SK94493A3 - Polymers and polymeric dispersions process for their production and their use - Google Patents

Abstract

These are polymers and aqueous polymer dispersions which consist of monomers from the group of unsaturated carboxylic acids, unsaturated carboxylic acid esters, unsaturated sulfonic acids and unsaturated N- substituted cyclic dicarboxylic acids as well as their use as admixtures to hydraulic binders.

Description

Polymers and polymer dispersions, process for their preparation and their use

Technical field

The invention relates to polymers and polymer dispersions based on carboxylic acids, carboxylic acid esters, sulfonic acids and cyclic imides, processes for their preparation and their use as admixtures in hydraulic binders.

The state of the art

Polymers based on butadiene, vinyl acetate, vinyl chloride, vinylidene chloride and esters of acrylic acid are known as additions to hydraulic binders such as cement, gypsum or anhydrite. Soft polymers with a transformation temperature (Tg) or with a freezing temperature below 0 ° C are predominantly used for the production of flexible, ductile cement-polymer plastics for protective coatings, sealants, sealants, elastic sealants and adhesives.

DE-OS 15 69 910 describes in the examples polymers of ethylenically unsaturated compounds which have been used as binders in sealants and sealants. It was a mixture of emulsion polymers and unsaturated esters of carboxylic acids, butadiene, isoprene, isobutene, vinyl alkyl ethers and the like. whose Tg was even below -5 ° C. DE-OS 25 24 064 discloses aqueous dispersions of copolymers having a Tg of from -15 ° C to -75 ° C, which consist of at least one ester of acrylic or methacrylic acid, vinyl acetate, of a maximum of 1.5% by weight. 2-acrylamido-2-methylpropane sulfonic acid and other monomers such as acrylonitrile, styrene and vinyl chloride. These plastic dispersions serve as adhesives for pressure-sensitive adhesive tapes that adhere well to rough surfaces, paper and metals. The disadvantage of these dispersions lies mainly in their low compatibility with cement.

DE-OS 32 20 384 discloses aqueous dispersions of plastics based on copolymers of olefinic unsaturated monoraers, monomers with amide groups and unsaturated sulfo acids in which the sulfo group is bonded either via an ester group or an amide group to acrylic or methacrylic acid. Particularly suitable monomers are 2-ethylhexyl acrylate as a softening base monomer as well as acrylonitrile, methyl methacrylate or styrene as hard comonomers. Dispersions are compatible with cement and exhibit good mechanical properties such as tensile strength and elongation in conjunction with cement. However, the water resistance of the polymer films as well as the durability of the hardened cement-polymer building materials after 28 days of alternating air-water placement are unsatisfactory, especially their long-term flexibility in several areas of application is insufficient.

Dispersions of plastics obtained by emulsion polymerization of esters of unsaturated carboxylic acids, styrene or vinyltoluol and an olefinically unsaturated sulfo acid based on acrylic or methacrylic acid are known from DE-OS 38 38 294. A particularly suitable combination is 2-ethylhexyl acrylate 2-acrylamido-2,2-dimethylethanesulfonic acid. These polymers are used in mixtures with cement, sand and other additives for protective layers, for filling cracks and joints. However, the building materials thus prepared exhibit only very low strength.

JP-A 62-108 759 discloses polymeric compositions based on aqueous copolymer dispersions containing a maleimide derivative containing on the N-atom a C1- to C4-alkyl-, cyclohexyl- or at most monosubstituted phenyl moiety as well as monomers such as e.g. -ethylhexyl acrylate, n-butyl acrylate, vinylidene chloride, styrene, acrylonitrile, methyl methacrylate, vinyl acetate, ethylene, vinyl chloride, acrylic acid, acrylamide, methacrylamide, which are combined so that the lowest film forming temperature of the copolymer obtained is below 0 θθ. The maleimides used are N-phenyl maleimide, N-methylphenyl maleimide, N-hydroxyphenylmaleimide, N-methoxyphenylmaleimide, N-benzoic acid maleimide,

N-methylmaleimide, N-tert. butylmaleimide etc. Polymer dispersions are used in the form of mixtures as coatings for protective layers of wood of concrete, mortar, slate, steel, aluminum, fibers, glass and plastics. Their disadvantage is that the polymerizates contain acrylamide or methacrylamide, which in a highly alkaline environment releases ammonia, which causes extreme odor.

Japanese abstract, Kokai 63-81177 and Kokai 63-117056 disclose polymers of unsaturated carboxylic acids, alkyl (meta) crystals and N-substituted maleimides, which are used to reduce seaweed, or together with halogenated phosphorus compounds as fireproof plastic materials.

It is an object of the present invention to prepare polymers or dispersions thereof based on elastic copolymers which do not exhibit the above-mentioned disadvantages. Such polymers are obtained by the process of the invention by means of various unsaturated monomers based on carboxylic acids, carboxylic acid esters, sulfoacids and cyclic imides.

SUMMARY OF THE INVENTION

According to this invention, polymers are obtained which are obtained by polymerization

(a) 70 to 98% by weight. alpha, beta-monoethylenically unsaturated carboxylic acid esters and alcohols with C1 to C12 carbon numbers, or mixtures of such esters,

b) 0.05-15% by weight. N-substituted cyclic imides of alpha, beta-monoethylenically unsaturated dicarboxylic acids,

c) 0.05 to 5% by weight. alpha, beta-monoethylenically unsaturated carboxylic acids,

(d) up to 28% by weight. other copolymerizable monomers,

e) 0.05 to 1% by weight. olefinically unsaturated sulfonic acids.

The polymers preferably consist of

(a) 85 to 96% by weight. alpha, beta-monoethylenically unsaturated carboxylic acid esters and alcohols having carbon numbers in the range of C1 through C12, or mixtures of such esters,

b) 0.5 to 7.5% by weight. N-substituted cyclic imides of alpha, beta-monoethylenically unsaturated dicarboxylic acids,

c) 0.25 to 2.0% by weight. alpha, beta-monoethylenically unsaturated carboxylic acids,

d) up to 13% by weight. other copolymerizable monomers,

e) 0.25-0.75 new acids.

A further object of the invention are aqueous polymer dispersions in which the polymers of the invention are dispersed so that they may optionally contain dispersants, emulsifiers, protective colloids or other conventional additives. The polymer dispersions have a solid phase content of from about 40 to 75% by weight. %, preferably from 50 to 70% by weight.

The choice of monomers as well as their weight ratio is preferably chosen so that the mixed polymer has a transition temperature of not more than 0 ° C, but preferably -20 ° C, and the hydraulic binder mixture formed thereby exhibits high flexibility even at very low temperatures. According to T. G. Fox (Bull. Am. Phys. Soc., Ser. II, 1, 1956, p. 123), the blended polymers were determined in good transition of the homopolymers of the individual components. The monomers of group a), whose homopolymers have a transition temperature normally below 0 ° C, but mostly below -40 ° C, with monomers from groups b) to d) or e), whose transition temperatures normally lie, can then be combined to set the desired temperature. above +20 ° C, but mostly above + 60 ° C.

The C1 to C12 alkyl esters of olefinic unsaturated mono- or dicarboxylic acids having from 3 to 5 carbon atoms are preferably used as monomers of group a). Examples of such esters are ethyl acrylate, n-butyl acrylate, n-hexyl acrylate. The olefinically unsaturated sulphonates may have a temperature transition temperature of the substances 2-ethylhexyl acrylate, n-dodecyl acrylate, 2-ethylhexyl methacrylate, methyl methacrylate, n-butyl crotonate, dibutyl fumarate, dioctyl fumarate, dibutyl maleate, dioctyl malonate, dioctyl malonate, dioctyl malonate, dioctyl malonate. Particularly preferred are 2-ethylhexyl acrylate and n-butyl acrylate, optionally combined with methyl methacrylate.

The monomers of group b) are cyclic imides which are derived from alpha, bexa-monoexylenically unsaturated dicarboxylic acids, preferably maleic acid, and on the imide nitrogen contain an alkyl, cycloalkyl or aryl radical which may be optionally substituted. Examples of such monomers are N-methylmaleinimide, N-ethylmaleinimide, N-t-butylmaleinimide, N-cyclohexylmaleinimide, N-hydroxyethylmaleinimide, N-phenylmaleinimide, N-methylphenylmaleinimide, N-hydroxyphenylmaleimimide and N- (2,6-dimethylphenylmaleimide) maleimide. Preferably, N-phenylmaleimide as well as N- (2,6-dimethylphenyl) -maleimide can be used.

In particular, the monomers of group c) improve the stability of the aqueous dispersions of the polymers. These are water-soluble alpha, beta-monoethylenically unsaturated carboxylic acids such as acrylic acid, crotonic acid, fumaric acid, maleic acid and itaconic acid. Acrylic acid has proven to be particularly suitable. and methacrylic. Optimization of the utility properties of the polymer dispersions can be achieved by the possible co-use of other monomers (group d). Examples of such polymers are acrylonitrile, styrene, vinyl chloride, vinyl acetate, vinyl propionate, 2-chlorobutadiene as well as di- or trifunctional monomers such as ethylene glycol diacrylate, divinylbenzole, pentaerythritri triacrylate, triethylene glycol dimethyl methacrylate, 2-chloromethyl acrylate, methacrylate, methacrylate.

The monomers of group e) containing sulfonic acid groups provide, in particular, the additional stabilizing effects of dispersed polymer particles in the preparation and storage of polymer dispersions. In addition, they improve the compatibility of the dispersions with the cement and hence the homogeneity of the mixture consisting of organic and mineral components. Examples of suitable olefinically unsaturated sulfonic acids are 4-styrene sulfonic acid, 3-sulfopropylacrylate, 3-sulfopropylmethacrylate, 2-sulfoethyl acrylate, 2 sulfoethyl methacrylate, acrylamido-2,2-dimethylethanesulfonic acid, and 2-methacrylamido-2,2-dimethylethanes. Particularly preferred is the use of 2-acrylamido-2,2-dimethylethanesulfonic acid.

The preparation of the polymers according to the invention or of polymer dispersions is carried out by polymerizing the monomers of groups a) to d) or e) in aqueous medium in the presence of free-radical initiators, dispersants, emulsifiers and possibly also regulating and protective colloids. The polymerization can take place as a batch as well as an addition process, and in both cases a progressive polymerization can take place. An addition process is preferred in which one part of the polymerization batch is introduced, heated to the reaction temperature, and finally the monomers in pure or emulsified form, the initiator as well as the optional co-initiator as well as other polymerization auxiliaries from the separate runs are continuously added. The monomer feed is preferably in the form of an aqueous emulsion. Upon completion of the addition, polymerization is effected by heating to 5-10 ° C, the reaction mixture is cooled to room temperature, and a water-soluble base, preferably an alkali metal hydroxide, is added to adjust the pH to 8-9. between about 20 and 100 ° C, preferably between 40 and 90 ° C. The polymerization temperature is set so that the initiators entering the batch decompose sufficiently rapidly to give reactions capable of converting the radicals. The content of free-radical initiators is approximately 0.1 to 1% by weight. based on the total monomers used. Particularly suitable are water-soluble inorganic peroxides of which peroxodisulfates, sodium peroxodisulfate and potassium peroxodisulfate are particularly preferred. Alternatively, the initiators may be combined with reducing agents such as sodium formaldehyde sulfoxylate, sodium sulfite, Na-hydrogen sulfite,

Na-pyrosulfite, Na-ditionite, Na-thiosulfate and / or ascorbic acid, hydroxylamine or hydrazine. The reducing agents may additionally be combined with a certain amount of heavy metals serving as accelerators such as salts of iron, cobalt, cerium, or vanadium. An additional amount of initiator or combined system may also be added at a later time to complete the conversion and reduce residual monomer moieties.

The average molecular weight of the dispersed polymers is, in addition to the transition temperature and the crosslinking density, the most important molecular variable in terms of the critical user-technical properties of the product, such as tack, adhesion, viscosoelasticity and tensile strength.

If the desired molecular weight range cannot be achieved by varying the polymerization temperature, additional molecular weight regulating agents such as n-butyl mercaptan, tert. butyl mercaptan, n-dodecyl mercaptan, thioglycol, thioglycolic acid and others in amounts up to about 2% by weight. based on the total content of monomers used. Preferably, tert. dodecyl. The addition of the regulators may follow any advantageously, however, together with the monomers.

Conventional anionic, nonionic, or cationic emulsifiers or suitable mixtures thereof can be used in the emulsion polymerization process of the present invention, provided that they are sufficiently soluble in the aqueous phase as well as in the monomeric phase and that there are no interfering effects with the monomers because Anionic surfactants are preferably used as emulsifiers. Emulsifying agents prepared by ethoxylating and sulphating C8 to C10-alkylphenols have proven particularly useful. For example, they can be derived from nonylphenol, isononylphenol, isooctylphenol, triisobutylphenol and triter-butylphenol and contain 3 to 30 units of ethylene oxide. Further examples of possible anionic emulsifiers are the alkali metal salts of alkylsulfonic acids, for example sodium dodecylsulfonate, alkylarylsulfonic acids, for example sodium dodecylbenzolsulfonate, as well as ethoxylated and sulfated fatty alcohols having an alkyl residue of C8 to C25 and a degree of ethoxylation of sodium from 5 to 10 esters.

Preferred nonionic emulsifiers are C8-C16 alkylphenol conversion products with 3 to 30 moles of ethylene oxide. in addition, ethoxylation products with C1 -C2 alkanols such as, for example, lauryl alcohol polyglycol ether and conversion products of polypropylene glycol and ethylene oxide are also suitable.

The desired average particle size of the dispersion phase can be controlled in some way by the type and amount of emulsifier used, the polymer particles being all the smaller the amount of emulsifying agent used. About 0.2 to 10% by weight is used. % of an emulsifying agent, preferably 0.5 to 7% by weight. emulsifying agent based on the monomer content, mixtures of one of the excess anionic and one nonionic emulsifier used have proved to be particularly suitable.

To prepare anhydrous polymers, the polymer dispersions are dried by heating, optionally using vacuum. Drying by spray drying or freeze drying is particularly preferred.

The polymers according to the invention as well as polymers composed according to claims 6 and 8 only from monomer components a) to d), in particular in the form of polymer dispersions, are preferably used as admixtures to hydraulic binders. They are well tolerated with hydraulic binders such as cement, lime, gypsum or anhydrite and therefore can be advantageously used in building materials which additionally contain other inorganic and / or organic components, for example sand, gravel and reinforcing fibers. Such building materials with a hydraulic binder containing the polymer in question exhibit increased flexibility or flexibility even at very low temperatures and are suitable, for example, as building materials, fungicides, sealants, repair materials, soundproofing materials, road coatings, building adhesives, interlayers and leveling layers.

Polymers and polymer dispersions according to the invention as well as polymers and polymer dispersions based on monomer components

(a) to (d) are particularly suitable as binders in cement-containing mixtures. For the preparation of such mixtures, for example, an appropriate amount of a hydraulic binder, for example cement, preferably Portland cement, is used and is dry-mixed with optional other admixtures such as sand, polymer fibers and the like. This is followed by the addition of a polymer dispersion optionally diluted with digesting water. Common mixtures of anhydrous polymers with hydraulic binders can also be used.

An advantage of the products according to the invention is, inter alia, that the N-substituted cyclic imide alpha, beta-monoethylenically unsaturated dicarboxylic acid contained as a comonomer component under strongly alkaline conditions of the freshly prepared cement slurry hydrolyzes to the corresponding carboxylic acid monoamide, resulting in thixotropy therefrom. prepared building material during its processing. Soft-plastic, well workable fresh compositions are obtained without the tendency to settle and mix with a homogeneous distribution of the polymer particles in the mineral matrix. The addition of rheology aids, in particular thixotropy, which often leads to undesirable side effects such as increased water intake or leads to swelling of the hardened building mass is not necessary.

Preferably 10 to 150, particularly preferably 50 to 100% by weight are used. polymer for the amount of hydraulic binder calculated as a solid resin. This corresponds to a plastic-binder coefficient of 0.1 to 1.5, particularly preferably 0.5 to 1.0.

The binder containing the compositions based on the polymers and polymer dispersions according to the invention is odorless and is particularly suitable for the production of flexible crack-bonding layers for the rehabilitation of, for example, concrete, which inter alia act as corrosion protection and also as a carbonization inhibitor for example in sealing and preserving facades and roofs. They can also be used as sound absorbing building materials, for example for rails to reduce the traffic noise of rail vehicles.

EXAMPLES

Example 1:

260 g of deionized water are introduced into a double wall polymerization apparatus of 2.5 liters, with stirrer, reflux condenser, thermometer and separate connections for dosing vessels, which is heated to 80 ° C under a nitrogen inlet. While stirring, 17.5 g of monomer emulsion prepared in advance in a separate nitrogen purged vessel consisting of:

418.1 g 16.5 g

27.2 g 25.7 g

1.6 g

32.1 g

1.6 g

1223.4 g of demineralized water of a% aqueous solution of ethoxylated and sulphated nonylphenol (E0-step 4) ethoxylated nonylphenol (EO-step 10) methacrylic acid monomer C (2.0% w / w) 2-acrylamido-2,2 acid -dimethylethanesulfone monomer E, Fy. Lubrizol (0.12% w / w)

N-phenylmaleimide monomer B, fy. Chemistry Linz (2,5% w / w) triethylglycol dimethacrylate - monomer D (0,12% w / w)

2-ethylhexyl acrylate (95.26 wt%)

After adjusting the constant start temperature to 80 ° C, a solution of 6.42 g of sodium persulfate in 64 g of deionized water was added over 5 hours, as well as the remainder of the monomer emulsion from the separate attached vessel over a period of three hours. After the addition was complete, the system polymerized for another two hours at 80 ° C.

The polymer dispersion obtained was cooled to room temperature and adjusted to pH 8.0 by addition of 10% aqueous sodium hydroxide solution. The dispersion is coagulate-free and has a dry matter content of 62.0% by weight. Further data on the polymer dispersion obtained are summarized in Table 2.

Examples 2-5:

Analogously to Example 1, polymer dispersions were prepared using the monomers listed in Table 1. The dry matter content, transition temperature and dynamic viscosity of the obtained polymer dispersions are shown in Table 2.

Comparative Example VI:

In analogy to Example 1 of DE-OS 32 20 384, a polymer dispersion was prepared which consists of the following monomer composition:

27.0 g methacrylamide (2.75% w / w) .18.0 g 2-acrylamido-2,2-dimethylethanesulfonic acid (1.83% w / w)

900.0 g 2-ethylhexyl acrylate (91.61% w / w)

18.0 g methacrylic acid (1.83% w / w)

9.0 g of 2-hydroxyethyl methacrylate (0.92% w / w)

1,4 g butanediol (1,4) dimethacrylate (0,14% w / w)

The properties of the polymer dispersion are shown in Table 2.

Table 1: Polymer compositions (in% by weight) monomers *

example and b C D e 1 95.26 A 2,5 B 2.0 C 0.12 I) 0.12 E 2 92.78 A 5,0 B 2.0 C 0.12 I) 0,1 E 3 94.88 A 2,5 B 2.0 C 0.12 I) 0.5. E 4 80.28 And 15.00 AA 2,5 B 2.0 C 0.12 I) 0,1 E 5 95.38 A 2,5 B 2.0 C 0.12 I>

* Used monomers

A 2-ethylhexyl acrylate

AA n-butyl acrylate

B N-phenylmaleimide (Fy. Chemie Linz)

C methylacrylic acid

D triethylene glycol dimethacrylate

E 2-Acrylamido-2,2-dimethylethanesulfonic acid (Fy. Lubrizol)

Table 2: Properties of polymer dispersions

T

At- | Contents súši- 1 temperature for- - «μοκιγ * Dynamic Vis- EXAMPLE ¹ (% I) materials.) | I running ^ C) 1 I viscosity measured as ^ (MPa.s) 1 1 I 62,0 1 1 -55 1 1 52 2 I 59,9 1 -52 1 54 3 | 59.5 1 -54 1 135 4 I 62,2 1 -53 595 5 i 61.7 1 -56 1 58 VI I 62,4 1 1 1 1 1 -45 1 1 l 248 1 designed acc. to DIN 53 189 2 by T .G.Foxa. 3ull. Am. Phys. Soc., Ser. II

(1956), 123 according to DIN 53 019, ISO 3219 at 23 ° C and 250 s ¹

Polymer properties

1. Water absorption of polymer films

In order to prove the durability of the polymer according to the invention according to Examples 1 to 5 compared to the polymer according to Comparative Example VI, in particular with regard to its use in the function of preservation and sealing elements in screeds containing binder for concrete and buildings for water absorption. Very high water absorption causes poor resistance of building materials to weathering.

The test was carried out according to DIN 53 495 on a polymer film having a surface of 63 cm and a layer thickness of 0.6 mm. The polymer film was obtained from a polymer dispersion diluted to approximately 50%, which was poured into a mold and dried to constant weight in an oven at 40 ° C. After determining its weight, the polymer film was stored for 24 hours in deionized water at 23 ° C. Then, gently squeezing the pulp tissue, freed the adhered water, weighed it and expressed the weight gain in percent. The test results are shown in Table 3.

2. Shear strength and ductility of building materials containing polymer

In order to determine the mechanical properties of the polymer according to the invention in comparison with known polymers as constituents of cement-containing building materials for flexible protective layers and their durability in connection with cement, the determination of shear strength and extensibility was used for both air and water placement of test specimens.

To prepare the test specimens, 50 g of polymer dispersion diluted to a solids content of 58% by weight was added to 50 g of cement (PZ 375 H, Mannersdorf) with stirring. After stirring for two minutes at 350 rpm, the obtained cementitious mixture with an aqueous coefficient ( V / Z) 0.42 and a polymer / cement (P / Z) ratio of 0.58 were cast into a rectangular plastic mold and allowed to air dry for 24 hours. The elastic test pieces were demolded and left in the air for a further 3 days. This was followed by the separation of the bodies 2 mm thick, half of which remained stored in the air until day 28 (23 ° C). The other half was subjected to a combined inhalation-water storage with a 12 hour rotation (water hardness: 6 ° dH) by day 28. Drying of the stored samples in air at 23 ° C for 24 hours was followed by testing both halves of the test pieces stored in different ways. The shear or tensile strength was determined as a measure of the strength of the cement / polymer bundle against the dynamic demands in dependence on tensile or deformation.

The assay was performed using a commercial plate / plate rotary rheometer of fy. Physica (UM-MC) at 23 ° C using 5mm diameter test specimens. The test specimens were exposed to an increasing deformation load at a constant frequency of 0.2 Hz using an MP 5 PP measuring system. The projecting shear strength against deformation was recorded and converted to ductility as known from DIN 53455. The test results are given in Table 3.

Comparative measurements show a low water absorption and thus better weathering resistance of the polymers of the present invention as compared to known polymers. In addition, the cementitious compositions containing the polymers of the invention due to their higher shear strength and ductility, especially after long-term alternating wet / dry weather simulation, exhibit improved durability and long-term flexibility compared to cementitious compositions containing known polymeric admixtures.

Table 3: Properties of polymers and polymer cements

1- 1 | shear strength | polymeric | cements (Nmm "^) I ~ I-- | ductility polymer- | | nych c 1 I (%) Ex. vodopohlti- | dry and AC | dry AC | the size of the polymer (% by weight) 1 save 1 1 I deposit | storage 1 1 imposition 1 7.7 1 | 0.66 0.47 1 | 90 117 | 2 6.9 1 1.05 0.84 | 77 107 3 11.0 1 0.59 0.48 | 134 180 | 4 8.8 | 0.55 0.82 | 120 213 | 5 7.8 1 0.41 0.61 | 90 123 | I VI 56.9 1 0.37 1 1 0.27 | 44 1 1 32 - I

Claims (8)

PATENT CLAIMS 1. Polymers obtainable by polymerization (a) 70 to 98% by weight. esters of alpha, beta-monoethylenically unsaturated carboxylic acids with C1 to C12 alcohols, or mixtures of such esters, b) 0.05-15% by weight. N-substituted cyclic imides of alpha, beta-monoethylenically unsaturated dicarboxylic acids, c) 0.05 to 5% by weight. alpha, beta-monoethylenically unsaturated carboxylic acids, (d) up to 28% by weight. other copolymerizable monomers, e) 0.05 to 1% by weight. olefinically unsaturated sulfonic acids. 2. Polymers according to claim 1, obtainable by polymerization (a) 85 to 96% by weight. esters of alpha, beta-monoethylenically unsaturated carboxylic acids with C1 to C12 alcohols, or mixtures of such esters, b) 0.5 to 7.5% by weight. N-substituted cyclic imides of alpha, beta-monoethylenically unsaturated dicarboxylic acids, c) 0.25 to 2.0% by weight. alpha, beta-monoethylenically unsaturated carboxylic acids, d) up to 13% by weight. other copolymerizable monomers, e) 0.25 to 0.75% by weight. olefinically unsaturated sulfonic acids. 3. Aqueous polymer dispersions characterized in that the polymer according to claims 1 and 2 is dispersed in an aqueous medium. 4. A process for the preparation of a polymer, characterized in that: a) 70 to 98, preferably 85 to 96% by weight. esters of alpha, beta-methylenically unsaturated carboxylic acids with C1 to C12 alcohols, or mixtures of such esters, b) 0.5 to 15, preferably 0.5 to 7.5% by weight. N-substituted cyclic imides of alpha, beta-monoethylenically unsaturated dicarboxylic acids, c) 0.05 to 5, preferably 0.25 to 2.0% by weight. alpha, beta-monoethylenically unsaturated carboxylic acids, d) up to 28, preferably up to 13% by weight. other copolymerizable monomers, e) 0.05 to 1, preferably 0.25 to 0.75% by weight. of olefinically unsaturated sulfonic acids is polymerized in an aqueous emulsion. Use of the aqueous dispersions according to claims 1 to 4 as additives to hydraulic binders. 6. Use of aqueous dispersions obtainable by polymerization (a) 70 to 98% by weight. esters of alpha, beta-methylenically unsaturated carboxylic acids with C1 alcohols. to C12, or mixtures of such esters, - 18 (b) 0.05 to 15% by weight. N-substituted cyclic imides of alpha, beta-monoexylenically unsaturated dicarboxylic acids, c) 0.05 to 5% by weight. alpha, beta-monoethylenically unsaturated carboxylic acids, (d) up to 28% by weight. other copolymerizable monomers, such as additions to hydraulic binders. A hydraulic binder comprising a polymer according to claims 1 to 4. 8. A hydraulic binder comprising a polymer obtainable by polymerization. (a) 70 to 98% by weight. esters of alpha, beta-methylenically unsaturated carboxylic acids with C1 to C12 alcohols, or mixtures of such esters, b) 0.05-15% by weight. N-substituted cyclic imides of alpha, beta-monoethylenically unsaturated dicarboxylic acids, c) 0.05 to 5% by weight. alpha, beta-monoethylenically unsaturated carboxylic acids, (d) up to 28% by weight. other copolymerizable monomers