Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2000-02-17
2003-04-22
Henderson, Christopher (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S258000, C526S307100
Reexamination Certificate
active
06552142
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for preparing aqueous dispersions of copolymers from hydrophilic and hydrophobic monomers. The present invention relates in particular to a process for preparing aqueous copolymer dispersions starting from monomers which exhibit different reaction behavior in different pH ranges.
In addition, the present invention relates to the aqueous copolymer dispersions prepared by the process of the invention and to their uses.
Further subjects of the present invention are the copolymers obtainable starting from these copolymer dispersions, and their uses.
It is known that for certain monomers there is an optimum pH range for homo- and/or copolymerization. For example, vinyl esters, i.e., the carboxylic esters of vinyl alcohol, such as vinyl acetate, vinyl chloroacetate or vinyl propionate, for example, refuse to undergo emulsion polymerization at a pH above 7.0. Below 7.0, the polymerization rate increases as the pH is progressively lowered. In contrast, at pH levels dropping below 7.0, aliphatic vinyl ethers increasingly undergo hydrolysis to acetaldehyde and aliphatic alcohols (F. Hölscher, Dispersionen synthetischer Hochpolymerer, Teil I: Eigenschaften, Herstellung and Prüfung, Springer Verlag, Berlin, 1969). The same applies to N-vinyllactams such as vinylpyrrolidone and vinylcaprolactam, for example.
Patent DE 1 089 930 B describes sprayable hairsetting compositions comprising a copolymer of from 90 to 25% by weight of an N-vinylpyrrolidone and from 10 to 75% by weight of a vinyl ester, e.g., vinyl acetate, and also a process for preparing these copolymers in low-boiling alcohols. According to that process, the initiator is added to monomers in the form of an initial charge in a low-boiling alcohol. The Fikentscher K values achieved in this way are relatively low, at merely from 15 to 60. The generation of high K values with this process is difficult, or even impossible, to achieve, owing to the chain transfer effect of the alcohols. In addition, there is no provision for pH control of the reaction medium during the polymerization reaction.
U.S. Pat. No. 5 319 041 discloses the copolymerization of N-vinylpyrrolidone (VP) and vinyl acetate (VAc) with a VP/VAc weight ratio of from 25 to 75% by weight/from 75 to 25% by weight in alcoholic solution. The K values achieved with this process are low, at from 25 to 40. Here again, high K values can be achieved only with great difficulty, if at all, for the reasons mentioned above and for technical reasons associated with the process.
DE-C-25 28 068 describes crosslinked, water-insoluble hydrophilic gels consisting of copolymers comprising (a) from 30 to 90% by weight of certain water-soluble monoolefins, such as polyethylene oxide alkyl acrylate, N-vinylpyrrolidone, acrylic acid, methacrylic acid or hydroxyalkyl acrylates, or of a mixture of these monomers with from 1 to 50% by weight of water-insoluble monoolefins; and (b) from 10 to 70% by weight of certain hydrophobic macromers as crosslinkers, having an average molecular weight of from 400 to 8000 g/mol. Water-insoluble monomers specified are alkyl acrylates and alkyl methacrylates, in which alkyl can have up to 18 carbon atoms; vinyl esters (e.g., vinyl acetate) derived from alkanecarboxylic acids having up to 5 carbon atoms; styrene; and alkyl vinyl ethers, in which alkyl contains up to 5 carbon atoms. The polymerization is conducted in bulk or in solution.
WO-A-97/31041 discloses a process for preparing pulverulent, nonhygroscopic, highly swellable, weakly crosslinked copolymers of vinylpyrrolidone (VP) and vinyl acetate (VAc) of high viscosity, prepared by precipitation polymerization in an organic solvent, preferably heptane, hexane or cyclohexane. The monomers are polymerized in-a VP/VAc ratio of from 90/10 to 60/40. Crosslinkers are used in amounts of from 0.1 to 2% by weight, based on monomers. The polymers described can be used in pharmaceutical applications, such as in the controlled release of active principles from tablets, for example. An unfavorable aspect of this process is, in particular, the use of very highly flammable organic solvents, which are technically difficult to separate from the product.
U.S. Pat. No. 5 395 904 describes a process for preparing homogeneous copolymers from vinylpyrrolidone and vinyl acetate, forming clear aqueous solutions. For this process, the monomers are introduced as initial charge in a defined proportion and are supplied separately to the reaction medium over a time which is a function of their reactivity. The solvent used is either alcohol or a water-alcohol mixture containing up to 50% by weight of water. The polymers obtained in this way have low K values of between 10 and 40. High molecular weights are impossible to obtain owing to the use of the regulating alcohols. Likewise, there is no provision for pH regulation in order to control the K value.
U.S. Pat. No. 3 166 525 mentions a process for copolymerizing N-vinylpyrrolidone and vinyl esters of acids and fatty acids in water, in which the entire monomer amount is introduced initially in the reaction solution. The K values of the polymers obtained in this way, although they are higher (up to 55) than those of polymers prepared in the same way in organic solvents, are nevertheless still too low for.certain applications. Fikentscher K values above 55 are not obtained with this process. Careful control of pH change over the entire reaction period with the aim of achieving higher K values is not mentioned.
Finally, DE-A-197 12 247 describes a process for the free-radical 20 aqueous emulsion polymerization of water-soluble monomers containing N-vinyl groups, such as, for example, N-vinylpyrrolidone with hydrophobic monomers such as, for example, vinyl acetate. A disadvantage of this process is the relatively high residual level of vinylpyrrolidone monomer, which generally in the examples is markedly above 50 ppm and can attain levels of up to 2000 ppm of N-vinylpyrrolidone (Examples 13 and 14). Fikentscher K values of more than 100 are likewise not achieved with this process. Careful control of pH change over the entire reaction period is, again, not envisaged here. Instead, the pH is to be kept constant over the duration of the reaction.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for preparing aqueous dispersions of high molecular mass copolymers from hydrophilic and hydrophobic monomers having different reaction behavior in different pH ranges. The products obtained are intended to have a low residual monomer content and high Fikentscher K values.
We have found that this object is achieved by conducting the polymerization as a free-radical polymerization in aqueous solution using a water-soluble initiator system and controlling the pH of the reaction medium such that it is reduced during the reaction.
The present invention accordingly provides a process for preparing an aqueous copolymer dispersion by free-radical polymerization of ethylenically unsaturated monomers in an aqueous medium comprising
(a) from 10 to 90% by weight of at least one preferably nonionic, hydrophilic, nitrogen-containing monomer A which has an N-vinyl group and whose polymerization rate possesses a maximum at pH levels above 7;
(b) from 10 to 90% by weight of at least one monoethylenically unsaturated hydrophobic monomer B whose polymerization rate possesses a maximum at pH levels below 7;
(c) from 0.01 to 20% by weight, based on the total monomers present in the reaction medium, of at least one water-soluble or water-insoluble free-radical initiator or of a mixture of water-soluble with water-insoluble free-radical initiators,
the pH being controlled in the course of the polymerization reaction in such a way that in the course of the polymerization it decreases by from 2 to 5 units between start and finish of the reaction. The residual monomer content in the resulting copolymer dispersion after the polymerization reaction has been ended is not more than 50 ppm.
In accordance with the inven
Angel Maximilian
Meffert Helmut
BASF Akeiengesellschaft
Henderson Christopher
Keil & Weinkauf
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