Aqueous polymer dispersion for coatings and adhesives

Abstract

The invention relates to a dispersion of polymer particles in aqueous medium, which comprises: (a) a first vinyl polymer, which first vinyl polymer has a glass transition temperature (Tg1) of from 20°C to 120°C, a number average molecular weight (Mn1) of from 500 to 50,000 g / M and an acid number of at least 20; and (b) a second vinyl polymer, which second vinyl polymer has a glass transition temperature (Tg2) of from -60°C to 100°C and an acid number lower than the acid number of the first vinyl polymer; wherein Tg1 is greater than Tg2; and wherein the first vinyl polymer comprises a branching point derived from a chain transfer agent having three or more reactive moieties. It also relates to a process for producing the dispersion; to a coating composition or an adhesive composition comprising the dispersion; and to a coated substrate or composite material comprising the dispersion.

Description

[0001] AQUEOUS POLYMER DISPERSION FOR COATINGS AND ADHESIVES

[0002] Field of the Invention

[0003] The present invention relates to an aqueous polymer dispersion comprising a first vinyl polymer and a second vinyl polymer, useful in compositions for coatings and adhesives.

[0004] Background of the Invention

[0005] Aqueous polymer dispersions are commonly used as resins for paints and coatings. They are desirable, compared with solvent-based paints and coatings, because the resulting products have low volatile organic compound (VOC) content, which is beneficial for environmental and health reasons. Moreover, legislation continues to limit the permissible amount of VOC in products.

[0006] Film formation of coating compositions is an important consideration. Minimum film forming temperature (MFFT) is a critical parameter in polymer dispersion-based coatings. It determines the lowest temperature at which the composition can coalesce into a continuous film during drying. Generally, there is an objective to improve film forming of aqueous polymeric dispersions. There are three common approaches to do this. One is to provide a polymer having a lower glass transition temperature (Tg). However, polymers with lower Tglead to coatings with compromised hardness and blocking resistance. A second approach is to increase the polarity of the polymer. This however will reduce water resistance of the coating formed. A third approach is to add a cosolvent to the composition. However, this will increase the VOC content. In summary, achieving proper film formation of aqueous polymer dispersion-based and coatings while maintaining good film properties of the dried coating is a long existing difficult balance.

[0007] Currently many commercial binders use oligomeric stabilization as a means of producing binders with exceptionally low levels of low molecular weight surfactants. The synthesis of these oligomeric surfactants relies solely on monofunctional thiols as chain transfer agent and binders made therewith exhibit good performance.

[0008] EP0758364B2 describes a process for producing organic solvent-free, aqueous crosslinkable polymer compositions. The compositions may be used for coatings and having an acceptably high hardness and a low minimum film forming temperature. The process involves forming a hydrophobic polymer from olefinically unsaturated monomer by emulsion polymerization in the presence of an acid-functional oligomer built from olefinically unsaturated monomers, and subsequently combining with a cross-linking agent. A chain transfer agent may be used in the formation of the oligomer. 3-mercaptopropionic acid and dodecyl mercaptane are exemplified.

[0009] US2023 / 287235A1 concerns a process for producing an aqueous vinyl polymer dispersion from two vinyl oligomers and a vinyl polymer, each of which components may comprise a chain transfer agent. US2023 / 142644A1 describes an aqueous vinyl polymer dispersion comprising an aqueous dispersion of three vinyl polymers, each of which may comprise a chain transfer agent. In US8975332B2 a process for preparing a multiphase particle dispersion from two monomer mixtures is described. Each of these references describe sulphonated and non-sulphonated mono and bi-functional chain transfer agents.

[0010] US2012 / 157551A1 concerns use of branched addition copolymers possessing melt or solution viscosities lower than the linear polymer analogues of equivalent or greater weight. Chain transfer agents described include mono- and multi-functional thiol chain transfer agents. US2003 / 013822 describes a method of preparing a branched polymer comprising reacting together a monofunctional vinylic monomer with a polyfunctional vinylic monomer in the presence of a chain transfer agent, which may be chosen from a range of thiol compounds including monofunctional and polyfunctional thiols.

[0011] US 5,820,925 concerns two-component paints based on a hydroxyl-functional polymer and a polyisocyanate cross-linker or hardener for automotive applications. The hardener component uses a tin catalyst and a polyfunctional thiol having at least three thiol groups; the nature of the thiol in the catalyst influences the rate of increase in viscosity of the paint when mixed, or “potlife”. In addition, mono-functional mercapto functional compounds are described as chain terminating agents in the acrylic polymer described.

[0012] There remains a need for a low VOC-content aqueous coating composition which has both a low minimum film forming temperature and leads to a coating having sufficient hardness and chemical resistance. The present inventors have found that in a dispersion of multiple vinyl polymers, employing a multifunctional chain transfer agent in the polymerization to form a vinyl polymer leads to a reduction in MFFT, without negatively affecting chemical resistance or hardness of the resulting aqueous polymer dispersion. This finding allows three key advantages: i) binders with reduced acid value can be made at comparable MFFT, which leads to improved water resistance; ii) higher Tgbinders can be produced at lower MFFT leading to increased hardness; and iii) lower MFFT allows for a reduction or elimination of the amount of cosolvent needed to obtain proper film formation.

[0013] In a first embodiment, the present invention provides dispersion of polymer particles in aqueous medium, which comprises:

[0014] (a) a first vinyl polymer, which first vinyl polymer has a glass transition temperature (Tgi), calculated according to the Flory Fox equation, of from 20°C to 120°C, a number average molecular weight (Mni), measured by gel permeation chromatography, of from 500 to 50,000 g / M and an acid number, calculated from the amount of acid groups present in the polymer, of at least 20; and

[0015] (b) a second vinyl polymer, which second vinyl polymer has a glass transition temperature (Tg2), calculated according to the Flory Fox equation, of from -60°C to 100°C and an acid number lower than the acid number of the first vinyl polymer; wherein Tgiis greater than Tg2; and wherein the first vinyl polymer comprises a branching point derived from a chain transfer agent having three or more reactive moieties.

[0016] In a second embodiment, the present invention provides a process for producing a dispersion of polymer particles in aqueous medium as defined herein, said process comprising:

[0017] (a) preparing a first monomer mixture, said first monomer mixture comprising: i. a first mixture of vinyl monomers; ii. a carboxylic acid-functional monomer; and iii. a chain transfer agent having three or more reactive moieties;

[0018] (b) carrying out emulsion polymerization of the first monomer mixture in water, in the presence of at least one surfactant and at least one initiator, to form an aqueous dispersion of a first vinyl polymer;

[0019] (c) preparing a second monomer mixture, said second monomer mixture comprising a second mixture of vinyl monomers; (d) carrying out emulsion polymerization of the second monomer mixture in the presence of the aqueous dispersion of a first vinyl polymer and an initiator to form an aqueous dispersion of polymer; and

[0020] (e) adding after either or both of steps (b) and (d) a base to, at least partly, neutralize acid groups present in the polymer(s).

[0021] In a third embodiment, the present invention provides a coating composition or an adhesive composition, which coating composition or adhesive composition comprises a dispersion of polymer particles in aqueous medium as defined herein and at least one additive selected from a color pigment, extender pigment, coalescing solvent, co-solvent surfactant, plasticizer, pH modifier, defoaming agent, thickener, leveling agent, matting agent, anti-setting agent and biocide.

[0022] In a fourth embodiment, the present invention provides a coated substrate comprising a substrate and a coating, which coated substrate is obtainable by applying a coating composition as defined herein to the substrate and allowing the coating composition to dry.

[0023] In a fifth embodiment, the present invention provides a composite material comprising a first component, a second component and an adhesive layer between the first component and the second component, wherein the composite material is obtainable by applying an adhesive composition as defined herein to at least one surface of the first component; bringing the coated surface of the first component into contact with the second component and allowing the adhesive composition to dry.

[0024] Detailed Description

[0025] As used herein, vinyl polymer is any polymer comprising olefi nically unsaturated monomers, i.e. monomers comprising a C=C bond. The term polymer in this context includes low molecular weight polymers which might also be termed oligomers. In particular the first vinyl polymer referred to herein may be considered conventionally to be either a polymer or an oligomer. The vinyl polymer may be a homopolymer but is typically a copolymer. Each of the first vinyl polymer and the second vinyl polymer comprise olefinically unsaturated monomers. Any suitable olefinically unsaturated monomer may be used. Typical monomers include non-acid-functional monomers, for example acrylate and methacrylate ester, styrene, vinyl ester, unsaturated halide, diene, vinyl amide and nitrile; acid-functional monomers or monomers bearing an acid-forming group; and cross-linking monomers.

[0026] Methacrylate includes an optionally branched alkyl ester of a C1-12 alcohol and methacrylic acid, for example methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate and cycloalkyl methacrylate, for example isobornyl methacrylate or cyclohexyl methacrylate. Acrylate includes an optionally branched alkyl ester of a C1-12 alcohol and acrylic acid, for example methyl acrylate, ethyl acrylate, n-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, and cycloalkyl acrylate, for example isobornyl acrylate, or cyclohexyl acrylate. Styrene includes styrene itself and a substituted styrene, for example a-methyl styrene, o-, m- and p-methylstyrene, o-, m- and p-ethylstyrene, p-chlorostyrene, p-bromostyrene and t-butyl styrene. Vinyl ester includes vinyl acetate and vinyl alkanoates. Unsaturated halide includes vinyl halide, for example vinyl chloride and vinyl fluoride and vinylidene halide, for example vinylidene chloride. Diene includes 1,3-butadiene and isoprene. Vinyl amide includes- N-vinyl pyrrolidone and N-vinyl caprolactam. Nitrile includes acrylonitrile and methacrylonitrile.

[0027] The non-acid functional monomer may be selected from one or more of methyl methacrylate, styrene, ethyl acrylate, n-butyl methacrylate and n-butyl acrylate. An acidfunctional monomer may be for example methacrylic acid and / or acrylic acid. A polymer may comprise a non-acid functional monomer selected from one or more of methyl methacrylate, styrene, ethyl acrylate, n-butyl methacrylate and n-butyl acrylate and an acid-functional monomer selected from methacrylic acid and / or acrylic acid.

[0028] Acid-functional monomer, or a monomer bearing an acid-forming group, includes an anhydride, for example methacrylic anhydride or maleic anhydride, or an acid chloride. It may also have cross-linking functionality. Typically the acid-functional monomer is a carboxyl-functional acrylic monomer or other ethylenically unsaturated carboxyl bearing monomer, for example acrylic acid, methacrylic acid, maleic acid, itaconic acid or fumaric acid. A sulphonic acid-bearing monomer could also be used, for example styrene p- sulphonic acid (or correspondingly styrene p-sulphonyl chloride). An acid bearing monomer could be polymerised as the free acid-or as a salt, for example the ammonium or alkali metal salt of ethylmethacrylate-2-sulphonic acid or 2-acrylamido-2- methylpropane sulphonic acid, or the corresponding free acids. Acid functional monomers can have cross-linking functionality. For example carboxylic acids can act as cross-linkers.

[0029] In one embodiment the first vinyl polymer comprises one or more alkyl-methacrylate; one or both of methacrylic acid and acrylic acid; and one or both of diacetone acrylamide and acetoacetoxy ethyl methacrylate. Typically it comprises methyl methacrylate; methacrylic acid; diacetone acrylamide and optionally butyl methacrylate. Typically the first vinyl polymer comprises at least 50 wt.% alkyl methacrylate, for example methyl methacrylate and butyl methacrylate.

[0030] In one embodiment the second vinyl polymer comprises an alkyl-methacrylate; an alkylacrylate; and one or both of diacetone acrylamide and acetoacetoxy ethyl methacrylate. Typically it comprises butyl methacrylate, butyl acrylate and diacetone acrylamide. Typically the second vinyl polymer comprises at least 50 wt.% alkyl methacrylate, for example butyl methacrylate.

[0031] The first vinyl polymer has an acid number of at least 20 mg KOH / g on solids and therefore comprises sufficient acid-functional monomers to achieve such an acid number. The first vinyl polymer may be derived from a monomer system which contains from 1 to 45 wt.% of acid functional monomers; typically from 3 to 30 wt.%; preferably from 3 to 20 wt.%; from 0.5 to 20 wt.%; from 1 to 15 wt.%; particularly from 1 to 10 wt.% of crosslinking monomers; and from 98.5 to 50 wt.% of non-acid functional, noncrosslinking comonomer; typically from 96 to 65 wt.%; preferably from 96 to 75 wt.%. The second vinyl polymer has an acid number lower than that of the first vinyl polymer. Accordingly, the second vinyl polymer typically will comprise fewer acid-functional monomers than the first vinyl polymer. For example, the second vinyl polymer may comprise less than 5 wt.% of any acid-functional monomer; for example less than 2 wt.%; in particular none at all.

[0032] The second vinyl polymer has a glass transition temperature (Tg2) of -60°C to 100°C, and which is lower than that of the first vinyl polymer (Tgi) of 20°C to 120°C. Monomers may therefore be selected to help achieve such a lower Tg2 compared with Tgi. The purpose of using monomer comprising a cross-linkable moiety is to provide subsequent crosslinkability in the resulting polymer system. A monomer comprising a cross-linkable moiety may be an acid-functional group or a non-acid functional monomer. A monomer comprising a cross-linkable moiety includes epoxy- and hydroxyalkylmethacrylate and acrylate, for example hydroxyethyl methacrylate, hydroxyethyl acrylate, glycidyl methacrylate and glycidyl acrylate; keto- or aldehyde functional monomers, for example acrolein, methacrolein and vinyl methyl ketone; acetoacetoxy ester of hydroxyalkyl, acrylate and methacrylate, for example acetoacetoxyethyl methacrylate and acrylate; and keto-containing amides such as diacetone acrylamide (DAAM).

[0033] In one embodiment the second vinyl polymer further comprises a monomer comprising a cross-linkable moiety. In another embodiment the first vinyl polymer comprises a monomer comprising a cross-linkable moiety.

[0034] A cross-linking agent may be added to the dispersion. The cross-linking agent is a component which reacts with at least two cross-linking functionalities in polymer chains to connect one polymer chain to another. For example, the incorporation of diacetone acrylamide (DAAM) into the polymer is typically accompanied by the addition of adipic acid dihydrazide (ADH) into the dispersion which, on drying of the dispersion reacts with the keto-functionality to cross-link the polymer. DAAM-ADH is a well-established cross-linking system for acrylic polymers. Preferably the amount of crosslinking agent is such that the ratio of the number of crosslinker groups present in the polymer to the number of reactive groups (for cross-linking purposes) in the crosslinking agent is from 10:1 to 1 :3, for example from 2:1 to 1 :1.5

[0035] The vinyl polymer may further comprise an adhesion promoting monomer. An adhesion promoting monomer is a functional monomer which enhances the bonding of the molecule to the surface of a substrate. Typical adhesion promoting monomers include phosphate and carboxylic acids (metal adhesion) and silyl ethers (glass / silaceous adhesion) which hydrolyze to give reactive Si-OH bonds.

[0036] Number average molecular weight, Mnis measured by gel permeation chromatography. The first vinyl polymer has a number average molecular weight (Mni) of from 500 to 50,000. In one embodiment, the first vinyl polymer has a number average molecular weight (Mni) of at least 1000; typically at least 5000, for example at least 10,000. In one embodiment, the second vinyl polymer has a number average molecular weight (Mn2) of at least 50,000; typically at least 100,000, for example at least 200,000. In an embodiment the second vinyl polymer has a number average molecular weight (Mn2) of at most 5,000,000. Typically, it is at most 3,000,000 for example at most 1 ,000,000.

[0037] Glass transition temperature, Tg, is calculated according to the Flory Fox equation. The Tg, in degrees Kelvin, of a copolymer having "n" copolymerised comonomers is given by the weight fractions Wof each comonomer type and the Tg’s of the homopolymers (in degrees Kelvin) derived from each comonomer according to the equation. The first vinyl polymer has a glass transition temperature (Tgi) of from 20°C to 120°C. In one embodiment Tgiis at least 40°C. Typically it is at least 60°C, for example at least 80°C. Typically Tgiis at most 110°C. Preferably it is at most 100°C, for example at most 90°C. The second vinyl polymer has a glass transition temperature (Tg2) of from -60°C to 100°C. In one embodiment Tg2 is at least -40°C. Typically it is at least -20°C, for example at least 0°C, or even at least 20°C. Typically Tg2 is at most 90°C. Preferably it is at most 80°C, for example at most 60°C, 40°C or even 20°C. In one embodiment Tg2 is -40°C to 20°C. In one embodiment Tgiis at least 20°C greater than Tg2. Typicality, Tgiis at least 40°C greater than Tg2; for example Tgimay be at least 60°C greater than Tg2.

[0038] Acid number is determined by standard calculation based on the number of acid functional monomers in the polymer. This is the amount in mg of KOH it takes to neutralize all the acid groups present in 1 g of dry (corrected for solid content) polymer. Acid number is calculated from the amount of acid groups in the polymer. It can be calculated as follows: mmoles acid I g solid polymer * 56 (mw KOH) gives the acid value in mgKOH / g. In one embodiment, the acid number of the first vinyl polymer is at least 20. Typically it is at least 40; preferably at least 50, or even at least 60. In one embodiment the acid number of the second vinyl polymer is less than 20. Typically it is less than 10. Preferably it is less than 5; most preferably it is less than 1 , for example 0.

[0039] In one embodiment, the acid number of the first vinyl polymer is at least 20 higher than the acid number of the second vinyl polymer. Typically, the acid number of the first vinyl polymer is at least 30 higher than the acid number of the second vinyl polymer, preferably at least 40 higher. In one embodiment, the polymer particles are multiphase particles and comprise both a phase a) comprising the first vinyl polymer as defined in claim 1 and a phase b) comprising the second vinyl polymer as defined in claim 1. In one embodiment the polymer particles have the form of a shell of a first vinyl polymer at least partially surrounding a core of the second vinyl polymer.

[0040] As used herein a chain transfer agent means a compound which brings about the termination of the original polymer chain with the simultaneous initiation of another polymer chain. Any chain transfer agent having at least three reactive moieties and suitable for producing vinyl polymers may be used. By branching point derived from a chain transfer agent, is meant that the chain transfer agent is present in the vinyl monomer. Without wishing to be bound by any theory, it is thought that the branching in the first vinyl polymer caused by a chain transfer agent having at least three reactive moieties leads to a reduction in MFFT of the resulting polymer dispersion.

[0041] The chain transfer agent has three or more reactive moieties. The chain transfer agent may comprise thiol reactive moieties or halogen reactive moieties.

[0042] In one embodiment the chain transfer agent has three or more thiol groups. Typically it has from three to eight thiol groups, preferably three, four, five or six thiol groups. Preferably, it has three or four thiol groups. The chain transfer agent may have further reactive moieties in addition to thiol groups.

[0043] In one embodiment, the chain transfer agent is one wherein the chain transfer agent is a 2-mercaptoproprionate ester, a 3-mercaptoproprionate ester, a mercaptoglycolate ester, a pentaerythritol tetra-(3-mercaptoacylate) or a derivative of dipentaerithrytol hexakis (3- mercaptopropionate).

[0044] Typically, the chain transfer agent is pentaerythritol tetra-(3-mercaptoacetate), pentaerythritol tetra-(3-mercaptopropionate), trimethylol propane trithioglycolate trimethylol propane trimercaptoglycolate, trimethylol propane tri(3-mercapto propionate), pentaerythritol tetra-(3-mercapto acetate), pentaerythritol tetra-(3-mercaptopropionate), dipentaerithrytol hexakis (3-mercaptopropionate), dipentaerithrytol hexakis (3- mercaptoglycolate), dipentaerithrytol hexakis (3-mercaptoacetate), tris 2-(3- mercaptopropionyloxy)ethyl isocyanurate, trimethylolpropane tris (2-mercaptopropionate), pentaerythritol tetrakis (2-mercaptopropionate) or dipentaerithrytol hexakis (2- mercaptopropionate). Preferably, the chain transfer agent is trimethylolpropane tris(3- mercaptopropionate) (TMPTMPA),, pentaerythritol tetrakis(3-mercaptopropionate) (PESH4).

[0045] The chain transfer agent may be present in any amount of at least 0.1 wt.% of the first vinyl polymer. Typically, the chain transfer agent is present in an amount of from 0.2 to 20 wt.% of the weight of the first vinyl polymer. Preferably, the chain transfer agent is present in an amount of from 0.5 to 15 wt.% of the weight of the first vinyl polymer; more preferably it is present in an amount of from 1 to 10 wt.%.

[0046] The chain transfer agent is typically introduced into the first vinyl polymer during its formation by emulsion polymerization. A chain transfer agent is typically added in a concentration of from 0.005 to 0.1 mol / Kg, for example in an amount of from 0.01 to 0.05 mol / Kg.

[0047] In addition to the chain transfer agent having three or more reactive moieties, the first vinyl polymer may comprise one or more further chain transfer agents selected from those commonly use in vinyl polymers, for example 3-mercaptopropionic acid and dodecyl mercaptane.

[0048] The second vinyl polymer may also comprise a branching point derived from a chain transfer agent having three or more reactive moieties a chain transfer agent. Typically however it does not. It may however comprise a chain transfer agent selected from those commonly used in vinyl polymers, for example 3-mercaptopropionic acid and dodecyl mercaptane.

[0049] The polymer particles are typically multiphase particles and comprise both a phase a) comprising the first vinyl polymer as defined above and a phase b) comprising the second vinyl polymer as defined above. Such multiphase particles may be core-shell particles.

[0050] The dispersion of polymer particles is in aqueous medium. Accordingly, it comprises a continuous phase which is aqueous. The polymer particles form a discrete phase. In its simplest form the aqueous medium is water. The process for producing a dispersion of polymer particles is typically a semi-continuous emulsion polymerization (where reagents are fed to the reactor over a defined time period) or a batch process (where all reagents are present in the reactor at the beginning of the reaction). Alternatively it may be continuous process. Each of steps (b) and (d) may be independently semi-continuous, batch or continuous. Preferably both steps (b) and (d) are semi-continuous. Typically the process is carried out in a manner analogous to EP0758364B2. Typically the process is free from organic solvents.

[0051] Step (a) comprises preparing a first monomer mixture, said first monomer mixture comprising: i. a first mixture of vinyl monomers; ii. a carboxylic acid-functional monomer; and iii. a chain transfer agent having three or more reactive moieties.

[0052] Typically step (a) comprises mixing the components in water, such that the first monomer mixture is an aqueous mixture. Typically the first monomer mixture of step (a) additionally comprises: iv. a monomer containing a reactive group for further crosslinking. Suitable monomers include DAAM. Typically the first monomer mixture of step (a) additionally comprises: v. a monomer containing a functional group for adhesion promotion. Typical adhesion promoting monomers include phosphate and carboxylic acids (metal adhesion), ureido functionality (wood / aged alkyd) and silyl ethers (glass / silaceous adhesion) which hydrolyze to give reactive Si-OH bonds.

[0053] In step (b) polymerization is typically carried out for a period of from 30 minutes to 3 hours. Typically this is carried out at a temperature of from 30°C to 120°C, for example from 50°C to 90°C. Reaction is typically under inert atmosphere, for example nitrogen or argon and under stirring. Step (b) may be carried out by introducing a seed of first monomer mixture from step (a) to the reactor, followed by introducing the bulk of the first monomer mixture from step (a) over a period of time, for example over from 5 minutes to 2 hours.

[0054] An initiator is used in step (b) and is typically added in aqueous solution. An initiator may be a conventional free radical initiator, for example hydrogen peroxide, t-butyl- hydroperoxide, cumene hydroperoxide, persulphates such as ammonium persulphate, potassium persulphate and sodium persulphate. Alternatively, it may be a redox system, for example combinations such as t-butyl hydroperoxide isoascorbic acid and iron-EDTA. The amount of initiator is generally 0.05 to 3 wt.% based on the total weight of the monomers. A dispersion of a first vinyl polymer results from step (b).

[0055] Step (b) is carried out in the presence of a surfactant. Suitable surfactants are any surfactant that imparts colloidal stability to the system either by incorporating ionic functional groups or steric stabilization. A surfactant may be a conventional anionic and / or non-ionic surfactant, for example a sodium, potassium or ammonium salt of a dialkylsulphosuccinate; sodium, potassium or ammonium salt of a sulphated oil; sodium, potassium or ammonium salt of an alkyl sulphonic acid; sodium, potassium or ammonium sulphate, for example sodium lauryl sulphate; an alkali metal salt of a sulphonic acid, a fatty alcohol, ethoxylated fatty acid, or fatty amides; sodium, potassium or ammonium salt of a fatty acid, for example sodium stearate or sodium oleate. Aryl- containing analogues of the alkyl-containing surfactants and phosphates are alternative surfactants. Examples include sulphonated hydrocarbons or phosphonated hydrocarbons, for example Lansurf LKD 1293, Maxemul 6106 (a polymerizable surfactant) and sodium dodecyl sulfate SDS. Typically the components are mixed as aqueous mixtures. The amount of surfactant is typically from 0.3 to 2 wt.% by weight, for example from 0.3 to 1 wt.% based on the weight of total monomers.

[0056] Typically step (c) comprises simply mixing the components. It may alternatively be carried out in water, such that the second monomer mixture is an aqueous mixture. The second monomer mixture may also comprise a monomer containing a reactive group for further crosslinking. Suitable monomers include DAAM. Typically the first monomer mixture of step (c) additionally comprises: v. a monomer containing a functional group for adhesion promotion. Typical adhesion promoting monomers include phosphate and carboxylic acids (metal adhesion), ureido functionality (wood / aged alkyd) and silyl ethers (glass / silaceous adhesion) which hydrolyze to give reactive Si-OH bonds.

[0057] In step (d) of the process typically polymerization of the second monomer mixture takes place in the presence of the first vinyl polymer produced in step (b). Step (d) may be any known type of polymerization. In one embodiment of the process of the present invention polymerization in step (b) is emulsion polymerization. Typically this is carried out at a temperature of from 30°C to 120°C, for example from 50°C to 90°C. Reaction is typically under inert atmosphere, for example nitrogen or argon and typically under stirring. Step (d) may be carried out by introducing a seed of second monomer mixture from step (c) to the reactor containing the aqueous dispersion of first vinyl polymer from step (b), followed by introducing the bulk of the first monomer mixture from step (c) over a period of from 5 minutes to 2 hours. An initiator may be used in step (d), typically added in aqueous solution. Initiators are as described above. A surfactant may be used in step (d). Surfactants are as described above. The first vinyl polymer may act as a surfactant in the reaction of the second monomer mixture. Accordingly, typically no additional surfactant is used in step

[0058] (d).

[0059] Neutralization in step (e) is typically carried out by addition of an aqueous solution of base. Reaction is typically over a period of time, for example from five minutes to two hours and at elevated temperature, of from 30°C to 120°C for example from 50°C to 90°C. A suitable base includes an organic or inorganic base, for example an organic amine such as a trialkylamine, for example triethylamine, tributylamine), morpholine and alkanolamines, and inorganic bases such as ammonia, NaOH, KOH, and LiOH. Neutralization step (e) may be carried out after step (b) or after step (d) or after both steps (b) and (d). Typically, step (e) occurs directly after step (b).

[0060] The process of the present invention may further comprise a step (f) purifying the aqueous dispersion of polymer particles. Purification may comprise multiple steps and is typically carried out by techniques known in the art, for example it may include one or more of oxidation and reduction steps and filtration.

[0061] The first vinyl polymer can be described as an ‘alkali soluble resin’ (ASR). The first polymer will dissolve and / or form micelles when it is neutralized in water, for example when step

[0062] (e) is carried out directly after step (b). When the second polymerization step (d) is performed, the first vinyl polymer can act as a surfactant for the second monomer mixture and form a shell of the second vinyl polymer. The resulting polymer particles are soluble shell polymer particles, with the first vinyl polymer acting as a soluble shell for the second vinyl polymer. The resulting polymer particles are present as an aqueous dispersion. Therefore, in one embodiment, the first vinyl polymer at least partially encapsulates the second vinyl polymer. The resulting polymer particles are known as core-shell polymer particles.

[0063] To produce a coating composition or adhesive composition, additives may be added to the aqueous dispersion of polymer particles by known techniques. Suitable additives include a color pigment, extender pigment, coalescing solvent, co-solvent surfactant, plasticizer, pH modifier, defoaming agent, thickener, leveling agent, matting agent, anti-setting agent and biocide.

[0064] A coated substrate is obtained by applying a coating composition to the substrate and allowing the coating composition to dry. The layer may be applied using any suitable technique known in the art, for example by brush, roller or spaying. Drying may be supplemented by applying radiation, for example UV or electron beam radiation, or applying heat to effect curing of the coating. Typically curing is carried out at ambient temperature. Preferably, the composition is free from cross-linking reagents which require curing at elevated temperatures or by applying radiation. The substrate may be any suitable substrate, such as for example wood, polymer, composite, metal or mineral substrate. The substrate may be a primed or bare substrate. A single layer or multiple successive layers of coating composition may be applied to the substrate.

[0065] The adhesive composition is particularly suitable for bonding components to form structural products. The components may be any suitable material such as wood, metal, plastic, paper, glass, concrete, gypsum or other mineral material such as stone or brick. The first and the second components may be of identical materials or different materials. Preferably, the components are pieces of wood. The composite material may be any suitable object. Preferably it is a structural wood product; preferably a timber product, for example a laminated timber product.

[0066] The invention is further illustrated by the following non-limiting examples.

[0067] Examples

[0068] Raw materials

[0069] Components used in the Examples are listed below:

[0070] Methyl methacrylate (MMA)

[0071] Methacrylic acid (MAA) (Thermo Fisher)

[0072] Diacetone acrylamide (DAAM) (Thermo Fisher)

[0073] Butyl methacrylate (BMA)

[0074] Butyl acrylate (BA)

[0075] Lauryl methacrylate (LM) (Merck) 3-mercaptopropionic acid (3MPA) (Merck)

[0076] Butyl 3-mercaptopropionate (BUM PA) (Merck)

[0077] Trimethylolpropane tris(3-mercaptopropionate) >95% (TMPTMPA) (Merck) Ethylene glycol bis (3-mercaptopropionate) (GDMPA) (TCI) Pentaerythritol tetrakis(3-mercaptopropionate) (PESH4) Adipic dihydrazide (ADH) (Merck)

[0078] Purity of monomers was at least 97% unless otherwise stated. Where no specific supplier is listed, materials were sourced internally from AkzoNobel.

[0079] Test methods

[0080] The following test methods were used.

[0081] The glass transition temperatures of hydrophobic polymers in the examples were calculated by means of the Flory-Fox equation. Thus the Tg, in degrees Kelvin, of a copolymer having "n" copolymerised comonomers is given by the weight fractions W of each comonomer type and the Tg’s of the homopolymers (in degrees Kelvin) derived from each comonomer according to the equation. The calculated Tgin degrees Kelvin may be readily converted to °C. (if the hydrophobic polymer is a homopolymer, its Tg is simply that of the polymerised monomer - normally available from the literature).

[0082] The minimum film forming temperature (MFFT) of a composition as used herein is the temperature where the composition forms a smooth and crack free coating or film using ISO 2115, except that an application bar of 75 micron is used (where 100 micron is standard); and when applied using a Rhopoint MFFT 90 apparatus.

[0083] Solids content was determined by placing, in triplet, the samples for 1 hour in a 125°C oven and measuring the weight loss, according to ISO 3251.

[0084] Viscosity was measured by on an Anton Paar physica MCR-301 rheometer according to ISO 3219. The reported value was obtained by first applying a shear rate of 10 s'1for 15 seconds and then measuring at a shear rate of 10 s-1for 30 seconds acquiring 15 datapoints (1 every 2 seconds). The average of these datapoints was reported. Konig Hardness as used herein is a standard measure of hardness, being a determination of how the viscoelastic properties of a film formed from the composition slows down a swinging motion deforming the surface of the film, and is measured according to ISO1522 on glass substrate.

[0085] Gloss at 20°C and Haze are measured with a BYK HAZE-Gloss meter on sandblasted glass panels according to ISO 13803.

[0086] Water resistance, ethanol resistance and coffee resistance are measured according to IKEA R2, which is based on EN12720 (modified for coffee resistance). Test results are presented as a grade of from 1 to 5, wherein 5 is no change, 4 is minor change, 3 is moderate change, 2 is significant change and 1 is strong change.

[0087] MEK double rubs were measured on a metal substrate according to ASTM D5402-15.

[0088] Preparation of Dispersion

[0089] A dispersion was prepared by the following process. The reagents of this semi continuous emulsion polymerization were added in two subsequent steps, each consisting of a different monomer / additive composition. In the first step the semi-continuous emulsion polymerization to form the first vinyl polymer was carried out by addition of a pre-emulsion consisting of monomers, including acid functional monomers, surfactant, chain transfer agent (first monomer mixture). After addition and hold time, the mixture was neutralized using a volatile base. This was followed by a second stage in which monomers of the second vinyl polymer (second monomer mixture) were polymerized in presence of the first stage polymer.

[0090] The composition of the dispersions in each of the Examples is given in Table 1. In Table 1 , mmol CTA stands for the amount in millimoles of chain transfer agent added to the reaction mixture. The monomer compositions were kept constant except for when the acid value was increased, in which case the monomer composition was adjusted to maintain the Tgdefined below. In these experiments the amount of MMA and BMA was adjusted to ensure that the Tgin each case was identical. In each Example, the first vinyl polymer to second vinyl polymer weight ratio was 1 :1.3 and Tgas calculated using the Flory Fox equations was 31 °C, based on a shell Tgof 60.8°C and a core Tgof -0.4. To all examples 2 wt.% of the total solids content of the dispersion of ADH was added at 40°C over 30 mins before application. Table 1 :

[0091] Coating Properties

[0092] The polymer dispersion of each Example was deposited as such on clean glass substrate to achieve a wet layer thickness of 100pm. The films were allowed to dry for 1 day at room temperature. Kdnig hardness, gloss at 20°, haze and water resistance were tested. The results are given in Table 2, where the number of thiol groups, calculated by simply multiplying the amount of CTA by the functionality (number of thiol groups) of the CTA, is also given. Table 2:

[0093] Note: CTA’s are: L = LM / 3MPA; T = TMPTMPA; B = BUMPA; G = GDMPA and P = PESH4; for water resistance, 5 is no change, 4 is minor change (test area is only visible when light source is angled towards observer, no structural change to surface), 3 is moderate change 5 (test area is visible, no structural change to coating), 2 is significant change (test area clearly visible, slight structural change, e.g. swelling, fiber raising, cracking, blistering) and 1 is strong change (structure is distinctly changed).

[0094] The compositions of the Examples (employing a multifunctional chain transfer agent) each 10 exhibited a lower MFFT than the comparative examples (each employing a monofunctional chain transfer agent). In some cases the MFFT of the Example is significantly lower than the Comparative Example. The compositions performed comparably well in terms of hardness, gloss, and haze. Water resistance was generally higher in the Examples.

[0095] 15 Table 3:

[0096] Rec = recovery (the sample is exposed to the solvent for the first specified period, wiped clean then left to recover for the second specified period)

[0097] 5 *** = no damage

[0098] ** = noticeable damage (marking, swelling, discoloration, blistering, tackiness)

[0099] * = significant damage

[0100] The compositions of the Examples show a significant reduction in MFFT and equal or 0 better performance in water resistance, ethanol resistance and chemical resistance than the Comparative Examples.

Claims

CLAIMS1 . A dispersion of polymer particles in aqueous medium, which comprises:(a) a first vinyl polymer, which first vinyl polymer has a glass transition temperature (Tgi), calculated according to the Flory Fox equation, of from 20°C to 120°C, a number average molecular weight (Mni), measured by gel permeation chromatography, of from 500 to 50,000 g / M and an acid number, calculated from the amount of acid groups present in the polymer, of at least 20; and(b) a second vinyl polymer, which second vinyl polymer has a glass transition temperature (Tg2), calculated according to the Flory Fox equation, of from -60°C to 100°C and an acid number lower than the acid number of the first vinyl polymer; wherein Tgiis greater than Tg2; and wherein the first vinyl polymer comprises a branching point derived from a chain transfer agent having three or more reactive moieties.

2. A dispersion according to claim 1 , wherein the chain transfer agent has three or more thiol groups.

3. A dispersion according to claim 1 or claim 2, wherein the chain transfer agent is a 2-mercaptoproprionate ester, a 3-mercaptoproprionate ester, a mercaptoglycolate ester, a pentaerythritol tetra-(3-mercaptoacylate) or a derivative of dipentaerithrytol hexakis (3-mercaptopropionate).

4. A dispersion according to any one of claims 1 to 3, wherein the chain transfer agent is pentaerythritol tetra-(3-mercaptoacetate), pentaerythritol tetra-(3- mercaptopropionate), trimethylol propane trimercaptoglycolate, trimethylol propane tri (3- mercapto propionate), pentaerythritol tetra-(3-mercapto acetate), pentaerythritol tetra-(3-mercaptopropionate), dipentaerithrytol hexakis (3-mercaptopropionate), dipentaerithrytol hexakis (3-mercaptoglycolate), dipentaerithrytol hexakis (3- mercaptoacetate), tris 2-(3-mercaptopropionyloxy)ethyl isocyanurate, trimethylolpropane tris (2-mercaptopropionate), pentaerythritol tetrakis (2- mercaptopropionate) or dipentaerithrytol hexakis (2-mercaptopropionate).

5. A dispersion according to any one of claims 1 to 4, wherein the chain transfer agent is present in an amount of from 0.5 to 15 wt.% of the weight of the first vinyl polymer.

6. A dispersion according to any one of claims 1 to 5, wherein the second vinyl polymer further comprises a monomer comprising a cross-linkable moiety.

7. A dispersion according to any one of claims 1 to 6, wherein Tgiis at least 40°C greater than Tg2.

8. A dispersion according to any one of claims 1 to 7, wherein the acid number of the first vinyl polymer is at least 20 higher than the acid number of the second vinyl polymer.

9. A dispersion according to any one of claims 1 to 8, wherein the polymer particles are multiphase particles and comprise both a phase a) comprising the first vinyl polymer as defined in claim 1 and a phase b) comprising the second vinyl polymer as defined in claim 1.

10. A dispersion according to any one of claims 1 to 9, wherein the first vinyl polymer comprises one or more alkyl-methacrylate; one or both of methacrylic acid and acrylic acid; and one or both of diacetone acrylamide and acetoacetoxy ethyl methacrylate.

11. A dispersion according to any one of claims 1 to 10, wherein the second vinyl polymer comprises an alkyl-methacrylate; an alkyl-acrylate; and one or both of diacetone acrylamide and acetoacetoxy ethyl methacrylate.

12. A process for producing a dispersion of polymer particles in aqueous medium as defined in any one of claims 1 to 11 , said process comprising:(a) preparing a first monomer mixture, said first monomer mixture comprising: i. a first mixture of vinyl monomers; ii. a carboxylic acid-functional monomer; and iii. a chain transfer agent having three or more reactive moieties;(b) carrying out emulsion polymerization of the first monomer mixture in water, in the presence of at least one surfactant and at least one initiator, to form an aqueous dispersion of a first vinyl polymer;(c) preparing a second monomer mixture, said second monomer mixture comprising a second mixture of vinyl monomers;(d) carrying out emulsion polymerization of the second monomer mixture in the presence of the aqueous dispersion of a first vinyl polymer and an initiator to form an aqueous dispersion of polymer particles; and(e) adding after either or both of steps (b) and (d) a base to, at least partly, neutralize acid groups present in the polymer(s).

13. A composition, which composition is a coating composition or an adhesive composition, and which composition comprises a dispersion of polymer particles in aqueous medium as defined in any one of claims 1 to 11 and at least one additive selected from a color pigment, extender pigment, coalescing solvent, co-solvent surfactant, plasticizer, pH modifier, defoaming agent, thickener, leveling agent, matting agent, anti-setting agent and biocide.

14. A coated substrate comprising a substrate and a coating, which coated substrate is obtainable by applying a coating composition as defined in claim 13 to the substrate and allowing the coating composition to dry.

15. A composite material comprising a first component, a second component and an adhesive layer between the first component and the second component, wherein the composite material is obtainable by applying an adhesive composition as defined in claim 13 to at least one surface of the first component; bringing the coated surface of the first component into contact with the second component; and allowing the adhesive composition to dry.

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