IE50287B1 - Ethylene copolymers capable of being processed into films and production process for these copolymers - Google Patents

Abstract

1. Process for the copolymerization of ethylene and propylene in at least one reactor comprising at least one zone, at a temperature between 180 degrees C and 320 degrees C and at a pressure between 300 and 2, 500 bars, by means of a Ziegler-type catalytic system comprising on the one hand at least one activator selected from the hydrides and the organo-metallic compounds of the metals of the groups I through III of the Periodic Table and on the other hand at least one transition-metal halogenated compound in such a proportion that the atomic ratio of the metal of the activator to the transition metal be comprised between 0,5 and 20, characterized in that on the one hand the gaseous flow feeding the reactor consists in the steady conditions of 65 to 85% by weight of ethylene and of 15 to 35% by weight of propylene and on the other hand the transition-metal halogenated compound comprises a complex compound of formula (MCla ) (MgX2 )y (AlCl3 )z (RMgX)b where M is a metal selected from titanium and vanadium, X is a halogen, R is a hydrocarbon radical, 2 =< a =< 3, 2 =< y =< 20, 0 =< z =< 1/3, and 0 =< b =< 1.

Description

The invention relates to ethylene copolyers capable of being converted into films and to a production process for said copolymers.

The literature contains a great many examples of ethylene and an alpha-olefin copolymer. Thus the catalysts capable of copolymerizing ethylene generally are capable of copolymerizing ethylene with an alpha-olefin. However the results of such a copolymerization are very much dependent on the catalyst used and most of all, as regards product quality, on the nature of the alpha-olefin. The copolymers of this category that have been researched roost in the literature indubitably are the copolymers of ethylene and of an alpha-olefin with at least 4 carbon atoms. Thus the French Patent No. 2 303 030 describes ethylene/hexene-1 copolymers with a density between 0.924 and 0.935 g/cm and a fluidity index between 0.1 and 0.7 and of which the methyl content (measured by infrared analysis) is between 17 and CHg per 1,000 carbon atoms. The French Patent No. 1 604 980 describes on one hand ethylene/butene-1 copolymers with a density between 0.921 and 0.928 g/cm^ and a fluidity index between 0.75 and 1.52 and of which the number of methyl groups is between 7.5 and 16.5 per 1,000 carbon atoms in the molecule, on the other hand- ethyl ene/octene-1 copolymers with a density between 0.914 and 0.929 g/cm and .a fluidity index between 0.11 and 1.56 and of which the number of methyl groups is between 5.5 and 21 per 1,000 carbon atoms in the molecule.

However, the copolymers-of ethylene and of an alpha-olefin with at least 4 carbon atoms, even though they have been known in principle for at least twenty years only recently have been successfully marketed in view of their adverse cost of manufacture affected by the prices for such comonomers as butene-1, hexene-1 and octene-1.

Furthermore, the ethylene/propylene copolymers, even though in a more favourable condition than the previous ones because of the lower price of the propylene comonomer, have been less researched. However, British Patent Specification No. 1 355 245 describes ethylene-propylene copolymers of a density between 0.910 and 0.938 g/cm and a fluidity index between 1 and 2 and of which the number of methyl groups is between 9 and 28 per 1,000 carbon atoms in the molecule.

These copolymers, of which the molecular weight is between 35,000 and 50,000, can be moulded.

On the other hand the French Patent No. 1 604 980 cited above offers two very valuable teachings regarding the research of the physical properties of the ethylene copolymers: - on one hand the importance of a narrow distribution of the molecular weights as regards improving the properties - at least with respect to the copolymers of ethylene and of an alpha-olefin with at least 4 carbon atoms, - on the other hand the observation that for constant comonomer content, the homogeneity of the copolymer lowers the density.

Of all of this literature, it is most important to remember that a copolymer of ethylene and of an alpha-olefin meant for a specific transformation mode into finished objects is genuinely defined only if there are provided simultaneously the following 6 characteristics: 1) density 2) nature of the comonomer 3) fluidity index 4) number of methyl groups per macromolecule ) average molecular weight 6) polydispersity index measuring the distribution of the molecular weights and defined below.

S0387 When there are copolymers with equal or equivalent values for five of the six above characteristics, and if the sixth characteristic has clearly different values, then it must be expected that such copolymers exhibit clearly different affinities for a given mode of transformation into finished objects, and therefore that these finished objects will have very different properties.

The present invention provides a process for the copolymerization of ethylene and propylene in at least one reactor comprising at least one zone,at a temperature between 180°C and 320°C and at a pressure between 300 and 2,500 bars, by means of a Ziegler-type catalytic system comprising on the one hand at least one activator selected from the hydrides and the organo-metallic compounds of the metals of the groups I through III of the Periodic Table and on the other hand at least one transition-metal halogenated compound in such a proportion that the atomic ratio of the metal of the activator to the transition metal is comprised between 0.5 and 20, wherein on the one hand the gaseous flow feeding the reactor consists in the steady conditions of 55 to 85% by weight of ethylene and of 15 to 35% by weight of propylene and on the other hand the transitionmetal halogenated compound comprises a complex compound of formula (MCla) (MgX2)y (A1C13)z (RHgX)b where M is a metal selected from titanium and vanadium, X is a halogen, R is a hydrocarbon radical, 2 The invention further provides ethylene/propylene copolymers comprising from 87.6 to 95.6% by moles of ethylene and from 4.4 to 12.4% by moles of propylene, having a density between 0.905 and 0.935 g/cm3 and a melt index between 0.4 and 2 dg/min, wherein they contain from 22 to 62 methyl groups per 1,000 carbon atoms in the molecule and in that their density p expressed in g/cm and their proportion of methyl groups are related by 0.9530 < P + 0.83m< 0.9568, and in that they are capable of being obtained by means of the process described above. 50387 Lastly the present invention provides films with improved properties manufactured from ethylene/propylene copolymers.

The ethylene/propylene copolymers may have a density between 0.905 and 0.935 g/cm3, a fluidity index between 0.4 and 2, with 22 to 62 methyl groups per 1,000 carbon atoms in the molecule, an average molecular weight between 10,000 and 28,000 and a polydispersity index between 7 and 14. The average molecular weight in the above definition is to be understood in conventional polymer engineering manner as being the number average molecular weight Hn and the polydispersity index being the ratio H/M„ of the weight average molecular weight of the number average molecular weight. Essentially the copolymers of the invention comprise ethylene units from 87.6 to 95.6% in moles and propylene units from 4.4 to 12.4% in moles, but they may also contain a minor amount of units at most 2% in moles from' another alpha-olefin such as butene-1, methyl-4pentene-1, hexene-1, octene-1. In the latter case terpolymers therefore will be involved.

The copolymers of the invention may be defined more rigorously by the relation between their density and their proportion of methyl groups; this relation differentiates in particular the copolymers of the invention from the ethylene/propylene copolymers described in British Patent Specification 0 No. 1 355 245. This relation, in which the density (in g/cm ) is symbolized by p and the proportion of the methyl groups by m, is the following: 0.9534 ·4 P + 0.83 m C 0.9568.

This relation is shown schematically in the attached drawing which shows the graph of the density (ordinate) against the proportion of the methyl groups (abscissa) of ethylene/propylene copolymers. In this drawing curve A represents the relation between the density and the number of methyl groups of the copolymers described in British Patent Specification No. 1 355 245 in examples 3, 7 and 11. The shaded region B represents the set of the copolymers of the invention. 0287 The copolymers of the invention thus defined exhibit remarkable properties and are capable of transformation into films, exhibiting a set of engineering properties superior to the ethylene polymers recognized as suitable for film making. The main properties affected by the improvement are the longitudinal elongation at rupture, tear resistance, and, as regards film preparation proper, the drawing force, the limit rate of drawing and industrial drawability. These latter three properties are of capital significance to the plastics processing industry since they determine both the power of the extruder required for the making of the film, the production rate, the material consumption required for making a film of predetermined qualities, and therefore as a result, the economic conditions in which the production tool is employed. The copolymers of the invention thus are characterized by a longitudinal elongation at rupture between 800% and 1,000% approximately for a film 50 pm thick, ty a drawing force equal to or less than 1.5 g for a drawing rate of 3.14 m/min, by a limit drawing rate exceeding or equal to 80 m/min, and by an industrial drawability equal to or less than 12 microns. The conditions in which these various properties are measured will be specified below in the examples cited. As regards the longitudinal elongation at rupture, it must absolutely be borne in mind, as regards the comparisons with the prior art, that the thickness of the sample measured be known because, as can be seen from table 3 of French Patent No.2 303 030, this property increases with thickness in the range of thicknesses between 30 and 150 pm, the linear coefficient of increase being between 2.4 and 5.3% pm approximately depending on the nature of the polymer.

The complex compound used in the process of the invention can be prepared by various methods: reducing titanium tetrachloride or vanadium tetrachloride by an organo-magnesium compound RMgX for instance results in a complex compound in which 0 The process ot the invention does not exclude the transition-metal halogenated compound comprising in addition to the complex compound of the formula specified above one or several other compounds such as in particular vanadium or titanium trichloride, where the titanium compound may be combined as a mixed crystal with aluminium chloride in the form TiClgi/SAlClg. In the latter case it will De advantageous as a rule to use this or these compounds in a different reactor (or in a different reaction zone) than the reactor (or the reaction zone) where said complex compound is being used.

The invention also relates to films of a thickness equal to or less than 12 ym. obtained from the above defined ethylene/propylene copolymers. These films are obtained by the conventional blow-extrusion techniques and exhibit improved properties as regards lb tear resistance and longitudinal elongation at rupture. Considering that their other physical properties in particular rupture strength, are not degraded, these films offer the exceptional advantage of providing the same strength and reliability in use as the conventional low-density polyethylene films, but at a much lower weight, the industrial drawability of the latter most often being equal to or larger than 20 microns. The films thus made have many applications, such as in particular large capacity bags, as films for agricultural uses, as rigid films for automated wrapping. The films of the invention offer adequate optical properties such as a gloss generally between 50% and 80%, a clearness generally between 15% and 50%, a cloudiness generally between 15% and 25% as determined by the measurements specified hereunder. These properties permit achieving a full range of films, including in particular matte films, lacking botn transparence and gloss.

In the process of the invention the halogen X preferably is selected from fluorine, chlorine and bromine. The radical R is selected from tne saturated aliphatic radicals (methyl, ethyl, propyl, n-butyl) and the unsaturated aliphatic radicals (allyl) and the aromatic radicals (phenyl). ihe complex compound of the formula specified above can be well applied within the entire temperature range of the invention. One skilled inthe art shall also evidently take care of selecting the other constituents of the catalytic systems as a function of their thermal stability and of their reactivity to ethylene at the selected temperature. Similarly the dwell time of the catalytic system will be selected by those skilled in the art as a function of the temperature of the reaction zone in which it is used and as a rule will be between 5 and 80 seconds approximately; this time will be the shorter, the higher the effective temperature of the catalyst.

The composition of the gaseous flow feeding the reactor in the stationary mode as characterized in the process of the invention must be understood to be an average composition across the entire reactor, and may vary within it, particularly when it comprises several zones. It is even especially advantageous that the content in propylene in the flow feeding a reactor zone shall be the higher as the temperature maintained in this zone is higher: this particularity of the process results in increasing the polydispersity index of the copolymer and in increasing the proportion of very high molecular weights in the copolymer and an appreciation of this effect is feasible by measuring the average molecular weight of a higher order Mz defined in the examples below. In the case of a reactor with several zones, it is even possible to have a zero content in propylene in the gas flow feeding one of the zones while the flow feeding the others have contents adequately high to observe the mean composition characteristic of the process of the invention. On the other hand, one skilled in the art will easily select that mean composition from the proposed range as a function of density of the copolymer he desires to obtain: the propylene content in this composition will be the higher, the lower the desired density. 50387 The process of the invention can be implemented by copolymerizing ethylene and propylene in the presence of a hydrocarbon such as propane, butane etc. used up to 50% by weight. In order to precisely control the fluidity index of the copolymer, it may also be advantageous to carry out the copolymerization in the presence af up to 2% in moles of hydrogen. In order to apply the process of the invention to the production of terpolymers as specified above, is is also possible to carry out the copolymerization in the presence of a minor amount up to 10% by weight of another alpha-olefin such as butene-1, hexene-1, etc. The process of the invention is implemented in continuous manner by using autoclave or else tubular reactors as conventionally employed in high-pressure polymerization engineering.

In case it is desired to produce copolymers of the invention with a particularly high polydispersity index, for instance exceeding , it is advantageous to use a first reactor operating at a pressure from 800 to 2,000 bars and at a temperature from 180 to 260°C and a second reactor operating a pressure from 300 to 600 bars and at a temperature of 240 to 320°C, the copolymerization taking place in each of the reactors in the total absence of hydrogen and the gas mixture flow (ethylene + propylene) feeding the second reactor representing from 17 to 69% of the total gas mixture supply flow.

The invention shall be better understood in the light of the following examples which are of illustrative intent.

EXAMPLES 1 through 3: Ethylene and propylene are copolymerized in an autoclave reactor of cylindrical shape and operating at a pressure of 1,200 bars and provided internally with an agitator and metal screens which define three zones of identical volume. Zone 1 is kept at a temperature of 220°C and fed by a flow of 40 kg/h of ethylene and P kg/'n of propylene; it receives a catalytic system comprising on one hand titanium trichloride violet TiCl^il/SJAlCl^ and on the other hand trioctyl-aluminium in such an amount that the atomic ratio Al/Ti is 3. Zone 2 is kept at a temperature T? and fed by a flow of 20 kg/h of ethylene and receives no catalyst at all. Zone 3 is kept at a temperature of 250°C and fed by a flow of 20 kg/h of ethylene and receives a catalystic system comprising on on hand a complex compound of formula TiCl3(l/3)AlCl3,6MgCl2 obtained by grinding together titanium trichloride violet and anhydrous magnesium chloride, and on the other hand dimethylethyldiethylsiloxalane in such an amount that the atomic ratio Al/Ti is 3; the reaction mixture containing the copolymer is evacuated from the discharge of zone 3 to a separation and recycling apparatus. The mean weight content of propylene in the reactor is : ir = P/(P+80). The copolymerization is carried out in the absence of hydrogen for examples 1 and 3, and in the presence of 0.3% in moles of hydrogen in example 2.

The obtained copolymer is characterized by the following properties: a) fluidity index (FI) measured according to ASTM D 1238-73, in dg/mn b) density p in g/cm c) number average molecular weight Mn measured by gel-permeation chromatography d) polydispersity index determined from Mn and the weight average molecular weight Mw measured by the same method e) proportion m of methyl groups per 1,000 carbon atoms in the molecule, determined by infrared radiation absorption analysis according to ASTM 0 2238-64T described in French Patent No. 1 604 980 f) average molecular weight of higher order Mz defined from the distribution C(M) of the molecular weights obtained by gel-permeation chromatography, by the formula M = Jo M2· cWdH z ,g M · C{M) dM The values of these properties are summarized together with the values of P, Tr, and π in the table 1 below. The values of Mn and Mz are in thousands.

TABLE 1 Example P π % T2°c I.F. PMn ^/Mn Kl/C Mz 1 27 26 240 1.9 0.908 15.5 8.7 58 449 2 16.4 17.5 240 1.9 0.929 21 7.0 33 456 3 21.6 21.2 190 1.7 0.914 15.5 9.4 47 492 EXAMPLE 4 Ethylene and propylene are copolymerized in the same reactor as previously. Zone 1 is kept at 220°C and receives the same catalytic system as previously and is fed by a flow of 39 kg/h of ethylene. Zone 2 is kept at 235°C and receives no catalyst at all. Zone 3 is kept at 260°C and receives the same catalytic system as previously. Zones 2 and 3 are each fed by a f1ow of 21 kg/h of ethylene and of 11.4 kg/h of propylene.

The mean weight content in propylene in the reactor therefore is 21.8%.

The copolymerization is carried out in the absence of hydrogen. The obtained copolymer is characterized by the same properties as in the previous examples, the values being listed in table II below.

EXAMPLES 5 and 6 Ethylene and propylene are copolymerized in the same reactor as previously, operating at a pressure of 1,000 bars. Zones 1, 2 and 3 are respectively kept at temperatures ot 180°C, 220°C and 255°C and are fed with equal flows of gas mixtures ot the same composition, namely 25% by weight of propylene for example 5 and 26% by weight of propylene for example 6 and tne ethylene complement in each case. The three zones receive the same catalytic system consisting on one hand of the complex compound liC l3(l/3)AlCl3, 2HgCl2 and on the other hand of dimethyl ethyl dietyl siloxalane in such an amount that the atomic ratio of Al/Ti is 2.5.

The copolymerization is carried out in the absence of hydrogen. The obtained copolymers are characterized by the same properties as in the preceding examples; the values are listed in table II.

EXAMPLES 7 and 8 Ethylene and propylene are copolymerized in a system of reactors comprising a first toroidal autoclave reactor with a volume of 0.91 litre constituting a single reaction zone and a second cylindrical autoclave reactor with a volume of 3 litres internally divided by means of metal screens into three zones of identical volume. These two reactors are arranged in parallel between the compressor supplying them, on the one hand, and the separator into which are evacuated their products, on the other hand.

The first reactor operates at a temperature of 290°C and a pressure of 4/0 bars and receives a catalytic system comprising on the one hand a complex compound of formula TiCl3(l/3)AlC13, 6MgC12 obtained by grinding together titanium trichloride violet and anhydrous magnesium chloride and on the other hand dimethylethyldiethyl siloxalane in such an amount that the atomic ratio Al/Ti is 3. The first and second reactor zones are kept at 180°G and 240°C respectively, and receive a catalytic system consisting of titanium trichloride violet and trioctylaluminium in such an amount that the atomic ratio Al/Ti is 3; the third zone is kept at a temperature of 242°C (example 7) or 248°G (example 8) and receives the same catalytic system as the first reactor. The second reactor operates at a pressure of 1,200 bars.

The first reactor is fed by a flow of 30 kg/h of ethylenepropylene mixture the zones 1, 2 and 3 of the second reactor are respectively fed with flows of 40 kg/h, 20 kg/h and 20 kg/h of ethylene-propylene mixture. The proportion of propylene is identical in all the feed zones and is 16% (example 17) or 15% (example 8) by weight. Copolymerization is carried out in the total absence of hydrogen. The obtained copolymers are characterized by the same properties as in the preceding examples; the values are listed in table II below; l° TABLE II Example I.F. PMn Vn m %o Mz 4 1.5 0.921 15 10.1 40 577 5 1.7 0.922 19.5 7.3 40 487 6 1.8 0.914 24 7.0 51 591 15 7 0.4 0.931 8 0.8 0.934 11.5 13.9 EXAMPLE 9 The copolymers of examples 1 through 6 are transformed by blowextrusion films of 50 pm thick. In order to compare their properties with those of a low-density polyethylene obtained by the conventional polymerization process in the presence of free radical initiators, a film of the same thickness is made from an ethylene homopolymer commercially known as PETEN B 1822 (PEKEMA, Finland). This reference polymer is characterized by the following properties measured by the methods described above (Mn in thousands): F.I. = 1.65 dg/min m = 19% Mw+fV 5·9 p = 0.922 g/cni3 Mn = 20.5 The properties determined for the films are as follows: a) rupture strength RR (in kg/cm ) and the elongation-atrupture AR (in %) in the longitudinal sens L and the transverse direction T as determined by the standard ASTM D 882-67 b) tear-resistance RD (in g/pm) in the longitudinal and transverse directions L and T respectively, as determined by the standard ASTM 0 1922-67 c) the optical properties of gloss B, clearness C and cloudiness T (in %) as respectively determined by the standards E 2431, E 2412 and E 2421 d) the rate of limit drawing VTL (in m/min) and the drawing force FT at a draw speed of 3.14 m/min (expressed in gf) as determined at 190°C using a TOYOSEIKi Extensometer e) the engineering drawability El (in microns) defined as that film thickness which allows a continuous blow-extrusion fabrication for 2 hours without difficulties.

Considering the very large number of measuremenents to be performed, only some given properties were ascertained for each product. The results of these measurements are shown in table III.

As regards the products obtained in example 4, the following results are obtained limit drawing rate 100 m/min drawing force 1.1 g engineering drawability 7 pm The product obtained in example 8 exhibits a limit drawing rate of 125 m/min.

TABLE III Product 1 2 3 5 6 PETEN RR L 140 185 195 174 190 T 100 150 149 166 170 L 910 900 900 980 520 AR T 1060 850 875 1210 710 L 6.4 5.6 5.2 6.8 RD T 14.1 15.4 10.2 6.0 B 57 53 61 80 52 93 C 22 48 15 39 T 17 23 19 16 24 8 VTL 85 80 80 160 35 π 0.65 1.1 1.7 1.4 4.6 El 9 7 9 22

Claims (12)

1. Process for the copolymerization of ethylene and propylene in at least one reactor comprising at least one zone, at a temperature between 180°C and 320°C and at a pressure between 300 and 2,500 bars, by means 5 of a Ziegler-type catalytic system comprising on the one hand at least one actiyator selected from the hydrides and the organo-metallic compounds of the metals of the groups I through III of the Periodic Table and on the other hand at least one transition-metal halogenated compound in such a proportion that the atomic ratio of the 10 metal of the activator to the transition metal is comprised between 0.5 and 20, wherein on the one hand the gaseous flow feeding the reactor consists in the steady conditions of 65 to 85% by weight of ethylene and of 15 to 35% by weight of propylene and on the other hand the transition-metal halogenated compound comprises a complex compound 15 of formula (MCl a ) (MgX 2 ) y (A1C1 3 ) Z (RMgX) b where M is a metal selected from titanium and vanadium, X is halogen, R is a hydrocarbon radical,

2. ^a ^3, 2 ^y £20, 0 20 2. Process according to claim 1, wherein the copolymerization is carried out in a reactor comprising at least two zones and in that the content of propylene in the flow feeding one zone is the higher as the temperature kept in said zone is the higher.

3. Process according to one of claims 1 and 2, wherein the 25 copolymerization is carried out in the presence of an additional alpha-olefin in such an amount that its proportion in the gaseous flow feeding the reactor in the steady conditions is equal to or less than 10% by weight.

4. Process according to one of claims 1 through 3, wherein the copolymerization is carried out in a reactor comprising at least two zones and by means of a catalytic system comprising in addition to said complex compound, a second transition-metal halogenated compound, and said complex compound is used in a zone of said reactor whereas said second halogenated compound is used in another zone of said reactor.

5. Process according to one of claims 1 through 4, wherein the copolymerization is carried out in a system of two reactors, the 10 first reactor operating at a pressure of 800 to 2,000 bars and at a temperature between 180°C and 260°C and the second reactor operating at a pressure of 300 to 600 bars and at a temperature between 240°C and 320°C, the copolymerization taking place in both reactors in the total absence of hydrogen and the flow of gaseous mixture feeding the 15 second reactor representing 17 to 69% of the total supply of gaseous mixture.

6. Ethylene/propylene copolymers comprising from 87.6 to 95.6% by moles of ethylene and from 4.4 to 12.4% by moles of propylene, having a density between 0.905 and 0.935 g/cm and a melt index between 20 0.4 and 2 dg/min, wherein they contain from 22 to 62 methyl groups per 1,000 carbon atoms in the molecule and in that their density p expressed in g/cm^ and their proportion of methyl groups are related by 0.9530 < p + 0.83 m 0.9568, ano they are capable of being obtained by means of the process 25 according to one of claims 1 to 5.

7. Copolymers according to claim 6, wherein their average molecular weight is between 10,000 and 28,000.

8. Copolymers according to one of claims 6 and 7, wherein their polydispersity index is between 7 and 14. 30

9. Films obtained from the copolymers according to one of claims 6 through 8.

10. Films according to claim 9, wherein their thickness is equal to or less than 12 micrometers.

11. A process for the copolymerization of ethylene and propylene substantially as described herein with reference to the Examples.

12. An ethylene propylene copolymer whenever produced by a process according to any of claims 1 to 5 or 11.