GB2349934A

GB2349934A - Pipes

Info

Publication number: GB2349934A
Application number: GB9910644A
Authority: GB
Inventors: David John Hill
Original assignee: Glynwed Pipe Systems Ltd
Current assignee: Glynwed Pipe Systems Ltd
Priority date: 1999-05-10
Filing date: 1999-05-10
Publication date: 2000-11-15
Also published as: GB9910644D0

Abstract

A pipe is formed from a polyolefin, the polyolefin being substantially unpigmented over the greater part (4 ) of the wall thickness of the pipe, but being pigmented to a depth of up to 1 mm at the outer surface (6) of the pipe. It may be produced by coextrusion the substantially unpigmented layer and outer pigmented layer being coextruded such that they fuse together. Such a pipe may be joined using an electrofusion method, by scraping an end of the pipe to remove the pigmented region thereby revealing the underlying non-pigmented region, inserting the scraped pipe end into an electrofusion coupling sleeve and energising the electrofusion coupling sleeve to fuse the pipe to the coupling sleeve.

Description

1 2349934 1 PIPES 5 This invention relates to pipes, and in particular to

plastics pipes and methods for joining such pipes.

The use of plastics pipes for conveying mains services such as water and gas is well known and pipes formed from various grades of polyethylene have been used for such purposes for many years. Plastics pipes offer the advantage of being joined together by fusion techniques, rather than by mechanical pipe couplers, and one technique commonly used to join pipes is electrofusion. Electrofusion jointing of pipes involves insertion of a pipe end into an electrofusion coupling sleeve, the inner surface of which is provided with a heating wire coil or other heating means which can be heated to melt the plastics material on the surface of the pipe, and also the inner surface of the coupler thereby leading to fusion.

One of the problems with electrofusion joints is that if the end surface of the pipe becomes contaminated with impurities, such as dirt or grease, or if the pipe surface has a relatively high concentration of polar or hydrophilic compounds or groups present, for example oxidised polyethylene, then this can lead to an imperfect bond between the pipe and coupler during fusion. In recognition of this, for many years, it has been conventional practice to scrape the surface of a pipe to remove any surface impurities.

However, scraping the surface of a pipe has been perceived to be a disadvantage owing to the time factor involved and the occasional poor 2 quality of scraping, resulting in poor joints of low strength, and more recently pipes have been produced which have a peelable surface layer that protects the underlying pipe material from contamination or oxidation, the peelable layer being removed in the vicinity of the ends of the pipe to reveal an apparently pristine surface prior to electrofusion. Such pipes are disclosed, for example, in British Patent No: 2263524 and documents cited therein.

The use of a peelable surface coating, although advantageous in some respects, nevertheless does have a number of significant disadvantages. Firstly, the methods of pipe manufacture and the manner of use of the pipe means that it is generally necessary to coat the entire pipe with the coating when all that is really required for most uses is that a small portion of the end of the pipe when cut be removable to allow electrofusion to take place. The exception to this is when it is required to weld an electrofusion saddle tee fitting at some point along a pipe, when it is necessary to remove a small patch or length of coating. Thus, in general, such coated pipes are wasteful of materials in circumstances where protection to the pipe is not required along its entire length.

Secondly, the addition of a separate peelable coating to the exterior of a pipe will lead to an increase in the stiffness of the pipe which in turn will make it less easy to bend or coil the pipe, particularly under winter conditions. For example, with 110 mm ODSDR 11 pipe, the underlying pipe will need to have a wall thickness of approximately 10 mm. It will be appreciated therefore that a 1 mm coating will increase the wall thickness by 10% with a commensurate increase in the stiffness and material cost of the pipe. This problem is particularly enhanced where the outer coating is made from an inherently stiffer material such as polypropylene, applied to an underlying polyethylene (for example a medium density polyethylene) pipe.

A further disadvantage of a peelable coating arises when it is 3 required to fuse a tapping tee to an existing pipe to make a service connection. The ground may be excavated by means of a narrow hole, exposing a short section of the buried pipe. To the operative in the depth of the hole, it may be unknown and non-obvious that the exposed pipe has a peelable coating of a different material (any markings may not be exposed or may be obscured by soil). If the operative employs conventional scraping means, the thickness of the peelable coating may be much more than the thickness of scraping removed, therefore not divulging the presence of a peelable layer. The operative may then attempt to carry out a weld using a top loading saddle tee fitting. Such a weld to the surface skin only, whether made from polypropylene or polyethylene, will inevitably result in a failed joint, which may or may not become obvious until subjected to service or test pressure.

Pipes for use with utilities such as water or gas are generally pigmented in order to increase their resistance to degradation induced by ultra-violet light, and to discourage microbial or algae growth within the pipe, for example in the case of water pipes used above ground. In general, the pigment is dispersed relatively evenly throughout the pipe.

This gives rise to several real or potential disadvantages. Firstly, the pigment is dispersed throughout the thickness of the pipe when in reality it is needed most at the surface of the pipe. Thus, the pigment below the surface can be regarded as wasted. Conversely, if the density of pigment is reduced in order to lower the overall cost of the pipes, the local density at any given point may be insufficient to prevent some light-induced oxidative degradation taking place.

Pipes which make use of natural polyethylene, that is to say polyethylene containing no pigment, and which are coated with a peelable coating of polypropylene in the manner described above, are known.

However, such coated pipes possess all the disadvantages associated with the additional peelable coating.

4 It is an object of the present invention to overcome or at least alleviate the aforementioned problems.

Accordingly, in a first aspect, the invention provides a pipe formed from a polyolefin, the polyolefin being unpigmented over the greater part of the wall thickness of the pipe, but being pigmented to a depth of up to 1 mm at the outer surface of the pipe.

In contrast to the separate peelable coatings disclosed in for example GB 2263524, the pigmented outer surface of the pipes forms an intrinsic part of the pipe wall. Typically, therefore, it is removed by scraping using a conventional pipe scraping tool prior to joining by electrofusion. In this context, the pigmented outer region of the pipe wall may act as an indicator showing visually when the required amount of polymer material has been scraped from the surface of the pipe, the underlying unpigmented polymer serving as the indicator.

It is preferred that the pigmented region extends to a depth of no more than 0. 5 mm from the outer surface of the pipe, for example no more than 0.4 mm, eg approximately 0.3 mm or less.

The level of pigmentation of the outer layer can be much higher than is the case when the pigment is dispersed throughout the wall of the pipe.

Therefore, for example, the outer pigmented region of the pipe wall can contain up to about 20% (w/w) of pigment, for example at least 1 % (w/w), more usually at least 2% (w/w) and preferably more than 5% (w/w) (in the case of pigments othe r than carbon black). Pigment strength and opacity typically vary according to pigment type, particle size and dispersion quality. Therefore the concentration of a given pigment sufficient to provide the required level of UV protection and/or opacity at the specific pigmented coating thickness to be used may need to be determined on a case by case basis. For example, the high strength and opacity of carbon black means that it can be used at concentrations of 2% (w/w) or more. In any event, the concentration of pigment will be an amount effective to prevent or substantially reduce UV damage to the pipe, and render the pipe sufficiently opaque to discourage microbial/algal growth in the pipe bore.

The pigment used in the pipes of the present invention can be selected from pigments such as carbon black, titanium dioxide, ultramarine blue, phthalocyanine blue, cadmium orange and yellow diazo yellow and other organic dyes, e.g. diazo dyes.

The pipes of the invention enjoy a number of significant advantages over conventional plastics pipes. Thus, for example, the pigmented outer region serves as an indicator, the removal of which to reveal an unpigmented layer underneath provides a visual means of determining when sufficient polymer has been removed by scraping prior to electrofusion taking place. Furthermore, by concentrating the pigment in the surface region, rather than dispersing the pigment throughout the pipe, an improved protective effect can be produced thereby reducing the susceptibility of the polymer in the surface region of the pipe to light induced oxidative degradation. Furthermore, by avoiding the need to disperse pigment throughout the entire thickness of the pipe wall, the overall concentrations of pigment used can be reduced, thereby making manufacture more economical.

Further advantages of the pipes of the invention include the ability to improve extrusion properties and provide other product benefits to the surface appearance and/or quality of the finished pipe without disturbing the bulk pipe properties. Thus, for example, the coating layer may contain a higher level of lubricants or other additives which modify the rheological and/or surface release properties in order to confer a preferred smooth surface finish. Such additives can include metal soaps, such as zinc 6 stearate, natural and petroleum based oils and waxes, fluorinated polymers and copolymers, for example. By providing these additives in the surface layer alone, where they are required, or at higher concentrations than in the bulk of the pipe material, the surface properties may be modified whilst the bulk properties of the extruded material are unaffected, thereby reducing or eliminating any concerns that the overall pipe properties or jointing of the pipes could be impaired by the presence of such additives.

Since the outer coating layer is highly pigmented, a further advantage is that the material from which the coating is extruded can be readily identified, controlled and limited in use in the factory environment.

Furthermore, for the purpose of reusing scrap pipe, the surface coating can either be stripped from the pipe surface e.g. using a (wood) bark cutter, or may be left intact and reprocessed into a lower grade application, in which case the additives will aid dispersion of the pigmented material into the bulk.

Thus, in a further aspect, the invention provides a pipe formed from a polyolefin, the polyolefin being unpigmented over the greater part of the wall thickness of the pipe, but being pigmented to a depth of up to 1 mm at the outer surface of the pipe, and wherein the pigmented polyolefin contains one or more additives selected from lubricants and other additives which modify the rheological and/or surface release properties of the pipe, the concentrations of such additives being higher than any concentrations of such additives in the unpigmented polyolefin.

The pipes of the present invention are formed by means of co extrusion, the unpigmented layer and outer pigmented region being co extruded together such that they fuse together. The resulting fusion boundary between the inner and outer regions is such as to resist delamination by, for example, peeling.

In a further aspect, the invention provides a method of joining a 7 plastics pipe using an electrofusion method, which method comprises selecting a pipe having an inner non-pigmented region and an outer pigmented region as hereinbefore defined; scraping an end of the pipe to remove the pigmented region thereby revealing the underlying non pigmented region; inserting the scraped pipe end into an electrofusion coupling sleeve; and energising the electrofusion coupling sleeve to fuse the pipe to the said coupling sleeve.

The invention will now be illustrated by way of example only with reference to the particular embodiment as shown in the accompanying drawings of which:

Figure 1 is a longitudinal sectional elevation through a pipe according to one embodiment of the invention; Figure 2 is a longitudinal sectional elevation illustrating the pipe after an end thereof has been scraped; and Figure 3 illustrates the connection of the pipes of Figure 1 and Figure 2 and electrofusion coupling sleeve.

Referring now to the drawings, Figure 1 illustrates a pipe 2 formed from medium density polyethylene. The pipe has a main region 4 formed from a non-pigmented or natural polyethylene, for example of the "Finathene 3802" type and an outer surface region 6 of a substantially similar or identical polymer but including a pigment such as titanium dioxide and phthalocyanine blue. In this embodiment, the thickness of the outer region is approximately 0.2 mm, although this could be varied if desired within a range of about 0.1 to 0.2 mm.

The pipe shown in figure 1 is formed by co-extrusion using a Battenfeld 75mm single screw extruder for the natural unpigmented 8 material, a Stork 25mm co-extruder for the pigmented layer and a two layer spiral mandrel co-extrusion tool.

After extrusion, the pipe is cooled by means of a water spray, and marked in conventional fashion.

In use, the pipe can be joined by means of electrofusion jointing as shown in Figure 3. Prior to jointing, a conventional scraping tool is used to remove the 0.2 mm region of pigmented polymer. This can be achieved by setting the scraping tool to the desired thickness. As will be appreciated, once the pigmented layer has been removed, the underlying colourless or non-pigmented material is revealed, thereby indicating visually to the user that the pipe has been scraped to an appropriate depth.

The pipe end is thereafter inserted into an electrofusion coupling sleeve of known type which is then energized to fuse the wall of the electrofusion coupling sleeve to the pipe wall.

It will readily be apparent that numerous modifications and alterations can be made to the pipe and pipe jointing methods shown in the accompanying drawings without departing from the principles underlying the invention, and all such modifications and alterations are intended to be embraced by this application.

9

Claims

CLAIM$

1 A pipe formed from a polyolefin, the polyolefin being substantially unpigmented over the greater part of the wall thickness of the pipe, but being pigmented to a depth of up to 1 mm at the outer surface of the pipe.

2. A pipe according to claim 1 wherein the pigmented region extends to a depth of no more than 0.5mm from the outer surface of the pipe.

3. A pipe according to claim 2 wherein the pigmented region extends to a depth of no more than 0.4mm e.g. 0.3mm or less.

4. A pipe according to any one of claims 1 to 3 wherein the outer pigmented region of the pipe wall contains up to about 20% (w/w) of pigment.

5. A pipe according to claim 4 wherein the level of pigment is more 20 than 5%.

6. A pipe according to claim 4 wherein the pigment is carbon black and is present at a concentration of at least 2% (w/w). 25

7. A pipe formed from a polyolefin, the polyolefin being unpigmented over the greater part of the wall thickness 'of the pipe, but being pigmented to a depth of up to 1 mm at the outer surface of the pipe; the pipe having been formed by means of coextrusion, the substantially unpigmented layer and outer pigmented layer being coextruded together 30 such that they fuse together.

8. A pipe substantially as described with reference to the accompanying drawings.

9. A method of joining a plastics pipe using an electrofusion method, which method comprises selecting a pipe having an inner substantially non pigmented region and an outer pigmented region as defined in any one of claims 1 to 8; scraping an end of the pipe to remove the pigmented region thereby revealing the underlying non pigmented region; inserting the scraped pipe end into an electrofusion coupling sleeve; and energising the electrofusion coupling sleeve to fuse the pipe to the said coupling sleeve.