Small particle size non-surface active protective colloid-stabilized latexes derived from monomers of high aqueous phase grafting tendencies

Abstract: Disclosed are aqueous polyacrylate dispersions of acrylic or methacrylic polymers, having improved rheological and stability characteristics, prepared by the aqueous poly­ merization of acrylic esters, methacrylic esters or their mixtures in the presence of a from about 0.05 to about 5% of a non-surface active protective colloid and a free-radical polymerization initiators that comprises a combination of from about 0.01 to about 1% of an azo initiator or an organo-­ soluble, water-insoluble peroxide initiator and between 0.01% to about 0.05% of a water-soluble peroxide initiator, by weight of total monomer content. The resultant aqueous poly­ acrylate dispersions have a particle size less than about 1000 nm and are useful in the manufacture of latex paints, binders for nonwoven materials, water-based inks, paper coat­ ings and water-borne adhesives, such as pressure-sensitive adhesives.

Description: This invention relates to aqueous polyacrylate dispersions, i.e., latices of acrylate polymers, that are useful for instance in the manufacture of latex paints, binders for nonwoven materials, water-based inks, paper coatings and water-based adhesives, and to processes for their preparation. Conventional acrylate latices are produced by emulsion polymerisation carried out in aqueous media. According to classical analysis, an emulsion polymerisation is considered to contain a three-phase reaction system consisting of large (about 1500 nm) droplets of the monomer, an aqueous phase containing a dissolved initiator, and colloidal particles of monomer-swollen polymer (usually 50-150 nm diameter). See, "Polymerization Mechanisms and Processes", 18 Encyclopedia of Chemical Technology pages 742-43 (John Wiley & Sons, Inc., 3rd Ed. 1982). It is known that the presence of protective colloids during the manufacture of polyacrylate dispersions or latices, from reaction mixtures having a high acrylate or methacrylate content, usually results either in coagulated dispersions or in extremely viscous dispersions which are unsuitable for use as paint hinders or any other use. Although the presence or more than about 50% vinyl acetate, by weight of the total monomer, enables a latex having good stability and rheological characteristics to be obtained in the presence of a protective colloid such as hydroxyethyl cellulose ("HEC") or polyvinyl alcohol, and permits good control of viscosity, when the vinyl acetate comonomer is not used or is used in amounts less than 50%, a stable latex cannot be obtained in the presence of a protective colloid. A number of methods have been developed to permit the successful incorporation of protective colloids during the emulsion polymerization of reaction mixtures having a high acrylate or methacrylate content. For example, British Patent 1,278,813 discloses the use of emulsion stabilizers, such as allyl alcohols, allyl halides and low molecular weight aliphatic alcohols, and several other patents disclose the use of other additives, for instance molecular weight regulators, such as mercaptoacetic acid and cyclo-hexylamine (U.S. Patent 4,265,796), or small amounts of water-soluble monomers having conjugated unsaturation such as furoic acid, or styrene sulfonic acid (U.S. Patent No. 4,659,771). Such methods are all disadvantageous in that they require the use of additives. Other known processes provide for the gradual addition of the protective colloid during the polymerization reaction (for instance, U.S. 3,876,596), but the resultant latices have a high viscosity, coarse particle size and poor processing characteristics. EP-A-0213895 discloses polyacrylate dispersions, but the nature of the polymerization initiator is not considered as deciding. There is need for a process for producing an aqueous polyacrylate dispersion by the aqueous polymerization of acrylic or methacrylic monomers that does not require the presence of additives and produces latices with small particle sizes, that is, a weight average particle diameter less than about 200 nm, high mechanical stability, low viscosity, and good processing characteristics. According to the invention, a process for making an aqueous polyacrylate dispersion by the aqueous polymerization of acrylic esters, methacrylic esters or their mixtures in the presence of a protective colloid and a free-radical polymerization initiator, is characterized in that the polymerisation is carried out in the absence of an emulsifier and the protective colloid is a non-surface active protective colloid from the group of hydroxyethylcellulose, carboxymethylhydroxyethylcellulose, carboxymethylcellulose, ethoxylated starch derivative, water-soluble starch, water-soluble alginate and gum arabic and prior to the start of polymerisation the aqueous dispersion comprises 0.05 to 5% non-surface active protective colloid, 0.01 to 1% of an azo initiator or organo-soluble, water-insoluble peroxide initiator, and 0.01% to 0.05% water-soluble peroxide initiator where all weight percentages are based on the total weight of monomer, and where the resultant polyacrylate latex has a particle size less than 200nm. Acrylic esters and methacrylic esters suitable for use in the practice of this invention include alkyl acrylates and methacrylates wherein the alkyl group contains less than 22 carbon atoms. Preferably at least 50%, by weight, of the total monomers are monomers selected from the group consisting of alkyl acrylates and methacrylates having less than 10 carbon atoms. Typical acrylic esters and methacrylic esters include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, t-butyl acrylate, t-butyl methacrylate, 3,3-dimethyl butyl acrylate and 3,3-dimethyl butyl methacrylate. Suitable C₁₀₋₂₂ alkyl esters of acrylic and methacrylic acid include decyl acrylate, decyl methacrylate, hendecanyl methacrylate, lauryl methacrylate, tridecanyl methacrylate, myristyl acrylate, myristyl methacrylate, pentadecanyl methacrylate, arachidyl methacrylate, behenyl methacrylate, 10-methyl hendecanyl methacrylate and 2-ethyllauryl methacrylate. These monomers can be homopolymerized or copolymerized with each other. In addition to being copolymerized with each other, the aforementioned acrylates and methacrylates can be copolymerized with other types of olefinically unsaturated monomers capable of addition type polymerization, including unsubstituted and substituted styrene, such as alpha-methylstyrene, acrylic or methacrylic acid, vinyl and vinylidene halides, allyl esters of saturated monocarboxylic acids, vinyl esters, vinyl ethers, vinyl ketones, dialkyl esters of monoethylenically unsaturated dicarboxylic acids, amine containing ethylenically unsaturated monomers, ethylenically unsaturated monocarboxylic and polycarboxylic acids and the anhydrides, nitriles, unsubstituted and substituted (including N-substituted) amides of all said acids. Suitable vinyl and vinylidene halides include vinyl chloride, vinylidene chloride, 2-cloropropene, and 3,3,3-trichloro-2-chloropropene. Suitable vinyl esters include aliphatic vinyl esters, such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl valerate, and vinyl caproate, and suitable allyl esters include allyl esters of saturated monocarboxylic acids, such as, allyl acetate, allyl propionate and allyl lactate. Although these monomers may be used in the general ranges described below, they are generally used in amounts ranging up to about 10% by weight of the total weight of monomers used. The allyl esters of saturated mono-carboxylic acids are generally used in relatively small amounts together with larger amounts of one or more different vinyl monomers, particularly with aliphatic vinyl esters such as vinyl acetate. Typical vinyl ethers include methylvinyl ether, ethylvinyl ether and n-butylvinyl ether and typical vinyl ketones include methylvinyl ketone, ethylvinyl ketone and iso-butylvinyl ketone. Suitable dialkyl esters of monoethylenically unsaturated dicarboxylic acids include diethyl maleate, dibutyl maleate, dioctyl maleate, diisooctyl maleate, dinonyl maleate, diisodecyl maleate, ditridecyl maleate, dipropyl fumarate, dibutyl fumarate, dioctyl fumarate, diisooctyl fumarate, didecyl fumarate, dibutyl itaconate and dioctyl itaconate. Monoethylenically unsaturated monocarboxylic acids, such as acrylic acid, methacrylic acid, ethacrylic acid and crotonic acid; monoethylenically unsaturated dicarboxylic acids, such as maleic acid, fumaric acid, itaconic acid and citraconic acid; and monoethylenically unsaturated tricarboxylic acids, such as aconitic acid and the halogen-substituted derivatives, e.g., alpha-chloroaorylic acid, and anhydrides of these acids, such as, e.g., maleic anhydride and citraconic anhydride are suitable for use as comonomers in the practice of this invention. Acrylonitrile, alpha-chloro-acrylonitrile and methacrylonitrile are among the corresponding nitriles of such acids which can be used as comonomers. Suitable amides of such acids include unsubstituted amides such as acrylamide, methacrylamide and other alpha-substituted acrylamides and N-substituted amides obtained by conventional reaction of the amides of the aforementioned mono- and poly-carboxylic acids with an aldehyde, such as formaldehyde. Typical N-substituted amides include N-methylolacrylamide, N-methylolmethacrylamide, alkylated N-methylolacrylamides and N-methylolmethylmethacrylamide. Amine containing ethylenically unsaturated monomers which can be used in the practice of this invention include substituted and unsubstituted aminoalkyl acrylates, hydrochloride salts of amino monomers and methacrylates, such as beta-aminoethylacrylate, beta-aninoethylmethacrylate, beta-dimethylaminoethylacrylate, beta-dimethylaminoethylmethacrylate. Typical hydroxy-containing monomers include beta-hydroxyethylacrylate, beta-hydroxypropylacrylate, gamma-hydroxypropylacrylate and beta-hydroxyethylmethacrylate. Styrene, styrene derivatives, and acrylonitrile comonomers can be present in amounts greater than 50%, for example, in amounts up to about 75%. However, the comonomer is typically present in an amount from about 0.5% to less than 50%, by weight of the total monomer content, preferably from about 0.5% to about 25%, most preferably from about 1% to about 10%. Non-surface active protective colloids useful in this invention include all water-soluble protective colloids which do not significantly lower the surface tension of water when present at 0.1%, based on the total weight of the solution. Typical non-surface active protective colloids include HEC, carboxymethyl hydroxyethyl cellulose, carboxymethyl cellulose (sodium and other salts), ethoxylated starch derivatives, water-soluble starch, water-soluble alginates such as sodium or potassium alginate, and gum arabic. Preferred protective colloids are water-soluble starch, HEC and sodium carboxymethyl cellulose. Although the amount of non-surface active protective colloid used will vary depending on the desired viscosity, particle size and latex properties, from 0.05 to 5%, by weight of the total monomer content, is employed, preferably from about 0.2% to about 1%. Surface active protective colloids, such as hydrophobically modified hydroxyethyl cellulose, polyvinyl alcohol and polyvinylpyrrolidone, may also be present during polymerization so long as they are not used in an amount that in combination with the non-surface active protective colloid will lower the surface tension of water below about 65 dynes/cm at 0.1% concentration by weight, based on the weight of the total solution. Typically, they are present in an amount less than or equal to about 0.1%, by weight, of the total monomer content. The free radical polymerization initiators useful in this invention are azo initiators, including organo-soluble and water-soluble azo initiators such as azobis isobutyronitrile and azobis cyanovaleric acid dipotassium salt or azobis isobutyramidine dihydrochloride, respectively, and organo-soluble, water-insoluble peroxide initiators such as benzoyl peroxide and lauroyl peroxide. The solubility of organo-soluble, water-insoluble peroxide initiators per this invention is less than about 0.1% (based on the aqueous phase) soluble in water in the presence of the above monomers. In order to produce a latex with suitable mechanical stability in the presence of non-surface active protective colloids, it is necessary to carry out polymerization in the presence of a relatively small amount of water-soluble peroxide initiator such as hydrogen peroxide, persulfate salts, etc. Typically, polymerization is carried out in the presence of one or more of these polymers in the aforementioned amounts. Preferably about 0.02 to about 0.04%, most preferably 0.025 to 0.03%, by weight of the total monomer content, are used. Anionic, cationic, nonionic and amphoteric emulsifiers and mixtures thereof known in the art may be used in the practice of this invention, but are not necessary for the successful use of this invention. Suitable emulsifiers include polyglycol ethers, sulfonated paraffin hydrocarbons, higher alkyl sulfates such as lauryl sulfate, alkali metal salts of fatty acids such as sodium stearate and sodium oleate, sulfuric acid esters of fatty alcohols, ethoxylated C₄₋₁₂ alkyl phenols and their sulfonation products and also sulfosuccinic acid esters in amounts usually of from 0.1 to 5.0% by weight based on the total weight of the monomers. Although this invention cannot be considered to be strictly limited to "emulsion polymerization" since organo-soluble initiators can be used, by definition emulsion polymerization requires the presence of a water-soluble initiator, and the polymerization mechanisms by which polymerization takes place are not fully understood, nevertheless the process of polymerization of this invention can be carried out using any of the known methods of emulsion polymerization including batch, semi-batch, continuous, thermal, or redox processes with batch, semi-batch or continuous addition of any of the aforementioned ingredients in any combination so long as the constraints mentioned above on the ingredients, in particular the initiator, are maintained. It is to be noted that latex paints formulated with the dispersions of this invention exhibit good abrasion resistance. The invention is evaluated and illustrated in the following examples indicated either as examples of the invention or comparison examples, in which all parts and percentages, (like all parts and percentages used in this specification) are by weight, and weight percentages are based on the total latex weight, unless otherwise indicated. Example 1 (Comparison) This comparative example illustrates polymerization in the presence of HEC as non-surface active protective colloid and a water-soluble azo initiator (without a water-soluble peroxide). A reaction vessel equipped with a stirrer, reflux condenser, thermometer, two feed ports, a nitrogen inlet and a water jacket was charged with 48.65 parts distilled water, 1.15 part nonylphenol ethoxylate mixed phosphate ester containing 9 moles of ethylene oxide surfactant, 0.2 parts hydroxvethyl cellulose having a Brookfield viscosity of 93 mPas (cps) in a 5% aqueous solution at 25°C and a hydroxyethyl molar substitution of 2.5, under a nitrogen blanket. Stirring was continuous throughout the reaction. The temperature of the reactor was raised to 85°C, via circulating hot water, at which time 6.0 parts of a 1% solution of 4,4'azobis(4-cyano-valeric acid) dipotassium salt and 44.0 parts of a monomer mixture comprising, by weight, 42.7% butyl acrylate, 56.8% methyl methacrylate, and 0.5% methacrylic acid were added separately, but concurrently, in a slow but continuous fashion over a 3-hour period. The reaction temperature was maintained at 85°C for 1 hour after the addition of the two continuous feeds at which time the reaction mixture was cooled to room temperature, filtered through cheese cloth, and characterized. Example 2 (Comparison) This comparative example illustrates polymerization in the presence of an organo-soluble azo initiator (without a water-soluble peroxide). The procedure and formulation of Example 1 were used except that the initiator was 0.05 part of azobis isobutyronitrile which was dissolved in the monomer feed, and the mixed phosphate ester surfactant was replaced by 2.3 parts Aerosol A-102 surfactant (Aerosol is a trademark of American Cyanamid) for ethoxylated alcohol half ester of disodium salt of sulfosuccinic acid and 0.7 part octylphenol ethoxylate surfactant containing 10 moles of ethylene oxide. Example 3 (Comparison) This comparative example illustrates polymerization in the presence of an organo-soluble peroxide initiator (without a water-soluble peroxide). The procedure and formulation of example 2 were used except that azobis isobutyronitrile was replaced by 0.1 part benzoyl peroxide. Example 4 (Comparison) This comparative example illustrates polymerization with a styrene comonomer and a water-soluble azo initiator. The procedure and formulation of example 1 were used except that the acrylic monomer mixture was replaced with a 50/50/1 (parts by weight) mixture of styrene/methacrylate/methacrylic acid at the same concentration. Example 5 (Invention) This example illustrates polymerization, per this invention, in the presence of a combination of a water-soluble azo initiator and potassium persulfate. The procedure and formulation of example 1 were used except that 0.035 part potassium persulfate was mixed with the 6.0 parts of aqueous solution containing 1% 4,4'azobis(4-cyanovaleric acid) dipotassium salt. Example 6 (Invention) This example illustrates polymerization, per this invention, in the presence of a combination of organo-soluble azo initiator and potassium persulfate. The procedure and formulation of example 5 were used except that 4-4'-azobis cyanovaleric acid) dipotassium salt was replaced by 0.05 parts azobis isobutyronitrile, which was dissolved in the monomer before addition. Example 7 (Invention) This example illustrates polymerization, per this invention, in the presence of a combination of an organo-soluble peroxide initiator and potassium persulfate. The procedure and formulation of example 6 were used except that azobis isobutyronitrile was replaced by 0.1 part benzoyl peroxide. Example 8 (Comparison) This is a comparative example which illustrates use of an organo-soluble azo initiator in vinyl acetate copolymer latex manufacture. The procedure and formulation of example 2 were used except that the acrylic monomer mixture was replaced by a mixture of 85 parts vinyl acetate and 15 parts butyl acrylate, and 1.15 parts of the surfactant of example 1 was used in place of the 2.3 parts Aerosol A-102 and 0.7 part octylphenol ethoxylate-10. Example 9 (Comparison) This is a comparative example illustrating emulsion polymerization in the presence of a water-soluble peroxide initiator alone. The procedure and formulation of example 1 were used except that the azobis cyanovaleric acid dipotassium salt was replaced by 6.0 parts of 2% potassium persulfate. Example 10 (Comparison) This is a comparative example showing polymerization without a protective colloid. The procedure and formulation of example 2 were carried out, except that hydroxyethyl cellulose was omitted completely. Example 11 (Comparison) This is a comparative example showing polymerization without a protective colloid. The procedure and formulation of example 1 were used except that hydroxyethyl cellulose was omitted completely. The resulting polymer dispersions were evaluated and the results are shown in the following Table. Latex Data Example pH Solids wt. % Viscosity mPas (cps) Particle Size µm (Microns) Particle Size After Mechanical Shear µm (Microns) 1(Comparison) 5.1 48 6,400 0.10 0.28 2(Comparison) 5.1 46 265 0.10 0.32 3(Comparison) 4.2 42 370 0.15 0.35 4(Comparison) 5.1 48 385 0.12 0.22 5(Invention) 4.7 48 120 0.16 0.20 6(Invention) 4.7 48 135 0.17 0.21 7(Invention) 4.6 46 156 0.18 0.20 8(Comparison) 3.1 42 6,800 0.10 0.55 9(Comparison) 2.8 32 >20,000 >4.0 - 10(Comparison) 5.1 46 122 0.10 0.33 11(Comparison) 5.3 48 11,200 0.10 0.30 All of the samples, except the comparative sample produced in example 9, had particle sizes of 200 nm or less which is preferable for use in many applications. In fact, it is somewhat surprising that emulsions with such small particle sizes resulted since polyacrylate polymers produced by emulsion polymerization techniques in the presence of protective colloids typically have particle sizes greater than 1000 nm. Particles sizes greater than 1000 nm are undesirable and, therefore, the dispersion produced in example 9, using a water-soluble peroxide initiator was not suitable for most purposes, in particular, for use in latex paint formulations. All of the dispersions had suitable viscosity for use in latex paints. All of the dispersions were examined using transmission electron microscopy to determine whether agglomeration occurred. All of the samples, except that of comparative example 9, were found to have no agglomeration. The sample produced in example 9 agglomerated. Mechanical stability was evaluated by comparing particle size before and after application of shear. The above data show that latices polymerized per this invention have superior mechanical stability over all of the comparison samples, as illustrated by their relatively small increase in average particle diameter as compared to that of the comparative samples. The rheological properties of the samples were also compared, by observing their properties when hand mixed. The dispersions of this invention exhibited the improved rheological properties (i.e., less pseudoplasticity) generally attributed to use of protective colloids. Aqueous dispersions of this invention are protective colloid containing polyacrylate latex polymer dispersions having improved rheological properties which have less tendency to agglomerate and greater mechanical stability than polyacrylate emulsions prepared by conventional emulsions polymerization techniques in the presence of protective colloids. Polymer latices of this invention have particle sizes less than about 1 µm(micron),in fact, in many instances smaller than 0.2 µm(micron),and are useful in the manufacture of latex paints, binders for nonwoven materials, water-based inks, paper coatings and water-borne adhesives, such as pressure-sensitive adhesives.

Claims: A process for making an aqueous polyacrylate dispersion by the aqueous polymerization of acrylic esters, methacrylic esters or their mixtures in the presence of a protective colloid and a free-radical polymerization initiator; characterized in that the polymerization is carried out in the absence of an emulsifier and the protective colloid is a non-surface active protective colloid from the group of hydroxyethylcellulose, carboxymethylhydroxyethylcellulose, carboxymethylcellulose, ethoxylated starch derivative, water-soluble starch, water-soluble alginate and gum arabic and prior to the start of polymerization the aqueous dispersion comprises 0.05 to 5% non-surface active protective colloid, 0.01 to 1% of an azo initiator or organo-soluble, water-insoluble peroxide initiator, and 0.01% to 0.05% water-soluble peroxide initiator where all weight percentages are based on the total weight of monomer, and where the resultant polyacrylate latex has a particle size less than 200nm. The process of claim 1, further characterized in that the esters polymerized are one or more alkyl acrylates or methacrylates in which the alkyl group contains less than 22 carbon atoms and that at least 50% by weight of the alkyl acrylates or methacrylates contain less than 10 carbon atoms. The process of claim 1, further characterized in that the ester or esters are copolymerized with a monomer or monomers capable of addition polymerization in an amount of from 0.5% to 25% by weight based on the total weight of monomer. A process as claimed in any of the proceeding claims, further characterized in that the initiator includes from 0.01 to 1% azobis isobutyronitrile, azobis cyanovaleric acid dipotassium salt, azobis isobutyramidine dihydrochloride, benzoyl peroxide or lauryl peroxide. A process as claimed in any of the proceeding claims, further characterized in that the initiator includes from 0.01 to 1% of hydrogen peroxide or a persulfate salt. A process as claimed in any of the proceeding claims, further characterized in that the initiator includes from 0.025 to 0.03% by weight of the total monomer content of a water-soluble peroxide initiator.

Inventors: CRAIG DANIEL HORACE, CRAIG, DANIEL HORACE
Applicants: HERCULES INC, HERCULES INCORPORATED
Application year: 1988
Main classification: C08F220
IPC codes: C08F4, C08F2, C08F20