There are various options for forming the foam when extruding foamed articles made from thermoplastic. For example, it is possible to mix the plastic, before extrusion, with a chemical foaming agent which decomposes in the extruder, such as for example azodicarbonamide.

Another option is firstly to melt the plastic in the extruder and then to add a chemical or physical foaming agent and to mix this with the molten mass. In order to obtain a sufficiently homogeneous plastic foam with small cells, which generally has better mechanical qualities than in homogeneous or coarse plastic foam, it is essential for the plastic to be intimately mixed with the foaming agent.

Especially when using a physical foaming agent, which may be added in the form of a liquid (such as pentane or a chlorofluorohydrocarbon), a gas (such as nitrogen) or a supercritical liquid (such as carbon dioxide), it is desirable to obtain a molten mass which is as homogeneous as possible and in which the foaming agent is dissolved in the plastic. Then, however the mixture was originally formed, it is necessary, by changing the thermodynamic equilibrium, to bring the foaming agent out of solution so that, as a gas, it can form the foam in the mixture, which is then a two-phase mixture. This process is referred to as "nucleation".

To form a so-called microfoam, with cell dimensions of 50 gm and smaller, it is important to start from a very homogeneous initial molten mass which is at the correct temperature and pressure, and it is also important for the thermodynamic equilibrium to be changed very rapidly.

A method of the type described in the introduction is known from WO 98/08667. In this publication, it is stated (Fig. 5 and the associated description on pages 25 et seq) that the nucleation of the molten mass takes place in an

extrusion head, by allowing the molten mass to flow through a large number of channels, resulting in a substantial pressure drop. Then, the molten material is guided through the remaining section of the extrusion head and the plastic, which during this time has been foamed, is cooled.

It has been found that this method does not lead to the desired result. It does not produce a very fine microfoam, but rather a coarse foam with irregularly shaped cells.

This problem can be explained by the fact that the foam which is formed in and immediately downstream of the nucleator, as it flows through the extrusion head, is subjected to shear forces which are so great that the foam structure formed is damaged. The walls between the foam cells, which are still hot and consequently relatively weak, can easily crack under the influence of the shear forces, resulting in the formation of larger and less regular foam cells. Moreover, the small part-streams of foamed plastic which are formed in the nucleator are not welded together to a sufficient extent. This too gives rise to large, irregularly shaped foam components between the small part-streams.

WO 98/08667 furthermore describes a method (Fig. 14 and the associated description on pages 31 et seq) in which a separate nucleator is not employed, but rather a simple narrowed section of the gap in the extrusion head serves as a nucleator. This method is only suitable for forming thin- walled articles, such as a wire insulation. When this method is used for products with thicker walls, however, it does not produce a microfoam.

The object of the invention is to provide a method for extruding foamed articles from thermoplastic, by means of which it is possible to extrude articles with wall thicknesses from one to several tens of millimetres and with a microfoam structure.

This object is achieved by means of the method according to claim 1.

It has been found that when this method is used, the drawbacks of the known method do not arise, and the extruded objects acquire a very fine microfoam structure.

Particularly good results are achieved if the measures according to claim 2 are employed.

The invention also relates to an apparatus for extruding foamed articles made from thermoplastic material according to claim 9.

Preferred embodiments of the method and apparatus according to the invention are defined in the dependent claims.

The invention is explained in the following description with reference to the drawing, in which: Fig. 1 shows a longitudinal section through the end part of an extrusion head with a nucleator mounted thereon, Fig. 2 diagrammatically depicts, on a larger scale, detail A from Fig. 1 with a specific embodiment of the nucleator, Figs. 3 and 4 diagrammatically depict, on a larger scale, detail A from Fig. 1 with another embodiment of the nucleator, with different flow patterns of the molten mass, Fig. 5 shows a highly diagrammatic plan view of an extrusion line for the extrusion of foamed articles, and Figs. 6 and 7 show foam structures which are formed respectively without and with the use of the method according to the invention.

Fig. 1 shows a longitudinal section through an end part of an extrusion head, which is denoted overall by 1, for forming a hollow section, in particular a pipe, from foamed plastic. The extrusion head 1 is intended to be mounted on an extruder (not shown here).

The extrusion head 1 comprises an outer cone 2 and an inner cone 3, which delimit an annular channel 4. A molten material which comes out of the extruder and comprises heated, pressurized plastic which is mixed with a foaming agent is pressed through this channel 4.

A nucleator assembly 5 is mounted on the outlet side of the extrusion head 1. This nucleator assembly 5 comprises an annular nucleator 6 and a nucleator holder 7, comprising two plates 8 and 9 between which the nucleator 6 is clamped in place. The entire assembly is attached concentrically to the extrusion head 1 by means of bolts

10.

The nucleator 6 comprises a multiplicity of fine channels and preferably comprises 1 or more screens with a mesh width of from 50-500 ym, preferably 100-300 ym. If appropriate, coarser screens may be arranged on the outlet side for reinforcement purposes. The number of screens is dependent on the viscosity of the molten material and on the pressure drop which is desired. In the case of a highly viscous molten mass, fewer screens are generally required than in the case of a low-viscosity molten mass.

The nucleator 6 may also comprise one or more perforated plates with holes of from 50-500 ßm, preferably 100-300 ym. The holes may be formed by means of laser perforation. The nucleator 6 will often be of planar design, since this leads to the most simple structure.

Other structures of the nucleator, for example those described in WO 98/08667, may also be used. When the molten mass comprising heated, pressurized plastic which has been mixed with a foaming agent is pressed through the nucleator 6, a foam structure with very small cells is formed in the molten mass. Since the nucleator 6 is located downstream, as seen in the direction of extrusion, of the orifice which is formed by the outer cone 2 and the inner cone 3 of the extrusion head 1 and shapes the article to be extruded, molten material which comes out of the nucleator 6 is now subjected to scarcely any shear forces which could damage the walls of the foam cells.

A particularly good and very fine microfoam structure is obtained if the molten material which comes out of the nucleator 6 is subjected to brief compression, during which it is ensured that no shear forces, or only slight shear forces, are generated in the molten mass. This brief compression, which helps the small streams of foamed plastic coming out of the nucleator 6 to weld together, can be realized in various ways.

For example, instead of a planar nucleator it is possible to use a curved nucleator, as shown in Fig. 2.

This nucleator 6 is of concave design on the outlet side.

As a result, the small streams of foamed plastic which come

out of the nucleator 6 are pressed forcefully against one another.

The welding together of the small streams of foamed plastic which come out of the nucleator 6 can also be improved by guiding the molten mass which comes out of the nucleator 6 through a narrowed section 11 which is relatively short, as seen in the direction of flow. The desired effect can only be achieved without damage to the foam structure if the narrowed section 11 is short. Good results are achieved with a narrowed section of a length of from 3-5 mm for a diameter of the extruded plastic pipe of 50 mm.

In Figs. 3 and 4, the narrowed section 11 is used in combination with a planar nucleator 6. However, it is also possible to employ the narrowed section with a curved nucleator, as shown in Fig. 2.

Another possibility for improving the welding together of the small streams of foamed plastic which come out of the nucleator 6 without damaging the foam structure consists in axially stretching the molten mass which comes out of the extrusion head 1 and the nucleator 6. This obviously causes the wall thickness of the extruded section to decrease, as depicted in Fig. 4. The axial stretching of the section may, if appropriate, be combined with guiding the extruded article through a narrowed section 11.

While the molten mass is being pressed through the nucleator 6, it is ensured that a pressure drop takes place in the nucleator at a rate of at least 0.05 GPa/sec, and preferably between 0.2 and 1.0 GPa/sec. The pressure itself has to fall from the pressure which prevails in the extrusion head 1, and is often between 10 and 50 MPa, to (virtually) atmospheric pressure.

The foamed, optionally stretched section which comes out of the extrusion head and the nucleator 6 is cooled in the conventional way in order to obtain the desired dimensional stability.

An extrusion line which is used in tests comprises two single-screw extruders 21 and 22 which are arranged in tandem and have a screw diameter of 120 mm, as extremely

diagrammatically depicted in plan view in Fig. 5. The first extruder 21 is used to plasticize the molten mass and to inject the foaming agent, in this case a gas (nitrogen or carbon dioxide). A feed hopper 23 for feeding the plastic material is mounted on the first extruder 21. The outlet of the first extruder 21 is connected to the second extruder 22 for cooling the molten material and dissolving the gas in the molten material. An extrusion head 24, which is normally used to extrude PVC pipes, is provided, on the outlet side, with a nucleator in the form of three screens with a mesh width of 250 ym and a wire thickness of 200 ym.

A narrowed section corresponding to Fig. 3, with a length of 4 mm, is arranged immediately downstream of the nucleator.

During the production of the extruded section, in this case a pipe, the extrusion line is followed by the conventional equipment such as vacuum calibration and cooling, stretching unit and saw.

The extrusion line described above is used to produce a polypropylene microfoam pipe with a diameter of 50 mm and a wall thickness of 4 mm. The conventional polypropylene grade was a standard copolymer. The foaming agent used was nitrogen. At a production rate of 50 kg/h of polypropylene, 0.75% by weight of nitrogen was used. The temperature of the mixture coming out of the first extruder 21 was approx.

210°C. In the second extruder 22, the mixture was homogenized and cooled to approx. 180°C. After leaving the extrusion head 24, the pipe was stretched by running the stretching unit at double speed. The diameter of the foam cells varied between 10 and 50 ym.

Figs. 6 and 7 show foam structures which are obtained respectively without and with the use of the method according to the invention. The enlargement factor is 25.

It can clearly be seen in Fig. 6 that if the method according to the invention is not used, the result is a coarse, irregular foam structure, while if the method according to the invention is used, the result is a very fine, regular foam structure.

It will be clear that the method and apparatus

described can be used for extrusion of various types of articles, such as pipes, sections, sheets, panels, pipe coatings, etc. The method can be used for all possible thermoplastic materials, such as polypropylene, polyethylene, polyvinyl chloride, polystyrene, etc.