Flame retardant, low smoke, cold resistant electric cable

Abstract

The present invention concerns an electric cable (10; 20) comprising: —an electric conductor (11, 21) surrounded with an electrically insulating layer (12, 22) made from a first polymer composition comprising: (a) polyvinyl chloride as a first polymer matrix, (b) an amount up to 45 phr of a plasticizing system consisting of a phthalate plasticizer and a second plasticizer selected from: dialkyl-adipate, dialkyl-sebacate or dialkyl-azelate, wherein the alkyl group contains from 4 to 10 carbon atoms; the first polymer composition having a limiting oxygen index (LOI) up to 30%; and—an outer sheath (13, 24), surrounding the insulating layer (12, 22), made from a second polymer composition comprising: (i) a halogen-free second polymer matrix; and (ii) a halogen-free inorganic flame-retardant filler in amount suitable to impart flame retardant properties to the outer sheath (13, 24). The cable of the present invention exhibits flame retardancy, low smoke and cold resistance properties and is particularly suitable for low voltage power transmission or for telecommunications.

Claims

1. An electric cable comprising: an electric conductor surrounded with an electrically insulating layer of a first polymer composition comprising: (a) polyvinyl chloride as a polymer matrix, (b) an amount up to 45 phr of a plasticizing system consisting of a phthalate plasticizer and a second plasticizer selected from: dialkyl-adipate, dialkyl-sebacate or dialkyl-azelate, wherein the alkyl group contains from 4 to 10 carbon atoms; the first polymer composition having a limiting oxygen index (LOI) up to 30%; and an outer sheath, surrounding the insulating layer, made from a second polymer composition comprising: (i) a halogen-free polymer matrix; and (ii) a halogen-free inorganic flame-retardant filler in amount suitable to impart flame retardant properties to the outer sheath.

2. The cable according to claim 1 wherein the phthalate plasticizer is selected from the group consisting of di(2-ethylhexyl) phthalate (DEHP), diisononyl phthalate (DINP), diisodecyl phthalate (DIDP), diisotridecyl phthalate (DITP) and a mixture thereof.

3. The cable according to claim 1, wherein the alkyl group of the second plasticizer contains from 6 to 10 carbon atoms.

4. The cable according to claim 1 wherein the plasticizing system is present in the first polymer composition in a total amount of from 30 phr to 45 phr.

5. The cable according to claim 1 wherein the first polymer composition further comprises a halogen-free and antimony-free flame retardant agent.

6. The cable according to claim 1 wherein the polyvinyl chloride (a) has a K value of at least 65 or higher.

7. The cable according to claim 1 wherein the first polymer composition further comprises a heat stabilizer.

8. The cable according to claim 7 wherein the heat stabilizer is selected from the group consisting of a zinc salt a calcium/zinc stabilizer and a mixture thereof.

9. The cable according to claim 1 wherein the insulating layer has a LOI of at least 20%.

10. The electrical cable according to claim 1, wherein the halogen-free flame-retardant filler of the second polymer composition is selected from the group consisting of magnesium hydroxide, aluminum trihydrate and mixtures thereof.

11. The electrical cable according to claim 1 wherein the halogen-free flame retardant filler is present in a total amount of from 80 to 500 phr.

12. The cable according to claim 1 wherein the second polymer composition comprises a polymer selected from the group consisting of polyethylene; polypropylene; polybutene; poly(4-methylpentene-1); ethylene copolymers with C.sub.3 to C.sub.8 alpha-olefins; copolymers of C.sub.2 to C.sub.8 alpha-olefin/s and a diene; ethylene-acrylate copolymer; ethylene-vinyl acetate copolymers; ethylene-butyl acetate copolymers and mixtures thereof.

13. The cable according to claim 1 wherein the second polymer composition of the outer sheath has a LOI greater than that of the first polymer composition of the insulating layer.

14. The cable according to claim 1 wherein the second polymer composition has a LOI greater than 35%.

15. The cable according to claim 1 wherein the second polymer composition has a LOI lower than 70%.

16. The electrical cable according to claim 1 wherein the halogen-free flame retardant filler is present in a total amount of from 120 to 300 phr.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) With reference to FIG. 1, the electric cable 10 is of the unipolar type comprising a single conductor 11 covered by and in direct contact with an electrically insulating layer 12. The insulating layer 12 is encircled by and in direct contact with an outer sheath 13. The insulating layer 12 and the outer sheath 13 are made of a PVC based first polymer composition and of a second polymer composition according to the present invention, respectively. The conductor 1 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.

(5) With reference to FIG. 2, the electric cable 20 according to the present invention may be of the tripolar type comprising three electrically conducting elements or conductors 21 each covered by and in direct contact with an electrically insulating layer 22. The three conductors 21 with the relevant insulating layers 22 are encircled by an outer sheath 24. The three coated conductors 1 are stranded together forming interstitial zones defined as the spaces between the coated conductors and the outer sheath 24) enveloping them.

(6) A bedding or interstitial filler 23 fills said interstitial zones. The bedding 23 can be made of a thermoplastic like material, for example polyethylene, preferably added with a zero-halogen flame-retardant filler like magnesium hydroxide in an amount, for example, of from 50 phr to 200 phr.

(7) In an alternative embodiment, the cable 20 may comprise two or four conductors 21.

(8) Both the cable 10 and 20 of FIGS. 1 and 2, respectively, may further comprise a mica tape layer interposed between each of the conductor 11 and 21 and the respective insulating layer 12 and 22.

(9) Both the cable 10 and 20 of FIGS. 1 and 2, respectively, may further comprise an armour surrounding the outer sheath 13 and 24. The armour can be made of metal wires, for example steel wires, preferably galvanized. In turn, the armour may be surrounded by a polymeric jacket, preferably made of a LS0H polymeric composition. A tape, for example a polyester tape, is advantageously interposed between the armour and the polymeric jacket.

(10) The cable 20 of FIG. 2 may further comprise a central strength element around which the conductors 22 are stranded, said central element being made of, for example, polyethylene, preferably added with a zero-halogen flame-retardant filler like magnesium hydroxide in an amount, for example, of from 50 phr to 200 phr.

(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.

(13) Preparation of the First Polymer Composition (Insulating Layer)

(14) The first polymer compositions of the examples reported in Table 1 were prepared in a closed turbomixer (volume of the mixing chamber: 3000 cm.sup.3). The amounts are reported as phr (parts by weight with respect to 100 parts by weight of the PVC polymer). PVC and stabilizer, in form of powder, were added in the turbomixer and thoroughly mixed very well. Then plasticizers and the other mixture components were added and the composition mixed to 180° C.

(15) The examples marked by the asterisk (*) are the comparative ones.

(16) The compositions of Examples 1-9 of Table 1 were printed in form of plaques by printing at 180° C. using a mechanical press and then tested.

(17) Electric cables were prepared using the compositions of examples 1-9 as insulating layers of tripolar cables (as the cable of FIG. 2) wherein each cable conductor was 2.5 mm.sup.2. The outer sheath of each tested cable was made of a second polymer composition comprising LLDPE as second polymer matrix and 230 phr of magnesium hydroxide as flame-retardant filler. Each sample of electric cable tested was 60 m long.

(18) A number of properties were evaluated both on the plaques and the cables and the results are set forth in Table 2.

(19) TABLE-US-00001 TABLE 1 Composition of samples 1 2 3 4 5 6* 7* PVC.sup.(a) (polymer 100 100 100 100 100 100 100 matrix) DIDP.sup.(b) (phthalate 12.5 12.5 12.5 12.5 20.0 17.0 12.5 plasticizer) Dioctyl adipate — 25.0 — — — — — (second plasticizer) Bis(2-ethylhexyl) 25.0 — 25.0 30.0 25.0 30.0 30.0 azelate (second plasticizer) Epoxidized soya — — — — — — 2.7 bean oil (plasticizer) Chalk (CaCO.sub.3) — — 13.0 13.0 — 13.0 13.0 (filler) Mg(OH).sub.2 (stearic 13.0 13.0 — — 13.0 — — acid coated) Ca/Zn (heat 3.5 3.5 3.5 3.5 3.5 3.5 3.5 stabilizer) Total amount of 37.5 37.5 37.5 42.5 45.0 47.0 45.25 plasticizers .sup.(a)K value: 70 .sup.(b)DIDP: diisodecyl phthalate

(20) TABLE-US-00002 TABLE 2 Measured properties of samples 1 2 3 4 5 6* 7* Volume resistivity (on 2.98 × 10.sup.13 —  0.8 × 10.sup.13 0.68 × 10.sup.13 0.54 × 10.sup.13 0.33 × 10.sup.13 0.30 × 10.sup.13 plaque) (ohm .Math. cm) Volume resistivity (on 2.98 × 10.sup.13 1.71 × 10.sup.13 1.65 × 10.sup.13 1.15 × 10.sup.13 1.16 × 10.sup.13 — — cable) (ohm .Math. cm) Limiting Oxygen Index 25 25 24 24 24 — 22 Bending Test at −52° C./4 Positive Positive — — — — — hours (IEC 60811-504, (no cracks) (no cracks) 2012) Impact Test at −52° C./4 Positive Positive — — — — — hours (IEC 60811-506, (no cracks) (no cracks) 2012) Elongation Test at −52° C./ 84.5% 81% — — — — — 4 hours (IEC 60811- 505, 2012)

(21) The volume resistivity was measured according to IEC 60502-1 (2004).

(22) The mechanical tests at −52° C. and the smoke density test were performed on cables. The experimental data show that the compositions 1-5 according to the invention had a volume resistivity equal to or higher than 1×10.sup.13, fulfilling the requirement for insulating layer according to IEC 60502-1 (2004). On the contrary, comparative compositions 6 and 7 had a very low volume resistivity. Without wishing to be bound to any theory, the unacceptable volume resistivity of comparative composition 6 could be due to an excessive amount of plasticizing system (47 phr) and the unacceptable volume resistivity of comparative composition 6 could be due to the presence of epoxidized soya bean oil in the plasticizing system.

(23) The cables comprising an insulating layer according to the invention (compositions 1 and 2) maintained adequate mechanical properties at temperatures of −50° C. or lower.