FIELD OF THE INVENTION

[0001] The present disclosure relates to coated electrical cables, these cables having low- smoke self-extinguishing properties, and to the flame-retardant compositions used therein.

BACKGROUND OF THE INVENTION

[0002] Due to the relatively high risk of fire being generated by electrical cables, it is common practice to coat them with self-extinguishing and flame-retardant polymer composition to which fire-resistant properties have been given by adding a suitable additive.

[0003] Polyolefin-based compositions based, for example, on polyethylene or ethylene/vinyl acetate copolymers, containing an organic halide combined with antimony trioxide as flame-retardant additive can, for example, be used for this purpose. However, halogenated flame- retardant additives have many drawbacks since they partially decompose during processing of the polymer, giving rise to halogenated gases that are toxic to workers and corrode metal parts of the polymer- processing equipment. In addition, when they are placed directly in a flame, their combustion gives rise to large amounts of smokes containing toxic gases. Similar drawbacks are encountered when polyvinylchloride (PVC) supplemented with antimony trioxide is used as base polymer.

[0004] Therefore, in recent years the production of self-extinguishing flame retardant cables has been directed toward halogen free compositions, using as flame-retardant agents inorganic oxides, preferably in hydrate or hydroxide form, in particular magnesium hydroxide or aluminum trihydrate.

[0005] In addition to fire and heat resistant properties, the compositions used for cable coating should also show excellent mechanical properties, particularly as to tensile strength at break and flexibility, particularly in terms of elongation at break.

[0006] At the same time, the compositions would also be required to have very high values of Limited Oxygen Index (LOI), generally of at least 35, measured on compression molded plates according to ASTM Standard D-2863. [0007] Typical compositions used in this field are made of a polymer matrix, added with an inorganic flame retardant such as aluminum trihydrate or magnesium dihydrate (either natural or functionalized for better compatibility), and an additive package comprising antioxidants, stabilizer, processing aids and optionally further heat/flame resistant agents. The polymer matrix in general comprise an ethylene homopolymer or copolymer having a density of from 0.905 to 0.970 g/cm ³ , and a copolymer of ethylene with an alpha-olefin, and optionally with a diene, and an ethylene copolymer with a polar comonomer; to help with compatibilization, a grafted PE copolymer is also usually found.

[0008] The state of the art and most commercially spread out solution comprises the combination of aluminum trihydrate as inorganic flame retardant and ethylene- vinyl acetate as ethylene-polar comonomer component which is chosen in view of the performances in terms of flame retardancy and mechanical properties. However, said compositions are characterized by a narrow operative window due to the relatively low melting temperature of EVA and the low decomposition temperature of Al(OH)3. Also, the flow characteristics and therefore the speed with which they can be extruded to coat the cables is not satisfactory.

[0009] Mg(OH)2 on the other hand has a much higher decomposition temperature and can be used as a replacement of Al(OH)3. In order to improve the mechanical properties, one of the suggested way (see CN102108148) is to crosslink the polymer matrix of the composition. By so doing however, the composition are no longer thermoplastic and, as a consequence, not recyclable.

[0010] EP3083791 B1 suggests to use magnesium hydroxide in combination with EVA.

However, the performances in terms of smoke release and LOI are not satisfactory. On the other hand, US2010/0101822 also reports that the use of natural and coated magnesium hydrate can cause increased cracking potential.

[0011] It would be therefore important to have available thermoplastic compositions for the preparation of coated cable having the suited combination of flame retardancy, low smoke generation and mechanical properties.

SUMMARY OF THE INVENTION

[0012] It is therefore an embodiment of the present disclosure a thermoplastic composition for electrical cable coating comprising a polyolefin portion comprising (a) an ethylene copolymer being selected ethylene/alkyl acrylate (b) a polyolefin elastomer (POE) being selected from copolymer of ethylene with at least one C3-C15 alpha-olefin, and optionally with a diene, said copolymer (b) having a density (ASTM D505) of from 0.860 to 0.904 g/cm ³ , (c) a polymeric coupling agent selected from an ethylene polymer grafted with carboxyl groups or organic silane groups and optionally (d) a copolymer of ethylene with at least one C4-C10 alpha-olefin, said copolymer (d) having a density of from 0.910 to 0.924 g/cm ³ , and a mineral portion comprising (e) magnesium hydroxide said components being in amount such that the weight ratio (e)/polyolefin portion ranges from 0.8:1 to 1.75:1, the weight ratio (a)/(b) ranges from 0.75:1 to 1.25:1 and the weight ratio (a)/(e) ranges from 0.15:1 to 0.4:1; said composition being further characterized by a melt flow rate (MFR) at 190°C with a load of 21.6 kg, according to ISO 1133-2:2011, of at least 2 g/10’, preferably at least 2.5 g/10’.

[0013] A further embodiment is represented by an electrical cable coated with at least one layer being comprised of the above described thermoplastic composition.

DETAILED DESCRIPTION OF THE INVENTION

[0014] All features of the copolymers (a)-(d) are not inextricably linked to each other. This means that a certain level of preference of one the features should not necessarily involve the same level of preference of the remaining features.

[0015] The term “copolymer” as used herein refers to both polymers with two different recurring units and polymers with more than two different recurring units, such as terpolymers, in the chain

[0016] Polymer component (a) is preferably selected from ethylene/butyl acrylate (EBA), ethylene/ethyl acrylate (EEA) or ethylene/methyl acrylate (EMA). The content of alkyl acrylate in the polymers preferably ranges from 5 to 25% wt. Preferably, in the (EBA) and (EEA) it ranges from 10 to 20% wt.

[0017] The MFR of the component (a) preferably ranges from 0.5 to 25 g/10’ more preferably from 1 to 20 g/10’. The density ranges from 0.920 to 0.935 g/cm ³ .

[0018] Component (a) can be prepared produced according to known techniques, usually by high-pressure polymerization where ethylene and comonomer are polymerized in the presence of oxygen or a peroxide as initiator.

[0019] In the polyolefin elastomer (POE) component (b), the alpha-olefin can be selected, for example, from propylene, 1 -butene, 1-pentene, 4-methyl- 1- pentene, 1 -hexene, and 1- octene. 1 -butene and 1-octene being particularly preferred. [0020] When a diene comonomer is present, it generally has from 4 to 20 carbon atoms, and is preferably selected from: linear, conjugated or non-conjugated diolefins, for example 1,3- butadiene, 1,4-hexadiene or 1,6-octadiene; monocyclic or polycyclic dienes, for example 1 ,4-cyclohexadiene, 5-ethylidene-2-norbornene, 5-methylene-2-norbornene, and the like.

[0021] Preferably, the POE contains at least one C4-C12 alpha-olefin, preferably 1-octene. The amount of alpha olefin preferably ranges from 3-25% by mole, preferably from 5-10% by mole.

Preferably, the density ranges from 0.870 and 0.90 g/cm ³ ; the MFR at 190°C with a load of 2.16 kg, according to ISO 1133-2:2011, preferably ranges from 0.1 and 30 g/10 min, preferably between 0.5 and 5 g/10 min.

[0022] Component (b), generally it is characterized by a narrow molecular weight distribution, with a Molecular Weight Distribution (MWD) index, defined as the ratio between the weight-average molecular weight Mw and the number-average molecular weight Mn, of less than 5, preferably between 1.5 and 3.5. The MWD index can be determined, according to conventional methods, by Gel Permeation Chromatography (GPC).

[0023] The POE component (b) can be produced by copolymerization of ethylene with an alpha-olefin, and optionally with a diene, in the presence of a single-site catalyst, for example a metallocene catalyst, as described, e. g., in patent applications WO 93/19107 and EP-A-632,065 or in patents US-5,246,783 and US-5,272,236.

[0024] Catalysts which are suitable for obtaining the copolymers (b) according to the present invention are also the so-called "Constrained Geometry Catalysts "described, for example, in patents EP-416,815 and EP-418,044.

[0025] Component (c) is preferably selected from maleic anhydride grafted polyethylene (MAHg-PE). These type of products are obtained by modification of ethylenic resins by a chemical compound containing maleic acid or maleic anhydride. The ethylenic resins (e.g., PE resins), in unmodified form, can have a melt index in the range of about 0.1 to about 50 g/10 min and a density in the range of about 0.860 to 0.950 g/cm ³ . They can be any ethylene/alpha-olefin copolymer produced by conventional methods using Ziegler-Natta catalyst systems, Phillips catalyst systems, or metallocene-based transition metal catalyst systems. Thus, the copolymer can be a very low density polyethylene (VLDPE), a linear low density polyethylene (LLDPE), a medium density polyethylene (MDPE) having a density in the range of 0.926 to 0.940 g/cm ³ , or a high density polyethylene (HDPE) having a density greater than 0.940 g/cm ³ . These ethylenic resins can also be such resins as EVA, EEA, high pressure low density polyethylene (HP-LDPE) (HP-LDPE is a homopolymer), or ethylene/alpha-olefin copolymers produced by employing single site metallocene catalysts. These various ethylenic resins can be referred to generically herein as polyethylenes.

[0026] The content of grafted organo-functional group is preferably in the range of about 0.05 to about 10 weight percent based on the weight of the resin. Modification can be accomplished by, for example, solution, suspension, or melting methods. The solution method is effected by mixing an organo-functional group containing chemical, an ethylenic resin, a non-polar organic solvent and a free radical initiator such as an organic peroxide, and then heating the mixture to about 100 to about 160°C to perform the modification reaction.

[0027] When present the component (d) is preferably a copolymer of ethylene with at least one C4-C10 alpha-olefin, said copolymer (d) preferably having a density of from 0.912 to 0.922 g/cm ³ . This product may be prepared according to known low-pressure processes in the presence of a Ziegler-Natta catalyst, a chromium-based catalyst or a metallocene-based catalyst. Preferably, a metallocene based catalyst is used. In component (d) the alpha-olefin is preferably 1 -butene, 1 -hexene or 1-octene, and is present in the copolymer in an amount of from 1 to 12%, preferably 3-10% by moles. The melt flow rate (MFR) at 190°C with a load of 2.16 kg, according to ISO 1133-2:2011, preferably ranges from 0.5 and 20 g/10 min, preferably from 1.0 and 10 g/10 min.

[0028] The magnesium hydroxide component (e) can be natural magnesium hydroxide, synthetic magnesium hydroxide or treated magnesium hydroxide.

[0029] With the term natural magnesium hydroxide it is meant magnesium hydroxide obtained by grinding minerals based on magnesium hydroxide, such as brucite and the like.

[0030] Brucite was formed in the deposits of magnesium containing minerals where was the thermal activity and it is found in combination with other minerals such as magnesite, dolomite, serpentine and calcite.

[0031] The grinding can take place according to known techniques, under wet or dry conditions, preferably in the presence of grinding coadjuvants, for example polyglycols or the like. The specific surface area of the ground product is generally between 5 and 20 m2/g, preferably between 6 and 15 m2/g. The magnesium hydroxide thus obtained can then be classified, for example by sieving, to obtain an average particle diameter generally of between 1 and 15 um, preferably between 1.5 and 5 um, and a particle size distribution such that not more than 10% of the total number of particles have a diameter lower than 1.5 um, and not more than 10% of the total number of particles have a diameter greater than 20 um.

[0032] Natural magnesium hydroxide generally contains three main impurities like CaO, Si02 and Fe203 based compounds derived from its mineral origin. Amount and nature of the impurities can vary depending on the source of the starting mineral. The degree of purity is generally between 80 and 98% by weight.

[0033] Treated magnesium hydroxide is the product obtained by treating natural magnesium hydroxide surface with agents able to increase compatibility with the polymer matrix. Said agents can be saturated or unsaturated fatty acids containing from 8 to 24 carbon atoms, or metal salts thereof, such as, for example: oleic acid, palmitic acid, stearic acid, isostearic acid, lauric acid ; magnesium or zinc stearate or oleate; organic silanes or titanates such as vinyltriethoxysilane, vinyltriacetylsilane, tetraisopropyltitanate, tetra-n- butyltitanate, and the like. Particularly preferred is the surface treatment with organic silanes.

[0034] The synthetic magnesium hydroxide is obtained by precipitation techniques and is characterized by the presence of flattened hexagonal crystallites that are substantially uniform both in size and morphology.

[0035] Preferably, the components (a) to (e) of the composition of the present disclosure are present in amount such that the weight ratio (e)/(a)-(d) ranges from 1 : 1 to 1.65: 1 , the weight ratio (a)/(b) ranges from 0.85:1 to 1.15:1 and the weight ratio (a)/(e) ranges from 0.25:1 to 0.35:1.

[0036] In a preferred embodiment of the present disclosure the thermoplastic composition has the following composition expressed as parts per hundred resin (phr):

[0037] Component (a) from 25 to 60; preferably 30 to 50;

[0038] Component (b) from 25 to 60; preferably 30 to 50;

[0039] Component (c) from 5 to 20; preferably 7 to 15;

[0040] Component (d) from 5 to 20; preferably 7 to 15;

[0041] Component (e) from 120 to 180; preferably 130 to 170. [0042] As previously mentioned, the composition of the present disclosure has a melt flow rate (MFR) at 190°C with a load of 21.6 kg, according to ISO 1133-2:2011, of at least 2 g/10’, preferably at least 2.5 g/10’ and more preferably in the range from 4 to 8 g/10’.

[0043] In a further preferred embodiment, an additive package is present in amount from 1 to lO phr.

[0044] The additive package may comprise conventional components such as antioxidants, processing coadjuvants, lubricants, pigments, other fillers and the like can be added to the compositions of the present invention.

[0045] Conventional antioxidants which are suitable for this purpose are, for example: polymerized trimethyldihydroquinoline, 4,4'-thiobis (3-methyl-6- tert-butyl) phenol; pentaerythritol tetrakis 3- (3,5-di- tert-butyl-4-hydroxyphenyl) propionate, 2,2'-thio- diethylene-bis- 3- (3,5-di-tert-butyl-4-hydroxy-phenyl) propionate and the like, or mixtures thereof.

[0046] Fillers which may be used in the present invention include, for example, calcium carbonate, glass particles, glass fibres, calcined kaolin, talc and the like, or mixtures thereof. Processing co-adjuvants usually added to the polymer base are, for example, calcium stearate, zinc

[0047] As mentioned, the compositions of the present disclosure are thermoplastics, i.e, are not cross-linked. The coating of the cable made from the said composition is not cross-linked as well. This means that the compositions, even in the form of cable coating, are fully recyclable.

[0048] The flame-retardant compositions according to the present disclosure can be prepared by mixing the polymer components (a)-(d), the magnesium hydroxide (e), and other possible additives according to methods known in the art. The mixing can be carried out, for example, using an internal mixer of the type with tangential rotors (Banbury) or with interpenetrating rotors, or alternatively in continuous mixers such as those of the type Ko-Kneader (Buss), Farrel continuous mixer or of the type co-rotating or counter-rotating twin-screw.

[0049] As mentioned, it constitute another embodiment of the present disclosure an electrical cable coated with the said thermoplastic flame retardant compositions. The said composition can coat directly the conductor of the cable or coat a previously coated insulating or bedding layer. [0050] Cable coating can be carried out, for example, by extrusion. When two layers are present, the extrusion can be carried out in two separate stages, extruding the inner layer onto the conductor in a first run and the outer layer onto this inner or bedding layer in a second run, or alternatively, by co-extrusion using a single extrusion head.

[0051] The following examples are given in order to illustrate, but not limit the present disclosure.

EXAMPLES

CHARACTERIZATIONS

[0052] Melt Flow Rate (MFR)

[0053] Measured according to ISO 1133 at 190°C with a load of 21.6 kg, unless otherwise specified.

[0054] Samples for the mechanical tests

[0055] The samples were prepared on a roll mill with a gap of 1.2-1.3 mm between the rolls; the samples were homogenized at a temperature in the range 130-140°C and a mixing time of around 10 min.

[0056] Elongation at break: measured according to ISO 527 1-2

[0057] Tensile Stress at break: measured according to ISO 527 1-2

[0058] Cone calorimeter test

[0059] Carried out according to ISO 5660-1 2015

[0060] Measurement of Limited Oxygen Index (LOI).

[0061] The LOI was measured according to ISO 4589-2 on specimen with a thickness of 3 mm, a width of 6,5 mm and a length of 100 mm. The specimen were obtained by compression moulding and cutting..

[0062] MATERIALS

[0063] Component (a):

[0064] (al) Lucalen LC A2700H commercialized by Lyondellbasell. It is an EBA copolymer containing 15% of BA with a MFR of 1.4 g/10’ and a density of 0.922.

[0065] (a2) Lucalen LC A2700M commercialized by Lyondellbasell. It is an EBA copolymer containing 17% of BA with a MFR of 7.2 g/10’ and a density of 0.924. [0066] (a3) Elvax 265 commercialized by Dow. It is an EVA copolymer containing 28% of

VA with a MFR of 3.0 g/10’ and a density of 0.951.

[0067] Component (b)

[0068] Lucene LC180 commercialized by LG Chem. It is an ethylene/octene-1 copolymer obtained by polymerization in the presence of a metallocene catalysts having a density (ASTM D1505) of 0.885 g/cm ³ with a MFR of 1.2.

[0069] Component (c)

[0070] Compoline CO/UL05 commercialized by Auserpolimeri. It is a maleic anhydride grafted ethylene copolymer.

[0071] Component (d)

[0072] Exceed 3518CB commercialized by ExxonMobil Chem. It is an ethylene/hexene- 1 copolymer obtained by polymerization in the presence of a metallocene catalysts having a density of 0.918 g/cm ³ with a MFR of 3.5 g/10’

[0073] Component (e)

[0074] (el) Ecopiren 3.5 distributed by Europiren B.V and produced by Russian Mining Chemical Company LLC. It is a natural Mg hydroxide obtained by brucite grinding. It has aD50 particle size of 3-4 pm, a surface area of 11-13 m ² /g and a minimum Mg(OH)2 content of 92.8%.

[0075] (e2) Ecopiren 3.5NP distributed by Europiren B.V and produced by Russian Mining

Chemical Company LLC. It is a natural Mg hydroxide obtained by brucite grinding followed by surface treatment with alkyl silanes. It has a D50 particle size of 3-4 pm a surface area of 9-11 m ² /g and a minimum Mg(OH)2 content of 92.8%.

[0076] (e3) Apyral aluminum trihydrate commercialized by Nabaltec. It has a D50 particle size of 1-2 pm and a surface area of 3-6 m ² /g

[0077] Antioxidants: Silmastab AE1527 commercialized by Silma Sri

[0078] Processing aid: Silmaprocess ALl 142 commercialized by Silma Sri

[0079] Example 1-4 and comparative 1-2

[0080] The samples were prepared on a roll mill with a gap of 1.2-1.3 mm between the rolls; the samples were homogenized at a temperature in the range 130-140°C and a mixing time of around 10 min. [0081] The amount and type of components are provided in Table 1. Tests result are provided in Table 2.

Table 1 - Composition

Table 2 - Properties [0082] The above data show that the polymer compositions of the present disclosure, thanks to the presence of specific components in specific ratio, are able to provide excellent properties in terms of flame retardancy, heat release and low smoke generation combined with mechanical properties aligned with most spread out commercial solutions.