Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1999-01-22
2001-09-25
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
Reexamination Certificate
active
06294632
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a biaxially oriented polypropylene film. More particularly, the present invention relates to a biaxially oriented polypropylene film which, when subjected to heat sealing/cutting, has a high sealing strength at the heat-sealed part and reduced fluctuations in sealing strength, and which has excellent stiffness suitable for bag-forming and filling and sealing, and has excellent transparency and gloss.
BACKGROUND ART
Biaxially oriented polypropylene films (hereinafter referred to as OPP films) are typical of polyolefin films. Because of their excellent transparency and gloss, OPP films are used as packaging materials in a wide range of fields. OPP films further have high rigidity, and hence have excellent printability and excellent suitability for bag-forming and filling operations. However, when the conventional OPP films are used in bag-forming by heat sealing/cutting, the heat-sealed parts have a low sealing strength and large fluctuations in sealing strength. Conventional OPP films therefore are disadvantageous in, for example, that bag breakage is apt to occur during filling and sealing, and also apt to occur due to vibration during the transportation or handling of the packages.
Under these circumstances, a multilayered OPP film comprising an OPP film having a heat-sealable layer has been proposed (see JP-B-7-17043). (The term “JP-B” as used herein means an “examined Japanese patent publication”.) However, the proposed film having a low-melting heat-sealable layer also has large fluctuations in heat-sealing strength and are apt to develop pinholes in parts having a low heat-sealing strength. Thus, the proposed film has a problem that it is unsuitable for use in food packaging for which airtightness is required.
In addition, an OPP film for heat sealing, employing an ethylene/propylene copolymer or an ethylene/propylene/butene copolymer has been proposed (see JP-A-7-241906). (The term “JP-A” as used herein means an “unexamined published Japanese patent application”.) However, the OPP films employing either of these copolymers are both inferior in stiffness as compared to conventional OPP films and hence have a problem that the suitability thereof for bag-forming and filling operations is reduced.
DISCLOSURE OF THE INVENTION
The present invention has been made taking the above described problems in conventional techniques into consideration and, therefore, an object thereof is to provide a biaxially oriented polypropylene film which, when subjected to heat sealing/cutting, has a high sealing strength at the heat-sealed part and reduced fluctuations in sealing strength, and which has excellent stiffness suitable for bag-forming and filling and sealing, and has excellent transparency and gloss.
The present invention relates to the following biaxially oriented polypropylene films (1) to (3).
(1) A biaxially oriented polypropylene film which has a melt flow rate of from 1 to 12 g/10 min and in which the melt viscosity thereof as measured with a rotational rheometer at a shear rate of 0 rad/sec (&eegr;0), the melt viscosity thereof as measured with the rheometer at a shear rate of 10 rad/sec (&eegr;10), and the MFR thereof satisfy the following relationship (1):
log[(&eegr;0-&eegr;10)/&eegr;10]+0.7logMFR≦0.7 (1).
(2) The biaxially oriented polypropylene film as described in (1) above, which has a half-crystallization time at 120° C. of from 80 to 200 seconds as measured with a crystallization rate analyzer based on depolarization intensity.
(3) The biaxially oriented polypropylene film as described in (1) above, which has a density of from 0.890 to 0.920 g/cm
3
.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is explained in detail below.
The biaxially oriented polypropylene film of the present invention has a melt flow rate (hereinafter often abbreviated as an “MFR”) (measured at 230° C., 21.18 N) of from 1 to 12 g/10 min, and further the melt viscosity of the film as measured with a rotational rheometer at a shear rate of 0 rad/sec (&eegr;0), the melt viscosity thereof as measured with the rheometer at a shear rate of 10 rad/sec (&eegr;10), and the MFR thereof satisfy the following relationship (1):
log[(&eegr;0-&eegr;10)/&eegr;10]+0.7logMFR≦0.7 (1).
OPP films having an MFR far lower than 1 g/10 min are undesirable in that they have a reduced heat-sealing strength. OPP films having an MFR far higher than 12 are undesirable in that heat sealing thereof results in sealed parts having a poor appearance (flashes are generated in the heat sealing).
The value of MFR is determined by finely cutting the biaxially oriented polypropylene film and analyzing the cut film in accordance with JIS K 7210 under conditions 14 (230° C., 21.1 N). The unit thereof is expressed by g/10 min.
Relationship (1) given above means that when OPP films having the same MFR are compared, the film which has an &eegr;0/&eegr;10 ratio closer to 1, in other words, which exhibits less change in melt viscosity with respect to shear rate change, has better heat sealability. If the value of
log[(&eegr;0-&eegr;10)/&eegr;10]+0.7logMFR
is far larger than 0.7, fluctuations in heat-sealing strength become increased and this causes pinhole generation.
&eegr;10 indicates the value of melt viscosity as measured at a shear rate of 10 rad/sec with rotational rheometer RDS-7700 Spectrometer (manufactured by Rheometrics Inc.). &eegr;0 is the value determined by using the Ellis's nonlinear equation described in Christoher W. Macosko,
Rheology
from melt viscosity values obtained in the shear rate range of from 0.1 to 500 rad/sec with RDS-7700.
The biaxially oriented polyolefin film of the present invention is obtained from a polypropylene while controlling the MFR, &eegr;0, and &eegr;10 thereof. Examples of the methods for controlling these properties include the following.
(1) A method in which when a polypropylene to be used is produced through polymerization, the MFR and molecular weight distribution of the polypropylene are controlled to thereby obtain from the polypropylene an OPP film satisfying the requirements concerning MFR, &eegr;0, and &eegr;10.
(2) A method in which when the polypropylene to be used is melt-kneaded together with additives such as an antioxidant and then pelletized, the MFR of the polymer is controlled by regulating the conditions for melt kneading, e.g., the screw design of the extruder and the temperature for extrusion, or by adding an organic peroxide or the like, whereby an OPP film satisfying the requirements concerning MFR, &eegr;0, and &eegr;10 is obtained. When the ratio of the MFR as measured after the melt kneading to the MFR of the polypropylene used becomes high, the &eegr;0/&eegr;10 ratio becomes close to 1.
(3) A method in which MFR is controlled during OPP film production by regulating the screw design of the extruder, the temperature for extrusion, etc. to thereby obtain an OPP film satisfying the requirements concerning MFR, &eegr;0, and &eegr;10. Specifically, an OPP film obtained by using an extruder with employing a high-shear screw at a high extrusion temperature has a high MFR and an &eegr;0/&eegr;10 ratio close to 1.
The biaxially oriented polyolefin film of the present invention preferably has a half-crystallization time at 120° C. of from 80 to 200 seconds, because such a film is excellent in heat-sealing strength and in the appearance of heat-sealed parts.
The half-crystallization time of a film is determined as follows. A film is analyzed with crystallization rate analyzer MK-801 based on depolarization intensity (manufactured by Kotaki Seisakusho K.K., Japan) to measure the change of the quantity of transmitted light at 120° C. (isothermally) until the termination of crystallization. The time at which the change of the quantity of transmitted light becomes the half of the change up to the termination of crystallization is taken as the half-crystallization time. The unit thereof is second.
Examples of methods for controlling the half
Ando Yuji
Saito Noriaki
Shiraishi Yasuhiro
Cheung William K
Chisso Corporation
Leydig , Voit & Mayer, Ltd.
Wu David W.
LandOfFree
Biaxially oriented polypropylene film does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Biaxially oriented polypropylene film, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Biaxially oriented polypropylene film will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2442904