NO803380L - BLOCK COPOLYMES AND PROCEDURES FOR ITS MANUFACTURING - Google Patents

Description

The present invention relates to a block copolymer and a process for its production.

Block polymerization essentially results in a combination of the best physical and chemical properties of two or more polymers, e.g. the combination of those for polypropylene with those for polyethylene. Thus, while polyethylene does not have melting points or tensile strengths as high as these properties of polypropylene, polyethylene actually possesses excellent low temperature properties such as brittleness and impact resistance.

When the excellent properties of both mentioned polymers are combined by block polymerization, a heteropolymer is obtained at once which is useful in many applications for which neither homopolymer was practically usable.

A group of block copolymers that have excellent physical properties are the ethylene-propylene block copolymers, e.g. those of the P-EP type, where P denotes a propylene homopolymer preblock and EP is a postblock of ethylene-propylene copolymer. By varying the properties of the blocks and the ethylene content of the polymer, the physical properties can be carefully regulated to suit the particular intended use of the polymer products. At constant melt flow rates, the impact strength of the block copolymer at room temperature is usually substantially directly proportional to the amount of polymerized ethylene in the total product.

Block copolymers are advantageously produced on a commercial scale by the method described in US patent no. 3,514,501. Briefly, this process involves the production of the preblock, preferably in the liquid phase, by catalytic polymerization of propylene in a hydrocarbon solvent such as liquid propylene to form a slurry. After preparation of the slurry, the prepolymer, which still contains active catalyst residues, is introduced into at least one reaction zone where it is reacted with monomer vapor for a sufficient period of time to form the polymer-post-block on the polymer-pre-block•in the desired quantity ratios. In the past, the conventional catalyst system used in such a polymerization process has been an unmodified or an electrode-donor-modified titanium halide component, activated with an organoaluminum cocatalyst. Typical examples of conventional catalyst systems for propylene polymerization include co-crystallized tantrichloride-aluminum trichloride catalysts with the general formula n«TiCl3•AlCl3 activated with diethylaluminum chloride or triethylaluminum. The cocrystallized titanium chloride-aluminum trichloride may have been subjected to a modification cation treatment with a suitable electron donor compound to increase its activity or stereospecificity. Such compounds are phosphorus compounds, esters of inorganic and organic acid ethers and several other compounds.

However, a main disadvantage of using the previously mentioned conventional catalysts has been the low catalyst productivity which has necessitated subsequent washing of the product in order to reduce the content of catalyst residues, which would otherwise have a detrimental effect on the product quality.

In recent times, new catalysts have been developed which are far more active than the previously mentioned conventional catalysts in the polymerization of alpha-olefins. Briefly, these catalysts comprise a titanium halide catalyst component supported on magnesium dihalide and an alkyl aluminum compound, which may be present as a complex with an electron donor compound. These catalyst components are described in the patent literature, e.g.

in US patents no. 3,830,787, 3,953,414, 4,051,313, 4,115,319 and 4,149,990.

The productivities achieved with these new catalysts are very high, resulting in polymers containing such small amounts of residual catalyst that the conventional washing step can be omitted. The catalyst works well in the homopolymerization of propylene and in the copolymerization of a mixture of propylene and another alpha-olefin, such as ethylene, provided that the polymer in the sation reaction is carried out in a liquid diluent, e.g. liquid propylene mono mer. However, in the vapor phase polymerization used in the preparation of the EP block copolymer of P-EP block copolymer described above, using conventional operating conditions, the product quality of the resulting block polymer has been found to be rather poor. In particular, in order to achieve a desired impact strength at a desired melt flow, it was found that significantly more ethylene had to be incorporated into the total polymer than is the case when using conventional catalysts. The necessary increase

in ethylene content to achieve impact resistance has an adverse effect on other desired properties in the final product, such as stiffness, deformation temperature, tensile strength, etc.

It is therefore an object of the present invention to provide a very efficient process for the vapor phase polymerization of ethylene-propylene blocks on a formed propylene polymer while forming products that have improved impact resistance without particularly affecting other desired physical polymer properties.

Another object of the invention is to provide a method for the production of ethylene-propylene block copolymers in which the polymerized ethylene content in the total polymer product is made as low as possible in order to achieve a desired impact strength.

A further object of the invention is to provide new ethylene-propylene block copolymers which exhibit improved processability in the extruded or injection-molded state compared to conventional ethylene-propylene block copolymers with the same total ethylene content.

Another object of the invention is to arrive at a new ethylene-propylene block copolymer which can be processed at lower extrusion or casting temperatures and/or lower extrusion or casting pressure than conventional resins with the same melt flow and total ethylene content.

Further purposes will be apparent from the following.

The above-mentioned purposes are achieved by a continuously sequenced vapor phase block copolymerization process which comprises: (A) providing a formed propylene polymer in finely divided form, said formed polymer containing active catalyst residues and is produced by polymerizing propylene in the presence of a catalyst composition containing the components (a) an aluminum trialkyl or an aluminum trialkyl 1 which is at least partially complexed with an electron donor compound, and (b) titanium trihalide or tetrahalide supported on magnesium dihalide or a complex of a titanium trihalide or tetrahalide with an electron donor -compound supported on magnesium dihalide; (B) introducing said formed polymer into at least one continuously stirred reaction zone, . (C) introducing ethylene and propylene monomers into the reaction zone in a molar ratio between ethylene and propylene of from approx. 0.15 to approx. 0.3,

(D) polymerisation of said ethylene and propylene monomers

in the vapor phase in the reaction zone of said formed propylene prepolymer.

The terms used herein shall have the following meanings: (a) "formed polymer" means a propylene polymer which is suitable for independent use but which contains active catalyst residues; (b) by "active catalyst residues" is meant here catalytic components in the polymer, which polymerize added monomers. substances without the need to add additional amounts of catalyst. The mentioned active ca ta lysator residues are preferably those which are initially used in the polymerization to form the formed polymer; (c) "block polymer" has the same meaning as heretofore

used in the technique, i.e. a polymer molecule consisting of a single part of an alpha-olefin polymer or copolymer attached to a single part of another alpha-olefin polymer or copolymer.

Block polymers are intended to comprise two or more copolymers that are sequentially polymerized, one on top of the other; a homopolymer followed by a copolymer; or alternating homo- or copolymer blocks of two or more alpha-olefin monomers;

(d) "volatile constituents" include non-polymerized alpha-olefin monomers as well as inert hydrocarbon diluents such as ethane, propane, butane, pentane, hexane, heptane, octane, aromatic hydrocarbons, diesel oils and the like; (e) by polymer in sation in a "hydrocarbon diluent" it is meant that the polymerization can proceed in the presence of inert hydrocarbon diluents such as those mentioned above under point (d) or polymer in sation in which the monomer, i.e. propylene, under relevant temperature and pressure conditions are kept in liquid form during polymerization, thereby serving as its own dispersing medium or mixture of intertied hydrocarbons and olefin monomers in liquid form; (f) by "vapor phase" block polymerization and "substantially dry prepolymer" is meant that a formed polymer contains 5% or less of volatile constituents, and is reacted with gaseous monomers in the absence of added inert hydrocarbon diluents.

Propylene, possibly in a mixture with smaller amounts of other alpha-olefins of from about 2 to 10 carbon atoms or more, can be used to form a prepolymer. Such other alpha-olefins are ethylene, butene-1, isobutene-1, pentene-1, hexene-1, and higher, as well as branched alpha-olefins, such as 2-methylbutene-1, 4-methylpentene-1 and higher. Of these monomers, propylene and mixtures of propylene and ethylene are of particular interest and are most preferred. When ethylene is a component, it is preferred that it is limited to a concentration of from approx. 0.3 to approx. 2% by weight of the total monomer supply.

The prepolymer is formed in a reaction zone using a hydrocarbon diluent and a catalyst for polymer in the batch, the polymerization being carried out to a solids content of from 5 to 60%, but preferably 20 - 40%. The preferred diluent is liquid propylene.

In the preferred method for prepolymer formation, i.e. the well-known "liquid pool" process, the propylene acts as liquid diluent as well as feed for the reaction, with the exception of small amounts of inert hydrocarbons, e.g. hexane, mineral oil, petrolatum, etc., which can be used for introducing the catalyst components into the reaction zone.

The reaction is continuous and monomerti 1 before see 1 and catalyst components are continuously supplied to the reactor and a slurry of polymer product and liquid propylene is removed, preferably through a cyclic discharge valve which simulates continuous operation. Various modifiers such as hydrogen are added to change the properties of the polymer product. Such modifiers are well known, and it should not be necessary to describe them in detail because they do not form any part of this invention.

The catalyst components used in the process for producing the prepolymer can be any of recently developed, highly active magnesium-supported catalyst components and organoaluminium coca catalyst components described e.g. in US patents no. 3,953,414, 4,051,313, 4,115,319.

Such a catalyst composition is typically in a two-component composition where the components are introduced separately into the polymerization reactor. Component (a) in such a composition is advantageously selected from trialkylaluminum compounds containing 1-8 carbon atoms in the alkyl group, such as triethylaluminum, trimethylaluminum, tri-n-butylaluminum, tri-isobutylaluminum, triisohexylaluminum, tri-n-octylaluminum and triisooctylaluminum. The trialkyl aluminum compound is preferably complexed with an electron donor before introduction into the polymerization reactor. The best results are obtained when esters of carboxylic acids or diamines, in particular

esters of aromatic acids are used as electron donors.

Some typical examples of such compounds are methyl and ethyl benzoate, methyl and ethyl p-rnetoxybenzoate, di-ethyl carbonate, ethyl acetate, dimethyl maleate, triethyl borate, ethyl o-chlorobenzoate, ethyl naphthenate, methyl p-toluate, ethyl toluate, ethyl p -butoxy-benzoate, ethyl cyclohexanoate, ethyl pivalate, N,N,N',N1-tetramethylenediamine, 1,2,4-trimethylpiperazine, 2,5-dimethylpiperazine and the like. The molar ratio of the aluminum alkyl compound to the electron donor can vary between 1 and 100, preferably between 2 and 5. The solution of the electron donor and the trialkyl aluminum compound in a hydrocarbon such as hexane or heptane is preferably pre-reacted for a certain period of time, usually less than 1 hour before feeding the mixture to the polymerization reaction zone.

The second component in the catalyst composition is either a titanium tri- or tetra-halide supported on magnesium dihalide, or a complex of a titanium tri- or tetrahalide with an electron donor supported on magnesium dihalide. The halogen in the respective halides can be chlorine, bromine or iodine, the halogen preferably being chlorine. If it is used in a complex form, the electron donor is suitably selected from the esters of inorganic and organic oxygenated acids and the polyamines. Examples of such compounds are the esters of aromatic carboxylic acids, such as benzoic acid, p-rncto sybenzoic acid and p-toluene acids and especially alkyl esters of said acids; the alkylenediarnines, e.g. N<1>,N",N"',N""-tetramethylethylenediamine. The molar ratio between magnesium and electron donor is equal to or higher than • 1 and preferably between 2 and 10. The titanium content expressed as titanium metal usually varies between 0.1 and 20% by weight

in the supported catalyst component and preferably between

1 and 3% by weight.

The manufacture of such supported catalyst components has previously been described and are commercially available.

Catalyst components (a) and (b) are added to the prepolymer reaction zone in amounts such that the Al/Ti molar ratio is kept in the wide range between approx. 1 and approx. 10,000 and preferably between about 10 and 200.

Temperatures at which the prepolymer formation can be carried out are as indicated in the known technique, e.g. 10 - 121.1°C, preferably 46 , 1 - 73 .9°C, and preferably 5.1.7 - 68.3°C. The pressures during the prepolymer formation can vary from atmospheric pressure or below this, where usually liquid inert hydrocarbon diluents are used (heptane or hexane) to pressures up to 35 kg/cm^ manometer pressure or higher, where propylene is used as its own dispersant or the propylene in mixture with a normally gaseous hydrocarbon diluent such as propane or butane, which is liquid under the reaction conditions.

The prepolymer from the reaction zone is fed to a separation zone, such as a cyclone or a bag filter, where the volatile components are separated from the polymer and treated according to known techniques and recycled to the reaction zone, the amount of volatile substances being removed being sufficient so that less than 10% and preferably no more than 5% content of volatile substances remains in the prepolymer.

In vapor phase block polymerization, recovered polymer from the separation zone and containing active catalyst residues is led to a continuously stirred reaction zone containing measures for introducing the ethylene monomer and the propylene monomer at one or more points along the length of the zone (and inert gases such as nitrogen) so that they active catalyst residues in the prepolymer polymerize said monomers into a block and thereby modify the final properties of the produced resin. The polymerization in the continuously stirred reaction zone is usually carried out at pressures lower than those used for the prepolymer preparation 1, i.e. a pressure of 0.7 - 3.5 kg/cm^ manometer pressure or somewhat higher. Polymerization temperatures can vary, e.g. from approx. 10 - approx. 98.9°C, but preferably from approx. 54.4 -

about. 93.3°C.

The ethylene and propylene monomers do not require premixing before introduction into the vapor phase zone; it is actually most advantageous to separately introduce each of the monomers at one or preferably several points along the length of the reactor. Liquid propylene can be introduced and this will, by evaporation, remove some of the polymerization heat developed in the reaction zone. The molar ratio between total introduced ethylene and total introduced propylene in the reaction zone should, however, be limited in the range from approx. 0.15 - approx. 0.3. If higher ratios are used, it has been found that the effectiveness of the ethylene content in the total polymer product on impact resistance properties is greatly reduced. By e.g. a ratio of 0.5, it is necessary to incorporate about 2 times the amount of ethylene in the total polymer to achieve the same impact strength as that of a final product produced at a ratio of about 0.2.

From approx. 5 - approx. 40 wt% block based on the weight of the total polymer is typically prepared in the total block k polymerization reactor system.

Suitable continuously stirred reaction zones include those described in US patent no. 3,514,501. The reaction zone can be one or more tube reactors in series and possibly provided with jackets for removing heat and suitable monomer introduction points as well as stirring devices. According to the preferred embodiment of the present invention, one or more horizontal helical mixer reactors are arranged for the continuous operation. Such reactors are equipped internally with a series of motor-driven helix blades and/or vanes. By suitable arrangement of the stirring equipment, the polymer can be moved continuously from the inlet to the outlet. The polymer powder behaves essentially independent of any agitation very much like a fluid and "flows" or moves from the inlet end of the reactor to its outlet end, i.e. flows along the length of the reactor like a fluid as a liquid would.

The propylene is provided at least at the inlet of the reactor, and if liquid propylene monomer is used, it is preferably also provided through intake spray nozzles situated at a distance along the upper part of the reactor. Ethylene monomer supply in vapor form can be introduced in a similar way at locations along the length of the reactor. The reactor is advantageously provided with an outer cooling jacket for removing heat through the reactor wall. Additional vapor phase reactors can be arranged in series with the block polymerization reactor in order to increase the residence time. If desired, any of the known modifiers may be added to one or more reactors for their intended conditions.

Due to the generally high productivity of the supported catalyst system expressed as kg polymer produced per kg of titanium metal, which productivity is further advanced by the present invention, there is no need to remove catalyst residues from the polymer in a de-ashing step as is the case with conventional catalyst.

The polymer products provided according to the invention and produced by the preferred "liquid pool" method have a melt flow range between approx. 0.1 and approx. 10 g/10 min., ratio between weight-average molecular weight and number-average molecular weight of over approx. 6.5, ethylene content of at least approx. 1, preferably over approx. 4% by weight, Ti content of no more than approx. 3ppm, Mg content of no more than approx. 40ppm, Cl content of no more than approx. lOOppm and a total ash content of no more than approx. 400 ppm.

Special advantages of the polymers according to the invention compared to conventional polymers include greater processability range, lower manufacturing complexity, superior ability to fill thin sections and molds with multiple mold spaces, better "draw-down", easier drawability and higher processability speeds with continuous filament - and staple fiber production.

Based on measurements of spiral melt flow, it was e.g. found that polymers according to the invention with melt flow (ASTM-1238 Condition L) in the range of approx. 2-10 g/10min., can be processed at temperatures that are 27.8 - 16.7°C, lower, or at molding pressures that are 24.5 - 10.5 kg/cm lower than what is the case for conventional polymers with the same melt flow values (ASTM-1238) and total ethylene content.

It is assumed that the molecular weight distribution Mw/Mn is that

a

property that best relates to the improvements in impact resistance as well as polymer-pheological properties and processability. Polymerization with a conventional catalyst system would typically result in a polymer product with an Mw/Mn ratio of at most 6.5 and usually below 6, while the polymers according to the invention have Mw/Mn ratios of at least 6.5, e.g. . between approx. 7 and approx. 10.

Various additives can, if desired, be incorporated into the polypropylene resin, such as fibers, fillers, antioxidants, nuclear deactivators, heat and light stabilizers, dyes, pigments, lubricants and the like.

The polymers can be advantageously used in the production of fibres, filaments and films by extrusion; of rigid objects by injection molding; and of bottles by pre-blowing.

The following examples further illustrate the advantages achieved by the present invention.

EXAMPLES 1-7.

The experiments were carried out on a large scale by continuous operation of a test plant. For the prepolymer preparation, propylene and catalyst components were continuously supplied to a stirred reactor, the monomer supply rate being set corresponding to a 2-hour residence time in the reactor. The organoaluminum compound in the cat lysator system was a hexane solution of tr i isobutyl aluminum (TIBA) which had been treated before introduction into the reactor with a hexane solution of methyl p-toluate (MPT), an electron donor compound. The solid supported titanium halide catalyst component was a commercially available catalyst rf-^I-T-l" The supported catalyst component contained approximately 1.5 wt% titanium, 20.3 wt% magnesium, 60.0 wt% chlorine and 9.6% by weight volatile hydrocarbons. Ethyl benzoate had been used in the preparation of the supported catalyst component. The two catalyst components were added at rates directly proportional to the polymer production rates and in amounts sufficient to maintain a concentration of polymer solids in the reactor slurry at a nominal value of about 40%. The catalyst productivity (kg polymer/

kg Ti metal) was calculated in each case from the discharge rate of polymer slurry, solids content in the slurry and addition rate for titanium catalyst component.

After separation of the prepolymer from unreacted propylene, said prepolymer, which still contained active catalyst residues, was fed sequentially to two series-connected, water-cooled horizontal jacketed reactors, each provided with helix blades as agitator. Propylene was introduced near the inlet to each of the reactors, and ethylene monomer through three inlets situated at an equal distance from each other above each of the reactors. The block copolymer product was recovered from the effluent of the second reactor. The operating conditions in each of the reactors were essentially the same unless otherwise stated. Relevant operating conditions and results are indicated in Table I. In the figure, the ratio between weight-% ethylene in the product is plotted against Izod notch layer strength (at room temperature). Curve A denotes the typical ratio obtained when preparing the prepolymer with conventional catalyst, e.g. Stauffer AA catalyst (3TiCl3•AICI3) with diethylaluminum chloride as a catalyst (at an Al/Ti molar ratio of approx. 3), and under conditions for achieving a melt flow for the end product of approx. 2 g/lOmin. In such a conventional process, it has been found that the ethylene/propylene molar ratios used in the steam phase reaction zone can be varied considerably, e.g. from approx.

0.2 - 0.8, without having any material effect on the ratio shown by curve A.

Curve B shows the ethylene content-Izod impact strength results obtained in Comparative Examples 1-6. The polymers in these examples were all prepared with a catalyst with a composition as required in the present invention, but the vapor-phase block copolymerization reactions were however carried out at ethylene/propylene ratios outside the limits of the invention. As can be seen from curve B in the graph, the ethylene introduced into each of the block copolymers was not very effective in achieving impact resistance; about the double ethylene introduction is actually necessary to obtain products with a desired impact strength compared to conventionally produced block copolymers (curve A).

Example 7, where the preparation was according to the present invention, however, resulted in a block copolymer which had a much improved ethylene content impact resistance (point is indicated in the figure).

EXAMPLES 8 and 9.

The polymer products from two continuous polymerization experiments carried out substantially according to the technique described in Examples 1-7 were subjected to detailed analysis, the exception being that 1.4 moles of ethylene was present in the prepolymer reactor feed stream in Example 9 and triethylaluminum was used as cocatalyst. The results are shown in table 2 together with relevant operating conditions.

As indicated in Table 2, standard ASTM test methods were used to determine the main properties of the polymer products.

The Mw/Mm ratio was determined by liquid chromatography using o-dichlorobenzene as solvent.

The contents of Ti, Mg and Al were determined by atomic absorption analysis of polymer ash dissolved in hydrochloric acid and the chlorine content by coiorimetric determination of a burnt polymer sample using a Parr oxygen bomb.

Claims (10)

1. Propylene-ethylene block copolymer, characterized in that it has a melt flow range between approx. 0> 1 and 10 g/10 min., a ratio between weight-average molecular weight and number-average molecular weight (Mw/Mn) above approx. 6.5, polymerized ethylene content above approx. 1.0, Ti content of a maximum of approx. 3 ppm, Mg content of a maximum of approx. 40 ppm, Sl content of at most approx. 100 ppm and a total ash content of approx. 400 ppm. 2. Polymer according to claim 1, characterized in that it has been obtained by a method including: (A) providing a formed propylene polymer in finely divided form, said formed polymer containing active catalyst residues and is produced by polymerisation of propylene in the presence of a catalyst composition containing the components: (a) an aluminum trialkyl or an aluminum trialkyl which is at least partially complexed with an electron donor compound, and (b) titanium trihalide or tetrahalide supported on magnesium dihalide, or a complex of a titanium trihalide or tetrahalide with an electron donor compound supported on magnesium dihalide; (B) introducing said formed polymer into at least one continuously stirred reaction zone; (C) introducing ethylene and propylene monomers into said reaction zone in a molar ratio between ethylene and propylene of from approx. 0.15 to approx. 0.3; (D) polymer in sation of said ethylene and propylene monomers in the vapor phase in the reaction zone of said formed propylene prepolymer. 3. Block copolymer according to claim 2, characterized in that the formed propylene polymer is produced in a polymerisation zone under sufficient pressure to maintain propylene in the liquid phase. 4. Block copolymer according to claim 2, characterized in that the propylene polymer formed is propylene homopolymer. 5. Block copolymer according to claim 2, characterized in that the propylene polymer formed is an arbitrary ethylene-propylene copolymer. 6. Block copolymer according to claim 2, characterized in that the alkyl group in the aluminum trialkyl catalyst in component (a) contains 1-8 carbon atoms. 7. Block copolymer according to claim 2, characterized in that the electron donor is an ester of a. aromatic acid. 8. Block copolymer according to claim 2, characterized in that component (a) is prepared by pre-reaction of the aluminum trialkyl compound with the electron donor for a period of less than 1 hour before polymer injection. 9. Block copolymer according to claims 1-8, characterized in that the electron donor is an ester of an aromatic carboxylic acid. 10. Block copolymer according to claim 9, characterized in that the ester is ethyl benzoate.