Process for preparing pulverulent polymers by drying aqueous...

Details Process for preparing pulverulent polymers by drying aqueous... Process for preparing pulverulent polymers by drying aqueous...

2000-07-06

2002-09-03

Szekely, Peter A (Department: 1714)

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

Synthetic resins

Processes of preparing a desired or intentional composition...

C525S221000, C525S223000

Reexamination Certificate

active

06444729

ABSTRACT:

DESCRIPTION
The present invention relates to a process for preparing pulverulent polymers by drying aqueous polymer dispersions in the presence of copolymers or salts thereof which in copolymerized form comprise ethylenically unsaturated C
3
-C
6
carboxylic acids (monomers A) and hydroxyalkyl esters of ethylenically unsaturated C
3
-C
6
carboxylic acids (monomers B), with or without further monomers C, as drying assistants, especially in connection with the spray drying of aqueous polymer dispersions. In addition, the present invention relates to a process for preparing polymer powders which can be redispersed in an aqueous medium, and to the redispersible polymer powders and their use.
Aqueous polymer dispersions are used in a large number of fields, for example as binders, in particular for synthetic resin renders, highly pigmented interior paints, and coating compositions, and as adhesives or modifiers for mineral building materials. The term mineral building materials is used hereinbelow to denote building materials which essentially comprise a mineral binder, such as lime, gypsum and/or, in particular, cement, and additions such as sand or stones, and which are converted by stirring them up with water into their ready-to-use form (mineral building material preparation), which over time, in air or under water, solidifies to form a stonelike structure. Accordingly, aqueous polymer dispersions for modifying mineral building materials can be added only at the site of use. In contrast, polymer powders can be formulated together with the mineral binder, thus simplifying the preparation of the mineral building materials. Moreover, polymer powders have the advantage over dispersions that they do away with the need to transport the dispersion medium, mainly water, from the place of preparation to the site of use.
The water-redispersible polymer powders that are required for the abovementioned purposes are obtainable in principle by drying the corresponding aqueous polymer dispersions. Examples of such drying techniques are freeze drying and spray drying. In spray drying, which is found to be particularly judicious when preparing relatively large amounts of powder, the aqueous polymer dispersion is sprayed, and the water is removed, in a stream of hot air; the air used for drying, and the sprayed dispersion, are preferably passed in cocurrent through the dryer (see for example EP-A 262 326 or EP-A 407 889).
However, the resulting polymer powder has the disadvantage that its redispersibility in an aqueous medium is in general not entirely satisfactory, since the polymer particle diameter distribution which results from redispersion generally differs from that in the initial aqueous dispersion. The reason for this is that aqueous polymer dispersions, unlike polymer solutions, do not form thermodynamically stable systems. Instead, the system tends to reduce the polymer/dispersion medium interface by combining small primary particles to form larger, secondary particles (gel specks, coagulum). In the state of disperse distribution in the aqueous medium, this can be prevented even for a relatively long time by the addition of dispersants, such as emulsifiers and protective colloids. In the course of the drying of aqueous polymer dispersions, however, the action of the dispersants is in many cases inadequate, and irreversible formation of secondary particles takes place to a certain extent. In other words, the secondary particles are retained on redispersion and impair the performance characteristics of the aqueous polymer dispersions that are obtainable on redispersion.
To prevent or at least reduce the formation of secondary particles in the course of drying, it has long been known to employ drying assistants. These substances are referred to as spray assistants, since spray drying in particular promotes the formation of irreversibly agglomerated secondary particles. This effect is all the more pronounced the lower the glass transition temperature (and thus the softening point or minimum film-forming temperature) of the polymer particles, especially if it is below the drying temperature. At the same time, drying assistants generally reduce the formation of polymer coating which remains adhering to the dryer wall, and thereby bring about an increase in the powder yield.
Examples of customary drying assistants are condensates of formaldehyde with aromatic hydrocarbons containing sulfonate groups (see for example DE-A 24 45813, EP-A 784 449, EP-A 407 889). Such condensates, although effective spray assistants, nevertheless have the disadvantage that as a result of the action of oxygen they tend to undergo discoloration, especially in an alkaline medium, for example in alkaline mortar. EP-A 467 103 discloses spray assistants comprising fully or partly neutralized, water-soluble copolymers which in addition to hydrophobic monomers comprise from 50 to 80 mol % of carboxyl-containing monomers.
EP-A-629 650 describes the use of polymers obtainable by free-radical polymerization, in an aqueous medium, of monomer mixtures comprising from 15 to 80% by weight of a monomer of the formula
where R
1
, R
2
and R
3
independently are H or C
1
-C
3
-alkyl, R
4
is C
1
-C
5
-alkylene and X is O or NH, and from 20 to 85% by weight of a free-radically copolymerizable monomer, as spray assistants in the spray drying of aqueous polymer dispersions.
JP-A 59-162161 describes water-soluble copolymers which comprise in copolymerized form from 10 to 90 mol %, based on the overall number of monomers, at least one hydroxyalkyl ester of an ethylenically unsaturated monocarboxylic acid and from 10 to 90 mol % of at least one ethylenically unsaturated mono- or dicarboxylic acid. The use of the copolymers as drying assistants is not proposed.
When using drying assistants a fundamental consideration is that they may influence the properties of the polymer powders and thus also the properties of preparations comprising such polymer powders. It is known, for example, that the water resistance of synthetic-resin renders based on polymer powders which are free from spray assistant is higher than that of synthetic-resin renders based on polymer powders which have been prepared using vinylpyrrolidone-vinyl acetate copolymers of EP-A 784 449 or polyvinyl alcohol of DE-A 221 4410 as drying assistants.
When using polymer powders dried with the aid of drying agents in mineral building materials it is often a disadvantage that the drying assistants retard the setting process (low flexural and compressive strength) and may cause a reduction in the ultimate hardness. Conversely, the duration of processability of polymer-modified mineral building material preparations which do not have the abovementioned disadvantages is often inadequate. Moreover, it is desirable to improve the adhesion of polymer-modified building material preparations on critical substrates, for example highly absorbent substrates or hydrophobic surfaces, for example rigid foams based on polyurethane or polystyrene, or plastics films.
It is an object of the present invention, therefore, to provide drying assistants for the drying of aqueous polymer dispersions which do not have the disadvantages of the prior art and which in particular do not retard the increase in hardness when used in mineral building materials. At the same time it is intended that they should improve the processing duration and the flowability of the mineral building material preparations. The polymer-modified mineral building materials should, moreover, possess high ultimate hardness and good adhesion to critical substrates.
We have found that this object is achieved, surprisingly, by the use of copolymers based on ethylenically unsaturated carboxylic acids having 3 to 6 C atoms and hydroxyalkyl esters of these carboxylic acids as drying assistants for aqueous polymer dispersions.
The present invention accordingly provides for the use of copolymers or salts thereof which comprise in copolymerized form, based in each case on the overall number of copolymerized monomers, from 20 to 95 mol %

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