Lightweight and flexible impact resistant power cable and process for producing it

Abstract

The present disclosure relates to an impact resistant, multipolar power cable (10) comprising, a plurality of cores (1), each core (1) comprising at least one conductive element (3) and an electrical insulating layer (5) in a position radially external to the at least one conductive element (3). The cores (1) are stranded together so as to form an assembled element providing a plurality of interstitial zones (2). An expanded polymeric filler (6) fills the interstitial zones (2) between the plurality of cores (1). An expanded impact resistant layer (7) is in a position radially external to the expanded polymeric filler (6) and comprises a polymer that differs from the expanded polymeric filler (6).

Claims

1. An impact resistant multipolar power cable comprising, a) a plurality of cores, each core comprising at least one conductive element and an electrical insulating layer in a position radially external to the at least one conductive element, the cores being stranded together so as to form an assembled element providing a plurality of interstitial zones; b) an expanded polymeric filler filling the interstitial zones, and comprising a polymer with a shore D hardness ranging from 30 to 70, a flexural modulus of from 50 MPa to 1500 MPa at 23 C., and a LOI of from 27 to 95% before expansion, wherein the expanded polymeric filler contains expanded microspheres; c) an impact resistant layer in a position radially external to and in contact with the expanded polymeric filler, wherein the layer comprises an expanded polymer that differs from the polymer for the filler and has, before expansion, a flexural modulus greater than that of the polymer for the filler; and d) a solid polymeric jacket surrounding the impact resistant layer.

2. The cable according to claim 1, wherein the expanded polymeric filler comprises polymers chosen from thermoplastic vulcanizates (TPV), thermoplastic olefins (TPO), flame retardant polypropylene, polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), and combinations thereof.

3. The cable according to claim 1, wherein the expanded polymeric filler has an expansion degree ranging from 15% to 200%.

4. The cable according to claim 3, wherein the expanded polymeric filler has an expansion degree ranging from 25% to 100%.

5. The cable according to claim 1, wherein the impact resistant layer comprises a polymer chosen from polyvinylidene fluoride (PVDF), polypropylene (PP), polyethylene (PE), and mixtures thereof.

6. The cable according to claim 1, wherein the impact resistant layer has an expansion degree ranging from 20% to 200%.

7. The cable according to claim 6, wherein the impact resistant layer has an expansion degree ranging from 20% to 50%.

8. The cable according to claim 1, wherein the impact resistant layer contains expanded microspheres.

9. The cable according to claim 1, wherein both the expanded polymeric filler and the impact resistant layer contain expanded microspheres.

10. The cable according to claim 1, further comprising a chemical barrier layer.

11. The cable according to claim 1, wherein the expanded polymeric filler fills the interstitial zones and forms an annular layer overlaying the interstitial zones and the stranded cores.

12. The cable according to claim 11, wherein annular layer has a thickness of about 1 mm to about 6 mm.

13. Process for producing an impact resistant multipolar power cable comprising a plurality of cores, each core comprising at least one conductive element and an electrical insulating layer in a position radially external to the at least one conductive element, the cores being stranded together so as to form an assembled element providing a plurality of interstitial zones; an expanded polymeric filler filling the interstitial zones; an impact resistant layer in a position radially external to and in contact with the expanded polymeric filler; and a solid polymeric jacket surrounding the impact resistant layer, the processing comprising: a) providing to an extruder a first polymer material with a shore D hardness ranging from 30 to 70, a flexural modulus of from 50 MPa to 1500 MPa at 23 C., and a LOI of from 27 to 95% for producing the expanded polymeric filler; b) providing to an extruder a second polymer material for producing the impact resistant layer, said second polymer having a flexural modulus greater than that of the first polymer; c) adding a foaming agent to the first and second polymer material, the foaming agent for at least the first polymer being thermally expandable microspheres; d) triggering the foaming agent of the first and second polymer material to expand the relevant polymer; e) coextruding the expanded first and second polymer material to form the polymeric filler filling the interstitial zones and the impact resistant layer; and f) extruding a solid polymeric jacket around the impact resistant layer.

14. Process according to claim 13, wherein the foaming agent for the second polymer comprises thermally expandable microspheres.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Further details will be illustrated in the following, appended drawing, wherein:

(2) FIG. 1 shows, in cross-section, an embodiment of a cable according to the present disclosure;

(3) FIG. 2 shows, in cross-section, another embodiment of a cable according to the present disclosure.

DETAILED DESCRIPTION

(4) The power cables of the present disclosure are multipolar cables. For the purposes of the present description, the term multipolar cable means a cable provided with at least a pair of cores. For example, if the multipolar cable has three cores, the cable is known as a tripolar cable.

(5) As used herein, and unless otherwise specified, the term core relates to a conductive element (typically made of copper or aluminium in form of wires or rod), an electrical insulation and, optionally, at least one semiconducting layer, typically provided in radial external position with respect to the electrical insulating layer. A second (inner) semiconducting layer can be present and typically provided between the electrical insulating layer and the conductive element. A metal screen, in form of wires or braids or tapes of conductive metal can be provided as outermost core layer.

(6) FIG. 1 illustrates a sketched view of a transversal cross-section of a tripolar cable according to an embodiment of the present disclosure. This cable (10) contains three cores (1) and three interstitial zones (2). Each core (1) comprises a conducting element (3), an inner semiconducting layer (4a), an electrical insulating layer (5), which may be crosslinked or not, and an outer semiconducting layer (4b).

(7) The three cores (1) are stranded together forming interstitial zones (2) defined as the spaces between the cores (1) and the cylinder enveloping such cores. The external perimetral profile of the stranded cores cross-section is, in the present case, trilobate as there are three cores.

(8) An expanded polymeric filler (6) fills the interstitial zones (2) interdisposed between the cores (1). The expanded polymeric filler (6) extends beyond and overlays the stranded cores (1) and interstitial zones (2) as defined by annular region (6a).

(9) Alternatively, as shown in FIG. 2, the polymeric filler (6) only fills the interstitial zones (2) interdisposed between the stranded cores (1). It does not form any significant annular layer overlaying the interstitial zones (2) and the stranded cores (1).

(10) In order to confer a multipolar cable with a suitably substantially circular transversal cross-section, the expanded polymeric filler expands to fill and, optionally, overlays the interstitial zones and the cores.

(11) The expanded polymeric filler (6, 6a) is surrounded by and in contact with an expanded impact resistant layer (7).

(12) As used herein, and unless otherwise specified; the term expanded refers to a polymer wherein the percentage of void volume is typically greater than 10% of the total volume of said polymer. As used herein, and unless otherwise specified, the term void refers to the space not occupied by the polymer but by gas or air. A not-expanded polymer is also referred to as solid.

(13) As used herein, and unless otherwise specified, the term expansion degree refers to the percentage of free space in an expanded polymer. The expansion degree of an expanded polymer may be defined according to the following equation:
G=(d.sub.0/d.sub.e1)100
wherein d.sub.0 indicates the density of the unexpanded polymer and d.sub.e represents the measured apparent density of the expanded polymer.

(14) The expanded polymeric filler (6) and impact resistant layer (7) were selected to meet the earlier discussed requirements. The cable (10) lacks a solid containment layer in contact with the expanded polymeric filler (6) and capable of providing the filler with the desired circularity.

(15) The cable (10) of FIGS. 1 and 2 are further provided with an optional metal (e.g. aluminium or copper) or metal/polymer composite (e.g. aluminium/polyethylene) layer (8) with overlapping edges (not shown) and an adhesive coating (not shown). The layer (8) can act as water or moisture barrier, has a thickness typically of from 0.01 mm to 1 mm, and has a negligible or null performance as impact resistant layer.

(16) A polymeric jacket (9), typically made of PE, PVC or chlorinated polyethylene optionally added with anti-UV additives, is provided, such as by extrusion, as the outermost cable layer. The polymeric jacket has a thickness typically of from 1.0 mm to 3.0 mm or more, depending on the cable size.

(17) Optionally, cable (10) further comprises a chemical barrier (not illustrated) in the form of a polymeric layer provided in radially internal position with respect to the jacket (9) and in radially external position with respect to the expanded impact resistant layer (7). For example, the chemical barrier may be as disclosed in U.S. Pat. No. 7,601,915. The barrier may comprise at least one polyamide and copolymers thereof, such as a polyamide/polyolefin blend, or TPE, and have an exemplary thickness of 0.5 mm to 1.3 mm. In at least one embodiment, when the impact resistant layer is made of PVDF, it can also perform as chemical barrier layer without changing the thickness, thus providing a cable with reduced diameter. In another embodiment, the chemical barrier layer is a polyimide.

(18) The expansion to form an expanded polymer filler and of the expanded impact resistant layer takes place during extrusion, more specifically before the polymeric material passes through the extrusion die. Expansion of the impact resistant layer may be by chemical agents, e.g., through the addition to the polymeric composition of a suitable expanding agent, which is capable of producing a gas under specific temperature and pressure conditions. Examples of suitable expanding agents are: azodicarbamide, paratoluene sulphonylhydrazide, mixtures of organic acids (citric acid for example) with carbonates and/or bicarbonates (sodium bicarbonate for example), and the like.

(19) In another embodiment, expansion to form an expanded impact resistant layer may take place due to microspheres that may be chosen from thermally expandable microspheres. The expansion of the polymer filler is carried out by thermally expandable microspheres. Thermally expandable microspheres are particles comprising a shell (typically thermoplastic) and a low-boiling point organic solvent encapsulated therein. With increasing temperature, the organic solvent vaporizes into a gas which expands to produce high internal pressures. At the same time, the shell material softens with heating so the whole particle expands under the internal pressure to form large bubbles. The microspheres have relative shape stability and do not retract after cooling. A suitable example of a thermally expandable microsphere is the commercial product sold under the name Expancel from Eka Chemicals.

(20) The polymer material is substantially fully expanded while it is still in the extruder crosshead and no significant expansion of the material occurs after it exits the extrusion die. This allows for controlled expansion with a circular cross-section.

(21) The use of thermally expandable microsphere as foaming agent was found particularly suitable for expanding the polymeric filler, while the choice of the foaming agent for the impact resistant layer is less critical. In one embodiment, the thermally expandable microspheres are used in both the polymeric filler and the impact resistant layer.

(22) According to the present disclosure, the polymer suitable for the interstitial filler has a shore D hardness ranging from 30 to 70, a flexural modulus (at 23 C. according to ASTM D 790) ranging from 50 MPa to 1500 MPa, and a limiting oxygen index (LOI) ranging from about 25% to 95%. As polymer properties may differ when expanded or non-expanded, the properties of the polymeric material are measured before expansion.

(23) Examples of the polymer suitable for the interstitial filler include, but are not limited to thermoplastic polymers selected, for example, from thermoplastic vulcanizates (TPV), thermoplastic olefins (TPO), flame retardant polypropylene, polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), and combinations thereof. Flame retardant polypropylene comprises added halogenated (e.g. brominated) flame retardant organics, as already mentioned above. Thermoplastic polyurethane and thermoplastic polyester elastomers are unsuitable as expandable material for the interstitial filler and impact resistant layer of the cable of the invention. Thermoplastic polyurethane and some thermoplastic polyester elastomers showed poor flame retardancy, while other thermoplastic polyester elastomers were found very difficult to be properly expanded.

(24) A non-limiting example of a TPV is Santoprene available from Exxon Mobil. Non-limiting examples of TPO's include polymers that are available from DuPont, Heraflex TPC-ET polymers available from RadiciPlastics.

(25) As used herein, and unless otherwise specified, the term containment layer refers to non-expanded layer, whether polymeric or otherwise, that functions to maintain the concentricity of the expanded polymeric filler surrounding cores of a multipolar cable. Without being limited to a particular theory, expanded layers are incapable of maintaining the concentricity of an expanded polymeric filler.

(26) In at least one embodiment, the polymer suitable for the interstitial filler reaches an expansion degree ranging from 15% to 200%, for example from 25% to 100%. The expanded polymeric filler expands to fill the interstitial zones and, optionally, to overlay and protect the plurality of cores. In at least one embodiment, the filler overlays the plurality of cores and the interstitial zones with a thickness of from about 0.5 mm to about 6 mm, yielding a substantially circular cross-section.

(27) According to the present disclosure, the impact resistant layer is not a containment layer but an expanded polymeric layer. The polymer suitable for the impact resistant layer has a flexural modulus higher than that of the polymer in the interstitial filler. The flexural modulus of the impact resistant layer can ranges from 500 to 2500 MPa.

(28) Examples of the polymer in the impact resistant layer include, but are not limited to polyvinylidene fluoride (PVDF), polyprolylene (PP), such as ethylene-propylene copolymer, and polyethylene (PE), and mixtures thereof. In one embodiment the polymer is an ethylene-propylene copolymer.

(29) A non-limiting example of polyethylene (PE) is low density PE (LDPE), medium density PE (MDPE), high density PE (HDPE), linear low density PE (LLDPE), ultra-low density-polyethylene (ULDPE).

(30) In at least one embodiment, the polymer suitable for the impact resistant layer reaches an expansion degree ranging from 20% to 200%, for example from 20% to 50%.

(31) In at least one embodiment, the expanded polymeric filler and the impact resistant layer are made from different polymeric materials. In particular, the material for the expanded impact resistant layer has a flexural modulus higher than that of the material for the interstitial filler.

(32) The cables according to the present disclosure may be produced by any well-known methods of manufacture for multipolar cables. The polymeric filler and the impact resistant layer are provided to surround the stranded cable cores by co-extrusion or by tandem extrusion.

(33) Preferably coextrusion of interstitial filler and impact resistant layer materialshaving different processing temperaturesis carried out in a single extrusion crosshead by pressure extrusion for the interstitial filler and sleeving extrusion for the impact resistant layer.

(34) Illustrative, non-limiting, examples are given herein-below in order to describe the present disclosure in further detail.

EXAMPLES

(35) Preparation of Cables with Expanded Filler

(36) A series of tripolar cables according to the present disclosure as well as comparatives were constructed. These cables are identified in the following text by the letters A to R and are detailed in Table 1. For each of cable A to R, a triplexed core was insulated with cross-linked polyethylene (XLPE). The cable construction is specified in Table 1.

(37) Comparative cables E and F were prepared based on known cable designs. Cable E has no filler, just an impact resistant layer in form of metallic armour (Mylar tape surrounded by a welded aluminium armour) surrounded by a PVC jacket, extruded over the cable core to complete the construction. Cable F has a solid PVC filler extruded over the triplexed core. While Cable F has an impact resistant layer in form of corrugated aluminium armour and an overall PVC jacket, extruded over the cable core to complete the construction.

(38) TABLE-US-00001 TABLE 1 Cable Construction Insulated Impact Metallic Chemical Cable Core Filler Resistant layer layer barrier Jacket A 3 5.3 mm.sup.2 + PVC + 3% fE PVDF.sup.1 + 3% fE yes PVC 0.8 mm XLPE 1.1 mm overlaid 1 mm 1.6 mm G = 75% G = 32% B 3 107 mm.sup.2 + PVC + 2% fE PP + 0.65% fH PVC 2 mm XLPE 2.5 mm overlaid 1.7 mm 2.8 mm G = 75% G = 33% C 3 107 mm.sup.2 + PVC + 2% fE PP + 0.8% fH PA PVC 2 mm XLPE 4.1 mm overlaid 1.7 mm 1.2 mm 2.8 mm G = 75% G = 33% D 3 107 mm.sup.2 + PVC + 3% fE PP + 0.8% fH Polylam PA PVC 2 mm XLPE 2.5 mm overlaid 1.7 mm 1.2 mm 2.8 mm G = 75% G = 33% E* 3 5.3 mm.sup.2 + Welded PVC 0.8 mm XLPE Al armor 1.6 mm F* 3 5.3 mm.sup.2 + PVC (solid) Corru- PVC 0.8 mm XLPE gated Al 1.6 mm armor M 3 5.3 mm.sup.2 + TPV + 3% fE PVDF.sup.2 0.8% fE yes PVC 0.8 mm XLPE 2 mm overlaid 1.3 mm 1.6 mm G = 66% G = 31% N 3 5.3 mm.sup.2 + PVC + 3% fE PP + 1.5% fE PA PVC 0.8 mm XLPE 1.2 mm overlaid 1 mm 0.7 mm 1.7 mm G = 75% G = 37% O 3 5.3 mm.sup.2 + PVC + 3% fE PP + 1.5% fE TPE PVC 0.8 mm XLPE 1.1 mm overlaid 1 mm 0.6 mm 1.6 mm G = 75% G = 37% P 3 5.3 mm.sup.2 + PVC + 3% fE PP + 1.5% fE PVDF PVC 0.8 mm XLPE 1.1 mm overlaid 1.2 mm 0.7 mm 1.7 mm G = 75% G = 37% Q 3 5.3 mm.sup.2 + PVC + 3% fE + PVDF.sup.1 + 3% fE yes PVC 0.8 mm XLPE skin (0.13 mm) 1.1 mm 1.5 mm 1 mm overlaid G = 32% G = 75% S* 3 107 mm.sup.2 + TPE + 7% fE + PP + 0.65% fH PVC 2 mm XLPE skin (0.7 mm) 1.7 mm 2.8 mm 3.4 mm overlaid G = 33% G = 254% *Comparative cables G = expansion degree PVC (filler) = polyvinylchloride (Shore D = 40, Flexural Modulus @ 23 C. = 70 MPa, LOI = 28.5%) TPV = thermoplastic vulcanizates (Shore D = 32, Flexural Modulus @ 23 C. = 152 MPa, LOI = 27%) PVDF.sup.1 = polyvinylidene fluoride (Shore D = 54, Flexural Modulus @ 23 C. = 356 MPa; LOI = 42%) PVDF.sup.2 = polyvinylidene fluoride (Shore D = 46, Flexural Modulus @ 23 C. = 607 MPa; LOI = 42%) PP = polypropylene (Shore D = 55, Flexural Modulus @ 23 C. = 475 MPa LOI = 42%) TPE = thermoplastic polyethylene (Shore D = 44; Flexural Modulus @ 23 C. = 145 MPa; LOI= 26%) fE = microsphere foaming agent (AkzoNobel Expancel) fH = citric acid foaming agent Polylam = aluminum/polyethylene laminate as moisture barrier (it does not impart any impact resistance) skinP = Polyvinylchloride skin skinH- thermoplastic polyethylene skin PA = Polyamide PVC (jacket) = Polyvinylchloride

(39) In cables A, M and Q, the impact resistance layer also performs as a chemical barrier.

(40) Skin present in cable Q and S is a layer co-extruded with filler to provide a better surface on the filler. The skin does not provide a containment function.

(41) The filler/impact resistant layer co-extrusion of comparative cable S was troublesome due to difficulties in controlling the dimension, especially in term of circularity of the cross-section, and in obtaining a smooth surface. Also, the cable did not pass impact resistance test.

(42) In order to evaluate the multipolar cables prepared in Table 1, impact, flame, flexibility and crush tests were conducted.

(43) Impact tests. The effect of impacts on a cable was evaluated by an impact test based on the standard IEC61901 (1.sup.st edition, 2005-07). The effects of an impact at various forces (J) were evaluated by means of measuring the depth of damage (mm). The cables were subjected to impact levels of 25 J to 70 J or to more severe conditions (from 150 J to 300 J) depending on their intended use. The depth of damage gives an indication of the degree of protection provided by the expanded impact resistant layer. Tables 2a and 2b set forth the values of the various energy levels analysed, depth of damage (mm) measured for samples A-F and M-Q.

(44) TABLE-US-00002 TABLE 2a Impact Strength Test Results Energy Levels Cable 25J 30J 40J 50J 60J 70J A 0.63 0.67 0.88 0.96 0.86 0.98 E* 0.53 0.76 0.91 1.18 1.18 1.26 F* 6.61 0.42 0.85 1.06 1.24 1.25 M 0.21 0.29 0.27 0.61 0.49 0.64 N 0.59 0.70 0.63 0.85 1.03 0.91 O 0.60 0.80 0.70 0.75 0.85 1.04 P 0.59 0.57 0.80 0.69 1.02 0.84 Q 0.41 0.59 0.84 0.72 0.94 0.84

(45) TABLE-US-00003 TABLE 2b Impact Strength Test Results Energy Levels Cable 150J 200J 250J 300J B 1.27 1.64 0.87 1.42 C 0.56 1.18 1.02 1.11 D 0.44 0.60 1.31 1.45

(46) This testing shows that the cables according to the invention resisted to impact in a way at least comparable to that of armoured cable E and F.

(47) Other tests: The flexibility and the effects of flame and crushing on certain multipolar cables were also evaluated. The flame test is a pass/fail test that follows the IEEE-1202 standard for 60 inch (about 1.5 m) length. The flexibility test is a three point bend test, recorded at 1% secant modulus according to ASTM D-790. The crush test applies the procedure of UL-1569 setting 5340N (1200 lbfas minimum load, and the table reports the maximum load bore by the cables. Table 3 gives the values for these test results.

(48) TABLE-US-00004 TABLE 3 Flame, Flexibility, Crush Test Results Flexibility Crush Cable Flame (MPa) (N) A Pass 91.0 5430 E* 338.0 14100 M Pass 114.0 6400 Q Pass 101.0 5750

(49) This testing shows that the cables of the invention performed favorably when compared to prior art cables. Their crush resistance is according to the standard requirements and goes along with a remarkably improved flexibility and to the capability of withstanding flame.

(50) The cables of the invention provide a solution for a cable which is light weight, flexible, impact resistant, crush resistant, flame resistant and chemical resistant.