DK144513B - PROCEDURE FOR THE PREPARATION OF CROSS-BONDED FORMATED SUBSTANCES BASED ON CROSS-BONDABLE OLEPHINE POLYMERS OR VINYL CHLORIDE POLYMER MATERIALS - Google Patents

Description

(19) DENMARK ·) § \ Ά & /

(12) PRESENTATION WRITING ου 144513 B

DIRECTORATE OF THE PATENT AND TRADEMARKET SYSTEM

(21) Application No. 55 ^ 5/74 (51) lnt.CI.3 B 29 C 25/00 (22) Filing date 14 Oct. 1974 C 08 J 7/00 (24) Running day 14 Oct. 1974 (41) Aim. available 1 Apr 6 1975 (44) Submitted Mar 22 1982 (86) International application # - (86) International filing day - (85) Continuation day - (62) Master application no -

(30) Priority Oct 15 1975, 47995/75, GB

(71) Applicant to the SHARE COMPANY NORDIC CABLE AND WIRE FACTORIES, Copenhagen F, DK.

(72) Inventor Peter Carstens and, DK: Francesco Paolo Procida, DK:

Svend Sandholm Pedersen, DK.

(74) Associate Engineer Hofman-Bang & Boutard.

(54) Process for preparing cross-linked molded articles on the basis of cross-linkable olefin polymer or vinyl chloride polymeric materials.

The present invention relates to a process for producing crosslinked shaped articles whose dimension in at least one direction significantly exceeds the dimension in one or the other direction, on the basis of crosslinkable olefin polymers or vinyl chloride.

OQ

polymeric materials, preferably HD polyethylene, whereby the material rO

heat-formed to the desired item, which is then treated with an organic peroxide under heat.

= 3r

Examples of shaped items whose dimension in at least one direction ^ significantly exceeds the dimension in the other direction (s) may be mentioned pipes, cables, sheets and profiles. Thus, the invention is not limited to items made, for example, by injection molding, pressing, or injection molding, but also includes all other conventional manufacturing methods for polymeric materials such as extrusion, calendering, lamination, casting, blowing, etc.

The shaped blanks need not consist solely of polymeric material. Of course, the polymers can of course contain conventional fillers and other additives, but they can also contain reinforcing agents, and in particular, they can be cables with a metal core coated with insulating and / or semiconducting layers of polymeric materials.

Several million tonnes of synthetic polymeric materials are used annually for various purposes, and it therefore seems unnecessary to try to improve the properties of these materials. However, certain restrictions on the physical properties of these materials will prevent them from being used in a number of areas where their other properties make their use ideal. Pores, for example, polyethylene, may need to be mentioned the following properties which need improvement: (1) Resistance to plastic flow at room temperature and elevated temperatures.

(2) Resistance to solvents at elevated temperatures.

(3) Resistance to voltage cracking.

(4) Resistance to brittle fractures occurring in non-cross-linked PE pipes after long-term pressure testing at elevated temperature.

The emergence of high density (or high density) polyethylene (HD-PE) is a good step on the road to solving these problems, but cross-linking of HD-PE, a process similar to rubber vulcanization, provides even greater improvements in these properties.

In developing crosslinking in crosslinkable polymeric materials, it is a known technique to use free radical generators, usually organic peroxides, to chemically by thermal treatment and / or to induce crosslinking in connection with accelerators, activators and / or coagents. . In most known methods 3 U4513 for the manufacture of shaped crosslinked blanks of crosslinkable polymeric materials, the free radical generator is required to be blended or incorporated into the polymer prior to shaping.

Such incorporation of free-radical generators is usually done on conventional mixing apparatus. For example, low density polyethylene (LD-PE), two-roll mills or Banbury mixers can be used. The mixing apparatus is heated to approx. 110 ° C and polyethylene, which may contain fillers, antioxidants and coagents, are added and treated for an appropriate time until the material is homogeneously melted. Then the free-radical generator is added, after which an additional mixing time of 2 - 5 minutes is usually sufficient. The temperature should usually not exceed 120 ° C after the addition of the free-radical generator. When the mixing process is complete, the material is granulated in a known manner and is then ready to be applied to extruders or molding machines where the finished blanks are molded at such low temperatures as to avoid premature crosslinking or scorch.

The heat treatment, which is then necessary to induce crosslinking in the now finished articles, can be carried out in a number of ways:

For extruded workpieces, the heat can in principle be applied in continuous processes from water vapor / 1 CV (continuous vulcanization) or CCV (catenary (ie chain line) CV) systems, from salt baths and from fluidized beds or in the form of infrared light. or microwave energy.

The four first mentioned heat sources have the disadvantage that they all require that the article which is desired to be cross-linked must be supported in some way or other than to deform at the high temperatures at which the cross-linking takes place and where the non-cross-linked material would be in the molten state. Therefore, these methods have not been used, for example, in the production of profiles or pipes of cross-linked polymeric materials, but only in the manufacture of cables where the conductor is able to hold the material in place during the vulcanization. The fifth method (microwave energy) to supply the required heat to the finished workpieces is limited to polymeric materials which themselves have large dielectric loss factors (tg $) or which are added to filler by compounding, e.g. carbon black, which gives rise to large dielectric losses.

Instead of these very complex heat transfer methods, as mentioned above, the direct method has the extrusion of the granule containing the free radical generator extruded through the extruder head or a special tool while providing sufficient heat to cause cross-linking in the material, the disadvantage that the cross-linked material readily clogs the extruder and that the extruded blank is deformed and often torn, and a smooth surface is not obtained.

For example, the aforementioned methods for crosslinking crosslinkable polymeric materials where the free radical generator is added prior to shaping the finished blanks cannot be applied to HD-PE, since HD-PE requires such high processing temperatures, both during the mixing of the free radicals. radical-forming and during shaping that premature cross-linking or scorch cannot be avoided.

Another known method which can be used to prepare cross-linked extruded HD-PE blanks is in principle by mixing powdered HD-PE with peroxide and various auxiliaries in a mixing auger which feeds a high pressure hydraulic press a heated tool designed in the profile the finished item should have. This method has two major drawbacks: (1) The process can be performed in specially designed machines; (2) the process is unsuitable for producing cross-linked HD-PE blanks containing filler.

In order to mold items of cross-linked polymeric material according to the known methods of injection molding, pressing or injection molding, it is necessary to incorporate or mix the free radical former before the material is supplied to the forming machine. This causes the molding conditions to become unfavorable, since premature cross-linking or scorch can only be prevented at low machining temperatures, and furthermore, the actual cross-linking process must take place in the tool in which the workpiece is designed. This means that the material does not get optimum properties, and the long residence time of the workpieces. the design tool becomes the production limiting factor which, for most products, makes the process unprofitable.

c U4513 5

To avoid these drawbacks, the blanks have been made of polymeric materials without the addition of crosslinking agents and then ionizing radiation has been utilized to induce the crosslinking. However, such processes are limited to applications on small cross-sectional items. Furthermore, extensive expensive equipment and large energy consumption are required.

U.S. Patent No. 5,198,868 and the corresponding British Patent No. 940,876 disclose a method of vulcanizing molded articles by impregnating with a vulcanizing agent. In practice, the cooled item is immersed in a bath at 30 - 50 ° C containing an organic peroxide dissolved in an organic solvent such as acetone or alcohol. After a residence time of 1 hour to a few days, the blank is removed and the absorbed solvent is evaporated in vacuo at low temperature, whereupon the blank is heat treated at 100 - 220 ° C in an autoclave and later optionally in a press to obtain the vulcanization.

This process is complicated in that it involves a number of treatment steps and further has the disadvantage of being limited to elastomeric materials such as rubber, styrene rubber, butyl rubber and silicone rubber, but in particular linear amorphous copolymers of ethylene with α-olefins, especially propylene. . The process is less useful for treating olefin homopolymer blanks, especially polyethylene, especially because it is not possible to penetrate the peroxide vulcanizing agent. If one wishes to prepare blanks of polyethylene or high crystalline polypropylene, it is necessary to mix the homopolymers with linear ethylene-propylene copolymers to ensure uptake of the peroxide crosslinking agent.

German Patent Specification No. 1,056,822 discloses a process for surface refining of polyethylene made by the low pressure method by spraying a crosslinking agent, especially organic peroxides, in the form of a finely divided solution on the still thermoplastic blank. Immersion of polyethylene blanks made by the high pressure method in a solution containing crosslinking agent or in liquid crosslinking agent is disclosed as known in the art. In the method according to the present disclosure, only one surface crosslink is obtained and the penetration is conditional on a swelling of the surface. It is not necessary to perform a subsequent heat treatment under pressure.

German Patent No. 1,072,803 discloses a process for cross-linking moldings of polyethylene by treatment with dicarboxylic acid dichlorides, optionally in the presence of swelling agents such as hydrocarbons. However, the use of these acid chlorides requires working in an absolutely anhydrous environment, and the acid chlorides are otherwise both easily accessible and difficult to handle.

French Patent No. 2,068,882 discloses a crosslinking of blanks of special vinyl chloride copolymers, the crosslinking being conditional on the incorporation of specified silanes into the polymer molecular chains. The crosslinking is elicited by treatment in an acidic environment at 0 - 150 ° C, and the process is particularly aimed at films and fibers formed in the acidic environment and extracted therefrom during simultaneous crosslinking.

German Patent Specification No. 2,204,655 describes a method of making electric cables with coating and / or insulating crosslinked polyethylene, wherein crosslinked polyethylene containing peroxide is extruded together with the cable core supporting the polyethylene during the subsequent crosslinking under heat effect. The method is limited to polyethylene having a density of 0.935 or less and, as mentioned, assumes that the crosslinking agent is incorporated prior to extrusion.

German Publication No. 2,204,658 discloses a similar method using polyethylene having a density of 0.935 or greater. Here, too, the cross-linking agent is preferably incorporated into polyethylene during extrusion, but the introduction can also be made by treating the molded blank with gas phase peroxide and subsequent heat treatment at 250 ° C in a salt bath. The method assumes the presence of a cable core to support the polyethylene during the cross-linking.

U.S. Patent No. 3,705-257 describes cables made up of multiple layers, with one or more of the layers being cross-linked. The innermost insulating layer preferably consists of cross-linked PE which is extruded containing cross-linking agent and cross-linked in a subsequent vulcanization chamber. After extrusion of the outer semiconducting layer crosses the outer portions of this layer by bonding preferably by spraying dicumyl peroxide dissolved in acetone or in molten form. After application, the cable with the layer of crosslinking agent applied is transferred to a special curing chamber where crosslinking is induced by heating. Also, the method described herein is limited to the manufacture of cables.

German Publication No. 2,214,628 discloses a process for preparing foamed cross-linked polymer particles, whereby the polymer particles (granules) are suspended in an aqueous emulsion containing crosslinking agent and heated until the crosslinking has taken place. Only then is the foaming and shaping done. The foaming agent can be added to the particles either after or simultaneously with the crosslinking.

About the particles, it is stated in the document that it can be any particle shape with a dimension of 1 - 10 mm and more generally, there is no restriction on the shape and size of the particles, which must however be adapted to the purpose of use. However, since the particles are later to be foamed in the ratio of 5 to 30 times the size of the particle in a mold, however, there are quite narrow limits to the shape and size of the particles, and the skilled artisan's thoughts are not immediately directed to subjects whose dimension in at least one direction significantly exceeds the dimension of the particle. the other directions, such as pipes and cables.

In the process of the disclosure disclosure, an insoluble inorganic metal salt is used as the dispersant for the particles to counteract their adhesion, and a surfactant is also added to dissolve or disperse the homogeneous crosslinking agent and facilitate penetration into the polymer particles.

Regarding the useful polymeric materials, a large number of polymers have been listed, but the examples given are all based on the unspecified high-pressure (LD) polyethylene "Mirason-9" and exceptionally in a particle diameter of 3 "4 mm.

The desired gel percentages are indicated as approx. $ 10 - $ 70, stating that higher gel percentages make foaming difficult. However, there is nothing in the text which makes it probable that these "undesirable" higher gel percentages can be obtained at all, in the examples not exceeding 12%.

Therefore, if the average skilled person who had recognized the above-mentioned disadvantages of the prior art and wanted to provide a method for producing homogeneously cross-linked shaped articles whose dimension in at least one direction significantly exceeds the dimension in the other or other directions, with high gel percentages without risk of deformity, should assess whether the method described in the disclosure could be applied to this / would he come to the conclusion that significant prejudice would be overcome, especially if he wanted to work with HD-PE.

What he could know for sure from the disclosure was that particles of the order of 3 - 4 mm can be crosslinked to gel percentages of up to 70 # without bonding if heated in an aqueous medium containing dispersed peroxide by a surfactant. , and further comprising an insoluble inorganic metal salt.

About the stability of the mold in the process and about the possibility of a higher gel percentage, especially for items of thickness of more than a few mm, he could not deduce anything with certainty.

In contrast, he would not be able to escape the prejudice expressed in the aforementioned United States Patent No. 3,198,868, which describes the vulcanization of molded articles, such as cables, tubes and the like, of elastomeric materials, and where it is expressly stated that crosslinking of ethylene homopolymers is impossible unless EP copolymers are incorporated.

However, even though German disclosure 2 214 628 disproves this claim when the speech is about unspecified LD-PE particles of small dimensions, the prejudice of the United States patent must weigh heavily, since these are subjects of the same kind as in the application, not least when talking about HD polyethylene, whose molecular structure could in no way imply easier penetration than for LD-PE.

The object of the present invention is to provide a process of the kind set out in the preamble of claim 1 which is not subject to the limitations of known methods in workable items and in material selection, and which is particularly suitable for the production of HD polyethylene parts. without deformation.

This is achieved in accordance with the invention by proceeding as set forth in claim 1, which is considered surprising on the basis of the above-mentioned analysis of the prior art, in particular German Publication No. 2,214,628.

In particular, the implementation of the process according to the invention had to be considered impossible if it would not be found that the articles adhere together and cross-link irregularly, while no insoluble inorganic metal salts are added, but as shown in the examples, on the contrary, polyvinyl alcohol shown as present. insufficient in German Publication No. 2,214,628.

In the process of the invention, the blanks are prepared in the usual manner from the usual starting materials, with no organic peroxide added, but the starting materials may contain fillers and / or antioxidants and / or any coagent for the crosslinking. The choice of an olefin polymer or vinyl chloride polymeric starting material is germ. limited by two factors, namely the properties of the finished product and the processing machines available. Thus, if during the processing a voltage-free blank is not obtained, which may be due to the concrete extruder or other processing machine, according to the invention it is advantageous to make the blank tension-free before heating in the presence of peroxide by making an intermediate voltage equalization, e.g. by heating the blank before introducing it into the peroxide-containing medium.

For example, if the item is to be HD-PE, for example, the HD-PE type will be chosen, which gives the optimum driving conditions, so that the production of workpieces is smooth and error-free and the products have optimum properties. Furthermore, the process of the invention is not limited to blanks made by extrusion and injection molding, but can be used in connection with all methods of manufacturing blanks of polymeric materials.

When the blank is formed by one of the known methods, one or 10 m 513 more of the usual organic peroxides such as di-tert-butyl peroxide and diumyl peroxide or organic hydroperoxides such as tert-butyl hydroperoxide are introduced. A particularly preferred peroxide is 1,1-bis- (tert-butyl peroxy) -3,3,5-trimethylcyclohexane, which causes particularly deformation during the crosslinking, inducing crosslinking already at fairly low temperatures (ca. 100 ° C). The choice of crosslinking agent for a given system depends, inter alia, on the olefin polymer or vinyl chloride polymeric material to be crosslinked, the process being applicable in principle to all crosslinkable olefin polymer or vinyl chloride polymeric materials provided that the crosslinking agent or agents are soluble in the polymer.

Finally, the process according to the invention can fulfill the following conditions which are of particular importance in the preparation of articles for special purposes, either individually or in combination: (1) The process can be controlled to achieve the desired degree of cross-linking, ie. a cross-linking, for example, HD-PE, which is sufficiently high but not so strong as to achieve such a decrease in density that it is no longer HD-PE.

(2) The process can be controlled so that the items can exhibit an acceptable small dimensional change.

(3) The process can be controlled so that the blanks do not contain peroxide residues in appreciable amount.

For example, the introduction of the peroxide crosslinking agent or crosslinking agents can be effected by immersing the blanks in an aqueous dispersion containing a surfactant and one or more peroxides in the form of an emulsion prepared either by mechanical processing, e.g. in a high pressure homogenizer, or using surfactants such as emulsifiers, protective colloids or the like.

Surprisingly, it has been found that under the temperature conditions set out in claim 1, the peroxide will distribute relatively homogeneously in the blank, so that after the crosslinking process a crosslinking with a very good penetration depth is observed.

The crosslinking of the subject itself is elicited by an appropriate treatment, e.g. thermally, either in the design and / or in the introduction of the peroxide or peroxides or in a separate process step. In the first case, the heat needed for the design itself may be wholly or partially incorporated into the subsequent crosslinking process.

A particularly preferred variant of the process according to the invention consists in carrying out the introduction of the peroxide or peroxides and the crosslinking process in one and the same aqueous medium in a closed or pressure-tight system. Preferably, the aqueous medium has approximately the same density as the fabricated article, ensuring that the blank does not deform during the heat treatment. Therefore, if desired, the density of the medium can be corrected by adding one or more salts.

In order to achieve a particularly safe control of the degree of cross-linking, it may be appropriate, after the part of the heating during which the peroxide penetrates into the workpiece, to replace the peroxide-containing medium with an aqueous medium containing no peroxide, so that during the subsequent heating penetrates further peroxide.

Especially in the manufacture of cables, in order to prevent an adhesion in the winding of the cross-linked cable, a winding of the cable can be carried out after the penetration of the peroxide, but before the final treatment to produce the cross-linking.

Surprisingly, it has been found that the process according to the invention achieves a crosslinking with an extremely good penetration depth and homogeneity without deforming the blanks. A standard test for the extent of crosslinking is the determination of the insoluble portion of the crosslinked polymeric material by extraction in boiling stabilized solvent. For HD-PE extraction, in boiling stabilized decalin extraction, 80-100¾ of the material was found to be insoluble after crosslinking by the process of the invention.

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The treatment time, both with respect to the introduction of the crosslinking agent (s) and the crosslinking process itself, depends, inter alia, on the temperature course of the treatment, the olefin or vinyl chloride polymeric material or materials used, the thickness, peroxide or peroxides nature and the desired degree of crosslinking, but will usually be from some a few hours up to a day.

similar considerations apply to the treatment pressure, as one can work both at atmospheric pressure and at a greater or lesser overpressure. In general, the parameters temperature, processing time and pressure can be varied within very wide limits. Crucial to the choice are the desired properties of the final product, e.g. crosslinking degree, dimensional stability and permissible content of peroxide residues. These properties as well as the process process are also determined by the selected peroxide, whose properties in turn impose wider or narrower limits on temperature, time and pressure.

In particular, it can be mentioned that the blistering, e.g. caused by volatile constituents from any fillers or by decomposition products of the crosslinking agents known, e.g. from the continuous vulcanization described above is avoided by the method of the invention. This is achieved by a suitable choice of the above parameters, for example, a sufficiently long processing time allows the degradation products to diffuse out, while also, e.g. by a pressure increase, may fully or partially suppress such blistering.

As mentioned above, coagent agents can also be used in the process according to the invention. These can either be introduced during the shaping of the workpiece or they may be included in the peroxide-containing liquid medium used. Useful coagents are polyunsaturated organic compounds such as triallyley anurate, sorbic acid and ethylene glycol dimethacrylate.

Furthermore, the crosslinking of the workpiece after the introduction of the crosslinking agent or agents can be induced instead of heat treatment by treating the workpiece at a lower temperature with activators or accelerators for the peroxides.

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The process according to the invention is further illustrated by the following examples, where the symbol 0 denotes a trademark registered in Denmark.

EXAMPLE 1 515 g of polyethylene ("Hostalen® GM 5010", Melt Index MFI 0.3 g / 10 min density 0.955 g / cm · containing 2.5 - 3.0 i carbon black) in the form of two outer diameter tubes of 17.0 mm and a thickness of 3.0 mm and a plate with dimensions 600 x 65 x 11.3 mm are placed in a tube octave with a diameter of 80 mm and a length of 680 mm.

Clay is prepared an emulsion of 20 g of di-tert-butyl peroxide ("Trigonox B" Noury van der Lande) 0.1 g of polyvinyl alcohol ("Elvanol 7260", du Pont)

2.6 g oleic acid 0.3 g NaOH

20 g of anhydrous Na 2 SO 4.

The autoclave is filled with distilled water and the above emulsion. The liquid is then circulated in the autoclave by means of a plunger pump which simultaneously acts as a homogenizer.

During 2 1/2 hours, the liquid temperature is brought up to 120 ° C and held for 4 hours, then raised to 133 ° 0 and maintained for 14 hours.

Following the cross-linking thus developed, the outer diameter of the tubes is increased from 17.0 to 17.2 mm, while their density has decreased from 0.955 to 0.925 g / cm 2.

For decalin extraction, a gel content of 97 i ° is found for the tubes. When removed from the autoclave, the tubes had no tendency for adhesion.

For the 113 mm plate a gel percentage of 92 is measured in material up to 1 mm from the surface. In material that is 3 -4 mm from the surface, the gel content is determined at $ 80.

EXAMPLE · 2 The 495 g polyethylene consisting of 7 pcs. polyethylene tube ("Hostalen IS 'GM 50IO", MFI 0.3 g / 10 min, density 0.955 g / cm, containing 2.5 - 3.0 $ carbon black), 60 - 65 cm long, outer diameter 17.0 mm and thickness of goods 2.8 mm.

An emulsion of the same composition as mentioned in Example 1 is prepared.

The autoclave is filled with distilled water and the above medium. Then the liquid circulates in the autoclave by means of a gear pump at a speed of 6 l / min.

Over 2 hours, the liquid temperature is brought up to 115 ° C and held for 3 hours, then raised to 158 ° C over the next 5 hours and kept at this temperature for an additional 7 hours.

Following the cross-linking thus developed, the outside diameter of the tubes is increased from 17.0 to 17.1 mm, while the density has decreased from 0.955 to 0.918 g / cm 3.

Yed decalin extraction has a gel percentage of 99 - 100.

When removed from the autoclave, the tubes show no tendency to adhere.

EXAMPLE 3 In the stirring autoclave mentioned in Example 1, 475 g of polyethylene consisting of a 100 x 70 x 12.5 mm pressed polyethylene plate containing no fillers is placed ("Hostalen® GM 6050, MEI 0.3 g / 10 min, density"). 0.946 g / cm 3) and an extruded 600 x 60 x 11.3 mm polyethylene sheet (Hostalen® GM 5010 ")

An emulsion of the same composition as mentioned in Example 1 is prepared.

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After filling the autoclave with the above emulsion and distilled water, the polyethylene is crosslinked under the same conditions as described in Example 2.

After the cross-linking thus developed, a gel percentage of 77 »4 is measured in the polyethylene plate of" GM 6050 "at a distance of approx. 6 mm from the surface.

In the polyethylene plate of "GM 5010", a gel percentage of 99.7 is measured at a distance of maximum 2.5 mm from the surface. In material 5 to 6 mm from the surface, the gel percentage is 86.2.

Example 4 crosslinking of enameled PVC profile approx. 1.5 mm thick.

An emulsion of the same composition is prepared as in Example 1. It is dosed in an amount equal to 10% by weight of peroxide to the weight of the PVC article and then filled into a 2 liter stainless steel autoclave together with the PVC article. The temperature in the autoclave was slowly raised to 125 ° 0 (about 2 hours), after which this temperature was kept for 48 hours. Following this treatment, the following changes could be observed with the PVC article: (a) At a Soxhlet extraction with tetrahydrofuran, an insoluble residue of 25 (against $ 0 in an untreated reference) was found.

(b) In a heat impact test (EMKO (10) 5 102/71 §15, a), however, for 1 hour at 95 ° C an impression of 2.25 mm was found (against 4.00 mm in an untreated reference).

(c) By a mechanical measurement in an Instron tensile tester, a Young module of 70 bar / $ was measured (versus about 140 barHo in an untreated reference).

EXAMPLE 5 In the tube autoclave used in Example 1, 569 g of polyethylene consisting of 7 polyethylene tubes are placed 17 mm x 2.5 mm (diameter ^ thickness). The pipes used were made from "Hostalen GM 5010" or "LupolenvS / 5261Z" (density 0.950, melt index (DIN 55735) at 21.6 kg load; 1.7 - 2.3). Ted some of the tubes, the coagents listed below were used and two of the tubes were voltage equalized.

An emulsion is prepared from 10 g of 1,1-bis (t-butyl peroxy) -3,3,5-trimethylcyclohexane 30 g of 2,5-dimethyl-2,5-di (t-butylpexoxy) hexane 32.6 g of oleic acid

4.7 g NaOH

1.3 g p o ly viny 1 alko lio 1 50 g Na2S0 ^ 5 1 boiled water.

The emulsion is filled with the autoclave and circulated with a centrifugal pump. The temperature is kept at 82 ° C for 7 hours, after which the emulsion is replaced with boiled water containing 50 g of Na 2 SO 2.

Over a period of 12 hours, the temperature increases from 105 ° C to 150 ° C by 3-5 ° C. The temperature rise is at least at the lowest temperatures.

After the crosslinking, the following mass fill, gel percentage and shrinkage are measured in the longitudinal direction of the tubes.

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Material Gel Percent Density Shrinkage% "Hostalen® GM 5010" 76 0.947 3.9 "Hostalen® GM 5010" + 2% Ethylene-Glyole Dimethacrylate 87 0.946 4.0 "Hostalen®GM 5010" + 2% Sorbic Acid 83 0.946 4.0 "Hostalen ® GM 5010 "+ 2% triallyl cyanurate 85 0.945 2.8" Hostal ® GM 5010 "heat treated to remove internal stresses 80 0.946 2.3" Lupolen®526l Z "99 0.931 4.2" Lupolen®5261 Z "heat treated 99 0.931 2.7 EXAMPLE 6 In the tube autoclave of Example 1, 420 g of polyethylene consisting of 8 polyethylene tubes are placed 17 mm x 2.5 mm. The tubes ^ were made partly of "Lupolen ® 5261Z" and partly of "Rigidex ® 20 02054P" (density 0.954} melt index (LIN53735) at 21.6 kg load: 2.0).

An emulsion is prepared as in Example 5, however, with 20 g of 1,1-bis (t-butyl peroxy) -3} 3} 5-trimethylcyclohexane as the sole peroxide.

The emulsion is filled with the autoclave and circulated with a centrifugal pump.

The temperature is kept at 94 ° C for 6.5 hours, after which the temperature is increased to 150 ° 0 during 9 hours, with the temperature increase up to 110 ° C being limited to 2 - 40 C per day. hour.

The temperature is maintained at 150 ° C for 2 hours.

Following the crosslinking, the following is measured:

Material Gel Percent Density Shrinkage% "Lupolen ® 5261" 97 0.930 3} 1 "Rigidex © HO2054P" 82 0.933 5.8