GB2048941A - Readily cross-linkable anodically electrodeposited polybutadiene compositions - Google Patents

Abstract

In the production of a resinous coating on a metal surface by anodically electro-depositing a film on the surface from an aqueous solution or dispersion of derivatives of liquid polybutadienes and cross-linking at elevated temperature, the solution or dispersion comprise anions derived from one or more liquid polybutadienes which have been combined with from 17.5 to 42.5 per cent by weight of maleic anhydride and subsequently half-esterified with one or more hydroxyalkyl or epoxyalkyl acrylates or methacrylates sufficient to half-esterify at least one quarter of the anhydride residues, sufficient carboxy groups being formed thereby to render the composition anodically electro- depositable after neutralisation and/or solution or dispersion in water. The anodically electro-deposited film is cross-linked by heating the metal surface at a temperature not less than 120 DEG C and not more than 180 DEG C. The presence of the hydroxyalkyl or epoxyalkyl acrylates or methacrylates in the composition enables cross- linking to be effected at significantly lower temperatures than previously proposed anodic electrodeposition compositions based on polybutadienes.

Description

SPECIFICATION Improved compositions for anodic electrodeposition This invention relates to a process for producing resinous coatings on metal surfaces of derivatives of liquid polybutadienes which are readily cross-linked at elevated temperatures after anodic electrodeposition on said metal surfaces from aqueous solutions or dispersions comprising said derivatives of liquid polybutadienes, and to the cross-linked coatings formed thereby.

It has long been known in the art to deposit resinous coatings on metal surfaces by first immersing the metallic article to be coated in an electrically conductive aqueous solution or dispersion containing dissolved or dispersed polymeric anions of one or more resins, and by then applying a voltage to the metallic article such that it becomes the anode of an electrocoating bath. Other constituents appropriate to surface coatings or paints rnay also be either dissolved or dispersed in the aqueous phase, including pigments, nonionised resins, water-miscible organic solvents, catalysts and co-reactive substances for promoting cross-linking of the deposited film, buffer agents and cations of the ionised resins.When a continuous film of the resinous coating, pigmented or unpigmented as the case may be, has been deposited on the metallic surface, the article is withdrawn from the bath, usually rinsed with water and then subjected to a treatment which will cross-link or cure the deposited film. This last need arises from the fact that the resin or resins comprising the discharged polymeric anions generally possess relatively low molecular weights until cross-linked and the films are correspondingly weak and lacking in solvent resistance etc.The requirement to utilise relatively low molecular weight resins in electrocoating processes stems from the ease of preparation of fluid aqueous solutions of low molecular weight resins, from their superior performance during the deposition process and from the ease with which they form adherent films of adequate strength to withstand the cross-linking operation.

The usual method of effecting cross-linking is to raise the temperature of the coated article by holding it in an oven at an elevated temperature for a prescribed period of time. High energy irradiation has also been proposed for inducing cross-linking of an essentially low molecular weight film, for example according to Japan Kokai No. 66186/1973, but heating has the advantage of inducing flowout and smoothness of the deposit on the metal surface, and of cross-linking the coatings on box sections and other areas which would be screened from radiation.

Derivatives of liquid polybutadienes have frequently been employed for use in electro-coating processes, especially their reaction products with maleic anhydride. Intermediate reaction products are formed by reacting liquid polybutadiene with maleic anhydride at elevated temperäture in order to form a resin containing combined anhydride groups which is not cross-linked. A subsequent ring-opening reaction may then be applied to the cyclic anhydride structure in order to provide carboxyl groups and ester groups. The carboxyl groups may be either simultaneously or subsequently neutralised and the resulting product may be diluted with water to form more or less fluid aqueous solutions or dispersions comprising anions of the maleinised liquid polybutadiene.Solutions are generally preferred, but the precise colloidal nature of the resin contained in such a coating composition will depend upon a combination of various parameters, such as the molecular weight and molecular weight distribution of the polybutadiene, the concentration of ionised carboxyl groups in the polymer, the degree of ionisation etc.

In order to prepare efficient anodically electrodepositable resins according to the prior art, the reaction products of maleic anhydride and liquid polybutadienes are usually themselves liquid; the viscosities of the reaction products are more or less considerably greater than the viscosities of the liquid polybutadienes used as starting materials, the magnitude of the increase in viscosity being dependent upon a number of parameters including the microstructure and other properties of the particular liquid polybutadiene employed and especially on the average proportion of maleic anhydride residues combined therewith.

It is general practice to half-esterify all or at least a major proportion of the combined anhydride groups via a ring-opening reaction with a suitable hydroxyl group-containing compound such as a low molecular weight monohydric alcohol, and subsequently to neutralise all or a proportion of the carboxyl groups formed thereby as the next step in the preparation of an electrodeposition bath. For example, methanol may be very conveniently employed during the half-esterification step, although many other hydroxyl group-containing substances including the half-esters and the half-ethers of dihydric alcohols may be used. Different hydroxyl group-containing compounds may of course be used in combination together or in sequence. The reaction is carried out at temperatures up to about 1200 C., generally at about 800 C. for 2 or 3 hours.Catalysts may be employed to promote the rate of half-esterification.

Before the subsequent step of neutralisation of part-neutralisation, it is often found desirable to add a small quantity of an organic liquid which is a solvent for the polybutadiene derivative and which is also water-miscible. The presence of such a substance serves to provide compatibility of the polybutadiene derivative with the neutralising agent which may be added in aqueous solution, and to facilitate mixing with water and dilution with water generally, during and after neutralisation.

Neutralisation or part-neutralisation may be carried out with one or more organic amines such as triethylamine and triethanolamine, and/or one or more inorganic bases such as sodium and potassium hydroxide.

Either only part or all of the total quantity of water required to prepare an eiectrocoating composition may be added at the time of neutralisation. The final concentration of an aqueous composition used for electrodeposition is generally in the range ofl O to 20 percent with respect to solids content, at which concentration the viscosity is low. Sufficient water may be added at the time of neutralisation to form a solution or dispersion of higher concentration, which would be more viscous, into which the other ingredients if any may be more efficiently dispersed. Finally the remainder of the necessary water of dilution may be added in order to form a composition suitable for an electrocoating bath.

An alternative procedure, which is less favoured in the art, involves simultaneous hydrolysis of the anhydride groups and neutralisation or part-neutralisation of the carboxyl groups formed thereby, that is to say, without previously half-esterifying any of the anhydride groups.

In order to cross-link or cure the film of anodically electrodeposited resin derived from the halfesterified maleinised liquid polybutadiene, the usual practice is to subject the metal article to a stoving temperature of about 1 800C., in an oven for a period of from about 5 to about 30 minutes, the duration depending upon a number of considerations such as the size of the object which has been subjected to electrodeposition. Cross-linking of the deposited film takes place by a mechanism which essentially involves reaction with atmospheric oxygen.

In the preparation of aqueous anodic electrodeposition compositions comprising derivatives of liquid polybutadiene, it is customary to employ other resins as additives for a variety of purposes, including those of accelerating the rate of cross-linking and of promoting a uniform degree of crosslinking throughout the thickness of the layer ("through-cure"). It will be appreciated that uniform crossiinking is of great significance in ensuring the efficient performance of the coating, and is difficult to achieve especially in thick films bearing in mind'that the reaction mechanism involves the presence of atmospheric oxygen. Modified phenolic resins, maleinised natural drying oils, maleinised drying oil fatty acids, modified alkyd resins, maleinised polypentadiene, maleinised polycyclopentadiene etc., have all been either used or proposed as starting materials.Such resins may themselves be anodica!ly depositable even in the absence of the ionised polybutadiene derivative, but they must of course in any case, be co-depositable therewith. They may or may not be reacted in the presence of the polybutadiene or the polybutadiene derivative atany stage of preparation, even as early as the maleinisation step. Thus such resins may be in a state of chemical combination with the liquid polybutadiene derivative, or they may simply be added by mixing. Finally, the possible use of non-ionisable co-resins may be mentioned, since many such are co-depositable together with the polymeric anions even in the absence of previous chemical combination therewith.

Although the presence of such resins confers many valuable improvements to the rate of cure of anodically depositable liquid polybutadiene derivatives, the need nevertheless remains for an anodically electrodepositable composition comprising a liquid polybutadiene derivative as the major constituent which crosslinks more readily than such compositions employed in the prior art comprising liquid polybutadiene derivatives, The nature and proportions of co-resins and other ingredients in the electrodeposition bath, especially of pigments, can affect the physical properties of the cross-linked film after stoving to a critical degree.These properties include smoothness and gloss; hardness and scratch resistance; flexibility and impact resistance or resistance to chipping; adhesion to the substrate and to any subsequently applied coatings; electrical resistivity; resistance to organic solvents; water resistance and corrosion resistance of the metal surface. These properties determine the efficiency of the coating in its particular end-use. Since a large proportion of anodically electrodeposited coatings comprising liquid polybutadiene derivatives is employed to provide a corrosion-resistant primer for steel car bodies, in which end-use resistance to corrosion of the steel by salt solutions is of critical importance, such test procedures as resistance to prolonged aqueous salt sprays are frequently applied.With particular respect to salt spray resistance, anodically deposited coatings derived from liquid polybutadiene as the principal constituent have proved outstandingly successful.

Besides affecting the physical properties of the cross-linked coatings, the nature and proportion of co-resins and other ingredients in the aqueous electrodeposition composition comprising liquid polybutadiene derivatives can also strongly influence the deposition behaviour of the composition and the properties of the film before cross-linking. Among the properties which require careful optimisation and balancing which may be mentioned are the average rate of deposition with time; the so-called throwing power of the composition, which determines the rate of deposition on areas of metal surface which are shielded from the cathode; the rupture voltage of the film, which determines the maximum voltage which may be applied to the anode without creating flaws in the film; and the tendency for the film to sag and run off vertical surfaces while the metal article is being stoved.

We have now made the surprising discovery that by partially or completely half-esterifying the maleic anhydride residues which have been combined with liquid polybutadiene by means of one or more hydroxy- or epoxyalkyl acrylates or methacrylates, such that sufficient carboxyl groups become available for neutralisation to form water-soluble or water-dispersible polymeric anions, films formed on metal surfaces by anodic electrodeposition of such resins cross-link much more readily than those deposited from aqueous liquid polybutadiene derivatives which have not been thus half-esterified.It is not only surprising that cross-linking occurs much more rapidly at elevated temperatures corresponding to those used in said prior art processes and that the cross-linking coatings can have superior technological properties, but that the deposition performance of the compositions can also be superior.

It is also surprising that significantly lower cross-linking temperatures may be employed while still attaining at least equivaient coating properties compared with those resulting from prior art processes involving the use of liquid polybutadiene derivatives; resulting in substantial savings in energy and reducing the emission of noxious fumes.

Accordingly the invention provides a process for producing resinous coatings on metal surfaces by a method of anodic electrodeposition employing an aqueous solution or dispersion comprising anions derived from one or more liquid polybutadienes which have been combined with between 17.5 and 42.5- percent by weight of maleic anhydride and subsequently half-esterified with one or more hydroxyalkyl or epoxyalkyl acrylates or methacrylates sufficient to half-esterify at least one-quarter of the anhydride residues, sufficient carboxy groups being formed thereby to render the composition anodically electrodepositable after neutralisation and/or solution or dispersion in water, the electrodeposited film being cross-linked by heating the metal surface in air to a temperature not less than 1 200C and not exceeding 1 800 C., preferably between. ? 300C and 1 55 C.

The invention also provides for cross-linked resinous coatings formed on metal surfaces by a process of anodic electrodeposition followed by cross-linking by means of heat as defined above.

Among the properties of the cross-linked coatings which can be significantly improved compared with those formed by prior art anodic electrodeposition processess involving the use of compositions comprising liquid polybutadiene may be mentioned corrosion resistance and the tendency for the film to run-off a vertical surface during stoving; while among the deposition properties which may be significantly increased may be mentioned throwing power and/or the rupture voltage.

The reaction between maleinised liquid polybutadiene, containing a proportion of combined maleic anhydride residues as previously defined, and one or more hydroxyalkyl acrylates or methacrylates may be carried out as previously described for the case of reaction with monohydric alcohols. Temperatures up to about 1 200C may be used, but it is preferred to use a temperature of about 800C. for about 2 hours. Catalysts may be used in order to promote the rate of the halfesterification reaction, especially when temperatures lower than about 800 C. are used. Alternatively higher temperatures may be used for a shorter reaction period, so long as the formation of gelled or insoluble products is avoided.

If it is desired to react only a proportion of the total number of maleic anhydride residues present with one or more hydroxyalkyl acrylates or methacrylates and to half-esterify part or all of the remainder with other hydroxy compounds, then the reactants may be caused to combine simultaneously or they may be added in sequence in any order.

In the case of reaction with one or more epoxyalkyl acrylates or methacrylates, such as glycidyl acrylate of methacrylate, it may be necessary to open the anhydride ring in order to promote a halfesterification reaction between the carboxyl groups thus formed and the epoxy groups. This ringopening reaction may be carried out by hydrolysis; alternatively a half-esterification reaction with one or more other hydroxy compounds may be carried out. The reaction to provide carboxyl groups may be carried out before their half-esterification with epoxyalkyl acrylate or methacrylate, or all the different reactions may be carried out at the same time. The esterification reaction Inay be catalysed as previously mentioned. In any event, sufficient free carboxy groups must be allowed to remain in order to provide for subsequent solution or dispersion in water.

The liquid polybutadiene containing combined maleic anhydride residues may be reacted with both, on one hand, a hydroxyalkyl acrylate or methacrylate or more than one such compou.nd, and, on the other hand, an epoxyalkyl acrylate or methacrylate or more than one such compound. Such reactions may be carried out simultaneously or in sequence in any order, in the presence of other hydroxy compounds if desired. In any case sufficient carboxy groups must be formed in order to provide for sufficient water-solubility or dispersibility to enable anodic electrodeposition to be carried out.

Particularly desirable results are obtained by half-esterifying the combined anhydride residues with both a hydroxyalkyl acrylate or methacrylate and an epoxyalkyl acrylate or methacrylate in approximately equimolar proportions of hydroxy group - and epoxy group -- containing esters.

The reaction between liquid polybutadiene and maleic anhydride is well known, and may be carried out by any method to provide a proportion of combined an hydride groups as defined previously.

In the preparation of the electrodeposition composition, we do not exclude the possibility of utilising any of the- co-resins used in the prior art such as those previously exemplified. These may also be reacted or modified further by reaction with hydroxyalkyl or epoxyalkylacrylates or methacrylates, either in the presence of the liquid polybutadiene containing combined maleic anhydride residues, or separately followed by mixing of the ingredients. Thus for example, we do not exclude the possibility of mixing the liquid polybutadiene containing combined maleic anhydride residues with the reaction products of alkyl resins and/or drying oils with maleic anhydride, and of reacting the mixture with hydroxyalkyl acrylates or methacrylates in order to form a water-soluble or water-dispersible anodic electrocoating composition for use in the present process.

Liquid polybutadienes which may be used as starting materials in the present process comprise substances having a molecular weight up to about 20000 and exhibiting a viscosity not exceeding about 2500 poise at a temperature of about 500 C. when in a substantially pure condition. Copolymers comprising up to about 50 percent of units derived from one or more other copolymerisable monomers expressed on a molar basis may be employed, and the structure of the polymers may comprise linear, branched, radial and star-shaped macromolecules.The terminal units of the macromolecules or the macromolecular branches may be linked to units containing functional groups which are capable of combining with maleic anhydride and/or with hydroxyalkyl or epoxyalkyl acrylates or methacrylates, but the composition and macrostructure of the liquid polybutadiene or polybutadienes selected must be such that the sequence of reactions may be carried out without incurring the danger of forming crosslinked and insoluble products.

In general, linear liquid polybutadiene homopolymers not containing functional groups co-reactive with maleic anhydride and having a molecular weight up to about 5000 are preferred on grounds of both cost and efficiency. A liquid polybutadiene formed by anionic polymerisåtion involving the use of a chain-transfer or telomerisation step with toluene and initiated by an organo-lithium compound is found to be highly efficient and easy to use for producing anodic electrodeposition compositions. Such a polymer would have a number-average molecular weight of from about 800 to about 5000 and a microstructure comprising about 55% by number of units in 1 4-configuration and about 45% by number of units in a 1,2-configuration.Many statements have been made in literature describing specific advantages to be derived from the use of polymers having this or that particular microstructure, such as predominantly cis-1 ,4-configuration, or a predominant vinyl or 1,2-configuration with carboxyl groups attached terminally to the macromolecules. It is generally agreed nowadays by those skilled in the art that by suitable experimentation designed to optimise the nature and degree of the chemical modification of the liquid polybutadiene including optimisation of the nature and proportion of co-resins and other ingredients, a liquid polybutadiene having more or less any microstructure may be employed and excellent-results still obtained.

The invention will now be illustrated by means of the following Examples.

EXAMPLE 1 (COMPARATiVE) A quantity of 400 g. of maleinised liquid polybutadiene was charged to a round-bottomed flask equipped with stirrer, nitrogen inlet, condenser and-thermometer, surrounded by an electrically-heated mantle.

The liquid polybutadiene used was a commercial product sold as "Lithene" PM ("Lithene" is a registered Trade Mark). It was reacted with maleic anhydride in a conventional procedure, employing proportions. of 100 g. of liquid polybutadiene and 25 g. of maleic anhydride. For convenience, the reaction product with maleic anhydride is designated "Lithene" PM 25 MA.

"Lithene" PM has a number-average molecular weight of about 1300, and a microstructure corresponding'to about 4050% units in the 1,2 configuration; 3040% trans-1,4; and 1 5-25% is1 ,4.

The molar proportion of maleic anhydride units in "Lithene" PM 25 MA is about 0.816 moles combined maleic anhydride per 400 g. product. To this charge was added 40 g. methanol and the temperature of the reaction mixture raised to 800 C. with continuous stirring, under an atmosphere of nitrogen.

After about 1 hour the reaction-product was cooled and neutralised with triethylamine, employing a proportion of 14 g. triethylamine per 100 g. reaction-product, and with the aid of mechanical stirring.

Water was added until the solids content was about 10%. Further small additions of triethylamine were made until the pH was about 8.5, forming a clear solution.

EXAMPLE 2 Using the same apparatus as in Example 1 and the same procedure 47 g. (0.408 moles) of hydroxyethyl acrylate were reacted with the "Lithene" PM 25 MA instead of the 40 g. methanol, and with the further addition of 2 g. of triethylamine as catalyst for this reaction. After 1 hour at 800C., 13 g.

of methanol (0.408 moles) were added, and the reaction was allowed to continue for 1 hour further at 800 C.

When this reaction period was over, a solution was prepared exactly as described in Example 1.

The solution at pH 8.5 was clear (solids content about 10%).

EXAMPLE 3 The same procedure and reactants were used as in Example 1, except that 94.2 g. (0.816) moles of hydroxyethyl acrylate were used instead of methanol, together with 2 g. of triethylamine as catalyst.

The solution obtained at pH 8.5 and 10% solids content was opaque.

EXAMPLE 4 The same procedure and reactants were used as in Example 1 except that 47.1 g. (0.408 moles) of hydroxyethyl acrylate were used together with 2 g. of triethylamine as catalyst, instead of methanol, and the reaction conditions were for 2 2 hours at 500 C. At the end of this period, 58.3 g. (0.408 moles) of glycidyl methacrylate were added, and the reaction continued for a further 26 hours at 500C. The solution of 10% solids content was slightly turbid at pH 8.5.

The resins in the solutions prepared, as described in Examples 1 to 4, were anodically deposited on panels of degreased bare steel as anodes. The panels were rinsed with tap water, dried with compressed air currents at room temperature, and then stoved in a convection oven at the temperatures shown in Table 1. After stoving, the panels were stored at room temperature for a minimum of 12 hours before testing.

Test methods -- notes on Table I 1. Rupture Voltages The rupture voltage recorded is that maximum voltage at which a film can be deposited without evidence of discontinuities appearing during deposition.

2. Throwing Power A sandwich-type cell was employed, using parallel plates of equal length as electrodes. The electrodes were separated by a polymethyl methacrylate plate of the same length, 3 mm. from the anode. The throwing power was expressed as the length of the shielded face of the anode, measured from its lowest point having a thickness of deposit not less than 0.2 x 10-3 inches (5.1 x 10-3 mm).

A higher value therefore denotes superior throwing power. The deposition voltages used were just smaller than the previously determined rupture voltage for each resin.

3. Minimum cure temperature Expressed as the lowest temperature required to cure the resin using an oven stoving period of 30 minutes, before the hardness and corrosion resistance of the coating became unacceptably low.

4. Run down and sag during stoving Assessed visually after stoving the plate in a vertical position in the convection oven at the minimum cure temperature.

5. Salt spray resistance Determined according to ASTM B.1 17, after 240 hours, with the resin deposited on degreased bare steel and cross-scribed.

Order of performance: 1 = less than 2 mm. creep from the scribe and less than 5% loss of adhesion 2 = less than 5 mm. creep, less than 109/0 loss of adhesion 3 = less than 10 mm. creep, less than 20% loss qf adhesion 4 = less than 20 mm. creep, less than 50% loss of adhesion 5 = total loss of adhesion.

6. Erichsen impact resistance The coatings were first cured at 1 600 C. except coating No. 1 which was cured at 1800 C.

The test was carried out using an 80 ft.lb. 0.5 inch (12.5 mm.) diameter spherical hammer, reverse impact.

7. Scratch hardness British Standard BS 3900, using the same cure conditions as specified for 6 above, and the same film thickness.

8. Pencil hardness Again as for test 6 above.

TABLE

Example Test 1 2 3 4 results 1. Rupture voltage (v.) 90 120 90 160 2. Throwing power 2.1 2.2 1.5 3 (inches and mm.) 52.5 55 37.5 75 3. Minimum cure temp., 180 130 130 130 dig. C 4. Run down and sag Severe Absent Absent Absent during storing 5. Salt spray resistance 3 1 4 1 6. Erichson impact Slight Slight Slight Slight resistance crazing crazing crazing crazing 7. Scratch hardness (g.) 1600 1700 2200 1300 8. Pencil hardness 6H 6H 9H 7H

Claims (10)

1. A process for producing a resinous coating on a metal surface, wherein a film of a composition is anodically electrodeposited on said surface from an aqueous solution or dispersion, and wherein the anodically electrodeposited film is cross-linked by heating the metal surface in air to a temperature not less than 1 200C and not exceeding 1800 C.;; said aqueous solution or dispersion comprising anions derived from one or more liquid polybutadienes which have been combined with from 17.5 to 42.5 per cent by weight of maleic anhydride and subsequently half-esterified with one or more hydroxyalkyl or epoxyalkyl acrylates or methacrylates sufficient to half-esterify at least one quarter of the anhydride residues, sufficient carboxy groups being formed thereby to render the composition anodically electrodepositable after neutralisation and/or solution or dispersion in water.

2. A process as claimed in claim 1, wherein the film is cross-linked by heating the metal surface to a temperature of from 130 to 1 550C.

3. A process as claimed in claim 1 or 2, wherein said aqueous solution or dispersion has a solids content of 10 to 20 per cent by weight.

4. A process as claimed in any one of claims 1 to 3, wherein the aqueous solution or dispersion additionally comprises one or more conventional constituents selected from pigments, nonionised resins, water-miscible organic solvents, catalysts and substances for promoting cross-linking and buffer agents.

5. A process as claimed in any one of claims 1 to 4, wherein the half-esterification of the liquid polybutadiene which has been combined with maleic anhydride is conducted at a temperature up to 1 200C., optionally in the presence of a-reaction-promoting catalyst.

6. A process as claimed in any one of claims 1 to 5, wherein the said acrylate or methacrylate is hydroxyethyl acrylate, hydroxyethyl methacrylate, glycidyl acrylate or glycidyl methacrylate.

7. A process as claimed in any one of claims 1 to 5, wherein the half-esterification is effected with hydroxyalkyl and epoxyalkyl acrylates and/or methacrylates with substantially equimolar proportions of hydroxy group- and epoxy group-containing esters.

8. A process as claimed in any one of claims 1 to 7, wherein the liquid polybutadiene has a number-average molecular weight of from 800 to 5000 and a microstructure comprising 55% by number of units in a 1,4-configuration and 45% by number of units in a 1,2-configuration.

9. A process for producing a resinous coating on a metal surface substantially as hereinbefore described in any one of Examples 2 to 4 of the foregoing Examples.

10. A heat-cross-lined resinous coating on a metal surface produced by the process claimed in any preceding claim.