WO2001089785A1

WO2001089785A1 - Recycled pipe

Info

Publication number: WO2001089785A1
Application number: PCT/GB2001/002206
Authority: WO
Inventors: Steven Robert Wood; Jeremy Bowman
Original assignee: Uponor Limited
Priority date: 2000-05-22
Filing date: 2001-05-21
Publication date: 2001-11-29
Also published as: AU7420301A; GB0012281D0

Abstract

A pressure pipe comprises recycled multi-layer pipe of at least 80 % polyethylene and at least 2 % polypropylene, the polyethylene and polypropylene being in separate layers of multi-layer pipe. The multi-layer pipe is granulated to granules of less than 20 mm maximum dimension. The granules are then melted and subjected to melt-phase mixing prior to extrusion to form the pressure pipe. The melt phase mixing is achieved by melting said granules in a pelletising extruder, chopping the extrudate therefrom into pellets of less than 10 mm maximum dimension; and supplying said pellets to an extruder for extruding said pipe. Alternatively the melt phase mixing is achieved by the granules being supplied to an extruder having intensive melt-phase mixing prior to an extrusion die for the pressure pipe.

Description

Recycled Pipe

This invention relates to plastics pipe made from recycled plastics material.

WO-A-9622485 discloses a dual layer plastics pipe comprising a polyethylene ("PE") core layer and a polypropylene ("PP") skin. Such pipe is successful for a number of reasons explained in that patent and also explored to a greater or lesser extent in GB- A-2263524, GB-A-2300456, EP-A-0604907 and others.

In WO-A-9622485, a particular feature of that invention is that the degree of adhesion between the PP skin and PE core is controlled to a relatively high level. The PE core therefore presents a truly virgin surface when the PP skin is peeled off. This means that no further preparation of the PE is required, once the PP skin has been peeled from the end of the pipe, before it can be joined using an electrofusion coupler. Normally, PE pipes have to be meticulously scraped to avoid oxidation layers interfering in the fusion process.

However, a disadvantage of this degree of adhesion is experienced in the manufacturing process when dealing with the inevitable amounts of waste pipe.

It is inherent in any extrusion process that, until the extrusion is up and running and all variables have stabilised, the output will not have consistent properties and must be scrapped. This is not normally a problem since the scrap is usually merely recycled. But in the present case where there are two materials, this problem is exacerbated. Firstly, simply because there are two materials in a eoextrusion process, the setting up is more complicated, resulting in more scrap; and, secondly, different materials cannot simply be recycled, but must be separated from one another first. This is where the adhesion problem is experienced, because the scrap value of the plastics material could be exceeded by the cost of separation, and indeed, in the current economic climate, this is probably the present case.

There are two major forms of polyethylene resins used for making pressure pipes. PE80 polyethylene resins give pipes having a strength at 20°C, and extrapolated to 50 years, of between 8.00 and 9.99MPa as a 97.5% lower confidence limit strength. PE100 polyethylene resins give pipes having a strength at 20°C, and, extrapolated to 50 years, of between 10.00 and 11.19MPa as a 97.5% lower confidence limit strength. For these polyethylene pipes there are at least two essential tests which must pass in order to qualify for pressure pipe applications. The two tests reflect two different failure modes, the first being ductile rupture, which is tested via 20°C pressure tests, and the second being slow crack growth, which is tested via 80°C pressure tests. Table 1 below summarises the critical tests defined in the CEN and ISO gas and water pipe standards for polyethylene pipes.

Test Stresses for PE80 and PE100 Pressure Pipes

It is also well known that contamination of one plastics material with another (sometimes even of the same family, let alone of an entirely different chemical composition) results in a radical drop-off in 80°C lifetime performance tests. For example, "Factors controlling the lifetimes of polyethylene pipes in the slit mode failure regime" by WRC Wright and J Bowman; Proceedings VII International Conference on Plastics Pipes (The Plastics and Rubber Institute), Bath, England, September 1988, explored the effects of addition of high density polyethylenes (non- pipe grade) with a pipe grade medium density polyethylene. The results obtained showed that a contamination of as little as 2% results in a significant loss of 80°C pressure pipe performance.

It is an object of the present invention therefore to employ waste twin- walled pipe in an economic and effective manner.

In accordance with the present invention there is provided a pressure pipe comprising recycled multi-layer pipe: wherein said multi-layer pipe comprises at least 80% polyethylene and at least 2% polypropylene; wherein said multi-layer pipe has been granulated to granules of less than 20mm maximum dimension; and, wherein the granules are melted and subjected to melt-phase mixing prior to extrusion to form said pipe.

Said melt phase mixing may be achieved by melting said granules in a pelletising extruder; chopping the extrudate therefrom into pellets of less than 10 mm maximum dimension; and supplying said pellets to an extruder for extruding said pipe.

Alternatively, said granules may be supplied to an extruder having intensive melt- phase mixing prior to an extrusion die for said pipe.

In another aspect, the present invention provides a pressure pipe comprising recycled multi-layer pipe, wherein said multi-layer pipe comprises at least 80% polyethylene and at least 2% polypropylene, and wherein the polyethylene and polypropylene have been melted and mixed to such an extent that the polypropylene disappears, as a separate phase under optical microscopy, in said pressure pipe.

Put another way, the present invention provides a method of forming a thermoplastics pressure pipe, comprising the steps of: a) granulating to granules of less than 20mm maximum dimension a multi-layer pipe which comprises at least 80% polyethylene and at least 2% polypropylene; b) melting the granules; c) mixing the granules melt; and, d) extruding to form said pipe.

Said mixing step c) may be performed in a pelletising extruder, whereupon the method further comprises, before step d), the step of chopping the extrudate from said pelletising extruder into pellets of dimension less than 10 mm.

Alternatively, step c) may be effected in an extruder for said pipe which includes an intensive melt phase mixer. For example, the extruder may comprise a screw feed for pressurising the granules as they are melting, and having vanes on a front end thereof to effect mixing of the melt prior to extrusion.

In any event, the surprising result is that a pressure pipe can be made which meets requisite standards from the recycled multi-layer pipe, and without separation of the PP from the PE. Indeed there appears to be no significant loss of performance in resistance to slow crack growth when compared with the pure PE of the original pipe. However, in ductile mode there is some loss of performance noted so that with a multi-layer pipe made with a PE100 polyethylene, the recycled material now meets the standards of a PE80 material, rather than PE100.

Without being tied to any particular theory, it is believed that with sufficient mixing in liquid form the PP appears to "dissolve" in the PE so that the PP disappears as a separate phase.

Preferably said multi-layer pipe has an outer layer of PP and an inner core of PE where the ratio of wall thickness of the outer layer to the outside diameter of the pipe is less than 1 :70 and preferably less than 1 : 100.

Preferably said multi-layer pipe comprises colourant filler only in said protective outer layer.

Preferably said polypropylene is a copolymer of propylene and ethylene.

Preferably said granulation is to less than about 10 mm maximum dimension, more preferably less than about 5 mm maximum dimension.

Preferably said pellets are round section cylinders. Preferably the pellets have dimensions of about 3 to 10 mm diameter and length of about 1 to 5 mm.

Preferably said pressure pipe comprises substantially nothing more than said recycled multi-layer pipe. Alternatively, colourant, for example carbon black, may be added during pelletisation.

Best results are obtained when the temperature of the melt in the extrusion of the pipe is greater than 200C, and preferably about 235C. At temperatures less than 220C defects in the pipe bore surface have been observed, in the form of voids and surface blemishes. Surprisingly, increasing the temperature of the melt has removed the occurrence of such blemishes.

The invention is further described hereinafter with reference to the following example.

Example

Various diameters and pressure classes of Profuse (trade mark) twin walled pipe were granulated to a size distribution of less than 20mm and typically less than 5 mm. Such Profuse pipe is made and sold by the present applicant and comprises a PE100 polyethylene core pipe having a polypropylene skin. The granules were heated in an extruder and the melt extruded as a round section rod of about 3 mm diameter and chopped to lengths of about 3 mm. The pellets so formed were then fed to a second extruder to extrude 125mm outside diameter pipe having a minimum wall thickness of 11.4mm, with this pipe being made without additional pigmentation. The pellets so-formed were also fed to a second extruder to extrude 160mm outside diameter pipe having a minimum wall^' thickness of 14.6mm, this pipe being made with black additional pigmentation.

Table 2 below gives the results achieved for samples of reworked pipe made according to the Example given above. TABLE 2 RESULTS OF TESTS ON PROFUSE RE-WORK WITH THE PP SKIN & PE CORE NOT SEPARATED, BUT THE FEEDSTOCK GRANULATED

AND PELLETISED PRIOR TO THE EXTRUSION OF THE PIPES

σv

In Table 2, it can be seen that, at 20°C, the reworked pipe, which was made form PE100 ProFuse pipe, did not achieve the 100 hours survival at a stress of 12.4 MPa required in order to achieve PE100 ranking. Nevertheless, at 9.0 MPa, it did achieve the 100 hours required to reach PE80 ranking. Moreover, at 80°C notched pipe life test, the reworked pipe achieved the same specification as the virgin material. Indeed, in other standard tests such as oxidation induction time (OIT), tensile yield stress and tensile elongation, the reworked pipe performed to the requirement. In addition, and surprisingly, we have found that the quality of butt joint and electrofusion joints, made from the reworked pipe performed to the required specification for the virgin pipe material, but only if the reworked material had undergone the pelletisation stage, that is, the extra melt processing. If the reworked material was not pelletised, butt fusion joints made between reworked pipe were brittle on the tensile test specified in the UK water industry standard WIS4-32-17. If the rework material was pelletised, then the joints were ductile.

During a production run at which the melt pressure was 220 bar, the temperature was 200C and the melt throughput was 400 kg/hr, it was discovered that surface blemishes, in the form of voids formed on the core surface of the finished pipe. However, these blemishes were surprisingly eliminated by raising the melt temperature to 235 C (while reducing the pressure to 201 bar in order to maintain the same throughput. Subsequently, the temperature could be reduced to 220C (with 227 bar of pressure) to extrude 90 mm outside diameter pressure pipe of good quality. Later, 125 mm outside diameter pressure pipe was satisfactorily extruded at 236C and 204 bar (throughput of 419 kg/hr).

Claims

1. A pressure pipe comprising recycled multi-layer pipe: wherein said multi-layer pipe comprises at least 80% polyethylene and at least 2% polypropylene, the polyethylene and polypropylene being in separate layers of the multi-layer pipe; wherein said multi-layer pipe has been granulated to granules of less than 20mm maximum dimension; and, wherein the granules are melted and subjected to melt-phase mixing prior to extrusion to form said pressure pipe.

2. A pressure pipe as claimed in claim 1, in which said melt phase mixing is achieved by melting said granules in a pelletising extruder; chopping the extrudate therefrom into pellets of less than 10 mm maximum dimension; and supplying said pellets to an extruder for extruding said pipe.

3. A pressure pipe as claimed in claim 1, in which said melt phase mixing is achieved by said granules being supplied to an extruder having intensive melt- phase mixing prior to an extrusion die for said pressure pipe.

4. A pressure pipe comprising recycled multi-layer pipe, wherein said multi-layer pipe comprises at least 80% polyethylene and at least 2% polypropylene, the polyethylene and polypropylene being in separate layers of the multi-layer pipe, and wherein the polyethylene and polypropylene have been melted and mixed to such an extent that the polypropylene disappears as a separate phase under optical microscopy in said pressure pipe.

5. A pipe as claimed in claim 4, having also the features of a pipe as claimed in any of claims 1 to 3.

6. A pipe as claimed in any preceding claim, in which said multi-layer pipe has an outer layer of polypropylene ("PP") and an inner core of polyethylene ("PE") where the ratio of wall thickness of the outer layer to the outside diameter of the pipe is less than 1:70 and preferably less than 1:100.

7. A pipe as claimed in claim 6, in which said multi-layer pipe comprises colourant filler only in said protective outer layer.

8. A pipe as claimed in claim 7, in which said polypropylene is a copolymer of propylene and ethylene.

9. A pipe as claimed in any preceding claim, in which said granulation is to less than about 10 mm maximum dimension, more preferably less than about 5 mm maximum dimension.

10. A pipe as claimed in claim 2, or any of claims 5 to 9 when dependent on claim 2, in which said pellets are round section cylinders.

11. A pipe as claimed in claim 10, in which the pellets have a diameter of about 3 to 6mm and length of about 1 to 3mm.

12. A pipe as claimed in any preceding claim, which comprises substantially nothing more than said recycled multi-layer pipe.

13. A pipe as claimed in any of claims 1 to 11, which comprises colorant added during pelletisation.

14. A method of forming a thermoplastics pressure pipe, comprising the steps of:

a) granulating to granules of less than 20mm maximum dimension a multi-layer pipe which comprises at least 80% polyethylene and at least 2% polypropylene, the polyethylene and polypropylene being in separate layers of the multi- layer pipe; b) melting the granules; c) mixing the granules melt; and, d) extruding to form said pipe.

15. A method as claimed in claim 14, in which said mixing step c) is performed in a pelletising extruder, the method further comprising, before step d), the step of chopping the extrudate from said pelletising extruder into pellets of dimension less than 10 mm. A method as claimed in claim 14, in which step c) is effected in an extruder for said pipe which includes an intensive melt phase mixer.

A method as claimed in claim 16, in which the extruder comprises a screw feed for pressurising the granules as they are melting, and has vanes on a front end thereof to effect mixing of the melt prior to extrusion.

A method as claimed in any of claims 14 to 17, in which the temperature of the melt in step c) is greater than 200C.

A method as claimed in claim 18, in which the temperature of the melt in step c) is about 235C.