NO831998L - CROSS-BONDED POLYMER MIXTURES, PROCEDURE FOR PREPARING SUCH AS, AND USE OF THEM - Google Patents

Abstract

1. Crosslinked material, having improved properties, obtained from copolymers of ethylene and of at least one alpha-olefine, characterized in that its gel factor is between about 95 and 99% and in that the said copolymers are chosen from amongst : (a) ethylene/propylene copolymers which comprise, if appropriate, up to 4 mol% of units derived from another alpha-olefine, have a density of between 0.905 and 0.930 g/cm**3 and a melt index of between about 0.4 and 3 dg/min, possessing from 32 to 62 methyl groups per 1,000 carbon atoms and are such that their density rho, expressed in g/cm**3 , and the proportion m of methyl groups therein are related to one another as follows : 0.9530 =< rho + 0.83 m =< 0.9568, (b) copolymers of ethylene and of at least one alpha-olefine containing at least 4 carbon atoms, the said copolymers having a density of between 0.905 and 0.940 g/cm**3 , a melt index of between 0.2 and 20 dg/min and an average proportion of alpha-olefine units of between 1 and 8 mol%, comprising crystalline fractions and amorphous fractions and having a heterogeneous distribution of the alpha-olefine units, and having a proportion of alpha-olefine units varying between 0.2 and 5 times their average proportion, depending on the fractions in question, (c) compositions comprising up to 40% by weight of an ethylene polymer prepared by free radical polymerization and at least 60% by weight of a copolymer of type (a) and/or of a copolymer of type (b), (d) copolymers of ethylene and of at least one alpha-olefine having from 3 to 12 carbon atoms, the said copolymers comprising from 0.5 to 10 mol% of units derived from the alpha-olefine, modified by the introduction of long-chain branches in such a quantity that their measured limiting viscosity is between 1.3 and 10 times their limiting viscosity calculated from the molecular weight distribution, (e) compositions comprising at least 60% by weight of at least one copolymer of type (a) and/or of type (b) and up to 40% by weight of an ethylene polymer having a specific gravity of between 0.956 and 0.970 and a melt index of between 0.1 and 1 dg/min, (f) compositions containing a mixture of at least one copolymer of type (a) and at least one copolymer of type (b), and (g) compositions containing several copolymers of type (b), more particularly several copolymers having markedly different melt indices, it being possible for the copolymers of types (a) and (b) to be obtained by copolymerization of ethylene and at least one alpha-olefine in at least one reactor comprising at least one zone, at a temperature of between 180 degrees and 320 degrees C and under a pressure of between 300 and 2,500 bars, by means of a Ziegler-type catalyst system comprising, on the one hand, at least one activator chosen from amongst the hydrides and organometallic compounds of the metals of groups I to III of the Periodic Table, and, on the other hand, at least one transition metal halogen.

Description

The present invention relates to cross-linked polymer mixtures of ethylene and at least one oC-olefin, a method for producing such, and their use in the production of cables for electric current.

French patent document No. 2,432,535 describes highly cross-linked polymer mixtures comprising a linear ethylene copolymer having a density, before cross-linking, of less than 0.940 g/cm 3 and comprising less than 30 protruding CH 2 groups per 1000 carbon atoms, in reality totally absent in long chains. Their cross-linking ratio, expressed in terms of the gel ratio, is at least 40% and can be 95%.

U.S. Patent No. 3,105,057 describes compositions comprising an ethylene/but-1-ene copolymer containing from 5 to 30% by weight of but-1-ene, a finely divided filler and an organic peroxide cross-linking agent. U.S. Patent No. 3,173,903 describes crosslinking of ethylene/propylene copolymers having a specific gravity of less than 0.92 and containing from 7 to 20 mol% propylene, prepared in a homogeneous liquid phase under a pressure of less than 100 bar and at a temperature below 85°C, using a mixture of peroxide and carbon black.

We have found that if known materials are replaced, by a cross-linking process, with a copolymer of ethylene and at least one oC-olefin containing a high proportion of units derived from ethylene, as defined below, the gel ratio of the cross-linked polymer obtained is increased .

An object of the present invention consists in cross-linked polymer mixtures which have improved properties and are obtained from copolymers of ethylene and at least one oO-olefin, characterized in that their gel ratio is between 95 and 99% and in that said copolymer is selected from: (a) Ethylene /propylene copolymers which, if appropriate, comprise up to 4 mol% of units derived from another o£-olefin, have a density of between 0.905 and 0.930 g/cm 3 and a melt index of between approx. 0.4 and 3 dg/minute, have from 32 to 62 methyl groups per 1000 carbon atoms and is such that their density p, expressed in g/cm 3, and the proportion m of methyl groups therein are related to each other in the following way: 0 .9530 ^f+ 0.83 m^0.9568, (b) copolymers of ethylene and at least one <?^-olefin containing at least 4 carbon atoms, said copolymers having a density of between 0.905 and 0.940 g/cm<3>, a melt index of between 0.2 and 20 dg/minute and an average proportion of <<.-olefin units of between 1 and 8 mol%, includes crystalline fractions and amorphous fractions and has a heterogeneous distribution of the <*j-olefin units, and has a proportion of c^-olefin units varying between 0.2 and 5 times their average proportion, depending on the fractions concerned, (c) mixtures comprising up to 40% by weight of an ethylene polymer produced by free radical polymerization and at least 60% by weight of a copolymer of type (a) and/or of a copolymer of type (b), (d) modified copolymers of ethylene and of at least one oC-olefin having from 3 to 12 carbon atoms, wherein said modified copolymers comprise from 0.5 to 10 mol% of units derived from the cC olefin, and wherein said copolymers are modified by the introduction of long-chain branches in such an amount that their measured intrinsic viscosity is between 1.3 and 10 times their intrinsic viscosity calculated from the molecular weight distribution, (e) mixtures comprising at least 60% by weight of at least one copolymer of type (a) and/or of type (b) and up to 40% by weight of an ethylene polymer having a specific gravity between 0.956 and 0 .9 70 and a melt index of between 0.1 and 1 dg/minute, (f) mixtures comprising a mixture of at least one copolymer of type (a) and at least one copolymer of type (b), and (g) mixtures which comprises several copolymers of type (b), and more particularly several copolymers having melt indices which are significantly different.

It is advantageous that the second oC-olefin included in the structure of the copolymers of type (a), and the o<L-olefin used in the preparation of the copolymers of type (b), contain from 4 to 10 carbon atoms. They are selected, for example, from but-1-ene, 4-methylpent-1-ene, hex-1-ene, oct-1-ene and dec-1-ene.

It is advantageous that the copolymers of types (a) and (b) are obtained by copolymerization of ethylene and at least one cC~olefin in at least one reactor comprising at least one zone, at a temperature between 180 and 320°C and under a pressure of between 300 and 2500 bar, using a Ziegler-type catalyst system comprising, on the one hand, at least one activator selected from the hydrides and organometallic compounds of the metals in groups I to III of the periodic table, and on the other hand, at least one transition metal -halogen compound.

The copolymers of type (a) advantageously have a number average molecular weight of between 10,000 and 28,000 and a polydispersity index (ratio between weight average molecular weight and number average molecular weight, both determined by gel permeation chromatography) of between 7 and 14. They are advantageously obtained by the copolymerization of ethylene and propylene in at least one reactor comprising at least one zone, at a temperature between 180 and 320 °C and under a pressure of between 300 and 2500 bar, using a Ziegler-type catalyst system which on the one hand comprises an activator selected from the hydrides and the organometallic compounds of the metals in groups I to III of the periodic table, and on the other hand at least one transition metal-halogen compound, where the gas stream feed to the reactor, in stable operation, consists of 65 to 85 wt% ethylene and 15 to 35 wt% propylene, and the transition metal halogen compound comprises

a complex compound with the formula (MCI ) (MgX0), -(A1C1.J -

Cl£. yOZ (RMgX)khvori M is a metal selected from titanium and vanadium, X is a halogen atom, R is a hydrocarbon radical, 2^a^3, 2^y^-20, 0-S 1/3 and O^b^l .

The copolymers of type (b) advantageously have a number average molecular weight of between 15,000 and 60,000 and a polydispersity index of between 3 and 9. Their crystalline fractions may have a single melting point, determined by differential thermal analysis, of between 118 and 130°C, and kai constitute from 20 to 50% by weight of the total copolymer of type (b). The copolymers of type (b) may comprise units originating from two C 2 -olefins, the respective average proportions of which have a ratio between 0.25 and 4. In the latter case, the copolymers of type (b) preferably have a polydispersity index of between 6 and 12. The copolymers of type (b) are advantageously produced by the copolymerization of ethylene and at least one °C,-olefin having at least 4 carbon atoms, in at least one reactor comprising at least one zone, at a temperature of between 180 and 320°C and under a pressure of between 300 and 2500 bar, by means of a Ziegler-type catalyst system comprising, on the one hand, an activator selected from the hydrides and the organometallic compounds of the metals in groups I to III of the periodic table tem, and on the other hand of at least one transition metal-halogen compound, where the atomic ratio between the metal in the activator and the transition metal is between 1<p>g 10 and the average residence time of the catalytic system in polymerization the reactor is between 2 and 100 seconds, where the gas stream feed to the reactor, in stable operation, consists of 10 to 80% by weight ethylene and 20 to 90% by weight °C-olefin, and where the catalyst system has a reactivity with respect to ethylene which is much better than its reactivity with respect to C 2 -olefins.

A non-limiting example of such compounds has the formula:

(TiCl3,jA1C13)(MX3)x(MgCl2)y,

in which 0.3<x<3, 0<y«$ 20, M is a transition metal selected from groups VB, VIB and VIII of the periodic table and X is a halogen atom. The mixture of gas-stream feed to the reactor must, during stable operation, be understood as an average for the mixture over the entire reactor, and it must be understood that this mixture is not necessarily uniform and that it can vary along the reactor, especially if the reactor comprises several zones . This mixture varies according to the nature of the c^-olefin in question. For a binary copolymer, the weight proportion of ω-olefin in this gas stream is thus preferably between 15 and 70% for but-1-ene and between 35 and 90% for hex-1-ene. If the reactor used in the method according to the invention comprises several zones, it is most often preferred to introduce the main amount of the <o>^-olefins in the first zone, which operates at temperatures between 180 and 240°C, while the last zone operates at a temperature between 240 and 320°C, without the introduction of any significant additional quantity of oC.-olefins.

The copolymers of ethylene and at least one oC-olefin of type (d) are advantageously prepared by a process which consists in copolymerizing the ethylene with at least one (Xroolefin in at least one reactor, in the presence of a catalyst system of the Ziegler type, at a temperature of 180 to 320°C and under a pressure of .300 to 2500 bar, where the average residence time for the catalyst system in the reactor is between 1 and 120 seconds, and then bringing the obtained copolymer into contact with 0.01 to 1 millimol per kg copolymer of at least one free radical initiator, at a temperature between 220 and 320°C for a period of between 5 and 200 seconds under a pressure of less than 1000 bar. The latter pressure is chosen to avoid polymerization of the unreacted monomer and consequently formation of low-density polyethylene by free-radical polymerization. The o£-olefin is either introduced or formed by the oligomerization of the ethylene. The proportion of o(_-olefin in the reaction mixture depends on the nature of said oC_olefin. It is advantageous between 15 and 35% by weight for propylene, between 5 and 65% by weight for but-1-ene and between 5 and 80% by weight for hex-1-ene. It is not necessarily constant along the reactor. The obtained copolymer can be brought into contact with at least one free radical initiator by introducing a solution or suspension of the free radical initiator in the last reaction zone in the reactor or set of reactors, and it should be understood that the pressure in said reactor is less than 1000 bar. They can also be brought into contact in a separator which operates under a pressure of 50 to 500 bar, and which is placed downstream of the reactor and has the purpose of separating the formed copolymers from the unreacted monomers. They can also be brought into contact in the copolymer conversion machine, such as for example an extruder or a plasticizing machine, suitably under a pressure of between 1 and 25 bar. It is well known that the pressure in the extruder can reach 250 bar, depending on the profile of the extruder screw. The free radical initiator is advantageously brought into contact with the copolymer in the oil forming machine using a solution of the initiator or a dry concentrate mixture.

In a particular embodiment, they can be cross-linked mixtures

according to the invention is obtained from mixtures which on the one hand comprise a copolymer of ethylene and a c£-olefin of type (a), as defined above, and on the other hand comprise an ethylene polymer produced by free radical polymerization. It is advantageous that the ethylene polymer produced by free radical polymerization has a density between 0.91 and 0.94 g/cm 3. It is obtained by polymerizing ethylene under high pressure and at high temperature, in the presence of a free radical initiator. The polymerization of ethylene can be carried out in the presence of small amounts of at least one monomer that is copolymerizable with ethylene, such as, for example, carbon monoxide, ethylenically unsaturated carboxylic acids, the esters derived from said acids and from an alcohol having from 1 to 8 carbon atoms, maleic anhydride, vinyl acetate or lig-

nende.

According to a special embodiment, the crosslinked mixtures according to the invention can be obtained from mixtures of type (e) which on the one hand comprise at least 60% by weight of at least one copolymer of type (a) and/or of type (b), and on the other hand comprises up to 40% by weight of an ethylene polymer having a specific gravity of between 0.956 and 0.970 and a melt index of between 0.1 and 1 dg/minute. Said polymer is usually an ethylene homopolymer, called high-density polyethylene, which can be produced by various polymerization processes that are already known, especially the processes at high pressure and high temperature, and several embodiments of which are described in the French patents documents Nos. 2,346,374, 2,399,445, 2,445,343 and 2,445,344. High-density polyethylene preferably has a broad molecular weight distribution, and its polydispersity index is, for example, between 8 and 18. The mixtures of type (a) have a specific gravity of between 0.910 and 0.940.

The mixtures of type (g) are, for example, a mixture of 2 copolymers of ethylene and but-1-ene which have a melt index between 5 and 10 dg/min, and which are obtained from copolymers which have melt indices of respectively approx. 1 and approx. 20 dg/min.

As is known, the cross-linked mixtures according to the invention can contain fillers, such as carbon black, metal oxides, carbonates or sulphates, antioxidants, lubricants and pigments.

They are obtained by reacting, with the copolymers, of ethylene and at least one oC-olefin which are used as starting materials, a peroxide compound selected from benzoyl peroxide, dicumyl peroxide, 1,3-bis-(tert-butylperoxyisopropyl)-benzene , di-tert-butyl peroxide, di-tert-amyl peroxide, lauroyl peroxide and mixtures thereof, in an amount of 0.5 to 2.5% by weight, with respect to the polymer to be cross-linked. The crosslinking is usually carried out at a temperature between 180 and 250°C for a period of between 5 and 20 minutes. It can be carried out under pressure or in molten salt baths.

The crosslinked mixtures according to the invention are advantageously used in the production of cables for electric current.

The purpose of the non-limiting examples that follow is

to illustrate the invention. Unless otherwise stated, parts means parts by weight.

The measurements carried out with the crosslinked mixtures consist of: - the proportion of insoluble materials (gel ratio), determined according to ASTM Standard specification D-2765/72 and expressed in weight% - the creep ratio, determined according to UTE Standard specification 33209 and expressed in % - the elongation at break EB (expressed in %), according to NFC Standard specification 32101 - the percentage residual elongation, according to HN Standard specification 33523 and CEI Standard specification 538.

OAK SAMPLE 1

An ethylene/propylene copolymer of type (a), and with the following characteristics, was used: 0.905 g/cm 3 , MI = 3 dg/minute, mol% with propylene units: 12, proportion of methyl groups: 60 per 1000 carbon atoms.

It was produced by copolymerization of ethylene and propylene in an autoclave reactor of cylindrical shape, which was operated at a pressure of 1200 bar, and was internally provided with an agitator and metal screens delimiting 3 zones of identical volume. Zone 1 was maintained at a temperature of 220°C and was fed with a flow of 40 kg/hr" of ethylene and 30 kg/hr of propylene. It received a catalytic system comprising on the one hand violet titanium trichloride TiCl^,(1/3)AlCl^ and on the other hand trioctyl aluminum in such an amount that the atomic ratio Al/Ti was 3. Zone 2 was maintained at a temperature of 240°C and was fed with a flow of 20 kg/hr of ethylene and it received no catalyst at all. Zone 3 was maintained at a temperature of 260°C and was fed with a flow of 20 kg/hour of ethylene and it received a catalytic system comprising on the one hand a complex compound of the formula TiCl.j" (1/ 3) AlCl3,6MgCl2 obtained by grinding together violet titanium trichloride and anhydrous magnesium chloride, and on the other hand dimethylethyldiethylsiloxane in such an amount that the atomic ratio Al/Ti was 3.

The reaction mixture containing the copolymer was evacuated from the outlet of zone 3 to a separation and recycling apparatus. The copolymerization was carried out in the presence of 0.1

% hydrogen by volume.

100 parts of the copolymer obtained was mixed with 50 parts of carbon black in a mixer at 80°C for 3 minutes. Then 1 part of 2,2,4-trimethyl-1,2-dihydroquinoline polymerizate (antioxidant A) and 5 parts of [1,3-phenylene-bis-(1-methylethylidene)]-bis-C(1,1 -dimethylethyl)peroxide](peroxide P) containing 40% by weight of active substance (that is, 2 parts of pure peroxide) added in a mixer at 120°C for 5 minutes.

The resulting mixture was crosslinked under a pressure of 35 bar for 12 minutes at 180°C.

Measurements were made with respect to the percentage of insoluble materials after cross-linking for 12 minutes at 180°C, the creep ratio at 150°C for 15 minutes under 2, 4 and 7 bar, and the percentage residual elongation. A measurement was also made of the time, in seconds, required to achieve a degree of cross-linking of 90% at 185°C using I.F.C. optio-meter of type M.I.E.

EXAMPLE 2

A heterogeneous ethylene/but-l-ene copolymer of type (b) described above was used, and with the following characteristics: = 0.918 g/cm 3 , MI = 0.8 dg/minute, mol% of but-l- en units = 3.4%, number average molecular weight: 43,000, pclydispersity index Mw/Mn = 3.6, proportion of lateral methyl groups: 17. per 1000 carbon atoms, melting point of the crystalline fraction = 122°C, heterogeneity of but-1-ene distribution = 0.5 to 2.2 times the average proportion, depending on the fractions considered.

This copolymer was produced in an autoclave reactor of cylindrical shape, which operated under a pressure of 900 bar, and which was internally provided with a stirrer and metal screens which defined three zones. Zone 1, which was maintained at a temperature of 210°C, had twice the volume of each of the following two zones, it was fed with a flow of 200 kg/hr of a mixture comprising 36 wt% but-l- one and 64% by weight of ethylene, and it received a catalytic system comprising on the one hand dimethylethyldiethylsiloxalane and on the other hand a compound with the formula TiCl3, 1/3A1C13, VC13 in such respective amounts that the atomic ratio Al/Ti was equal to 3. Zone 2, which was maintained at a temperature of 240°C, was fed with a flow of 55 kg/hour of the same mixture as above and it received the same catalytic system. Finally, zone 3, at the end of which the reaction mixture containing the copolymer was carried out and fed into a separation and recycling device, was maintained at a temperature of 280°C and received neither monomer nor catalyst. The average residence time for the catalytic system in the reactor was 43 seconds.

The obtained copolymer was crosslinked under the same conditions as in example 1.

EXAMPLE 3

An ethylene/propylene/but-1-ene terpolymer of type (a) was used, which was prepared under the conditions described in example 1, but a fraction of the propylene gas stream was replaced with but-1-ene, and it had the following characteristics: ^ = 0.906 g/cm 3 , MI = 0.7 dg/minute, mol% of propylene units: 7, mol% of but-1-ene units: 2.4, proportion of lateral alkyl groups: 47 per 1000 carbon atoms.

This terpolymer was crosslinked under the conditions from example 1.

The results of the measurements regarding Examples 1 to 3 are shown in Table I.

EXAMPLES 4 and 5

The copolymer and terpolymer, described in example 2 and example 3 respectively, were crosslinked under the following conditions.

100 parts of polymer were mixed in a mixer at 120°C for 5 minutes with 0.55 parts of antioxidant A and 32 parts of peroxy

syd P which titrates 40% active substance (ie 1.28 parts of pure peroxide P). The mixture was crosslinked under a pressure of 35 bar for 12 minutes at 180°C.

The results of the measurements are shown in table II.

EXAMPLES 6 and 7

To 100 parts of copolymer of type (a), described in example 1, under the conditions of examples 4 and 5, 0.5 part of antioxidant A and the amounts, in parts by weight, of pure peroxide P indicated in table were added III.

The crosslinking was carried out at a temperature of 180°C for a contact time of 12 minutes. The characteristics of the cross-linked compounds obtained were measured. The creep was measured under 2 bar at 200°C.

The results of the measurements are shown in table III.

Claims (11)

1. Cross-linked polymer mixtures which have improved properties and which are obtained from copolymers of ethylene and at least one °C -olefin, characterized in that their gel ratio is between 95 and 99% and in that said copolymers are selected from: (a) ethylene/propylene copolymers which, if appropriate, comprise up to 4 mol% of units derived from another C-olefin, have a density of between 0.905 and 0.930 g/cm 3 and a melt index of between about 0.4 and 3 dg/minute, have from 32 to 62 methyl groups per 1000 carbon atoms and is such that their density £, expressed in g/cm , and the proportion m of methyl groups therein are related to each other in the following way: 0.9530i£+ 0.83 m <<> 0.9568, (b) copolymers of ethylene and at least one c<.-olefin containing at least 4 carbon atoms, said copolymers having a density of between 0.905 and 0.940 g/cm <3> , a melt index of between 0.2 and 20 dg/minute and an average proportion of cC-olefin units of between 1 and 8 mol%, comprises crystalline fractions and amorphous fractions and has a heterogeneous distribution of the oC-olefin units, and has a proportion of o£_-olefin units that varies between 0.2 and 5 times their average share, depending on the fractions concerned, (c) mixtures comprising up to 40% by weight of an ethylene polymer produced by free radical polymerization and at least 60% by weight of a copolymer of type (a) and/or of a copolymer of type (b), (d) modified copolymers of ethylene and of at least one o(_-olefin having from 3 to 12 carbon atoms, wherein said modified copolymers comprise from 0.5 to 10 mol% of units derived from the o^-olefin, and wherein said copolymers are modified by the introduction of long-chain branches in such an amount that their measured intrinsic viscosity is between 1.3 and 10 times their intrinsic viscosity calculated from the molecular weight distribution, (e) mixtures comprising at least 60% by weight of at least one copolymer of type (a) and/or of type (b) and up to 40% by weight of an ethylene polymer having a specific gravity of between 0.956 and 0.970 and a melt index of between 0.1 and 1 dg/minute, (f) mixtures comprising a mixture of at least one copolymer of type (a) and at least one copolymer of type (b), and (g) mixtures comprising several copolymers of type (b), and more particularly several copolymers having melt indices which are significantly different. 2. Mixtures according to claim 1, characterized in that the o^-olefin contains from 4 to 10 carbon atoms. 3. Mixtures according to one of claims 1 and 2, characterized in that the copolymers of type (a) have a number average molecular weight of between 10,000 and 28,000. 4. Mixtures according to one of claims 1 and 2, characterized in that the copolymers of type (b) have a number average molecular weight of between 15,000 and 60,000. 5. Mixtures according to one of claims 1 to 3, characterized in that the copolymers of type (a) have a polydispersity index of between 7 and 14. 6. Mixtures according to one of claims 1, 2 and 4, characterized in that the copolymers of type (b) have a polydispersity index of between 3 and 9. 7. Mixtures according to one of claims 1, 2, 4 and 6, characterized in that the crystalline fractions of the copolymers of type (b) have a single melting point at between 118 and 130°C and constitute from 20 to 50 by weight % of the total copolymer of type (b). 8. Mixtures according to one of claims 1, 2, 4, 6 and 7, characterized in that the copolymers of type (b) contain units originating from two oC-olefins whose respective average proportions stand in a ratio between 0 ,25 and 4. 9. Mixtures according to claim 8, characterized in that the copolymers of type (b) have a polydispersity index of between 6 and 12. 10. Process for producing the mixtures according to one of claims 1 to 9, characterized by the effect of a peroxide compound on a copolymer of ethylene and at least one oC-olefin in an amount of 0.5 to 2.5 parts by weight for 100 parts by weight of copolymer. 11. Use of the mixtures according to one of claims 1 to 9, in the production of cables for electric current.