EP3784703A1

EP3784703A1 - Method for producing aqueous polymer dispersions in a tube reactor

Info

Publication number: EP3784703A1
Application number: EP19721593.2A
Authority: EP
Inventors: Hans-Peter Weitzel; Robert Braunsperger
Original assignee: Wacker Chemie AG
Current assignee: Wacker Chemie AG
Priority date: 2019-04-30
Filing date: 2019-04-30
Publication date: 2021-03-03
Also published as: US20220041774A1; WO2020221435A1; US11591423B2; CN112513106A

Abstract

The invention relates to a method for producing aqueous polymer dispersions by means of radically initiated emulsion polymerisation of ethylenically unsaturated monomers in the presence of protective colloids and/or emulsifiers in a continuously operated tube reactor, characterised in that the direction of flow of the reactor contents is reversed along the longitudinal axis of the rector at regular time intervals.

Description

Process for the production of aqueous polymer dispersions in one

Tubular reactor

The invention relates to a process for the preparation of aqueous polymer dispersions by means of free-radically initiated emulsion polymerization of ethylenically unsaturated monomers in the presence of protective colloids and / or emulsifiers in a continuously operated tubular reactor, and the use of the process products obtained therewith.

Aqueous polymer dispersions are used as binders in a wide range of applications, for example in adhesives,

Coating applications, as a binder in carpet, textile and paper applications as well as in construction chemical products such as tile adhesives, plasters and sealants. These aqueous dispersions are usually prepared by aqueous emulsion polymerization in stirred batch reactors or also continuous stirred tank cascades, as described in EP 1 323 752 B1. The process efficiency is limited by the dissipation of the released heat via cooling surfaces, for example cooling coils and reactor wall. For this reason, there have been repeated investigations into transferring the process from stirred kettles to tubular reactors without internals in order to benefit from the greater surface to volume ratio and correspondingly better cooling. Unfortunately, this shows that with a correspondingly narrow tube cross-section, these tube reactors quickly become clogged by deposits, known as fouling (polymer fouling), and the system operating time is drastically reduced.

As a solution, tubular reactors with built-in stirrers were used. But even here polymer fouling cannot be avoided and limits the availability of the system. This polymer fouling also occurs in traditional stirred tanks, but the impairment of the system availability is not as serious as with tubular reactors due to the smaller cooling surface and changed reactor geometry. In stirred kettles, one helps to slow down the build-up of deposits by coating the surfaces with antifouling agents, as described in EP 3 256 497 B1. In However, this effect is not sufficient for tubular reactors. In DE-AS 1137216, a tubular reactor is described with a stirrer or scraper that goes through the wall. With this procedure, the wall covering is not avoided, but mechanically removed by a subsequent measure. Problems must therefore be expected on a production scale, due to the narrow distances between the stirrer or scraper and the wall, mechanical stress and corresponding blockages due to worn wall covering. In EP 0 029 701 B1, the wall coverings which form in a tubular reactor through which liquid flows are removed from the reactor again by pulsating flow.

The object was to provide an improved process for the production of aqueous polymer dispersions that at the same time ensures high plant availability with high space-time performance (process efficiency).

The invention relates to a process for the preparation of aqueous polymer dispersions by means of free-radically initiated emulsion polymerization of ethylenically unsaturated monomers in the presence of protective colloids and / or emulsifiers in a continuously operated tubular reactor, characterized in that the flow direction of the reactor contents (polymerization mixture) along the longitudinal axis of the reactor at regular time intervals is reversed.

The process according to the invention can in principle be applied to all reactors with a pipe-like geometry. This includes, for example, stirred tubular reactors, unstirred tubular reactors, tubular reactors with internals such as static mixing elements, and also Taylor reactors with a cylindrical stirrer. Stirred tubular reactors are preferred.

The tubular reactor preferably has a cylindrical geometry. The tubular reactor is largely characterized by the ratio of length to diameter. The ratio of length to diameter is preferably from 8: 1 to 40: 1, particularly preferably from 10: 1 to 25: 1. With smaller ratios one approaches too closely the Geometry of traditional stirred kettles, with strong back-mixing of the medium to be reacted. With larger values, the reactor becomes very long, which limits the practical implementation, since the tubular reactor is preferably vertical and is limited by the building dimensions.

The tubular reactor can - viewed in the longitudinal direction or in the flow direction - be stored vertically, horizontally or in a position between these two directions. The longitudinal direction of the tubular reactor is generally the distance from the reactor bottom to the reactor cover. Vertical storage is preferred. If the tubular reactor is not mounted horizontally, the reaction medium can flow through it with gravity from top to bottom or, preferably, against gravity from bottom to top.

A stirred tubular reactor can use any stirrer technology, preferably single-dimensioned paddle stirrers are used as stirring elements. The speed of the stirrer depends on the overall dimensions of the reactor and is between 200 and 2000 revolutions per minute, preferably between 500 and 1500 revolutions per minute. One revolution stands for one revolution of the stirrer around its longitudinal axis or around the axis that is parallel to the direction of flow. The stirrer can be driven in a conventional manner, for example via a mechanical gear, and sealed via a magnetic coupling or a mechanical seal.

The mean residence time of the polymerization mixture in the tubular reactor is generally from 10 minutes to 5 hours, preferably from 15 minutes to 2 hours, particularly preferably from 20 minutes to 1 hour and most preferably from 25 minutes to 45 minutes. The mean residence time can be set, for example, via the metering speed or the dimensions of the tubular reactor.

Preferably, the direction of flow of the tubular reactor contents is reversed at a time interval (time interval) of 60 minutes to 48 hours, particularly preferably the direction of flow is reversed at a time interval (time interval) of 6 hours to 12 hours. A preferred embodiment of a tubular reactor for carrying out the process according to the invention is shown in FIG. 1 as a tubular reactor (1). The embodiment in FIG. 1 is purely illustrative for explaining the method and in no way restrictive for the present invention.

The tubular reactor (1) in which the polymerization takes place is constructed from a steel tube, preferably a jacketed tube (2), which has an axially arranged agitator shaft (3) inside which is equipped with several agitator elements (4). The tubular reactor (1) is equipped with a metering line (5) which is connected to an upstream mixing unit (6) for mixing the starting materials.

The tubular reactor (1) is connected via a withdrawal line (7) to a downstream container (8) in which the polymerization product is collected and, if necessary, after-treated.

The starting materials can be introduced into the tubular reactor (1), preferably continuously, via the metering line (5). The polymerization product can be discharged from the tubular reactor (1), preferably continuously, through the withdrawal line (7). Via one or more additional metering devices (9a) to (9e), further substances, preferably initiators, can be fed to the tubular reactor along the tubular reactor.

To reverse the direction of flow according to the invention, in the embodiment according to FIG. 1, for example, the procedure can be such that the educt mixture (pre-emulsion) from the mixing unit (6) via the branch (5a) of the metering line (5) and via the open valve (10) to the tubular reactor (1) is supplied. The polymerisation product is fed to the container (8) via the opened valve (11) and the section (7a) of the removal line (7). The valve (12) and the valve (13) are closed in this phase of operation. To reverse the direction of flow, valves (10) and (11) are closed, and then valve (12) and valve (13) are opened. In the following phase of operation, the educt mixture is discharged from the mixing unit (6) via the branch (5b) of the metering line (5) via the open Valve (12) fed to the tubular reactor (1), and the polymerisation product removed via the now open valve (13) and fed to the container (8) via section (7b) of the removal line (7). To reverse the direction of flow again, valves (12) and (13) are closed and valves (10) and (11) are opened again.

The polymerization takes place according to the emulsion polymerization process in an aqueous medium, preferably no organic solvents are used. The polymerization temperature of the polymerization mixture in the tubular reactor is preferably between 40.degree. C. and 120.degree. C. and particularly preferably between 50.degree. C. and 110.degree. The pressure in the tube reactor depends on whether the monomers to be polymerized are liquid or gaseous at the respective polymerization temperature and is preferably 1 to 110 bar _abs. In the copolymerization of gaseous comonomers such as ethylene, 1,3-butadiene or vinyl chloride, under Pressure, and particularly preferably at 10 to 80 bar _abs . polymerized.

The constituents of the reaction mixture (starting materials) can be mixed beforehand in a mixing unit and fed continuously to the tubular reactor. The constituents of the reaction mixture are preferably mixed continuously in a mixing unit to form a pre-emulsion and this is transported into the tubular reactor. In the case of thermal initiation, the procedure is preferably such that no oxidation catalyst is added to the pre-emulsion. In the case of initiation with a redox initiator combination, the procedure is preferably such that the reduction initiator is added to the pre-emulsion and the oxidation initiator is preferably added to the tubular reactor. The transport takes place by means of pumps or via the pure mass flow when the mixing unit is completely filled.

The mixing unit can be, for example, a stirred tank or a static mixing section. The mixing unit can be provided with a double jacket in order to cool or heat, if necessary, during mixing.

The starting materials can be tempered before being introduced into the tubular reactor. For example, one or more When introduced into the tubular reactor, the starting materials must be kept at a temperature between 10 ° C. and the polymerization temperature. A mixture (pre-emulsion) comprising one or more ethylenically unsaturated monomers, one or more protective colloids and / or one or more emulsifiers, and optionally one or more initiators, in particular reduction initiators, before or when being introduced into the tubular reactor at a temperature just below the Polymerization temperature or tempered to polymerization temperature. The aforementioned mixtures are preferably heated to a temperature between the polymerization temperature and 20 ° C. below the polymerization temperature, in particular to 10 ° C. below the polymerization temperature. Initiators, in particular oxidation initiators, are particularly preferably added to a mixture heated in this way directly before entry into the tubular reactor or metered directly into the tubular reactor. The temperature control can take place before, during or after their thorough mixing. Common heat exchangers can be used for this purpose.

The tube reactor can be tempered with common cooling and / or heating devices, such as jacket coolers or jacket heaters. Cooling and / or heating devices can, for example, be attached to the tubular reactor on the wall or to built-in cooling coils. For example, the outer reactor wall can be provided with a cooling or heating jacket (double-jacket tube), the intermediate space of which is traversed by a temperature control liquid. A tubular reactor with a jacketed tube is preferably used.

Before the start of the polymerization, the tubular reactor is preferably filled with a polymer dispersion which preferably corresponds to the end product of the polymerization in terms of polymer composition, type and amount of protective colloid, and particle size and solids content. Alternatively, before the start of the process according to the invention, that is to say before the start of the polymerization, the tubular reactor can be filled with a mixture which comprises the starting materials but no initiators, in particular no oxidation initiators. Finally can the tubular reactor must be filled with water, preferably exclusively with water, before the start of the process according to the invention.

The tubular reactor is generally operated continuously. In continuous operation, the starting materials, in particular ethylenically unsaturated monomers, protective colloids and / or emulsifiers and / or initiators, are introduced into the tubular reactor during the emulsion polymerization and the polymerization product is removed from the tubular reactor. In the case of continuous operation, the entering mass flows should correspond to the exiting mass flows.

The polymerization is generally carried out up to a conversion of at least 85% by weight, preferably up to a conversion of 90 to 99% by weight, of the monomers which are liquid under polymerization conditions. The polymerization product is then transferred to a collecting tank (expansion tank). The transport takes place by means of pumps or due to the pressure difference between the tubular reactor and the collecting tank. Post-polymerization can optionally be carried out in the collecting tank using known methods, for example by post-polymerization initiated with a redox catalyst. The volatile residual monomer content is then optionally removed by passing over or preferably passing inert entrainment gases such as air, nitrogen or preferably water vapor over / through the aqueous polymerization mixture in a manner known to the person skilled in the art (stripping). After its aftertreatment, the polymerization product is removed from the collecting tank and stored in a silo, for example.

The ethylenically unsaturated monomers are preferably selected from the group comprising vinyl esters, (meth) acrylic acid esters, vinyl aromatics, olefins, 1,3-dienes and vinyl halides and optionally other monomers copolymerizable therewith.

Suitable vinyl esters are those of carboxylic acids having 1 to 18 carbon atoms. Vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methyl vinyl acetate, vinyl pivalate and vinyl esters of alpha-branched monocarboxylic acids with 9 to 11 carbon atoms are preferred Atoms, for example VeoVa9 ^® or VeoVa10 ^® (trade names of Hexion). Vinyl acetate is particularly preferred.

Suitable monomers from the group of acrylic acid esters or methacrylic acid esters are, for example, esters of unbranched or branched alcohols having 1 to 15 carbon atoms. Preferred methacrylic acid esters or acrylic acid esters are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate, t-butyl methacrylate, 2-ethylhexyl acrylate. Methyl acrylate, methyl methacrylate, n-butyl acrylate, t-butyl acrylate and 2-ethylhexyl acrylate are particularly preferred.

Preferred vinyl aromatics are styrene, methyl styrene and vinyl toluene. The preferred vinyl halide is vinyl chloride. The preferred olefins are ethylene, propylene and the preferred dienes are 1,3-butadiene and isoprene.

If necessary, 0 to 10% by weight, based on the total weight of the monomer mixture, of auxiliary monomers can also be copolymerized. It is preferred to use from 0.1 to 5% by weight of auxiliary monomers. Examples of auxiliary monomers are ethylenically unsaturated mono- and dicarboxylic acids, preferably acrylic acid, methacrylic acid, fumaric acid and maleic acid; ethylenically unsaturated carboxamides and nitriles, preferably acrylamide and acrylonitrile; Mono- and diesters of fumaric acid and maleic acid such as the diethyl and diisopropyl esters and maleic anhydride; ethylenically unsaturated sulfonic acids or their salts, preferably vinyl sulfonic acid, 2-acrylamido-2-methyl-propane sulfonic acid. Further examples are pre-crosslinking comonomers such as poly-ethylenically unsaturated comonomers, for example dialyl phthalate, divinyl adipate, diallyl maleate, allyl methacrylate or triallyl cyanurate, or post-crosslinking comonomers, for example acrylamidoglycolic acid (AGA), methyl acrylamido-methyl ester (AGA), methyl acrylamidoglycolate N-methylol methacrylamide, N-methylol allyl carbamate, alkyl ethers such as isobutoxy ether or esters of N-methylol acrylamide, N-methylol methacrylamide and N-methylol allyl carbamate. Epoxy-functional comonomers such as glycidyl methacrylate and glycidyl acrylate are also suitable. Other examples are silicon-functional comonomers such as acryloxypropyltri (alkoxy) and methacryloxypropyltri (alkoxy) silanes, vinyltrialkoxysilanes and vinylmethyldialkoxysilanes, it being possible for example to contain ethoxy and ethoxypropylene glycol ether radicals as alkoxy groups. Monomers with hydroxy or CO groups may also be mentioned, for example methacrylic acid and acrylic acid hydroxyalkyl esters such as hydroxyethyl, hydroxypropyl or hydroxybutyl acrylate or methacrylate, and compounds such as diacetone acrylamide and acetylacetoxyethyl acrylate. One or more monomers are preferably selected from the group comprising vinyl esters; Vinyl ester mixtures containing several monomers from the group comprising vinyl esters, olefins, vinyl aromatics, vinyl halides, acrylic acid esters, methacrylic acid esters, fumaric and / or maleic acid mono- or diesters; (Meth) acrylic acid esters; (Meth) acrylic acid ester mixtures containing one or more monomers from the group comprising methacrylic acid esters, acrylic acid esters, olefins, vinyl aromatics, vinyl halides, fumaric and / or maleic acid mono- or diesters; Monomers or monomer mixtures of dienes such as butadiene or isoprene, and of olefins such as ethene or propene, it being possible for the dienes to be polymerized, for example, with styrene, (meth) acrylic esters or the esters of fumaric or maleic acid; Monomers or monomer mixtures of vinyl aromatics, such as styrene, methyl styrene, vinyl toluene; Monomers or monomer mixtures composed of vinyl halogen compounds such as vinyl chloride, it being possible for the monomer mixtures to include auxiliary monomers.

Monomer mixtures of vinyl acetate with 1 to 50% by weight of ethylene are particularly preferred; Monomer mixtures of vinyl acetate with 1 to 50% by weight of ethylene and 1 to 50% by weight of one or more further comonomers from the group of vinyl esters with 3 to 12 carbon atoms in the carboxylic acid radical, such as vinyl propionate, vinyl laurate, vinyl esters of alpha-branched ones Carboxylic acids with 9 to 11 carbon atoms such as VeoVa9 ^® , VeoValO ^® ; Monomer mixtures of one or more vinyl esters, 1 to 50% by weight of ethylene and preferably 1 to 60% by weight of (meth) acrylic acid esters of unbranched or branched alcohols having 1 to 15 carbon atoms, in particular n-butyl acrylate. or 2 ethylhexyl acrylate; Monomer mixtures with 30 to 75 wt .-% vinyl acetate, 1 to 30 wt .-% vinyl laurate or Vinyl esters of an alpha-branched carboxylic acid with 9 to 11 carbon atoms, and 1 to 30% by weight (meth) acrylic acid esters of unbranched or branched alcohols with 1 to 15 carbon atoms, in particular n-butyl acrylate or 2-ethylhexyl acrylate, which still contain 1 to 40% by weight of ethylene; Monomer mixtures with one or more vinyl esters, 1 to 50% by weight of ethylene and 1 to 60% by weight of vinyl chloride; wherein the monomer mixtures mentioned can each also contain the auxiliary monomers mentioned in the amounts mentioned, and the data in% by weight add up to 100% by weight in each case.

(Meth) acrylic acid ester monomer mixtures are particularly preferred, such as monomer mixtures of n-butyl acrylate or 2-ethylhexyl acrylate or copolymers of methyl methacrylate with n-butyl acrylate and / or 2-ethylhexyl acrylate; Styrene-acrylic acid ester monomer mixtures with one or more monomers from the group consisting of methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate; Vinyl acetate / acrylic acid ester monomer mixtures with one or more monomers from the group consisting of methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate and, if appropriate, ethylene; Styrene-1,3-butadiene monomer mixtures; where the monomer mixtures mentioned can also contain auxiliary monomers in the amounts mentioned, and the data in% by weight are based on 100% by weight in each case. - add up%.

Examples of particularly preferred comonomers for vinyl chloride monomer mixtures are α-olefins such as ethylene and propylene, vinyl esters such as vinyl acetate, acrylic acid esters and methacrylic acid esters of alcohols with 1 to 15 carbon atoms such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate , t-butyl acrylate, n-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl acrylate, fumaric and maleic acid mono- or diesters such as the dimethyl and diethyl esters of maleic acid and fumaric acid.

Most preferred are monomer mixtures with vinyl acetate and 5 to 50% by weight of ethylene; Monomer mixtures with vinyl acetate and 1 to 50% by weight of ethylene and 1 to 50% by weight of a vinyl ester of -branched monocarboxylic acids having 9 to 11 carbon atoms; Monomer mixtures with 30 to 75% by weight of vinyl acetate, 1 to 30% by weight of vinyl laurate or a vinyl ester an α-branched carboxylic acid having 9 to 11 carbon atoms, and 1 to 30% by weight (meth) acrylic acid esters of unbranched or branched alcohols having 1 to 15 carbon atoms, which optionally also contain 1 to 40% by weight ethylene ; Monomer mixtures with vinyl acetate, 5 to 50 wt .-% ethylene and 1 to 60 wt .-% vinyl chloride; as well as monomer mixtures containing 60 to 98% by weight of vinyl chloride and 1 to 40% by weight of ethylene, the monomer mixtures each also being able to contain auxiliary monomers in the stated amounts, and the data in% by weight are based on 100% by weight in each case add up.

The selection of monomers and the selection of the proportions by weight of the comonomers are carried out in such a way that, in general, a glass transition temperature Tg of -50 ° C to + 50 ° C, preferably -20 ° C to + 30 ° C, results. The glass transition temperature Tg of the polymers can be determined in a known manner by means of differential scanning colorimetry (DSC). The Tg can also be approximately calculated in advance using the Fox equation. According to Fox T. G. / Bull. Am. Physics Soc. 1, 3, page 123 (1956), the following applies: 1 / Tg = xl / Tgl + x2 / Tg2 + ... + xn / Tgn, where xn stands for the mass fraction (% by weight / 100) of the monomer n and Tgn is the glass transition temperature in Kelvin of the homopolymer of the monomer n. Tg values for homopolymers are given in Polymer Handbook 2nd Edition, JU. Wiley & Sons, New York (1975).

The polymerization is initiated with the initiators customary for emulsion polymerization, in particular redox initiator combinations of oxidation initiator and reduction initiator. Examples of suitable oxidation initiators are the sodium, potassium and ammonium salts of peroxodisulfuric acid, hydrogen peroxide and azobisisobutyronitrile. The sodium, potassium and ammonium salts of peroxydisulfuric acid and hydrogen peroxide are preferred. The initiators mentioned are generally used in an amount of from 0.01 to 2.0% by weight, based on the total weight of the monomers.

Suitable reducing agents (reduction initiators) are the sulfites and bisulfites of alkali metals and of ammonium, for example sodium sulfite; the derivatives of sulfoxylic acid such as zinc or Alkali formaldehyde sulfoxylates, for example sodium hydroxymethanesulfinate (Bruggolite) and ascorbic acid, isoascorbic acid or its salts, or formaldehyde-free reducing agents such as 2-hydroxy-2-sulfinato-acetic acid disodium salt (Bruggolite FF6). The amount of reducing agent is preferably 0.015 to 3% by weight, based on the total weight of the monomers.

To control the molecular weight, regulating substances can be used during the polymerization. If regulators are used, they are usually used in amounts between 0.01 and 5.0% by weight, based on the monomers to be polymerized. Examples of such substances are n-dodecyl mercaptan, tert. -Dodecylmer- captan, mercaptopropionic acid, ethyl mercaptopropionate, isopropanol and acetaldehyde. Preferably no regulating substances are used.

Polymerization is preferably carried out in the presence of protective colloids. Suitable protective colloids are partially saponified polyvinyl alcohols; Polyvinylpyrrolidones; Polyvinyl acetals; Polysaccharides in water-soluble form such as starches (amylose and amylopectin), celluloses and their carboxymethyl, methyl, hydroxyethyl, and hydroxypropyl derivatives; Proteins such as casein or caseinate, soy protein, gelatin, lignosulfonates; synthetic polymers such as poly (meth) acrylic acid, copolymers of (meth) acrylates with carboxyl-functional comonomer units, poly (meth) acrylamide, polyvinylsulfonic acids and their water-soluble copolymers; Melamine formaldehyde sulfonates, naphthalene formaldehyde sulfonates, styrene maleic acid and vinyl ether maleic acid copolymers; cationic polymers such as polydiallyldimethylammonium chloride (poly-DADMAC).

Preferred protective colloids are partially saponified or fully saponified polyvinyl alcohols. Partially saponified polyvinyl alcohols with a degree of hydrolysis of 80 to 95 mol% and a Höppler viscosity, in 4% aqueous solution, of 1 to 30 mPas (Höppler method at 20 ° C., DIN 53015) are particularly preferred. Partially saponified, hydrophobically modified polyvinyl alcohols with a degree of hydrolysis of 80 to 95 mol% and a Höppler viscosity of 4% are also particularly preferred iger aqueous solution, from 1 to 30 mPas. Examples of these are partially saponified copolymers of vinyl acetate with hydrophobic comonomers such as isopropenyl acetate, vinyl pivalate, vinyl ethylhexanoate, vinyl esters of saturated alpha-branched monocarboxylic acids with 5 or 9 to 11 carbon atoms, dialkyl maleate and dialkyl fumarates such as diisopropyl, vinyl fumarate ethers such as vinyl fumarate ethers, such as vinyl fumarate and diisopropyl chloride Vinyl butyl ether, olefins such as ethene and decene. The proportion of the hydrophobic units is preferably 0.1 to 10% by weight, based on the total weight of the partially saponified polyvinyl alcohol. Mixtures of the polyvinyl alcohols mentioned can also be used.

Most preferred are polyvinyl alcohols with a degree of hydrolysis of 85 to 94 mol% and a Höppler viscosity, in 4% aqueous solution, of 3 to 15 mPas (Höppler method at 20 ° C., DIN 53015). The protective colloids mentioned are accessible or commercially available by means of processes known to the person skilled in the art.

The protective colloids are generally added in the course of the polymerization in a total amount of 1 to 20% by weight, based on the total weight of the monomers.

If appropriate, emulsifiers, for example anionic and / or nonionic emulsifiers, can be used in the polymerization, for example 0.1 to 2.0 wt. -%, based on the total weight of the comonomers. Examples of anionic emulsifiers are alkyl sulfates with a chain length of 8 to 18 carbon atoms, alkyl or alkylaryl ether sulfates with 8 to 18 carbon atoms in the hydrophobic radical and up to 40 ethylene oxide or propylene oxide units, alkyl or alkylaryl sulfonates with 8 to 18 C atoms, esters and half esters of sulfosuccinic acid with monohydric alcohols. Examples of nonionic emulsifiers are C12-C14 fatty alcohol ethoxylates with a degree of ethoxylation of 2 to 20 ethylene oxide units.

The aqueous dispersions obtainable with the process according to the invention have a solids content of 30 to 75 wt. -%, preferably from 50 to 60 wt. -%. The aqueous dispersions can be used to produce polymer powders that are redispersible in water. For this purpose, the aqueous dispersions, optionally after the addition of protective colloids as an atomizing aid, are dried by means of fluidized bed drying, freeze drying or, preferably, spray drying.

The aqueous polymer dispersions and the water-redispersible polymer powders can be used in the areas of application typical for them. For example in construction chemical products, possibly in connection with hydraulically setting binders such as cements, gypsum and water glass, for the production of construction adhesives, in particular tile adhesives and thermal insulation adhesives, plasters, leveling compounds, floor leveling compounds, leveling compounds, sealing slurries, grout and paints. Also as binders for coating agents and adhesives or as coating agents or binders for textiles and paper.

The following examples serve to further explain the invention:

General description of the experiment:

The polymerization was carried out in a tubular reactor (1) with a length of 1600 mm and an internal diameter of 100 mm. The reactor volume was approx. 12.5 liters. The reaction mixture was mixed transversely to the longitudinal axis by a stirrer (3) with 8 stirrer blades (4) measuring 50 mm x 50 mm, the distance between the stirrer blades and the reactor wall was 25 mm, thus avoiding contact with the reactor wall. Along the axis of the reactor there were 5 further addition options (9a) to (9e) for initiator.

The tubular reactor (1) was continuously supplied with the mixture of substances to be polymerized from an upstream pressure vessel (6) with a volume of 16 liters. The upstream pressure vessel (6) was continuously filled with the relevant substances via pumps.

After emerging from the tubular reactor (1), the product was transferred via a pressure-maintaining valve (14) or (15) to a pressureless container (8) with a volume of 1000 liters and collected. At the end of the experiment, the product mixture was aftertreated and bottled.

Composition to be polymerized:

The following substances were continuously fed into an upstream pressure vessel (stirred tank) (6) and premixed:

4.4 kg / h water, 4.0 kg / h of a 20% strength by weight aqueous solution of a partially saponified polyvinyl alcohol with a degree of hydrolysis of 88 mol% and a Höppler viscosity of 4 mPas (determined according to DIN 53015 at 20 ° C. in 4 wt.% Aqueous solution), 10.4 kg / h vinyl acetate, 1.15 kg / h ethylene, 195 g / h 5 wt.% Aqueous ascorbic acid solution, 1.5 g / h formic acid and 4 g / h h 1% by weight aqueous iron ammonium sulfate solution.

This mixture was transferred to the tubular reactor (1) at a rate of 20 kg / h. The initiator potassium persulfate was metered in as a 3% strength by weight aqueous solution at the metering points (9a) to (9e).

The finished product left the tubular reactor (1) with a conversion of approx. 92% and was collected in a pressureless container (8) in vacuo.

The dispersion was then used to remove excess. Ethylene is transferred to a further pressureless container, in which a pressure of 0.7 bar was applied, and there by adding 0.4 kg of a 10% by weight aqueous t-butyl hydroperoxide solution and 0.8 kg of a 5% by weight igen aqueous ascorbic acid solution based on 100 kg dispersion post-polymerized to a value of <1000 ppm residual vinyl acetate. The pH was adjusted to 4.5 by adding sodium hydroxide solution (10% strength by weight aqueous solution). Finally, the batch was filled from the pressureless container through a 250 mm sieve. Comparative example 1 (comparative experiment):

The mixture to be polymerized was introduced at the lower end of the tubular reactor (1) and the product removed at the upper end. The valves (10) and (11) were open, the valves (12) and (13) closed. The flow rate was approx. 20 liters / h. The stirrer speed was 800 revolutions / minute. The pressure in the reactor (1) was adjusted to 55 bar via the transfer valve (14). The initiator dosing rates were

(9a) 0.11 kg / h

(9b) 0.11 kg / h

(9c) 0.21 kg / h

(9d) 0.30 kg / h

(9e) 0.40 kg / h

After 24 hours, the polymerization was ended and the free volume of the reactor (1) was determined by filling it up with water and weighing the amount of water. With this procedure, the reactor volume was determined to be 11.2 liters, that is, in "normal operation" according to the prior art, the reactor loses approx. 1.3 liters volume after 24 hours, equivalent to a corresponding build-up of wall covering.

Example 2 (experiment according to the invention):

The mixture to be polymerized was introduced at the lower end of the reactor (1) and the product was removed at the upper end. The valves (10) and (11) were open, the valves (12) and (13) closed. The flow rate was approx. 20 liters / h. The stirrer speed was 800 revolutions / minute. The pressure in the reactor (1) was adjusted to 55 bar via the transfer valve (14). The initiator dosing rates were

(9a) 0.11 kg / h

(9b) 0.11 kg / h

(9c) 0.21 kg / h

(9d) 0.30 kg / h

(9e) 0.40 kg / h After 12 hours, the direction of flow was reversed and the mixture to be polymerized was added from above and the product was removed at the lower end. Here, the valves (10) and (11) were closed, the valves (12) and (13) opened. The pressure in the reactor was adjusted to 55 bar via the transfer valve (15). The initiator dosing rates are

(9a) 0.40 kg / h

(9b) 0.30 kg / h

(9c) 0.21 kg / h

(9d) 0.11 kg / h

(9e) 0.1 kg / h

This change was carried out every 12 hours and the system was operated for a total of 72 hours. With this procedure, the reactor volume was determined to be 12.4 liters at the end of the experiment, that is, with the procedure according to the invention, the reactor loses only 0.1 liters volume after 72 hours, which is equivalent to a negligible build-up of wall covering.

In both examples an end product with the following properties was obtained:

Solids content 58.5%

pH 4.5

Viscosity (Brookfield at 23 ° C and 20 rpm) 1500 mPas

Particle size distribution Dw (Beckmann Coulter) 1300 nm

Glass transition temperature (DSC according to ISO 11357) 16 ° C

Claims

Patent claims:

1. A process for the preparation of aqueous polymer dispersions by means of radically initiated emulsion polymerization of ethylenically unsaturated monomers in the presence of protective colloids and / or emulsifiers in a continuously operated tubular reactor, characterized in that the direction of flow of the reactor contents is reversed along the longitudinal axis of the reactor at regular time intervals.

2. The method according to claim 1, characterized in that the

The direction of flow of the reactor contents is reversed in a time interval of 60 minutes to 48 hours.

3. The method according to claim 1, characterized in that the

The direction of flow of the reactor contents is reversed in a time interval of 6 hours to 12 hours.

4. The method according to claim 1 to 3, characterized in that stirred tubular reactors or unstirred tubular reactors or tubular reactors with internals such as static mixing elements or Taylor reactors with a cylindrical stirrer are used as the tubular reactor.

5. The method according to claim 1 to 4, characterized in that the components of the reaction mixture are mixed beforehand in a mixing unit to form a pre-emulsion and are continuously fed to the tubular reactor.

6. The method according to claim 1 to 5, characterized in that in the case of thermal initiation, no oxidation catalyst is added to the pre-emulsion, and the oxidation initiator is preferably added to the tubular reactor.

7. The method according to claim 1 to 5, characterized in that in the case of initiation with a redox initiator combination, the reduction initiator is added to the pre-emulsion and the oxidation initiator is added to the tubular reactor.

8. The method according to claim 1 to 7, characterized in that the addition of the reaction mixture is carried out so that the residence time of the polymerization mixture in the tubular reactor is 10 minutes to 5 hours.

9. The method according to claim 1 to 8, characterized in that one or more ethylenically unsaturated monomers from the group comprising vinyl esters, (meth) acrylic acid esters, vinyl aromatics, olefins, 1,3-dienes and vinyl halides are polymerized.

10. Use of the process products from claims 1 to 9 in construction chemical products, optionally in conjunction with hydraulically setting binders such as cements, gypsum and waterglass, or as binders for coating agents and adhesives, or as coating agents or binders for textiles and paper, or for production of polymer powders redispersible in water.