Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2002-04-10
2003-09-02
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...
C264S176100, C264S328100, C264S349000, C525S063000, C525S064000, C528S480000, C528S50200C
Reexamination Certificate
active
06613871
ABSTRACT:
PRIOR ART
German Patent 3842796 C2 describes clear impact-resistant molding compounds comprising A) 10 to 90 wt % of a hard phase, formed from 80 to 100 wt % (relative to A) of methyl methacrylate and 20 to 0 wt % of a lower alkyl acrylate, B) 90 to 10 wt % of a elastomeric phase, distributed in the hard phase, with a glass transition temperature below −10° C., an average particle size of the elastomeric phase smaller than 130 nm and an inhomogeneity of the elastomeric-phase particle size of less than 0.5, formed from at least 50 wt % (relative to B) of a lower alkyl acrylate and 0.5 to 5 wt % of a graft cross-linking agent or of a cross-linking monomer with three or more ethylenically unsaturated groups capable of radical polymerization and, if necessary, with further ethylenically unsaturated aliphatic comonomers capable of radical (co)polymerization, and C) not more than 0.05 wt % of water-soluble constituents, wherein at least 15 wt % of the hard phase is covalently bonded to the tough phase. Processing of the emulsion polymer to the molding compound takes place in the extruder, especially in a twin-screw vented extruder, wherein the water phase is separated in liquid form. Details about the polymer concentration in the wastewater are not provided.
European Patent A 0683028 describes a method for dewatering a two-phase liquid mixture comprising a thermoplastic plastic melt and an aqueous phase in a counter-rotating twin-screw extruder. Therein coagulation of the plastic latex can take place directly in the extruder under the shear action in the coagulation zone at a temperature in the thermoplastic range of the plastic. The melt is then conveyed in partly filled screw channels, and compressed to a cohesive melt cake in at least one of these screw channels while developing a steep, local narrowly limited pressure gradient. In this way the water drained off from the boundary of the melt cake flows downward under the influence of gravity in such a way that the melt cake is not in contact with a coherent water phase. When this method is used, the water content of an emulsion polymer having a starting water content of 55 wt % can be reduced, for example, to only 8 wt %. The residual quantities of volatile constituents can then be largely separated in a vented extruder by means of a forward-and-backward vent zone. The granules discharged at the granulating nozzle ultimately have a residual moisture content of only 0.06 wt %. In the case of three-phase emulsion polymers, residual polymer concentrations in the range of 0.4 wt % are found as organic substance in the pressed-out water.
German Patent 19718597 C1 describes a method for dewatering a two-phase liquid mixture comprising a thermoplastic plastic phase and an aqueous phase by coagulating the two-phase liquid mixture in a first extruder, dewatering the coagulated substance in a twin-screw extruder with counter-rotating screws with a vent zone, and separating volatile constituents by devolatilization, wherein a single-screw extruder or a twin-screw extruder is used as the first extruder, this twin-screw extruder being equipped with screws rotating in the same direction. In the case of three-phase emulsion polymers, residual polymer concentrations in the range of 0.35 wt % are obtained in the pressed-out water.
OBJECT AND ACHIEVEMENT
Dewatering of two-phase liquid mixtures comprising a thermoplastic plastic melt and an aqueous phase by coagulating the plastic melt and squeezing the water out of the melt cake in an extruder system suffers from the problem that the wastewater often has an undesirably high polymer concentration. This problem occurs especially in emulsion polymers with small particle size, since they usually can be coagulated only poorly. In emulsion polymers with relatively large particle diameters, which otherwise can be coagulated and during standard operation exhibit only low polymer concentrations in the wastewater, the problem also exists if the throughput is increased. In this case the coagulation efficiency generally deteriorates, with the result that the polymer concentration in the wastewater increases once again. This constitutes a particular disadvantage, since the separated aqueous phase thus contains the polymer as an impurity and cannot be used further. The polymer content in the wastewater leads to high loads (COD values) on the clarifying plant during wastewater treatment. Furthermore the polymer loss reduces the yields of the thermoplastic plastic melt. It was therefore necessary to find a method that leads to a reduction of the polymer concentration in the wastewater.
The object is achieved by a method for reduction of the polymer content in the wastewater from dewatering of two-phase liquid mixtures comprising a thermoplastic plastic melt and an aqueous phase by coagulating the plastic melt and squeezing out the water from the melt cake in an extruder system, characterized in that superheated steam with a temperature of at least 280° C. is injected immediately after introduction of the two-phase liquid mixture into the extruder system.
OPERATION OF THE INVENTION
The method of the invention is suitable for reduction of the polymer concentration in the wastewater from dewatering of two-phase liquid mixtures comprising a thermoplastic plastic melt and an aqueous phase by coagulating the plastic melt and pressing the water out of the melt cake in an extruder system.
The method is suitable in particular for emulsion polymers with an average particle size of 50 to 130 nm (weight-average value), for example as are described generally in German Patent 3842796 C2 as well as specifically in the examples. The inhomogeneity of the particle size, can, for example, be less than 0.2. The emulsion polymers in this case comprise latex particles with a thermoplastic hard phase and a cross-linked elastomeric phase, which can be used as two-shell impact modifiers.
The method is also suitable for emulsion polymers that can otherwise be readily coagulated and that have average particle sizes of larger than 130 nm up to 500 nm, especially from 250 to 400 nm, when the systems are operated at higher throughput and as a result the polymer concentrations in the wastewater increase undesirably once again. Worth mentioning in particular here are emulsion polymers for impact modifiers of three-shell structure for polymethyl methacrylate, as described generally in European Patent A 0683028 and German Patent 19718597 C1 and specifically in the examples. If the polymer throughput in the methods described in European Patent A 0683028 and German Patent 19718597 C1 is increased, for example doubled, the heat input and in turn the coagulation efficiency usually deteriorates. Thereby the initially very low polymer concentrations in the wastewater increase to levels higher than 1 wt % or even higher than 2 wt %. In combination with the present invention, the values even at high throughputs can be lowered once again to levels below 1 wt %, especially below 0.5 wt %.
The method is therefore suitable in particular for latices. These usually contain 30 to 50 wt % of dispersed plastic particles, whose average particle size can be, for example, 100 to 500 nm. Accordingly the aqueous phase corresponds to 70 to 50 wt %; it generally contains dissolved emulsifiers, and possibly coagulating agents or other auxiliary and foreign substances.
The latex particles comprise thermoplastic plastics that can be processed in an extruder in molten condition. These comprise thermoplastic plastics with glass transition temperatures of 50 to 300° C. or a temperature range in the molten condition in which they are sufficiently resistant to decomposition. The melt temperature in the twin-screw extruder usually lies between 100 and 250° C.
Important classes of thermoplastic plastics are copolymers based on butadiene, styrene and possibly acrylonitrile, as well as polyvinyl chloride, polyacrylates or polymethacrylates. A further important class includes latices of multi-phase thermoplastic plastics containing latex particles with a thermoplastic hard ph
Hoess Werner
Sado Krzysztof
Schikowsky Hartmut
Boykin Terressa M.
Roehm GmbH & Co. KG
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