Method of and apparatus for manufacturing articles of thermoplastic cross linked polymeric materials

POLAND PATENT DESCRIPTION OF THE REPUBLIC

PEOPLE _

Additional Patent to Patent No. ——

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Reported: 25.02.78 (P. 204925)

OFFICE

PATEHTOWY

PRL

Priority: 28.02.77 for claims 1-6 and 9-12 01.08.77 for claims 7-8 and 18-21 France

Int.

. Cl.² B29F 5/00 B29D 23/04

The application was announced on 06.11.78

Patent description published: 30.06.1982

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Creators of the Invention: Patent Holder: Pont-A-Mousson SA, Nancy (France)

Method and device for producing products from cross-linked thermoplastic material and

The invention relates to a method for producing products from cross-linked thermoplastic material and to a device for carrying out this method. The invention relates in particular to the production of cross-linked high-density polyethylene products.

The cross-linking process, i.e. the so-called vulcanization, in which two adjacent polymer chains join together, forming under the influence of either radiation or activated carbon, a three-dimensional network is known and finds application in the treatment of polyolefins, vinyl polymers and elastomers, especially ethylene-propylene elastomers.

When treating certain polyolefins, such as low-density polyethylene, or ethylene-propylene elastomers, using activated carbon peroxide as crosslinking agent, the final product can easily be obtained by first forming a powdered starting mixture using known techniques for working with plastic materials such as extrusion or injection molding, and secondly by carrying out the crosslinking process, i.e. vulcanization, for example by passing the formed product through an autoclave or through a liquid bed. The temperature at which the product is formed in the extruder or in the injection press is much lower than the temperature necessary to cause cross-linking by the decomposition of organic peroxides commonly used, such as di-dimethyl peroxide, di-tertiobutyl peroxide, 4-4bis-t-butyl valerate peroxide, peroxide 2 , 5-30

112 756

-dimethyl-2-5-di- (t-butyl) hexane, 2,5-dimethyl-2-5-di- (t-butyl) -3-hexine peroxide.

Between the forming phase, during which the plastic material has a viscosity enabling its processing by known methods, and the crosslinking phase, there is a temperature difference sufficient for both phases to be sufficiently separated, so as to avoid the risk of premature peroxide degradation.

However, it is different when it is necessary to cross-link high-density polyethylene, when this material has a low viscosity that allows it to be extruded or injection-molded only at 170-220 ° C, depending on the molecular weight of high-density polyethylene, i.e. at a temperature above the onset temperature of the peroxide decomposition (about 140 ° G for the least active peroxides). Therefore, if high-density cross-linked polyethylene was used, methods for producing low-density polyethylene or ethylene-propylene elastomer components would cause a cross-linking reaction to be initiated in the molded mixture, which is not indicated due to the fact that especially In the case of an extruder, the material would be subjected to tangential stresses hindering the proper course of the crosslinking process and giving the final product a very poor quality.

The method of producing high density polyethylene products is known from Swiss Patent No. 477 973. The molded material under the action of two pistons is subjected to very high pressure, in the range of 8,000-10,000 bars, in the coupling space,

112 756 from which, after passing through the manifold, is fed through a side line to the mold. Great pressure ensures homogeneity of the material and causes a rise in material temperature before it is introduced into the mold. This increase in temperature initiates the process of crosslinking the material.

In the apparatus of this Swiss patent, two pistons compressing the molded material operate perpendicular to the mold axis. The material under the pressure of the pistons, already in a gelatinous state, changes the direction of movement twice, for the first time at the exit of the compression space and for the second time in the manifold, which is detrimental to good homogenization of the polyethylene mixture and crosslinking component, because material stagnations occur in the places where the road breaks material transitions. This causes a heterogeneous structure of the manufactured pipes.

In addition, a process for the continuous production of pipes of high density, thermoplastic crosslinked materials having a molecular weight of 200,000-1 OOdOOO containing a crosslinking component is known. According to this method, the cross-linking mixture is carried out under a pressure of 500-1800 bar along a straight line through a zone of simultaneous forming and sintering, in this zone the mixture is simultaneously introduced into the mold and subjected to sintering by heating the mold, to a sintering temperature of 140-200 ° C, and then, it passes through a non-cross-linking zone, where the mixture contained in the 'mold is cross-linked by heating the mold to a crosslinking temperature of 200-300 ° C, so higher than sintering temperature.

The device for applying this method comprises powdered material feeding successively along one axis, two forms of heat treatment of the material, associated with the coaxial core with those forms that limit, together with the core, the annular space in which the material formed under the pressure of the piston is located, which is coaxial with the core - sliding and displaced by the material of the molded pipe extruded through the piston. In this device, the outer surface of the core and the inner surface of the walls of the mold in which crosslinking takes place are provided with a liner to prevent the crosslinked material from sticking. In the device, the material is formed, sintered and cross-linked during rectilinear shifting inside the device. '.

Good quality products were obtained by known methods, but in continuous production it was found that after some time passing the material in the crosslinking zone, a deposit is formed despite the form having a special coating of non-stick material, detrimental to the quality of the product. This deposit is formed of cross-linked polyethylene particles adhering to the walls of the mold and is formed as a result of the activity of the radicals released during the cross-linking reaction, since these radicals are sufficiently active to in turn activate the surface of the liner made of non-adhesive material and cause it to stick.

Attempts have been made to prevent this disadvantage by injecting an additional lubricant into the mold before the crosslinking zone, but regardless of the technical difficulties associated with the unstable flow of this liquid, this treatment does not completely eliminate the formation of sediment in the mold in the crosslinking zone. ,

The subject of the invention is a method of producing products made of thermoplastic material, which is a powdered mixture of thermoplastic material, and^; '4. '' cross-linking agent. The method is carried out by compacting the powdered mixture by applying pressure, which causes it to be introduced and moved in a heated matrix, in which, by creating pressure oriented according to the axis of the matrix, a sintering zone of the mixture is created and it is characteristic that the matrix in the sintering zone is subjected to heating at a temperature lower than the temperature at which the decomposition of the crosslinking agent becomes palpable, and the cross-linking process of the formed material is carried out in the cross-linking zone behind the matrix by moving the intermediate in a molten salt bath, the temperature of which causes the cross-linking agent to decompose.

In the process according to the invention, the process takes place in three phases, a first phase of pre-forming the powdered mixture is used, which ensures a direct, very favorable introduction of the mixture into the matrix, a second phase of proper sintering and a clearly separated third crosslinking phase.

The concept of sintering includes the process of thickening and melting the grains of a mixture to obtain a compact product.

The use of a molten salt bath to carry out the cross-linking reaction has the advantage that after gelation the product moves freely in the cross-linking zone, whereby heat exchange is ensured by the bath, and the product during cross-linking does not rub against the walls of any channel, because simple mechanical guidance is sufficient, there is therefore no risk of material stagnation. This route, in the case of pipe production, is obtained by its linear contact with the toroidal surface of the rings mounted in the bath tank and lubricated by this liquid.

To ensure proper product calibration, preferably in the bath entrance zone, a shaping device is mounted whose useful cross-section can be constant or variable. When producing a pipe, its internal diameter decreasing in front of the shaping device can be maintained in the desired tolerances by introducing an inert gas, such as nitrogen, injected inside the pipe.

When applying the method according to the invention to high density polyethylene pc products, no shear stress occurs during crosslinking, which allows obtaining optimal product properties. The total elongation of the samples tested was 400-500%. These results are independent of the time elapsed since the start of production by the device, because in the crosslinking zone they do not. no sludge was created that could reduce product quality * Therefore, it is possible to carry out continuous production on an industrial scale, since the necessity of frequent replacement of elements damaged by surface sediments was eliminated, which was a disadvantage of known methods.

The method according to the invention finds application in the production of pipes and profiles, as well as in the formation of cable coatings or coatings of metal arrriatures. "By stretching is meant not only elongation, but also any longitudinal stretching obtained especially using a matrix, or under the action of a liquid under pressure, either inside the mold or without the mold. .

Thus, pipes with different diameters can be produced in one device, with constant time capacities, regardless of the pipe diameters, it is also possible to manufacture _Kjoints, profiles, car body components

112 , dishes, packaging / corrugated or shaped plates, etc. \.

The invention also relates to a method device comprising a heated matrix for forming and sintering, a stem arranged inside the matrix providing, in cooperation with the matrix, the formation of an annular space and means for applying, along the axis of the matrix, pressure on the powdered mixture to preform this mixture by compression * and moving this mixture through the matrix, characteristic and that behind the matrix constituting the sintering zone there is a tank containing a bath of molten salt for crosslinking the sintered intermediate coming out of the matrix. '

The device according to the invention may also comprise and between the die exit and the crosslinking tank means for stretching the formed material.

The subject of the invention is presented in the examples of the drawing, in which Fig. 1A shows the upper part of the device for producing pipes 2 of cross-linked high-density polyethylene in cross-section, Fig. IB - the bottom of this device, Fig. 2 - another embodiment of the shaping system used in the part of the device shown in Fig. IB, in axial section, Fig. 3 - a detail of the shaping system of Fig. 2, in section along 2 lines 3-3, Fig. 4 - yet another embodiment of the shaping system used in the part of the device shown in Fig. IB, in axial section, Fig. 5 - another embodiment of the outlet of the reservoir shown in Fig. IB containing the crosslinking salt bath, Fig. 6 - diagram of the device 31 containing stretching means pipes, Figure 7 - cross-section means for stretching pipes and for cross-linking used in the device of Figure 6.

The device, whose upper part A- is shown in Fig. 1Α, comprises powdered polyethylene and a crosslinking agent 3i formed into the shape of a pipe and sintered, and in the lower horizontal part B of the device shown in Fig. IB, the material is crosslinked.

Part A of the device with a vertical axis Χ-Χ, includes a horizontal base 1 resting on brackets 2 and constituting 41 support for the matrix 4 positioned in the axis Χ-Χ, «equipped with a foot 5 pressed by the projections of threaded rods 6 and attached to the base 1 by means of 7. The matrix 4 consists of a cylindrical body 8 in which a hole 9 is made coaxial with the central hole 41 3 of the base 1, the matrix 4 has a heating oil circulation channel 8a. The upper end of the die 4 is provided with a flange 10 through which the rods β pass and whose upper surface supports the supply tank 11, cooled through the cooling water channel 12. The diameter of ₅], the container 11 at its base corresponds to the diameter of the opening 9. Between the container 11 and the flange 10 there is a metal ring 11a, which neither undergoes heating nor cooling, and whose contact surfaces are formed in a way limiting heat conduction. To the 'foot 5 of the matrix 4, inside the opening 3 of the base 1, a second matrix 13 coaxially adheres vertically to the matrix 4 held in place by the bottom support plate 14 attached by means of threaded rods 6. 6 (

Plate 14 has a hole 15 with a diameter larger than the diameter of the die hole 13. The system thus formed from the base of the reservoir 11 up to the hole 15 of the support plate 14, forms a cylindrical, rectilinear channel ^ Matrix 13 is also equipped with annular heating means formed

756 . 6th ; . ' for example, via external electric umbilicals 17.

Based on base 1, outside of matrix 4, two cylinders 18 and 19 containing pistons 20, 21 with background 5 pins 22, 23 are supported, to which a horizontal plate 24 fastened with nuts 25, 26 is attached. Plate 24 slides along the rods 6 and in its center has a cylindrical opening 27 extended inside the vertical annular projection 28 of the plate 24, cooled and near the plate 24 through the water flow channel 29. The projection 28 is a piston cooperating with the cylindrical segment 36 formed by the lower edge of the opening 11a and by the upper part of the opening 9 of the die 4.

The upper ends of the rods 6, above the plate 24, constitute the support and for the support plate 30, which in turn rests on the cylinder of the cylinder 31 containing the piston 32 connected from below with the vertical stem 33 slidingly inserted into the hole 27 of the protrusion-piston 28 and additionally guided through the plate support 30 and through a horizontal plate 34 connected to it, slidingly sliding along bars 6. The mandrel 33 slides inside the matrix 4 and matrix 13, making the upper return point of the lower end of the mandrel 33 located approximately at the level of the lower end of the matrix 13. The mandrel 33 is also provided with a heating element ί 35, in the form of an oil channel or electrical resistor.

The device has a dosing tank 37 connected to the reservoir 11 through a charging mouth 38 ensuring feeding of the sample mixture to the space between the mandrel 33 and the matrix 4. The cooling line 12 of the reservoir 11 prevents polyethylene from reaching the melting temperature at which it becomes coherent, regardless of the time polyethylene in the tray

11. The cooled zone is the pre-forming zone. In this zone, the mixture must remain in powdered form so that it can then be formed into a tube under the effect of the compaction caused by the piston 28 in the cylindrical part 36 formed above the matrix 4 and / or in the inlet zone of the matrix 4.. Before the start of the cycle, the piston 28 and the piston 33 together with the piston 32 occupy the upper position. The piston 32 of the actuator 31 is not subject to any pressure. The operation of the cylinders 18 'and 19 causes the piston 28 to lower, which passes through the powdered mixture located in the storage tank li and maintained by the cooling channel 12 at ambient temperature. Further lowering of the piston 28 causes the mixture to thicken in the cylindrical space 36. A pressure of 1000 bar is necessary to thicken the mixture. The mixture, which is simultaneously formed and thickened, then moves to the matrix 4. '

Under the influence of the pulling force of the material, the mandrel 33 is simultaneously lowered, in the matrix 4 the material is subject to an increase in temperature created by its heating element 8a and the heating element 35 of the mandrel 33. The temperature is adjusted so that the material melts, but remains below the temperature at which decomposition of the crosslinker becomes palpable. Stacks of 150-180 ° C are used. When the piston 28 reaches its lower turning point, the pressure exerted on the pistons 20 and 21 of the cylinders 18.19 is reduced and while the piston 28 remains in its lower position,. the spindle 33 returns to the upper position due to the action of the piston 32. When the spindle 33 reaches the upper position, the piston 28 is pulled up and a new cycle begins.

V

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The matrix 4 is thus a sintering zone from which the material as a result of the piston 28 descending along the mandrel 33 is pushed into the matrix 13, in which the same temperature is maintained and from which the material is discharged in the form of an intermediate. After leaving the matrix 13, the material is pushed rapidly through further movements of the piston 28 into the space constituting the coating 15a, for example having the form of a bellows, in which the temperature is maintained approximately the same as before, or possibly slightly elevated. Coating 15a is a transition zone in which the semi-finished product, free from the coating walls, may undergo lateral deformations, absorbing unevenness resulting from step displacement, with a continuous pushing force exerted towards the horizontal part B constituting the crosslinking zone.

Part B of the device is a closed horizontal tank 41, preferably cylindrical, divided along two vertical partitions 42, 43, containing a bath 44 of molten salt and heated, for example, by electric resistors 46 to a temperature enabling crosslinking, i.e. to a temperature above 200 ° C. Into the front compartment 47 of the tank 41 is connected a hose constituting the connector 48, the upper wider end of which is attached to the tank, and the other end with a slightly smaller internal diameter, corresponding to the external diameter of the pipe being crosslinked, is attached to the entrance of the forming die 49 attached to the walls of the tank 41 using fastening elements 51. The pipe, after sintering, is introduced with some looseness into the inlet opening of the connector 48 with a decreasing cross-section, and then after changing the direction by 90 °, into the cylindrical opening 52 of the forming die 49, which ensures uniformity of the cross-section and surface of the pipe subjected to simultaneously commencing crosslinking, at least in its outer peripheral part. The pipe is then moved through two further compartments 53, 54 of the tank 41 in which the cross-linking process is completed and in which the pipe is guided through rings 56 connected to the walls of the tank 41 with brackets 57, the inner hole 55 of the rings 56 being toroidal for sliding and straight contact with pipe. The cross-linked pipe is led out of the tank 41 through the end wall 58 provided with a sealable replaceable ring 59.

The pressure inside the gas pipe, introduced through the conduit 61 via the axial channel 62 in the stem 33, also contributes to the calibration of the tube immersed in the bath 44. Nitrogen is preferably used as the gas.

The molten bath salt 44 preferably consists of a eutectic mixture having the following composition: 53 parts by weight of K NO₃, 40 parts by weight Na NO₂, 7 parts by weight Na NO₃.

Dividing the tank 41 into compartments allows them to maintain different temperatures. The temperature in the compartment 47 containing the forming die 49 is usually higher, for example 250-300 ° C, so as to achieve a rapid material change in the peripheral zone of the pipe. In the remaining ranges 53 and 54 the temperature is lower, for example it does not exceed 250 ° C to avoid thermal decomposition of the product.

When machining a pipe, for example, 3 mm thick, the length of the tank 41 is about 2 m, at a pipe speed of 150 m / h.

and

Behind the tank 41 is the device 63 shown in Figure IB only partially, consisting of two movable tracks tangential to the cross-linked pipe and exerting a constant tensioning force on it.

An additional forming die located between the tensioning device 63 and the tank 41 can also be used.

In accordance with the invention, satisfactory attempts have been made to use, as a thermoplastic material, high density polyethylene with a molecular weight of 300,000—500,000, known as Manolene 56020 with a density of 0.956 and a viscosity index of 2 under a load of 20 kg / cm². Additives containing, in addition to organic peroxide, dyes, antioxidants, UV-resistant agents, and lubricants were added to this material, all of which are known products used in the plastics processing industry.

The crosslinking agent used was a catalytic compound containing 0.5% of di-thiobutyl peroxide, known under the name Trigonox B. A frequency of 70 piston strokes per minute was used in these tests, with each complete stroke of the piston being 40 mm, the length of the pipe produced was about 20 mm, and the mandrel travel was 20-25 mm.

Further tests were also carried out, during which the elongation was measured according to the French standard NF T 51 034, between two benchmarks of the standardized sample by means of a strain gauge, using a drawing speed of 100 mm / min. The results of the tensile tests, showing the mechanical characteristics measured on the pipe, depending on the extruded length and showing the average value for 5 samples, are presented in Table I.

Table I

<td></td><td>Extruded Length (m)</td><td> 100</td><td> 500</td><td> 1000</td><td> 1500</td><td> 2000</td>

<td> 40</td><td>Stress point (Kg / / cm²)</td><td> 180</td><td> 184</td><td> 170</td><td> 190</td><td> 182</td>

<td></td><td>Burst stress (Kg / cm²)</td><td> 300</td><td> 280</td><td> 290</td><td> 270</td><td> 280</td>

<td> 45</td><td>Total elongation (%)</td><td> 520</td><td> 490</td><td> 460</td><td> 500</td><td> 470</td>

<td></td><td>Pressure causing sample rupture (bars)</td><td> 55</td><td> 52</td><td> 57</td><td> 50</td><td> 52</td>

Figs. 2 and 3 show another embodiment of the shaping die used instead of the drawing die 49 shown in Fig. IB. The drawing die consists of several sets 64, 65, 66 profiling discs 67 with a concave cross section, giving the pipe a gradually decreasing cross section.

To reduce the diameter of the pipe, it is also possible to use in the device a drawing die as shown in Fig. 4, formed by a series of shaping rings 68a, 68b, characterized by a cylindrical bore with variable diameter. These rings made of Teflon or polished steel are separated from each other by struts 69 and held in position by pressure means 70, possibly supplemented₆₅ slack adjustment measures as illustrated.

112 756 . ⁹

Fig. 5 shows how the final product in the form of, for example, an unclosed profile is discharged from the container 41 not through the sealing ring, but through the upper opening 71 in the container 41. The product 71 is fed through the bent guide 72.

When processing a non-tubular product, the mandrel 33 shown in Fig. 1A is eliminated from the device. When coating a cable or metal fitting, the cable or fitting is inserted in place of the mandrel 33.

In the diagram of Fig. 6, the use of a forming device 101 corresponding to Part A of Fig. 1A producing intermediate 102 at vl, this intermediate then passes through tank 103, in which it is subjected to stretching and then crosslinking processes, after which it is moved by a draft tractor 104 at a speed v2 higher than speed vl, to a cooling bath 105 and with this same speed v2 or approximately equal - to the tractive puller 106 and then to the winder 107.

Fig. 7 shows an intermediate 102 which slides through the opening 15 in the lower support plate 14 from the forming device 101 and then moves to. of the assembly eliminating the effects of step work resulting from discontinuities in the movement of the intermediate, and then continuously to the tank 103. In this assembly, the intermediate 102 is subject to a change of direction around the Grooved disc 108 loosely mounted in the stirrup 109 rotatably mounted on the axis 110 attached to the bracket 111. The groove of disc 108 has a depth at least equal to the radius of intermediate 102.

The intermediate 102 is then moved through a pulley 112 also provided with a groove connected via a speed variator 113 (shift gear) to the electric motor 114 shown schematically in the drawing. The intermediate undergoes a new direction change and is fed into the stretching and crosslinking tank 103. This tank contains the same mixture of molten salts as tank 41 of Fig. IB, located in two compartments, in the stretching compartment 115 and in the crosslinking compartment 116, separated by a diaphragm 117 with an opening 118 through which intermediate 102 passes.

Inside the tank 103, in the compartment 115, several profiling rollers 121a, 121b, 121c, 121d are mounted, the cross section of which corresponds to the geometry of the product under tension, the rollers being connected to a maneuvering strip 119 actuated with articulated levers 120a and 120b. The crosslinking compartment 116 is equipped with retaining discs 122. If desired, the profiling discs can be replaced with other suitable profiling elements. The number of sets of discs or profiling elements is changed depending on the desired degree of stretching. Retention rings can be replaced by another guide and support system, such as gutters, rings, etc.

After passing through tank 103, intermediate 102 pulled fest by a drawing machine 104 of known type, for example by a belt device moved at speed v2 by an assembly consisting of engine 11 * 4a and speed variator 113a, after which the pipe is fed to cooling tank 105 and then to winder 107, after possibly passing through an additional drawing machine 106, of the same type '10 ˚o drawing machine 104, which rotates at a speed equal to or approximate to speed v2. Puller 106 is designed to facilitate winding of the pipe and is not intended to reduce its cross-section.

Intermediate 102 is discharged from the shaping device 101 by a continuous, reciprocating movement, the stopping time being the time required to lift the forming mandrel, after which the intermediate is pulled continuously to the remaining installations and by means of a drive pulley 112 with a linear speed vl. Free tilt around the axis 110 of disc 108 compensates for inaccuracies that arise when the intermediate is stopped. The diameter of the intermediate reaches at this point dl and is maintained due to the gas pressure inside the pipe that prevents the walls from being crushed.

The intermediate product with a diameter dl is introduced at a speed vl into the stretching interval 115. Due to the operation of the drawing machine 104 at a linear speed v2 higher than the speed vl 20, the intermediate product is stretched longitudinally. The diameter of the pipe is then reduced when it reaches the first set of discs 121a to a value dl slightly less than dl, with the diameter at the exit of 115 reaching a value d2 slightly less than dl as a result of extension. Profiles 121a-121d support the intermediate before crosslinking occurs. In the stretching interval 115, the temperature is chosen so as to ensure that the catalyst starts working. This temperature is approximately equal to the forming temperature in the forming part of the device, for example in the order of 160-190 ° C.

^s In the hole 118 of the diaphragm 117, the intermediate exhibits dimensional characteristics, diameter, thickness, uniform in relation to the characteristics at the entrance to the stretching interval 115. The intermediate 102 is then introduced into the crosslinking compartment 116, in which the temperature reaches values ​​above 200 ° C, preferably within 220 —250 ° C. The decomposition of the catalyst then causes crosslinking of the material and the stretching phenomenon is stopped as a result of crosslinking.

The location of the diaphragm 117 in the tank 103 depends on the desired diameter of the pipe at the exit from the crosslinking compartment 116, this diameter being determined by the ratio of speed v2 / vl. Similarly, individual discs 121a, 121b, 121c, 121d are selected according to the intended profile of the finished product, in case this profile is different from the profile of the intermediate 102 discharged from the forming part of the device.

In the event that the planned tube profile remains the profile of the smooth tube of intermediate 102, the rollers 121a, 121b, 50 121c, 121d no longer have a calibrating or profiling role but only serve as supporting and guiding elements, these elements can be replaced by other appropriate means. In each case, however, pulleys 122 constituting only support and guide elements₅₅ during the crosslinking process they can be replaced by appropriate support and guide means. After leaving the tank 103 and after passing through the drawing machine 104, the pipe cools down in the tank 105 containing for example water, after which it is drawn by the drawing machine 106,₆₀ and then wound on winder 107.

; As an excellent property of such a stretched product, dimensional stability and total irreversibility of deformations are emphasized.

Stretched polyethylene, for example ₆₅ four times in the 115 range, then

112 756 ii crosslinked in the crosslinking compartment 116 does not undergo any post-deformation return except thermal return. Crosslinking blocks one chain from the other. The cross-linked product is birefringent and solid in its sizes above and below the melting point of polyethylene crystallites.

In contrast, the same section stretched four times in the compartment 115, then cooled but not crosslinked, regains its original dimensions when it is reheated to a temperature of about 140 ° C, i.e. higher than the melting point of polyethylene.

Examples of pipe production in the device according to the embodiment shown in Figs. 6 and 7.

EXAMPLE 1 High-density polyethylene with a molecular weight of 300,000-55,000, such as the product called Monolene 56 020, was used to make the pipe under the conditions described above, where cross-linking agents containing stabilizing and coloring agents of known type were introduced. as

2,5-dimethyl-2,5- (dimerti-butyl-peroxy) -3-hexin, known as Luperox 130. Table II presents the parameters of the process carried out for various diameters, starting from the diameter of the intermediate dl = 27.5 mm.

¹² .

The crosslinking index was * 87% at both the beginning and the end of the pipe, and the post-deformation return at 120 ° C, measured according to NF T 54 021, was 4%. Post-deformation recovery at 160 ° C was 6% measured according to the same standard. '

The process according to the invention finds application in the case of polyolefins susceptible to crosslinking by peroxides, especially and preferably in the case of products with high molecular weight.

The process of the invention is also applicable to low density polyethylene, crosslinked by the addition of peroxide. ^from