WO2020157150A1

WO2020157150A1 - Aqueous primer

Info

Publication number: WO2020157150A1
Application number: PCT/EP2020/052192
Authority: WO
Inventors: Yajun Zhu; Dongping LIU
Original assignee: Jotun A/S
Priority date: 2019-01-30
Filing date: 2020-01-29
Publication date: 2020-08-06
Also published as: CN113366065A; CN111500191A

Abstract

An aqueous primer composition comprising: (A) a polysiloxane sol; (B) an accelerator selected from BaSO4, Feldspar, Talc, mica, CaSO4, or kaolin; (C) Zn metal; and optionally (D) an organic Zn salt or zinc chloride.

Description

Aqueous primer

This invention relates to a new aqueous primer composition for application to substrates such as steel. In particular, the invention relates to a primer composition comprising a polysiloxane sol, a particular accelerator, optionally zinc metal and optionally a water-soluble organic zinc salt or zinc chloride. The primer of the invention is fast curing, abrasion resistant, has a low volatile organic content (VOC) and, importantly, can be supplied as a two component kit for easy mixing before application. The kit of the invention gives rise to a smooth, homogeneous blend on mixing without the formation of lumps or gels.

Shop primers are preferred. The primer can also be overcoated with further coating layers, without extensive pretreatment and without loss of intercoat adhesion.

Background

Organic solvent based primer coating systems are known. However, with ever-increasing environmental awareness, there is a strong desire to develop improved technology that would allow the use of water-based coating systems as primers and avoid organic solvent based solutions. In this regard, a leading water bom primer technology is based on alkali metal silicates. These systems cause a very high pH on the surface of the substrate which causes the next coating, usually an epoxy type of paint, to lose adhesion and blister when the system is exposed to water. Any steel primed with an alkali silicate shop primer has to either be removed completely or thoroughly washed repeatedly with fresh water before over coating to circumvent said issues.

Furthermore, alkali silicates require specialised production equipment and application equipment where all parts in contact with the wet paint have to be made out of non-metal materials.

There is a significant need therefore to develop new primers that are water borne but do not rely on alkali metal silicates. Some aqueous based shop primers are known. WO2017/129784 describes an aqueous shop primer comprising a polysilane sol, an accelerator, an anticorrosive pigment and microspheres.

One problem that has been identified with prior art aqueous primers is incomplete mixing of the components of the kit of parts in which the primer is supplied. This leads to the formation of lumps in the composition and these can block the spraying apparatus that is used to apply the primer composition to a substrate. Whilst this problem could be overcome with a special mixing apparatus, such a device is not normally available at the point of application of the composition to a substrate and that is where the mixing of the components occurs.

It is also important that the components of the primer are stable. The Zn metal component of the primer composition must be kept apart from the

polysiloxane sol component until application of the composition to a substrate is desired. One problem which has also been encountered is that the compositions that make up the kit of parts that is mixed to form the primer composition lack storage stability.

The present inventors have devised a water borne primer with very low VOC, that is water thinnable and that dries quickly to give water resistance. The primer also resists weathering, water, chemicals and solvents and can be applied by conventional equipment such as via airless spray. The kit from which the primer is prepared is storage stable and the components are readily mixed without complex equipment to give a homogeneous blend.

The primer is based on an aqueous polysiloxane binder in conjunction with an accelerator and Zn metal. It has been surprisingly found that the specific combination of components described in claim 1 gives rise to a composition that meets all the requirements for a primer, especially a shop primer, in particular in terms of mixing quality, storage stability, drying time, water and chemical resistance, abrasion resistance and which allows overcoating with new paint layers without the need to remove or repeatedly wash the primer.

The polysiloxane sol of use in the primer of the invention is not new and is described, inter alia, in US2011/0268899, WO2018202472 and US2014/0106176. These documents however, primarily consider the structure of the binder rather than target particular primer compositions that perform advantageously. The present inventors supplement the knowledge in these documents by identifying particular primers that are remarkably useful in the art, particularly as shop primers.

The invention in particular relates to shop primers. Shipping containers and other marine metal objects are typically assembled by welding together a number of individual metal components. In marine and industrial construction, it is usually desirable to pre-paint steel with a zinc-containing primer before fabrication, and many such coating compositions, known as shop primers or pre-construction primers, are known. To prevent corrosion during welding, the components are surface prepared (e.g. cleaned and roughened by, for example, shot-blasting) and then coated with shop primer compositions that provide temporary corrosion protection. Such compositions allow primed components to be welded through the shop primer layer without having to remove the primer coating near the weld.

After welding, the primer coating may be over-coated with a further primer layer and optionally, a topcoat to provide extended corrosion protection and the desired aesthetic appearance.

Shop primers can be organic or inorganic and can be supplied in water or in organic solvent. Some shop primer compositions include zinc powder in an organic resin such as an epoxy resin, an epoxy ester, a polyurethane, a polystyrene resin or a silicone resin. Coatings based on these organic binders are not well suited for coating steel that must ultimately be welded because the binder tends to decompose from the heat of the weld, resulting in pores in the weld seams. Primers according to the present invention may be particularly suitable as shop primers.

Summary of invention

Thus, viewed from one aspect the invention provides an aqueous primer composition comprising:

(A) a polysiloxane sol;

(B) an accelerator selected from BaSO₄, Feldspar, Talc, mica, CaSCri, or kaolin;

(C) Zn metal; and optionally (D) an organic Zn salt or zinc chloride.

Viewed from another aspect the invention provides an aqueous primer composition which is free of Zn metal comprising:

(A) a polysiloxane sol;

(B) an accelerator selected from BaSO₄, wollastonite or Feldspar; and optionally

(D) an organic Zn salt or zinc chloride.

Viewed from another aspect the invention provides an aqueous primer composition comprising:

(A) at least 5.0 wt%, such as 5 to 50 wt% of a polysiloxanesol;

(B) O.lto 50 wt% of an accelerator selected from BaSO₄, Feldspar, Talc, mica, CaSCri, or kaolin;

(C) 10 to 90 wt% of Zn metal; and optionally

(D) 0.1 to 5.0 wt% an organic Zn salt or zinc chloride.

(A) 0.5 to 35 wt% dry weight of a polysiloxanesol;

(B) 0.1 to 90 wt% dry weight of an accelerator selected from BaSO₄, Feldspar, Talc, mica, CaSCri, or kaolin;

(C) 10 to 95 wt% dry weight of Zn metal; and optionally

(D) 0.1 to 5.0 wt% dry weight an organic Zn salt or zinc chloride.

(A) 20 to 90 wt% of a polysiloxane sol;

(B) at least 10 wt% of an accelerator selected from BaSO₄, wollastonite and Feldspar; and optionally

(D) 0.1 to 5.0 wt% of an organic Zn salt or zinc chloride.

It is preferred if any primer composition of the invention is free of ZnO.

Viewed from another aspect the invention provides a kit of parts comprising:

(I) a first component comprising (A) a polysiloxane sol, (B) an accelerator as herein defined and optionally (D) an organic Zn salt or zinc chloride; and (II) a second component comprising Zn metal.

The parts of the kit are mixed shortly before application to a substrate, such as within one or two hours of application. It will be appreciated that the components (I) and (II) will be supplied separately for mixing.

Viewed from another aspect the invention provides a kit of parts comprising:

(I) a first component comprising 20 to 70 wt% of (A) a polysiloxane sol, 20 to 75 wt% of (B) an accelerator as herein defined and optionally 0.5 to 10 wt% of an organic Zn salt or zinc chloride;

(II) a second component comprising at least 60 wt% of Zn metal.

Viewed from another aspect the invention provides a kit of parts comprising:

(I) a first component comprising 0.5 to 70 wt% dry weight of (A) a polysiloxane sol, 20 to 95 wt% dry weight of (B) an accelerator as herein defined and optionally 0.5 to 10 wt% dry weight of an organic Zn salt or zinc chloride;

(II) a second component comprising at least 60 wt% dry weight of Zn metal.

Viewed from another aspect the invention provides a process for protecting a substrate from corrosion comprising applying to that substrate a primer composition as hereinbefore defined or a blend of components (I) and (II) as herein before defined and allowing said primer composition or blend to cure on said substrate.

Viewed from another aspect the invention provides a substrate coated with a cured primer composition or cured blend of components (I) and (II) as herein before defined.

Viewed from another aspect the invention provides the use of a primer composition as hereinbefore defined or a blend of components (I) and (II) as herein before defined to provide temporary corrosion protection to a substrate.

Detailed description of invention

The present invention relates to a primer composition which is suitable for application to a substrate to provide corrosive protection to said substrate, such as temporary corrosive protection. The primers of the invention can be applied by spraying and possess low VOC and can be thinned with water to achieve an ideal application viscosity. Nevertheless, the primers are fast drying and rapidly are able to withstand mechanical abrasion and treatment with chemicals, water and the like. They are easy to apply to a substrate and they resist sagging. The primers of the invention may be shop primers and are weldable producing a minimum of weld pores, weld spatter, weld smoke and back burning. As well as being corrosion resistant, the primers are capable of being overcoated with most types of paint without any extra pre-treatment of the primer coat (other than cleaning to remove detritus such as dust, salt and/or grease that have been acquired after primer application and prior to applying the next coat).

Finally but importantly, the components (I) and (II) that make up the primer composition are storage stable and are readily mixed without any complex equipment and give rise to a blend that does not block spraying apparatus.

Without wishing to be limited by theory it is envisaged that ensuring that any basic components are present in the component (II) of the kit, such as any glass based microspheres, maximises blending homogeneity. Whilst an alternative may have been to employ a plurality of mixing components, the use of a two component system is advantageous in terms of user compliance. Kits of the invention preferably comprise components (I) and (II) only.

Binder - Component (A)

The primer composition of the invention or component (I) of the kit of the invention contains a polysiloxane sol (also known as the binder component) that is capable of curing upon application of the coating composition to a substrate, typically a steel substrate. The polysiloxane sol component is a component formed by the condensation reaction of at least one silane to form a highly branched polysiloxane sol which contains a plurality of free silanol functionalities. When this sol is combined with the other components of the primer composition, and applied to a substrate, a coating is formed and the free silanol groups present in the sol form links to groups present on a substrate surface. This curing reaction takes place spontaneously at room temperature to form the primer. The polysiloxane sol is explicitly a sol rather than a gel. It can be regarded as a solution, a colloidal solution, an emulsion or a suspension. The polysiloxane is therefore present in an aqueous form before application. The polysiloxane gels during the curing process upon mixing with the other components of the primer.

The polysiloxane sol of the present invention is ideally derived from at least one alkoxysilane precursor that has been subjected to hydrolysis to form the corresponding silanol. It will be appreciated that in many alkoxysilanes there are multiple alkoxy groups (typically up to three such groups) and hence there are multiple hydrolysis products possible depending on the hydrolysis procedure. Fully hydrolysed and partially hydrolysed products can be formed. This hydrolysis reaction results in the formation of alcohol.

The hydrolysed silanes/partially hydrolysed silanes can then be condensed together as is well known to form complex oligomers/polymers. Due to the various different monomers present when a hydrolysis reaction is effected, a complex polysiloxane sol forms which cannot be easily characterised by a general formula. For example, due to partial hydrolysis, two partially hydrolyzed molecules can link together in a condensation reaction to form a siloxane and so on. This condensation process obviously causes the formation of alcohol and water by products.

Thus, condensation is associated with the formation of a 2, or 3- dimensional network of siloxane [Si-O-Si] bonds accompanied by the production of water and alcohol species.

The polysiloxane sol of the present invention is typically provided in aqueous form in component (I). Moreover, it is preferably essentially VOC free (volatile organic compounds). It is easy therefore to remove the formed alcohols by evaporation during sol formation. Importantly, as the sol presents with free silanol groups, no further alcohol is liberated on curing keeping VOC content low.

The sol is preferably room temperature curable.

It is preferred if the polysiloxane sol of the invention is based upon the condensation of at least one bis-aminofunctional alkoxysilane which is subject to hydrolysis or epoxyfunctional alkoxysilane which is subject to hydrolysis.

Surprisingly, it was found that stable aqueous, substantially alcohol-free

compositions based on silicon compounds can be obtained from bis-aminofunctional alkoxysilanes or epoxyfunctional alkoxysilane optionally together with

organofunctional alkoxysilanes. In particular, the sol gel chemistry relies on bis- aminofunctional alkoxysilanes or epoxyfunctional alkoxysilanes together with alkyl- functional alkoxysilanes. The silane compounds are ideally present in substantially completely hydrolyzed form but partially hydrolysed form is also possible. These compositions crosslink at low temperatures.

Thus, the polysiloxane sol materials of the present invention are preferably based on condensates of bis-aminofunctional alkoxysilanes, such as

bis(triethoxysilane)amine or bis(trimethoxysilane)amine. These monomers can be subject to hydrolysis to obtain fully or partially hydrolysed analogues of bis- aminofunctional alkoxysilanes, such as bis(triethoxysilane)amine or

bis(trimethoxysilane)amine. Epoxyfunctional alkoxysilanes can be subject to hydrolysis to obtain fully or partially hydrolysed analogues as well. Such silanes include 3 -glycidyloxypropyltri ethoxy silane (GLYEO), 3- glycidyloxypropyltrimethoxysilane (GLYMO).

These treated monomers may be used alone or combined with other monomers to form the sol. Other preferred monomers include

bis(triethoxysilane)amine, bis(trimethoxysilane)amine, n-propyltriethoxysilane, n- propyltrimethoxy silane (PTMO), 3 -glycidyloxypropyltri ethoxy silane (GLYEO), 3- glycidyloxypropyltrimethoxy silane (GLYMO), 3 -aminopropyltri ethoxy silane (AMEO), 3 -aminopropyltrimethoxy silane (AMMO),

methacryloxypropyltriethoxysilane (MEEO), methacryloxypropyltrimethoxysilane (MEMO), N-(n-butyl)-3 -aminopropyltri ethoxy silane, vinyltrimethoxy silane

(VTMO), N-(n-butyl)-3-aminopropyltrimethoxysilane (Dynasylan(R) 1189), 3- mercaptopropyltrimethoxy silane (MTMO), 3 -mercaptopropyltri ethoxy silane (MTEO), N-2-aminoethyl-3 -aminopropyltrimethoxy silanes (DAMO), polyethylene glycol-functionalized alkoxysilanes, tetraethoxysilane (Dynasylan A),

tetramethoxy silane (Dynasylan M), methyltriethoxysilane (MTES),

methyltrimethoxysilane (MTMS), bis(triethoxysilylpropyl)tetrasulfane (Si 69), bis(triethoxysilylpropyl)disulfane (Si 266), bis(trimethoxysilylpropyl)disulfane, bis(trimethoxysilylpropyl)tetrasulfane, vinyltriethoxysilane (VTEO), 1- aminomethyltri ethoxy silyne, 1-aminomethyltrimethoxysilyne, 1- methacryloxymethyltrimethoxy silane, 1 -methacryloxymethyltri ethoxy silane, 1 - mercaptomethyltriethoxysilane, 1-mercaptomethyltrimethoxysilane,

isobutyltrimethoxysilane, isobutyltriethoxysilane, octyltriethoxysilane

(Dynasylan(R) OTEO), octyltrimethoxysilane, hexadecyltriethoxysilane, hexadecyltrimethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, 2- aminoethyl-3-aminopropylmethyldimethoxy silanes, 2-aminoethyl-3- aminopropylmethyldiethoxysilanes, ureidopropyltrimethoxysilane,

ureidopropyltriethoxysilane, tridecafluorooctyltriethoxysilane,

tridecafluorooctyltrimethoxysilane, Dynasylan(R) 1151 (alcohol-free aminosilane hydrolysis product), Dynasylan(R) HS 2627 (alcohol-free cocondensate of aminosilane and alkylsilane), Dynasylan(R) HS 2776 (aqueous, alcohol-free cocondensate of diaminosilane and alkylsilane), Dynasylan(R) HS 2909 (aqueous, alcohol-free cocondensate of aminosilane and alkylsilane), Dynasylan(R) HS 2926 (aqueous, alcohol-free product based on epoxysilane), and Dynasylan(R) SIVO 110 (aqueous, alcohol-free product of epoxysilane).

Any of these monomers can also be subjected to hydrolysis to generate hydrolysed or partially hydrolysed monomers for reaction with the bis- aminofunctional alkoxysilanes or epoxyfunctional alkoxysilanes, which may also be hydrolysed or partially hydrolysed. The use of a hydrolysed bis-aminofunctional alkoxysilanes or epoxyfunctional alkoxysilanes along with a non-hydrolysed alkyl alkoxysilane monomer is preferred. A further preferred option is epoxyfunctional alkoxysilanes in combination with aminofunctionalalkoxysilanes.

Epoxyfunctional alkoxysilanes of use in the invention are most preferably epoxyalkylfunctional alkoxysilanes.

The use of epoxyfunctional alkoxysilanes is especially preferred as starting materials for sol formation. Any alkoxy group in the polysiloxane sol of the invention will preferably have C1-6 carbon atoms, such as C1-4 carbon atoms, especially C1-3 carbon atoms, e.g. methyl, ethyl, n-propyl, i-propyl or n-butyl.

In a most preferred embodiment the sol is formed from a co-condensate based on an w-glycidyloxyalkylalkoxysilane of formula I

wherein X is a 2-(3, 4-epoxy cyclohexyl)ethyl, 1-glycidyloxymethyl, 2- glycidyloxy ethyl, 3-glycidyloxypropyl or 3-glycidyloxyisobutyl group,

R¹ and R each independently are a linear or branched alkyl group comprising from 1 to 4 C atoms; and

x is 0 or 1,

or is a bis(alkoxysilylalkyl)amine of formula II

wherein each R¹ independently is a linear or branched alkyl group comprising from 1 to 4 C atoms and A is a bis-amino-functional group of formula Ila

wherein i and i* each independently are an integer of 1, 2, 3 or 4, f and f* each independently are an integer of 1 or 2, and g and g* each independently are an integer of 0 or 1.

To prevent any VOC content alcohol liberated during sol gel formation is distilled off.

Hydrolysis of the starting alkoxysilanes can be effected using formic acid or other mild hydrolysing agent, e.g. as shown in US2011/0268899. The condensation reaction is preferably effected at low pH e.g. less than 7, such as 3 to 5. The reaction time is preferably controlled to ensure formation of a sol rather than a gel. Reaction times might be up to 3 hrs. Too long reaction times may allow a curing reaction to begin. The synthesis of the necessary sol is summarised in

US2011/0268899, WO2018202472 or US2014/0106176.

Viewed therefore from another aspect the polysiloxane sol of use in the invention is one that is obtainable by a process comprising: (i) hydrolysing at least one bisaminofunctional alkoxysilane or epoxyfunctional alkoxysilanes;

(ii) reacting the resulting hydrolysed product with itself, with at least one optionally hydrolysed alkylalkoxysilane or with at least one optionally hydrolysed aminofunctional alkoxypolysiloxane.

Any alcohol formed can be removed so that the alcohol content of the sol is less than 3 wt%.

Alternatively viewed, the polysiloxane sol of use in the invention is one that is obtainable by a process comprising hydrolysing a bisaminofunctional

alkoxysilane or epoxyfunctional alkoxysilane and allowing the resulting hydrolysed product to self condense to form a sol. Any alcohol formed can be removed so that the alcohol content of the sol is less than 3 wt%.

It will be appreciated that more complex sols can be prepared by adding further reactants into the mix. Whilst therefore there must be at least one bisaminoalkoxysilane or epoxyfunctional alkoxysilane reactant as hereinbefore defined, other possible reactants include vinylsilanes, alkylsilanes, alkoxysilanes and so on.

The ratio of bis-amino functional alkoxysilanes or epoxyfunctional alkoxysilanes to alkyl alkoxysilanes may be 10: 1 to 1 : 10, such as 5: 1 to 1 :5 by wt.

The formed sol should be dispersible in water at room temperature of 23°C. By water dispersible is meant that the sol may form an emulsion, suspension or colloidal solution in water.

Alternatively viewed, the polysiloxane sol of use in the invention is one that is obtainable by the reaction of at least the components (i) a glycidyloxypropyl alkoxysilane of the general formula (I) :

X-Si(OR)₃, where X represents a 3 -glycidyloxypropyl group and R represents a methyl or ethyl group,

(ii) an aqueous silica sol with an average particle size ranging from 5 to 150 nm and a solids content of ³ 45 to £ 55 wt.%, (iii) at least one acid selected from the group consisting of nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid, formic acid, and acetic acid, and

(iv) a bis-amino silane of the general formula (II)

where R¹ is a methyl or ethyl group, and optionally

(v) at least one additional alkoxysilane of the general formula (III)

where Y represents a propyl-, butyl-, octyl-, 3-mercaptopropyl-, 3- ureidopropyl-, or 3-isocyanatopropyl group, R3 represents a methyl or ethyl group, and n equals 0 or 1.

It is important that the sol has free silanol groups so that on curing, no further alcohol is released thereby keeping VOC levels low.

Whilst the sols of the invention are provided in water, the polysiloxanes are often diluted with water before use to achieve an ideal application viscosity.

Notably, commercially available polysiloxanes of use in this invention are available as aqueous solutions.

Where a wt% of polysiloxane sol is given herein, that wt% is derived from the content of water and polysiloxane present unless otherwise stated. The polysiloxane sol (i.e. including the weight of any water in the primer composition) may form at least 5 wt% of the primer composition, such as at least 10 wt%, more preferably at least 30 wt%. In some embodiments the polysiloxane sol may form 5 to 50 wt% of the primer composition, preferably 10 to 45 wt%, such as 20 to 40 wt%. If the primer composition is thinned with water during its preparation then this water may be considered to become part of the polysiloxane sol. This makes identifying specific percentages challenging.

Preferably, the amount of water present in the primer composition of the invention is at least 5 wt%, such as at least 10 wt%. In some embodiments, the primer composition of the invention comprises 5 to 50 wt%, such as 10 to 35 wt% water.

The polysiloxane sol component A typically contains 50 to 98 wt% water, such as 65 to 95 wt% water. Often the solids content of the polysiloxane sol is 2 to 50% by weight, preferably 5 to 35% by weight. Solids content of the sol is especially 10 to 25 wt%. The sol, which may be purchased from a supplier, may however be diluted by the user as necessary which makes percentages difficult to specify. The skilled person will be able to devise appropriate contents of polysiloxane sol in the composition.

The water used for dilution is preferably deionized water. Any water used for dilution is, for wt percentage terms, regarded as part of the polysiloxane sol herein. Whenever percentages of sol are presented in the text, that refers to the weight of sol and water total in the primer.

The condensation products formed by the reaction of, inter alia, bis- aminofunctional alkoxysilanes or epoxyfunctional alkoxysilanes and alkyl alkoxysilanes may be oligo/polymeric. Both are termed a polysiloxane herein.

They may have a particle size between 0.5 and 350 nm, or preferably between 0.5 to 130 nm. The weight average molecular weight may range from 1000-150000 g/mol, preferably between 4000 to 30000 g/mol, more preferably between 1000 to 50000 g/mol, even more preferably between 1000 to 5000 g/mol. The viscosity of the polysiloxane (in water) may be 5 to 20 mPa s at 20°C.

The polysiloxane component is preferably VOC (volatile organic

compounds) free. This means that it contains less than 3 wt% volatile organic compounds (according to ASTM D5201), such as 2 wt% or less, ideally 1 wt% or less. Alternatively, in contains 100g/L VOCs or less, preferably 40 g/L of VOCs or less.

A polysiloxane sol should not release further alcohol on crosslinking.

Curing of the sol should be possible at temperatures which might be experienced where the substrate is coated, such as 0 to 40°C.

The polysiloxane sol preferably has a pH of 1.0 to 8.0., such as 3.0 to 5.5. The silane may have a pH range of 3.2 to 4.0. The formation of the sol generates cross-linked structural elements, which are chain like, cyclic, or 3D but the structures are difficult to define via way of general formula due to the number of potential structures that form.

The polysiloxane of use in the invention is not new and these can be purchased from commercial sources.

The volumetric median particle size is preferably 2 to 10 nm, such as 5.4 nm.

Defining the content of the polysiloxane sol in the primer of the invention is difficult as the amounts vary depending on the water content and on the content of Zn metal (it is heavy and therefore takes a disproportionate percentage of the weight of the primer). The dry weight of the binder (i.e. the weight of polysiloxane solids ignoring water) may form 0.5 to 35 wt%, such as 1.0 to 20 wt%, especially 2.0 to 10 dry wt% of the primer. More especially, dry weight in the primer for the polysiloxane is 2.0 to 12 wt%, especially 4.0 to 10 wt%. These percentages refer to the amount of polysiloxane calculated as the total of all of the components mixed but ignoring the weight of water (dry weight).

If not explicitly specified as“dry weight” a weight percentage quoted in the text/claims relates to the primer including any water content, unless context permits otherwise.

Component (A) is present only in kit component (I). Component (II) should be free of component (A).

Accelerator - Component B

The water borne primers of the invention contain an accelerator selected from BaSO₄, Feldspar, Talc, mica, CaSCri, or kaolin, preferably BaSO₄, Feldspar, Talc, mica, or kaolin. Preferably the accelerator is BaSO₄ or Feldspar, especially BaSO₄.

The amount of accelerator in the primer compositions of the invention may be 0.1 to 50 wt%, such as 1.0 to 40 wt%, preferably 3.0 to 30 wt%.

In dry weight terms, component (B) may form 0.1 to 90 wt%, such as 1.0 to 75 wt%, more preferably 3.0 to 50 wt%, such as 5.0 to 30 wt%, especially 10 to 20 wt%. The accelerator (B) is present in component (I) of the kit of the invention. It is preferred if the accelerator is not present in component (II).

The primer composition of the invention should not contain zinc oxide. Our compositions are preferably free of ZnO. It is believed that ZnO causes gelling of the component (I) of the kit of the invention and should not be used.

It is also preferred if the primer composition is free from iron oxide, such as Fe₃O₄. Iron oxides are believed to detrimentally affect the colour of the composition and should not be used.

The optimum wt ratio of polysiloxane (dry wt) to accelerator is 1 :5 to 1 : 1.

Zn metal - Component C

Preferred primer compositions of the invention contain Zn metal, preferably zinc powder. The Zn metal is not present in the component (I) of the kit of the invention but is present in kit Component (II).

The zinc used in the composition can be metallic zinc in the form of a powder or flakes, hollow spheres embedded with zinc on the surface, minerals embedded with zinc on the surface, and polymers embedded with zinc on the surface. Alternatively, the zinc can be surface-treated metallic zinc chemically inert to the aqueous environment.

Preferably, the zinc has a mean particle size in the range of 0.5-20 microns, such as 1-15 microns, preferably 2-5 microns or 6-9 microns. In particular, the zinc is in the form of zinc powder, especially of the stated particle size ranges. The use of zinc dust as an anticorrosive pigment is well known and its use will be familiar to the person skilled in the art.

The content of Zn metal in the primer of the invention can vary over broad ranges depending on the anti-corrosive requirements of the composition in question. In some embodiments, the Zn content of the primer composition may range from 10 to 90 wt%, such as 30 to 80 wt%, such as 50 to 80 wt%, such as 60 to 80 wt%. In dry weight terms, the Zn metal may represent 10 to 95 wt%, preferably 20 to 90 wt%, such as 30 to 85 wt% (dry weight basis).

In some embodiments of the invention, Zn metal is absent. Water-soluble zinc salt - Component D

The presence of a water-soluble organic zinc salt or zinc chloride is preferred for providing the primer composition with the necessary quick drying time. The organic Zn salt or zinc chloride is part of component (I) of the kit of the invention.

The presence of the organic zinc salt or zinc chloride accelerates the curing process. It works in conjunction with the accelerator component to minimise the curing time. Excessive quantities of this component can however reduce the abrasion resistance of the primer composition.

The water-soluble organic zinc salt is one having a solubility in deionised water of at least 0.5 g/L at 25 °C, preferably at least 5.0 g/L, such as at least 10 g/L or at least 20 g/L. More preferably, the water solubility should be at least 50 g/L. For the avoidance of any doubt, as used herein a solubility of, for instance, 10 g/L means that the solubility of the salt is 10 g of salt in 1000 g of water at 25 °C.

Typically organic zinc salts will have a relatively low molecular weight such as up to 600 g/mol. Suitable salts include as zinc butyrate, acetate, gluconate, glycerate, glycolate, lactate, propionate, salicylate, citrate, oleate, benzoate, laurate, tartrate, stearate, valerate, formate, picolinate, ascorbate, bisglycinate, lysinate, malate, mono-L-methionine sulphate, pidolate and mixtures thereof.

In some embodiments the organic zinc salt may be a hydrate, for example zinc gluconate dihydrate or zinc citrate trihydrate.

It is preferred if the organic anion comprises at least two carboxyl groups or contains at least one carboxyl group and at least one hydroxyl group.

In the kits according to the invention, the organic zinc salt or zinc chloride may be included as part of kit Component (I) and hence with the sol (A).

Component (D) should not be present in component (II).

Particularly preferred salts are zinc gluconate, zinc lactate and zinc citrate, and the respective hydrates thereof. Especially preferred is zinc gluconate.

Especially preferred is barium sulphate as accelerator and zinc gluconate as organic Zn salt.

Mixtures of organic zinc salts can also be used. In a preferred aspect component (D) does not include zinc acetate. This means that the composition comprises less than 0.1 wt% of each of zinc acetate, preferably less than 0.05 wt% of zinc acetate, preferably no zinc acetate.

In a preferred aspect component (D) is zinc gluconate, zinc citrate, zinc chloride, zinc acetate, or mixtures thereof. It is preferred that component (D) is zinc gluconate, zinc citrate, zinc chloride, or mixtures thereof. It is further preferred that component (D) is zinc gluconate, zinc citrate, or mixtures thereof.

The zinc salt component (D), if present, preferably forms 0.1 to 5.0 wt% of the primer as a whole, preferably 0.25 to 5.0 wt%, preferably 0.5 to 5.0 wt%, such as 0.75 to 4.0 wt%, especially 1.0 to 3.0 wt%. Levels of 2 wt% ± 0.5 wt% are particularly preferred.

The zinc salt component (D), if present, preferably forms 0.1 to 5.0 wt% dry weight of the primer as a whole, preferably 0.25 to 5.0 wt% dry weight, preferably 0.5 to 5.0 wt% dry weight, such as 0.75 to 4.0 wt% dry weight, especially 1.0 to 3.0 wt% dry weight. Levels of 2 wt% ± 0.5 wt% dry weight are particularly preferred.

In the case of a hydrate these levels exclude any water associated with the salt. Where a mixture of zinc salts are used as Component (D), these levels refer to the combined level of zinc salts.

The optimised content of Zn salt, such as a Zn gluconate, is such that the weight ratio of polysiloxane sol to organic Zn salt is 30: 1 to 15 : 1. If the ratio is lower than 15: 1 then there is too much organic Zn salt present and this reduces the abrasion resistant of the coating.

(A) a polysiloxane sol;

(B) BaSO₄;

(C) Zn metal; and

(D) an organic Zn salt such as Zn gluconate.

(A) at least 5.0 wt%, such as 5 to 50 wt% of a polysiloxane sol;

(B) 0.1 to 50 wt% of BaSO₄; (C) 10 to 90 wt% of Zn metal; and

(D) 0.1 to 5.0 wt% an organic Zn salt.

(A) 0.5 to 35 wt% dry weight of a polysiloxane sol;

(B) 0.1 to 90 wt% dry weight of BaS04;

(C) 10 to 95 wt% dry weight of Zn metal; and optionally

(D) 0.1 to 5.0 wt% dry weight an organic Zn salt such as Zn gluconate. Viewed from another aspect the invention provides an aqueous primer composition comprising

(A) 0.5 to 20 wt% dry weight of a polysiloxane sol;

(B) 5.0 to 30 wt% dry weight of an accelerator selected from BaSO₄, Feldspar, Talc, mica, CaSCri, or kaolin;

(C) 10 to 90 wt% dry weight of Zn metal; and

(D) 0.1 to 5.0 wt% dry weight an organic Zn salt or zinc chloride.

Viewed from another aspect the invention provides an aqueous primer composition comprising

(A) 0.5 to 20 wt% dry weight of a polysiloxane sol;

(B) 5.0 to 30 wt% dry weight of BaS04;

(C) 10 to 90 wt% dry weight of Zn metal; and

(D) 0.1 to 5.0 wt% dry weight an organic Zn salt such as Zn gluconate.

In some embodiments of the invention, there is no Zn metal present in the primer. In this embodiment, the accerlator is BaSO₄, Wollastonite and Feldspar, preferably BaSO₄ or feldspar, most preferably BaSO₄. It is an option to supply the Zn free composition as a kit in which the accelerator is part of the component (II) of the kit.

In this embodiment, the invention preferably provides an aqueous primer composition which is free of Zn metal comprising:

(A) 25 to 80 wt% of a polysiloxaane sol;

(B) 15 to 70 wt% of an accelerator selected from BaSO₄, Wollastonite and Feldspar; and optionally

(D) 0.1 to 5.0 wt% an organic Zn salt or zinc chloride. Alternative, the invention preferably provides an aqueous primer composition which is free of Zn metal comprising:

(A) 5 to 50 wt% (dry weight) of a polysiloxaane sol;

(B) 30 to 90 wt% (dry weight) of an accelerator selected from BaSO₄, wollastonite and Feldspar; and optionally

(D) 0.1 to 5.0 wt% dry weight an organic Zn salt or zinc chloride.

Other Constituents

The primer layer composition may also contain various other components, e.g. to enhance its anticorrosive properties and so on.

The primer composition may contain colour pigments such as inorganic pigments. Examples of the color pigments include titanium white, chrome oxides and carbon black. Others inclue iron oxide, such as red or black iron oxide.

The addition of one or more thickening agents/thixotropic agents improves the anti-settling properties, film formation and spraying properties of the primer. Examples of suitable thickening agents are bentonite, fumed/colloidal silica, natural thickeners (e.g. alginates), cellulosic thickeners, saccharides, and polysaccharides. When present, the thickening agents are preferably present in kit part (I), preferably only in kit part (I). The use of fumed silica is especially preferred. Silica may also be used in comp. II together with Zn to improve the flow properties of the Zn powder. Cellulosic thickeners are also a preferred option. Cellulosic thickeners are believed to improve wet film sag resistance, even at low levels.

Both as thixotropic agent and as flow enhancer, it is used in rather small amounts (additive level) from 0.01 to 2.0 wt%, preferably 0.05 to 1.0 wt%, preferably less than 0.5 wt%, less than 0.2 wt% of the final composition. When present, cellulosic thickeners may form 0.01 to 2.0 wt% of the final composition, such as 0.03 to 1.0 wt%, e.g. 0.15 to 0.6 wt%.

Welding enhancers may also be used. Titanium dioxide acts simultaneously as a welding enhancer and as a colour pigment.

The primer composition may also contain glass or ceramic particles or microspheres to improve the hardness and weldability of the coating. Examples of such particles/microspheres are mentioned in WO2017/129784. Glass and in particular glass microspheres are preferred. Microspheres may form 1.0 to 20 wt% of the primer composition, such as 1.0 to 15 wt%, especially 3.0 to 15 wt%. In dry weight terms, microspheres may form 1.0 to 20 dry wt%, such as 2.0 to 15 wt% dry weight.

Microspheres, if present, will be part of component (II) of the kit of the invention. They are preferably absent from component (I).

In a preferred embodiment, part of the zinc powder is combined with glass such as a silicate glass, in component (II) of the kit of the invention. A suitable glass is a borosilicate glass. This blend component might be a 20-80 wt% such as about 50:50 blend of zinc powder and glass/microspheres.

When present, welding enhancers are preferably present in kit part (I), preferably only in kit part (I). Colours are preferably present in kit part (II), preferably only in kit part (II).

The primer composition as a whole preferably comprise 0.1 to 10 wt%, preferably 1.0 to 5.0 wt% of other additives (other than microspheres), e.g. titanium dioxide and thixotropic material.

The primer composition as a whole may comprise 0.1 to 10 wt%, preferably 1.0 to 5.0 wt% (dry weight) of additives (other than microspheres), e.g. titanium dioxide and thixotropic material.

As noted previously, the primers of the invention are designed to avoid the problems associated with certain waterborne silicate primers. It is therefore preferred if the primers of the invention contain no alkali metal silicates, such as lithium silicate, potassium silicate or sodium silicate.

It is also preferred if the primer is free of organic solvent.

Biocide(s) may also be used. If present, biocides may be used in rather small amounts such as from 0.01 to 1.0 wt%, preferably 0.05 to 0.5 wt%, preferably less than 0.25 wt%, less than 0.2 wt% of the final composition. Biocides, if present are preferably part of component (I).

An adhesion improver may also be used. If present, an adhesion improver may be present in amounts from 0.01 to 10wt%, preferably 0.05 to 5 wt%, preferably less than 2.5 wt%, less than 2 wt% of the final composition. Adhesion promoters may improve the adhesion to, e.g. epoxy primers.

Suitable adhesion promoters may be water borne silanes such as amino/ silanols such as those sold by Evonik under the tradename Dynasylane Hydrosil.

Kit

The primer composition of the invention is formed by mixing the various components shortly before the primer is applied to a substrate to avoid premature curing. The primer is therefore preferably supplied as a two component kit for mixing by the user.

Component (I) of such a kit contains the polysiloxane sol component (A), accelerator (B), organic Zn salt or zinc chloride (D) and additives that are

conventionally present, e.g. titanium dioxide and thixotropic material. A mildew preventative and/or adhesion improver may also be present.

Component (II) comprises the Zn metal component (C) of the primer of the invention and microspheres/glass if present. Ideally no other component is present in this component (II) of the kit other than optional silica flow enhancer.

Components (I) and (II) can be mixed in a defined ratio depending on the contents of each component. A mixing wt ratio of 1 : 1 to 1 :3 is common, such as about 1 :2, component (I) to (II).

Component (I) preferably comprises 20 to 70 wt% of (A) a polysiloxane sol,

20 to 75 wt% of (B) an accelerator as herein defined and optionally 0.5 to 10 wt% of an organic Zn salt or zinc chloride;

Component (II) preferably comprises at least 60 wt%, such as at least 70 wt% of Zn metal and, if present, 5 to 30 wt% microspheres/glass. It is preferred if no other components are present in component (II) other than optional silica.

Component (I) preferably comprises 0.5 to 70 wt% dry weight, such as 20 to 70 dry wt% of (A) a polysiloxane sol, 20 to 95 wt% dry weight, preferavly 20 to 75 dry wt% of (B) an accelerator as herein defined and optionally 0.5 to 10 wt% dry weight of an organic Zn salt or zinc chloride; It is preferred if the composition of the invention (i.e. the mixed primer) has a VOC less than 100 g/L, preferably less than 50 g/L, most preferred below less 30, e.g. less than 15 g/L. In one embodiment, the primer of the invention is provided in aqueous form. In a second embodiment, the primer is provided with an amount of water but is designed to be thinned further before use.

The composition of the invention when applied preferably dries within 10 minutes at 23°C 50% RH, preferably within 2 minutes following the test protocols set out in the tests section of the application.

Following the test protocol set out below, it is also preferred if the composition dries through and provides a mechanically abrasion resistant surface within 20 minutes at 23°C 50% RH, preferably within 5 minutes.

The final primer of the invention is resistant to water.

In this regard, it is preferred if the primer has early water resistance, i.e. is water resistant within 24 hours, preferably 12 hours curing time at 23 °C 50% RH, preferably within 6 hours following the test protocols set out in the tests section of the application (stable water rub test). Curing can be accelerated by higher humidity conditions.

The primers of the invention also have good humidity resistance. Primers according to the invention are considered to be“stable” if they exhibit a humidity resistance of at least 24 hours when measured according to the resistance to humidity test (ISO 6270-2:2005). Primers of the invention preferably have a stability resistance of at least 36 hours, especially at least 48 hours.

In one preferred embodiment the primer of the invention is a shop primer. It is important that the shop primer can be welded. The main purpose of the shop primer is to provide temporary corrosion resistance whilst all welding of parts takes place. The shop primers of the invention can be welded on with a speed of at least 70 cm/minute with MIG or similar welding technique producing less than 125 mm² pores each 1 meter weld. Moreover, welding should produce a minimum of weld spatter, weld smoke and back burning.

It is a final and important goal that the shop primer can be overcoated without any extra pre-treatment of the shop primer coat. It will be appreciated that any surface to be painted is cleaned before overcoating, e.g. to remove dust, salt and/or grease that have been acquired after shop primer application and prior to applying the next coat. That is not a pre-treatment step. In particular, the application of an epoxy primer is possible.

The pigment volume concentration (PVC) is an important parameter of the shop primer of the invention. The critical volume concentration is also important. PVC is the volume concentration of pigment. CPVC is the maximum volume concentration of pigment fully covered by the resin. PVC/CPVC>1 means that not all the pigments are covered with resin. The paint will be porous. The ratio is a balance between barrier protection and cathodic protection. Too high ratio results in other issues like popping and cohesion loss. Preferably PVC/CPVC is in the range of 0,8-1, 2, more preferably 0,9-1, 1, most preferably 0,95-1,05.

At anytime before during or after the mixing process, water can be added to thin the composition. Thinning of the composition can extend pot life but slows drying time. Thinning by 10 to 35 wt% of the total weight of the composition is possible. Obviously, the amount of water thinning required is partially dictated by the amount of water present in the stating polysilxane sol (A).

Applications

The composition of the invention is applicable by all types of application equipment, as brush and roller, conventional spray gun, airless spray and air-assisted airless spray. Ideally, the paint is applied by airless spray to give rise to a primer that spontaneously cures on application to the substrate.

The substrate to be coated is preferably a metal substrate, ideally a steel substrate. That substrate may be one that is used in a marine environment. Typical substrates therefore include parts of a ship, metallic containers like shipping containers, bridges and so on.

After final mixing, the primer is typically applied onto a steel surface ideally to a final thickness of 5-200 microns, such as 50-150 microns (dry film thickness (DFT) as measured according to the test methods section). Such a coating will provide a temporary protection to the steel surface. In the case of a shop primer, the final thickness is preferably 5-50 microns, such as 15-25 microns. This being said, the present invention also relates to a method for

temporarily protecting a steel surface with an anti-corrosive coating, the method comprising coating the steel surface with a primer composition as defined herein.

The thus coated steel surfaces will typically be stored for 2-40 weeks such as up to around 6 months, where after the steel surfaces can be used in the manufacture of steel constructions where the coated steel surfaces are surfaces of steel body parts of which the steel construction is constituted. The steel construction is assembled by welding and importantly, the steel body parts coated with the composition according to the invention can readily be welded to provide high quality junctions between the steel body parts of the steel constructions.

Thus, the present invention also relates to a method for manufacturing a steel construction, said steel construction being constituted by a plurality of steel body parts, said method comprising the steps of: a) coating at least one of the steel body parts with a primer composition according to claim 1; b) storing the thus coated steel body part(s) for a period of 2-40 weeks; c) assembling at least a part of the steel construction by welding together at least two of the steel body parts, at least one of said at least two steel body parts being coated as in step (a).

Further steps and details in the manufacturing of a steel construction will be obvious for the person skilled in the art.

The primer can be overcoated using any kind of paint without any pretreatment of the primer coat (other than to clean it to remove dust, grease etc).

The invention will now be described with reference to the following non limiting examples.

Test procedures Storage Stability test

Component (I) was stored in a closed can at 50 °C for 1 month. The content was checked at intervals (once a week). The paint was stirred by hand with a wooden stick and visually inspected.

1 : Pass, paint homogeneous without lumps/gels or significant color change.

0: Not pass, paint is inhomogeneous, contains lumps/gel or change color.

Determination of dry film thickness (DFT)

Dry film thickness is measured using an elcometer on a smooth steel substrate.

Calculation of the volatile organic compound (VOC) content, volume solids and solids content of the coating compositions

The volatile organic compound (VOC) content, volume solids and solids content of the coating compositions is calculated in accordance with ASTM D5201-05.

Curing rub test

The panels were prepared by airless spray application (15-25mm DFT) on SA2.5 blasted steel panels (615nozzle, 16MPa output pressure). After curing for specific time (24h, 1 week or 1 month) in 23C 50RH% in good ventilation. The panels were placed in a container with water at 23 °C so that half of the panel was immersed.

The panels were immersed for 5 minutes; a clean woven cotton cloth soaked in water was rubbed back and forth (double rubs) with medium pressure 80 times. The coating and the cotton cloth were then evaluated by a water resistance value from rate 0 to 5. Rate 0 means penetration to the substrates in 80 rubs or less. Rate 1 means heavy depression in the film but no actual penetration to the substrates after 80 rubs. Rate 2 means heavy marring and heavy depression in the film after 80 rubs. Rate 3 means some marring and apparent depression of the film after 80 rubs. Rate 4 means burnished appearance in rubbed area and slight amount of film on cloth after 80 rubs. Rate 5 means film has fully cured, no effect on surface and no film on cloth after 80 rubs. Surface dry time

The panels were prepared by airless spray application (15-25mm DFT) on SA2.5 blasted steel panels. The panels were placed in good airflow or ventilation. After a specific time, the films were touched to check whether the film transferred to the finger. If no film on finger, the film has already surface dried, which was according to GB 1728-79.

Film forming properties

The exposure area or unclosed area on film on Sa 2.5 blasting panel was checked by visual inspection.

Anti-sagging

The shop primer was thoroughly mixed and thinned by 5% by wt. The panels (Sa2.5 blasted) were placed close to vertical at a minimum angle of 80 degrees, 50% overlapping. The distance from nozzle to panel to achieve a constant film thickness by airless spray(615nozzle, 16MPa output pressure) was 35-40 cm. Immediately after application, the film thickness is measured using a comb. Spray 2-3 panels with increasing film thickness and confirm the maximum wet film thickness which has good sagging resistance.

Examples

Table 1 -Materials used

Example 1 - Stability of Kit Component (I)

The stability of kit component (I) was tested with various accelerators. Accelerators are needed in the coating to allow the coating to cure.

Table 2. Accelerators for storage stability (all components are in parts by weight)

These results demonstrate that the presence of ZnO together with the binder causes gelling. The accelerators of the invention do not gel.

Example 2

Components (I) and (II) are combined in the amounts shown in table 3. Curing properties are determined.

Table 3. all components are shown in overall parts by weight (i.e. after mixing)

These results demonstrate that the best curing performance is observed for BaSO₄. Example 3 - BaSO₄

In order to determine an optimum BaSO₄ content (the preferred accelerator) the following experiments are conducted. Compositions as described in Table 4 are prepared. The volume solid is the same (68%) in each example by adjusting the content of Zn dust and BaSO₄ which lead the BaSO₄% in dry film to be 15.1%, 31.7% and 45.3%. Corresponding Zn weight content in dry film is 68%, 50%, 35%. Other ingredients were kept constant.

Table 4. Testing dosage of BaSO₄

All components are shown in overall parts by weight (i.e. after mixing)

(Sivo 165 solid content is 20wt%)

With the amount of BaSO₄ increasing and zinc metal decreasing, the surface dry time is longer but curing and film forming are not influenced and remain good in all three examples. The optimal amount for BaSO4 provides a shop primer surface dry time (<5min). Example 4 Ex.21 to 23 were prepared with 4 parts or 1 part of fumed silica to test anti-sagging properties.

Table 5 - Anti-sagging - All components are shown in overall parts by weight (i.e. after mixing)

Example 5

Examples 25 to 34 further demonstrate the anti-sagging behaviour of the claimed invention. The presence of a cellulosic thickener may improve sagging of a wet film. In particular, cellulosic thickeners increase sag resistance at comparative low levels. Table 6 - All components are shown in overall parts by weight (i.e. after mixing)

Claims

1 An aqueous primer composition comprising:

(A) a polysiloxane sol;

(C) Zn metal; and optionally

(D) an organic Zn salt or zinc chloride.

2. An aqueous primer composition which is free of Zn metal comprising:

(A) a polysiloxane sol;

(B) an accelerator selected from BaSO₄, wollastonite, or Feldspar; and optionally

(D) an organic Zn salt or zinc chloride.

3. An aqueous primer composition as claimed in claim 1 comprising:

(A) at least 5.0 wt% of a polysiloxane sol;

(B) 0.1 to 50 wt% of an accelerator selected from BaSO₄, Feldspar, Talc, mica, CaSCri, or kaolin;

(C) 10 to 90 wt% of Zn metal; and optionally

(D) 0.1 to 5.0 wt% an organic Zn salt or zinc chloride.

4. An aqueous primer composition as claimed in claim 1 comprising:

(A) 0.5 to 35 wt% dry weight of a polysiloxane sol;

(B) 0.1 to 90 wt% dry weight of an accelerator selected from BaSO₄,

Feldspar, Talc, mica, CaSCri, or kaolin;

(C) 10 to 95 wt% dry weight of Zn metal; and optionally

(D) 0.1 to 5.0 wt% dry weight an organic Zn salt or zinc chloride.

5. An aqueous primer composition as claimed in claim 2 which is free of Zn metal comprising:

(A) 20 to 90 wt% of a polysiloxane sol; (B) at least 10 wt% of an accelerator selected from BaSO₄, wollastonite and Feldspar; and optionally

(D) 0.1 to 5.0 wt% of an organic Zn salt.

6. An aqueous primer according to any preceding claim wherein the polysiloxane sol is one that is obtainable by a process comprising:

(i) hydrolysing at least one bisaminofunctional alkoxysilane or epoxyfunctional alkoxysilanes;

7. An aqueous primer according to any preceding claim wherein said water- soluble organic zinc salt is zinc gluconate.

8. An aqueous primer composition as claimed in any preceding claim free of alkali metal silicate.

9. An aqueous primer composition as claimed in any preceding claim which is free of ZnO.

10. An aqueous primer composition as claimed in any preceding claim wherein the accelerator is barium sulphate.

11. An aqueous primer composition as claimed in any preceding claim wherein the primer composition comprises titanium dioxide.

12. An aqueous primer composition as claimed in any preceding claim wherein the primer composition comprises silica, such as fumed silica, preferably in an amount of 0.1 to 5.0 wt%.

13. An aqueous primer composition as claimed in any preceding claim wherein the primer is a shop primer.

14. An aqueous primer composition as claimed in any preceding claim wherein the primer composition comprises microspheres.

15. An aqueous primer composition as claimed in any preceding claim wherein the primer composition comprises a cellulosic thickener.

16. A kit of parts comprising:

(I) a first component comprising (A) a polysiloxane sol, (B) an accelerator selected from BaSO₄, Feldspar, Talc, mica, CaSCri, or kaolin,; and optionally (D) an organic Zn salt; and

(II) a second component comprising Zn metal.

17. A kit of parts comprising

(I) a first component comprising 20 to 70 wt% of (A) a polysiloxane sol, 20 to 75 wt% of (B) an accelerator selected from BaSO₄, Feldspar, Talc, mica, CaSCri or kaolin, and optionally 0.5 to 10 wt% of an organic Zn salt;

(II) a second component comprising at least 60 wt% of Zn metal.

18. A kit as claimed in claim 17 where in component (II) comprises a blend of Zn metal and borosilicate glass.

19. A kit as claimed in claim 17 where in component (II) comprises Zn metal and microspheres.

20. A kit as claimed in claim 17 where in component (I) comprises a Zn gluconate, titanium dioxide and silica.

21. A process for protecting a substrate from corrosion comprising applying to that substrate a primer as claimed in any one of claims 1 to 15 or a blend of components (I) and (II) as defined in claim 16 to 20 and allowing said primer or blend to cure on said substrate.

22 . A substrate coated with a cured primer as claimed in any one of claims 1 to 15 or a cured blend of components (I) and (II) as defined in claim 16 to 21.