Patent Application
20070218230
The invention relates to strips reinforced with steel cords that are used for the reinforcement of pipes or tubes, and to tubes reinforced with such strips.
Pipes or tubes made from high-density polyethylene (HDPE) are becoming more and more common and are replacing traditional steel tubing for water supply and gas distribution in both domestic and industrial applications. Other polymers like fluoropolymers or polyamids adapted for the use in special applications where temperature, sourness, gas permeability or other fluid properties are important can equally be used. Although these polymers have excellent physico-chemical properties with respect to the fluida being transported, in many cases they lack intrinsic mechanical strength in order to be useful when the pressure of the transported fluid is raised.
To this end plastic pipes have been reinforced with spirally wound strips containing a reinforcing material in their longitudinal direction. These strips are helicoidally wound around the tube at an angle of between 50 to 600 to the axis of the tube. Lower than 50° angles are used in case a longitudinal reinforcement of the tube is needed. Higher than 60° angles are used in case a stronger radial reinforcement is needed. While in the past the preferred reinforcing material for such strips was an aramid type of material, the use of steel cords or wires has been suggested (see e.g. WO 02/090812).
The use of steel cord or wire in strips for the reinforcement of tubes presents some special challenges that have to be overcome. One particular problem that is the subject of the present invention is the lack of anchoring between the steel cord and the polymer matrix. Such anchoring is particularly important in the region of a pipe-to-pipe connection or pipe-to-end-fitting because there the steel reinforcement is interrupted. At these ends all tensional forces in the strip of the first pipe end have to be brought over to the second pipe end (or end-fitting as the case may be) through the intermediate polymer matrix such that the connection does not break due to the pressure on the fluid. The transfer of force can for example be achieved by welding a sleeve over both tube ends in the case of a tube-to-tube connection. This sleeve must be limited in length because a connection has to be made in the field with portable (hence short) equipment, sometimes under harsh conditions (with limited shelter) and within a reasonable time period (to limit its cost).
For clarity: with ‘anchoring’ something different than ‘adhesion’ is meant. Adhesion systems provide transfer of forces between two materials by mediation of a chemically reacting interface layer. Although adhesion systems are possible between steel and various polymers they require the presence of specials coating—e.g. brass in case the polymer matrix is rubber—that add to the total cost of the system. When thermoplastic polymers are used the adhesion system can even become more complicated—due to the saturated nature of the molecules—requiring two or more intermediate layers adding more to the overall cost. This cost is even more exuberant when considering that the adhesion is mainly needed in the connection region: outside these regions the cord-to-polymer adhesion is less important. Within the context of this application only mechanical transfer of forces between cord and polymer is thus considered and this transfer of forces is called ‘anchoring’.
It is the object of the present invention to provide a solution to the connector problem. It is also an object of the present invention to provide those cords that exhibit superior anchoring properties in a polymer matrix. It is a further object of this invention to provide such an anchoring without the need of any adhesion system.
Cords used for the reinforcement of pipes, tubes or hoses that have to withstand a design burst pressure Pburst in general must obey a simple relation when wound at the neutral angle of arc tan √2:
n·(Fb/dcord)·φstrip·≧Pburst·D·(¾)
where the symbols have the following meaning
In the design of the cord, one will always seek to maximise the ratio Fb/dcord i.e. transversal breaking load per unit diameter as this can reduce the number of layers n. Fb/dcord can also be written as:
Fb/dcord=σin·ψc·φc·dcord·(π/4)
wherein:
Within the technical field of rubber hoses, these requirements have lead to the use of cords that are thick, have a high metallic fill factor and a low cabling loss. Cords that are typically used to this end have a diameter of between 2.4 and 4.5 mm, are of the Warrington type, have a metallic fill factor of at least 75%. There is no need for anchoring here, because there is enough chemical adhesion between the zinc or brass coated cord and the rubber.
The inventors have found that these conventionally known cords are not fit for use in a pipe or tube reinforced with strips. For example the thickness of the cord cannot be increased at will when using a strip to reinforce a tube because:
Typically one strives to keep the diameter of the cord below 2.4 mm, although it is more preferred to keep it below 2.0 mm. Most preferred is to keep it between 1.0 to 1.65 mm.
Moreover, the inventors identified a class of steel cords that do solve the anchoring problem. The characterising features to which such a problem solving strip with steel cord must obey are disclosed in the independent claim 1 and the claims 2 to 7 depending thereof.
In the connection of one pipe to the next (or to an end-fitting as the case may be) all forces must be transmitted over the anchoring between steel cord and polymer. Such a connection comprises a sleeve that is slid over the end of the first tube to be connected and thereafter retracted to cover the end of the second tube, so that the sleeve is shared between both ends of the tube. Thereafter the sleeve is welded to the polymer of both tube ends. The anchoring should be sufficient that the connection or the mounting of the end-fitting can be done over a sufficiently small distance.
As mechanical anchoring is an interplay between the surface structure of the steel cord (the amount of openness, the angle between filaments and cord direction, the smoothness of the outer surface) and polymer matrix (its hardness, its ability to flow into the cord) the degree of anchoring or anchoring strength is best quantified as the pull-out force per unit length fa that is needed to pull a single cord out of a strip in the longitudinal direction of the strip (expressed in N/mm). The inventors have found that the transversal breaking load per unit diameter (Fb/dcord) must not be larger than 300 times the pull-out force per unit length fa in order to allow for a conveniently small connector length (claim 1). Better is when this value is not larger than 150 times fa (claim 2). As such pull-out forces can also be reached by using chemical adhesion means between steel cord and polymer, the pull-out force is expressly limited not to be too high. Indeed, 30 times the pull-out force per unit length fa must be smaller than Fb/dcord.
This criterion can also be expressed in terms of a ‘critical length Lc’. The critical length is that embedment length where the total pull out force is equal to the breaking load of the cord. The requirements can then be reformulated in that the critical length Lc must be smaller than 300·dcord, respectively must be smaller than 150·dcord. Likewise, the critical length must not be smaller than 30·dcord i.e. a number that is easily achievable with chemical mediated adhesion. The critical length is proportional to the connector length.
fa is determined by embedding the steel cord in a block of polymer matrix material over a length of 25.4 mm. After proper cooling, the steel cord is longitudinally pulled out and the maximum force is registered. fa is then equal to the force divided by the embedment length and is expressed in N/mm. The test will be described in more detail in the ‘Description of the preferred embodiments of the invention’ section.
By preference the strips according the invention must be reinforced with cords that are sufficiently open i.e. within their perimeter enough polymer must be able to ingress in order to mechanically lock the polymer into the cord. The ratio of the metallic area Ametal to the area of the circumscribed circle (πdcord2/4)—or φc as defined above—must be less than 0.70 or 70% (claim 3). This is a measure for the degree of ‘openness’ of the cord. The lower the number, the more polymer can ingress into the cord, and if more polymer can enter the cord it will better anchor the cord.
As will be clear from the above a lower metallic fill factor must be compensated by a higher initial strength of the wires. Preferably the tensile strength as expressed over the total circumscribed area Fb/(πdcord2/4) must be larger than 1500 N/mm2. Even more preferred is that this tensile strength is above 1700 N/mm2. Most preferred is that this value is above 2000 N/mm2.
A subclass of cords that reach a sufficient mechanical anchoring in combination with the required strength are characterised in that each and every of the filaments constituting the cord comes into contact with the polymer matrix at regular or irregular intervals (claim 4). In this way all filaments carry an equal anchoring load enhancing the balanced loading of the filaments.
The anchoring capacity can be further improved by giving the filaments permanent bends in directions perpendicular to the initial axis of the filament resulting in a wavy or helical shape prior to twisting them together (claim 5). This increases locally the angle between the filaments and cord axis, leading to an improved anchoring of the cord in the polymer.
The mechanical anchoring is also determined by the stiffness of the polymer matrix. A soft material will more easily let go the steel strand embedded in it compared with a hard material. Thermoplastic polymers are the material of preference (claim 6). More preferred are thermoplastic polymers with a shore D hardness between 45 and 75 Most preferred are polymers with a shore D value between 50 and 65. Shore D is measured according the ISO 868 (or the equivalent ASTM D2240-03) standard and is well known to the person skilled in the art in the field of polymer physics.
There are a number of thermoplastic polymers that either in pure form or blended with co-polymers obtain the desired mechanical hardness. The most prominent one's are high density polyethylene (HDPE) (claim 10) and polypropylene (PP). Other polymers like polyvinylchloride (PVC), polyamide (PA11) or cross linked polyethylene (PEX) or fluoropolymers like ethyl-tetra-fluoro-ethylene (ETFE), fluorinated ethylene propylene (FE), hexa-fluoro-propylene (HFP), poly (vinylidene fluoride) (PVDF), poly (phenylene sulfide) (PPS), poly (etheretherketone) (PEEK), or perfluoroalkoxy (PFA) could be considered as well.
According a second aspect of the invention, a pipe is claimed (claim 8). This pipe comprises a centre tube and at least one layer of reinforcing material in the form of a strip. The strip is helically wound around said centre tube. The strip comprises a polymer matrix and a plurality of steel cords, the steel cords being arranged parallel in the strip. The steel cords have a breaking load Fb and a diameter dcord, hence they have a transversal breaking strength per unit diameter of Fb/dcord expressed in N/mm. The pipe characterises itself from the state of the art in that the cords are mechanically anchored in the polymer matrix of the strip as characterised in claim 1 i.e. that the steel cords are mechanically anchored in the matrix with a longitudinal pull-out force per unit length of fa in N/mm, such that the transversal breaking load per unit diameter Fb/dcord is smaller than 300 times fa or in formula:
(Fb/dcord)<300·fa
A higher degree of anchoring of the steel cord in the strip of the tube is claimed in claim 9 namely that
(Fb/dcord)<150·fa
According a third aspect of the invention, a method to shorten the covered length of a pipe-to-pipe or a pipe-to-end-fitting sleeve connection is claimed (claim 10). With covered length is meant that length at the end of the pipe that is covered with the connector sleeve. As above the pipe comprises a thermoplastic center tube and at least one layer of reinforcing material in the form of strip, where the strip comprises a polymer matrix and a plurality of steel cords that are arranged parallel in the strip. The connection is different from the existing art in that the steel cords are purely mechanically anchored in the polymer matrix and that the covered length per pipe end is more than 30 times and less than 300 times the diameter of the steel cord.
The invention will now be described into more detail with reference to the accompanying drawings wherein
a depicts the mould to test the anchoring property of the cords in the polymer matrix.
b shows how the steel cord is pulled out of a polymer block.
a is a schematic presentation of cord Nr. 2.
b is a schematic representation of cord Nr. 3.
c is a schematic representation of cord Nr. 1.
d is a schematic representation of cord Nr. 7.
Table 1 on page 16 enumerates a number of cords that have been investigated for their suitability to be used in a strip for the reinforcement of a pipe or tube. The columns (A) through (K) contain the following:
(B) A short cord type description. The two letter codes behind the formula categorises the breaking strength of the filaments used for the construction:
Note that within each strength class, the tensile strength diminishes with increasing filament diameter.
The cord built-up (column ‘B’) is explained in what follows:
Only a few of the constructions above are fitted to solve the anchoring problem of tube reinforcement strips as will be demonstrated below.
To determine the anchoring strength, the pull out force per unit length fa is determined on steel cords embedded in blocks of polymer with a height of 12.5 mm, a width of 24.5 mm and a length of approximately 200 mm. The preparation of these is clarified in
The pull-out testing is performed after a resting time of 16 hours in the following way (depicted in
The test is adapted from the ASTMD 2229-85 ‘Standard Test Method for Rubber Property—Adhesion to Steel Cord’.
The anchoring results of table 1, column ‘J’ have been obtained with ‘Eltex® TUB 172’ which is a high-density polyethylene copolymer designed for the extrusion of gas pressure pipes, produced by Solvay Polyolefins Europe. The measured Shore D hardness of the blocks was between 54 and 55.5.
The test method can easily be extended—as depicted in
The ratio of (Fb/dcord)/fa is calculated in column K. Cords 1, 5, 6, 7, 8, 9 fulfil claim 1. When interpreted in terms of the critical length Lc this would mean that a length of 300×dcord is sufficient to anchor the cord up to its breaking load. Hence these types of cords are best suited to obtain a short connection between pipes. For cords Nr. 1, 5, 6, 7, 8 the critical length is even shorter than 150×dcord (claim 2). Cords 2, 3 and 4 have a (Fb/dcord)/fa ratio that is larger than 300 (i.e. outside the scope of claim 1). Apparently these cords have too much metallic area inside the circumscribed circle and do not allow enough polymer ingress although they yield a very favourable tensile strength (column ‘H’ in table 1). A subclass of the class of cords defined by claim 1 is thus defined by claim 3 wherein all cords in addition have a metallic fill factor smaller than 0.70.
The cords that distinguish themselves as having a higher anchoring capability also share the characteristic of claim 4. Each and every filament constituting the steel cord for the reinforcement of the strip makes partial and intermittent contact to the matrix. With ‘partial’ is meant that the contact is at least over a part of the circumference of the filament. With ‘intermittent’ is meant that the contact does not have to be over the full length of the filament. This is illustrated in
c depicts the cord Nr. 1 where all filaments of the outer strands periodically come into partial contact with the matrix. Only the core strand never—or extremely rarely-comes into contact with the matrix.
Over and above the cords Nr. 5, 6 and 7 contain filaments that have been deformed prior to cabling. The filaments of the strand with four (4) filaments have been crimped in two directions perpendicular to one another by leading them through two sets of gear pairs. The filaments of the strand with six (6) filaments have been bend in one plane in one direction only prior to twisting into the strand. The final shape of the latter filaments is a polygonal helix i.e. the projection of the filament on a plane perpendicular to its axis is a—not necessarily closed—polygon where for an undeformed filament the projection of the is a circle. Cord 7 is a cord where only the six (6) filaments have been bend in one plane prior to twisting while the remaining group of four (4) filaments has not been deformed. In that respect the series of cords 6, 7 and 8 form a series of cords with decreasing degree of deformation. Cord 6 (all filaments deformed) yields better adhesion results than cord 8 (no deformed filaments). This influences also the critical length that increases with less deforming (see column K). Considering all of the above, it will be clear to the person skilled in the art that the constructions that are best suited to reinforce a strip for pipe reinforcement are of type Nr. 6 to Nr. 9.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04101731.0 | Apr 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/51834 | 4/25/2005 | WO | 10/25/2006 |
