RU2171948C2 - Multi-layer pipe - Google Patents

Abstract

FIELD: plastic pipes. SUBSTANCE: multi-layer plastic pipe consists of at least two layers of different plastic materials. Plastic materials consist of polyolefin plastic. Pipe is resistant of quick propagating of cracks expressed in form of critical temperature of pipe which is below weighted mean value of critical temperature of single-layer pipes made from separate plastic materials included in multi-layer pipe having similar size. Critical temperatures of single-layer pipes made from separate plastic materials differ by at least 5 deg. and critical temperature of pipe is lowest temperature at which crack formed in pipe propagates through distance equal to 4 diameter of pipe at pressure differential between inner and outer sides equal to 0.5 MPa. It is good practice to have critical temperature of pipe below -15 C and composition of material including olefin plastics, for example, polyethylene plastic and polypropylene plastic. EFFECT: enhanced resistance to crank propagation. 8 cl

Description

 The invention relates to a multilayer pipe, in particular to a multilayer plastic pressure pipe with improved resistance to rapid crack propagation.

 The term "pressure pipe" means a pipe which, when used, is subjected to an internal pressure higher than atmospheric, that is, the pressure inside the pipe exceeds the pressure outside the pipe.

Currently, for various purposes, for example, for transporting fluids, i.e. liquids or gases, for example, water or natural gas, in which the fluid may be pressurized, pipelines made of polymer materials are often used. The transported fluid may also have a different temperature, usually in the range of -40 ^° C to about 100 ^° C. Such pipelines are preferably made of a polyolefin plastic material, for example, ethylene plastic (HDPE, PESP) or polyvinyl chloride (PVC) or other materials, optionally related to polymers.

 The disadvantage of such pipes at low temperatures is that when they are subjected to stresses, at which cracks are formed, they can spread very quickly over a considerable distance in the pipe, so the pipe becomes unusable. The crack propagation velocity in the longitudinal direction of the pipe can be significant, for example, about 100 - 300 m / s. This type of crack formation is commonly referred to as rapid crack propagation (RCP). In order for a plastic pressure pipe to be used, it is necessary that, at a certain temperature and internal pressure, it does not have the rapid propagation of cracks exceeding more than 4 times the diameter of the pipe in the longitudinal direction.

The crack propagation resistance of a given pipe (RCP) can be determined in various ways. In one method, known as the S4 test (steady state laboratory test) developed at King's College London and described in Draft International Standard ISO 13477, a pipe was tested with an axial length of at least 7 pipe diameters. The pipe had an outer diameter of about 110 mm or more and a wall thickness of about 10 mm or more. To determine the resistance of the pipe to rapid crack propagation (RCP), in accordance with the present invention, a pipe with an outer diameter and a wall thickness of 110 and 10 mm, respectively, was selected. Although the pipe is externally under ambient pressure, an internal pressure is created in the pipe, which is maintained at 0.5 MPa, which is higher than atmospheric pressure. One end of the pipe or the so-called crack formation zone is cooled. To avoid pressure drop, a number of discs are mounted on the shaft inside the pipe during the test. In order to achieve rapid propagation of the crack in the axial direction in the so-called crack formation zone near one end of the pipe, bombarding particles sharp like a knife are “shot” at it. To avoid unnecessary pipe deformation, support is provided for the crack formation zone. The testing equipment is adjusted so that crack formation begins in the test material, and a series of tests are carried out at various temperatures. During each test, measure the axial length of the crack in the measurement zone having a total length of 4.5 diameters and plot it against the measured temperature. In the event that the crack length exceeds 4 diameters, the propagation of the crack is evaluated. If the pipe passes the test at a given temperature, the temperature is successively lowered until a temperature is reached at which the pipe no longer passes the test, but the crack propagation exceeds 4 times the diameter of the pipe. The critical temperature (T _crit ) is the lowest temperature at which the pipe passes the test. The lower the critical temperature, the better, as the result is an increase in pipe life. It is desirable that the critical temperature is about -5 ^° C or lower.

In this regard, it should be mentioned that some plastic materials have such poor crack propagation resistance (RCP) properties (high critical temperature T _crit ) that the value of T _crit exceeds the normal measurement limit for T _crit , which is +23 ^o C. In such cases, it is usually used, to achieve the result of the present invention, a T _crit value of +23 ^° C for these materials when a weighted average T _crit value is calculated as described below.

For bimodal polyethylene plastic intended for pipes that are resistant to rapid crack distribution (RCP), the T _crit value is in the range from about 0 to about -25 ^° C, while the T _crit value for polypropylene plastic is above about +23 ^o C .

Another way to determine the properties of RCP for a pressure pipe made of plastic is to determine the critical pressure P _crit . This method corresponds to the described method, but instead of maintaining a constant pressure and sequentially lowering the temperature, a constant temperature is maintained at 0 ^° C ± 2 ^° C, and the pressure in the pipe is gradually increased. The highest pressure at which the pipe still passes the test is called the critical pressure, P _crit . In connection with the development of materials, the present method for determining the critical pressure is less preferable, since the test pipe must usually be subjected to such high pressure as, for example, 1 MPa or higher, which leads to risk and difficulties. Therefore, it is preferable to determine the critical temperature T _crit , which is used in the present invention.

This would be of great advantage if pressure pipes could be made of plastic material having improved resistance to rapid crack propagation. If pressure pipes made of plastic material having a critical temperature T _crit below -5 ^° C, preferably below -15 ^° C could be obtained, a particular advantage would be achieved.

 It has been found that, according to the present invention, pressure pipes of plastic material with improved resistance to rapid crack propagation can be obtained by manufacturing pipes with several layers of various polyolefin plastic materials.

According to the present invention, the pipe can be made of suitably selected and joined layers of various plastic materials to achieve resistance to rapid crack propagation, measured as T _{crit of a} multilayer pipe, which is better than the weighted average T _crit for single-layer pipes of individual plastic materials included in multilayer pipe.

According to a particularly preferred embodiment of the present invention, there is provided a multilayer pipe made of polyolefin plastic, whose resistance to rapid crack propagation, measured as T _crit , is better than the resistance of a pipe of the same size, but made of the individual plastic materials used.

 The distinguishing features of the present invention are apparent from the appended claims.

Under the "weighted average value", T _crit.value , for a multilayer pipe, which consists, for example, of layers A, B and C, and the layers are, for example, x _A %, x _B % and x _C % respectively the mass of the pipe ( x _A + x _B + x _C = 100%), it means the value of T _crit , which is obtained if T _crit is determined for single-layer pipes A, B and C, each of which has the same dimensions as a multilayer pipe, and then these values critical temperature T _crit is determined in accordance with parts by weight, according to which A, B and C are included in the multilayer pipe and added to each other. In this case, the weighted average for the above example will be (x _A • T _{crit. A} • x _B • T _{crit. B} + x _C • T _{crit. C} ) / 100. According to the present invention, T _{crit of a} multilayer pipe (T _{crit. Total} ) below the above weighted average, T _crit .

Согласно особенно предпочтительному варианту настоящего изобретения, достигается стойкость к быстрому распространению трещины, измеренная как Т_крит, которая лучше стойкости к быстрому распространению трещины у каждого из включенных материалов, то есть Т_{крит.общ} ниже критической температуры (Т_крит.i) каждого из материалов, включенных в различные слои многослойной трубы. Это условие можно выразить, в общем, следующей зависимостью
Т_{крит.общ} < Т_крит.i,
где i может иметь значение от 1 до n.For a multilayer pipe according to the present invention having an n-number of layers, the critical temperature of this layer was determined as T _crit , and the mass of the layer was designated x, with the relationship between the critical temperature (T _crit.total ) of the multilayer pipe and the weighted average critical temperature of the single layer pipes from individual materials included in a multilayer pipe can be determined by the formula

According to a particularly preferred embodiment of the present invention, resistance to rapid crack propagation is achieved, measured as T _crit , which is better than resistance to rapid crack propagation in each of the included materials, i.e., T _crit.total below the critical temperature (T _crit.i ) of each of the materials included in various layers of a multilayer pipe. This condition can be expressed, in general, by the following dependence
T _crit.total <T _crit.i ,
where i can have a value from 1 to n.

 Pressure pipes according to the present invention are made of polyolefin plastic, especially polyethylene and polypropylene plastic materials.

 By polyethylene plastic or ethylene plastic is meant plastic based on polyethylene or ethylene copolymers, in which ethylene monomer makes up the largest part of the mass. So, polyethylene plastic may consist of ethylene homopolymers or copolymers, in which case graft copolymers or copolymers of ethylene and one or more other monomers capable of copolymerizing with ethylene can be copolymers. Polyethylene plastic can be of the type of low density polyethylene (HDPE, MDPE, LDPE), that is, obtained using a coordination catalyst at low pressure. A particularly preferred type of plastic plastic is the so-called bi- or multimodal polyethylene, that is, polyethylene, the molecular weight distribution curve of which has two or more peaks or maxima.

 Similar to what has been described for polyethylene plastic, polypropylene plastic or propylene plastic means a plastic based on polypropylene or propylene copolymers, the content of propylene monomer being the largest part of the mass. Thus, the polypropylene plastic may consist of propylene homopolymers or copolymers, in which case the copolymers may be graft copolymers or copolymers of propylene and one or more other monomers copolymerizable with propylene.

As already mentioned, it has been found that if a pipe is made according to the present invention from several layers of different polymers, its resistance to rapid crack propagation (RCP) is improved. The expression "different polymers" means polymers with different basic monomers, for example, polyethylene plastic compared to polypropylene plastic, as well as different qualities of the same basic polymer, which differ, for example, with respect to the RCP value (rapid crack propagation), i.e. which have different values of T _crit . The difference in the values of T _crit should be at least 5 ^o C, preferably at least 10 ^o C.

 In its simplest form, the pipe according to the present invention contains two layers of different polymers, for example, one layer of polyethylene plastic and one layer of polypropylene plastic. However, it is preferable that the pipe is made of more than two layers of polymers, for example, of three or more layers. At present, it is most preferable to produce a pipe according to the present invention from three layers, for example, the inner layer is made of polypropylene plastic, the middle layer is made of propylene plastic, and the outer layer is made of polyethylene plastic. Layers of different polymers can be placed on top of each other or a thin intermediate layer can be applied between these polymer layers, if necessary. When calculating the weighted average value of critical temperatures for the materials of individual layers, the influence of these intermediate layers in many cases is so insignificant that it can be ignored.

 Different layers in the pipe may have the same or different thickness.

 The present invention is also not limited to multilayer pipes, which consist of a combination of two different polymeric materials. The present invention also includes combinations of three or more polymeric materials.

 Improved crack propagation resistance (RCP) can be used in a variety of ways.

According to one aspect, this resistance of pipes of a polymeric material having a relatively low resistance to rapid crack propagation, RCP (high T _crit ), is improved relatively cheaply by bonding the starting polymer material with a layer of another polymer material having a higher resistance to rapid spreading, RCP (lower value of T _crit ). Both of these polymeric materials can then be represented as two or more alternating layers. In this regard, of interest is the option in which cheap polymer waste is used as a single layer in combination with one or more layers of the best polymer materials having a lower T _crit value.

When polymer materials having different T _crit values are combined, according to the present invention, it is preferable to place material having a lower T _crit at least on the outside of the pipe.

 According to another and particularly preferred embodiment, resistance to rapid crack propagation (RCP) is achieved, which exceeds the resistance of the included individual materials. So, by combining layers of, for example, polyethylene plastic and polypropylene plastic, it is possible, according to the present invention, to obtain pipes having a crack propagation resistance (RCP) that is greater than the resistance of both included polyethylene and polypropylene plastics.

 This is an unexpected effect and provides the present invention with a particular advantage.

 Multilayer pipes according to the present invention are made by methods known per se, for example, by extrusion or injection molding, preferably by co-extrusion. For a more detailed description of such methods, reference is made, for example, to a report by D. Georgievich, “Joint Extrusion,” Rapra Review Reports, Volume 6, No. 2, 1992, p. 51-53.

 To further simplify the understanding of the present invention, an example follows. An example of the present invention is intended to be illustrative only and should not be construed as limiting the present invention. All parts and percentages of the ratio in the example relate to mass, unless otherwise indicated.

Example 1
Multilayer pipes having an outer diameter of 110 mm and a diameter / wall thickness ratio (SDR) of 11 were extruded on a conventional pipe extruder, Cincinnati Milacron CMS 60, having a screw diameter of 60 mm, and on two conventional side extruders having screw diameters of 50 and 30 mm, respectively, connected to a multilayer tool of conventional design, capable of extruding 1-5 layers of different thicknesses of materials and composition of the layers. Operations regulations have been adapted to the instructions of the respective material suppliers and to the conditional distribution of the layers. It was found that the maximum melting points were between 200 and 240 ^o C. The linear velocity was approximately 0.5-1.0 m / min. Calibration and cooling of the pipes was carried out using conventional equipment supplied by Cincinnati and installed downstream.

 After being conditioned for 2 weeks, the pipes were tested on an S4 test rig designed to test for crack propagation resistance, which was originally manufactured at King's College London, but was then modified to meet new requirements in accordance with ISO / DIS 13477 (revised and revised edition of ISO / TS / 138 / SC4 WG RCP - Doc. N 048). The tests were carried out in accordance with ISO / DIS 13477 (revised standard ISO / TC / 138 / SC4 WG RCP - Doc. N 048).

 The test results and tested combinations of materials are presented in table. 1 and 2.

When examining the test results, it should be noted that the total critical temperature (T _{crit. Total} ) of the multilayer pipes according to the present invention is lower, and in many cases it is significantly lower than the weighted average T _crit (T _{crit. Value} ) of the individual materials included. So, the values of T _crit (T _crit.value ) for testing NN 1, 2, 3, 7 and 8 below -25 ^o C, although none of the included materials has a critical temperature, T _crit , lower than -15 ^o C This should be considered as a very unexpected and significant technical advantage, since the possibility of using pipes increases to a considerable degree.

 It can also be noted that materials B and C have very low resistance to rapid crack propagation (RCP), but nevertheless, through their use, very good results can be obtained for multilayer pipes.

In this regard, it should be noted test N 9, which represents a comparative test. During this test, the outer layer (material B) cracked and it was impossible to make any significant measurements on it. Therefore, the value of T _crit.total was found to be less than +23 ^o C, which exceeds the value of T _crit.value . This test can be compared with the result of test No. 7, which contains, however, a higher content of material B (71%), but all of them have a T _crit value of _less than -25 ^o C. layers of materials in a specific order, and more precisely so that the material having the lowest critical temperature (T _crit ) is located at least on the outside of the pipe.

In addition, the results presented in table. 2 show that with an increased content of material with a low critical temperature, T _crit , the value of T _{crit overall} decreases (see, for example, tests NN 3-5 and 6-8). In addition, for example, test N 11 shows, when compared with test N 28, that the T _crit value can also significantly improve the relatively thin intermediate layers of another material, although the material of the intermediate layers has a worse T _crit than materials in other layers.

Claims (8)

1. A multilayer plastic pipe, consisting of at least two layers of different plastic materials, with improved resistance to rapid crack propagation, characterized in that the plastic materials consist of polyolefin plastic, and the pipe has resistance to rapid crack propagation, expressed as the critical temperature of the pipe, which is below the weighted average value of the critical temperature of single-layer pipes of individual plastic materials included in a multilayer pipe having the same size er, moreover, the critical temperatures of single-layer pipes of individual plastic materials differ by at least 5 ^° C, and the critical temperature of the pipe is the lowest temperature at which a crack formed in the pipe extends essentially equal to 4 pipe diameters, with a pressure difference between the inner side and the outer side of the pipe 0.5 MPa.  2. The pipe according to claim 1, characterized in that it has a critical temperature that is lower than the temperature of all included plastic materials separately. 3. The pipe according to claim 1 or 2, characterized in that it has a critical temperature below -5 ^° C, preferably below -15 ^° C, measured on a pipe having an outer diameter of 110 mm and a wall thickness of 10 mm.  4. A pipe according to any one of claims 1 to 3, characterized in that the layers consist of at least one plastic, which is selected from polyethylene plastic and polypropylene plastic. 5. The pipe according to claim 4, characterized in that the layers consist of at least two layers of polyethylene plastic, the values of Tkrit which differ by at least 10 ^o C. 6. The pipe according to claim 4 or 5, characterized in that the layers consist of at least two layers of polypropylene plastic, Tkrit values, which differ by at least 10 ^o C.  7. The pipe according to any one of paragraphs, characterized in that the plastic material having a low critical temperature Tkrit is located at least on the outside of the pipe.  8. The pipe according to claim 7, characterized in that it contains at least three layers, and the material having the lowest critical temperature Tkrit is located on the outer and inner sides of the pipe.

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