AU2023219868B1

AU2023219868B1 - Paint formulation

Info

Publication number: AU2023219868B1
Application number: AU2023219868A
Authority: AU
Inventors: Gregory Mitchell Wilson
Original assignee: SURFACE SOLUTIONS Pty Ltd
Current assignee: SURFACE SOLUTIONS Pty Ltd
Priority date: 2023-02-22
Filing date: 2023-08-23
Publication date: 2024-03-28
Anticipated expiration: 2043-08-23

Abstract

The present invention relates to a paint formulation that can be used to paint steel tubes used in housing and construction, agricultural fabrication, and general fabrication. The paint formulation is substantially alkaline resistant on contact and yet is removable in a process comprising immersion in an alkaline solution followed by immersion in an acid solution. The paint formulation comprises a solvent system; a styrene acrylic polymer solvated in the solvent system, the styrene-acrylic polymer having free carboxylate groups available for cross-linking; a cross-linker that forms cross-linked bonds between free carboxylate groups in the styrene-acrylic polymer; wherein the cross-linked bonds allow the styrene-acrylic polymer to resist alkaline degradation on contact, while the cross-linker is alkaline reactive upon immersion in alkaline solution to break the formed cross-linked bonds.

Description

Paint formulation

This document claims priority to AU2023900451 the contents of which are hereby incorporated by reference in their entirety.

Technical field

The present invention relates to a paint formulation that can be used to paint steel tubes used in housing and construction, agricultural fabrication, and general io fabrication.

Background

Steel tubes are painted during manufacture to prevent corrosion of the steel tubes prior to use. The paint coating is thought to assist in ensuring that the steel tubes do not significantly corrode until they are finally coated in either an organic coating e.g. paint, or a metal coating such as zinc. Typically, the steel tubes are purchased by organisations that want to fabricate something e.g. caravan bases, agricultural equipment, etc. The fabricators that use the steel tubes prefer to weld the ungalvanized painted steel. Once the welding is complete and a final product is produced, the product can be galvanised. A large proportion of painted steel tubes that go into fabricated goods need to be galvanised to give the finished product protection from corrosion.

Prior to galvanising the painted steel tubes, the paint on the outside surfaces needs to be removed. A typical galvanising plant will take painted steel products and put them through an alkali/degreasing bath to remove or soften the paint coating. The treated steel will then be rinsed in deionised water and finally dipped in an acid pickling bath to remove rust and in an attempt to further remove the paint.

A problem that the current galvanisers have is that they cannot easily remove some paints using these pre-galvanising conditions. Typically, what happens is that the paint that is removed comes off in big pieces, sometimes in sheets, and then that paint waste contaminates the alkali bath and or the acid bath. There is a significant cost associated with cleaning up the alkali degreasing baths and the acid pickling baths when pieces, such as sheets, of paint come off the steel tubes and remain partially or fully undissolved in the baths.

An exception to this problem is solvent-based alkyd resin paints that are more readily soluble in the acid/alkaline conditions of the baths. However, it is desirable to move away from solvent-based paints due to health and safety and environmental considerations.

io Where the paint is not solvent-based and is a water-based paint, despite the challenges, it is important to remove as much of the paint from the steel tube as possible because paint that is not removed gets hot zinc placed on top of it during the galvanising process. This unremoved paint with zinc on top provides a poor surface for the zinc to adhere to in the galvanising process, sometimes leading to failure of the galvanising process.

It would be desirable to provide a system, method and or product that overcomes at least some of the problems of the prior art, and or which at least provides a useful alternative.

Summary of invention

In a first aspect there is provided a paint formulation that is substantially alkaline resistant on contact and yet is removable in a process comprising immersion in an alkaline solution followed by immersion in an acid solution, the paint formulation comprising: a solvent system; a blend of acrylic polymers solvated in the solvent system, the acrylic polymers each having free carboxylate groups available for cross-linking; a cross-linker that forms cross-linked bonds between free carboxylate groups in the blend of acrylic polymer; wherein a first type of acrylic polymer in the blend is an alkali/acid resistant water-based polymer; and a second type of acrylic polymer in the blend is an alkali soluble water-based polymer.

While others have focused on processes for removing the paint, the present inventors have realised that focusing on the formulation of the paint to make it easier to remove in the alkali/acid baths of the pre-galvanising process can be advantageous. In embodiments, the present invention may provide a paint that can both protect the underlying steel tube from rusting, and a paint that is more easily removed from the steel than prior paints during the degreasing and picking bath steps found at the known galvanising plants.

Advantageously, the paint is not removed by water at normal pH in the range of from about 6 to about 10. However, the paint is substantially removed by alkali degreasing bath(s) (alkali) and pickling bath(s) (acid) used at known galvanising plants. A benefit of the present paint, in embodiments, is that galvanisers can substantially completely remove the paint prior to galvanising, ensuring that they have a pristine steel surface onto which they can adhere the zinc. The paint of embodiments of the present invention tends not to be removed in large sheet-like pieces and therefore does not circulate as waste in the alkali bath and acid bath. This improvement reduces free paint flowing in the bath vessels and piping, and may subsequently increase the life of the baths and may therefore reduce the amount of cleaning and dumping/refilling of the baths required during lifespan.

While steel tubes for e.g. general construction are described herein as exemplary surfaces from which the present paint is removable, it should be understood that the paint formulation of the invention is also usefully applied to any surface where the paint may later need to be removed.

It is not straightforward to develop a paint that can be removed by an alkaline material such as an alkali degreasing bath. This is particularly the case for paint intended to coat steel tubes, because any paint applied to a steel tube needs to be somewhat resistant to degradation under the alkaline conditions that the tube is subject to during production as described below. In the steel mill where the paint is applied to the tube, a tube rolling coolant is applied to the steel tube. This tube rolling coolant is for lubrication of the tubes during the rolling process and it is also used as a corrosion inhibitor. The tube rolling coolant is alkaline with a pH of about

9.0. Therefore, when the tube rolling coolant either gets onto the freshly applied paint or lodges itself onto the steel tube prior to painting, it will dissolve the paint and or allow the steel tube to corrode. Accordingly, any paint applied needs to be able to at least withstand contact by the alkaline tube rolling coolant. It is therefore necessary to carefully formulate the paint to cause it to be at least substantially resistant to the alkaline contact of the tube rolling coolant during production, but still be able to be substantially removed in immersion in the alkali and the acid baths in the galvanising plant.

io The paint can be a water-based paint. By water-based it is meant that the pigments and binders in the paint are present with water as the carrier. Water-based paints can still comprise some solvents other than water, but the predominant solvent is water, for example at least or at most about 20, 30, 40, 50 or 60% of the total solvent in the paint is water. The solvent is the liquid component of the paint that vaporises as the paint dries. By removing as much water from the system as possible, the paint can be faster drying. In an embodiment in which the paint can be described as not being water based there is at most about 5, 10, 15 or 20% water in the paint as a % of the total solvent. By fast drying it is meant that the paint will dry upon application to the product in a few seconds or minutes.

Other solvents can also be added as would be understood by the skilled person familiar with paint formulation. For example, a coalescing co-solvent can also be present. The coalescing cosolvent may help to optimise the film forming properties of the paint formulation. The coalescing solvent if present can be present in an amount in the range of from about 6 to about 20% of the overall composition. Preferably, the amount of coalescing solvent is reduced to less than about 6% such as about 4 or about 2 %. Preferably the paint is fast drying. The fast drying is preferably of the order of about 5 seconds. The characteristic of being fast drying is particularly advantageous for time-sensitive projects and industrial applications where efficiency and productivity are paramount.

The primary polymer in the formulation is a styrene-acrylic polymer. Styrene-acrylic polymers are known as acrylates. The acrylate group of polymers includes acrylic acid and its esters: methyl acrylate (or meth acrylic), butyl acrylate, ethyl acrylate, and 2-ethylhexyl acrylate. The term (meth) acrylic polymer describes polymers predominantly composed of alkyl acrylate and methyl methacrylate. The polymers can also comprise some styrene in the formulation. These acrylic monomers are highly reactive chemicals, which means they readily combine with themselves or other monomers to form polymers. One of the most important polymer reactions, occurring via emulsion polymerization, involves acrylic-based monomers combining with styrene to form a styrene-acrylic emulsion polymer. This class of polymers is versatile due to the wide-ranging family of acrylic monomers, which, when combined with styrene, can build random copolymers with specific glass transition io temperatures (Tg).

In embodiments, the styrene acrylics can be substituted with pure acrylic water based emulsions. The pure acrylic water-based emulsions are typically not used in the present commercial formulations as they are more expensive than the styrene acrylics. Pure acrylics are sometimes used where durability in UV light is required. However, the present paints are temporary primer coatings with a longevity of 12 months, so the added expense of a pure acrylic is not required.

Without wishing to be bound by theory, it is thought that the paint of an embodiment of the present invention will not actually come off in the alkali bath. For example, during initial trials, the paint did not appear to be removed in the alkali bath but went on to dissolve completely in the acid bath at the galvanising plants. The assumption is that the paint softened in the alkali bath and then the final bonds in the paint polymer were broken by the acid conditions in the acid bath.

A challenge with the paint being softenable in an alkali bath (where bonds are broken by reaction between the paint and the alkali base) is that the paint is also more readily washed away by alkaline materials during the application process. If the paint is too softenable (reactive) in the presence of an alkali material, then the paint will be washed away by e.g. the tube rolling coolant during the paint application process. During production of the steel tube, when paint is applied, the tubes inevitably get alkaline tube rolling coolant onto the bare steel tube surfaces prior to the mix of polymers in the paint being applied. Furthermore, the tube rolling coolant even mixes with the solvent of the paint. Accordingly, the paint needs to be at least partially stable in an alkaline solution.

Initial laboratory testing shows that this property of partial stability is achieved with styrene acrylics. There can be a blend of styrene acrylics. At least one (and preferably not all) of the styrene acrylics in the blend can be an alkali/acid resistant water-based polymer. At least one (and preferably not all) of the styrene acrylics in the blend can be an alkali soluble water-based polymer.

Without wishing to be bound by theory, it is thought that the resistance or solubility io in alkaline solution is related to the number of carboxylic groups COOH on the styrene acrylic polymer. The resistance to alkaline being provided by fewer free carboxylic acid groups; the solubility being provided by more free carboxylic acid groups.

In order to achieve the desired consistency, it is preferred that at least one of the styrene acrylics has a relatively large number of free carboxy groups. It is thought that at least one of the styrene acrylic polymers should have a relatively low number of free carboxy groups. These carboxy groups are available for cross linking as described below. The skilled person will understand the mechanisms available for reacting with an acid group in order that is no longer a free to donate its proton in subsequent reaction. For example, esterification ties up the carboxylic acid group so that it is no longer a free carboxylic acid group.

The carboxylic acid groups are thought to increase the solubility of the styrene acrylic in the alkaline solution, since when they are charged (in alkaline pH) the chains repel and untangle. Upon the addition of base, e.g. NaOH or KOH, the carboxylic acid groups are deprotonated and water diffuses into the styrene acrylic polymer particles resulting in a swelling of the particles. The polymer chains with the lowest molar mass and highest acid value will dissolve. For the polymers with a low acid value, the large particles do not disintegrate but stay intact as water swollen polymer particles.

The styrene acrylic polymers can have an acid number. The acid number indicates the amount of potassium hydroxide in mg, being necessary to neutralize the free fatty acids contained in 1 g of the polymer. The acid number is a measure of the number of carboxylic acid groups within the polymer. Thus, the acid value of the styrene acrylic polymers can be measured in terms of mg/g KOH emulsion. A polymer with a mg/g KOH of about 0.5 mg/g KOH will be an alkali/acid resistant water-based polymer. By adding a polymer with this low value of mg/g KOH to the composition, the cured paint may stay intact during the tube manufacturing process yet be more easily removable during the galvanising process. However, with such a low mg/g KOH value the polymer will be difficult to solubilise. By adding a polymer with a high value of mg/g KOH, which is more soluble, the paint may wash io off too easily. The result is that the preferred composition is a blend of both properties. The polymer with the lower mg/g KOH of about 0.5 mg/g KOH is added with at least one other styrene acrylates that have a mg/KOH/g of around 40 mg/g KOH.

In an embodiment, there is a blend of polymers comprising:

* Styrene acrylic A = 41.2 mg/g KOH; and or * Styrene acrylic B = 38.1 mg/g KOH and * Styrene acrylic C = 0.47 mg/g KOH

In an embodiment, there is acrylate polymer:

* Styrene acrylic D = 9.77 mg/g KOH.

This polymer can be blended with styrene acrylic C.

Accordingly, preferably in the formulation there is at least one styrene acrylic polymer with an acid number in the range of from about 0.4 to 0.5 (alkaline resistant). There is also at least one styrene acrylic polymer with an acid number in the range of from about 10 to about 50 such as about 9 to about 45 (alkaline soluble). In an embodiment there are no other styrene polymers other than these.

The alkaline resistant polymer (A, B or D) can have about 70, 80, 90 or 100 times, about 50 times or about 20 times fewer carboxylic acid groups than the alkaline soluble polymer (C).

Optionally, the styrene acrylates can be sold under the trade mark Hydrocry TM

. Each of the styrene acrylic polymers can be added in an amount in the range of from about 5 to about 40% of the overall composition. In an embodiment, there is less of the alkaline/acid resistant water-based polymer (e.g. styrene acrylic C) than the alkali soluble water-based polymer (e.g. styrene acrylic A). In an embodiment there is about 5wt% of the alkaline/acid resistant water-based polymer (e.g. styrene io acrylic C). In an embodiment there is about 40wt% of the alkali soluble water based polymer (e.g. styrene acrylic A and or B). In an embodiment the ratio of the alkaline resistant to alkali soluble styrene acrylate is about 1:5 to about 1:8.

In an embodiment there is about 70 to 80wt% of the alkali soluble water-based polymer (e.g. styrene acrylic D). In an embodiment, there is less of the alkaline/acid resistant water-based polymer (e.g. styrene acrylic C) than the alkali soluble water based polymer (e.g. styrene acrylic D).

In order to achieve the desired consistency of the present paint formulation, it is preferred that there is cross-linking between the free carboxylic acid groups. The cross linker can be any chemical that complexes with the free carboxylic acid groups on the polymers. The cross linker can be added as a separate component to the paint. The cross linker can be using groups within the polymer itself. In this latter embodiment, the polymer can be referred to as self-cross linking. The cross linker can be provided with the polymer. There can, therefore, be a component in the polymer, as supplied, that acts as a cross-linker.

In an embodiment there can be an amine or imine crosslinker added to the formulation. One such example is a polyethylenimine solution. However, in a preferred embodiment, there is metal cross-linking. The metal based crosslinker is preferably one in which the polymer crosslinks formed are able to breakdown in the presence of an amine compound, such as ammonia or monoethanolamine. This will allow the cross-linked paint polymer to be solvated in an alkaline liquid and be removed. In an embodiment, the cross-linker is a metal salt such as a metal ammonium carbonate. In a preferred embodiment the cross-linker is Zinc Ammonium Carbonate (ZAC). With ZAC, the zinc metal complexes with free carboxylate groups to reduce the tendency of the polymers to dissolve in water and effects their relative resistance to pH either under alkaline or acidic conditions. However, upon immersion in alkaline solution the cross-links can be broken down for removal of the paint.

The cross-linker can be present in an amount in the range of from about 0.1 to 4% of the overall composition. In a preferred embodiment, at least about 2 % of the io formulation comprises ZAC. However, there can be any amount as required to effect cross-linking.

At least one of the styrene acrylics can have a glass transition temperature of about 38 degC. The glass transition temperature can assist in reducing the amount of co-solvent in the formulation. It can be desirable to have less co-solvent to allow the paint to dry quickly. In an embodiment the blend of styrene acrylics comprises one or more and preferably all of the following three:

• Styrene acrylic A (Tg 22C) • Styrene acrylic B (Tg 96C) • Styrene acrylic C (Tg 38C)

In an embodiment

• Styrene acrylic D (Tg 28C) • Styrene acrylic C (Tg 38C)

In some embodiments there are other additives in the paint. The other additives can include bulking agents such as fillers (including calcium carbonates, diatomaceous earth and similar), biocides, anti-foaming agents and anti-scratch agents.

Where calcium carbonate is added, it can be used in conjunction with the acrylate polymer to provide the dry film for the final paint. By substituting in the CaCO3 and reducing polymer content, there can be a reduction in the amount of water present in the formulation. Less water can reduce the drying time of the paint. However, if there is not enough polymer and too much CaCO3, the finished paint will not properly adhere to the underlying substrate. Accordingly, it is preferred that the ratio of polymer to calcium carbonate (or similar salt) is in the range of from about 4:1 to about 2:1. Fillers such as diatomaceous earth can increase the viscosity of the paint composition and make it too viscous to apply using airless spray systems.

If calcium carbonate is added to the paint, then a dispersing agent can be added to disperse it. A high-molecular surfactant can be used to disperse the CaCO3. The high molecular surfactant can be a polycarboxylic acid. The dispersing agent may also lower the viscosity of the resultant product. The dispersing agent can be sold under the brand Carrybon L400. The dispersing agent can be used in a range of from about 0.2 to about 1 %.

Foaming of paint is principally caused by surfactants used in the polymer manufacturing process. To a lesser extent, foaming is caused by the ingress of the surface active materials from the tube rolling process. Antifoam can be added to control foaming particularly in the vacuum coating systems used to apply paint to steel tubes. The antifoamer can be a siloxane. The siloxane can be a siloxane based silicate. The antifoamer can be sold under trademark DC 8610. There can be about 0.005 to 0.1% of the antifoamer measured as a percentage of the overall composition.

In an embodiment, the antifoamer can be Volatile Organic Carbon free and mineral oil-free. Such a foamer can be sold under the brand BYK 1641 and comprises an emulsion of foam-destroying polymers and hydrophobic particles as well as silicone. The polymer can be a mixtures of 1,2-Benzisothiazol-3-one and a reaction mass of: 5-chloro-2- methyl-4-isothiazolin-3-one [EC no. 247-500-7] and 2-methyl 2H - isothiazol-3- one [EC no. 220-239- 6] (3:1). The antifoamer sold under the brand BYK 1641 is a polymeric defoamer which requires high shear mixing to give maximum antifoam properties for the final paint formulation. This antifoamer can be useful in paint formulations as it is thought to minimise disruption to the finished wet paint surface on application to steel. Other equivalent antifoams can be used to give maximum foam reduction in the final paint formulation and minimal effect of the surface tension of the paint.

The viscosity of the resultant paint formulation is about 18 to about 22 seconds. The viscosity is preferably about 20 seconds. The cup test is a common method used to measure the viscosity or flow characteristics of paint. It involves pouring a specified volume of paint into a cup and measuring the time it takes for the paint to flow through an orifice or opening at the bottom of the cup.

There are different cup types and standards used for viscosity measurements, but one widely used cup is the Ford cup. The Ford cup comes in different sizes (e.g., Ford #4, Ford #3) and has a small orifice at the bottom. The cup is filled with paint, and the time it takes for the paint to completely flow out through the orifice is measured using a stopwatch. The present method for measuring viscosity was AS1580 214.2 with a viscosity range of 18-22 seconds @25°C. A Ford number 4 cup was used for all measurements.

The viscosity of the paint is inversely related to the flow time. A shorter flow time indicates lower viscosity or thinner consistency, while a longer flow time indicates higher viscosity or thicker consistency.

The freshly painted tubes will come in contact with hard surfaces within the steel mill during transfer from production lines and when the freshly painted steel tubes encounter forklifts. It is possible for the paint to be scratched which is undesirable because those scratches can be a source of corrosion. In order to achieve resistance to scratching the surface properties of the paint can be modified. An aim of modifying the surface properties of the paint is to reduce the surface friction in the paint, so that when two surfaces are rubbing together, stick slip is prevented or reduced and hence the likelihood of scratching is reduced. Advantageously, anything added to the paint to e.g. improve scratch resistance does not adversely affect the surface properties of the paint.

The present invention also comprises a method for preparing the paint formulation.

The method can comprise adding a blend of styrene acrylic polymers to a water based solvent comprising water optionally co-solvated with a coalescing solvent and ammonia; adding a cross-linker to the blend of styrene acrylic polymers and allowing free carboxylic acid groups in the polymer to cross-link. The styrene acrylic polymers can be those described herein.

The method can comprise providing a blend of styrene acrylic polymers in a substantially non-aqueous solvent comprising ammonia; causing activation of a cross-linker to cause free carboxylic acid groups in the polymer to cross-link.

The method can also comprise the steps of adding additives such as a bulking agent, an emulsifier, a biocide and or antifoamer. Pigments can be added for colour. The paint tends to be blue for use commercially. The method can also comprise further paining a steel product with the paint. The steel product can be a rolled steel tube.

The present invention also comprises steel products when painted using the formulation according to the present invention. The steel product can be a rolled steel tube.

Brief Description of the Figures

Embodiments of the invention will now be described with reference to the accompanying drawings which are not drawn to scale and which are exemplary only and in which:

Figure 1 is an example formulation according to an embodiment of the invention.

Figure 2 is a further example formulation according to a preferred embodiment of the invention.

Figure 3 shows specifications of an embodiment of a painted steel tube.

Figure 4 is a formulation of another embodiment.

Detailed Description of Embodiments of the Invention

1. Example of formulation preparation

A first formulation is prepared according to an embodiment as follows: To a vessel there was added water which is stirred at room temperature. Some water can be held back for flushing out lines and tank.

To the water, a co-solvent of butyl Icinol was added. Butyl Icinol or 2-Butoxyethanol is an organic compound with the chemical formula BuOC 2H 40H. This colorless liquid has a sweet, ether-like odor, as it derives from the family of glycol ethers, and is a butyl ether of ethylene glycol. The co-solvent can assist in how the paint dries. The co-solvent can be a coalescing solvent to optimise the film forming process of the polymers in the paint. The butyl ether of ethylene glycol can be substituted with a range of other coalescing solvents such as Texanol (Isobutyric acid, monoester with 2,2,4-trimethylpentane-1,3-diol), butyl diglycol (2(-2butoxyethoxy) ethanol) and Dowanol DPnB (dipropylene glycol n-butyl ether).

Diatomaceous earth was added to the mixture over about 45 minutes. Diatomaceous earth can extend primary pigments, adds bulk and strength, and may enhance coating adhesion in paints. The uniform particle size of diatomaceous earth can ensure perfect batch-to-batch matching of paint color and sheen. When adding the diatomaceous earth is a preferred that there is a vortex in water. The diatomaceous earth can be sold under the brand Diafil 530. At this stage, the appearance of the mixture was checked to ensure there are no lumps (Quality Control Test 1)

An anti-foamer was added. The anti-foamer can be polydimethylsiloxane Silicone

Oil sold under the brand Silfax D7160B. After addition the solution was mixed for 5 min (Ensure Silfax D7160B is stirred prior to addition).

Ammonia 25% was added with mixing. At this stage, the pH can be tested to ensure there is an alkaline pH above the pKa of polymers (QC Test 2)

The following acrylic polymers were added:

• Hydrocryl 6030 (styrene acrylic, Tg 22C) and • Hydrocryl 6040 (styrene acrylic, Tg 96C). • Hydrocryl 122 (styrene acrylic, Tg 38C)

The Hydrocryl 6040 and Hydrocryl 6030 could be substituted with alykd resins. The alkyd resins are a family of resins (as are the styrene acrylic resins). Furthermore, the Hydrocryl polymers could all be replaced with one single polymer. The single polymer could be Hydrocryl 6099, however, the blend has been found to be better than the use of the single polymer.

• Hydrocryl 6040 Mix 5 min • Hydrocryl 6030 Mix 5 min * Hydrocryl 122 Mix 30 min

At this stage, more ammonia 25% was added to adjust the pH if needed to ensure an alkaline pH above the pKa of the acrylic polymers.

The crosslinker was added. A ZAC crosslinker was added sold under the brand Zinplex 15. The solution was mixed for 5 minutes.

A broad-spectrum biocide was added for the preservation of the paint against spoilage from bacteria, yeasts and fungi. A biocide sold under the brand Proxel GXL can be added with mixing for 5 min. The pH was about 9.7.

A thickener was added. The thickener was a Hydroxyethylcellulose. The thickener can be a sold under the brand Natrosal 250HHR. The Natrosal can be mixed with water in separate container for 15 min, make sure there are no lumps. The Natrosol mixture can be added slowly to the main tank with stir for 20min before sending the sample to QC. At the QC the pH and viscosity can be checked (QC Tests 3 - 4). It should be understood that the Natrosol can be substituted for a range of alternative thickening and stabilising materials.

Pigment was added prior to use. The pigments can comprise any known in the art. The resultant formulation is shown in Figure 2.

Figure 1 is a comparative formulation.

2. Further example of formulation preparation

In the first phase water was added to a vessel and then, under high shear mixing, was added an antifoamer sold under the brand BYK 1641.

Once the antifoam was completely mixed, Carrybon L400 was mixed in under high shear along. Carrybon L400 is a polycarboxylic acid type calcium carbonate dispersing agent. It aids in the dispersing of calcium carbonate into water whilst maintaining a low viscosity for the final slurry.

An emulsifier was added. The emulsifier can be a nonylphenol ethyoxylate with the formula C15H230.(C2H40) sold under the brand Teric GN9. It can be added in the range 0.1 to about 1 %.

Calcium carbonate (sold under the brand Omyacarb 100) can be added slowly over 1 hr until completely dispersed into the formula. Ammonia 25% can be added with mixing. At this stage, the pH can be tested to ensure there is an alkaline pH above the pKa of polymers.

The following acrylic polymers were added:

Hydrocryl 915 (styrene acrylic, Tg 28C) Mix10min

• Hydrocryl 122 (styrene acrylic, Tg 38C) Mix 30 min

Mixing times can vary to achieve the desired result. Hydrocryl 915 is a water based acrylic polymer that incorporates zinc ammonium carbonate to assist in curing the polymer as it dries. Hydrocryl 122 is a styrene acrylic polymer that is resistant to alkali materials, it is thought to help to modulate the finished paint's propensity to dissolve in various alkali solutions.

A pigment can be added. A suitable pigment can be the one sold under the brand Aquatal RAL 5002 which is a water-based mixture of white, blue and violet pigments that allow the finished paint formulation to be classified as a Ultramarine 5002 colour. The Aquatal RAL 5002 can be mixed until the colour is uniformly distributed into the paint mixture, typically this takes 30 min.

A broad-spectrum biocide was added for the preservation of the paint against spoilage from bacteria, yeasts and fungi. A biocide sold under the brand Actecide B20 can be added with mixing for 5 min. The final pH was about 9.7.

The resultant formulation is shown in Figure 4.

3. Applying the paint to the steel Paint was supplied in 200L drums and then prior to use the paint was stirred using pneumatically controlled stirrers and then transferred to a stirred holding tank.

From the holding tank the paint was sent to a smaller reservoir that allows for paint to be pumped to a spray system located around the steel tubes as they are being produced. They paint film thickness was controlled by a vacuum system that removes excess paint from the tube and then returns the excess paint to the small reservoir. The painted tube then went through a heated drying system and was then cut to length. The painted film was routinely checked for film thickness, uniform coverage and general appearance.

Various tests can be applied to the paint as outlined in Figure 3. Initial thickness readings indicated readings of approximately 8-10 pm such as 4-6 pm with a vacuum pressure of 5 kPa. The pressure was reduced by taking tape off the application head giving a pressure of approximately 3kPa. This gave thicker paint, however the surface finish was unacceptable with large bare patches and many scratch marks. The vacuum pressure was allowed to return to 5kPa and thickness readings of between 6-8 pm were obtained. The gloss level readings were approximately 10 at 60°C.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the io common general knowledge in the art, in Australia or any other country.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Any promises made in the present description should be understood to relate to some embodiments of the invention and are not intended to be promises made about the invention as a whole. Where there are promises that are deemed to apply to all embodiments of the invention, the applicant/patentee reserves the right to later delete them from the description and does not rely on these promises for the acceptance or subsequent grant of a patent in any country.

Claims

1. A paint formulation that is substantially alkaline resistant on contact and yet is removable in a process comprising immersion in an alkaline solution followed by immersion in an acid solution, the paint formulation comprising: a solvent system; a blend of acrylic polymers solvated in the solvent system, the acrylic polymers each having free carboxylate groups available for cross-linking; a cross-linker that forms cross-linked bonds between free carboxylate groups in the blend of acrylic polymer; wherein a first type of acrylic polymer in the blend is an alkali/acid resistant water-based polymer; and a second type of acrylic polymer in the blend is an alkali soluble water-based polymer.

2. The paint formulation according to claim 1, wherein the acrylic polymers in the blend are styrene-acrylic polymers.

3. The paint formulation according to claim 2, wherein the ratio of the first type of acrylic polymer to second type of acrylic polymer is in the range of from about 1.5 to about 1:8.

4. The paint formulation according to any one of the preceding claims, wherein the alkaline resistance is related to the number of free carboxylic acid groups; and the first type of acrylic polymer has about 100 to about 20 times fewer carboxylic acid groups than the second type of acrylic polymer.

5. The paint formulation according to any one of the preceding claims, wherein the first type of acrylic polymer has an acid number in the range of from about 0.4 to 0.5 and the second type of acrylic polymer has an acid number in the range of from about 10 to about 50 such as about 9 to about 45.

6. The paint formulation according to claim 5 wherein the acrylic polymers in the blend consist only of the said blended acrylic polymers.

7. The paint formulation according to any one of the preceding claims, wherein the first type of acrylic polymer has a glass transition temperature of at least 38C.

8. The paint formulation according to any one of the preceding claims, wherein the solvent system comprises at most 20% water.

9. The paint formulation according to any one of the preceding claims wherein the solvent system further comprises ammonia.

10. The paint formulation according to any one of the preceding claims, wherein the water based solvent system further comprises a coalescing solvent.

11. The paint formulation according to any one of the preceding claims, wherein the cross-linker is a metal ammonium carbonate.

12. The paint formulation according to claim 11, wherein the cross-linker is zinc ammonium carbonate.

13. The paint formulation according to claim 11 wherein the cross-linker is provided with the second acrylic polymer.

14. The paint formulation according to any one of the preceding claims, wherein the paint formulation further comprises an antifoam agent.

15. The paint formulation according to any one of the preceding claims, wherein the paint formulation further comprises an anti-scratch agent.

16. The paint formulation according to any one of the preceding claims, wherein the paint formulation further comprises a filler comprising calcium carbonate.

17. The paint formulation according to any one of the preceding claims, wherein the paint formulation further comprises one or more of bulking agent, biocide, thickener and pigment.

18. The paint formulation according to any one of the preceding claims, wherein the viscosity is about 20 seconds measured using Standard AS1580 214.2 @25°C in a Ford number 4 cup.

19.A method for preparing the paint formulation of any one of the preceding claims comprising adding at least two styrene acrylic polymers to a solvent system; adding a cross-linker or causing a cross linker to be activated thereby allowing free carboxylic acid groups in the polymer to cross-link.

20.A metal article when painted with paint formulation according to any one of claims 1 to 18.