GB1604545A

GB1604545A - Coating compositions

Info

Publication number: GB1604545A
Application number: GB23776/78A
Authority: GB
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1978-05-30
Filing date: 1978-05-30
Publication date: 1981-12-09
Also published as: IT7923045A0; PL215924A1; AU4733879A; NZ190540A; ZA792636B; IT1121516B; CA1148678A; CS209809B2; FR2427368A1

Description

(54) COATING COMPOSITIONS (71) We, IMPERIAL CHEMICAL INDUSTRIES LIMITED, Imperial Chemical House, Millbank, London SW1P 3JF, a British Company do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 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 current 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 of film-forming polymer at a cathodic substrate has certain advantages over deposition at an anodic substrate since, for example, there is 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 phosphoric acid, which is capable of ionising at least some of the basic nitrogen-containing groups.

One suitable type of synthetic polymer containing basic nitrogen-containing groups which can be applied to a cathodic substrate is described in British Patent 1,461,823 and the disclosure of this patent is incorporated herein by this reference to it. The polymers described in this patent are essentially ionisable epoxy-amine abducts which are dispersible in aqueous medium in the presence of an acid, and which may be crosslinked with crosslinking agents such as phenol/formaldehyde or aminoplast resins.

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 to variation in pH or temperaure. In general, the useful polymers having nitrogen-containing groups do not usually form true solutions in an aqueous medium but instead 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, for example 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 synthetic polymers having basic nitrogen-containing groups, 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 higher pH is often desired. In general the stability of most polymer dispersions is satisfactory at high acid concentrations but stability at higher pH values, for example a pH 5 - 7 is desirable since the coulombic yield of polymer deposited at a cathodic substrate is thereby increased due to the smaller amount of neutralising acid required.

We have now found that there is an improvement in the stability of aqueous coating compositions, particularly those 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 together before they are dispersed in an aqueous medium 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 as the sole film forming components of a mixture and heating, i) an epoxy-amine adduct having ionisable nitrogen-containing groups, and ii) a crosslinking agent for the adduct, which agent is free from & arboxyl groups selected from phenoUformaldehyde and aminoplast resins or blends thereof to effect an increase in viscosity of the initial mixture of (i) and (ii); b) an acid which is capable of ionising the nitrogen-containing groups in the product defined in (a), there being present sufficient of the acid to neutralise at least 30% of the nitrogen-containing groups and form a dispersion of said product in the aqueous medium.

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

Preferably the increase in viscosity achieved by heating is at least 10% and preferably is in the range of 10 - 70%. The adduct and the crosslinking agent may be heated in the absence of a solvent but preferably one or both of them is or are dissolved or substantially dissolved in a 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 hydrocarbons.

Preferably, in order to increase its viscosity by the desired amount, the mixture of adduct and crosslinking agent is heated at a temperature of at least 50"C and more preferably of 100"C or higher, for example in the range 100 - 150"C. Heating may be continued for any desired penod of time, for example over 1/2 to 5 hours.

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

The reaction product obtained by mixing and heating the adduct 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 compositons of the invention include phosphoric, acetic, lactic acids, dimethylolpropionic, methane sulphonic and gluconic acids.

Particularly suitable crosslinking agents include phenol/formaldehyde, urea/formaldehy de 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 - B2 or (b) B1 - Al - C1 - A2 - C2 - A3 - 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.

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 electrodeposion, for example in spraying or brushing.

The invention is illustrated by the following Example in which parts and percentages are by weight: (a) Preparation of an 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 - 1000C with stirring until dissolved.

Diethylamine 1.6 parts was added to the above solution at 500C and after 2 hours at 55"C there were added: Ethylene diamine 0.4 parts The temperature was maintained at 550C for 3 hours, raised to 15"C over 1 hour and heating under reflux was continued for 1 hour. 1.2 parts of volatile materials were removed by distillation.

(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 - 1200C 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 71.7% by titration) 4.1 parts and water 77.5 parts The translucent emulsion produced was stable for at least 3 months and its particle size was too small to resolve by optical microscopy.

(ii) Not according to the invention Epoxy-amine adduct from (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.

WHAT WE CLAIM IS: 1. An aqueous coating composition which comprises (a) a dispersion in an aqueous medium of a product obtained by mixing as the sole film-forming components of a mixture and heating, (i) an epoxy-amine adduct having ionisable nitrogen-containing groups, and (ii) a crosslinking agent for the adduct, which agent is free from carboxyl groups, selected from phenol/formaldehyde and aminoplast resins, or blends thereof 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 the acid to neutralise at least 30% of the nitrogen-containing groups 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 claim 1 or claim 2 wherein the mixture of (i) and (ii) is heated to at least 500C.

4. An aqueous coating composition according to any one of claims 1-3 wherein the acid is selected from phosphoric, lactic, acetic, and dimethylol propionic acids.

5. An aqueous coating composition according to any one of Claims 1 - 4 wherein the crosslinking agent is a urea/formaldehyde, or melamine/formaldehyde resin.

6. An aqueous coating composition according to claim 5 wherein the crosslinking agent is a butylated urea/formaldehyde resin.

7. An aqueous coating composition according to any one of claims 1-6 wherein the epoxy-amine adduct is a compound of formulae (a) B1 - A1 - C1 - A2 - B2 or (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 of p b 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.

8. An aqueous coating composition according to claim 1 substantially as herein defined and with reference to the Example.

9. A process of a preparing an aqueous coating composition which comprises (a) mixing as the sole film forming components of a mixture and heating (i) an epoxy-amine adduct having ionisable nitrogen-containing groups, and (ii) a crosslinking agent for the adduct which agent is free from carboxyl groups selected from phenol/formaldehyde and aminoplast resins or blends thereof, to effect an increase in viscosity of the initial mixture of (i) and (ii), including any 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 the nitrogen-containing groups and form a stable dispersion of said product in the aqueous medium.

10. A process of prepanng an aqueous coating composition according to claim 10 substantially as herein defined and with reference to the Example.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. (ii) Not according to the invention Epoxy-amine adduct from (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. WHAT WE CLAIM IS:

1. An aqueous coating composition which comprises (a) a dispersion in an aqueous medium of a product obtained by mixing as the sole film-forming components of a mixture and heating, (i) an epoxy-amine adduct having ionisable nitrogen-containing groups, and (ii) a crosslinking agent for the adduct, which agent is free from carboxyl groups, selected from phenol/formaldehyde and aminoplast resins, or blends thereof 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 the acid to neutralise at least 30% of the nitrogen-containing groups and form a stable dispersion of said product in the aqueous medium.