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...
Reexamination Certificate
2000-06-12
2002-02-19
Yoon, Tae H. (Department: 1714)
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...
C524S320000, C524S819000, C524S822000, C523S340000, C523S342000
Reexamination Certificate
active
06348532
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the preparation of vinylaromatic copolymer redispersion powders redispersible in water and the use of the products of the process.
2. The Prior Art
Polymer powders are known which are dispersible in water and are obtainable by drying the corresponding polymer dispersions. These powders have been successfully used for many years, in particular in the construction sector. They improve the properties of hydraulically setting systems, such as cement mortars. For example they improve their abrasion resistance, flexural strength and adhesion. Usually, such products are prepared on the basis of a polyvinyl acetate, vinyl acetate/ethylene copolymers, vinyl acetate/vinyl ester copolymers and vinyl chloride/ethylene copolymer and are commercially available.
The requirements which an industrially usable dispersion powder has to meet are very high. Specifically, this powder must be free-flowing, and it may not cake during storage; i.e.—it may not lose its flowability in the course of time. If the powder cakes, it becomes virtually impossible to handle. Large lumps can no longer be mixed into the pulverulent formulation. To display its full effectiveness, the powder must be very readily redispersible in water so that the initial particles of the dispersion are obtained.
It is desirable to avoid caking under pressure and temperature during storage of the redispersion powder and at the same time to achieve very good redispersion properties in water. Hence considerable amounts of antiblocking agents and protective colloids are added to the so-called powder-based dispersions. U.S. Pat. No. 3,784,648 discloses adding melamine formaldehyde sulfonate condensates to the dispersion prior to drying in order to obtain polymer powders which are easily redispersible. DE-A 3,143,070 recommends the addition of naphthalene formaldehyde sulfonates, while EP-A 78,449 suggests the addition of vinylpyrrolidone/vinyl acetate copolymers; and WO-A 97/25371 suggests the addition of amino acids or their water-soluble salts prior to drying of the dispersion.
In specific applications, in particular for aqueous dispersions of styrene copolymers, the presence of protective colloids is not desirable. This is because they adversely affect the performance characteristics of the styrene copolymers.
SUMMARY OF THE INVENTION
It is therefore an object to provide a process for the preparation of emulsifier-stabilized, protective colloid-free redispersion powders based on vinylaromatic copolymers, by means of which a redispersible, agglomerate-free redispersion powder stable to blocking and having a long self life can be obtained even without the addition of protective colloids as an atomizing aid.
The above object is achieved according to the present invention by adding hydroxycarboxylic acid to the polymer dispersion to be dried. To date, EP-A 234,393 (U.S. Pat. No. 4,774,271) had only disclosed that tricarboxylic acids or their derivatives can be added for drying dispersions which contain impact modifiers in the form of graft polymers on a rubber base. Redispersion powders and their preparation are not discussed in this document. For the preparation of redispersion powders for cosmetic or pharmaceutical applications, WO-A 97/42255 discloses drying these powders in pH ranges of from 2.0 to 6.5 or from 7.5 to 12, the pH being established by adding buffer systems. DE-C 3,917,646 describes the use of hydroxycarboxylic acids as agglomeration assistants for the agglomeration of the fine fractions of synthetic, pulverulent polymers.
The present invention relates to a process for the preparation of vinylaromatic copolymer redispersion powders redispersible in water by means of free-radical emulsion polymerization of a comonomer mixture comprising
a) one or more vinylaromatic comonomers; and
b) one or more 1,3-diene comonomers (b1) or one or more comonomers (b2) selected from the group consisting of the alkyl esters having 1 to 8 C atoms of acrylic acid or methacrylic acid; and
c) 0.1 to 10% by weight, based on the total weight of the comonomer mixture, of one or more comonomers selected from the group consisting of the ethylenically unsaturated monocarboxylic acids or dicarboxylic acids or the anhydrides thereof, the ethylenically unsaturated carboxamides, the ethylenically unsaturated sulfonic acid compounds and in each case the salts thereof;
and drying of the aqueous copolymer dispersion obtainable thereby, with one or more hydroxycarboxylic acids or the salts thereof being added in an amount of 3% to 40% by weight, based on the total weight of copolymer, either before the polymerization or during the polymerization or after the polymerization but before the drying.
Suitable vinylaromatic comonomers a) are styrene, methyl-styrene and vinyltoluene, with styrene being preferred. Suitable 1,3-diene comonomers (b1) are 1,3-butadiene and isoprene, with 1,3-butadiene being preferred. Suitable comonomers (b2) from the, group consisting of the-alkyl esters of (meth)acrylic acid are, for example, methyl methacrylate, methyl acrylate, n-butyl methacrylate, n-butyl acrylate, ethyl methacrylate, ethyl acrylate, 2-ethylhexyl methacrylate and 2-ethylhexyl acrylate. Methyl methacrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate are preferred; and n-butyl acrylate is particularly preferred.
Examples of comonomers c) from the group consisting of the carboxylic acids and the anhydrides thereof are acrylic acid, methacrylic acid, fumaric acid, itaconic acid, crotonic acid, maleic acid, mesaconic acid, glutaric acid and maleic anhydride. Examples of comonomers from the group consisting of the carboxamides are acrylamide and methacrylamide. Examples of comonomers from the group consisting of the sulfonic acids are vinylsulfonic acid and 2-acrylamidopropanesulfonic acid. Acrylic acid, methacrylic acid, fumaric acid, maleic acid and maleic anhydride are preferred.
The choice of monomers or the choice of the amounts by weight of the comonomers is made in such a way that in general a glass transition temperature Tg of −70° C. to +68° C., preferably −50° C. to +50° C., in particular 0° C. to +25° C., results. The glass transition temperature Tg of the polymers can be determined in a known manner by means of differential scanning calorimetry (DSC). The Tg can also be calculated approximately beforehand by means of the Fox equation. According to Fox T. G.,
Bull. Am. Physics Soc
. 1, 3, page 123 (1956), the following applies: 1/Tg =x
1
/Tg
1
+x
2
/Tg
2
+. . . +x
n
/Tg
n
, where x
n
represents the mass fraction (% by weight/100) of the monomer n and Tg
n
is the glass transition temperature in degrees Kelvin of the homopolymer of the monomer n. Tg values for homopolymers are shown in
Polymer Handbook
2
nd Edition
, J. Wiley & Sons, New York (1975).
In general, the comonomer mixture contains 15.0% to 85.0% by weight, preferably 30% to 80.0% by weight, of vinylaromatic a) and 15.0% to 85.0% by weight, preferably 30% to 69.9% by weight, of 1,3-diene (b1) or (meth)acrylate (b2). The amount of comonomer c) is 0.1% to 10% by weight, preferably 2% to 5% by weight, based in each case on the total weight of the comonomer mixture.
Suitable examples of hydroxycarboxylic acids are organic acids which contain one or more OH groups in the molecule, in addition to one or more COOH groups. Monohydroxy and dihydroxy derivatives of monocarboxylic acids, dicarboxylic acids, or tricarboxylic acids having in general 2 to 10 C atoms are preferred. Mandelic acid (hydroxyphenylacetic acid), lactic acid (2-hydroxypropionic acid), malic acid (hydroxysuccinic acid), tartaric acid (2,3-dihydroxybutanedioic acid) and citric acid are particularly preferred. Citric acid (2-hydroxy-1,2,3-propanetricarboxylic acid) is most preferably used. The hydroxycarboxylic acids can also be used in the form of their salts, and in this case in particular in the form of water-soluble salts or salts soluble in aqueous alkaline media. Th
Collard & Roe P.C.
Wacker Polymer Systems GmbH & Co. KG
Yoon Tae H.
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