U.S. patent application number 10/515387 was filed with the patent office on 2005-10-06 for container of biodegradable heat-resistant hard resin molding.
Invention is credited to Kawakami, Yukichika, Sato, Tomoaki, Suzuki, Takehisa, Tobita, Hisanori, Yamane, Kazuyuki.
Application Number | 20050221032 10/515387 |
Document ID | / |
Family ID | 29561401 |
Filed Date | 2005-10-06 |
United States Patent
Application |
20050221032 |
Kind Code |
A1 |
Yamane, Kazuyuki ; et
al. |
October 6, 2005 |
Container of biodegradable heat-resistant hard resin molding
Abstract
A shaped container of biodegradable, heat-resistant and rigid
resin having a laminate structure including a glycolic acid polymer
layer and another biodegradable resin layer, is formed by shaping
under heating so that another biodegradable resin forms an outer
and/or inner layer. The container is excellent in biodegradability,
stiffness and heat resistance as well as excellent see-through of
contents, and is suitable as a temporary preservation container for
food, etc.
Inventors: |
Yamane, Kazuyuki;
(Fukushima-Ken, JP) ; Kawakami, Yukichika;
(Fukushima-Ken, JP) ; Sato, Tomoaki; (Ibaraki-Ken,
JP) ; Tobita, Hisanori; (Ibaraki-Ken, JP) ;
Suzuki, Takehisa; (Ibaraki-Ken, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
29561401 |
Appl. No.: |
10/515387 |
Filed: |
November 23, 2004 |
PCT Filed: |
May 28, 2003 |
PCT NO: |
PCT/JP03/06704 |
Current U.S.
Class: |
428/34.9 ;
428/35.7 |
Current CPC
Class: |
B32B 2307/7163 20130101;
B32B 2439/70 20130101; Y02W 90/13 20150501; B65D 65/466 20130101;
B32B 27/36 20130101; Y10T 428/1328 20150115; Y10T 428/1352
20150115; Y02A 40/90 20180101; B32B 27/08 20130101; Y02A 40/961
20180101; Y02W 90/10 20150501 |
Class at
Publication: |
428/034.9 ;
428/035.7 |
International
Class: |
B65D 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 29, 2002 |
JP |
2002-155111 |
Claims
1. A shaped container of biodegradable, heat-resistant and rigid
resin, having a laminate structure including a glycolic acid
polymer layer and another biodegradable resin layer, which has been
shaped under heating so that said another biodegradable resin layer
forms an outer and/or inner layer.
2. A shaped container according to claim 1, wherein said another
biodegradable resin layer is disposed at least as the outer layer
outside the glycolic acid polymer layer.
3. A shaped container according to claim 2, wherein said outer
layer of another biodegradable resin comprises a lactic acid
polymer.
4. A shaped container according to claim 2, further including said
another biodegradable resin layer also as an inner layer disposed
inside the glycolic acid polymer layer.
5. A shaped container according to claim 4, wherein said inner
layer of another biodegradable resin comprises a lactic acid
polymer.
6. A shaped container according to 5 claim 1, further including an
adhesive layer between the glycolic acid polymer layer and the
outer and/or inner biodegradable resin layer.
7. A shaped container according to claim 1, wherein the glycolic
acid polymer layer and another biodegradable resin layer have a
total thickness of 100-5000 .mu.m, of which the thickness of the
glycolic acid polymer layer occupies 2-98%.
8. A shaped container according to claim 1, which has been treated
for heat-setting after the shaping under heating.
9. A shaped container according to claim 1, exhibiting a flexural
modulus Ef of at least 100 kg/mm.sup.2 as measured in a state where
a load is applied from the outer resin layer.
10. A shaped container according to claim 9, exhibiting a flexural
modulus Ef (as measured in a state where a load is applied to the
outer resin layer) and a container thickness t giving a product
Ef.times.t of at least 1 kg/mm.
11. A package, comprising a shaped container of biodegradable,
heat-resistant and rigid resin according to claim 1 and a
biodegradable film.
12. A package according to claim 11, wherein the biodegradable film
is heat-shrinkable.
13. A package according to claim 11 wherein the biodegradable film
includes at least one glycolic acid polymer layer.
14. A package according to claim 12, wherein the biodegradable film
has a gas-barrier property.
Description
TECHNICAL FIELD
[0001] The present invention relates to a shaped container of a
biodegradable, stiff, heat-resistant and rigid resin, suitable as a
temporary preservation container for contents, such as food.
BACKGROUND ART
[0002] Polylactic acid and succinic acid-based aliphatic
polyesters, etc., have drawn interest as biodegradable resins
harmonizable with environment. If they can be used to form a
temporary preservation container for food, the container is
expected to be disposed without causing garbage by biodegradation
thereof after the use, and also expected be composted together with
contents such as food after expiry of the relishable period
thereof, thus eliminating an operation, such as separation of
materials. However, as these resins are inferior in heat
resistance, a container made thereof can be softened or deformed
when heated for re-warming the food and can sometimes cause
overflow of the contents, thus being liable to cause soiling of the
surroundings or scald of the user. Further, a container made of
these resins is liable to cause oxidative degradation of the
contents such as food, due to gas penetration therethrough.
[0003] On the other hand, polyglycolic acid is known as a
crystalline biodegradable resin having a melting point of
180.degree. C. or higher and a high gas-barrier property, but a
relatively thick sheet thereof is liable to cause whitening when
shaped under heating, so that the see-through of the resultant
shaped container is impaired. Accordingly, though a glycolic acid
polymer as represented by polyglycolic acid has been proposed to
form a gas-barrier composite film by lamination with another
thermoplastic resin, such as polylactic acid and succinic
acid-based aliphatic polyester (Japanese Laid-Open Application
(JP-A) 10-80990), but the use thereof as a material for a shaped
container of a rigid resin has not been substantially
practiced.
DISCLOSURE OF INVENTION
[0004] In view of the above-mentioned circumstances, a principal
object of the present invention is to provide a shaped container of
rigid resin which is excellent in biodegradability, stiffness and
heat resistance and is suitable as a temporary preservation
container of contents, such as food. More specifically, the present
invention aims at realizing such a shaped container of rigid resin
by a laminate composite shaped container including a glycolic acid
polymer as an essential component resin layer.
[0005] According to the inventors' study, it has been found that a
glycolic acid polymer layer is a resin very suited for providing a
shaped container of composite rigid resin as mentioned above
excellent in biodegradability, stiffness and heat resistance, if it
is disposed together with another biodegradable resin layer in an
appropriate positional relationship.
[0006] More specifically, according to the present invention, there
is provided a shaped container of biodegradable, heat-resistant and
rigid resin, having a laminate structure including a glycolic acid
polymer layer and another biodegradable resin layer, which has been
shaped under heating so that said another biodegradable resin layer
forms an outer and/or inner layer.
[0007] The reason why the glycolic acid polymer layer is extremely
suited for providing a composite shaped container having the
above-mentioned properties is because it has the following
advantageous properties compared with other biodegradable resins,
such as lactic acid polymers and succinic acid-based aliphatic
polyesters. More specifically, (a) glycolic acid polymer is
crystalline and particularly its crystallization speed is
remarkably rapid. Moreover, accompanying the crystallization
thereof, it provides a resin layer exhibiting a remarkably larger
stiffness compared with other biodegradable resins. This property
is of course remarkably preferable for providing a rigid resin
container. (b) However, because of its crystallinity, if a single
layer of glycolic acid polymer is shaped under heating for
providing a shaped container by, e.g., deep drawing or blow
molding, the shaped container is defectively whitened as described
above. As a result of the inventors' further study, however, it has
been found that the whitening of the single layer of glycolic acid
polymer is principally caused as a result of roughening of the
surface at which a tension stress is concentrated, and the
whitening can be remarkably alleviated if it is laminated with
another biodegradable resin layer thereover and then subjected to
shaping under heating. The whitening due to roughening during
shaping of the glycolic acid polymer layer surface is caused
particularly remarkably on its outer surface subjected to a larger
degree of deformation, and accordingly a larger degree of
whitening-prevention effect can be attained if another
biodegradable resin layer is disposed on an outer surface than an
inner surface of the glycolic acid polymer layer. However, a
further better whitening-prevention effect can be attained when
such another biodegradable resin layer is disposed on both the
outer and inner surfaces of the glycolic acid polymer layer. (c)
The glycolic acid polymer layer forming the shaped container in
lamination with another biodegradable resin layer is crystallized
to increase its stiffness during the shaping under heating and
optionally performed heat-setting treatment, thereby providing a
rigid resin shaped container suitable for temporary preservative
storage of food. (In contrast thereto, a lactic acid polymer
exhibits a very slow crystallization speed, so that the
crystallization thereof hardly proceeds during the heat-setting
treatment.) (d) Further, glycolic acid polymer is caused to have a
remarkably increased heat resistance as high as not to cause a
substantial deformability in lamination with another bio-degradable
resin layer during 1 minute or longer of heating in a microwave
heater. (Incidentally, a succinic acid-based polyester has a very
low melting point (of ca. 100.degree. C.) and also a low
crystallinity, thus being poor in both heat resistance and rigidity
(or stiffness). (e) Glycolic acid polymer has a much higher
gas-barrier property compared with not only other biodegradable
resins as a matter of course but also EVOH (ethylene-vinyl alcohol
copolymer) which is a conventionally used representative
gas-barrier resin, as high as ca. 3 times or higher (i.e., ca. 1/3
or lower in terms of an oxygen transmission co-efficient) as that
of EVOH, so that it can provide, e.g., a bottle shaped therefrom,
exhibiting a remarkably enhanced effect of preserving the contents
in the bottle. (f) Glycolic acid polymer has a biodegradability
comparable to or even higher than those of other biodegradable
resins such as lactic acid polymers and succinic acid-based
aliphatic polyester resins. Accordingly, a shaped container having
a laminate structure consisting essentially of these biodegradable
resin layers can be disposed without causing garbage, by
biodegradation thereof after the use as a temporary preservation
container for food, etc., and can also be composed together with
contents such as food after expiry of the relishable period
thereof.
[0008] The shaped container of biodegradable, heat-resistant and
rigid resin according to the present invention has been completed
based on the above-mentioned findings.
EMBODIMENTS OF THE INVENTION
[0009] (Glycolic Acid Polymer)
[0010] Glycolic acid polymer is a biodegradable (hydrolysable) and
crystalline polyester having a recurring unit represented by a
formula (1) below:
--(OCH.sub.2CO)-- (1)
[0011] It is preferred to use glycolic acid homopolymer (PGA)
consisting only of the above recurring unit, but another recurring
unit can be contained provided that a structure having a main chain
which can be cut by biodegradation (or hydrolysis) is
preferred.
[0012] Preferable structures may include ester structures including
carboxylic acid esters and carbonic acid esters, and amide
structure. Particularly, an aliphatic ester structure is preferred
in view of biodegradability. Examples thereof may include the
following:
--(OCHCH.sub.3CO)-- (2)
--(OCH.sub.2CH.sub.2CH.sub.2OCO)-- (3)
--(OCH.sub.2CH.sub.2CH.sub.2CH.sub.2CO)-- (4)
[0013] The proportion of such another recurring unit structure is
below 50 wt. %, preferably below 30 wt. %, further preferably below
15 wt. % in order to retain the effect of increasing stiffness and
heat-resistance due to crystallization.
[0014] Further, it is also possible to incorporate another
thermoplastic resin in the glycolic acid polymer layer for the
purpose of controlling the crystallizability thereof in a
relatively small amount (e.g., up to 20 wt. %) within an extent of
not adversely affecting the stiffness and heat-resistance.
[0015] As for such another thermoplastic resin, it is not
impossible to use a general-purpose resin, such as polyethylene,
polypropylene, polyvinyl chloride or polystyrene, but in order to
increase the content of biodegradable resin, it is preferred to use
another biodegradable resin, such as a lactic acid polymer,
succinic acid-based aliphatic polyester which is a poly-condensate
of succinic acid and ethylene diol or butane diol,
polycaprolactone, .omega.-hydroxyacetic acid polycondensate and
Biomax (registered trade mark, available from Du Pont), cellulose
or starch.
[0016] (Another Biodegradable Resin)
[0017] As for a constituent resin of said another biodegradable
resin for forming the rigid resin shaped container of the present
invention together with the glycolic acid polymer layer it is
possible to use biodegradable resins, such as a lactic acid
polymer, succinic acid-based aliphatic polyester which is a
poly-condensate of succinic acid and ethylene diol or butane diol,
polycaprolactone, .omega.-hydroxyacetic acid polyconden sate and
Biomax (registered trade mark, available from Du Pont), cellulose
or starch, raised above as examples of another biodegradable resin
which can be incorporated in the glycolic acid polymer layer. Among
these, it is preferred to laminate a layer of lactic acid polymer
which has a relatively good heat-resistance. As an example of such
another biodegradable resin, it is also possible to use a regrind
(i.e., recovered and re-pulverized product) of a rigid resin shaped
container of the present invention. Such a regrind may comprise
biodegradable resins, such as glycolic acid polymer, lactic acid
polymer and succinic acid-based polyester, can further contain an
adhesive resin in some cases, and can be used within an extent of
not remarkably lowering the transparency of the shaped container of
the present invention.
[0018] In the shaped container of the present invention, such
another biodegradable resin is disposed as at least an outer layer
or an inner layer, preferably as an outer layer, with respect to
the glycolic acid polymer layer, but may more preferably be
disposed as both an outer and an inner layer so as to provide a
structure wherein the glycolic acid polymer layer is sandwiched
between a pair of other biodegradable resin layers which can be not
identical to each other but most preferably each comprise a lactic
acid polymer layer. The resultant shaped container may have a layer
structure including at least two layers. Examples of such a layer
structure may include: another biodegradable resin/glycolic acid
polymer/another biodegradable resin (possibly containing a
regrind), and another biodegradable resin/regrind/glycolic acid
polymer/another biodegradable resin (possibly containing a
regrind). The above-mentioned another biodegradable resin layer can
have a two-layer structure of different resins, and in this case,
the entire layer structure may comprise, for example, succinic
acid-based polyester/lactic acid polymer/glycolic acid
polymer/lactic acid polymer, which structure may be provided with
easy sealability because the succinic acid-based polyester has a
relatively low melting point. In any case, an adhesive layer can be
inserted, as desired, between layers.
[0019] (Thickness)
[0020] In order to be a rigid resin shaped container having a good
stiffness after an appropriate degree of heat treatment, the shaped
container of the present invention is required to have a thickness
(a total thickness of the glycolic acid polymer layer and another
biodegradable resin layer) of averagely at least 100 .mu.m,
preferably at least 150 mm, particularly preferably 200 .mu.m or
larger. Below 100 .mu.m, when the container is caused to receive a
relatively heavy food, such as fried food, daily dishes or cooked
rice, the container is liable to be warped and the handling thereof
is liable to be awkward. The upper limit may be determined
principally in view of economical factors and generally 5000 .mu.m
or smaller.
[0021] Further, in order to ensure a necessary stiffness while
retaining the whitening prevention effect owing to the lamination,
it is preferred the glycolic acid polymer layer has a thickness
which is 2-98%, more preferably 5-80% of the total thickness of the
glycolic acid polymer layer and another bio-degradable resin
layer.
[0022] (Adhesive Layer)
[0023] The shaped container of the present invention can be
composed of only the above-mentioned glycolic acid polymer layer
and another biodegradable resin layer, and this is preferred in
order to increase the biodegradability of the entire container. In
the case of once forming a multilayer sheet and then shaping the
sheet by secondary processing, such as (deep) drawing or blow
forming, etc., however, it is possible to insert an adhesive resin
layer in order to enhance the inter-layer bonding strength. As the
adhesive resin, epoxy-modified polyolefin, crosslinked
ethylene-vinyl acetate copolymer, etc., may preferably be used. The
biodegradability of these resins is inferior to the above-mentioned
various biodegradable resins, but the load thereof to the
environment can be alleviated due to a small amount thereof because
the adhesive layer is used in a small thickness of, e.g., ca.
0.5-30 .mu.m. If an adhesive resin having a better biodegradability
is developed, such an adhesive resin may suitably be used in the
present invention, of course.
[0024] (Shaping Under Heating)
[0025] The shaped container of the present invention can be
directly formed by a melt resin forming method, such as multilayer
injection molding, by blow molding (stretch blow molding) of a
laminate preform of glycolic acid polymer layer and another
biodegradable resin layer formed by such a melt-resin forming
method, by direct blow molding, inflation, melt-vacuum forming, or
by vacuum forming or deep drawing of a once-formed laminate sheet,
as a suitably adoptable technique. According to the vacuum forming,
the sheet may be pre-heated for 0.5 sec. to 3 min., preferably 1
sec. to 2 min., at 60-120.degree. C., and shaping the sheet so as
to fit to a mold by placing the mold under vacuum. The shaping by
the melt vacuum forming may be effected by heating at
160-240.degree. C., preferably 170-230.degree. C.
[0026] (Heat Treatment)
[0027] Through the above-mentioned shaping under heating, the
shaped container is provided with increased stiffness and
heat-resistance, principally owing to the crystallization of the
glycolic acid polymer layer included therein, but can be subjected,
as desired, to an additional heat-treatment (heat-setting) for
causing further crystallization to increase the stiffness and heat
resistance. The heat-treatment is performed at a temperature equal
to or higher than a heat-resistant temperature usually required of
the shaped container, preferably 100-210.degree. C., more
preferably 150-200.degree. C.
[0028] The heat-treatment time is not particularly restricted but
may ordinarily be 1 sec. to 60 min., preferably 2 sec. to 10 min.,
particularly preferably 5 sec. to 5 min. Heat-treatment for less
than 1 sec. may be insufficient in some cases, and a period longer
than 60 min. does not provide a substantially different
heat-treatment effect but merely results in a longer processing
time.
[0029] (Stiffness, Heat Resistance)
[0030] Through the above-mentioned shaping under heating and
optional heat-treatment, the shaped container of the present
invention is provided with necessary level of stiffness and
heat-resistance.
[0031] A desirable level of stiffness of the shaped container may
be represented by a flexural modulus Ef of at least 100
kg/mm.sup.2, particularly at least 150 kg/mm.sup.2 as measured in a
state where a load is applied from the outer resin layer, and also
a factor Ef.times.t of at least 1 kg/mm.sup.2, particularly at
least 2 kg/mm, taking the contribution of the thickness t [mm] into
consideration. These values can be also measured, e.g., in the case
of sheet forming (shaping), by subjecting a sheet before the
shaping to a quantity of heat provided to the sheet during the
actual shaping under heating and heat-treatment, then subjecting
the heated sheet to the flexure test and applying a correction to
the measured values corresponding to a thickness reduction after
the shaping.
[0032] A desired level of heat-resistance of a shaped container of
the present invention may be represented by no visible deformation
of the container after placing cooked and cooled rice of ca. 180
cm.sup.3 in terms of a dry state volume before the cooking and
subjecting the rice in the container to 1 min. of microwave heating
at a power of 500 W.
[0033] (Whitening)
[0034] The shaped container of the present invention is required to
exhibit such a level of whitening resistance as to allow
seeing-through of the contents after the shaping under heating.
More specifically the shaped container of the present invention is
required to exhibit a haze (measured with respect to a cut sheet
piece cut out from a side wall of the shaped container according to
JIS K6714) of at most 50%, preferably 20. % or below, more
preferably 10% or below. If the haze is above 50%, the shaped
container is like a frosted glass sheet so that the contents are
difficult to judge by seeing-through. In contrast thereto, a haze
of 20% or below represents a state of frosted glass sheet not
providing a difficulty for determination of the contents, and a
haze of 10% or below represents a good see-through of the
contents.
[0035] (Use)
[0036] The thus-obtained rigid resin shaped container of the
present invention is extremely suitably used as a temporary
preservation container for food which should desirably have
heat-resistance, bio-degradability desirable for disposal,
stiffness desirable for handling of the container and see-through
of the contents, and is also suitably used as a container for
medical appliances for which similar properties are desirable,
inclusive of heat-resistance for heat-sterilization. Further, in
case where the container is shaped into a bottle, the bottle is
also suitably used as a container for contents, such as a beverage,
disliking degradation with oxygen. Further, a regrind of the rigid
resin shaped container of the present invention may be utilized,
because of its stiffness, for providing chopsticks or tooth picks
(though these can be made of young wood lumbered for decreasing the
wood population), disposable forks, small blown containers for
seasonings, small pouches (which be provided with an easy
sealability if laminated with a succinic acid-based polyester),
"baran" (i.e., a green sheet provided with a pattern of bamboo
leaf), etc., attached to a container for box lunch frequently
available in convenience stores, and it becomes possible to compose
an entire box lunch set of bio-degradable resins. In this instance,
these adjuncts can be poor in transparency.
[0037] Incidentally, the rigid resin shaped container of the
present invention is formed in a shape suitable for accommodating
contents, whereas a flat sheet or film having an identical laminate
structure can be used as a lid member to be combined with a
container of the present invention formed as a bowl or
parallelepiped container to form a container accommodating food,
etc., capable of microwave heating, by principally utilizing
excellent properties, such as gas-barrier property, heat resistance
and biodegradability, of the flat sheet or film.
[0038] However, such an open bowl or parallel-piped-shaped
container of the present invention can also be used in such a
manner as to form a temporarily packaged product together with an
ordinary food wrapping film, etc., adapted to microwave heating.
Further, such a container may also be used as a deep-drawn
packaging material for storing a stacked sliced ham utilizing its
property of heat-resistance, pinhole-resistance or label adhesion,
etc., as desired properties. If such a container is required of
sealability with a lid material, it is possible to dispose a layer
of succinic acid-based polyester outside or inside thereof.
[0039] The shaped container of the present invention can be
combined with a biodegradable film provided separately to provide
an entirely biodegradable package. Examples of specific structures
thereof may include the following:
[0040] 1) A shaped container of the present invention together with
contents is covered with a bio-degradable film, and the edges of
the film are superposed (wrapped) or further sealing the
super-posed edges. The sealing may be performed with opposite edges
of one inner surface (palm-to-palm sealing) or edges of inner and
outer surfaces (envelope sealing or back seaming). (More details of
such packaging embodiments are shown in, e.g., JP-A 3-162262 and
JP-B 2991526.)
[0041] 2) The shaped container of the present invention is formed
as a container bottom having a flange portion surfaced with a
sealable resin, and after contents being placed therein, a lid
member comprising a biodegradable film is sealed onto the flange
portion of the container bottom to form a package. (Details of this
embodiment are shown in, e.g., JP-A 4-72135.)
[0042] In either of the above-mentioned embodiments 1) and 2), if
the biodegradable film is heat-shrinkable, the package formed in
the above-described manner may be passed through a shrink tunnel to
shrink the film, thereby providing a beautiful package.
[0043] The biodegradable (heat-shrinkable) film may have a layer
structure of, e.g., lactic acid polymer/glycolic acid
polymer/succinic acid-based polyester. An anti-fog agent can be
applied on or incorporated in the biodegradable film. Such a
biodegradable film may be used instead of a wrapping film
("KUREWRAP", made by Kureha Chemical Industry Co., Ltd). used in
Examples described hereinbelow.
EXAMPLES
[0044] Hereinbelow, the present invention will be described more
specifically based on Examples and Comparative Examples.
Example 1
[0045] 2 g of pellet form polylactic acid ("LACTY", made by
Shimadzu Seisakusho K.K.) were placed on a 15 cm-dia. and 200
.mu.m-thick amorphous sheet of poly-glycolic acid (exhibiting a
melt viscosity of 2000 Pa.multidot.s at 240.degree. C. and a shear
rate of 100/s), and melted in a heat press at 240.degree. C. by
preheating for 1 min. and pressing at 5 MPa for 1 min., followed
immediately by cooling in iced water to form a 300 .mu.m-thick
transparent laminate sheet. After being dried, the thus-obtained
sheet was shaped into a 200 .mu.m-thick bowl with an outer layer of
the polylactic acid by air-pressure forming. The bowl was supported
by a jig so as to retain its shape and, in this state, was
heat-treated at 120.degree. C. for 1 min. After the heat treatment,
the bowl retained its shape even after the jig was removed. The
bowl was then placed in an oven at 100.degree. C. but caused no
change in outer appearance or shape whereby heat-resistance thereof
was confirmed. The bowl exhibited a haze of 10% or below throughout
the shaping, heat-treatment and oven-treatment.
Example 2
[0046] Cooked and cooled rice was placed in the bowl-shaped product
of Example 1, and surface-covered with a wrapping film ("KUREWRAP",
made by Kureha Chemical Industry Co., Ltd.), and in this state, was
heated for 1 min. in a microwave heater. After the heating, the
bowl caused no change in outer appearance or strength and could be
taken out together with the heated rice while holding the bowl by
hands, so that its heat resistance could be confirmed.
Comparative Example 1
[0047] 7 g of pellet-form polylactic acid (trade name: "LACTY
9030", made by Shimadzu Seisakusho) was melted in a heat press at
240.degree. C. by preheating for 1 min. and pressing at 5 MPa for 1
min., and then immediately cooled in iced water to form a 300
.mu.m-thick single layer sheet. After being dried, the
thus-obtained sheet was heated at 240.degree. C. and shaped into a
200 .mu.m-thick bowl by air-pressure forming. The bowl was
supported by a jig so as to retain its shape and, in this state,
heat-treated at 120.degree. C. for 1 min. As a result, the bowl was
softened at 120.degree. C. and resulted in a shape change after the
jig was removed.
Comparative Example 2
[0048] A bowl of polylactic acid subjected to heat-treatment in the
same manner as in Comparative Example 1 was cooled to room
temperature while being supported by the jig, whereby the bowl
shape could be retained. Cooked and cooled rice was placed in the
bowl after cooling, surface-covered with a wrapping film and then
heated for 1 min. in a microwave heater. After the heating, the
bowl was deformed, and the heated rice overflowed out of the
deformed bowl.
Comparative Example 3
[0049] Polyglycolic acid (melt viscosity: 2000 Pa.multidot.s at
240.degree. C. and a shear rate of 100/s) was melted in a heat
press at 240.degree. C. by preheating for 1 min. and pressing at 5
MPa for 1 min., and then immediately cooled in iced water to form a
250 .mu.m-thick single-layer sheet. After being dried, the
thus-obtained sheet was heated at 240.degree. C. and shaped into a
150 .mu.m-thick bowl by air-pressure forming. The bowl was
supported by a jig so as to retain its shape and, in this state,
heat-treated at 120.degree. C. for 1 min. The shaped bowl exhibited
a haze of 60%.
Example 3
[0050] Polyglycolic acid (PGA), polylactic acid (PLA) (trade name:
"LACTY 9030", made by Shimadzu Seisakusho) and ethylene-glycidyl
methacrylate copolymer adhesive resin ("BOND FAST EF-E", made by
Sumitomo Kagaku Kogyo K. K.) were extruded through a 5-layer T-die
extruder to form a trans-parent multiplayer sheet having a layer
structure of PLA/adhesive resin/PGA/adhesive resin/PLA (having
thicknesses from the left of 90/10/100/10/90 .mu.m).
[0051] The thus-obtained multilayer sheet was heated at 80.degree.
C. for 1 min. and then shaped into a 200 .mu.m-thick bowl by
air-pressure forming. After the shaping, the bowl was supported by
a jig so as to retain its shape and, in this state, heat-treated at
150.degree. C. for 1 min. In the bowl-shaped rigid container,
cooked and cooled rice was placed and surface-covered with a
wrapping film, followed by heating for 1 min. in a microwave
heater. After the heating, the bowl caused no change in outer
appearance or strength and could be taken out together with the
heated rice with hands, thus confirming its heat resistance. The
bowl exhibited a haze of 9% after the microwave heating.
Example 4
[0052] The 300 .mu.m-thick multilayer sheet obtained in Example 3
was shaped under vacuum at 100.degree. C. for 2 sec. by using a
continuous deep drawing machine, whereby a sufficiently transparent
160 .mu.m-thick lunch box-shaped container could be formed. Similar
vacuum forming was confirmed to be possible by heating in a range
of 80-110.degree. C. for 2 sec. After the shaping, the container
was supported by a jig so as to retain a rectangular lunch box
shape and heat-treated at 150.degree. C. for 1 min. In the lunch
box-shaped rigid container after the heat-treatment, cooked and
cooled rice was placed and surface-covered with a wrapping film,
followed by heating for 1 min. in a microwave heater. After the
heating, the lunch box-shaped rigid container caused no change in
outer appearance or strength and could be taken out together with
the heated rice with hands, thus confirming its heat resistance.
The container exhibited a haze of 9% after the microwave
heating.
Example 5
[0053] The 300 .mu.m-thick multilayer sheet obtained in Example 3
was examined with respect to melt-vacuum formability. As a result,
it was confirmed possible to prepare a 160 .mu.m-thick lunch
box-shaped container by using an infrared heater set at 200.degree.
C. while adjusting the heating time. After the shaping, the
container was supported by a jig so as to retain the rectangular
lunch box shape and heat-treated at 150.degree. C. for 1 min. In
the lunch box-shaped rigid container after the heat-treatment,
cooked and cooled rice was placed and surface-covered with a
wrapping film, followed by heating for 1 min. in a microwave
heater. After the heating, the lunch box-shaped rigid container
caused no change in outer appearance or strength and could be taken
out together with the heated rice with hands, thus confirming its
heat resistance. The container exhibited a haze of 9% after the
microwave heating.
INDUSTRIAL APPLICABILITY
[0054] As described above, according to the present invention,
there is provided a rigid resin shaped container, which has a
laminate structure including a glycolic acid polymer layer and
another biodegradable resin layer, is excellent in
bio-degradability, stiffness and heat-resistance as well as
excellent see-through of contents, and is thus suitable as a
temporary preservation container for food, etc.
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