Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Adhesive outermost layer
Reexamination Certificate
2002-09-03
2004-03-30
Chen, Vivian (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Adhesive outermost layer
C428S346000, C428S347000, C428S480000, C428S910000, C528S271000, C528S354000, C528S359000, C528S361000
Reexamination Certificate
active
06713175
ABSTRACT:
TECHNICAL FIELD
This invention relates to a biodegradable biaxially stretched film.
BACKGROUND ART
Most of conventional plastic products, especially plastic packaging materials are discarded soon after use, and their disposal problems are pointed out. Among general purpose packaging plastics, as representative ones, polyethylene, polypropylene, polyethylene terephthalate (“PET”), etc. can be cited. But these materials are high in heat buildup when burned and there is a possibility of damaging the incinerator during burning treatment. Further, polyvinyl chlorides, which are large in the amount of use even now, cannot be burned due to their self-extinguishing properties. Also, in many cases, plastic products including such materials which cannot be burned, are buried. But due to their chemical and biological stability, they scarcely decompose but remain, thus causing a problem that they shorten the life of burial sites. Therefore, plastic products that are low in heat buildup during burning, decompose in soil, and safe are desired, and many researches are being made.
As one example, there are polylactic acids. For polylactic acids, the heat buildup during burning is less than half that of polyethylene, and hydrolysis proceeds naturally in soil or water and then they are decomposed by microorganisms into unharmful materials. Now researches are being made for obtaining molded products, specifically film sheets and containers such as bottles using polylatic acids.
Polylactic acid is a polymer formed by condensation-polymerizing a lactic acid. Lactic acids have two kinds of optical isomers, i.e. L-lactic acid and D-lactic acid. Their crystallizability varies with the ratio between these two. For example, a random copolymer of which the L-lactic acid to D-lactic acid ratio is 80:20 to 20:80 has no crystallizability. In other words, it is a transparent, completely amorphous polymer which softens near its glass transition point of 60° C. On the other hand, for a homopolymer made up of only L-lactic acid or D-lactic acid, although its glass transition point is likewise 60° C., it becomes a semicrystalline polymer having a melting point of 180° C. or over. The semicrystalline polylactic acid turns into an amorphous material that excels in transparency, by rapidly cooling after melt extrusion.
It is known that it is possible to improve its strength and shock resistance of a polylactic acid by biaxially stretching it during forming into film. Further, it is known to manufacture a film which will not substantially shrink by suppressing the heat shrinkage of the film by heat-treating it after biaxial stretching. The heat shrinkage rate is determined by the heat treatment temperature and time of the film and the properties of the raw materials used. So the heat treatment temperature and time are suitably adjusted according to the properties of the film material.
After packaging an article in a film, clear fold lines are sometimes formed by slightly fusing the film by applying a hot plate so as not to easily rise. Specifically, this is done for packaging folded neatly in end faces of a cubic article such as video tapes and cassette tapes, packaging of cubic gums, and packaging of boxed tobacco. For them, stretched polypropylene film, cellophane, etc. are used. For them, K-coat film and K-coat cellophane having its surface coated with vinylidene chloride are used. Tacking is usually done by melting the vinylidene chloride layer by pressing a hot plate.
But such vinylidene chlorides are said to be one of today's environmental pollution sources, and they cause various problems. For example, they promote production of dioxin if burned at low temperature. Thus, for a polylatic acid biaxially stretched film too, it is not preferable to perform K-coating.
To polylactic acid biaxially stretched film, by adjusting the manufacturing method, it is possible to impart heat setting properties while suppressing heat shrinkage. This is because a polylatic acid is low in crystallizability compared with polypropylene or polyethylene terephthalate. Further, by setting it to a film having a suitable crystallizability, it is possible to solve the above problems.
An object of the present invention is to provide a biaxially stretched film which is decomposable in the natural environment, and by solving the above problems, allows tacking, has heat stickability, suppresses thickness unevenness, break, whitening and unevenness, and which has stretching stability.
DISCLOSURE OF THE INVENTION
In order to solve the above problems, the present invention provides a biodegradable biaxially stretched film comprising a polylatic acid-family polymer as the major component, and having a storage elastic modulus at 120° C. of 100-230 MPa as measured using test method concerning temperature dependency of the dynamic viscous elasticity under JIS K7198.
Preferable embodiments of this invention include a biodegradable biaxially stretched film having an area stretching ratio of 6.8 times or over, biaxially stretched at a longitudinal stretching temperature of 70-90° C. and a lateral stretching temperature of 70-80° C., and after biaxial stretching, heat-set at a temperature of 100° C. to melting point (Tm) in a gripped state, a biodegradable biaxially stretched film having an area stretching ratio of 6.8 times or over, simultaneously biaxially stretched at a stretching temperature of 70-80 ° C., and after biaxial stretching, heat-set at a temperature of 100° C. to melting point (Tm) in a gripped state, a biodegradable biaxially stretched film having a tensile strength of 1000-2000 kgf/cm
2
and a tensile elongation of 5-150% as measured at a tensile speed of 200 mm/min using a No. 2 test piece under JIS K 7129, and a biodegradable biaxially stretched film having the weight-average molecular weight of the polylactic acid-family polymer of 60000-700000.
BEST MODE FOR EMBODYING THE INVENTION
Hereinbelow, embodiments of this invention will be described.
The biodegradable biaxially stretched film according to this invention is a film comprising a polylactic acid-family polymer as its major component and having a storage elastic modulus E′ at 120° C. of 100-230 MPa.
The polylactic acid-family polymer is a homopolymer of D-lactic acid or L-lactic acid, or a copolymer of D-lactic acid or L-lactic acid. It may contain other hydroxy-carboxylic acid units as a small amount of copolymeric components and may also contain a small amount of chain extender residual groups.
As the polymerizing method, a known method such as condensation polymerization or ring opening polymerization may be used. For example, in the condensation polymerization, it is possible to obtain a polylactic acid having any desired composition by directly subjecting L-lactic acid, D-lactic acid or mixture thereof to dehydration condensation polymerization.
Also, in the ring-open polymerization method (lactide method), by polymerizing a lactide, which is a cyclic dimer of a lactic acid, it is possible to obtain a polylactic acid using a selected catalyst and a polymerization adjusting agent or the like as necessary.
The weight-average molecular weight of the polylactic acid-family polymer is preferably 60000-700000, more preferably 80000-400000, and most preferably 100000-300000. If the molecular weight is less than 60000, practical physical properties such as mechanical properties and heat resistance will scarcely reveal. If higher than 700000, the melt viscosity will be too high, so that molding workability is poor.
As other hydroxy-carboxylic acids as the small-amount copolymer components, it is possible to cite optical isomers of lactic acids (D-lactic acid for L-lactic acid and L-lactic acid for D-lactic acid), 2-functional aliphatic hydroxy carboxylic acids such as glycolic acid, 3-hydroxy butyric acid, 4-hydroxy butyric acid, 2-hydroxy-n-butyric acid, 2-hydroxy-3,3-dimethyl butyric acid, 2-hydroxy-3-methyl butyric acid, 2-methyl lactic acid, and 2-hydroxy caproic acid: lactones such as caprolactone, butyrolactone and valerolactone.
Further, if necessary, as a sma
Takagi Jun
Terada Shigenori
Chen Vivian
Mitsubishi Plastics Inc.
Wenderoth , Lind & Ponack, L.L.P.
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