AU2011201842A1

AU2011201842A1 - Coating compositions for metal substrates, methods for coating metal substrates and metal substrates coated with the compositions

Info

Publication number: AU2011201842A1
Application number: AU2011201842A
Authority: AU
Inventors: Valentino De Fazio
Original assignee: Acquos Pty Ltd
Current assignee: Acquos Pty Ltd
Priority date: 2011-04-21
Filing date: 2011-04-21
Publication date: 2011-05-12

Description

1 COATING COMPOSITIONS FOR METAL SUBSTRATES, METHODS FOR COATING METAL SUBSTRATES AND METAL SUBSTRATES COATED WITH THE COMPOSITIONS 5 FIELD OF THE INVENTION The present invention relates generally to compositions for coating metal substrates. Particularly, the invention relates to an improved polymer system for coating compositions used in direct to metal applications, including 10 pretreated or untreated steel. More particularly, the invention relates to a water based polymer system that can be used in the formulation of conventional paint coatings for metal substrates (including a pretreated metal) or as a primer coat in Direct to Metal applications (where the metal or steel is treated or untreated). The present invention also relates to methods for coating metal substrates with 15 the improved coating compositions and metal substrates coated with the compositions. BACKGROUND TO THE INVENTION 20 Pretreated metal substrates and in particular, pretreated steel, are widely used in applications such as wire coating and coil coatings to enable the treated metal substrate to have a higher corrosion resistance relative to the untreated metal substrate. These type of treatments are also known as conversion coatings. 25 The types of treatments or conversion coatings that are generally used are: 1) Galvanised zinc coatings 2) Aluminium Zinc coatings 3) Zinc iron alloy coating 30 4) Electro galvanised coating Another type of coating is known as a Direct to Metal primer, which is a primer coating used on either treated or untreated metal which may then be coated 2 further with a topcoat. These primers have traditionally required solvent based finishes, such as finishes based on volatile organic compounds (VOC), as water based systems have proven inadequate for adhering the coating to the metal substrate. 5 Once a metal substrate is primed, a further coating such as a conventional paint coating, must usually be applied (on one or both sides of the coil) for a number of reasons including additional protection, improving corrosion resistance better physical properties or for aesthetic reasons such as improving 10 the colour. Examples of topcoats include: Polyester Polyamide modified polyester Polyurethane 15 Silane containing polymers have been used extensively in coating applications. For example, US 5,244,950 disclose the use of silanes in contact adhesives. US 6,274,671 teach the use of silanes in solvent based coating systems. US 5,204,404 disclose a water based composition mixed with either a polyurethane or polyester composition for coatings applied to automobiles and trucks. US 20 4,336,309 teach coating compositions incorporating a hydroxyl group with various different silane coupling agents for plastics. However, there remains a need for improved coating compositions for metal substrates. 25 SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a coating composition for a metal substrate comprising a polymer manufactured by a 30 method comprising reacting a hydroxyl-substituted polymer with an epoxy silane.

3 As used herein, the term "metal substrate" is not intended to be limited in any way. For example, the term includes metals, metallic elements, alloys, sheet metal and wires any or all of which may be pretreated, precoated or uncoated. 5 Any suitable epoxy silane may be used in the present invention. Preferred epoxy silanes will have an epoxy functional group on one end of the silane molecule and an alkoxysilane, dialkoxysilane or trialkoxysilane on the other end of the silane molecule. The epoxy functional group may be substituted or unsubstituted. In use it is believed the alkoxysilane, dialkoxysilane or 10 trialkoxysilane group hydrolyses to a silanol group. Preferred epoxy silanes include epoxy trimethoxysilane or epoxy triethoxysilane. Other preferred epoxy silanes include Silane Z6040 (as available from Dow Corning), Momentive A187 or Evonik Glymo silanes. Even more preferably, the epoxy silane is 3 g lycidyloxypropyltrimethoxysilane (DYNASYLAN GLYMO). 15 The hydroxyl-substituted polymer is not particularly limited. In one embodiment, the hydroxyl-substituted polymer is a copolymer manufactured from a hydroxyl containing monomer and at least one compatible monomer. Hydroxyl containing monomers having a water solubility greater than 10% may be 20 utilised in the manufacture of the hydroxyl-substituted polymer. Alternatively, the hydroxyl containing monomer is selected from the group consisting of hydroxyl ethyl methacrylate (HEMA), hydroxyl ethylacrylate (HEA), hydroxylpropyl acrylate (HPA), and hydroxylpropylmethacrylate (HPMA). 25 Preferably, the hydroxyl containing monomer is present in an amount of up to 15% of the total monomer combination i.e. the total amount of the hydroxyl containing monomer and the compatible monomer. The at least one compatible monomer is not intended to be limited in any way. 30 Preferably, the compatible monomer is selected from the group consisting of styrene, methyl methacrylate, butyl acrylate, 2-ethylhexyl acrylate, ethyl acrylate, methyl acrylate, vinyl acetate, butadiene and ethylene. More than one 4 compatible monomer may be used. Preferably, two or three compatible monomers are utilised. It is preferred the hydroxyl-substituted polymer is manufactured at slightly 5 acidic or neutral pH. Furthermore, it is also preferred that the hydroxyl substituted polymer is manufactured in the absence of unsaturated carboxylic acids such as acrylic acid and methacrylic acid, unsaturated amides such as acrylamide and methacrylamide, polymerisable carboxylic acids, polymerisable amides and any other water soluble monomers or potentially soluble water 10 soluble monomers, other than the hydroxyl containing monomer described above. Without wishing to be bound by any theory, it is believed the presence of residual acrylic acid and / or methacrylic acid undesirably interferes with the performance of the final polymer. Further, manufacturing the hydroxyl substituted polymer in the absence of unsaturated carboxylic acids like acrylic 15 acid and methacrylic acid, unsaturated amides such as acrylamide and methacrylamide, or other water soluble monomers avoids any unnecessary neutralisation step. The hydroxyl-substituted polymer is preferably manufactured in an aqueous 20 based medium. For example, the hydroxyl-substituted polymer is manufactured in water. Even more preferably however, the hydroxyl-substituted polymer is manufactured in water using emulsion polymerisation. The polymerisation may be conducted using conventional water based 25 polymerisation techniques such as seeded polymerisation using a monomer emulsion feed. Alternatively, the hydroxyl-substituted polymer may be manufactured by any conventional latex polymer dispersion polymerisation technique. For example, it is proven advantageous to use a seeded polymerisation technique for consistent particle size reproducibility and to 30 reduce batch to batch variation. Generally, part of the monomer is introduced into the reaction vessel at the start along with a shot dose of initiator for initial particle formation. The reaction vessel will usually contain some of the aqueous phase (usually water) along with the initial surfactants, buffer systems, 5 chelating agents along with some of the monomer (DMIR techniques or "direct monomer feed into reactor" has proven very advantageous as it will give a more consistent particle size) and the initiator solution as a shot dose at the desired seed temperature. The remainder of the monomer feed is generally 5 added as a "pre-emulsion" whereby the remaining monomer, surfactant for stabilisation and water is continually added with the initiator over a course of 2 8 hours as regulated by the exotherm (i.e. temperature). The surfactants used in the polymerisation are generally conventional 10 surfactants (anionic and non-ionic) that are stable at the both the reaction pH and the final pH. One of ordinary skill in the art will appreciate that surfactants that are not compatible with long term storage of silane coupling agents like the epoxy silanes used in the present invention should not be used. Minimal surfactants should be used to aid water resistance. 15 The initiator systems used are generally either persulfate types (ammonium persulfate, potassium persulfate sodium persulfate) or organic types such as TBHP (tert-butyl hydroperoxide). Hydrogen peroxide may also be used as well as reducing systems such as sodium metabisulfite (usually as a separate feed). 20 However, other initiator systems are also possible. Once the hydroxyl-substituted polymer is formed, it is cooled and further reacted if desired to lower residual monomers that may remain. Adjustments such as biocide addition may also be made if desired. 25 The reaction between the epoxy silane and the hydroxyl-substituted polymer may be performed in any suitable manner. For instance, the epoxy silane may be added to the hydroxyl-substituted polymer either neat, diluted or as a hydrolysed solution. Preferably, the epoxy silane is added at temperatures 30 lower than the temperatures at which the polymerisation reaction is performed. It is also preferred the amount of epoxy silane reacted with the hydroxyl substituted polymer is up to the same weight of the hydroxyl-substituted polymer.

6 Preferably, the polymer manufactured by the reaction of the hydroxyl substituted polymer with the epoxy silane is a water based polymer. It is also preferred the final polymer has a pH that is approximately neutral. 5 The final composition according to the present invention is stable at around pH 7.5 or below to pH 4 and will usually be supplied as a stable latex dispersion ready to be formulated further for coating applications if desired. The final composition or latex emulsion preferably has a pH near neutral (i.e. 7), more 10 preferably a pH between about 6 and 7 and even more preferably the composition has a pH between about 6.5 and 7. The final composition or latex emulsion generally has a glass transition temperature (Tg) between about -10 to 90. Preferably, the glass transition 15 temperature (Tg) of the compositions will be between about -5 to 5, 5 to 30, 30 to 50, 50 to 70 or 70 to 90. Even more preferably, the glass transition temperature (Tg) will be between about -5 to 5, 40 to 50 or 50 to 60. The final coating composition or latex emulsion may, but need not, be 20 formulated up with extra silanes, for example any silane that may further react with the hydrolysed epoxy silane present or attached onto the back bone of the polymer. It may also be advantageous to add other components to the coating composition or latex emulsion such as chromate-containing substances and phosphate-containing substances, pigments such as zinc oxide, coloured 25 pigments, titanium dioxide as well as other organic acids to formulate a primer or a one step primer coat / wash coat for application onto precoated steel, untreated steel or other metal substrate prior to further powder coating or painting of the steel or metal substrate using conventional methods. The selection of said additional components will of course, be appreciated by one of 30 ordinary skill in the art and will depend on the particular application.

7 As noted above, the coating composition may be used on any suitable metal substrate. The metal substrate may be pretreated, precoated or may be uncoated. 5 The coating composition may be a latex polymer dispersion and when manufactured as described above exhibits outstanding adhesion onto metal substrates that have been precoated with a pretreatment consisting of zinc, aluminium or zinc alloys. 10 Whilst not wanting to be bound to any specific theory, it is believed the epoxy functional group of the epoxy silane forms the basis of the reactive group for reaction onto the hydroxyl functional groups on the backbone of the hydroxyl substituted polymer (which has a hydrophilic nature). This type of mechanism, conducted after the polymerisation reaction as opposed to previous systems 15 that use a silane during polymerisation has the advantage of not being aggressive on the hydrolysed silanol group and thus avoiding silane condensation within itself (a problem with conventional methods of manufacture). The hydroxyl groups may also act as potential reactive sites for reaction with the hydrolysed silanol groups of the hydrolysed silanes whether 20 from the extra silane described above (if this is added to the final composition) or from the epoxy silane that is reacted with the hydroxyl-substituted polymer. According to a second aspect of the invention there is provided a method for coating a metal substrate comprising applying a coating composition as 25 described above in relation to the first aspect of the invention to the metal substrate. If desired, the coating composition may be formulated with a paint system prior to application to the metal substrate. Of course, this will depend on the 30 particular application.

8 The coating composition may be applied to the metal substrate at a wet film thickness of up to 2mm, more preferably 1 mm or less. Upon drying, a cured film is formed over the metal substrate. 5 To assist drying, the method may further comprise the step of applying heat to the coated metal substrate. Whilst heat applied to the coated metal substrate or cured film is not essential, it has further been proven beneficial, especially when applied as a basis of a primer coating for coil coatings in replacing conventional solvent based primer systems on existing lines or systems. 10 According to a third aspect of the invention there is provided a metal substrate coated with a coating composition as described above in relation to the first aspect of the invention. 15 According to a fourth aspect of the invention there is provided a metal substrate obtained by the method as described above in relation to the second aspect of the invention. DETAILED DESCRIPTION OF THE INVENTION 20 A more detailed description of the invention will now be provided with reference to the accompanying experimental results. It will be appreciated that the experimental results are provided for illustration only and should not be construed as limiting on the invention in any way. 25 Example 1 A 1 Litre laboratory reactor equipped with a heating bath and stirrer at 300rpm was charged (in order) with 192 grams of distilled tap water, 0.9 grams of 30 sodium bicarbonate, 5 grams of surfactant composed of a 20% solution of a sodium salt partially sulphated surfactant of a Nonyl Phenol ethoxylate (N30 with 40% sulphated content 60% non-sulphated content approx.) and 0.05 grams of EDTA sodium salt. The contents are heated to 85 degrees and 9 seeded with 16 grams of prefeed and 1.1 grams of initiator solutions and held for 15 minutes whereby the remaining prefeed was dosed over 3.5 hours and the remaining initiator was dosed over 4 hours. The prefeed consisted of 173 grams of water, 22 grams of surfactant composed of a 20% solution of a 5 sodium salt partially sulphated surfactant of a Nonyl Phenol ethoxylate (N30 with 40% sulphated content 60% non-sulphated content approx.), 10 grams of hydroxyl ethyl methacrylate, 166 grams of butyl acrylate and 111 grams of styrene monomer. The initiator solution consisted of 2 grams of sodium persulfate in 26 grams of water. The contents of the reactor once complete 10 were held at 85 degrees C for 30 minutes and then cooled to below 30 degrees C whereupon 0.4 grams of tertiary butyl hydroperoxide (70% solution) was added mixed in 2 grams of water then further mixed for 10 minutes before a final solution of 10 grams hydrolysed epoxy silane coupling agent Dynasylan* Glymo in water (50% solution) was slowly added whilst stirring and mixed for a 15 further 10 minutes. The latex dispersion was filtered and tested to have a solids content of 40.7%. The pH was 6.7. Comparative example 2 20 Example 1 (above) was repeated but the final silane addition was omitted. The latex dispersion was filtered and tested to have a final solids content of 40.8%. The pH was 6.8 Comparative example 3 25 A 1 Litre laboratory reactor equipped with a heating bath and stirrer at 300rpm was charged (in order) with 192 grams of distilled tap water, 0.9 grams of sodium bicarbonate, 5 grams of surfactant composed of a 20% solution of a sodium salt partially sulphated surfactant of a Nonyl Phenol ethoxylate (N30 30 with 40% sulphated content 60% non-sulphated content approx.) and 0.05 grams of EDTA sodium salt. The contents are heated to 85 degrees and seeded with 16 grams of prefeed and 1.1 grams of initiator solutions and held for 15 minutes whereby the remaining prefeed was dosed over 3.5 hours and 10 the remaining initiator was dosed over 4 hours. The prefeed consisted of 173 grams of water 22 grams of surfactant composed of a 20% solution of a sodium salt partially sulphated surfactant of a Nonyl Phenol ethoxylate (N30 with 40% sulphated content 60% non-sulphated content approx.), 5 grams of acrylic acid, 5 166 grams of butyl acrylate and 111 grams of styrene monomer. The initiator solution consisted of 2 grams of sodium persulfate in 26 grams of water. The contents of the reactor once complete were held at 85 degrees C for 30 minutes and then cooled to below 30 degrees C whereupon 0.4 grams of tertiary butyl hydroperoxide (70% solution) was added mixed in 2 grams of 10 water then further mixed for 10 minutes before a final solution of 10 grams hydrolysed epoxy silane coupling agent Dynasylan* Glymo in water (50% solution) was slowly added whilst stirring and mixed for a further 10 minutes. The latex dispersion was filtered and tested to have a solids content of 39.9%.The pH was 5.5. The latex dispersion was split into two equal portions 15 and one of the portions was used in comparative example 4 below. Comparative example 4 A portion (345 grams) of the latex dispersion made in example 3 was neutralised with 13 grams of aqueous ammonia (10% in water W/V) and a pH 20 of 8.3. Testinci Galvanised steel panels that were washed and dried were coated with the four 25 polymer latex dispersions as made with a wet film thickness of 1 mm and allowed to air dry in an oven at 60 degrees for 24 hours. Once cured, the panels were removed and placed in water for 1 week under total water immersion. 30 11 Results Example 1- Full adhesion onto the galvanised panels although slight water whitening of the film was evident. Film shows full adhesion onto panels and 5 fully intact. Comparative example 2- Adhesion failure of the film evident in areas. Slight water whitening similar to example 1. 10 Comparative example 3- Adhesion failure of the film evident in areas. Slight water whitening similar to example 1. Comparative example 4 - Worst of the 4 samples for adhesion. Water whitening more evident. 15 As can be seen from the results, the latex dispersion made according to the present invention exhibits superior performance compared to prior art latex dispersions. The examples also show that the epoxy silane coupling agent when used according to prior art methods does not give the as much improved 20 performance. Unless the context requires otherwise or specifically stated to the contrary, integers, steps, or elements of the invention recited herein as singular integers, steps or elements clearly encompass both singular and plural forms of the 25 recited integers, steps or elements. Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated step or element or integer or group 30 of steps or elements or integers, but not the exclusion of any other step or element or integer or group of elements or integers. Thus, in the context of this specification, the term "comprising" is used in an inclusive sense and thus 12 should be understood as meaning "including principally, but not necessarily solely". It will be appreciated that the foregoing description is has been given by way of s illustrative example of the invention and that all such modifications and variations thereto as would be apparent to persons of skill in the art (for example change in Tg values and monomer combinations) are deemed to fall within the broad scope and ambit of the invention as herein set forth. 10 The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the referenced prior art forms part of the common general knowledge in Australia.

Claims

1. A coating composition for a metal substrate comprising: a polymer manufactured by a method comprising reacting a hydroxyl 5 substituted polymer with an epoxy silane.

2. The coating composition according to claim 1, wherein the epoxy silane is an epoxy trimethoxysilane or an epoxy triethoxysilane. 10

3. The coating composition according to claim 1 or claim 2, wherein the epoxy silane is 3-glycidyloxypropyltrimethoxysilane.

4. The coating composition according to any one of the preceding claims, wherein the hydroxyl-substituted polymer is a copolymer manufactured from a 15 hydroxyl containing monomer and at least one compatible monomer.

5. The coating composition according to claim 4, wherein the hydroxyl containing monomer has a water solubility greater than 10%. 20

6. The coating composition according to claim 4 or claim 5, wherein the hydroxyl containing monomer is selected from the group consisting of hydroxyl ethyl methacrylate (HEMA), hydroxyl ethylacrylate (HEA), hydroxylpropyl acrylate (HPA), and hydroxylpropylmethacrylate (HPMA). 25

7. The coating composition according to any one of claims 4 to 6, wherein the hydroxyl containing monomer is present in an amount of up to 15% of the total monomer combination.

8. The coating composition according to any one of claims 4 to 7, wherein the 30 at least one compatible monomer is selected from the group consisting of styrene, methyl methacrylate, butyl acrylate, 2-ethylhexyl acrylate, ethyl acrylate, methyl acrylate, vinyl acetate, butadiene and ethylene. 14

9. The coating composition according to any one of claims 4 to 8, wherein two or three compatible monomers are utilised.

10. The coating composition according to any one of the preceding claims, 5 wherein the hydroxyl-substituted polymer is manufactured at slightly acidic or neutral pH.

11. The coating composition according to any one of the preceding claims, wherein the hydroxyl-substituted polymer is manufactured in the absence of 10 unsaturated carboxylic acids, preferably acrylic acid and methacrylic acid, unsaturated amides, preferably acrylamide and methacrylamide, polymerisable carboxylic acids and polymerisable amides.

12. The coating composition according to any one of the preceding claims, 15 wherein the hydroxyl-substituted polymer is manufactured in an aqueous based medium, preferably water, and even more preferably water using emulsion polymerisation.

13. The coating composition according to any one of the preceding claims, 20 wherein the polymer is a water based polymer.

14. The coating composition according to any one of the preceding claims, wherein the amount of epoxy silane reacted with the hydroxyl-substituted polymer is up to the same weight of the hydroxyl-substituted polymer. 25

15. The coating composition according to any one of the preceding claims, wherein the metal substrate is precoated or pretreated.

16. The coating composition according to any one claims 1 to 14, wherein the 30 metal substrate is uncoated or untreated.

17. The coating composition according to any one of the preceding claims, further comprising at least one component selected from the group consisting 15 of silanes, chromate-containing substances, phosphate-containing substances, pigments, zinc oxide, titanium dioxide, and organic acids.

18. A method for coating a metal substrate comprising applying a coating 5 composition according to any one of claims 1 to 17 to the metal substrate.

19. The method according to claim 18, wherein the coating composition is formulated with a paint system prior to application to the metal substrate. 10

20. The method according to claim 18 or claim 19, further comprising applying heat to the coated metal substrate.

21. A metal substrate coated with a coating composition according to any one of claims 1 to 17. 15

22. A metal substrate obtained by the method according to any one of claims 18 to 20.

23. A coating composition for a metal substrate substantially as hereinbefore 20 described with reference to the examples but excluding the comparative examples.

24. A method for coating a metal substrate substantially as hereinbefore described with reference to the examples but excluding the comparative 25 examples.