Process for manufacture of an acrylic polymer powder

Abstract: A method for preparing an acrylic polymer powder which is redispersible in water by atomizing an aqueous polymer dispersion containing a dispersing aid, the improvement which comprises employing an acrylic polymer containing 1-15 wt% olefinically unsaturated carboxylic acid comonomer and an effective amount of a dispersing aid consisting essentially of polyvinyl alcohol.

Description: The present invention relates to a process wherein polymer emulsions are spray dried into redispersible powders which are useful as additives in cementitious compositions. BACKGROUND OF THE INVENTION Polymer modified hydraulic cement compositions are widely used in industry. There has been extensive investigations of polymer modified hydraulic cement which shows that such polymer modifications of cementitious compositions improve strength, adhesion, flexibility and workability. Much of the modifier admixtures have been in the form of aqueous dispersions. The disadvantage to using a liquid latex is that it requires the end user to mix the liquid polymer composition with the cement at the job site, thereby requiring a two-component mixture. In addition, the two components have to be packaged separately, a wet container for the latex and a dry container or bag for the cement. The concept of using a redispersible, dry polymer for cement modification is such that the powdered polymer is dry mixed with the cement and aggregate premixture at the job site; all that is required is the addition of water. During the wet mixing with water, the powder polymer becomes re-emulsified, or redispersed, in the modified cement mixture and performs similarly to the liquid latex. Redispersible polymer powders are polymer emulsions which have been converted by spray drying (atomizing the aqueous polymer dispersions in a stream of hot air) to free flowing powders. When blended with water, these powders redisperse back into liquid emulsions with essentially identical properties to the original emulsions. With dispersions whose polymers have glass transition temperatures (Tg) below about 50° C, it is necessary to add dispersing aids to coat the individual spray dried particles to prevent them from sticking together and to promote redispersion of the particles into an emulsion. Sticking can occur in the spray drying apparatus when the powder is at elevated temperatures, or upon long term storage under the influences of heat, humidity and pressure. U.S. 3,409,578 discloses the preparation of a dry cementitious composition modified by an acrylic polymer powder. The acrylic polymer powder is prepared from a latex and contains carboxylate groups in the dispersed polymer, or the dispersing agent for the polymer, or in both. The carboxylate groups may be in the acid form or in the form of a monovalent metal salt or an ammonium salt. U.S. 3,196,122 discloses cementitious compositions prepared using acrylic polymer emulsions. U.S. 3,547,853 discloses cementitious compositions comprising a dry redispersible acrylic polymer, an aliphatic hydroxycarboxylic acid set-retarding agent that also serves as a sequestrant, and trimethylol propane, trimethylol ethane or mixture of the latter two compounds. The acrylic polymer may be any water-insoluble emulsion polymer of one or more (meth)acrylic esters or such a polymer containing about 1 to 10 wt% olefinically unsaturated acid. JP-A-41 85 606 discloses the manufacture of a water-redispersible acrylic resin emulsion polymer powders. According to this document an emulsion polymerisation is carried out in the presence of a protective colloid. One specific example of a protective colloid is polvinylalcohol. By way of the protective colloid the acrylic polymer dispersion is stabilized. Similarly EP-A-0 450 437 discloses use of polyvinylalcohols in copolymerisates of acrylic polymers. These copolymerisates are stated to be obtained by emulsion or solvent polymerization. The products obtained are used as surface finish as they are. No polymer powders are obtained. SUMMARY OF THE INVENTION The present invention provides a process for manufacture of an acrylic polymer powder as set out in the claims. The acrylic polymer powder is readily redispersible in water and is obtained by spray drying an aqueous acrylic polymer dispersion containing a polyvinyl alcohol (PVOH) dispersing aid. The acrylic polymer comprises 1-15 wt%, based on total monomer, of an olefinically unsaturated carboxylic acid comonomer. The redispersible polymer powders obtained according to the process are particularly suited as additives to cementitious compositions, such as gypsum, cement, mortar and magnesium cement. One of the challenges in making an acceptable, redispersible acrylic powder is to incorporate a water-sensitive monomer into the polymer so that it has hydrophilicity and/or alkaline sensitivity. However, the presence of water soluble monomers capable of hydrogen bonding in the cement matrix inhibits the hydration of the cement and the set time may be significantly retarded. It is important to incorporate the minimum level of water-soluble monomer into the emulsion to insure that the set time is not affected. The present invention does not retard the set time of cement. DETAILED DESCRIPTION OF THE INVENTION The polymer dispersions for the preparation of the redispersible polymer powders can be obtained in a conventional manner by aqueous emulsion polymerization of acrylic monomers in the presence of conventional thermal and/or redox polymerization initiators and surfactants as stabilizing agents at elevated temperatures, for example, up to 90° C. The mean particle size can be adjusted by conventional measures, for example, via the type and amount of the surfactants. Thus the surfactants composing the stabilizing system which is used in the polymerization recipe for making the acrylic polymer dispersions include ionic and/or nonionic surfactants which are well known to those skilled in the polymerization art. Protective colloids, such as, for example, PVOH, polyvinyl pyrrolidone and cellulosics, frequently have an advantageous effect and are often used in combination with the surfactants. The concentration range of the total amount of surfactants useful is from 0.5 to 10 wt%, preferably 1 to 6 wt%, based on total emulsion. In the preparation of the polymer dispersions that are spray dried to yield the polymer powders, the polymer content of the dispersion may vary from 40 to 65 wt% and particularly from 45 to 55 wt%. The polymers generally have glass transition temperatures (Tg) of from +50 to -60° C, polymers having a Tg of less than +25° C preferably being used. The acrylic polymers generally contain 85 to 99 wt% esters of acrylic acid or methacrylic acid and 1 to 15 wt%, based on total monomer weight, of an olefinically unsaturated acid comonomer to render the latex redispersible in the aqueous medium upon alkaline treatment. Suitable ester monomers include esters of acrylic acid or methacrylic acid with C 1 -C 18 alcohols, especially C 1 -C 8 alcohols. The polymer must contain acid functional comonomers at 1-15 wt%, preferably 3-10 wt% to give the polymer water sensitivity. Any olefinically unsaturated carboxylic acid may be used, including mono-, di- and polycarboxy, mono- and polyolefinic acids. Illustrative of such acid comonomers are acrylic acid, itaconic acid, citraconic acid, methacrylic acid, crotonic acid and cinnamic acid. It is preferred to use these comonomer acids at 5 to 10 wt%. In addition, the acrylic polymers may contain other comonomers, including, for example, vinyl aromatic monomers, such as styrene; vinyl esters, in particular of acetic and propionic acids, as well as vinyl laurate and vinyl esters of versatic acids; vinyl chloride and/or vinylidene chloride; ethylene and diolefins, in particular butadiene; acrylonitrile and amides, such as acrylamide, methacrylamide, N-methylol-(meth)acrylamide. The amount of such comonomers can be varied within a wide range (preferably up to 20 wt %). The polymer may contain hydrophilic or polar groups in concentrations below that which would render the polymer soluble in water. Prior to spray drying, the aqueous polymer emulsion is neutralized with a base to at least pH 7. The carboxylate groups of the polymer are neutralized with a mono- or polyvalent metal ion. Examples of suitable ions include calcium, magnesium, sodium, potassium, lithium, strontium, barium, aluminum, lead, zirconium, cerium, cobalt, chromium, zinc, iron, manganese, nickel and tin. The ion can be added in the form of a basic salt, as a hydroxide, oxide or carbonate. The ion source should have sufficient solubility in aqueous conditions so crosslinking and/or neutralization of the carboxylate groups with the metal ion can take place. This neutralized latex gives the polymer water-sensitivity that, when the dispersing aid is added, will prevent coalescence of the particles upon drying. The salts may be added stoichiometrically or in excess amounts if needed for redispersibility. Calcium hydroxide or sodium hydroxide, for example, is typically added to the dispersion to adjust the pH to at least 7. In the preparation of the acrylic polymer powders according to the invention, PVOH is added to the dispersion in the form of an aqueous solution. Suitable PVOHs include those having a degree of hydrolysis ranging from 87 to 99 mole% and a degree of polymerization (DPn) ranging from 200 to 5000. An amount of PVOH effective for redispersing the dried acrylic polymer powder in water, such as from 2-25 wt%, preferably 5-15 wt%, based on the polymer content of the aqueous polymer dispersions, is utilized. The PVOH-dispersing aid is added after neutralization step b) prior to the spray drying step. In addition to the PVOH, other dispersing aids may be used, including melamine formaldehydes, lignosulfonate salts, polyvinyl pyrrolidone, poly (vinylformamide), hydroxyethylcellulose and poly(meth)acrylates. Spray drying of the aqueous polymer dispersions which contain PVOH can be carried out in a conventional manner well known in the art, in particular using one-material or multi-material nozzles. In general, the temperature of the warm air stream used for spray drying is from 100-200° C, in particular from 100-170° C. The dry, redispersible acrylic polymer powders can be separated off in a conventional manner, in particular using cyclones or filtered evaporators. Polymer powders having good blocking resistance are obtained in a high yield and can be readily stored at room temperature, without caking. The polymer powders are readily redispersible in water. As modifiers for cementitious mixtures, the polymer powders at 5-30 wt% are mixed with cement. It may be advantageous in some systems to add an inert filler material such as clay, chalk, talc, silica or other fine sized particles, to reduce the tendency for the redispersible powder to block over long-term storage. The useful range of this filler is from 0-40 wt%, based on the redispersible polymer powder, preferably from 8-30 wt%. The amount of filler material used depends on the particular polymer and its Tg. The invention provides a process for manufacture of a dry powder polymer which may be redispersed to form a latex for cementitious applications. The dry powder redisperses back into the same dispersion form with respect to particle size distribution. Since the dry powder can be blended with sand, cement and other dry ingredients and transported directly to the site of usage, the shipping costs associated for the aqueous medium is eliminated. For cementitious applications where dry materials like cement are used, a one-step mixing process carried out by the end user can be advantageous. Example 1 To 86 parts by weight (pbw) solids of an aqueous butyl acrylate/-methyl methacrylate/methacrylic acid (45.5/45.5/7.5 wt ratio) terpolymer latex containing 0.5% sodium lauryl ether sulfate there was added 5 pbw calcium hydroxide in aqueous solution. After neutralization and mixing, 9 pbw of a 20% aqueous solution of polyvinyl alcohol (87-89 mole% hydrolyzed; Mw 31,000-50,000) was added. The polymer mixture was mixed at room temperature and spray dried using conventional techniques at 100° C to form a fine, free-flowing powder of 0.31 g/ml bulk density. These powders were readily dispersible in water. The redispersed latex was stable and performed in cementitious applications as would an acrylic latex. Example 2 A latex premix formulation was prepared using three components: (1) 85 pbw solids of an aqueous butyl acrylate/methyl methacrylate/acrylic acid/methacrylic acid (41/54/2.5/2.5) copolymer latex at 50% solids containing 1% sodium lauryl ether sulfate; (2) 5 pbw of sodium hydroxide added in 50% aqueous solution; and (3) 10 pbw of PVOH (87-89 mole% hydrolyzed; Mw 31,000-50,000). This premix was spray dried at 100° C with introduction of kaolin clay at 5% based on the powder weight to yield a fine, free-flowing polymer powder. The powder redispersed instantaneously at 25° C in water. Example 3 The polymer powder of Example 1 was used in the ASTM C109 cement patching application and compared to Rohm & Haas MC76 acrylic latex, a commercial aqueous acrylic latex. The polymers were evaluated at use level of 15% based on weight of cement with the water level adjusted to obtain equivalent flow. Polymer powder Acrylic latex Type I cement (g) 900 900 C109 sand (9) 2475 2475 superplasticizer 8 --- powder (g) 127 --- latex (g) --- 281 (48%) water (g) 375 190 defoamer (g) 2.5 4.0 compress. strength (psi;MPa) 1 day 1633; 10.9 1632; 10.9 7 day 3619; 24.2 2928; 19.6 tensile strength (psi;MPa) 7 day dry 661; 4.4 630; 4.2 7+1 day wet 347; 2.3 365; 2.4 STATEMENT OF INDUSTRIAL APPLICATION The present invention provides a process for manufacturing spray dried acrylic polymer powders that are useful as additives in cementitious compositions.

Claims: Process for manufacture of an acrylic polymer powder which is redispersible in water, comprising the steps of: a) forming an aqueous dispersion of an acrylic polymer containing 1-15 wt% olefinically unsaturated carboxylic acid comonomer based on total monomer weight by emulsion polymerization of the monomers in presence of a surfactant, the concentration range of the total amount of surfactants being from 0.5 to 10 wt% based on total emulsion weight, whose carboxylic groups are neutralized with a mono- or polyvalent metal ion by b) neutralizing said aqueous dispersion with a base to at least pH 7, and c) spray drying said aqueous dispersion, wherein the dispersion also contains a dispersing aid consisting essentially of polyvinyl alcohol which is added to the dispersion in the form of an aqueous solution, wherein said dispersing aid is added after neutralization step b) prior to the spray drying step, wherein the polyvinyl alcohol is 87-99 mole% hydrolyzed, and the polyvinyl alcohol has a degree of polymerization (DPn) of 200-5000, and the dispersion contains 2-25 wt% polyvinyl alcohol, based on polymer content of the dispersion. The process according to claim 1 in which the dispersion contains 5-15 wt% polyvinyl alcohol, based on polymer content of the dispersion. The process according to claims 1 to 2 wherein the acrylic polymer powder also contains melamine formaldehyde, lignosulfonate salt, polyvinyl pyrrolidone, poly(vinylformamide), hydroxyethylcellulose or poly(meth)acrylate. The process according to claims 1 to 3 wherein the acid comonomer is acrylic acid, methacrylic acid, itaconic acid, crotonic acid, citraconic acid and/or cinnamic acid. The process according to claim 4 in which the polymer contains 3-10 wt% acid comonomer. The process according to claim 5 wherein the acrylic polymer powder comprises 5-15 wt% polyvinyl alcohol.

Inventors: MAO CHUNG-LING, PAK-HARVEY HELEN HE YON, MAO, CHUNG-LING, PAK-HARVEY, HELEN HE YON
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1993
Main classification: C08J3
IPC codes: C08J3, C04B24, C04B40, C08L29, C09D133