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The present invention relates to a process for the production of flame retardant compositions, particularly suited to the production of self- extinguishing cables.

Self-extinguishing cables can be produced by coating the cable itself with a sheath having flame- retardant properties, consisting of a polymeric material to which a product suitable to achieve flame-retardant properties (i.e. flame retardants) has been added, that has the capability of preventing the spread of the flame along the cable in case of fire. Usually, in case of cables with more conductive elements, the flame retardant material is also used as a filler of the spaces present among said conductive elements.

For the production of a sheath or flame-retardant bedding, a composition based on polyolefins is usually used, for example polyethylene or ethylene/vinylacetate copolymers added with an organic halide combined with antimony trioxide, which work as flame retardants. However, the use of halogenated flame retardant additives has many drawbacks, for example during the production of the composition and/or of the cable they can partially decompose, forming halogenated gases that may be toxic or otherwise harmful to production workers and can corrode the metal components of the production plant. In addition, when exposed to a flame action, their combustion generates large amounts of smoke also containing toxic halogenated gases. The same drawbacks are noticed using, as flame -retardant composition, polyvinylchloride (PVC) , possibly added with antimony trioxide.

In recent years flame retardant compositions free of halogenated components (HFFR=Halogen Free Flame Retardant) have been developed, using as flame retardant mineral fillers, particularly hydrated oxides or aluminum and/or magnesium hydroxides, for example, hydrated alumina or magnesium hydroxide, which, when exposed to high temperatures, decompose through endothermic reaction and release large quantities of water, so as to stop the flame propagation .

However, in order to obtain a satisfactory flame retardant effect, it is generally necessary to use large amounts of mineral filler (for example, for magnesium hydroxide, about 120-250 parts by weight with respect to 100 parts by weight of polymer base) . These large quantities lead to a significant reduction of the processability of the composition and of the mechanical properties of the resulting sheath, in particular as regards impact resistance, elongation and stress at break.

Patent US-5.707.732 refers to an electrical cable or for telecommunications coated with a flame- retardant composition comprising 100 parts by weight of a polymer mixture and 5 to 250 parts by weight of flame retardant filler. This latter can be magnesium hydroxide or aluminum trihydrate, while the polymer mixture consists of: (i) a polyethylene synthetized by using a metallocene single-site catalyst and having a M _w /M _n ratio not greater than 3; (ii) a polyethylene synthetized by using a transition metal catalyst other than a metallocene single-site catalyst and having a M _w /M _n ratio greater than 4, and optionally (iii) a copolymer of ethylene with an unsaturated ester or a very low density polyethylene having a density of not greater than 0.915 g/cm ³ wherein the polymers (i) or (ii) are modified with an unsaturated aliphatic diacid anhydride through grafting or copolymerization .

Patent application WO 99/05688 describes self- extinguishing cables and related flame retardant compositions, wherein these compositions comprise:

(a) a crystalline propylene homopolymer or copolymer;

(b) a copolymer of ethylene with at least one alpha- olefin, and optionally with a diene, characterized by a composition distribution index (CDI) greater than 45%, said index being defined as the percentage by weight of the copolymer molecules having a content of alpha-olefin within 50% with respect to the average total mole content of alpha-olefin; (c) natural magnesium hydroxide in an amount sufficient to impart flame-retardant properties.

As regards the crystalline propylene homopolymer or copolymer, this generally has a melting enthalpy greater than 75 J/g, preferably greater than 85 J/g, and ensures the final composition sufficient resistance to thermopressure , an essential characteristic for the realization of a self- extinguishing cable to meet the market specifications. As regards the copolymer of. ethylene with at least one alpha-olefin (b) , this is obtained by ethylene copolymerization with at least one alpha- olefin in the presence of a "single-site" catalyst, such as a metallocene catalyst, and it is characterized by a narrow distribution of molecular weights, in particular by a molecular weight distribution index (MWD) lower than 5.

Patent application WO 00/19452 describes self- extinguishing cables coated with a flame-retardant composition which includes: (a) an ethylene homopolymer or copolymer having a density of from 0.905 to 0.970 g/cm ³ , selected from: ethylene homopolymers , copolymers of ethylene with an alpha- olefin, copolymers of ethylene with an ethylenically unsaturated ester, or mixtures thereof; (b) a copolymer of ethylene with an alpha-olefin, and optionally with a diene, said copolymer (b) having a density ranging from 0.860 to 0.904 g/cm ³ , and being characterized by a composition distribution index (CDI) higher than 45%; (c) natural magnesium hydroxide in a quantity sufficient to impart flame retardant properties; wherein at least one of the polymeric components (a) and (b) contains hydrolysable organic silane groups grafted on the polymer chain. The silane groups are preferably added during the production of the flame-retardant composition by addition of an appropriate silane compound and a radical initiator.

The Applicant has realized that, for the production on an industrial scale of HFFR compositions as those described above, it is generally necessary to use devices having a high mixing efficiency, such as co-rotating twin-screw extruders, characterized by high rotation speed of the screws (for example, for a cylinder with a diameter of about 90 mm, the rotation speed can also be higher than 1000 rpm) and by high length, both to ensure homogeneous mixing of the filler in the polymer base and also to carry out grafting reactions during the extrusion (the so-called "reactive extrusion"), in particular to modify the polymers through in-situ grafting of coupling agents such as silanes or maleic anhydride. Typically the length of such extruders is such as to obtain a L/D value higher than 40 (L = cylinder length; D = cylinder diameter) . Moreover, they are usually provided with different feeding zones, one or two degassing zones, and high power engines (for example: for a cylinder of about 90 mm diameter, powers also over 1000 kW) .

Thanks to these features, this machinery allows to obtain a good mixing both of dispersive and distributive type. "Distributive mixing" means the capability of the mixer to distribute homogeneously the filler particles evenly within the polymer matrix, thus avoiding the presence of filler aggregates in the finished product. On the contrary, "dispersive mixing" means the capability of the mixer to disgregate up the particles having a size greater than a prefixed value. In this type of extruders, mixing is carried out in the interpenetration area of the screw threads, while the flow channel of the material is longitudinally open, so reflux phenomena (back-flow) may occur, with consequent difficulties in temperature control and homogeneous flowing of the material. This process, however, presents the advantage of high productivity thanks to the high flow rates which may, however, be limited by the maximum temperature at which the material can be processed. For this reason, in order to reach the best productivity of such extruders, it is recommended to use materials that can be processed at high temperatures .

Another kind of continuous mixing machines is based on single screw co-kneader extruders where, simultaneously with the rotational movement, the screw is making an alternative movement in the axial direction. On the wall of the cylinder a series of pins are fitted: the interaction between these and the screw movement ensure a good level both of dispersive and distributive mixing, even for limited cylinder lengths (L/D between 8 and 30) and for low screw speeds (for example: for a cylinder having diameter of about 100 mm, the screw speed can reach 500 rpm) . In this way a gentle mixing is also guaranteed, without application of high powers (for example: for a cylinder of about 100 mm diameter, generally the power does not exceed 300 kW) and a high control of the mixing temperature. Thanks to this high versatility, these machines are suitable to the processing of materials of very different nature (such as plastics or rubbers) and also of heat- sensitive materials (for example, the mixing of rubber in the presence of thermo-sensitive crosslinking agents such as peroxides or PVC, where often machines with a L/D ratio lower than 10 are used) .

For the production of PVC-based flame-retardant compositions, less performing machines are widely used, such as counterrotating twin-screw extruders, characterized by relatively low screw rotation speed (for example: for a cylinder of 90 mm diameter, the rotation speed is generally lower than 100 rpm) and by limited length, with a L/D value not higher than 30, typically of 20-25 or, for upgraded machines, to L/D values up to 40. They are also equipped with a single feeder positioned in the initial part of the extruder and a single degassing zone; as these machines have simply to gel the composition, they are equipped with low power engines (for example: for a cylinder having a diameter equal to about 90 mm, the power is generally lower than 150 kW) . In this type of extruders, mixing is carried out mainly by the action of calendering between the screws, thus obtaining a good dispersion thanks to the high mechanical work due to the elongational flow, while the distributive mixing is quite poor. This hinders the possibility of correcting, during the passage in the extruder, any composition dishomogeneity due to feeding irregularities. Furthermore, the output of these extruders is limited as the movement of the material between the screws produces a force, orthogonal to the screw axis, which can push these latter towards the internal walls of the cylinder, and therefore the flow-rate cannot be too high for avoiding contact between screws and cylinder, which would cause a premature machinery wear. On the other hand, the counter- rotating twin screw extruders have the advantage of allowing a good temeprature control and therefore the material to be extruded does not overheat, the energy transmitted by the engine being efficiently utilized for the transport of the material itself. Moreover, in such extruders the flow channel is longitudinally closed, since at each step the thread of a screw is closed by the thread of the other screw: this prevent the material from reflowing (back flow) . Therefore, the counter-rotating twin-screw extruders are particularly used for the production of PVC-based compositions or other thermo- sensitive materials in which a high quantity of mixing energy is not required; all the ingredients are typically pre-mixed in the form of powders thus ensuring a substantially homogeneous feeding composition.

The Applicant has now faced the problem of providing low cost halogen- free flame retardant compositions, comprising a flame-retardant inorganic filler (HFFR compositions) , which have the required flame-retardant and mechanical properties in order to meet the specific demands of the market, and at the same time, can be produced at industrial scale even through relatively simple conventional machinery, such as those normally used for the production of PVC-based compositions, without requiring more complex and expensive mixing equipment, such as co- rotating twin screw extruders or single co-kneader extruders with high L/D, having characteristics as indicated above, as well as control systems for the continuous dosing of the required raw materials. This simplification would allow manufactures of flame retardant compositions, including both composition manufacturers or cable makers with equipment in its facility for the production of compositions, to use the same mixing machines for the production of both PVC-based compositions and HFFR compositions, in order to allow greater production flexibility without expensive investments for purchase of different machineries for the production of flame-retardant compositions. Therefore: a) manufacturers, already provided with machinery for the production of PVC as described above, would have the possibility to produce HFFR compositions on the same plant with a very limited investment;

b) manufacturers which wish to have a single type of machinery, suitable both for the production of PVC and HFFR compositions, could invest in cheaper machinery, also of second hand, as those described above for PVC, thus greatly shortening the Return on Investment (ROI) .

The Applicant has now found that these objectives and others better indicated hereinafter can be achieved by a process as defined hereafter, which provides for:

use of counterrotating twin screw extruders characterized by screws rotation speed and relatively low length of the same (L/D not greater than 40) or single-screw co-kneader extruders with low L/D (not greater than 16), thanks to a rheological behaviour of the composition such as to minimize the elastic component and thus limiting the torque of the extruder engine in order to allow the use of machines with relatively low powerful engines;

use of raw materials, both inorganic and polymeric, in the form of powders in order to improve the homogeneity of the final mixture thanks to a maximization of the distributive mixing deriving from a reduction of the energy used for the dispersive mixing, which, as explained above, is difficult to implement in a counterrotating twin screw extruder or in a single screw co-kneader extruder with low L/D values, typically used for the production of PVC- based compositions;

use of a polymeric matrix consisting of at least 10% by weight, with respect to the total weight of the polymeric matrix, of at least one polyolefin having a melting temperature (T _f ) not greater than 100°C and a melting enthalpy (ΔΗ _£ ) not greater than 110 J/g.

According to a first aspect, the present invention therefore relates to a process for the production of a flame-retardant composition including a polymeric matrix and at least one inorganic filler with flame retardant properties, said process comprising :

feeding an extruder with said polymeric matrix in a powder form and said at least one inorganic filler in a powder form, said polymeric matrix comprising at least 10% by weight,- with respect to the total weight of the polymeric matrix, of at least one polyolefin having a melting temperature (T _f ) not higher than 100°C and a melting enthalpy (ΔΗ _£ ) not higher than 110 J/g;

inside said extruder, melting said polymeric matrix and mixing it with said at least one inorganic filler in the presence of at least one coupling agent ;

discharging the so obtained composition from the extruder ;

said extruder being selected from: counterrotating twin-screw extruders having a L/D value ( length/diameter of the extrusion cylinder) not higher than 40; single screw co-kneader extruders with a L/D value not higher than 16. In a preferred embodiment, said polymeric matrix and said at least one inorganic filler are pre-mixed in a powder form in a mixer before being fed to the extruder .

Preferably, said polymeric matrix in a powder form has an average particle size ranging from 50 i to 1500 μπι, preferably from 100 μπι to 500 μιη.

Preferably, said at least one inorganic filler in a powder form has an average particle size ranging from 0.5 μπι to 20 μπι, preferably from 1.5 μπι to 5 μπι.

Preferably, said at least one polyolefin having a melting temperature ( Tf ) not greater than 100°C and a melting enthalpy (ΔΗ _£ ) not greater than 110 J/g, is present in the polymeric matrix in an amount of at least 20% by weight. Preferably, this amount is lower or equal to 80% by weight.

Preferably, said at least one polyolefin having a melting temperature ( T _f ) not higher than 100°C and a melting enthalpy (AH _f ) not higher than 110 J/g, is selected from:

(a) copolymers of ethylene with at least one alpha-olefin C ₃ - Ci ₂ , having a density of from 0.860 to 0.905 g/cm ³ , preferably from 0.865 to 0.900 g/cm ³ , and a molecular weight distribution index (MWDI) not greater than 4, preferably from 1.5 to 3.5;

(b) copolymers of ethylene with at least one ester having ethylenic instauration;

(c) ethylene/propylene copolymers including 5 to 25% by weight of ethylene, 75 to 95% by weight of propylene, and optionally a quantity not exceeding 10% by weight of a diene, said copolymers having a molecular weight distribution index (MWDI) ranging from 1 to 5 ; (d) heterophasic propylene/ethylene copolymers, comprising: a thermoplastic phase consisting of a propylene homopolymer or copolymer with ethylene and/or an alpha-olefin C ₄ - Ci ₂ , and an elastomeric phase consisting of a copolymer of ethylene with an alpha-olefin C ₃ -C _12;

or mixtures thereof .

Preferably, said at least one polyolefin has a melting temperature (T _f ) ranging from 45° to 95 °C and a melting enthalpy (ΔΗ _£ ) from 20 to 100 J/g.

In the present description and in the attached claims, except where otherwise indicated, the percentages by weight of the different components are indicated with respect to the total weight of the polymeric matrix.

In the present description and in the attached claims, with "alpha-olefin C ₃ - Ci ₂ " it is meant an olefin of formula CH ₂ =CH-R, wherein R is a linear or branched alkyl having from 1 to 10 carbon atoms. Preferably, the alpha-olefin is a C ₄ -C ₈ alpha-olefin. The alpha-olefin is preferably selected from: 1- butene, 1-pentene, 4 -methyl - 1 -pentene , 1-hexene, 1- octene, 1-dodecene.

The melting temperature (T _f ) and the melting enthalpy ( AHf ) are determined by Differential Scanning Calorimetry (DSC) measurements, according to conventional methods. The melting temperature is measured at the maximum peak of the DSC curve .

The molecular weight distribution index (MWDI) can be determined, according to conventional methods, by means of Gel Permeation Chromatography.

As regards the copolymer of ethylene with at least one alpha-olefin (a) , this preferably has the following monomer composition: 75-97% by mole, preferably 90-95% by mole, of ethylene; 3-25% by mole, preferably 5-10% by mole, of at least one alpha-olefin C ₃ -C ₁₂ ; 0-5% by mole, preferably 0-2% by moles, of at least one diene.

Preferably, said at least one diene is selected from: conjugated or non-conjugated linear C ₄ -C ₂ o diolefins, (for example 1 , 3 -butadiene , 1 , -hexadiene , 1 , 6 -octadiene) ; monocyclic or polycyclic dienes (e.g. 1 , 4 -cyclohexadiene , 5-ethylidene-2-norbornene, 5- methylene-2 -norbornene) .

Said copolymer of ethylene with at least one alpha-olefin (a) can be produced by copolymerization of ethylene with at least one alpha-olefin, and optionally with a diene, in the presence of a single- site catalyst, in particular a metallocene catalyst as described, for example, in patents US-5,246,783 and US-5 , 272 , 236. The metallocenes used as olefins catalysts are generally coordination complexes between a transition metal, usually of Group IV, in particular titanium, zirconium or hafnium, and two cyclopentadienylic ligands, which are optionally substituted, used in combination with a co-catalyst, for example an alumoxane, preferably a methylalumoxane , or a boron compound (see, for example, J. Organometallic Chemistry, 479 (1994), 1- 29, US-5, 414, 040, US- 5 , 229 , 478 , WO 93/19107, EP- 889,091, EP-632,065). Other single-site catalysts usable for the production of component (c) are the so-called "Constrained Geometry Catalysts", described, for example, in patents EP-416,815, EP- 418,044, US-5, 703, 187.

Examples of ethylene copolymers (a) are the commercial products: Engage™ of Dow Chemical, Exact™ of ExxonMobil Chemical and Tafmer™ of Mitsui Chemicals .

As regards the ethylene copolymers with at least one ester having ethylenic instauration (b) , they are generally copolymers of ethylene with at least one ester selected from: C _x -C ₃ (preferably Cx-Gj) alkyl acrylates, C _x -C ₈ (preferably Ci-C ₄ ) alkyl methacrylates , and vinyl C ₂ -C ₈ (preferably C ₂ -C ₅ ) carboxylates . The quantity of ester contained in the copolymer may generally vary from 5% to 50% by weight, preferably from 15% to 40% by weight. Examples of Ci-C ₈ acrylates and methacrylates are: ethyl acrylate, methyl acrylate, methyl methacrylate , tert-butyl acrylate, n-butyl acrylate, n-butyl methacrylate, 2-ethylhexyl acrylate, and the like. Examples of vinyl C ₂ -C ₈ carboxylates are: vinylacetate , vinylpropionate , vinylbutanoate , and the like.

Particularly preferred are ethylene-n- butylacrylate copolymers (EBA) , preferably having a content of n-butylacrylate ranging from 15 to 20%, which, compared to other copolymers such as EVA (ethylene-vinylacetate) having an equivalent comonomer content, have a decomposition temperature higher than about 30-50°C, therefore they allow the use of higher processing temperatures.

Ethylene copolymers (b) can be produced according to known processes, usually by high pressure copolymerization analogous to that used for LDPEs . Examples of ethylene copolymers of the type (b) as described above are commercial products: Lucofin™ of Lucobit, Escorene™ of ExxonMobil Chemical and Lotryl™ of Arkema .

As regards ethylene/propylene copolymers (c) , these preferably include from 5 to 15% by weight of ethylene, from 85 to 95% by weight of propylene, and optionally a diene quantity not exceeding 5% by weight. Dienes suitable as possible termonomers can be preferably selected from: 1 , 4 -hexadiene , 1,6- octadiene, 5-methyl-l, 4-hexadiene, 3 , 7-dimethyl-l , 6- octadiene, dicyclopentadiene (DCPD) , ethylidene norbornene (ENB) , or mixtures thereof. Particularly preferred is ethylidene norbornene (ENB) .

Preferably, the ethylene/propylene copolymers (c) have a triad tacticity value measured by ¹³ C-NMR spectroscopy, of at least 75%, more preferably at least 85%.

Preferably, the ethylene/propylene copolymers (c) have a melting enthalpy (AH _f ) of from 0.5 to 70 J/g, more preferably from 1 to 35 J/g. As regards the melting temperature (T _f ) , this corresponds to the maximum point in the DSC curve, which shows a single large widened peak, and possibly minor peaks which can be substantially neglected. The melting temperature (T _f ) of the copolymer (c) is preferably from 25° C to 100°C, more preferably from 30°C to 80°C.

Copolymers (c) as indicated above, are available on the market under the brand names Vistamaxx™ (ExxonMobil Chemical Co.) and Versify™ (The Dow Chemical Co . ) . Further details about these products can be found, for example, in the text of the patent application U.S. 2007/0134506.

As regards the heterophase propylene/ethylene copolymers (d) , these are usually produced by sequential copolymerization of: (a) propylene, optionally containing small quantities of at least one olefin comonomer selected from ethylene and alpha-olefin C ₄ -C ₁₂ , and subsequently of (b) a mixture of ethylene with an alpha-olefin C ₃ - Ci ₂ , preferably propylene, and optionally with small quantities of a diene. The so obtained copolymers are also known as "reactor thermoplastic elastomers". The production of said heterophasic copolymers is usually made with Ziegler-Natta catalysts based on halogenated titanium compounds supported on magnesium chloride. Processes of this type are described, for example, in patents EP 0 400 333 Al , EP 0 373 660 Al , US 5,286,564.

The thermoplastic phase of said heterophasic copolymers is essentially formed by a propylene homopolymer or a propylene copolymer with an olefin comonomer selected from ethylene and alpha-olefin C ₄ - Ci2 (preferably ethylene) . The quantity of comonomer is preferably not higher than 10% by mole with respect to the total moles of monomers forming the thermoplastic phase.

The elastomeric phase is preferably equal to at least 40% by weight, more preferably at least 50% by weight, with respect to the total weight of the heterophasic copolymer. It is essentially formed by an ethylene elastomeric copolymer with an alpha- olefin, preferably propylene, and optionally a diene. In a preferred embodiment, the composition of the elastomeric phase is the following: from 15 to 50% by weight, more preferably from 20 to 40% by weight, of ethylene; from 50 to 85% by weight, more preferably from 60 to 80% by weight, of propylene, with respect to the total weight of the elastomeric phase. Heterophase copolymers (d) as defined above are available on the market under the brand names Adflex™, Hifax™ and Softell™ (Lyondell Basell) .

In a preferred embodiment, said at least one polyolefin is mixed with at least another polyolefin having a melting temperature (T _f ) greater than 100°C and/or a melting enthalpy (ΔΗ _£ ) greater than 110 J/g, more preferably a melting temperature (T _f ) greater than 110° C and/or a melting enthalpy (AH _f ) greater than 115 J/g.

Said at least another polyolefin is preferably selected from:

(e) ethylene homopolymers or copolymers with at least one alpha-olefin C ₃ - Ci ₂ , said homopolymers or copolymers having a density of from 0.910 to 0.940 g/cm ³ , preferably from 0.915 to 0.930 g/cm ³ ;

(f) copolymers of ethylene with at least one alpha-olefin C ₃ - Ci ₂ , said copolymers having a density of from 0.870 to 0.909 g/cm ³ preferably from 0.880 to 0.905 g/cm ³ , and a molecular weight distribution index ( DI) greater than 4, preferably greater than 5.

As regards the ethylene homopolymer or copolymer (e) , this is preferably selected from: medium density polyethylene (MDPE) having a density of from 0.926 to 0.970 g/cm ³ ; low density polyethylene (LDPE) , linear low density polyethylene (LLDPE) , having a density of from 0.910 to 0.926 g/cm ³ and linear high density polyethylene (HDPE) having a density greater than 0.940 g/cm ³ .

Similarly to the component (f ) , the component (e) generally has a molecular weight distribution index (MWDI) higher than 4, preferably higher than 5.

This type of polymers is available on the market under different brand names (for example Flexirene™ of Polimeri Europa, ExxonMobil LLDPE LL of ExxonMobil Chemical) and they can be produced according to processes well known in the art. In particular, MDPE is generally produced by homopolymerization of ethylene at medium-low pressure in the presence of a Ziegler-Natta catalyst, with formation of a homopolymer having a very low branching degree. LDPE is generally produced by a high-pressure process wherein ethylene is homopolymerized in the presence of oxygen or a peroxide as initiator with formation of polyethylene chains having long branching (long branched PE) . LLDPE is a copolymer with short branchings, resulting from copolymerization of ethylene with an alpha-olefin C3-C12, preferably 1- butene, 1-hexene or 1-octene, and it is usually produced by means of low-pressure processes in the presence of a Ziegler-Natta catalyst or a chromium catalyst. The alpha-olefin C ₃ -Ci ₂ is preferably contained in the copolymer in a quantity ranging from 1 to 15% by mole.

As regards the component (f ) , this is preferably a very low density polyethylene (VLDPE) having a density of from 0.870 to 0.909 g/cm ³ , preferably from 0.880 to 0.905 g/cm ³ . These products are typically obtained by copolymerization of ethylene with an alpha-olefin C ₃ -C ₁₂ , such as 1-butene, 1-hexene, 1- octene, in the presence of: (i) a catalyst based on chromium and titanium; or (ii) a catalyst based on magnesium, titanium, a halogen and an electron donor; or (iii) a catalyst based on vanadium, an electron donor, an aluminum halide and a halogenated hydrocarbon as promoter. In a preferred embodiment, the process according to the present invention is used for the production of a flame-retardant composition including:

- from 10 to 80% by weight, preferably from 15 to 60% by weight, of at least one polyolefin, having a melting temperature (T _f ) not greater than 100°C and a melting enthalpy (ΔΗ _£ ) not greater than 110 J/g, selected from the products (a) , (b) , (c) and (d) indicated above;

- from 20 to 90% by weight, preferably from 40 to 85% by weight, of at least another polyolefin, having a melting temperature (T _f ) greater than 100°C and/or a melting enthalpy (ΔΗ _£ ) greater to 110 J/g, selected from the products (e) and (f) indicated above.

As regards the inorganic filler with flame retardant properties, this is generally selected from hydroxides, hydrated oxides, metal salts or hydrated salts, in particular of calcium, aluminum or magnesium, such as: magnesium hydroxide, alumina trihydrate, magnesium carbonate hydrate, magnesium carbonate, calcium and magnesium carbonate hydrate, calcium and magnesium carbonate, or mixtures thereof.

The flame-retardant inorganic filler is typically used in the form of particles having a granulometry ranging within wide limits, which may be untreated or surface-treated with saturated or unsaturated fatty acids containing from 8 to 24 carbon atoms, or salts thereof, such as for example: oleic acid, palmitic acid, stearic acid, isostearic acid, lauric acid, magnesium or zinc oleate or stearate, and mixtures thereof. To increase the compatibility with the polymeric base, the flame- retardant inorganic filler can also be treated with a coupling agent selected, for example, from silanes or organic titanates such as vinyltriethoxysilane , vinyltriacetylsilane , tetra- isopropyltitanate , tetra-n-butyl titanate, and the like.

The flame retardant inorganic filler is present in the composition according to the present invention in an amount ranging preferably from 70% to 280% by weight, more preferably from 100% to 220% by weight, with respect to the total weight of the polymeric components .

As regards said at least one coupling agent, it is typically a compound having an ethylenic unsaturation and at least one functional group able to interact with the hydroxyl groups present on the flame retardant filler. When, in the flame-retardant composition, at least one ethylene copolymer with at least one ester having ethylenic unsaturation (b) is used, said coupling agent, instead of an ethylenic unsaturation, may contain a polar functional group able to interact with the groups of the polar ethylene copolymer of type (b) .

The coupling agent may be selected in particular from :

(i) silane compounds containing an ethylenic unsaturation;

(ii) silane compounds containing a polar functional group ;

(iii) epoxy compounds containing an ethylenic unsaturation;

(iv) monocarboxylic or, preferably, dicarboxylic compounds, or derivatives thereof (preferably esters or anhydrides) containing an ethylenic unsaturation.

Silane compounds (i) are preferably selected from compounds of formula RR'SiY ₂ , wherein R is a hydrocarbon group containing an ethylenic unsaturation, Y is a hydrolyzable organic group, and R' is a saturated hydrocarbon group or is equal to Y. Preferably, Y is a C1-C12 alkoxide, for example: methoxy, ethoxy, propoxy, hexoxy, dodecoxy, methoxyethoxy, and the like; R is vinyl, allyl, acryl, methacryl, acryloxyalkyl or metacryloxyalkyl . Examples of silane compounds (i) are:

gamma-methacryloxypropyl -trimethoxysilane ,

allyltrimetoxysilane ,

vinyl -tris ( 2 -methoxyethoxy) silane,

vinyltrimethoxysilane ,

vinyltriethoxysilane ,

allyltriethoxysilane ,

vinylmethyldimethoxysilane ,

allylmethyldimethoxysilane ,

allylmethyldiethoxysilane ,

vinylmethyldiethoxysilane ,

or mixtures thereof .

Silane compounds (ii) are of the same type as described in (i) wherein the group R is a polar functional group, preferably amino, both of primary, secondary or tertiary type. An example of silane compounds (ii) is aminosilane.

Examples of epoxy compounds (iii) are: glycidyl acrylate, glycidyl methacrylate , monoglycidyl ester of itaconic acid, maleic acid glycidyl ester, vinyl glycidyl ether, allyl glycidyl ether, or mixtures thereof .

Examples of monocarboxylic or dicarboxylic compounds or derivatives thereof (iv) are: maleic acid, maleic anhydride, fumaric acid, citaconic acid, itaconic acid, acrylic acid, methacrylic acid, and anhydrides or esters derived therefrom, or mixtures thereof. Particularly preferred is maleic anhydride.

Said at least one coupling agent can be added to the composition related to the present invention, according to at least three different methods.

Preferably, the coupling agent is pre-grafted on at least one ethylene homopolymer or ethylene copolymer with at least one alpha-olefin C ₃ -C ₁₂ , which is added to the composition as an additional component. The quantity of pre-grafted coupling agent is preferably between 0.5 and 10% by weight, preferably from 1 to 5% by weight, with respect to the total weight of said at least one ethylene homopolymer or copolymer. Pre-grafted polyolefins with maleic anhydride are commercially available, for example, under the trademarks Fusabond™ (Du Pont) , Orevac™ (Arkema, Exxelor™ (ExxonMobil Chemical) , Yparex™ (DSM) , Tecnobond™ (Tecnofilm) .

In a second embodiment, the coupling agent is grafted on at least one of the polymer components through a so-called "reactive extrusion", i.e. by means of a grafting reaction carried out inside the extruder by adding said at least one coupling agent and at least one radical initiator. The quantity of said at least one radical initiator, for example an organic peroxide, added to achieve the reactive extrusion, is generally from 0.01 to 1% by weight, preferably from 0.1 to 0.5% by weight, with respect to the total weight of the polymeric matrix.

In a third embodiment, suitable for the coupling agent of type (ii) , the latter is added to the HFFR composition as any other ingredient, without special treatments as described above.

The compositions according to the present invention may include other components known in the art, such as antioxidants, light stabilizers, processing aids, lubricants, pigments, other fillers. For example, suitable antioxidants are: polymerized trimethyldihydroquinoline , 4,4' -thiobis (3-methyl-6- tertbutyl) phenol, pentaerythrityl tetra- [3- (3 , 5- ditertbutyl -4 -hydroxyphenyl ) ropionate] , 2,2- thiodiethylene bis [3- (3 , 5 -ditertbutyl -4 -hydroxyphenyl propionate], and the like, or mixtures thereof.

Other fillers usable in the compositions according to the present invention can be selected for example from: glass particles or fibers (short or long), kaolin (calcined or not calcined), talc, calcium carbonate or mixtures thereof. As processing aids may be used, for example: calcium stearate, zinc stearate, stearic acid, paraffin wax, silicone rubbers, polyalkyleneglycols or mixtures thereof.

Preferably, the composition according to the present invention may further include at least one dehydrating agent, according to what described in patent application WO 00/39810, which is able to absorb moisture trapped in the flame retardant filler and released during extrusion. Dehydrating agents suitable for the purpose are, for example: calcium oxide, calcium chloride, anhydrous alumina, zeolites, magnesium sulfate, magnesium oxide, barium oxide, or mixtures thereof .

The compositions according to the present invention are preferably used in uncrosslinked form, in order to obtain coatings having thermoplastic properties and thus recyclable. In order to improve the homogeneity of the produced composition, in particular as regards the dispersive mixing, the components forming the composition are fed to the extruder in a powder form, possibly pre-mixed in a mixer (for example a turbomixer or a "dries" helix stirrer) before the feed hopper, in order to ensure a pre-dispersion of the components making the subsequent homogenization inside the extruder easier. For this purpose not only the flame retardant filler, but also the polymeric components are previously reduced in a powder form, for example by means of disc or blade pulverizers, optionally by pre-cooling the granules of the less crystalline polymeric components in order to facilitate their pulverization and increase the process yield.

The present invention is further illustrated below by means of some working examples also with reference to the enclosed figures, in which:

Fig. 1 is a sectional view of a self- extinguishing low voltage unipolar electrical cable according to a first embodiment;

Fig. 2 is a sectional view of a self- extinguishing low voltage unipolar electrical cable according to a second embodiment;

Fig 3 is a sectional view of a self-extinguishing low voltage three-polar electrical cable.

With reference to Fig. 1, a self-extinguishing cable (11) includes a metallic or bimetal conductor (e.g. copper/aluminum, Copper Clad Aluminum (CCA), copper/steel, Copper Clad Steel CCS) (12), an internal electrically insulating layer (13) and an external layer (14) (sheath) consisting of the composition according to the present invention.

The internal layer (13) can be constituted by a polymeric composition known in the art, crosslinked or uncrosslinked, preferably halogen- free , which can for example include: polyolefins (particularly polyethylene, polypropylene, ethylene/propylene thermoplastic copolymers, ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) elastomers, ethylene/ ethylenically unsaturated ester copolymers (for example EVA, EBA, EMA, EEA) , ethylene/alpha-olefin copolymers, or mixtures thereof.

With reference to Fig. 2, a self -extinguishing cable (21) includes a metallic or bimetallic conductor (22) as described above directly coated with an external layer (23) consisting of a composition according to the present invention, without interposition of an electrically insulating layer. In this case, the external layer (23) also acts as electrical insulation. A thin layer (not shown on Fig 2) acting as anti-abrasive coating, can also be externally applied to the external layer (23) .

With reference to Fig. 3, a three-polar cable

(31) includes three metallic or bimetallic conductors

(32) as described above, each one coated with an insulating layer (33), two of which are phase conductors, while the third one is the neutral conductor. The insulating layers (33) can be constituted by an electrically insulating polymeric material as described above for Fig 1, or by a flame- retardant composition, in particular according to the present invention. The so insulated conductors (32) are stranded together and the interstices present among them are usually filled with a filling material (35) in order to form a continuous structure having a substantially cylindrical shape. The filler material (35) is preferably a material having flame retardant properties, usually a low viscosity and low cost material containing a flame retardant filler as described above, or even a composition according to the present invention. On the so obtained structure an external sheath (36) is applied, formed by the composition according to the present invention. The different layers of the above described cables are usually produced by means of an extrusion process. The following working examples are now provided only in order to illustrate the present invention, but without limiting it.

EXAMPLES 1-5

The compositions shown on the following Table 1 were produced using a counterrotating twin-screw extruder having a cylinder of 115 mm diameter, a L/D value (length/diameter of the extrusion cylinder) equal to 25, the cylinder being electrically thermostated into 6 separate zones (temperature setup increasing from 160 to 185°C) , thermostatization of the screw with oil (temperature set-up at 80°C) . All components in a powder form were dosed and fed into a dries mixer to be premixed and then pneumatically transferred to the forced hopper, the only feeding zone of the extruder. At the outlet of the extruder there was a filter-holding head (to which a filter mesh with a width size of 300 μπι was applied) and cutting in air; the so produced granules were cooled by means of a fluidized bed system by means of air cooling and then collected in bags of 25 kgs . During the composition production, the screw was set at 17 rpm and 180 bar pressure on the head and 130 A of current absorption of the extruder motor were reached. The composition temperature when discharged from the head was of 210 °C. The quantities of the single components in Table 1 are indicated as % by weight with respect to the total weight of the polymeric components .

Each composition was used to produce a single- core cable according to Fig 1, by extrusion of the same on a copper conductor of 1.5 mm ² insulated with crosslinked polyethylene (XLPE) , in order to obtain a thermoplastic flame- retardant layer having average thickness of about 0.7 mm.

TABLE 1

EXAMPLE 1 (*) 2 (*) 3 4 5

Flexirene™ CL 10 45.5 35.5 35.5 44.5 25.5

Clearflex™ CL DO 45.5 35.5 35.5 35.5 35.5

Riblene™ FL30 20.0 . - -

Engage™ 8003 20.0 20.0

Lucofin™ 1400 HN 30

Tecnobond™ CFA-S 9.0 9.0 9.0 9.0

Dynasylan™ 6498 2.0

Peroximon™ DC 40 0.2

Hydrofy™ G 2.5 160.0 160.0 160.0 160.0 160.0

Polyplastol™ 51 0.5 0.5 0.5 0.5 0.5 Αηοχ™ 20 0.5 0.5 0.5 0.5 0.5

Total 261.0 261.0 261.0 263.2 261.0

Tensile Strength 12.2 13.5 13.1 12.7 12.3 (MPa)

Elongation at 70 95 155 165 145 Break (%)

LOI (% Oxygen) 30 31 31 32 32

(*) comparative examples

Flexirene™ CL 10 : LLPDE, ethylene/1 -butene copolymer obtained with Ziegler-Natta catalysis having density = 0.918 g/cm ³ , Melt Flow Index (190°C/2.16 kg) = 2.7 g/10', T _f = 124 °C; AH _f = 115 J/g (Polimeri Europa);

Clearflex™ CL DO : VLPDE, ethylene/1 -butene copolymer obtained with Ziegler-Natta catalysis having density = 0,900 g/cm ³ , Melt Flow Index (190°C/2.16 kg) = 3 g/10', T _f = 115 °C; AH _f = 95 J/g (Polimeri Europa);

Engage™ 8003 : ethylene/l-ottene copolymer obtained by metallocenic catalysis having: ratio by weight ethylene/l-ottene = 82/18, density = 0.885 g/cm ³ , Melt Flow Index (190°C/2.16 kg) = 1.0 g/10'; T _f = 77 °C; AH _f = 98 J/g (Dow Chemical);

Riblene™ FL30 : LDPE, ethylene homopolymer obtained with radical initiator at high pressure, density = 0.924 g/cm ³ , Melt Flow Index (190°C/2.16 kg) = 2.2 g/10', T _f = 110 °C; AH _f = 98 J/g (Polimeri Europa);

Lucofin™ 1400 HN: ethylene/butylacrylate copolymer, ethylene/butyl-acrylate weight ratio 84/16, density = 0.930 g/cm ³ , Melt Flow Index (190°C/2.16 kg) = 1.4 g/10'; T _f = 96 °C (Lucobit) ;

Tecnobond™ CFA-S : LLDPE polyethylene grafted with maleic anhydride having: density = 0.930 g/cm ³ , Melt Flow Index (190°C/2.16 kg) = 1.75 g/10 ¹ , T _f = 120 °C (Tecnofilm) ;

Dynasylan™ 6498: coupling agent, concentrated vinylsilane (oligomeric siloxane) containing vinyl and epoxy groups, density about 1 g/cm ³ , boiling point 242°C, flash point 75°C (Evonik) ;

Peroximon™ DC40: radical initiator, cumyl peroxide in 40% calcium carbonate (Arkema) ;

Hydrofy™ G 2.5 : natural magnesium hydroxide obtained by grinding brucite, having d ₅₀ = 2.9 μιτι, surface area BET = 7.02 g/cm ³ (Nuova Sima)

Polyplastol™ 51 : processing aid based on amide derivatives (Eigenmann & Veronelli);

Anox 20™ : antioxidant, penthaerytrityl tetra-[3- (3 , 5-diterbutyl-4 hydroxyphenyl ) propyonate]

(Chemtura) .

The flame-retardant coatings so obtained were evaluated according to their tensile properties, in accordance with standards CEI 20-34, § 5.1. or CEI EN 60811-1-1 (2001) .

The self-extinguishing properties of the compositions, were evaluated by measuring the Limited Oxygen Index (LOI), determined according to ASTM D 2863 standard on a sample obtained from a plate, 3 mm thick, molded by compression of the tested material for 5 minutes at a temperature of 170-190°C and pressure of 200 bar. The results are reported in Table 1. As can be seen, the compositions according to the present invention allow obtaining flame- retardant coatings having high flexibility (elongation at break greater than 100%) without compromising tensile strength and flame-retardant properties .

EXAMPLES 6-8

The compositions reported in the following Table 2 were produced, using the same equipment and the same conditions as described above for Examples 1-5. The quantities of the single components in Table 2 are expressed as % by weight with respect to the total weight of the polymeric components.

Each composition was used to produce a unipolar cable according to Fig 1, as indicated above for Examples 1-5. The so obtained flame-retardant coatings were evaluated according to their tensile properties, in accordance with standards CEI 20-34, § 5.1. or CEI EN 60811-1-1 (2001). The self- extinguishing properties of the compositions were evaluated by measuring the Limited Oxygen Index (LOI) , determined according to ASTM D 2863 as described above for Examples 1-5. The results are reported in Table 2.

As can be seen, the compositions according to the present invention allow obtaining flame-retardant coatings having high flexibility (elongation at break greater than 100%) without compromising tensile strength and flame- retardant properties.

TABLE 2

Example 6 7 8

Escorene™ UL 00119 62.5 Lucofin™ 1400 HN 47.7

Riblene™ FL30 26.7 39.6 39.6

Engage™ 8003 47.7

Tecnobond™ CFA-S 10.8 12.7 12.7

Martinal™ OL-104 LEO 134.6

Hydrofy™ G 2.5 144.1 144.1

CaC0 ₃ coated 26.9 34.5 34.5

Irganox™ 1010 0.9 1.0 1.0

Stearic acid 1.5 1.6 1.6

CaO 1.4 1.4

Total 263.9 282.6 282.6

Tensile Strength 13.0 12.5 9.0

(MPa)

Elongation at break 150 135 172

(%)

LOI (% Oxygen) 32.5 32.0

Escorene™ UL 00119 : ethylene/vinylacetate copolymer, ethylene/vinylacetate weight ratio 81/19; density = 0.942 g/cm ³ , Melt Flow Index (190°C/2.16 kg) = 0,65 g/10'; T _f = 88 °C; AH _f = 65 J/g (ExxonMobil Chemical); Lucofin™ 1400 HN : ethylene/butyl -acrylate copolymer, ethylene/butyl-acrylate weight ratio 84/16; density = 0.930 g/cm ³ ; Melt Flow Index (190°C/2.16 kg) = 1.4 g/10'; T _f = 96 °C (Lucobit) ;

Riblene™ FL30 : LDPE, ethylene homopolymer obtained with radical initiator at high pressure, density = 0.924 g/cm ³ ; Melt Flow Index (190°C/2.16 kg) = 2.2 g/10' ; T _f = 110 °C; AH _f = 98 J/g (Polimeri Europa) ; Engage™ 8003 : ethylene/1 -octene copolymer obtained by metallocene catalysis having: ethylene/l-octene weight ratio = 82/18; density = 0.885 g/cm ³ ; Melt Flow Index (190°C/2.16 kg) = 1.0 g/10 ¹ ; T _f = 77 °C; AH _f = 98 J/g (Dow Chemical) ;

Tecnobond™ CFA-S : LLDPE polyethylene grafted with maleic anhydride having: density = 0.930 g/cm ³ ; Melt Flow Index (190°C/2.16 kg) = 1.75 g/10' ; T _f = 120 °C (Tecnof ilm) ;

Martinal™ OL-104 LEO : precipitated aluminum hydroxide, average particle size (d ₅₀ ) = 1.7-2.1 μπ\; specific surface area (BET) = 3-5 m ² /g; density = 2.4 g/cm ³ (Albemarle) ;

Hydrofy™ G 2.5 : natural magnesium hydroxide obtained by grinding brucite, having average particle size (d ₅₀ ) = 2.9 ]im, surface area BET = 7.02 g/cm ³ (Nuova Sima)

Irganox™ 1010 : antioxidant - penthaerytrytol tetrakis (3- (3 , 5 -di-tert-butyl -4 -hydroxyphenyl)

propionate) (Ciba) .