Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2000-03-24
2001-11-27
Chen, Vivian (Department: 1773)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S271000, C528S359000, C528S361000, C428S480000, C428S910000
Reexamination Certificate
active
06323308
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an aliphatic polyester film. More particularly, the present invention relates to an aliphatic polyester film which is useful as a packaging film and a tape for fresh food, processed food, pharmaceutical products, medical equipment, electronic parts and the like, and which has superior properties in the above use, such as hand tearability, twist retention, processability and film forming property.
BACKGROUND OF THE INVENTION
Films made from aliphatic polyester, inclusive of polylactic acid, have been developed to acquire natural decomposability upon disposal in a natural environment. For example, a polylactic acid film is naturally hydrolyzed in soil and decomposed into harmless substances with the aid of microorganisms.
On the contrary, the preferential aim of acquiring natural decomposability has prevented sufficient expression of the superior properties that aliphatic polyester inherently has. To be specific, advanced orientation and crystallization of the film inhibits hydrolysis and decomposition by microorganisms. To suppress orientation and crystallization of the film, therefore, strength and thermal dimensional stability have been sacrificed to a certain degree.
In the meantime, attempts have been made to improve degree of orientation and crystallinity of the film, thereby to increase strength and thermal dimensional stability. For example, JP-A-7-207041 discloses polylactic acid made from a polylactic acid polymer, which has a plane orientation &Dgr;P of not less than 3.0×10
−3
, and which shows a difference value (&Dgr;Hm−&Dgr;Hc) between the amount of crystallization melting heat &Dgr;Hm upon elevation of the temperature of the film, and the amount of crystallization heat &Dgr;Hc generated by crystallization during temperature elevation of not less than 20 J/g.
This polylactic acid film does not aim at improving hand tearability, twist retention and post-processing stability, such as printability, required of a packaging film. Moreover, the polylactic acid polymer film exemplified therein has insufficient properties, particularly post-processing stability, such as dimensional resistance at 120° C. necessary for printing, laminating and the like, such that this film is insufficient to achieve the above-mentioned goal. In the prior art technique relating to other aliphatic polyester films, no attention has been paid to the reduction of viscosity or properties of polylactic acid polymer even when attempting to improve hand tearability, twist retention and post-processing stability, such as printability, of a packaging film.
Conventionally, polyethylene terephthalate oriented films have been used for packaging bags, adhesive tapes and the like because of the superior properties they have in toughness, heat resistance, water resistance, transparency and the like. In general terms, a packaging film for fresh food, processed food, pharmaceutical products, medical equipment, electronic parts and the like should permit easy tearing with hand, i.e. “hand tearability”, and also should be able to stay twisted upon twisting, i.e. “twist retention”. Polyethylene terephthalate oriented films have the above-mentioned superior properties but they are poor in hand tearability. As a result, the film causes difficulty in tear opening a packaging bag or cutting an adhesive tape. Further, the film is defective in that it cannot be used for twist packaging due to its poor twist retention.
A film having superior hand tearability is cellophane. It is superior not only in hand tearability but also in properties such as transparency, twist retention and the like, and is very useful for various packaging materials and adhesive tapes. However, cellophane shows moisture-adsorption characteristic, which prevents provision of constant quality of the film due to seasonal variations in these properties.
As a film that satisfies hand tearability and twist retention, for example, a uniaxially oriented polyester film (JP-B-55-8551) and a polyethylene terephthalate film (JP-B-56-50692) obtained by copolymerization of diethylene glycol component and the like, have been proposed.
The above-mentioned uniaxially oriented polyester film permits linear splitting in the orientation direction but otherwise in other directions. The polyethylene terephthalate film obtained by copolymerization of diethylene glycol component and the like does not show properties which polyethylene terephthalate inherently has. This film is also unsatisfactory in terms of hand tearability and twist retention.
SUMMARY OF THE INVENTION
The present invention solves the above-mentioned problems and provides an aliphatic polyester film, which is useful as a packaging film and a tape, degradable when disposed in a natural environment, superior in hand tearability and twist retention, and superior in processability and film forming property as evidenced by the following aspects:
(a) the film hardly suffers from printing misalignment, wrinkling and the like during film processing such as printing, laminating and the like when producing a packaging film, a tape and the like, and
(b) the film hardly suffers from snaking during running.
According to the present invention, the above object has been achieved by using an aliphatic polyester having a specific repeating unit and a reduced viscosity (&eegr;sp/C) in a specific range. The use of such polyester results in improved hand tearability, twist retention, processability and film forming property. By setting the thermal shrinkage and thickness unevenness to a given range, a packaging film more superior in the above-mentioned properties can be obtained.
Accordingly, the present invention provides an aliphatic polyester film made from an aliphatic polyester having a primary repeating unit of the following formula
—O—CHR—CO—
wherein R is hydrogen or alkyl having 1 to 3 carbon atoms, and a reduced viscosity (&eegr;sp/C) of not less than 0.50 dl/g and not more than 2.50 dl/g, wherein the film has a thermal shrinkage in the longitudinal direction at 120° C. of not more than 5.0%.
In one embodiment of the present invention, the film has a thickness unevenness in the longitudinal direction of not more than 10.0%.
In another embodiment of the present invention, the film has a thickness unevenness in the width direction of not more than 10.0%.
In yet another embodiment of the present invention, the film has a thermal shrinkage in the width direction at 120° C. of not more than 5.0%.
In one embodiment of the present invention, the film has a refractive index (Nz) in the thickness direction of not less than 1.4400 and not more than 1.4550.
In one embodiment of the present invention, the film shows a difference value (Nx−Ny) between the refractive index (Nx) in the longitudinal direction and the refractive index (Ny) in the width direction of not less than −0.0200 and not more than 0.0000.
In one embodiment of the present invention, the aliphatic polyester is a polylactic acid.
In one embodiment of the present invention, the film is a biaxially oriented film.
DETAILED DESCRIPTION OF THE INVENTION
As used in the present specification, by “primary” and “main” are meant that other copolymers and other resin components may be mixed to the extent that the effect of the present invention is not impaired.
The aliphatic polyester in the present invention has a primary repeating unit of the formula
—O—CHR—CO—
wherein R is hydrogen or alkyl having 1 to 3 carbon atoms. The alkyl groups having not less than 4 carbon atoms have been excluded because heat stability during melt extrusion, orientation and crystallizability of the film are degraded. In the above-mentioned formula, the alkyl having 1 to 3 carbon atoms may be, for example, methyl, ethyl, n-propyl, isopropyl and the like.
The above-mentioned aliphatic polyester preferably contains the above-mentioned repeating unit in a proportion of not less than 90 mol %, more preferably not less than 95 mol %. Other units that may be contained besides the above-mentioned repeati
Kobayashi Hisato
Kumano Katsufumi
Mori Keiji
Oda Naonobu
Okudaira Tadashi
Chen Vivian
Morrison & Foerster / LLP
Toyo Boseki Kabushiki Kaisha
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