WO2004016420A1

WO2004016420A1 - Plastics pipe

Info

Publication number: WO2004016420A1
Application number: PCT/GB2003/003613
Authority: WO
Inventors: Jeremy Bowman
Original assignee: Uponor Innovation AB
Current assignee: Uponor Innovation AB
Priority date: 2002-08-19
Filing date: 2003-08-19
Publication date: 2004-02-26
Anticipated expiration: 2005-02-19
Also published as: AU2003269094B2; GB0219272D0; US20060013980A1; AU2003269094A1; EP1530511A1; GB2392222A; GB2392222B; GB0319435D0; US8685508B2

Abstract

A plastics pipe which comprises an inner core and an outer removable skin layer bonded thereto, the outer removable skin layer comprising a polymeric material, chosen for its physical and mechanical properties, and an adhesion-modifying additive, the adhesion-modifying additive being present in the skin layer in an amount such that the adhesion of the skin layer to the inner core is sufficient to prevent substantial undesired relative movement between the skin layer and the core during installation, but insufficient to prevent the outer skin layer from being cleanly removed by peeling, at least at the ends of the pipe, and insufficient to cause a substantial reduction in the impact strength of the inner core.

Description

PLASTICS PIPE

This invention relates to plastic pipes and more parti cularly to a novel composite plastics pipe , a method for ±ts . manufacture, and a method for making joint s in such a pipe .

In the handling, installation. and connection of plast ics pipes, the pipe surface is easily damaged. In

"no-dig" plastics pipe installation techniques , for examp le, a tunnel is bored in the ground for the pipe and the pipe is then pusried or pulled th_xough the tunnel into an excavated hole where the next pipe - j oint is t o be made . Installation techniques such as pipe-bursting and slip lining can also place extreme stress on the pipe surface .

Other modern pi-pe laying methods can also sulbject the pipe to substant ±al bending and tensile forces , both of which can result in a deterioration of the mecha nical strength of the pipe . In addition , the useful life of the pipe may be reduced by diffusi ble materials in the ground, or by environment conditions , for example, exposure to direct si nlight for long periods .

Of greatest concern, is that modern pipe L aying methods can result in the pipe becoming scratched and dirt}_/ . This is disadvantageous firstly as the pipe material may be notch sensitive , in which case^ any scratches may cause greater damage to occur in the pipe during subsequent handling or use . Secondly, dirt and/or oxidation on the pipe surface ma y prevent successful welding . The main reason for failu re of j oints using an elec rofusion fitting is that the s irface of the pi-pe is dirty or has become oxidised. For this reason, until recently, the pipe ends always have had to 3oe cleaned and scraped before jointing, for example, with a hand or mechanical scraper. In practice, the cleaning and scraping is often uneven (the underside of the pipe in particular may be treated less carefuliy) , and the quality of the end result depends upon the professional skill of the installer.

In recent years there have been proposals to provide the pipe -with a non-adherent skin layer which can be removed in order to permit jointing. Composite pipe constructions of this type are described, for example, in JP3-24392, EP0474583, EPO 604907, GB2323556, GB2300456, and WO93/00212. The entire disclosures of all these patents are incorporated herein by reference for all purposes .

All of these prior art pipe constructions suffer from the disadvantage that modern pipe laying techniques tend to cause wrinkling, trucking, or at least undesired movement of the non-adherent skin layer relative to the core when the pipe is pusried through the cjround. These proposals have therefore not proved commercially acceptable.

More traditional proposals, wherein a protective skin layer is provided which is strongly adherent to the pipe, do not, of course, overcome the problem of dirt and oxidation on the outer surface, since such skin layers are very difficult to remo ze without elaborate equipment. The presence of a tightly adherent skin layer can also dramaticalLy lower the impact strength of the plastics pipe. The first appreciation that the above problems could, be solved by using a protective skin layer which is only- lightly adherent to the core pipe occurs in GB2297137 ancL GB2297138 , the entire disclosures of which are 3 incorporated herein by reference for all purposes.

In GB2297138, for example, there is provided a plastics pipe which comprises an inner core and an outer protective layer bonded thereto, in which the dimensions

) of the pipe and the protective layer are such that the ratio of the external diameter of the pipe to the thickness of the protective layer is at least 70, preferably at least a 1O0, and the cohesive strength of the outer protective iayer, excluding any lines of

3 weakness, at least at the ends of the pipe, is greater than the strength of the adhesive bond between the outer protective layer and the inner core. According to this specification, by a correct choice of the material of the skin layer and the extrusion conditions, it is possible

) to provide a level of adhesion which stiil permits clean removal of the skin layer by peeling, wϋlst preventing rucking or wrinkling of the skin layer during installation and without substantially adversely affecting- the mechanical properties of the pipe.

3

The composite pipe of UK patents GB2297137 and GB2297138 has been commercially extremely- successful, but it has been found that under specific conditions of temperature and loading it is difficult to provide a skin.

D layer which has both the required toughness and limited- adhesion to the core pipe. Quality control of the base polymer material of the skin layer, and control of the extrusion. conditions during manufacture, need to be rigorousiy maintained iff undesirable quantities of scrap

5 are to be avoided. This substantially ir creases both raw- material and manufacturing costs .

2 novel composite plastics p ipe has now >een developed which has the advantages of a plastics p>ipe with a removable oute r protective layer, but which gives improved, flexibility in choice of materials and manufacturing conditions without substantially adversely affecting the mechanical properties off the pipe .

In a first aspect , the invention provides a plastics pipe -which comprises an inner core and an outer removable skin layer bonded thereto, the outer removable skin layer comprising a polymeric material, chosen for its physical and mechanical properties , and an adhesion-modifying addit ve ,

the adhesion-modifying additive being present in the skin layer in an amount such that the adhesion of the skin layer to the inner core is su fficient to present subst antial undesired relative moveme nt between the s kin layer and the core during installation, but insufficient to prevent the oute r skin layer from being cleanly removed by peeling, at least at the ends of the pipe, and insuf ficient to caus e a substantial reduction in the impact strength of the inner core .

In a further aspect , the invention provides a method for t he production of a plastics pipe comprising an inner core and an outer removable skin layer bonded thereto, the outer removable skin layer comprising a polymeric mater ial chosen fo r its physical and mechanical prope rties , and an effective amount of an adhesion- modif ying additive, -which method comprises co-extruding molten polymeric materials forming the inner core and the out er removable skin layer from one or more ext ruder die s , bringing the molten polymeric materials tog-ether and allowing them to cool, such that , on cooling _^ the adhxesion of the skin layer to the inner co e is suf ficient to prevent substantiai undesired rei ative movement , between the skin layer and the core during ins tallation of the pipe , but insuf fficient to prevent the skin layer from be ing cleanly removed by peelingg, at lea st at the ends of the pipe, and insufficient to cause a substantial reduction in the impact strength of the inner core .

In another aspect the invention, provides a methtod of making a j oint to a plastics pipe a ccording to the first aspect of the invention, or of j oin ing two such plastics pipes , which compri ses peeling the skin layer from the region or regions o f the pipe to be j oined, to exp ose a clean surface suitable for electrofusion j ointing, installing an electrofusion fitt ing over the clean surface or surfaces of the pipe or pipes and activating the electrofusion f itting to fuse "the region or regions of the pipe or pipes thereto .

By "undesired. relative movement" in this specification is meant movement or de-bonding of the skin layer relative to the core during directional drilling, pipe bursting, slip lining, or otϊier conventional pipe installation procedures .

By selecting the material of -the skin layer on the basis of its physical and mechanical properties and then modifying its adhe sive properties . with an appropriate additive, we have found that it is possible to improve greatly the consistency of the resultant pipe without sacrificing quality and performance. Thus the properties of the material of the skin layer no longer have to be a compromise between conflicting requirements .

The strength of the adhesive bond between the skin layer and the inner core is prefearably at least 0.1 N/mm, more preferably at least 0.2 N/mm, when measured by a rolling drum peel test as described in Appendix L. The adhesive bond between the skin layer and the inner core is preferably less than 2.0 N/mm, more preferabiy less than 1.5 N/mm. Very good results have been achieved using an adhesion between the skin layer and the inner core within the range of from 0.3 to 1.5 N/miτL, when measured by the above-mentioned rolling drum peei test. Without wishing to be bound to any particular theory, it is believed that the adhesion between the high molecular weight polymers of the inner bonding layer and thxe core is as a result of Van der Waals and/or diffusive bonding, or similar forces .

It is likely that any adhesion between the skin layer and the inner core will have some effect upon the impact strength of the plastics pipe, and it is presumably for this reason that prior art proposals (other than GB2297137 and GB2297138) have always sought to avoid adhesion between the skin layer and the core. Nevertheless, it has been found that the combination of the tough skin lawyer and the light bonding used in the present invention can still produce a plastics pipe having sufficient impact strength to meed; the requirements of all available standards. Moreover improved impact strength- over the products of GB 2297137 and GB2297138 caϊi also be obtained. Preferabiy. the strength of the adhesive bond between the skin layer and the inner core is such that the impact strength of the composite plastics pipe is a least 50%, preferably at least 75%, more preferably at least 90% of the impact strength of the inner core without the skin layer.

The inner core and the skin layer of the composite plastics pipe of the present invention can comprise any suitable thermoplastic polym_eric materials, consistent with the maintenance of the required properties. Suitable polymeric materiais include, for example, olefinically-unsaturated polymers and co-polymers, for example, polyolefins such as polyethylene, polypropylene, polybutene and polybutylene; ethylene and propylene co- polymers, for example, ethyl ene-vinyl acetate polymers, and propylene—vinyl acetate polymers; halogenated-vinyl polymers such as vinyl chloride polymers and co-polymers; polyamides, fo example, nylon 6, nylon 11 and. nylon 66; polycarbonates ; ABS polymers and ionomer polymers such as Surlyn (RTM) .

The inner core of the pipe comprises a polymeric material chosen to be compatible with the particular application, and in particular with the fluid material to be conveyed by the pipe. For many applications polyethylene is the preferred material for the inner core. The grade of polyethylene chosen, that is to say, high density, medium density,, low density, or linear low density, will depend upon the particular application. Suitable grades of polyethylene for pressure pipe applications, preferably meet the requirements of at least one of prEN 12201-1 (except clause 4.2.1 and the associated pigment or carbon black requirements if an unpigmented m&terial is used) , prEN12201-2 (eixcept clause 5.2 and th≤ associated pigment or carbon black requirements if an unpigmented material is used) , prENl555-l (^except clause 4.2.2 and the associated pigment or carbon black requirements if an unpigmented material is used) and prEN1555-2 (except ciause 5.2 and the associated pigment or carbon, black requirements if an unpigmented material is used ) .

Any. suitable equivalent grade of polyethylene may, of course, also be used.

The skin layer is formed from a polymeric material or a blend of polymeric materials having good mechanical and physica.1 properties, especially toughness and low temperature impact strength, Preferred polymeric materials for the skin layer; comprise propylene homo- and co-polymers _r propylene block co-polymers, and propylene random co-polymers. Preferably the skin layer has a notched Charpy impact strength of at least 1 kJ/m², more preferably at least 2 kJ/m² and most preferably at least 4 kJ/m², when measured using the method of ISO 179/16A at a temperature of -20°C.

Under typical co-extrusion conditions, many polymer material combinations tend to adhere strongly together, and this is generally true for many grades of polyethylene and propylene polymers. In this specification, polymeric materials of the sfJcih layer are termed "adherent polymeric materials", if, but for the presence of the adhesion reducing additive, they would adhere to the core pipe to an extent such that the outer skin layer cannot be peeled from the core pipe, or such i that the impact strength of the composite plastics pipe is substantially reduced, or both, when the core pipe and the skin layer are extruded under, typical extrusion conditions. In such cases the skin layer will need to comprise ar appropriate amount . of an adhesion-reducing i additive. The quantity of adhesion-reducing additive in the polymeric material of the skin layer is preferably such_ that the adhesion to the inner core is below 2.0N/mm ani preferably below 1. ON/mm when measured using a rolling- drum peel test as described in Appendix 1. The actuai quantity of adhesion-reducing additive in the skin layer depends on the additive used, but in general is within, the range of from 0.0001% to 15%. h>y weight, mores preferably from 0.5% to 10% by weight and most preferably from 1.0% to 8% by weight .

The adhesion-reduci g additive is desirably one that has little or no effect on the mechanical and physicai properties of the polyme ic material of the skin layer. Preferred adhesion-reducing additives include esters, and_/, for example, one class of preferred esters., includes esters of polyhydric alcohols such as ethane-1, 2-diol, 2, 2-dimethyl-l, 3-propanediol (neopentyl glycol) , 2 — methyl~2-propyl-l, 3-propanediol, 2-bτ_ιtyl-2-ethyl-l, 3— propanediol, 2-butyl-2-methyl-l, 3-propanediol, 1_/3— propanediol, 2-methyl-l 3-propanediol, 1, 2-pentanediol, 1, 6-hexanediol, 3-hexyne — 2, 5-diol, 2, 5-dimethyl-3-hexyne— 2,5-diol, 1,10— decanediol, 2,2-bis(4— hydroxyoyclohexyl) propane , 1, — bis (hydroxymethyl) cyclohexane, 2,2, 4-trimethyl-l, 3— pentanediol, ,glycerol (propane-1, 2, 3-triol) _r trimethylolethane, trimethylolpropane_^ ditrimethylolpropane, perntaerythritol, dipentaerythritol_^ tripentaerythritol, sorbitol and mannitol .

Another class of preferred esters includes esters off fatty acids such as decanoic acid, docosa oic acid,- dodecanoic acid (lauric acid) , hexadecanoic acid

(palmitic acid) , octadecanoic acid, octanoic acid ( capr lic acid) , oleic acid, linoleic acid, linoleni c acid, octadecanoic acid ( stearic acid) , behenic acid, erucic acid, myristic acid, nonanoic ac id, and ricinoleic acid.

Examples of preferred esters of p-olyhydric alcohois include glycerol ester s with erucic acid, stearic acid, lauric acid, linoleic acid, myristic acid, oleic acid, palmitic acid, ricinoleic acid and behenic acid . Particularly good re sults have been achieved with glycerol mono - stearate, which is one of the most preferred adhesion-reducing additives for use in tb e present invention .

Examples of prefe rred esters of ffatty acids include hexadecanoic acid, 2 , 3 , -bis [ (trimeth^ lsilyl ) oxy] prop^yl ester and octadecanoic acid , 2 , 3, -bis

[ (trirnethylsilyl) oxy] propyl ester .

.Another class of preferred esters includes cycl ic ester s of hydroxycarboxylic acids, for example, γ- butyr olactone, which is the cycl ic ester of 4- hydro xybutyric acid.

Yet another class of preferred adhesion reducing addit ives includes acid anhydrides , for exampie, propanoic acid 2-methyl anhydride .

A still further class of adhesion-reducing additives that can be used includes fatty acid amides , for example oleamide , stearamide, erucamide , behemamide and ethylene - bis - stearamide . Excellent results have been obtained using stearamide, erucamide and ethylene - bis - stearamide, and these are further examples of most pref rred adhesion-reducing additive s for use in -the present invention.

Yet another class of adhesion-reducing additives that can be used includes ethoxylated amines and esters .

Yet another adhesion-reducing additive that c an be used is polybutylen .

Mixtures of any of the above entioned adhesion- reducing additives may be used where compatible i n any suitable combinations , proportions and amounts .

A particularly preferred plas-fcics pipe accordi ng to the present invention comprises an inner cor e of polyethylene and a skin layer of a propylene bloc k co- polymer comprising from 2% to 4% of a glycerol ester as an adhesion-reducin<g additive .

Preferably the impact strength of a 90 mm outside diameter plastics pipe having a -p olyethylene inner core ~* and a polypropylen e skin layer with an SDR of 17 . 0 is greater than 300 j oules when measured using the method of EN1411 ..1996 at a temperature of -10°C using a 90mm diameter tup for impacting the pipe .

The adhesion-modifying additi^ve is preferably added to the polymeric material of the skin layer as a masterbatch . The masterbatch pre ferably comprises from 1% to 50% by weight of the adhesion-modifying additive, more preferably from 2% to 30% by weight .

In general, the use of processing aids, incILuding metal stearates , smch as calcium stearate, has been found to be ineffective in reducing . adhesion and shou ld be avoided . While the invention has been found to be particularly applicable to combinations of adherent polymeric materials and adhesion-reducing additives, in principle it could also be applied to no-adherent polymeric materials, whereupon the adhesion-modifying additive would need to be an adhesion-promoting additive.

An advantage of the plastics pipes o f the present invention is that the normal UV stabiliser and colorant package need not be included in the plastics material of the inner core, provided that sufficient quantities of these materials are included in the skin layer. This enables the inner core to comprise a natural polymeric material, free or substantially free from additives which add to the cost of the core material and which, in certain ci cumstances, . may impair the .mechanical or physical properties of the core material. Alternatively, stabilisers can be included in the core material, but the outer protective skin layer can be coloured to indicate" the underlying grade of pipe and/or the fluid being transported by the pipe.

Suitable stabiliser or ultra-violet blocking additives for the outer protective skin include, for example, titanium dioxide, carbon blacl, and other fillers. Whilst carbon. black is an excellent UV stabiliser and reinforcing filler, buried pipes are frequently colour coded and its use in the outer

I protective layer is therefore not possible for many applications. Titanium dioxide is, therefore, the preferred filler and UV stabiliser since this is also compatible with many colorant packages. Other filler materials, such as chalk and talc, may also be used. The j preferred tiller particle size is dependent on the filler being used, but for titanium dioxide_^ for example, the average particle size range is prefer able from 0 . 003 to 0 . 025 microns .

The skin layer and the inner core can, of course, each comprise more than one layer of polymeric materiaal , although in practice this is not usually necessary.

The relative thickness of the s kin layer and the dimens ions of the pipe have also been found to affect the impact resistance of the pipe . This is discussed in GB 229713 8 . Preferably the skin layer has a thickness of greater than 0 . 1 mm, more preferably greater than 0.2 mm, and mo st preferably within the range of from about 0. 3 mm to 2. 0 mm.

-The dimensions of - the pipe- and the protective la^ er are preferably such that the ratio of the external diameter of the pipe to the thickness of the skin la^ er is at least 70 , more preferably at least 100 , most' preferably in the range 100 to 800 . From this it can be seen that it is possible to use a thic er skin layer orn a pipe o f greater diameter .

When stripping the s kin layer from the pipe, it is import ant that no residue or holidays should be left on the pipe surface that could interfere with -the electrofusion j ointing process . Thus conventional adhesives and skin layers that are prone to tearing or fragmentation should he avoided . In general the force required to rupture the skin layer shoταld be greater tϊhan the force required to peel the skin layer from the inaner core .

By ^λ a clean surface" in this specification is meant a pipe surface that can be subjected to electrofusion jointing without further preparation or treatment . Such surfaces should give rise to a -quality of electrofusion joint that meets the requirements of one or more of pr EN12201 part 3, pr EN1555 parf 3 and WIS 04-32-14.

The composite plastics pipe of the present invention is preferably produced by co-extrusion, -wherein the polymeric materials are brought together in the pressure area of the die and exit as a single extrudate. For example, the die may be connected to one, two, or more extruders and fed with separate streams of molten material. Alternatively, the die may be provided with concentric die outlets fed with separate streams of molten polymeric materials which are to fo rm the inner core and the skin layer. In this case, the extrudates, --on- leaving -the extruder die outlets, can be brought into' contact with each other whilst still molten, preferably in a sizing die which simuitaneously adjusts the outer diameter of the pipe.

In a further alternative, the inner core extrudate may be passed through a sizing die before applying the skin layer. In this case it may be necessary to re-heat or flame-brush the surface of the inner core extrudate to create a surface ready to receive the skin layer. Because of the difficulty of maintaining a consistent adhesion between the inner core and the skin layer, and of keeping the core surface clean (prior to coating with the skin), this method is not presently preferred.

The method of the invention is illustrated by the following Examples:

EXAMPLE 1 A polyethylene core pipe of nominal outer diameter 90mm was co-extruded with a propylene random copolymer skin layer. The experiment was repeated replacing the propylene random copolymer with (I) a propylene block copolymer and (II) with a blend of the copolymers, in each case with the addition of 15% by weight of a masterbatch comprising 20% by weight of a glycerol ester.

Skin adhesion was measured using a rol ling drum peel test as described in Appendix 1.

The shin layers of all of the resultant pipes could be peeled readily using a simple hand tool, exposing a clean surface of the core pipe. Electrofusion jointing tests gave "very good results in conformance with prEN12201 part 3, prEN1555 part 3 and WIS 04-32-14. in a comparison test, a pipe was extruded with a propylene block copolymer skin layer without the glycerol ester. The skin layer firmly adhered to the core pipe and could not be removed from the pipe by peeling.

The impact strength of the pipes was measured at -10°C, with and without the skin layer, usinςj the method of EN 1411: 1996. In further experiments the pipes were notched at 90° to the point of impact prior to testing to simulate service conditions. The results are cgiven in Table 1.

TABLE 1

*Passed if the skin breaks but the core remains intact ,

The results show that by using an adhesion-r-educing additive to reduce the bond strength of a propylene block copolymer s in layer to a polyethylene core , the impact strength of the pipe can be substantially in creased when compared to a similar pipe having a propylen e random copolymer s in layer, whilst the peelability of the pipe is retained . EXAMPLE 2

The procedure of Example 1 was repeated, by extruding a polyethylene inner core and a polypropylene skin layer including a range of adhesion-reducing additives. All percentages are by weight. The same tests were carried out with the following results:

00

EBS = Ethylene-Bis-Stearamide PP = Polypropylene block copolymer GMS = Glycerol Mono-Stearate

Tti ese results further demonstrate the improvement i_n impact strength and peelability obtained using the method off the present invention .

APPENDIX 1

DETERMINATION OT THE ADHESION STRENGTH OF PIPE SKI - CORE PEEL

Apparatus

A tensile testing machine accurate to grade A o f BS5214 : Part 1: 1975 ox grade 1 of BS1610 : Part 1 : 1985, for example, a Lloyds tensile test machine, usi_ng a 100N load cell.

Test Specimens

Two test pieces are cut one from each end of the sample pipe, 25mm +/- 2mm wide, the two sample rings of pipe are trimmed around the circumference to remove the jagged edge. The pipe is marked along top dead centr-e (TDC) of the extrusion Line (if known) .

The two ring specimens are marked with an indelible marker at quarterly points around the circumference beginning at TDC (if known) as illustrated in figure 1,-

Procedure

Cut through the skin along mark at TDC & prise edge of skin from pipe, peel skin off to 30 — 40mm length, freed peeled skin through the jig as shown. & clamp in up>per j aws .

Mount the test piece d_n the jig as shown in Figure 2 and 2a.

The s in is then peeled from the pipe at a separation rate of lOOmm/min and a trace recorded of load versus time .

The average value of the load reguired to peel the skin sample is calculated (Newtons), and divided by the true width of the peel sample to obtain the test result (Newtons/milli etre) .

The average of the 10 peak load values recorded^' is calculated (Newtons) , and divided by the true width of the peel sample to obtain the test result (Newtons/millimetre) .

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incoxporated herein b>y reference.

All of the features disclosed, in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any metiod or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive .

Each featμre disclosed in this specification, (including any accompanying claims, abstract and drawings), may toe replaced by alternative features serving the same, equivalent, or similar purpose, unless expressly stated otherwise. THnus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar f atures .

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination,^' of the steps of any method or process so disclosed.

Claims

1. A plastics pipe which comprises an inner core and an outer removable skin layer bonded thereto, the outer removable skin layer comprising a polymeric material, chosen for its pohysical and mechanical properties, and an adhesion-modifying additive,

the adhesion-modifying additive being p>resent in the skin layer in an amount sxαch that the adriesion of the skin layer to the inner core is sufficiemt to prevent substantial undesired relative movement between the skin layer and the core during installation, but insufficient to prevent the outer skin layer from hoeing cleanly removed by peeling, at least at the ends of the pipe, and insufficient to- cause a .substantial, .reduction in the impact strength of the inner core.

2. A plastics pipe according to claim 1 , wherein the strength of the adhesive bond between the skin layer and the inner core is from 0.2 N/mm to 2.0 N/mm, when measured by a rolling drum p^eel test as described in Appendix 1 .

3. A plastics pipe according to claim 1 or 2, wherein the strength of the a.dhesive bond between the skin layer and the inner core is from 0.3 N/mm to 1.5 N/mm, when measured by a rolling drum peel test as described in Appendix 1.

4. A plastics pipe according to any one of the preceding claims, in which the st ength of' the adhesive bond between the skin layer and the inner core is such that the impact strength of the composite pipe is at least 5C*%, preferably 75%, of the impact strength of the inner core wit oirt the skin layer.

5. A plastics pipe according to any one of the preceding claims, in which the inner core comprises polyethylene -

6. A plastics pipe according to any one of the preceding claims, wherein the≤ skin layer comprises a propylene homo-or co-polymer, a propylene block co-polymer, or a propylene random copolymer.

7. A plastics pipe according to claim 6, wher in the skin layer comprises a propylene block co-polymer.

8. A plastics pipe according to any one of the preceding claims, in which th_e inner core comprises polyethylene and the skin. layer co pri sesf-¹ a propylene co-polymer and wherein the impact strength of the pipe is greater than 300 joules, when measured using the method of EN1411:199>6 at a temperature of -10°C using a 90mm tup for impacting the pipe.

9. A plastics pipe according to any one of the preceding claims, wherein the skin layer comprises an adherent polymeric materi_al and an appropriate amount of an adhesion-reducing additive.

10. A plastics pipe according to claim 9, wherein the adhesion-red.'ucing additive in the skin layer is p'resent in an amount of from. 0.5% to 10% by weight.

11. A plastics pipe according to claim S or 10, wherein the adhesion reducing additive is an ester of a polyhydric alcohol.

12. A plastics pipe according to claim 11, wherein the polyhydric alcohol is glycerol.

13. A plastics pipe according to claim 11 or 12, wherein the ester is a fatty acid ester.

14. A plastics pipe according to claim 11 or 12, wherein the adhesion-reducing additive is glycerol mono- stearate .

>

15. A plastics pipe according to any one of the preceding claims, which comprises en inner core of polyethylene, and. a skin layer of a propylene block copolymer comprising from 2% to 4 -% of a glycerol ester.

)

16. A plastics pipe according to any one of claims 1 to 10, wherein the adhesion-reducing additive is a fatty acid amide.

3 17. A plastics pipe according to claim 16, wherein the fatty acid amide is stearamide or errucamide.

18. A plastics pipe according to claim 16, wherein the fatty acid amide is ethylene-bis-stearamide. )

19. A plastics pipe according to any one of the preceding claims, wherein the skin layer has a thickness within the range of from 0.2 mm to 2.0 mm.

20. A plastics pipe according to any one of the preceding claims, wherein the ratio of the external diameter of the pipe to t_he thickness of the skin layer is from 100 to 800.

21. A plastics pipe according to any one of the preceding claims, wherein the strength (in INI per mm of width) of the skin layer exceeds tlhe peel strength (in N per mm width) of the adhesive bond between the outer removaDole skin layer and the inner core.

22. A plastics pipe substantially as hereinbefore described.

23 . A method for the product ion of a plasti cs pipe comprising - an - inner core and ' an outer r emovable skin layer loonded thereto , the outer remova ble skin layer comprising a polymeric material chosen for its physical and mechanical properties , and an effective amount of an adhαesion-modif ying a_dditive , which met hod comprises co-extruding molten polymeric materials forming the inner core and the outer removable skin layer from one or more extruder dies , bringing the molten oolymeric materials t ogether and allowing, them to co ol , such that , on co oling, the adhesion of the skin layer to the inner core is sufficient to prevent substantial undesired relative movement between the s kiin layer and the core during installation of the ρ>ipe , but insufficient to prevent the skin layer fr om being cleanly removed by peeling , at least at the ends of the pipe, and insufficient to cause a substantial reduction in the impact st rength of the inner core .

24. A method according to claim 23, wherein tine polymeric materials of the inner core and the outer removable skin layer are- extruded simultaneously arid brought together whilst still molten.

25. A method according to claim 23 or 24, substantially as described in the Examples.

26. A method of making a joint to a plastics pipe according to any one of claims 1 to 22, or of joining two such plastics pipes _r which comprises peeling the skin layer from the region or regio ns of the pipe to be joined, to expose a clean surf a ce suitable for electrofusion jointing, installing an electrofusion fitting over the clean surface" or surfaces of the pipe or pipes and activating he ^•electrofusion fitting to fuse the- region or regio-ns - of the pipe or pipoes thereto.