Cables with a halogen-free recyclable coating comprising polypropylene and an ethylene copolymer having high structural uniformity

A cable, in particular for power transmission, for telecommunications or for data transmission, or also combined power/telecommunications cables, wherein at least one coating layer consists of a recyclable material which is halogen-free and has superior mechanical and electrical properties. This material consists of a polymer mixture comprising: (a) a crystalline propylene homopolymer or copolymer; and (b) a copolymer of ethylene with at least one alpha-olefin having from 4 to 12 carbon atoms, and optionally with a diene; the said copolymer (b) being characterized by a density of between 0.90 and 0.86 g/cm.sup.3 and by a Composition Distribution Index, defined as the weight percentage of copolymer molecules having an alpha-olefin content within 50% of the average total molar content of alpha-olefin, of greater than 45%.

CROSS-REFERENCES TO RELATED APPLICATIONS
 Applicant claims the right of priority under 35 U.S.C. .sctn. 119(a)-(d)
 based on patent application No. MI97A 001739, filed Jul. 23, 1997, in
 Italy.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to cables, in particular for power
 transmission, for telecommunications or for data transmission, or also
 combined power/telecommunications cables, wherein at least one coating
 layer consists of a recyclable material which is halogen-free and has
 superior mechanical and electrical properties.
 2. Description of the Related Art
 There is currently a great need for highly environmentally friendly
 products, consisting of materials which are not harmful to the environment
 either during their production or when in use, and which are readily
 recyclable at the end of their working life. However, the option of using
 ecological materials is, in all cases, subject to the need to keep costs
 within acceptable limits, while still guaranteeing performances which are
 at least equivalent to those of conventional materials and which are, in
 any case, satisfactory under the most common conditions of use.
 In the cables sector, in particular power transmission cables, the various
 coatings surrounding the conductor commonly consist of crosslinked polymer
 materials, in particular polyethylene or ethylene copolymers suitably
 crosslinked during extrusion, so as to give satisfactory mechanical
 performances even under heating in continuous use and under conditions of
 current overload, while at the same time maintaining a high level of
 flexibility. These materials are crosslinked and therefore cannot be
 recycled since they are devoid of thermoplastic properties, hence they can
 only be disposed of at the end of their working life by means of
 incineration. Moreover, in certain cases the outer protective sheath
 consists of polyvinyl chloride (PVC) which is difficult to separate by
 conventional methods (for example in water by density differences) from
 the crosslinked polyolefins containing inorganic fillers (for example from
 ethylene/propylene rubbers containing inorganic fillers), and, on the
 other hand, PVC cannot be incinerated together with crosslinked
 polyolefins since this produces highly toxic chlorinated products by
 combustion.
 In U.S. Pat. No. 4,948,669 cable-coating compositions are described
 comprising from 29 to 50% by weight of low-density polyethylene,
 containing as comonomer an alpha-olefin having from 4 to 12 carbon atoms,
 in particular 1-octene, in an amount such as to obtain a density of
 between 0.90 and 0.92 g/cm.sup.3, in admixture with: (a) a propylene
 homopolymer; (b) a non-elastomeric copolymer of propylene with ethylene;
 or (c) heterogeneous copolymers of propylene with ethylene, obtained in
 reactor. As polyethylene it is particularly suggested using product
 Dowlex.RTM. 4000E from Dow Chemical, containing about 17% of 1-octene and
 having a melt index equal to 3.3 and a density of 0.912 g/cm.sup.3. These
 are products obtained using titanium-based Ziegler-Natta catalysts, having
 a relatively high density and thus little flexibility.
 In patent application WO 96/23311 a low-voltage high-current cable is
 described, wherein the insulating coating, the inner sheath and the outer
 sheath are made of the same non-crosslinked polymer-based material which
 is black coloured by addition of carbon black. Using the same base
 material would allow recycling without the need to separate different
 materials. As polymer material for the outer sheath, it is suggested
 using, in place of PVC, ultra-low-density polyethylene (ULD-PE), for
 example products Engage.RTM. from DuPont-Dow Elastomers and Exxpol.RTM.
 from Exxon. Inorganic fillers such as aluminium or magnesium hydroxide are
 added to these materials in order to give them flame-retardant properties.
 In U.S. Pat. No 5,246,783 cables are described, having as insulating and/or
 semiconductive coatings polymer materials based on copolymers of ethylene
 with at least one C.sub.3 -C.sub.20 alpha-olefin, with a density of from
 0.86 to 0.96 g/cm.sup.3, known commercially under the tradename Exact.RTM.
 from Exxon, preparable using metallocene catalysts. These copolymers are
 used in crosslinked form, achieved by chemical means (for example with
 dicumyl peroxide) or by irradiation.
 BRIEF DESCRIPTION SUMMARY OF THE INVENTION
 The Applicant has perceived that the technical problem of obtaining a cable
 with a coating made of a non-crosslinked, and thus recyclable, polymer
 material which also has mechanical and electrical properties suitable to
 the usual conditions of use is dependent on the use of a crystalline
 propylene homopolymer or copolymer mixed with a copolymer of ethylene with
 an alpha-olefin having a low density and a high structural uniformity, in
 particular having a highly homogeneous distribution of the alpha-olefin
 between the polymer molecules. This high structural uniformity is
 obtainable in particular by copolymerization of the corresponding monomers
 in the presence of a single-site catalyst, for example a metallocene
 catalyst.
 In particular, the Applicant has found that excellent performances, both in
 terms of mechanical properties, in particular elongation at break, stress
 at break and modulus, and in terms of electrical properties, may be
 obtained by using, as non-crosslinked base material for at least one of
 the coating layers of the cable, a mixture as defined hereinbelow,
 comprising polypropylene and a copolymer of ethylene with at least one
 C.sub.4 -C.sub.12 alpha-olefin and optionally with a diene comonomer,
 having a density of from 0.90 to 0.86 g/cm.sup.3 and a Composition
 Distribution Index, defined as the weight percentage of copolymer
 molecules having an alpha-olefin content within 50% of the average total
 molar content of alpha-olefin, of greater than 45%.
 Therefore, according to a first aspect, the invention relates to a cable
 comprising a conductor and one or more coating layers, wherein at least
 one of the said coating layers comprises, as non-crosslinked base polymer
 material, a mixture comprising: (a) a crystalline propylene homopolymer or
 copolymer; and (b) a copolymer of ethylene with at least one alpha-olefin
 having from 4 to 12 carbon atoms, and optionally with a diene; the said
 copolymer (b) being characterized by a density of from 0.90 to 0.86
 g/cm.sup.3 and a Composition Distribution Index, defined as the weight
 percentage of copolymer molecules having an alpha-olefin content within
 50% of the average total molar content of alpha-olefin, of greater than
 45%.
 According to a further aspect, the invention relates to a cable comprising
 a conductor and one or more coating layers, wherein at least one of the
 said coating layers has electrical insulating properties and comprises a
 mixture as defined above as non-crosslinked base polymer material.
 According to a further aspect, the invention relates to a cable comprising
 a conductor and one or more coating layers, wherein at least one of the
 said coating layers has semiconductive properties and comprises a mixture
 as defined above as non-crosslinked base polymer material.
 According to a further aspect, the invention relates to a cable comprising
 a conductor and one or more coating layers, wherein at least one of the
 said coating layers is an outer protective sheath and comprises a mixture
 as defined above as non-crosslinked base polymer material.
 According to a further aspect, the invention relates to a cable comprising
 a conductor and one or more coating layers, wherein at least 70%,
 preferably at least 90%, by weight relative to the total weight of the
 base polymer material of the said coating layers consists of the mixture
 as defined above.
 The Composition Distribution Index provides a measure of the distribution
 of the alpha-olefin between the copolymer molecules (the higher the value
 of this index, the more homogeneous is the distribution of the comonomer
 between the copolymer molecules) and can be determined by techniques of
 Temperature Rising Elution Fractionation, as described, for example, in
 patent U.S. Pat. No. 5,008,204 or in Wild et al., J. Poly. Sci. Poly.
 Phys. Ed., Vol. 20, p.441 (1982).
 The copolymers (b) have a molecular weight distribution index, defined as
 the ratio between the weight-average molecular weight M.sub.W, and the
 number-average molecular weight M.sub.n, which is generally low, usually
 between 1.5 and 3.5. The molecular weight distribution index can be
 determined by conventional methods, by means of Gel Permeation
 Chromatography (GPC).
 The copolymers (b) are also generally characterized by a melting enthalpy
 of from 30 to 60 J/g.
 Copolymers of ethylene with at least one C.sub.4 -C.sub.12 alpha-olefin,
 and optionally with a diene, having these characteristics are obtainable
 by copolymerization of ethylene with the alpha-olefin, and optionally with
 the diene comonomer, in the presence of a single-site catalyst, for
 example a metallocene catalyst, as described, for example, in U.S. Pat.
 Nos. 5,246,783 and 5,272,236, or alternatively they may be obtained
 commercially under the trademarks Engage.RTM. from DuPont-Dow Elastomers
 and Exact.RTM. from Exxon Chemical. The metallocenes used to polymerize
 the olefins are coordination complexes of a transition metal, usually from
 Group IV, in particular titanium, zirconium or hafnium, with two
 optionally substituted cyclopentadienyl ligands, used in combination with
 a co-catalyst, for example an alumoxane, preferably methylalumoxane, or a
 boron compound (see for example J. M. S.-Rev. Macromol. Chem. Phys.,
 C34(3), 439-514 (1994); J. Organometallic Chemistry, 479 (1994), 1-29, or
 alternatively patents U.S. Pat. Nos. 5,414,040, 5,229,478, WO 93/19107 and
 EP-A-632,065, or the already mentioned U.S. Pat. Nos. 5,246,783 and
 5,272,236). 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.
 With the term alpha-olefin it is meant an olefin of formula
 CH.sub.2.dbd.CH--R, where R is a linear or branched alkyl having from 2 to
 10 carbon atoms. The alpha-olefin may be selected, for example, from
 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-dodecene
 and the like. 1-hexene and 1-octene are particularly preferred.
 When a diene termonomer is present, this 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.
 Ethylene/alpha-olefin or ethylene/alpha-olefin/diene copolymers which can
 be used according to the present invention generally have the following
 composition: 75-97 mol %, preferably 90-95 mol %, of ethylene; 3-25 mol %,
 preferably 5-10 mol %, of alpha-olefin; 0-5 mol %, preferably 0-2 mol %,
 of a diene.
 The crystalline propylene homopolymer or copolymer (a) generally has a
 melting enthalpy of greater than 75 J/g, preferably greater than 85 J/g.
 It may be selected in particular from:
 (1) isotactic propylene homopolymers with an isotactic index of greater
 than 80, preferably greater than 90, even more preferably greater than 95;
 (2) propylene homopolymers obtainable using metallocene catalysts, having a
 pentad mmmm content of greater than 90% (determined by .sup.13 C-NMR
 analysis);
 (3) crystalline copolymers of propylene with ethylene and/or an
 alpha-olefin having from 4 to 10 carbon atoms, with an overall content of
 ethylene and/or alpha-olefin of less than 10 mol %;
 (4) heterogeneous propylene copolymers obtainable by block polymerization
 of propylene and of mixtures of propylene with ethylene and/or an
 alpha-olefin having from 4 to 10 carbon atoms, containing at least 70% by
 weight of polypropylene homopolymer or of crystalline propylene/ethylene
 copolymer, with an isotactic index of greater than 80, the remainder
 consisting of an elastomeric ethylene/propylene copolymer with a propylene
 content of from 30 to 70% by weight;
 (5) crystalline propylene homopolymers or copolymers of syndiotactic
 structure, obtainable using metallocene catalysts.
 According to the present invention, the ethylene/alpha-olefin or
 ethylene/alpha-olefin/diene copolymer (b) as described above is present in
 admixture with the crystalline propylene homopolymer or copolymer (a) in a
 predetermined amount, such as to make the resulting polymer mixture
 sufficiently flexible, and in particular so as to give it a elongation at
 break value, measured according to CEI standard 20-34, .sctn.5.1, of at
 least 100%, preferably of at least 200%, and a 20% modulus value, measured
 according to CEI standard 20-34, .sctn.5.1, of less than 10 MPa,
 preferably less than 7 MPa.
 In general, these characteristics are obtainable using mixtures comprising
 from 10 to 60%, preferably from 15 to 50%, by weight of crystalline
 propylene homopolymer or copolymer (a) and from 40 to 90%, preferably from
 50 to 85%, by weight of ethylene/alpha-olefin or
 ethylene/alpha-olefin/diene copolymer (b), the percentages being relative
 to the total weight of the polymeric components (a) and (b).
 In accordance with the present invention, the use of non-crosslinked
 polymer mixtures as defined above makes it possible to obtain a
 recyclable, flexible coating which has excellent mechanical properties,
 both in terms of modulus and in terms of elongation and stress at break.
 In particular, it is possible to obtain mechanical performances under
 heating, that is at 90.degree. C. for continuous use and at 130.degree. C.
 in the case of current overload, which are comparable with the typical
 performances of the polyethylene-based crosslinked coatings currently on
 sale, making the above-mentioned mixtures suitable not only for low
 voltage but also for medium- and high-voltage cables.
 The mechanical properties mentioned above are accompanied by excellent
 electrical properties, such as insulation constant (Ki) and dielectric
 loss (tan delta), both under dry conditions and when the cable is
 submerged in water. In particular, it has been found that the
 non-crosslinked material according to the present invention has a very
 high insulation constant which is maintained within acceptable values even
 after prolonged immersion in water.
 The fact that an insulating material has low water absorption makes it
 possible to reduce dielectric loss remarkably and thus to achieve lower
 energy dissipation levels, in particular during high power transmission.
 In the case of low-voltage high-current power transmission, low water
 absorption avoids an excessive reduction of electrical resistivity of the
 insulating material and thus of its electrical performance.
 The polymer mixtures according to the present invention are also capable of
 containing inorganic fillers without an unacceptable reduction in their
 mechanical and elastic properties, in particular as to elongation at
 break, which remains well above 100%. It is thus possible to produce
 compositions with flame-retardant properties which are endowed with high
 flexibility and high mechanical strength.
 Thus, according to a further aspect, the present invention relates to a
 flame-retardant polymer composition, comprising:
 (a) a crystalline propylene homopolymer or copolymer;
 (b) a copolymer of ethylene with at least one alpha-olefin having from 4 to
 12 carbon atoms, and optionally with a diene; the said copolymer (b) being
 characterized by a density of between 0.90 and 0.86 g/cm.sup.3 and by a
 Composition Distribution Index, defined as the weight percentage of
 copolymer molecules having an alpha-olefin content within 50% of the
 average total molar content of alpha-olefin, of greater than 45%;
 (c) an inorganic filler in an amount such as to impart flame-retardant
 properties.
 Moreover, a further aspect of the present invention resides in a cable
 comprising a conductor and one or more coating layers, wherein at least
 one of the said coating layers comprises a flame-retardant polymer
 composition as defined above.
 The inorganic filler is generally an inorganic oxide, preferably in hydrate
 or hydroxide form. Examples of suitable compounds are aluminium, bismuth,
 cobalt, iron, magnesium, titanium or zinc oxides and the corresponding
 hydroxides, or mixtures thereof. Magnesium hydroxide, aluminium hydroxide
 and alumina trihydrate (Al.sub.2 O.sub.3.3H.sub.2 O) or mixtures thereof
 are particularly preferred. One or more inorganic oxides or salts such as
 CoO, TiO.sub.2, Sb.sub.2 O.sub.3, ZnO, Fe.sub.2 O.sub.3, CaCO.sub.3 or
 mixtures thereof may advantageously be added to these compounds in minor
 amounts, generally less than 25% by weight. Preferably, the
 above-mentioned metal hydroxides, in particular magnesium and aluminium
 hydroxides, are used in the form of particles having sizes which can range
 from 0.1 to 100 .mu.m, preferably from 0.5 to 10 .mu.m. In the case of
 hydroxides, these may advantageously be used in the form of coated
 particles. Saturated or unsaturated fatty acids containing from 8 to 24
 carbon atoms, and metal salts thereof, are usually used as coating
 materials, such as, for example: oleic acid, palmitic acid, stearic acid,
 isostearic acid, lauric acid; magnesium or zinc stearate or oleate; and
 the like.
 The amount of inorganic filler which is suitable for imparting
 flame-retardant properties may vary within a wide range, generally between
 10 and 80% by weight, preferably between 30 and 70% by weight, with
 respect to the total weight of the composition.
 A coupling agent selected from those known in the art, for example silane
 compounds or carboxylic derivatives having at least one ethylenic
 unsaturation can be added to the mixture in order to enhance the
 compatibility between the inorganic filler and the polymer matrix.
 Examples of silane compounds which are suitable for this purpose are:
 .gamma.-methacryloxypropyltrimethoxysilane, methyltriethoxysilane,
 methyltris(2-methoxy ethoxy)silane, dimethyldiethoxysilane, vinyltris
 (2-methoxyethoxy)silane, vinyltrimethoxysilane, vinyl triethoxysilane,
 octyltriethoxysilane, isobutyl triethoxysilane, isobutyltrimethoxysilane
 and mixtures thereof.
 Carboxylic derivatives with ethylenic unsaturation which may advantageously
 be used as coupling agents are, for example, unsaturated carboxylic
 anhydrides or, preferably, unsaturated dicarboxylic anhydrides; maleic
 anhydride is particularly preferred. Alternatively, it is possible to use
 polyolefins as compatibilizing agents, these polyolefins optionally
 containing ethylenic unsaturations, on which carboxylic groups have been
 grafted by reaction with the above-mentioned carboxylic derivatives having
 at least one ethylenic unsaturation.
 The coupling agent, either of silane type or of carboxylic type, can be
 used in its normal state or can be grafted to at least one of the polymer
 components of the mixture.
 The amount of coupling agent to be added to the mixture may vary mainly
 depending on the type of coupling agent used and on the amount of
 inorganic filler added, and is generally between 0.05 and 30%, preferably
 between 0.1 and 20%, by weight, relative to the total weight of the base
 polymer mixture.
 Other conventional components such as antioxidants, fillers, processing
 co-adjuvants, lubricants, pigments, water-tree retardant additives and the
 like are usually added to the base polymer material. In the case of the
 semiconductive layers 3 and 5, the polymer material is preferably filled
 with carbon black in an amount such as to give this material
 semiconductive properties (namely, so as to obtain a resistivity of less
 than 5 ohm.m at room temperature).
 Suitable conventional antioxidants are, for example: polymerized
 trimethyldihydroquinoline, 4,4'-thiobis(3-methyl-6-tert-butyl)phenol;
 pentaerythryl-tetra[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate],
 2,2'-thiodiethylene-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]
 and the like, or mixtures thereof.
 Other fillers which may be used in the present invention include, for
 example, 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 stearate, stearic
 acid, paraffin wax and the like, or mixtures thereof.

DETAILED DESCRIPTION OF THE INVENTION
 In FIG. 1, the electrical cable 1 comprises a conductor 2; an inner layer 3
 with semiconductive properties; an intermediate layer 4 with insulating 5
 properties; an outer layer 5 with semiconductive properties; a screen 6;
 and an outer sheath 7.
 The conductor 2 generally consists of metal wires, preferably made of
 copper or aluminium, which are braided together using conventional
 techinques.
 At least one of the layers 3, 4 and 5, and preferably at least the
 insulating layer 4, comprises polypropylene as non-crosslinked base
 polymer material, mixed with a copolymer of ethylene with at least one
 alpha-olefin, and optionally with a diene, as defined above. In a
 preferred embodiment of the present invention, all of the insulating and
 semiconductive layers 3, 4 and 5 comprise a polymer mixture as defined
 above as non-crosslinked base polymer material.
 A screen 6, generally consisting of helically wound electrically conductive
 wires or strips, is usually placed around the outer semiconductive layer
 5. This screen is then covered with a sheath 7, consisting of a
 thermoplastic material such as polyvinyl chloride (PVC), non-crosslinked
 polyethylene (PE) or, preferably, a mixture comprising polypropylene and
 an ethylene/alpha-olefin or ethylene/alpha-olefin/diene copolymer, as
 defined above.
 FIG. 1 shows only one possible embodiment of a cable according to the
 present invention. It is clear that suitable changes known in the art may
 be made to this embodiment without thereby departing from the scope of the
 present invention. In particular, the recyclable polymer mixtures
 according to the present invention may advantageously also be used for
 coating telecommunications cables or data transmission cables, or
 alternatively combined power/telecommunications cables.
 The properties of the polymer materials used according to the present
 invention (Cop. 1 and 2) and of the material used for comparative purposes
 (Cop. 3) are given in Table 1. As melting enthalpy the second melting
 value (.DELTA.H.sub.2m) is given, obtained by DSC at a scan speed of
 10.degree. C./min. The melt flow index (MFI) was measured according to
 ASTM standard D 1238/L (at 230.degree. C. and 21.6 N for polypropylene,
 and at 190.degree. C. and 21.6 N for ethylene/1-octene copolymers). The
 Composition Distribution Index (CDI) was determined by Temperature Rising
 Elution Fractionation techniques.
 TABLE 1
 Polymer Density MFI CDI .DELTA.H.sub.2m
 material (g/cm.sup.3) (dg/min) (%) (J/g)
 PP 1 0.900 1.6 -- 98
 PP 2 0.900 1.8 -- 90
 Cop. 1 0.885 1.0 &gt;70 55.6
 Cop. 2 0.868 0.5 &gt;70 34.4
 Cop. 3 0.902 3.0 -- 78.0
 PP 1 (Moplen.RTM. S30G--Montell): isotactic poly propylene (homopolymer);
 PP 2 (Moplen.RTM. EP2S30B--Montell): random crystalline propylene/ethylene
 copolymer;
 Cop. 1 (Engage.RTM. 8003--DuPont-Dow Elastomers): ethylene/1-octene
 copolymer with 82/18 weight ratio (5.5 mol % of 1-octene), obtained by
 metallocene catalysis;
 Cop. 2 (Engage.RTM. 8150--DuPont-Dow Elastomers): ethylene/1-octene
 copolymer with 75/25 weight ratio (7.6 mol % of 1-octene), obtained by
 metallocene catalysis;
 Cop. 3 (Stamylex.RTM. TMX 1000--DSM): ethylene/1-octene copolymer (4.6 mol
 % of 1-octene), obtained using a titanium Ziegler-Natta catalyst.
 The polymer materials in Table 1 were used to prepare the mixtures given in
 Table 2.
 The mixtures 1-3a were prepared in a Brabender mixer (volume of the mixing
 chamber: 80 cm.sup.3), filled to 95% of volume. Mixing was carried out at
 a temperature of 170.degree. C. for a total time of 10 min (rotor speed:
 40 rpm). At the end of the mixing, the final torque (reported in Table 2)
 was measured under the abovementioned conditions.
 Mixtures 4, 5 and 6 were prepared in a 20 mm-diameter counter-rotatory
 Brabender twin-screw mixer with a rotor speed of 50 rpm and with the
 following temperature profile: 1st zone=100.degree. C., 2nd
 zone=160.degree. C., 3rd zone=190.degree. C., 4th zone=190.degree. C.
 For the filled systems there were used:
 Hydrofy.RTM. GS-1.5: Mg(OH).sub.2 coated with stearic acid from SIMA
 (average particle diameter: 2 .mu.m; specific surface: 11 m.sup.2 /g);
 Rhodorsil.RTM. MF175U: silicone rubber from Rhone-Poulenc acting as
 processing co-adjuvant/lubricant.
 The following were used as antioxidants:
 Irganox.RTM. 1010: pentaerythritol
 tetra[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (Ciba-Geigy);
 Irganox.RTM. PS802 FL: distearyl thiodipropionate (DSTDP) (Ciba-Geigy).
 The compositions are given in Table 2 as phr (i.e. parts by weight per 100
 parts of polymer matrix).
 The mixtures thus obtained were subjected to mechanical tensile strength
 tests according to CEI standard 20-34, .sctn.5.1, on test specimens
 obtained from 1 mm-thick plates obtained by compression moulding at
 190-195.degree. C. and 200 bar after preheating for 5 min at the same
 temperature. The pulling speed of the clamps was 250 mm/min for mixtures 1
 -3a, and 50 mm/min for mixtures 4, 5 and 6. The results are given in Table
 2.
 TABLE 2
 EXAMPLE 1 1a 2 2a 3(*) 3a(*) 4 5 6(*)
 PP 1 -- -- -- -- -- -- 40 40 40
 PP 2 35 35 35 35 35 35 -- -- --
 Cop. 1 65 65 -- -- -- -- 60 -- --
 Cop. 2 -- -- 65 65 -- -- -- 60 --
 Cop. 3 -- -- -- -- 65 65 -- -- 60
 Hydrofy .RTM. GS-1.5 -- 160 -- 160 -- 160 -- -- --
 Rhodorsil .RTM. MF175U -- 1.5 -- 1.5 -- 1.5 -- --
 --
 Irganox .RTM. PS 802FL -- -- -- -- -- -- 0.2 0.2
 0.2
 Irganox .RTM. 1010 -- 0.5 -- 0.5 -- 0.5 0.1 0.1 0.1
 Final torque (N .multidot. m) 6.2 9.8 7.8 11.2 6.1 7.3 --
 -- --
 Stress at break (MPa) 16.7 10.5 17.5 10.4 6.9 5.5 15.1 20.4 9.1
 Elongation at break (%) 662 567 713 621 711 54 702 695
 33
 10% modulus (MPa) -- -- -- -- -- -- 4.1 4.5 8.3
 20% modulus (MPa) 6.0 5.6 4.8 4.7 8.0 6.6 -- -- --
 (*)comparative