FR2500789A1 - EXTRUSION PROCESS AND DEVICE FOR THE MANUFACTURE OF CROSSLINKED PLASTIC PRODUCTS - Google Patents

Abstract

METHOD AND DEVICE FOR PRODUCING CROSSLINKED PLASTIC MATERIALS. THE DEVICE INCLUDES A MAIN EXTRUDER 1-4 AND AN AUXILIARY EXTRUDER 5-8 WHICH IS PLACED AT THE EXTERIOR END OF THE FEEDING SCREW 4 OF THE MAIN EXTRUDER. FOLLOWING THE PROCESS, USING THE MAIN EXTRUDER 1-4, A RAW PLASTIC MATERIAL SUBSTANTIALLY FREE OF RETICULATING AGENT IS EXTRUDED AT A HIGH TEMPERATURE AND, BY MEANS OF THE AUXILIARY EXTRUDER 5-8, A QUANTITY OF INJECTION IS INJECTED OF A CONCENTRATED MIXTURE OF RAW PLASTIC MATERIAL AND OF A CROSSLINKING AGENT IN MOLTEN RAW PLASTIC MATERIAL AT A LOWER TEMPERATURE. IN THE MASS EXTRUDED BY MEANS OF THE MAIN EXTRUDER 1-4, A CERTAIN QUANTITY OF AN ACTIVATOR ACCELERATING CROSS-LINKING AND / OR AN INHIBITOR DELAYING THE SAID CROSS-LINKING, MAY BE ADDED.

Description

Extrusion method and device for manufacturing products made of

  The invention relates to a method for manufacturing crosslinked plastic products, by means of an extrusion device having a feed screw comprising a threaded part, said method comprising - the extrusion of a raw plastic material substantially free of crosslinking agent at a first high temperature, - the injection of a relatively small amount of a mixture of raw plastic material and at least one crosslinking agent into the circulating melt, - l operation which consists in allowing the raw plastic material and the mixture to mix before they leave the

extrusion device.

  The invention also relates to an extrusion device

  intended for the implementation of the above method.

  By means of this process and this device, it is possible to manufacture, from a thermoplastic resin,

crosslinked plastic products.

  The most common thermoplastic resin which must be crosslinked is polyethylene, whose molecular structure -2- is crosslinked by the crosslinking effect where it results that the thermal resistance of the product is considerably improved. Crosslinking of plastic products which must be

  manufactured by an extrusion process, is currently-

  accomplished using a large number of different processes

  rents. The cheapest and also the most commonly used is to use peroxides, by adding an appropriate peroxide which does not decompose at the extrusion temperature, to the raw plastic material, before the extrusion process and in connection with him. After shaping which takes place at the extrusion head, the product is heated to a temperature for which the peroxide decomposes, this

  which gives the crosslinking of the product.

  A disadvantage of the above process is its slowness. The properties of the flux in the molten state of a crosslinked material, even if it is only partially crosslinked, are very poor, from which it results that a decomposition, even partial, of the peroxide, must not be perform in the extruder. It follows that the peroxide to be used must be such that it decomposes only at high temperatures, or that the extrusion temperatures must be low. In both cases, the product temperature must be increased considerably after shaping, so that the peroxide can decompose at least at reasonable speed. As a result of the well known low thermal conductivity of plastics, crosslinking, especially of objects with thick walls, is slow and requires long heating lines

after the extruder.

  The invention proposes to eliminate the drawbacks

  above and to provide a method and a device for extruding

  more efficient and practical.

  In the process of the invention, the crosslinking is always

  carried out by means of peroxides, but, thanks to

  appropriate extrusion temperature is sufficient

  te to obtain rapid completion of the reaction of reti-

  culation, and no additional heating is required after

  the extruder, at least the need for additional heating

  is considerably reduced.

  More precisely, the method according to the invention is mainly characterized in that the injection takes place at a location located, seen in the direction of flow, in the vicinity of the outer end of the threaded portion of the screw. power supply and at a second high temperature

weaker than the first.

  The extrusion device according to the invention is mainly

  Lement characterized in that the opening is located, looking in the axial direction, between the outer end of the threaded portion of the main screw and the head

mixed.

  The invention will be better understood using the description

  below relating to a certain number of modes of execution

given as an example.

  In the accompanying drawing Figure 1 is a partial sectional view of an extruder intended for the implementation of the method of the invention, Figure 2 is a partial sectional view of a second extruder intended for the implementation of the method according to the invention, Figure 3 is a sectional view on an enlarged scale of the

  Figure 2 extruder screw device.

  - 4 - According to Figure 1, the raw plastic is dosed in the hopper 2 of the main extruder 1. By effect

  joint of the heat introduced through the cylinder wall

  drique 3 and shear produced by the rotational movement of the feed screw 4, the mixture melts and becomes homogeneous. At the same time, an amount of the same raw material is metered into the hopper 6 of an auxiliary extruder 5 placed in the vicinity of the outer end of the cylinder 3 together with an excess of peroxide. By the effect of the auxiliary screw 7 and the heat introduced from the outside, the mixture melts and becomes homogeneous, before its introduction into the mixture of the main extruder via the channel 8 located in the vicinity of the outer end of the threaded portion of the extruder

main.

  When the temperature of the mixture of the main extruder 1 is set at a sufficiently high level and when the temperature of the mixture of the auxiliary extruder 5 is kept low enough that the decomposition of the peroxide does not take place inside from the auxiliary extruder 5, a mixture is obtained whose temperature is

  determined on the basis of the temperatures of the feed materials

  ted by the main extruder 1 and the auxiliary extruder 5, in accordance with the ratio of the respective flows of materials. After the mass flows have combined, the mixture is homogenized by the effect of the outer end of the feed screw 4 of the main extruder 1, but, in the equalization of heat, it occurs a slight delay which results from the slowness of the thermal conduction, delay during which the mass reaches the extrusion head 9. Under these conditions,

  cross-linking takes place quickly, because the temperature

  mass mass is extremely high.

  A preferred embodiment of the invention is shown in FIGS. 2 and 3. According to this embodiment, the point of contact 8-11 of the two flows of molten material is located after -5- the threaded portion 12 of the main screw 4, looking in the direction of flow. By means of this arrangement, the disadvantageous effect of threading on the melt

  containing peroxide is eliminated.

  It is known that two types of flow exist in the threaded part of an extruder: a direct flow due to the rotation of the screw and a reverse flow due to the back pressure caused by the extrusion head. The sum of these flows transports the mass as a whole in the direct direction, but the reverse flow causes a considerable dispersion of the delay times of the various particles melted in the threaded part. If said auxiliary flow containing peroxide reaches the main flow at a point where there is a reverse flow, it remains longer in the threaded part than a portion of auxiliary molten material which would reach the direct component of the flow of the melt in the part threaded 'of the main screw. In order to prevent crosslinking inside the extruder in such a case, the mass temperature of the main extruder should be kept below that which would be reached in the case where only a direct flow would exist after

combination of molten fluxes.

  Such a situation is achieved when the flow coming from the auxiliary extruder 5 can come into contact with the flow from the main extruder at a point 8-11 located after the threaded part 12 of the main screw 4. At the point of contact 8-11, the main screw 4 is formed by a portion of thinner cylindrical shaft 10 which defines, at the same time as the cylinder wall 3, a

ring mix 11.

  The external part of the main screw 4 which follows the thinner shaft portion 10 is a conventional mixing head 13 provided with axial feed grooves 14 and grooves

exhaust 15.

  -6- The arrangement is such that the mass being in the mixing chamber it can only penetrate into the supply grooves 14. Between each of the supply grooves 14 and an adjacent exhaust groove 15, there is a peripheral passage 17 which allows the mass contained in each

  supply bleed 14 to enter an exhaust bleed

  adjacent pement 15. Between each pair of supply and exhaust grooves 14, 15, there is an axial edge 16 in sliding contact with the internal surface of the cylinder 3, thus preventing any flow between the different couples of grooves d 'inlet and outlet 14, 15. The peripheral passage 17 has a small radial height with respect to the grooves 14, 15 on its two sides, which ensures a mixture

full of circulating masses.

  From the outer ends of the escape grooves

  ment 15, the mixed mass enters an annular channel 19 defined by the conical inner surface of the head

  extrusion 9 and through the corresponding conical outer end

  laying 18 of the mixing head 13. Finally, the extruded mass emerges through the central channel 20, giving

thus its shape to the extruded product.

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

extrusion 9.

Example

  Using the main extruder 1 (inner cylinder diameter 40 mm), LD polyethylene was treated with a density of 920 kg / m3 and a melt index 1.8 g over ten minutes with a weight of 2.16 kg. The temperature of the mixture was set at 210WC. By means of the auxiliary extruder 5, a mixture of the same raw material and of a dicoumyl peroxide was carried out in an 86-14 ratio. The temperature of the mixture was fixed at 135 ° C. The output from the main extruder was set at 30 kQ / h and the output from the auxiliary extruder at 5 k / h. After mixing the mass flows, the temperature of the combined mass measured was 2050C. The total mass flow was not 35 kg / h and the share of peroxide in the total amount was about 2%. After the extrusion head 9, the product was crosslinked for approximately 15 seconds, the degree of crosslinking being 82%, by dissolution in xylene. The appearance and

  mechanical properties of the product were excellent.

  In addition to the above embodiments, it is conceivable, without departing from the spirit of the invention, to use a static mixing head, that is to say a non-rotating head. In this case, the main screw 4 - 10, according to FIG. 3, will rotate relative to the mixing head 13 which will be fixed to the

extruder cylinder 3.

  The co-rotating and static mixing heads are good

known per se.

-8-

Claims (8)

Patent claims.   1. Process for the manufacture of plastic products   crosslinked tick by means of an extrusion device (1-9) having a feed screw (4) provided with a threaded portion, said method (12) comprising: - the extrusion of a raw plastic raw material sensi-   free of crosslinking agent at an initial temperature   high ture, - the injection of a relatively small quantity of a mixture of raw plastic material and at least one crosslinking agent in the circulating melt, and - the operation which consists in letting the material mix   raw plastic and the mixture before they leave the disposi- extrusion tif,   characterized in that the injection takes place in a place   located (in the direction of flow, in the vicinity of the former   outer end of the threaded portion <12) of the feed screw (4), and at a second elevated temperature which is weaker than the first.   2. Method according to claim 1,   characterized in that the injection takes place in a place   cement located in the direction of flow, behind the portion   threaded (12) of the feed screw (4>.   3. Method according to claim 1 or 2, characterized in that a small amount of an activator   accelerating the crosslinking, is added to the extrusion mass   der by means of a main extruder (1 - 4).   4. Method according to claim 1 or 2, characterized in that peroxide is used as a retipulating agent. - g- 5. Method according to claim 1 or 2, characterized in that the first high temperature is of the order of 2100C and that the second high temperature is around 1350C.   6. Extrusion device (1 - 20) used for placing   in the process according to claim 2 for the manufacture of   cation of crosslinked plastic products, comprising: - a main extruder (1 - 4) used to extrude a molten raw plastic material and comprising an extrusion cylinder (3) with an injection opening (8) through its jacket, - a main screw (4) rotatably mounted inside said cylinder (3) and having a threaded part (12), - an extrusion head! 9) connected to the external end of the cylinder (3), - a head mixing (13 - 18) arranged inside the   cylinder (3), between the outer end of the main screw   blade (4) and the extrusion head (9), - and an auxiliary extruder (5 7) mounted at the opening (8) for injecting a molten raw plastic material together with additives into the cylinder (3),   characterized in that the opening (8) is located opposite   in the axial direction between the outer end of the threaded part (l21 of the main screw (4) and the mixing head (13 - 18).   7. Extrusion device according to claim 6, characterized in that the main screw (4) is provided with a thinner shaft portion (10) defining, at the same time as the jacket (3), a annular mixture (11) located opposite the opening (8).   8. Extruder according to claim 6, characterized in that the mixing head (13) is coupled to   the main screw (4) in a co-rotating manner.