For the polymerization of olefins the Ziegler-Natta-catalyst system is generally used, which consists of a so-called procatalyst and a cocatalyst. The procatalyst is based on a compound of a transition metal belonging to any of the groups IV(A) to VIII(A) (Hubbard) of the periodical system of elements and the cocatalyst is based on an organometallic compound of a metal belonging to any of the groups I (A) to III (A) (Hubbard) of the periodical system of elements.
Nowadays, the procatalysts typically comprise an inert carrier, on which the actual active catalyst component i.e. the transition metal compound or the mixture of the complex formed by the catalytical compounds has been layered. The morphology and size distribution of the particles of such a carrier are most significant for the activity of the catalyst and the properties of the polymer obtained through the catalyst. With an active catalyst you are, namely, able to produce a polymer, from which, due to its purity, no catalyst residues need to be removed. The morphology of the carrier, again, influences the morphology of the polymer product itself for it has been noticed that the morphology of the catalyst is repeated in the structure of the polymer (the so-called replica phenomenon). When a flowing product polymer having the desired morphology and a narrow particular size distribution is wanted, which is desirable in view of the many aims of use of the processing processes, the properties of the carrier shall by the aid of the replica phenomenon be made similar.
Nowadays, the Ziegler-Natta type procatalysts typically comprise a magnesium based carrier, such as magnesium chloride, which has been treated with a transition metal compound of titanium halide, such as titanium tetrachloride, and sometimes also with an electron donor compound. It is also known that the carrier can be brought to an advantageous and equal-size crystal form by letting it crystallize as a complex from any of its crystal solvents.
The treatment of such a carrier with a crystal solvent has been disclosed among others in the U.S. Pat. No. 4,071,674, in which the procatalyst based on a transition metal compound has been prepared by bringing a titanium or vanadium compound to react with a reaction product that has been formed when magnesium dihalide and the addition product of alcohol are reacting with an organometallic compound of a metal of any of the groups I-III. The preparation of the procatalyst begins with the addition of alcohol dropwise to the suspension of magnesium dihalide, after which the organometallic compound is added dropwise to the reaction mixture. After agitation the preactivated carrier is activated by adding titanium tetrachloride to the mixture. The adding stages of this kind of a method are primitive and do not at all allow regulation of the morphology of the procatalyst in the way desired.
Treatment with a crystal solvent has also been described in patent application JP-59-215301. In this publication the carrier complex (10 g of MgCl.sub.2 and 24.2 g of EtOH) have been prepared by an emulsifying technique. The carrier complex melt has been dispersed into n-decane as spheroidal melt particles. Thereafter the carrier particles in the emulsion have been shock coagulated by transferring the emulsion into cold hydrocarbon medium. A drawback of this method is, among others, that such components are needed in the preparation of the carrier that are not useful at later stages of the catalyst preparation and this presupposes the existance of a purifying and recirculation equipment for this purpose.
The patent family comprising, among others, the EP publication 65700 and the U.S. Pat. No. 4,421,674 which claims priority from the Italian application IT 2,188,181 (810521), deal with a method for the preparation of a catalyst, which is particularly active in the polymerization of gaseous ethylene.
In the process titanium halide is brought to react with a magnesium chloride catalyst carrier being in the form of microballs, after which the reaction product particles are recovered by physical means and are mixed together with an organometallic cocatalyst product.
Characterizing of this method representing the prior art is that:
According to FI patent publication 80055 (Neste Oy) the above-mentioned carrier complex formed by the carrier and the crystal solvent can be melted to clear liquid. When this kind of a liquid is conducted through the spray nozzle to the spray space cooled with cold nitrogen gas it crystallizes to spheroidal small particles of the carrier complex which are very flowable and loose. The process took in practice place so that MgCl.sub.2 and C.sub.2 H.sub.5 OH were melted at the temperature 110.degree. to 130.degree. C. to a clear melt. Then the clear homogenized mixture was fed through a nozzle dispersing it into a cooled spray chamber. The spraying gas used in the spraying was dry nitrogen having the temperature +130.degree. C. and as cooling medium dry nitrogen was conducted to the spraying chamber, the temperature of which was 20.degree. C. As the nozzle a gas fluidizing nozzle was used.
This kind of a method produces very flowable and loose particles. Moreover, the carrier complex crystallizes without any evaporation of the crystal solvent. When such a carrier is brought into contact with titanium compound, abundantly for catalytically active complexes between MgCl.sub.2 and the titanium compound are formed on the surface of the carrier when the crystal solvent leaves.
Accordingly, two methods of preparation based on the spraying are prior known in the field. The spray drying method patents are based on a fairly complete drying of the carrier liquid from ethanol (C.sub.2 H.sub.5 OH) after the spraying. Hereby, the carrier has usually been dried at a temperature exceeding 150.degree. C., whereby a major portion of the alcohol of the complex is evaporated. In the spray-drying method a carrier product is typically obtained, the alcohol concentration of which is between 15 to 25% by weight and anyway below 30% by weight.
One central drawback of spray-drying is the bad morphology of the carrier obtained and the wide particle size distribution, which is caused by the breaking of the particles during the process. In FIGS. 1 and 2 an electron microscope picture of the carrier is presented. The close-up picture of FIG. 2 shows that the spheroidal particles are either compressed or then they have been broken into crust fragments of the hollow ball. On the basis of the physical process and the particles created in it, it can be supposed that the removal of C.sub.2 H.sub.5 OH at temperatures exceeding +150.degree. C. results in the formation of a gas state inside the particles, which then results in the disintegration of the spheroidal particles into crust fragments or their becoming compressed when the gas cools down in the hollow space. The spray-drying will, anyhow, result in a very poor carrier morphology.
It can thus be said that while the spraying steps of prior spray-drying methods were easy to carry out and their yield of produced carrier is good, the activity of the procatalyst obtained by activation with TiCl.sub.4 when polymerizing PP (polypropylene) is only between 10 to 12 kg of PP/g of catalyst and the morphology of the polymer obtained by it is very poor producing even up to 60 to 70% by weight of finely-divided material (d&lt;1 mm), the bulk density being of the order of about 0.2 g/ml. The above-mentioned activity of the procatalyst obtained by spray-drying is lower than that of the procatalysts obtained by other methods, which is due to the fact that in spray-drying the major portion of the C.sub.2 H.sub.5 OH participating in the TiCl.sub.4 activation leaves the carrier before it is reacted with TiCl.sub.4.
The spray-crystallization of such a melt MgCl.sub.2 --C.sub.2 H.sub.5 OH complex that contains on average 3.5 or more C.sub.2 H.sub.5 OH molecules as per each MgCl.sub.2 molecule is easy and gives a relatively good yield, whereby the result is a procatalyst having a good activity i.e. of the order of about 15 kg of PP/g of catalyst. It has, however, been noticed that the C.sub.2 H.sub.5 OH amount of the carrier is the same as the respective amount of the melt and too high causing during the TiCl.sub.4 activation heavy formation of HCl gas, which breaks the procatalyst particles and results in a poor procatalyst morphology and a poor particle size distribution. In the polymerization polypropylene having a poor morphology and about 25 to 55% by weight of finely-divided substance (d&lt;1 mm) is obtained. The bulk density of the polymer obtained is 0.40 to 0.44 g/ml.
In the spray-crystallization of melt MgCl.sub.2 --C.sub.2 H.sub.5 OH complex which contains less i.e. in the average about 2.9 molecules of C.sub.2 H.sub.5 OH as per each MgCl.sub.2 molecule, big difficulties are again encountered when spraying melt into the crystallization chamber. Melt with little alcohol apparently is too viscous and causes clogging of the nozzle and formation of too big particles e.g. as a result of solidification. The carrier yield is very low remaining below 10% in the polymerization of propylene a good activity of the procatalyst is obtained i.e. about 15 kg of PP/g of catalyst, but the morphology of the polypropylene is poor and it contains 20 to 40% of finely-divided material (d&lt;1 mm), the bulk density being between 0.40 to 0.44.
Accordingly, it seems that the spray-crystallization results in a procatalyst whose activity is medium, and whose polymer products contain a very great portion of finely-divided material and possess a polymer morphology which is poor.