful as curing agents:

(A) Schreiber et al., U.S. Patent 3,996,175 (December 7, 1976) describes curing agents made by reacting low molecular weight epoxy resins (such as the diglycidyl ether of bisphenol A) with low molecular weight aromatic diamines (such as diaminodiphenylmethane).

(B) Lin et al., U.S. Patent 5,091,574 (February 25, 1992) describes curing agents that are made by reacting excess diglycidyl ether of bisphenol A with a polyoxyalkylene diamine, and then reacting the product of that first step with excess polyoxyalkylene diamine.

The coating solutions that contain known amine-epoxy adducts usually contain an organic solvent to keep the component in a solution with a useful viscosity. Aqueous coating formulations could minimize the need for organic solvents. However, the presence of water increases the tendency of coating resins to blush, particularly at ambient temperature curing conditions. See Tess, "Epoxy Resin Coatings". Epoxy Resins (2d Edition), at 743-744 (Marcel Dekker Inc. 1988). What is needed is a curing agent for epoxy coating compositions that has improved resistance to blushing or carbonation in aqueous formulations.

One aspect of the present invention is a process to make an amine-terminated adduct by reacting a core epoxy resin with an excess of polyamine resin at a temperature of 50°C to 140°C, characterized in that the core epoxy resin contains an adduct made by reacting excess lower-molecular-weight epoxy resin with an amine chain extender that contains on average 1.8 to 2.2 amine hydrogen atoms per molecule. A second aspect of the present invention is an amine-terminated adduct made by the process described above.

A third aspect of the present invention is a coating composition that contains: (1) an epoxy resin, (2) an amine-terminated curing agent for the epoxy resin,

(3) a solvent,

(4) optionally a catalyst to catalyze the reaction of the epoxy resin and the curing agent, and

(5) optionally an emulsifying agent, characterized in that the amine curing agent is an amine-terminated adduct as previously described.

A fourth aspect of the present invention is the use of a coating composition as previously described to coat a substrate by applying the composition to the substrate and curing it or permitting it to cure. The adducts of the present invention and compositions containing them meet the objectives of the invention in that they form stable aqueous dispersions and are substantially resistant to carbonating or blushing when they are applied or cured. Moreover, preferred adducts can be selected to provide the following additional benefits:

(1) stable emulsions can be formed in aqueous solutions having a pH of greater than 6, whereas many prior art curing agents will only form stable emulsions in low pH solutions that can corrode metallic substrates;

(2) all components used to make the adduct can be selected from nonvolatile components or from components that are approved for use under clean air laws, so that lengthy qualification of new materials for use is not necessary; (3) the adduct and epoxy resin can be selected to cure without liberating formaldehyde or other volatile toxic materials; (4) the adduct can cure with epoxy resins at ambient temperatures, so that it is useful for coating large outdoor structures that are impractical for heat-cured coatings; and (5) the adduct can be selected to act as an emulsif ier for the epoxy resin, so that a separate emulsif ier is not needed.

The present invention uses a core epoxy resin, which contains an adduct made by reacting an equivalent excess of lower molecular weight epoxy resin with an amine chain extender and optionally with a lipophilic phenol or a lipophilic carboxylic acid. (By " lower molecular weight", we mean that the epoxy resin has an epoxy equivalent weight (EEW) of no more than 350.) The lower molecular weight epoxy resin is preferably the diglycidyl ether of an aromatic or aliphatic diol. Examples of suitable aromatic diols include bisphenol A, bisphenol F, resorcinol, and hydroquinone. The diglycidyl ether of an aromatic bisphenol preferably has an epoxy equivalent weight (EEW) of 156 to 210. Aliphatic diols preferably contain cycloaliphatic groups, such as bis(hydroxymethyl)-cyclohexane(perhydrobisphenol A). The diglycidyl ether of an aliphatic epoxy resin preferably has an EEW of 100 to 240.

The chain extender is an amine compound that can react to advance the molecular weight of the epoxy resin without causing it to gel prematurely. It is theorized that the chain extender should promote substantially linear polymerization without substantial cross-linking. The chain extender preferably contains on average between 1.9 and 2.1 amine hydrogen atoms per molecule, and more preferably contains on average about 2 amine hydrogen atoms. Moreover, the chain extender preferably does not substantially catalyze the reaction of epoxy groups with epoxy groups to form gelled structures. It preferably has an average molecular weight of at least 73, more preferably at least 90, and more preferably at least 105. The molecular weight is preferably not so high that it hurts the ability of the advanced resin to form an emulsion. It is preferably no more than 230. The chain extender preferably contains benzylamine or phenylamine moieties.

Examples of useful primary or secondary amines include benzylamine, methoxybenzylamine, phenylethylamine, 4-methoxyphenyl ethylamine, 2-methyl-6-ethylaniline, 2,6-xylidine, 2,6-diisopropyl-aniline, 2,6-diethylaniline and n,n'-bis-(t-butyl)ethylene-diamine.

The reaction mixture for making the core epoxy resin preferably further contains a lipophilic compound having a single carboxylic acid group or a single phenolic hydroxyl group. The lipophilic compound preferably contains at least 10 carbon atoms, or preferably at least 12 carbon atoms and most preferably at least 14 carbon atoms. It preferably contains no more than 24 carbon atoms, more preferably no more than 18 carbon atoms and most preferably no more than 16 carbon atoms. It may be aliphatic, aromatic or aliphatic-aromatic. It is preferably alkyl or alkyl-aromatic. Examples of suitable lipophilic compounds include nonylphenol, dodecylphenol, t-butylphenol and saturated or unsaturated monocarboxylic fatty acids.

The ratios of low molecular weight epoxy resin, amine chain extender, and lipophilic compound are preferably selected to provide an epoxy-terminated adduct having an average EEW of 450 to 650. The equivalent ratio of epoxy resin to amine chain extender is preferably between 2.5:1 and 15: 1, and more preferably between 4: 1 and 6: 1. The equivalent ratio of epoxy resin to lipophilic compound (comparing epoxy groups to acid groups and/or phenolic hydroxyl groups) is preferably between 12: 1 and 3: 1, and more preferably between 8: 1 and 4: 1. The epoxy-terminated adduct preferably contains between 1.35 and 1.7 epoxy groups per molecule on average. Its number average molecule weight is preferably between 600 and 1200.

The core epoxy resin reacts with a stoichiometric excess of polyamine. The polyamine and reaction conditions should be chosen to prevent the reaction mixture from gelling, which may result from uncontrolled temperature rise due to the exothermic reaction. Examples of methods to control the reaction include selecting amines which have a lo er reactivity and mixing the reagents in a staged or continuous fashion.

For instance, primary amine groups which are bonded to secondary or tertiary carbon atoms have lower activity than primary amine groups bonded to primary carbon atoms. The majority of the amine groups in the polyamine are preferably primary amine groups which are bonded to secondary or tertiary carbon atoms. Preferably more than 50 percent of primary amine groups are bonded to a secondary or tertiary carbon, more preferably at least 75 percent, more highly preferably at least 90 percent, and most preferably about 100 percent. More active polyamines are preferably mixed with the core epoxy resin in a staged or continuous manner in order to control the reaction. The selection of which reagent is added to which is not critical, but it is usually more practical to add the polyamine to the core epoxy resin.

Examples of suitable polyamines include isophorone diamine, xylylene diamine, diaminocyclohexane, diethylenetriamine (DETA) and triethylenetetramine (TETA). The polyamine is preferably a higher molecular weight, nonvolatile polyamine, such as an amine- terminated polymer. Examples of suitable amines are amine-terminated poly(alkylene oxide) polymers, such as poly(tetramethylene ether) or the polymers illustrated in Formula III.

Formula III

H₂N- ₂

wherein:

(1) each R' is hydrogen or an alkyl group containing 1 to 6 carbon atoms;

(2) R" is an alkyl group containing 2 to 6 carbon atoms; and

(3) "m" is an average number of repeating units greater than 1.

Each R' is preferably methyl or ethyl, and each R" is preferably a propylene or butylene group, "m" is preferably between 2 and 35. The polyamine preferably has an amine hydrogen equivalent weight (AHEW) of at least 35. It's AHEW is preferably no more then 1250, more preferably no more than 900, and most preferably no more than 500.

The molar ratio of primary amine groups to the epoxy groups is preferably at least 4: 1 and more preferably at least 6: 1. It is preferably no more than 10: 1 and more preferably no more than 8:1. The ratio is preferably chosen so that the resulting amine-terminated adduct has an AHEW of at least 90, and more preferably at least 150. The amine-terminated adduct preferably has a AHEW of no more than 2500, and more preferably no more than 1200. Its number average molecular weight is preferably between 800 and 10,000. The temperature of the reaction preferably reaches at least 80°C, more preferably at least 100°C, and most preferably at least 115°C. The temperature of the reaction preferably does not exceed 130°C, and more preferably does not exceed 125°C. The amine-terminated adduct can be used as a curing agent in organic solutions, but it is preferably used as a curing agent in aqueous solutions. It is converted to an aqueous- soluble adduct by neutralizing at least some of the amine groups with organic acids (such as formic acid, acetic acid or lactic acid) or with inorganic acids (such as hydrochloric acid, hydrobromic acid or phosphoric acid). The equivalent ratio of acid to amine groups is preferably no more than 1 : 2.45 and more preferably no more than 1 :2.75. The equivalent ratio of acid to amine is preferably at least 1 : 10. The partially neutralized amine-terminated adduct is dissolved in water or an aqueous solution. The concentration of adduct in the solution is preferably between 25 and 50 weight percent, and more preferably between 35 and 45 weight percent. Such solutions preferably have a pH between 6 and 9, more preferably between 7 and 8.

Coating compositions of the present invention further contain an epoxy resin. The resin should contain more than 1 epoxy group per molecule on average. The epoxy resin is preferably either a liquid epoxy resin or a solvent-borne epoxy resin having an EEW of no more than 600. Examples of suitable epoxy resins include bisphenol A epoxy resins, bisphenol F epoxy resins, poly(alkylene oxide) epoxy resins and aliphatic epoxy resins. Suitable epoxy resins are known and commercially available. Examples of suitable organic solvents include xylenes, glycol ethers, and isopropanol.

The epoxy resin preferably forms an emulsion in the aqueous coating composition. Certain epoxy resins form stable emulsions in water, but most epoxy resins require the presence of an emulsifying agent such as a surfactant. Suitable emulsifying agents are commercially available and are familiar to persons of ordinary skill in the epoxy resin arts.

The composition preferably further contains a catalyst for the reaction of the epoxy resin and the amine-terminated adduct. Suitable catalysts include tertiary amines and quaternary ammonium salts, salicylic acid, phosphines and phosphonium salts, and nitrogen- -containing heterocyclic compounds such as benzoxazoles, benzothiazoles and benzimidazoles. Such catalysts are commercially available and familiar to persons of ordinary skill in the art.

The equivalent ratio of epoxy resin to amine groups in the coating composition is preferably between 1 : 1.2 and 1 :0.6, and more preferably between 1 : 1 and 1 :0.9. The composition (excluding the water) preferably contains 0 to 8 percent emulsifying agent, and more preferably between 0 and 3 percent emulsifying agent. The composition (excluding the water) preferably contains 0J to 5 percent catalyst, and more preferably between 0J and 1.0 percent catalyst. The composition (excluding the water) preferably contains 0 to 8 percent organic solvent, and more preferably between 2 and 5 percent organic solvent. The amount of water used may vary depending upon the intended use of the formulation. The composition may optionally contain other additives, such as stabilizers, flow-modifiers, wetting agents and pigments. The formulation preferably contains 20 to 85 percent water, more preferably 30 to 70 percent water.

The composition may be used in an ordinary manner for ambient-temperature or high-temperature cured coating formulations. It is applied to a substrate, such as metal or concrete, by known means such as spraying, dipping, brushing or rolling. If high-temperature curing is desired, then the coating is baked at a temperature and for a time sufficient to cure it. o The temperature is preferably between 60°C and 250°C. The curing time is preferably between 10 minutes and 2 hours. If ambient temperature curing is desired, then the formulation is simply allowed to cure. The temperature is preferably high enough, and the relative humidity is preferably low enough, for solvent in the formulation to evaporate off before the composition cures. The temperature is preferably at least 10°C and more preferably at least 5 20°C. The relative humidity is preferably no more than 80 percent and more preferably no more than 60 percent. The coating is preferably allowed to cure for at least 2 days, and more preferably for at least 7 days. It should not usually require more than 14 days to cure at temperatures of 10°C to 25°C.

The resulting coatings protect the substrate from weather and aggressive 0 chemicals.

Certain hardeners within the scope of the present invention may provide the following additional advantages over existing hardeners when used in coating formulations: excellent gloss; good flexibility; reduced viscosity in organic high solids formulations; good emulsif ication in aqueous solutions with epoxy resin (even with no added emulsif ier); and 5 rapid hardening at ambient temperature and under high humidity.

The invention is further exemplified by the following working examples: The following examples are for illustrative purposes only. They should not be taken as limiting the scope of either the Specification or the Claims. Unless otherwise stated, all parts and percentages are by weight: 0 Example 1 - Preparation of Adduct 1

A core epoxy resin was made by the following procedure: A 1 -liter reaction flask was loaded with 50.00 g of benzylamine; 388.00 g of a mixture containing 75 percent D.E.R.* 330 epoxy resin and 25 percent D.E.R.* 354 epoxy resin; and 61.50 g of nonylphenol. The flask was purged with nitrogen and heated to 100°C over a period of 15 minutes. The heat was 5 discontinued as an exotherm raised the temperature to 135°C over 8 minutes. Then the temperature was raised to 150°C for 2.5 hours. A sample of the resulting core epoxy resin had an EEW of 475, a number average molecular weight of 680 and a weight average molecular weight of 1300. The core epoxy resin was converted to an amine-terminated adduct by the following procedure: The reaction mixture from the previous step was cooled to 95°C A 552.6 g quantity of JEFFAMINE D400 polyoxypropylene diamine (AHEW = 115) was added, and the temperature was reduced to 61 °C. After stirring for 20 minutes, the temperature was raised to 81°C, and an exotherm carried the temperature to 116°C. The temperature was raised to 120°C, and the reaction was continued for 30 minutes. The resulting adduct had a viscosity of 120 Pa-s a 23°C. Its calculated AHEW was 280.

A solution containing 360 g of deionized water and 36 g of acetic acid at room temperature with stirring was added incrementally to a 361 g quantity of the adduct, which o was at 80°C The resulting clear aqueous solution had a calculated AHEW of 587, a viscosity of 39 Pa s at 23°C and a pH of 7.5. The solution is referred to as Adduct 1. Example 2 - Preparation of Adduct 2

A core epoxy resin was made as in Example 1 using 47.6 g of benzylamine, 369.4 g of epoxy resin mixture and 58.5 g of nonylphenol. The core epoxy resin was adducted as 5 described in Example 1 with 274.3 g of JEFFAMINE D400 polyoxypropylene diamine (AHEW = 115) and 250.2 g of JEFFAMINE D230 polyoxypropylene diamine (AHEW = 60). The calculated AHEW of the adduct is 180. A 258.7 g quantity of the adduct was added incrementally at 80°C to a mixture of 404.2 g deionized water and 37.1 g acetic acid at about room temperature. Example 3 - Preparation of Adduct 3 The procedure of Example 2 was repeated up to the point that the adduct of the core resin and the amine mixture was formed. The adduct was diluted by heating to 80°C for 30 minutes with 20 weight percent 1-methoxy-2-propanol and mechanically mixing for a few hours. The resulting mixture had a viscosity of 3300 mPa-s at 23°C. It is hereinafter called Adduct 3. 5 Example 4 - Preparation of Adduct 4

The procedure of Example 2 was repeated up to the point that the adduct of the core resin and the amine mixture was formed. The adduct was diluted by heating to 80°C or 30 minutes with 20 weight percent JEFFAMINE D230 polyoxypropylene diamine and mechanically mixing for a few hours. The resulting mixture had a viscosity of 7700 mPa-s at 23°C. It is hereinafter called Adduct 4.

Example 5 - Preparation of Adduct 5

The procedure of Example 4 was repeated using JEFFAMINE D400 polyoxypropylene diamine. The resulting mixture had a viscosity of 13,000 mPa-s at 23°C. It is hereinafter called Adduct 5. Example 6 - Coating Compositions

The adducts from the previous examples were mixed with deionized water and epoxy resin in the quantities shown in Table I to form coating compositions. The coating compositions were cast on Bonder steel panels at a thickness of 150 μm. The panels were baked for the time and at the temperature shown in Table I. The resulting coated panels were tested using the following tests: Persoz pendulum hardness, Erichsen indentation, reverse direct impact, and MEK double rubs. The results of the tests are set out in Table I. The following epoxy resins were used in Table I: Epoxy resin A - XZ-92400.01 * water-emulsif iable epoxy resin made by The Dow Chemical

Company.

Epoxy resin B - D.E.R.* 331 liquid diglycidyl ether of bisphenol A having an EEW of 182 to 192, available from The Dow Chemical Company.

Epoxy resin C - A mixture containing 87.7 percent D.E.R.* 337 epoxy resin (partially advanced diglycidyl ether of bisphenol A having an EEW of 230 to 250) dissolved in 1-methoxy-2- propanol, available from The Dow Chemical Company.

Epoxy resin D - A mixture containing 87 percent D.E.R.* 337 epoxy resin (partially advanced diglycidyl ether of bisphenol A having an EEW of 230 to 250) dissolved in 1-methoxy-2- propanol (10 percent) with an emulsif ier (3 percent), available from The Dow Chemical Company.

Epoxy resin E - A mixture containing 85 percent D.E.N. * 438 epoxy novolac resin dissolved in 1- methoxy-2-propanol.

Epoxy resin F - A mixture of Epoxy resin B with 5 weight percent diglycidyl ether of polytetrahydrofuran having an EEW of 210.

TABLE 1

Sample

1

2

3

4

5

I 6 I

7

8

9

10

11

12

13

Miscellaneous Comments: a - Solvent resistance improved after standing at room temperature. b - The experiment shows that the adduct can emulsify ordinary bisphenol A epoxy resins in aqueous solutions without an external emulsifying agent. c- Curing humidity 80 percent. d - 1 percent DMP 30 curing catalyst, available from Rohm & Haas, was added. e - Cast film thickness was 120 μm. f - 5 weight percent methoxypropanol was added to the formulation to reduce its viscosity to 4600 mPa-s. Front and reverse impact resistance are 160 Ib-in (18 J). g - Viscosity for the entire system was about 7000 mPa-s.

In testing Adhesion, "GtO" means that after Crosshatch and tape pull, about 100 percent coating subjected to the Crosshatch remains. The (+) indicates that the test sample performed a little better than average, and the (-) indicates that the test sample performed a little worse than average.

JO-

Claims

1. A process to make an amine-terminated adduct by reacting a core epoxy resin with an excess of polyamine resin at a temperature of 50°C to 140^CC, characterized in that the core epoxy resin contains an adduct made by reacting excess lower molecular weight epoxy resin with an amine chain extender that contains on average about 1.8 to 2.2 amine hydrogen atoms per molecule.

2. A process as described in Claim 1 wherein the core epoxy resin is an adduct made by reacting excess lower molecular weight epoxy resin with a benzylamine or aniline compound.

3. A process as described in any of the preceding claims wherein the low o molecular weight epoxy resin is further reacted with a lipophilic carboxylic acid compound or a lipophilic phenolic compound.

4. A process as described in any of the preceding claims wherein the core epoxy resin has an epoxy equivalent weight of 450 to 650.

5. A process as described in any of the preceding Claims wherein the 5 polyamine is added to the core epoxy resin in a staged or continuous fashion.

6. A process as described in any of the preceding Claims wherein more than 50 percent of the primary amine groups in the polyamine are bonded to secondary or tertiary carbon atoms.

7. A process as described in any of the preceding claims wherein the 0 polyamine compound has an a ine-hydrogen equivalent weight of 90 to 300.

8. A coating composition that contains:

(1) an epoxy resin,

(2) an amine-terminated curing agent for the epoxy resin,

(3) a solvent, (4) optionally a catalyst to catalyze the reaction of the epoxy resin and the curing agent, and (5) optionally an emulsifying agent, characterized in that the amine curing agent is an amine-terminated adduct made by a process as described in any of Claims 1-7.

9. A coating composition as described in Claim 7 wherein the solvent is an aqueous solvent.

10. Use of a coating composition as described in Claim 7 to coat a substrate by applying the composition to the substrate and curing it or permitting it to cure.