Fire resistant cable with ceramifiable layer

Abstract

A fire resistant cable comprising: a conducting element (2; 21); a layer, surrounding the conducting element, made of a ceramifiable composition comprising: a thermoplastic polymer mixture comprising: (a) a copolymer of ethylene with a C.sub.4-C.sub.12 alpha-olefin, having a density of from 0.860 to 0.910 g/cm.sup.3, a melt flow index not higher than 3 g/10 min and a melting point of 105 C. at most; (b) an ethylene homopolymer or copolymer of ethylene with a C.sub.4-C.sub.12 alpha-olefin, having a density of from 0.900 to 0.985 g/cm.sup.3, a melt flow index not higher than 5 g/10 min and a melting point of at least 110 C.; and (c) a polyethylene grafted with an ethylenically unsaturated monomer; at least 25 wt % of silica; a fluxing agent selected from alkaline metal oxides or precursors thereof; an inorganic hydroxide compound selected from magnesium hydroxide, aluminium hydroxide and mixtures thereof; a stabilizing agent comprising a hydrated magnesium silicate in an amount of at least 5 wt %; weight percentages being based on the total weight of the ceramifiable composition. Upon exposure to elevated temperatures such as those encountered in case of fire, the ceramifiable composition is transformed into a ceramic material capable of protecting the conducting element from fire and mechanical stresses. The fire resistant cable of the present invention can continue operating under fire conditions for a certain period of time.

Claims

1. A fire resistant cable (10; 20) comprising: a conducting element (2; 21); a layer, surrounding the conducting element, made of a ceramifiable composition comprising: a thermoplastic polymer mixture comprising: (a) a copolymer of ethylene with a C.sub.4-C.sub.12 alpha-olefin, having a density of from 0.860 to 0.910 g/cm.sup.3, a melt flow index not higher than 3 g/10 min and a melting point of 105 C. at most; (b) an ethylene homopolymer or copolymer of ethylene with a C.sub.4-C.sub.12 alpha-olefin, having a density of from 0.900 to 0.985 g/cm.sup.3, a melt flow index not higher than 5 g/10 min and a melting point of at least 110 C.; and (c) a polyethylene grafted with an ethylenically unsaturated monomer; at least 25 wt % of silica; a fluxing agent selected from alkaline metal oxides or precursors thereof; an inorganic hydroxide compound selected from magnesium hydroxide, aluminium hydroxide and mixtures thereof; a stabilizing agent comprising a hydrated magnesium silicate in an amount of at least 5 wt %; weight percentages being based on the total weight of the ceramifiable composition.

2. The fire resistant cable according to claim 1 wherein the thermoplastic polymer mixture comprises: (a) from 65 parts to 90 parts of the copolymer of ethylene with a C.sub.4-C.sub.12 alpha-olefin, having a density of from 0.860 to 0.910 g/cm.sup.3, a melt flow index not higher than 3 g/10 min and a melting point of 105 C. at most; (b) from 10 parts to 30 parts of the ethylene homopolymer or copolymer of ethylene with a C.sub.4-C.sub.12 alpha-olefin, having a density of from 0.900 to 0.985 g/cm.sup.3, a melt flow index not higher than 5 g/10 min and a melting point of at least 110 C.; and (c) from 5 parts to 15 parts of the polyethylene grafted with an ethylenically unsaturated monomer the parts being parts by weight of the component with respect to 100 parts by weight of the thermoplastic polymer mixture.

3. The fire resistant cable according to claim 1 wherein the thermoplastic polymer mixture is present in an amount of at least 25 wt % based on the weight of the ceramifiable composition.

4. The fire resistant cable according to claim 1, wherein the silica is an amorphous silica made of substantially spherical particles.

5. The fire resistant cable according to claim 1, wherein the stabilizing agent further comprises at least one of MgO, CaO, PbO or a precursor thereof.

6. The fire resistant cable according to claim 1, wherein the hydroxide compound is aluminium hydroxide.

7. The fire resistant cable according to claim 1, wherein the silica is present in an amount up to 40 wt %, based on the weight of the ceramifiable composition.

8. The fire resistant cable according to claim 1, wherein the fluxing agent is present in an amount of at least 5 wt % with respect to the weight of the ceramifiable composition.

9. The fire resistant cable according to claim 1, wherein the hydroxide compound is present in an amount of from 0.1 wt % to 5 wt % based the weight of the ceramifiable composition.

10. The fire resistant cable according to claim 1, wherein the hydrated magnesium silicate as stabilizing agent is present in an amount lower than 13 wt % based on the weight of the ceramifiable composition.

11. The fire resistant cable according to claim 1, wherein the ceramifiable composition comprises from 6 wt % to 25 wt % of stabilizing agent based on the weight of the ceramifiable composition.

12. The fire resistant cable according to claim 1 which is a power cable (10) where the layer made of a ceramifiable composition is a bedding layer (4) and/or an outer sheath (5).

13. The fire resistant cable according to claim 1 which is a telecommunication cable (20) where the layer made of a ceramifiable composition is a jacket layer (24).

14. An extrudable ceramifiable composition comprising: a thermoplastic polymer mixture comprising: (a) a copolymer of ethylene with a C.sub.4-C.sub.12 alpha-olefin, having a density of from 0.860 to 0.910 g/cm.sup.3, a melt flow index not higher than 3 g/10 min and a melting point of 105 C. at most; (b) an ethylene homopolymer or copolymer of ethylene with a C.sub.4-C.sub.12 alpha-olefin, having a density of from 0.900 to 0.985 g/cm.sup.3, a melt flow index not higher than 5 g/10 min and a melting point of at least 110 C.; and (c) a polyethylene grafted with an ethylenically unsaturated monomer; at least 25 wt % of silica; a fluxing agent selected from alkaline metal oxides or precursors thereof; an inorganic hydroxide compound selected from magnesium hydroxide, aluminium hydroxide and mixtures thereof; a stabilizing agent comprising a hydrated magnesium silicate in an amount of at least 5 wt %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further characteristics will be apparent from the detailed description given hereinafter with reference to the accompanying drawing, in which:

(2) FIG. 1 is a cross section view of a cable according to the invention for power transmission at low voltage.

(3) FIG. 2 shows a cross section view of a cable according to the invention for telecommunication.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) With reference to FIG. 1, the fire resistant power cable 10 according to the present invention may be of the tripolar type comprising three electrically conducting elements or conductors 2 each covered by an insulating layer 3 to form a core 1. The three conductors 2 with the relevant insulating layers 3 are encircled by an outer sheath 5. The three cores 1 are stranded together forming interstitial zones defined as the spaces between the cores 1 and the cylinder (the outer sheath 5) enveloping such cores. A bedding or interstitial filler 4 fills said interstitial zones.

(5) The insulating constant k.sub.i of the electrical insulating layer 3 is such that the required electric insulating properties are compatible with the standards (e.g. IEC 60502-1, 2004 or other equivalent thereto). For instance, the electrical insulating layer 3 has an insulating constant k.sub.i equal to or greater than 3.67 MOhm.km at 90 C.

(6) The conductors 2 can be in form of a solid rod or of bundled wires made of electrically conductive metal such as copper or aluminum or composite thereof.

(7) According to a first embodiment, the outer sheath 5 is made of the ceramifiable composition of the present invention.

(8) According to a second embodiment, the bedding 4 is made of the ceramifiable composition of the present invention.

(9) With reference to FIG. 2, a fire resistant telecommunication cable 20 according to the present invention comprises a plurality of optical fibres 21 (comprising a glass core and cladding a polymeric protecting coating system) grouped and housed into modules 22 in polymeric material, optionally further containing water-blocking material (not shown) in form of gel or filaments. The modules 22 are stranded around a central strength member 23 and a jacket 24 surrounds modules and strength member.

(10) According to an embodiment of the invention, the jacket 24 and/or modules 22 is/are made of the ceramifiable composition of the present invention.

(11) The present description shows only some embodiments of a cable according to the invention. Suitable modifications can be made to these embodiments according to specific technical needs and application requirements without departing from the scope of the invention.

(12) The following examples are provided to further illustrate the invention.

EXAMPLES

(13) Samples of the ceramifiable composition according to the present invention were prepared by mixing all of the components in a Banbury internal mixer (volume: 1.6 l; filling factor 85%; speed rotation: 50-75 rpm; discharge temperature of the compound: 220 C.). Comparative samples were also prepared using the same apparatus and process. The samples were prepared using components and amounts (expressed as wt % based on the total weight of the composition) as set forth in Table 1.

(14) TABLE-US-00001 TABLE 1 Composition of samples 1 2 3* 4* 5* 6* 7* 8* 9 Polymer (a) 29.2 28.1 27.2 26.7 26.7 30.6 28.1 28.1 28.7 Polymer (b) 7.3 7.0 6.8 8.4 7.7 7.0 7.0 7.2 Polymer (c) 3.7 3.5 3.4 2.0 3.2 4.6 3.5 3.5 5.4 Comparative Polymer 9.6 SiO.sub.2 32.9 31.6 30.6 42.0 42.0 23.0 31.6 31.6 32.2 Al(OH).sub.3 1.8 1.8 1.7 1.9 1.9 3.8.sup.a 1.8 1.8 1.8 CaO 3.2 3.1 2.7 3.4 3.4 3.1 3.1 3.1 Talc 11 10.5 13.6 10.7 (Mg.sub.3Si.sub.4O.sub.10(OH.sub.2) PbO 3.9 3.8 4.2 4.2 3.9 3.9 MgO 0.6 0.6 Zn borate 24.1 Kaolin 10.5 Hydromagnesite + 10.5 Huntite Na.sub.2CO.sub.3 8.9 8.6 8.3 7.5 7.5 4.1 8.6 8.6 8.8 Additives 2.0 1.9 1.9 2.1 2.1 2.1 1.9 1.9 2.1 .sup.aAs Al.sub.2O.sub.3 Polymer (a): ethylene-octene copolymer (30 wt % of octane with respect to the copolymer weight); density=0.885 g/cc (ASTM D792), MFI=1.00 g/10 min (190 C./2.16 kg; ASTM D1238), melting point=78.0 C.); Polymer (b): linear low density polyethylene; density=0.911 g/cm.sup.3 (ASTM D1505), MFI=3.00 g/10 min (190 C./2.16 kg; ASTM D1238), melting point=118.0 C.); Comparative Polymer: linear low density polyethylene; density=0.911 g/cm.sup.3 (ASTM D1505), MFI=13.00 g/10 min (190 C./2.16 kg; ASTM D1238), melting point=116.0 C.); Polymer (c): maleic anhydride grafted polyethylene; density=0.93 g/cm.sup.3 (ASTM D792), MFI=1.75 g/10 min (190 C./2.16 kg; ASTM D1238), melting point=120.0 C.); SiO.sub.2: amorphous silica, BET=20 m.sup.2/g, D.sub.50=150 nm; Hydromagnesite: Mg.sub.3Ca(CO.sub.3).sub.4; Huntite: Mg.sub.4(CO.sub.3).sub.3(OH).sub.2.3H.sub.2O; Additives: stearic acid (processing aid), pentaerythritol tetrakis (3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate) (antioxidant), tetrakismethylene (3,5-di-tert-butyl-4-hydroxyhydrocinnamate) methane (antioxidant).

(15) The compositions marked with an asterisk are comparative examples.

(16) Each composition was printed in form of plaques by printing at 180 C. using a mechanical press and then tested. In particular, mechanical properties, i.e. elongation at break (EBexpressed as percentage) and tensile strength (TS expressed in Mpa) were evaluated at room temperature (20 C.) on 2002001 mm plaques, while the fire-tests were performed on tablets obtained from 1501003 mm plaques. The mechanical properties tests were repeated after ageing the samples in air oven (168 hours, 100 C.) and in mineral oil (4 hours, 70 C., Oil IRM 902), and the variations are reported in Table 2 (TS and EB).

(17) The fire tests were carried out by placing the tablets in a muffle furnace at temperatures of 400 C., 600 C., 800 C. and 1000 C. The tablets behavior under heating were evaluated by visual inspection and, when cooled down, by soft and hard hammering to assay the char integrity.

(18) The results of the mechanical and fire tests are reported in Table 2.

(19) TABLE-US-00002 TABLE 2 Mechanical and fire tests. 1 2 3* 4* 5* 6* 7* 8* 9 TS 9.9 11.7 <9 4.8 6.9 11.0 11.1 10.5 11.2 EB 481.3 491 <120 131 235 391 566 513 463 TS @ 10.0 11.4 11.9 11.3 10 12.5 100 C. EB @ 452.5 377 295 509 481 416 100 C. TS @ 3 8 2 4 100 C. EB @ 6 23 25 10 6 100 C. TS in 6 15 3 10 19 oil EB in 35 12 25 12 12 oil Fire test YES YES YES YES NO NO NO YES

(20) According to IEC 60092-359:SHF1 (1994), EB should be greater than 120% and TS should be greater than 9.0 MPa.

(21) According to IEC 60502/2 ST8 (2005), after air oven ageing the TS should be greater than 9 MPa.

(22) According to IEC 60502/2 SE1 (2005), after air oven ageing the EB should be greater than 250%.

(23) According to UNE 21123-4 (2014), after ageing in mineral oil, the TS should not differ from the TS measured prior to ageing (TS) more than 40% (difference referred to the values measured prior to ageing).

(24) According to UNE 21123-4 (2014), after ageing in mineral oil, the EB should not differ from the EB measured prior to ageing (EB) more than 40% (difference referred to the values measured prior to ageing).

(25) A YES fire test means that the sample maintained its integrity and shape with no cracks compromising its mechanical resistance or swellings at temperatures up to 1000 C.

(26) From the experimental data reported in Table 2, it can be seen Samples 1-2 and 6-9 had mechanical features according to the standard.

(27) The mechanical behavior of comparative sample 3* was impaired by an excessive amount of talc (greater than 12 wt %). The mechanical behavior of comparative sample 4* was impaired by the use of a polymer (b) having a MFI greater than 5 g/10 min and by the lack of a hydrated magnesium silicate (talc). Comparative sample 5*, lacking a hydrated magnesium silicate too, has poor a mechanical behavior, though slightly better than that of comparative sample 4*, because comparative sample 5* contains a polymer (b) according to the invention.

(28) Comparative samples 6*-8* showed that replacement of a hydrated magnesium silicate (talc) in the compositions with zinc borate, kaolin or hydromagnesite/huntite does not provide a composition with satisfactory fire resistant properties.

(29) The samples prepared with the compositions of the present invention (samples 1, 2 and 9), besides having a mechanical behaviour making them suitable for the manufacturing of a cable layer, ceramified by heating up to 1000 C. and gave place to a solid ceramified layer with only superficial cracks not compromising their integrity.