GB2103992A

GB2103992A - Shrink moulding

Info

Publication number: GB2103992A
Application number: GB08221265A
Authority: GB
Inventors: Dimitri R Stein
Original assignee: Kabelmetal Electro GmbH
Current assignee: Kabelmetal Electro GmbH
Priority date: 1981-08-21
Filing date: 1982-07-22
Publication date: 1983-03-02
Also published as: BE894141A; DE3133091A1; NL8203114A; IT1189333B; FR2520477A1; IT8248936A0

Abstract

A cross-linked (extruded) shrink moulding of short length, preferably a shrink tube (2), comprises elements (3) of greater tensile strength and lower elongation than the tube wall material, which are embedded in the wall (2), and extend in the longitudinal direction and are distributed over the periphery, and which have an axial length which essentially corresponds to the axial length of the moulding before distention (Figure 1). The elements (3) prevent change in the axial length of the tube, when it is radially shrunk. <IMAGE>

Description

SPECIFICATION Shrink moulding The present invention relates to a shrink moulding of short length, such as a shrink cap, shrink tube, shrink sleeve and the like, made of an extruded and crosslinked material.

Such shrink mouldings are as a rule produced by injection-moulding or extruding a so-called preform, then crosslinking it and distending it in this crosslinked state, and cooling this distended moulding so as to fix its distended state by "freezing".

It is already known (Austrian Patent 188,510) to manufacture shrink tubes from thermoplastic compositions by heating the extruded or injection moulded tube of low diameter, distending it by means of compressed air and fixing this changed condition by subsequent cooling. However, it is a disadvantage of this process that such shrink tubes of thermoplastic, for example including those of polyvinyl chloride, are insufficiently heatstable for present-day requirements and moreover do not exhibit the desired "elastic shape memory", i.e. no longer resume all details of their original shape on shrinkage, A remedy is provided by a technique, also known, for the production of heat-shrinkable products, sold under the trademark Thermofit, in which a high density polyolefine material is used for the production of injection mouldings.These mouldings are subsequently exposed to high intensity electron irradiation so that a crosslinked three-dimensional network of the molecules is achieved. This results in a mechanically resistant moulding, which does not creep, does not split open and exhibits "elastic shape memory". If, for example, a shrink tube which has been processed in this way is drawn over the article to be covered and is heated briefly to above the crystallite melting point, namely to above 1350 in the known instance, it shrinks rapidly to its original shape and size and a firm, resistant covering results.

Any desired base polymers, which may also be modified, can be employed for the process described above, to suit the specific requirements.

However, the precondition in every case is crosslinking by irradiation before the mouldings are distended, or stretched, in the heated condition.

Another possibility is to use, as base materials for the shrink articles, polymers which are crosslinked after grafting-on of crosslinking aids, such as organosilanes, before or during the moulding of the preform, under exposure to moisture. In this process it is moreover important that the exposure to moisture should either be effected using a special apparatus downstream of the production of the preform or should be effected in the mould itself, as the result of a certain amount of moisture inherently adhering to the polymers and additives, or should be effected by simple storage exposed to the atmosphere.

This proposal is based on the thought that in contrast to peroxide crosslinking with heat exposure, which has long been known and is being used, for example, in the cable industry, the grafting of reactive low molecular compounds, for example of organosilanes, as crosslinking aids onto the macromolecules of the base materials, which in turn produce, in the course of secondary reactions, a polyfunctional chain linking, will result in "bundle-like" crosslinking points, where a plurality of macromolecules are fixed to one another via one crosslinking node. This special chemical crosslinking mechanism leads to high bonding forces at the molecular level; though these forces are loosened on heating, when the material is in the thermoplastic state, and thereby permit distention of, for example, the moulding, resumption of the original shape occurs after reheating and rapid shrinkage.

Shrink articles produced from such materials accordingly also have an "elastic shape memory", and are therefore suitable for a variety of possible uses, for example as tubes and caps, for instance for pressure-tight and moisture-resistant closing of the ends of electric cables or for use as single or multi-part sleeves for protecting the connection or coupling points of electrical cables or of tubebundle cables. The properties of the articles are only insignificantly, if at all, different from radiation crosslinked shrink articles, but the process of manufacture is substantially simplified.

Regardless of the nature of the base materials or the crosslinking technique, which also includes, for example, chemical crosslinking by means of peroxides, there have hitherto, however, always been difficulties due to the fact that, for example in the manufacture of shrink tubes, i.e. shrink mouldings of short length, distention of the preform, for example by inflation, converts a part of its "axial length" to "radial length". This ultimately has the consequence that such a tube, on shrinkage, shortens by the proportion of axial stretching induced during expansion. Accordingly, dimensional accuracy is difficult to maintain.

It is therefore the object of the invention to provide a possible way of in particular producing shrink tubes or shrink sleeves which guarantee dimensional accuracy, i.e. which do not exhibit any axial shortening in the shrunk state.

This object is achieved, according to the invention, if elements of increased tensile strength and low elongation, which run in the lengthwise direction and are distributed over the periphery, are provided in the wall of the shrink moulding.

This has the consequence that a radial distention, for example inflation, is possible as hitherto, but that the conversion of "axial length" into "radial length" during distention does not occur and accordingly no longer comes into play on shrinkage. The shrink mouldings produced according to the invention are dimensionally accurate and lengthwise shrinkage does not occur when the moulding is shrunk onto an article.

It is known per se (from German Patent 2,344,577) to strengthen the sheath of electrical cables by embedded tensile elements. The object is to increase the mechanical strength of an elongate product of great length, so as to produce, in this way, so-called self-supporting overhead cables. In this way, when the cable is suspended between the pylons, the tensile forces which arise are taken up by the embedded endless tensile elements and the conductor is relieved of tension.

Against this, the invention is based on the discovery that the elements which are embedded in the wall of the preform of short length ensure stabilisation of the possible movement of the shrinkable material in the axial direction during the distention process whilst, surprisingly, the possible movement, for example of the polymer material, in the radial direction can occur unhampered, as before. Thus, the measure provided by the invention not only increases the lengthwise tensile strength of the end product, in the present instance the shrink moulding, but also, on distention, directs the material flow in a controlled manner into the desired direction and only into this direction.

In a further development of the invention, it is advantageous if the tension-resistant elements are embedded in the form of fibres, individually or bundled in groups, in the wall. These fibres can, for example, be those based on glass or those based on polyamide, for example those known under the trademark Kevlar. These elements, for example bundles produced from massive profile strands or from stranded-together or entangled individual strands, can also additionally be coated and/or impregnated with a pressure-sensitive adhesive.

The mechanical bonding to the polymer material can thereby be increased and, moreover, a sealing effect can be achieved, especially in the case of bundled individual strands.

The wall of the shrink moulding can be made of individual layers, and these layers can also consist of different materials. The elements of increased tensile strength are in that case embedded in one of the layers or between a pair of layers.

However, in implementing the invention it has proved particularly advantageous to make the wall of the shrink moulding of a single material and constructed as a single layer, with the elements of increased tensile strength embedded in the polymer material. In this case it has proved advantageous to locate the elements of increased tensile strength in the outer phase of the wall. This is true of cases where, because of the material employed or because of the particular end use, an increased notching action is to be expected.

In the conventional cables with so-called "wavy" tension-relief elements in the outer sheath, the waviness of these elements is intended to serve to produce the closest possible frictional bond between the tension-relief elements and the sheath material (East German Patent 124,152), so that the elements may be used as tensile members.

In contrast, in the case of the present invention it is important that the axial length of the elements of increased tensile strength should substantially correspond to the axial length of the shrink moulding before distention. This then ensures that, given the short length of the shrink mouldings, it is not the increased length corresponding to the waviness which results, but that the retaining elements actually perform their task of eliminating axial shrinkage. The conventional "waving" of the tension relief elements in contrast affects the "longitudinal rigidity" of the tensile elements.

To produce the shrink mouldings according to the invention an advantageous procedure to follow, where shrink mouldings having two open ends, such as tubes, sleeves or the like, are concerned, is that first an endless pipe or tube of the crosslinkable material is extruded and during the extrusion the elements of increased tensile strength are embedded in the pipe or tube wall, concentrically arranged in respect of the latter, and that the shrink mouldings of appropriate length are cut from this endless pipe or tube before or after crosslinking. This permits economical manufacture of the preforms and ensures that the elements of increased tensile strength, of the requisite axial length, are embedded in the wall of each individual part.

If, as a further concept of the invention also provides, the wall of the shrink mouldings is composed of several layers, it is advantageous if, on extrusion of the individual layers, the elements of increased tensile strength are embedded, in the course of the same process, preferably in the outermost layer.

The crosslinking of the preforms, endless tube or pipe or of the cut lengths of the latter can be effected by conventional methods. Chemical crosslinking by means of suitable peroxides is just as possible as crosslinking by use of high-energy radiation.

Another possibility is crosslinking by the action of moisture. In that case, it has proved advantageous to effect the crosslinking of the preform in the course of exposure to moisture at elevated temperatures of between 800C and 2000C, preferably between 1 400 and 1 800. The distention to the state which is to be frozen-in, is then carried out immediately afterwards, that is to say "in line", whilst the preform is still warm. If, as a further development of the inventive concept additionally provides, the crosslinking reaction is effected by the actual moulding or injection process during manufacture of the preform, a separate moisture treatment at an elevated temperature can be dispensed with. The temperature increase before distention can be effected independently of the moisture treatment, for example by ultra-high frequency heating in the case of mixtures capable of absorbing microwaves. The economics of the process of manufacture can thereby be further improved.

The crosslinking itself can be effected in a type of sauna, as is already employed in the moisture crosslinking of cables, that is to say in a water vapour atmosphere, at elevated temperatures.

However it is also advantageously possible, in carrying out the invention, to use, for the same purpose, a heated glycerine-water mixture or oil water mixture which on the one hand offers the advantage of uniform temperature conditions without special expendirue on control and on the other hand, by virtue of the components which are more compatible than water with the polymers of the shrink articles, ensures more rapid diffusion of the small amounts of water required for the crosslinking. As base materials it is possible to use all polymers for which an addition reaction of organosilanes by a radical-initiated grafting is possible.

Materials which have proved particularly advantageous, even on grounds of processability alone, for the purposes of the invention, are regardless of the type of crosslinking chosen, are polyethylene and copolymers of ethylene with vinyl acetate or with acrylate comonomers.

However, the base materials used can equally well be those based on ethylene-propylene rubber alone or blended with polypropylene.

To achieve stability to ultraviolet, but also to improve the mechanical and rheological properties of the mixture when producing the preforms, it has proved advantageous to add carbon black to the base material. Particularly suitable for this purpose are so-called acetylene blacks with nonhygroscopic properties. These carbon blacks moreover has a high conductivity and even small amounts, for example 1.5 to 3.0 parts per 100 parts of polymer, suffice to give the shrink article the requisite stability to ultraviolet. The use of non-hygroscopic acetylene blacks is of importance especially where the preforms are crosslinked by the action of moisture after silanes have been grafted onto the base materials.

If, amongst the possible types of crosslinking, it is crosslinking by moisture which is desired, then it is additionally important that the crosslinking aids, for example organosilanes, should be available in an amount which is adequate but well-balanced relative to the peroxides, so as to produce the molecular bonding forces required for shrinking back from a distended state. The molar ratio of peroxides, producing radical positions on the macromolecule, to silanes added is therefore advantageously about 1:10 when carrying out the invention.

The invention will be explained in more detail in relation to the shrink tube depicted in Figures 1 and 2.

The preform produced from crosslinked material, for example cut from an endless crosslinked pipe or tube, and shown in Figure 1, is marked 1. Elements 3 of increased tensile strength and low elongation are arranged in the pipe or tube wall 2; in the illustrative embodiment shown, these elements consist of bundled individual threads 4 based on glass fibres. As illustrated in the figure, the elements 3 run rectilinearly in the axial direction, that is to say the axial lengths of the preform 1 and elements 3 are substantially the same. The elements 3 are moreover arranged so as to be distributed over the periphery of the tube or pipe; in the case illustrated, there are four elements, but depending on the requirements made of the finished product the number of tension-resistant elements can of course also be increased.

In contrast to the illustrative embodiment shown in Figure 1, Figure 2 shows the preform 1 in the already distended or inflated state, that is to say in the shape of ready-to-use shrink mouldings, whose distended state is frozen-in. The bore 5 is increased compared to Figure 1 and the thickness of the pipe wall 2 is reduced, corresponding to the increase in diameter of the bore 5. The axial length is fixed by the elements 3 and remains, by virtue of the invention, unaffected by the process of distention in the radial direction. This has the consequence that the shrinking process also takes place exclusively in the radial direction, whilst the axial length of the shrink moulding 1 remains unchanged. Accordingly, high dimensional accuracy is ensured.

As can be seen from Figure 2, the elements 3 show a different cross-sectional shape compared to that of Figure 1, as a result of the multi-strand nature of the fibres. This changed cross-section illustrates the forces which act during the distention process and which in the prior art resulted in high axial shrinkages of the tubing and the like. These forces are now absorbed by the elements 3, which permit material to migrate in a radial direction but prevent a change in axial length during the distention process.

Claims

1. Shrink moulding composed of an extruded and crosslinked material, characterised in that elements (3) having a tensile strength higher than that of the said material and an elongation lower than that of the said material, which elements (3) run in the lengthwise direction and are distributed around the periphery of the moulding, are provided in the wall (2) of the moulding.

2. Shrink moulding according to claim 1, characterised in that the elements (3) are embedded in the form of fibres, individually or bundled in groups, in the wall (2).

3. Shrink moulding according to claim 1 or 2, characterised in that the wall (2) is composed of a plurality of layers.

4. Shrink moulding according to claim 1, 2 or 3, characterised in that the elements (3) are coated and/or impregnated with a pressure-sensitive adhesive.

5. Shrink moulding according to any of claims 1 to 4, characterised in that the elements (3) are disposed in an outer zone of the wall (2) of the moulding.

6. Shrink moulding according to any one of claims 1 to 5, characterised in that the axial length of the elements (3) is substantially equal to the axial length of the moulding before distention.

7. Shrink moulding in the form of a shrink tube or shrink sleeve, according to any of claims 1 to 6.

8. Shrink moulding in the form of a shrink cap, according to any of claims 1 to 6.

9. Shrink moulding according to claim 1, substantially as described with reference to the accompanying drawing.

10. Process for the manufacture of a shrink moulding according to claim 1 having two open ends, characterised in that an endless pipe or tube of the crosslinked material is extruded, the said elements (3) being introduced during the extrusion, and being arranged concentrically to the axis of the pipe or tube, and being embedded in the pipe or tube wall (2) as the latter is formed, and thereafter shrink mouldings of the requisite length are cut from the endless pipe or tube so produced.

11. Process for the manufacture of a shrink moulding according to claim 1 having two open ends, which is of multi-layer construction, characterised in that after extrusion of one or more inner layers the elements (3) are embedded in the outer layer.

12. Process according to claim 10 or 1 1, characterised in that the extrudate is crosslinked by means of high-energy radiation.

13. Process according to claim 10 or 1 1, characterised in that the extrudate is crosslinked in the course of exposure to moisture at temperatures of between 800 and 2000 C, the distention to the state which is to be "frozen-in" being carried out whilst the preform is still at an elevated temperature.

14. Process according to claim 13, characterised in that the said temperatures are between 1400 and 1 80 C.

15. Process according to any of claims 10 to 14, characterised in that the said elements (3) are elements comprising glass fibres.

16. Process according to any of claims 10 to 1 5, characterised in that an ethylene polymer or copolymer composition is used to provide the material for the shrink moulding.

17. Process according to claim 16, characterised in that the said composition comprises an ethylene vinyl acetate copolymer or an ethylene acrylate copolymer.

18. Process according to claim 16, characterised in that the said composition comprises an ethylene-propylene rubber with or without polypropylene or another polyolefine.

19. Shrink moulding manufactured by a process according to any of claims 10 to 18.