WO1994004616A1 - Segregating water-borne coating systems - Google Patents

Abstract

The present invention provides a water-borne coating system comprising a top-coat polymer and a base-coat polymer each in dispersion, and including a component that results in increased mobility of the top-coat polymer relative to the base-coat polymer. Preferably, excess surfactant is used to increase the mobility of the top-coat polymer by the formation of micelles, and the curing agent acts to aggregate the base-coat polymer before the curing reaction and whilst segregation occurs. Suitable surfactants include ethylene oxide copolymers and ethoxylated nonylphenol. Preferably non-ionic stabilisation is used for the top-coat dispersion.

Description

Segregating Water-borne Coating Systems

The present invention relates to water-borne coating systems, more especially to segregating water-borne coating systems. The desirable protective properties of coatings can only be achieved where integrated continuous films are formed on a substrate where the applied paint has dried. Film formation from water-borne systems is a complex operation, but is known to be strongly dependent on the rigidity of the polymer used in the system, a property characterized by the glass transition temperature, Tg.

Various alkyd resins, acrylic and other vinyl polymers, epoxy resins and polysiloxanes can be produced with suitable Tgs and acceptable other properties, and may be used as film-forming polymers in water-based systems, although often different monomers are used in a copolymer system to achieve a compromise between flex¬ ibility and surface hardness in the paint film. The coating should not only have good appearance and dur- ability, but should have good chemical and UV resistance, good stain or dirt resistance and good weatherability. Polymers with such properties are often expensive and/or provide poor adhesion to substrates, so that mixed systems are often used, and sometimes two different coatings are applied: a base coat chosen for its adhesion, general protection and, over metals, corrosion resistance, and a top coat to provide chemical resis¬ tance, durability, stain resistance, non-stick properties etc. It would be advantageous, however, to have a single coating system which segregates to provide an enrichment of one polymer, having suitable top-coat properties, near the air interface, and/or of a polymer having suitable base-coat properties, near the substrate.

There is a considerable body of work in the area of inferfacial segregation for solvent-based and powder coatings which has identified the major driving forces giving rise to these phenomena. In both systems the differential critical surface energy of the two polymers is most often cited as the single most important para¬ meter driving interfacial segregation. In solvent-based systems the second most important parameter is believed to be the use of a two-component solvent system, paying particular attention to the solubility and evaporation rates, such that the least soluble polymer is with the faster evaporating solvent. Molecular weight has also been shown to have a significant effect on extent of segregation.

In contrast to the above, water-based systems involve the use of colloidal dispersions, and in this case we are dealing with particulate polymer particles rather than polymer chains. Surprisingly, in contrast to the situation for interfacial segregation in solvent- based and powder systems, where the top layer is composed of the lowest surface energy component, in water-based systems we have found that it is possible to produce coatings where the top layer consists of the higher surface energy component. Furthermore, molecular weight does not appear to have the same decisive importance in determining the extent of segregation as it does in solvent-based systems.

We have, however, identified other physicochemical properties of the system as important to achieving inter- facial segregation.

Our work has shown that segregating systems can be obtained with base-coat and top-coat polymers in col¬ loidal dispersions when there is preferential mobility of the top-coat polymer. Accordingly, the present invention provides a water- borne coating system comprising a top-coat polymer and a base-coat polymer, each in dispersion, and including at least one component that results in selective mobility of the top-coat polymer in the presence of the base-coat polymer.

Advantageously, the top-coat polymer and the base- coat polymer are present in the system as separate dispersions, and the dispersions are then mixed when preparing a composition for application to the substrate. Thus, preferably, the present invention provides a water-borne coating system comprising two distinct polymeric dispersions and including one or more components that result in differential mobility of the two polymers in the mixed system.

As the solids level increases with evaporation of the water, the stabilised polymer molecules in the mixed system have a natural tendency to aggregate with other molecules of the same kind and, in some cases, depending on the nature of the stabilisers, with one another. Preferential mobility may, for example, be achieved by enhancing the aggregation of the base-coat polymer par- tides with one another and/or by reducing the tendency to aggregation of the top-coat polymer particles, that is by promoting preferential aggregation. An increase in the mobility of the top-coat polymer relative to that of the base-coat polymer may, for example, be brought about by the lubrication of the top-coat polymer particles relative to those of the base-coat polymer particles; increase in viscosity of the base-coat polymer relative to that of the top-coat polymer or decrease in the viscosity of the top-coat polymer relative to that of the base-coat polymer would also bring about the necessary differential mobility of the two polymers.

Advantageously, two segregation-promoting components are present in addition to the top-coat and base-coat polymers in order to produce selective mobility of the top-coat polymer: one component as a dispersant for the top-coat polymer, and one component as an aggregating agent for the base-coat polymer, but the invention is not restricted to this. For example, selective mobility or selective aggregation may be achieved by employing a sufficiently large ratio of mean particle size of the base-coat polymer to that of the top-coat polymer. Preferably, however, for the base-coat polymer an aggregating or mobility-reducing agent is used, and/or for the top-coat polymer a mobility-increasing agent is used.

The present invention also provides a water-borne coating system comprising

(A) a dispersion of a top-coat polymer, and

(B) a dispersion of a base-coat polymer, the system including a component that results in the selective aggregation of the base-coat polymer in the presence of the top-coat polymer and/or a component that results in higher mobility of the top-coat polymer.

Such segregation-promoting component(s) may each be present in the dispersion of base-coat polymer or may be separate from this, as appropriate; each may, for example, be present in a dispersion of the top-coat polymer for subseguent mixing with a dispersion of the base-coat polymer dispersion.

The component producing an increase in mobility of the top-coat polymer may, for example, be a surfactant, which may be non-ionic, cationic, or anionic. The surfactant may, for example, be a block copolymer having, as well as a hydrophobic moiety, a hydrophilic moiety which may be chemically the same as or similar to or different from the surface layer on the top-coat polymer molecules, that surface layer being the hydrophilic moiety of the stabiliser used. Such copolymers may also be used for stabilisation of the top-coat polymer dispersion; often, about l -3% of the copolymer is required for stabilisation purposes (calculated on the top-coat monomer content) and generally about 2 to 3% is used commercially. Our findings suggest that for polyethylene oxide-polypropylene oxide block copolymers at least about 1% (± 0.5%) of the stabiliser is chemi¬ cally bound to the top-coat polymer molecule resisting, for example, removal by centrifugation at 20,000 revolutions per minute (and C¹³NMR appears to show that the hydrophobic moiety is held within the polymer molecule) . The remainder of the about l -3% of the stabilising polymer is, we believe, physically bound to or associated with the chemically-bound stabilising polymer. When excess (i.e. free) stabilising polymer is present in a concentration above the critical micelle concentration, excellent segregation has been obtained; in contrast, segregation can be more difficult or unreliable when no excess stabiliser is present, as in the case of most commercially formulated top-coat polymer dispersions. If desired, different copolymers may be used as stabiliser and as segregation-promoting agent, each in the apropriate concentration. (It will be evident that the term "polymer" is used herein, in relation to the stabiliser, in a broad sense, and is not restricted in terms of molecular weight; indeed, high molecular weight polymers may be inappropriate since hydrophilic and hydrophobic components that are too long may prevent micelle formation.) Non-polymeric surfac¬ tants may also be used as segregation-promoting agents, provided these are also present above the critical micelle concentration. Surfactants having a relatively high surface tension are, for example, to be particularly mentioned.

Examples of suitable surfactants are AB and ABA block copolymers, advantageously having ethylene oxide chains, for example any block copolymer of ethylene oxide with a relatively hydrophobic monomer, for example polyethylene oxide-polypropylene oxide block copolymers, polyethylene oxide-styrene block copolymers and adducts of ethylene oxide and nonylphenol. Commercially avail¬ able surfactants include Pluronic (BASF) and Synperonic (ICI) polyethylene oxide-polypropylene oxide block copolymers, more especially Synperonic PE/F68 and PE/F88, Fluorad FC171 (3M) , Gafac PE510 (GAF) , Fenopen C0436 (Rhone Poulenc) , Aerosol A-102 and A-103 (American Cyanamid) , Triton X405 (Rohm & Haas) , Agrilan AEC178, AEC145 and F513 (Harcros Chemicals Ltd.) and the non- polymeric surfactant Dehyquart (Henkel Organics) . Some of these have ionic groups, for example Gafac PE510; it is important to emphasise that if these are also used as the primary top-coat polymeric stabiliser the segregation may not be as good.

The critical micelle concentration is dependent on a number of factors, including molecular weight, ionic strength of the solution and type and size of polar groups and/or non-polar groups. We have found that total amounts of stabiliser of at least 3%, for example at least 4%, or at least 5%, give segregation, and advantageously such polymers used both as stabiliser and segregation-promoting agent should be present in an amount of up to 6%, for example up to 7%, or up to 8%, calculated on the weight of top-coat polymer (that is, on the weight of monomer(s) reacted). Corresponding totals should be considered when the stabiliser and surfactant are different; the surfactant itself may be present in an amount of, for example, at least 1%, for example, at least 1 %, or at least 2%, and amounts of at least 2 %, or at least 3%, and up to 3 % or 4%, should be mentioned. As an alternative, or in addition, preferential aggregation of the base-coat polymer may be brought about, for example by reaction or inter-reaction of that polymer with a third component or with itself, or by destabilisation of the dispersion of base-coat polymer. Thus, for example, a base-coat polymer may be a network- forming polymer which is self-cross-linking, or a curing agent therefor may be present in the system, and selective aggregation may be achieved when the cross- linking reaction begins. Hydrogen bond formation between the base-coat polymer and the "aggregating" component, for example a water-soluble flocculating agent or the stabiliser for the dispersion, may also bring about selective aggregation. Alternatively, if the base-coat polymer dispersion has some ionic stabilisation, charge neutralisation of that dispersion will cause destabilisa- tion. In another embodiment, the dispersion of the base- coat polymer may be prepared with the aid of a water- miscible solvent more volatile than water, for example a suitable alkoxyalcohol, e.g. Dowanol, so that the polymer dispersion becomes less stable as the solvent evaporates during film-formation. Destabilisation of the disper- sion may also be brought about by selective depletion flocculation by addition of a water-soluble polymer. Modification of the surface of the base-coat polymer particle, for example by an associative thickener, causing viscous drag, may also be used, leading to reduced mobility of the base-coat polymer. Preferably, however, the base-coat polymer is a network-forming polymer and an aggregating component which is also a curing agent is used. Where the curing reaction occurs at room temperature, the curing agent is of course kept separate from the base coat component until application to the substrate; thus, it may be a separate component or present in a dispersion of the top-coat polymer. Preferably, the top-coat polymer dispersion has no or substantially no electrostatic stabilisation; that is, this dispersion is characterised by the substantial absence of ionic stabilisation and substantial absence of ionic functionality.

Accordingly, the present invention especially provides a water-borne coating system comprising A) a dispersion of a top-coat polymer having preferably substantially no electrostatic stabilisation, and B) a dispersion of a network-forming, base-coat polymer, and wherein the network-forming polymer of component B is self-cross-linking or a curing agent therefor is present in the system, and wherein preferably excess surfactant is also present.

These polymers are used in the form of dispersions; that is, the polymer is present as a disperse phase in a continuous phase of water. Such systems may be differentiated as lyophobic or lyophilic and are often colloidal; these are characterized by the size of the polymer particles, which may lie in the range 1 nm to 1,000 nm.

Lyophilic dispersions that are true solutions in the thermodynamic sense may be looked upon as one-phase systems in the sense that the polymers are giant molecu¬ les playing the solute role; lyophobic dispersions are characterized by the presence of a two-phase (at least) system where the disperse phase consists of sub-divided bulk matter.

These dispersions may be prepared in a variety of ways. For example, a polymer solution (a lyophilic dispersion) may be prepared from water-insoluble polymers by modifications that introduce sufficient ionic character to render the polymer water-soluble, for example by incorporating acid groups and neutralising these by salification or quaternisation forming a poly(electrolyte) . A latex (a lyophobic colloid system) is prepared by emulsion polymerization from one or more monomers; alternatively, a lyophobic colloid system may be prepared from a pre-for ed polymer in cases where the polymer can not be prepared from its monomer(s) by emulsion polymerization; such systems are known as pseudo-latexes, and their preparation is disclosed, for example, in U.S. 4 177 177. A further lyophobic colloid system is a dispersion formed from a water-insoluble polymer that has been treated to render dispersions of it in water stable to aggregation, for example by grafting on a hydrophilic species such as poly(ethylene oxide). Although any one of these lyophilic and lyophobic dispersions may be used in a system of the present inven¬ tion, the polymer solutions, having ionic character, are least preferred for the top-coat dispersion, although they may be advantageous for the base-coat dispersion. Stable lyophobic colloids, dispersions of polymers insoluble in their continuous phase, can only be main¬ tained when a repulsive interaction is provided to overcome the Van der Waals' attraction between the polymer particles. In conventional systems, stabilisation is generally achieved by incorporation of ionic material, either by adding an ionic stabiliser during or after polymerisation or by incorporating ionic functionality into the polymer; electrostatic forces keep the polymer particles separate. In some instances, an ionic stabiliser and another stabiliser, which is non- ionic, are used; the combination of ionic (electro¬ static) and non-ionic (steric) stabilisation is known as electrosteric stabilisation.

Contrary to most conventional aqueous coating systems, however, in a system of the present invention there is preferably no electrostatic stabilisation of the top-coat polymer dispersion, the dispersion being essen¬ tially non-ionic. This may be achieved by employing a steric stabilisation and using non-ionic initiators for the top-coat polymer such as are disclosed, for example, in GB 2124636 A and GB 2127835 A. Steric stabilisation is brought about by added polymer. The best stabilisa¬ tion and top-coat mobility increase is observed when the polymer is an amphipathic copolymer, that is one that consists of a component capable of adsorption and/or partial absorption and/or attachment through covalent binding to the disperse-phase polymer particle and another component that is well solvated by the continuous phase. Such stabilisation has been described, for example, in British Patents 1,196,247 and 1,544,335. Electrosteric stabilisation should, however, preferably be avoided.

Suitable top-coat polymers are acrylic and other vinyl polymers and, indeed, polymers of any suitable α,β- ethylenically-unsaturated monomer, polyesters, more especially alkyd resins, and polysiloxanes and polyurethanes. The polymers may be homo-or co-polymers, and include fluorinated polymers, for example poly- (trifluoroethyl methacrylate) , and analogous polymers of the type produced by Du Pont, especially the Zonyl range. Acrylic and methacrylic polymers should especially be mentioned, and, as examples of siloxane polymers, poly- (y-methacroyloxy-propyltrialkoxysilanes) , vinyl-tipped polydimethylsiloxane polymers (e.g. Sylgard 182 and 184, available from Dow Corning) , and silicon-modified epoxies. Any other suitable polydimethylsiloxane which is capable of forming a coherent film may also be used. Air-drying systems are preferred; an example is an oxime-tipped polydimethylsiloxane used with tetraethyl- orthosilicate as curing agent. Suitable copolymers are styrene/acrylic ester copolymers, copolymers of vinyl acetate with vinyl esters of versatic acids, vinyl acetate/2-ethylhexyl acrylate copolymers (e.g. 70-80% VAc, 30-20% EHA) , and butyl acrylate/methyl methacrylate copolymers (e.g. 40-60% BA, 60-40% MMA) .

Suitable base-coat polymers are epoxy resins including, for example, resins based on the dipropargyl ethers of bisphenol A, alkyd resins, vinyl/styrene copolymers, or, for example where a top coat is fluorine- or silicon-containing, the base coat may be a conven¬ tional acrylic resin.

We have shown that, surprisingly, it is not neces¬ sary to utilise a top-coat polymer with lower surface energy than the base-coat polymer.

We have determined the critical surface tensions of particular epoxy and acrylic polymers used in our experiments. Films were applied to glass plates using a lOOμm cube applicator, and then allowed to dry at room temperature for 2 days, before finally drying at 50°C in a vacuum oven for 2 hours. The determination of critical surface tension was carried out using a set of Sherman surface tension test pens: these are felt-tipped pens, each of which contains solvents with a specified surface tension. The pens are used to apply solvent to each film, and the film is observed to see whether the solvent has wetted the surface to give a continuous layer of solvent on the film surface. A series of pens is used, starting with one having a low surface tension, then working up to higher surface tensions until a pen is found which does not wet the film. The critical surface tension of the film is then determined as being between the value of the highest pen which wets the film and the value of the lowest pen which fails to wet it.

The results for the epoxy resin, Beckopox EP384, and for a methyl methacrylate/butyl acrylate polymer (Latex 1 in the Examples) showed that the critical surface tension of the epoxy resin was less than that of the acrylic polymer. Surface energy arguments would have suggested that an epoxy resin, of lower surface energy, would segregate over the acrylic resin; this is not, however, the case in segregating systems of the present invention where, in the presence of a curing agent and/or excess surfactant, segregation of acrylic resins over epoxy resins can be achieved. We have also shown that this result is independent of substrate: the same segregation was achieved for both a metal (aluminium) substrate and a plastics sheet (polytetrafluoroethylene) , a much lower surface energy substrate. Thus, surface energy considerations are not crucial for segregation in systems of the present invention. Contrary to many segregating systms, also, incom¬ patibility between the two polymers is not required. For example, an acrylic latex containing UV stabiliser or having UV-resistant properties may be made to segregate over another acrylic latex. It will thus be evident that the terms "top-coat polymer" and "base-coat polymer" are used merely for ease of reference to the film-forming polymers present in the system. The terms are not intended to divide polymers into rigid categories: a particular polymer may, for example, be a "top-coat polymer" in one system and a "base-coat polymer" in another system. The polymers must of course be capable of minimum film formation under the conditions used for paints. The minimum film forming temperature (MFT) , the minimum temperature at which film coalescence occurs, is in¬ directly related to Tg, and may be higher or lower than the Tg depending on the polarity of the polymer; the presence of ionic surfactants can also have some effect but, as explained above, these should preferably be avoided for stabilising the top-coat polymer dispersion. The resulting coating appears as one or two layers, in either of which cases the appropriate surface (air interface or substrate interface) is enriched with the appropriate polymer; that is the "top-coat polymer" is enriched at the top surface (air interface) and/or the "base-coat polymer" is enriched at the bottom surface (substrate interface) . Preferably, the present invention is operated with a top-coat polymer in dispersion, preferably colloidal dispersion form, free from electrostatic stablisation, to produce an enrichment of that polymer at the air interface. We have achieved, for example, segregation of acrylic, vinyl and siloxane polymers over epoxy resins such that the top surfaces contain, respectively, considerably more acrylic, more vinyl and more siloxane polymer than the initial mixtures.

The content of the top 2 microns, especially the top 5 microns, or more, or (in the case of an anti- corrosive coating where segregation to the substrate is important) the bottom 2 microns, especially the bottom 5 microns, or more, is important, although the critical depth will also vary with the desired use: for example, for an anti-stain or siloxane anti-fouling coating, the composition of only the top 1/2 micron may be important, whereas, for UV-durability purposes, the content of at least the top 5 microns is of importance.

The amount of the preferred polymer at the surface - in the depth of film specified above - will depend inter alia on the amount of that polymer used in the system, and on the thickness of the film. With films of 100 micron nominal wet film thickness (wft) , and with equal mixtures of acrylic and epoxy, an amount of acrylic > 55%, preferably > 60%, more especially > 65%, at the top surface, may be said to represent an acceptable enrichment; with a 200 micron film (wft) a greater enrichment would be expected, and with equal amounts of acrylic and epoxy in the coating composition, we have obtained, for example, films that appear as two layers with > 80% acrylic in the top layer. With such mixtures, advantageously at least 60%, preferably at least 65%, especially at least 70%, more especially at least 75%, very especially at least 80%, of top- or base-coat polymer is preferred at the top or lower surface respec¬ tively. Even for films as thin as 50 microns (wft) , we have achieved enrichment of the desired polymer at the top surface. Alternatively, with an initial mixture containing a lower proportion of the relevant polymer, for example with a mixture containing 10% acrylic and 90% epoxy, enrichment may be said to be achieved, even though there is less than 50% of acrylic or other polymer concerned at the relevant surface. Complete separation, so there is a continuous phase of top-coat polymer, containing no base-coat polymer, over a continuous phase of base-coat polymer, containing no top-coat polymer, may not be ideal: in the absence of a gradient interface, such films may delaminate. Thus, a graduation in composi¬ tion of the resultant film may be preferred.

The monomer(s) starting materials for the top-coat polymer may be non-functional or may contain hydroxy, amide or ester groups, the polymer generally having no significant ionic functionality. It is important also that any functional groups on the polymer should not interact with the base-coat polymer or the curing agent therefor, or, if reaction is possible, reaction should not occur while the applied paint film is still wet. Thus, for example, a functional polymer in which the functional group (e.g. NH₂) is relatively inaccessible ("buried") could be used so that reaction between the top-coat polymer and the base-coat polymer or curing agent occurs when the paint film is dry, to give, for example, adhesion between the layers. Alternatively, a highly-functional top-coat polymer which reacts slowly can be used to provide a highly cross-linked surface for solvent resistance or an anti-stain barrier. N-methylol- acrylamide polymers, for example, are self-cross-linking on exposure to acid, and their use as a top-coat polymer with subsequent acid treatment of the dry paint film should especially be mentioned.

A curing agent may be used to provide cross-linking of a top-coat polymer. The curing agent may be able to react only with top-coat polymer, or may be able to react both with top-coat polymer and base-coat polymer. An example of the second case would be the use of an a ine curing agent when both top-coat and base-coat polymers contain epoxy functionality. In this type of system it is important to ensure that the reaction of top-coat polymer is slower than, or, in general, occurs after that of base-coat polymer. As mentioned above, curing after segregation may be advantageous; if desired this may be brought about by the use of a curing agent that reacts only at high temperature so that the film is air-dried first, then cured. Acrylic and vinyl polymers may be prepared by emulsion polymerisation to give latexes, and when these are to be used as top-coat polymer in the present invention the polymerisations should preferably be carried out in the presence of a steric stabiliser and non-ionic initiator. Top-coat polymers may also be prepared in non-aqueous conditions to give a dispersion that may be added to water containing a suitable stabiliser, preferably a steric stabiliser, and the resulting system subjected to distillation that removes solvent, the result being a (sterically) -stabilised dispersion of polymer. Such a procedure is disclosed in GB 2006229 A.

In the case of top-coat polymers that cannot be prepared by conventional emulsion polymerisation tech¬ niques the method disclosed in US 4,177,177 may be used, whereby the preferred polymer is dissolved in a suitable solvent, the solution dispersed in water containing suitable surfactants (preferably non-ionic) and the resulting dispersion homogenised. Solvent may be distilled off to give an aqueous dispersion containing polymer averaging <0.5μ in size. Top-coat polymer dispersions may also be prepared by the grafting of suitable hydrophilic monomers to the backbone of the polymer to be dispersed. This may be achieved by the selection of grafting initiators such as are disclosed in British Patent 1,101,983. Steric stabilisers are preferably block or graft polymers, or 'blocky' polymers, the components of which have different affinities towards the disperse and continuous phases of the system. The block polymers are usually of the AB or ABA type, whilst the grafts may be closely spaced (comb type) or randomly placed on the polymer backbone. Stabilisation may also be achieved by the use of polymerisable hydrophilic compounds that will covalently bond to the polymer particles. Such materials, known as macromonomers, are exemplified by the methacrylic ester of the mono-methyl ether of polyethyl¬ ene glycol and by unsaturated block polymers. The term "macromonomers" is used herein to denote linear macro- molecules ("pre-polymers" or "semi-polymers") that have a polymerisable function at one end only of the macro- molecule; hence chain extension can only be by branching. The polymerisable function may be an active double bond or other moiety that can undergo addition polymerisation, but may also be a group that can undergo polycondensation. A review of macromonomer synthesis and application may be found in, for example, Rempp P. , Franta E. et al., Advances in Polymer Sci. 58(1), 1985. Macromonomers that may be used as stabilisers in emulsion polymerisation are disclosed in GB 2127835 A, and include macromonomers of the type tipped with methacrylol or acrylol groups.

Preferred polymeric stabilisers are, for example, ABA block polymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) containing at least 40% PEO. We have found, for example, that 80% ethylene oxide and 20% propylene oxide combinations give excellent results. It is advisable not to fall below an overall molecular weight of 4,000 and not to exceed an upper limit of 16,000. Commercially available block polymers of ethylene oxide and propylene oxide are sold under the brand names of Pluronic (BASF) and Synperonic (ICI) , and such polymers are particularly well suited to act as steric stabilisers for the dispersions of this invention. Other non-ionic stabilisers may be used to prepare the dispersions, and examples include polyoxyethylene alkylphenols, e.g. ethoxylated nonyl phenol, polyoxyethyl¬ ene alcohols, polyoxyethylene esters of fatty acids, poly¬ oxyethylene alkylamines, polyoxyethylene alkylamides and polybutylene oxide - polyethylene oxide block poly- mers. Further examples may be found in "Non-ionic Surfactants", ed. M.J. Schick, Marcel Dekker, New York 1967.

Steric stabilisation results from the repulsive forces between particles that occur from loss of con- figurational entropy when the particles approach one another, and the solvated chains start to compress or overlap. The resultant loss in entropy is sufficient to overcome Van der Waals' - London forces of attraction, thus imparting stabilisation to the colloid by preventing close contact that would lead to flocculation. A full account of the steric stabilisation mechanism is given in Polymeric Stabilisation of Colloidal Dispersions, D.H. Napper, Academic Press 1983. As mentioned above, the stabiliser used to prepare the top-coat polymer dispersion may be used in excess, to provide the free surfactant that acts as segregation- promoting agent, although this is probably inappropriate with the macromonomers described above.

Suitably, therefore, a stabiliser is used in an amount of at least 0.1% by weight, for example at least 0.5% by weight, and, for example, up to 5% or 6% by weight or more, e.g. up to 2%, up to 3% or up to 8% by weight, advantageously 2 to 3% by weight if further surfactant is to be added, or at least 3%, preferably at least 4%, if no further surfactant is present, the amounts in each case being based on the total amount of top-coat monomer(s) . Higher amounts of stabiliser may also be suitable, for example up to 25% by weight, for example when methods of preparation of the dispersion other than emulsion polymerisation are used.

Free surfactant (whether a stabilising polymer or not) can be present during formation of the top-coat polymer dispersion or can be added subsequently.

The surfactant may contain, for example, a long chain hydrocarbon group, having for example at least 8 or at least 9 carbon atoms, and for example up to 20 carbon atoms, for example at least 12 carbon atoms, being for example a straight chain hydrocarbon group, and/or a chain of at least 3 ethylene oxide units, advantageously at least 4 ethylene oxide units, for example at least 5, especially at least 6, such units, and for example up to 40 or up to 30 such units. Polyethylene glycol units (hydrophilic) should also be mentioned. Various ethylene oxide surfactants are easily available commercially: Gafac PE510, for example has 6 ethylene oxide units, and Triton X405 has about 40 units, Gafac PE510 has a C_g hydrocarbon group and Dehyquart has a C₁₂ hydrocarbon group. Copolymers having at least 70 % of ethylene oxide units should be mentioned. Synperonic PE/F88 and PE/F68, which have given especially good results have, respectively, 214 and 154 ethylene oxide units (chains of 107 and 77 ethylene oxide units) and 40 and 30 propylene oxide units. These hydrophilic/hydrophobic ratios may also be used in other surfactants but, if desired, the hydrophile/lipophile balance may be altered by varying the percentage of ethylene oxide, and if necessary, the length of each hydrophilic and hydrophobic chain can be varied to assist segregation. The selection of the surfactant may, if appropriate, also be made by reference to surface tension.

The top-coat polymer dispersion preferably has low ionic strength and the initiator is preferably non-ionic. Suitable initiators are based, for example, on hydrogen peroxide or other peroxy compounds, with reducing agents such as, for example, ascorbic or tartaric acids, thiourea or tertiary amines (see, for example, GB 2127835 A) . Azo initiators of the type disclosed in GB 2120261 A may also be used. Thus the polymer produced will have hydroxy or other non-ionic functional groups. In the case where the top-coat polymer is prepared by emulsion polymerisation, the molecular weight of this polymer is suitably in the range of from 250,000 to

10,000,000. For example, an acrylic polymer may have a molecular weight up to 1,000,000, a non-acrylic polymer a molecular weight up to 10,000,000 and an acrylic/non- acrylic copolymer a molecular weight up to 2,000,000. With other polymerisation methods, molecular weight may be the same or may be lower, in which case the polymer is advantageously cross-linked to achieve suitable top-coat properties. Alkyds, for example, may have a molecular weight in the range 10,000 to 100,000, and siloxanes may have a molecular weight <50,000.

The non-volatiles content of the top-coat disper¬ sion A may be, for example, in the range of from 35 to 45%, e.g. 40%, by weight.

Preferably the mean particle size of the top-coat polymer is smaller than that of the base-coat polymer. The mean particle size of the polymers is preferably in the range of from 50 to 1000 nm, for example 50 to 800 nm, advantageously 100 to 800 nm, for example 200 to 800 nm, and for a component A polymer the size is preferably nearer the lower end of these ranges, advantageously up to 500 nm, for example up to 450 nm, especially up to 400 nm, for example in the range 150 to 400 nm, advantageously 200 to 400 nm, especially up to 350 nm, for example up to 300 nm, more especially in the range 200 to 350 nm, for example 200 to 300 nm; we have found a range 170 to 300 nm to be very suitable. The use of a micro-gel for component A should also be mentioned. To achieve such sizes reproducible emulsion poly¬ merisation techniques are preferred. Control of particle size may be achieved by varying the quantity of initiator or stabiliser used, by the type of stabiliser used, or by varying the conditions under which the polymerisation is carried out. For example, we have found a "seed and feed" method of emulsion polymerisation gives suitable particle sizes but may lead to a lower concentration of free polymer in comparison to other methods, and if this concentration is below the critical micelle concentration poor results could be obained; compensation may therefore be required in terms of the use of a higher quantity of stabiliser or other surfactant, and use of aggregating agent. The mean particle size of the base-coat polymer is preferably larger, for example at least 300 nm, more especially at least 350 nm, advantageously at least 400 nm, for example at least 450 nm, especially at least 500 nm. Reducing the ratio of the particle size of the top-coat polymer to base-coat polymer favours segregation by increasing the mobility of the top-coat polymer compared to the base-coat polymer, and preferably this ratio is in the range of from 1:1.5 to 1:4.

Accordingly, the present invention provides a water- borne coating system comprising a top-coat polymer and a base-coat polymer each in dispersion, preferably colloidal dispersion, the particle sizes of the two polymers being such that when the system is applied to a substrate and a film of the desired thickness is formed enrichment of the top polymer at the air interface or of the base polymer at the substrate interface is obtained. Advantageously, the polymers are monomodal and have a narrow size distribution.

Epoxy and alkyd polymers, which may be used as base coat, are not prepared by emulsion polymerisation. Suitable dispersions may be prepared by addition of a stabliser (surfactant) after polymerisation, and the polymer dispersed subsequently in water, for example as described in ICI commercial brochures. Alternatively, a modified epoxy or other base-coat polymer starting material may be used; for example, an epoxy resin may be reacted with an amine-tipped polyethylene oxide, or an acid-tipped polyethylene glycol polymerised with bis- phenol A and/or with bis-phenol A diglycidyl ether (e.g. Epikote 828) may be used. Stabilisation of a base-coat polymer colloidal dispersion may be ionic or non-ionic; steric stabilisation, for example as discussed above, optionally together with ionic stabilisation, should especially be mentioned. Functional groups present on base-coat polymers may be ionic or non-ionic. Advantage¬ ously, however, the base-coat polymer dispersion has some ionic character. We have, for example, prepared an epoxy resin stabilised with an acid-tipped polyethylene oxide chain. The resin therefore has electrosteric stabilisa¬ tion, with a small proportion of acid functionality. The acid functions were then converted to a salt to provide water dispersibility and the dispersion has shown good segregation. Similarly, alkyd dispersions may be prepared by stabilisation of one alkyd, with low acid number, with a solution of a pre-reacted alkyd having a high acid number (that alkyd being reacted with an amine to provide polar end groups) .

We have surprisingly found that segregation is promoted by aggregation of the base-coat polymer. For example, when a colloidal dispersion of an acrylic polymer with no electrostatic stabilisation is mixed with a suitable epoxy colloidal dispersion in the absence of a curing agent and with no or limited amount of free surface active compound, no segregation occurs. In the presence of a curing agent and especially with excess surfactant, however, we have obtained enrichment of the acrylic material in the top layer. However, such segregation occurs before any substantial curing occurs and while a significant quantity of water remains in the system. For example, we have obtained segregation in about 40 minutes or less, after initial lay-down, whereas drying of the film generally takes several hours and complete curing takes several days. We believe that the stabilisation of the top-coat dispersion is an important factor, and that the presence of curing agent leads to selective aggregation of the epoxy layer.

Thus, the system may contain, for example, a film- forming polymer in colloidally-stabilised form capable of network-formation to give a cross-linked film, together with a further component that causes selective aggregation of that film-forming polymer. This further component, the aggregating agent, may or may not give rise to curing of the base-coat polymer. The situation where the aggregating agent acts both to aggregate and to cure the base-coat polymer should especially be mentioned.

We have monitored the degree of segregation occur¬ ring during film-formation as a function of time, both under normal conditions, where there is evaporation of the water from the composition, and in the case where there are periods during which evaporation is prevented by covering the film. The increase in segregation as the water evaporated, observed under normal conditions, was temporarily suspended during such periods. We believe that as the water disappears from the system after lay- down, the solids content, and the density, increases, but this increase will be greater for the base-coat polymer. This is certainly the case for the 3 micron volume element nearest the substrate, as measured by FTIR-ATR. Thus, segregation is believed to occur due to selective aggregation of the base-coat polymer, causing it to be less mobile than the top-coat polymer. Top-coat polymer particles are then able to be carried towards the upper surface of the film by convection currents set up within the film due to evaporation of water.

While not wishing to be bound by theory, it is believed that the effect of excess stabiliser or other surfactant, which is not adsorbed on the top-coat polymer particles, is predominantly on the mobility on the top¬ coat polymer. We believe, that micelle formation, with hydrophilic moieties at the surface of the micelles increases mobility of the top-coat polymer, the micelles acting like ball bearings to "dilute" the interactions between the adsorbed stabilised polymers. Advantageous¬ ly, the hydrophilic moieties of the surfactant are the same as or chemically similar to those of the stabiliser. This increase in mobility of the top-coat polymer occurs during the initial period of water loss after the two dispersions are mixed (for example, until about 30 % of the water has evaporated) , and before substantial curing has occurred but while the aggregation of the base-coat polymer occurs. That is, the micelle formation keeps the top-coat polymer more mobile longer, as the solids content increases.

The longer the film remains mobile the greater the time available for segregation to occur: thus, for example, we have found that when the temperature at which film-formation occurs was increased from room temperature to 40°C the same level of segregation was not achieved. Advantageously, the top-coat polymer should be more mobile than the base-coat polymer at all stages during the film-forming process.

The identity of the top-coat polymer and the base- coat polymer, especially their stabilisation and mobility/viscosity before addition to the system, may affect the ease of segregation and hence affect the number and amount of segragation-promoting agent(s), for example, the need for an aggregating agent and/or the amount and identity of any free surfactant (a stabiliser or non-stabilising surfactant) that may be present. The amount of free surfactant, its identity and the need for a curing agent are also believed to be inter-dependent. The amount of aggregating agent added may also be used to control segregation. Preferably a curing agent is present in approximately stoichiometric amount compared with the base-coat B polymer. Curing reactions that allow ambient, sub-ambient and stoving conditions to be used may be chosen.

Suitable curing agents for epoxy polymers are, for example, polya ines and polyamides, for example aliphatic polyamines, modified amine curing agents (for example amine adducts with mono- and di- epoxies) , aromatic primary amines, secondary amines, and amides, including amidopolyamines or polyamine-polyamides or polyamido- amines; a mixture of a tertiary amines with a primary amine and/or secondary amine may also be used. Thus, for example, an excess of aliphatic amine is often reacted with an epoxy resin to provide an amine-functional adduct, or alternatively amines can be reacted with dimer acids to provide amine-functional polyamides. These products are often preferred to simple amines as they are less volatile, are less likely to cause skin sensitisa- tion, and have a more moderate rate of reaction with epoxies, which prevents excessive exothermic reactions occurring during curing. Multi-functional curing agents, for example amines having a functionality of 3 or more, are advantageous, and the curing agent may, for example, be linear or branched-chain. Advantageously the curing agents are water-soluble, and high molecular weight compounds should especially be mentioned. Curing agents with a molecular weight of at least 300, for example at least 400, advantageously at least 450, and especially at least 1200, preferably at least 1700, should especially be mentioned; these are typical of curing agents for epoxy resins. We have prepared suitable curing agents for epoxy resins by modification of an epoxy resin, for example Epi ote 828 by reaction with diamines, for example a Jeffamine and optionally isophorone diamine, advantageously with subsequent conversion to a salt to provide water solubility.

Such curing agents, for example polyamines, preferably polyamine-polyamides or polyamido-amines, act both as curing agent and aggregating agent. It is possible that with a sufficiently thin stabilising layer or a sufficiently long curing agent molecule a functional group on the curing agent can approach an epoxy molecule allowing association, while also allowing part of the curing agent molecule to project beyond the stabilising layer to allow association of a further functional group on that molecule with a different epoxy molecule, leading to selective aggregation. Thus, where there is steric stabilisation of the base-coat polymer dispersion the chain length of the hydrophilic part of the stabiliser is preferably less than the chain length of the aggregating agent. Alternatively, some functional stabilisation of the epoxy resin can associate with the curing agent, allowing aggregation; hydrogen bonding of an acid or hydroxy function with a polyamine-polyamide curing agent (which is a good hydrogen bond former) should especially be mentioned.

Acrylic resins used as base-coat polymers may, if desired, also have a common curing and agglomerating agent, and the principles and preferred features men- tioned above in conection with epoxy resin curing and agglomerating agents apply.

In the case of a self-curing base-coat polymer, aggregation of the particles may be brought about by use of an additional material. With alkyd resins, for example, which are oxidatively cured, the aggregating agent may be non-polymeric, for example a telechelic acid, or a polymer with hydrophobic and hydrophilic parts may be used to bridge between reactive groups and other sites on the resin; an associative thickener is such an example.

Base-coat polymers tested have included the epoxy resins Beckopox EP384 and two Epirez resins WJ3520 +

WJ55-5520 supplied by Rhone-Poulenc. These are described by their manufacturers as being non-ionically stabilised dispersions of solid bis-phenol A epichlorohydrin epoxide resins. Beckopox has a particle size »450 nm, Epirez WJ55-5520 «500 nm and Epirez WJ3520 ~250 nm as measured by light-scattering techniques. Beckopox EP384 gave excellent results with various acrylic latices, including those of acrylic particle size ^200 nm and ^300nm, and segregation was also obtained with the larger-sized Epirez polymer, but not with the smaller-sized one, which is consistent with our view that reduction in the ratio of top-coat polymer to base-coat polymer brings about an increase in the relative mobility of the top-coat polymer, and hence promotes segregation; other factors, such as the stabilisation may, however, be influential.

Suitably, a stabiliser for the base-coat component is used in an amount of at least 0.1% by weight, for example at least 0.5% by weight, and, for example, up to 5% by weight, e.g. up to 3% by weight, advantageously 2 to 3% by weight, calculated on the weight of the base-coat monomer(s) , especially if the polymer is prepared by emulsion polymerisation. Higher amounts of stabiliser may also be suitable. An amount of stabiliser in the range of from 0.1 to 25% by weight may, for example, be used, for example for other methods of preparation of the dispersion. The molecular weight of the base-coat polymer may be, for example, in the range of from 370 to 8000 for an epoxy resin, from 5000 to 100,000 for an alkyd resin, or from 3000 to 1,000,000 for an acrylic polymer.

The non-volatiles content of the base-coat colloidal dispersion B may be, for example, in the range of from 30 to 75, usually 40 to 60, weight percent.

The proportion of top-coat polymer to base-coat polymer may vary widely. Advantageously the top-coat polymer is 0.1 to 95%, for example at least 5%, and for example up to 90%, especially 10 to 90% by weight of the total weight of polymer in the composition. For example, there may be 10 to 20% by weight of top-coat polymer used to give enrichment at the air interface; formation of a continuous layer is more likely, however, with a higher proportion of this polymer, and an amount of from 10 to 50% of top-coat polymer is advantageous. The use of a mixture containing 80 to 99.9% of the top-coat polymer, and 0.1 to 20% of base-coat polymer acting as adhesion promoter or corrosion inhibitor should also be mentioned. In general, it is preferred that sufficient levels of top-coat polymer are used to provide enrichment of top- coat polymer at the upper surface compared to both the bulk composition and to the composition at the lower surface, and/or that a sufficient amount of base-coat polymer is used to provide enrichment of base-coat polymer at the lower surface compared to both the bulk composition and the composition at the top surface.

Accordingly, the present invention also provides a water-borne coating system comprising a top-coat polymer and a base-coat polymer, each in dispersion, for example, colloidal dispersion, and a component that brings about aggregation of the base-coat polymer, there also being free surfactant in the system, such that when the system is applied to a substrate selective enrichment of top¬ coat polymer at the air interface and/or of base-coat polymer at the substrate interface is obtained, the degree of this segregation being controlled if desired by adjustment of the proportion of the free surfactant in the system, or of the relative mean particle size of the top-coat and base-coat polymers, or of the proportions of the top-coat and base-coat polymers, or of the functionality, the water-solubility and/or the molecular weight of the aggregating agent, or of the ratio of aggregating agent to base-coat polymer, or by adjustment of two or more of these factors.

The present invention further provides a water-borne coating system comprising a colloidal dispersion of a top-coat polymer preferably having essentially no elec- trostatic stabilisation, a colloidal dispersion of a base-coat polymer preferably having some degree of electrostatic stabilisation, and an aggregating agent, preferably a curing agent, for the base-coat polymer, there also being free surfactant in the system the identity and concentration of the surfactant, the relative mean particle size and the proportions of the two polymers, the ratio of aggregating agent to base-coat polymer, and the water-solubility and/or the molecular weight of the aggregating agent being such that when the system is applied to a substrate and a film of the desired thickness is formed, enrichment of one polymer at the air or substrate interface is obtained.

The present invention further provides a water-borne coating system comprising a colloidal dispersion of a top-coat polymer having cationic or preferably steric stabilisation (with essentially no electrostatic stabil¬ isation) , a colloidal dispersion of a base-coat polymer, and an aggregating agent, preferably a curing agent, for the base-coat polymer, there also being free surfactant in the system, the relative mean particle size of the two polymers, the nature of the base-coat stabilisation, the identity and concentration of the free surfactant, and the ratio of aggregating agent to base-coat polymer being such that when the system is applied to a substrate and a film of the desired thickness is formed, enrichment of one polymer at the air or substrate interface is obtained.

A composition of the present invention may be pre¬ pared, for example, by simple mixing of components A and B, or, where component B includes a curing agent, by addition of that curing agent to dispersion A together, if desired, with additional surfactant, and/or with water to adjust the non-volatiles content, and subsequent mixing with the dispersion B. Alternatively, the dispersions A and B may be mixed, if appropriate, with additional surfactant, and the curing agent added subsequently: we have found the order of mixing is not important.

To achieve performance with respect to UV photo- degradation, various conventional (and usually expensive) additives may be incorporated into the top coat, either into the polymer itself or into the polymer particles. These include UV absorbers (e.g. benzotriazoles) , UV stabilisers (e.g. Hindered amines (HALS) , antioxidants, peroxide decomposers, metal deactivators, excited state quenchers, and photostabilised pigments. To achieve adhesion and corrosion protection, expensive additives may be confined to the base-coat, thus giving price performance. Examples include adhesion promoters (e.g. aminosilanes) , wetting agents (e.g. alkylureido wet adhesives) , anti-corrosive pigments (e.g. molybdates, tungstates and chromates) , corrosion inhibitors (e.g. phosphates and phosphonates) and corrosion cosmetic additives of the type used in the Interfine product such as, for example, calcium etidronate.

Other conventional materials may also be present in each dispersion, for example one or more additives selected from pigments, coalescence additives, rheology modifiers, defoaming agents and pigment dispersants.

One or more pigments may, for example, be present in each of the top-coat or base-coat dispersions, or in a dispersion containing both top- and base-coat polymers, and/or may be present as a separate component in the system, pre-dispersed in a mixture of water and a small amount of solvent. Typically, pigments have a particle size of around 200 nm. Control of pigment distribution in the resulting film may be achieved, if desired, for example by variation of pigment size and/or quality of dispersion and/or by selection of pigment to correspond in polarity to the relevant polymer: smaller particle sizes will favour pigmentation of the surface layer, as will a higher degree of dispersion; and a non-polar pigment, such as siloxane-treated Ti0₂, may, for example, be employed with an essentially non-ionic top-coat poly¬ mer dispersion, to provide a pigmented top-coat layer. A polar pigment such as Fe₂0₃ may be dispersed in the base- coat polymer dispersion to give a pigmented base layer.

The compositions of the present invention may be applied to the substrate by conventional means, for example by roller, brush, or spray. A substrate may be, for example, wood, metal, plastics or any other conventional substrate. Coatings are preferably 50-500 microns in thickness, for example 100-300 microns, advantageously 100-200 microns. Thinner coatings, for example of 5-100 micron thickness, may also be used.

The present invention further provides a process for coating a substrate using a mixture of two polymers, wherein there is applied a water-based coating composi¬ tion comprising a mixture of two polymers each in colloidal dispersion, the composition including a component that results in selective mobility of the top¬ coat polymer in the presence of the base-coat polymer, or selective aggregation of the base-coat polymer in the presence of the top-coat polymer, and wherein the rate of evaporation of the composition after application to the substrate is such that at at least one surface of the resulting film there is enrichment of one of the polymers compared with the mixture.

The system must, of course, contain two polymers or polymer dispersions that differ in some respect. Usually the polymers are different, but they are not necessarily so; for example, the dispersions may differ in the additives present, and in particle size of the polymer and/or in stabilisation of the dispersions.

Advantageously a composition of the present invention is air-dryable, but film formation may also be brought about by stoving, for example if the top-coat and base-coat polymers were present in the same dispersion in the coating system.

The following Examples illustrate the invention. All parts and percentages are by weight unless otherwise specified.

EXAMPLES The following products were used in the Examples.

STABILISERS/SURFACTANTS/POLYETHYLENE OXIDE ADDITIVES

1. Non-ionic

Synperonic PE/F38. PE/F68. PE/F88 and PE/F108 are poly¬ ethylene oxide-polypropylene oxide ABA block copolymers (PEO/PPO/PEO) (steric stabilisers) manufactured by ICI containing 80% PEO and having, respectively, molecular weights approximately 4800, 8350, 11,800 and 14,000, surface tensions at 20°C and at 0.1% w/v of 41.6, 43.8, 39.0 and 38.0 dyne/cm, and hydroxy values of 26-22, 15.5- 11.5, 11.8-7.8 and 8.9-6.5 mg KOH/g.

Ethoxylated nonylphenol NP30 is an adduct of 30 moles of ethylene oxide to 1 mole of nonylphenol, equivalent to a BA block copolymer. Fluorad FC171 is an ethoxylated perfluoroalkylsulphonate available from 3M.

Polyethylene glycol is polyethylene oxide of molecular weight 1000.

2. Cationic

Dehvσuart LT: lauryltrimethyla monium chloride available from Henkel Organics, UK. Fluorad FC135: perfluoroalkyl quaternary ammonium iodides available from 3M. 3. Anionic

Gafac PE510 is nonylphenol ethoxylate phosphate with 6 moles of ethylene oxide supplied by GAF.

Fenopen C0436 is nonylphenol ethoxylate sulphate having 3 or 3 moles of ethylene oxide, available from Rhone

Poulenc.

Aerosol MA-80: sodium dihexyl sulphosuccinate available from American Cyanamid.

Aerosol A-OT75 is dioctylsulphosuccinate supplied by

American Cyanamid.

Fluorad FC99: amine perfluoroalkylsulphonates available from 3M. Fluorad FC129: potassium perfluoroalkyl carboxylates available from 3M.

Aerosol A-102: Disodium ethoxylated alcohol half ester of sulphosuccinic acid available from American Cyanamid.

CURING AGENTS Beckopox EH623 is an internally modified aliphatic polyamidoamine curing agent manufactured by Hoechst, supplied as an 80% aqueous solution.

Epilink 8088 is an amine adduct supplied by Akzo as a 70% solution in water. CMD JT60-8536 is a modified polyamidoamine supplied by

Rhone-Poulenc as a 60% solution in 60% butoxyethanol/20% propoxyethanol/20% toluene.

CMD J60-8290 is a water-reducible amine adduct supplied by Rhone-Poulenc as a 60% solution in 95.5% propoxyethanol/4.5% acetic acid. SL RES 1084 is a modified aliphatic polyamine supplied by Shell as an 80% solution in water.

Jeffa ine D400 is a polyoxypropylene diamine, molecular weight approximately 400, supplied by Texaco. Jeffamine ED600 and ED4000 are diamine-tipped polyoxy- alkylenes, with molecular weights of approximately 600 and 4000 respectively. They are represented by the generalised structure:

CH₃ CH-, CH₃ H₂N-C IH-CH₂-(OCIHCH₂)_a-(OCH₂CH₂)_b-(OCH₂CIH)_C~NH₂

ED 600 : a+c approximately = 2.5, b approximately = 8.5 ED 4000 : a+c approximately = 2.5, b approximately = 86.0 Sylgard 184 curing agent is a curing agent for Sylgard 184 siloxane resin supplied by Dow Corning.

CURING AGENT SYNTHESIS Curing Agent 1

A solution of Epikote 828 (31.33g) in methoxy- propanol (13.46g) was added to a solution of Jeffamine ED600 (99.97g) in methoxypropanol (19.42g), over 1 hour while the temperature was maintained between 50 and 55°C. The temperature was then raised to 80°C and this temperature was maintained for 2 hours. This provided Curing Agent 1, which was completely water-soluble and which nominally contained 80% by weight curing agent, with a nominal active hydrogen equivalent weight of 263, and a nominal number average molecular weight, Mn, of 1576.

Curing Agent 2

A solution of Epikote 828 (41.67g) in methoxy¬ propanol (18.0g) was added to a solution of Jeffamine ED600 (99.91g) in methoxypropanol (17.4g), over 1 hour while the temperature was maintaind between 50 and 55°C. The temperature was then raised to 80°C and this tempera¬ ture was maintained for 2 hours. This provided Curing Agent 2, which was completely water-soluble and which nominally contained 80% by weight curing agent, with a nominal active hydrogen equivalent weight of 319, and a nominal Mn of 2552. Curing Agent 3

A solution of Epikote 828 (48.22g) in methoxy¬ propanol (20.83g) was added to a solution of Jeffamine ED600 (100.06g) in methoxypropanol (16.36g), over 1 hour while the temperature was maintained between 50 and 55°C. The temperature was then raised to 80°C and this tempera¬ ture was maintained for 2 hours. This provided Curing Agent 3 which was completely water-soluble and which nominally contained 80% by weight curing agent, with a nominal active hydrogen equivalent weight of 361, and a nominal Mn of 3853. Curing Agent 4

A solution of Epikote 828 (52.18g) in methoxy¬ propanol (15.73g) was added to a solution of Jeffamine ED600 (100.03g) in methoxypropanol (22.35g), over 1 hour while the temperature was maintained between 50 and 55°C. The temperature was then raised to 80°C and this tempera¬ ture was maintained for 2 hours. This provided Curing Agent 4 which was completely water-soluble and which nominally contained 80% by weight curing agent, with a nominal active hydrogen equivalent weight of 391, and a nominal Mn of 5480. Curing Agent 5

A solution of Epikote 828 (7l.56g) in methoxy¬ propanol (30.01g) was added to a solution of Jeffamine ED600 (37.82g) and isophoronediamine (50.8g) in methoxy¬ propanol (9.96g), over 1 hour while the temperature was maintained between 50 and 55°C. The temperature was then raised to 80°C and this temperature was maintained for 2 hours. This provided Curing Agent 5 which was not water- soluble. (This nominally contained 80% by weight curing agent, with a nominal active hydrogen equivalent weight of 150, and a nominal Mn of 930.) Curing Agent 5a

5g of Curing Agent 5 was mixed with distilled water (5g) and acetic acid (O.lg, 2.5% by weight on curing agent) to provide 5a, an opaque dispersion. Curing Agent 5b

5g of Curing Agent 5 was mixed with distilled water (5g) and acetic acid (0.2g, 5% by weight on curing agent) to provide 5b, an opaque dispersion. Curing Agent 5c

5g of Curing Agent 5 was mixed with distilled water (5g) and acetic acid (0.3g, 7.5% by weight on curing agent) to provide 5c, which was a semi-opaque dispersion. Curing Agent 5d 5g of Curing Agent 5 was mixed with distilled water (5g) and acetic acid (0.4g, 10% by weight on curing agent) to provide 5d, which was a slightly hazy solution.

The nominal active hydrogen equivalent weights were calculated assuming that each amine active hydrogen can react with one epoxy group, and that no side reactions occur.

_AHFW = Weight of amine + weight of epoxy

No. of equivalents of amine active H - no. equivalents epoxy

The nominal number average molecular weights (Mn's) of the curing agents were calculated in a similar way, assuming for simplicity that all of the amine active hydrogens are equally able to react with epoxy groups and that no side reactions occur. _ Weight of airline + weight of epoxy

Total moles of reagents - no. of equivalents of epoxy groups

EPOXY RESINS/DISPERSIONS Beckopox EP384 is a dispersion of a solid epoxy resin manufactured by Hoechst, supplied at 53% non-volatiles in 40:7 water: ethoxypropanol, with epoxy equivalent weight 525 and particle size 450nm. Epirez WJ3520 and WJ55-5522 are solid epoxy resin disper- sions manufactured by Rhone-Poulenc, supplied at 55% non- volatiles in water/2-propoxyethanol. WJ3520 has an epoxy equivalent weight of 535 and particle size 252nm; WJ55- 5522 has epoxy equivalent weight 625 and particle size 50θnm. Epikote 828 is a liquid bisphenol A-epichlorohydrin epoxy resin manufactured by Shell.

Epoxy Resin DER361 is an epoxy resin available from Dow Chemical Company, with an epoxy equivalent weight of 188.

SYNTHESIS OF EPOXY DISPERSIONS Epoxy Dispersion 1 (stabilised with acid-tipped polyethylene glycol (PEG) M. wt. 8000)

Polyethylene glycol (Mw 8000) (615g) , phthalic anhydride (22.8g), Ektasolve diacetate (789.8g) (an aromatic solvent available from Eastman Kodak) and tri-n- butylamine (14.7g) were combined and heated together for one hour at 130°C under nitrogen and with total return reflux.

194.7g of this pre-reacted PEG, 616.5g of epoxy resin DER361 (epoxy equivalent weight 188) and 314.9 g of bisphenol A were charged to a 3 litre flask equipped with a dispersion blade, nitrogen purge and reflux condenser. The vessel was heated to 130°C; an exothermic reaction occurred. The temperature was then held at 154°C and the reaction was continued to an epoxy equivalent weight of 800. Heating was stopped and Ektasolve EP (32.3g) was added. When the temperature had reached 90°C de-ionised water was added slowly under high shear. If necessary, that is, if an oil-in-water dispersion was achieved, a homogeniser was used to improve emulsification. The mixture was homogenised for 30 minutes whilst adding further water to a total of lOlOg of water. The emulsion was then neutralised with triethanolamine (9.2g) to stabilise the emulsion, thereby preventing settling.

PREPARATION OF PIGMENTED EPOXY DISPERSIONS. Pigmented Dispersions 1 and 2

A pigmented millbase was prepared by adding micronised red iron oxide Grade MRU complying to BS3981: 1976 (1985) Red, Category B, Type 1, Grade 1, Class B (35.8g) to Beckopox EP384 (72.4g) while stirring using a Dispersmat. The mixture was then stirred for a further 20 minutes to provide thorough dispersion of the pigment .

A pigmented epoxy dispersion was prepared by adding 23.8g of the millbase to 43.3g of Beckopox EP384 while stirring with a mechanical stirrer, for 10 minutes, to give Pigmented Dispersion 1, containing 20% w/w pigment, calculated on epoxy non-volatiles.

A pigmented epoxy dispersion was prepared by adding 38.4g of the millbase to 12.lg of Beckopox EP384 while stirring with a mechanical stirrer for 10 minutes, to give Pigmented Dispersion 2, containing 50% w/w pigment, calculated on epoxy non-volatiles. Pigmented Dispersions 3 and 4

A pigmented millbase was prepared by adding zinc phosphate complying to BS 5193:1975 (66.lg) to Beckopox EP384 (78.9g) while stirring using a Dispersmat. The mixture was then stirred for a further 20 minutes to provide thorough dispersion of the pigment.

A pigmented epoxy dispersion was prepared by adding 17.6g of the millbase to 48.4g of Beckopox EP384 while stirring with a mechanical stirrer, for 10 minutes, to give Pigmented Dispersion 3, containing 20% w/w pigment, calculated on epoxy non-volatiles.

A pigmented epoxy dispersion was prepared by adding 43.8g of the millbase to 14.3g of Beckopox EP384 while stirring with a mechanical stirrer for 10 minutes, to give Pigmented Dispersion 4, containing 50% w/w pigment, calculated on epoxy non-volatiles. SILOXANE RESINS

Sylgard 184 is a vinyl-tipped siloxane elastomer suppied by Dow Corning.

Magnasoft TP405 is an amine-functional silane emulsion supplied by Union Carbide.

PREPARATION OF SILOXANE DISPERSIONS Siloxane Dispersion 1

Sylgard 184 elastomer (36.4g) , Sylgard 184 curing agent (4.06g) and platinum-divinyl tetramethylsiloxane complex (2 drops) were briefly stirred together. The mixture was then dispersed by stirring it together with Synperonic PE/F68 (2.0g) and distilled water (58.lg), using a Silverson stirrer/emulsifier for 5 minutes. Siloxane Dispersion 2 A functional siloxane polymer was prepared by blending 100 parts of an α,ω-hydroxy-functional dimethyl- siloxane polymer (viscosity 7.5 poise, 25°C) and 4 parts of methyl-tris(methylethylketoxime)εilane in the absence of atmospheric moisture. The resulting fluid siloxane polymer had a viscosity of 15.2 poise at 25°C. 38.73g of this material was mixed with tetraethylorthosilicate (1.19g), and then the mixture was dispersed by stirring together with Synperonic PE/F68 (1.99g) and distilled water (58.01g) using a Silverson stirrer/ emulsifier for 5 minutes. LATEX SYNTHESIS

Methyl methacrylate/butyl acrylate latices Steric stabilisation Latices 1-6 Method

The polymerisation was carried out in a 51 flanged reaction vessel equipped with stainless steel stirrer, reflux condenser, nitrogen inlet, thermocouple and two peristaltic pump inlets. The vessel was immersed in a waterbath maintained at 60°C. The latex was prepared by a semicontinuous method in which the feed rate is intended to be lower than the rate at which the slowest polymerising monomer is consumed, to prevent build-up of monomer and consequent composition drift, but adapted to establish a population of "seed" particles, grown during the main feed stage. The following reagents were used (Latex 1) :

Note:

5% surfactant calculated on monomer weight 0.5% hydrogen peroxide "100 volume" calculated on monomer weight 0.25% ascorbic acid calculated on monomer weight Method

Items 1 and 2 were charged to the vessel then stirred for 60 minutes under a nitrogen purge for 1 hour to allow the temperature to equilibrate. The "seed" stage was initiated by the addition of 24g of items 7 and 8, followed by 23g of items 5 and 6 and 37g of items 3 and 4. The polymerisation was allowed to proceed for 15 minutes prior to beginning the "feed" stage. 83.4g of items 3 and 4 were added to the remaining items 5 and 6 to provide item 9. The remainder of 7 and 8 was added over 180 minutes. Item 9 was added over 90 minutes, followed by the addition of the remainder of 3 and 4 over a further 90 minutes. The reaction mixture was kept in the waterbath for a further 60 minutes, then allowed to cool before filtering through a 50 μm nylon mesh. 3774g of filtered latex was obtained, containing 40.7% non- volatiles. The average particle size, determined by a dynamic light scattering technique, was 231nm.

Latices 2 and 3 were prepared using the identical procedure as for Latex 1.

Latices 4 and 5 were prepared also using the Latex 1 procedure but, for Latex 4, using 1% ascorbic acid (calculated on the monomer weight) ; for Latex 5, with replacement of the Synperonic PE/F68 with 5% Synperonic PE/F88; and for Latex 6, with replacement of the Synperonic by 5% ethoxylated nonylphenol NP 30. The products had the following characteristics:

Latices 7-13 Method

The polymerisation was carried out in a 1 litre flanged reaction vessel equipped with stainless steel stirrer, reflux condenser, nitrogen inlet, thermocouple and two peristaltic pump inlets. The vessel was immersed in a waterbath maintained at 60°C. The latex was prepared by a semi-continuous process. The feed rate is intended to be lower than the rate at which the slowest polymerising monomer is consumed, to prevent a build-up of monomer and consequent composition drift, but differed from the method of Latex 1 in having a single feed stage, and no seed stage.

Latex 7 was prepared as follows:

Distilled water (263g) was charged to the vessel and heated with stirring under a nitrogen purge for 1 hour to allow the temperature to equilibrate. Meanwhile, Synperonic PE/F88 (103.6g) was dissolved in distilled water (72.7g). Methyl methacrylate (144.4g) and butyl acrylate (118.lg) were mixed. The Synperonic PE/F88, distilled water, methyl methacrylate and butyl acrylate were charged to a beaker and formed into an emulsion using a Silverson mixer set to maximum speed. The process was continued until the viscosity of the emulsion exceeded 25 seconds as measured using a Ford Cup 4 [according to ASTM method D1200-88]. Hydrogen peroxide (5.0g) was added to the emulsion which was mixed for a further one minute. Ascorbic acid (2.1g) was dissolved in distilled water (75. Og) . The reaction was conducted by adding simultaneously at 60°C feeds of the Synperonic PE/F88, distilled water, methyl methacrylate, butyl acrylate and hydrogen peroxide, and the ascorbic acid and distilled water. The feed rates were arranged such that both feeds were complete in 3 hours; however, the feeds were suspended if the reaction temperature exceeded 60°C, and continued when it had cooled to 60°C. The latex was held at 60°C for one hour after completion of the feeds, and then cooled to below 30°C. The product was filtered through a 80μm mesh. 712.5g of filtered latex was obtained, containing 33.8% non-volatiles. The average particle size, determined by a dynamic light scattering technique, was 460nm.

Latices 8-13 were prepared by the same procedure as for Latex 7, but using different types and levels of stabiliser, and with appropriate variation of the amount of distilled water used.

The products had the following characteristics:

Latex Stabiliser Non-volatiles Particle size content range

6% PE/F88 33.8? 350-550nm Av 460nm weighted >450nm

Cationically stabilised latex Latex 19

Dehyquart L.T. (lauryltrimethylammonium chloride) (15g) , distilled water (432.6g) and ascorbic acid (2.1g) were charged into a 1 litre flask and heated at 60°C whilst purging with nitrogen. Butyl acrylate (11.8g) and methyl methacrylate (14.4g) (5% of the monomers) were charged to the reactor and left for 15 minutes. Hydrogen peroxide (0.26g) and distilled water (1.9g) (5% of the initiator) were added to initiate the feed stage and left for 15 minutes. Methyl methacrylate (129.98g) and butyl acrylate (106.32g) were then added to the reactor over 3 hours whilst the temperature was maintained at 60°C; hydrogen peroxide (4.89g) and distilled water (35.6g) were added to the reactor simultaneously with the monomer feed. The mixture was held at 60°C for 1 hour, after all the additions, before cooling to 30°C and filtering through a 80 micron mesh.

Anionicallv-stabilised latex Latex 20

Distilled water (3l8g) was charged to a 1 litre flask whilst purging with nitrogen at 60°C. Aerosol A OT75 (7g) and distilled water (129.5g) were added to a 700 ml flask followed by methyl methacrylate (I44.4g) and butyl acrylate (118. lg) added under vigorous stirring.

Hydrogen peroxide (5.15g) was then added and the emulsion was left for 1 minute. The initiator, ascorbic acid (2.1g) was dissolved in distilled water (27.9g). The two feeds were added simultaneously to the reactor over 3 hrs. The system was then held at 60°C for 1 hr, and then cooled to 30°C before filtering through a 80 micron mesh.

Vinyl acetate homopolymer latex Latex 21

A vinyl acetate homopolymer latex was synthesised using the same procedure as for Latex 1, giving a product containing 39.0% non-volatiles, with an average particle size of 194 nm.

Styrene/acrylic latex Latex 22

A styrene/acrylic latex was synthesised using the same procedure as for Latex 1, but using a waterbath temperature of 80°C. Butyl acrylate (314.lg), methyl methacrylate (174.5g) and styrene (134.6g) were polymeri¬ sed using Synperonic PE/F68 (31.2g), distilled water (894.6g), hydrogen peroxide (3.12g of "100 volume") and ascorbic acid (1.56 g) . 30 minutes after all of the monomer had been charged, a further 0.170g of "100 volume" hydrogen peroxide in 1ml of water was added, followed after another 30 minutes by a final addition of 0.163g hydrogen peroxide in 1ml of water. The polymer- isation was allowed to continue for another 30 minutes before cooling and filtering. A latex containing 31.7% non-volatiles, with an average particle size of 200 nm was obtained.

PREPARATION OF PIGMENTED ACRYLIC LATICES Pigmented Dispersions 5 and 6

A pigmented millbase was prepared by adding rutile titanium dioxide (Tioxide RH D2 type) (66.Og) to Latex 4 (64.6g) while stirring using a Dispersmat. The mixture was then stirred for a further 20 minutes to provide thorough dispersion of the pigment.

A pigmented acrylic latex was prepared by adding 15.4g of the millbase to 92.3g of Latex 4 while stirring with a mechanical stirrer for 10 minutes, to give Pigmented Dispersion 5, containing 20% w/w pigment calculated on acrylic non-volatiles.

A pigmented acrylic latex was prepared by adding 38.9g of the millbase to 92.3g of acrylic Latex 4 while stirring with a mechanical stirrer for 10 minutes, to give Pigmented Dispersion 6, containing 50% w/w pigment calculated on acrylic non-volatiles.

CONDUCTIVITY MEASUREMENTS ON EPOXY EMULSION

The conductivity of the water-based epoxy emulsion Epoxy Dispersion 1 (as prepared) , having a solids level of 50.6%, was measured. The effect upon the conductivity of changing the solids level of the emulsion was examined, as was the effect of the addition of Synperonic (F88) (solids level 15.2%) . The results are summarised in the following Table:

Epoxy % Synperonic Conductivity

% solids (calculated as solids (μS cm^-1)

Synperonic to solids epoxy)

50.6 409 50.6 2.0 410 25.3 384 25.3 0.5 370 25.3 2.0 376 25.3 5.0 371 12.6 238 12.6 2.0 245

It can be seen that the conductivity falls as the emulsion solids level is reduced (as expected) . The addition of Synperonic PE/F88 does not appear to have any significant effect upon the conductivity of the emulsion. Any adsorption of the Synperonic onto the epoxy surface would reduce the mobility of the epoxy particles and thus reduce the conductivity. As evidenced by these results this does not appear to happen, which is consis¬ tent with the view that in mixed systems the role of the additional surfactant is to influence the mobility of the top-coat polymer.

FILM PREPARATION

Unless otherwise stated, all films were prepared at 3mm nominal wet film thickness, were allowed to dry at room temperature, and were evaluated for segregation when dry. Films which appeared to the naked eye as two distinct layers showed gross segregation. Determination of the extent of enrichment in these and other films (thin films, 100 microns, were also prepared in some instances) was carried out in each case by Attenuated Total Reflectance Fourier Transform Infra Red analysis (ATR-FTIR) to establish the composition of the upper surface and optionally also the lower surface, each to a depth of approximately 2μm, the acrylic content being calculated from the ratio of IR bands at 1730cm"^*1 and 1508cm"¹.

Example 1 Top coat polymer: methyl methacrylate/butyl acrylate Beckopox EH623 (0.5g) was stirred with distilled water (15.6g) , Latex 3 (4.8 g) , containing Synperonic PE/F68, was added, followed by the addition of Beckopox EH384 (2.6g), and the mixture was stirred thoroughly with a spatula to provide a blend containing nominally 15% non-volatiles, with equal weights of acrylic copolymer and epoxy+curing agent. An aliquot of the blend (7.1g) was weighed into a 5.5cm diameter aluminium dish, to provide a nominal wet film thickness of 3mm. The film was allowed to dry at ambient temperature over 4 days. The film contained two distinct layers: an upper translucent flexible layer and an opaque brittle lower layer. ATR-FTIR showed the upper surface to contain approximately 80% by weight acrylic copolymer; the lower surface contained 20% acrylic copolymer.

Example 2

Top coat polymer: methyl methacrylate/butyl acrylate

Beckopox EH623 (l.Og) , distilled water (3.0g), Latex 5 (8.4g), containing Synperonic PE/F88, and Beckopox EP384 (5.0g) were mixed as described above, to provide a blend containing nominally 40% non-volatiles and equal weights of acrylic copolymer and epoxy+curing agent. Films with nominal wet film thickness of 3mm and 100 microns were prepared in aluminium dishes, then allowed to dry at ambient temperature. Visual examination of the dry film indicated two layers of approximately equal thickness: a translucent flexible upper layer, over an opaque brittle lower layer. ATR-FTIR analysis of the thick film indicated that the upper surface contained 90% acrylic and the lower contained 30% acrylic. The upper surface of the thin film contained 65% acrylic and the lower surface contained 55% acrylic.

Example 3

Top coat polymer: vinyl acetate homopolymer

Beckopox EH623 (l.Og), distilled water (2.41g), and Latex 21 (8.84g) , containing Synperonic PE/F68, were stirred together before adding Beckopox EP384 (5.03g) to give a mixture containing nominally 40% non-volatiles. A sample of the mixture was placed in an aluminium dish to give a nominal wet film thickness of 3 mm, then allowed to dry. The dry film contained two layers. FTIR indicated that the top surface contained 80% vinyl acetate and the lower surface contained approximately equal amounts of epoxy and vinyl acetate.

Example 4 TOP coat polymer: styrene/acrylic polymer

Beckopox EH623 (0.99g) and Latex 22 (11.30g), containing Synperonic PE/F68, were stirred together before adding Beckopox EP384 (5.05g). A sample of the mixture was placed in an aluminium dish to give a nominal wet film thickness of 3 mm, then allowed to dry. The dry film was uniformly opaque, with no evidence of two layers being present. FTIR indicated that the top surface contained 70% styrene/acrylic.

Example 5 Siloxane/epoxy segregating systems

(a) 8.77g of the Sylgard Siloxane Dispersion 1 prepared above, containing Synperonic PE/F68 stabiliser, was mixed with Beckopox EP384 (5.10g) , distilled water (2.68g) and Beckopox EH623 (0.99g) . A film was applied to a glass slide using a 200μm cube applicator, and a thick film (nominal wet film thickness 3mm) was also prepared by placing a sample into an aluminium dish. The thin film dried to give a layer of essentially pure siloxane over an epoxy-rich lower layer. The thick film gave two distinct layers, a lower brittle epoxy-rich layer and a soft siloxane-rich upper layer; the top surface was more than 90% siloxane.

(b) Beckopox EH623 (0.98g), distilled water (2.64g), Siloxane Dispersion 2 (8.80g), containing Synperonic PE/F68 stabiliser, and Beckopox EP384 (4.98g) were stirred together. A thick film (nominal wet film thickness 3mm) was prepared. After drying at room temperature two distinct layers were visible: an upper siloxane-rich layer above an epoxy-rich lower layer. In this case, the final coating was better from the point of view of overall mechanical properties than that of Example 5a, having a harder siloxane surface; i.e. it was a fully cured, but nevertheless flexible, coating.

Example 6 (a) Silane/epoxy segregating system

Magnasoft TP405 (9.06g) (38.1% NVs; average particle size 18lnm determined by dynamic light scatter¬ ing) , Beckopox EP384 (5.0g), Beckopox EH623 (l.Og) and distilled water (l.Og) were stirred together. Thick (approx 3mm wet film thickness) and thin (approx. 200μm wet film thickness) films were prepared as described above. Upon drying, the thick film had two distinct layers of approximately equal thickness. The upper layer appeared to be silicone-rich, as sellotape would not adhere to the film. ATR-FTIR confirmed that the top 2μm was very silicone-rich (estimated 90%+ silicone) The thin film was uniformly opaque, but also contained a silicone-rich upper surface as shown by the sellotape test and by FTIR (which showed a silicone content in the upper 2μm similar to that observed in the thick film) .

Example 7

Preparation of pigmented epoxy/acrylic blends Pigmented Blend 1

Beckopox EH623 (4.2g), distilled water (11.2g) , Pigmented Dispersion 3 (23.9g) (epoxy) and Pigmented Dispersion 5 (acrylic, containing Synperonic PE/F68 stabiliser) (40.Og) were mixed to provide Pigmented Blend 1, nominally containing 44% non-volatiles. 7.1g of the pigmented blend was placed in a 5.5cm diameter aluminium dish to provide a thick film, and a film was also applied to a glass plate using a 400 μm draw-down cube. Both films were allowed to dry at ambient temperature, then examined visually. The thick film was analysed by ATR- FTIR to determine the composition of the resin in the surface 2μm. The upper surface contained 68% acrylic and the lower surface contained 40% acrylic. Pigmented Blend 2a Beckopox EH623 (3.8g), distilled water (10.Og), Pigmented Dispersion 4 (27.6g) (epoxy) and Pigmented Dispersion 6 (40.lg) (acrylic, Synperonic PE/F68 stabiliser) were mixed to provide Pigmented Blend 2a, containing nominally 50% non-volatiles. Thick and thin films were prepared, and the thick film was analysed by ATR-FTIR. The upper surface contained 69% acrylic and the lower surface contained 55% acrylic. Pigmented Blend 2b 20g of Pigmented Blend 2a was diluted with distilled water (5.0g), to provide Pigmented Blend 2b, containing nominally 40% non-volatiles. Thick and thin films were prepared, and the thick film was analysed by ATR-FTIR. This showed that the upper surface contained 90% acrylic and the lower surface contained 66% acrylic. Pigmented Blend 3a

Beckopox EH623 (4.2g) was stirred with distilled water (23.9g) before adding Pigmented Dispersion 5 (40.Og) and Pigmented Dispersion 1 (23.9g). The blend was stirred thoroughly to provide Pigmented Blend 3a, containing nominally equal weights of acrylic copolymer and epoxy+curing agent, with an overall nominal non¬ volatile content of 44%.

The thick film showed a pink and white upper surface (approximately equal areas of each colour) ; the thin film upper surface was uniformly pink. ATR-FTIR analysis showed that the upper surface of the thick film contained 84% acrylic and the lower surface contained 45%, and the upper surface of the thin film contained 72% acrylic, the lower surface 55%. Pigmented Blend 3b 20g of Pigmented Blend 3a was diluted with distilled water (2.2g) , to provide Pigmented Blend 3b, containing nominally 40% non-volatiles. Thick and thin films were prepared as for Pigmented Blend 1.

The dried thick film had a uniformly white upper surface; the surface of the thin film was white in the centre with pink edges.

ATR-FTIR analysis of the thick film showed that the upper surface contained 87% acrylic and the lower surface contained 55% acrylic. The upper surface of the thin film contained 80% acrylic and the lower surface contained 55% acrylic. Pigmented Blend 4a

Beckopox EH623 (3.8g), distilled water (10.lg), Pigmented Dispersion 2 (27. lg) and Pigmented Dispersion 6 (40. lg) were mixed to provide a blend containing nominally 50% non-volatiles and 50% pigment on resin weight. Thick and thin films were prepared as for Pigmented Blend 1.

The upper surfaces of both the films appeared uniformly pink.

ATR-FTIR of the thick film showed that the upper surface contained 77% acrylic and the lower surface con- tained 55% acrylic. The upper surface of the thin film contained 66% acrylic and the lower surface contained 58% acrylic.

Pigmented blend 4b 20g of Pigmented Blend 4a was diluted with 5g of distilled water to provide a pigmented blend containing nominally 40% non-volatiles. Thick and thin films were prepared as for Pigmented Blend 1. The thick film had an almost uniformly white surface, with a small region (approximately 5% of the total surface area) containing pink flecks. The surface of the thin film was mainly white, turning to pink at the edges.

ATR-FTIR of the thick film showed that the upper surface contained 87% acrylic and the lower surface con- tained 60% acrylic. The upper surface of the thin film contained 85% acrylic and the lower surface contained 57% acrylic. Layers were cut through the thin film, using a microtome, to allow determination of the composition profile throughout the film depth. The results are tabulated below:

Depth from top surface/μm

25

32

39

44 59 69 85

There appeared to be a strong correlation between the colour of the layers and the resin composition.

These results show that the composition of the film was constant over the top 30 microns.

Pigmented Blend 5 (pigmented epoxy + unpigmented acrylic latex

Beckopox EH623 (4.2g), distilled water (11. Og), Pigmented Dispersion 3 (16.4g) and Latex 4 (37.lg) (containing Synperonic PE/F68 stabiliser) were mixed to provide Pigmented Blend 5. Thick and thin films were prepared. Both of the films showed a clear top layer above a pigmented lower layer. ATR-FTIR of the thick film showed the upper surface contained 84% acrylic and the lower surface contained 45% acrylic. The upper surface of the thin film contained 85% acrylic and the lower surface contained 40% acrylic. Pigmented Blend 6 (pigmented epoxy + unpigmented acrylic latex

Beckopox EH623 (4.2g), distilled water (11.Og), Pigmented Dispersion 1 (18.7g) and Latex 4 (37.0g) (containing Synperonic PE/F68 stabiliser) were mixed to provide Pigmented Blend 10. A thick film was prepared; this showed a clear top layer above a pigmented lower layer. ATR-FTIR showed the upper surface contained 86% acrylic and the lower surface contained 50% acrylic.

The results show that it is possible to achieve segrega¬ tion in pigmented films to give significant enrichment of the top-coat polymer at the upper surface.

Example 8

Segregation behaviour with different epoxy resins 3mm films were prepared as above from 1:1 acrylic:epoxy mixtures using Latex 11 (containing 5% Synperonic PE/F88) and three different epoxy resins. In each case the curing agent was Beckopox EH623 used in a 1:1 stoichiometry. The experiment was repeated to check replication of results. These are given below. EPOXY ACRYLIC COMPOSITION TOP SURFACE

1ST RUN REPEAT

a. Beckopox 80 75 b. Epoxy Dispersion 1 90 80 c. Epirez WJ55-5522 85 85

Example 9 Effect of increasing the proportion of top-coat stabiliser

Films were prepared and analysed in the usual way using blends of different epoxy resins and acrylic latices containing different amounts of Synperonic PE/F88 stabiliser.

With only 2% of stabiliser results were variable: segregation could not be achieved reliably. However, good segregation was achieved with higher amounts of the stabiliser. Results are given below.

EPOXY LATEX % PE/F88 TOP SURFACE ANALYSIS IN LATEX

RATIO OF % ACRYLIC BANDS AT 1730cm^-1 & 1508cm^"1

g. Epoxy Disper¬ sion 1 10 85 h. Epoxy Disper¬ sion 1 7 i. Epoxy Disper¬ sion 1 12 85

Example 10

Segregation behaviour with different top-coat stabilisers In experiments a-f, EH623 curing agent (lg) was added to distilled water and the mixture was stirred into solution before adding 4g of latex and stirring into solution; the weight of water used was such as to maintain a non-volatile content of 40%. 5.7g of Beckopox EP384, or equivalent of other epoxy dispersion, to give a 1:1 ratio of acrylic:epoxy was then stirred in. 6.9g were weighed into an aluminium dish and left to cure at room temperature at 25°C. 3mm films were prepared, and the acrylic content of the top coat determined. Films were also prepared using latices stabilised with Synperonic PE/F88 and PE/F68. The results are shown in the Table below.

Segregation was possible with each of Synperonic PE/F38, 68 and 88 and NP30. Segregation was best with Latices 13, 14, 15 and 16, containing 2% and 5% of PE/F38 and PE/F68, although the difference between the result with 2% PE/F88 and 2% PE/F38 was not significant. Latex 6, with 5% NP30, gave less segregation than expected, probably attributable to its different method of prepara¬ tion: the seed and feed procedure appears to lead to binding of more of the stabilising polymer, leaving less free polymer in solution. sSegregation did not occur with the 5% PE/F 108 stabiliser, probably because the longer polyethylene oxide chain precluded the formation of micelles in solution.

Example 11

Effect of different post-added stabilisers

3mm films were prepared from three different acrylic latices and three different post-added stabilisers, used with the same epoxy and curing agent (Beckopox EP384 and Beckopox EH823) , and at the same solids level as normal (40%) . In each case the ratio of acrylic:epoxy was 1:1, and the stabiliser was added to the stabilised latex in an amount of 3% calculated on the weight of acrylic monomer. The compositions of the blends were as follows:

Latex 18 19.δg¹ Stabiliser 0.7g ¹ 19.7g when NP30 Curing agent 2.0g post-added Water 2.2g stabiliser used Epoxy 10.Og

Latex 6 8.4g Stabiliser 0.7g ² 9.9g when NP30 Curing agent 2.0g post-added Water 3.4g stabiliser used Epoxy 10.Og²

Latex 8 19.2g 2.65g when NP30 Stabiliser 0.7g post-added Curing agent 2.0g stabiliser used

Water 2.6g³

Epoxy 10.Og⁴ 9.9g when PE/F88 post-added stabiliser used

Results were as follows:

* PEG = polyethylene glycol mw 1000

In experiments a-f, segregation occurred in all cases, although Latex 6 (seed and feed) did not show as excellent segregation as the others. Comparison of the results with the 2% PE/F88 stabiliser in Examples 9, 10 and 11 (Latex 8 in this Example and Latex 9 in Example

10) clearly showed the improvement in segregation in this Example attributable to the addition of further stabiliser preparation of the latex. PEG, which has no hydrophobic moiety, and is not a surfactant, failed to provide segregation in experiments h and i. Segregation was obtained in experiment g, but probably attributable to the presence of 5% NP30 in the latex itself (and note the difference between experiments g and h, attributable solely to the method of preparation) .

Post-addition of 3% of Synperonic PE/F88 to a latex stabilised with 2% of the same stabiliser in a cor- responding experiment also gave excellent results: 80% acrylic in the top surface with Beckopox, 90% with Epoxy Dispersion l and 85% with Epirez WJ55-5522. Good segregation was also obtained when the ratio of epoxy:Beckopox curing agent was 1:0.5.

Example 12

Effect of post-added surfactants of different ionic character

Films were prepared from a blend of Latex 8 (con¬ taining Synperonic 2% PE/F88) , Beckopox EP384 epoxy resin and Beckopox E8623 curing agent using a range of post- added surfactants, non-ionic, cationic and anionic. The blends were made up to correspond to the composition of Example 11(c), except that the amounts of surfactant and water were adjusted as near as possible to account for the varying concentrations of the surfactants as supplied. The compositions prepared and the acrylic content of the top surface of 3mm films prepared therefrom are listed below; the components for each composition are listed in the order of their addition.

Theoretical composition

Curing agent 2g

Surfactant (0.68g of 30% solution)

Water (2.66g + or - adjustment)

Latex 19.16g

Epoxy log

a. Gafac PE510 (100% active: 0.17g surfactant + 0.41g water, of which O.lg is concentrated ammonia to bring the surfactant into solution) .

Acrylic content of top surface Curing agent 2.02g

Latex 19.20g

Surfactant 0.19g 80'

Water 3.08g (O.lg ammonia) Epoxy 9.99g

b. Fenopen C0436 (58% active: 0.3g of surfactant + 0.28g of water) .

70!

c. Dehyquart LT (35% active: 0.5g surfactant + 0.08g water) .

75-^*

d. Aerosol MA80 (80% active: 0.22g surfactant + 0.36g water) .

25%

e. Aerosol A-102 (31% active: 0.56g surfactant + 0.02g water) .

90%

f. Fluorad FC99 (25% active: 0.7g surfactant, subtract 0.12g of water) .

Curing agent 2.0ig Latex 19.14g 55%

Surfactant 0.7lg Water 1.61g

Epoxy lO.OOg g. Fluorad 129 (50% active: 0.35g surfactant + 0.23g water) .

60?

h. Fluorad FC 135 (50% active: 0.35g surfactant + 0.23g water) .

60?

i. Fluorad FC 171 (100% active: 0.17g surfactant + 0.41g water) .

70%

The best results were obtained with the ethoxylated surfactants:

- Gafac PE510 and Fenopen C0436

where R = P0₄ ²~2NH₄ ⁺ and n = 6 (Gafac) and S0 ~NH₄ ⁺ and n = 3 or 4 (Fenopen) (compare NP30 where n = 30 and R = OH) Aerosol A-102

where R = H and n = 8 and Fluorad FC171 ^cn^F2n+l^so2^N(^c2H5) (^C 2 H₂0)_χCH₃ (n ^8) as well as with Dehyquart, which possesses a long-chain hydrocarbon group.

Comparison of results of Fluorad FC171 with the other Fluorad surfactants, for example Fluorad FC129

C_nF_2n+_iS0₂N(C₂H₅)CH₂C00^~K⁺ (n -^ 8) and

Fluorad FC135 C_nF_2n+1S0₂NHC₃H₆N⁺(CH₃)₃I^" (n 8) strongly suggests the importance of ethylene oxide units, and comparison of Gafac PE510 and Fenopen C0436 shows that a 6-unit ethylene oxide chain is apparently better than a 3-unit chain.

Example 13

Effect of ionic stabilisation of the top-coat dispersion Films were prepared in the usual way from a 1:1 epoxy:acrylic blend containing cationically stabilised Latex 19 or anionically stabilised Latex 20, each with addition of 5% of Synperonic PE/F88 added after prepara- tion, and containing Beckopox curing agent: (1:1 stoichiometry) . Results were as follows:

Epoxy

a. Epirez WJ55-5522 b. Epoxy Dispersion 1 c. Epirez WJ55-5522

Although segregation was obtained, the results did not appear as favourable as with non-ionic stabilisation of the top coat, more especially with the anionically- stabilised latex where extensive coagulation occurs with amine epoxy curing agents.

Example 14

Effect of variation in curing agent type on segregation Several commercial, in-house, and simple amine curing agents were examined for their effect on segrega¬ tion. The curing agent was mixed with distilled water, and Latex 1 and then Beckopox EP384 were added to give a 1 : 1 stoichiometric ratio of active H : epoxy (based on the nominal active hydrogen equivalent weight of the curing agents) , a 1 : 1 weight ratio of (epoxy + curing agent) : acrylic, and an overall nominal non-volatiles content of 40%. The mixtures were placed in aluminium dishes to give nominally 3 mm thick wet films. Films were examined visually and by FTIR. (i) The results for the commercial and simple amine curing agents are summarised in the Table below:

SEGREGATION BEHAVIOUR

GROSS % ACRYLIC (BY FTIR) APPEARANCE TOP BOTTOM

Two layers 80 20

Single layer 60 50

Two layers 65 20

Two layers 60 20

Two layers 70 25

Single layer 60 50

Single layer 60 50

Single layer 60 50

Single layer 60 50

* Weight-average molecular weight as determined by GPC. The results for the five commercially available water-reducible curing agents show that only one of them, CMD JT60-8536, failed to provide segregation. The CMD JT60-8536 has a lower molecular weight (determined by GPC of the salicylaldehyde derivative, relative to poly¬ styrene standards) than the other curing agents. This curing agent was insoluble in water; i.e. it was water- dispersible rather than water-soluble.

The low molecular weight amines diethylenetriamine and 1,6-hexanediamine also failed to give bilayer films. These amines are able to react with epoxy resins at room temperature, but are expected to be too short to provide "bridging" between epoxy particles which is believed to provide selective aggregation of the epoxy. Jeffamine ED4000 is a higher molecular weight water-soluble amine, but has a lower functionality than the commercial curing agents. Texaco report that the reaction of ED4000 with epoxy resins is sluggish at room temperature in the absence of a cure accelerator. Jeffamine D400 was water-insoluble, and failed to give adequate segregation.

(ii) Films prepared using Curing Agents 1, 2, 3 and 4 prepared above gave uniformly opaque films with no sign of gross stratification. Films prepared with curing agents 5a, 5b and 5c also gave uniformly opaque films with no evidence of gross segregation, and the composition of the upper and lower surfaces were the same: not markedly different. The film prepared with curing agent 5d, however, showed two distinct layers of approximately equal thickness; the upper surface contained 80% acrylic, and the lower surface contained 35% acrylic.

The curing agents 1-4 were all completely soluble in water, and had nominal Mn's greater than that of Beckopox EH623, but failed to provide stratification. As indi¬ cated above, Texaco state in their commercial literature that the reaction between epoxy resins and the Jeffamine ED series amines (on which the curing agents 1-4 were based) is too sluggish at room temperature to allow these materials to be used as curing agents unless a cure accelerator is used.

The curing agents 5a-5c contain isophorone diamine, which is capable of reacting with epoxy resins at ambient temperatures. However, the curing agents were not soluble in water, and these materials failed to give segregation. The curing agent 5d was water-soluble and did provide a segregating system.

These results suggest that it is preferable to use a high molecular weight, water-soluble curing agent with sufficient reactivity to react with the epoxy component while the film is still mobile. Example 15

Effect of variations in ratio of curing agent : epoxy dispersion a) Beckopox EH623 (0.5g) was stirred together with distilled water (3.41g) until dissolved, Latex 2 (5.34g) was added, then thoroughly mixed prior to adding Beckopox

EP384 (5.0g). This provided a mixture containing a 1:1 weight:weight ratio of acrylic polymer : epoxy+curing agent, and a 0.5 : 1 ratio of curing agent active hydrogens : epoxy functional groups. After thorough stirring, 7.1g of the mixture was weighed into an aluminium dish to give a nominal wet film thickness of 3mm. The film was allowed to dry at room temperature, then examined by visual inspection of a cross-section cut through the film. The film was uniform in appearance throughout the film depth; that is, it showed no gross segregation. The compositions of the upper and lower surfaces were analysed using ATR-FTIR. The upper surface was found to contain 80% acrylic polymer, the lower surfaces containing 50% acrylic polymer. b) Beckopox EH623 (0.75g), distilled water (3.66g), Latex 2 (5.34g) and Beckopox EP384 (5.0g) were mixed as in a) ; this provided a ratio of curing agent active hydrogens : epoxy functional groups of 0.75 : 1. The film was found to have stratified into two layers approximately equal in thickness: an upper flexible layer with a lower brittle layer. The upper surface contained 80% acrylic polymer and the lower surface con¬ tained 57% acrylic polymer. c) Beckopox EH623 (l.Og) , distilled water (3.91g), Latex 2 (5.34g) and Beckopox EP384 (5.0g) were mixed as in a) giving a l : 1 ratio of curing agent active hydrogens : epoxy functional groups. The film was found to have stratified into two layers approximately equal in thickness: an upper flexible layer with a lower brittle layer. The upper surface contained 80% acrylic polymer and the lower surface contained 25% acrylic polymer. d) Beckopox EH623 (2.0g), distilled water (4.91g), Latex 2 (5.34g) and Beckopox EP384 (5.0g) were mixed as in a) to give a 2 : 1 ratio of curing agent active hydrogens : epoxy functional groups. The film was uniformly opaque throughout the film thickness, i.e. the film had not stratified to provide two distinct layers visible to the naked eye. The upper surface of the dry film had the same composition as the bulk composition of the film: that is, there was no enrichment of acrylic at the upper surface. However, the lower surface contained 10% acrylic: an enrichment of epoxy at the lower surface.

In all four cases segregation was achieved, but the maximum segregation was obtained, giving bilayer films, when an approximately 1 : 1 ratio of curing agent active hydrogens : epoxy groups was used. At the lowest level of curing agent (0.5 : 1) only a single layer was obtained, with no visual evidence of segregation, and at a higher level of curing agent (2 : 1) a single layer was also obtained. This shows that the amount of curing agent can be used to control the extent of segregation.

Example 16

Omission of curing agent

(a) A separate film was prepared from a blend containing Latex 3 (3.43g), Beckopox EP384 (2.5g) and distilled water (11.8g) , using the same procedure as described above, to prepare a blend containing nominally 15% non-volatiles and equal weights of acrylic copolymer and epoxy. The dried film was uniformly opaque, with no evidence of gross differences in appearance throughout the film depth. ATR-FTIR analysis indicated that the composition of the upper and lower surfaces were the same; no segregation had occurred.

(b) Magnasoft TP405 (6.96g), Beckopox EP384 (5.0g) and distilled water (l.29g) were stirred together, then cast to provide thick and thin films. The thick film was uniformly opaque and showed no sign of a bilayer structure. The behaviour of the Magnasoft TP405/epoxy system appears similar to that of the acrylic/epoxy system (a) in that stratification was only observed in the presence of curing agent.

(c) Following the method of Example 10, 3mm films were prepared from Beckopox EH623, distilled water, Latex 10 (containing 4% PE/F88) and Beckopox EP384 epoxy dispersion.

(i) With a 1:1 stoichiometry of curing agent to epoxy resin (approximately 1 part curing agent to 5 parts epoxy resin) , 85% acrylic was found in the top surface;

(ii) with half the amount of curing agent a similarly excellent stratification was observed; (iϋ) when the curing agent was omitted, however, the acrylic content of the top surface dropped to 60%.

Example 17

Effect of variations in non-volatiles content on segregation Beckopox EH623 (4.29g) and Latex 3 (21.34g) were thoroughly stirred together prior to the addition of Beckopox EP384 (37.96g), and the mixture was stirred again to provide a blend containing equal weights of acrylic copolymer and of epoxy+curing agent non- volatiles. The overall nominal non-volatile content of the blend was 40%.

A series of blends containing 20, 25, 30 and 35% non-volatiles was prepared by the dilution of aliquots of the initial blend with distilled water. Films of each of the blends, of 3mm nominal wet film thickness, were prepared in aluminium dishes, and allowed to dry at ambient temperature. All of the dried films were similar in appearance, with two distinct layers: a translucent flexible layer over an opaque brittle lower layer; segregation had occurred in each case.

Example 18

Effect of variation of acrylic:epoxy ratio on segregation

A series of blends of Latex 2, Beckopox EH623, Beckopox EP384 and distilled water were prepared, each contained an overall nominal non-volatile content of 37%. A constant weight ratio of 5:1 Beckopox EP384 : Beckopox EH623 was employed in each blend. The weight % of acrylic copolymer as a percentage of the overall non- volatiles. was varied from 7% to 28%. Films of nominal 3mm wet film thickness were prepared as described above. The composition of the upper and lower surfaces were analysed by ATR-FTIR; all films showed an enrichment of acrylic in the upper surface. The results are tabulated below:

% Acrylic in % Acrylic in % Acrylic in initial blend upper surface lower surface 28 66 ιo

17 30 18 7 19 10 The results show that even at 7% acrylic polymer in the initial blend there was an enrichment of acrylic at the upper surface compared to the bulk composition, and compared to the composition of the lower surface.

Example 19

Effect of variations in film thickness on segregation

Beckopox EH623 (l.Og), distilled water (3.91g), Latex 2 (5.34g) and Beckopox EP384 (5.0g) were mixed as in Example a). 1.4g of the mixture was weighed into an aluminium dish to provide a nominal wet film thickness of lmm. Films were also applied to glass plates using different cube applicators, providing nominal wet film thicknesses of 200 and 100 μm. Films were allowed to dry, then carefully removed from the glass using a razor blade. The compositions of the upper and lower surfaces were determined using ATR-FTIR. The results are tabulated below:

Segregation is believed to occur due to selective aggregation of the base-coat polymer, causing it to be less mobile than the top-coat polymer. Top-coat polymer particles are then able to be carried towards the upper surface of the film by convection currents set up within the film due to evaporation of water. The longer the film remains mobile the greater the time available for segregation to occur. It might therefore be expected that thicker films, which take longer to dry, would provide the greatest opportunity for segregation to occur. Thick films (3mm wet film thickness) were used in other Examples; these films took up to 24 hours or more to dry. Clearly, for practical coatings use, such thick films and long drying times are not preferred, and film thicknesses for coatings use may lie, for example, in the range 50-200μm. These films would dry much more rapidly, but the above results demonstrate that significant segregation can be obtained for films in this range. At lmm wet film thickness the extent of segregation was similar to that for films of 3mm wet film thickness. Thinner films showed an enrichment of acrylic at the upper surface compared to the bulk composition or to the composition at the lower surface.

Example 20

Segregation behaviour with dispersion of differently- sized epoxy resin Beckopox EH623 (l.Og), distilled water (2.74g),

Latex 2 (8.86g) and Epirez WJ3520 (4.93g) were mixed to provide a blend containing nominally 40% non-volatiles. A film with a nominal wet film thickness of 3mm was prepared in an aluminium dish. The dried film appeared uniformly opaque, with no evidence of any difference in the appearance of the film throughout the film depth. The composition of the upper and lower surfaces were analysed by ATR-FTIR and found to be the same.

Example 21

Investigation of the mechanism by which segregation occurs

(a) A 50:50 blend of the epoxy and acrylic resins with curing agent, as in Example 1, was cast onto an optical prism at 40 wt % solids. During the evaporation process the composition of the coating was monitored at the substrate coating interface by FTIR.ATR. The results shown in Figure 1 are a quantitative estimate of the epoxy and water concentration in the bottom 3μm of the coating. In this particular system it can be seen that the epoxy resin concentration, relative to the acrylic resin concentration, increased from the initial value of 50:50 to 75:25, over a period of 1 hour. During this period the concentration of water at the interface decreased from 60 wt % to below 50 wt %.

The experiment was repeated with the coating covered; the result is shown in Figure 2: by preventing evaporation segregation was inhibited. These experiments show that the mechanism by which segregation occurs is directly related to the evaporation process, and in the absence of evaportion the blend is a stable dispersion. (b) A 50:50 blend of siloxane:epoxy resins with curing agent, as detailed in Example 5a was applied to a substrate and the composition of the bottom 3μm of the coating (or more accurately the 3μm adjacent to the optical prism) was monitored during film-formation. Figure 3 shows a series of FTIR.ATR spectra taken at different time intervals during the evaporation process (A = 2 min, B = 15 min C = 120 min and D = 1100 min, after the initial lay-down process) . The broad band at 1650 cm^"1 is representative of water and is observed to decrease with increasing evaporation time. The bands at 1180, 1230 and 1510 cm"¹ are diagnostic for the epoxy resin and are clearly shown to be increasing during the evaporation process. The band at 1255 cm^-1 is diagnostic for the siloxane resin and is clearly shown to be decreasing relative to the epoxy bands. In this case too the segregation process can be halted by preventing further evaporation. This would not be the case if segregation were attributable solely to preferential sedimentation of the epoxy resin; rather, there appears to be preferential mobility of the siloxane resin/prefer¬ ential sedimentation of the epoxy resin as a function of water content.

Claims

1. A water-borne coating system comprising a top¬ coat polymer and a base-coat polymer each in dispersion, and including a component that results in increased mobility of the top-coat polymer relative to the base- coat polymer.

2. A water-borne coating system comprising a top¬ coat polymer and a base-coat polymer each in dispersion, and including a component that results in selective aggregation of the base-coat polymer in the presence of the top-coat polymer.

3. A water-borne coating system comprising a top- coated polymer and a base-coated polymer each in disper¬ sion, and including free surfactant in an amount above the critical micelle concentration, and optionally an aggregating agent for the base-coat polymer.

4. A coating system as claimed in claim 2 or claim 3, wherein the aggregating agent is a curing agent for the base-coat polymer.

5. A water-borne coating system comprising a top¬ coat polymer and a base-coat polymer each in dispersion, and including excess surfactant and a curing agent for the base-coat polymer.

6. A coating system as claimed in any one of claims 1 to 5, wherein the top-coat polymer and base-coat polymer are present in separate colloidal dispersions.

7. A coating system as claimed in any one of claims 1 to 6, wherein there is substantially no electro¬ static stabilisation of the top-coat polymer dispersion.

8. A water-borne coating system comprising

(A) a dispersion of a top-coat polymer having substan¬ tially no electrostatic stabilisation,

(B) a dispersion of a base-coat polymer, and including a curing agent for the base-coat polymer and sufficient non-adsorbed surfactant to form micelles.

9. A coating system as claimed in any one of claims 1 to 8, wherein the dispersion of the top-coat polymer is prepared by emulsion polymerisation.

10. A coating system as claimed in any one of claims 1 to 9, wherein the total of top-coat polymer stabiliser and any additional free surfactant is at least 3% by weight, calculated on the weight of the top-coat momomer(s) .

11. A coating system as claimed in claim 10, which includes at least 3% of the top-coat polymer stabiliser, calculated on the weight of the top-coat monomer ( s ) .

12. A coating system as claimed in any one of claims 1 to 11, which includes excess of the top-coat polymer stabiliser.

13. A coating system as claimed in any one of claims 1 to 12, which includes as free surfactant a block copolymer of ethylene oxide with a relatively hydrophobic monomer.

14. A coating system as claimed in claim 13, wherein the block copolymer is an ABA block copolymer of ethylene oxide and propylene oxide containing 80% of ethylene oxide and having a molecular weight less than 12000.

15. A coating system as claimed in any one of claims 1 to 12, which includes as free surfactant an adduct of ethylene oxide and nonylphenol.

16. A coating system as claimed in any one of claims 13 to 15, wherein the free surfactant contains from 6 to 30 units of ethylene oxide.

17. A coating system as claimed in any one of claims 1 to 16, wherein the dispersion of the top-coat polymer is stabilised by a polyethylene oxide/polypropyl- ene oxide block copolymer having at least 40% polyethyl¬ ene oxide or by a macromonomer.

18. A coating system as claimed in any one of claims 1 to 17, wherein the top-coat polymer has substan¬ tially no ionic functionality.

19. A coating system as claimed in any one of claims 1 to 18, wherein the base-coat polymer has some ionic stabilisation.

20. A coating system as claimed in any one of claims 1 to 19, wherein the dispersion of the base-coat polymer is sterically or electrosterically stabilised.

21. A coating system as claimed in claim 20, wherein there is no or minimal electrostatic stabilisa¬ tion of the base-coat polymer dispersion.

22. A coating system as claimed in claim 20 or claim 21, wherein the stabiliser for the base-coat polymer dispersion comprises a polyethylene oxide/ polypropylene oxide block copolymer having at least 40% polyethylene oxide, or a macromonomer, provided the stabilisation is different from the stabilisation of the top-coat polymer dispersion.

23. A coating system as claimed in any one of claims 1 to 22, which includes an aggregating agent for the base-coat polymer that is water-soluble.

24. A coating system as claimed in any one of claims 20 to 23, which includes an aggregating agent for the base-coat polymer in which the chain length is greater than the chain length of the hydrophilic part of the steric stabiliser used for the base-coat polymer dispersion.

25. A coating system as claimed in any one of claims 1 to 24, wherein there is used a curing agent for the base-coat polymer having a molecular weight of at least 1500.

26. A coating system as claimed in any one of claims 1 to 25, wherein a curing agent for the base- coat polymer is present in approximately stoichiometric amount compared with the base-coat polymer.

27. A coating system as claimed in any one of claims 1 to 26, wherein the dispersions are colloidal dispersions.

28. A coating system as claimed in any one of claims 1 to 27, wherein the mean particle size of the top-coat polymer is less than that of the base-coat polymer.

29. A coating system as claimed in claim 28, wherein the ratio of mean particle size of the top-coat polymer to mean particle size of the base-coat polymer is in the range of from 1:1.5 to 1:4.

30. A coating system as claimed in any one of claims 1 to 29, wherein the top-coat polymer has a mean particle size up to 500 nm.

31. A coating system as claimed in any one of claims 1 to 30, wherein the base-coat polymer has a mean particle size of at least 500 nm.

32. A coating system as claimed in any one of claims 1 to 31, wherein the top-coat polymer is 10 to 90% by weight of the total polymer in the composition.

33. A water-borne coating system comprising a dispersion of a top-coat polymer having steric stabilisa¬ tion and essentially no electrostatic stabilisation, a dispersion of a base-coat polymer having some electro¬ static stabilisation, and an aggregating agent for the base-coat polymer,and including excess surfactant, such that when the system is applied to a substrate and a film of the desired thickness is formed, enrichment of one polymer at the air or substrate interface is obtained.

34. A coating system as claimed in any one of claims 1 to 33, wherein the system is air-dryable and/or curable, at ambient temperature or below.

35. A coating system as claimed in any one of claims 1 to 34, wherein a UV absorber, a UV stabiliser, an antioxidant, a peroxide decomposer, a metal deacti- vator, an excited state quencher or a photostabilised pigment is incorporated into the top-coat polymer or into its dispersion.

36. A coating system as claimed in any one of claims 1 to 35, wherein an adhesion promoter, a wetting agent, an anti-corrosive pigment, a corrosion inhibitor or corrosion cosmetic additive is incorporated into the base-coat polymer or into its dispersion.

37. A coating system as claimed in any one of claims 1 to 36, wherein the top-coat polymer is an acrylic or other vinyl polymer or copolymer of any two α,β-ethylenically unsaturated monomers, or is an alkyd resin or a polyεiloxane.

38. A coating system as claimed in any one of claims 1 to 37, wherein the base-coat polymer is an epoxy or alkyd resin or an acrylic or other vinyl polymer or is a copolymer of any two α,β-ethylenically unsaturated monomers.

39. A coating system as claimed in any one of claims 1 to 38, wherein the top-coat polymer is a polysiloxane and the base-coat polymer is an epoxy.

40. A process for the preparation of a coating composition, which comprises combining the components specified in any one of claims 1 to 3, 5, 8 and 33.

41. A coating composition whenever prepared by a process as claimed in claim 40.

42. A process for coating a substrate, wherein there is used a coating composition as claimed in claim 41.

43. A process for coating a substrate using a mixture of two polymers, wherein there is applied a water-based coating composition comprising a mixture of two polymers each in colloidal dispersion, the composi¬ tion including a component that results in selective mobility of the top-coat polymer in the presence of the base-coat polymer, and wherein the rate of evaporation of the composition after application to the substrate is such that, at at least one surface of the resulting film, there is enrichment of one of the polymers compared with the mixture.

44. A substrate coated by a coating system as ^r1 C 2

claimed in any one of claims 1 to 39 and 41.

45. A substrate coated by a process as claimed in claim 42 or claim 43.