WO2007068647A1 - Method for the treatment of powder or granulate of a thermoplastic material or a rubber in a screw-type machine - Google Patents

Abstract

Disclosed is a method for treating powder or granulate of a thermoplastic material or a rubber which is semicrystalline or amorphous and can be heated by means of a high-frequency electric field in a screw-type machine. According to said method, the powder or granulate of the thermoplastic material or rubber is preheated to a temperature lying below the crystallite melting point of semicrystalline plastics or rubbers and below the melting range of thermoplastic materials or rubbers before being fed to the screw-type machine, is fused and homogenized in the screw-type machine, and is then pressed through an extrusion nozzle and granulated or processed into molded products. The inventive method is characterized in that the thermoplastic material or rubber is preheated by means of a high-frequency electromagnetic field.

Description

Process for treating powder or granules of a thermoplastic or rubber in a screw machine

The invention relates to a method for treating powder or granules of a thermoplastic or a rubber, which can be heated by a high-frequency electric field, in a screw machine, wherein the powder or granules of the thermoplastic material or the rubber before being fed to the screw machine on a Temperature below the crystallite melting temperature in semi-crystalline thermoplastics or rubbers and preheated below the Schmelzberei- ches in amorphous thermoplastics or rubbers, the screw machine is melted and homogenized and then pressed through an extrusion die, granulated or processed into moldings.

As screw machines apparatuses are referred to in a known manner, having one or more screws for melting, conveying and pressure build-up in a polymer. Continuously operating screw machines are generally referred to as extruders. Further screw machines are, for example, injection molding machines which operate semicontinuously.

The granulation of thermoplastics or rubbers is known and serves to homogenize the thermoplastics or rubbers and possibly to incorporate additives and additives such as stabilizers, colorants, agents for improving the mechanical properties, fillers and the like in the thermoplastics or rubbers. In addition, the handling of the thermoplastics or rubbers during transport and further processing by granulation can be considerably improved compared to the handling of powder.

In addition to the direct coupling of polymerization and granulation, in which the powder of the thermoplastic or rubber often still has residual heat from the polymerization process and is therefore fed to the extruder at elevated temperature, the addition of powder is especially in compounding the thermoplastic or rubber to the screw machine at a temperature which corresponds to the ambient temperature, is common. This is due, in particular, to the intermediate storage of the powder in silo plants and to the transport methods via pneumatic conveying systems, with generally complete cooling of the powder to ambient temperature occurring.

In compounding, powders of the thermoplastic or rubber of the screw machine are generally supplied as bulk material at ambient temperature. The powder must be melted after the feed zone of the screw machine: this takes place for the most part on the drive energy the screw machine, which is dissipatively converted into heat energy. The melting takes place predominantly by the conversion of mechanical energy into heat and is supported, especially in large screw machines only to a lesser extent, by an external heating. Nevertheless, an external heater for starting the screw machine is necessary.

The melting of the powder on the drive energy of the screw machine means a high energy expenditure and, moreover, the material is subjected to a high shear and tensile stress during the melting process, which can lead to degradation and / or crosslinking reactions depending on the chemical nature of the thermoplastic or rubber ,

Furthermore, especially with large extruders there is the danger of the formation of so-called semolina nests, that is to say areas of unmelted material which emerge from the screw machine in this state.

The above disadvantages associated with melting powder of a thermoplastic or rubber in the screw machine supplied at low temperature, especially at ambient temperature, can be largely reduced by subjecting the powder to a heat treatment prior to feeding it to the extruder becomes. Such a method is proposed in WO-A 01/70473. In particular, the powder should be heated with steam and then dried with hot air or passed through an externally heated tube.

It was accordingly an object of the invention to provide a method according to which the feed stream to the screw machine, in particular even at high mass flows of the same, compared to known methods warmed up uniformly, and thus in particular an improved product quality is achieved. In addition, the warming up of thermoplastic or rubber in granular form is also to be made possible.

The invention consists in a process for treating powder or granules of a thermoplastic or a rubber which is partially crystalline or amorphous and which can be heated by a high-frequency electric field, in a screw machine, wherein the powder or granules of the thermoplastic material or the rubber prior to feeding to the screw machine to a temperature below the crystallite melting point of semicrystalline plastics or rubbers and below the melting range of thermoplastics or rubbers, melted in the screw machine and homogenized and then pressed through an extrusion die, granulated or processed to form parts, which is characterized It is characterized in that the preheating of the thermoplastic or rubber is effected by means of an electromagnetic high-frequency field.

It has been found that it is possible to heat thermoplastics or rubber very evenly by means of a high frequency electromagnetic field. For this it is a prerequisite that the thermoplastic material or rubber can be heated by an electromagnetic high-frequency field. These include, in particular, those which have polar structures, that is to say a dipole moment greater than 0. It may, for example, be individual structural chemical units of the thermoplastic or rubber, for example CH ₂ O units in polyoxymethylenes.

The polar structures make it possible to polarize the polarization of the powder or granules by means of an electromagnetic high-frequency field, in particular by microwave radiation. Other possible heating mechanisms for the powder or granules are electrical resistance heating, heating by Maxwell-Wagner effect or electron polarization.

Thermoplastics with polar structures which are directly heatable in a radio frequency electromagnetic field include e.g. Polyamide (PA), polyetheretherketone (PEEK), polyethylene terephthalate (PET), polyvinylchloride (PVC), polyurethane (PU), polyoxymethylene (POM), polybutyleneterephthalate (PBT), polyethersulphone (PES), poly-n-butylmethacrylate (PBMA) , Polymethyl methacrylate (PMMA), polyimides or styrene acrylonitrile (SAN). Rubbers with polar structures are in particular chloroprene rubber, ethylene-acrylate rubber, ethylene-vinyl acetate rubber, chlorosulfonated polyethylene rubber, chlorinated polyethylene rubber, acrylate rubber, fluorine rubber, oxygen rubber or silicone rubber.

In this context, a high-frequency electromagnetic field is any electromagnetic field suitable for the dielectric heating of a powder or granulate. The electromagnetic high-frequency field used in the method according to the invention preferably has a frequency in the range of 100 kHz to 900 GHz. According to a preferred embodiment of the present invention, the high frequency field has a frequency in the range of 0.3 to 300 GHz. This is microwave radiation, which is preferably used for preheating. For example, microwave radiation has been used in the art for microwave welding of polymers, thermoforming of polymer parts or hot drawing of polymer strands, rods or fibers. Such applications are described, for example, in H. Potente et al., "Microwave Field Heatability - Microwave Welding Studies", Welding and Cutting 55 (11), 616-623 (2003), M. Chen et al., "Basic Ideas of Microwave Processing of Polymers "Polymer Engineering and Science, 33 (17) 1092-1109 (1993); Y. Takeuchi, "Morphological changes of wide polyoxymethylene rod during a microwave heating drawing process", Polymer, 26 (13) 1929 to 1934 (1985) or in EP 0 084 274 B1 or DE 197 18 505 A1.

DE 10 2005 032 700.1 describes the use of an electromagnetic high-frequency field for reducing stresses in the powder or granules from a polymer melt.

According to a preferred embodiment of the present invention, the powder or granules are preheated in the post-treatment continuously by the application of microwave radiation. Suitable microwave heaters are known in the art. For example, a microwave heating device with variable frequency can be used for the method according to the invention, as described in the document EP 0 801 879 B1.

The design and operation of the microwave sources are preferably adjusted in the inventive method on the material properties and on the shape of the thermoplastic material or rubber. In order to avoid local overheating, the power input can be distributed over several microwave devices, possibly also by cyclically switching them on and off.

According to a preferred embodiment of the present invention, the microwave source used is a round waveguide which concentrically surrounds the powder or granules. Here, a particularly favorable distribution of the microwave field is given, which leads to a particularly uniform heating.

Preferably, a heat output is introduced into the powder or granules by the high-frequency field, which is controlled by means of a non-contact sensor that detects changes in properties of the powder or granules during preheating. Such property changes that can be detected by the sensor are, for example, changes in temperature or refractive indices of the powder or granules.

The irradiation density of the electromagnetic high-frequency field can be constant during the preheating of the powder or granules or can be controlled in a time-dependent manner (for example stepwise, sinusoidally or linearly increasing or decreasing).

The symmetry of the high-frequency field can be adapted to the geometry of the powder or granules. The powder or granulate preferably passes the electromagnetic high-frequency field continuously in the method according to the invention. Prefers the high frequency field is designed so that the most homogeneous possible heating of the powder or granules occurs in the high frequency field.

The preheating of the powder or granules of the thermoplastic or rubber is advantageously carried out at a temperature in the range of 5 to 30 K below the crystallite melting point for semicrystalline plastics or rubbers and below the melting range for amorphous thermoplastics or rubbers. Particularly preferably, the upper limit of the preheating temperature is in the range of 10 to 20 K below the crystallite melting temperature for semicrystalline plastics or rubbers and below the melting range for amorphous thermoplastics or rubbers.

In processes for treating powder or granules of a thermoplastic or rubber, the screw machine is frequently additionally supplied with fillers or reinforcing materials. Advantageously, the same, before being supplied to the melting machine in an electromagnetic high-frequency field, in particular by means of microwave radiation, are preheated. With regard to the upper limit of the temperature, which must be observed here, it should be noted in particular that the preheated fillers or reinforcing materials do not damage the thermoplastic or rubber in the melting machine or are themselves damaged.

The material supplied to the screw machine in the form of powder or granules of a thermoplastic material or a rubber is melted and homogenized in the screw machine and then pressed through an extrusion die and granulated or pressed through an extrusion die and processed into a molded part. As a molded part, a semi-finished product, for example a pipe, hose or plastic profile or a finished part is referred to in a known manner.

In the method according to the invention, the screw machine is preferably an extruder.

The thermoplastic used in the process according to the invention is preferably polyoxymethylene, a polyamide or a polyester.

The inventive method has a number of advantages. The preheating in the electromagnetic high-frequency field requires a small amount of time and energy consumption, since the heat energy is generated immediately inside the powder or granules by means of the electromagnetic high-frequency field. The energy input can be selectively controlled, for example by varying the number and power of microwave sources. The sources can be positioned so that the energy is entered as evenly as possible in the powder or granules. By the method according to the invention, thermally homogeneous melts are achieved; they show a more uniform flow behavior, which proves to be particularly advantageous in the flow through Granulierlochplatten, with the result of a more uniform shape and a narrower size distribution of the granules obtained as a product.

Claims

claims Anspruch [en] A process for treating a powder or granules of a thermoplastic or a rubber which is semi-crystalline or amorphous, and which is heatable by a high frequency electric field, in a screw machine, wherein the Powder or granules of the thermoplastic material or of the rubber before being fed to the screw machine are preheated to a temperature below the crystallite melting temperature in semicrystalline plastics or rubbers and below the melting range in the case of amorphous thermoplastics or rubber, melted and homogenized in the screw machine and then through an extrusion die is pressed and granulated or processed into shaped parts, characterized in that the preheating of the thermoplastic or rubber takes place by means of an electromagnetic high-frequency field. 2. The method according to claim 1, characterized in that the electromagnetic high frequency field generates microwave radiation. 3. The method according to claim 1 or 2, characterized in that the screw machine filling and reinforcing materials, of which at least a part is heated by a high-frequency electric field supplied, melted in the screw machine and mixed in the melt of the thermoplastic material or rubber , 4. The method according to claim 3, characterized in that the filling and reinforcing materials are preheated prior to the supply thereof to the screw machine in an electromagnetic high-frequency field. 5. The method according to claim 3, characterized in that the heating is effected by entry of microwave radiation. 6. The method according to any one of claims 1 to 5, characterized in that the preheating of the powder or granules of the thermoplastic material or of the rubber to a temperature in the range of 5 to 30 K below the Kristal- litschmelztemperatur at partially crystalline plastics or rubbers and below the Melting range in amorphous thermoplastics or rubbers takes place. 7. The method according to claim 6, characterized in that the preheating of the powder or granules of the thermoplastic material or of the rubber a temperature in the range of 10 to 20 K below the Kristallitschmelztempera- temperature in semicrystalline plastics or rubbers and below the melting range in amorphous thermoplastics or rubbers takes place. 8. The method according to any one of claims 4 or 5, characterized in that the fillers and reinforcing materials are preheated to a temperature at which they do not damage the thermoplastic material or the rubber or even thermally damaged. 9. The method according to any one of claims 1 to 8, characterized in that the screw machine is an extruder. 10. The method according to any one of claims 1 to 9, characterized in that the thermoplastic material is a polyamide or a polyester.