WO2020030428A1

WO2020030428A1 - An aqueous multi-stage copolymer dispersion, and a water whitening resistant paint containing the same

Info

Publication number: WO2020030428A1
Application number: PCT/EP2019/069885
Authority: WO
Inventors: Rui DENG; Zhao Bin Zhang; Zhong Zeng; De Qin FAN
Original assignee: Basf Se
Priority date: 2018-08-06
Filing date: 2019-07-24
Publication date: 2020-02-13
Also published as: CN112543794A; CN112543794B

Abstract

The present inventions relates to an aqueous multi-stage copolymer dispersion, particularly an aqueous multi-stage copolymer dispersion useful for paints which need water whitening resistance, wherein the multi-stage copolymer consisting of a first stage copolymer having a Fox Tg in the range of -10 to 20º C resulted from first stage monomers and a second stage copolymer having a Fox Tg in the range of 60 to 120º C resulted from second stage monomers, and relates to a paint comprising the aqueous multi-stage copolymer dispersion.

Description

An Aqueous Multi-Stage Copolymer Dispersion, And

A Water Whitening Resistant Paint Containing the Same

Field of the Invention

The invention relates to an aqueous multi-stage copolymer dispersion useful in paints which need water whitening resistance, such as architectural exterior paints, interior paints and stone-finish paints.

Background

Certain paints are generally formulated with an aqueous copolymer dispersion as a binder, particles of natural mineral, such as sand, stone or particles of synthesized stone-like materials (e.g. ceramics), and various additive components. Water whitening resistance is an important property for paints which may be subjected to snow and rain, and is also desirable for paints which may be subjected to vapor or moisture. Various technical solutions have been proposed to endow such paints with improved water whitening resistance, especially from the perspective of emulsion binders.

It is believed that hydrophilic components in those paints promote the adsorbing of water which can easily migrate from the paint film surface into the interstitial areas and thus cause the water- whitening problem, as discussed in CN102746601 B.

CN 102746601 B provides aqueous copolymer dispersions for stone-finish paint and architectural exterior or interior coatings, which require water whitening resistance. The dispersion comprises a copolymer and a polyamine, whereas the copolymer comprises at least one nonionic monomer selected from C6-C22-alkyl (meth)acrylate or Versatic vinyl ester. The amount of the nonionic monomer ranges from 15 % to less than 50 % based on dry weight of the copolymer, and the polyamine ranges from 0.1 to 2 percent. The copolymer further comprises, as copolymerized units, 0.05 wt% to 3 wt% of a monomer having at least one alkoxysilane functionality. Water whitening resistance was tested for films of compositions consisting of the copolymer dispersions and 10 % coalescent agent (based on polymer dry weight) on vinyl plate and glass plate. With the high dosage of coalescent agent and long curing time of 24 hours, the latex films performed well. No water whitening resistance was tested for any architectural paint containing the copolymer dispersion.

CN104356287A discloses an internal and external cross-linking pure acrylic emulsion with high water whitening resistance for stone-finish paints. The pure acrylic emulsion for stone-finish paints is synthesized from raw materials including a hard monomer, a soft monomer, a composite emulsifier, an internal cross-linking monomer, an organic silicon coupling agent, a thermal decomposition initiator, a pH regulator and an external cross-linking agent by a semi-continuous seed emulsion polymerization method. The emulsion particles as prepared have a structure with a soft core and a hard shell, and it was required that the soft core has Tg (glass transition temperature) of 0 to 22 °C and the hard shell has Tg of 25 to 60 °C. No water whitening resistance was tested for any film of the emulsion or for any real stone paint containing the emulsion.

CN102977257A discloses a water whitening resistant emulsion with core-shell structure for stone- finish paints. The water whitening resistant emulsion with core-shell structure is prepared from raw materials including styrene, an alkyl methacrylate, an alkyl acrylate, and an alkenyl carboxylic acid, a room-temperature self-crosslinking monomer, a silane coupling agent, a composite emulsifier and a thermal initiator by a semi-continuous seed emulsion polymerization method. It was disclosed that the emulsion particles as prepared have a structure with a soft core and a hard shell. No water whitening resistance was tested for any film of the emulsion or for any real stone paint containing the emulsion.

CN101665650A discloses a pure acrylic emulsion for stone-finish paints, which is prepared from a formulation including core monomers consisting of 5 to 20 % of methyl methacrylate, 5 to 20 % of ethylhexyl acrylate, 5 to 10 % of butyl acrylate and 1 to 5 % of a high-density cross-linking monomer, and shell monomers consisting of 5 to 20 % of methyl methacrylate, 5 to 10 % of ethylhexyl acrylate, 5 to 10 % of butyl acrylate, 1 to 3 % of acrylic acid, 1 to 5 % of a polymerizable emulsifier, 1 to 3 % of a persulfate, and 50 % of water by a core-shell emulsion polymerization method. No water whitening resistance was tested for any film of the emulsion or for any real stone paint containing the emulsion.

It remains a challenge to provide a suitable aqueous copolymer dispersion as the binder for paints which require water whitening resistance property. Additionally, a good balance between bonding strength and water whitening resistance is desirable for those paints.

There is a need for an aqueous copolymer dispersion useful as binder in paints, such as architectural exterior paints, architectural interior paints and stone-finish paints, which need water whitening resistance, the aqueous copolymer dispersion endowing the paints with improved water whitening resistance. There is a further need for an aqueous copolymer dispersion endowing the paints with improved water whitening resistance and acceptable bonding strength.

Summary of the Invention

It was found that the above objectives can be addressed with an aqueous multi-stage copolymer dispersion wherein the multi-stage copolymer consists of a first stage copolymer having a Fox Tg in the range of -10 to 20 °C resulted from first stage monomers and a second stage copolymer having a Fox Tg in the range of 60 to 120 °C resulted from second stage monomers.

Accordingly, the present application provides an aqueous multi-stage copolymer dispersion wherein the multi-stage copolymer consists of a first stage copolymer having a Fox Tg in the range of -10 to 20 °C resulted from first stage monomers and a second stage copolymer having a Fox Tg in the range of 60 to 120 °C resulted from second stage monomers, and further provides a paint, such as architectural exterior paints, interior paints and stone-finish paints, comprising the aqueous multi-stage copolymer dispersion.

Detailed Description of the Invention

The present application relates to an aqueous multi-stage copolymer dispersion, particularly an aqueous multi-stage copolymer dispersion useful for paints which need water whitening resistance, such as architectural exterior paints, interior paints and stone-finish paints, wherein the multi-stage copolymer consists of a first stage copolymer having a Fox Tg in the range of -10 to 20 °C resulted from first stage monomers and a second stage polymer having a Fox Tg in the range of 60 to 120 °C resulted from second stage monomers.

Within the context of the present application, the term (meth)acrylic acid is to be understood to mean acrylic acid and/or methacrylic acid. Likewise, the term (meth)acrylate is to be understood to mean acrylic ester and/or methacrylic ester, the term (meth)acrylamide is to be understood to mean acrylamide and/or methacrylamide, and the term (meth)acrylonitrile is to be understood to mean acrylonitrile and/or methacrylonitrile.

Within the context of the present application, the term Fox Tg refers to a glass transition temperature Tg as calculated according to the following Fox equation as disclosed in T.G. Fox, Bulletin of the American Physical Society, Volume 1 , Issue No. 3, page 123 (1956):

1/Tg = Wi/Tgi + W₂/Tg₂ + ··· + W_n/Tg_n wherein

Wi, W₂, ... W_n are the mass fractions of the monomers 1 , 2, . . . n, respectively, and

Tg-i, Tg₂, ...Tg_n are the glass transition temperatures of homopolymers of the monomers 1 , 2, . . . n in degrees Kelvin, respectively.

The Tg values for homopolymers of the majority of monomers are known and are listed in, for example, Ullmann's Ecyclopedia of Industrial Chemistry, Vol. 5, Vol. A21 , page 169, VCH Weinheim, 1992. Other sources of glass transition temperatures of homopolymers include, for example, J. Brandrup, E. H. Immergut, Polymer Handbook, 1 st Edition, J. Wiley, New York 1966, 2nd Edition, J. Wiley, New York 1975, and 3rd Edition, J. Wiley, New York 1989.

The aqueous multi-stage copolymer dispersion according to the present invention comprises one or more multi-stage copolymers in an aqueous medium, which is, for example, an aqueous multi- stage copolymer emulsion. The one or more copolymers may have, for example, a core/shell structure, resulted from the polymerization of first stage monomers and subsequent second stage monomers. There is no particular limitation to the species of the first stage monomers and the second stage monomers, with the provision that both first stage copolymer and second stage copolymer resulted from the corresponding first stage and second stage monomers shall have respective Fox Tgs. It is essential for the first stage copolymer to have a Fox Tg in the range of -10 to 20 °C, preferably in the range of -8 to 20 °C, more preferably -5 to 15 °C. It is also essential for the second stage copolymer to have a Fox Tg in the range of 60 to 120 °C, preferably 75 to 120 °C, more preferably 80 to 1 15 °C, most preferably 85 to 1 10 °C.

The first stage monomers and the second stage monomers may each independently be selected from monomers known to be useful for preparing aqueous polymer dispersions in the field of paints, especially paints which need water whitening resistance such as stone-finish paints. Species of the first stage monomers and second stage monomers may be the same or different, provided that the resulted first stage and second stage copolymers shall have respective Fox Tgs in the ranges as discussed above.

The first stage monomers and the second stage monomers each independently comprise,

(A) at least one hydrophobic monoethylenically unsaturated monomer (a) in an amount of at least 80 % by weight, preferably at least 85 % by weight, more preferably at least 90% by weight, and mostly preferably at least 95 % by weight; and

(B) at least one hydrophilic monoethylenically unsaturated monomer (b) in an amount of at least 0.1 % by weight and no more than 20 % by weight, preferably no more than 15 % by weight, more preferably no more than 10 % by weight, and mostly preferably no more than 5 % by weight; each based on the total weight of the respective stage monomers.

Preferably, the at least one hydrophobic monoethylenically unsaturated monomer (a) may be selected from the group consisting of (meth)acrylate monomers, (meth)acrylonitrile monomers, styrene monomers, vinyl alkanoate monomers and monoethylenically unsaturated di-and tricarboxylic ester monomers.

Particularly, the (meth)acrylate monomers may be Ci-Cig-alkyl (meth)acrylates, for example, but not limited to, methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate, n-octyl (meth)acrylate, n-decyl (meth)acrylate, n- dodecyl (meth)acrylate (i.e. lauryl (meth)acrylate), tetradecyl (meth)acrylate, oleyl (meth)acrylate, palmityl (meth)acrylate, stearyl (meth)acrylate, isobornyl (meth)acrylate, benzyl (meth)acrylate, and phenyl (meth)acrylate; Ci-Cio-alkoxyalkyl (meth)acrylates, for example, but not limited to, ethoxyethyl (meth)acrylate. Preferably, the (meth)acrylate monomers are Ci-Ci2-alkyl (meth)acrylates, such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, 2- ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate, n-octyl (meth)acrylate, lauryl (meth)acrylate or a mixture thereof.

Particularly, the styrene monomers may be unsubstituted styrene or Ci-C₆-alkyl substituted styrenes, for example, but not limited to, styrene, a-methylstyrene, ortho-, meta- and para- methylstyrene, ortho-, meta- and para-ethylstyrene, o,r-dimethylstyrene, o,p-diethylstyrene, ispropylstyrene, o-methyl-p-isopropylstyrene or any mixtures thereof.

Particularly, the vinyl alkanoate monomers may be vinyl esters of C2-Cn-alkanoic acids, for example, but not limited to, vinyl acetate, vinyl propionate, vinyl butanoate, vinyl valerate, vinyl hexanoate, vinyl versatate or a mixture thereof.

The monoethylenically unsaturated di-and tricarboxylic ester monomers may be full esters of monoethylenically unsaturated di-and tricarboxylic acids, for example, but not limited to, diethyl maleate, dimethyl fumarate, ethyl methyl itaconate, dihexyl succinate, didecyl succinate or any mixture thereof.

In a preferred embodiment according to the present invention, one or more Ci-Ci2-alkyl (meth)acrylates, styrene or a mixture thereof is chosen as the at least one hydrophobic monoethylenically unsaturated monomer (a). More preferably, one or more C-i-Cs-alkyl (meth)acrylate are used as the hydrophobic monoethylenically unsaturated monomer (a). More particularly, the C-i-Cs-alkyl (meth)acrylate is selected from the group consisting of methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate and 2-ethylhexyl (meth)acrylate, especially the group consisting of methyl methacrylate, n-butyl acrylate and 2-ethylhexyl acrylate.

Preferably, the at least one hydrophilic monoethylenically unsaturated monomer (b) may be monoethylenically unsaturated monomers containing at least one functional group selected from the group consisting of carboxyl, carboxylic anhydride, sulfonic acid, phosphoric acid, hydroxyl and amide. Examples of the hydrophilic monoethylenically unsaturated monomer (b) include, but are not limited to, monoethylenically unsaturated carboxylic acids, such as (meth)acrylic acid, itaconic acid, fumaric acid, citraconic acid, sorbic acid, cinnamic acid, glutaconic acid and maleic acid; monoethylenically unsaturated carboxylic anhydride, such as itaconic acid anhydride, fumaric acid anhydride, citraconic acid anhydride, sorbic acid anhydride, cinnamic acid anhydride, glutaconic acid anhydride and maleic acid anhydride; monoethylenically unsaturated amides, especially N-alkylolamides, such as (meth)acrylamide, N-methylol (meth)acrylamide, 2- hydroxyethyl (meth)acrylamide; and hydroxyalkyl esters of monoethylenically unsaturated carboxylic acids, such as hydroxyethyl (meth)acrylate and hydroxypropyl (meth)acrylate.

In a preferred embodiment according to the present invention, acrylic acid, methacrylic acid, itaconic acid, acrylamide or a mixture thereof is preferred as the at least one hydrophilic monoethylenically unsaturated monomer (b).

The first stage monomers and the second stage monomers may each independently further comprise one or more monoethylenically unsaturated monomers (c) containing at least one alkoxysilane functional group, preferably hydrolyzable alkoxysilane functional group, in an amount of no more than 5% by weight, preferably no more than 3% by weight, more preferably in the range of 0.05 to 1.5% by weight, and still preferably 0.1 to 1 % by weight, based on the total weight of the respective stage monomers. Within the context of the present application, the monoethylenically unsaturated monomer containing at least one alkoxysilane functional group, which is also called alkoxysilane functional monoethylenically unsaturated monomer (c) hereinafter, does not belong to above mentioned categories of monomer (a) and monomer (b). The addition of monomer (c) can improve the bonding strength of the aqueous multi-stage copolymer dispersion and/or the paints containing the dispersion with target objects, especially architecture walls.

Suitable Examples of the alkoxysilane functional monoethylenically unsaturated monomer (c) include, but are not limited to, vinyltrialkoxysilanes such as vinyltrimethoxysilane; alkylvinyldialkoxysilanes such as dimethoxymethylvinylsilane (for example Silquest A-2171 ); (meth)acryloxyalkyltrialkoxysilanes such as (meth)acryloxyethyltrimethoxysilane, (3- acryloxypropyljtrimethoxysilane and (3-methacryloxypropyl)trimethoxysilane (for example Silquest A-174); and derivatives thereof.

Optionally, the first stage monomers and the second stage monomers may each independently further comprise one or more monoethylenically unsaturated monomers (d) in an amount of no more than 10% by weight, preferable 2% to 10% by weight, more preferably 2% to 8% by weight, based on total weight of the respective stage monomers. The unsaturated monomers (d) include, but not limited to, vinyl esters, monoethylenically unsaturated monomer with one or more acetoacetoxy or acetoacetamide functional groups and other suitable monomers. The vinyl esters can be vinyl esters of aliphatic, saturated or unsaturated Ci-C24-carboxylic acids, such as, formic acid, acetic acid, propionic acid, butyric acid, valeric acid, isovaleric acid, caproic acid, caprylic acid, capric acid, undecylenic acid, lauric acid, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, arachidic acid, behenic acid, lignoceric acid, cerotic acid and melissic acid. Within the context of the present application, the monoethylenically unsaturated monomer (d) does not belong to above mentioned categories of monomer (a) and monomer (b).

Suitable monoethylenically unsaturated monomer with one or more acetoacetoxy or acetoacetamide functional groups includes, but are not limited to, acetoacetoxyethyl (meth)acrylate, acetoacetoxypropyl (meth)acrylate, acetoacetoxybutyl (meth)acrylate, 2,3- di(acetoacetoxy)propyl (meth)acrylate, allyl acetoacetates and vinyl acetoacetates.

In a particular embodiment according to the present invention, the first stage monomers and the second stage monomers may each independently comprise

(A) at least 80 % by weight of a hydrophobic monoethylenically unsaturated monomer (a), selected from the group consisting of Ci-Cig-alkyl (meth)acrylates, preferably Ci-Ci2-alkyl (meth)acrylates, more preferably C-i-Cs-alkyl (meth)acrylates, styrene, Ci-C₆-substituted styrenes, and any mixtures thereof;

(B) at least 0.1 % by weight and no more than 20 % by weight of a hydrophilic monoethylenically unsaturated monomer (b), selected from the group consisting of monoethylenically unsaturated carboxylic acids, monoethylenically unsaturated amides, especially N-alkylolamides, hydroxyalkyl esters of monoethylenically unsaturated carboxylic acids, and any mixtures thereof;

(C) optionally, no more than 5% by weight of an alkoxysilane functional monoethylenically unsaturated monomer (c) other than monomers (a) and (b), selected from the group consisting of vinyltrialkoxysilanes, alkylvinyldialkoxysilanes, (meth)acryloxyalkyltrialkoxysilanes, and any mixtures thereof;

each based on the total weight of the respective stage monomers.

In a more particular embodiment according to the present invention, the first stage monomers and the second stage monomers may each independently comprise

(A) at least 80 % by weight of a hydrophobic monoethylenically unsaturated monomer (a), selected from the group consisting of C-i-Cs-alkyl (meth)acrylates, styrene, and any mixtures thereof;

(B) at least 0.1 % by weight and no more than 20 % by weight of a hydrophilic monoethylenically unsaturated monomer (b), selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, acrylamide, and any mixtures thereof;

(C) optionally, no more than 5% by weight of an alkoxysilane functional monoethylenically unsaturated monomer (c), selected from the group consisting of vinyltrimethoxysilane, dimethoxymethylvinylsilane, (meth)acryloxyethyltrimethoxysilane, (3-acryloxypropyl)- trimethoxysilane, and any mixtures thereof; and

each based on the total weight of the respective stage monomers.

Optionally, the first stage monomers and the second stage monomers may each independently further comprise no more than 5% by weight of a multi-ethylenically unsaturated monomer such as allyl methacrylate, diallyl phthalate, 1 ,4-butylene glycol dimethacrylate, 1 ,2-ethylene glycol dimethacrylate, 1 ,6-hexanediol diacrylate, divinyl benzene or any mixtures thereof, based on the total weight of the respective stage monomers.

It is to be understood, within the context of the present application, all monomers included as the first stage monomers account for 100 % by weight. Likewise, all monomers included as the second stage monomers account for 100 % by weight.

The first stage monomers may account for, as a percentage of the total weight of first stage monomers over the total weight of first stage monomers and second stage monomers, 64 to 95% by weight, preferably 70 to 95 % by weight or 64 to 90% by weight, more preferably 70 to 90% by weight, more preferably 70 to 85% by weight, even more preferably 70 to 80 % by weight and most preferably 70 to 75% by weight.

The aqueous multi-stage copolymer dispersion according to the present invention may be prepared by a polymerization process including polymerization of the first stage monomers resulting the first stage copolymer and subsequent the second stage monomers resulting the second stage copolymer. Multi-stage polymerization techniques well known in the art may be used for preparing the aqueous multi-stage copolymer dispersion according to the present invention. For example, an emulsion polymerization in an aqueous medium is applicable for the purpose of the present invention.

In the emulsion polymerization process, most conventional surfactants known to the skilled person in the art may be used.

Surfactants may be non-reactive anionic and/or nonionic surfactants. Suitable non-reactive anionic surfactants, for example, include, but are not limited to, alkyl, aryl or alkylaryl sulfate salts, sulfonate salts or phosphate salts; alkyl sulfonic acids; sulfosuccinate salts; fatty alcohol ether sulfate salts and fatty acids. Suitable non-reactive nonionic surfactants for example include alcohol or phenol ethoxylates such as polyoxyethylene alkylphenyl ether. Among others, fatty alcohol ethoxylates, fatty alcohol sulfates such as sodium salt of oleyl cetyl alcohol sulfate (for example, Disponil® OCS 27), alkylbenzene sulfonates such as alkali metal or ammonium salts thereof, salts of alkyl ether sulfosuccinates such as sodium salt of isotridecanol ethoxylate sulfosuccinate, oleic acid alkyl ester sulfonates such as sodium oleic acid methyl ester sulfonate, alkyl ether phosphoric acid mono/diester such as Hostaphat 1306, disodium salt of fatty alcohol polyglycol ether sulfate, sulfonate and sulfosuccinate, disodium salt such as Disponil® FES 993.

Surfactants may also be polymerizable surfactants, also called a reactive surfactant, containing at least one ethylenically unsaturated functional group. Suitable polymerizable surfactants for example include, but are not limited to, allyl polyoxyalkylene ether sulfate salts such as sodium salts of allyl polyoxyethylene alkyl ether sulfate, allyl alkyl succinate sulfonate salts, allyl ether hydroxyl propanesulfonate salts such as sodium salts, polyoxyethylene styrenated phenyl ether sulfate salts such as ammonium salts, for example DKS Hitenol^® AR 1025 and DKS Hitenol^® AR 2020, polyoxyethylene alkylphenyl ether sulfate ammonium salts, and polyoxyethylene allyloxy nonylphenoxypropyl ether.

Surfactant to be used according to the present invention may be a non-reactive surfactant, a reactive surfactant or a combination thereof. Preferably, a combination of a non-reactive surfactant and a reactive surfactant was used for preparing the aqueous multi-stage copolymer dispersion according to the present invention. It is believed that water whitening resistance performance of an aqueous multi-stage copolymer dispersion and/or a paint containing the dispersion may be improved if a reactive surfactant is used for preparing the dispersion. If a combination of non-reactive and reactive surfactants is used in the polymerization, the two types of surfactants may be used in suitable weight ratio known to the skilled person in the part, such as at a weight ratio in the range of 1 :20 to 20:1 , more preferably 1 : 10 to 10:1 , and most preferably 1 :4 to 4:1 .

Surfactants may be formulated together with the first stage monomers and the second stage monomers respectively. In such a case, the surfactants for the two stage monomers may be same or different. Alternatively, the surfactants may be added into the reaction medium. Both methods may be applied individually or collectively, as known in the art. If there is any surfactant that is introduced into the reaction medium separately from the monomers, such a surfactant may be same as or different from those formulated together with the first and/or second stage monomers.

Surfactants may be used in a suitable amount known to the skilled person in the art, for example, in a total amount of 0.1 % to 6% by weight, based on the total weight of the two stage monomers.

The emulsion polymerization may be carried out in the presence of various common initiating systems, including but not limited to a thermal or redox initiator. The initiator is usually used in an amount of no more than 10% by weight, preferably 0.02 to 5% by weight, more preferably 0.1 to 1.5 wt%, based on the total weight of the two stage monomers.

Thermal initiators, such as peroxides, persulfates and azo compounds, are generally used. Peroxides, which may be used include, but are not limited to, inorganic peroxides such as hydrogen peroxide, or peroxodisulfates such as mono- or di-alkali metal or -ammonium salts of peroxide disulfuric acid, for example, its mono- and di-sodium, -potassium or -ammonium salts, or organic peroxides, such as alkyl hydroperoxides, for example tert-butyl, p-menthyl or cumyl hydroperoxide, tert-butyl perpivalate, and dialkyl or diaryl peroxides, such as di-tert-butyl or di- cumyl peroxide, 2,5-dimethyl-2,5-di-t-butylperoxyhexane or dibenzoyl peroxide. Azo compounds which may be used are, in principle, 2,2^'-azobis(isobutyronitrile), 2,2^'-azobis(2,4- dimethylvaleronitrile), 2,2^'-azobis(amidinopropyl) dihydrochloride (AIBA, such as V-50™ from Wako Chemicals), 1 ,T-azobis(1 -cyclohexanecarbonitrile), 2,2’-azobis(2-amidinopropane)salts, 4,4’-azobis(4-cyanovaleric acid) or 2-(carbamoylazo)isobutyronitrile. Among others, sodium persulfate (SPS), potassium persulfate (KPS), ammonium persulfate (APS), 2,2 - azobis(amidinopropyl) dihydrochloride (AIBA, V-50™), and 4,4'-azobis(4-cyanovaleric acid) (ACVA , V501 ) are preferred as the thermal initiator.

A redox initiator usually comprises an oxidizing agent and a reducing agent. Suitable oxidizing agents are the abovementioned peroxides. Suitable reducing agents may be alkali metal sulfites, for example potassium and/or sodium sulfite, or alkali metal hydrogensulfites, for example potassium and/or sodium hydrogensulfite, or alkali metal metabisulfites, for example potassium and/or sodium metabisulfite, or formaldehyde sulfoxylates, for example potassium and/or sodium formaldehyde sulfoxylate, alkali metal salts, specifically potassium and/or sodium salts, of aliphatic sulfinic acids, or alkali metal hydrogen sulfides, for example, potassium and/or sodium hydrogen sulfide, or acetone bisulfites, for example sodium acetone bisulfite (2-hydroxy-2- propanesulfonic acid monosodium salt), ascorbic acid, isoascorbic acid. Preferable redox initiators include an oxidizing agent selected from the group consisting of t-butylhydroperoxide and hydrogen peroxide, and a reducing agent selected from ascorbic acid, sodium formaldehyde sulfoxylate, sodium acetone bisulfite and sodium metabisulfite (sodium disulfite).

The polymerization may be carried out and maintained at a temperature lower than 100 °C throughout the course of the reaction. Preferably, the polymerization is carried out at a temperature between 60 °C and 95 °C, more preferably between 80 °C and 90 °C. Depending on various polymerization conditions, the polymerization may be carried out for several hours, for example 2 to 8 hours, especially 2 to 6 hours.

According to the present invention, it is preferable that the copolymer particles contained in the aqueous multi-stage copolymer dispersion according to the present invention have a particle size in a reasonable range, as larger particle size will result in worse water whitening resisting performance. The copolymer particles contained in the dispersion preferably have an average particle size in the range of 50 to 200 nm, preferably 70 to 150 nm, more preferably 90 to 130 nm, still preferably 100 to 120 nm, as measured by dynamic light scattering (DLS) as described in detail hereinafter.

The aqueous multi-stage copolymer dispersion according to the present invention may also contain an organic base and/or inorganic base which have been added into the polymerization system as a neutralizer. Suitable neutralizers include, but are not limited to, inorganic bases such as ammonia, sodium hydroxide, potassium hydroxide, zinc oxide, sodium borate, potassium borate, aluminum hydroxide or a combination thereof, and organic bases such as dimethyl amine, diethyl amine, triethyl amine, monoethanolamine, triethanolamine, dimethylaminoethanol, 2- amino-2-methyl-1 -propanol, N-butyldiethanolamine (for example, Vantex™-T), ethanediamine, propane diamine, diethylene diamine, triethylenetetramine, diethylenetriamine, or a combination thereof. Among others, sodium hydroxide, ammonia, dimethylaminoethanol, 2-amino-2-methyl-1 - propanol, N-butyldiethanolamine, or any mixtures thereof are preferable as the neutralizer useful for the polymerization process. Particularly, the aqueous multi-stage copolymer dispersion according to the present invention may have a pH in the range of 7.0 to 10.0, preferably in the range of 7.5 to 9.5, more preferably in the range of 8.0 to 9.0.

The aqueous multi-stage copolymer dispersion according to the present invention may have a solid content in the range of 10% to 70% by weight, preferably 35% to 60% by weight, more preferably 40 to 55% by weight.

The aqueous multi-stage copolymer dispersion according the present invention optionally contains or may be formulated with pigment to prepare a paint composition, as described in CN 102746601 A. Examples of the pigment may include, but are not limited to, zinc oxide, antimony oxide, zirconium oxide, chromium oxide, iron oxide, lead oxide, zinc sulfide, lithopone, and titanium dioxide such as anatase and rutile titanium dioxide. It may also be contemplated that the paint composition optionally contains opaque polymer particles, for example, Ropaque™ opaque polymers (Rohm and Haas Co., Philadelphia, PA., USA); and/or extenders including calcium carbonate, calcium sulfate, barium sulfate, mica, clay, calcined clay, feldspar, nepheline, syenite, wollastonite, diatomaceous earth, alumina silicates, aluminum oxide, silica, and talc; and/or colorants known in the art.

The paint composition prepared from the aqueous multi-stage copolymer dispersion according the present invention is useful as paint which needs water whitening resistance, such as architectural exterior paints, interior paints and stone-finish paints.

The aqueous multi-stage copolymer dispersion according the present invention may be formulated into a paint composition by various processes well known in the art. There is no particular preference for the preparation of the paint composition.

For example, the paint composition may be formulated by a process as generally described in CN 102746601 A. First, optionally, at least one pigment is well dispersed in an aqueous medium under high shear such as is afforded by a COWLES mixer or, in an alternative, at least one predispersed pigment may be used. Then the aqueous multi-stage copolymer dispersion is added under low shear stirring along with other adjuvants, as desired. Alternatively, the aqueous multi- stage copolymer dispersion may be included in an optional pigment dispersion step. The paint composition may contain conventional paint adjuvants such as, for example, tackifiers, emulsifiers, coalescent agents such as Texanol® (from Eastman Chemical Co.), cosolvents such as glycols and glycol ethers, buffers, neutralizers, thickeners or rheology modifiers, humectants, wetting agents, biocides, plasticizers, defoamers, colorants, waxes, and anti-oxidants.

The present invention is further demonstrated and exemplified in the Examples, however, without being limited to the embodiments described in the Examples.

Examples

The average particle diameter of the copolymer particles as referred herein relates to the Z average particle diameter as determined by means of dynamic light scattering (DLS) method. The measurement method is described in the ISO 13321 :1996 standard. For this purpose, a sample of the aqueous copolymer dispersion will be diluted and the obtained aqueous dilution will be analysed. In the context of DLS, the aqueous dilution may have a polymer concentration in the range from 0.001 to 0.5 % by weight, depending on the particle size. For most purposes, a proper concentration will be 0.01 % by weight. However, higher or lower concentrations may be used to achieve an optimum signal/noise ratio. The dilution can be achieved by addition of the aqueous copolymer dispersion to water or an aqueous solution of a surfactant in order to avoid flocculation. Usually, the dilution is performed by using a 0.1 wt % aqueous solution of a non-ionic emulsifier, e.g. an ethoxylated C16/C18 alkanol (with ethoxylation degree of 18), as a diluent.

Measurement configuration: High-performance particle sizer (HPPS) from Malvern Instruments, UK, automated, with continuous-flow cuvette and Gilson autosampler.

Parameters: measurement temperature 20.0°C; measurement time 120 seconds (6 cycles, each of 20 s); scattering angle 173°; laser wavelength 633 nm (HeNe); refractive index of medium 1.332 (aqueous); viscosity 0.9546 mPa-s. The measurement gives an average value of the second order cumulant analysis (mean of fits), i.e. Z average. The "mean of fits" is an average, intensity-weighted hydrodynamic particle diameter in nm.

Description of commercially available materials used in the following Examples:

Disponil® LDBS 23 IS: surfactant, Sodium n-alkyl-(C10C13) alkylbenzene sulfonate, from BASF, Germany;

Disponil® SUS 87 SPEZIAL: surfactant, Sulfosuccinate based on fatty alcohol polyglycol ether, disodium salt, from BASF, Germany;

ADEKA Resoap SR-1025: surfactant, Oxirane, [(2-propenyloxy)methyl]-, reaction products with alcohol C10-14-branched-, oxirane and sulfamic acid, from ADEKA CORPORATION, Japan;

ADEKA Resoap ER-10: surfactant, Oxirane, [(2-propenyloxy)methyl]-, reaction products with alcohol C10-14-branched- and oxirane, from ADEKA CORPORATION, Japan;

Silquest A-171 : comonomer, Vinyl trimethoxysilane, from Momentive Performance Materials Inc., USA.

All experiments described hereinafter were performed at a temperature of 20 °C unless otherwise specified.

Example 1 (Inventive):

A pre-emulsion of first stage monomers was prepared by combining monomers composed of 136.4 g methyl methacrylate (MMA), 133.2 g 2-ethylhexyl acrylate (EHA), 35.1 g butyl acrylate (BA), 4.4 g methacrylic acid (MAA), 16.6 g acrylamide (Am, 30 wt% aqueous solution) and 1.2 g Silquest A-171 , surfactants composed of 14.0 g ADEKA Resoap SR-1025, 1 .0 g ADEKA Resoap ER-10 and 3.2 g Disponil SUS 87 SPEZIAL, and 153.4 g deionized water in a vessel, and emulsified under stirring for 10 min with a magnetic stirring bar at a speed of 500 rpm.

A pre-emulsion of second stage monomers was prepared in the same way as described above, except that the pre-emulsion of second stage monomers was composed of 1 17.9 g methyl methacrylate (MMA), 1.6 g methacrylic acid (MAA), 5.7 g acrylamide (Am, 30 wt% aqueous solution), 0.5 g Silquest A-171 , 5.2 g ADEKA Resoap SR-1025, 0.4 g ADEKA Resoap ER-10 and 1.2 g Disponil® SUS 87 SPEZIAL, and 57.0 g deionized water.

A mixture of 4.9 g Disponil® LDBS 23 IS and 157 g Dl water was charged to a five-liter multi-neck flask equipped with a mechanical stirring device under nitrogen atmosphere protection. The mixture was heated to a temperature of 86 °C under nitrogen atmosphere and the condition was maintained for following operations, unless otherwise specified. To the flask, 23.2 g of the pre- emulsion of first stage monomers (the first portion) and 2.9 g of a 3wt% aqueous ammonium persulfate solution were added under stirring within 1 min in parallel. Five minutes after the addition of the first portion, the remaining pre-emulsion of first stage monomers was fed over 135 min. Twenty minutes after the addition of the pre-emulsion of first stage monomers, the pre- emulsion of second stage monomers was added over 50 min. Separately, 47.4 g of a 3 wt% aqueous ammonium persulfate solution feeding was started once the addition of the first portion was finished, which lasted 205 min. Sixty minutes after the completion of ammonium persulfate feeding, the reaction temperature was decreased to 50 °C with cooling water, and 3.7 g of a 20 wt% aqueous ammonium hydroxide solution was added.

The obtained aqueous multi-stage copolymer dispersion has a pH of 8.5, a solid content of 45.0 wt%, and a particle size of 1 12 nm.

Example 2 (Comparative):

An aqueous copolymer dispersion was prepared in the same way as described above for Example 1 , except that the two pre-emulsions of first stage monomers and second stage monomers were combined as one pre-emulsion in the preparation of pre-emulsion step.

A mixture of 4.9 g Disponil® LDBS 23 IS and 157 g Dl water was charged to a five-liter multi-neck flask equipped with a mechanical stirring device under nitrogen protection. The mixture was heated to a temperature of 86 °C under nitrogen atmosphere and the condition was for following operations unless otherwise specified. To the flask, 23.2 g of the pre-emulsion (the first portion) and 2.9 g of a 3 wt% aqueous ammonium persulfate solution were added under stirring within 1 minute in parallel. Five minutes after the addition of the first portion, the remaining pre-emulsion feeding was started and finished in 185 min, and 47.4 g of a 3 wt% aqueous ammonium persulfate solution feeding was started simultaneously and finished in 205 min. Sixty minutes after the completion of ammonium persulfate feeding, the reaction temperature was decreased to 50 °C with cooling water, and 3.7 g 20 wt% aqueous ammonium hydroxide solution was added.

The obtained aqueous copolymer dispersion has a pH of 8.3, a solid content of 45.1 wt%, and a particle size of 1 10 nm.

Example 3 (Inventive):

An aqueous multi-stage copolymer dispersion was prepared in the same way as described above for Example 1 , except that the pre-emulsion of first stage monomers was composed of 365 g methyl methacrylate (MMA), 321 g 2-ethylhexyl acrylate (EHA), 89 g butyl acrylate (BA), 1 1 .2 g methacrylic acid (MAA), 42.3 g acrylamide (Am, 30 wt% aqueous solution), 3.23 g Silquest A- 171 , 35.9 g ADEKA Resoap SR-1025, 2.65 g ADEKA Resoap ER-10, 8.1 g Disponil® SUS 87 SPEZIAL, and 391 g deionized water; and the pre-emulsion of second stage monomers was composed of 300 g methyl methacrylate (MMA), 4.2 g methacrylic acid (MAA), 15.7 g acrylamide (Am, 30 wt% aqueous solution ) and 1.2 g Silquest A-171 , 13.3 g ADEKA Resoap SR-1025, 1.0 g ADEKA Resoap ER-10, 3.0 g Disponil® SUS 87 SPEZIAL, and 144 g deionized water.

The obtained aqueous multi-stage copolymer dispersion has a pH of 8.4, a solid content of 45.1 wt%, and a particle size of 1 10 nm.

Example 4 (Inventive):

An aqueous multi-stage copolymer dispersion was prepared in the same way as described above for Example 1 , except that the pre-emulsion of first stage monomers was composed of 210.7 g methyl methacrylate (MMA), 133.2 g 2-ethylhexyl acrylate (EHA), 35.1 g butyl acrylate (BA), 5.4 g methacrylic acid (MAA), 20.5 g acrylamide (Am, 30 wt% aqueous solution), 1.6 g Silquest A- 171 , 17.4 g ADEKA Resoap SR-1025, 1.3 g ADEKA Resoap ER-10, 3.9 g Disponil® SUS 87 SPEZIAL and 189.4 g deionized water; and the pre-emulsion of second stage monomers was composed of 43.6 g methyl methacrylate (MMA), 0.6 g methacrylic acid (MAA), 2.3 g acrylamide (Am, 30 wt % aqueous solution), 0.2 g Silquest A-171 , 1 .9 g ADEKA Resoap SR-1025, 0.1 g ADEKA Resoap ER-10, 0.4 g Disponil® SUS 87 SPEZIAL and 21.0 g deionized water.

The obtained aqueous multi-stage copolymer dispersion has a pH of 8.6, a solid content of 45.3 wt%, and a particle size of 1 14 nm.

Example 5 (Inventive):

An aqueous multi-stage copolymer dispersion was prepared in the same way as described above for Example 1 , except that the pre-emulsion of first stage monomers was composed of 167.2 g methyl methacrylate (MMA), 133.2 g 2-ethylhexyl acrylate (EHA), 35.1 g butyl acrylate (BA), 4.8 g methacrylic acid (MAA), 18.2 g acrylamide (Am, 30 wt% aqueous solution), 1.4 g Silquest A- 171 , 15.5 g ADEKA Resoap SR-1025, 1.1 g ADEKA Resoap ER-10, 3.5 g Disponil® SUS 87 SPEZIAL and 168.3 g deionized water; and the pre-emulsion of second stage monomers was composed of 87.1 g methyl methacrylate (MMA), 1.2 g methacrylic acid (MAA), 4.6 g acrylamide (Am, 30 wt% aqueous solution), 0.4 g Silquest A-171 , 3.9 g ADEKA Resoap SR-1025, 0.3 g ADEKA Resoap ER-10, 0.9 g Disponil® SUS 87 SPEZIAL and 42.1 g deionized water.

The obtained aqueous multi-stage copolymer dispersion has a pH of 8.6, a solid content of 45.2 wt%, and a particle size of 1 1 1 nm.

Example 6 (Inventive):

An aqueous multi-stage copolymer dispersion was prepared in the same way as described above for Example 1 , except that the pre-emulsion of first stage monomers was composed of 145.4 g methyl methacrylate (MMA), 133.2 g 2-ethylhexyl acrylate (EHA), 35.1 g butyl acrylate (BA), 4.5 g methacrylic acid (MAA), 17.1 g acrylamide (Am, 30wt % aqueous solution), 1.3 g Silquest A- 171 , 14.5 g ADEKA Resoap SR-1025, 1.1 g ADEKA Resoap ER-10, 3.3 g Disponil® SUS 87 SPEZIAL and 157.8 g deionized water; and the pre-emulsion of second stage monomers was composed of 108.9 g methyl methacrylate (MMA), 1 .5 g methacrylic acid (MAA), 5.7 g acrylamide (Am, 30 wt% aqueous solution), 0.4 g Silquest A-171 , 4.8 g ADEKA Resoap SR-1025, 0.4 g ADEKA Resoap ER-10, 1 .1 g Disponil® SUS 87 SPEZIAL and 52.6 g deionized water.

The obtained aqueous multi-stage copolymer dispersion has a pH of 8.8, a solid content of 45.3 wt%, and a particle size of 1 15 nm.

Example 7 (Inventive):

An aqueous multi-stage copolymer dispersion was prepared in the same way as described above for Example 1 , except that the pre-emulsion of first stage monomers was composed of 123.6 g methyl methacrylate (MMA), 133.2 g 2-ethylhexyl acrylate (EHA), 35.1 g butyl acrylate (BA), 4.2 g methacrylic acid (MAA), 16.0 g acrylamide (Am, 30 wt% aqueous solution), 1.2 g Silquest A- 171 , 13.5 g ADEKA Resoap SR-1025, 1.0 g ADEKA Resoap ER-10, 3.1 g Disponil® SUS 87 SPEZIAL and 147.3 g deionized water; and the pre-emulsion of second stage monomers was composed of 130.6 g methyl methacrylate (MMA),1 .1 g methacrylic acid (MAA), 6.8 g acrylamide (Am, 30 wt% aqueous solution), 0.5 g Silquest A-171 , 5.8 g ADEKA Resoap SR-1025, 0.4 g ADEKA Resoap ER-10, 1 .3 g Disponil® SUS 87 SPEZIAL and 63.2 g deionized water.

The obtained aqueous multi-stage copolymer dispersion has a pH of 8.2, a solid content of 44.9 wt%, and a particle size of 108 nm.

Example 8 (Inventive):

An aqueous multi-stage copolymer dispersion was prepared in the same way as described above for Example 1 , except that the pre-emulsion of first stage monomers was composed of 101.9 g methyl methacrylate (MMA), 133.2 g 2-ethylhexyl acrylate (EHA), 35.1 g butyl acrylate (BA), 3.9 g methacrylic acid (MAA), 14.8 g acrylamide (Am, 30 wt% aqueous solution), 1.1 g Silquest A- 171 , 12.6 g ADEKA Resoap SR-1025, 0.9 g ADEKA Resoap ER-10, 2.8 g Disponil® SUS 87 SPEZIAL and 136.8 g deionized water; and the pre-emulsion of second stage monomers was composed of 152.4 g methyl methacrylate (MMA), 2.1 g methacrylic acid (MAA), 8.0 g acrylamide (Am, 30 wt% aqueous solution), 0.6 g Silquest A-171 , 6.8 g ADEKA Resoap SR-1025, 0.5 g ADEKA Resoap ER-10, 1 .5 g Disponil® SUS 87 SPEZIAL and 73.6 g deionized water.

The obtained aqueous multi-stage copolymer dispersion has a pH of 8.5, a solid content of 45.1 wt%, and a particle size of 1 13 nm.

Example 9 (Comparative): An aqueous multi-stage copolymer dispersion was prepared in the same way as described above for Example 1 , except that the pre-emulsion of first stage monomers was composed of 80.1 g methyl methacrylate (MMA), 133.2 g 2-ethylhexyl acrylate (EHA), 35.1 g butyl acrylate (BA), 3.6 g methacrylic acid (MAA), 13.7 g acrylamide (Am, 30 wt% aqueous solution), 1 .0 g Silquest A- 171 , 1 1 .6 g ADEKA Resoap SR-1025, 0.9 g ADEKA Resoap ER-10, 2.6 g Disponil® SUS 87 SPEZIAL and 126.3 g deionized water; and the pre-emulsion of second stage monomers was composed of 174.2 g methyl methacrylate (MMA), 2.4 g methacrylic acid (MAA), 9.1 g acrylamide (Am, 30 wt% aqueous solution), 0.7 g Silquest A-171 , 7.7 g ADEKA Resoap SR-1025, 0.6 g ADEKA Resoap ER-10, 1 .7 g Disponil® SUS 87 SPEZIAL and 84.2 g deionized water.

The obtained aqueous multi-stage copolymer dispersion has a pH of 8.6, a solid content of 45.3 wt%, and a particle size of 1 10 nm.

Example 10 (Inventive):

An aqueous multi-stage copolymer dispersion was prepared in the same way as described above for Example 1 , except that methyl methacrylate (MMA) in the pre-emulsion of second stage monomers was replaced by 1 17.9 g styrene (St).

The obtained aqueous multi-stage copolymer dispersion has a pH of 8.3, a solid content of 44.9 wt%, and a particle size of 108 nm.

Example 1 1 (Inventive):

An aqueous multi-stage copolymer dispersion was prepared in the same way as described above for Example 1 , except that methyl methacrylate (MMA) in the pre-emulsion of first stage monomers was replaced by 136.4 g styrene (St).

The obtained aqueous multi-stage copolymer dispersion has a pH of 8.5, a solid content of 45.2 wt%, and a particle size of 1 13 nm.

Example 12 (Comparative):

An aqueous multi-stage copolymer dispersion was prepared in the same way as described above for Example 1 , except that the first stage pre-emulsion was prepared by combining monomers composed of 194.7 g methyl methacrylate (MMA), 74.8 g 2-ethylhexyl acrylate (EHA), 35.1 g butyl acrylate (BA), 4.4 g methacrylic acid (MAA), 16.6 g acrylamide (Am, 30 wt% aqueous solution), and 1 .2 g Silquest A-171 , surfactants composed of 14.0 g ADEKA Resoap SR-1025, 1 .0 g ADEKA Resoap ER-10 and 3.2 g Disponil® SUS 87 SPEZIAL, and 153.4 g deionized water in a vessel, and emulsified with stirring.

The preparation of the second stage pre-emulsion and the subsequent reaction were carried out in the same way as in Example 1. The obtained aqueous multi-stage copolymer dispersion has a pH of 8.4, a solid content of 45.0 wt%, and a particle size of 1 10 nm.

Example 13 (Comparative):

An aqueous copolymer dispersion was prepared in accordance with Example 4 of CN 102977257 A. The obtained aqueous copolymer dispersion has a pH of 7.0, a solid content of 45.3 wt%, and a particle size of 163 nm.

Performance Tests

I. Water Whitening Test for Latex Films of the Aqueous Copolymer Dispersions

To an aqueous copolymer dispersion as prepared above, 4% of coalescent agent Texanol® based on the dry weight of the dispersion was added. The dispersion was stirred for 5 min with a mechanical stirring machine at a speed of 500 rpm. The obtained dispersion was drawn down on a glass plate with doctor blading to obtain a wet film having a thickness of 100 pm (microns). Then the coated plate was dried in an oven at a temperature of 50 °C for 30 min. After the coated plate was cooled to 20 °C, the obtained panel was immersed in water for 96 hr. Water whitening resistance (WWR) was rated by visual evaluation according to the following rating criteria. Each panel was evaluated independently by two different technical experts and an average was taken as the rating score.

Typically, a rating score“3” is regarded as“acceptable”, and a higher score is preferable.

II. Water Whitening Test for Paints Obtained from the Aqueous Copolymer Dispersions

To 100 g of an aqueous copolymer dispersion as prepared above, following components were added by a disperser at 200 rpm with the indicated order: 50 g Natrosol® 250HBR (hydroxyethylcellulose, 2wt%), 1 .5 g biocide (Acticide® M, from THOR, Germany), 1 g of a 25wt% aqueous ammonia solution, 2 g Foamaster® MO NXZ, 8 g propylene glycol, 1 .5 g Rheovis® HS 1 152 (a hydrophobic modified acrylic swellable copolymer emulsion), 71.2 g water, 4.8 g coalescent agent Texanol®, 375 g sand (40-80 mesh, black : white = 1 :1 ), 375 g sand (80-120 mesh, black : white = 1 :1 ), and further stirred at a speed of 500 rpm for 5 min to obtain a stone finish paint. The stone finish paint was coated on a cement panel (2 mm thickness) and dried in air for 2 weeks. The dried panel was dipped into water for 24 hr. Water whitening resistance (WWR) was rated by visual evaluation according to the following rating criteria. Each panel was evaluated independently by two different technical experts and an average was taken as the rating score.

Typically, a rating score“4” is regarded as“acceptable”, and a higher score is preferable. III. Wet bonding strength for Paints Obtained from the Aqueous Copolymer Dispersions

The wet bonding strength was tested in accordance with JG/T 157-2009. A bonding strength of at least 0.8 MPa is regarded as acceptable. Test results for all aqueous copolymer dispersions according to Examples were summarized in Table 1 below.

Table 1

^*1^st stage ratio means the percentage of the total weight of first stage monomers over the total weight of first and second stage monomers;

n.a.: not measured.

It can be seen from Table 2 below, acceptable water whitening resistance is achieved for both latex film and paint when the weight ratio of the first stage monomers accounts for 64 wt% or higher based on the total weight of first and second stage monomers. It can also be seen that successive copolymerization of the two stage monomers is essential for achieving an acceptable bonding strength of a paint containing the aqueous copolymer dispersion. Table 2

It can be seen from Table 3 below, acceptable water whitening resistance and bonding strength may be achieved only when the first stage copolymer and the second stage copolymer have Fox Tgs in the ranges as required herein.

Table 3

It can be seen from Table 4 below, difference in species of monomers within the pools as required herein has minor influence on the performances of aqueous multi-stage copolymer dispersions and paints in the case that the two stage copolymers have the required Fox Tgs. Comparison between Example 1 and Examples 10 and 11 showed that replacement of MMA with St, both having same Tg, resulted in slightly change in WWR and bonding strength performances. Table 4

Claims

1. An aqueous multi-stage copolymer dispersion, particularly an aqueous multi-stage copolymer dispersion useful for paints which need water whitening resistance, wherein the multi-stage copolymer consisting of a first stage copolymer having a Fox Tg in the range of -10 to 20 °C resulted from first stage monomers and a second stage copolymer having a Fox Tg in the range of 60 to 120 °C resulted from second stage monomers.

2. The aqueous multi-stage copolymer dispersion according to claim 1 , wherein the Fox Tg of the first stage copolymer is in the range of -8 to 20 °C, preferably -5 to 15 °C.

3. The aqueous multi-stage copolymer dispersion according to claim 1 or 2, wherein the Fox Tg of the second stage copolymer is in the range of 75 to 120 °C, preferably 80 to 1 15 °C, more preferably 85 to 1 10°C.

4. The aqueous multi-stage copolymer dispersion according to any of claims 1 to 3, wherein the total weight of first stage monomer accounts for, as a percentage of the total weight of first stage monomers over the total weight of first stage monomers and second stage monomers, 64 to 95% by weight, preferably 70 to 95 % by weight or 64 to 90% by weight, more preferably 70 to 90% by weight, more preferably 70 to 85% by weight, even more preferably 70 to 80 % by weight and most preferably 70 to 75% by weight.

5. The aqueous multi-stage copolymer dispersion according to any of claims 1 to 4, wherein the first stage monomers and the second stage monomers each independently comprise,

(A) at least one hydrophobic monoethylenically unsaturated monomer (a) in an amount of at least 80 % by weight, preferably at least 85 % by weight, more preferably at least 90% by weight, mostly preferably at least 95 % by weight; and

(B) at least one hydrophilic monoethylenically unsaturated monomer (b) in an amount of at least 0.1 % by weight and no more than 20 % by weight, preferably no more than 15 % by weight, more preferably no more than 10 % by weight, mostly preferably no more than 5 % by weight, each based on the total weight of the respective stage monomers.

6. The aqueous multi-stage copolymer dispersion according to claim 5, wherein the hydrophobic monoethylenically unsaturated monomer (a) is selected from the group consisting of (meth)acrylate monomers, preferably Ci-Cig-alkyl (meth)acrylate monomers, (meth)acrylonitrile monomers, styrene monomers, vinyl alkanoate monomers, and any mixture thereof.

7. The aqueous multi-stage copolymer dispersion according to claim 5, wherein one or more Ci- Ci2-alkyl (meth)acrylates, more preferably C-i-Cs-alkyl (meth)acrylates, are used as the hydrophobic monoethylenically unsaturated monomer (a).

8. The aqueous multi-stage copolymer dispersion according to claim 7, wherein the Ci-Ci2-alkyl (meth)acrylate is selected from the group consisting of methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate n- octyl (meth)acrylate, lauryl (meth)acrylate and any mixtures thereof, especially from the group consisting of methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and any mixtures thereof.

9. The aqueous multi-stage copolymer dispersion according to any of claims 5 to 8, wherein the hydrophilic monoethylenically unsaturated monomer (b) is a monoethylenically unsaturated monomer containing at least one functional group selected from the group consisting of carboxyl, carboxylic anhydride, hydroxyl and amide.

10. The aqueous multi-stage copolymer dispersion according to claim 9, wherein the hydrophilic monoethylenically unsaturated monomer (b) is selected from the group consisting of monoethylenically unsaturated carboxylic acids, such as (meth)acrylic acid, itaconic acid and maleic acid; monoethylenically unsaturated amides, especially N-alkylolamides, such as (meth)acrylamide, N-methylol (meth)acrylamide, 2-hydroxyethyl (meth)acrylamide; and hydroxyalkyl esters of monoethylenically unsaturated carboxylic acids, such as hydroxyethyl (meth)acrylate and hydroxypropyl (meth)acrylate, and any mixtures thereof.

1 1 . The aqueous multi-stage copolymer dispersion according to claim 10, wherein the hydrophilic monoethylenically unsaturated monomer (b) is selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, acrylamide, and any mixtures thereof.

12. The aqueous multi-stage copolymer dispersion according to any of claims 5 to 1 1 , wherein the first stage monomers and the second stage monomers each independently further comprise a monoethylenically unsaturated monomer (c) containing at least one alkoxysilane functional group, preferably hydrolyzable alkoxysilane functional group, in an amount of no more than 5% by weight, particularly no more than 3% by weight, preferably in the range of 0.05 to 1.5% by weight, more preferably 0.1 to 1 % by weight, based on the total weight of the respective stage monomers.

13. The aqueous multi-stage copolymer dispersion according to claim 12, wherein the monoethylenically unsaturated monomer (c) containing at least one alkoxysilane functional group is selected from the group consisting of vinyltrialkoxysilanes such as vinyltrimethoxysilane; alkylvinyldialkoxysilanes such as dimethoxymethylvinylsilane; (meth)acryloxyalkyltrialkoxysilanes such as (meth)acryloxyethyltrimethoxysilane and (3-acryloxypropyl)trimethoxysilane, and any mixtures thereof.

14. The aqueous multi-stage copolymer dispersion according to any of claims 1 to 13, which is obtainable or obtained by a process including polymerization of first stage monomers resulting the first stage copolymer and subsequent second stage monomers resulting the second stage copolymer, preferably in the presence of a reactive surfactant.

15. The aqueous multi-stage copolymer dispersion according to claim 14, wherein the reactive surfactant is selected from the group consisting of allyl polyoxyalkylene ether sulfate salts such as allyl polyoxyethylene alkyl ether sulfate sodium salts, allyl alkyl succinate sulfonate salts, allyl ether hydroxyl propanesulfonate salts such as sodium salts, polyoxyethylene styrenated phenyl ether sulfate salts such as ammonium salts, polyoxyethylene alkylphenyl ether sulfate ammonium salts, polyoxyethylene allyloxy nonylphenoxypropyl ether, and any mixtures thereof.

16. The aqueous multi-stage copolymer dispersion according to any of claims 1 to 15, wherein the copolymer contained in the dispersion is in form of particles having an average particle size in the range of 50 to 200 nm, preferably 70 to 150 nm, more preferably 90 to 130 nm, still preferably 100 to 120 nm, as measured by dynamic light scattering method.

17. A paint which needs water whitening resistance, such as architectural exterior paints, interior paints and stone-finish paints, comprising the aqueous multi-stage copolymer dispersion according to any of claims 1 to 16.