ITMI20111572A1 - FIRE-FIGHTING COMPOSITIONS AND RELATIVE PRODUCTION PROCESS - Google Patents

Description

DESCRIPTION

Attached to a patent application for INDUSTRIAL INVENTION having the title

â € œFLAME FIGHTING COMPOSITIONS AND RELATED PROCESS

PRODUCTIONâ €

The present invention relates to a flame-retardant composition, the related production process and its use for the production of self-extinguishing cables.

Self-extinguishing cables can be produced by coating the cable itself with a sheath having flame-retardant properties which includes a polymeric material to which a product has been added to impart flame-retardant properties (i.e. flame retardants), i.e. the ability to prevent the propagation of the flame along the cable in case of fire. Usually, in the case of cables with several conductive elements, the flame-retardant material is also used as a filler for the spaces present between said conductive elements.

A composition based on polyolefins, for example polyethylene or ethylene / vinyl acetate copolymers, added with an organic halide combined with antimony trioxide, which act as flame retardants, is usually used for the production of a sheath or a flame-retardant filler. However, the use of halogenated flame retardant additives has numerous drawbacks, for example during the production of the mixture and / or the cable they can partially decompose, forming halogenated gases which can be toxic or in any case harmful for the production workers and can corrode the metal parts of the production plant. Furthermore, when exposed to the action of a flame, their combustion generates large quantities of fumes also containing toxic halogenated gases. The same drawbacks can be found when using polyvinyl chloride (PVC) as a flame retardant composition, possibly added with antimony trioxide.

In recent years, flame retardant compositions without halogenated compounds have been developed (HFFR = Halogen Free Flame Retardant), which use mineral fillers as flame retardants, in particular hydrated oxides or hydroxides of aluminum and / or magnesium, for example hydrated alumina or magnesium hydroxide, which, when exposed to high temperatures, decompose through an endothermic reaction and release large quantities of water, so as to stop the spread of the flame.

As far as the hydrated alumina is concerned, this begins to decompose at relatively low temperatures (around 180 ° C), causing various drawbacks during the preparation of the mixture, for example the formation of bubbles and defects in the semi-finished composition and in the finished product. Therefore, the use of hydrated alumina is limited to compounds that use polymers that do not require high process temperatures. On the contrary, magnesium hydroxide begins to decompose at higher temperatures (around 300 ° C) and is therefore characterized by greater thermal stability and higher enthalpy of decomposition than hydrated alumina. These properties make magnesium hydroxide particularly suitable for the production of sheaths for self-extinguishing cables, which require high compounding and extrusion temperatures and limited defects.

However, the use of magnesium hydroxide as a flame retardant filler presents some difficulties, mainly related to the high quantity required to obtain a satisfactory flame retardant effect, generally about 120-250 parts by weight compared to 100 parts by weight of polymer base. These large quantities lead to a significant reduction in the processability of the compound and the mechanical properties of the resulting sheath, in particular as regards impact resistance, elongation and breaking load.

Numerous efforts have been made to try to improve the processability of flame retardant compounds containing magnesium hydroxide by modifying the magnesium hydroxide itself, for example in terms of crystallinity, particle size distribution, surface properties (see for example patents US-4.145.404 , EP 0780425, US-4.098.762). These modifications have been obtained both by developing different methods of synthesis of magnesium hydroxide, and by superficially treating the particles of synthetic or natural magnesium hydroxide, the latter being generally obtained by grinding minerals such as brucite.

Another way to make magnesium hydroxide suitable as a flame retardant filler for the production of self-extinguishing cables is to use polymers particularly suitable for incorporating large quantities of filler without excessively worsening the mechanical and elastic properties.

For example, US-5,707,732 refers to an electrical or telecommunications cable coated with a flame retardant composition comprising 100 parts by weight of a polymer blend and from 5 to 250 parts by weight of a flame retardant filler. . The latter can be magnesium hydroxide or aluminum trihydrate, while the polymeric mixture consists of: (i) a polyethylene synthesized using a metallocene single-site catalyst and having an Mw / Mn ratio not higher than 3; (ii) a polyethylene synthesized using a catalyst of a transition metal other than a single-site metallocene catalyst and having an Mw / Mn ratio greater than 4; and optionally (iii) a copolymer of ethylene with an unsaturated ester or a very low density polyethylene having a density not exceeding 0.915 g / cm <3>; wherein the polymers (i) or (ii) are modified by grafting or copolymerization with an anhydride of an aliphatic unsaturated diacid.

In patent application WO 99/05688 self-extinguishing cables and related flame-retardant compositions are described, in which these compositions comprise: (a) a homopolymer or crystalline copolymer of propylene; (b) a copolymer of ethylene with at least one alpha-olefin, and optionally a diene, characterized by a composition distribution index (CDI) greater than 45%, this index being defined as the percentage by weight of molecules of the copolymer having an alpha-olefin content within 50% of the total mean molar alpha-olefin content; (c) natural magnesium hydroxide in an amount sufficient to impart flame retardant properties. As regards the homopolymer or crystalline copolymer of propylene, this generally has a melting enthalpy higher than 75 J / g, preferably higher than 85 J / g, and guarantees to the final compound a sufficient resistance to thermo-pressure, an essential characteristic for the creation of a self-extinguishing cable that meets market specifications. As regards the copolymer of ethylene with at least one alpha-olefin (b), this is obtained by copolymerization of ethylene with at least one alpha-olefin in the presence of a "single-site" catalyst, for example a catalyst metallocene, and is characterized by a narrow molecular weight distribution, 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 comprises: (a) a homopolymer or copolymer of ethylene having a density from 0.905 to 0.970 g / cm <3>, chosen from: homopolymers of ethylene, 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 from 0.860 to 0.904 g / cm <3>, and being characterized by a composition distribution index (CDI) greater than 45%; (c) natural magnesium hydroxide in an amount sufficient to impart flame retardant properties; wherein at least one of the polymeric components (a) and (b) contains hydrolyzable organic silane groups grafted onto the polymeric chain. The silane groups are preferably introduced during the production of the flame-retardant composition by adding a suitable silane compound and a radical initiator.

The Applicant has found that, for the production on an industrial scale of HFFR compositions such as those described above, it is generally essential 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 of high length, both in order to guarantee a homogeneous mixing of the filler in the polymer base, and to allow reactions to be carried out grafting during extrusion (the so-called "reactive extrusion"), in particular to modify the polymers present by in situ grafting of coupling agents, such as silanes or maleic anhydride. Generally the length of these extruders is such as to obtain a value of L / D greater than 40 (L = length of the cylinder; D = diameter of the cylinder). Furthermore, they are generally equipped with different feeding areas (at the machine mouth and side), with one or two degassing areas, and with high-power motors (for example for a cylinder with a diameter of about 90 mm, powers also above 1000 kW).

Thanks to these characteristics, this type of machinery allows to obtain a good mixing of both dispersive and distributive types. By "distributive mixing" we mean the ability of the mixer to distribute the filler particles homogeneously within the polymeric matrix, thus avoiding the presence of filler aggregates in the finished product. By "dispersive mixing" we mean, on the other hand, the ability of the mixer to break up particles having dimensions greater than a predetermined value. In this type of extruder, mixing takes place in the area of interpenetration of the screw threads, while the material flow channel is open longitudinally, so there may be back-flow phenomena, with consequent difficulties in controlling the temperature and in the regularity of advancement of the material. However, this processing has the advantage of high productivity thanks to the high flow rates, which can however be limited by the maximum temperature at which the material can be processed. For this reason, to make the most of the productivity of these extruders, it is advisable to use materials that can be processed at high temperatures.

Another category of machinery suitable for the production on an industrial scale of HFFR compositions such as those described above is constituted by single-screw co-kneader extruders where, simultaneously with the rotational movement, the screw is equipped with an alternating movement in the axial direction. A series of pegs are positioned on the cylinder wall: the interaction between these and the movement of the screw guarantees a good level of both dispersive and distributive mixing, even for limited cylinder lengths (L / D between 8 and 30 ) and for reduced screw speeds (for example for a cylinder with a diameter of about 100 mm, the speed of the screw can reach up to 500 rpm). In this way a "gentle" mixing is also guaranteed, ie without the application of high power (for example for a cylinder with a diameter of about 100 mm, the power generally does not exceed 300 kW) and a high temperature control. mixing. Thanks to this high versatility, these machines lend themselves to the processing of materials of a very different nature (such as plastics or rubbers) and also of heat-sensitive materials (for example the mixing of rubber in the presence of thermolabile cross-linking agents such as peroxides or PVC, where often machines with 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 counter-rotating twin-screw extruders, characterized by relatively low rotation speeds of the screws (for example, for a 90 mm diameter cylinder, the rotation speed is in typically less than 100 rpm) and of limited length, with an L / D value not exceeding 30, typically 20-25 or, for more recent machines, L / D values up to 40. They are also equipped with a single dispenser at the machine mouth and a single degassing area; simply having to gel the "compound", these machines are equipped with motors of limited power (for example for a cylinder with a diameter of about 90 mm the power is generally less than 150 kW). In this type of extruder, the mixing takes place mainly by calendering between the screws, thus obtaining a good dispersion thanks to the high mechanical work due to the elongational flow, while the distribution-type mixing is rather poor. This prevents any non-homogeneity of composition due to feeding irregularities to be corrected during the passage through the extruder. Furthermore, the capacity of these extruders is limited as the passage of the material between the screws determines a force, orthogonal to the axis of the screws themselves, which tends to push the latter towards the internal walls of the cylinder, therefore the capacity cannot be too high to avoid contact between screws and cylinder, which would cause premature wear of the machinery. On the other hand, counter-rotating twin-screw extruders have the advantage of allowing good temperature control, so that they do not cause overheating of the material to be extruded, the energy transmitted by the motor being used efficiently for transporting the material itself. Furthermore, in these extruders the flow channel is closed longitudinally, as at each step the thread of one screw is closed by the thread of the other screw: this prevents the material from flowing back (back flow).

Therefore, counter-rotating twin-screw extruders are particularly popular for the production of compositions based on PVC or other heat-sensitive materials where a high amount of energy is not required for mixing; all the ingredients are generally pre-mixed in the form of powders so as to guarantee a substantially homogeneous feeding composition.

The Applicant has now posed the problem of producing low-cost, halogen-free flame retardant compositions comprising an inorganic flame retardant filler (HFFR compositions), which are endowed with the necessary flame retardant and mechanical properties such as to meet the specific market demands. , and at the same time they can also be produced industrially using counter-rotating twin-screw extruders or co-kneader single-screw extruders with short L / D (for example less than 10), such as those normally used for the production of PVC-based compositions, without therefore requiring more complex and expensive mixing equipment, such as co-rotating twin screw extruders or co-kneader single screw extruders with high L / D (for example greater than 15), having characteristics as described above, in addition to all the control systems and continuous dosing of the necessary raw materials. This simplification would allow the producers of flame retardant compositions, whether they are companies producing "compounds" or cables and internally equipped with equipment for the production of "compounds", to use the same mixing equipment for the production of both PVC-based compounds. and HFFR compounds, so as to allow greater production flexibility without the need for large investments for the purchase of different machinery for the production of flame-retardant compounds. Therefore:

a) companies already equipped with machinery for the production of PVC as described above would have the possibility of producing HFFR compounds on the same plant with a very limited investment;

b) companies that intend to equip themselves with a single type of machinery, suitable for both the production of PVC and HFFR compounds, could invest in cheaper machinery, even already used, such as those for PVC described above, thus significantly shortening the Return of the Investment.

The Applicant has now found that these objectives and others that will be better illustrated below can be achieved by means of a flame-retardant composition as defined below, which allows to:

- use relatively high process temperatures both in the production phase of the mixture and during the extrusion for the realization of self-extinguishing cables;

- use counter-rotating twin-screw extruders characterized by relatively low rotation speed and length of the screws or single-screw co-kneader extruders with short L / D (for example less than 10), thanks to a rheological behavior of the compound such as to minimize the elastic component and therefore to limit the effort of the extruder motor, so as to allow the use of machines with relatively low power motors; - use raw materials, both inorganic and polymeric, in the form of powders so as to improve the homogeneity of the final mix thanks to a maximization of the distribution type mixing, which derives from a reduction in the amount of energy destined for dispersive mixing , which, as explained above, is difficult to implement in a counter-rotating twin-screw extruder or in a single-screw co-kneader extruder with short L / D (for example less than 10), typically used for the production of PVC-based compounds.

According to a first aspect, the present invention therefore relates to a flame-retardant composition which comprises:

(a) from 20 to 60% by weight, preferably from 30 to 50% by weight, of at least one homopolymer of ethylene or copolymer of ethylene with at least one C3-C12 alpha-olefin, said homopolymer or copolymer having a density from 0.910 to 0.940 g / cm <3>, preferably from 0.915 to 0.930 g / cm <3>;

(b) from 20 to 60% by weight, preferably from 30 to 50% by weight, of at least one copolymer of ethylene with at least one C3-C12 alpha-olefin, said copolymer 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 (MWDI) higher than 4, preferably higher than 5;

(c) from 0 to 40% by weight, preferably from 10 to 35% by weight, of at least one ethylene copolymer selected from:

(c1) copolymers of ethylene with at least one C3-C12 alphaolefin, having a density 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 higher than 4, preferably from 1.5 to 3.5;

(c2) copolymers of ethylene with at least one ester having an ethylene unsaturation; and their mixtures; and (c3) ethylene / propylene copolymers comprising from 5 to 25% by weight of ethylene, from 75 to 95% by weight of propylene, and optionally a quantity not exceeding 10% by weight of a diene, said copolymers having an index of molecular weight distribution (MWDI) from 1 to 5, and a melting enthalpy (∠† Hf) not exceeding 75 J / g and a melting temperature (Tf) not exceeding 105 ° C;

(d) at least one inorganic filler with flame retardant properties;

(e) 0.5 to 10% by weight, preferably 1 to 5% by weight, of at least one coupling agent.

Within the scope of the present description and the attached claims, except where otherwise indicated, the weight percentages of the various components are expressed with respect to the total weight of the polymeric components (a), (b) and (c).

In the context of the present description and the attached claims, with "alpha-olefin C3-C12" is meant an olefin having the formula CH2 = CH-R, where R is a linear or branched alkyl having from 1 to 10 carbon atoms . Preferably the alpha-olfine is a C4-C8 alphaolefin. The alpha-olefin is preferably selected from: 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-dodecene.

As regards component (a), this is preferably selected from: medium density polyethylene (MDPE) having a density from 0.926 to 0.970 g / cm <3>; low density polyethylene (LDPE), linear low density polyethylene (LLDPE), having a density from 0.910 to 0.926 g / cm <3> and linear high density polyethylene (HDPE) having a desity greater than 0.940 g / cm <3 >

Similarly to component (b), component (a) generally has a molecular weight distribution index (MWDI) greater than 4, preferably greater than 5.

As known, the molecular weight distribution index (MWDI) can be determined, according to conventional methods, by gel permeation chromatography.

This type of polymers are available on the market under different brands (for example Flexirene ï ›› from Polimeri Europa, ExxonMobil LLDPE LL from ExxonMobil Chemical) and 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 the formation of a homopolymer having a very low degree of branching. LDPE is generally produced through a high pressure process in which ethylene is homopolymerized in the presence of oxygen or a peroxide as an initiator, with the formation of long branched polyethylene chains (long branched PE). LLDPE is a short branched copolymer resulting from the copolymerization of ethylene with a C3-C12 alpha-olefin, preferably 1-butene, 1-hexene or 1-octene, and is generally produced by low pressure processes in the presence of a Ziegler-Natta catalyst or a chromium catalyst. The C3-C12 alpha-olefin is preferably present in the copolymer in an amount from 1 to 15% by moles.

As regards component (b), this is preferably a very low density polyethylene (VLDPE) having a density from 0.870 to 0.909 g / cm <3>, preferably from 0.880 to 0.905 g / cm <3>. These are products generally obtained through the copolymerization of ethylene with a C3-C12 alpha-olefin, such as for example 1-butene, 1-hexene, 1-octene, in the presence of: (i) a chromium-based catalyst and titanium; or (ii) a catalyst based on magnesium, titanium, a halogen and an electron donor; or (iii) a vanadium-based catalyst, an electron donor, an aluminum halide and a halogenated hydrocarbon as promoter.

As regards the copolymer of ethylene with at least one alpha-olefin (Cl), this preferably has the following monomer composition: 75-97% by moles, preferably 90-95% by moles, of ethylene; 3-25% by moles, preferably 5-10% by moles, of at least one C3-C12 alpha-olefin; 0-5% by moles, preferably 0-2% by moles, of at least one diene.

Preferably, said at least one diene is selected from: linear C4-C20 diolefins, conjugated or non-conjugated (for example 1,3-butadiene, 1,4-hexadiene, 1,6-octadiene); monocyclic or polycyclic dienes (for example 1,4-cyclohexadiene, 5-ethylidene-2-norbornene, 5-methylene-2-norbornene).

Said copolymer of ethylene with at least one alphaolefin (c1) can be produced by copolymerization of ethylene with at least one alphaolefin, and optionally with a diene, in the presence of a "single-site" catalyst, in particular a metallocene catalyst, as described, for example in US-5,246,783 and US-5,272,236. The metallocenes used as olefin catalysts are generally coordination complexes between a transition metal, usually of group IV, in particular titanium, zirconium or hafnium, and two cyclopentadienyl ligands, which are optionally substituted, used in combination with a co- catalyst, for example an aluminoxane, preferably a methylaluminoxane, 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 that can be used 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 (c1) copolymers are the commercial products Engage ï £ ̈ from Dow Chemical and Exact ï £ ̈ from Exxon Chemical.

As regards the copolymers of ethylene with at least one ester having an ethylene unsaturation (c2), they are generally copolymers of ethylene with at least one ester selected from: C1-C8 (preferably C1-C4) alkyl acrylates, C1 -C8 (preferably C1-C4) alkyl methacrylates, and vinyl C2-C8 (preferably C2-C5) carboxylates. The quantity of ester present in the copolymer can generally vary from 5% to 50% by weight, preferably from 15% to 40% by weight. Examples of C1-C8 acrylates and methacrylates are: ethyl acrylate, methyl acrylate, methyl methacrylate, tert-butyl acrylate, n-butyl acrylate, nbutyl methacrylate, 2-ethylhexyl acrylate, and the like. Examples of vinyl C2-C8 carboxylates are: vinyl acetate, vinyl propionate, vinyl butanoate, and the like.

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

The copolymers of ethylene (c2) can be produced according to known processes, usually by high pressure copolymerization similar to that used for LDPE. Examples of ethylene copolymers of the type (c2) as described above are commercial products Lucofin ï £ ̈ by Lucobit and Lotryl ï £ ̈ by Arkema.

As regards the ethylene / propylene copolymers (c3), these preferably comprise from 5 to 15% by weight of ethylene, from 85 to 95% by weight of propylene, and optionally a quantity not exceeding 5% by weight of a diene. Suitable dienes as possible termonomers can be preferably selected from: 1,4-hexadiene, 1,6-octadiene, 5-methyl-1,4-hexadiene, 3,7-dimethyl-1,6-octadiene, dicyclopentadiene (DCPD), ethylidiene norbornene (ENB), or mixtures thereof. Particularly preferred is ethylidiene norbornene (ENB).

Preferably the ethylene / propylene copolymers (c3) have a triad tacticity value, measured by <13> C-NMR spectroscopy, equal to at least 75%, more preferably equal to at least 85%.

Preferably the ethylene / propylene copolymers (c3) have a melting enthalpy (∠† Hf) from 0.5 to 70 J / g, more preferably from 1 to 35 J / g. Fusion enthalpy is typically determined by differential scanning calorimetry (DSC) measurements. As for the melting temperature (Tf), which can also be determined by DSC, this corresponds to the maximum point in the DSC curve, which has a single large widened peak, and possibly smaller peaks which can be substantially neglected. The melting temperature (Tf) of the copolymer (c3) is preferably comprised from 25 ° C to 105 ° C, more preferably from 30 ° C to 80 ° C.

Copolymers (c3) as defined above are available on the market under the brands Vistamaxx ï ›› (ExxonMobil Chemical Co.) and Versify ï ›› (The Dow Chemical Co.). Further details on these products can be found, for example, in the text of US patent application 2007/0134506.

As for the inorganic filler with flame retardant properties (d), this is generally chosen from hydroxides, hydrated oxides, salts or hydrated salts of metals, in particular calcium, aluminum or magnesium, such as for example: magnesium hydroxide, alumina trihydrate, magnesium carbonate hydrate, magnesium carbonate, calcium and magnesium carbonate hydrate, calcium and magnesium carbonate, or mixtures thereof. Magnesium hydroxide is particularly preferred, as it is characterized by a decomposition temperature higher than 300 ° C, so that it allows to use high extrusion temperatures. Magnesium hydroxide of natural origin is particularly preferred for obtaining low cost HFFR compounds, even if it can begin to decompose already around 250 ° C, in any case well above those of aluminum hydroxide; it is obtained by grinding minerals containing magnesium hydroxide, for example brucite (see the aforementioned patent applications WO 99/05688 and WO 00/19452).

The flame retardant inorganic filler (d) is generally used in the form of particles of variable particle size within wide limits, which can be untreated or superficially treated with saturated or unsaturated fatty acids containing from 8 to 24 carbon atoms, or their salts, such as for example: oleic acid, palmitic acid, stearic acid, isostearic acid, lauric acid, oleate or magnesium or zinc stearate, and their mixtures. To increase compatibility with the polymer base, the flame retardant inorganic filler (d) can also be treated with a coupling agent selected for example from organic silanes or titanates, such as vinyl triethoxysilane, vinyltriacetylsilane, tetra-isopropyltitanate, tetran-butyltitanate, and similar.

The flame retardant inorganic filler (d) is present in the composition according to the present invention in an amount preferably from 70% to 280% by weight, more preferably between 100% and 220% by weight, with respect to the total weight of the polymeric components ( a), (b), and (c).

As regards said at least one coupling agent (e), this is generally a compound having an ethylenic unsaturation and at least one functional group capable of interacting with the hydroxyl groups present on the flame retardant filler. If component (c) is chosen from among those of type (c2), said coupling agent (e), in addition to the above, as an alternative to an ethylene unsaturation, may contain a polar functional group capable of interacting with polar groups of the ethylene copolymer of type (c2).

The coupling agent can be selected in particular from:

(i) silane compounds containing ethylene unsaturation;

(ii) silane compounds containing a polar functional group;

(iii) epoxy compounds containing ethylene unsaturation;

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

Silane compounds (i) are preferably selected from the compounds of formula RR'SiY2, where R is a hydrocarbon group containing an ethylene unsaturation, Y is a hydrolyzable organic group, and R 'is a saturated hydrocarbon group or à ̈ 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, methacrylate, acryloxyalkyl or methacryloxyalkyl. Examples of silane compounds (i) are:

gamma-methacryloxypropyl-trimethoxysilane, allyltrimethoxysilane,

vinyl-tris (2-methoxyethoxy) silane, vinyltrimethoxysilane,

vinyl triethoxysilane,

allyltriethoxysilane,

vinylmethyldimethoxysilane, allylmethyldimethoxysilane, allylmethyldietoxysilane, vinylmethyldietoxysilane,

or their mixtures.

Silane compounds (ii) are of the same typology as those described in (i) where the R group is a polar functional group, preferably amino, of either 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, glycidyl ester of maleic acid, vinyl glycidyl ether, allyl glycidyl ether, or mixtures thereof.

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

Said at least one coupling agent (e) can be added to the composition according to the present invention in at least three different ways. According to a first embodiment, the coupling agent is pre-grafted onto at least one homopolymer of ethylene or copolymer of ethylene with at least one C3-C12 alpha-olefin, which is added to the composition as an additional component. The amount of pre-crimped coupling agent is preferably comprised 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 homopolymer or copolymer of ethylene. Pre-crimped polyolefins with maleic anhydride are commercially available, for example, under the brands Fusabond ï ›› (Du Pont), Orevac ï ›› (Arkema), Exxelor ï ›› (Exxon Chemical), Yparex ï ›› (DSM) , Tecnobond ï ›› (Tecnofilm).

In a second embodiment, the coupling agent is grafted onto at least one of the polymeric components (a), (b) and (c) through a so-called â € œreactive extrusionâ €, i.e. by means of a grafting reaction which is carried out inside the extruder by adding said at least one coupling agent and at least one radical initiator. The amount of said at least one radical initiator, for example an organic peroxide, added to achieve the â € œreactive extrusionâ € 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 components (a), (b) and (c).

In a third embodiment, valid for the couplant type (ii), this is added to the HFFR composition like any other ingredient provided, without the need for special treatments as described above.

The compositions according to the present invention can comprise other components known in the art, such as antioxidants, light stabilizers, process aids, lubricants, pigments, other fillers. For example, antioxidants suitable for this purpose are: polymerized trimethyldihydroquinoline, 4,4'-thiobis (3-methyl-6-terbutyl) phenol, pentaerythrityl tetra- [3- (3,5-diterbutyl-4-hydroxyphenyl) propionate], 2 , 2'-thiodethylene bis [3- (3,5-diterbutyl-4-idorssiphenyl) 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 their mixtures. As processing aids, for example: calcium stearate, zinc stearate, stearic acid, paraffin wax, silicone rubbers, polyalkylene glycols or their mixtures can be used.

Preferably, the composition according to the present invention can also comprise at least one dehydrating agent, in accordance with what is described in the patent application WO 00/39810, which is capable of absorbing the humidity that can be trapped in the flame retardant filler. and released during extrusion. Suitable dehydrating agents are, for example: calcium oxide, calcium chloride, anhydrous alumina, zeolites, magnesium sulfate, magnesium oxide, barium oxide, or their mixtures.

The compositions according to the present invention are preferably used in non-crosslinked form, so as to obtain coatings having thermoplastic properties and therefore recyclable.

The compositions according to the present invention can be produced according to known techniques, by mixing the polymeric components, the flame retardant filler and the other additives in a discontinuous (batch) or, preferably, continuous mixer. Among the batch mixers, for example internal mixers equipped with tangential (Banbury) or interpenetrating rotors can be used, while the preferred continuous mixers are the Buss co-kneader mixers or the single screw or twin screw extruders.

Preferably, the compositions according to the present invention can be produced by mixing the components by means of a counter-rotating twin-screw extruder having an L / D value (length / diameter of the extrusion cylinder) not exceeding 40 or in a single-screw co-kneader extruder with Short L / D (e.g. less than 10).

In order to improve the homogeneity of the composition produced, in particular as regards the dispersive type mixing, the components that form the composition are fed to the extruder in the form of powder, possibly premixed in a mixer (for example a turbomixer or a propeller agitator â € œdriesâ €) before the feeding hopper, in order to guarantee a pre-dispersion of the components which facilitates the subsequent homogenization inside the extruder. For this purpose, not only the flame retardant filler, but also the polymeric components are previously reduced into powder form, for example by means of disc or blade pulverizers, possibly pre-cooling the granules of the less crystalline polymeric components in order to facilitate their pulverization. and increase the yield of the process.

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

Fig. 1 is a sectional view of a low voltage self-extinguishing electric cable of the unipolar type according to a first embodiment; Fig. 2 is a sectional view of a low voltage self-extinguishing electric cable of the unipolar type according to a second embodiment;

Fig. 3 is a sectional view of a low voltage self-extinguishing electric cable of the three-pole type.

With reference to Fig. 1, a self-extinguishing cable (11) comprises a metallic or bimetallic conductor (eg copper / aluminum, "Copper Clad Aluminum" (CCA), copper / steel, "Copper Clad Steel" CCS) (12), an electrically insulating inner layer (13) and an outer layer (14) (sheath) constituted by the composition according to the present invention.

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

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

With reference to Fig. 3, a cable (31) of the three-pole type comprises three metallic or bimetallic conductors as described above (32), each covered with an insulating layer (33), of which two are phase conductors, while the third 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 in accordance with the present invention. The conductors (32) thus insulated are stranded together and the interstices present between them are generally filled with a filling material (35) so as to form a continuous structure having a substantially cylindrical shape. The filling 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 also a composition in accordance with the present invention. An external sheath (36) constituted by the composition in accordance with the present invention is applied to the structure thus obtained. The various layers of the cables illustrated above are usually produced through an extrusion process.

The following embodiments are now provided purely for illustrative but not limitative purposes of the present invention.

EXAMPLES 1-3

The compositions shown in the following Table 1 were produced, using a counter-rotating twin-screw extruder having a cylinder with a diameter of 115 mm, an L / D value (length / diameter of the extrusion cylinder) equal to 25, with electric thermostating of the cylinder divided into 6 zones (increasing set temperature from 160 to 185 ° C), oil thermostating of the screw (set temperature of 80 ° C). All the components in the form of powder were dosed and fed into a â € œdreisâ € to be premixed and subsequently pneumatically transferred to the forced hopper, single feed of the extruder. At the exit from the extruder there is a filter holder head (to which a filtering net with free light corresponding to 300 microns has been applied) and cut in the air; the granules thus produced were cooled by means of a fluidized bed system with air cooling and finally collected in 25 kg bags. During the production of the compound the screw was set at 17 rpm and 180 bar of pressure on the head and 130 A of current absorption of the extruder motor were reached. The temperature of the compound leaving the head was 210 ° C.

Each composition was used to produce a single-core cable in accordance with Fig. 1, by extruding it on a copper conductor with a section of 1.5 mm <2> insulated with cross-linked polyethylene (XLPE), so as to obtain a thermoplastic flame retardant layer with an average thickness of approximately 0.7 mm. The quantities of the individual components in Table 1 are expressed as% by weight with respect to the total weight of the polymeric components.

TABLE 1

Example 1 (*) 2 (*) 3 4 5 Flexirene ï ›› CL 10 45.5 35.5 35.5 44.5 25.5 Clearflex ï ›› CL D0 45.5 35.5 35.5 35.5 35.5 Riblene ï ›› FL30 - 20.0 - - --Engage ï ›› 8003 - - 20.0 20.0 --Lucofin ï ›› 1400 HN - - - - 30 Tecnobond ï ›› CFA-S 9.0 9.0 9.0 - 9.0 Dynasylan ï ›› 6498 - - - 2.0 -Peroximon ï ›› DC 40 - - - 0.2 --Hydrofy ï ›› G 2.5 160.0 160.0 160.0 160.0 160.0 Polyplastol ï ›› 51 0.5 0.5 0.5 0.5 0.5 Anox ï ›› 20 0.5 0.5 0.5 0.5 0.5 Total 261.0 261.0 261.0 263.2 261.0

Breaking load 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 ï ›› 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 ', melting point = 121 ° C (Polymers Europa);

Clearflex ï ›› CL D0: 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 ', melting point = 115 ° C (Polymers Europa);

Engage ï ›› 8003: ethylene / 1-octene copolymer obtained with metallocene catalysis having: ethylene / 1-octene weight ratio = 82/18, density = 0.885 g / cm <3>, Melt Flow Index (190 ° C / 2 , 16 kg) = 1.0 g / 10 '(Dow Chemical);

Riblene ï ›› FL30: LDPE, ethylene homopolymer obtained with high pressure radical initiator, density 0.924 g / cm <3>, Melt Flow Index (190 ° C / 2.16 kg) = 2.2 g / 10 ', point melting point = 114 ° C (Polymers Europe); Lucofin ï ›› 1400 HN: ethylene / butyl acrylate copolymer, weight ratio ethylene / butyl acrylate 84/16, density = 0.930 g / cm <3>, Melt Flow Index (190 ° C / 2.16 kg) = 1.4 g / 10 '(Lucobit);

Tecnobond ï ›› CFA-S: LLDPE polyethylene crimped with maleic anhydride having: density = 0.930 g / cm <3>, Melt Flow Index (190 ° C / 2.16 kg) = 1.75 g / 10 ', point of melting = 120 ° C (Tecnofilm);

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

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

Hydrofy ï ›› G 2.5: natural magnesium hydroxide obtained by grinding brucite, having d50 = 2.9 µm, surface area BET = 7.02 g / cm <3> (Nuova Sima) Polyplastol ï ›› 51: processing aid based on amide derivatives (Eigenmann &Veronelli);

Anox 20ï ››: antioxidant, pentaerythrityl tetra- [3- (3,5-diterbutyl-4 hydroxyphenyl) propionate] (Chemtura).

The flame retardant coatings thus obtained have been evaluated in relation to their tensile properties, in accordance with the provisions of the CEI 20-34, § 5.1 standard. or CEI EN 60811-1-1 (2001).

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

Claims (16)

CLAIMS 1. Flame retardant composition which includes: (a) from 20 to 60% by weight, preferably from 30 to 50% by weight, of at least one homopolymer of ethylene or copolymer of ethylene with at least one C3-C12 alpha-olefin, said homopolymer or copolymer having a density from 0.910 to 0.940 g / cm <3>, preferably from 0.915 to 0.930 g / cm <3>; (b) from 20 to 60% by weight, preferably from 30 to 50% by weight, of at least one copolymer of ethylene with at least one C3-C12 alpha-olefin, said copolymer 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 (MWDI) higher than 4, preferably higher than 5; (c) from 0 to 40% by weight, preferably from 10 to 35% by weight, of at least one ethylene copolymer selected from: (c1) copolymers of ethylene with at least one C3-C12 alphaolefin, having a density 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 higher than 4, preferably from 1.5 to 3.5; (c2) copolymers of ethylene with at least one ester having an ethylene unsaturation; and their mixtures; and (c3) ethylene / propylene copolymers comprising from 5 to 25% by weight of ethylene, from 75 to 95% by weight of propylene, and optionally a quantity not exceeding 10% by weight of a diene, said copolymers having an index of molecular weight distribution (MWDI) from 1 to 5, and a melting enthalpy (∠† Hf) not exceeding 75 J / g and a melting temperature (Tf) not exceeding 105 ° C; (d) at least one inorganic filler with flame retardant properties; (e) 0.5 to 10% by weight, preferably 1 to 5% by weight, of at least one coupling agent. 2. Composition according to claim 1, wherein component (a) is selected from: medium density polyethylene (MDPE) having a density from 0.926 to 0.970 g / cm <3>; low density polyethylene (LDPE), linear low density polyethylene (LLDPE), having a density 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> 3. Composition according to claim 1 or 2, wherein component (b) is a very low density polyethylene (VLDPE) having a density from 0.870 to 0.909 g / cm <3>, preferably from 0.880 to 0.905 g / cm < 3>. Composition according to any one of the preceding claims, wherein the copolymers of ethylene with at least one alpha-olefin (Cl) have the following monomer composition: 75-97% by moles, preferably 90-95% by moles, of ethylene ; 3-25% by moles, preferably 5-10% by moles, of at least one C3-C12 alpha-olefin; 0-5% by moles, preferably 0-2% by moles, of at least one diene. 5. Composition according to any one of the preceding claims, wherein the copolymers of ethylene with at least one ester having an ethylene unsaturation (c2) are selected from: C1-C8 (preferably C1-C4) alkyl acrylates, C1-C8 (preferably C1-C4) alkyl methacrylates, and vinyl C2-C8 (preferably C2-C5) carboxylates, the amount of ester present in the copolymer ranging from 5% to 50% by weight, preferably from 15% to 40% by weight. 6. Composition according to claim 5, wherein the copolymers of ethylene with at least one ester (c2) are selected from ethylene-nbutylacrylate (EBA) copolymers, preferably having a content of n-butylacrylate from 15 to 20%. 7. Composition according to any one of the preceding claims, wherein the ethylene / propylene copolymers (c3) comprise from 5 to 15% by weight of ethylene, from 85 to 95% by weight of propylene, and optionally an amount not exceeding 5% by weight of a diene. 8. Composition according to any one of the preceding claims, wherein said at least one inorganic filler with flame retardant properties (d) is chosen from: hydroxides, hydrated oxides, salts or hydrated salts of metals, in particular calcium, aluminum or magnesium . 9. Composition according to any one of the preceding claims, wherein said at least one inorganic filler is magnesium hydroxide. 10. Composition according to any one of the preceding claims, wherein said at least one inorganic filler (d) is present in an amount from 70% to 280% by weight, preferably between 100% and 220% by weight, with respect to the total weight of the polymeric components (a), (b), and (c). 11. Composition according to any one of the preceding claims, wherein said at least one coupling agent (e) is selected from: (i) silane compounds containing ethylene unsaturation; (ii) silane compounds containing at least one polar functional group; (iii) epoxy compounds containing ethylene unsaturation; (iv) monocarboxylic or, preferably, dicarboxylic compounds, or derivatives thereof, containing an ethylene unsaturation. Composition according to any one of the preceding claims, wherein said at least one coupling agent (e) is pre-grafted onto at least one homopolymer of ethylene or copolymer of ethylene with at least one C3-C12 alpha-olefin. 13. Composition according to any one of claims 1 to 8, wherein said at least one coupling agent (e) is grafted onto at least one of the polymeric components (a), (b) and (c) by reactive extrusion. 14. Process for producing a composition according to any one of the preceding claims, which comprises mixing the components by means of a counter-rotating twin-screw extruder having an L / D value (length / diameter of the extrusion cylinder) not exceeding 40, or in a single screw cokneader extruder with short L / D (for example less than 16). 15. Process according to claim 14, in which the components are fed to the extruder in powder form, optionally premixed in a mixer. Use of a composition according to any one of claims 1 to 13 for the production of a flame retardant coating, in particular of a self-extinguishing cable.