Products of crosslinked thermoplastics material

INVENTORS: GENARO MARIE DENIS, with address at: 7? rué Pie rre Curie, 5 ^ 520 Laxou, FRANCE, BERNARD MARIE GINGLINGER, residing at: 14 Le Point du - Jóur Route de Montauville, 54? 00 Pont-A-Mousson, FRANCE and CLAUPE MAURICE ROTHARHEL, residing at: Lotissement de Prochéville 5 ^ 700 Pont-AMousson, FRANCE, all of French nationality.

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The present invention relates to a method of manufacturing products of thermoplastic lattice material and gives a device for its implementation. More particularly it relates to the manufacture of crosslinked high density polyethylene products.

Crosslinking or vulcanization, whereby two neighboring polymer chains are joined together to form a three-dimensional network, under the effect of either radiation or activated carbon, is a known operation that can be applied to polyolefins, vinyl polymers and t · elastomers, in particular ethylene-propi elastomers,

- 2 log.

In the case of any chickenlefins, such as · low-density 11-ethylene and the nylon-propylene turners »when an activated carbon is used as a crosslinking agent that produces an organic peroxide» the final product can be obtained from the starting powder mixture without difficulty proceeding successively to a shaping "by practical realization of the classic toonies of transformation of plastic materials" such as extrusion and injection "and to the crosslinking operation" or vulcanization »which is carried out, for example, by passing the product formed by an autoclave through a fluidized bed · In effect» the temperature at which the forming is operated in the extruder or injection press »is clearly lower at the temperature necessary to achieve the provocation of crosslinking by decomposition of the classically used organic peroxides »such as i

- dieumyl peroxide;

- ditereiobutyl peroxide;

- Ψ-bie-t-butyl valerate peroxide;

- 2'5dimethyl-2-5di- (t-butyl) -hexane peroxide;

- peroxide of 2,5 "dlmetll-2,5" di- (t-butyl) -hexlno 3 · Between the forming phase »during which the plastic material has a viscosity that allows its transformation according to the classic procedures» and The clone * reticulum phase has a sufficient temperature interval so that the two operating alons are perfectly separated and avoids a * b (the risk of premature decomposition of peroxide.

It occurs differently when it is necessary to crosslink high-density polletllone since this material does not reach a dcfbll viscosity for its conformation by extrusion or injection more than at a temperature or between 170 and 220 ° C # depending on the molecular mass of the high-density polyethylene * that is, a temperature higher than the temperature at the beginning of the peroxide composition (approximately 140 ° C for the less active peroxides) so that it is applied in the High-density re-lattice of the procedures used for the production of objects in low-density polyethylene or ethylene-propylene elastomer is varied in the mezola subjected to the forming operation, a crosslinking removal that would be undesirable because * in particular in in the case of an oxtruder *, the material would then be subjected to a cut that would prevent the correct lime scale from crosslinking and give the final product a very mediocre quality. has proposed to suppress this cut using a heated row of forming and crosslinking in which the mixture is densified and forced to progress in the tooling under the effect of a pressure crooked by a piston * but it is necessary ontonoea * to avoid em sin the

- 4 harmful deformation material, provide on the surfaces of the row with which the material comes into contact a coating of an anti-adherent material such as polytetrafluoroethylene.

Indeed, in this case an excellent quality of the products is observed but this t / enloa does not allow to consider a practically continuous production since it is found that afterwards it gives a relatively short time of passage of the material * it airs in the crosslinking zone of the spinneret a deposit that damages the quality of the product · This dapósl, to estí constituted by partioulas of polietlleno retoulado that adhere to the wall and pot is due to the radicals 11, During the reaction, they are active enough to activate the surface of the anti-adherent material and give it a bond.

Attempts have been made to inject a significant amount of lubricant into the swath before the crosslinking zone * but this tonic * in addition to the fact that it poses difficulties of realization due to the unstable sliding regime of the fluid in the swath * does not allow the tank to be removed in the crosslinking zone.

To solve the problem posed * the Invention proposes a procedure for the manufacture of products of thermoplastic material removed from a * essentially pulverulent * mixture that contains the thermoplastic material

- 5 and Tin crosslinking agent according to which the densification of the mixture is ensured by exerting on it a pressure that makes it penetrate and progress in a heated row and causes crosslinking of the material, characterized in that the mixture agglomerates <£ powdery by exerting on £ Lla a pressure - oriented along the axis of the row, the mixture is caused to melt in the row by subjecting the latter to a temperature lower than the temperature at which the decomposition of the crosslinking agent is sensitive, and, downstream of the row, the crosslinking of the shaped material is caused by cutting it go through a molten salt bath whose temperature allows the decomposition of the crosslinking agent.

In addition to the fact that the proposed procedure foresees a pre-shaping phase of the powdery mixture, which allows a very favorable direct dimension in the die, it ensures that there is a net separation of the shaping phase itself, by agglomeration of the phase Crosslinking. The term agglomeration encompasses the operation of densification and fusion 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 once the gelled product can, as it were, follow a free path in the crosslinking zone, in the sense that, while Once the thermal exchange by the bath liquid is assured, the: The product does not have to slide in contact with the walls and, in order to allow its progression to the outlet, a simple mechanical gaffing that * in the case of a tube * can be obtained by a linear oontac or with the toroidal superitele of anillog supported by 1 «cuba quo containing the bath and otherwise benefiting from a lubrication effect in the liquid. However, it may be advantageous * to assure the calibration of the product * to provide a conforming gauge on the bath entrance screed, the iftll of which can be constant or variable. IF it is a tube * its internal diameter * -to the deoreoer at the height of the gauge * -can be kept within the desired tolerances * by the effect of an inert gas * such as nitrogen¿ injected into the tube ·

In the application of the process to high-density polyethylene products * the absence of any cut during retloulaolóh allows to obtain optimal properties. The elongation at nptpture of test specimens subjected to tensile tests is between 400 and 5 ° 0 and this result is independent of the time elapsed from the beginning of production of the device *, since no crosslinking occurs in the crosslinking zone. deposit whose effect is to lower the quality of the products. Therefore it is possible to consider a continuous industrial production since the need to frequently change degraded organs for a superficial deposit * that is a disadvantage of the known procedures * is suppressed ·

The proposed procedure can be used for ¥

- 7 manufacture of pipes and profiles, as well as for the reverse, cable gutting OR of metal armor.

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

By stretch, it is understood here not only a longitudinal extension or an elongation, but even everything formed by longitudinal and radial expansion, obtained in particular under the action of a punch d of a die, 6 under the action of a fluid a pressure, operation that can take place inside or without intervention of a mold ·

Thus, it can be manufactured from a single device made of tubes of different diameters, with constant flow rates, whatever the diameter of the shaped bars, etc. Likewise, body parts, containers, packaging corrugated plates OR thermoformed plates, etc. can be manufactured.

The device for carrying out the procedure is characterized in that it comprises upstream of a row of agglutinate means to exert, along the axis of the row, a pressure in a crosslinkable powdery mixture, to pre-shape this mixture by compaction and to make it - add a tank destined to contain a molten salt bath for the reticu in the row and downstream from the row -

- 9 portion of the agglutinated product leaving the row ·

The device according to the invention may further comprise means for stretching the shaped material between the die outlet and the crosslinking tank.

Other characteristics of the invention may be stated below with the following description and with re. Reference to the attached drawings, in which:

Figures 1A and IB together represent a device for the manufacture of crosslinked high-density polyethylene pipes.

Figure 2 is an axial sectional view of a variant of the forming gauge used in the device of Figure IB,

Fig. 3 ^is a sectional view according to line 3-3 of figure 2.

Figure 4 is an axial sectional view of another variant of the forming gauge used in the device of figure IB.

Figure 5 is an axial section of a variant shape of the outlet end of the molten salt bath of Figure IB used for crosslinking of tube material.

Figure 6 is a schematic view of a device comprising stretching means.

Figure 7 is a longitudinal section view • 9 of the stretching and crosslinking means used in the device of Figure 6 ·

The device depicted in Figures Id and IB comprises a vertical part d in which a powdery mixture containing a polletlen and a homogeneously distributed crosslinking agent is tube-shaped and calcined * and a horizontal part B where it is operated the crosslinking of matter ·

The part d with vertical axis XX comprises a horizontal frame 1 that was supported by uprights 2 and serves as a support for a row 4 with axis XX * that has a base plate 5 θη which supports the stirrup 6d of threaded ties 6 which are fastened by nuts 7, thus ensuring a fljaoión in the frame · The row 4 comprises, parallel to its axis *, a hollow body 8 in which the gauge 9 of the row 4¿ is formed coaxial to a central gauge 3 of the frame 1 * and it can get hot by oil circulation in 6a · The upper end of row 4 has a collar 10 crossed by tie rods 6 * and whose upper face has a feed hopper 11 that can be cooled by means of a 12 ”water circulation and whose diameter is opening in the base corresponds to that of calibration 9 «Between the hopper 11 and the collar 10 there is a metallic crown that is neither cooled nor heated and whose contact surfaces are notched so as to reduce heat transmission.

- 10 Against the base plate 5 of row 4 and Inside frame 3 gauge 1, a row 13 * is arranged to calmly pour coaxial to row k and is kept in position thanks to a lower support plate 14 fixed on the braces 6 ·

This plate is perforated by an opening 15 whose diameter is slightly larger than that of the row 13 gauge · The assembly thus forms *. From the base of the feed hopper 11 to the opening 15 of the support plate 14 * a rectilinear duct cylindrical · Row 13 is also provided with annular heating means * for example made up of external elotory resistances 1? «

The frame 1 serves as support * outside the row 4¿ to two jack cylinders 18 * 19 containing pistons 20 * 21 in whose vertical rods 22 * ​​23 a horizontal plate 24 is fixed by bolting in 25 * 26 · The plate 24 Can it slide on the straps 6 and has 2 in its opening center? which is prolonged I poured warmly inside an annular projection 28 that forms body with plate 24 and that constitutes a piston that is cooled * near plate 24 * by a circulation of water 29 «and that cooperates with the cylinder portion formed by the opening of the crown and the upper part of the wall of the row 9 of row 4.

The straps 6 support at their upper extremity

- 11 by means of plate 24 * a plate? 0 on which a jack cylinder 31 is supported, which contains a track 32 integral with a mandrel and vertical punch 33 Which fits in sliding contact in opening 27 of piston 28 and it is guided by stage 30 and by a horizontal plane. 34 supportive of the drill and sliding on the braces 6 ·

The mandrel 33 extends into the row and of the row 13 "the limit of the ridge being situated upwards from its lower end approximately at the height of the lower end of this row 13 * The mandrel also comprises a heating element 35" of olive oil or electric resistance.

The device is fed from a tank-doser 37 connected to the hopper 11 by a 3-channel which must ensure a homogeneous distribution of the powder in the interval between the mandrel 33 7 la lera 4. The cooling duct 12 of the hopper makes it possible to avoid that the polletlleno * whatever its time in the hopper * cannot reach a temperature at which it would become coherent under the effect of a beginning of fusion ·

The refrigerated zone is in effect a pre-forming zone whose height is of the order of magnitude twice the diameter of the mandrel 33 " ⁱⁿ I^to That the mixture must remain powdery so that it can be shaped into

- 12 ma of tube under the densifying action exerted by the piston 28 "in the tubular part 36 took over the row 4" and * eventually; at the entrance of this last »

At the beginning of a circle * track 28 is in a high position and mandrel 33 is likewise. No pressure is exerted on the piston 32 of the jack 31 · Thanks to the aoolon of the jacks 18 «19 *, the piston 28 is lowered, which passes through the pulverulent mezzola placed in the hopper 11 where it is kept at room temperature by the duct cooling 12 · The piston 28 continues its descent and compacts the plunger in the tubular part 36 · The pressure required for this compaction is in the order of 1000 bar · The mixture is therefore shaped and simultaneously densified · Next is dragged to the row. The punch 33 simultaneously undergoes a downward movement under the effect of dragging the material which is then subjected in row 4 to a rise in temperature created by the heating elements 8 of row 4 and 35 of the mandrel 33 · temperature is regulated to a value that causes the melting of the material but remains below the temperature at which the decomposition of the crosslinking agent is sensitive · This temperature was * for example * between 150 and 180 ° C · When the piston 28 reaches the lower point of its stroke «the pressure that acides in the pistons 20« 21 of the jacks, 18 «19 is reduced * and the piston 28 is kept in the lower position *

L

-13 with the mandrel 33 being retracted with the help of the piston 32 · When the mandrel has reached its upper position, the piston 28 re rides in turn and starts from. again a cycle.

The row 4 therefore constitutes an agglutination zone from where the matter is pushed, thanks to the downward movements of the piston 28 along the punch 33 »in row 13 where the temperature is maintained at the same level as previously, and of where it comes out in the form of an outline. At the exit of the row 13, the outline is pushed, thanks to the downward movements of the piston 28, in an envelope 15 & for example in the form of a bellows, where the temperature is maintained approximately at the same level as previously or, eventually, it is brought to a higher value, and it constitutes a transition in which the outline, free with respect to the wall of the envelope 15a, can undergo lateral displacements that absorb the shocks of its progression, taking into account the continuous traction that is exerted on it, downstream, towards part B that constitutes a cross-linking zone.

Part B consists essentially of a closed horizontal tank 41, preferably cylindrical, longitudinally compartmentalized by two vertical partitions 42, 43 containing a bath 44 of molten salt, and heated for example by means of electrical resistances46 to a temperature that allows crosslinking, that is to say higher than - 14 200 ° C. A tubular joint 48 penetrates into the anterior compartment 47 whose wide * upper end is fixed on the * * ouba 41 * fixing its other end * whose internal diameter; narrower * corresponds to the external seoolóh of the tube to be pulled * to the inlet end of a forming gauge 49 supported by the wall of the ouba 41 by means of element a 51 «The calcined tube adjusts tolerance to the inlet end of joint 48 * ouya secoióh tapers to penetrate after a 9 mb dlreoolon ball in gauge $ 2 of gauge 49 thus ensuring profiling and uniform surface state in the tube * undergoing simultaneously a crosslink at least on its peripheral surface »

The tube then passes through the other two compartments 53 * 54 in which the crosslinking is oonoluldo and where it is guided by rings 56 attached to the wall of the ou ba by supports 57 and whose opening 55 Internal has a toric shape that secures with the linear contact sliding tube. The crosslinked tube emerges from the terminal wall 58 of the ouba through a sealed ring through an added ring 59.

Also contributing to the calibration of the tube immersed in the bath 44 is the pressure of a gas kept under pressure inside the calibration of the tube and introduced from a nozzle 61 by a half-tube of an axial conduit 62 provided in the mandrel 33 · The gas used it is preferably nitrogen.

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

- 53 part in

- 40 parts in

-? parta by weight of K NO ^ weight of NaNO ^ weight of NaNO ^

The compartment of the tub 41 allows different temperatures to be set in the different compartments · The highest temperatures are normally highest »for example between 250 and 300® C» in the compartment 47 containing the gauge 49 »in order to achieve a quick transformation in the matter in the peripheral ruler of the tube. It will be less elevated in the other compartments 53 * 54 »at most equal to 250 ° C for example in order to avoid the risk of thermal degradation of the product.

For the treatment of a tube of 3 ®md © thickness, the length of the tube 41 can be approximately 2 m »approximately» with a circulation speed of the tube which in this case is 150 mA.

Reference 63 designates a device »schematically and partially represented» consisting of two mobile tires in contact with the cast tube · to exert continuous traction on this tire ·

- 16 A supplementary forming gauge may be provided beyond the pulling device 63 or between this device, the tank and the tub 41.

Satisfactory tests have been carried out using as thermoplastic material a high-density polyethylene * with a molecular mass of between 300,000 and 600,000 *, for example a PSILIPS polystyrene * sold under the brand name MANOLENE 56 020^w, of density 0 * 956 and viscosity index! dad equal to 2 under a load of 20 kg / cmS

* Additives have been incorporated into the mixture * in a homogeneous way, which include * in addition to an organic peroxide * colorants * anti-oxidizers * anti-UV agents * lubricants »which are all known and used products in the transformation industry of plastic materials. As crosslinking agent, in particular, a 0 * 5% catalyzed formula of diterclobutllo * peroxide sold under the name of^rtTRIGONOX B^H.

In this case * it works at a rate of 70 piston strokes per minute and * for a total piston stroke of 40 mm * the length of tube produced is approximately 20 mm * with the chuck displacement being 20 to 25 mm.

Tests have been carried out * during which the elongation between two references of an ISO 1/2 standard specimen is measured »with the aid of an external gauge» according to NP T 51 034 *, the pulling speed being

- 17 100 mm / min. The results of the tensile tests that show the mechanical characteristics measured in a extruded length of cast iron tube and that reproduce the average value for five specimens * appear in the following table:

<td>Extruded length (m)</td><td> 100</td><td> 500</td><td> 1.000</td><td>I.5OO</td><td>2X00</td>

<td>Effort at the threshold of displacement (in kg / cm2)</td><td> 180</td><td> 184</td><td> 170</td><td> 190</td><td> 182</td>

<td>Effort to seizure (in kg / cm2)</td><td>J00</td><td> 280</td><td> 290</td><td>27O</td><td> 280:</td>

<td>Elongation at break (X)</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></td>

Figures 2 and 3 represent a forming gauge »used in place of gauge 49 in Figure IB» and made up of several groups 64 »65» 66 of profile rollers 67 with concave profile »that give the tube a progressively decreasing seooidh.

It is also possible to achieve a decrease in the desired diameter »use a gauge such as that shown in figure 4» consisting of a succession of forming rings 68a »68b» of oil-gauge calibration, of

7'

- 18 different diameter. These polished steel δ teflon rings are separated by spacers 69 and held in place by a clamping device 70 'eventually supplemented by a tolerance recovery system, not shown.

In figure 5 »the final product, which for example is an unclosed profile, instead of leaving the tank 41 through a sealing device on the end face, is extracted through an upper opening? 1 of the wall of the tank to which it is taken by means of a guide 72..

In the event that the product does not have a tubular shape, the mandrel 33 of figure 1A is deleted. If it is about wrapping a cable or armor, this cable or this armature replaces the mandrel 33 ·

In the scheme of figure 6, reference 101 designates a forming device according to part 1A and IB, which produces a tubular outline or parison 102 at a speed vi; this sketch goes through a tank 103 in which it is subjected to stretching operations and after crosslinking, later and successively penetrating into a drive stretching machine 104 at a speed v2 higher than speed vi, a cooling tank 105 »a drive stretcher 106 at the same speed v2 as above or substantially the same, and a reel 107 As shown in FIG. 7 »the parison 102 emerges from the forming device 101 through the opening 15-19 of the lower support plate 14. It then passes to a device that allows to suppress the shaking due to the discontinuity in the parison advance and enter the tank 103 continuously. An example of the device is shown in figure 7 · In this device, the parison 102 undergoes a change of direction around a pulley 108 mounted wildly on a bracket 109 which is in turn pivoting in a vertical plane f

about 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.

EX parison 102 then passes over a driving pulley. 112 also with a throat, connected by means of a speed variator 113 to an electric motor 114 (schematically represented) * The parison thus undergoes a new change of direction that takes it to a tank de.stretching and crosslinking 103 * This tank contains a mixture of molten salts of the same type as that in tank 41 of figure IB, split into two 115 X crosslinking drawing compartments 116 separated by a diaphragm 117 »having an opening 118 that allows the parison 102 to pass.

Inside the tub 1031 in the compartment 115, several profiled sheaves 121a, 121b, 121c and 121d are mounted, the profile of which adopts the geometry of the stretched product, all of them integral with a manipulation rod Ϊ19 mounted on joints 120a and '120b. The compartment of re

- 20 tic 116 has maintenance rollers 122. If desired | profiling rollers can be replaced by other appropriate profiling devices. The number of Roller Sets 6 profiling devices can vary according to the degree of stretch desired. The maintenance sheaves can be, if desired, replaced by any other guide and support device, such as gutters, rings, etc.

After having passed through tank 103, parisún 102 is drawn by a stretcher 104 of a known type, for example of belts, at the speed v2 I saw by a motor group 114a-variator 113a schematically represented and then enters the cooling tank 104., before being wound on the reel 107 after having possibly passed through a new stretcher 106 which may be of the same type as the stretcher 104 and which rotates at a speed equal to or close to v2. This stretcher is intended to facilitate removal of the tube but in no way to produce a further reduction in section.

The parisén 102 leaves the forming device 101 according to a discontinuous movement, the stopping times corresponding to the rise of the forming mandrel and then it is continuously dragged towards the other devices of the installation by the driving pulley 112 whose speed linear is vi. Swinging idler pulley 108 around

- 21 of its axis 110 then compensates for the lack of material supply during the stop time of the parison. The diameter of the parison at that time reaches a di value and is maintained thanks to a gas pressure inside it, to avoid the collapse of the parison.

The parison of diameter di therefore penetrates at the speed vi into the drawing compartment 115 · The effect of the stretching machine 104 below, whose linear speed v2 is chosen to be greater than vi, is then felt and the parison is stretched longitudinally. Its diameter is reduced to a value of dll slightly less than di, from the moment it reaches the first set of sheaves 121a, and its diameter allocates a value d2 significantly less than di, at the outlet of tank 15, due to the longitudinal stretching. The profile rollers 121a to 121d support parison 102, while crosslinking has not yet occurred. The temperature of the drawing compartment 115 is chosen so that the lifetime of the catalyst is still important and is close to that of forming in the forming device, for example in the order of 160 to 1902C.

At the height of the opening 118 of the diaphragm 117, the parison has dimensioned characteristics (diameter, thickness) homothetic to that which it presented at its entry into the drawing compartment 115 · Next it penetrates the crosslinking compartment 116 where the temperature - 22 reaches values ​​higher than 2002C and preferably between 220 and 2502 C. The decomposition of the catalyst then causes the crosslinking of the material and the stretching phenomenon is stopped by this crosslinking.

The location of the diaphragm 117 in the tub 103 is chosen as a function of the desired diameter of the tube at the exit of the crosslinking compartment 116, this diameter being determined by the speed ratio v2 / vl. Likewise, the different sheaves 121a, 121b, 121c, and 121d are chosen -, consequently, since their geometry must adapt as precisely as possible to the profile to be achieved in the case that this profile is different from the parison 102 that comes out of the device of conformation. But in the case that the profile to be obtained is a smooth homothetic tube of the tube parison 102, the sheaves 121a, 121b, 121c and 121d, no longer serve for calibration or profiling but only for support and guiding and they can be substituted by any equivalent means. In all cases, the sheaves 122, since they are not used to support and guide the tube during its crosslinking, can also be replaced by any equivalent support and guide means. '

At the outlet of tank 103 and after passing the tedger 104, the tube is cooled in tank 105-i, which contains, for example, water. Below is taken by

- 23 the stretcher 106 and wound on the winder 107 ·

A notable property of the product thus stretched and re-gouged is its dimensional stability and elastic memory ausenola &lt;S contraction ·

For example * a four-way stretch polyfill profiling in stretch compartment 115 and then retouched in crosslinking compartment 116 undergoes no other shrinkage than simple thermal shrinkage. The crosslinking blocks the stretched chains with respect to each other. · The retouched product is blinding but stable in its dimensions due to enzyme and below the melting point of the polletllel orlstalitos * sal voldmlca dilation.

On the contrary * the same stretched profile four times in compartment 115 7 afterwards simply refrigerated instead of being retouched * it comes back · it was dying elastic * with its original dimensions from the moment it takes its temperature again haoia 140 ° C · temperature above that of melting the polletlleno.

The following non-limiting examples are given by way of illustration of the invention with reference to the process of Figures 6 and 7

EXAMPLE 1

High-density pullet * with a molecular mass between 300,000 and 500 · θ00 * such as the oomerolall used with maroa ^mMANOLENE '¹ 56 020 has been used to

- 24 mold a tube in the conditions above ·

Additives have been added which comprise * in addition to stabilizers and colorants of known type * a crosslinking agent which is 2 * 5-dlmetll-2¿5 ~ (ditertibtttil-peroxy) -hexlno-3f sold under the name of LUPEROX 130 " · The operating conditions for different diameters * from a diameter of paral are di ³ 27 * 5 or m * are given in table I * below ·

TABLE I

<td>Shows</td><td>Speed</td><td>Velooldad 2^δ</td><td>Flow</td><td>T. esti-</td>

<td></td><td>saw</td><td>stretcher v2</td><td>ex exit</td><td>rado ^toC</td>

<td></td><td>mh</td><td>mh</td><td>thunder-</td><td></td>

<td></td><td></td><td></td><td>dora kg / h</td><td></td>

<td> 1</td><td> 35,4</td><td> 140</td><td> 9.1</td><td> 185</td>

<td> 2</td><td> 35,4</td><td> 140</td><td> 9.1</td><td> 180</td>

<td> 3</td><td>35Λ</td><td> 140</td><td> 9.1</td><td> 175</td>

<td> 4</td><td> 35,4</td><td> 210</td><td> 9.1</td><td> 175</td>

<td rowspan="2">T. oompart.</td><td colspan="4">CONTINUATION TABLE I</td>

<td>External vi</td><td>6Í external d3 tube</td><td>thickness</td><td>weight</td>

<td>reticulaoidn</td><td>d2 tuto a la sa</td><td>in the curl</td><td>Him</td><td>g / m</td>

<td>®C</td><td>lida tank mm</td><td>τη τη</td><td></td><td></td>

<td> 220</td><td> 18</td><td> 17.30</td><td> 1.03</td><td> 60,6</td>

<td> 220</td><td> 17</td><td> 16.68</td><td> 1,02</td><td> 54</td>

<td> 230</td><td> 16.5</td><td> 15</td><td> 0,90</td><td> 48</td>

<td> 240</td><td> 15.4</td><td> 14.50</td><td> 0,90</td><td> 41,6</td>

- 25 Diameter d3 is slightly less than diameter d2; this does not result from stretching simply from a cooling contraction at the exit of the salt bath of the tank 103 *

EXAMPLE 2

Other tests have been carried out with the same mixture as in example 1 "with the different .parameters having the following value vi" 36 * 6 m / h v2 "132 m / h di - 27 * 5 mi d2 ³ 18 * 5 aim compartment temperature. stretched180® C cross-link compartment temperature = 230^fiC 'Then a tube with a diameter of mm was obtained at the beginning and at the end of the extrusion of one length; of 2,000 mj this tube has undergone tensile tests whose

<td colspan="5">results are given in table II below</td>

<td rowspan="2"></td><td rowspan="2">Shows one</td><td colspan="3">TABLE II</td>

<td>Effort at sliding threshold kg / cm2 173</td><td>Effort at break kg / om2 214</td><td>Extend to break X 390</td>

<td>I ate at twenty</td><td> 2</td><td> 172</td><td> 166</td><td> 325</td>

172

213

400

- 26 CONTINUATION TABLE II

<td>Our</td><td>Effort at the threshold of dis- ₂lizamiento kg / cnr</td><td>Effort at break &lt;kg / cm^¿</td><td>Elongation at break%</td>

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

<td>Final 5</td><td> 176 .</td><td> 219</td><td> 370</td>

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

The degree of crosslinking was $ 87 at the beginning as well as the end and the contraction at 1202C measured according to the NF T 5 ^ 021 standard of 4%. The contraction at 1602C measured according to the same standard was 6%.

The process of the invention is particularly applicable to peroxidically crosslinkable polyolefins and is particularly interesting for products with a high molecular weight; for example for tiers below 2. ·

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

The present embodiment thus exemplified is illustrative and not limiting, whereby modifications, modifications or improvements may be introduced to the previously detailed embodiment, without thereby escaping the scope of the sphere of protection of the present invention patent, which remains basically defined by the claims that follow · '