Polymerization processes and products

Details Polymerization processes and products Polymerization processes and products

1999-08-12

2001-08-21

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...

C526S201000, C526S240000, C526S303100, C526S317100

Reexamination Certificate

active

06277932

ABSTRACT:

This invention relates to beads which have a narrow size range distribution and which are formed of water soluble or water swellable polymeric material, and to methods of making these beads by reverse phase bead polymerization.
High molecular weight water soluble or water swellable polymers are currently made mainly by either the gel polymerization and comminution process or by reverse phase polymerization.
In the gel process, an aqueous solution of the monomer or monomer blend is polymerised in bulk to form a rigid gel, which is then dried and comminuted. The product has a wide particle size distribution of irregularly shaped particles and includes a large amount of fines.
The reverse phase polymerization process comprises forming droplets of an aqueous solution of water-soluble ethylenically unsaturated monomer or monomer blend and polymerising the monomer or monomer blend, while the droplets are suspended in a non-aqueous liquid, to form aqueous polymer droplets. If the droplets are very small the product is a reverse phase emulsion. If the droplets are beads, the bead polymerization process is completed by drying the resultant polymer beads and separating the polymer beads from the non-aqueous liquid.
The usual way of performing a reverse phase bead polymerization process comprises charging a reaction vessel with non-aqueous liquid and dispersing the aqueous monomer or monomer blend in bulk into the liquid with sufficient agitation to form the aqueous monomer beads, and conducting the polymerization while stirring the suspension vigorously so as to keep the beads suspended in the non-aqueous liquid. The resultant particle size distribution is much narrower, and the amount of fines is much less than when the polymer is made by the widely used gel polymerization and comminution process and this is advantageous. However the reverse phase bead polymerization process does have a tendency to produce beads having a wider size distribution than would be desired including fines and some mis-shapen beads. This is due to the inevitable collisions and shearing forces applied to the monomer droplets and to the polymerising beads, especially in large scale commercial processes.
Although the product can be sieved or otherwise classified according to size, this inevitably still leads to a product having a fines fraction and a significant spread of particle sizes. For instance fines become trapped on the surface of larger beads and sieving does not separate them. Attempts to obtain a narrower size distribution by sieving to a very narrow size range are not practicable on a large commercial scale (for instance above 1 kg or above 10 kg) and again the final product still contains fines.
The products, contaminated with fines, have been widely used on a commercial scale and are considered satisfactory for many purposes. For instance water soluble beads are used on a large scale for dissolution in water to form a flocculant or viscosifying solution and cross linked, water swellable, beads are used for delivering, by a sustained release mechanism, active ingredient distributed through the beads. Because of their more regular shape and narrower size distribution and lower fines production, they are often preferred over products made by gel polymerization and comminution. However it would be desirable to produce beads which have better performance properties, for instance handling and dissolution or release properties.
It is known to conduct oil-in-water emulsion and bead polymerization processes, using water-insoluble monomer or monomer blend dispersed in water, under conditions whereby collisions between the beads during polymerization can be reduced. For instance in GB 1,124,610 it is proposed to form a monomer emulsion, having a particle size below 5 &mgr;m, and to feed this into a simple tubular loop reactor in which the tubular reactor has an upwardly extending tubular leg which leads, at its top, into a downwardly extending tubular leg. Monomer emulsion is fed into the base of the upwardly extending leg and polymer emulsion is taken, when appropriate, from the base of the downwardly extending leg. The upward and downward movement of the emulsion is due, at least in part, to changes in specific gravity as the polymerization proceeds. The polymerization period is suggested as 0.5 to 20 hours, preferably 1 to 10 hours.
In U.S. Pat. No. 3,922,255 a blend of water-insoluble monomers is fed through orifices (to form non-aqueous beads) into the base of a vertical column along with an aqueous medium containing a stabiliser such as gelatin. This aqueous medium and the non-aqueous monomer beads travel together upwardly through the column and thereby form a dispersion of beads in water in the column. In an example, the time of travel through this column averages 3.5 minutes. The dispersion is taken from the top of this column through a line and fed to the top of a downwardly extending column heated to a temperature at which polymerization is initiated. Accordingly there is no initiation of polymerization until a considerable period after contact of the beads with the continuous phase, including passage through a feed line. The beads and the aqueous medium are caused to flow slowly down this column, with a residence time of 150 minutes in an example. The resultant slurry of partially polymerised beads is taken from the base of the column, some of the aqueous medium recycled to the top of the column, and the beads and the remainder of the aqueous medium are fed to a reactor where they are subjected to further reaction for, in an example, four hours.
In EP 67,415, water-insoluble monomer is fed through a droplet generator into an aqueous suspension medium containing a stabiliser so as to form a suspension of droplets in the aqueous medium. This suspension is then fed through a line to the top of a column where polymerization is initiated and the aqueous medium flows downwardly at a rate such that the droplets initially reside at the top of the column but sink, concurrent with the downflowing liquid, as polymerization progresses. In an example, the residence time in this column is 170 minutes. The aqueous medium and the droplets are then reacted under plug flow conditions in another reactor, and the resultant suspension of partially polymerised beads in aqueous medium is then fed into a third reactor which is another column and wherein aqueous medium flows upwardly and the polymer beads, when they are completely polymerised, sink to the base of the column and are recovered. The total polymerization time is around 5 hours in an example.
Other disclosures of polymerising water-insoluble is monomer beads include JP 51-150592, EP 271,922 and U.S. Pat. No. 4,579,718.
In U.S. Pat. No. 4,444,961 a particular system is described for forming a dispersion of monomer beads in an immiscible liquid, This comprises a perforated plate separating a monomer supply from a vertical column of the immiscible liquid, and a vibrating pump for pulsing beads through the perforated plate into the column. In the preferred embodiments, the monomer is water-insoluble monomer and the beads are pulsed into the base of an upwardly flowing column of water. However it is also proposed that the beads could move countercurrent to the flow of the column. It is also proposed that a water-soluble monomer blend could be pumped as beads into a column of water-immiscible liquid, in similar manner. The monomer droplets flow through this column and emerge from it as a dispersion in the immiscible liquid, after about 100 seconds in an example. The dispersion is then passed through a line into a separate vessel in which a vessel which is separate from the column in which the agitation is provided to maintain a dispersion of the droplets and polymerization is initiated.
None of these methods are capable of giving the improvement that we desire in the bead polymerization of a water-soluble monomer or monomer blend or in the properties of the resultant beads. For instance in U.S. Pat. No. 4,444,961 the agitation during polymerization will cause bead co

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