GB2093760A

GB2093760A - Heat-shrinkable articles

Info

Publication number: GB2093760A
Application number: GB8201505A
Authority: GB
Original assignee: KM Kabelmetal AG
Current assignee: KM Kabelmetal AG
Priority date: 1981-03-02
Filing date: 1982-01-19
Publication date: 1982-09-08
Also published as: DE3107907A1; GB2093760B; DE3107907C2; IT8149762A0; CA1169218A; JPS57144726A; IT1142939B; FR2500840B1; BE892335A; FR2500840A1

Abstract

Copolymers of ethylene having a comonomer content of 20 to 40% by weight, or polyolefins or polyolefin mixtures blended with one or more elastifying components, are used as base materials in the production of cross-linked heat-shrinkable articles which have elastomeric behaviour, e.g. shrink caps, shrink sleeves, or shrink tubes.

Description

SPECIFICATION Production of shrink articles This invention relates to a process of a kind useful for the production of shrink tubes, shrink sleeves, shrink caps and other shrink articles, in particular those of short length, for extrudable materials, in which a parison is injection-moulded, extruded, or otherwise formed, is thereafter cross-linked, and is distended in its cross-linked state, this distended parison thereupon being cooled sufficiently to fix it in its distended state.

It is already known (cf. Austrian Patent Specification 1 88,510) to produce shrink tubes from thermoplastic compositions by heating an extruded or injection-moulded tube of relatively small diameter, distending it by means of compressed air, and fixing it in this distended state by subsequent cooling. However, this process has the disadvantage that shrink tubes of this type, made from thermoplastic compositions, for example those made from polyvinyl chloride compositions, are not sufficiently temperature-resistant for present-day requirements; also, they do not have the desired "elastic memory", that is to say, they do not resume their original shape, in all details, in the shrinking step.

A remedy for this disadvantage is offered by a process, also known, for the production of certain heat shrink products which are marketed under the trade name of Thermofit. In this process, a highdensity polyolefin material is used for injection-moulding shaped articles. These articles are then subjected to high intensity electron radiation, so that a crosslinked three-dimensional molecular network is achieved. This produces a mechanically resistant shaped article, which is creep-resistant and does not split, and which has an "elastic memory". For example, if a shrink tube thus produced is pulled over an object which is to be covered, and is then heated for a short period above the crystallite melting point, which is above 1350 in the case under consideration, it shrinks back rapidly to its original shape and dimensions, and a tough resistant coating is produced.

Any desired polymers, and even modified polymers, can be used in the process described above, depending on the specific use requirements. However, in each case crosslinking has to be carried out by irradiation, prior to the mouldings being distended or stretched while warm. This requires great care (to provide effective radiation protection), and also entails a large outlay in respect of equipment, so that production is undesirably difficult and expensive.

It has therefore already been proposed to make shrink articles from polymeric materials which, after grafting-on of crosslinking auxiliaries, e.g. organo-silanes, will crosslink, under the influence of moisture, prior to or during the shaping of the parison. In this process it is necessary to be able to arrange for the action of moisture to occur (i) by means of a special moisture-supplying device downstream of the point of production of the parison, or (ii) in the mould itself, utilising those small quantities of moisture which in practice are inevitably present on polymers and additives, or (iii) by simple storage under ambient conditions.This proposal is based on the idea that, in contrast to the method of peroxidic crosslinking under the influence of heat, which has been known for a long time and which is used in (e.g.) the cable industry, one could graft reactive low-molecular weight compounds, e.g.

organosilanes, as crosslinking auxiliaries on to the macromolecules of the polymeric materials, which in turn, in the course of secondary reactions, would lead to a polyfunctional chain bonding, with the formation of "bundle-type" crosslinking points, several macromolecules being fixed to one another at a crosslinking knot. This particular chemical crosslinking mechanism produces high bonding forces at the molecular level, which, though they may be loosened on warming to a thermoplastic state and hence permit the distending for example of the moulding, do enable the original shape to be resumed after reheating and rapid shrinkage.

Shrink articles prepared from such materials therefore also have an "elastic memory", so that they are suitable for very varied uses, for instance as tubes and caps, e.g. for pressure-tight and moistureproof sealing of the ends of electric cables or as sleeves, one-piece or multi-piece, for the protection of connection points or junction points of electric cables or tube bundle cables. Compared wit radiation-crosslinked shrink articles, the properties are unchanged or substantially unchanged, but the production process is considerably simplified.

Regardless of the polymeric materials used, or of the crosslinking technique employed, it can ofter advantageous if the shrink articles produced, for example shrink tubes, can be deformed eiastomerically.

The shrink articles produced by the known processes do have a sufficient flexibility for ordinary uses, that is to say they are flexible and readily deformable, but these properties are really due to the fact that the wall thickness of the stretched article is relatively small. If however, a shrink article is required to be deformable without the use of great force relatively readily in a longitudinal direction and also radially, these properties cannot be achieved with the customary shrink articles. For example, crosslinked polyethylene exhibits only small reversible elongations, and under further loading it then yields irreversibly until the tear limit is reached.

Setting out from shrink articles produced by known processes, therefore, the invention is based on the object of producing shrink articles which, though they can be stretched prior to heat-shrinking in a longitudinal direction and radially, just like various others, will spring back into their original shape on removal of the stretching forces. The ability to be stretched elastically widens the field of application of the shrink articles, in respect of geometrical considerations, and makes them easier to put in position.

According to the invention, a process of the kind first mentioned herein is characterised in that the shrink articles are produced from a polyolefin material having elastomeric properties, this material comprising a copolymer of ethylene which by virtue of a comonomer content of 20 to 40 (preferably 25 to 35) % by weight has elastomeric properties, or comprising a polyolefin or polyolefin mixture provided with elastomeric properties by blending with one or more elastifying components. Shrink articles thus produced, for example shrink tubes, are elastic in their behaviour, in that, even after shrinking-on, the shrink article exerts pressure, by means of elastomeric forces, on the shrink-covered object.

The above-mentioned high comonomer content of 20 to 40 and preferably 25 to 35% enables the degree of crystallinity of the ethylene copolymer and also its melting point to be reduced, so that the material has enhanced elastomeric properties. it is also important that depending on the composition or the choice of polymeric material the shrink process can be effected at various, but adjustable, temperatures. For example, it is possible that, if ethylene copolymer incorporating vinyl acetate, ethyl acrylate or butyl acrylate as comonomers are used, shrinkage of a tube produced from such materials can be effected even at temperatures not exceeding 1 000C, for example at 750 C.

Shrink articles which have elastomeric properties, however, can also be produced, in accordance with the invention, using polymeric materials which have a melting range not reaching above 1200 C, these being polyolefins or polyolefin mixtures in a blend with one or more elastifying components. If, in this case, for example, a low-density polyethylene is employed, i.e. one with a density not more than 0.94 g/cm3, an article produced from such a material will have a shrinkage temperature of about 1 1 OOC.

The shrinkage temperature can also be altered, in accordance with the invention, by employing a polymeric material having a melting range not reaching below 1 200C, which may be one using a highdensity polyethylene, i.e. one with a density not less than 0.94 g/cm3. Shaped articles produced therefrom are shrunk on to the object to be protected at temperatures not less than 1200 C, preferably at 125 to 13506.

Without foregoing the elastomeric properties, shrink tubes can be produced, the shrinkage temperature of which is still above the region indicated hitherto, for certain applications. For example, this is the case if, instead of polyethylene, isotactic polypropylene or a partially crystalline propylene copolymer having a melting region above 1 500C is used in combination with at least one elastifying component. A high molecular weight polyisobutylene, i.e. one with a molecular weight of 100,000--150.000, can be advantageously used in this case as the elastifying component. Shrinkable shaped articles thus produced only begin to shrink, in this case, at 150 to 1 600 C.

Another possible means of providing the polymeric material with elastomeric properties comprises using an ethylene-propylene copolymer as an elastifying component. For example, this copoylmer can be an ethylene-propylene rubber (EPR), or again Levapren, for instance.

Shaped articles produced by the process according to the invention and having elastomeric properties can of course also contain fillers. Depending on the chemical or electrical properties desired, carbon blacks, silicates or chalks can be employed in various proportions and finenesses of division.

As indicated above, the present polymeric materials are crosslinked. One crosslinking process or another may be preferred, depending on the equipment available and the properties required. In each case, the shrink articles which are being produced by the process of the invention are formed, in the desired configuration, by injection-moulding, extruding, blow-moulding, or otherwise, and are thereafter crosslinked. They are then distended at an appropriate elevated temperature into the desired shape, and this distended shape is fixed by cooling. The presence of residual crystallinity in the material is of particular importance for making durable the exterior shape of the shrink article, which was conferred upon it by an expansion step in a heated condition.

As already indicated, shrinkable shaped articles can be produced by the process of the invention which shrink on to the object concerned at differing response temperatures. These properties can be used particularly advantageously, in accordance with the invention, to produce shrink articles wherein individual functional parts shrink at different temperatures. Thus, shaped articles having elastomeric properties can be produced wherein one half or one side shrinks at an extremely low temperature, for example at 700 C, whereas the other half or side initially retains a stable shape up to considerably higher temperatures and only begins to shrink at, for example, 1 50 to 1 600 C. This provides the possibility of constructing shaped articles having a plurality of functional elements which shrink at differing temperatures. Shaped articles which have elastomeric properties and which are composed in the manner just described can be produced by joining parisons representing the desired elements together by means of appropriate plastic-welding or polymer-welding equipment, these being parisons having different shrinkage temperatures, and by expanding them, after crosslinking, to the desired shape.

Examples of compositions which inter alia can be used for shrink articles produced by the process ol the invention are given below.

EXAMPLE I (shrinkage temperature 110-1 200 C) Polyethylene homopolymer (density not more that 0.94 g/cm3, melt index 0.2-2.5) 10-20 parts Ethylene-propylene rubber 80-90 parts Vinyltrimethoxysilane 1.0-1.5 parts Dicumyl peroxide 0.03-0.05 parts Catalyst (Naftovin SN/L) 0.05 parts Carbon black (acetylene black Y) 15 parts EXAMPLE II (shrinkage temperature 70-85 0C) Polyethylene copolymer having 25-35 mol % of vinyl acetate 100 parts Calcined clay (hard kaolin M 100) 10 parts Carbon black 10 parts Vinyltrimethoxysilane 2.0 parts Peroxide 0.05-0.1 parts Catalyst (dibutyl-tin dilaurate (Naftovin SN/L)) 0.05 parts EXAMPLE III (shrinkage temperature 130--1350C) High-density polyethylene (density over 0.94 g/cm3) 70 parts Polyisobutylene (molecular weight 100,000-1 50,000) 30 parts Non-hygroscopic acetylene black (Noir Y) 70 parts Anti-ageing agent (for example Vulkanox HS) 0.4 parts EXAMPLE IV (shrinkage temperature 1 55-1 650C) Polypropylene 65 parts Isotactic EPM (ethylene-propylene rubber of EPDM (unsaturated 35 parts ethylene-propylene tubber) Non-hygroscopic carbon black 30 parts Anti-ageing agent (Vulkanox HS) 0.25 parts The polymeric materials of Examples I and II can be cross-linked under the influence of moisture, by virtue of the addition of appropriate proportions of a silane compound, which is grafted on.

Alternatively, they could be converted into a cross-linked product by means of a peroxidic crosslinking reaction, using an amount of peroxide increased approximately 10-fold.

The polymeric materials of Examples Ill and IV can be crosslinked not only by silanes, as in Examples I and II, but also by means of high-energy radiation. A peroxidic crosslinking too would be possible, although in making a choice of peroxides one has to take into account the higher processing temperatures of the polymeric materials of Examples Ill and IV.

Other elastomers can also be used as elastifying components. In cases, where, for example, flame resistance is of particular interest, a chlorinated polyethylene, such as is commercially available, inter alia, under the trade names Bayer CM and/or CPE (Hoechst and Dow Chemical), can be used as the elastomeric component, as in the following Example V.

EXAMPLE V (shrinkage temperature 11 50C) Polyethylene homopolymer (density not less than 0.94 g/cm3) 50 parts Chlorinated polyethylene (Bayer CM) 50 parts Dicumyl peroxide 1.5 parts Anti-ageing agent (Flectol H), e.g. with fillers 0.5 parts (carbon blacks and calcined clays) Crosslinking is carried out here by the peroxidic method.

If the sheat-shrink shaped article is to exhibit flame-resistant behaviour but halogens are to be absent, one expedient is to employ, exclusively, hydrated aluminium oxide, instead of carbon blacks or light-coloured fillers. This possibility is illustrated by the following Example Vl.

EXAMPLE Vl (shrinkage temperature 130-1 350C) High-density polyethylene (density over 0.94 g/cm3) 70 parts Polyisobutylene (molecular weight 100,000-1 50,000) 30 parts Al(OH)3 70 parts Anti-ageing agent (for example Vulkanox HS) 0.4 parts

Claims

1. A process for the production of shrink tubes, shrink sleeves, shrink caps and other shrink articles, from extrudable materials, in which a parison is injection-moulded, extruded, or otherwise formed is thereafter crosslinked, and is distended in its crosslinked state, this distended parison thereupon being cooled sufficiently to fix it in its distended state, characterised in that the shrink articles are produced from a polyolefin material having elastomeric properties, this material comprising a copolymer of ethylene which by virtue of a comonomer content of 20~40% by weight (preferably 25~35% by weight) has elastomeric properties, or comprising a polyolefin or polyolefin mixture provided with elastomeric properties by blending with one or more elastifying components.

2. Process according to claim 1, characterised in that the said material comprises an ethylene copolymer incorporating vinyl acetate, ethyl acrylate or butyl acrylate as a comonomer.

3. Process according to claim 1, characterised in that the said material comprises a blend of a polyolefin or a polyolefin mixture with one or more elastifying components, which has a melting range not reaching above 1200 C.

4. Process according to claim 3, characterised in that a polyethylene of low density (i.e. not more than 0.94 g/cm3) represents the (or a) polyolefin.

5. Process according to claim 1, characterised in that the said material comprises a blend of a polyolefin or a polyolefin mixture with one or more elastifying components, which has a melting range not reaching below 1200 C.

6. Process according to claim 5, characterised in that a polyethylene of high density (i.e. not less than 0.94 g/cm3) represents the (or a) polyolefin.

7. Process according to claim 5, characterised in that an isotactic polypropylene or a partially crystalline propylene copolymer having a melting region above 1500C represents the (or a) polyolefin.

8. A process according to claim 5, 6 or 7, characterised in that a polyisobutylene having a molecular weight of 100,000-1 50,000 represents the (or an) elastifying component.

9. Process according to claim 5, 6, 7 or 8, characterised in that an ethylene-propylene copolymer represents the (or an) elastifying component.

10. Process according to claim 9, characterised in that the ethylene-propylene copolymer is an ethylene-propylene rubber or an unsaturated ethylene-propylene rubber (EPDM).

11. Process according to claim 5, 6, 7, 8, 9 or 10, characterised in that a chlorinated polyethylene, which provides the article with flame-resistant properties, represents the (or an) elastifying component.

12. Process according to any of the preceding claims, characterised in that hydrated aluminium oxide is employed as a filler in the said material.

1 3. Process according to any of the preceding claims, characterised in that the said material comprises one or more polymers which, by the grafting-on of one or more silane compounds, have been made crosslinkable by the action of moisture.

14. Process according to claim 1, wherein the composition of the said material is substantially as set forth in any of the foregoing Examples I to Vl.

1 5. Shrink articles produced by a process according to any of claims 1 to 14.

1 6. Shrink articles as claimed in claim 1 5, wherein individual functional parts shrink at different temperatures.