Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1999-05-20
2001-08-07
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S062000, C526S064000, C526S079000, C526S080000, C526S078000, C526S329100, C526S328000
Reexamination Certificate
active
06271320
ABSTRACT:
The present invention relates to a process for the preparation of a polymer dispersion by free radical polymerization of an aqueous monomer emulsion.
In the preparation of polymer dispersions by emulsion polymerization, the distinction is generally made between batch, semibatch and continuous processes, various methods for the addition of monomers to the reaction containers being described.
Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Vol. 1, page 397 et seq. describes a semibatch process for emulsion polymerization, taking the large-scale industrial polymerization of acrylates as an example. A monomer emulsion prepared in a separate kettle is introduced continuously into the polymerization reactor, mixed there with an aqueous initiator solution and polymerised.
Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Vol. 14, page 92 et seq. gives a comparative overview of batch, semibatch and continuous processes for emulsion polymerization. In a general procedure for a semibatch process, the emulsion feed may contain all ingreduents used for the emulsion polymerization, for example monomers, surfactants, water and regulator, the monomer emulsion being prepared in a separate kettle, ie. the feed tank. Here, however, the initiator may not be mixed into the monomer emulsion since polymerization could then take place in the feed tank.
Encyclopedia of Polymer Science and Engineering, Vol. 6, page 10 et seq. (1986) likewise describes batch, semibatch and continuous reactors for emulsion polymerization. As above, the preparation of a monomer emulsion in a separate kettle is described as a possible variant of the batch and semibatch process, which emulsion-may contain all emulsion components with the exception of the initiator. This is added directly to the polymerization reactor in a separate feed. In the continuous processes described, the monomers, either without prior emulsification together with the aqueous phase or in the form of a monoemulsion which is prepared separately and stored, are fed continuously to the reactor or reactors.
Semibatch processes in which a continuous feed is introduced into a stirred reactor but no product is removed during the reaction are preferable to the batch processes. Thus, the composition of the feed and the first filling can be varied in a simple manner and a large number of different dispersions can therefore be prepared in one and the same reactor. In addition to these advantages, further benefits of the semibatch process are its greater safety owing to the smaller amounts of monomers in the reactor compared with the batch process and better removal of the heat of reaction, in particular in the case of dispersions in which there is a sharp increase in viscosity during the polymerization. Furthermore, the possible regulation of the feed rates of the monomers or of the monomer emulsion and hence improved control of the reaction are advantageous.
In the semibatch process, the monomers can be fed into the polymerization reactor, for example, separately from the initiator feed in pure form as a single feed, as described, for example, in EP-A-0 575 625. Further methods for adding the monomers to the polymerization reactor are, as also described above, the addition together with the aqueous phase without prior emulsification and the addition of a monomer emulsion prepared in a separate kettle.
Prior art processes with the addition of the monomers in nonemulsified or pure form have the following disadvantages:
a) The energy required for emulsifying the monomers must be applied by the stirrer of the polymerization reactor. This requires either the use of special, expensive stirrers, some of which are complicated and difficult to clean, or an increase in the power supply to the reaction material by increasing the stirrer speed or using larger stirrers. The latter measures lead to increased shearing of the dispersions and hence to greater coagulum formation.
b) In the case of insufficient emulsification in the reactor, a pure monomer phase may form in the reactor, ie. the specifically lighter monomer which has not yet reacted forms a cohesive layer floating on the remaining reaction material. After the end of the emulsion feed, there is no cooling effect of the cold feeds and there may be a sharp temperature increase in the interior of the kettle as a result of the reaction of this concentrated monomer phase. Moreover, the reaction of such a concentrated monomer phase leads to increased coagulum formation and to severe contamination of the reactor.
The prior art semibatch processes described above, in which a monomer emulsion is fed continuously to the polymerization reaction, all require the preparation and provision of this emulsion in a separate kettle before the beginning of the reaction. However, this process variant has the following disadvantages:
a) The initiator cannot be mixed into the monomer emulsion since polymerization in the feed vessel is then to be expected. However, separate initiator feed into the reactor results in a locally increased electrolyte concentration at the feed point, which leads to a higher level of undesirable coagulum formation.
b) The emulsions used for the aqueous free radical emulsion polymerization are thermodynamically unstable, ie. separation of the monomer emulsion as a result of coalescence of the monomer droplets may occur and a monomer phase may form in the feed vessel. At the same time, emulsifier is released and forms micelles in the reactor and may thus initiate the growth of a further undesirable particle generation. Moreover, the introduction of a formed pure monomer phase at the end of the feed give rise to the problems described above, resulting from a sharp temperature increase, increased coagulum formation and contamination of the reactor.
c) In the preparation of more highly concentrated dispersions, the monomer emulsion fed to the polymerization reactor must have a high monomer content. This leads in general to emulsions having a high viscosity and hence to problems with the conveying of these emulsions. Furthermore, particularly highly concentrated aqueous monomer emulsions tend to separate.
None of the abovementioned publications indicates the use of a continuously prepared aqueous monomer emulsion in the free radical aqueous emulsion polymerization for the preparation of polymer dispersions.
U.S. Pat. No. 5,250,576 describes a continuous process for the preparation of a special water-in-oil emulsion (high internal phase emulsion HIPE) having a high content of disperse aqueous phase and its subsequent polymerization and the dewatering for the preparation of absorber foams. The process comprises:
a) preparation of an oil phase from monomers, such as styrene and p-methylstyrene, comonomers, eg. alkyl acrylates and alkyl methacrylates, butadiene, etc., crosslinking agents and emulsifiers;
b) preparation of an aqueous phase from a water-soluble electrolyte and a water-soluble initiator;
c) simultaneous introduction of oil phase and water phase into a dynamic mixing zone;
d) preemulsification;
e) HIPE formation in the dynamic mixing zone by increasing the flow rate of the aqueous phase and/or reducing the flow rate of the oil phase;
f) continuous conveying from the dynamic into a static mixing zone through which the mixture flows, a stable water-in-oil emulsion being formed;
g) polymerization and dewatering.
In contrast to the free radical aqueous emulsion polymerization in which the polymerization takes place in the micells and not in the monomer droplets (oil phase), in this process the polymerization takes place in the oil phase so that it may be considered as a special case of mass polymerization. Furthermore, the process is not intended for the preparation of a stable aqueous polymer dispersion but serves for the preparation of a polymer foam which can be dewatered to give an absorber. Accordingly, the problems described above, which are specific to the preparation of aqueous polymer dispersions, for example coagulum formation, play no role in the process of U.S. Pat. No.
Bächer Reinhard
Bauer Gerhard
Hartmann Jurgen
Keller Andreas
Lawrenz Sven
BASF - Aktiengesellschaft
Cheung William K.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wu David W.
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