WO2000063313A1

WO2000063313A1 - Aqueous coating composition

Info

Publication number: WO2000063313A1
Application number: PCT/EP2000/000800
Authority: WO
Inventors: Mannish Sarkar
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1999-04-20
Filing date: 2000-04-14
Publication date: 2000-10-26
Anticipated expiration: 2001-10-20
Also published as: GB9908900D0; AU4286900A

Abstract

An aqueous coating composition (especially a paint intended for application under ambient conditions) comprising silanol-functionalised resin and particulate non-film-forming solids such as pigments and extenders where the improvement lies in using the silanol-functionalised resin in acidic aqueous solution together with a high pigment volume concentration (PVC) of at least 65% so that dried coatings obtained from the composition have the high opacities characteristic of high PVC, yet the resin is sufficiently crosslinked to give good abrasion resistance with any tendency to excessive cohesion in the coating being offset by the air voids and other discontinuities created by the high PVC.

Description

AQUEOUS COATING COMPOSITION

This invention relates to an aqueous coating composition containing silanol-functionalised resin and particulate non-film- forming solid. The compositions enable both silanol-functionalised resin and pigment and/or extender to be used in a new and more efficient way, resulting in increased opacity for a~ dried coating and also a quicker achievement of an often better scrub-resistance after application and drying of the coating on a substrate. The coating compositions may be paints intended for application at ambient temperatures (e.g. 5 to 35°C) to surfaces found in buildings and one embodiment of the invention allows paints to be formulated which are less heavy to carry and so easier to transport . Conventional aqueous paints contain colloidal dispersions (sometimes called "emulsions" or "latexes") of inherently film- forming particulate resins such as vinyl, acrylic, alkyd, epoxy, polyurethane or polyester polymers. As the paint dries, these particles of resin coalesce to form films which bind together other components of the paint and in particular they bind any non-film-forming particulates such as pigments, matting agents or extenders giving the dried coating scrub resistance and other useful properties. "Extenders" are particulate solids which may be added to a paint formulation for various reasons. For example they may improve the performance of a particulate pigment by inhibiting its agglomeration by spacing apart the pigment particles, or they may alter the rheology of the paint to make it easier to apply or they may simply reduce the cost of the formulation.

The film-forming resin also serves to cause a dried coating of paint to be cohesive and to adhere to a substrate to which it has been applied. Good cohesion and adhesion are both important in achieving good abrasion resistance and good water resistance as reflected by a good scrub resistance. The article "Use of Silane Promoters in Coating Formulations for improving their Adhesion to a Substrate" by M N Sathyanarayana et al in "Paint India" November 1995 (see pages 25 to 32) reports that adhesion can be improved by the use of silanes which contain hydrolysable oxy groups. Hydrolysis of the oxy groups subsequently results in the formation of the silanol-functionalised resin. Sathyanarayana et al particularly refer to hydrolysable silanes containing hydrolysable methoxy groups such as

or its analogues in which the trimethylene glycidyl ether is replaced by various alternative organo functional groups specified in Table 1 on page 26 of the article. They report that such silanes have been used as adhesion promoters for over 30 years. Sathyanarayana et al refer to the use of

"appropriate" amounts of silane and add that the silanes "are normally added in the range of 0.5-2% of the weight of the [film-forming] resin" where the upper limit of 2% is important because (paradoxically) "An increase in the content reduces the adhesive strength" of the bond between the paint and the surface.

European Patent Specification EP 0 327 376A is even more cautious than Sathyanarayana et al about the need to use. appropriately low levels of hydrolysable silanes. EP 0 327 376A relates to the use of such silanes to improve the scrub resistance (i.e. the adhesiveness and cohesiveness) of paints comprising emulsions of particulate vinyl film-forming resins. It warns that if the amount of hydrolysable silane used exceeds even 0.8wt% of the film-forming resin, then the scrub resistance falls dramatically.

We have now discovered that the introduction of an inappropriate amount of hydrolysable silane into a conventional aqueous dispersion paint leads to a loss of scrub-resistance in at least one of two different ways. Firstly, a loss of adhesion can arise from the fact that increasing the amount of hydrolysable silane also leads to the formation of very much stronger cohesive forces within the coating as it dries and unpublished work suggests that these forces are excessive relative to the adhesive forces which form between the coating and the substrate. Coatings shrink as they dry and it has been found that the shrinkage of a strongly cohesive film will seriously disrupt the less strong adhesion of the film to the substrate. Secondly, inappropriately large amounts of hydrolysable silane on the surface of particles of film-forming resin in a conventional aqueous paint inhibit the ability of the particles to coalesce so they are less able to form a uniform dried coating. Instead they are likely to form weakly interconnected zones of strongly coherent coating. In addition, high concentrations of hydrolysable silane can make the dispersion of particles unstable and likely to form gels in the can during storage .

Hitherto, attempts to use large amounts of non- film- forming materials in a paint formulation have led to unwelcome and usually unacceptable losses in the abrasion resistance and the water-resistance of the dried coating of paint, owing to losses in the adhesion and cohesion of the dried coating. This is because the large volume of non- film-forming material creates air voids and other discontinuities which disrupt the film-forming resin, so reducing its ability to form both cohesive and adhesive bonds. The volume of pigment, including any extenders which may be present, in a dried coating of paint is often expressed as a percentage of the total volume of such pigment material in the total volume of the entire dried coating. The percentage is usually called the "pigment volume content" or "PVC". Generally a paint will not have a PVC of above 70% because abrasion resistance, water resistance, adhesion and cohesion are poor. Often even at 65% PVC, the loss of adhesion and cohesion is unwelcome. When the term "PVC" is used in connection with a liquid paint it means that the liquid paint is such that a coating of the paint when fully dry will have that PVC. A PVC of above 65% and preferably above 70% or above 75% would be desirable in a paint intended to have a good opacity, that is to say, a good ability to hide marks on a surface. This is because the air voids created in the dried coating increase its opacity. It has now been discovered that by using an aqueous solution (including a micellar or similar quasi -solution) of silanol-functionalised resin in the formulation of a coating composition having a PVC of at least 65%, it is possible to achieve a more uniformly highly cohesive and highly adhesive dried coating, which additionally gives improved opacity for a given concentration of opacifying material. It is presumed that this is possible because the coating composition is more stable during storage, it avoids the use of the coalescence of particles to form its dried coatings and it employs the disruption caused by the air voids and other discontinuities to offset any tendency to excessive cohesion within the dried coating owing to an excess of condensed silanol moieties.

Accordingly, this invention provides an aqueous coating composition containing silanol-functionalised resin and particulate non-film- forming solid, wherein a) the silanol-functionalised resin is in the form of an acidic aqueous solution (including micellar or similar quasi-solution) and contains sufficient silanol moieties to enable resin to crosslink when the composition dries; b) the silanol-functionalised resin is present in an amount such that the content thereof in the dried coating is 3 to 30 vol% of the dried coating; and c) the PVC of the coating composition is at least 65% (and is preferably at least 70%, more preferably from 75 to 87 or even up to 90%) . The present invention also provides a concentrate which on dilution with water provides an aqueous coating composition as defined above, which concentrate has a solids content of from 15 to 35 vol% based on the total volume of the concentrate .

The present invention also provides the use of a silanol-functionalised resin as a film- forming agent in a coating composition as defined above.

The present invention further provides a process for preparing a composition or concentrate as defined above which comprises mixing the particulate non-film- forming solid with an aqueous solution of the silanol-functionalised resin. The present invention makes it possible to employ silanol-functionalised resin in a new way which enables a paint to be less sensitive to the amounts of silanol present and which in particular produces a paint which is relatively stable during storage, which is better able to form uniform coatings and which avoids serious loss of adhesion on drying. The paints take advantage of the strong cohesive forces obtainable from silanol condensation, so as to contain large amounts of non-film-forming solids. Strong cohesive forces combined with the good adhesion give rise to improved abrasion and water resistance in the dried coat of paint, as manifested by an improved performance in the "ICI Scrub Test" described later in this specification. The use of large amounts of non-film-forming material is essential in the performance of this invention, yet it is unusual for the above reasons. The present invention provides an improvement to high quality paints . It can also be used to improve lower quality paints such as distempers where scrub resistance is not of primary or even great importance, but where a certain level of scrub resistance is required. It is speculated that when the aqueous coating composition is applied to a surface and allowed to dry at ambient temperatures and humidities, at some stage in the process, silanol moieties i) form Si-O- bonds which crosslink adjacent molecules of resin by means of multi-silicon oxy links such as -Si-O-Si- (disiloxy) , -Si-O-Si-O-Si- (trisiloxy) or even polysiloxy, (Si-0-)_n, ii) bind particles of co-reactable non-film- forming solids by means of either multi-silicon oxy links where the solid is siliceous such as a silica or a silicate comprising hydroxy groups and especially a silica coated rutile pigment or by means of a siloxy bond Si-O-M where M is a metal moiety in the non- film- forming solid which is associated with moieties (especially hydroxy moieties, e.g. aluminium or magnesium hydroxy moieties) which are co-reactable with silanol, and iii) bind to co-reactable groups on the surface in much the same way as in (ii) above.

Although the crosslinked resin forms a film which binds the non-film- forming particles, the crosslinked resin is sufficiently sparse between the particles that the strong cohesive forces created by the silicon oxy bonds are partially offset by numerous air voids and other discontinuities created in the dried coatings by the large volume of non- film-forming solids present. The result is a useful balance between cohesive and adhesive forces to give a dried coating which has good cohesion yet adheres well to a substrate, so having excellent abrasion-resistance and water-resistance, all combined with improved opacity or equivalent opacity achieved with less opacifying pigment. Additionally and very importantly, it has been discovered that the relatively high speed of formation of the -Si-O-Si- link relative to speed of coalescence in a conventional paint means that abrasion resistance is developed much more quickly than with conventional aqueous matt paints. For example, a conventional degree of abrasion resistance can be achieved often one day and certainly 7 days after application of the coating to a substrate, whereas 21 to 28 days are usually needed by a conventional paint having a similar solvent level .

Improvements in opacity occur because of the ability to formulate at high PVC while retaining good abrasion resistance which causes a large number of air voids in the dried coating. Alternatively, an equivalent opacity may be obtained with use of less than 65% of the usual amount of pigment and often with use of less than 50% of the usual amount of the pigment . A further advantage of the aqueous paints of this invention is that they can be formulated to have a much lower ratio of resin and non-filming- forming solid to water in a liquid paint. The percentage of the total amount of non-volatile material (most importantly resin and non- film-forming solids) in a liquid paint is usually expressed as the "volume percentage solids content" of the paint and it is the volume percentage of all non-volatile ■ material based on the total volume of the liquid paint, that is to say, the combined volume of non-volatile material plus the water and any other liquid ingredient of the paint.

Conventional aqueous paints have solids contents of from 25 to 50 vol% and if their solids contents fall below about 25 vol%, the opacity of the paint becomes unacceptable. The solids contents of paints according to this invention are preferably from 5 to 20 vol% and most preferably from 8 to 15 vol%. This makes the paints less dense, which in turn means that they are less heavy to carry and transport. However, it is also possible to supply paints as concentrates for subsequent dilution with water by the user. Such concentrates offer the skilled user the opportunity to adjust the viscosity of the paint by dilution, so as to suit local conditions of temperature and humidity. The concentrates would generally have a solids content of from 15 to 35 vol% and preferably from 15 to 30% vol%.

It is essential that in the aqueous coating composition, the silanol functionalised resin should be present in the form of a solution, because it has been found that dispersions, i.e. emulsions or latexes, of particulate film-forming resins do not produce the same high levels of scrub-resistance at high PVC. Dispersions are also less tolerant of high concentrations of silanol and can be less stable. In paint chemistry, and in this specification, the term "solution" includes not only thermodynamically perfect solutions but also micellar or similar quasi -solutions, which are transparent to 600nm light like perfect solutions, although they do scatter light to a small extent and may therefore appear hazy in the absence of organic cosolvent . Quasi-solutions are similar to micellar solutions to the extent that they both contain lightly associated aggregates of resin molecules. Preferably the silanol-functionalised resin should be dissolved to the extent that a 7 wt% solution of the resin in water should after standing for at least 48 hours at 20 C, without precipitate forming, have a light transmission of at least 50% and preferably at least 90% when a beam of light of wavelength 600nm is passed along a 10mm path through the solution. Such a measurement may be conveniently made using a Perkin Elmer "Lambda 10" spectrophotometer . Such high transmissions indicate that any micellar or similar structure has an average unit size of below 100 nm and a transmission above 95% and especially above 98% indicates a most preferred average unit size of below 50nm. A latex generally has a transmission of less than 10%.

The silanol-functionalised resin is present in the composition in an amount such that the content thereof in the dried coating is at least 3vol%, preferably at least 8vol%, most preferably at least 10vol% of the dried coating. The silanol-functionalised resin is present in the composition in an amount such that the content thereof in the dried coating is no more than 30vol%, preferably no more than 20vol%, most preferably no more than 13vol% of the dried coating.

The silanol-functionalised resin can be provided with its silanol moieties by bonding a non-silane functionalised resin to various co-reactable silanes containing hydrolysable oxy groups, which hydrolyse spontaneously in water to form the silanol moieties. The silane may contain, for example, from 1 to 3, preferably 1 or 2 , silicon atoms and, for example, from 1 to 3 , preferably 1 or 2 , co-reactable groups such as primary or secondary a ino groups . The most preferred silanes contain 1 silicon atom and 1 co-reactable group, preferably separated by from 2 to 4, most preferably 3, methylene (-(CH₂)-) groups. Useful mono-silanes contain the grouping

-Si-⁽OR¹⁾ _n ⁽R²⁾ ₃-_n where; n is 1 to 3 (preferably 3 giving a silane species containing the grouping -Si(0-)₃ which can hydrolyse to give three silanol moieties i.e. -Si(OH)₃); OR¹ is a hydrolysable oxy group; -R¹ is any group which can be removed from the oxy group by hydrolysis to leave a silanol moiety -Si-OH and; R² is any benign group which does not prevent hydrolysis or bonding of the silane to the film-forming resin. Preferred hydrolysable oxy groups (OR¹) are alkoxy (usually to C₄) groups (e.g. methoxy or ethoxy groups) , or alkanoate groups (especially C₂ to C₅ alkanoates) or oximes and especially straight chain groups which do not sterically hinder hydrolysis. Essentially, the hydrolysable oxy group may be any group which on hydrolysis produces a silanol moiety which can spontaneously condense with similar silanol moieties to crosslink adjacent resin molecules, or which may bond with hydroxy or other suitable groups on a substrate or on a non-film-forming solid particle. Benign groups (R²) are present when n is 1 or 2 and are preferably C_x to C₆ alkyl groups, especially methyl or ethyl.

The co-reactable silanes containing hydrolysable oxy groups may be caused to bond to the unfunctionalised resin by for example co-reacting a group in the silane with a co-reactable group in the resin. Examples of silanes containing such co-reactable groups are:

epoxy

ammo H₂NCH₂CH₂Si(ORl)_rl(R2)₃._{n 0}r

H₂NCH₂CH₂CH₂Si(ORl)_n(R2)₃._n or alkyl amino analogues diamino H₂NCH₂CH₂NHCH₂CH₂CH₂Si(ORl)_n(R2)₃._n and alkyl amino analogues

mercapto HSCH₂CH₂CH₂Si(ORl)_n(R2)₃__{n an}

chloropropyl ClCH₂CH₂CH₂Si(ORl)_n(R2)₃__n Such silanes may be used individually or in combination, for example an amino- or diamino-silane with an epoxy silane.

Alternatively, the silane may be included in a polymeric resin by choosing a silane having a group which is copolymerisable with the resin monomers so that the silane can be introduced as a co-monomer. Examples of silane co-monomers for use in copolymerising with ethylenic monomers are: vinyl CH₂=CH Si (OR¹),, (R²)₃._n ethacrylate or CH₂=C (R³) C0₂CH₂CH₂CH₂Si (OR¹) _n (R²) ,._n

acrylate [R³ = H or CH₃]

cationic styryl

CH₂=CHC_SH₄CH₂NH₂CH₂CH₂N⁺H₂ (CH₂) ₃Si (OR¹) _n(R²) ₃._n ^"Cl

The silanol-functionalised resin may be zwitterionic or cationic in character when in solution, but the best results are obtained from cationic resins . The pH of the paint is less than 7, generally 4 to 6 , for example 4.5 to 5.5.

Typical resins include the so-called "epoxy resins", that is to say resins comprising epoxy functional (preferably) aromatic compounds. A useful simple epoxy resin is the diglycidyl ether of bisphenol A (whose graphic formula is shown later) or the bisphenol F analogue which is (4 -hydroxypheny1) methane .

More complex epoxy functional resins comprise those prepared by reacting an epihalohydrin, such as epichlorhydrin, with a plurality of dihydroxy phenolic compounds such as bisphenol A or bisphenol F. Many such simple and complex epoxy functional resins are commercially available in a range of epoxy equivalent weights, for example under the trademark "Epikote" from Shell Chemicals Limited, Carπngton, England and they have the general formula:

where R is H or alkyl, preferably methyl and m is preferably 0 to 3 and can on average be non-integral , e.g. 1.2. More preferably m is 0 to 1 and most preferably just above 0. A simple resin which m is about 0 is available as "Epikote" 880 (epoxy equivalent weight 192) . A resin in which m is about 1 is available as "Epikote" 1001 (epoxy equivalent weight .475). Extended epoxy functional resins comprise the epoxy functional reaction product of an epoxy functional resin as described above with a chain extender such as a diol, for example polyethylene glycol, or a diamme. It is also possible to use the hydrogenated analogues of the above epoxy resins.

The epoxy functional resins preferably have an average epoxy functionality of at least 1.1 preferably at least 1.5 and most preferably between 1.8 and 5 mol kg¹. In addition, suitable epoxy functional resms have epoxy equivalent weights of less than 2,000, preferably less than 500, most preferably less than 250. The lower limit of an epoxy equivalent weight is less important, but the epoxy functional resms generally have an epoxy equivalent weight of at least 96. The epoxy functional resms advantageously have a molecular weight of less than 1,000. This has been found to provide enhanced stability.

The silane containing hydrolysable oxy groups may be bonded to an epoxy resin by for example co-reacting an ammo, diamino or thio group silane with the epoxy functional compound during or after the formation of the epoxy resin. Both the formation of the epoxy resin and its introduction to the silane may be achieved simultaneously by choosing an amino or diamino silane as the sole amino co-monomer in the formation of the epoxy resin.

A typical reaction scheme for imparting silane functionality can be illustrated by the co-reaction of diglycidyl ether of bisphenol A and methyl- N-amino-propyl-triethoxy silane as follows:

Silane-functionalised epoxy resins are most effectively used with PVCs over 75% and preferably in the range 80 to 87% PVC. Other useful epoxy functional resins include extended epoxy functional glycidyl derivatives of terephthalic acid, isocyanurates, sorbitol and novalac resins, plus epoxy functionalised resms derived from ethylenically unsaturated monomers .

Examples of suitable ethylenically unsaturated monomers include vmyl esters (especially v yl acetate or vmyl "Versatate"^*1) and also alkyl (especially methyl, ethyl and n-butyl) esters of unsaturated carboxylic acids such as acrylic or methacrylic or fumar c or maleic acids, unsaturated carboxylic acids such as acrylic or methacrylic acids, unsaturated acid anhydrides such as maleic anhydride, monovmylidme aromatics especially styrene, vmyl toluene or vmyl pyπdme , alkenes and halogenated alkenes such as ethylene, propylene, vmyl chloride, vinylidene chloride and tetrafluorethylene, unsaturated nitπles, dienes and (fqr use only in copolymerisations) minor amounts of hydroxyl or ammo alkyl (especially ethyl) esters of unsaturated carboxylic acids such as acrylic or methacrylic acids, epoxy compounds such as glycidyl methacrylate and also sulphonate. The number average molecular weights are preferably from 400 to 1 000 000 usually below 30,000 especially for acrylics, and most preferably 3,000 to 5,000 as measured by gel permeation chromatography . Such resms can be given silane functionality by copolymerismg their ethylenically unsaturated monomers with at least one of the ethylenically unsaturated silanes described earlier. Alternatively, co-monomers may be included which provide a functional group which can be co-reacted with a co-reactable group m the silane. Such co-monomers co-reactable with the silane may contain epoxy functionality as mentioned above, which can be introduced by copolymerismg with for example glycidyl methacrylate or allyl glycidyl ether. Alternatively, the co-reactable co-monomers may contain hydroxy groups (e.g.

¹ ' Vmyl "Versatate" is the vmyl ester of so-called "Versatic" acid which is a mixture of aliphatic monocarboxylic acids each containing an average of 9, 10 or 11 carbon atoms and is commercially available from the Shell Chemical Company of Camngton, England hydroxyethyl acrylate), carboxylic acid groups (e.g. acrylic or methacrylic acids) , ammo groups (ammoethyl acrylate, dimethyl ammoethyl acrylate or methacrylamide) , or dienes (e.g. butadiene or divinyl benzene) . Other typical resms include those having a polyethylene glycol backbone. Advantageously, such resms have a molecular weight of less than 2,000, for example from 1,000 to 1,500. Such resms have been found to have particularly enhanced stability, although possibly at the expense of increased water sensitivity. They may, therefore, advantageously be used in a blend with other resms such as epoxy resms. The epoxy res s may also include a polyethylene glycol backbone, either part or completely. The polyethylene glycol res s have end groups to which the silanol functionality can be incorporated. For instance they may have diglycidyl, dnsocyanate or acrylate/methacrylate end groups .

Useful silanol-functionalised resms can also be made by the Michael addition of Michael nucleophiles (such as the ammo-, alkylammo-, or mercapto-silanes described earlier m this specification) to ethylemc unsaturation which is activated for Michael addition. A typical reaction would be as follows :

-CH₂-C H-NHCH2CH₂CH₂Sι(ORl )₃ aqueous acetic acid

© Θ

-C H₂-C H-NHCH₂C H₂CH₂Sι(OH)₃ . AcO It is beneficial, though not essential, to employ more than one silicon atom per molecule of resin and so one or more than one ethylenic unsaturation per molecule should be used. Examples of suitable resins include polyol esters of unsaturated carboxylic acids such as methyacrylic or preferably acrylic acids or polyol (usually diol) esters of unsaturated polycarboxylic acids or anhydrides such as maleic anhydride or fumaric acid. Particular examples include trimethyl propane triacrylate and ethylene di (hydrogen maleate) . The backbone may be, for example, a polyethylene glycol resin.

The Michael addition may also be performed on the condensate obtained by reacting a polyisocyanate (for example a tri-isocyanurate) with a hydroxy alkyl carboxylate (for example a hydroxy butyl acrylate) in the presence of a tin catalyst .

Ethylenic silanol-functionalised resins are most effectively used with PVCs in the range 65 to 77% with PVCs of 70 to 75% being preferred. Other Michael adducts which can be used are water soluble Michael adducts of unsaturated polyesters and aminosilanes neutralised by aqueous acids, for example those Michael adducts described in US 4 122 074.

Other suitable resins include water-soluble alkyd, polyurethane or polyester resins. Again the number average molecular weights are usually from 400 to 1,000,000 with less than 30,000 being preferred.

Crosslinking is essential and so the silanol-functionalised resin must contain sufficient silanol groups to form the Si-O-Si crosslinks. Theoretically each resin molecule should contain at least one, for example at least two, crosslinkable silanol group, either involving the same silicon atom (eg. -Si (OH) ₃ , which presents an unwelcome risk of trimersation) or very preferably involving different silicon atoms, that is to say two -Si-OH groups. Some silanol groups will be consumed by attachment to non-film- forming particles and some may form intramolecular links even when on different silicon atoms, so in fact a typical initial distribution of two silanols per molecule could be reduced to below 2. Useful degrees of crosslinking can occur if the notional average of silicon atoms per molecule is as low as 1, but in general a minimum of 1.1 would be useful, for example a minimum of 1.5.

It is also important to ensure that a sufficient number of silanol groups remain available for bonding to the substrate. For this reason it is preferred that the silanol groups are derived from a silane species containing the grouping -Si(0-)₃ such as trialkoxy silane and that the average number of silicon atoms per molecule is from 1 to 15, for example from 1 to 2. Simple epoxy resins will tend to average 1 to 2 or 1 to 4 silicon atoms per molecule with extended epoxy resins averaging from 1 to 2 or 1 to 10. Ethylenic copolymers (such as polyvinyls and polyacrylics) have longer molecular chains and so they can accommodate a larger number of silanol moieties, for example 5 to 15 silicon atoms per chain. To some extent, the number of silanol moieties per molecule is governed by the preferred concentrations of silicon in the resin or paint.

Because the silanol-functionalised resins are in solution and form coherent coatings by crosslinking instead of coalescence, and because their effect on cohesiveness is moderated by the high PVC, the resins and paints used in this invention can tolerate higher concentrations of silanol moiety where necessary. The preferred level of hydrolysable silicon in the silanol-functionalised resin is from 0.5 to 20 wt% based on the total weight of the silanol-functionalised resin. The best scrub resistances have been achieved with levels of from 3 to 15 wt% with a range of 4 to 8 wt% being most preferred. The level of hydrolysable silicon in the total liquid paint formulation is preferably from 0.02 to 0.5 (especially 0.03 to 0.3) wt% based on the total weight of the liquid paint. It is preferred to match the content of silanol-functionalised resin in the composition with the silicon content in the silanol-functionalised resin. Generally as the silicon content increases, the silicon-functionalised resin is required in a lesser amount.

The non-film-forming particulate solids will generally have a number average particle size of from 50nm to 50μm depending on the type of solid. Solid inorganic opacifying pigments include rutile and anatase titanium dioxides which are usually coated with silica, alumina and or zirconia and have particle sizes of from 150 to 500nm as measured by electron microscopy. Typical organic non-film-forming white pigments are the voided particles known as "Ropaque" (particle size about 400nm) and "Spindrift" (particle size 1 m to 15 m as measured by electron microscopy) described respectively in European Patent Specifications EP 0 113 435B and EP 0 041 508B, or corresponding United States Patent Specifications US 4 474 909 and US 4 321 332, the contents of which are herein incorporated by reference. Useful extenders are associated with hydroxy groups and include oxy compounds of magnesium, aluminium and silicon, all of which usually have some presence of hydroxy groups, even though they may be essentially present as oxides or silicates. Examples include silicates such as clays (eg kaolin or bentonite) and talcs. The most common matting agent is silica. Rutile titanium dioxide is often used in amounts of 10 to 20 wt% based on the total weight of the liquid paint, but it has been found that using paints according to this invention, corresponding opacities can be achieved with as little as 3 to 13 wt% of rutile. Analogous savings can be made with other pigments or extenders . The paints may also contain other conventional additives such as extenders which do not bond to silanol moieties, (eg chalk, limestone or barytes) and also fungicides, dispersants, antifoaming agents, flow improvers and thickeners . Thickeners include the various celluloses and gums including xanthan and guar gums and Ti or Zr chelates . However, it is a further advantage of the solutions of the silanol-functionalised resins of this invention that they can often serve as their own dispersant for solid particles -of pigments and extender. The coating compositions can tolerate conventional aqueous dispersions of non-silanol functionalised film- forming resins of the type conventionally used in making aqueous paints such as the polyvinyl or polyacrylic resins mentioned earlier. The volume percentage of these non-silanol functionalised resins based on the total volume of the coating composition is preferably not greater than that of the silanol-functionalised resin. These dispersions may be added to contribute an extra film forming capability or simply to act as organic extenders. A particularly effective way of using a combination of pigment and extender to achieve high opacity (albeit at the cost of some loss of scrub-resistance) is to contact an aqueous dispersion of particles of one of them with a cationic silanol-functionalised resin, for example an amino silanol-functionalised epoxy resin, having a charge of one particular sign, whilst an aqueous dispersion of particles of the other is contacted with an anionic dispersant and then mixing the oppositely charged dispersions, so that electrostatic attraction brings the particles and resin into well-ordered contact leading to a coating composition having even further improvements in opacity. The selection of charge for the particles and the order and manner of addition of the compounds should be explored on a case-by-case basis. For example, an aqueous dispersion of particles of pigment (especially rutile) may be contacted with an anionic dispersant whilst an aqueous dispersion of particles of a clay extender may be contacted with a cationic silanol-functionalised resin and then the modified rutile is added to the modified extender. The two dispersions are mixed together to produce a well-ordered combination of pigment extender, resin and air voids thereby further enhancing the opacity of the coating composition so obtained.

For the purposes of this Specification, Abrasion-Resistance (ICI Scrub Test) , and Opacity (or covering power) were measured as follows:

Abrasion-Resistance: Measured using the ICI Scrub Test which is indicative of the adhesiveness and cohesiveness of a dried coating of paint. The ICI Scrub Test was performed as follows :

Each paint was first passed through an 80μm nylon mesh to remove any bittyness and was then de-aerated by centrifuging it at 1500 rpm for 7 minutes under vacuum. A 200μm thick wet coating of the paint was applied using a block-spreader to a black surface provided on a specially made polyvinyl chloride sheet 165mm wide by 432mm long obtainable as "Scrub Test Panel" P121-ION from the Leneta Company of New Jersey, USA. Each painted surface was allowed to dry and age for either 1, 7 or 28 days (as specified with the result) at room temperature, (i.e. about 18°C) . The aged panel was cut into eight equal rectangles 160mm long and 54mm wide and the abrasion-resistance of the painted surface of each rectangle was determined as follows :

A felt pad was mounted on a stem which in turn was mounted 5 to 10mm off-centre in a rotary drive, so that operation of the drive caused the pad to sweep out eccentrically a circular area of 1250mm². The pad was moistened with a 0.5wt% solution in water of the non-ionic surfactant "Synperonic" N supplied by Imperial Chemical Industries Pic of Wilton, Cleveland, England and kept moist throughout the test by adding more of the surfactant solution. Each painted rectangle was in turn laid on a platen with its painted surface facing upwards. The platen was carried on one arm of a lever pivotally mounted about a horizontal pivot. The other arm of the lever carried a 0.5kg weight . The distances of the centre of the platen and the weight from the pivot were 150mm and 180mm respectively. The weight rotated the platen into a horizontal plane where it was stopped by the pad from any further upwards rotation. The pad was then caused to rotate against the painted surface until a complete circle of the surface of the Scrub Test Panel was exposed. The number of rotations needed to expose the complete circle was counted.

Counts were made for all eight rectangles and then the entire procedure was repeated to provide a further eight counts. The average of these sixteen counts was reported as the abrasion-resistance of the paint.

Opacity: Measured as the ICI Opacity Factor which indicates the ability of a dried coat of paint to hide marks. The ICI Opacity Factor is measured as follows:

Each paint is screened and tested as in the ICI Scrub Test and then applied to a fully transparent polyethylene terephthalate foil using a 150μm block spreader. The foil should be 280 by 10mm and 50μm thick and is of "Melinex" supplied by Imperial Chemical Industries Pic of Teeside, England. The applied coating is allowed to dry for 7 days at room temperature (18°C) . The coated foil is then inspected over a light box and four defect-free zones identified.

Each defect-free zone is placed in turn on a white ceramic tile with its paint coating uppermost. The CIE tristimulas value Y for each coating is measured using a spectrophotometer fitted with a CIE Standard Illuminant C and 2 degree observer conditions . Each zone is then placed on a black ceramic tile and the CIE Y values determined as above.

After measurement of the Y values, the thickness of the paint coating in each defect-free zone is determined and an average of the thicknesses is taken. Thickness is determined by stamping a 38mm disc from each zone, weighing the disc, completely removing the coating using acetone, re-weighing the disc and then using weight of the coating, the area of the disc and the density of the solids content of the paint to calculate the volume "V" of paint which would be applied to an area of lm² if the paint were to be spread on it at the same rate .

The Y values and the volume "V" divided by lm² to obtain a notional film thickness "t" are then substituted into the Kubelka-Munk Equations to arrive at a value for the

Scattering Coefficient "S" in accordance with the procedures specified in Paragraph 4 of British Standard BS 3900 Part D7 1983 (also known as 150 6504/1-1983) . The contents of BS 3900: Part D: 1983 are herein incorporated by reference. The value for "S" divided by the film thickness and then multiplied by the volume percent solids and 1000 is quoted as the "ICI Opacity Factor". The invention is further illustrated by the following Examples of which Examples A to D are comparative. All "parts" mentioned in the Examples are parts by weight.

COMPARATIVE EXAMPLES A TO D Abrasion Resistance and Opacity of Conventional Aqueous Matt pamts :

The Abrasion Resistance and Opacity of each of four conventional aqueous white matt pamts were measured in order to illustrate the standards associated with conventional products. Each pamt comprised an aqueous dispersion of a conventional particulate polyacrylate film-forming resin, particles of silica-coated rutile titanium dioxide pigment and particles of extender. The results are given m Table 1. The pamts were: Comparative Example A:

A high quality white matt having a high rutile content of 17 parts by weight and containing a mixture of extenders consisting of kaolin, chalk, ' Ropaque ' , talc and silica n a weight ratio of 28: 6: 5: 14: 2.5 giving an overall extender PVC of 40.0%. The total PVC was only 57.5% which allows a good abrasion resistance but sacrifices opacity derived from the air voids which would be created by a high PVC. Therefore, high opacity is obtained by use of the very high rutile content of 17 parts by weight. Rutile is, however, an expensive ingredient.

Comparative Example B:

A low cost commercial matt white having a low rutile content of 8.5 parts by weight and containing a mixture of extender consisting of kaolin, chalk and "Ropaque" but this time in a weight ratio of 47: 14: 10 to give an overall extender PVC of 63.3%. The total PVC was 71.9% which gives fairly good opacity for a low rutile content but at the cost of poor abrasion resistance.

Comparative Example C:

An experimental aqueous matt having a moderately rutile content of 13 parts by weight and an overall PVC of 70%. In order to improve adhesion at this fairly high PVC, the polyacrylic resin was modified by a commercial cyclic ureido adhesion promoter of the type described in European Patent Specification EP 0 078 169A. The opacity achieved was good and the ureido promoter produced a useful improvement in

Abrasion Resistance at 28 days. However, the ureido promoter is also an expensive ingredient and the improvement is not as much as would be expected by following the present invention.

Comparative Example D: An experimental aqueous matt white as in Comparative Example C except that the PVC was increased to 85% producing an extremely good opacity at the cost of very poor abrasion resistance despite the use of the Ureido Promoter.

TABLE 1

None of the dispersions of film-forming resins would permit the transmission of 600nm light across a 10mm path through the dispersion which is as expected for particles of undissolved resin.

EXAMPLES 1 TO 4 Illustration of the Invention using Cationic Silanol-Functional Epoxy Resin: Example 1 ( PVC 89% ) :

In Example 1, a silanol -functional epoxy resin was derived from bisphenol A diglycidyl ether and

N-methyl-γ-amino propyltrimethoxy silane and then used to make a paint having a PVC of 89% and the formulation shown in Table 2.

Firstly, the silanol-functionalised resin was made as follows :

106.5g of "Epikote" 880 (bisphenol A diglycidyl ether of epoxy equivalent weight 192) were stirred into an organic solvent at 25 °C under nitrogen, together with 127g of aminosilane, which was N-methyl-γ-amino propyltrimethoxy silane. The organic solvent was Dowanol PM (available from Dow Chemical Company) which is propylene glycol monomethyl ether. Stirring was continued for one hour and then the temperature was raised to 80°C and stirring continued for a further three hours, after which it was stopped and the reaction product allowed to cool at 25°C. A silane-functionalised epoxy resin was obtained which had a weight average molecular weight of 1,700.

The silane-functionalised epoxy resin was converted to a clear stable aqueous solution of cationic silanol- functionalised epoxy resin by taking 239g of it (still at 25 °C) and adding it slowly with rapid stirring to a mixture of 45g of glacial acetic acid and l,424g water. The solution obtained has been shown to remain stable at ambient temperature for at least one year. It was in solution to an extent that 95% of a beam of 600nm light passing through 10mm of the solution is transmitted. The silanol-functionalised resin contained 7.2 wt% silicon based on the total weight of the resin. The aqueous solution of cationic silanol-functionalised resin was then used to make an aqueous paint, having a PVC of 89%, using amounts of ingredients as specified in Table 2.

Sufficient water need for the formulation specified in Table 2 was added to a high speed disperser, together with the aqueous silanol-functionalised resin which also served as a dispersant. The disperser blade was rotated slowly (300 rpm) to mix the water and resin and then the rutile (which was coated with silica) was added. Next the other non- film-forming solids were added and the blade was rotated at 3,000 rpm for 30 minutes so as to achieve a good dispersion.

The paint obtained was allowed to stand for at least 24 hours and was then subjected to the Abrasion-Resistance and Opacity measurements described above. The results are shown in Table 2.

Examples 2 to 4 (PVC 84%, 80% and 76%) :

The procedure of Example 1 was repeated except that the silane used was γ-amino propyltriethoxy silane and the amounts of reactants used in making the silanol-functionalised resin were as follows:

"Epikote" 880 159g

Silane 127g

The silane-functional resin is then dissolved in aqueous acetic acid as follows:

*Silane- functional resin added to the acetic acid 485g

Acetic Acid 90g

Water 2,875g *Molecular Weight 1,700 A similarly clear and stable aqueous solution of the silanol-functionalised resin was obtained, but this time the resin contained only 6.4 wt% silicon based on the total weight of the resin. The aqueous solution was then used to make the three aqueous paints of PVCs 84%, 80% and 76%, as specified in Table 2, using the same procedure as used in Example 1, except that thickener, guar gum, was added after all the other non- film- forming solids had been added. The guar gum was added in three equal amounts, with rotation of the disperser blade at 300 rpm for 5 minutes after each addition.

The paints obtained were allowed to stand for at least 24 hours and were then subjected to the Abrasion-Resistance and Opacity measurements described above. The results are shown in Table 2.

EXAMPLES 5 TO 9

Use of other silanol-functionalised resins:

Aqueous solutions of cationic silanol-functionalised ethylenic resins based on either alkyl acrylate copolymer resin (Example 5) or on trimethylol propane triacrylate polymer resin (Example 6) and of Michael addition products (Examples 7 to 9) were made as follows:

Example 5:

A silane-functionalised alkyl acrylate resin was made by adding over a period of two hours a mixture consisting of

36.5g methyl methacrylate, 43.5 butyl acrylate, 20g glycidyl acrylate and 4.7g azodi-isobutylronitrile initiator to stirred "Dowanol PM" (Dow Chemical Company) organic solvent maintained at 100°C under nitrogen. The reaction was continued for a further hour to complete the copolymerisation. The resulting copolymer had a glass transition temperature (Tg) of 5°C and a number average molecular weight of 3,500 as measured by gel permeation chromatography using a polystyrene standard.

A 360g portion of the copolymer resin solution was taken and to it was added 66g of N-methyl-γ-amino propyltrimethoxy silane. The resultant mixture was stirred and heated to 80°C under nitrogen for one hour, whereupon a silane-functionalised acrylate resin was produced. The functionalised resin was then converted to a stable clear aqueous solution of cationic silanol-functionalised resin by adding it slowly with rapid stirring to a mixture of 60g of glacial acetic acid and 2,334g of water.

A stable, clear aqueous solution of cationic silanol-functionalised acrylic resin was obtained which was dissolved to the extent that over 95% of 600nm light passing through 10mm of the solution was transmitted. It contained 2.5 wt% of silicon based on the weight of the functionalised resin.

Example 6 : A cationic silane-functionalised acrylate resin was made by adding 335g of amino-γ-propanetriethoxy silane with stirring and under nitrogen to 296g of trimethyol propane triacrylate over a period of one hour whilst ensuring that the temperature of the mixture did not exceed 52 °C. Stirring was then continued for a further five hours to ensure completion of the reaction to produce a silane-functionalised acrylic resin.

The silane-functionalised resin was converted to a stable clear aqueous solution by taking 50g of the above reaction product and adding it slowly to 33g of glacial acetic acid followed by 428g of water, all with rapid stirring. The solution of cationic silane-functionalised acrylic resin obtained was dissolved to an extent that over 95% of 600nm light passing through 10mm of the solution was transmitted. It contained 8.7 wt% silicon based on the 5 weight of the functionalised resin.

Examples 7 to 9 :

A mixed Michael adduct resin was made. 221g of Silquest A-1100 silane (γ-aminopropyl-triethoxysilane obtainable from OSi specialities) was charged to a pot and heated to 80°C. A

10 solution of 156g D.E.R.331 epoxy resin (a liquid reaction product of epichlorohydrin and bisphenol A from Dow Chemical Co.) in 304.8g Sartomer 344 (PEG 400 diacrylate from Cray Valley, France) was fed with a dropping funnel with stirring. The temperature during the preparation was maintained at

15 80 °C. On completion of feed the contents of the flask were further stirred at 80°C for 1 hour.

The above adduct, which had an epoxy :PEGDA ratio of 40:60, was added rapidly with stirring to a solution of 170.45g acetic acid in 5959g water to give an aqueous 20 solution polymer.

COMPARATIVE EXAMPLES 1 TO 3

Comparative Example 1

An aqueous phase consisting of deionised water (893.54 g) and the surfactant SDBS (sodium dodecyl benzene 25 sulphonate) (8.83g) was added to a reaction flask fitted with a stirrer and raised to a temperature of 86°C. To this was fed over a period of 3 hours a mixture of methyl methacrylate (126.24g), butyl acrylate (132.78g), methyacrylic acid (5.89g), gamma- methacryloxypropyltrimethoxy silane (Silquest 30 A174) (29.44g) and the initiator Vazo 67 (2.94g) (obtainable from Du Pont (UK) Ltd) while maintaining the temperature at 84-86°C. After the addition was over the flask was maintained at the same temperature for a further period of 30 minutes. The temperature was then reduced to 60°C and t-butyl hydroperoxide (0.04g) was added and the temperature held at 60°C for 30 minutes. Following this sodium metabisulfite (0.04g) in water (0.25g) was added and stirred for a further 30 minutes. The flask was then cooled down and contents discharged to obtain to obtain silane functional latex.

Comparative Example 2 A silane-containing latex described in US 5,543,445 was prepared based on silane containing stabiliser 2 described in the same patent .

A mixture of aqueous stabiliser 2 from the above patent (800g) was added to a flask purged with nitrogen and raised to 40 °C. A mixture of methyl methacrylate (40g) and butyl acrylate (40g) was added and stirred for 10 minutes. To this was added hydrogen peroxide (100% vol) (0.2g) and ascorbic acid (0.2g) and the mixture stirred at 40 °C for 1 hour. The reaction mixture was then cooled down to obtain a silane functional dispersion.

Comparative Example 3

A silane-containing latex described in US 5,543,445 was prepared based on silane containing stabiliser 2 described in the same patent . A mixture of aqueous stabiliser 2 from the above patent (800g) and deionised water (600g) was added to a flask, purged with nitrogen and raised to 40 °C. A mixture of methyl methacrylate (80g) and butyl acrylate (80g) was added and stirred for 10 minutes. To this was added hydrogen peroxide (100% vol) (0.2g) and ascorbic acid (0.2 g) and the mixture stirred at 40 °C for 1 hour. The reaction mixture was then cooled down to obtain a silane functional dispersion.

The aqueous solutions of silanol-functionalised acrylic resins made for the purposes of Examples 5 to 9 and latexes made for the purposes of Comparative Examples 1 to 3 were each in turn used to make paints having the formulations specified in Table 2. This was done by adding the requisite amounts of water to a high speed disperser together with the silanol- functionalised resins which again also served as dispersants. The other ingredients of the paints were added using the same procedure as employed for Examples 1 to 4.

EXAMPLE 10

Use of Opposite Charge Attraction on Non- filming Particles

The procedure of Examples 1 to 4 was repeated except that kaolin was added to the disperser together with the cationic silanol-functionalised resin so that it acquired a positive charge and the rutile was pre-treated with 0.13 parts of an aqueous solution of an anionic dispersant known as "Dispex" and supplied by Allied Colloids Limited of Bradford, England, and then added to the mixture of resin and kaolin. Again, the ingredients, abrasion resistance and opacity are shown in Table 2. Despite a PVC of 83%, the abrasion resistance was still comparable with a conventional adhesion-promoted resin, but an opacity substantially greater than that of Example 2 was achieved. Table 2

Example Type PVC% Abrasion Abrasion ICI Rutile Kaolin Thickener Resin Resin Volume% resistance resistance Opacity t% wt% (guar solution Volume% in solids in 1 day 1 week factor gum) wt% wt% dry film liquid paint solution 89 200 3.3 32.7 0 21.9d 11.45 19.62 polymer solution 84 >1000 >1000 11,700 12.4 14.9 0.69 18.98a 10.5 13 polymer solution 80 >1000 >1000 10,800 12.5 13.8 0.75 17.75a 10.61 12.9 polymer solution 76 >3000 >3000 10, 100 12.5 12 0.56 21.9a 16.5 12.4 polymer solution 87 130 580 8,000 12.6 14 0.7 10.5a 6.95 12 polymer solution 84 105 11,300 14.2 14.3 0.6 23.75d 11.5 13 polymer solution 65 400 1500 5,840 5.56 11.43 0.56 47.39 29.93 14.12 polymer solution 72.5 300 300 7,390 6.1 12.56 0.61 34.06 21.85 14.12 polymer solution 81 250 800 9,300 6.7 13.74 0.67 20.12 13.12 14.12 polymer

10 solution 83 300 330 14,400 12.7 14 0.7 17.2a 11 12.6 polymer

|Comp 1 latex 84 17 25 7,600 6.8 14 0.68 11.96 9.29 13.88 lcomp 2 latex 83 66 71 16,600 13.9 15.4 0.75 8.97 11.02 14.2 jComp latex 77 46 51 9,500 11.4 12.6 0.62 14.82 17.64 12

Claims

- λ34CLAIMS

1. An aqueous coating composition comprising silanol-functionalised resin and particulate non-film-forming solid wherein a) the silanol-functionalised resin is in the form of an acidic aqueous solution and contains sufficient silanol moieties to enable resin to crosslink when the composition dries ; b) the silanol-functionalised resin is present in an amount such that the content thereof in the dried coating is 3 to 30 vol% of the dried coating; and c) the PVC of the coating composition is at least 65%.

2. A coating composition according to Claim 1 wherein the PVC is from 75 to 90%.

3. A coating composition according to Claim 1 or Claim 2 wherein the hydrolysable silicon content of the silanol-functionalised resin is from 0.5 to 20 wt%.

4. A coating composition according to any one of the preceding claims wherein the coating composition has a solids content of from 5 to 20 vol% of the volume of the aqueous composition.

5. A coating composition according to claim 4 wherein the solids content is from 8 to 15 wt%

6. A coating composition according to any one of the preceding Claims wherein the silane-functionality on the silanol-functionalised resin is derived from a silane species containing the grouping -Si(0-)₃.

.

7. A coating composition according to any one of the preceding Claims wherein the silanol-functionalised resin is an amino silanol-functionalised resin.

8. A coating composition according to Claim 7 wherein the

5 silanol-functionalised resin bears at least one amino silanol group having from 2 to 4 carbon atoms between the nitrogen and silicon atoms.

9. A coating composition according to Claim 8 wherein the amino silanol group is γ-amino propyl silanol.

10 10. A coating composition according to any one of the preceding Claims wherein the resin part of the silanol-functionalised resin comprises an epoxy resin.

11. A coating composition according to Claim 10 wherein the epoxy resin has a molecular weight of less than 1,000.

15 12. A coating composition according to any one of Claims 1 to 9 wherein the resin part of the silanol-functionalised resin is derived from ethylenically unsaturated material .

13. A coating composition according to any one of the preceding Claims wherein the resin part of the

20 silanol-functionalised resin comprises a polyethylene glycol resin having a molecular weight of less than 2,000.

14. A dried coating obtained from a coating composition as claimed in any one of the preceding Claims wherein the coating has a PVC of over 65% and contains a crosslinked resin provided

25 with from 3 to 15 wt% of silicon atoms.

15. A concentrate which on dilution with water provides an aqueous coating composition according to any one of Claims 1 to 13, which concentrate has a solids content of from 15 to 35 vol% based on the total volume of the concentrate. -,

36

16. Use of a silanol-functionalised resin as a film- forming agent in a coating composition as defined in any one of Claims 1 to 13.

17. A process for preparing a composition as defined in any one of claims 1 to 14 or a concentrate as defined in claim 15 which comprises mixing the particulate non- film-forming solid with an aqueous solution of the silanol-functionalised resin.

18. A process according to Claim 17 wherein the non- film- forming solid comprises pigment and extender, which process comprises contacting an aqueous dispersion of particles of one of the pigment and extender with a cationic silanol-functionalised resin, contacting an aqueous dispersion of particles of the other of the pigment and extender with_^an anionic dispersant, and then mixing the oppositely charged dispersions.