Products of crosslinked thermoplastics material

The present invention relates to a method for manufacturing products made of crosslinked thermoplastic material and to a device for its implementation. It relates more particularly to the manufacture of products in crosslinked high density polyethylene.

Crosslinking or vulcanization, by which two neighboring polymer chains are linked together by forming a three-dimensional network, under the effect of either radiation or activated carbon, is a known operation which can be applied to polyolefins, vinyl polymers and elastomers, in particular, ethylene-propylene elastomers.

In the case of certain polyolefins, such as low density polyethylene and ethylene propylene elastomers, when a pro15 activated carbon from an organic peroxide is used as the crosslinking agent, the final product can be obtained without difficulty. successively forming the starting powder mixture, by using conventional techniques for processing plastics, such as extrusion and injection, and in the crosslinking, or vulcanization operation, which is carried out for example by passing the shaped product through an autoclave or through a fluidized bed. Indeed, the temperature at which the forming takes place in the extruder or in the injection press, is much lower than the temperature which it is necessary to reach to cause crosslinking by decomposition of organic peroxides conventionally. employees, such as:

- dicumyl peroxide;

- ditertiobutyl peroxide;

- 4-4bis-t-butyl-valerate peroxide;

- 2,5-dimethyl-2-5-di- (t-but.yl) -hexane peroxide

- 2,5-dimethyl-2,5-di- (t-butyl) -hexyne peroxide: Between the forming phase, during which the plastic material has a viscosity allowing its transformation according to conventional methods, and the phase of crosslinking, there is therefore a sufficient temperature interface so that the two operations are well separated and that the risk of premature decomposition of the peroxide is avoided. /

It is different when it is necessary to crosslink high density p ^ yethy2 lene, because this material reaches a sufficiently low viscosity for its shaping by extrusion or injection only at a temperature between 170 and 220 ° C, depending the molecular mass of high density polyethylene, that is to say a temperature higher than the temperature at which decomposition of peroxides begins (around 140 ° C. for the least active peroxides), so that the application to crosslinked high density polyethylene of the processes used for the production of low density polyethylene or ethylene propylene elastomer would initiate in the mixture subjected to the forming operation a crosslinking reaction which would be undesirable because, in particular in the case of an extruder extruder, the material would then be subjected to a shear preventing the correct development of crosslinking and giving the final product a very poor quality.

It has already been proposed to eliminate this shearing by using a heated forming and crosslinking die in which the mixture is densified and forced to progress in the tooling under the effect of a pressure created by a piston, but it is then necessary , to avoid prejudicial deformations within the material, to provide on the surfaces of the die with which the material comes into contact a coating of a non-stick material, such as polytetrafluoroethylene.

In this case, good quality of the products is actually observed, but this technique does not allow practically continuous production to be envisaged, since it is found that after a relatively short time for the material to pass, it is created in the crosslinking zone. of the sector a deposit which harms the quality of the product. This deposit consists of particles of crosslinked polyethylene adhering to the wall and is due to the radicals released during the reaction, which are sufficiently active to activate the surface of the non-stick material and cause sticking.

An attempt has been made to inject a large quantity of lubricant into the die before the crosslinking zone, but this technique, apart from raising difficulties in implementation, due to the unstable flow regime of the fluid in the die , does not remove the deposit in the crosslinking area. /

To solve the problem posed, the invention proposes dn / pro3 ceded for the manufacture of products on crosslinked thermoplastic material from a mixture, essentially pulverulent, containing the thermoplastic material and a crosslinking agent according to which the mixture is densified. by exerting on it a pressure which makes it penetrate and progress in a heated die and one causes the crosslinking of the matter, characterized in that the powder mixture is sintered by exerting on it a pressure oriented along the axis of the die, the mixture is caused to melt in the die by subjecting the latter to a temperature below the temperature at which the decomposition of the crosslinking agent becomes sensitive and, downstream of the sector, the shaped material is crosslinked by passing it through a bath of molten salt whose temperature allows the decomposition of the crosslinking agent.

In addition to the fact that the proposed method provides for a preliminary phase of preforming the powder mixture, which allows direct, very favorable admission into the die, i] ensures a clear separation of the actual forming phase, by sintering, from the crosslinking phase. . The term sintering includes the operation of densification and melting of the grains to obtain a compact product. As for the use of a molten salt bath to create the crosslinking reaction, it has the advantage that the product once gelled can, so to speak, follow a free path in the crosslinking zone, in the sense that the heat exchange being ensured by the liquid of the bath, the product does not have to slip in contact with walls, and that, to allow its progression until the exit, it is enough of a simple mechanical guidance, which, in the case of a tube, can be obtained by linear contact with the toric surface of rings supported by the tank containing the bath and benefiting from the rest of a lubrication effect. within the liquid. It can however be advantageous, to ensure the calibration of the product, to provide in the region of entry of the bath a template template whose useful section can be constant or variable. In the case of a tube, its internal diameter, decreasing in line with the gauge, can be maintained within the desired tolerances, by the effect of an inert gas, such as nitrogen, injected into the inside the tube. ,

In the application of the process to high density polyethylene products, the absence of any shearing during lXy4é4 ticulation makes it possible to obtain optimal properties. The elongation at break of test pieces subjected to tensile tests is between 400 and 500 and this result is independent of the time elapsed since the start of production of the device, because in the crosslinking zone it does not occur deposit whose effect would be to lower the quality of the products. It is therefore possible to envisage continuous industrial production, since the need to frequently change the organs degraded by a surface deposition, which is a drawback of the known methods, is eliminated.

The proposed process can be used for the manufacture of tubes and profiles, as well as for the sheathing of cables or metal reinforcements.

According to another characteristic of the process according to the invention, the crosslinking of the shaped material can be preceded by drawing at a temperature close to that of the shaping.

By stretching here is meant not only a longitudinal extension or an elongation, but also any forming by longitudinal and radial expansion, obtained in particular under the action of a punch or a die, or under the action of a fluid. under pressure, operation which can take place inside a mold or without the intervention of a mold.

Tubes of different diameters can therefore be manufactured from a single device, with constant hourly flow rates, whatever the diameter, rods, profiles, etc. We can also manufacture body parts, containers, packaging, corrugated or thermoformed sheets, etc.

The device for implementing the method is characterized in that it comprises, upstream of a sintering die, means for exerting, along the axis of the die, a pressure on a crosslinkable pulverulent mixture, to preform this mixture by compression and to make it progress in the die and, downstream of the die, a tank intended to contain a bath of molten salt, for the crosslinking of the sintered product leaving the die.

*

The device according to the invention may also comprise between the outlet of the die and the crosslinking tank desytfoyens to stretch the shaped material. / î /

Other characteristics of the invention will appear during the description which follows with reference to the appended drawing, in which;

- Rig. IA and IB together represent a device for the manufacture of crosslinked high density polyethylene tubes;

- Fig. 2 is a view in axial section of a variant of the shaping template used in the device of FIG. IB;

- Fig. 3 is a sectional view along line 3-3 of FIG. 2;

jO - Fig. 4 is an axial sectional view of another variant of the shaping template used in the device of Fig.lB;

- Fig. 5 is an axial sectional view of an alternative form of the outlet end of the molten salt bath of FIG. IB used for crosslinking the material of the tube;

- Fig. 6 is a schematic view of a device comprising drawing means;

- Fig. 7 is a view in longitudinal section of the stretching and crosslinking means used in the device of FIG. 6.

The device shown in Figs. IA and IB comprises a vertical part A in which a pulverulent mixture containing a polyethylene and a crosslinking agent distributed in a homogeneous manner is formed into a tube and sintered, and a horizontal part B where the crosslinking of the material takes place.

Part A, of vertical axis XX, comprises a horizontal frame 1 which is supported by uprights 2 and serves to support a die 4 of axis XX, having a sole 5 in contact with which the clamping of the shoulder 6A of threaded tie rods 6 by means of nuts 7 ensures a fixing on the frame. The die 4 has pa30 parallel to its axis, a cylindrical body 8, in which is formed the bore 9 of the die 4, coaxial with a central bore 3 of the frame 1, and which can be heated by circulation of oil in 8a . The upper end of the die 4 has a flange 10, crossed by the tie rods 6, and the upper face of which supports a feed hopper 11 which can be cooled, by a circulation of water 12, and the diameter of which opening at the base corresponds to that of the bore 9. Between the hopper 11 and the flange 10 is interposed a metal crown 11a which is neither chilled, nor. heated, and the contact surfaces of which are notched so as to reduce heat transmission.

Against the sole 5 of the die 4 and inside the bore 3 of the frame 1 is disposed vertically a die 13, coaxial with the die 4 and which is held in place by means of a lower support plate 14 fixed on tie rods 6.

This plate is pierced with an opening 15 whose diameter is slightly greater than that of the bore of the die 13. The assembly thus forms, from the base of the feed hopper 11 to the opening 15 of the support plate 14, a straight cylindrical conduit. The die 13 is also provided with annular heating means, for example constituted by external electrical resistors 17,

The frame 1 serves as a support, externally to the die 4, to two jack cylinders 18, 19 containing pistons 20, 21 on the vertical rods 22, 23 of which is fixed a horizontal plate 24 by bolting at 25, 26. The plate 24 can slide on the tie rods 6 and has in its center an opening 27 which extends vertically inside an annular boss 28 integral with the plate 24 and constituting a piston which is cooled, in the vicinity of the plate 24, by a circulation of water 29, and which cooperates with the portion of cylinder formed by the opening of the ring 11a and by the upper part of the wall of the bore 9 of the die 4.

The tie rods 6 support at their upper end, above the plate 24, a plate 30 on which rests a cylinder cylinder 31 containing a piston 32 integral with a mandrel or vertical punch 33 which is engaged in sliding contact in the opening 27 of the piston 28 and guided by the plate 30 and by a horizontal plate 34 integral with the mandrel and sliding on the tie rods 6.

The mandrel 33 extends inside the die 4 and the die 13, the limit of the upward stroke of its lower end being situated approximately at the level of the lower end of this die 13. The the mandrel also comprises a heating element 35, with oil circulation or electrical resistance.

The device is supplied with power from the dosing unit 37 connected to the hopper 11 by a chute 38 of a feeder which must ensure a homogeneous distribution of the powder in the interval existing between the mandrel 33 and the die 4. The conduit cooling 12 of the hopper makes it possible to prevent the polyethylene, whatever its residence time in the hopper, from reaching a temperature at which it would become coherent under the effect of the beginning of melting.

The cooled zone is in fact a preforming zone whose height is of the order of magnitude of twice the diameter of the mandrel 33, in which the mixture must remain pulverulent so that it can be shaped into a tube, under the densification action exerted by the piston 28, in the tubular part 36 formed above the die 4 and, if necessary, at the entry of the latter.

At the start of a cycle, the piston 28 is in the high position and the mandrel 33 also. A. no pressure is exerted on the piston 32 of the jack 31. Thanks to the action of the jacks 18, 19, the piston 28 is lowered which passes through the pulverulent mixture placed in the hopper 11 where it is kept at the temperature ambient by the cooling duct 12. The piston 28 continuing to. compact the mixture down into the tubular part 36. The pressure required for this compaction is of the order of 1000 bars. The mixture is therefore shaped and simultaneously densified. It is then entrained in the die. The punch 33 simultaneously undergoes a downward movement, under the effect of the entrainment of the material, which is then subjected in the die 4 to a rise in temperature created by the heating elements 8 of the die 4 and 35 of the mandrel 33. The temperature is adjusted to a value which causes the material to melt but remains below the temperature at which the decomposition of the crosslinking agent becomes sensitive. This temperature will, for example, be between 150 and 180 ° C. When the piston 28 reaches the low point of its stroke, the pressure acting on the pistons 20, 21 of the cylinders 18, 19, is reduced and, the piston 28 being held in the low position, the mandrel 33 is raised using the piston 32. When the mandrel has reached its high position, the piston 28 is raised in turn and a new cycle begins.

The die 4 therefore constitutes a sintering zone from which the

- material is pushed, thanks to the downward movements of the piston 28^Zalong the punch 33, in the die 13 where the temperature is maintained at the same level as before, and from which it emerges in the form of a blank. At the outlet of the die 13, the blank is pushed, thanks to the downward movements of the piston 28, into a sheath 15a, P £ .r example in the form of a bellows, where the temperature is maintained at approximately the same level as previously or, where appropriate, is raised to a higher value, and which constitutes a transition zone in which the blank, free with respect to the wall of the sheath 15a, can undergo lateral deflections absorbing the jolts of its progression , taking into account the continuous traction which is exerted on it, downstream, towards part B which constitutes a crosslinking zone.

Part B consists essentially of a closed horizontal tank 41, preferably cylindrical, compartmentalized longitudinally by two vertical partitions .42, 43, containing a bath 44 of molten salt, and heated for example by means of electrical resistors 46 at a temperature allowing crosslinking, that is to say greater than 200 ° C. In the upstream compartment 47 penetrates a tubular connector 48 whose upper end, wide; is fixed on the tank 41, its other end, whose duller diameter 20, narrower, corresponds to the external section of the tube to be crosslinked, being fixed to the entry end of a shaping template 49 supported by the wall of the tank 41 by means of elements 51. The sintered tube engages with play in the inlet end of the connector 48, the section of which will shrink, to penetrate, after a change of direction by 90 ° in the cylindrical bore 52 of the template 49 which thus ensures a profiling and a uniform surface finish to the tube, which simultaneously undergoes crosslinking at least in its peripheral surface.

The tube then passes through the two other compartments 53,

54 in which the crosslinking is completed and where it is guided by rings 56 secured to the wall of the tank by supports 57 and whose internal opening 55 has a toric shape ensuring with the tube a sliding contact of a linear nature. The crosslinked tube comes out of the end wall 58 of the tank by passing through a sealed ring 59 in sealed contact.

Also contributes to the calibration of the tube immersed in the bath 44 the pressure of a gas kept under pressure at the i of the bore of the tube and introduced from a manifold

<img file="LU79128A1_D0001.tif" />

the Intermediate of an axial duct 62 provided in the mandrel 33.

The gas used is preferably 3'nitrogen.

The molten salt constituting the bath 44 is advantageously a eutectic mixture of mineral salts, for example nitrates and nitrites, such as the eutectic having the following composition:

- S3 parts by weight of K N0_

- 40 parts by weight of Na NO ^ parts by weight of Na NO ^

The partitioning of the tank 41 makes it possible to establish different temperatures in the different compartments. The temperature is normally higher, for example between 250 and 300 ° C, in the compartment 47 containing the template 49, so as to obtain a rapid transformation of the material in the peripheral region of the tube. It will be lower in the other compartments 53, 54, at most equal to 250 ° C. for example, to avoid the risk of thermal degradation of the pioduit.

For the treatment of a tube 3 mm thick, the length of the tank 41 can be approximately 2 m, with a circulation speed of the tube which is in this case 150 m / h.

PO The reference 63 designates a device, shown schematically and partially, constituted by two movable tracks in contact with the crosslinked tube, to exert a continuous traction on the latter.

An additional shaping template may be provided beyond the traction device 63 or between this device and the tank 41.

Satisfactory tests have been carried out using as thermoplastic material a high density polyethylene, of molecular mass between 300,000 and 500,000, for example a polyethylene PHILIPS, marketed under the brand "MANOLENE 56 020 / 'with density 0.956 and viscosity index equal to 2 under a load 2 of 20 kg / cm.

In the mixture have been incorporated, in a homogeneous manner, additives comprising, in addition to an organic peroxide, dyes, antioxidants, anti-UV agents, lubricants, all of which are known products used in the materials processing industry. plastics. As crosslinking agent, a formula catalyzed with 0.5% of ditertiobutyl4 peroxide has been used,

<img file="LU79128A1_D0002.tif" />

sold under the name of ”ÏRIGONOX B

In this case, one works at the rate of 70 strokes of the piston per minute and, for a total stroke of the piston of 40 mm, the length of tube produced is approximately 20 mm, the displacement of the mandrel é5 being from 20 to 25 mm .

Tests have been carried out, during which the elongation between two marks of a test piece standardized ISO 1/2 is measured, using an extensometer, in accordance with standard NE T 51 034, 1 at traction speed. being 100 mm / min. The results of the tensile tests showing the mechanical characteristics measured on a tube as a function of the extruded length, and reproducing the average value for 5 test pieces, appear in the following table:

<td>Extruded length (m)</td><td> 100</td><td> 500</td><td> 1 000</td><td> 1 500</td><td> 2 000</td>

<td>1 Stress at flow threshold (in Kg / crn ^)</td><td> 180</td><td> 184</td><td> 170</td><td> 190</td><td> 182</td>

<td>Breaking stress (in Kg / cm ^) 1</td><td> 300</td><td> 280</td><td> 290</td><td> 270</td><td> 280</td>

<td>Elongation at break (%)</td><td> 520</td><td> 490</td><td> 460</td><td> 500</td><td> 470</td>

<td>Burst pressure of tubular specimens (bars)</td><td> 55</td><td> 52</td><td> 57</td><td> 50</td><td> 52</td>

The Eig. 2 and 3 represent a conforming template, used in place of the template 49 of the Eig. IB, and constituted by several groups 64, 65, 66 of profiteer rollers 67 of concave profile, which give the tube a progressively decreasing section.

It is also possible to obtain a reduction in the desired diameter, to use a template as shown in FIG. 4, constituted by a succession of shaping rings 68a, 68b, with cylindrical bore, of different diameter. These teflon or polished steel rings are separated by dividers 69 and now bare 35 in place by a clamping device 70, possibly supplemented by a play take-up system, not shown. /

At Eig. 5, the final product, which is for example an unclosed profile, instead of leaving the tank 41 by passing through a sealing device on the end face, is extracted through an upper opening 71 of the wall of the tank to which it is brought by means of a guide 72.

In the case where the product does not have a tubular shape, the mandrel 33 of FIG. IA is deleted. If it is a question of sheathing a cable or an armature, this cable, or this armature, replaces the mandrel 33.

• 35

In the diagram of FIG. 6, the reference 101 designates a forming device in accordance with part A of FIGS. IA and IB, which produces a tubular blank or parison 102 at a speed v1; the latter passes through a tank 103 in which it is subjected to drawing and then crosslinking operations, then it enters successively into a drawing drawing machine 104 at a speed v2 greater than the speed VI, a cooling tank. 105, a 10 ό drive drawing machine at the same speed v2 as above or substantially equal, and a rewinder 107.

As shown in Fig. 7, the parison 102 emerges from the forming device 101 through the opening 15 of the lower support plate 14. It then passes through a device making it possible to eliminate the jolts due to the discontinuity in the advance of the parison and entering the tank 103 continuously. An example of such a device is shown in FIG. 7. In this device, the parison 102 undergoes a change of direction around a grooved pulley 108 mounted madly on a stirrup 109 itself pivoting in a vertical plane around an axis 110 mounted on a support 111. The groove of the pulley 108 has a depth at least equal to the radius of the parison 102.

The parison 102 then passes over a driving pulley 112 also with a groove, connected by means of a variable speed drive 113 to an electric motor 114 (schematically represented). The parison thus undergoes a new change of direction which brings it into a drawing and crosslinking tank 103. This tank contains a mixture of molten salts of the same type as that of the tank 41 of FIG. IB, distributed in two stretching 115 and crosslinking 116 compartments separated by a diaphragm 117, having an opening 118 allowing the parison 102 to pass. /. VS '

Inside the tank 103, in the compartment 115, / are

I 2 assembles several profiling rollers 121a,] 2] b, 121c and 121d, the profile of which follows the geometry of the drawn product, all integral with an operating rod 119 mounted on articulations 120a and

120b. The crosslinking compartment 116 has holding rollers 122. If desired, the profiling rollers can be replaced by other suitable profiling devices. The number of sets of rollers or profiling devices can vary depending on the desired draw rate. The retaining rollers can be, if desired, replaced by any other guide and support device, such as gutters, rings, etc.

After passing through the tank 103, the parison 102 is driven by a drawing machine 104 of known type, for example with belts, driven at the speed v2 y vl by a motor group 114a-variator 113a schematically represented, then it enters the tank of cooling 105, before being wound on the reel 107 after having possibly passed over a new drawing machine 106, which can be of the same type as the drawing machine 104 and which rotates at a speed equal to, or close to, v2. This drawing machine is intended to facilitate the extraction of the tube, but in no way to produce a new reduction in section.

The parison 102 leaves the forming device 101 in a discontinuous movement, the downtime corresponding to the ascent of the forming mandrel, then it is driven continuously to the other devices of the installation by the driving pulley 112 whose linear speed is vl. The swaying of the idler pulley 108 around its axis 110 then compensates for the lack of material supply during the downtime of the parison. The diameter of the parison reaches a value dl at this moment and it is maintained by means of a gas pressure inside it, to avoid collapse of the parison.

The parison of diameter dl therefore penetrates at speed vl into the drawing compartment 115. The effect of the drawing machine 104 which follows, whose linear speed v2 is chosen to be greater than vl, is then felt and the parison is stretched longitudinally. Its diameter is reduced to a dll value slightly less than dl, as soon as it reaches the first set of rollers 121a, and its diameter at. dyes a d2 value significantly lower than dl, at the exit of the baj

15, due to the longitudinal stretch. The profiling rollers

121 cl support parison 102, while crosslinking has not yet occurred. The temperature in the drawing compartment 115 is chosen so that the half-life of the catalyst is still long; it is close to that of shaping in the forming device, for example of the order of 160 to 190 ° C. . .

At the opening 118 of the diaphragm 117, the parison has dimensional characteristics (diameter, thickness) homothetic to those which it exhibited when it entered the stretching compartment 115. It then enters the crosslinking compartment 116 where the temperature reaches values ​​greater than 200 ° C and preferably between 220 and 250 ° C. The decomposition of the catalyst then causes crosslinking of the material and the stretching phenomenon is stopped by this crosslinking.

The location of the diaphragm 117 in the tank 103 is chosen as a function of the desired diameter of the tube at the outlet of the crosslinking compartment 116, this diameter being determined by the ratio of the speeds v2 / vl. Similarly, the different rollers 121a, 121b, 121c, 121d are chosen accordingly since their geometry must adapt as precisely as possible to the profile to be obtained in the case where this profile is different from that of the parison 102 leaving the device. forming. But in the case where the profile to be obtained remains a smooth homothetic tube of the tubular parison 102, the rollers 121a, 121b, 121c, I21d, are no longer used for calibration or profiling but only for support and guidance and can be replaced by all equivalent means. In all cases, the rollers 122 serving only for supporting and guiding the tube during its crosslinking, can also be replaced by any equivalent support and guiding means.

At the outlet of the tank 103 and after passing through the drawing machine 104, the tube is cooled in the tank 105 containing for example water. It is then taken up by the drawing machine 106 and wound on the winder 107.

A remarkable property of the product thus stretched and crosslinked is its dimensional stability and the absence of elastic memory or shrinkage.

For example, a polyethylene profile stretched four times .Z.

<img file="LU79128A1_D0003.tif" />

in the stretching compartment 115, then crosslinked in the crosslinking compartment 116 undergoes no other shrinkage than simple thermal shrinkage. Crosslinking blocks strings stretched relative to each other. The crosslinked product is birefringent but stable in its dimensions above and below the melting temperature of the polyethylene crystallites, except for the volume expansion.

On the contrary, the same section stretched four times in compartment 115, then simply cooled instead of being crosslinked, regains, by elastic memory, its original dimensions as soon as the temperature is again brought to 140 ° C., temperature higher than that of polyethylene melting.

The following nonlimiting examples are given by way of illustration of the invention, with reference to the process of FIGS. 6 and

7.

EXAMPLE 1

High density polyethylene, of molecular weight between 300,000 and 300,000, such as that sold under the brand MANOLENE 56,020 was used to shape a tube under the above conditions.

Additives have been added which include, in addition to stabilizers and dyes of known type, a crosslinking agent which is 2,5-dimethyl-2,5- (ditertiobutyl-peroxy) -hexyne-3, so ld under the name of LUPEROX 130. The operating conditions for different diameters, from a parison diameter dl = 27.5 mm, are given in Table I below.

TABLE I

<td>i Exchanged ple</td><td>" tn o P i— ( • H&gt; &gt; mh</td><td>0) W Q) 3 ü) Q) WM (U -H P p cm H 'Φ&gt; &gt; * 0) CM mh</td><td>Output flow</td><td>extrude</td><td>P M &lt;udaadem 0 · Η 0 P • O H ° C</td><td>ο T.compart, crosslinking</td><td>0 external gd 2 tube A. la ³ exit bac '115</td><td>M 0) 3 CW , not G) G) P Ό. X 3 G) + J m p mm</td><td>i winder</td><td>ί 1 thickness ί</td><td>weight Has</td>

<td> 1</td><td> 35,4</td><td> 140</td><td> 9,</td><td> 1</td><td> 185</td><td> 220</td><td> 18</td><td colspan="2"> 17,30</td><td> 1,03</td><td> 60,6</td>

<td> 2</td><td> 35,4</td><td> 140</td><td> 9,</td><td> 1</td><td> 180</td><td> 220</td><td> 17</td><td colspan="2"> 16,68</td><td> 1,02</td><td> 54</td>

<td> 3</td><td> 35,4</td><td> 180</td><td> 9,</td><td> 1</td><td> 175</td><td> 230</td><td> 16,5</td><td> 15</td><td></td><td> 0,90</td><td> 48</td>

<td> 4</td><td> 35,4</td><td> 210</td><td> 9,</td><td> 1</td><td> 175</td><td> 240</td><td> 15,4</td><td colspan="2"> 14,50</td><td> 0,90</td><td>41.6, Λ ⁷_lllBM</td>

<img file="LU79128A1_D0004.tif" />

] 5

The diameter d3 is slightly smaller than the diameter d2; this does not result from a stretching but simply from a cooling withdrawal at the outlet of the salt bath from the tank 103.

EXAMPLE 2

Other tests were carried out with the same mixture as in Example 1, the different parameters having the following value:

vl = 36.6 m / h _V2 = 132 m / h dl = 27.5 mm d2 = 18.5 mm temperature of the stretching compartment = 180 ° C temperature of the crosslinking compartment - 230 ° C.

A tube with a diameter of 16 mm was then obtained at the start and at the end of the extrusion with a length of 2000 m; this tube has undergone fraction tests, the results of which are given in Table II below.

TABLE II

<td></td><td></td><td>sample</td><td>constraint at the listening threshold</td><td>breaking stress</td><td>elongation at</td>

<td> 20</td><td></td><td></td><td>lemegt kg / cm</td><td> . / ²kg / cm</td><td>breaking %</td>

<td></td><td></td><td> 1</td><td> 173</td><td> 214</td><td> 390</td>

<td></td><td>beginning</td><td> 2</td><td> 172</td><td> 166</td><td> 325</td>

<td></td><td></td><td> 3</td><td> 172</td><td> 213</td><td> 400</td>

<td> 25</td><td></td><td></td><td></td><td></td><td></td>

<td></td><td></td><td> 4</td><td> 172</td><td> 209</td><td> 360</td>

<td></td><td>End</td><td> 5</td><td> 176</td><td> 219</td><td> 370</td>

<td></td><td></td><td> 6</td><td> 169</td><td> 223</td><td> 380</td>

The crosslinking rate was 87% at the start as at the end and the shrinkage at 120 ° C measured according to standard NF T 54 021 of 4%. The shrinkage at 160 ° C measured according to the same, standard was 6%.

The process of the invention is in particular applicable to polyolefins crosslinkable by the peroxidic route and particularly advantageous for products of high molecular weight; for example for grades lower than 2.

It is also applicable to low polyethylene made crosslinkable by the addition of a peroxide.