GB2177703A

GB2177703A - Extruded products and the manufacture thereof

Info

Publication number: GB2177703A
Application number: GB08606143A
Authority: GB
Inventors: Jillian Anne Furr; Anthony Edward Williamson
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1985-07-12
Filing date: 1986-03-12
Publication date: 1987-01-28
Also published as: GB8517665D0; GB2177703B; GB8606143D0

Abstract

A method of forming an extruded product of a cross-linked organic polymer in which the extrudate is formed of a mixture of the polymer and a compound cross-linkable therewith, and has subsequently applied to at least one surface a layer of a material incorporating a catalyst which infiltrates into the extrudate so as to enhance or expedite the cross-linking process. This avoids the problem of pre-cure.

Description

SPECIFICATION Extruded products and the manufacture thereof This invention relates to extruded products formed of a cross-linked organic polymer such as polyethylene, more especially though not exclusively, to tubular products such as cable sheathing, and also to methods of forming such products.

In order to achieve cross-linking of polyethylene it is known to incorporate within it a suitable organic compound such as an organosilane, the reaction being expedited by the addition of a silanol condensation catalyst.

Commonly the polyethylene is compounded with an organosilane to give granules of a grafted product. These are subsequently mixed with a catalyst in granular form, and the whole cross-linkable composition extruded. Cross-linking occurs spontaneously on exposure to moisture, and this reaction is accelerated by heating, e.g. in a hot-water bath.

However it has been found that some pre-curing of the material can occur during extrusion, resulting in the presence of cured lumps within the extrudate thereby affecting its properties and/or appearance, or causing blockage or partial blockage of the extruder head.

In order to alleviate this problem, it has been proposed to extrude the grafted product without catalyst, and to achieve cure by submersing the extrudate for a time (dependant on thickness) into a bath of water containing the catalyst and an emulsifying agent. Whilst the product can be successfully cross-linked using this technique -handling of large quantities of the product can be difficult.

An object of the invention is to provide an alter native way of overcoming the problem, of pre-cure during extrusion and although it is mainly applicable to extruded products formed of cross-linked polyethylene it could also be employed in the production of extrudates of other cross-linkable polymer compositions.

According, therefore, to the present invention a method of forming an extruded product of a crosslinked organic polymer comprises forming an extrudate of a mixture of the polymer and a com pound cross-linkable therewith, and applying to at least one surface of the extrudate a layer of a material incorporating a catalyst which infiltrates into the extrudate and enhances or expedites the cross linking process.

Preferably the said layer is formed of a thermo plastic material which is not cross-linkable and is extruded on to the said surface during the extrusion of the main extrudate.

The layer material is preferably one which can be extruded at approximately the same temperature as the main extrudate, and is bondable to the material of the latter.

The main extrudate and/or the said layer or layers may incorporate additives, as is well known in the art, for example as stabilizers, fire retardants or smoke suppressants.

In the formation of cable sheathing a said catalyst-containing layer is conveniently applied to both the inner and the outer surface of the main extrudate, which preferably consists of an oleofin polymer such as polyethylene.

At least the outer layer, in such a case, which can be relatively thin compared with the main extrudate, may incorporate a colouring additive, in addition to a fire retardant. This can in some cases avoid the need to incorporate such materials in the main extrudate.

One method of forming a tubular sheath of an electric cable in accordance with the invention will now be described by way of example with reference to the accompanying drawing.

In the drawing 1 represents, in diagrammatic form, am extruder having a three layer extruder head of known kind arranged to apply to the exterior of a plurality of mutually insulated stranded and sheathed conductors 2 an over sheath 3 comprising inner and outer layers 4, 5 respectively sandwiching between them a polyethylene tube 6.

The latter, which has a wall thickness of approximately 2.6mm, incorporates an organosilane and usually contains a fire retardant additive, such as Aluminium Trihydrate, but is uncatalysed.

The inner and outer layers each have a thickness of approximately 0.5mm, and are formed of a thermoplastic material containing approximately 1.0% by weight of catalyst, such as dibutyltin dilaurate (DBTDL), heavily loaded with a fire retardant material such as Aluminium Trihydrate. The thermoplastic material, which is conveniently a copolymer of polyethylene, is not cross-linkable and is such as to permit the catalyst to migrate into the tube material to expedite the cross-linking reaction.

The sheathed assembly is subsequently heated in a system environment between 60 and 65 C, during which time the cross-linking process is completed.

Although the tube 5 is shown coated both internally and externally by catalyst loaded thermoplastic material, it may be adequate to coat one of its surfaces only, for example its external surface.

The outer layer 5 may also incorporate a colouring pigment the colouring of the main tube in such a case being unnecessary.

It will be appreciated that although the process is especially suitable for the production of extruded sheaths for electric cables it can also be applied to other extruded sections, and other cross-linkable materials in which cross-linking is expedited by a catalyst.

It will also be appreciated that although the above example has been described with reference to a cross-linking reaction in which the temperature of the steam or water environment lies between 60 and 60 and 65 C, the cross-linking reaction can also be achieved by heating in a steam or water environment in which the temperature is above 65"C. The maximum allowable temperature is approximately 1000C for steam, and slightly lower than 100 C for water.

Claims

1. A method of forming an extruded product of a cross-linked organic polymer comprising forming an extrudate of a mixture of the polymer and a compound cross-linkable therewith, and applying to at least one surface of the extrudate a layer of a material incorporating a catalyst which infiltrates into the extrudate and enhances or expedites the cross-linking process.

2. A method of forming an extruded product of a cross-linked organic polymer according to Claim 1, wherein the said layer is formed of a thermoplastic material which is not cross-linkable and is extruded onto the said surface during the extrusion of the main extrudate.

3. A method of forming an extruded product of a cross-linked organic polymer according to Claim 2, wherein the said layer material is extruded at approx-imately the same temperature as the main extrudate.

4. A method of forming an extruded product of a cross-linked organic polymer according to Claim 3, wherein the said later material is bondable to the material of the main extrudate.

5. A method of forming an extruded product of a cross-linked organic polymer according to any preceding claim, wherein the main extrudate is of tubular shape and a said catalyst-containing layer is applied to both surfaces of the main extrudate.

6. A method of forming an extruded product of a cross-linked organic polymer according to Claim 5, wherein the main extrudate consists of an oleofin polymer.

7. A method of forming an extruded product of a cross-linked organic polymer according to Claim 6, wherein the said oleofin polymer is polyethylene or a co-polymer thereof.

8. A method of forming an extruded product of a cross-linked organic polymer according to any preceding claim, wherein the main extrudate and/ or the said layer or layers incorporate additives.

9. A method of forming an extruded product of a cross-linked organic polymer according to Claim 8, wherein the said additives include one or more of stabilizers, fire retardants, smoke suppressants and colouring additives.

10. A method of forming an extruded product of a cross-linked organic polymer according to Claim 9, wherein the main extrudate is in the form of a tube at least the outer surface of which has a said catalyst-containing layer, which layer incorporates a colouring additive and a fire retardant.

11. A method according to any preceding claim wherein the catalyst comprises dibutyltin dilaurate.

12. A method according to any preceding claim wherein the cross-linking reaction is achieved by heating the extrudate, following the application of the catalyst-containing layer or layers, in a steam or water environment at a temperature of between 60 C and 100 C.

13. A method according to Claim 12 wherein the temperature is between 600C and 65"C.

14. An electric cable wherein the conductor elements are surrounded by a tubular sheath formed by a method according to any preceding claim.

15. A method of forming an extruded product of a cross-linked organic polymer substantially as herein described with reference to Figure 1 of the drawing.