EP4232488A1 - Coating composition - Google Patents

Abstract

A curable coating composition comprising: a) at least one curable polysiloxane-based binder; b) an acrylic polymer; and c) a crosslinking agent; wherein said acrylic polymer comprises amino, hydroxyl and carboxylic acid functional groups.

Description

Coating composition

The present invention relates to curable anticorrosive coating compositions, more specifically to coating compositions comprising at least one curable polysiloxane, a crosslinking agent and an acrylic polymer having certain functional groups. The invention further relates to substrates coated with the coating compositions, and the use of the coating compositions to coat substrates.

Background

Approximately 85% of all steel produced is carbon steel, and it is prone to corrosion over time if no measures have been taken to protect the steel. The most common method to protect steel structures from corroding is by application of traditional solvent based or water based organic coatings.

In low corrosive environments, alkyd-based coatings are typically used, as these are single component, surface tolerant and low-cost materials. They are generally however not well suited for use in highly corrosive environments because they do not give very good anticorrosive protection. In addition, their appearance is easily faded by UV exposure. Solvent-based alkyds are also high in VOC.

In order to obtain better corrosion resistance, BPA-epoxies are sometimes used instead of alkyd-based coatings. However, these coatings generally require a topcoat in order to maintain a nice colour and gloss during the lifetime of the object due to the rapid degradation of epoxies when exposed to UV irradiation.

In addition, BPA-epoxy coatings are generally two-component coatings i.e. requiring mixing of separately provided components before application. The complexity of 2-component coatings demands some skills from the paint applicator. Unskilled applicators are often the cause of paint failures due to wrong mixing of the components, use of wrong coatings etc. Single component coatings (i.e. which are curable without the addition of further components to the composition before application) on the other hand are easier to handle and reduce the chance of paint failure due to component mixing issues and pot life issues.

WO1 993/013179A1 describes coating compositions comprising a polymer (A) carrying pendant and/or terminal curable functional groups, at least a major proportion of the repeating units in the polymer of (A) being other than siloxane units, and (B) a curable organohydrogen polysiloxane or polydiorganosiloxane, the curable functional groups in component (A) being capable of undergoing a condensation curing reaction with component (B). There is however no disclosure of a coating composition comprising a polysiloxane and an acrylic polymer having amino, hydroxyl and carboxylic acid functional groups.

W0 1998/023691 A1 describes a curable coating composition having a binder comprising a compound or polymer (A) containing at least one primary or secondary amine group, a compound or polymer (B) containing at least one ethylenically unsaturated double bond activated by an adjacent electronwithdrawing group, and a polymer (C) containing at least two silicon-bonded alkoxy groups. There is however no disclosure of a coating composition comprising an acrylic polymer having amino, hydroxyl and carboxylic acid functional groups.

W02004/067576A2 describes an ambient temperature curing coating composition comprising a certain branched alkoxy-functional polysiloxane, a catalyst, and a certain acrylic polymer. The acrylic polymer is substantially free of functional groups that can react with the polysiloxane or with the catalyst in the coating composition. There is however no disclosure of the use of an acrylic polymer having amino, hydroxyl and carboxylic acid functional groups.

There thus remains a need for a new curable coating composition having good corrosion resistance and weathering resistance (e.g. having good adhesion in both low and high humidity environments). It would also be desirable for any such coating not to require the use of an additional topcoat i.e. to be suitable for use as both a primer and a topcoat, in order to reduce the number of coats needed thus saving time and money and reducing the number of products in the paint locker. Good storage stability and low VOC are also desirable, as is fast and reliable curing (in both low and high humidity environments).

The present inventors have now provided a coating composition that solves the afore-mentioned problems. Specifically, the inventors have established that the use of an acrylic polymer comprising amino, hydroxyl and carboxylic acid functional groups in a coating composition additionally comprising at least one curable polysiloxane and a crosslinking agent provides a coating having excellent corrosion resistance, adhesion and weathering resistance. Surprisingly, the inventors have also established that these properties are significantly improved when compared to the properties of a comparative coating which was prepared using an acrylic polymer comprising none, or only one or two, of said functional groups. Without wishing to be bound by theory, there thus appears to be some kind of synergistic effect associated with the presence of all three functional groups on the properties of the composition.

In addition to having excellent corrosion resistance, adhesion and weathering resistance, the coating compositions according to the present invention are curable single component compositions (i.e. that don’t require mixing with additional components before application) and can be used both as an anticorrosive primer and topcoat, which reduces the complexity for the applicator and the number of coats needed. The coating also has good storage stability, low VOC, and offers fast and reliable curing in both low and high humidity environments.

Summary of the Invention

In one aspect, the present invention relates to a curable coating composition comprising: a) at least one curable polysiloxane-based binder; b) an acrylic polymer; and c) a crosslinking agent; wherein said acrylic polymer comprises amino, hydroxyl and carboxylic acid functional groups.

In a further aspect, the present invention relates to a substrate coated with a coating composition according to any aspect described herein, optionally wherein the coating composition is cured.

In a still further aspect, the present invention relates to the use of a coating composition according to any aspect described herein to coat a substrate, preferably wherein the coating composition is applied to the substrate as a primer and/or topcoat.

In a further aspect the invention provides the use of an acrylic polymer comprising amino, hydroxyl and carboxylic acid functional groups in an anticorrosive coating composition.

In a further aspect the invention provides the use of an acrylic polymer comprising amino, hydroxyl and carboxylic acid functional groups to prepare a single component curable coating composition. In a further aspect, the present invention relates to a substrate coated with one layer only, said layer comprising a coating composition according to any aspect described herein, optionally wherein the coating composition is cured.

Definitions

The following terminology is used throughout unless context allows otherwise.

The term acrylic polymer encompasses both acrylate and acrylamide polymers. Such acrylic polymers may be methacrylics or acrylics.

The term (meth)acrylic means acrylic or methacrylic. Thus the term (meth)acrylate covers methacrylate or acrylate.

Percentages of components may be calculated as dry solids unless otherwise stated. This means that the weight contribution of any solvents in the coating composition is ignored.

As used herein the term “polysiloxane” refers to a polymer comprising siloxane, i.e. -Si-O- repeat units.

As used herein the term “polysiloxane-based binder” refers to a polysiloxane that comprises at least 30 wt%, preferably at least 50 wt% and more preferably at least 70 wt% repeat units comprising the motif -Si-O-, based on the total weight of the polysiloxane. Polysiloxane-based binders may comprise up to 99.99 wt% repeat units comprising the motif -Si-O-, based on the total weight of the polymer. The repeat units, -Si-O- may be connected in a single sequence or alternatively may be interrupted by non-siloxane parts, e.g. organic-based parts.

As used herein the term “alkyl” refers to saturated, straight chained, branched or cyclic groups.

As used herein the term “cycloalkyl” refers to a cyclic alkyl group.

As used herein the term “alkylene” refers to a bivalent alkyl group.

As used herein the term “alkenyl” refers to unsaturated, straight chained, branched or cyclic groups.

As used herein the term “aryl” refers to a group comprising at least one aromatic ring. The term aryl encompasses fused ring systems wherein one or more aromatic ring is fused to a cycloalkyl ring. An example of an aryl group is phenyl, i.e. CeHs. As used herein the term "substituted" refers to a group wherein one or more, for example up to 6, more particularly 1 , 2, 3, 4, 5 or 6, of the hydrogen atoms in the group are replaced independently of each other by the corresponding number of the described substituents.

As used herein the term “arylalkyl” group refers to groups wherein the bond to the Si is via the alkyl portion.

As used herein the term “volatile organic compound (VOC)” refers to a compound having a boiling point of 250 °C or less.

Detailed Description of the Invention

The present invention relates to curable anticorrosive coating compositions, more specifically to coating compositions comprising at least one curable polysiloxane-based binder, a crosslinking agent and an acrylic polymer having certain functional groups.

Polysiloxane-based binder - component a)

The coating compositions of the present invention comprise at least one curable polysiloxane-based binder. The polysiloxane-based binder present in the coating composition of the present invention can generally be any curable polysiloxane. As used herein when referring to the polysiloxane, the term “curable” means that the polysiloxane comprises functional groups that enable a crosslinking reaction to take place either directly between polysiloxane molecules or via a crosslinking agent. Preferably the polysiloxane-based binder is moisture curable.

The polysiloxane-based binder is preferably an organopolysiloxane with terminal and/or pendant curing-reactive functional groups. A minimum of two curing-reactive functional groups per molecule is preferred. Ideally, all curing- reactive functional groups are the same. Preferably the coating composition comprises a polysiloxane having a functionality of more than two, preferably more than 3. Examples of curing-reactive functional groups are silanol, alkoxy, acetoxy, enoxy, ketoxime, alcohol, amine, epoxy and/or isocyanate, such as silanol, alkoxy, acetoxy, enoxy, ketoxime, alcohol, amine, and/or epoxy. Preferred curing-reactive functional groups are selected from silanol, alkoxy or acetoxy groups. Alkoxyfunctional polysiloxanes, especially methoxy-functional polysiloxanes are particularly preferred. As used herein, the term “alkoxy-functional polysiloxane” includes but is not limited to alkoxy-silyl-functional polysiloxanes. Optionally the polysiloxane-based binder comprises more than one type of curing-reactive functional group. Preferably the at least one polysiloxane-based binder comprises a single type of curing-reactive functional group.

The curing reaction is typically a condensation cure reaction. The polysiloxane-based binder optionally comprises more than one type of curing- reactive group and may be cured, for example, via both condensation cure and amine/epoxy curing.

The polysiloxane-based binder present in the coating compositions of the present invention typically comprises at least 30 wt% polysiloxane parts, preferably more than 50 wt% polysiloxane parts and still more preferably more than 70 wt% polysiloxane parts such as 99.99 wt% polysiloxane parts or more.

The polysiloxane parts are defined as repeat units comprising the motif -Si- O- based on the total weight of the at least one polysiloxane. The wt% of polysiloxane parts can be determined based on the stoichiometric wt ratio of starting materials in the polysiloxane synthesis. Alternatively, the polysiloxane content can be determined using analytical techniques such as IR or NMR. Information about the wt.% polysiloxane parts in a commercially available polysiloxane is easily obtainable from the supplier.

It is to be understood that the polysiloxane-based binder can comprise a polysiloxane consisting of a single repeating sequence of siloxane units or be interrupted by non-siloxane parts, e.g. organic parts.

The organic parts may comprise, for example, alkylene, arylene, poly(alkylene oxide), amide, thioether or combinations thereof, preferably the organic parts may comprise, for example, alkylene, arylene, poly(alkylene oxide), amide, or combinations thereof.

In one embodiment the polysiloxane-based binder comprises alkoxy (e.g. methoxy) functional groups in a terminal and/or pendant position. The use of a polysiloxane-based binder comprising alkoxy (e.g. methoxy) functional groups in a terminal position is preferred. In a preferred embodiment, the at least one polysiloxane-based binder is a linear or branched alkoxy-functional polysiloxane- based binder, preferably a linear or branched methoxy-functional polysiloxane- based binder, preferably a branched methoxy-functional polysiloxane-based binder. The coating composition may comprise only one type of polysiloxane-based binder or may comprise a mixture of different polysiloxane-based binders. In one embodiment, component a) of the coating composition comprises a mixture of two or more polysiloxanes-based binders, preferably a mixture of a first branched or linear alkoxy-functional polysiloxane-based binder and a second branched or linear alkoxy-functional polysiloxane-based binder having the same or lower Mw than the first polysiloxane-based binder.

In a preferred embodiment, the coating composition comprises a branched polysiloxane-based binder, more preferably a branched alkoxy-functional polysiloxane based binder. A branched methoxy-functional polysiloxane based binder is most preferred. By branched is meant that the polysiloxane chain is branched. In one preferred embodiment the branched polysiloxane-based binder comprises cage-like polysiloxane structures.

In a most preferred embodiment, the coating composition comprises a branched polysiloxane-based binder comprising methyl, phenyl and methoxy groups. In one embodiment, the coating composition comprises a first branched alkoxy-functional polysiloxane-based binder and a second branched alkoxyfunctional polysiloxane-based binder having the same or lower Mw than the first polysiloxane-based binder,

A preferred curable polysiloxane-based binder present in the coating compositions of the present invention is represented by formula (D1) below: wherein each R¹ is independently selected from a hydroxyl group, Ci-6-alkoxy group, Ci-6-hydroxyl group, Ci-6-epoxy containing group, Ci-e amine group, C1.10 alkyl, Ce- aryl, C7-10 alkylaryl or O-Si(R⁵)s-z (R⁶)z each R² is independently selected from C1.10 alkyl, Ce- aryl, C7-10 alkylaryl or C1.6 alkyl substituted by poly(alkylene oxide) and/or a group as described for R¹; each R³ and R⁴ is independently selected from C1.10 alkyl, Ce- aryl, C7-10 alkylaryl or Ci-e alkyl substituted by poly(alkylene oxide); each R⁵ is independently a hydrolysable group such as Ci-e alkoxy group, an acetoxy group, an enoxy group or ketoxy group; each R⁶ is independently selected from an unsubstituted or substituted Ci-e alkyl group; z is 0 or an integer from 1-2; x is an integer of at least 2; y is 0 or an integer of at least 1.

A more preferred curable polysiloxane-based binder present in the coating compositions of the present invention is represented by formula (DT) below: wherein each R¹ is independently selected from a hydroxyl group, Ci-6-alkoxy group, Ci-6-hydroxyl group, Ci-6-epoxy containing group, Ci-e amine group, or O-Si(R⁵)s-z (R⁶)z each R² is independently selected from Ci-w alkyl, Ce- aryl, C7-10 alkylaryl or C1.6 alkyl substituted by poly(alkylene oxide) and/or a group as described for R¹; each R³ and R⁴ is independently selected from C1.10 alkyl, Ce- aryl, C7-10 alkylaryl or Ci-e alkyl substituted by poly(alkylene oxide); each R⁵ is independently a hydrolysable group such as Ci-e alkoxy group, an acetoxy group, an enoxy group or ketoxy group; each R⁶ is independently selected from an unsubstituted or substituted Ci-e alkyl group; z is 0 or an integer from 1-2; x is an integer of at least 2; y is 0 or an integer of at least 1.

It is preferred if formula D1 or DT contains a minimum of at least two curing- reactive functional groups per molecule.

Preferably R¹ is selected from a hydroxyl group and O-Si(R⁵)3-z(R⁶)z, wherein R⁵ is a Ci-Ce alkoxy group, R⁶ is Ci-e alkyl and z is 0 or an integer from 1-2. More preferably R¹ is selected from a hydroxyl group and O-Si(R⁵)3-z(R⁶)z, wherein R⁵ is a C1-C3 alkoxy group, R⁶ is C1.3 alkyl and z is 0 or an integer from 1-2. Most preferably R¹ is O-Si(R⁵)3-z(R⁶)z, wherein R⁵ is a C1-C3 alkoxy group, R⁶ is C1.3 alkyl and z is 0 or an integer from 1-2.

Preferably R² is a Ci- 10 alkyl group, Ce- aryl, C7-10 alkylaryl or O-Si(R⁵)3-z (R⁶)z

Preferably R³ is a Ci- 10 alkyl group or Ce- aryl. More preferably R³ is a C1.4 alkyl group or a Ce aryl group, still more preferably a C1-2 alkyl group or a Ce aryl group, and yet more preferably a methyl group or a phenyl group.

Preferably R⁴ is a Ci-w alkyl group or Ce- aryl. More preferably R³ is a C1.4 alkyl group or a Ce aryl group, still more preferably a C1.2 alkyl group or a Ce aryl group, and yet more preferably a methyl group or a phenyl group.

The weight average molecular weight (Mw) of the at least one polysiloxane- based binder present in the coating compositions of the present invention is typically in the range of 200 to 50,000 g/mol, preferably 200 to 10,000, more preferably 400 to 5000 g/mol, most preferably 500 to 2000 g/mol.

Preferred coating compositions of the present invention comprise 10-95 wt% component a), more preferably 20 wt% or more, such as 20-80 wt%, e.g. 20 to 60 wt%, still more preferably 25-60 wt%, based on the total dry weight of the composition. In one particularly preferred option the coating composition comprises 20 to 40 wt%, preferably 25 to 40 wt.% of component a) based on the total dry weight of the coating composition. In one particularly preferred option the coating composition comprises 28 to 35 wt.% of component a) based on the total dry weight of the coating composition.

When including the contribution of solvents to the overall weight of the composition, the coating compositions of the present invention typically comprise 0.1-50 wt% component a), preferably 10-40 wt%, more preferably 20-40 wt% based on the total weight of the composition. Greater than 30 wt.% component a) is particularly preferred.

It is possible to use a blend of polysiloxane-based binders. In that scenario, these percentages refer to the combined weight of all polysiloxane-based binders present.

Where the coating composition comprises a mixture of two or more polysiloxane-based binders, e.g. a mixture of a first polysiloxane-based binder and a second polysiloxane-based binder, the weight ratio of the first polysiloxane-based binder to the second and optional further polysiloxane-based binders is typically in the range of 10:1 to 1:1, preferably 5:1 to 1:1 , such as 5:1 to 2:1.

In one embodiment, there is a large difference in viscosity between the two polysiloxane-based binders. For example, a second polysiloxane-based binder may have a viscosity of 5 to 30 cSt, preferably 8 to 20 cSt, and the first polysiloxane-based binder may have a viscosity of 70 to 250 cSt, preferably 90 to 180 cSt.

Suitable polysiloxane-based binders for use in the coating composition of the present invention are commercially available. Representative commercially available polysiloxane-based binders include REN 50 and REN 80 from Wacker, Silikophen P50X and Silikophen P80X from Evonik. In a preferred embodiment, the composition comprises a polysiloxane-based binder selected from Dow Corning 3074, Dow Corning 3037, Silres IC 232, SY231 , SY550 and MSE 100.

Acrylic polymer

The coating compositions of the present invention comprise an acrylic polymer comprising amino, hydroxyl and carboxylic acid functional groups. As discussed above, the inventors have established that a coating composition formulated with an acrylic polymer having each of these three groups has surprisingly improved properties in terms of adhesion, weathering resistance and corrosion resistance.

As used herein, the term “amino” means any group having the structure - NX¹X², wherein each of X¹, and X² is independently selected from H or a linear or branched Ci-Ce alkyl group. The amino group may therefore be a primary, secondary or tertiary amine group, preferably a secondary or tertiary amine.

The term “hydroxyl” has its ordinary meaning in the art i.e. -OH.

Similarly, the term “carboxylic acid” refers to a group having the structure - COOH. In solution, the carboxylic acid group may however be deprotonated, or be present as a salt e.g. -COONa.

The functionality of the acrylic polymer may be provided by pendant groups and/or terminal groups. In a preferred embodiment the functionality is provided by pendant groups i.e. of the monomers themselves. Preferably, the acrylic polymer is a copolymer and comprises a monomer unit comprising an amino group, a monomer unit comprising a hydroxyl group, and a monomer unit comprising a carboxylic acid group.

In a preferred embodiment, the acrylic polymer comprises a monomer residue unit having the structural formula (A): wherein X is NH or O, preferably O;

R¹’ is H or Me; each R²’ is independently selected from H, C1-C4 linear or branched alkyl, preferably C1-C2 alkyl; and

L¹’ is a linear or branched Ci-Ce alkyl linker, preferably a linear C1-C4 alkyl linker.

The monomer unit can be derived from amino functional monomers such as 2-(dimethylamino)ethyl (meth)acrylate, 3-(dimethylamino)propyl (meth)acrylate, 2- (diethylamino)ethyl (meth)acrylate, 2-(diisopropylamino)ethyl (meth)acrylate, 2-(tert- butylamino)ethyl (meth)acrylate, N,N-dimethyl(meth)acrylamide, N,N-diethyl (meth)acrylamide, N,N-diisopropyl (meth)acrylamide and N-(3- (dimethylamino)propyl) (meth)acrylamide.

In a preferred embodiment, the monomer unit has a structural formula (A) and comprises a tertiary amine group e.g. -NMe2 or -NEt2. In a most preferred embodiment, the monomer unit is derived from the polymerisation of 2- (dimethylamino)ethyl (meth)acrylate.

In a preferred embodiment, the acrylic polymer comprises a monomer residue unit having the structural formula (B): L²’ is a linear or branched Ci-Ce alkyl linker, preferably a linear C1-C4 alkyl linker. One or more of the carbon atoms in the alkyl linker L²’ may be substituted with one or more hydroxyl groups.

The monomer unit can be derived from hydroxyl functional monomers such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxy-1 -methylethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate and hydroxyisobutyl (meth)acrylate.

In a most preferred embodiment, the monomer unit is derived from 2- hydroxyethyl (meth)acrylate.

In a preferred embodiment, the acrylic polymer comprises a monomer residue unit having the structural formula (C): wherein R⁴’ is H or Me;

L³’ is a linear or branched C1-C4 alkyl linker, preferably a linear C1-C2 alkyl linker; and n is an integer from 0 to 1 , preferably 0.

The monomer unit can be derived from carboxylic acid functional monomers such as acrylic acid, 2-carboxyethyl acrylate, methacrylic acid and carboxymethyl methacrylate:

Preferably the carboxylic acid functionality is provided by the use of methacrylic acid as a monomer.

In a preferred embodiment, the acrylic polymer comprises all three monomer units (A) to (C). Preferably, the acrylic polymer additionally comprises at least one monomer unit not comprising an amino, hydroxyl, or carboxylic acid functionality. Preferred are monomer units derived from monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, tert-butyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, 2-propylheptyl (meth)acrylate and isodecyl (meth)acrylate.

In one embodiment, the acrylic polymer comprises a monomer unit derived from the polymerisation of a linear or branched Ci-Ce alkyl (meth)acrylate monomer, e.g. n-butyl (meth)acrylate, or methyl (meth)acrylate.

In one embodiment, the acrylic polymer comprises 50 wt.% or more, preferably 60 wt.% or more, more preferably 70 wt.% or more, most preferably 80 wt.% or more monomer units not comprising amino, hydroxyl, or carboxylic acid functionality. Said monomer units not comprising amino, hydroxyl, or carboxylic acid functionality may in one embodiment be derived from the polymerisation of a linear or branched Ci-Ce alkyl (meth)acrylate monomer, e.g. n-butyl (meth)acrylate.

In one embodiment, the acrylic polymer comprises 1 to 50 wt.% of monomer units comprising amino, hydroxyl or carboxylic acid functional groups, preferably 1 to 40 wt.%, more preferably 2 to 30 wt.%, most preferably 5 to 25 wt.%. In a preferred embodiment, the acrylic polymer comprises 25 wt.% or less monomer units comprising amino, hydroxyl or carboxylic acid functional groups, more preferably 20 wt.% or less.

In one embodiment, the acrylic polymer comprises 0.1 to 20 wt.% of a monomer unit comprising an amino functional group, preferably 0.5 to 15 wt.%, more preferably 1.0 to 10 wt.%.

In one embodiment, the acrylic polymer comprises 0.1 to 20 wt.% of a monomer unit comprising a hydroxyl functional group, preferably 0.5 to 15 wt.%, more preferably 1.0 to 10 wt.%.

In one embodiment, the acrylic polymer comprises 0.05 to 10 wt.% of a monomer unit comprising a carboxylic acid functional group, preferably 0.1 to 10 wt.%, more preferably 0.5 to 5.0 wt.%.

In one embodiment the acrylic polymer has a glass transition temperature (Tg) of at least 0°C, preferably 10°C or more, e.g. 10-25°C, such as 15-25°C. In one embodiment, the acrylic polymer has a Tg greater than 15°C. In one embodiment, the acrylic polymer has glass transition temperature of less than 100°C, preferably less than 50°C. The glass transition temperature of the polymer can be measured according to ASTM method E1356-08.

In one embodiment, the acrylic polymer has a viscosity of 1000 cP or more, preferably 5000 cP or more, more preferably 10,000 cP or more.

In one embodiment, the acrylic polymer has a non-volatile matter content of greater than 50 wt.%, preferably greater than 60 wt.%, more preferably greater than 70 wt.%.

In one embodiment, the acrylic polymer has a weight average molecular weight (Mw) of at least 5000 g/mol, such as at least 7500 g/mol, preferably at least 10,000 g/mol. In one embodiment, the acrylic polymer has a weight average molecular weight (Mw) of up to 20,000 g/mol, such as up to 15,000 g/mol.

In one embodiment, the acrylic polymer has a number average molecular weight (Mn) of at least 3000 g/mol, such as at least 4500 g/mol, preferably at least 5000 g/mol. In one embodiment, the acrylic polymer has a number average molecular weight (Mn) of up to 15000 g/mol, such as up to 10000 g/mol.

The polydispersity index (PDI) of the acrylic polymer (defined as Mw/Mn) is preferably is the range of 1 to 10, preferably 2 to 5.

Typically the acrylic polymer is present in the coating composition in an amount of 0.1 to 30 wt.% of the total dry weight of the coating composition, preferably 0.1 to 25 wt%, more preferably 1 to 20 wt.%, most preferably 2 to 15 wt.% based on the total dry weight of the composition. In one particularly preferred option the acrylic polymer is present in 3 to 12 wt.% based on the total dry weight of the composition. In one particularly preferred option the acrylic polymer is present in 6 to 15 wt.% based on the total dry weight of the composition.

The acrylic polymer may be prepared using polymerization reactions known in the art. The polymer may, for example, be obtained by polymerizing a monomer mixture in the presence of a polymerization initiator and optionally a chain transfer agent by any of various methods such as solution polymerization, bulk polymerization, emulsion polymerization, dispersion polymerization and suspension polymerization in a conventional way or by controlled polymerization techniques. In preparing a coating composition using this polymer, the polymer is preferably diluted with an organic solvent to give a polymer solution having an appropriate viscosity. From this standpoint, it is desirable to employ solution polymerization.

Examples of suitable initiators for free-radical polymerization include azo compounds such as dimethyl 2,2’-azobis(2-methylpropionate), 2,2'-azobis(2- methylbutyronitrile), 2,2'-azobis(isobutyronitrile), and 1,1 - azobis(cyclohexanecarbonitrile); and peroxides such as tert-amyl peroxypivalate, tert-butyl peroxypivalate tert-amyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2- ethylhexanoate, 1 ,1,3,3-tetramethylbutyl peroxy-2-ethylhexanoate, tert-butyl peroxydiethylacetate, tert-butyl peroxyisobutyrate, tert-butyl peroxybenozate, 1,1- di(tert-amyl peroxy)cyclohexane, tert-amylperoxy 2-ethylhexyl carbonate, tertbutylperoxy isopropyl carbonate, tert-butylperoxy 2-ethylhexyl carbonate, polyether poly-tert-butylperoxy carbonate, di-tert-butyl peroxide, dibenzoyl peroxide, and 1,1- bis(tert-amylperoxy)cyclohexane. These compounds are used alone or as a mixture of two or more thereof.

Examples of the organic solvent include aromatic hydrocarbons such as xylene, toluene, mesitylene; ketones such as methyl ethyl ketone, methyl isobutyl ketone, methyl amyl ketone, methyl isoamyl ketone, diisobutyl ketone, cyclopentanone, cyclohexanone; esters such as butyl acetate, tert-butyl acetate, amyl acetate, ethylene glycol methyl ether acetate, propyl propionate, n-butyl propionate, isobutyl isobutyrate; ethers such as ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dibutyl ether, dioxane, tetrahydrofuran, alcohols such as n-butanol, isobutanol, methyl isobutyl carbinol, benzyl alcohol; ether alcohols such as butoxyethanol, 1-methoxy-2-propanol; terpenes such as limonene; aliphatic hydrocarbons such as white spirit; and optionally a mixture of two or more solvents. These compounds may be used alone or as a mixture of two or more thereof. Preferred are mixtures of aromatic hydrocarbons and one or more solvents selected from ketones, esters, ethers, alcohols and ether alcohols. A mixture of xylene and n-butanol is particularly preferred.

Crosslinking agent

In addition to the polysiloxane-based binder and acrylic polymer, the coating compositions of the present invention comprise a crosslinking agent. The crosslinking agent suitably reacts with the curing-reactive functional groups in the polysiloxane-based binder to form a crosslinked coating when exposed to moisture. The crosslinking agent may also suitably act as a base catalyst for the hydrolysation/condensation reaction.

The skilled person will appreciate that the appropriate crosslinking agents are chosen depending on the type of curing-reactive functional groups present in the polysiloxane-based binder. If the curing-reactive functional groups are silanol or alkoxy, a preferred crosslinking agent is an organosilicon compound represented by the general formula shown below, a partial hydrolysis-condensation product thereof, or a mixture of the two:

Rd-Si-K4-d wherein each R is independently selected from an unsubstituted or substituted monovalent hydrocarbon group of 1 to 6 carbon atoms or a Ci-e alkyl substituted by poly(alkylene oxide). each K is independently selected from a hydrolysable group such as an alkoxy group; and d is 0, 1 or 2, more preferably 0 or 1.

Preferred crosslinkers of this type include tetraethoxysilane, vinyltris(methylethyloximo)silane, methyltris(methylethyloximo)silane, vinyltrimethoxysilane, methyltrimethoxysilane and vinyltriisopropenoxysilane as well as hydrolysis-condensation products thereof. Suitable crosslinking agents are commercially available, such as Silcate TES-40 WN from Wacker and Dynasylan A from Evonik.

If the curing-reactive functional groups are amine, epoxy or isocyanate, the curing agents are preferably amine, sulfur or epoxy functional.

The crosslinking agents can also be dual crosslinking agents containing, for example, both amine/sulphur/epoxy/isocyanate and an alkoxysilane groups. Preferred dual crosslinking agents are represented by the general formula below: wherein

LL is independently selected from an unsubstituted or substituted monovalent hydrocarbon group of 1 to 6 carbon atoms; each M is independently selected from a hydrolysable group such as an alkoxy group; a is 0, 1 or 2, preferably 0 or 1 ; b an integer from 1 to 6; and Fn is an amine, epoxy, glycidyl ether, isocyanate or thiol group. Preferably Fn is an amine, epoxy, glycidyl ether or thiol group. More preferably Fn is an amine group. In a most preferred embodiment, the crosslinking agent is an aminosilane.

Examples of such dual crosslinking agents include 3- isocyanatopropyltrimethoxysilane, 3- isocyanatopropyltriethoxysilane, 3- aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, (3- glycidoxypropyl)trimethoxysilane, 3-mercaptopropyltrimethoxysilane. Preferred examples of dual crosslinking agents include 3-aminopropyltrimethoxysilane, 3- aminopropyltriethoxysilane, (3-glycidoxypropyl)trimethoxysilane, 3- mercaptopropyltrimethoxysilane. One particularly preferred crosslinking agent is 3- aminopropyltriethyoxysilane such as Dynasylan AMEO from Evonik.

In a most preferred embodiment, the crosslinking agent is an organosilane, preferably an aminosilane, such as an aminotrialkoxysilane, e.g. aminoalkyltrialkoxysilane. If the crosslinking agent is an aminosilane, it is preferred if the amino group is primary. The aminosilane crosslinker is ideally of low Mw such as 400 g/mol or less, e.g. 50 to 300 g/mol.

Preferred options include 3-aminopropyltrimethoxysilane, 3- aminopropyltriethoxysilane or N-(2-aminoethyl)-3- aminopropylmethyldimethoxysilane.

Preferred crosslinking agents are represented by the general formula below: (LL)_a - Si - (CH₂)_b - Fn

(M)₃-a wherein

LL is an unsubstituted Ci-Ce alkyl group such as methyl; each M is an alkoxy group such as C1.4 alkoxy group; a is 0 or 1 ; b an integer from 1 to 6; and

Fn is an amine, epoxy, glycidyl ether or thiol group.

In one embodiment, the crosslinking agent does not contain an isocyanate group. It is therefore also preferred if the curable polysiloxane binder is free of isocyanate groups. It is therefore also preferred if the acrylic polymer is free of isocyanate groups. As used herein the term “free of isocyanate groups” means that such groups are completely absent. It is preferred if the curable coating composition of the invention as a whole is substantially free, such as free, of any isocyanate functional groups. By substantially free, is meant that the curable coating composition of the invention does not contain isocyanate functional groups that can react with the curable polysiloxane binder or acrylic polymer.

It is preferred if the curable coating composition of the invention should contain less than 0.5 wt% dry weight, such as less than 0.1 wt% dry weight, especially 0.05 wt% or less dry weight of compounds comprising NCO groups.

It is thus preferred if no component of the curable coating composition of the invention comprises an isocyanate group.

It is therefore also preferred if the cured coating composition of the invention does not contain a urethane motif, i.e_-(NCOO-).

Preferably the coating composition is supplied in curable form but kept dry in order to prevent premature curing.

The crosslinking agent is typically present in an amount of 0.1 to 10 wt.% of the coating composition, preferably in the range of 0.5 to 6 wt.%, more preferably in the range of 1 to 6 wt.% based on the total dry weight of the composition.

Viewed from another aspect the invention provides a curable coating composition comprising: a) 10 to 95 wt%, more preferably 25-60 wt%, based on the total dry weight of the composition of at least one curable polysiloxane based binder; b) 0.1 to 30 wt.% of the total dry weight of the coating composition, preferably 0.1 to 25 wt%, more preferably 1 to 20 wt.%, most preferably 2 to 15 wt.% of an acrylic polymer; and c) 0.1 to 10 wt.% of the total dry weight of the coating composition, preferably in the range of 0.5 to 6 wt.%, more preferably 1 to 6 wt.%. of a crosslinking agent; wherein said acrylic polymer comprises amino, hydroxyl and carboxylic acid functional groups.

Viewed from another aspect the invention provides a curable coating composition comprising: a) 20 to 60 wt%, based on the total dry weight of the composition of at least one curable polysiloxane based binder; b) an acrylic polymer; and c) a crosslinking agent; wherein said acrylic polymer comprises amino, hydroxyl and carboxylic acid functional groups.

Viewed from another aspect the invention provides a curable coating composition comprising: a) 25 to 40 wt%, based on the total dry weight of the composition of at least one curable polysiloxane based binder; b) 2 to 15 wt.% based on the total dry weight of the composition of an acrylic polymer; and c) 1 to 6 wt.%. based on the total weight of the composition of a crosslinking agent; wherein said acrylic polymer comprises amino, hydroxyl and carboxylic acid functional groups.

Viewed from another aspect the invention provides a curable coating composition comprising: a) at least one curable polysiloxane based binder; b) 0.1 to 25 wt%, more preferably 1 to 20 wt.%, most preferably 2 to 15 wt.% (dry weight) of an acrylic polymer; and c) a crosslinking agent; wherein said acrylic polymer comprises amino, hydroxyl and carboxylic acid functional groups.

Viewed from another aspect the invention provides a curable coating composition comprising: a) 20 to 60 wt% based on the total dry weight of the composition of at least one curable polysiloxane based binder; b) 2 to 15 wt.% based on the total dry weight of the composition of an acrylic polymer; and c) a crosslinking agent; wherein said acrylic polymer comprises amino, hydroxyl and carboxylic acid functional groups.

Viewed from another aspect the invention provides a curable coating composition comprising: a) 10 to 40 wt%, such as 20 to 40 wt%, of at least one curable polysiloxane-based binder; b) an acrylic polymer; and c) a crosslinking agent; wherein said acrylic polymer comprises amino, hydroxyl and carboxylic acid functional groups.

The weight ratio of (total) polysiloxane-based binder to acrylic polymer is preferably 1:1 or more. It is therefore preferred if there is more polysiloxane-based binder than acrylic polymer in dry weight terms. More preferably, there may be a weight ratio of polysiloxane-based binder to acrylic polymer of 5:1 to 3:2, such as 4:1 to 2:1.

Catalyst

In order to assist the curing process, the coating composition of the invention preferably comprises a catalyst. Representative examples of catalysts that can be used include transition metal compounds, metal salts and organometallic complexes of various metals, such as, tin, iron, lead, barium, cobalt, zinc, antimony, cadmium, manganese, chromium, nickel, aluminium, gallium, germanium, titanium, boron, lithium, potassium, bismuth and zirconium. The salts preferably are salts of long-chain carboxylic acids and/or chelates or organometal salts.

Examples of suitable tin-based catalysts include for example dibutyltin dilaurate, dibutyltin dioctoate, dibutyltin diacetate or dioctyltin dilaurate. Examples of commercially available tin catalysts include BNT-CAT 400 and BNT-CAT 500 from BNT Chemicals, FASCAT 4202 from PMC Organometallix and Metatin Katalysator 702 from DOW.

Examples of suitable zinc catalysts are zinc 2-ethylhexanoate, zinc naphthenate and zinc stearate. Examples of commercially available zinc catalysts include K-KAT XK-672 and K-KAT670 from King Industries and Borchi Kat 22 from Borchers.

Examples of suitable bismuth catalysts are organobismuth compounds such as bismuth 2-ethylhexanoate, bismuth octanoate and bismuth neodecanoate. Examples of commercial organobismuth catalysts are Borchi Kat 24 and Borchi Kat 315 from Borchers. K-KAT XK-651 from King Industries, Reaxis C739E50 from Reaxis and TIB KAT716 from TIB Chemicals.

Examples of suitable titanium catalysts are organotitanium catalysts such titanium naphthenate, tetrabutyl titanate, tetrakis(2-ethylhexyl)titanate, triethanolamine titanate, tetra(isopropenyloxy)-titanate, titanium tetrabutanolate, titanium tetrapropanolate, titanium tetraisopropanolate and chelated titanates such as diisopropyl bis(acetylacetonyl)titanate, diisopropyl bis(ethylacetoacetonyl)titanate and diisopropoxytitanium bis(ethylacetoacetate). Examples of suitable commercially available titanium catalysts are Tyzor IBAY from Dorf Ketal and TIB KAT 517 from TIB Chemicals.

Other suitable catalysts are iron catalysts such as iron stearate and iron 2- ethylhexanoate, lead catalysts such as lead octoate and lead 2-ethyloctoate cobalt catalysts such as cobalt-2-ethylhexanoate and cobalt naphthenate, manganese catalysts such as manganese 2-ethylhexanoate and zirconium catalysts such as zirconium naphthenate, tetrabutyl zirconate, tetrakis(2-ethylhexyl) zirconate, triethanolamine zirconate, tetra(isopropenyloxy)-zirconate, zirconium tetrabutanolate, zirconium tetrapropanolate and zirconium tetraisopropanolate.

Further suitable catalysts are zirconate esters.

The catalyst may also be an organic compound, such as triethylamine, guanidine, amidine, cyclic amines, tetramethylethylenediamine, 1,4- ethylenepiperazine and pentamethyldiethylenetriamine. Further examples include aminosilanes, such as 3-aminopropyltriethoxysilane and N,N-dibutylaminomethyl- triethoxysilane.

In one preferred embodiment the catalyst is a tin, titanium, bismuth, guanidine and/or amidine catalyst, more preferably a tin, titanium, guanidine and/or amidine catalyst.

Preferably the catalyst is present in the coating composition of the invention in an amount of 0.01 to 5 wt% based on the total dry weight of the coating composition, more preferably 0.05 to 4 wt%, most preferably 0.1 to 3 wt.%. In a most preferred embodiment, the catalyst is a tin based catalyst such as dioctyltin dilaurate, and is present in an amount of 0.1 to 3 wt.% based on the total dry weight of the coating composition.

Further optional components

The coating composition according to the present invention may optionally further comprise one or more components selected among other binders, inorganic or organic pigments, extenders and fillers, additives, solvents and thinners. The pigments may be inorganic pigments, organic pigments or a mixture thereof. Inorganic pigments are preferred. Examples of inorganic pigments include titanium dioxide, red iron oxide, yellow iron oxide, black iron oxide, zinc oxide, zinc sulfide, zinc phosphate, lithopone and graphite. Examples of organic pigments include carbon black, phthalocyanine blue, phthalocyanine green, napthol red and diketopyrrolopyrrole red. Pigments may optionally be surface treated to be more easily dispersed in the coating composition. Inorganic pigments, especially anticorrosive pigments such as zinc phosphate, are preferred.

Examples of extenders and fillers are minerals such as dolomite, plastorite, calcite, quartz, barite, magnesite, silica, nepheline syenite, wollastonite, talc, chlorite, mica, kaolin, pyrophyllite and feldspar; synthetic inorganic compounds such as calcium carbonate, magnesium carbonate, barium sulphate, calcium silicate and silica; polymeric and inorganic microspheres such as uncoated or coated hollow and solid glass beads, uncoated or coated hollow and solid ceramic beads, porous and compact beads of polymeric materials such as poly(methyl methacrylate), poly(methyl methacrylate-co-ethylene glycol di methacrylate), poly(styrene-co-ethylene glycol dimethacrylate), poly(styrene-co-divinylbenzene), polystyrene, poly(vinyl chloride). The use of feldspar as an extender in the coating compositions of the present invention is particularly preferred.

Preferably the total amount of extender and/or pigment present in the compositions of the invention is 1-60 wt%, more preferably 5-50 wt% and still more preferably 10-50 wt%, based on the total dry weight of the composition. The skilled person will appreciate that the extender and pigment content will vary depending on the particle size distribution, the particle shape, the surface morphology, the particle surface-resin affinity, the other components present and the end use of the coating composition. In a preferred embodiment, the coating composition comprises an extender in an amount of 30-50 wt.% of the dry weight of the composition. In a preferred embodiment, the coating composition comprises a pigment in an amount of 1-20 wt.% of the dry weight of the composition, preferably 1 to 10 wt.%.

Examples of further additives that can be added to the coating composition are reinforcing agents, rheology modifiers, wetting and dispersing agents, defoamers and plasticizers.

Examples of reinforcing agents are flakes and fibres. Fibres include natural and synthetic inorganic fibres and natural and synthetic organic fibres Examples of rheology modifiers include thixotropic agents, thickening agents and anti-settling agents. Representative examples of rheology modifiers are silicas such as fumed silicas, organo-modified clays, amide waxes, polyamide waxes, amide derivatives, polyethylene waxes, oxidised polyethylene waxes, hydrogenated castor oil wax, ethyl cellulose, aluminium stearates and mixtures thereof. Rheology modifiers that need activation may be added to the coating composition as is and activated during the paint production process or they can be added to the coating composition in a pre-activated form, e.g. solvent paste. Preferably rheology modifiers are each present in the composition of the invention in an amount of 0-5.0 wt%, more preferably 0.2-3.0 wt% and still more preferably 0.5-2.0 wt%, based on the total dry weight of the of the coating composition.

Examples of plasticizers are polymeric plasticizers, chlorinated paraffins, phthalates, phosphate esters, sulphonamides, adipates, epoxidised vegetable oils and sucrose acetate isobutyrate. Preferably plasticizers are present in the compositions of the invention in an amount of 0-10 wt%, more preferably 0.5-7 wt% and still more preferably 1-5 wt%, based on the total dry weight of the of the coating composition.

Dehydrating agents and stabilizers improve the storage stability of the coating compositions. The dehydrating agent is preferably a compound which removes moisture and water from the coating composition. It is also referred to as water scavenger or drying agent. The dehydrating agents may be hygroscopic materials that absorb water or bind water as crystal water. These are often referred to as desiccants. Examples of such compounds include anhydrous calcium sulphate, calcium sulphate hemihydrate, anhydrous magnesium sulphate, anhydrous sodium sulphate, anhydrous zinc sulphate, molecular sieves and zeolites. The dehydrating agents may also be compounds that chemically react with water. Examples of dehydrating agents that react with water include orthoesters such as trimethyl orthoformate, triethyl orthoformate, tripropyl orthoformate, triisopropyl orthoformate, tributyl orthoformate, trimethyl orthoacetate, triethyl orthoacetate tributyl orthoacetate and triethyl orthopropionate; ketals; acetals; enolethers; orthoborates such as trimethyl borate, triethyl borate, tripropyl borate, triisopropyl borate, tributyl borate and tri-terf-butyl borate; organosilanes such as trimethoxymethylsilane, triethoxymethylsilane, tetraethoxysilane, phenyltrimetoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane and ethyl polysilicate; oxazolidines such as 3-ethyl-2-methyl-2-(3-methylbutyl)-1,3-oxazolidine and 3-butyl-2-(1-ethylpentyl)-1,3-oxazolidine; and isocyanates, such as p- toluenesulfonyl isocyanate. As noted above however, it is preferred if isocyanate based compounds are avoided.

Stabilizers are preferably acid scavengers. Examples of stabilizers are carbodiimide compounds, such as bis(2,6-diisopropylphenyl)carbodiimide, bis(2- methylphenyl)carbodiimide, 1,3-di-p-tolylcarbodiimide and others as described in WO20 14064049.

Preferably the dehydrating agents and stabilizers are each present in the compositions of the invention in an amount of 0-5 wt%, more preferably 0.5-2.5 wt% and still more preferably 1.0-2.0 wt%, based on the total dry weight of the of the composition.

It is highly preferred if the coating composition contains a solvent. This solvent is preferably volatile and is preferably organic. Examples of organic solvents and thinners are aromatic hydrocarbons such as xylene, toluene, mesitylene; ketones such as methyl ethyl ketone, methyl propyl ketone, methyl isobutyl ketone, methyl isoamyl ketone, methyl amyl ketone, diisobutyl ketone, cyclopentanone, cyclohexanone; esters such as butyl acetate, terf-butyl acetate, amyl acetate, isoamyl acetate, propyl propionate, n-butyl propionate, isobutyl isobutyrate; ether esters such as ethylene glycol methyl ether acetate, ethyl 3-ethoxypropionate; ethers such as ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dibutyl ether, dioxane, tetrahydrofuran; alcohols such as n-butanol, isobutanol, methyl isobutyl carbinol, benzyl alcohol; ether alcohols such as butoxyethanol, 1- methoxy-2-propanol; terpenes such as limonene; aliphatic hydrocarbons such as white spirit; and optionally a mixture of two or more solvents and thinners.

Preferred solvents are aromatic solvents, especially xylene and mixtures of aromatic hydrocarbons.

The amount of solvent is preferably as low as possible. The solvent content may be up to 45 wt% of the composition, preferably up to 40 wt% of the composition, such as up to 35 wt% but may be as low as 15 wt% or less, e.g. 10 wt% or less. In one embodiment, the solvent is present in an amount of at least 1 wt.%, preferably at least 5 wt.% of the total composition. Again, the skilled person will appreciate that some raw materials comprise solvent and contribute to the total solvent content as specified above and that the solvent content will vary depending on the other components present and the end use of the coating composition. Alternatively, the coating can be dispersed in an organic non-solvent for the filmforming components in the coating composition or in an aqueous dispersion.

Coating compositions

The coating composition of the invention preferably has a solids content above 40 vol%, e.g. above 45 vol%, such as above 50 vol%, preferably above 60 vol%. In one embodiment, the coating composition has a solids content of up to 80 vol%, such as up to 70 vol%. The method used for measuring volume solid is based on ISO 3233.

Preferably the coating composition should have a content of volatile organic compounds (VOC) below 500 g/L, preferably below 420 g/L, more preferably below 400 g/L, e.g. below 380 g/L. In a most preferred embodiment, the VOC content is less than 210 g/L. VOC content can be calculated (ASTM D5201-01) or measured, e.g. as described in US EPA Method 24 or ISO 11890-2.

In a preferred embodiment, the coating composition is supplied as a single component composition. As used herein, the term “single component” means that there is no need for mixing the composition with additional components before application to a substrate e.g. to ensure curing. Alternatively, the coating composition may be supplied as a kit of parts, e.g. with the crosslinking agent supplied separately from the polysiloxane and/or acrylic polymer.

Applications

In one aspect, the invention relates to a substrate coated with the coating composition of the present invention. The coating composition of the invention can be applied to a whole or part of any object surface which is subject to corrosion/weathering. The substrate will typically be metallic, such as floors and railings on ships or the surface of a fixed marine object such as an oil platform. Steel substrates are particularly preferred.

Application of the coating composition can be accomplished by any convenient means, e.g. via painting (e.g. with brush or roller) or spraying the coating onto the substrate. The application of the coating can be achieved as conventionally known in the art. The coating composition can be applied directly on the substrate. The coating composition can also be applied on top of a coating layer such as a primer layer. It is preferred if the coating composition is applied directly on the substrate, e.g. direct to metal.

A top coat may be applied on top of the coating composition of the invention. It is preferred there is no top-coat layer. It is preferred if the coating composition is applied directly on to the substrate, e.g. direct to metal, and that there is no top-coat layer. In one aspect the invention relates to the use of the coating composition as a single layer applied directly on to the substrate. The layer of the coating composition of the invention may be the only layer present.

It will be appreciated that the coating composition of the invention may need to be applied in multiple coats in order to build a layer on the substrate of sufficient thickness. We regard the application of multiple coats of the same coating composition as giving rise to a single layer herein.

In a preferred embodiment, the coating composition of the invention is curable at room temperature, i.e. when exposed to moisture the composition will cure at the temperature in the environment in question without the application of heat. That might typically be in the range of 0 to 50°C. Preferably, curing occurs at less than 40 °C, more preferably at room temperature, i.e. in the range 12 to 35 °C. It will be understood that since the coating compositions of the invention are curable they may be referred to as curable coating compositions. In a preferred embodiment, the coating composition is moisture curable. In a preferred embodiment the coating compositions is cured at a humidity of 30 - 85 %.

The layer formed using the coating composition of the invention preferably has a dry film thickness of 40 to 400 pm, more preferably 80 to 175 pm, such as 100 to 150 pm. It will be appreciated that any layer can be laid down using single or multiple applications of the coating.

The invention will now be further described by reference to the following Examples.

Determination Methods

Polymer solution viscosity

The viscosity of the acrylic polymers was determined in accordance with ASTM D2196 Test Method A using a Brookfield DV-I viscometer with LV-2 or LV-4 spindle at 12 rpm. The polymer solutions were tempered to 23.0 °C ± 0.5 °C before the measurements.

Determination of non-volatile matter content of the acrylic polymer solutions The non-volatile matter content in the polymer solutions was determined in accordance with ISO 3251. A test sample of 0.5 g ± 0.1 g was taken out and dried in a ventilated oven at 150 °C for 30 minutes. The weight of the residual material was considered to be the non-volatile matter (NVM). The non-volatile matter content is expressed in weight percent. The value given is the average of three parallel measurements.

Determination of molecular weights

The polymers were characterised by Gel Permeation Chromatography (GPC) measurement. The molecular weight distribution (MWD) was determined using a Malvern Omnisec Resolve and Reveal system with two PLgel 5 pm Mixed-D columns from Agilent in series, tetrahydrofuran (THF) as eluent at a constant flow rate of 1 ml/min and with a refractive index (Rl) detector. The columns were calibrated using narrow polystyrene standards Polystyrene Medium EasiVials (4 ml) Red, Yellow and Green from Agilent. The column oven temperature and the detector oven temperature were 35 °C. The sample injection volume was 100 pl. The data were processed using Omnisec 5.1 software from Malvern.

Samples were prepared by dissolving an amount of polymer solution corresponding to 25 mg dry polymer in 5 ml THF. The samples were kept for minimum 3 hours at room temperature prior to sampling for the GPC measurements. Before analysis the samples were filtered through 0.45 pm Nylon filters. The weight-average molecular weight (Mw) and the polydispersity index (PDI), given as Mw/Mn, are reported in the tables.

Determination of glass transition temperature

The glass transition temperature (Tg) was obtained by Differential Scanning Calorimetry (DSC) measurements. The DSC measurements were performed on a TA Instruments DSC Q200. The sample was prepared by drawdown of the polymer solution on a glass panel using an applicator with 100 pm gap size. The glass panel was dried over night at room temperature and subseguently 24 hours at 50 °C in a ventilated heating cabinet. The dry polymer material was scraped off the glass panels and approx. 10 mg of the dry polymer material was transferred to an aluminium pan. The pan was sealed with a non-hermetic lid. The measurement was performed by running a heat-cool-heat procedure, within a temperature range from -50 °C to 120 °C, with a heating rate of 10 °C/min and cooling rate of 10 °C/min and using an empty pan as reference. The data were processed using Universal Analysis software from TA Instruments. The inflection point of the glass transition range, as defined in ASTM E1356-08, of the second heating is reported as the Tg of the polymers.

Examples: Synthesis of acrylic polymers

General procedure for preparation of acrylic polymers IP1 and CP1 to CP5

A quantity of solvent was charged to a temperature-controlled reaction vessel equipped with a stirrer, a condenser, a nitrogen inlet and a feed inlet. The reaction vessel was heated and maintained at the reaction temperature. A pre-mix of monomers and initiator was prepared. The pre-mix was charged to the reaction vessel at a constant rate over 2 hours under a nitrogen atmosphere. The reaction vessel was maintained at the reaction temperature for a further 2 hours. Finally, the reactor was cooled to room temperature.

The ingredients for preparing the copolymers and the polymerization conditions are listed in Table 1 below. All amounts are given in parts by weight. An overview of the functional groups in the synthesised polymers is listed in Table 2.

Table 1: Acrylic polymer synthesis

Table 2: Overview of functional groups in acrylic polymer

Preparation of coating compositions

The coating compositions were prepared using a high speed dissolver. The acrylic polymer was added after the grinding phase. Inventive coating composition IE1 and comparative coating compositions CE1-CE5 were prepared comprising the different acrylic polymers listed in Table 2. The components of the compositions are shown in Table 3.

5 Table 3: Coating composition components (in weight %).

¹Mw: 700 - 1500 Daltons, Vise: 8 - 20 cSt (25 °C), Methoxy content: 15-18 wt%, Methyl/phenyl ratio 0.25/1.0.

²DC3074 from Dow Corning, Mw: 1200 - 1700 Dalton, Vise.: 90 - 180 cSt (25°C), Methoxy content: 15 - 18 wt.%.

10

Weathering resistance testing

The coating compositions in Table 3 were applied using brush directly to steel plates and the weathering resistance of the coatings was tested in a humidity test.

15 The method is based on the standard ASTM D2247-11 “Standard Practice for Testing Water Resistance of Coatings in 100% Relative Humidity” (2011). The pull- off strength of the paint system was recorded, and the type of break assessed as described in ISO 4624:2016. The degree of blistering was assessed as described in

ISO 4628-2:2016. The results are shown in Table 4 below.

Table 4: Humidity resistance testing results.

The MPa-value obtained in the test and the type of fracture was reported. An evaluation of the nature of the fracture after the pull-off test is performed:

A is cohesive failure of substrate;

A/B is adhesive failure between substrate and first coat;

B is cohesive failure of first coat;

B/C is adhesive failure between first and second coats; n is cohesive failure of the nth coat of a multicoat system; n/m is adhesive failure between the nth coat and the mth coat of a multicoat system;

Y is cohesive failure of adhesive

Adhesive failure mean failure in the interphase between the coating and substrate- or between two layers of coating, where cohesive failure is the failure within one coating layer coating or within the substrate.

The blistering is evaluated by the quantity (density) and the size of the blistering, where 0 is no visible blistering, and 5 is the highest degree of blistering. The characterizing is based on ISO 4628. Humidity test results show that IE1 did best overall. This formulation is the only one that got no blistering (higher numbers represent more blistering) and got a cohesion break. This indicates that all three functional groups must be present to get the best performance in humidity test.

Salt spray exposure resistance testing

The coating compositions IE1 and CE3 in Table 3 were tested for their resistance to neutral salt spray exposure according to ISO 12944. The panels were prepared by applying the coating on 3 mm blast-cleaned steel panels (Sa 21 ) 150 mm x 75 mm. The film thickness for each coat was 150 microns WTF. 2 coats were applied where the second coat was applied 1 day after the first coat.

Testing was conducted in salt spray chamber for 1440 hours (C5-high marine, ISO 12944). Results are shown in Table 5 below.

Table 5: Neutral salt spray exposure testing results.

The salt spray results show that corrosion creep for IE1 is slightly better than CE3 (lower numbers indicate better resistance to corrosion creep). However, regarding blistering, IE1 is significantly better than CE3 (higher numbers indicate more extensive blistering).

Compositions comprising an acrylic polymer having amino, hydroxyl and carboxyl functionality have thus been shown to have better humidity and corrosion resistance than compositions comprising an acrylic polymer not having these functionalities.

Claims

- 33 - Claims

1 . A curable coating composition comprising: a) at least one curable polysiloxane-based binder; b) an acrylic polymer; and c) a crosslinking agent; wherein said acrylic polymer comprises amino, hydroxyl and carboxylic acid functional groups.

2. The coating composition of claim 1 , wherein the acrylic polymer comprises a monomer residue unit having the structural formula: wherein X is NH or O, preferably O; wherein R¹’ is H or Me; each R²’ is independently selected from H, C1-C4 linear or branched alkyl, preferably C1-C2 alkyl; and

L¹’ is a linear or branched Ci-Ce alkyl linker, preferably a linear C1-C4 alkyl linker.

3. The coating composition of claim 1 or 2, wherein the acrylic polymer comprises a monomer residue unit having the structural formula: wherein R³’ is H or Me; and - 34 -

L²’ is a linear or branched Ci-Ce alkyl linker, preferably a linear C1-C4 alkyl linker, optionally substituted with one or more hydroxyl groups.

4. The coating composition of any preceding claim, wherein the acrylic polymer comprises a monomer residue unit having the structural formula: wherein R⁴’ is H or Me;

L³’ is a linear or branched C1-C4 alkyl linker, preferably a linear C1-C2 alkyl linker; and n is an integer from 0 to 1, preferably 0.

5. The coating composition of any preceding claim, wherein the acrylic polymer comprises 1 to 50 wt.% of monomer units comprising amino, hydroxyl or carboxylic acid functional groups, preferably 1 to 40 wt.%, more preferably 2 to 30 wt.%, most preferably 5 to 25 wt.%

6. The coating composition of any preceding claim, wherein the at least one curable polysiloxane based binder is a linear or branched alkoxy-functional polysiloxane-based binder, preferably a linear or branched methoxyfunctional polysiloxane-based binder, preferably a branched methoxyfunctional polysiloxane-based binder, especially a branched methoxy methylphenylsiloxane.

7. The coating composition of any preceding claim, wherein the at least one curable polysiloxane based binder has the structural formula (DT): wherein each R¹ is independently selected from a hydroxyl group, Ci-6-alkoxy group, Ci-6-hydroxyl group, Ci-6-epoxy containing group, Ci-e amine group, or O-Si(R⁵)₃-z (R⁶)_z; each R² is independently selected from C1.10 alkyl, Ce- aryl, C7-10 alkylaryl or Ci-e alkyl substituted by poly(alkylene oxide) and/or a group as described for R¹; each R³ and R⁴ is independently selected from C1.10 alkyl, Ce- aryl, C7-10 alkylaryl or Ci-e alkyl substituted by poly(alkylene oxide); each R⁵ is independently a hydrolysable group such as Ci-e alkoxy group, an acetoxy group, an enoxy group or ketoxy group; each R⁶ is independently selected from an unsubstituted or substituted Ci-e alkyl group; z is 0 or an integer from 1-2; x is an integer of at least 2; y is 0 or an integer of at least 1. The coating composition of any preceding claim, wherein the at least one curable polysiloxane based binder has a weight average molecular weight (Mw) of 200 to 50,000 g/mol (determined by GPC), preferably 200 to 10,000 g/mol, more preferably 400 to 5,000 g/mol, most preferably 500 to 2,000 g/mol. The coating composition of any preceding claim, wherein the crosslinking agent is an organosilane, preferably of formula wherein

LL is independently selected from an unsubstituted or substituted monovalent hydrocarbon group of 1 to 6 carbon atoms; each M is independently selected from a hydrolysable group such as an alkoxy group; a is 0, 1 or 2, preferably 0 or 1 ; b an integer from 1 to 6; and

Fn is an amine, epoxy, glycidyl ether, or thiol group, such as 3- aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane or N-(2-aminoethyl)-3- aminopropylmethyldimethoxysilane.

10. The coating composition of any preceding claim, wherein component a) of the coating composition is present in an amount of 0.1 to 50 wt%, preferably 10 to 40 wt%, more preferably 20-40 wt%, based on the total weight of the composition.

11. The coating composition of any preceding claim, wherein component a) of the coating composition is present in an amount 10-95 wt%, more preferably 20-80 wt%, still more preferably 25-60 wt%, based on the total dry weight of the composition, especially 25 to 40 wt.% of component a) based on the total dry weight of the coating composition.

12. The coating composition of any preceding claim, wherein the acrylic polymer is present in an amount of 0.1 to 30 wt.% of the total dry weight of the coating composition, preferably 0.1 to 25 wt%, more preferably 1 to 20 wt.%, more preferably 2 to 15 wt.%, most preferred 3 -12 wt.%

13. The coating composition of any preceding claim, wherein the crosslinking agent is present in an amount of 0.1 to 10 wt.% of the total dry weight of the coating composition, preferably in the range of 0.5 to 6 wt.%, more preferably 1 to 6 wt.%.

14. The coating composition of any preceding claim, wherein the coating composition further comprises an anticorrosive pigment, preferably an inorganic pigment such as zinc phosphate. - 37 -

15. The coating composition of any preceding claim, further comprising a catalyst, preferably a transition metal catalyst, more preferably a tin-based catalyst such as dibutyltin dilaurate, dibutyltin dioctoate, dibutyltin diacetate or dioctyltin dilaurate.

16. The coating composition of any preceding claim having a solids content greater than 60 vol% and/or a volatile organic compounds (VOC) content below 21 Og/L (e.g. determined using ISO 11890-2).

17. The coating composition of any preceding claim being wherein the curable polysiloxane-based binder and the crosslinking agent are free of isocyanate groups.

18. The coating composition of any preceding claim being free of isocyanate functional groups.

19. The coating composition of any preceding claim wherein the weight ratio of curable polysiloxane based binder to acrylic polymer is 1 :1 or higher (i.e. the curable polysiloxane based binder is preferably in excess).

20. A curable coating composition as claimed in any preceding claim comprising: a) 20 to 60 wt%, based on the total dry weight of the composition of at least one curable polysiloxane based binder; b) an acrylic polymer; and c) a crosslinking agent; wherein said acrylic polymer comprises amino, hydroxyl and carboxylic acid functional groups.

21. A substrate coated with the coating composition as claimed in any preceding claim, optionally wherein the coating composition is cured.

22. A substrate as claimed in claim 21 coated with one layer only, said layer comprising a coating composition according to claim 1 to 20. - 38 -

23. Use of a coating composition as claimed in claim 1 to 20 to coat a substrate, preferably wherein the coating composition is applied to the substrate as a primer and/or topcoat.

24. Use of an acrylic polymer comprising amino, hydroxyl and carboxylic acid functional groups in an anticorrosive coating composition.