Propylene-ethylene copolymers made from three different...

Details Propylene-ethylene copolymers made from three different... Propylene-ethylene copolymers made from three different...

1995-09-06

2001-10-09

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S348000, C526S347000, C526S351000, C526S352000, C526S158000, C526S124500, C525S053000, C525S247000, C525S268000, C525S270000, C525S322000

Reexamination Certificate

active

06300434

ABSTRACT:

Propylene-ethylene copolymers obtainable by three-step polymerization from the gas phase in an agitated fixed bed by means of Ziegler-Natta catalyst system which, in addition to a titanium-containing solid component, also contains, as cocatalyst, an aluminum compound, where, in a first polymerization step, propylene is polymerized at from 60 to 90° C. and at from 20 to 40 bar and at a mean residence time of the reaction mixture of from 0.5 to 5 hours, then, in a second polymerization step, a mixture of propylene and ethylene is polymerized onto the polymer obtained from the first polymerization step at from 40 to 110° C. and from 5 to 30 bar, this pressure being at least 7 below the pressure in the first polymerization step, and at a mean residence time of the reaction mixture of from 0.2 to 4 hours, and then, in a third polymerization step, ethylene or a mixture of ethylene and propylene is polymerized onto the polymer obtained from the second polymerization step at from 40 to 110° C. and from 5 to 30 bar and at a mean residence time of the reaction mixture of from 0.1 to 5 hours, the weight ratio between the monomers reacted in the first and second polymerization steps being adjusted to from 1:1 to 20:1 and the weight ratio between the monomers reacted in the first two polymerization steps and those reacted in the third polymerization step being adjusted to from 1:2 to 20:1.
In addition, the present invention relates to a process for the preparation of these propylene-ethylene copolymers and to films and moldings made from these copolymers.
Propylene-ethylene copolymers obtainable by polymerization on Ziegler-Natta catalyst have already been described in a number of patent specifications. U.S. Pat. No. 4,260,710 discloses the preparation of homopolymers and copolymers of alk-1-enes by polymerization in a stirred reactor with the aid of Ziegler-Natta catalysts. The catalyst components used here contain, inter alia, compounds of polyvalent titanium, aluminum halides and/or alkylaluminum compounds, and electron-donor compounds, where silanes, esters, ethers, ketones or lactones are usually used (EP-B 14 523, EP-B 45 977, EP-B 86 473, EP-A 171 200 and U.S. Pat. No. 4,857,613).
Furthermore, a number of processes have been disclosed, for the preparation of propylene-ethylene block copolymers with the aid of a Ziegler-Natta catalyst system (U.S. Pat. No. 4,454,299, U.S. Pat. No. 4,455,405, ZA-B 0084/3561, ZA-B 0084/3563, ZA-B 0084/5261 and GB-B 1,032,945), in which gaseous propylene is first polymerized in a first reaction step, and the resultant homopolymer is subsequently passed to a second reaction step where a mixture of ethylene and propylene is polymerized on. The process is usually carried out at superatmospheric pressure and in the presence of hydrogen as molecular weight regulator. The copolymers obtainable in this process usually have excellent impact strength, but, in addition to reduced rigidity, also have a relatively high tendency toward stress whitening. For the purpose of the present invention, stress whitening is taken to mean the white coloration of a previously transparent sample in individual areas which occur in many plastics during stretching.
A combination of various mechanical properties, in particular high impact strength, and still sufficiently high rigidity can be observed, in particular, in polymer blends. Thus, for example, the properties of rigid, heat-resistant polymers can be combined with those of soft, resilient polymers by mixing in such a manner that the resultant blends have an advantageous combination of the good properties of the two types of polymer (Saechtling, Kunstoff-Taschenbuch, Carl-Hanser-Verlag, Munich, page 8 [1986]).
The earlier applications DE-A 40 19 455 and DE-A 40 19 456 discloses, for example, blends of different types of polyolefin which have high impact strength, high rigidity and a low tendency toward stress whitening. Blends of this type are obtained by mixing certain propylene copolymers, which themeselves comprise propylene homopolymers and random propylene copolymers, with polyethylenes. However, the preparation of these blends is technically relatively complex. First, it is necessary to prepare propylene copolymers by a two-step process where propylene is polymerized in a first step and a mixture of propylene and ethylene is subsequently polymerized onto the resultant propylene polymer in a second step. Independently thereof, a polyethylene having a certain density is prepared in a further process step. The polymers obtained in this way, the propylene copolymer and the polyethylene, are then usually mixed with one another in an extruder. This preparation process gives products having good applicational properties, but, in addition to the actual polymerization steps, also requires an additional mixing step, which causes problems in coordinating the individual process steps in terms of time and increases the apparative complexity of the process.
It is an object of the present invention to overcome the above disadvantages and to develop propylene-ethylene copolymers which have good applicational properties and can be prepared in a very simple manner. Among the good applicational properties of the propylene-ethylene copolymers of this invention is a reduced amount of chlorine in the polymer. This increases the applicability of the polymers for use in food packaging.
We have found that this object is achieved by the novel propylene-ethylene copolymers defined at the outset.
The process which gives these copolymers can be carried out either batchwise or preferably continuously in the conventional reactors used for the polymerization of propylene. Suitable reactors are, inter alia, continuously operated stirred reactors, it also being possible to employ a series of stirred reactors connected in series. The reactors contain a fixed bed of finely divided polymer which is usually kept in motion by stirring.
The process can be carried out using the Zeigler-Natta catalyst which are conventional in polymerization technology. In addition to a titaninum-containing solid component, these also contain , inter alia, a cocatalyst. Suitable cocatalysts are aluminum compounds. In addition to this aluminum compound, an electron-donor compound is preferably also employed as a further constituent in the cocatalyst.
The titanium-containing solid component is generally prepared using, as titanium compound, a halide or alkoxide of trivalent or tetravalent titanium, preference being given to titanium chlorides, in particular titanium tetrachloride. The titanium-containing solid component advantageously contains a finely divided carrier, for which purpose silica, alumina and aluminosilicates have proven successful. Particularly preferred carriers are silica or an aluminosilicate of the formula SiO
2
. aAl
2
O
3
where a is from 0.001 to 2, in particular from 0.01 to 0.5.
The preparation of the titanium-containing solid component is also carried out using, inter alia, compounds of magnesium, in particular magnesium halides, alkylmagnesium compounds and arylmagnesium compounds, and alkoxymagnesium and aryloxmagnesium compounds, preferably magnesium dichloride, magnesium dibromide and magnesium di(C
1
-C
10
-alkyl) compounds. In addition, the titanium-containing solid component may also contain halogen, preferably chlorine or bromine.
Furthermore, the titanium-containing solid component also contains electron-donor compounds, for example monofunctional or polyfunctional carboxylic acids, carboxylic anhydrides and carboxylic esters, furthermore ketones, ethers, alcohols, lactones, and organophosphorus and organosilicon compounds. Preferred electron-donor compounds within the titanium-containing solid component are phthalic acid derivatives of the general formula I
where X and Y are each chlorine or C
1
- to C
10
-alkoxy or together are oxygen. Particularly preferred electron-donor compounds are phthalic esters, where X and Y are C
1
-C
8
-alkoxy, for example methoxy, ethoxy, propoxy or butoxy.
Other preferred electron-donor compounds within

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