IES64145B2 - Method of manufacturing cross-linked polyethylene - Google Patents

Abstract

A method of manufacturing cross-linked polyethylene consists of an extrusion stage followed by a cross linking stage. During the extrusion stage a relatively small quantity of relatively fine polyethylene particles of relatively high density is added.

Description

METHOD OF MANUFACTURING CROSS-LINKED POLYETHYLENE Field of the Invention The present invention relates to a method of manufacturing crosslinked polyethylene for use in covering low power cables up to 1 kV.

Prior Art - Overview The periodical Wire Industry, in the June, 1991 issue at page 313 includes an Article entitled The Monosil Process for Polymeric Power Cables. The Article describes three methods of manufacturing cross-linked polyethylene for use in power cables, firstly the Peroxide Cure Process, secondly the Sioplas Process and thirdly the Monosil Process. The Article describes the history of the developement of the Monosil Process, and favourably contrasts the Monosil Process with the earlier Sioplas Process.

The Sioplas Process is described in and is the subject of GB 1 286 460 to Dow Corning and involves three stages.

The Monosil Process is described in and is the subject of GB 1 526 398 jointly to BICC Limited (now BICC Pic.) and Etablissements Maillefer S.A. and involves only two stages. An extrusion stage is followed by a water curing stage. - 2 Prior Art - Detailed Description of Honosil Process As currently used the first stage of the Monosil Process comprises metering into a screw extrusion machine polymer in the form of cylindrical pellets having a length of 3 mm and a diameter of 4 mm, together with compounding ingredients comprising a hydrolysable unsaturated silane, a free-radical generator and a silanol condensation catalyst; blending the compounding ingredients with the polymer in the barrel of the extruder and raising the temperature sufficiently to effect grafting of silane groups to the polymer, the amount of free-radical generator being sufficiently low to limit direct free-radical crosslinking to a level below that which would prevent extrusion of the material; and extruding the reaction mixture from the extruder through an extrusion die to form an elongate shaped product.

For processing of olefin polymers, the preferred compounding ingredients are vinyl tri-methoxy silane (VTMOS), a peroxide that decomposes rapidly at the grafting temperature, and dibutyl tin dilaurate. Preferred peroxides are dicumyl peroxide and 1,3 bis (tertbutyl peroxy isopropyl) benzene.

Other compounding ingredients are added including anti-oxidants to resist weathering of the end product, fillers and pigments and an inhibitor to prevent reaction with copper wire.

The polymer and at least some of the compounding ingredients are metered into and premixed in the hopper of the extruder. Certain compounding ingredients are pre-mixed with the polymer. Peroxide is fed as a coating on the particles of polymer. Some fillers that are difficult to disperse, notably conductive carbon black, may be better predispersed in the polymer or some of it. Provided that satisfactory mixing can be achieved, it may be desirable not to add certain ingredients, mostly liquids, to the top of the extruder hopper, but to inject them to the base of the hopper or direct to the throat of the extruder. This applies in particular to ingredients that evolve unpleasant vapour and/or cause the material to stick to the hopper equipment, notably many of the silanes. - 3 The extruder itself has a single barrel with at least one screw extending from end to end of it; usually only one screw will be used, but the single screw may incorporate sections with different profiles, possibly including intermediate sections without helical flights (as for example in two stage vented extruders). The extruder provides a premixing zone, a homogenising zone and a metering zone. The homogenising zone is of the kind, described and claimed in Mai liefer S.A.'s GB 964428, in which the material to be extruded is forced over the flight of the extruder screw from a converging groove to another groove, not otherwise communicating with the converging groove. One advantage of this arrangement is that entrainment of insufficiently softened particulate material is inhibited. When the three zones referred to are present, the grafting reaction will normally take place in the metering zone.

From the metering zone of the extruder the homogenised and grafted mixture passes to the extruder die, which is mounted in a cross-head in the case of cable manufacture.

In the second stage the grafted polymer of the finished shaped product is cross-linked by exposure to water or a moist atmosphere at a suitable temperature, as in the three-stage silane grafting Sioplas Process.

The Monosil Process may be regarded as an improvement over the three-stage silane grafting Sioplas Process in that the Monosil Process avoids the need for two high-temperature processing steps and for storing moisture sensitive intermediates such as grafted polymer, whilst retaining the advantage of the silane grafting technique that the extruded product can be examined for dimensional accuracy in a very short period compared with vulcanising and chemical crosslinking methods. The latter involve high-temperature treatment of the extruded product under pressure to effect crosslinking.

A disadvantage of the Monosil process as described above is that the first component used in the extrusion stage is relatively expensive. Polyethylene pretreated to include an anti-oxidant to prevent weathering and an inhibitor to prevent reaction with copper - 4 wire is considerably more expensive, perhaps twice as expensive, as untreated polyethylene without these additives. This first component represents about 98% by weight of the input to the extruder and represents the majority of the raw material cost.

A further disadvantage of the Monosil. process as described above arises from the use of silane as the second component. Extruders rely on frictional contact between the working surfaces of the components of the extruder and the material being extruded. In modern extruders, this frictional contact has been enhanced by the provision of small grooves on the internal surface of the barrel of the extruder. The use of a highly viscous liquid such as silane may produce a film of liquid in the interior of the extruder, negating the frictional effect. In particular, the viscous liquid may fill the grooves, and may completely eliminate the frictional effect of the grooves. Feeding may stop, on restarting there may be excess silane in the extruder feed zone. As a result, grafting and cross linking may take place earlier than is desirable. Consequently, the output of the extruder may comprise a splurge of cross linked extrudate followed by a time gap in which no extrudate emerges. The splurge may be accompanied by a jet of flame, which is very dangerous.

This potential problem can in practice be overcome, but in order to do so the process conditions must be chosen with care and skill.

GB 2 202 537 A in the name BICC, published a decade after GB 1 526 398 mentioned above, describes a method of controlling grafting.

Prior Art - Recent Development Useful in the Honosil Process In a recent development, a proprietary silane product has been developed and put on the market which includes an anti-oxidant and an inhibitor. The proprietary product is sold by Union Carbide under the Trade Mark Silcat VS 735/1. The proprietary product is intended for use inter alia in the Monosil Process and is intended to remove the need to use polyethylene pretreated with an anti-oxidant and an inhibitor, and instead to allow untreated polyethylene without these additives to be used, thereby overcoming the first disadvantage mentioned above. The Preliminary Data Sheet issued by Union Carbide - 5 states that based on industrial experience the benefits observed relative to Silcat R, an earlier silane additive not including an anti-oxidant and inhibitor, are as follows: the use of a wide range of unstabilised polymers meeting the highest ageing requirements - 10 days/150°C - on copper conductor. higher onset temperature - 30°C higher than Silcat R preventing premature crosslinking or allowing higher setting temperatures of the first heating zones of the extruder barrel giving quicker melt of the resin, better homogenization and improved grafting efficiency with higher output. longer storage stability. higher SADT (self-accelerating decomposition temperature) over 90°C.

The Preliminary Data Sheet goes on to state that Silcat VS 735/1 crosslinking chemical has been specially developed for modification of unstabilised polyethylene or its copolymers and/or elastomers to yield a moisture crosslinking system. Silcat 735/1 crosslinking chemical is a vinyl-silane, incorporating an anti-oxidant - effective metal deactivator system, grafting and crosslinking catalysts in a ratio suitable for crosslinking unstabilised polymers in commercially available extrusion equipment.

In use of the newly developed Union Carbide proprietary product Silcat VS 735/1 it has been found that the claims made for the product have been justified but because of the oily nature of the product the process conditions need to be chosen with even greater skill and care than heretofore in order to provide a consistent end product.

Object and Summary of the Invention An object of the present invention is to provide an improved method of manufacturing cross-linked polyethylene. - 6 The invention is based on the experimental discovery that use of untreated polyethylene (i.e. without anti-oxidant and inhibitor) together with the addition of a small quantity of fine high density polyethylene particles and the proprietary product mentioned above yields a consistently high quality product without the requirement for critical process conditions.. By way of theoretical explanation it is believed that because the polyethylene is more finely divided than heretofore, the highly viscous silane is absorbed more quickly at an earlier stage in the extrusion process and further upstream in the extruder. As a result, the frictional effect of the extruder working surfaces, especially the frictional effect of the grooves on the internal surface of the extruder barrel, is restored. However, it should be appreciated that the invention is not to be limited or constrained or dependent in any way on the above theoretical explanation.

The invention provides a two stage method of manufacturing cross-linked polyethylene including an extrusion stage followed by a cross linking stage characterised by the addition during the extrusion stage of a relatively small quantity of relatively fine polyethylene particles of relatively high density.

Preferably the quantity of the fine high density particles is in the range of 1% to 5% by weight, even more preferably about 2% by weight.

Preferably the diameter of the fine high density particles is about 0.5 mm.

Preferably the density of the individual fine high density particles is in the range 0.95 to 0.97 gm/cm .

Preferably the polyethylene used in the process is untreated polyethylene.

Use of untreated polyethylene in place of treated polyethylene may reduce the cost of the process significantly. The additive of the invention is relatively expensive but amounts to only a small percentage - 7 of the total material used; untreated polyethylene which represents the bulk of the material used is commercially available at about half the cost of treated polyethylene.

Critical process conditions are no longer necessary, thus reducing the cost of carrying out the process and reducing wastage”arising from any failure to carry out the process correctly.

Having now described the invention in its broadest aspect, a number of specific Examples will now be given to illustrate the specific manner in which the invention may be put into effect.

Examples 1 to 5 Experiment Film grade PE resin HDPE Powder Silane V5735/1 Carbon black masterbatch Extrusion Barrel Zone Head Zone set 1 2 3 4 5 6 7 1 2 3 4 (°C) 150 150 160 170 180 190 200 210 210 210 210 Conductor : 50 mm compacted aluminium Diameter over Insulation : 11 .6 mm Line Speed : 71 m/min Extruder : 26 rpm Results Example Silane Level HDPE Level Hot Set Result Tensile Before Aaeina Strength After Aaeina 1 1.8% 2% 20% 21.1 19.6 2 1.6% 2% 50% 21.3 20.2 3 1.4% 2% 80% 20.8 18.8 4 1.6% 1% 80% 21.0 19.4 5 1.6% 0.5% 80% 21.6 20.2 - 8 The fine high density polyethylene powder used is a commercially available product intended for use in blow moulding or rotational moulding and is of fairly uniform diameter. An earlier experiment with a commercially available high density polyethylene having a broader range of product sizes produced unsatisfactory results with respect to the quality of the insulation surfaces.

The polyethylene pellets used are the type used in, for example, the manufacture of plastic bags or sacks, and are produced by Exxon Corporation of the United States of America.

Claims (5)

1. A method of manufacturing cross-linked polyethylene including an extrusion stage followed by a cross linking stage characterised by the addition during the extrusion stage of a relatively small quantity of relatively fine polyethylene particles of relatively high density.

2. A method according to claim 1 in which the quantity of fine high density particles is in the range of 1% to 5% by weight, even more preferably about 2% by weight. ft

3. A method according to claim 1 in which the diameter of the fine high density particles is about 0.5 mm.

4. A method according to claim 1 in which the density of the individual fine high density particles is in the range 0.95 to 0.97

5. A method according to claim 1 in which the polyethylene used in the process is untreated polyethylene. <-—7 |