GB2090559A

GB2090559A - Method and apparatus for the extrusion of cross-linked plastics products

Info

Publication number: GB2090559A
Application number: GB8137463A
Authority: GB
Original assignee: PROPLAST Oy
Current assignee: PROPLAST Oy
Priority date: 1980-12-19
Filing date: 1981-12-11
Publication date: 1982-07-14
Also published as: IT8149930A0; DE3149333A1; FR2500789A1; SE8107539L; ES508641A0; JPS57128529A; DK541781A; NL8105692A; ES8300570A1

Abstract

The apparatus comprises a main extruder (1-4) and an auxiliary extruder (5-8) placed at the outer end of the feeding screw (4) of the main extruder. By means of the main extruder (1-4), a plastics raw- material substantially free from cross- linking agents is extruded at an elevated temperature and, by means of the auxiliary extruder (5-8), a rather small quantity of a concentrated mixture of the molten plastic raw-material and of a cross- linking agent is injected into the molten plastic raw-material mass at a lower temperature. Into the mass extruded by means of the main extruder (1-4), some quantity of an activator accelerating the cross- linking and/or of an inhibitor retarding the cross-linking may be added. The injection is effected either adjacent the downstream end of the screw (4), or between the end of the screw (4) and a mixing means (14-18). <IMAGE>

Description

SPECIFICATION Method and extruder device for the manufacture of cross-linked plastic products The present invention relates to a method for the manufacture of cross-linked plastic products by means of an extruder device having a feeding screw with a thread portion, comprising: ~extruding a plastic raw-material substantially free from cross-linking agents at a first elevated temperature, ~injecting a relatively small quantity of a mixture of plastic raw-material and at least one cross-linking agent into the molten mass flow, ~allowing the plastic raw-material and the mixture to get mixed before leaving the extruder device.

The invention also relates to an extruder device for carrying out the method.

By means of this method and device, crosslinked plastic products can be manufactured out of thermoplastic resin.

The most common thermoplastic resin to be cross-linked is polyethene, whose molecular structure is reticulated by the effect of crosslinking, as a result of which the heat resistance of the product is considerably improved.

The cross-linking of plastic products to be manufactured by means of extrusion is to-day performed by quite a number of different methods.

The most inexpensive, and also most commonly used, method is cross-linking by means of peroxides, wherein an appropriate peroxide that does not decompose at the extrusion temperature is added to the plastic raw-material before or in connection with the extrusion process. After the shaping, taking place at the extrusion head, the product is heated to a temperature at which the peroxide decomposes, thereby producing crosslinking of the product.

A drawback of the process described above is its slowness. The melt-flow properties of a crosslinked material, even if it is only partly crosslinked, are very poor, because of which even partial decomposition of the peroxide must not take place in the extruder. It follows from this that the peroxide to be used must be such that it decomposes only at high temperatures, or the extrusion temperatures must be low. In both cases the temperature of the product must be raised extensively after the shaping in order that the peroxide should decompose at least with a reasonable speed. It follows from the well-known poor heat-conductivity of plastic materials that the cross-linking especially of objects with thick walls is slow and requires long heating lines after the extruder.

The object of the present invention is to eliminate the above drawbacks and to provide a more efficient and more practical extrusion method and device.

In the method in accordance with the invention, the cross-linking is still performed by means of peroxides but by means of appropriate arrangements, a situation is reached in which the extrusion temperature is sufficient for rapid completion of the cross-linking reaction and no additional heating is required after the extruder, or the need for additional heating is substantially reduced.

More specifically, the method in accordance with the invention is mainly characterized in that the injection is carried out at a location situated, as seen in the flow direction, near the outer end of the thread portion of the feeding screw and at a second elevated temperature, which is lower than the first temperature.

The extruder device according to the invention is mainly characterized in that the aperture is situated, as seen in the axial direction, between the outer end of the thread portion of the main screw and the mixing head.

The invention will be examined in more detail below, reference being made to the exemplifying embodiments in accordance with the attached drawings.

Figure 1 shows a partially sectional view of one extruder for carrying out the method in accordance with the invention.

Figure 2 shows a partially sectional view of a second extruder for carrying out the method in accordance with the invention.

Figure 3 shows a sectional view on an enlarged scale of the screw arrangement of the extruder according to Figure 2.

According to Figure 1, the plastic raw-material is dosed into the funnel 2 of the main extruder 1.

By the joint effect of the heat introduced through the cylinder wall 3 and of the shearing produced by the rotary movement of the feeding screw 4, the mix melts and becomes homogeneous. At the same time, an amount of she same raw-material is dosed into the funnel 6 oF an auxiliary extruder 5, placed near the outer end of the cylinder 3, together with an excess quantity of peroxide. By the effect of the auxiliary screw 7 and of heat introduced from outside, the mix melts and becomes homogeneous before it is introduced into the mix of the main extruder through the channel 8 situated near the outer end of the thread portion of the main extruder.

When the temperature of the mix of the main extruder 1 is now adjusted to a sufficiently high level and when the temperature of the mix of the auxiliary extruder 5 is kept so low that decomposition of peroxide does not takes place inside the auxiliary extruder 5, a mixture is produced whose temperature is determined on the basis of the temperatures of the materials fed by the main extruder 1 and the auxiliary extruder 5 in accordance with the ratio of respective material flows. After the mass flows have combined, the mixture is homogenized by the effect of the outer end of the feeding screw 4 of the main extruder 1, but in the equalization of the heat there is a slight delay, resulting from the slowness of the conduction of heat, during which delay the mass reaches the extrusion head 9.Now the crosslinking takes place fast, because the temperature of the mass is remarkably high.

A preferred embodiment of the invention is presented in Figures 2 and 3. According to this embodiment, the contact point 8, 11 of the two molten material flows is situated after the thread portion 12 of the main screw 4, as seen in the flow direction. By means of the arrangement, the disadvantageous effect of the thread on the peroxide-containing melt is eliminated.

It is known that two types of flow take place in the thread portion of an extruder: a forward flow due to the rotation of the screw, and a backward flow due to the counter pressure caused by the extrusion head. The sum of these flows carry the mass, as a whole, in the forward direction but the backward flow causes a considerable dispersion in the delaying times of different molten particles in the thread portion. If said peroxide-containing auxiliary flow hits the main flow at a point where a backward flow is taking place, it will remain much longer in the thread portion than an auxiliary melt portion hitting a forward flow component of the melt in the thread portion of the main screw.In order to prevent cross-linking within the extruder in such a case, the temperature of the main extruder mass must be kept lower than in a case where only forward flow takes place after the melt flows have combined.

Such a situation is achieved when the flow from the auxiliary extruder 5 is allowed to contact the flow in the main extruder at a point 8, 11 after the thread portion 12 of the main screw 4. At the contact point 8, 11, the main screw 4 is formed with a thinner, cylindrical shaft portion 10 defining, together with the cylinder wall 3, an annular mixing chamber 11.

The outer part of the main screw 4, following the thinner shaft portion 10 is a conventional mixing head 13 provided with axial inlet grooves 14 and outlet grooves 15.

The arrangement is such that the mass in the mixing chamber 1 1 can enter the inlet grooves 14 only. Between each one inlet groove 14 and one adjacent outlet groove 15 there is a peripheral passage 17 allowing the mass in each inlet groove 14 to enter one adjacent outlet groove 1 5.

Between each pair of inlet and outlet grooves 14, 1 5 there is an axial ridge 1 6 in sliding contact with the inner surface of the cylinder 3, thus preventing any flow between the different pairs of inlet and outlet grooves 14, 15. The peripheral passage 17 has a small radial height in relation to the grooves 14, 1 5 on either sides thereof, thus assuring a thorough mixing of the flowing masses.

From the outer ends of the outlet grooves 1 5 the mixed mass enters an annular channel 19 defined by the conical inner surface of the extrusion head 9 and the correspondingly conical outer end 1 8 of the mixing head 13. Finally, the extruded mass emerges through the central channel 20 which gives the extruded product its shape.

By means of the above arrangement, each particle of the combined mass has a substantially constant duration of flow from the mixing chamber 11 to the extrusion head 9.

EXAMPLE By means of the main extruder 1 (cylinder inner diameter 40 mm), LD-polyethene was run whose density was 920 kg/m3 and melt index 1.8 g/1 0 min with a weight of 2.16 kg. The temperature of the mix was adjusted at 21 OOC. By means of the auxiliary extruder 5, a mixture of the same raw-material and of dicoumyl peroxide at a ratio of 86:14 was run. The temperature of the mix was set at 1 350C. The output of the main extruder was set at 30 kg/h and the output of the auxiliary extruder at 5 kg/h. After mixing of the mass flows, 2050C was measured as the temperature of the combined mass. The total mass flow was not 35 kg/h, and the share of peroxide in the total quantity was about 2 per cent. After the extrusion head 9, the product was cross-linked in about 15 seconds, the cross-linking degree being 82% as dissolved in xylene. The appearance and the mechanical properties of the product were excellent.

In addition to the embodiments above, it is conceivable, within the scope of the invention, to use a static, i.e., non-rotating, mixing head. In that case, the main screw 4, 10 according to Figure 3 will rotate in relation to the mixing head 13 which is fixed to the extruder cylinder 3.

Both co-rotating and static mixing heads are known per se.

Claims

1. A method for the manufacture of crosslinked plastic products by means of an extruder device having a feeding screw with a thread portion, comprising: ~extruding a plastic raw material substantially free from cross-linking agents at a first elevated temperature; ~injecting a relatively small quantity of a mixture of plastic raw-material and at least one cross-linking agent at a second elevated temperature, which is lower than the first temperature, into the molten mass flow at a location situated, in the flow direction; near the outer end of the thread portion of the feeding screw; and ~allowing the plastic raw-material and the mixture to get mixed before leaving the extruder device (Figure 1).

2. A method as claimed in Claim 1, wherein the injection is carried out at a location situated, in the flow direction, behind the thread portion of the feeding screw (Figures 2 and 3).

3. A method as claimed in Claim 1 or 2, wherein a small quantity of an activator accelerating the cross-linking and/or an inhibitor retarding the cross-linking is added to the mass to be extruded by means of a main extruder.

4. A method as claimed in Claim 1 or 2, wherein an inhibitor retarding the cross-linking is added to the mixture to be injected by means of an auxiliary extruder.

5. A method as claimed in Claim 1 or 2, wherein peroxide is used as cross-linking agent.

6. A method as claimed in Claim 1 or 2, wherein the ratio of the quantities of material fed at the first elevated temperature and at the second elevated temperature is from 10/1 to 5/1.

7. A method as claimed in Claim 1 or 2, characterized in that the first elevated temperature is in the range 180 to 2200C and the second elevated temperature is in the range 120 to 1800C.

8. An extruder device for carrying out the method of Claim 2 for the manufacture of crosslinked plastic products, comprising: (a) a main extruder for extruding a molten plastic raw-material, comprising: ~an extrusion cylinder with a jacket and an outer end, - a main screw rotatable within the cylinder and having an outer end and a thread portion with an outer end, ~an extrusion head connected to the outer end of the cylinder, - a mixing head arranged within the cylinder between the outer end of the main screw and the extrusion head, and ~means defining an injection aperture through the jacket, the aperture being situated, in the axial direction, between the outer end of the thread portion and the mixing head; and (b) an auxiliary extruder arranged at the aperture for injecting a molten plastic rawmaterial, together with additives, into the cylinder.

9. An extruder device as claimed in Claim 7, wherein the main screw is formed with a thinner shaft portion defining, together with the jacket, an annular mixing chamber opposite the aperture.

.

10. An extruder device as claimed in Claim 7, wherein the mixing head is connected to the main screw in a fixed co-rotating manner.