Process for preparing aqueous polymer dispersions

Details Process for preparing aqueous polymer dispersions Process for preparing aqueous polymer dispersions

1999-10-06

2002-08-27

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S563000, C524S832000, C526S201000, C526S330000, C526S331000

Reexamination Certificate

active

06441082

ABSTRACT:

BACKGROUND OF THE INVENTION
1) Field of the Invention
The invention relates to a process for preparing aqueous polymer dispersions, and also to their use for preparing water-redispersible polymer powders.
2) Background Art
Protective-colloid-stabilized aqueous polymer dispersions, for example for construction sector applications, are known. To ensure that the aqueous polymer dispersions are stable, protective colloids, such as polyvinyl alcohol, polyvinylpyrrolidone, cellulose or starch, are used during the polymerization in an aqueous medium. EP-A 133899 (U.S. Pat. No. 4,532,295) has disclosed starch-stabilized polymer dispersions. WO-A 84/00369 (U.S. Pat. No. 4,528,315) and EP-B 62106 (U.S. Pat. No. 4,397,968) describe dispersions stabilized using polyvinyl alcohol.
However, the addition of significant amounts of protective colloids, which are necessary to ensure sufficient stability, is attended by disadvantages: protective colloids are relatively expensive, their addition reduces the binder content of the dispersion, rheological disadvantages result and, finally, the addition of protective colloids makes the polymer films produced from the dispersion susceptible to attack by water. The susceptibility to attack by water is, in particular, undesirable in many applications in the construction sector and coatings sector, and has to be counteracted by subsequent steps during formulation. Reactivity with water is particularly disadvantageous in highly polymer-filled cement applications, such as sealing slurries.
To improve the water resistance of polymer films which have been prepared using protective-colloid-stabilized polymer dispersions EP-A 727441 recommends the use of water-soluble protective colloids based on from 5 to 50% of comonomers with acid and/or anhydride functions, from 0.1 to 80% of long-chain (meth)acrylates and up to 94.9% of water-insoluble principle monomers, such as (meth)acrylates or vinyl esters. A disadvantage is that these protective colloids are prepared by polymerization in organic solvents, and complicated solvent removal by distillation is required.
DE-A 2618898 (U.S. Pat. No. 3,965,032) describes protective colloids based on copolymer polyelectrolytes having nonionic hydrophobic units, such as methyl methacrylate, and hydrophilic units, such as ethylenically unsaturated comonomers substituted with sulfonate groups and prepared by polymerization in organic solvents. These protective colloids have disadvantages which are the same as the abovementioned EP-A 727441.
DE-A 19608911, which is a subsequent publication, discloses crosslinkable protective colloids having sulfonate-containing, N-methylol-containing and hydrophobic monomer units. A disadvantage is that when polymers stabilized with protective colloids of this type are used in acid media or exposed to heat they crosslink and therefore become brittle. This is contrary to the properties of flexibility required in many applications (sealing slurries, sealing compositions, and paints for bridging cracks).
EP-B 206814 describes the use of terpolymers made from (meth)acrylic acid and from sulfonate-functional monomer and vinyl esters for stabilizing aqueous systems which comprise dissolved or suspended solids, for example cooling water or boiler water.
EP-A 671435 discloses that copolymers made from alkyl acrylates and from sulfonate- or carboxylate-substituted monomers are suitable spraying aids for spraying protective-colloid-stabilized dispersions. EP-A 629650 (U.S. Pat. No. 5,462,978) describes copolymers made from sulfonic-acid-functional comonomers and from water-insoluble comonomers as spraying aids in the spray drying of aqueous polymer dispersions.
SUMMARY OF THE INVENTION
The object on which the invention was based was to provide protective-colloid-stabilized aqueous polymer dispersions which have better water resistance when used in the construction sector or coatings sector than previously known polymer dispersions, for example those stabilized with polyvinyl alcohol.
The invention provides a process for preparing protective-colloid-stabilized aqueous polymer dispersions by free-radical polymerization of ethylenically unsaturated monomers by emulsion polymerization, which comprises carrying out the polymerization in the presence of one or more protective colloids comprising
a) from 5 to 95% by weight of monomer units containing sulfonic acid groups or sulfonate groups,
b) from 5 to 95% by weight of noncrosslinkable, water-soluble monomer units, and
c) from 0 to 5% by weight of hydrophobic monomer units selected from the class consisting of water-insoluble ethylenically unsaturated compounds, where the proportions in % by weight are based on the total weight of the copolymer.
DESCRIPTION OF THE REFERRED EMBODIMENTS
Suitable monomer units a) are water-soluble, free-radical-polymerizable, ethylenically unsaturated compounds which contain sulfonic acid groups and, respectively, sulfonate groups —SO
3
M, where M═H, an alkali metal ion, an ammonium ion or an alkaline earth metal ion. Preference is given to 2-acrylamido-2-methylpropanesulfonic acid (AMPS), styrenesulfonic acid, sulfoalkyl (meth)acrylates, sulfoalkyl itaconates, in each case preferably having a C
1
-C
6
-alkyl radical, vinylsulfonic acid and ammonium, alkali metal or alkaline earth metal salts thereof. Particular preference is given to 2-acrylamido-2-methylpropane-sulfonic acid (AMPS), styrenesulfonic acid, sulfopropyl acrylate, sulfopropyl itaconate, vinylsulfonic acid and ammonium, sodium, potassium and calcium salts thereof.
Preferred monomer units b) are water-soluble, free-radical-polymerizable, ethylenically unsaturated compounds which contain carboxyl groups —COOM, where M═H, an alkali metal ion, ammonium ion or alkaline earth metal ion, or contain amide groups —CONH
2
or contain hydroxyl groups. Particular preference is given to acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, hydroxyalkyl (meth)acrylates, such as hydroxyethyl acrylate or hydroxypropyl acrylate, hydroxybutyl acrylate, acrylamide and methacrylamide. Acrylamide and methacrylamide are most preferred.
For the purposes of the present invention, water-soluble generally implies solubility in water of at least 10% by weight at 23° C.
Suitable monomer units c) are free-radical-(co)polymerizable, ethylenically unsaturated compounds which have less than 4% by weight water-solubility at 23° C. Preference is given to esters of acrylic or methacrylic acid with alcohols having from 1 to 18 carbon atoms, such as methyl methacrylate, methyl acrylate, N-butyl acrylate, 2-ethylhexyl acrylate; vinylaromatics, such as styrene or vinyltoluene; olefins, such as ethylene, propylene or butadiene; vinyl halides, such as vinyl chloride; vinyl esters of aliphatic carboxylic acids having from 1 to 18 carbon atoms, such as vinyl acetate, vinyl propionate, isopropenyl acetate, vinyl laurate, and vinyl esters of &agr;-branched monocarboxylic acids having from 5 to 11 carbon atoms, such as VeoVa9® or VeoVa10®, Shell products which are vinyl esters of verstatic acid, or a saturated monocarboxylic acid mixture of highly branched C-9 and C-10 isomers.
Preferred protective colloids have
a) from 20 to 95% by weight of monomer units containing sulfonic acid groups and/or sulfonate groups and
b) from 5 to 80% by weight of monomer units containing carboxyl groups, amide groups or hydroxyalkyl groups, in particular protective colloids having
a) from 20 to 95% by weight of monomer units containing sulfonic acid groups or sulfonate groups and
b) from 5 to 80% by weight of monomer units containing amide groups, where the proportions in % by weight are in each case based on the total weight of the copolymer.
Particularly preferred protective colloids have
a) from 40 to 60% by weight of monomer units which derive from one or more monomers selected from the class consisting of 2-acrylamido-2-methylpropane-sulfonic acid, styrenesulfonic acid, sulfoalkyl (meth)acrylates, sulfoalkyl itaconates, in each case preferably with a C
1
-C
6
-alkyl radical, vinyl

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