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
1998-12-14
2001-09-18
Michl, Paul R. (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...
C525S057000, C526S202000
Reexamination Certificate
active
06291573
ABSTRACT:
The present invention relates to a process for preparing polymer powders from aqueous polymer dispersions in which the dispersed polymer P comprises in copolymerized form
i) at least one vinylaromatic monomer (a),
ii) 1,3-butadiene as monomer (b) and
iii) one or more monomers (c) different than the monomers (a) and (b).
The present invention also relates to the polymer powders obtainable by the process of the invention and to their use, especially for modifying mineral construction materials.
Aqueous dispersions based on styrene-butadiene polymers are important industrial products. They have a considerable cost advantage over dispersions based on styrene-butyl acrylate polymers because butadiene can be separated off directly from the C
4
cut of the steam cracker whereas butyl acrylate requires at least two steps for its preparation from steam cracker products. In addition to the principal application as synthetic rubber and as impact modifiers in engineering plastics, aqueous polymer dispersions based on styrene-butadiene polymers are employed in coating compositions, for example for paper, as adhesives, or as binders, in emulsion paints for example, or in synthetic resin bound plasters. EP-A-754 657 reports on the use of aqueous styrene-butadiene polymer dispersions for modifying mineral construction materials.
For a range of the above applications it is desirable to use not the aqueous polymer dispersions themselves but rather the polymer in powder form. To obtain the polymer in powder form the dispersion must be subjected to a drying operation—for example, spray drying or freeze drying. In the case of spray drying, the polymer dispersion is sprayed in a stream of hot air and so dewatered, the drying air and the sprayed dispersion preferably being guided cocurrently through the dryer.
When preparing polymer powders by drying aqueous polymer dispersions it should be noted that the polymer particles of the aqueous dispersion undergo aggregation with one another on drying; that is, they form what are known as secondary particles. The coarsening of the polymer powder which accompanies the formation of secondary particles generally results in poorer performance properties. A further consequence of increased secondary particle formation is the development of polymer deposits on the walls of the dryer, so reducing the powder yield in the drying step.
To reduce the formation of secondary particles on drying it is common to add what are known as drying assistants to the aqueous polymer dispersions. These drying assistants are in many cases also known as spraying assistants, since spray drying in particular promotes the formation of secondary particles.
Typical drying assistants (frequently referred to as spraying assistants) of the prior art are naphthalenesulfonic acid-formaldehyde condensates, as described in der DE-A-24 45 813 and in EP-A-407 889 A1. Further spraying assistants are melamine-formaldehyde condensates, copolymers and homopolymers of vinylpyrrolidone (see e.g. EP-A-78 449 and EP-A-576 844), copolymers of ethylenically unsaturated carboxylic acids with hydrophobic monomers (see e.g. EP-A-467 103) or with hydroxyalkyl esters (see e.g. JP-A-59-162161). Other spraying assistants include the sulfo-containing polymers of EP-A-629 650.
EP-A-62 106 describes the preparation of aqueous polymer dispersions based on (meth)acrylic esters and/or styrene in the presence of polyvinyl alcohol as protective colloid. The spray drying of the dispersions thus obtainable is proposed.
The use of polyvinyl alcohol in the drying of aqueous vinyl ester polymers, such as polyvinyl acetate dispersions, is known from EP-A-56 622, EP-A-680 993, DE-A-22 14 410 and DE-A-26 14 261.
EP-A-601 518 describes the drying of aqueous polyacrylate dispersions in the presence of polyvinyl alcohol.
In the preparation of polymer powders by drying aqueous polymer dispersions in the presence of drying assistants it should be noted that these assistants may have a massive influence on the performance properties of the polymer powders. Furthermore, not every drying assistant is suitable for the drying of a specific polymer dispersion.
In view of the great interest in styrene-butadiene polymers there was interest in preparing them in powder form.
It is an object of the present invention to provide a process for preparing polymer powders composed essentially of vinylaromatic monomers and butadiene.
We have found that this object is achieved by drying aqueous polymer dispersions composed essentially of vinylaromatic monomers and butadiene in the presence of polyvinyl alcohol as the drying assistant. Polymer powders of this kind are notable in particular for their advantageous performance properties in mineral construction materials.
The present invention accordingly provides a process for preparing polymer powders from aqueous polymer dispersions, in which the dispersed polymer P comprises in copolymerized form
i) at least one vinylaromatic monomer (a),
ii) 1,3-butadiene as monomer (b), and
iii)one or more monomers (c) different than the monomers (a) and (b),
which comprises drying the aqueous polymer dispersion in the presence of polyvinyl alcohol as the drying agent. The present invention also provides the polymer powders obtainable by this process and provides for the use of these polymer powders as binders in paints, varnishes and coating compositions generally, in adhesives, and for modifying mineral construction materials.
In the process of the invention it is possible in principle to employ all commercially available polyvinyl alcohols. These are generally products of the hydrolysis and/or transesterification of polyvinyl acetate or of similar, saponifiable polyvinyl esters or polyvinyl ethers, generally having a degree of saponification >70 mol%, based on the overall number of saponifiable monomer units. A comprehensive overview of commercially available polyvinyl alcohols is given, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th ed., Vol. 21A, pp. 743-747 and in C. Finch (ed.), Polyvinyl alcohol, Wiley Interscience, New York, 1973. The polyvinyl alcohol employed in the process of the invention generally has a degree of saponification in the range from 73.4 to 99.9 mol %, corresponding to an ester number of from 1 to 270 mg of KOH/g of polyvinyl alcohol, preferably in the range from 82.6 to 92.4 mol %, corresponding to an ester number of from 90 to 190 mg of KOH/g of polyvinyl alcohol. The degree of polymerization lies preferably within the range from 100 to 5000, in particular in the range from 200 to 1000. The degree of polymerization is to be understood as the average number of monomer units per polymer molecule. Polyvinyl alcohols of this kind generally feature a viscosity in the range from 1 to 40 mPa·s, preferably in the range from 2 to 20 mPa·s (determined by the Höppler method as a 4% strength by weight solution in water at 20° C.).
In the process of the invention the polyvinyl alcohol is generally employed in an amount of from 1 to 30% by weight, preferably from 3 to 20% by weight and, in particular, from 5 to 15% by weight, based on the overall weight of polymer P in the aqueous polymer dispersion. Together with the polyvinyl alcohol it is also possible to employ the known drying assistants of the prior art, with preference being given to hydroxyl-containing polymers. Suitable hydroxyl-containing polymers are, for example, products of the saponification of copolymers of vinyl acetate with ethylene, and copolymers of hydroxyalkyl esters of ethylenically unsaturated carboxylic acids with ethylenically unsaturated carboxylic acids. Among the latter preference is given to those which are composed of from 30 to 70 mol % of ethylenically unsaturated carboxylic acids, such as acrylic acid, and from 30 to 70 mol % of at least one hydroxyalkyl ester of an ethylenically unsaturated carboxylic acid, such as hydroxyethyl acrylate and/or hydroxypropyl acrylate, and whose number-average molecular weight M
n
lies within the range from 5000 to 15,000. Polymers of this kind are disclose
Claassen Peter
Denu Hans-Jürgen
Heibel Claudia
Pakusch Joachim
Reck Bernd
BASF - Aktiengesellschaft
Michl Paul R.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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