Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-11-15
2001-07-10
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S240000
Reexamination Certificate
active
06258893
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a film-shaped or sheet-shaped unoriented molding formed from a molding material comprising a polypropylene-based resin material. More precisely, it relates to an unoriented polypropylene-based film having excellent transparency, impact resistance at low temperature and heat resistance as well as excellent tear resistance, or to an unoriented polypropylene-based sheet having excellent transparency, impact resistance at low temperature and heat resistance as well as excellent stress whitening resistance against an impact such as a drop or a hit.
BACKGROUND ART
Films or sheets of polypropylene-based resins have such characteristics that those are inexpensive and have excellent chemical resistance, oil resistance, mechanical strength, transparency, and heat resistance. Accordingly, those are widely utilized as materials for packaging food, textiles, etc. and as food containers, industrial material parts or stationery such as files.
The conventional unoriented polypropylene-based resin films or sheets have excellent heat resistance, however, they are poor in impact resistance at low temperature and tear strength if a composition containing a homopolymer of a propylene is used for the unoriented polypropylene-based resin films or sheets. On the other hand, if a copolymer composition containing a propylene-&agr;-olefin random copolymer is used therefor, though they have excellent transparency, their heat resistance and impact resistance at low temperature are poor. In addition, in case where a composition of a block copolymer consisting of a propylene homopolymer and a propylene-&agr;-olefin copolymer is used therefor, impact resistance at low temperature is excellent, but there are such defects that transparency is low, tear strength is poor, whitening due to impacts such as a drop or a hit is remarkable.
Therefore, in the conventional unoriented films or sheets using a polypropylene, it has not been easy to obtain ones satisfying all of transparency, impact resistance at low temperature and heat resistance, while possessing excellent tear strength and/or stress whitening resistance against impact (property that is not liable to be white due to impact).
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide an unoriented polypropylene-based film or sheet having excellent transparency and heat resistance as well as excellent tear strength and/or stress whitening resistance, without deteriorating impact resistance at low temperature.
The present inventors earnestly conducted studies in order to achieve the foregoing object. As a result, they found that, in an unoriented molding composed of a polypropylene-based material comprising a propylene-&agr;-olefin copolymer is dispersed as particles in crystalline polypropylene, transparency and heat resistance are improved without the deterioration of impact resistance at low temperature, and in addition, tear strength and stress whitening resistance can also be improved by controlling the copolymer particles so as to have a specific dispersion state in a cross-section along an MD direction of the molding. Thus, they have accomplished the present invention.
That is, the present invention provides an unoriented polypropylene-based molding which is a film-shaped or sheet-shaped molding formed from a molding material comprising a polypropylene-based resin material, the material comprising 40 to 80% by weight of crystalline polypropylene and 60 to 20% by weight of propylene-&agr;-olefin copolymer containing 20 to 80% by weight of propylene polymerization units, the propylene-&agr;-olefin copolymer dispersed as particles in the crystalline polypropylene, wherein the particles of the copolymer have an aspect ratio (L/D) of mean dispersed particle length (L) to mean dispersed particle diameter along the thickness of the molding (D) of 30 or more in a cross section of the molding along the MD direction, and the mean dispersed particle diameter of 0.3 &mgr;m or less.
The unoriented molding having the above characteristics in form is preferably that a ratio of MFR of the crystalline polypropylene to that of the propylene-&agr;-olefin copolymer (MFR of the crystalline polypropylene/MFR of the propylene-&agr;-olefin copolymer) is 10 or less.
The above-mentioned unoriented polypropylene-based molding is preferably an unoriented film having a thickness of 10 to less than 100 4 &mgr;m.
The above-mentioned unoriented polypropylene-based molding is preferably a sheet having a thickness of 0.1 to 4 mm.
The unoriented molding of the present invention comprises the elongated copolymer particles dispersed in a matrix of the crystalline polypropylene in such a manner the particles should have an aspect ratio higher than a certain level, and an unoriented film of sheets having such a dispersion state has been made by the present invention for the first time.
According to the present invention, an unoriented film having excellent impact resistance at low temperature, transparency, heat resistance and tear strength can be obtained thanks to such characteristics in form.
Further, according to the present invention, a sheet having excellent impact resistance at low temperature, transparency, heat resistance and stress whitening resistance can be obtained thanks to such characteristics in form.
The unoriented film of the present invention is useful as a film for package, in particular, as a film for food package (for example, for packaging retort foods or for packaging frozen foods) that requires transparency as well as impact resistance at low temperature, heat resistance and tear strength.
Further, the sheet of the present invention is useful as sheets for stationary such as files, or industrial material parts, in particular, as sheets for food packaging containers, sheets for industrial material parts or the like (for example, sheets for cosmetic papers, sheets for civil engineering) that particularly requires transparency as well as stress whitening resistance and impact resistance at low temperature.
Preferred embodiments of the present invention will be explained hereinafter.
(1) Polypropylene-based Resin Material of the Present Invention
The molding material for forming the unoriented molding of the present invention comprises a polypropylene-based resin material that consists of crystalline polypropylene and propylene-&agr;-olefin copolymer, the copolymer being dispersed as particles in the crystalline polypropylene (the copolymer is dispersed as domains in a matrix of the crystalline polypropylene).
(i) Crystalline Polypropylene
The crystalline polypropylene used for the present invention is a crystalline polymer comprising principally of propylene units, and preferably comprises 90% by weight or more of the propylene units based on the whole polymer. Specifically, it may be a homopolymer of propylene, or it may be a random copolymer comprising 90% by weight or more of propylene units and less than 10% by weight of &agr;-olefin. When it is a copolymer, the &agr;-olefin may include ethylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 4-methyl-1-pentene, 3-methyl-1-pentene, and the like. It is preferable in view of the production cost to use a propylene homopolymer or propylene-ethylene random copolymer having a propylene unit content of 90% by weight or more.
The melt flow rate (abbreviated as “MFR” hereinafter) of the crystalline polypropylene is preferably in the range of 0.1-50 g/10 minutes in view of the stability upon film-forming.
(ii) Propylene-&agr;-olefin Copolymer
The propylene-&agr;-olefin copolymer used for the present invention is a random copolymer of propylene and an &agr;-olefin other than propylene. The content of propylene unit is preferably in the range of 20-80% by weight, more preferably 20-75% by weight, particularly preferably 20-70% by weight based on the whole copolymer. When the content of propylene unit exceeds 80%, the desired dispersed state of the copolymer particles (referred to as “copolymer domains” hereinafter) in the matrix of crystallin
Akitaya Shinichi
Ishimoto Yuya
Mochizuki Yasuhiro
Nakashima Takanori
Nakata Shinji
Chisso Corporation
Kilpatrick Stockholm LLP
Nutter Nathan M.
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