Two-step method for dehydrating plastic dispersions

Details Two-step method for dehydrating plastic dispersions Two-step method for dehydrating plastic dispersions

1999-12-13

2001-09-11

Hruskoci, Peter A. (Department: 1724)

Liquid purification or separation

Processes

Making an insoluble substance or accreting suspended...

C210S737000, C210S738000, C264S102000, C264S211230, C366S085000

Reexamination Certificate

active

06287470

ABSTRACT:

This invention relates to an improved process for dewatering resin melts containing water.
STATE OF THE ART
EP-A 0 534 235 describes a process for preparing high-impact strength modified thermoplastics. A resin latex is first coagulated by adding a coagulant (calcium formate solution) at 90° C. The precoagulate with a solids content of about 60 wt. % is then dewatered mechanically in an extruder. One of the advantages of the process lies in the removal of a portion of the water contained in the rubber. This achieves a higher throughput of rubber with simultaneously reduced energy consumption.
EP-A 0 006 503 describes the preparation of a coagulate from a graft polymer of a polybutyl acrylate latex by precipitation by adding a calcium chloride solution at 95° C.
U.S. Pat. No. 4,602,083 describes a process for the coagulation of polymers containing water by adding water-soluble non-oxidizing salts such as calcium hypophosphite or zinc methanesulfonate, for example.
When adding precipitants, there is generally the problem that the quality of the resin may be impaired since the auxiliary can cause unwanted side reactions, such as yellowing, for example. EP-A 0 683 028 describes a process for dewatering a two-phase fluid mixture of a thermoplastic resin melt and a water phase in a counter-rotating twin-screw extruder. The coagulation of the resin latex in this case occurs under the shear action in the coagulation zone at a temperature in the thermoplastic range of the resin. Alternatively, a precoagulate can also be used. The melt is then transported in partially filled screw grooves and is blocked into a cohesive melt cake in at least one of these screw grooves with the formation of a locally narrowly limited steep pressure gradient. In this way, the water flows downward in front of the boundary of the melt cake under the action of gravity, so that the melt cake is not in contact with a cohesive water phase. Using this process, the water content of an emulsion polymer with an initial water content of 55 wt. % can be reduced to only 8 wt. % water. The residual volatile fractions can then be largely separated in a degassing extruder through a forward and backward degassing zone. The granulate taken off at the granulation nozzle finally has a residual moisture content of only 0.06 wt. %.
EP-A 0 423 759 describes a process for preparing a particulate polymer in which a polymer latex is mixed with a coagulant. An organic solvent in which the polymer is insoluble, for example n-heptane, is then added to the mixture, whereby granular polymer particles are formed in the polymer slurry. The process is characterized by the fact that at least one of the mixing steps has to occur in a double-screw extruder with co-rotating screws.
PROBLEM AND SOLUTION
EP-A 0 683 028 makes possible very good dewatering of two-phase fluid mixtures of a thermoplastic resin melt and an aqueous phase. For example, water contents of only about 8 wt. % can be reached before transfer to the degassing extruder step. However, it has been found that the temperatures needed for coagulation in the coagulation zone of the extruder and the resultant extremely high temperatures on the outer cylinder wall of up to 350° C. can cause extreme materials stress. The energy has to be introduced by heat conduction through the cylinder wall. The heat flow density, of course, is limited by finite material strength, that must not be exceeded by thermal stresses. The thermal stresses occur because of the temperature gradient from the outer cylinder wall to the inner cylinder wall necessary for the heat transfer. This is all the more problematical when the cylinder has to be made of corrosion-proof steel to prevent corrosion, since high-alloy corrosion-resistant materials usually have poor heat conductivity. Because of the poor thermal conductivity, the limited material strength, and the finite heat exchange area, there are therefore limits on coagulability and thus on total product throughput. Especially, however, the problem exists that system wear is very high in counter-rotating twin-screw extruders. This causes a short service life of the cylinder and of the built-in extruder screws.
Therefore, the problem was seen to be to develop an improved process for dewatering two-phase fluid mixtures of a thermoplastic resin phase and an aqueous phase, with which extreme material stress on the coagulation extruder is avoided to the greatest possible extent. On the other hand, the total throughput of the resin melt to be dewatered should be as high as possible. The dewatering capacity should also be at least equal to or greater than in the process of EP-A 683 028. The process should also lead to the smallest possible residual polymer contents of the wastewater, since these are undesirable. At the same time, process steps such as adding precipitants for latex coagulation should also be avoided.
The problem has been solved by a process for dewatering a two-phase fluid mixture of a thermoplastic resin phase and a water phase by
a) coagulating the two-phase fluid mixture in a first extruder
b) dewatering the coagulate in a twin-screw extruder with counter-rotating screws with a dewatering zone
c) separating volatile constituents by degassing characterized by the fact that
a single-screw extruder or a twin-screw extruder is used in step a) as the first extruder, with the twin-screw extruder being equipped with co-rotating screws.
It was found, surprisingly, that the separation of process steps a) and b), which are performed in one step in EP-A 0 683 028, leads to an especially effective process when a coagulate is first produced in step a) in a single-screw extruder or in a twin-screw extruder with co-rotating screws, before performing the actual dewatering and degassing in steps b) and c). Producing the coagulate then occurs very effectively since the energy input necessary for coagulation can occur essentially by dissipation (shear). Since the coagulation is already effective in step a), in turn a lower and more readily controllable and thus more exact temperature setting is possible in step b). Coagulation in the extruder in process step a) is preferably performed at a melt temperature at least 30° C. higher than the subsequent dewatering in process step b), and it is especially preferred to be performed with a higher screw speed. This leads to lower material stress, particularly in the cylinder of the dewatering extruder, and at the same time makes possible better control and more stable operation of the dewatering step b). Overall, more effective dewatering of the two-phase mixture of the thermoplastic resin melt and the water phase is therefore achieved than in the process according to the state of the art. In this way, at least 94% of the aqueous phase can be separated in liquid form at the end of process step b).
FIELD OF USE OF THE INVENTION
The process pursuant to the invention is generally suitable for dewatering two-phase fluid mixtures of the thermoplastic resin phase and a water phase. For example they can be emulsion polymers for polymethyl methacrylate molding compositions (e.g., see EP-A 245 647) or latices, for example such as high-impact strength modifiers.
Corresponding listings of such two-phase mixtures that can be dewatered can be found, for example, in EP-A 0 534 235 or in EP-A 0 006 503.
Latices usually contain 30 to 50 wt. % dispersed resin particles whose average particle sizes, for example, can be from 100 to 500 nm. The water phase accordingly amounts to 70 to 50 wt. %; it generally contains dissolved emulsifiers or other auxiliaries and extraneous substances. The latex particles consist of thermoplastic resins that can be processed in an extruder in the molten form. Among them are thermoplastic resins with glass transition temperatures of 20 to 150° C. and a temperature range in the molten condition in which they are sufficiently resistant to decomposition. The melt temperature during processing in an extruder is usually between 50 and 250° C.
Important classes of thermoplastic resins are copolymers based on styrene,

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