GB2050381A

GB2050381A - Aqueous coating composition

Info

Publication number: GB2050381A
Application number: GB7917972A
Authority: GB
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1979-05-23
Filing date: 1979-05-23
Publication date: 1981-01-07

Abstract

An aqueous coating composition prepared by dispersing in an aqueous medium in the presence of an acid a product obtained by mixing and heating an epoxy-amine adduct and a phenol/formaldehyde or aminoplast crosslinking agent. The composition has improved stability at high pH values and at elevated temperatures and is suitable for cathodic electrodeposition.

Description

SPECIFICATION Coating compositions This invention relates to the preparation of aqueous coating compositions; in particular to such compositions which are useful for application to a conductive substrate by electrodeposition.

A conductive substrate may be coated with certain synthetic film-forming polymers by immersing the substrate in an aqueous dispersion of the polymer and making the substrate one electrode in an electrical circuit through which is passed a suitable electrical current. The current is passed between the substrate and a counter-electrode, the counter-electrode being in electrical contact with the dispersion. Suitable polymers usually contain ionisable groups. Those polymers which are to be deposited at an anodic substrate usually contain anionic groups and those which are to be deposited at a cathodic substrate usually contain cationic groups. Other materials may be co-deposited at the substrate-electrode with the polymer, for example pigment and/or a crosslinking agent for the polymer.

Deposition offilm-forming polymer at a cathodic substrate has certain advantages over deposition at at anodic substrate since, for example, there is little or no dissolution of metal from a cathodic metal substrate, such as a ferrous metal substrate. In the case of anodic deposition dissolution of metal can discolour the polymer coating. A further advantage of using a cathodic substrate is that a previously applied phosphate conversion coating is substantially unaffected by the passage of electric current.

Suitable synthetic film-forming polymers which may be applied by cathodic deposition are those polymers which contain basic nitrogen-containing groups. Suitable basic nitrogen-containing groups are amino or amine groups or residues of these groups. These polymers are preferably dispersed in an aqueous medium in the presence of an acid, for example lactic acid, which is capable of ionising at least some of the basic nitrogen-containing groups.

A particularly suitable group of synthetic polymers containing basic nitrogen-containing groups which can be applied to a cathodic substrate are the ionisable epoxy-amine adducts. By the term 'epoxy-amine adducts' we mean the product of reacting a compound containing at least two epoxy groups with a primary or secondary amine (including a diamine). The preparation of such epoxy-amine adducts is described in the standard chemical text books, for example in "The Chemistry of Organic Film formers" by D.H. Solomon (John Wiley & Sons Inc., 1967). Typical epoxy group-containing compounds which may be used include the linear epoxy resins obtained by reacting a polyhydric polynuclear phenol, e.g. bis-phenol A with an epichlorohydrin.

One important requirement in a coating composition comprising an aqueous dispersion of synthetic film-forming polymer, and particularly one which is to be applied to a substrate by cathodic deposition under industrial conditions, is good stability to prolonged storage or usage and variation in PH or temperature. In general, the useful polymers having nitrogen-containing groups do not usually form true solutions in an aqueous medium but rather at least part of the polymer exists in the aqueous medium as discrete liquid, solid, colloidal or other forms of disperse particles which owe their stability in the coating bath to various factors. Typical factors are the number and/or the nature of the ionisable basic groups in the polymer and the degree of neutralisation of such groups by an acid.

Whilst the aqueous dispersions of these synthetic polymers together with a desired crosslinking agent, may be of satisfactory stability for certain purposes, particularly when the pH is relatively low, good stability of the dispersions at a relatively high pH is often desired but is frequently not achieved. Good stability at higher pH values, for example at pH 5 - 7, is desirable since the coulombic yield of polymer deposited at a cathodic substrate is greater due to the smaller amount of neutralizing acid required.

We have now found that there is an improvement in the stability of aqueous coating compositions, particularly suitable for cathodic deposition, which comprise a dispersion of an epoxy-amine adduct and certain crosslinking agents for the polymer, when the adduct and the crosslinking agent are mixed and heated in the presence of an acid.

According to this invention we provide an aqueous coating composition which comprises: (a) a dispersion in an aqueous medium of a product obtained by mixing and heating, (i) an epoxy-amine adduct having ionisable nitrogen-containing groups and (ii) a crosslinking agent for the adduct selected from phenol/formaldehyde and aminoplast resins or blends of these resins to effect an increase in viscosity of the initial mixture of (i) and (ii); and (b) an acid which is capable of ionising the nitrogen-containing groups in the product defined in (a), there being present sufficient of said acid to neutralise at least 30% of the nitrogen-containing group in and form a dispersion of said product in the aqueous medium.

The increase in viscosity of the mixture may be determined by comparing the viscosity of the mixture before and after heating, after dilution with a suitable solvent for example 2-butoxyethanol to a solids content of 40% by weight.

Preferably the increase in viscosity achieved by heating is at least 10% and for convenience may be in the range 10 - 70%. The adduct and the crosslinking agent are preferably heated substantially in the absence of added water but preferably one or both of them is or are dissolved or substantially dissolved in a non-aqueous solvent before mixing together, and further suitable solvent may be added to the mixture, for example, to ensure homogeneity. Suitable solvents include alcohols, glycol ethers and esters, ketones, and aromatic hydro-carbons.

Preferably, in order to increase its viscosity by the desired amount, the mixture of adduct and cross-linking agent is heated at a temperature of at least 500C and more preferably of 100"C or higher, for example in the range 100 - 150"C. Heating may be continued for any desired period of time, for example over 1/2 to 5 hours.

The viscosity of the mixture before and after heating may be determined by those conventional procedures which are used in the coating art.

The reaction product obtained by mixing and heating the polymer and crosslinking agent may be dispersed in an aqueous medium in the presence of the acid by suitable procedures. Suitable acids to be used in the aqueous coating compositions of the invention include phosphoric, acetic, lactic, dimethylolpropionic, methane sulphonic and gluconic acids.

Particularly suitable crosslinking agents are phenol/formaldehyde, urea/formaldehyde and melamine/ formaldehyde resins. Urea/formaldehyde resins are particularly suitable especially when butylated.

Epoxy-amine adducts which are particularly suitable in this invention are those described in British Patent 1,461,823 which are selected from compounds of formula (a) B1-A1-C1-A2-B2or (b) B1-A1-C1-A2-C2-A2-B2, wherein A1 A2 and A3 are residues of essentially straight chain di-epoxides and are free of unreacted epoxide groups, B1 and B2 are residues of primary or secondary monoamines of pKb value not greater than 4 and C1 and C2 are residues of diamines or primary monoamines the adduct having a molecular weight of from 1,500 to 7,000.

These adducts are prepared by reacting essentially linear epoxy resins, for example those prepared by reacting a polyhydric polynuclear phenol with an epichlorhydrin, with suitable primary or secondary amines or diamines. Typical epoxy resins from which A1 A2 and A3 may be derived have a molecular weight in the range 400 - 4000 and contain two epoxy groups per molecule. Typical amines to provide the residues B1 and B2 are dimethylamine and diethylamine. Typical amines to provide the residues C1 and C2 are ethylamine and ethylene diamine.

Other epoxy-amine adducts which may be used in this invention will be known to those skilled in the art.

The good stability at high temperatures of the product obtained by premixing and heating is useful in that an emulsion of the product can be prepared at high temperature. There is the further advantage of stability at these higher temperatures that a smaller proportion of neutralizing acid is necessary and thus dispersions of higher solids content may be prepared.

The aqueous coating compositions of this invention may contain pigment and other conventional paint additives. The compositions may be used in application processes other than cathodic electrodeposition, for example in spraying or brushing.

The invention is illustrated by the following Examples in which parts and percentages are by weight: EXAMPLE 1 (a) Preparation ofan epoxy-amine adduct The following ingredients 2-ethoxy ethanol 10.0 parts n-butanol 3.3 parts epoxy resin (having epoxide equivalent of 475 and commercially available as "Epikote" 1001) 'Epikote' is a Registered Trade Mark 13.4 parts epoxy resin (having epoxide equivalent of 901 and commercially available as "Epikote" 1004) 13.4 parts were heated to 80 - 1 00'C with stirring until dissolved.

Diethylamine 1.6 parts was added to the above solution at 500C and after 2 hours at 55"C there was added: Ethylene diamine 0.4 parts The temperature was maintained at 55"C for 3 hours, raised to 11 50C over 1 hour and heating under reflux was continued for 1 hour. 1.2 parts of volatile materials were then removed by distillation at 115-130 C.

(b) Preheating of epoxy-amine adduct and urea formaldehyde resin A mixture of epoxy-amine adduct as obtained in (a) above 55.7 parts and urea/formaldehyde (a 53% solution in xylol/butanol of a butylated urea/formaldehyde resin (urea/formaldehyde = 1/2.25 molar) 18.9 parts were heated for 3 hours at 100 - 1 200C during which time the viscosity of a sample (diluted to 40% solids by weight with 2-butoxy ethanol) increased from 8.0 stokes to 10.0 stokes.

(c) Preparation of unpigmented coating compositions (i) According to the invention The total product of (b) above (74.7 parts) was cooled and emulsified by mixing with Commercial lactic acid (free lactic acid content of 71.4% by titration) 4.1 parts and water 77.5 parts The translucent emulsion produced had a pH of 5.5 - 6.5 and was stable for at least 3 months. Its particule size was too small to resolve by optical microscopy. A cathodic substrate was satisfactorily coated by electrodeposition using this emulsion.

(ii) Not according to the invention Epoxy-amine adductfrom (a) 55.7 parts Urea/formaldehyde resin as in (b) 18.9 parts were mixed without heating and emulsified in a solution of lactic acid in water in the manner described in (i).

The emulsion produced had a milky appearance. It settled out on storage after several days and discrete particles were just visible under an optical microscope X400.

A similar result was obtained when the urea/formaldehyde resin was replaced by an equivalent amount of a melamine/formaldehyde resin.

EXAMPLE 2 (a) Preparation ofan epoxy-amine adduct 1000 parts of an epoxy resin (of epoxide value approximately 900 and commercially available as "Epikote" 1004) were dissolved in 250 parts of methyl isobutyl ketone and warmed to 50"C. 40 parts of di-ethylamine was added and allowed to react for one hour. This was followed by the addition of 16 parts of ethylene diamine which was allowed to react for 3 hours at 50"C followed by one hour at 1000C.

(b) Preheating of epoxy-amine adduct andphenol-formaldeh yde resin To the product in (a) (which had an epoxide value equivalent to 2 mg KOH!g) were added 211 parts of a phenol/forma Idehyde resin commercially available as "U ranox L9". The mixture was heated at 100-1200C for 3 hours during which time there was a significant increase in viscosity (greater than 10%).

(c) The product of (b) was emulsified in water in the presence of lactic acid as described in Example 1.

The translucent emulsion has a pH of 5.5 - 6.5 and was stable for at least 3 months. A cathodic substrate was satisfactorily coated by electrodeposition using this emulsion. When the epoxy-amine adduct and phenol-formaldehyde resin were not preheated an emulsion produced in the same way was unstable.

Claims

1. An aqueous coating composition which comprises (a) a dispersion in an aqueous medium of a product obtained by mixing and heating, (i) an epoxy-amine adduct having ionisable nitrogen-containing groups, and (ii) a crosslinking agent for the adduct selected from phenol/formaldehyde and aminoplast resins, or blends of these resins to effect an increase in viscosity of the initial mixture of (i) and (ii), and (b) an acid which is capable of ionising the nitrogen-containing groups in the product defined in (a), there being present sufficient of said acid to neutralise at least 30% of the nitrogen-containing groups in and form a stable dispersion of said product in the aqueous medium.

2. An aqueous coating composition according to claim 1 wherein there is an increase in viscosity of the initial mixture of (i) and (ii) of at least 10% on a 40% by weight resin solids basis.

3. An aqueous coating composition according to any one of claims 1 and 2 wherein the mixture of (i) and (ii) is heated to a temperature of at least 50%.

4. An aqueous coating composition according to any one of claims 1 to 3 wherein (i) and (ii) are mixed and heated substantially in the absence of water.

5. An aqueous coating composition according to any one of claims 1 to 4 wherein the acid is selected from phosphoric, lactic and acetic acids.

6. An aqueous coating composition according to any one of claims 1 to 5 wherein the crosslinking agent is a urea/formaldehyde, or melamine/formaldehyde resin.

7. An aqueous coating composition according to claim 6 wherein the crosslinking agent is a butylated urea/formaldehyde resin.

8. An aqueous coating composition according to any one of claims 1 to 7 wherein the epoxy-amine adduct comprises a compound of formula (a) B1-A1-C1-A2-B2or (b) B1 -A1 - C1 -A2 - C2 - A3 - B2, wherein A1, A2 and A3 are residues of essentially straight chain di-epoxides and are free from unreacted epoxide groups, B1 and B2 are residues of primary or secondary monoamines or pKb value not greater than 4, and C1 and C2 are residues of diamines or primary monoamines, the adduct having a molecular weight of from 1,500 to 7,000.

9. An aqueous coating composition substantially as herein described and with reference to any one of the Examples.

10. A process for preparing an aqueous coating composition which comprises (a) mixing and heating, (i) epoxy-amine adduct having ionisable nitrogen-containing groups and (ii) a crosslinking agent for the adduct selected from phenol/formaldehyde and aminoplast resins or blends of these resins to effect an increase in viscosity of the initial mixture of (i) and (ii), including any non-aqueous solvent which is present, of at least 10%; and (b) dispersing the product of (a) in an aqueous medium in the presence of an acid which is capable of ionising the nitrogen-containing groups in the product of (a), there being sufficient of said acid to neutralise at least 30% of said basic groups and form a stable dispersion of said product in the aqueous medium.

11. A process of preparing an aqueous coating composition substantially as herein defined and with reference to the Examples.

12. A method of coating a conductive substrate by electrodeposition wherein an electric current is passed between the substrate as cathode and another electrode immersed in an aqueous coating composition according to any one of claims 1-11.