The present invention relates to a multilayer pipe, and more specifically to a multilayer pressure pipe of plastic with improved resistance to rapid crack propaga¬ tion. By pressure pipe is meant a pipe which, when used, is subjected to an internal positive pressure, i.e. the pressure inside the pipe is higher than the pressure out¬ side the pipe.

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 poly- olefin plastic, such as ethylene plastic (HDPE, MDPE), or of polyvinyl chloride (PVC) or alternatively of other materials that are not necessarily based on polymer.

A drawback of such pipes at low temperatures is that if they are subjected to stress which initiates a crack, this crack can be spread or propagated very fast a consi¬ derable distance in the pipe and make the pipe useless. The speed of propagation of the crack can be as high as about 100-300 m/s in the longitudinal direction of the pipe. This type of cracking is generally called rapid crack propagation (RCP). To make a pressure pipe of plastic material acceptable, it should, at a certain temperature and at a certain inner pressure, not have rapid crack propagation greater than 4 times the pipe diameter in the longitudinal direction of the pipe.

The RCP properties of a given pipe can be determin¬ ed in various ways. According to one method, called the S4 test (Small Scale Steady State), which has been deve¬ loped at Imperial College, London, and which is described in ISO DIS 13477, a pipe is tested, which has an axial

length not below 7 pipe diameters. The outer diameter of the pipe is about 110 mm or greater and its wall thick¬ ness about 10 mm or greater. When determining the RCP properties of a pipe in connection with the present in- vention, the outer diameter and the wall thickness have been selected to be 110 mm and 10 mm, respectively. While the exterior of the pipe is at ambient pressure (atmos¬ pheric pressure), the pipe is pressurised internally, and the internal pressure in the pipe is kept constant at a pressure of 0.5 MPa positive pressure. One end of the pipe, the so-called initiating zone, is cooled. A number of discs have been mounted on a shaft inside the pipe to prevent decompression during the tests. A knife projec¬ tile is shot, with well-defined forms, towards the pipe close to its one end in the so-called initiating zone in order to produce a rapidly running axial crack. The ini¬ tiating zone is provided with an abutment for avoiding unnecessary deformation of the pipe. The test equipment is adjusted in such manner that crack initiation takes place in the material involved, and a number of tests are effected at varying temperatures. The axial crack length in the measuring zone, having a total length of 4.5 dia¬ meters, is measured for each test and is plotted against the measured temperature. If the crack length exceeds 4 diameters, the crack is assessed to propagate. If the pipe passes the test at a given temperature, the tempe¬ rature is lowered successively until a temperature is reached, at which the pipe no longer passes the test, but the crack propagation exceeds 4 times the pipe diameter. The critical temperature ( _cr;L .t-) is the lowest tempera¬ ture at which the pipe passes the test. The lower the critical temperature the better, since it results in an extension of the applicability of the pipe. It is desir¬ able for the critical temperature to be around -5°C or lower.

In this context, it should be mentioned that some plastic materials have so poor RCP properties (high

^τ crit) that the value of _cr i _t exceeds the normal measur¬ ing limit for ^ _C τ±t ^wni ch is +23°C. In such cases, use is conventionally made, for the purpose of the present in¬ vention, of a Tc^t value of +23°C for these materials when calculating the below described weighted average value of _cr i^-.

For bimodal polyethylene plastic intended for pipes that are required to be resistant to RCP, _cr i _t generally is in the range of from about 0°C to about -25°C, whereas ^cτ± ° ^r polypropylene plastic is above about +23°C.

Another method of determining the RCP properties for a pressure pipe of plastic is to determine the critical pressure Pcrit instead. The method conforms with the above described method, but instead of keeping the pres- sure constant and successively lowering the temperature, the temperature is kept constant at 0°C±2°C and the pres¬ sure in the pipe is successively increased. The highest pressure at which the pipe still passes the test, is called the critical pressure, P _C rit ^{# Tne} method for determining P _r i is less preferred in connection with the development of materials, since the tested pipe must usually be subjected to such high pressures as 1 MPa or more, which involves risks and difficulties. Therefore the method of determining T _c -t is preferred and used in the present invention.

It would mean a great advantage if it was possible to provide pressure pipes of plastic material having improved resistance to rapid crack propagation, RCP. It would be particularly advantageous if it was possible to provide pressure pipes of plastic material having a T _cr it below -5°C, preferably below -15°C.

According to the present invention it has been found that pressure pipes of plastic material with improved resistance to rapid crack propagation can be achieved by making the pipes of several layers of different polyole- fin plastic materials.

According to the invention, it is possible, by mak¬ ing a pipe of suitably selected and combined layers of different plastic materials, to accomplish resistance to rapid crack propagation, measured as ^ _C τ t °f the multi- layer pipe, which is better than the weighted average value of T _C χ±t for single-layer pipes of the individual plastic materials that are included in the multilayer pipe.

According to a specially preferred aspect of the invention, a multilayer pipe of polyolefin plastic is provided, whose resistance to rapid crack propagation, measured as T _QJ -^+, is better than that of a pipe of the same dimension, built up of one of the individual plastic materials included. The characteristic features of the invention are evident from the accompanying claims.

By "weighted average value", _cr i _t v is meant, for a multilayer pipe, which is built up of e.g. the layers A, B and C, the layers constituting for instance ^%, X _β % and XQ%, respectively, of the weight of the pipe

(x^ + β + c = 100%), the T _cr i ₁ - value which is obtained if _cr it is determined for single-layer pipes of A, B and C, said pipes each having the same dimensions as the multilayer pipe, and subsequently these T _cr i-t values are weighted corresponding to that part by weight, by which A, B and C are included in the multilayer pipe and added to each other. The weighted average value for the above- mentioned example will then be (xA' ^τ crit _A ^{+ x} B* ^τ critB ⁺ of the multilayer pipe (T _cr itTot) ^s low© ¹ " than the , above-mentioned, weighted average value, ^cτ± . _{V '}

For a multilayer pipe according to the invention having n layers, the critical temperature of a given layer being designated T _cr i^- ^an< 3 the weight of the layer i being designated x^, the relationship between the critical temperature (T _cr i Tot) of the multilayer pipe and the weighted average value of the critical

temperatures of single-layer pipes of the individual materials included in the multilayer pipe, can be generally defined by the formula n 2 _J ^x i ^τ crit i

1 ^τ critTot ⁽ = ^τ critv ⁾ n

1 ^« ±

1

According to a specially preferred aspect of the present invention, a resistance to RCP is provided, mea- sured as _cr i _t , which is better than that of each of the materials included, i.e. _cr i-t-rrι ₀ -|- of the multilayer pipe according to the invention is lower than the critical temperature (T _cr i _t j_) of each of the materials included in the different layers of the multilayer pipe. This condition can be generally expressed by the following relationship:

TcritTot ^{< τ} crit i wherein i can assume values from 1 to n.

The pressure pipes according to the invention are made of polyolefin plastic, especially polyethylene plas¬ tic and polypropylene plastic.

By polyethylene plastic or ethylene plastic is meant a plastic based on polyethylene or on copolymers of ethy¬ lene, in which the ethylene monomer constitutes the greatest part of the mass. Polyethylene plastic may thus consist of homopolymers or copolymers of ethylene, in which case the copolymers can be graft copolymers or co¬ polymers of ethylene and one or more other monomers copo- lymerisable with ethylene. The polyethylene plastic can be of low pressure type (HDPE, MDPE, LLDPE), i.e. produced by using a coordination catalyst at low pressure. A particularly preferred type of polyethylene plastic is so-called bi- or multimodal polyethylene, i.e.

polyethylene whose molecular weight distribution curve has two or more peaks or maxima.

Similarly to that discussed above for polyethylene plastic, it is meant by polypropylene plastic or propy- lene plastic a plastic based on polypropylene or on copo¬ lymers of propylene, the propylene monomer constituting the greatest part of the mass. The polypropylene plastic may thus consist of homopolymers or copolymers of propy¬ lene, in which case the copolymers can be graft copoly- mers or copolymers of propylene and one or more other monomers copolymerisable with propylene.

As mentioned above, it has been found in the present invention that the RCP properties are improved if the pipe is made of several layers of different polymers. By the expression "different polymers" is meant polymers with different basic monomers, such as polyethylene plas¬ tic versus polypropylene plastic, as well as different qualities of the same basic polymer, which differ suffi¬ ciently, for instance in respect of the RCP value, i.e. which have different T _cr i^-. The difference in T _C τ±t should be at least 5°C, preferably at least 10°C.

In its simplest form, the pipe according to the in¬ vention comprises two layers of different polymers, such as one layer of polyethylene plastic and one layer of polypropylene plastic. It is preferred, however, that the pipe is built up of more than two polymer layers, such as three or more layers. Most preferred at present is that the pipe according to the invention is built up of three layers, such as an inner layer of polyethylene plastic, an intermediate layer of propylene plastic and an outer layer of polyethylene plastic. The different polymer layers can be juxtaposed or, if desirable, it is possible to arrange a thin intermediate layer between the polymer layers. The contribution of these intermediate layers when calculating the weighted average value of the criti¬ cal temperatures for the individual layer materials is in many cases so small that it may neglected.

The various layers in the pipe can have the same or different thickness. For instance, the layer or layers of one polymer may have one thickness, whereas the layer or layers of the other polymer may have another, greater or smaller thickness. If there are more layers of one poly¬ mer, also these layers can have the same or different thickness.

Furthermore the invention is not restricted to mul¬ tilayer pipes, which consist of a combination of two dif- ferent polymer materials. Also combinations of three or more polymer materials are comprised by the invention.

The improved resistance to RCP obtained in the in¬ vention can be used in various ways.

According to one aspect, it is possible in pipes of a polymer material having relatively low resistance to RCP (high T _cr £ _t ) to improve this resistance at a compa¬ ratively low cost by combining the original polymer mate¬ rial with a layer of another polymer material having higher resistance to RCP (lower T _C τ±t) . The two polymer materials may then be present as two or more alternating layers. An embodiment of interest in this context is the using of cheap polymer scrap as one layer and the combin¬ ing of this with one or more layers of better polymer materials having a lower _cr ^ _t . When combining polymer materials having different ςjj-i-, it is preferred according to the invention that the material having a lower T _cr .i _t is arranged at least on the outside of the pipe.

According to another and especially preferred aspect, resistance to RCP is achieved, which is higher than that of the individual materials included. By com¬ bining layers of e.g. polyethylene plastic and polypro¬ pylene plastic, it is thus possible according to the invention to provide pipes having resistance to RCP which is better than the resistance of both the included poly¬ ethylene plastic and the included polypropylene plastic.

This is surprising and confers a special advantage of the invention.

The inventive multilayer pipes are produced in a manner known per se by, for instance, extrusion or injec- tion moulding, preferably by coextrusion. For a more de¬ tailed description of such processes, reference is made to, for instance, D. Djordjevic, "Coextrusion", Rapra Review Reports, Vol. 6, No. 2, 1992, pp 51-53.

In order to further facilitate the understanding of the invention, an Example follows below. The Example of the invention is only intended to be illustrative and should not be interpreted as limiting to the invention. In the Example, all parts and percentages relate to weight, unless otherwise stated. Example 1

Multilayer pipes having an outer diameter of 110 mm and a diameter/wall thickness ratio (SDR) of 11 were ex¬ truded by a conventional Cincinnati Milacron CMS 60 pipe extruder having a screw diameter of 60 mm and 2 conven- tional side extruders having screw diameters of 50 and 30 mm, respectively, all connected to a multilayer tool of conventional design, having the possibility of extrud¬ ing 1-5 layers of varying thickness of the materials and composition of the layers. The operational adjustments were adapted to the instructions of the respective sup¬ pliers of material and the stipulated distribution of layers. Maximum melting temperatures were measured to be between 200 and 240°C. The line speed was about 0.5-1.0 m/min. The pipes were calibrated and cooled by means of conventional downstream equipment supplied by , Cincinnati.

After 2 weeks' conditioning, the pipes were test¬ ed in an RCP S4-test rig which was originally manufac¬ tured at Imperial College, London, but modified to satisfy new requirements according to IS0/DIS 13477

(revision ISO/TC/138/SC4 WG RCP - DOC N048) . The tests

were carried out according to ISO/DIS 13477 (revision ISO/TC/138/SC4 WG RCP - DOC N048).

The tested combinations of materials and the test results are shown in Tables 1 and 2.

Table 1

Mate¬ MFR ₂ MFR ₅ MFR ₂₁ rial Type Designation Density at 190° at 190° at 190°

(g/m ³ ) (g/10 min) (g/10 min) (g/10 min) high density polyethylene HE 2499 1) 950 0.1 0.4 for pressure pipe appli¬ cations

B polypropylene homopolymer HA 101 B 1) 901 0.4(230°C) 0.4 0.4 for discs and plates polypropylene block copo¬ RA 130 E 1) 902 0.2(230°C) 0.4 lymer containing 3.5% by weight ethylene comonomer and intended for indus¬ trial applications

D polybutylene of pipe qua¬ DP 4137 2) 930 0.4 lity

E 50/50 A/C F high density polyethylene 953 0.1 0.5 of experimental quality ¹

H high molecular high den¬ NCPE 2216 ^l) 952 sity polyethylene of blow moulding quality

1) supplied by Borealis A/S, Denmark ^'

2) supplied by Shell Plastics, England

Table 2

the outer layer was split

Table 2 ( cont )

Reference tests

Test Combination of materials

No. (outside-inside)

When studying the test results, it should be noted that T _cr - _L+;rot of the multilayer pipes according to the invention is lower, and in many cases considerably lower than the weighted average value of T _cr i^ ( _cr i-tv) ° the individual materials included. Thus, for Tests Nos 1, 2, 3, 7 and 8 is below -25°C although none of the included materials has a T _C ±-t lower than -15°C. This must be considered very surprising and means a considerable technical advantage by the applicability of the pipes being increased to a considerable extent.

Moreover, it may be mentioned that materials B and C have very low resistance to RCP and yet, by using them, it -• ^•• possible to obtain very good results for multilayer pipes. In this context, Test No. 9, which is a comparative test, should be noted. In this test, the outer layer (mate¬ rial B) was split in the testing and it was hardly possible to carry out any meaningful measurements. For this reason, ^τ critTot ^{nas Deen set} at >+23°C, which is higher than

^τ crit _V (+10°C). This test can be compared with the result of Test No. 7, which contains a still higher content of material B (71%) but all the same has a value of which is below -25°C. This indicates that it is sometimes also important that the layers of materials are arranged in a certain order and more specifically in such manner that the material having the lowest _cr i^- is arranged at least on the outside of the pipe.

Besides, the results in Table 2 demonstrate that ^τ critTot i ^s lowered at an increasing content of material having a low T _cr i+ (see for instance Tests Nos 3-5 and 6- 8). Moreover, e.g. Test No. 11 demonstrates, compared with Test No. 28, that also relatively thin intermediate layers of another material can produce a considerable improvement of T _cr t, although the material of the intermediate layers has a poorer T _C χ±t than the materials in the other layers.