Polypropylene resin composition and use thereof

Details Polypropylene resin composition and use thereof Polypropylene resin composition and use thereof

1999-03-31

2002-02-26

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...

C526S351000, C526S128000, C526S065000, C525S053000, C525S240000

Reexamination Certificate

active

06350828

ABSTRACT:

FIELD OF THE TECHNIQUE
The present invention relates to a polypropylene resin composition, to a process for the production of such resin composition and to uses thereof.
BACKGROUND OF THE TECHNIQUE
Polypropylene has widely been used in various fields including automobile parts, machine and electric appliances, household commodities, kitchen utensils and packaging films. However, problems have been brought about in that large-sized formed articles are difficult to obtain by, for example, extrusion molding, and in that a high speed molding can scarcely be attained, since polypropylene exhibits lower melt tension (abbreviated hereinafter sometimes as MT). Concretely, the following problems have been encountered:
(1) In blow molding, a phenomenon of “draw-down” due to stretching of the parison by its own weight, causing decrease in the film thickness may be apt to occur, whereby blow molding of large-sized articles, for example, automobile parts, such as bumper and spoiler; and others, such as bottles, is rendered difficult.
(2) In the case of production of sheet or film by a calendering technique, the resulting sheet or film may often suffer from thickness irregularity and, in addition, it has a lower surface gloss.
(3) In the case of production of formed articles by extrusion molding, a high-speed molding may scarcely be practised and, in addition, large-sized extrusion-molded articles may difficultly be obtained.
(4) In the case of production of vacuum- or pressure formings from a sheet by a vacuum or pressure forming technique, large-sized molded articles are difficult to obtain and, in addition, a deep drawing may difficultly be incorporated.
(5) In the case of production of sheet or film by an inflation molding technique, a poor surface condition may often be encountered, since the baloon may often become unstable.
(6) In the case of producing stretched films, the resulting film may be apt to suffer from occurrence of so-called surging, so that an accident of film breaking upon the stretching may occur and, in addition, the resulting stretched film exhibits a low thickness accuracy.
(7) In the case of producing foamed articles, foaming with a high foaming ratio may difficultly be attained and, in addition, the cells of foamed article are large and coarse with non-uniform cell size.
In order to avoid these problems, it has heretofore been practised to employ such polypropylene reins as given below in which the melt tension is increased:
1) A polypropylene resin composition prepared by blending a polypropylene with a high-pressure low-density polyethylene or with a high-density polyethylene
2) A polypropylene resin having a widely extended molecular weight distribution
3) A modified polypropylene resin which is obtained by slightly cross-linking a polypropylene resin using a peroxide, electron irradiation or maleic acid
4) A branched long chain polypropylene resin which is obtained by introducing long chain branching upon the polymerization of propylene.
However, these prior art polypropylene resins having improved melt tension exhibit disadvantages in that the formed article produced therefrom reveals inferior appearance and/or lower transparency and in that the stiffness of the resin is insufficient, though occurence of draw-down is made scarce for all these resins. Alternatively, if the molding temperature is elevated in order to effect a high speed molding, problems may be brought about that the resin will suffer from deterioration due to increased heat evolution in the resin, causing higher trend to gel formation (fish eye formation).
In Japanese Patent Kokai Sho-59-149907 A, there is disclosed a process for producing a polypropylene resin having higher melt tension and higher stiffness with superior moldability by a two-stage polymerization. This process comprises performing a 1st stage polymerization of propylene to build up 50-85%, based on the entire weight of the final polymer product, of a polypropylene product having an intrinsic viscosity [&eegr;] of 0.5-3.0 dl/g and, then, effecting a 2nd stage polymerization to build up 50-15%, based on the entire weight of the final polymer product, of a polypropylene product having an intrinsic viscosity [&eegr;] of at least 9 dl/g, to thereby produce a crystalline polypropylene resin composition having, as the entire polypropylene resin composition, an intrinsic viscosity [&eegr;] of 2-6 dl/g, a melt flow rate (MFR) of 0.01-5 g/10 min. and an isotactic pentad fraction of 0.940 or higher.
The polypropylene resin composition produced by this process exhibits, however, a wide molecular weight distribution, as seen, for example, from the Mw/Mn values given in Examples of the specification of this prior patent gazette in the range of 23.2-42.2, so that the moldability of this resin composition is worse, whereby the appearance of the molded articles therefrom becomes inferior. In addition, it exhibits a lower isotactic pentad fraction, as seen in Examples of the patent gazette in the range of 0.955-0.969, so that the stiffness of the resin is insufficient. Moreover, the polypropylene resin composition obtained by the above-mentioned two-stage polymerization suffers from a problem of high tendency to occurrence of gel formation which causes deterioration in the appearance of the molded article, since a polypropylene product exhibiting a low intrinsic viscosity [&eegr;] is produced in the first stage polymerization and a polypropylene product exhibiting a high intrinsic viscosity [&eegr;] is produced in the second stage polymerization. Furthermore, when performing the above-mentioned two-stage polymerization in a continuous way for the benefit of industrial production in order to produce a polypropylene product having a low intrinsic viscosity [&eegr;] in the first stage polymerization and to produce a polypropylene product having a high intrinsic viscosity [&eegr;] in the second stage polymerization, it is necessary to effect the first stage polymerization of propylene in the presence of hydrogen and to realize the second stage polymerization of propylene in the absence of hydrogen, so that it is required to reduce the excess hydrogen contained in the reaction product from the first stage polymerization as low as possible on subjecting it to the second stage polymerization and, thus, a complicated polymerization apparatus becomes necessary. There may occur a still further problem in that a sufficiently high intrinsic viscosity [&eegr;] of the polypropylene product resulting from the second stage polymerization is not obtained due to the presence of the unremoved hydrogen rest, which may bring about an insufficient melt tension and insufficient stiffness.
In Japanese Patent Kokai Sho-59-172507 A, a process for producing a polypropylene resin superior in the stiffness, moldability and heat resistance by polymerizing propylene in two stages is disclosed. This process comprises producing, in one stage, 35-65%, based on the total weight of the final resin, of a polypropylene product having an intrinsic viscosity [&eegr;] of 1.8-10 dl/g and an isotacticity of at least 97.5% by weight and producing, in the other stage, 65-35%, based on the total weight of the final resin, of a polypropylene product having an intrinsic viscosity [&eegr;] of 0.6-1.2 dl/g and an isotacticity of at least 96.5% by weight, so as to thereby obtain a polypropylene resin composition having, as a whole, an intrinsic viscosity [&eegr;] of 1.2-7 dl/g and a molecular weight distribution expressed by Mw/Mn of 6-20. However, the intrinsic viscosity [&eegr;] of the polypropylene product of higher intrinsic viscosity [&eegr;] side, namely, higher molecular weight side, of the polypropylene resin composition is relatively low, as seen from the values given in Examples of the above patent gazette lying in the range of 2.10-7.28 dl/g, so that a sufficient melt tension and sufficient stiffness will not be attained, resulting thereby sometimes in an

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