Flame- retardant electrical cable

Abstract

A flame-retardant electric cable has a core including at least one electric conductor, an electrically insulating coating and an outermost layer made from a substantially thermoplastic, low smoke zero halogen flame-retardant polymer composition. The composition includes a polymeric base made of at least one polyethylene homopolymer or copolymer having a density of 0.94 g/cm.sup.3 at most. The composition further includes 60-64% by weight of a metal hydroxide, at least 2% by weight of an ammonium coated montmorillonite having average particle dimensions of from 5 to 20 μm, and a polysiloxane.

Claims

1. A flame-retardant electric cable having a core comprising at least one electric conductor, an electrically insulating coating, and an outermost layer made from a thermoplastic, low smoke zero halogen flame-retardant polymer composition, wherein the substantially thermoplastic low smoke zero halogen flame-retardant polymer composition comprises: a) a polymeric base made of at least one polyethylene homopolymer or copolymer having a density of 0.94 g/cm.sup.3 at most; b) 60-64% by weight of a metal hydroxide; c) at least 2% by weight of an ammonium coated montmorillonite having average particle dimensions of from 5 to 20 μm; and d) a polysiloxane.

2. The flame-retardant electric cable according to claim 1, wherein the outermost layer is a jacket.

3. The flame-retardant electric cable according to claim 1, wherein the outermost layer is a skin layer.

4. The flame-retardant electric cable according to claim 3, wherein the skin layer has a thickness of from 0.05 to 0.5 mm.

5. The flame-retardant electric cable according to claim 1, wherein the polymeric base is made of at least one polyethylene copolymer.

6. The flame-retardant electric cable according to claim 5, wherein at least one polyethylene copolymer is a metallocene linear low density polyethylene.

7. The flame-retardant electric cable according to claim 1, wherein the polyethylene homopolymer or copolymer have a density in the range of 0.86 to 0.92 g/cm.sup.3.

8. The flame-retardant electric cable according to claim 1, wherein the metal hydroxide is magnesium hydroxide.

9. The flame-retardant electric cable according to claim 1, wherein the amount of montmorillonite is comprised in the range from 2 to 8% by weight.

10. The flame-retardant electric cable according to claim 1, wherein the ammonium coated montmorillonite contains dimethyl, di(hydrogenated tallow) ammonium.

11. The flame-retardant electric cable according to claim 1, wherein the polysiloxane is comprised in the range from 1 to 2% by weight.

12. The flame-retardant electric cable according to claim 1, wherein the polysiloxane is polydimethylsiloxane.

Description

(1) The features and advantages of the present disclosure will be made apparent by the following detailed description of some exemplary embodiments thereof, provided merely by way of non-limiting examples, description that will be conducted also by referring to the attached drawings, wherein

(2) FIG. 1 is a cross-sectional view of an electric cable according to the present disclosure; and

(3) FIG. 2 is a cross-sectional view of another electric optical cable according to the present disclosure.

(4) FIG. 1 shows a cable 10 according to a non-limiting embodiment of the disclosure. Cable 10 has a core comprising a conductor 11 made of an electrically conductive material, e.g. aluminium, copper, carbon nanotubes or composite thereof. The conductor 11 may be in the form of a solid bar or a of bundle of wires, preferably stranded.

(5) In the cable of the disclosure, the core may include a single conductor or preferably a plurality of conductors.

(6) The conductor 11 is electrically insulated by an insulating layer 12 in form of an extruded polymeric coating optionally having flame-retardant properties. For example, the insulating layer 12 can be made of an extruded polymeric material such as polyethylene or a polyethylene mixture, optionally filled with flame-retardant fillers, such as magnesium or aluminium hydroxide.

(7) In the embodiment shown in FIG. 1, the insulating layer 12 is extruded in direct contact with the conductor 11.

(8) Cable 10 comprises a jacket 13a as outermost layer, made of the polymeric material having flame-retardant properties according to the present disclosure. The jacket 13a surrounds the insulating layer 12 and, optionally, is in direct contact thereof. The jacket 13a is manufactured by extrusion. The jacket 13a has a thickness suitable for providing the cable with mechanical protection.

(9) FIG. 2 shows a cable 20 according to another non-limiting embodiment of the disclosure. In the cable 20, those features that are structurally and/or functionally equivalent to corresponding features of the cable 10 described above will be assigned the same reference numbers of the latter and will not be further described for conciseness.

(10) The cable 20 differs from the cable 10 described above in that the outermost layer is a skin layer 14, made of the polymeric material having flame-retardant properties according to the present disclosure. The skin layer 14 surrounds and directly contacts the jacket 13b. The skin layer 14 is manufactured by extrusion. The skin layer 14 has a thickness substantially smaller than that of the jacket 13b (of from 0.05 to 0.5 mm, for example of from 0.1 to 0.2 mm) and does not provide significant mechanical protection to the cable 20.

(11) In this embodiment, the jacket 13b is made of a LS0H polymer composition. A composition suitable for the jacket of the present cable is, for example, similar to that use for the outermost layer but lacking any ammonium coated montmorillonite.

(12) The outermost layer of the cable of the disclosure, being either a jacket (as in the case of the cable 10 of FIG. 1) or a skin layer (as in the case of the cable 20 of FIG. 2) is made from a flame-retardant polymer composition described above.

(13) A polyethylene homopolymer or copolymer as polymeric base of the present polymer composition can be a homopolymer of ethylene (such as a low density polyethylene, LDPE; very low density polyethylene, VLDPE; ultra low density polyethylene, ULDPE) or a copolymer of ethylene with one or more alpha-olefins having 3 to 12 carbon atoms, for example 4 to 8 carbon atoms, and, optionally, comprising a diene, such as an ethylene-propylene rubber (EPR) or a linear low density polyethylene (LLDPE). Examples of the alpha-olefins are propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene.

(14) In an embodiment, the polymeric base of the present polymer composition is made of at least one polyethylene copolymer, like a linear low density polyethylene (LLDPE), or of a mixture thereof.

(15) In an embodiment, at least one polyethylene copolymer of the polymeric base of the present polymer composition is a metallocene LLDPE.

(16) The polyethylene homopolymer or copolymer have a density of 0.94 g/cm.sup.3 at most. In an embodiment, the polyethylene homopolymer or copolymer have a density in the range of 0.86 to 0.92 g/cm.sup.3.

(17) The flame-retardant polymer composition of the present disclosure further comprises a metal hydroxide in amount of 60 to 64 wt % corresponding to about 180-220 phr.

(18) If the amount of the least metal hydroxide is below 60 wt %, the flame retardancy properties may be insufficient whereas if the amount of the least metal hydroxide is above 64 wt % the mechanical properties of the flame-retardant composition, especially its elongation characteristics, might be reduced to an inacceptable extent.

(19) In an embodiment, the metal hydroxide is selected from magnesium hydroxide, aluminium hydroxide or a combination thereof. An example of metal hydroxide suitable for the present cable is magnesium hydroxide, for example of natural origin (brucite), optionally surface-treated.

(20) The flame-retardant polymer composition of the present disclosure further comprises an ammonium coated montmorillonite having average particle size dimensions of from 5 to 20 μm as inorganic fillers.

(21) The montmorillonite having average particle size dimensions of from 5 to 20 μm may be naturally occurring and is preferably layered. In an embodiment, the naturally occurring montmorillonite may be purified according to conventional purification processes before its use in the flame-retardant polymer composition of the present disclosure.

(22) The amount of the ammonium coated montmorillonite in the flame-retardant polymer composition is at least 2 wt % (corresponding to about at least 5 phr).

(23) In an embodiment, the amount of ammonium coated montmorillonite in the flame-retardant polymer composition is in the range from 2 to 8 wt % (corresponding to about 6-26 phr). An amount of ammonium coated montmorillonite lower than 2 wt % brings no substantial effect in the flame-retardant polymer composition; while an amount of ammonium coated montmorillonite greater than 8 wt % can cause a decrease of the mechanical feature of the composition, especially in term of tensile strength and elongation at break, thus making the composition no more suitable for an electric cable.

(24) Some of the cations (for example sodium ions) in the ammonium coated montmorillonite are exchanged, by surface treating the montmorillonite, with an ammonium cation-containing compound, such as a salt. Suitable ammonium coated montmorillonites for the present cable contain alkyl ammonium and polyol ammonium. In an embodiment, the ammonium coated montmorillonite contains (is surface treated with) dimethyl, di(hydrogenated tallow) ammonium.

(25) The cationic coating allows to increase the compatibility of montmorillonite with the polymeric matrix.

(26) Applicant experienced that the presence of ammonium coated montmorillonite having micrometric particle dimensions as indicated above in the flame-retardant polymeric composition used for the manufacture of the outermost layer of the cable not only contributes to improve the flame-retardant properties in combination with the metal hydroxide, but also increases significantly the resistance to dripping of the polymeric material forming such coating. In particular, Applicant experienced that montmorillonite having micrometric particle dimensions as indicated above promotes the formation of cohesive carbon residues (“char”) of such outermost layer when it is exposed to a flame, for example in the event of a fire, which results in a reduction of the dripping of the polymeric material forming such coating and in a significant increase of the fall time of incandescent fragments of the polymeric material, e.g. during the flame test.

(27) Thus, when the flame-retardant composition of the present disclosure is used for the manufacture of the outermost layer of the cable (e.g. the jacket or a skin layer coating the jacket), ammonium coated montmorillonite as specified above promotes, under fire, the formation a surface crust made substantially of cohesive carbon residues (char) and having reduced gas permeability, which protects the underlying part of the insulating coating preventing its burning and dripping for a significantly extended period of time.

(28) Furthermore, ammonium coated montmorillonite being a component with flame-retardant properties allows achieving the desired flame retardancy performances, while contributing to maintain advantageously good mechanical and workability characteristics of the flame-retardant polymer composition.

(29) The Applicant observed that the presence of ammonium coated montmorillonite eases the extrusion of the polymer composition containing it. In particular, the addition of ammonium coated montmorillonite in the above-mentioned amount decrease by 15-20% the viscosity of the polymer composition with respect to a composition not containing it.

(30) The flame-retardant polymer composition for the outermost layer of the cable of the disclosure further comprises a polysiloxane as processing aid.

(31) The amount of polysiloxane in the present flame-retardant polymer composition can range from 1 to 2 wt % (about 3-7 phr).

(32) The polysiloxane may be any compound comprising a main chain of repeating —Si—O— unit and side chains chosen from a linear or branched alkyl group having from 1 to 6 carbon atoms, a linear or branched alkoxy group having from 1 to 6 carbon atoms, a linear or branched alkenyl group, e.g. vinyl group, having from 1 to 6 carbon atoms, a phenyl group, a phenoxy group and their combinations.

(33) In an embodiment, the polysiloxane is polydimethylsiloxane.

(34) The presence of at least one polysiloxane in the flame-retardant polymer composition for use in the manufacture of the outermost layer of the cable improves the compatibility between the flame-retardant fillers (in particular metal hydroxides such as magnesium hydroxide) with the polymer matrix of the composition by increasing the interactions between the hydroxyl groups of the fillers and the polyolefin chains thereby improving the dispersion of the inorganic filler in the polymer matrix. Moreover, the use of the aforementioned polysiloxanes in the polymeric composition helps to reduce the viscosity during extrusion thus improving the workability of the flame-retardant polymer composition.

(35) The flame-retardant polymer composition may further comprise further conventional components such as antioxidants, processing aids, stabilizers, pigments, coupling agents, etc.

(36) Conventional antioxidants which are suitable for this purpose are by way of example: polymerized trimethyldihydroquinoline, 4,4′-thiobis (3-methyl-6-tert-butyl) phenol, pentaerythritol tetrakis [3-(3,5-di-terz-butyl-4-hydroxyphenyl) propionate], 2,2′-thio-diethylene-bis-[3-(3,5-di-tert-butyl-4-hydroxy-phenyl) propionate] and the like or mixtures thereof.

(37) Process aids usually added to the base polymer are, for example, calcium stearate, zinc stearate, stearic acid, paraffin wax, silicone rubbers and the like, and mixtures thereof.

(38) The lubricants used are, for example, paraffin waxes of low molecular weight, stearic acid, stearammide, oleammide, erucamide.

(39) The coupling agent may be used with the aim of further improving compatibility between the flame-retardant inorganic fillers such as magnesium hydroxide and polymer matrix. This coupling agent can be selected from those known in the art, for example: saturated silane compounds or silane compounds containing at least one ethylenic unsaturation; peroxides or mixtures thereof. As an alternative, a monocarboxylic acids or dicarboxylic acids anhydrides, optionally grafted onto the polymeric base, may be used.

(40) The electrical cable according to the present disclosure may be produced based on cable manufacturing techniques known to those skilled in the art. In particular, the outermost layer may be formed using conventional processes with a thickness chosen to comply requirements and needs of the particular application for the cable.

(41) The cable according to the disclosure can be used particularly for the transport of electric energy or data. In one embodiment, the cable according to the disclosure is used for the transport of low voltage electric currents (LV), i.e. electric currents having a voltage not exceeding 1 kV.

(42) The present disclosure will now be described with reference to the following examples which are provided for purpose of illustration only and thus are not to be construed as limiting the scope of the present disclosure in any way.

EXAMPLE 1

(43) Preparation of Test Compositions According to the Disclosure and Comparative Composition.

(44) A comparative flame retardant polymer base composition (hereinafter referred to as Sample A) and test compositions according to the disclosure (hereinafter referred to as samples from B to E) have been prepared by mixing, in an open mixer, polymers, fillers and additives as indicated in the following Table 1.

(45) The polymeric base was made of a mixture of metallocene LLDPE (having a density of 0.885 g/cm.sup.3) and of LLPDE (having a density of 0.911 g/cm.sup.3).

(46) The metal hydroxide was natural magnesium hydroxide with no surface treatment. As ammonium coated montmorillonite, montmorillonite A had average particles dimensions of 7-9 μm, while montmorillonite B had average particles dimensions of 15-20 μm.

(47) Table 1 shows the amounts of polyethylene base polymers and other additives and fillers in the compositions used to produce the outermost layer of the insulating coating of both comparative and test samples, where the comparative sample is marked with an asterisk.

(48) The amounts are provided as percent by weight on the total weight of the composition.

(49) TABLE-US-00001 TABLE 1 Sample Sample Sample Sample Sample Component A* B C D E Metallocene 22.45 21.77 21.32 21.77 21.32 LLDPE LLDPE 9.05 8.77 8.59 8.77 8.59 Mg(OH).sub.2 64.13 62.21 60.92 62.21 60.92 Montmorillonite — 3.00 5.00 — — A Montmorillonite — — 3.00 5.00 B Polydimethyl 1.50 1.45 1.42 1.45 1.42 siloxane Stearic acid 2.00 1.94 1.90 1.94 1.90 Additives 0.88 0.85 0.83 0.85 0.83

EXAMPLE 2

(50) Tests on dripping under fire conditions.

(51) The samples obtained according to Example 1 were tested to determine their dripping behaviour under fire conditions and for mechanical properties.

(52) The dripping tests were aimed at detecting the time of falling the first piece of the samples (fall time) and the formation of cohesive carbon residues (chars) on them under fire conditions (anti-drop effect).

(53) With regard to the evaluation of fall time and mechanical properties, three specimens from plates having dimensions 35×150 mm and 2.7 thick were obtained from each sample.

(54) Each specimen of a sample was clamped vertically from an upper end portion with a clamp fixed on a support while the lower end was free. The specimen was then burned under the action of a flame produced by a bunsen fed with air at a flow rate of 3.8 ml/min and with liquid propane gas (LPG) at a flow rate of 0.65 ml/min, maintaining the ratio between mass flow rates used.

(55) The flame was oriented at about 90° with respect to the specimen and directed towards the lower edge of the specimen at the shorter side thereof.

(56) The flame was held in this position for all the time of the experiment and the time from approaching the flame at the lower edge of the specimen until the first piece from the specimen falls off (fall time) was recorded. The results are set forth in Table 2.

(57) TABLE-US-00002 TABLE 2 Sample Fall time (seconds)  A* 53 B 84 C 117 D 74 E 116

EXAMPLE 3

(58) Mechanical Properties and Viscosity

(59) The tensile strength of all the samples was evaluated according to IEC 60811-1-1 (1996). Samples B to E according to the disclosure had a tensile strength about 25% lower than that of sample A, but within the standard requirement anyway. The elongation at break of all the samples was evaluated according to IEC 60811-1-1 (1996). Samples B and D according to the disclosure had an elongation at break about 5% higher than that of sample A, while samples C and E according to the disclosure had an elongation at break about 10% lower than that of sample A but within the standard requirement anyway.

(60) The viscosity of all the samples was evaluated according to ISO 289-1 (2015). Samples B to D according to the disclosure had viscosity about 15-20% lower than that of sample A.