GB2310266A

GB2310266A - Thermoplastic pipe and method of making such

Info

Publication number: GB2310266A
Application number: GB9702780A
Authority: GB
Inventors: John Brennan; Bob Leevers; Detlef Ruegner; John Ackery; Darryl Stewart
Original assignee: James Hardie Research Pty Ltd
Current assignee: James Hardie Research Pty Ltd
Priority date: 1996-02-13
Filing date: 1997-02-11
Publication date: 1997-08-20
Also published as: ID16197A; ZA971040B; GB9702780D0; AUPN805296A0

Description

A THERMOPLASTIC PIPE The present invention relates to thermoplastic pipes and more particularly to thermoplastic pipes adapted for use in conditions involving at least partial exposure to the sun.

Thermoplastic pipes, such as those formed from polyethylene or polypropylene, are widely used in manly industries to transport fluids in both pressurised and non-pressurised applications. Carbon is often added to the formulation of the pipes in order to stabilise them against degradation from ultra violet radiation and this addition typically results in black colouration of the pipes.

It has been found that the tensile strength of thermoplastic pipes significantly decreases as they are heated and consequently, pipes exposed to sunlight must be designed to contain their rated pressure whilst the pipe is at its hottest expected temperature.

However, because the maximum temperature is only reached on hot days, and even then only for a relatively short period of the day, the exposed pipes are overrated for the majority of the time. This results in such pipes having a larger wall thickness than is normally necessary, with corresponding material wastage and extra expense. Furthermore, excessive thermal expansion due to heating can cause damage to the pipes and to the associated fittings and equipment.

In an attempt to reduce these problems, it has previously been proposed to provide a loose fitting reflective sheath over the pipes and to provide, by means of repulsive electrostatic charges, an insulative air gap between the innermost surface of the sheath and the outermost surface of the pipe. The reflective qualities of the sheath in combination with the insulative properties of the air gap to some extent reduce the temperature rise in the pipe caused by radiant heat from sunlight and alleviate the problems discussed above.

However, this approach also has seml inherent disadvaes. Firstly, the sheaths must be mantfrtured separately and subsequently drawn over each corresponding length of pipe, with the result that installation is relatively difficult, time consutning and expensive. Secondly, the loose sheaths are prone to mechanical damage both initially during transportation and installation, and subsequently by environmental factors such as contact with animals or wind blown debris. This incurs high maintenance costs as the sheaths must be intact in order to ensure that the pressure rating of the pipe is maintained.

The sheaths also increase the difficulty in connecting adjacent lengths of pipe as the separate layers must be individuallyjoined Accordingly, this system has significant practical and commercial limitations.

It is an object of the present invention to overcome or at least ameliorate one or more of the disadvantages of the prior art.

Accordingly, in a first aspect, the invention provides a thermoplastic pipe adapted for exposure to sunlight, said pipe including an inner thermoplastic layer having an inner surface and an outer surface, and an outer thermoplastic layer having an inner surface and an outer surface, wherein the inner and outer layers are disposed in close fitting coaxial relationship, and wherein the outer surface of the outer layer is substantially reflective to minimise temperature rise in the inner layer when the pipe is exposed to sunlight.

According to a second aspect, the invention provides a method of producing a thermoplastic pipe adapted for exposure to sunlight, said method including the steps of forming an inner thermoplastic layer having an inner surface and an outer surface, forming an outer thermoplastic layer having an inner surface and an outer surface, positioning the inner and outer layers in close fitting coaxial relationship, and adapting the outer surface of the outer layer to be substantially reflective thereby to minimise temperature rise in the inner layer when the pipe is exposed to sunlight.

In a first preferred embodiment of the invention, the outer surface of the inner layer is thermoplastically bonded directly to the inner surface of the outer layer in a softened state during a cos fusion process. Alternatively, the inner and outer layers may be formed separately, engaged tclescopically, and joined with an interference fit. In a further variation, the outer layer may be shrunk over the inner layer by localised heating.

In an alternative embodiment, the inner and outer layers are joined by means of an intermediate layer of barrier material. In one preferred embodiment, the barrier material is an adhcsive. Alternatively, however, the barrier layer is formed from a compatible thermoplastic material, in which case the inner layer, the outer layer and the intermediate barrier layer may be coduded to form the composite pipe.

Preferably, the entire outer layer is formed from a substantially reflective thermoplastic material. Alternatively, however, a reflective coating may be added to the outer surface of the outer layer. In the preferred embodiment, the inner and outer layers are formed from a suitable polyolefin such as polyethylene or polypropylene. The outer layer is preferably white in colour, and is desirably produced by adding titanium dioxide to a basic thermoplastic formulation.

Preferred embodiments of the pipe have an outer diameter ranging from 60mm to around 1200mm. The radial thickness of the outer layer is desirably between 0.5mm and around 5mm.

The inner layer is desirably formed from black polyethylene. However, non black polyethylene may also be used, as carbon is no longer necessary in the formulation because the inner layer is not exposed to direct sunlight.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is an end viewofathermoplastic pipe according to a first embodiment of the invention; Figure 2 is a sectional side view ofthe pipe shown in Figure 1; Figure 3 is a schematic side view of an extrusion dye used in producing the coextruded pipe shown in Figures 1 and 2; Figure 4 is an end view of a theenoplastic pipe according to a second embodiment of the invention including an intennediate barrier layer, and Figure 5 is a sectional side view ofthe pipe shown in Figure 4.

Referring firstly to Figures 1 and 2, there is shown a thermoplastic pipe 10 adapted for use in applications involving at least partial exposure to sunlight. The pipe includes an inner polyethylene layer 12 co-extruded with an outer polyethylene layer 14 such that the innermost peripheral surface 16 ofthe outer layer is fuses with the outermost peripheral surface 18 ofthe inner layer.

The outer layer contains additives, such as titanium dioxide, to give it a white appearance and to provide the desired reflective characteristics. The inner layer is preferably black in colour. The white outer layer reflects radiant heat from the sun and thereby minimises the temperature rise of the inner layer. As the composite thermoplastic pipe remains at a lower temperature than a standard black pipe under similar conditions, a relatively smaller wall thickness may be used to achieve the same pressure rating. This saves material and therefore provides a significantly less expensive pipe in any given exposed application.

Figure 3 shows an artnrrion hrd 20 adapted to produce the pipe shown in FiFes 1 and 2. The extrusion bead includes a fist extrusion pin or mandrel 22 and an outer die 24.

The inner pipe layer 12 is extruded between these two components. A second extrusion pin 26 is added to the extrusion head and has a corresponding outer die 28. A bolt 30, shown in phantom, is used to connect the second dye to the first. An adjustment plate 32 is attached to the second dye by longitudinal adjustment bolt 34. Rotation of bolt 34 adjusts the width of channel 36 through which flows the molten polyethylene 38 to form the outer layer 14. A radial adjustment scsew 40 is used to regulate the thickness ofthe extrudes outer layer 14 by varying the radial spacing between the innermost edge 42 ofthe second die and the outermost surface 18 ofthe inner layer.

In use, molten black polyethylene to fbrm the inner layer 12 of the pipe is extruded in the direction indicated by arrow 44 between the first extrusion pin 22 and the corresponding die 24. The molten polyethylene 38 for the outer layer 14 is simultaneously fed through the second dye 32 into channel 36 and thereafter onto the molten inner layer.

Upon cooling, the inner and outer layers bond together to form the unitary thermoplastic pipe 10.

Alternatively, the outer layer 14 may be extruded independently ofthe first and thereafter shrunk, under application of localised heat, onto the outermost peripheral surface of the inner layer 12. It is also possible to manufacture the inner and outer layers independently with appropriate dimcnsions to form an interference fit at the transitional surface or interface.

Figures 4 and 5 show another embodiment of thermoplastic pipe 10 which, in addition to inner layer 12 and outer layer 14, includes a barrier layer 50 therebetween. If the inner and outer layers are produced from thermoplastics which will not rcadily bond under extrusion then the inti barrier layer SO may be fbrmed from a suitable adhesive to fix the inner and outer layers together. Alternatively, the barrier layer SO may be formed from a third thermoplastic material having a melt flow index suitable for forming an extrusion bond with both the inner and outer layers. In this embodiment, the pipe is produced by an extrusion head similar to that shown in Figure 3 but including a third extrusion pin and corresponding die plate. In either case, the inner and outer layers are still close fitting in the sense of being effectively integral and substantially without an intermediate air gap or similar clearance space.

An example of a formulation used to produce the outer layer of the pipe according to the invention is shown below.

Outer Layer Composition 1. Polyethylene Resin lOOpartsbyweight 2. Titanium Dioxide Powder 2-10 parts by weight 3. Aluminium Powder 4-10 parts by weight 4. Chemical UV Absorber As required to meet the weathering resistance clause of Australian Standard AS4131 NB 1. Polyethylene Resin is pipe grade natural resin meeting the requirements of Australian Standard AS4131.

NB 2. Aluminium Powder is optional and may be added to increase the amount of infra-red radiation reflected.

In preferred embodimcnts, the reflective outer layer may include other additives such as calcium carbonate, talc, silica, hollow glass spheres, solid glass spheres, wollastonite, kaolin, UV absorbers, blowing agents, cross-linked polyethylene, inorganic fillers, metallic powders and/or any combinations of the foregoing to enhance the performance characteristics of the pipe in terns of particular properties such as reflectivity, conductivity, toughness, and abrasion resistance, dc-in. upon the intended application For example, the addition of a blowing agent reduces thermal conduction in the pip.

The addition of crostlinked polyethylene to the outer layer results in greter resistance to mechanical damage. Heat transfer through the outer layer can also be reduced by adding a filler with lower heat transfer properties than the surrounding polyethylene. Such additives include hollow or solid glass microspheres and inorganic fillers. The addition of metallic powder to the outer layer also improves the reflection of incident radiation thereby reducing the heat reaching the inner layer.

A particularly useful application of pips according to the invention is in above ground fluid tansport pipelines where the reflective outer layer may also act as a heat sinlr directing radiant heat from the upper exposed surface ofthe pipe to the cooler shaded or partially buried surface ofthe pipe.

The thermoplastic pipe according to the invention possesses many advantages over the prior art. In particular, the pipe may be used in exposed applications without the effective pressure rating being reduced due to solar heating and without the need for an excess of pipe material. The invention also provides a robust unitary pipe that may be handled in the same manner as those existing in industry. Further, the pipe may be cut and joined in a manner similar to conventional pipe and does not require special processes, tools or staff training.

Claims

1. A thermoplastic pipe adapted for exposure to sunlight, said pipe including an inner thermoplastic layer having an inner surface and an outer surface, and an outer thermoplastic layer having an inner surhce and an outer surface, wherein the inner and outer layers are disposed in close fitting coaxial relationship, and wherein the outer sure ofthe outer layer is substantially reflective to minimise temperaerre rise in the inner layer when the pipe is exposed to sunlight.

2. A thermoplastic pipe according to claim 1, wherein the outer suzfre ofthe inner layer is bonded directly to the inner surface ofthe outer layer.

3. A thermoplastic pipe according to claim 2, wherein the outer surface ofthe inner layer is thermoplastically bonded to the inner surface ofthe outer layer as part of a co extrusion process.

4. A thermoplastic pipe according to claim 1, wherein the outer layer is shrunk over the inner layer by the application of localised heat.

5. A thermoplastic pipe according to claim 1, wherein the inner layer and the outer layer are joined by means of an interference fit.

6. A thermoplastic pipe according to claim 1, wherein the inner layer and the outer layer are joined by means of an intermediate layer of barrier material.

7. A thermoplastic pipe according to claim 6, wherein said barrier material includes an adhesive compatible with the inner and outer layers.

8. A thermoplastic pipe according to claim 6, wherein the barrier layer is formed from a thermoplastic material, co-extruded with the surrounding inner and outer layers.

9. A thermoplastic pipe according to any one of the preceding claims, wherein the inner layer and the outer layer are formed from a polyolefin material.

10. A thermoplastic pipe according to claim 9, wherein said polyolefin material @is is polyethylene or polypropylene.

11. A thermoplastic pipe according to any one ofthe preceding claims, wherein said outer layer is substantially white in colour, exhibiting reflective characteristics.

12. A thermoplastic pipe according to claim 11, wherein said white colour is produced by the addition oftitanium dioxide to a basic thermoplastic formulation.

13. A thermoplastic pipe according to any one ofthe preceding claims, wherein said inner layer is substantially black in colour.

14. A thermoplastic pipe according to claim 13, wherein said black colour is produced by the addition of carbon to a basic thermoplastic formulation to reduce degradation from ultraviolet radiation 13. A thermoplastic pipe according to any one of the preceding claims, wherein the outer diameter of the pipe is between 60mm and around 1200mm.

16. A thermoplastic pipe according to any one of the preceding claims, wherein the outer layer is between 0.5mm and around 5mm in radial thickness.

17. A thermoplastic pipe according to any one ofthe preceding claims, wherein the outer layer includes one or more additives selected from the following: calcium carbonate; talc; silica; hollow glass spheres; solid glass spheres; wollastonite; kaolin; titanium dioxide; ultraviolet absorbers; blowing agents; cross-linked polyethylene; inorganic fillers; and metallic powders to enhance selected cbaracteristics including reflectivity, conductivity, toughness, and abrasion resistance according to the ifltded application.

18. A thermoplastic pipe according to any one ofthe needing claims, wherein the formulation used to produce the outer layer consists of 100 parts by weight of polyethylene resin, between 2 and around 10 parts by weight of titanium dioxide powder, and between 4 and around 10 parts by weight of aluminium powder.

19. A method of producing a thermoplastic pipe adapted for exposure to sunlight, said method including the steps of forming an inner thermoplastic layer having an inner surface and an outer surface, forming an outer thermoplastic layer having an inner surface and an outer surface, positioning the inner and outer layers in close fitting coaxial relationhhip, and adapting the outer surface of the outer layer to be substantially reflective thereby to minimise temperature rise in the inner layer when the pipe is exposed to sunlight.

20. A method according to claim 19, including the further step of bonding the outer surface of the inner layer directly to the inner surface of the outer layer.

21. A method according to claim 20, wherein the outer surface of the inner layer is thermoplastically bonded to the inner surface of the outer layer as part of a co-extrusion process.

22. A method according to claim 19, including the step of appiying localized heat to the outer layer and thereby shrinking the outer layerover the inner layer to provide intimate contact there between.

23. A method according to claim 19, wherein the inner layer and the outer layer are formed separately, and wherein the method includes the further step of subsequently positioning the layers in overlapping telescopic interengagement whereby the layers are joined by means ofan interference fit 24. A method according to claim 19, including the step ofjoining the inner layer and the outer layer by means of an intermediate layer of barrier material.

25. A method according to claim 24, wherein said barrier martial includes an adhesive compatible with the inner layer and the outer layer.

26. A method according to claim 24, wherein the barrier layer is formed from a thermoplastic material, and wherein the method includes the step of coeuding the barrier layer simultaneously with the surrounding inner and outer layers.

27. A method according to any one of claims 19 to 26, wherein the inner layer and the outer layer are formed from a polyolefin material.

28. A method according to claim 27, wherein said polyolefin material is polyethylene or polypropylene.

29. A method according to any one of clams 19 to 28, wherein said outer layer is substantially white in colour and formed by the step of adding titanium dioxide to a basic thermoplastic formulation.

30. A method according to any one of claims 19 to 29, wherein said inner layer is substantially black in colour, and formed by the step of adding carbon to a basic thermoplastic formulation.

31. A method according to any one of claims 19 to 30, including the step of adding one or more of the following additives to a basic thermoplastic formulation for the outer layer.- calcium carbonate; talc; silica; hollow glass spheres; solid glass spheres; wollastonite; kaolin; titanium dioxide; ultraviolet absorbers; blowing agents; cross-linked polyethylene; inorganic fillers; and metallic powders to enhance selected characteristics including reflectivity, conductivity, toughness, and abrasion resistance according to the intended application.

32. A method according to any one of claims 19 to 31, wherein the formulation used to produce the outer layer consists of 100 parts by weight of polyethylene resin, between 2 and around 10 parts by weight titanium dioxide powder, and between 4 and around 10 parts by weight of aluminium powder.

33. A thermoplastic pipe substantially as hereinbefore described with reference to the accompanying drawings.

34. A method of producing a thermoplastic pipe, substantially as bereinbefore described with reference to the accompanying drawings.