Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2001-05-31
2002-12-24
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
Reexamination Certificate
active
06498232
ABSTRACT:
The invention relates to the use of an apparatus with at least one shearing module for essentially salt-free coagulation of plastics dispersions, and also to the process carried out with this apparatus.
Many polymers are prepared by homo- or copolymerization of suitable monomers in a liquid medium, e.g. by emulsion, miniemulsion or microsuspension poly-merization. Here, the polymer precipitates in the form of a usually aqueous dispersion of solid, from which the polymer has to be separated out, unless the dispersion is to be used as such.
The polymers are usually separated out from the dispersion by coagulation. There is a wide variety of different known methods for this. For example, dispersions can be coagulated by adding strong electrolytes. This is mostly done using salts which contain polyvalent cations, such as Ca
2+
, Mg
2+
or Al
3+
. A disadvantage of this method is that relatively large amounts of precipitating agents remain in the product and impair important product properties. Downstream washing of the precipitated polymer with large amounts of water is therefore necessary, and this causes problems in terms of costs and the environment. Another disadvantage of precipitation with electrolytes is that the precipitated product is frequently produced as a clump which comprises unprecipitated material or excess precipitating agent, or as very finely divided material difficult to separate out by sedimentation or filtration.
It has also become known that polymer dispersions can be coagulated by subjecting them to high shear forces. Here, the respective polymer dispersion is subjected to high shear forces until the polymer particles agglomerate. If the solids content of the polymer is above 20%, the polymer coagulated in this way can become pasty to crumbly.
DE-A-196 54 169 discloses a process for coagulating graft-rubber dispersions, where coagulation is brought about using shear-precipitation in a stator-rotor arrangement. Both the stator and the rotor, which rotates within the stator, have slots through which the dispersion is passed radially from the inside to the outside as a result of the rotation of the rotor. The shear to which the dispersion is subjected here is strong enough for it to coagulate.
DE-A-29 17 321 discloses a process for separating out, from an aqueous emulsion, polymers which have a softening range above 100° C., where the aqueous emulsion is coagulated in an extruder by shearing and/or heating to temperatures above the softening range of the polymer, and the coagulated material is then melted and discharged hot from the extruder, under pressure. The water is then separated out in a subsequent step. The process is very energy-intensive and requires a counter-rotating non-intermeshing twin-screw extruder for the precipitation. In addition, ammonium acetate is used as auxiliary to accelerate the coagulation, and this is undesirable for environmental reasons.
U.S. Pat. No. 3,821,348 describes a process in which acrylonitrile-copolymer dispersions or acrylonitrile-graft-polymer dispersions with a high acrylonitrile content and a very low content of elastomeric butadiene-acrylonitrile rubber are coagulated to give a paste, using a Waring mixer as the shearing apparatus, and then extruded through a fine die to give thin lengths and passed into hot water. The product is then washed, dried and finally shaped into lengths in a compression molder at 150° C.
It is an object of the present invention, in the light of this prior art, to provide an apparatus and a process for coagulating plastics dispersions or rubber dispersions, with which cost-effective coagulation of dispersions of this type becomes possible without adding chemical coagulants.
We have found that this object is achieved by using an apparatus with at least one shearing module which has a stator and a rotor arranged within the stator, where the surfaces facing toward one another in the stator and in the rotor are in each case smooth, or at least the rotor exhibits a structure formed on its surface and facing from this in the direction of the stator, and between the stator and the rotor there is a gap of predetermined gap width.
For the purposes of the present invention, “gap” is a very general and inclusive term for any desired space between rotor and stator. The predetermined gap width may therefore also include the flight depth, defined as (outer diameter of a screw minus the diameter of the screw root)/2.
This apparatus has proven very reliable in the essentially salt-free coagulation of plastics dispersions or rubber dispersions. It is fundamentally very simple in construction, and no susceptibility to clogging has been found. If desired, additional conveying modules may be used to convey the dispersion to be coagulated to the apparatus and away from the apparatus after coagulation has taken place. However, the apparatus may also be freely operated without conveying modules of this type. In particular, there is no requirement to use, for example, pressure vessels or pumps to ensure the presence of a certain pressure in advance in order to supply the apparatus with the dispersion to be coagulated.
For the purposes of the present invention, plastics dispersions are dispersions in which the homo- and/or copolymers have a glass transition temperature above 0° C., whereas the glass transition temperatures for rubber dispersions are below 0° C.
The predetermined gap width may be constant, but may also in each case vary within each of the one or more shearing modules. The diameter of the rotor here may decrease or increase in the direction of conveying. This decrease or increase in the diameter in the direction of conveying may occur more than once.
It has proven advantageous for the diameter of the rotor to diminish in the direction of conveying, or for the predetermined gap width to decrease in the direction of conveying.
The rotor may have a toothed-wheel structure, the rows of teeth in which have a circular arrangement radially around the rotor. If desired, the stator may have one or more approximately complementary rows of teeth. In this arrangement the coagulation mechanism is different from that with smooth surfaces of the stator and rotor. Whereas in that case coagulation takes place as a result of exposure to a continuous shear field, the use of a stator-rotor combination whose rotor has a surface structure, or of a stator-rotor combination with complementary toothed wheel or, respectively, rows of teeth gives a constantly repeating shear stress. The dispersion experiences a reduction in pressure once one of the rotor teeth has passed by the stator, only to be subjected again to strong shear at the next tooth which follows. This arrangement gives very intensive shear action. Depending on the requirements relating to the dispersion to be coagulated, a selection may therefore advantageously be made between a smooth stator-rotor system, i.e. a stator-rotor system with a smooth surface, and one in which at least the rotor surface has a toothed-wheel structure.
The rows of teeth on the stator and on the rotor may be approximately rectangular. They may also have an approximately star-shaped arrangement on the rotor. A helical arrangement of teeth is also possible, but for this there can be no complementary shaping of the stator.
Upstream and/or downstream of the shearing module of the apparatus used according to the invention, there may be a conveying screw with one or more flights, preferably arranged on the same shaft as the shearing module. The feeding and transport of the dispersion to be coagulated in the apparatus, and also the to discharge of the coagulated dispersion, can be made to occur of their own accord if a conveying screw is used.
The gap width may vary within a relatively wide range, depending on the dispersion to be coagulated and the product quality desired. Gap widths of from about 0.05 to 20 mm give good results, and even if the gap width is in the lower region no susceptibility to clogging of the apparatus is found. Typical gap widths which may be mentioned f
Barghoorn Peter
Czauderna Bernhard
Ehrmann Gerd
Grabowski Sven
Güntherberg Norbert
Boykin Terressa M.
Keil & Weinkauf
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