Continuous process for producing ethylene/propylene copolymers

Abstract: Continuous process for producing high melting and viscoelastic ethylene-propylene copolymers is disclosed. This process is characterized (1) using two or more polymerizers connected in series; (2) supplying all of a catalyst and molecular weight regulator (hydrogen) to the first polymerizer; (3) removing gas from the second and/or subsequent (if any) polymerization at a rate of tanks being 0.5 to 10 V₁/Hr (V₁/Hr (V₁ being a volume of the polymerizer) calculated at NTP. Monomers having an ethylene content of 5 wt.% or less are supplied to a first step carried out as described above; after which, in a subsequent polymerizer, monomers containing at least 10 wt.% ethylene are introduced and polymerized.

Description: The present invention relates to a continuous process for producing ethylene/propylene copolymers. Sheets produced by finishing and moulding known general-purpose polypropylenes suffer from various defects. Thus upon moulding for secondary finishing, rapid sagging of the sheets occurs, the range of finishing conditions is narrow and moulding efficiency is poor. In addition, in the case of wide sheets, sagging is a serious problem, the thickness of the post-finished articles tends to be non-uniform and wrinkles due to sags laid on one another tend to occur. Accordingly, only moulded articles of small size can be produced. When finishing known general-purpose polypropylenes by blow moulding, the following problems may be encountered. Due to serious parison sagging upon moulding, the thickness of the moulded articles becomes non-uniform. Thus, blow moulding is applicable only to moulded articles of a small size. When polypropylene having a high molecular weight is used to prevent the above sagging, not only is there the possibility of giving poor fluidity to the melt, an increased load upon moulding or an increased energy loss and other mechanical troubles but marked chalking of the moulded articles may occur. In order to mitigate the above problems, polypropylene, low density polyethylene, etc. may be incorporated in the polypropylene, for example as proposed in JP-A-80614/72 and JP-A-8848/75. However, articles moulded from such mixtures tend to have chalk marks and, in order to prevent chalk marks, intense kneading is required upon melting and thus the use of these mixtures is restricted because of the need to select a kneader and the increased power consumption. In addition, the products obtained also generally suffer from a decrease in rigidity. JP-A-91954/78, 185335/82, 187337/82 and 7439/83, each propose methods for kneading together polypropylenes having different moleclar weights, using a granulator. However, articles moulded from the resultant mixtures tend to give more serious chalk mark problems than do the polypropylene/low density polyethylene mixtures described above. The use of the mixture of polypropylene is thus restricted by the kneading process and choice of different molecular weight materials. Further, in order to solve the problems involved in the formability of general-purpose polypropylenes, many proposals have been made for methods for broadening the molecular weight distribution of a polypropylene product by multiple polymerization, (see, for example, JP-A-185304/82, 190006/82, 7405/83, 7409/83 and 172507/84). According to examples of these specification, polypropylene having different molecular weights is formed in multiple steps by batch polymerization in most o them. However, batch polymerization suffers from the defect that productivity per unit plant is low when considered as a commercial process. In JP-A-185304/82 there is described continuous multistage polymerization and it is said that (a) when the order of stagewise production is from high molecular weight to low molecular weight material, the necessary difference in molecular weight can be achieved simply by adding hydrogen at a later stage of the multistage polymerization, so that such is a preferred process conversely, (b) when the order is from low molecular weight to high molecular weight material, unnecessary hydrogen must be removed from a vessel in which the polymerization mixture is retained, by lowering the pressure of by deaeration, prior to production of high molecular weight material and, therefore, process (b) is inferior to process (a). However, in duplicate investigations we have found that process (a) gives rise to the problem that the melt flow rate (MFR) of the high molecular weight portion can be measured only with difficulty or measurement is impossible when the MFR is low, resulting in difficulties in controlling the operation (it may be noted that it is possible to measure viscisity [η] but much time is required for this measurement so it is not a practical method for control of the operation). Furthermore, the polypropylene by process (a) shows an abnormally large difference between the MFR of the powders prior to granulation and that of the pellets obtained by granulation (the MFR value of the powders is low) and it has been noted that there are problems in controlling the difference in molecular weight at each step of the multiple polymerization and in controlling the MFR values of the products. Crystalline polypropylene while it has excellent physical properties such as rigidity, heat resistance, etc. suffers from the disadvantage that the impact strength, in particular the impact strength at low temperatures, is low. Accordingly, the field of practical use has been limited. In order to overcome this defect, many proposals have been made for processes for block copolymerizing ethylene or other α-olefins (see, for example, JP-A-142652/75, 8094/77 and 34112/82). In these proposed processes, however, after a first polymerization of propylene as the major component has been performed, hydrogen is removed and a second polymerization of monomers containing a relatively large amount of ethylene is performed, whereby some measures are elaborated to broaden the molecular weight distribution. In general, when adopting multiple continuous polymerization for a block copolymerization, some distribution (considered to be close to a full mixing tank distribution) in residence time of each catalyst particle in each step occurs so that the product becomes an aggregate of polymer particles in which the proportions of the polypropylene and polyethylene contents (note: a portion containing ethylene in a relatively large amount) vary with time and, as a result of this heterogeneity, give rise to quality defects. Particularly in the continuous process for producing copolymers according to the invention (described below in detail) a molecular weight difference is given in each stage in step (i) of the multiple polymerization of propylene as the major component so that when the prior art technique is adopted as it is, the molecular weight difference in each polymer particle is broadened as compared with conventional block copolymerization and problems due to heterogeneity could be more remarkable. Many methods for improving the above problems of multiple continuous polymerization have been proposed. For example, in JA-A-48916/83, 11616/80, 69215/83, etc., there is proposed a method in which a slurry produced by a polypropylene polymerization step (polymerization step 1) is classified by a cyclone and the finely divided particles are returned to the polypropylene polymerization step. However, classification based on polymer particle size is not necessarily identical with the residence time distribution and the effect is thus insufficient. In JP-A-195718/82, 29811/83, etc., there is proposed a method in which a slurry produced by a polypropylene polymerization step (polymerization step 1) is classified by a cyclone and the finely divided particles are returned to the polypropylene polymerization step. However, classification based on polymer particle size is not necessarily identical with the residence time distribution and the effect is thus insufficient. In JP-A-195718/82, 29811/83, etc., there are also proposed methods in which supply of the catalyst to a polymerizer and withdrawal of the slurry from the polymerizer are effected intermittently and a part of the catalyst going into a polyethylene polymerization step (polymerization step 2) is reduced while the residence time is short. These methods involve the problem that the polymerization is unstable. In addition, as in embodiment 8 of the present invention described below, there are also proposed methods in which the slurry from the polypropylene polymerization step is treated with an electron donor compound thereby to selectively inactivate the catalyst particle discharged in a short residence time. For example, in JP-A-145115/82 and 115417/80, various electron donor compounds are proposed but the compounds in the range used in the examples are insufficient to achieve the objects of the present invention as described below. Based on the results of various investigations to solve the problems noted above, we have proposed the process of Japanese Patent Application No. 283728/85. However, this process is somewhat complicated in that at least four polymerizers are required, there are limitations to a reaction weight ratio and polymerization conditions in each polymerizer, etc. As a result of various investigations to solve the technical problems described above, it was noted that when two or more polymerizers were connected in series, catalyst and hydrogen were fed only into a first tank and propylene was polymerized, the polymerization mixture in the first tank was sequentially transferred in a polymerizer at a later stage and accompanied thereby, the reaction progress, catalyst concentration and hydrogen concentration at later stages were reduced in order so that polymers having a high molecular weight can be produced and polypropylene having a broad molecular weight distribution could be obtained as a final product. Based on this finding, the process of Japanese Patent Application No. 264593/85 was proposed. In the process described above, however, problems were encountered in that operation and control were complicated because three or more polymerizers were required, and there are limitations to the reaction weight ratio and polymerization conditions in each polymerizer, etc. An object of the invention is to provide a continuous process for producing high molten viscoelastic ethylene-propylene copolymers which have good physical properties for sheets, blow moulding and injection moulding and stably produce ethylene-propylene block copolymers capable of preparing large sized moulded articles. It has now been found, in accordance with the present invention, that in a process wherein (1) polymerization of propylene as the major component is performed in at least two first polymerizers to which propylene optionally in admixture with up to 5 % by weight of ethylene (based on the total of propylene and ethylene) is supplied, using 3 or more polymerizers connected in series and polymerization of monomers containing ethylene in a relatively large amount is then performed in one or more remaining polymerizers (the ethylene content in the monomers supplied being 10 to 100 wt.%) and (2) wherein all the catalyst used and hydrogen (molecular weight regulator) are supplied to a first polymeric so that the catalyst and hydrogen (except for that consumed in the process) are transferred in order into the second and subsequent polymerizing, together with the reaction mixture (slurry), the molecular weight of the polymer produced in each polymerizer can be freely controlled on a practical level by continuously withdrawing gas from the gas phase of the second and subsequent polymerizers, without providing any special facilities such as for degasification. In accordance with the invention there is provided a continuous process for the production of an ethylene/propylene copolymer using a Ziegler-Natta catalyst, in which process: (a) the polymerization is effected in at least three polymerization vessels connected in series and is carried out as a two-stage polymerization namely:- (i) a first polymerization stage, in which propylene is polymerized as major component, carried out in at least the first two polymerization vessels to which propylene, optionally in admixture with up to 5% by weight of ethylene (based on the total of propylene and ethylene), is supplied to the said at least two first polymerization vessels, and (ii) a second polymerization stage carried out in at least one polymerization vessel subsequent to those in which the first polymerization stage (i) is carried out, the monomers supplied to the second polymerization stage comprising ethylene and, optionally, propylene and containing from 10 to 100% by weight of ethylene; (b) all the catalyst to be used is supplied to the first polymerization vessel, the catalyst being passed through the second and subseqent vessels together with the polymerization reaction mixture to from polymer in each polymerization vessel on the same catalyst, and is then discharged from the last vessel; (c) hydrogen gas is used as a molecular weight regulator in the continuous propylene polymerization stage (i), all the hydrogen being supplied to the first reaction vessel and the balance of unconsumed hydrogen gas being transferred from polymerization vessel to polymerization vessel together with the polymerization reaction mixture; (d) gas is continuously removed from the gas phase of the second and subsequent (if any) polymrization vessels in the continuous polymerization step (i) at a rate of 0.5 to 10 V₁/hr calculated at 0°C, 0 kg/cm²G, the volume of the polymerization vessel being V₁; and, (e) the amount of polymer formed in polymerization stage (i) is 60 to 95 wt.% of the total polymer formed. In the invention, the ratio of the MFR value of the polymers produced in the first and last polymerization vessel in at least two polymerization vessel connected in series is preferably that given by the equation: wherein MFR i is the MFR of polymer produced in the i order of polymerization, and MFR i is MFR of the polymer produced in the last polymerization vessel in the continuous propylene polymerization stage (i). Gas withdrawn from the gas phase of one or more of the second an subsequent polymerization vessels may be recycled to the first polymerization vessel, together with gas recovered from a subsequent degasification vessel. The continuous polymerization step (i) is preferably performed in the presence of an inert solvent or liquid propylene using a compound of the formula: AlR ℓ / mX 3-m (wherein R ℓ is a C₁-C₂₀ hydrocarbyl group; X is a halogen atom; and m is a number satisfying the relationship 3 ≧ m > 1.5) as an organic aluminium compound (B) in combination with a titanium-containing solid component (A) constituting the Ziegler Natta catalyst and a glycol ether (C) is continuously added to the slurry after completion of step (i) in a concentration of: (C)/Ti in (A) = 0.01 to 1.0 (mol/atom) [based on the titanium content of (A)] to carry out the continuous polymrization step (ii). In the following description reference will be made to the accompanying drawings which is a flow sheet of a polymerization device used in Examples 1 to 6 and Comparative Examples 1 to 5 described below. The apparatus shown basically comprises, connected in series, first and second polymerizers 1 and 2, a degasification vessel 3, a third polymerizer 4 and, finally, a further degasification vessel 5. Pumps 6 and control valves 7 are arranged in the lines to and from the various vessels as shown to permitted of control and operation of the apparatus. As catalyst for use in the present invention, a so-called Ziegler Natta catalyst may be sufficient but there is no particular limitation. Preferably, there are used catalysts basically composed of titanium compounds and organic aluminum compounds in combination. As the titanium compound, there can be advantageously used catalysts generally used for the stereoregular polymerization of propylene such as a titanium trichloride compositions obtained by reducing titanium tetrachloride with hydrogen or metallic aluminium; compositions obtained by further grinding the titanium trichloride composition with a ball mill, an oscillating mill. thereby to activate the titanium trichloride composition; compositions obtained by further treating the actived composition with an electron donor; titanium trichloride compositions obtained by reducing titanium tetrachloride with an organic aluminium compound followed by various treatments (for example, a titanium trichloride composition obtained by crystal transition with heating in titanium tetrachloride, highly activated titanium trichloride obtained by treating with an electron donating compound or an electron accepting compound, etc.); or a so called carrier type catalyst obtained by carrying titanium tetrachloride on a carrier such as magnesium chloride. As the organic aluminium compound, compounds of the formula: AlRnR′n′X₃ - (n + n′) can be preferably used. (In the formula, R and R′ each represents a hydrogen group such as an alkyl group, an aryl group, etc.; X represents a halogen such as fluorine, chloride, bromine and iodine; and, n and n' each represents an optional number shown by: 0<n + n'≧ 3 .). Specific examples are trialkyl aluminium or dialkyl aluminium monohalides which can be used singly or in admixture. Further electron donors ordinarily used as a third component of a catalyst can also be used in combination with the titanium compound and organic aluminium compoun described above. As a mode of polymerization, slurry polymerization using as solvent a hydrocarbon (such as propane, hexane, heptane, octane, benzene or toluene, or bulk polymerization), using propylene as a solvent, can be used. In the process of the present invention, as the polymerizers, tanks are preferably used. In the polymerization step (i) using propylene as the main component, two or more polymerizers are connected in series, in which the transfer of the polymerization reaction mixture (slurry) is performed by continuously withdrawing a part of the slurry (liquid phase) and continuously transferring it to a subsequent polymerizer, In the polymerization step (ii) containing ethylene in a relatively large amount, the step (ii) includes a first tank which continuously receives the polymerization reaction mixture from the terminal polymerizer in the first step. Preferably, using one or more polymerizers of tank type, ethylene and propylene are supplied in a definite ratio to perform continuous polymerization. In the process of the present invention, the catalyst to be used is supplied only in the first tank polymerizer throughout the whole steps described above. Thus, the solid catalyst supplied is passed in order through polymerizers such as the second to third tanks together with the other polymerization reaction mixture, during which the catalyst is withdrawn from the terminal polymerizer in such a state that the solid particles are covered by the polymer formed in each polymerizer thereon. If a catalyst is freshly supplied in the second and subsequent tanks, polymers having greatly different MFR from the polymer produced in the first tank are produced on the solid particles of the catalyst so that it is difficult to uniformly mix these polymers with each other, even though a graining step after collection of the products; molded articles obtained from these products cause poor appearance of the articles such as fish eye and such is not preferred. In the process of the present invention, hydrogen is used as the molecular weight regulator. The hydrogen is supplied only in the first tank polymerizer throughout the whole steps described above, like the catalyst described above. The supply to the first tank may be performed in any of the gaseous phase and the liquid phase in the tank. However, the supply of hydrogen to the second and subsequent tanks from each preceding tank is made in the form of dissolving hydrogen in the polymerization reaction mixture described above, i.e., in a solution state. Accordingly, in the case of supplying hydrogen in the liquid phase (polymerization reaction mixture) of the first tank, care should be taken that undissolved hydrogen which is in a foam state not be transferred to the second tank. If the hydrogen that is incorporated in the liquid phase in the foam state is transferred to a next tank (there is some possibility in the second and subsequent tanks, depending upon state of stirring), a degasified amount becomes larger upon degasification in the second and subsequent tanks than in the case that no foam of hydrogen is present and thus, an efficiency of withdrawal decreases. As described above, the polymerization reaction mixture is sequentially transferred from the first polymerizer to the terminal polymerizer. The polymerization reaction mixture, namely, slurry, withdrawn from the terminal polymerizer is transferred to a pressure drop tank for decreasing the pressure. The gas generated in the pressure drop tank contains large quantities of propylene so that the gas is generally compressed, recycled to the polymerizers and re-used. Further the gas withdrawn from the gaseous phases in the second and subsequent polymerizers are recycled and re-used, together with the gas generated in the pressure drop tank described above. Thus, the process can be performed without loss of raw materials. In the process of the present invention, an amount of gas to be withdrawn from the gaseous phase of the polymerizers is preferably performed in a range of 0.5 to 10 V₁ per an hour calculated as 0°C and 0 kg/cm²G, when the volume of the polymerizer is made V₁. It is preferred to continuously withdraw the gas through a control valve so as to keep the hydrogen concentration at the aimed concentration, while measuring the hydrogen concentration in the gaseous phase by process gas chromatography, etc. By the degree of this hydrogen withdrawal, MFR of the polymer produced in the tank can be reduced to a preferred degree, as compared to the case no gas is withdrawn. The whole amount or a part of the withdrawn gas can be recycled to the first tank. In the process of the present invention, propylene, ethylene and the solvent may be supplied to each polymerizer, depending upon necessity. The solvent is replenished to properly maintain the slurry concentration in each polymerizer depending upon the amount of polymer increased. By performing the polymerization step (i) of the present invention, the molecular weight of polymer produced in each tank stepwise increases from the first to second tanks and the second to third tanks, whereby a sufficient difference in molecular weight can be given as a whole. In the process of the present invention, a polymerization pressure in the polymerization step (i) using propylene as the major component is not limited but is generally under normal pressure to 50 kg/cm²G. The polymerization pressure of each polymerizer in accordance with the process of the present invention which is connected in series may be the same as or different from one another. In the process of the present invention, a polymerization temperature in the continuous polymerization step (i) performed using propylene as the major component is not limited but is generally in a range of 20 to 100°C, preferably 40 to 80°C. In the process of the present invention, a mean residence time of the reaction mixture in each polymerizer connected in series is not limited but the process is carried out generally for 30 minutes to 10 hours. Further the aforesaid various polymerization conditions, namely, pressure, temperature, residence time can be chosen depending upon quality of the objective polypropylene, catalyst used, or the like, whereby the objects of the second process of the present invention can easily be achieved. Furthermore, for transfer of the slurry between the polymerizers connected in series, conventionally used transportation through a pump, transportation utilizing a pressure difference and other methods can be adopted but there is no particular restriction. The MFR of polypropylene thus obtained via the polymerization step (ii) at a later stage of the process in accordance the present invention is generally 0.01 to 100, For sheet molding and blow molding, polypropylene having the MFR value of 0.05 to 10, preferably 0.10 to 5.0 is used. For reference, it is preferred that the molecular weight difference among polypropylene produced in the respective polymerizers of polymerization step (i) connected in series be in a range of equation [2] described below, when expressed as a MFR value: wherein MFR i : MFR of polymer produced in the i order of polymerizer MFR t : MFR of polymer produced in the terminal polymerizer in the continuous polymerization step (i) of propylene as the major component When the numerical value in the left side is less than 2.0, high melting viscoelasticity of the objective polymer in the second process of the present invention tends to be insufficient and such is not preferred. The monomer composition to be supplied in the polymerization step (i) in the process of the present invention is: When the amount of ethylene used exceeds 5 wt%, a tendency of lowering physical properties such as rigidity, heat resistance, etc. characteristic of polypropylene is noted with respect to the finally obtained copolymer and such is not preferred. In addition, 1-butene, 4-methylpentene-1, styrene or non-conjugated dienes, etc. may be added to the monomers as a third component in 0 to 10 wt% based on propylene. The ratio of polymerization in the polymerization step (i) of the process of the present invention is 60 to 95 wt%, preferably 75 to 90 wt%, based on the ethylene-propylene copolymer which is the product finally obtained. Polymerization in the polymerization step (i) which exceeds the upper limit described above results in reduced rigidity of the block copolymer product; when the polymerization degree is less than the lower limit described above, improvement in impact strength of the polymer at low temperatures is insufficient. The polymerization degree can be chosen in the range described above depending upon the desired quality. The slurry flown out after completion of the polymerization step (i) in the process of the present invention is continuously transferred to the first polymerizer in the polymerization step (ii), whereby the polymerization step (ii) containing ethylene in relative large quantities is performed. In the polymerization step (ii), ethylene is used at minimum twice (10 wt%) the amount, as compared to the polymerization step (i). In the polymerization step (ii), it is not necessarily required to use two or more polymerizers. In case that the amount of polymerization in the step (ii) is as relatively large as, for example, 20 to 40 wt%, however, the amount of the polymer produced in each polymerizer can be balanced by the use of two or more polymerizers. Further in the polymerization step (ii), the ethylene concentration in the monomers supplied differs more greatly than in the step (i). Therefore, once the slurry discharged from the polymerization step (i) is received in the pressure drop tank and subjected to degasification (note: the propylene, ethylene and hydrogen dissolved are removed), the slurry can also be supplied to the polymerizers in the polymerization step (ii). The polymerization step (ii) is carried out in a manner similar to the polymerization in the polymerization step (i) except for supplying the monomers in a definite weight ratio of ethylene/ethylene + propylene, a necessary amount of hydrogen and solvent. According to a preferred embodiment of the process of the present invention, a specific glycol ether (hereafter sometimes referred to as additive C) is added as the third catalyst component in the polymerization step (ii). The purpose of adding the additive lies directly in reducing the activity of the catalyst to be added to a considerable extend but substantially in selectively inactivating a short path catalyst (catalyst having a high activity) to homogenize the polymer produced in the second step. As the glycol ether, mention may be made of an ethylene glycol monoalkyl ether, an ethylene glycol dialkyl ether, a propylene glycol monoalkyl ether and a propylene glycol dialkyl ether. Specific examples are ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol monoethyl ether, ethylene glycol diethyl ether, ethylene glycol monopropyl ether, ethylene glycol dipropyl ether, ethylene glycol monobutyl ether, ethylene glycol dibutyl ether, propylene glycol monomethyl ether, propylene glycol dimethyl ether, propylene glycol monoethyl ether, propylene glycol diethyl ether, propylene glycol monopropyl ether, propylene glycol dipropyl ether, propylene glycol monobutyl ether, propylene glycol dibutyl ether; further as condensation products of glycol, a diethylene glycol monoalkyl ether, a diethylene glycol dialkyl ether, a triethylene glycol monoalkyl ether, a triethylene glycol dialkyl ether, tetraethylene glycol monoalkyl ether, a tetraethylene glycol dialkyl ether, a dipropylene glycol monoalkyl ether, a dipropylene glycol dialkyl ether, a tripropylene glycol monoalkyl ether, a tripropylene glycol dialkyl ether, tetrapropylene glycol monoalkyl ether, a tetrapropylene glycol dialkyl ether, a polyethylene glycol monoalkyl ether, a polyethylene glycol dialkyl ether, a polypropylene glycol monoalkyl ether, a polypropylene glycol dialkyl ether. As the alkyl group, mention may be a hydrocarbon chain having 1 to 20 carbon atoms. In addition, glycol ethers obtained by reacting ethylene oxide and propylene oxide may also be used. These ethers (C) are used in a molar ratio in (C)/Ti in (A) = 0.01 to 1.0, based on titanium in the titanium-containing catalyst component (A). It is also preferred to incorporate (C) in a range of 30 to 80%, based on the catalytic activity being made 100% in the case of incorporating no glycol ether, while the effect is different depending upon kind of the glycol ether. When the addition amount is too much, the effect of inactivating the short path catalyst is great but reduction of the total catalytic activity is also great; such is not preferred from an economical viewpoint and control in a ratio of polymerization (i) to polymerization (ii) is limited which is also not preferred. Conversely when (C) is too less, the aforesaid effect of selectively inactivating the short path catalyst is insufficient and such is not preferred. The reason why the glycol ethers used in the process of the present invention is remarkably excellent as compared to ketones, amines, amides, alkyl ethers, carboxylic acid esters or halogen compounds conventionally known is unclear but it is assumed that the glycol ethers would react with the organic aluminum compounds (B) to form complexes insoluble in inert solvents and the complexes might react with the catalyst inside the polymer particles only with difficulty so that the action of predominantly inactivating the short path catalyst would be markedly exhibited. Namely, it is also assumed that necessary conditions would be the formation of liquid complexes insoluble in inert solvents and that the complexes possess a viscosity difficult to easily permeate into the inside of the polymer particles. The addition of the aforesaid glycol ether in the polymerization step (ii) is performed to the slurry withdrawn from the degasified step (i) described above prior to initiation of polymerization in the polymerization step (ii). The glycol ether may also incorporated directly in the polymerizers. The mode of adding the glycol ether may be continuous or intermittent; in the case of the latter, however, an interval of the addition is within 1/8 the residence time of the slurry in the polymerizers of the polymerization step (ii). When the interval is long, the effect of adding the additive C becomes insufficient. General polymerization conditions for the polymerization step (ii) in the process of of the present invention are as follows. That is, a polymerization temperature is between 20 and 80°C, preferably between 40 and 70°C; a pressure is 0 to 50 kg/cm²G and a mean residence time is 20 minutes to 10 hours. In order to control the molecular weight, hydrogen is generally used; its amount to be used is 1 to 40 mol% as a concentration in the gaseous phase in the polymerizers. A ratio of the monomers used is 10 to 100 wt%, preferably 20 to 70 wt%, in a weight ratio of ethylene to ethylene + propylene. An amount of polymerization is 5 to 40 wt%, preferably 10 to 25 wt%, based on the finally obtained ethylene-propylene copolymer. Further in the polymerization step (ii), small quantities of other α-olefins or non-conjugated dienes may also be used in combination, as in the polymerization step (i). Major effects of the process in the present invention described above in detail can be summarized as follows. The ethylene-propylene copolymer in accordance with the process of the present invention has a broader molecular weight distribution than in conventional copolymers so that the fluidity is good upon extrusion molding and as a result, an amount extruded through an extruder can be increased and consumption power can be saved. Likewise, the ethylene-propylene copolymer has characteristics such as excellent fluidity upon injection molding and therefore, excellent results can be obtained in quality and finishing efficiency of molded articles obtained for use in a variety of molding fields. Secondly, in the process of the present invention, there is allowance in conditions for operation of each polymerizer as a definite multiple polymerization and therefore, control in the polymerization processes and control in polymerization conditions can be extremely easily done. As described above, the present invention can achieve the effects described above which were impossible to achieve in the prior art, by adopting the particular polymerization conditions and by the use of the particular additive (in preferred embodiments). The present invention will be described in more detail with reference to the examples below but is not deemed to be limited thereto. Analyse and measurement in the examples were performed as described below. 1) MFR (g/10 minutes) : ASTM D-1238, 230°C, 2.16 kg load 2) Ethylene content (wt%) : infrared absorption spectrum method 3) Polymerization ratio (wt/wt) of polymerization (i) to polymerization (ii) : Copolymers having various reaction ratios of ethylene/propylene were previously prepared. Using the copolymers as standard samples, a calibration curve was prepared by the infrared absorption spectrum method. A ratio of ethylene/propylene reacted in polymerization (ii) was determined and the polymerization ratio was calculated from the ethylene content (described above) of the whole copolymers. 4) Calculation of MFR of polymer produced in each polymerizer : MFR₁ : MFR of the polymer polymerized in the first polymerizer ( ) MFR₂ : MFR of the polymer polymerized in the second polymerizer ( ) MFR₁ + ₂ : MFR of the total polymers polymerized in the first and second polymerizers W₁ : ratio of the total polymers produced in the first polymerizer in the polymerization step (i) ( ) W₂ : ratio of the total polymers produced in the second polymerizer in the polymerization step (i) ( ) W₁ + W₂ : 1.0 MFR₂ was determined by the following relationship: 5) Measurement of physical properties of articles sheet molded: Young's modulus (kgf/mm²) : ASTMD-882 Blanking impact strength (kgf/mm²) : ASTMD-781 Heating behavior : CHISSO's method (described below) In order to evaluate heating vacuum properties of a sheet as a model, the sheet was fixed in a frame of 40 cm x 40 cm, which was put in a thermostat at 200°C to measure the following physical properties. Namely, a) a sagging amount (mm) at the initial stage of heating of the sheet, b) the maximum spring back amount (%) : [1/150 x (150 - maximum sagging amount (mm) when recovered x 100)] and c) residence time (second) from the maximum recovery to re-sagging. 6) Appearance of sheet : visual observation Example 1 (1) Preparation of catalyst Six liters of n-hexane, 5.0 mols of diethyl aluminum monochloride (DEAC) and 12.0 mols of diisoamyl ether were mixed at 25°C for 5 minutes. The mixture was reacted at the same temperature for 5 minutes to give reaction solution (I) (molar ratio of diisoamyl ether/DEAC, 2.4). In a reactor substituted with nitrogen were charged 40 mols of titanium tetrachloride followed by heating to 35°C. The whole amount of the reaction solution (I) described above was dropwise added thereto over 180 minutes. Then, the mixture was kept at the same temperature for 30 minutes. After elevating the temperature to 75°C, the mixture was reacted for further an hour. After cooling to room temperature, the supernatant was removed and 30 liters of n-hexane were added followed by decantation. The operation was repeated 4 times to give 1.9 kg of solid product (II). The whole amount of this (II) was suspended in 30 liters of n-hexane. In this state, 1.6 kg of diisoamyl ether was added to the suspension at 20°C and 3.5 kg of titanium tetrachloride at room temperature in about 5 minutes. The mixture was reacted at 65°C for an hour. After completion of the reaction, the mixture was cooled to room temperature (20°C). After the supernatant was removed by decantation, 30 liters of n-hexane were added and the mixture was stirred for 15 minutes and settled and then the supernatant was removed. After this operation was repeated 5 times, the system was dried under reduced pressure to give solid product (III). (2) Preparation of catalyst In a tank having an inner volume of 50 liters were charged 40 liters of n-hexane, 850 g of diethyl aluminum chloride, 360 g of the solid product described above and 3.8 g of methyl p-toluate. Next, while keeping at 30°C and stirring, propylene gas was supplied in 180 g/H for 2 hours to conduct a preliminary treatment. (3) Polymerization Polymerization was conducted in a polymerization device shown in the drawing. In polymerizer (1) were continuously supplied 26 ℓ/H of n-hexane and 120 mℓ/H of a catalyst slurry. Propylene was supplied and controlled in each polymerizer to show temperature of 70°C and pressure of 8 kg/cm²G in polymerizers (1) and (2), respectively. Further 500 Nℓ/H of gas was continuously discharged from polymerizer (2). The solution was withdrawn through a control valve so as to show 80% of solution level in each polymerizer. Gas from a degasification tank was compressed by a compressor and recycled to polymerizer (1). Amounts reacted in each polymerizer and analytical data of MFR are shown in Table 1. The slurry containing the polymer particles withdrawn from polymerizer (2) was degasified in a degasification tank (3) at 60°C and 0.5 kg/cm²G which was transferred to polymerizer (4) through pump 6. The gaseous phase hydrogen concentration was 10 mol% and the temperature was 60°C in polymerizer (4). Ethylene was supplied in 500 g/H and, propylene and hydrogen were supplied so as to have a gaseous phase gas composition of ethylene/(ethylene + propylene) = 0.40. The slurry discharged from polymerizer (4) was passed through degasification tank (15). After the catalyst was inactivated with methanol, the slurry was neutralized with an aqueous sodium hydroxide solution and washed with water. After separation and drying steps, about 5.5 kg/H of white copolymer powders were obtained. Analytical data are shown in Table 1. (4) Graining and sheet molding To 15 kg of the white polymer powders obtained above were added 15 g of BHT® (2,6-di-t-butyl-p-cresol), 7.5 g of Irganox 1010® (tetrakis[methylene(3,5-di-t-butyl-4-hydrocinnamate)]-methane) and 30 g of calcium stearate. The mixture was subjected to graining using a granulator of 40 mm⊘. Then, the granulated matters were finished with an extrusion molding machine of 50 mm⊘ at 225°C to prepare a sheet having a width of 60 cm and a thickness of 0.4 mm. Physical properties of the sheet were evaluated by the methods described above. The results are shown in Tables 3 and 4. Comparative Example 1 The procedures of Example 1 were carried out in a similar manner except that the gas in the gaseous phase was not discharged from polymerizer (2). The sheet made from polymer obtained was inferior in heating behavior. Comparative Example 2 The procedures of Example 1 were carried out in a similar manner except that he gaseous phase concentrations of polymerizers (1) and (2) were made identical by supplying together with hydrogen. Also in this case, the sheet was markedly inferior in heating behavior. Examples 2 through 4 and Comparative Examples 3 and 4 The procedures of Example 1 were carried out in a similar manner except that the gaseous phase hydrogen concentration and the amount of gas discharged were changed as shown in Table 1. When the amount of gas discharged is insufficient, the sheet is inferior in heating behavior and when the amount is too much, appearance was not good upon molding of the sheet (due to non-uniform fluidity). Example 5 The procedures of Example 1 were carried out in a similar manner except that diethyleneglycol dimethyl ether (a) was added to the degasification tank (3) in a/Ti = 0.3 (molar ratio) based on titanium in the solid catalyst and the gaseous phase hydrogen concentration in polymerizer (4) was made 2.0 mol%. Remarkable improvement was noted in low temperature blanking impact strength. Comparative Example 5 The procedures of Example 5 were carried out in a similar manner except that diethyleneglycol dimethyl ether was not added. Multiple occurrence of FE in sheet appearance and reduction in blanking impact strength were noted. Example 6 In Example 5, methyl p-toluate was supplied to polymerizer (1) in 1 g per 1 g of the solid product in the catalyst slurry. Further the procedures were performed, changing the amount of the catalyst slurry to 240 mℓ/H. As shown Table 4, the results reveal that Young's modulus was remarkably increased and heating behavior was improved with the catalyst system of the present invention. Comparative Example 6 The procedures of Example 6 were carried out in a similar manner except that diethyleneglycol dimethyl ether was not added. Multiple occurrence of FE in sheet appearance and reduction in blanking impact strength were noted.

Claims: A continuous process for producing an ethylene-propylene copolymer using a Ziegler-Natta catalyst, in which process:- (a) the polymerization is effect in at least three polymerization vessels connected in series and is carried out as a two-stage polymerization namely:- (i) a first polymerization stage, in which propylene is polymerized as major component, carried out in at least the first two polymerization vessels to which propylene, optionally in admixture with up to 5% by weight of ethylene (based on the total of propylene and ethylene), is supplied to the said at least two first polymerization vessels, and (ii) a second polymerization stage carried out in at least one polymerization vessel subsequent to those in which the first polymerization stage (i) is carried out, the monomers supplied to the second polymerization stage comprising ethylene, and optionally, propylene and containing from 10 to 100% by weight of ethylene; (b) all catalyst to be used is supplied to the first polymerizer, the catalyst being passed through the second and subsequent polymerizers together with the polymerization reaction mixture to form polymer in each polymerizer on the same catalyst and is then discharged from the last polymerizer; (c) hydrogen gas is used as a molecular weight regulator in the continous propylene polymerization step (i), all the hydrogen gas being supplied to the first polymerizer and the balance being transferred to subsequent polymerizer in series together with the polymerization reaction mixture; (d) gas is continuously removed from the gas phase of the second and subsequent polymerizers in propylene polymerization step (i) at a rate of 0.5 to 10 V₁/hr calculated at 0°C, 0 kg/cm²G (the volume of the polymerizer being V₁); and, (e) the amount of polymer formed in polymerization step (i) is 60 to 95 wt.% of the total polymer formed. A process as claimed in claim 1 in which the ratio of the MFR value of the polymer produced in the first polymerizer and the last polymerizer in at least two polymerizers connected in series is adjusted to be in the range given by the equation: [wherein MFR₁ is the MFR of the polymer produced in the i order o polymerizer, and MFR t is MFR of polymer produced in the last polymerizer in the propylene polymerization step (i)] A process as claimed in claim 1 or claim 2 in which the gas withdrawn from the gas phase of the second and/or subsequent polymerizers (if any) is recycled to the first polymerizer, together with gas recovered from a subsequent degasification tank. A process as claimed in any one of claims 1-3 in which the continuous polymerization step (i) is performed in the presence of an inert solvent or liquid propylene using a compound of the formula: AlR ℓ / mX 3-m (wherein R ℓ is C₁-C₂₀ hydrocarbyl group; X is a halogen atom; and m is a number of 3≧m>1.5) as an organic aluminium compound (B) in combination with a titanium-containing solid component (A) constituting the Ziegler-Natta catalyst, and a glycol ether (C) is continuously added to the slurry after completion of step (i) in a concentration given by the equation. (C)/Ti in (A) = 0.01 to 1.0 (mol/atom) to effect the continuous polymerization step (ii).

Inventors: CHIBA HIROMASA, KAWAZOE SHUNJI, OKA TAKAHIRO, CHIBA, HIROMASA, KAWAZOE, SHUNJI, OKA, TAKAHIRO
Applicants: CHISSO CORP, CHISSO CORPORATION
Application year: 1988
Main classification: C08F210
IPC codes: C08F297, C08F2