Process for the production of propylene block copolymers by a three step method

A process for producing a propylene block copolymer by a three-step reaction in the presence of a stereoregular catalyst. At the first step, propylene is polymerized at a temperature of 55.degree. C. or more to form polypropylene having an intrinsic viscosity of from 0.5 to 3.5 in the prescribed amount. At the second step, propylene is polymerized at a temperature of from 30.degree. to 90.degree. C. to form polypropylene having an intrinsic viscosity of at least 4 in the prescribed amount. And at the third step, ethylene and propylene are copolymerized at a temperature of from 30.degree. to 90.degree. C. to form an ethylene-propylene copolymer having an intrinsic viscosity of at least three in the prescribed amount. The propylene block copolymer formed by the three steps is superior and well balanced in impact resistance and stiffness.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for the production of propylene 
block copolymers. More particularly, it relates to a process for producing 
with high efficiency propylene block copolymers which are superior in 
physical properties, particularly in impact resistance and stiffness. 
Crystalline polypropylene is superior in, for example, stiffness, heat 
resistance and surface gloss, but has a disadvantage in that its impact 
resistance is poor. 
In order to overcome the disadvantage of poor impact resistance in the 
crystalline polypropylene, various methods in which propylene block 
copolymers containing an ethylene unit are produced have been proposed. 
One of the methods involves forming a propylene homopolymer at the first 
step, further polymerizing propylene at higher temperatures than in the 
first step at the second step, and then copolymerizing ethylene and 
propylene at the third step (see Japanese patent application Laid-Open No. 
71712/1980). In accordance with these conventional methods, however, the 
productivity of the desired polymers drops and there cannot be produced 
copolymers which are well balanced in impact resistance and stiffness. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a process for producing with high 
efficiency propylene block copolymers which are superior and well balanced 
in impact resistance and stiffness. 
It has been found that the object can be attained by producing propylene 
block copolymers by a three-step polymerization method while controlling 
the polymerization temperature at each step and also the intrinsic 
viscosity and amount of polymer formed at each step within specific 
ranges. 
The present invention relates to a process for producing a propylene block 
copolymer by a three-step reaction in the presence of a stereoregular 
catalyst, which comprises: 
polymerizing propylene at a temperature of at least 55.degree. C. to form 
polypropylene having an intrinsic viscosity of from 0.5 to 3.5 in an 
amount of from 50 to 94% by weight based on the total amount of copolymers 
formed finally (first step); 
polymerizing propylene at a temperature ranging between 30.degree. and 
90.degree. C. to form polypropylene having an intrinsic viscosity of at 
least 4 in an amount of from 25 to 3% by weight based on the total amount 
of copolymers formed finally (second step); and 
copolymerizing ethylene and propylene at a temperature ranging between 
30.degree. and 90.degree. C. to form an ethylene-propylene copolymer 
having an intrinsic viscosity of at least 3 in an amount of from 25 to 3% 
by weight based on the total amount of copolymers formed finally (third 
step). 
DETAILED DESCRIPTION OF THE INVENTION 
The term "stereoregular catalyst" as used herein refers to a catalyst which 
is generally used in a stereoregular polymerization reaction of, for 
example, ethylene and propylene, and it usually comprises a transition 
metal halogen compound component and an organoaluminum compound component. 
Suitable examples of transition metal halogen compounds are titanium 
halides, with titanium trichloride being especially preferred. Various 
types of titanium trichloride can be used, including (1) titanium 
trichloride prepared by reducing titanium tetrachloride by various 
techniques, (2) titanium trichloride activated by further subjecting the 
titanium trichloride (1) to ball mill treatment and/or washing with 
solvents (e.g., inert solvents and/or polar compound-containing inert 
solvents), and (3) titanium trichloride prepared by subjecting titanium 
trichloride or a titanium trichloride eutectic compound (e.g., TiCl.sub.3 
1/3AlCl.sub.3) to a copulverization treatment in combination with, for 
example, amines, ethers, esters, sulfur compounds, halogen compounds, and 
organic or inorganic nitrogen or phosphorus-containing compounds. In 
addition, titanium halides deposited on magnesium can be used. 
Suitable examples of organoaluminum compounds are represented by the 
general formula: AlR.sub.n X.sub.3-n (wherein R is an alkyl group 
containing from 1 to 10 carbon atoms, X is a halogen atom, and 
0&lt;n.ltoreq.3). These examples include dimethylaluminum chloride, 
diethylaluminum chloride, ethylaluminum sesquichloride, ethylaluminum 
dichloride, and triethylaluminum. These compounds can be used singly or in 
a mixture comprising two or more thereof. 
The molar ratio of the organoaluminum compound to the transition metal 
compound is usually from 1:1 to 100:1. 
At the first step of the process of the invention, propylene is polymerized 
at a temperature of 55.degree. C. or more, preferably from 61.degree. to 
70.degree. C. to form polypropylene having an intrinsic viscosity [.eta.] 
of from 0.5 to 3.5, preferably from 0.7 to 3.0 in an amount of from 50 to 
94% by weight, preferably from 70 to 90% by weight, based on the total 
amount of copolymers formed finally. If the polymerization temperature is 
lower than 55.degree. C., the productivity of polymer drops. If the 
intrinsic viscosity is less than 0.5, the impact strength of the copolymer 
formed is low, whereas if it is more than 3.5, its moldability is reduced. 
Furthermore, if the amount of the polypropylene formed in the first step 
is less than 50% by weight based on the total amount of copolymers formed 
finally, the stiffness of the copolymer drops, whereas if it is more than 
94% by weight, the impact strength of the copolymer drops. 
At the second step, propylene is polymerized at a temperature of from 
30.degree. to 90.degree. C., preferably from 40.degree. to 70.degree. C. 
to form polypropylene having an instrinsic viscosity of at least 4, 
preferably from 4.5 to 7.5 in an amount of from 25 to 3% by weight, 
preferably from 15 to 5% by weight based on the total amount of copolymers 
formed finally. If the intrinsic viscosity of polypropylene formed in the 
second step is less than 4, the impact strength of the copolymer formed 
finally drops. If the amount of the polypropylene formed in the second 
step is less than 3% by weight, the stiffness of the copolymer formed 
finally drops, whereas if it is more than 25% by weight, the impact 
strength of the copolymer undesirably drops. 
At the third step, ethylene and propylene are copolymerized at a 
temperature of from 30.degree. to 90.degree. C., preferably from 
40.degree. to 70.degree. C. to form an ethylene-propylene copolymer having 
an intrinsic viscosity of at least 3, preferably from 4 to 12 in an amount 
of from 25 to 3% by weight, preferably from 18 to 5% by weight based on 
the total amount of copolymers formed finally. If the intrinsic viscosity 
of the ethylenepropylene copolymer formed in the third step is less than 
3, the impact strength of the copolymer formed finally drops. If the 
amount of the ethylene-propylene copolymer is less than 3% by weight, the 
impact strength of the copolymer formed finally drops, whereas if it is 
more than 25% by weight, the stiffness of the copolymer formed finally 
undesirably drops. Furthermore it is preferable for the ethylene-unit 
content of the ethylene-propylene copolymer to be controlled within the 
range of from 1 to 10% by weight, preferably from 2 to 8% by weight. If 
the ethylene-unit content is less than 1% by weight, the impact strength 
of the copolymer formed finally drops, and the amounts of non-crystalline 
polymers formed increase. On the other hand, if it is more than 10% by 
weight, the stiffness of the copolymer formed finally undesirably drops. 
The intrinsic viscosity can be controlled by appropriately changing the 
concentration of a molecular weight modifier (e.g., hydrogen). The 
reaction pressure is from 1 to 30 kilograms per square centimeter and 
preferably from 2 to 15 kilograms per square centimeter at each step of 
the process of the invention. 
The process of the invention can be performed by various techniques such as 
a method in which at least three reactors are used and polymerization is 
performed continuously, a method in which at least one reactor is used and 
polymerization is performed batchwise, and a combination thereof. The 
polymerization method is not critical; any of suspension polymerization, 
solution polymerization, gas phase polymerization, and so forth can be 
employed. Inert solvents which can be used in the suspension 
polymerization include aliphatic hydrocarbons such as hexane and heptane, 
alicyclic hydrocarbons such as cyclohexane, and aromatic hydrocarbons such 
as benzene and toluene. 
The process of the invention offers various advantages. One of the 
advantages is that the productivity of the desired ethylene-propylene 
block copolymer is high. Another advantage is that the ethylene-propylene 
block copolymer is superior and further well balanced in impact resistance 
and stiffness. Another advantage is that the ethylene-propylene copolymer 
is superior in physical properties such as impact whitening, heat 
resistance, and surface gloss. 
Hence the propylene block copolymer produced by the process of the 
invention is a very useful material for use in the fabrication of, for 
example, car parts and domestic electrical appliances.

The present invention is described in greater detail with reference to the 
following Examples 1 to 7. 
EXAMPLES 1 TO 7 
Five liters of dehydrated n-heptane was placed in a 10-liter autoclave 
equipped with a stirrer, and 1.0 gram of diethylaluminum chloride and 0.3 
gram of titanium trichloride were added thereto. 
As the first step of the process of the invention, propylene was 
polymerized with stirring for 90 minutes while maintaining the temperature 
of the liquid layer at 65.degree. C., feeding a predetermined amount of 
hydrogen so that polypropylene had a given intrinsic viscosity, and also 
continuously feeding propylene so that the reaction pressure was 9 
kilograms per centimeter. At the end of the time, unreacted propylene was 
removed and the temperature of the liquid layer was lowered to 50.degree. 
C. 
As the second step, propylene was polymerized for 40 minutes while 
maintaining a temperture of 50.degree. C. and pressure of 9 kilograms per 
square centimeter, and continuously feeding predetermined amounts of 
hydrogen and propylene. 
Finally, as the third step, a propylene/ethylene mixture and a prdetermined 
amount of hydrogen were introduced in the autoclave and polymerized for 30 
minutes while maintaining the temperature at 50.degree. C. At the end of 
the time, unreacted propylene was removed, and 50 milliliters of n-butanol 
was added to the polymerization product and stirred at 65.degree. C. for 1 
hour to decompose the catalyst. Then the thus-produced polymer was 
separated, washed and dried to obtain a white powdery polymer. 
The physical properties of the polymer was measured, and the results are 
shown in the Table. 
COMATIVE EXAMPLES 1 TO 4 
The procedure of the Examples was repeated wherein the intrinsic viscosity 
and amount of the polymer formed at each step were changed. The results 
are shown in the Table. 
TABLE 
__________________________________________________________________________ 
First Step 
Second Step 
Third Step 
Physical Properties of Copolymer 
Formed Finally 
Amount of 
Amount of Amount of 
Ethylene-unit*.sup.2 
Impact*.sup.4 
Polymer Polymer Copolymer 
Content MI*.sup.3 
Strength 
Stiffness*.sup.5 
1 
Run No. 
[.eta.]*.sup.1 
(wt %) 
[.eta.]*.sup.1 
(wt %) 
[.eta.]*.sup.1 
(wt %) 
(wt %) (g/10 min.) 
(kg .multidot. cm/cm) 
(kg/cm.sup.2) 
__________________________________________________________________________ 
Example 1 
2.9 76 5.9 
10 4.6 14 2.9 0.27 41 9700 
Example 2 
2.4 78 7.2 
12 9.4 10 6.2 0.36 46 9900 
Example 3 
2.3 80 4.9 
10 4.6 10 4.0 0.80 24 9400 
Example 4 
1.3 83 5.3 
7 7.7 10 5.0 9.3 29 11700 
Example 5 
1.0 83 4.7 
7 5.2 10 4.7 28 5.3 12400 
Example 6 
0.75 
80 4.6 
10 4.4 10 4.3 45 3.8 13200 
Example 7 
2.8 75 5.5 
10 4.7 18 12 0.31 79 7300 
Comparative 
2.8 89 -- -- 4.9 11 3.0 0.30 31 8200 
Example 1 
Comparative 
4.0 76 5.9 
10 4.5 14 unmoldable 
Example 2 
Comparative 
2.9 76 3.0 
10 4.5 14 3.0 0.52 11 10700 
Example 3 
Comparative 
2.7 60 4.5 
10 3.2 30 3.5 0.29 85 6300 
Example 4 
__________________________________________________________________________ 
Note: 
*.sup.1 [.eta.] (Intrisic viscosity) as determined at 135.degree. C. in 
decalin. 
*.sup.2 Ethylene-unit content as determined by an infrared spectrum 
method. 
*.sup.3 MI (Melt index) as determined according to JIS K 7210. 
*.sup.4 Impact strength as determined according to JIS K 7110 (Izod Impac 
Strength Test). 
*.sup.5 Stiffness as determined according to JIS K 7106 (Olsen Stiffness 
Test).