Counter-rotating twin screw extruder

The invention relates to a counter-rotating twin screw extruder for compounding polymers.

When producing a polymer composition, the ingredients thereof, as different polymers, fillers and additives, as anti-oxidants, light stabilizers, etc. have to be mixed intimately in order to obtain a composition as homogenous as possible. This is done by compounding the ingredients in a compounding machine as a counter-rotating twin-screw extruder.

While on one hand, the compounding should be carried out at a high temperature and shearing rate in order to achieve a homogenous composition, degradation of the polymers is caused by too severe conditions.

Particular problems are encountered when compounding multimodal polymers, as multimodal polyethylene materials. Multimodal polymers are in many respects superior to corresponding mono-modal materials. Multimodal polyethylene materials and, more particularly, bimodal polyethylene materials have a widespread and increasing use as materials for various applications as pipes, wires and cables, films, blow-molded and injection-molded articles, etc.

Multimodal polymer compositions such as bimodal polyethylene materials consist of a low molecular weight polymer fraction and a high molecular weight fraction. The high molecular weight molecules are known to be most sensitive to the compounding conditions needed to achieve the desired degree of homogenization.

For instance, undispersed domains of high molecular weight molecules appear as white spots in colored materials. The white spots may adversely affect the strength of the article. Further, when compounding polymer compositions, e.g. for the production of a film, gel particles appear as disfiguring spots in the finished film which consist of high molecular weight polymer not adequately compounded. Although compounding at higher temperatures and shear rates may remove the white spots and gel particles, degradation of the high molecular weight molecules may occur which negatively effects the otherwise superior properties of the multimodal polymer material.

Thus, the white spots and gel particles are a serious problem in the polymer industry and a solution of the problem would mean the removal of a serious obstacle to use otherwise superior multimodal polymer compositions.

In EP-A-645 232 is described a way of reducing this problem by adding liquid nitrogen or solid carbon dioxide to the polymer feed. This is, however, a rather costly way. According to WO 98/15591, the problems may be tackled by compounding at a low shear rate so that the temperature of the polymer increases slowly. This requires a highly precise control of the process conditions of the counter-rotating twin screw extruder, however, and the production capacity is rather low.

U.S. Pat. No. 6,409,373 discloses a counter-rotating twin-screw extruder having a mixing section upstream of and a mixing section downstream of a throttle valve or gate plates, each mixing section comprising forward-conveying wings and backward-conveying wings downstream of the forward-conveying wings. Although in a monomodal polymer material the number of gels may be reduced, it is not possible to obtain multimodal polymer materials of high homogeneity without adversely affecting the superior quality of multimodal polymer materials with the known extruder.

PATENT ABSTRACTS OF JAPAN vol. 0050. no. 85 (C-057); 3 Jun. 1981-06-03) & JP 56 031433 A discloses a twin screw extruder having a throttle element between two supply sections with two supply ports, each supply section having a conveying section with screws and a mixing section.

PATENT ABSTRACTS OF JAPAN vol. 0060, no. 11 (C-088), 22 Jan. 1982 (1982-01-22) & JP 136633 A discloses a twin screw extruder according to the pre-amble of claim 1.

U.S. Pat. No. 6,280,074 discloses a twin screw extruder having screws with wings in the shape of “V” to form backward- and forward-conveying movements.

It is an object of the invention to obtain multimodal polymer materials of high homogeneity with a high production capacity at low cost.

This object is attained with the counter-rotating twin-screw extruder according to claim 1 for compounding.

The counter-rotating twin extruder according to the invention comprises a mixing section having at least two mixing zones, each mixing zone consisting of at least one, preferably at least two forward-conveying wings and at least one, preferably at least two backward-conveying wings downstream of the forward-conveying wings on each rotor.

The number of forward-conveying wings and the number of backward-conveying wings of each mixing zone preferably corresponds to the number of flights of the screws of the conveying section upstream of the mixing section. For instance, in each mixing zone two, three or four forward-conveying wings and backward-conveying wings, respectively, may be used.

To adjust the filling degree of the mixing section, a throttle valve or gate is usually provided between the mixing section and the discharge port of the extruder. Instead of a throttle valve, a gear pump connected to the discharge valve may be used to control the filling degree in the mixing section. As a throttle valve, a rotary slot bar may be used as disclosed in JP-A-3004647. In that solution two bars extend across the rotors which have a convex side rotating in a concave depression in the barrels, the throttle gap being defined by the distance between the rotors and the throttle edge of the bars.

The twin screw extruder of the present invention has two mixing sections, namely one upstream of the throttle valve and one downstream thereof. Whereas the first mixing section upstream of the throttle valve comprises at least two mixing zones, each mixing zone having at least two forward-conveying wings and at least two backward-conveying wings, the second mixing section downstream of the throttle valve has preferably a lower number of mixing zones, for instance only one mixing zone when the first mixing section has two mixing zones.

In the first mixing section, mainly dispersive mixing takes place, so that the particles of the powder material to be mixed are broken up, whereas in the second mixing section, mainly distributive mixing occurs. That is, in the first mixing zone of the first mixing section next to the screws of the conveying section, the polymer material starts to melt and continues to melt in the second mixing zone of the first mixing section, and then the polymer melt is charged through the throttle valve into the second mixing section. There, the melt is kneaded further to distribute the different polymers and optionally, fillers, additives and so forth homogeneously in the melt. Due to its high dispersive mixing efficiency of the at least two mixing zones of the first mixing section, the twin screw extruder of the invention is particularly effective for compounding multimodal polymer materials.

From the second mixing section, the polymer melt is discharged through the discharge port into a gear pump or a discharge extruder. From the gear pump or the discharge extruder, the melt is passed through a die plate, after which it is cooled and cut to pellets. Alternatively, the melt is discharged through a discharge port directly after the first mixing section.

Whereas the filling degree of the first mixing section is determined by the throttle valve, the filling of the second mixing section downstream of the throttle valve may be adjusted by the suction side pressure of the gear pump.

Preferably, the upstream ends of the forward-conveying wings of the first mixing zone of the first mixing section are positioned at the downstream end of the screws of the first conveying section, and it is also preferred that the upstream ends of the forward-conveying wings of the mixing zone of the second mixing section are positioned at the downstream ends of the of the screws of the second conveying section. That is, the screws of the first and second conveying sections and the forward-conveying wings of the first zone of the first mixing section and of the mixing zone of the second mixing section, respectively, preferably are positioned to form continuous closed flights.

In the first mixing section, the downstream end of the forward-conveying wings and the upstream end of the backward-conveying wings of each mixing zone are preferably offset to form a passage for the material to be mixed. Due to these passages, a mixing action in axial direction occurs, and it is avoided that material, in particular unmelted material, is pressed between the barrels and the wings on the rotors, which would cause a deflection of the rotors resulting in a non-uniform mixing due to a non-uniform gap between the wings and the barrels.