Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Shaping by extrusion
Reexamination Certificate
1998-09-04
2001-12-04
Dye, Rena L. (Department: 1772)
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Shaping by extrusion
C264S290200, C264S209600, C264S236000, C428S036910, C138S026000
Reexamination Certificate
active
06325959
ABSTRACT:
The present invention relates to use of at least partly cross-linked, biaxially oriented polyolefin plastic as material for pressure pipes.
By pressure pipe is meant a pipe which, when used, is subjected to a positive pressure, i.e. the pressure inside or outside the pipe is higher than the pressure outside and inside the pipe, respectively.
Nowadays, pipes of polymer material are frequently used for various purposes, such as fluid transport, i.e. transport of liquid or gas, e.g. water or natural gas, during which the fluid can be pressurised. Moreover, the transported fluid may have varying temperatures, usually not outside the temperature range from about −40° C. to about 100° C. Such pipes are now preferably made of polyolefin plastic, such as ethylene plastic (HDPE, MDPE), or of polyvinyl chloride (PVC) or alternatively of other materials that are not necessarily based on polymer.
It is known that the physical and mechanical properties of many polymer materials can be improved by orienting the material and its structure. It is also known that the properties which by orientation are improved in the direction of orientation in most cases become lower in a direction perpendicular to the direction of orientation, in many cases on the same level as the properties of an unoriented material or worse. In many cases, it is not satisfactory to have so different properties in two different directions, and a biaxial orientation of the material may then be applied. The biaxial orientation means that the polymer material is oriented in two directions, perpendicular to one another. By this technique, the physical and mechanical material properties can thus be adapted in the two main directions in dependence on load, requirements as to function, pipe manufacturing process etc.
For a pipe construction of the type involved having an internal positive pressure, the load in the pipe wall is, in the normal case, greatest in the circumferential or peripheral direction and lower in the axial direction.
In conventional manufacture of plastic pipes by extrusion, preferably a uniaxial orientation of the polymer material is obtained in the axial direction of the pipe owing to the shear, elongation etc., to which the molten mass of polymer is subjected on its way from the extruder, through the nozzle and by calibration and cooling to a ready-formed pipe in solid state.
Thus, pipes manufactured by conventional extrusion usually have better physical and mechanical properties in the axial direction of the pipe compared with its peripheral direction, at the same time as the load or tension in the pipe wall is greater in the peripheral direction than in the axial direction.
The properties of such conventional plastic pipes are quite sufficient for many purposes, but not satisfactory in some cases, for instance in applications requiring high pressure resistance, i.e. pipes that are subjected to an internal fluid pressure for a long and/or short period. For such pipes, improved compressive strength is desired.
With a view to reducing the restrictions with pressurised plastic pipes, it is known that the physical and mechanical properties can be improved by biaxial orientation of the pipe, i.e. the polymer material in the pipe is oriented in two directions which are perpendicular to one another. One of these two directions is the axial direction of orientation, i.e. the direction (direction of extrusion) in which the pipe is manufactured in the normal case, whereas the other direction is the circumferential or peripheral direction of the pipe. Thanks to biaxial orientation, a plurality of the properties of the pipe can be improved to a considerable extent, and especially the compressive strength, both for shorter and longer periods, should be mentioned.
The improved compressive strength means that with corresponding dimensions of the pipe material, it is possible to subject the pipe to higher internal pressure compared with a pipe that is not biaxally oriented. Alternatively, it is possible to use, with the same pressurisation, a smaller thickness of material in a biaxially oriented plastic pipe.
Other mechanical properties that can be improved by orientation is, for instance, rigidity (modulus of elasticity), strength (tensile strength) as well as impact toughness. Increased rigidity and strength result in improvement of the short-term strength of the material. The improved impact toughness means that the pipe is less sensitive to external influence in the form of impacts, i.e. the handling of the pipe is rendered more easy. Improvements of properties can also be achieved, for instance, in the form of improved chemical resistance, improved weather resistance and lower coefficient of thermal expansion.
As examples of known methods for biaxial orientation of plastic pipes, reference can be made to an article by W. E. Gloor, “Why biaxially oriented pipe?”, Modern Plastics, November 1960, pp. 111-114, 212, 214, an article by K. Richard, G Diedrich and E. Gaube, “Strengthened pipes from Ziegler Polythene”, Plastics, December 1961, pp. 111-114, and PCT Application WO 93/19924.
As mentioned above, plastic pipes are generally manufactured by extrusion, or, to a smaller extent, by injection moulding. A conventional plant for extrusion of plastic pipes comprises an extruder, a nozzle, a calibrating device, cooling equipment, a pulling device, and a device for cutting or for coiling-up the pipe. By the molten mass of polymer on its way from the extruder through the nozzle and up to calibration, cooling and finished pipe being subjected to shear and elongation etc. in the axial direction of the pipe, an essentially uniaxial orientation of the pipe in its axial direction will be obtained. A further reason that contributes to the orientation of the polymer material in the direction of material flow is that the pipe can be subjected to tension in connection with the manufacture.
To achieve biaxial orientation, the above-described plant can be supplemented, downstream of the pulling device, with a device for temperature control of the pipe to a temperature that is suitable for biaxial orientation of the pipe, an orienting device, a calibrating device, a cooling device, and a pulling device which supplies the biaxially oriented pipe to a cutting device or coiler.
The biaxial orientation can also be carried out in direct connection with the first calibration after extrusion, in which case the above-described supplementary equipment succeeds the first calibrating device.
The biaxial orientation of the pipe can be carried out in various ways, for instance mechanically by means of an internal mandrel, or by an internal pressurised fluid, such as air or water or the like. A further method is the orienting of the pipe by means of rollers, for instance by arranging the pipe on a mandrel and rotating the mandrel and the pipe relative to one or more pressure rollers engaging the pipe, or via internally arranged pressure rollers that are rotated relative to the pipe against an externally arranged mould or calibrating device.
To sum up, there are several prior art methods for biaxial orientation of plastic pipes, and all these methods can be used in the present invention.
For all prior art methods for biaxial orientation of crystalline and uncross-linked plastic materials, such as polyolefin plastic, the orientation must be carried out within a given, restricted temperature range, and the pipe must be cooled within a limited period of time to prevent the material from quickly returning to an unoriented state (relaxation). Moreover, the requirements for temperature accuracy in the pipe/pipe wall are great in order to obtain a uniform orientation both along the pipe and in the peripheral direction of the pipe, and to obtain satisfactory dimensional tolerances for the final product.
The upper limit of the restricted temperature range at which the process can be performed is determined by the crystalline melting point (T
m
) of the material, which for instance for HDPE is about 128-135° C. Orientation above this temperature no
Ek Carl-Gustaf
Höjer Lars
Borealis A/S
Dye Rena L.
Merchant & Gould P.C.
Miggins Michael C.
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