Multi-layered pipe

Description of the invention

The present invention relates to a multilayer pipe comprising layers of thermoplastic polyolefins of the same type but having different properties which are inseparably connected to each other.

A pipe of this type known from DE 299 06 998.2 U1 consists of an outer and an inner layer, the inner layer of which has an FNCT value or NPT value, of a single-layer discharge pipe and the outer layer has an FNCT value that is approximately a factor of 3 or an increased NPT value. .

Starting from the state of the art, the object of the present invention is to provide a multilayer pipe with increased strength during and after laying.

A multilayer pipe comprising layers of thermoplastic polyolefins of the same type but having different properties which are inseparably connected to each other, the first outer layer and the second layer adjacent thereto are layers of non-crosslinked material, the second layer has an FNCT value or an NPT value approximately corresponding to that of a single layer standard discharge pipe and the first layer has an FNCT value that is approximately at least a factor of 3 or a higher NPT value than the second layer. the second layer is adjacent to the third layer, which is also a layer of non-crosslinked material and has an FNCT value at least a factor of 3 higher or an NPT value of at least a factor 2 higher than the second layer.

Preferably, in the pipe of the invention, the first and third layers have an FNCT value that is at least a factor of 5 higher or an NPT value that is at least a factor 2.5 greater than the second layer.

All layers of the pipe are fused together and remain in relation to the pipe unit as a single layer pipe.

All layers of the pipe are joined by welding and remain in relation to the pipe unit as a single layer pipe.

Preferably, in the multilayer pipe according to the invention, the sum of the wall thicknesses of the first and third layers amounts to 50% of the total wall thickness of the pipe.

The wall thickness of the third layer is at least 20% of the total wall thickness of the pipe.

Particularly preferably, all layers of the pipe consist of high-density polyethylene (PEHD).

All pipe layers consist of PE 80 and / or PE 100.

The material of the first and third pipe layers consists of hexene-copolymerized PEHD with a two-module molecular weight distribution.

The material of the outer layer of the pipe contains an admixture of a material that increases the durability to ultraviolet.

The material of the outer layer of the tube is admixed with a coloring material.

The pipe according to the invention is composed of layers, all layers being made of a non-crosslinked material so that the ends of the pipes can be seamlessly and reliably joined to each other using all the usual joining techniques, such as electrofusion welding or butt welding. Moreover, the pipe can be recirculated without problems since no interfering cross-linked areas are present. In addition, the pipe exhibits great durability, since the crack growth of the outer layer subjected to cracking resistance during laying is very low, especially when using the trenchless laying technique, which is especially noted at low temperatures when laying water and gas pipes in the ground. However, the pipe is particularly resistant to a high external point load, which can be caused, for example, by stones, since due to the increased FNCT value or NPT value, the inner third layer can be subjected to tensile stresses under the point load without any damage.

If desired to overcome particular stresses, the first and third layers may have an FNCT value higher by at least a factor or an NPT value higher by at least a factor 2.5 than the second layer.

By fusing or welding, all the layers of the pipe form a pipe unit that behaves like a single-layer pipe, the pipe consisting of 3 layers having a standardized outer diameter and a normalized outer diameter to total wall thickness ratio (SDR) of generally 11 or 17.

Preferably, the sum of the wall thicknesses of the first and second layers is up to 50% of the total wall thickness of the pipe, the wall thickness of the third layer being respectively at least 20% of the total wall thickness of the pipe. The preferred embodiment provides that the thickness of each wall of the first and third layers is 25% and the thickness of the second layer is 50% of the total wall thickness of the pipe.

All pipe layers are preferably made of high-density polyethylene (PEHD), and the pipe layers can be made of both PE 80 and PE 100. Each layer may also consist of PE 100 or PE 80, depending on the choice.

The first and third layers preferably consist of hexene-copolymerized PEHD with a two-module molecular division. The material of at least the first layer of the pipe may contain an admixture of an ultraviolet (UV) stability enhancing material or an admixture of a coloring material.

The first and third layers have an FNCT (Full Notched Creep Test) value higher by at least a factor of 3 or an NPT (Notched Pipe Test) value higher by at least a factor of 2 than the second layer, which has an FNCT value or NPT value of the standard monolayer discharge pipe. The test to determine the FNCT value was described by SH Beech, S. Palmer and RW Burbidge in the dissertation entitled "Accelerated Laboratory Tests to Predict the Resistance to Slow Crack Growth of High Performance Polyethylene Pipe Resins in 1997 at the International Plastic Pipe Symposium (1997), p. 205 et seq.

The next description is presented in the dissertation by M. Fleissner entitled "Experience With a Full Notch Creep Test in Determining the Stress Crack Performance of Polyethylenes, published in Polymer Engineering and Science, February 1998, vol. 38, no. 2.

As it results from the described test conditions, at 80 ° C, 4.6 MPa and 2% N100 sheet metal as a crumb bed, the FNCT value of a single-layer standard discharge pipe made of PE 100 is about 200-500 ha, the FNCT value of the first and third layer material according to of the invention is about 600 to 1500 h. When using hexene-copolymerized PEHD with a bimodal molecular weight distribution, the FNCT value exceeds 2200 h and can reach about 3000 h.

The performance of the NCT test is described by WJ Allwood and SH Beech in The Noteched Pipe Tests for the Performance Assessment of Polyethylene Pipe, Plastic Fuel Gas Pipe Symposium, (1993), p. 339 et seq. Based on the test conditions described therein, the NPT value for a single-layer standard discharge pipe made of PE 100 is approximately 3000 h, at 80 ° C and 4.6 Mpa, and the NPT value of the first and third layer material according to the invention is at least approximately 6000 h. Hexene-copolymerized PEHD with two-module molecular weight distribution, the NPT value can reach a much higher value of up to 7,500 h.