WO2008064417A1

WO2008064417A1 - A coating material

Info

Publication number: WO2008064417A1
Application number: PCT/AU2007/001835
Authority: WO
Inventors: Mark Geoffrey Sceats; Connor James Horley
Original assignee: Calix Pty Ltd
Current assignee: Calix Pty Ltd
Priority date: 2006-11-28
Filing date: 2007-11-28
Publication date: 2008-06-05
Anticipated expiration: 2009-05-28

Abstract

A coating material, a composite material, and an anti-fouling paint. The coating material comprises a polymer matrix and embedded particles of caustic partially calcined dolomite or caustic magnesite.

Description

A COATING MATERIAL

FIELD OF INVENTION

The present invention relates broadly to coating materials and composites containing the coating material. The invention also relates to uses of the coating for applications that require one or more of the properties of a high flame resistance, thermal resistance, insolubility in water, UV resistance, flexibility, adhesion, and non-slip surface texture properties, and resistance to biofouling by virtue of being biocidal, biostatic or biorepellant to microorganisms, while being non-toxic to other organisms.

BACKGROUND

Coating materials are made for many applications, such as acrylic paints that can be applied to a surface by a brush or roller and generic coatings that can be applied to the surface by other means such as a spray. These coatings are generally characterized by a polymer matrix that provides the necessary flexibility, and sets and binds the fillers, which provide or enhance the desirable coating properties, and which binds to the substrate surface to provide the desired adhesion. The desirable properties are developed by the choice of the polymer matrix, including its fillers, stabilizers, accelerators and solvent (water or oil, such as linseed oil), and is broadly characterized by its gross properties such as an emulsion in the case of acrylic paints, its thermosetting or thermoplastic behavior, and its physical properties such as adhesion, texture, durability (scratch resistance), shelf life and weathering, which derive from the chemical composition.

The polymer matrix, when set, is generally comprised of organic materials, and the matrix phase is generally organic in nature. This first phase in the coating material binds the materials, provides the flexibility by the degree of polymerization, and provides the adhesion. The second phase is generally solids, which also include organic materials such as a natural or synthetic fiber or granules, or an inorganic material such as an oxide, generally as a powder. This second phase is typically added to provide the colour by pigments, and/or control the strength of the material, and influences the texture and hardness. The coating material may be prepared from a solvent based process, such as an oil based coating, or a water based coating commonly known as a latex paint, or by polymerization induced by initiators. There are various coating materials that have been developed to suit particular applications. In certain limits, the material may be sufficiently strong that it has applications as a standalone material in the sense that it may be prepared on substrate, and then lifted off as a flexible material when set or partially set, and then used for the desired application. Accordingly, the term 'coating', as used herein, does not limit the materials described to conventional coating-on-substrate applications, but may be used to describe a wide range of coating materials.

The use of lime, CaO(s), or hydrated lime Ca(OH)₂(S) as is formed in a slurry, as a whitewash coating has been well established in antiquity and has the property that it is biocidal and biostatic. Its modern day use as a biocide has been limited by the carbonation reaction of Ca(OH)₂(S) with atmospheric CO₂(g) to produce calcite CaCO₃(S), which limits its biocidal activity to several months. The incorporation of lime into modern paints, such as water-based coatings (such as acrylic paints) has generally not been successful because its high alkalinity leads to a breakdown of the emulsion. Mallow, in US Patent 6,231 ,650 describes the requirements of a hydrated lime coating biocidal paint as one which must retain the ability to pull in and substantially encapsulate a microorganism, and posses the alkalinity necessary to kill microorganisms. In order to kill microorganisms, the lime must have a pH between about 11-13. In order to retain the alkalinity necessary to kill microorganisms, the coating, or paint, must be protected from attack by carbon dioxide. Mallow describes that a biocidal coating can be made comprising hydrated lime and a binder comprising a cellulose derivative selected from the group consisting of an alkyl derivative, a hydroxyl derivative, and a carboxyl derivative. The binder has film properties of a barrier for carbon dioxide but not film properties of a barrier for water vapor. The binder also renders the coating durable and adhesive upon drying and prevents a substantial increase in friability. Mallow extends this approach in US 6,280,509, which describes a coating incorporating lime with a non-ionic polyolefinic ester latex. In US 20040194656 Mallow describes that the coagulation and phase separation that occurs upon blending with lime, can be overcome by a composition comprising hydrated lime, alkaline potassium salt, and a non-ionic polyolefinic latex. This formulation was for a paint that had a greater shelf life and reduced the hardening, thickening and cross linking of the paints during their useful life. Limestone itself, CaCO₃ is widely used a filler/extender/pigment in paints, and is usually inactive. However, in flame retardant applications, calcium carbonate is used to reduce the spread of flaming droplets (not flame-retardant per se) through its impact on flow properties.

Magnesium hydroxide, or hydrated magnesia, Mg(OH)₂, is found naturally as the mineral Brucite, and is widely used as a component in low smoke, self- extinguishing flame retardant polymers (such as cables) and coatings. These compositions replaced the earlier halogen based compositions, which release toxic gases on combustion. Magnesium hydroxide has a relatively high decomposition temperature (about 340° C.) and a satisfactory thermal stability (WO 99/05688). As the hydroxide decomposes with heat, the steam released interferes with oxygen gas, displacing it and reducing flammability. Additionally, the magnesium oxide ceramic is formed and acts as a thermal barrier, blocking and deflecting heat and flame from penetrating to the substrate below the coating. In these coatings, the preferred amount of metal hydroxide in the precursor composition is from about 25% to about 35% by weight of the total composition, such that a preferred amount of metal hydroxide in the final coating is from about 40% to about 60% by weight of the final coating.

Magnesia, MgO is widely used as an extender in coatings. This material is prepared by a number of high temperature processes that produce a "dead burned", unreactive material that is insoluble in water. It is found naturally as the mineral Periclase.

Dolomite, MgCO₃-CaCO₃ or dolomitic lime or magnesia, can be used as a

(white) pigment and/or extender (A.M.Youssef, Pigment & Resin Technology, 2002, 31 , 226-233) for generic coatings, with the dolomite replacing other materials such as ground limestone. Ground dolomite has a high natural whiteness and it's noted for its ease of dispersion. When used in coatings, it is known to improve properties such as weatherability, reducing shrinkage, fissure development and water absorption. A particular example is found in US Patent 6,103,360 (Caldwell and Fernando), which describes a coating composition for ceiling boards that has a high light reflectance and durability, and which describes the use, inter alia, of dolomite particles of various sizes, as filler. US Patent 5,167,705 (Coughlan) describes a super, high-opacity thin coat concept airport runway marking paint comprising 25- 75% by weight of titanium dioxide in the paint, with an extender that may include dolomite. US Patent 4,828,617 (Troszt 1987) describes a pigment comprising alumina/alumina and other materials that include powdered dolomite, with such a pigment providing anti-corrosion and self cleaning properties.

Partially calcined dolomite (sometimes called lowly calcined dolomite) MgO-CaCO₃ is generally formed by calcining dolomite in kilns, and this material is generally phase separated and comprises MgO(s) and CaCO₃(S). This material may be used to replace MgO in applications, including coatings, and MgO may be used as an extender in coatings. For example, US Patent 6,783,799 (Goodson, published 2004) describes that replacing highly calcined MgO with low calcined dolomite as the base, increases coating strength and reactivity of a phosphoric acid based sprayable coating. This coating sets very hard, like a cement, and is not flexible. Its description as a coating derives from its method of application, akin to sprayed concrete.

In another application environment for coatings, the factors that are known to effect the growth of organisms in ocean, estuary or fresh water river environments have been extensively studied. It is generally accepted that organisms will grow on any surface that is not toxic to that organism exposed to an aquatic environment. The growth of organisms is itself a complex process as an ecosystem develops from mono-cellular organisms to molluscs.

All phyla living in aquatic environments including algae, bacteria and invertebrates use materials to attach to the substrata of underwater surfaces. Larvae of invertebrates and spores of algae need to find and attach to a surface as part of their life cycle. This 'first touch' adhesion often takes place within minutes under water and can occur with a wide range of substrates and over a wide range of temperatures, salinities and turbulence.

On ships in a marine environment, it is barnacle contamination on which most interest has focused due to the serious adverse effect on performance. All successful compositions must control barnacle contamination. Adhesion of larger organisms such as mussels consists of protein tethers attached at one end of the organism and by an adhesive pad at the other. The adhesive pads are comprised of mussel adhesive proteins (MAPS), in which DOPA is the key protein residue.

Thus to be successful as an anti-foulant paint, the active biological agent must be able to resist attachment by the EPS from unicellular organisms as well as adhesive proteins, such as MAPs from mussels, and most importantly, those from barnacles. Since the 1960s, the biological agent has been either a toxin, or a low surface energy material that is not conducive to the generation of adhesion to the surface through weak physical adsorption.

Lime was used as the first anti-foulant paints before 1960 and has been used as an additive with other materials such as asphalt and tar as the binder. Lime is relatively insoluble, but it is sufficiently soluble through the formation of Portlandite Ca(OH)₂(S) that the lifetime is not competitive with biocide based systems. The generic anti-fouling property of lime is well established, and it has been deployed in porous bags as a barrier control around commercial oyster farms to control oyster drills (Urosalpinx and Eupleura) and other fouling species since the turn of the century.

In general terms lime is an irritant, but is otherwise not toxic to shellfish and humans. It is a bio-repellant through its irritation of soft tissue. Its impact is purely local, and derives from the basic properties of lime which generates a local pH that is higher than seawater or the body fluids of organisms. Generally, a higher local pH causes the protein or polysaccharide matrix to swell, and the organism detaches to limit this irritation. That is, the response, called 'lime softening" is not a toxic response. However, the solubility of lime is sufficiently high that the leaching occurs too quickly to form an effectively anti-fouling paint.

Magnesia in crystalline form is widely used in waste treatment and in many applications in which lime is used. It has a lower pH, and is more insoluble than lime. The surface of magnesia in water is essentially hydrated, and the dissociation creates a negatively charged surface. On a weight-by-weight comparison, MgO is more effective than CaO in terms of its alkalinity for waste treatment. In many water treatment applications, its low solubility is desirable because the reactions are rate limited by the dissolution of the MgO through the formation of Brucite Mg(OH)₂(S), leading to more manageable precipitates. Essentially, MgO is slow to dissolve and accommodates to the environment slowly. Dissolution in sea water is also slowed by the high concentration of magnesium in sea water. When an MgO surface is exposed to water, the local pH is that of the sea water (~ 8.3) because of this slow response. However, when the surface is encompassed by EPS or MAPS, the exchange with the sea water is cut off, and the local pH rises towards 10.3 of hydrated MgO in equilibrium. This triggers the swelling, and the organism ceases its attempts to colonise the surface. The reaction of magnesia with tissue is generally the same and causes irritation to marine organism, in the same way that magnesia irritates human eyes, throat and lungs, and in the hydrated form of milk of magnesia, causes an irritation of the stomach lining.

Marine growth can considerably affect a marine vessel's performance. A 10μ increase in average hull roughness for a bulk carrier can equate to as much as 1 ,600 tonnes of additional fuel use over the course of one year due to fouling and subsequent frictional surface resistance. Since fuel costs can amount to as much as 50% of the total operating costs for such ships, and a fouled hull can increase fuel consumption by as much as 30%, an effective anti-foul coating is critical to the performance of any ship. Removing this growth is a costly procedure, as the fouling is very strongly bonded. Dry-docking for the purpose of cleaning and recoating a hull with fresh anti-foul paint is costly not only because of the expenses associated with maintenance, but also the loss of business due to inactivity.

Prior to the 1960s vessel hulls were coated with lime and later with arsenic and mercury compounds to reduce fouling. The lime was applied with other materials, often as a mortar, and has a limited lifetime due to its solubility in sea water, its strength, and carbonation with CO₂ (as in a whitewash). During the 1960s anti-fouling paints were developed using metallic compounds, in particular the organometallic material tributyl tin (TBT). By the 1970s most ocean going vessels used TBT painted on their hulls. TBT was used on vessels that plied rivers and estuaries, and was used not only on commercial vessels but also pleasure boats.

While anti-fouling TBT paints have been found to be effective in killing marine life attached to vessel hulls, they have also killed and caused genetic alterations in other marine species, e.g., shell deformations in oysters. In the 1970s TBT contamination was linked to the high mortality of oyster larvae in the Arcachon Bay on the west coast of France. In the 1980s in south-west England, TBT poisoning was linked to the decline of the dog whelk.

There are presently three types of marine anti-foul paints:- Controlled Depletion Polymer (CDP) Coatings Self Polishing Coating (SPC) Fouling Release Coatings

CDP paints use a high loading of the biocide particles embedded in a binder which is a tough, impermeable, insoluble matrix. There is an adequate leaching rate because the soluble biocide particles are in continuous contact with the surface. When the water-soluble components have been dissolved a "dead" matrix will remain, normally referred to as the leached layer. This leached layer will increase in thickness over time dampening the release of biocides, effectively limiting the service life of the coating to around 36 months. The binder in CDP paints can be polyvinyl resins or other high molecular weight synthetic resins. These can be mixed with soluble rosins to control the leaching rates. An example of such paint is a vinyl acetate-vinyl chloride copolymer with 35% by volume of cuprous oxide as the biocide particles. In other examples copper metal may be used, for example at 20% loading. Inert non-toxic pigments, such as lime and diatomaceous silica, are generally added, and these can affect the leaching rate.

SPC paints are based on soluble binders, and there is a lower loading of the biocide particles. In these, the matrix itself is designed to dissolve at a desired rate to replenish the surface with the biocide. Being self-polishing, the performance is not only more predictable than CDP, but the smoothness of the coating will also be superior, reducing the hull roughness, and hence positively affecting the bunker consumption. A tin-free SPC paint system is reported to be about 150 percent more expensive than a tin-based SPC system and its effective service life is normally limited to 60 months. For SPC paints, rosin, or a similar soluble resin, is typically used as the binder. Additives such as plasticizers and tougheners are used to control physical properties such as brittleness and the solution rate. The volume fraction of the biocide particles is generally between 8- 24%. By contrast with CDP coatings, the very thin leached layer of an SPC coating, having formed, will be easily removed using normal high pressure washing at 150 - 200 bar.

The relatively new Fouling Release coatings contain no biocides, but instead have a very smooth surface where the fouling is not able to permanently attach when the vessel is in motion. Many types of biofouling can be reduced on surfaces that have a critical surface tension in the range of 20 to 30 dynes cm^'1. The adhesion strength to the surface scales as (γE)^1/2 where γ is the surface energy and E is the Young's modulus. Thus a Fouling Release coating will be one that minimises the adhesion strength. Such coatings can be made from silicone and from certain flurosilicone polymers. Fouling Release coatings appear to have a very long theoretical service life and 60-120 months is normally expected, but in practical terms, this is limited by the extent of mechanical damages caused to the coating.

A need therefore exists to provide a coating material that seeks to address at least one of the above mentioned problems.

SUMMARY

In accordance with a first aspect of the present invention there is provided a coating material comprising a polymer matrix and embedded particles of caustic partially calcined dolomite or caustic magnesite.

The coating material may further comprise particles of one or more of a group consisting of magnesite, dolomite or limestone.

The polymer matrix may be a polymer based on a cellulose or polysaccharide ester or ether, and the polymer matrix one or more of a group consisting of initiators, stabilizers, and fillers.

The polymer matrix may be a polymer based on acrylic resins forming an emulsion with water and a co-solvent.

The co-solvent may comprise propylene glycol.

The coating material may further comprise an atoxic filler.

The atoxic filler may comprise one or more of a group consisting of aluminium oxide, titanium dioxide, silicon dioxide, and mineral pigment powders.

The coating material may further comprise a humecant comprising a water soluble glycol incorporated as part of a composition of the co-solvent.

The glycol may comprise one or more of a group consisting of glycerol, polyethylene glycol and tripropylene glycol. In accordance with a second aspect of the present invention there is provided a composite material formed from the coating material as defined in the first aspect, and further comprising one or more of a group consisting of natural or synthetic fibres, metallic wires, and woven beds.

In accordance with a third aspect of the present invention there is provided antifouling paint formed from the coating material as defined in the first aspect.

The particles may be wholly or partially hydrated.

The polymer matrix may be insoluble in water and a volume of the particles to the polymer matrix may be in a range of about 30-80%.

The polymer matrix may be soluble in water and a volume of the particles to the polymer matrix may be in a range of about 5-40%.

The polymer matrix may be chosen such that the surface tension is in a range of about 20 to 30 dynes cm^"1.

A mean surface area of the particles may be in a range of about 60-250 m² gm^"1, with a distribution that maintains a constant surface area of the particles exposed to water as the particles are leached.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Figure 1 shows a schematic drawing illustrating an example application of a coating according to an example embodiment.

DETAILED DESCRIPTION

The example embodiments described provide a coating material, and a material that can be used as the basis for a composite material. Coating materials are described that can withstand extreme conditions, principally a resistance to degradation and fragmentation arising from the intermittent application of flames (that is a high flame resistance), such as from fires as well as the exhaust gases from combustion systems, and a high thermal resistance while maintaining the desirable properties for an exposed environment, such as water resistance, UV sunlight resistance, flexibility, strong adhesion to surfaces, and a texture that provides non-slip properties, whether dry or wet. In one embodiment, a feature of the material is that it may be anti-viral, anti-bacterial and anti-fungal. In another embodiment, the material or products of its thermal decomposition are not toxic compounds, emit only minimal Volatile Organic Compounds, and does not leach toxic materials into water. A preferred embodiment is able to meet a majority of these requirements in a single formulation.

A preferred embodiment of the coating material comprises a flash calcined caustic magnesite MgO(s) or caustic partially calcined dolomite MgO. CaCO₃(S), which is produced to have a high surface area and a high mesoporous volume. This caustic material will react with water to produce Mg(OH)₂, and with the polymer active sites, often mediated by the Mg(OH)₂. For example, caustic magnesite is a catalyst for the reactions of cellulose and polysaccharides that lead to polymerisation, which are generally alkali catalysed. More generally, there are other base catalysed reactions, such as esterification.

Sawii et al (J. Chem Eng Japan, 28,288-293) found that CaO and MgO powders exhibited strong antibacterial activity through a biocidal activity. Sawi and

Yoshikawa (J. Applied Microbiology 2004, 96, 803-809 extended these results to demonstrate anti-fungal activity. The term "anti-microbial" composition as used herein embraces compositions having biocidal and/or biostatic activity against various types of micro-organisms, for example gram negative bacteria such as Escherichia coli and Pseudomonas aeruginosa, gram positive bacteria such as

Staphylococcus aureus and Propionibacterium acnes, moulds such as Aspergillus niger and Penicillium funiculosum, yeasts such as Candida albicans, Saccharomyces cerevisiae and Pityrosporum ovale, dermatophytic fungi such as Trichophyton rubrum, microalgae such as Chlorella spp. and Spyrogyra spp. and viruses such as Herpes virus and Picornavirus. In the example embodiments it is exploited that the difficulty in the producing lime bases formulations for polymer based coatings resulting from the high pH of lime can be overcome for e.g. caustic magnesite, while maintaining the anti-microbial properties of the resulting coating. In other words, it is exploited that the pH for e.g. caustic magnesite is low enough to enable formulation of polymer based coatings, and yet high enough to achieve anti-microbial polymer based coatings.

The caustic properties generally scale with the high surface area of the materials, and flash calcination produces a material with the desired properties. However, the low solubility of MgO is such that these rates of reaction, and hence the caustic nature of the material in the example embodiments, can be controlled at the particle surface, and the surface can be controlled by thermomechanical and chemical processes. For example, recarbonisation from CO₂ in the atmosphere can reduce the reactivity, as well as sintering. A second mechanism of control of reactivity is to use a material that is only partially calcined so that the environment around the particle is buffered by the carbonate ion that is dissolved. A third mechanism of control of reactivity is to dilute the caustic material with carbonates such as magnesite, dolomite and limestone. That is, the filler may have a surface activity that provides the optimum balance between reactivity for strength of the coating and the degree of hydration for flame retardation.

In water-based polymer systems, such as emulsions, the reactivity is reduced by such means so that the emulsion does not phase separate, so as to provide a long shelf life. It is to be understood that the addition of water to such a material, without any polymer, forms a slurry that sets with dehydration to form a cement, the strength of which depends on the caustic properties. That is, a cementitious matrix is formed, or partially formed, in the aqueous phase of such an emulsion, and this is, in effect, an inorganic polymer. This cementitious matrix surrounds, and consumes, the particles. To that extent, the material described in the example embodiments gains many of its attributes from being a composite of an organic polymer and an inorganic polymer.

Conditions are preferably found such that the hydrated material is stable so as to give a long shelf life. The propensity for an emulsion to phase separate is a known problem when lime is incorporated into a water-based paint. Magnesium hydroxide has a lower pH than lime, and this reduces this propensity. The advantage exploited by using magnesia based materials in the example embodiments is that the reactivity of the material, and thus the propensity to phase separate, can be controlled by the thermomechanical and chemical processing.

Calcined magnesite and dolomite are both refractory materials when dead- burned (ie sintered), which have a high thermal resistance that increases with temperature. Magnesite and dolomite chemically decompose when the temperature exceeds about 900⁰C by the release of carbon dioxide, which is a flame suppressant, and forms the known refractory materials magnesia and dolime. Also, the caustic materials produced from calcining magnesite and dolomite in example embodiments advantageously also sinter to these refractory materials. These properties contribute to the flame resistance of the coatings in example embodiments.

Patent WO 2007-045050A (Sceats and Horley) describes a caustic partially calcined dolomite, which has not phase separated into CaCO₃(S) and MgO(s), and which has a high surface area and pore volume. This is a preferred material to be used in the coatings in embodiments described herein. Further, it can be exploited that mineral deposits of dolomite may have a wide range with respect to the relative amounts of calcium and magnesium, and that the desired properties of the material for use in example embodiments can be made by mixing the available calcined material with calcium and magnesium carbonates. Further, it can be exploited that the reactivity of the calcined material depends on its surface morphology, which can be controlled by the degree of sintering of the material. That is, the material preferred for example embodiments depends not only on its chemical composition, but also on its physical properties such as surface area and pore volume that are determined by the manufacturing process. The material as described in the example embodiments is an active component, which advantageously improves the performance of the coating. When used with water, these materials advantageously hydrate readily.

The preferred material for example embodiments having a desired degree of reactivity is generally a mixture of caustic magnesite and/or caustic partially calcined dolomite , with magnesite and/or dolomite. This material is herein referred to as "caustic material".

Another application of example embodiments of the coating material is for an anti-fouling paint comprising particles containing caustic magnesite based particles and a binder. The particles in the embodiments for such an application may be in the form of granules of pure caustic magnesite MgO, or a caustic partially calcined dolimite MgOOaCO₃ or combinations thereof. It is preferred that the caustic magnesite MgO content of the particle is as high as possible.

The role of the said granules in example embodiments is to be a bio- repellant, rather than a biocide. It is exploited that magnesium and hydroxide ions formed from the dissolution of MgO are non-toxic, that sea water has a high concentration of magnesium, and that the hydroxide ion concentration is controlled by the acidity of the water. Magnesium is an essential nutrient to all life. As will be described below, it is the high local concentration of magnesium that acts as the bio-repellant. In the example embodiments, caustic magnesite is preferable to use rather than lime because it is more insoluble, and thus gives a longer lifetime.

The reactivity of magnesite particles is strongly affected by the surface area/volume ratio, which is small for dead-burned magnesia, and higher for what is referred to herein as a caustic magnesite. Surface areas of 60 m² gm^"1 have been realized, but this is expected that the surface area can be readily increased to over 250 m² gm^'1. In contrast, the heating of the particles causes sintering which is exploited in conventional fabrication of inert MgO, with the resultant reduction of the surface area and a loss of the caustic properties. Thus a low dissolution rate is a characteristic of what is known in the art as "dead burned" magnesia, and a higher dissolution rate exploited in example embodiments is a characteristic of what is referred to herein as "caustic magnesite". Further control of the dissolution can be achieved by using particles that also contain CaO or CaCO₃ in different embodiments.

It an example embodiment, the MgO at the surface of the caustic magnesite particle is hydrated to Mg(OH)₂, which is the mineral Brucite. This hydration may occur prior to application of the paint, or as part of the curing process of the paint. With respect to the latter, there is a significant liberation of heat during hydration, and this may advantageously be used to set the paint through the heat transfer to the binder. Provided that such a process does not destroy the biorepellant nature of the particle surface, then such a thermosetting paint has many preferable properties. The binder may be formed from a number of materials that, by themselves, form a marine paint that is adequate for the purpose, including being environmentally safe. In the context of marine environment applications, it is preferable to use a polymer or a resin as the binder. As mentioned above, the particles preferably are sufficiently caustic that the reactivity enables the beneficial effects as described herein. The binder may be sufficiently soluble so that the paint is an SPC paint, or may be sufficiently insoluble so that the paint is a CRP paint as described in the background section.

The binder may comprise polymers and polymer solvent mixtures that set through a number of mechanisms understood in the art. In some formulations, the binder is suspended in the binder and supplied as a one-part paint, and in other formulations the paint is prepared prior to application by mixing the particles, which may be suspended in a fluid, with the binder. In some formulations, the particles may activate the binder and set the mixture, requiring a pre-mixing of the components.

A large number of suitable binders are commercially available for use in example embodiments so that they can bind with the caustic particles, without dissolving them, or without substantially modifying the properties of the caustic particles such that the caustic particles, when exposed to water on the surface, exhibit the desired bio-repellant properties of hydrated MgO. It is the MgO and its interaction with water that is the active ingredient on or near the surface that provides the desired anti-fouling action in preferred embodiments and the choice of binder preferably is such that the surface properties of exposed particles are essentially those of the MgO particles. The above does not mean to imply that the binder does not react with the particle, but rather that the particle, when exposed to the water, exhibits the required caustic properties of MgO.

The anti-fouling paint in an example embodiment may be comprised of

5%-25% by volume of the caustic particles, and the caustic particles may be of a size between 0.5-125 microns. There is a general correlation between the size of the particles and the roughness of the surface, and between the roughness of the surface and the deleterious viscous drag of the paint. However, it will be appreciated that there are means of treating the surface of the paint such that a smooth surface may result notwithstanding the initial particles size. Submicron size caustic particles may be used in example embodiments but the cost/benefit ratio in many applications may not generally warrant the use of such. However, where it is critical to have a non-toxic anti-fouling surface which minimises the viscous drag, exploiting that surfaces that have a finite nanoscale roughness, such as dolphin skin, have a lower drag than a perfectly smooth surfaces, the use of submicron size caustic particles may be warranted.

A preferred embodiment is a paint comprised of caustic MgO-containing particles of ~5 microns size in a binder that sets to give a smooth suface finish and is self polishing through the dissolution of the particles and the binder.

In another preferred embodiment a paint comprises of a binder and caustic particles comprised of a material which is substantially hydrated magnesite, namely magnesium hydroxide Mg(OH)₂. That is, the caustic particles based on MgO may be pre-hydrated to the extent of 5-100% conversion of MgO to magnesium hydroxide Mg(OH)₂. Such hydrated caustic particles would generally be used with binders that are different from those described for non-hydrated caustic MgO particles.

The pigment of a preferred embodiment may advantageously comprise various kinds of bauxite. For example, gibbsite, goethite, hematite and particularly siderite bauxites, boehmite AIO(OH), magnetite (Fe₃O₄), siderite (FeCO₃), lepidocrocite (γ-FeO(OH)), manganite (gamma.-MnO(OH)), rhodocrosite (MnCO₃), pyrolusite (β-MnO₂) as well as zinc phosphate. Natural mineral substances which comprise compounds of several metals are also considered to be advantageous for another embodiment. For example, jacobsite (MnFe₂O₄), franklinite (ZnFe₂O₄) or chromite (FeCr₂O₄) may also be used. The toxicity of the coating is largely determined by the toxicity of the pigment.

In one example embodiment of the present invention, a coating material is formed from the setting of a stable mixture of>

(a) a water based acrylic resin, or mixture of resins, formed as an emulsion with water and co-solvents such as propylene glycol, the resin being derived from ethyl acrylate, methyl acrylate, propyl acrylate, butyl acrylate, ethyl methacrylate, methyl methacrylate, butyl methacrylate, hydroxyethyl methylacrylate and mixtures thereof; and

(b) a powder of the caustic material which is mixed with water to form a slurry; and

(c) powdered fillers such as pigments, silicon dioxide, fly ash, silicon carbide, boron nitride, aluminum oxide, silicon nitride, hard technical ceramics or cermets, hard metals, aluminum nitride, stabilized ZrO and ZrO₂, diamond, ammonium meta-vanadate, vanadium oxides, tungsten carbide, molybdenum metal and/or oxide.

The proportions of the mixture, and the reactivity of the powder (b) are preferably such that a stable emulsion is formed which has a shelf lifetime of the order of years. The setting of these materials occurs by the release of water, as is common with all water based acrylic coatings. In this embodiment, the powder (b) may be mixed with the polymer resins first, and the emulsion then formed by adding water and co-solvents.

These materials, when combined and set, can provide a durable coating that has high flame retardation, high thermal resistance, high water impermeability, scratch resistance, flexibility, strong adhesion, wet or dry non-slip properties, and a high resistance to biofouling without being toxic. The composition of the acrylic polymers, and the relative proportions of the components determine the magnitude of these properties of the set material as required for various applications. The means of applying the material may be as paint by brushing or rolling, as spray, or by extrusion and other means known in the art of polymer based coatings. As an emulsion, the shelf life of the material is also dependent on the composition.

In another example embodiment similar to the above embodiment, the co- solvent propylene glycol is replaced by, or blended with other water soluble glycols, such as glycerol, polyethylene glycol and tripropylene glycol, which are recognised as humectants. A humectant draws water and water vapour into the material so as to stabilize the water content of the coating at a level sufficient to pull biological contaminants into the caustic material, such that the material's biocidal activity is maintained. A preferred humectant for water base coatings is glycerol. A another example embodiment of the present invention comprises a composite material compound formed from the setting of a stable mixture of:-

(a) a polymer matrix; and

(b) a powder of the caustic material; and

(c) powdered fillers such as pigments, silicon dioxide, fly ash, silicon carbide, boron nitride, aluminum oxide, silicon nitride, hard technical ceramics or cermets, hard metals, aluminum nitride, stabilized ZrO and ZrO₂, diamond, ammonium meta-vanadate, vanadium oxides, tungsten carbide, molybdenum metal and/or oxide, and the like.

In this example, the polymer matrix can be made from binders that include epoxies, urethanes, melamine, polyesters, natural and modified natural polymers, and vinyl polymers. For example, the material may be a adherent polymer such as polyurethane, polyester, poly-ether-ether ketone, styrene polybutadiene,polyvinylidene chloride, polycarbonate, and polyvinyl chloride. Another example material is hydroxypropyl-2-phosphatepropyl cellulose which has good adhesion on hydrophilic metallic surfaces. The polymer matrix itself should preferably have good adhesion to the surface. For example, sodium carboxymethylcellulose is a cellulose ether commonly used as an adhesive. The polymers can include one or more monomers such as vinyl acetate, vinyl propionate, vinyl butyrate, ethylene, vinyl chloride, vinylidine chloride, vinyl fluoride, vinylidene fluoride, styrene, butadiene, urethane, epoxy, melamine, an ester, or an alkyd. Examples of natural and modified natural polymers are protein and carbohydrate polymers such as starch.

Another example embodiment of the present invention comprises a composite material that is formed by the application of the coating material of one of the previous example embodiments, to further elements such as natural fibres from vegetable matter, or man made fibres such as Kevlar, or metal threads. The fibres and threads may be either mixed with the coating material before setting, and/or or formed as woven sheets to which the coating material is applied before setting. The principle purpose of this aspect is to provide a composite material that has additional strength because of the large surface area available for adhesion, without loss of the beneficial properties of the coating material as previously described.

A preferred embodiment of the coating will now be described in greater detail with reference to examples. This coating is formed by the setting of a water based emulsion of a mixture of acrylic resins, with a solvent comprising water and propylene glycol, a hydrated mixture of the caustic material and other non-toxic fillers, such as alumina and silicon dioxide powders. The proportions of the materials are variable depending on the application, from 30-50% of the polymer, 0-10% of propylene glycol, 30-50% of the caustic material and 0-30% of the fillers, not including water. The acrylate resin mixture and the solvent proportions are selected so as to give the desired spreading, adhesion, and hardness properties of the set material. The particle sizes of the caustic material and the fillers are selected to provide the required texture of the material, such that anti-slip properties are formed by using larger particles. The alumina is selected in this embodiment because it has anti- corrosion and self cleaning properties. The fraction of the caustic material is chosen to give the required flame retardation and thermal resistance, along with the correlated properties such as biological activity. Of these components, the proplylene glycol is the only Volatile Organic Compound (VOC). However, it is relatively benign and is in common use in many low VOC paints. The coating does not have the smell associated with the VOCs in water-based acrylate paints.

In confidetial tests, the coating material was applied to a thin metal substrate and dried over several hours, as expected for such a paint. The coating was relatively hard, scratch resistant and had a strong adhesion to the metal substrate. The coating was flexible and did not break with the flexing of the substrate.

The coating readily satisfied the basic requirement of a flame retardant material (surviving a 17O⁰C flame for 15 seconds), and did not materially decompose when a propane torch at about 800⁰C was applied to the surface for 30 seconds. There is no flame from the volatalization and burning of the resin and propylene glycol, meaning that combustion of these directly produces CO₂ and H₂O. At longer durations, the flame produced self extinguishes. It is noted that heating of the material below 800⁰C releases the water from the caustic material, and any CO₂ from the calcination of any residual carbonate bound to magnesium. The heating above 900⁰C releases further CO₂ from the calcination of any carbonate bound to calcium. These gases are known to have a propensity to quench flames. The advance of the heat front through the coating is retarded by the high thermal resistance of the caustic material (because of its MgO, a known refractory material) and the transformations described above produce additional MgO(s) and CaO(s) such that the thermal resistance increases with time. Dolime MgO.CaO is also a known refractory material. Further, the sintering of the MgO causes a structural rearrangement of the surface. The minerals AI₂O₃ and SiO₂ are non-volatile, and also form a barrier to the combustion. The polymer material without the caustic material, with SiO₂ and AI₂O₃ is a flame retardant in its own right. However, this performance is markedly improved with the inclusion of caustic material. The adhesion of the coating material to metals and other materials is generally enhanced by the caustic material component.

Panels, when coated with this material, were immersed in sea water for an extended period of nine months, and it was observed that there was no growth of organisms such as barnacles and the like on the panel, and that algae had not adhered. The adhesion of the coating on the panel was also unaffected by the immersion. That is, the panels do not exhibit weathering, and posses a marine anti- foul property.

Figure 1 shows a schematic drawing illustrating an example application of a coating 100 according to an example embodiment. The coating 100 is applied to the hull 102 of a ship 104. Typically, the coating 100 would be applied substantially up tp a maximum water line 106 of the ship 104. The coating 100 comprises a polymer matrix and embedded particles of caustic partially calcined dolomite or caustic magnesite.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1. A coating material comprising a polymer matrix and embedded particles of caustic partially calcined dolomite or caustic magnesite.

2. The coating material of Claim 1 , further comprising particles of one or more of a group consisting of magnesite, dolomite or limestone.

3. The coating material of claims 1 or 2, wherein the polymer matrix is a polymer based on a cellulose or polysaccharide ester or ether, and the polymer matrix one or more of a group consisting of initiators, stabilizers, and fillers.

4. The coating material of claims 1 or 2, wherein the polymer matrix is a polymer based on acrylic resins forming an emulsion with water and a co- solvent.

5. The coating material of claim 3, wherein the co-solvent comprises propylene glycol.

6. The coating material of any one of the previous claims, further comprising atoxic filler.

7. The coating material of claim 6, wherein the atoxic filler comprisies one or more of a group consisting of aluminium oxide, titanium dioxide, silicon dioxide, and mineral pigment powders.

8. The coating material of claim 4, further comprising a humecant comprising a water soluble glycol incorporated as part of a composition of the co- solvent.

9. The coating material of claim 8, wherein the glycol comprises one or more of a group consisting of glycerol, polyethylene glycol and tripropylene glycol.

10. A composite material formed from the coating material of any of claims 1-9 and further comprising one or more of a group consisting of natural or synthetic fibres, metallic wires, and woven beds.

11. An antifouling paint formed from the coating material of any of claims 1-9.

12. The antifouling paint of claim 11 , wherein the particles are wholly or partially hyd rated.

13. The anti-fouling paint of claims 11 or 12, wherein the polymer matrix is insoluble in water and a volume of the particles to the polymer matrix is in a range of about 30-80%.

14. The anti-fouling paint of claims 11 or 12, wherein the polymer matrix is soluble in water and a volume of the particles to the polymer matrix is in a range of about 5-40%.

15. The anti-fouling paint of claims 11 or 12, wherein the polymer matrix is chosen such that the surface tension is in a range of about 20 to 30 dynes cm^"1.

16. The anti-fouling paint of any one of claims 11-15, wherein a mean surface area of the particles is in a range of about 60-250 m² gm^"1, with a distribution that maintains a constant surface area of the particles exposed to water as the particles are leached.