U.S. patent application number 12/622878 was filed with the patent office on 2010-04-08 for foamed sheet of polylactic acid resin, foam molding of polylactic acid resin and method of preparing foam molding.
This patent application is currently assigned to JSP Corporation. Invention is credited to Akira Iwamoto, Takashi Kawada, Kenichi Takase.
Application Number | 20100086758 12/622878 |
Document ID | / |
Family ID | 38173918 |
Filed Date | 2010-04-08 |
United States Patent
Application |
20100086758 |
Kind Code |
A1 |
Takase; Kenichi ; et
al. |
April 8, 2010 |
FOAMED SHEET OF POLYLACTIC ACID RESIN, FOAM MOLDING OF POLYLACTIC
ACID RESIN AND METHOD OF PREPARING FOAM MOLDING
Abstract
A foamed sheet of a base resin containing 50 to 100% by weight
of a polylactic acid resin and having an apparent density of 63 to
630 kg/m.sup.3, a thickness of 0.5 to 7 mm and endothermic and
exothermic calorific values of .DELTA.H.sub.endo:2 and
.DELTA.H.sub.exo:2, respectively, as measured by heat flux
differential scanning calorimetry at a heating rate of 2.degree.
C./min. The endothermic calorific value .DELTA.H.sub.endo:2 is at
least 10 J/g and the difference
(.DELTA.H.sub.endo:2-.DELTA.H.sub.exo:2) between the endothermic
calorific value .DELTA.H.sub.endo:2 and the exothermic calorific
value .DELTA.H.sub.exo:2 is more than 20 J/g and less than 40 J/g.
A foam molding such as a receptacle is prepared by thermoforming
the foamed sheet.
Inventors: |
Takase; Kenichi;
(Kanuma-shi, JP) ; Kawada; Takashi; (Kanuma-shi,
JP) ; Iwamoto; Akira; (Utsunomiya-shi, JP) |
Correspondence
Address: |
BACON & THOMAS, PLLC
625 SLATERS LANE, FOURTH FLOOR
ALEXANDRIA
VA
22314-1176
US
|
Assignee: |
JSP Corporation
Tokyo
JP
|
Family ID: |
38173918 |
Appl. No.: |
12/622878 |
Filed: |
November 20, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11305045 |
Dec 19, 2005 |
7645810 |
|
|
12622878 |
|
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Current U.S.
Class: |
428/219 ;
264/292 |
Current CPC
Class: |
C08J 2367/04 20130101;
C08J 2203/14 20130101; C08J 2201/03 20130101; C08J 9/122 20130101;
Y10T 428/1397 20150115; C08J 9/141 20130101; C08J 9/0023
20130101 |
Class at
Publication: |
428/219 ;
264/292 |
International
Class: |
B32B 3/26 20060101
B32B003/26; B29C 55/00 20060101 B29C055/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 6, 2003 |
JP |
2003-163019 |
Oct 22, 2003 |
JP |
2003-362508 |
Oct 20, 2004 |
JP |
2004-305517 |
Claims
1. An extruded foamed sheet of a base resin comprising at least 50%
by weight of a polylactic acid resin, said foamed sheet having an
apparent density of 63 to 630 kg/m.sup.3, a thickness of 0.5 to 7
mm and endothermic and exothermic calorific values of
.DELTA.H.sub.endo:2 and .DELTA.H.sub.exo:2, respectively, as
measured by heat flux differential scanning calorimetry at a
heating rate of 2.degree. C./min, wherein the endothermic calorific
value .DELTA.H.sub.endo:2 is at least 10 J/g and the difference
(.DELTA.H.sub.endo:2-.DELTA.H.sub.exo:2) between the endothermic
calorific value .DELTA.H.sub.endo:2 and the exothermic calorific
value .DELTA.H.sub.exo:2 is more than 20 J/g and less than 40
J/g.
2. An extruded foamed sheet as claimed in claim 1, wherein the
extruded foamed sheet has an exothermic calorific value
.DELTA.H.sub.exo:10, as measured by heat flux differential scanning
calorimetry at a cooling rate of 10.degree. C./min, of at least 20
J/g.
3. An extruded foamed sheet as claimed in claim 1, wherein the
extruded foamed sheet has a melt tension of at least 2 cN at
190.degree. C.
4. An extruded foamed sheet as claimed in claim 1, wherein the
extruded foamed sheet has cells having an average cell diameter in
the extrusion direction of X mm, an average cell diameter in the
transverse direction of Y mm and an average cell diameter in the
thickness direction of Z mm, and wherein X, Y and Z satisfy the
following conditions: 0.05 mm<Z<0.8 mm 0.2<Z/X<0.8
0.2<Z/Y<0.65.
5. A method of producing an open-topped foam receptacle having a
draw ratio of S2/S1 where S1 represents an area of the top opening
thereof and S2 represents an inside surface area thereof,
comprising thermoforming in a mold an extruded foamed sheet of a
base resin containing at least 50% by weight of a polylactic acid
resin, said extruded foamed sheet having an apparent density of 63
to 630 kg/m.sup.3, a thickness of 0.5 to 7 mm and endothermic and
exothermic calorific values of .DELTA.H.sub.endo:2 and
.DELTA.H.sub.exo:2, respectively, as measured by heat flux
differential scanning calorimetry at a heating rate of 2.degree.
C./min, wherein the endothermic calorific value .DELTA.H.sub.endo:2
is at least 10 J/g and wherein the draw ratio S2/S1 and the
difference .DELTA.H.sub.x (.DELTA.H.sub.x=.DELTA.H.sub.endo:2
-.DELTA.H.sub.exo:2) between the endothermic calorific value
.DELTA.H.sub.endo:2 and the exothermic calorific value
.DELTA.H.sub.exo:2 satisfy the following equation:
S2/S1.ltoreq.-0.08.DELTA.H.sub.x+4.2.
6. An extruded foamed sheet as claimed in claim 1, wherein the
endothermic and calorific value .DELTA.H.sub.endo:2 is at least 20
J/g.
7. An extruded foamed sheet as claimed in claim 1 wherein the
polylactic acid resin has been modified by reaction with an organic
peroxide, an isocyanate, an epoxy compound, a metal complex, a
polyvalent carboxylic acid or a mixture thereof.
8. An extruded foamed sheet as claimed in claim 7 wherein the
polylactic acid resin has a melt tension of at least 3 cN.
9. An extruded foamed sheet as claimed in claim 1 wherein the
polylactic acid resin has been reacted with an organic
peroxide.
10. An extruded foamed sheet as claimed in claim 1 wherein the
polylactic acid resin has been crosslinked by reaction with an
organic peroxide to raise its melt tension to at least 3 cN.
11. An extruded foamed sheet as claimed in claim 1 wherein the
polylactic acid resin has an endothermic calorific value
.DELTA.H.sub.endo:Material of at least 10 J/g as measured by heat
flux differential scanning calorimetry at a heating rate of
2.degree. C./min.
12. An extruded foamed sheet as claimed in claim 9 wherein the
organic peroxide is dicumyl peroxide.
13. A method as claimed in claim 5 wherein the polylactic acid
resin has been modified by reaction with an organic peroxide, an
isocyanate, an epoxy compound, a metal complex, a polyvalent
carboxylic acid or a mixture thereof.
14. A method as claimed in claim 5 wherein the polylactic acid
resin has been reacted with an organic peroxide.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a foamed sheet of a base resin
containing a polylactic acid resin as its major component, to a
foam molding of such a polylactic acid resin-containing base resin
and to a method of preparing a foam molding by thermoforming such a
foamed sheet.
[0003] 2. Description of Prior Art
[0004] Foamed bodies of generally employed resins such as
polyethylene, polypropylene and polystyrene resins are now used in
various fields because of their good heat insulating properties,
cushioning properties and lightness in weight. These foam moldings,
which are generally stable, remain in our environment when disposed
of and cause environmental problems.
[0005] To cope with this problem, many studies are being made on
polymers which are decomposable by microorganisms. Among various
biodegradable polymers, polylactic acid resins have actually used
as, for example, surgical suture materials. Polylactic acid resins
are promising because they are safe to human bodies, because they
are decomposable, when left in the environment, by hydrolysis and
by biological degradation and because their starting material,
lactic acid, can be prepared with a high yield at a low cost by
fermentation of biomass such as corn. In particular,
environmentally friendly foamed bodies such as foamed sheets of a
polylactic acid resin are now being developed.
[0006] For example, Japanese Unexamined Patent Publication No.
JP-A-2002-322309 discloses a foamed sheet of a non-crystalline
polylactic acid resin. While the sheet is obtainable with ease by
extrusion, the heat resistance thereof is poor. Japanese Unexamined
Patent Publications Nos. JPA-2000-s 136259 and JP-A-2002-3709
disclose a foamed sheet of a crystalline polylactic acid resin.
While the crystalline polylactic acid resin has good heat
resistance, it is difficult to obtain a foamed sheet because of
poor moldability and poor foamability. Moreover, the foamed sheet
obtained has poor thermofoamability because of a high apparent
density, non-uniform cell shapes and a low closed cell content.
SUMMARY OF THE INVENTION
[0007] It is, therefore, an object of the present invention to
provide a foamed sheet of a polylactic acid resin which has both
good heat resistance and good thermoformability.
[0008] Another object of the present invention is to provide a foam
molding obtained by thermoforming a foamed sheet of a polylactic
acid resin.
[0009] It is a further object of the present invention to provide a
method of producing a foam molding using a foamed sheet of a
polylactic acid resin.
[0010] In accomplishing the above objects, there is provided in
accordance with the present invention a foamed sheet of a base
resin comprising at least 50% by weight of a polylactic acid resin,
said foamed sheet having an apparent density of 63 to 630
kg/m.sup.3, a thickness of 0.5 to 7 mm and endothermic and
exothermic calorific values of .DELTA.H.sub.endo:2 and
.DELTA.H.sub.exo:2, respectively, as measured by heat flux
differential scanning calorimetry at a heating rate of 2.degree.
C./min, wherein the endothermic calorific value .DELTA.H.sub.endo:2
is at least 10 J/g and the difference
(.DELTA.H.sub.endo:2-.DELTA.H.sub.exo:2) between the endothermic
calorific value .DELTA.H.sub.endo:2 and the exothermic calorific
value .DELTA.H.sub.exo:2 is less than 40 J/g.
[0011] In another aspect, the present invention provides a foam
molding obtained by thermoforming a foamed sheet of a base resin
containing at least 50% by weight of a polylactic acid resin, said
foam molding having endothermic and exothermic calorific values of
.DELTA.H.sub.endo:Mold and .DELTA.H.sub.exo:Mold, respectively, as
measured by heat flux differential scanning calorimetry at a
heating rate of 2.degree. C./min, wherein the difference
(.DELTA.H.sub.endo:Mold-.DELTA.H.sub.exo:Mold) between the
endothermic calorific value .DELTA.H.sub.endo:2 and the exothermic
calorific value .DELTA.H.sub.exo:Mold is not less than 10 J/g.
[0012] The present invention also provides a method of producing an
open-topped foam receptacle having a draw ratio of S2/S1 where S1
represents an area of the top opening thereof and S2 represents an
inside surface area thereof, comprising thermoforming in a mold a
foamed sheet of a base resin containing at least 50% by weight of a
polylactic acid resin, said foamed sheet having an apparent density
of 63 to 630 kg/m.sup.3, a thickness of 0.5 to 7 mm and endothermic
and exothermic calorific values of .DELTA.H.sub.endo:2 and
.DELTA.H.sub.exo:2, respectively, as measured by heat flux
differential scanning calorimetry at a heating rate of 2.degree.
C./min, wherein the endothermic calorific value .DELTA.H.sub.endo:2
is at least 10 J/g and wherein the draw ratio S2/S1 and the
difference .DELTA.H.sub.x
(.DELTA.H.sub.x=.DELTA.H.sub.endo:2-.DELTA.H.sub.exo:2) between the
endothermic calorific value .DELTA.H.sub.endo:2 and the exothermic
calorific value .DELTA.H.sub.exo:2 satisfy the following
equation:
S2/S1.ltoreq.-0.08.DELTA.H.sub.x+4.2.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Other objects, features and advantages of the present
invention will become apparent from the detailed description of the
invention which follows, when considered in the light of the
accompanying drawings, in which:
[0014] FIG. 1 is an example of a DSC curve of a polylactic acid
resin as measured with a heat flux differential scanning
calorimeter, explanatory of a method of determining an endothermic
calorific value of a polylactic acid resin;
[0015] FIG. 2 is an another example of a DSC curve similar to FIG.
1;
[0016] FIG. 3 is a graph explanatory of a method of determining the
melt tension of a base resin or a foamed sheet;
[0017] FIG. 4(a) and FIG. 4(b) are sectional views in the extrusion
direction and transverse direction, respectively, explanatory of a
method of measuring average cell diameters in the extrusion
direction, thickness direction and transverse direction of the
foamed sheet;
[0018] FIG. 5 is an example of a DSC curve of a foamed sheet as
measured with a heat flux differential scanning calorimeter,
explanatory of a method of determining a calorific value of each of
an endothermic peak and an exothermic peak;
[0019] FIG. 6 is an another example of a DSC curve similar to FIG.
5;
[0020] FIG. 7 is a further example of a DSC curve similar to FIG.
5; and
[0021] FIG. 8 is a graph explanatory of a method of determining the
half crystallization time of a foamed sheet.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
[0022] The foamed sheet according to the present invention
comprises a base resin containing at least 50% by weight of a
polylactic acid resin. The term "polylactic acid resin" as used
herein is intended to refer to a polymer or copolymer containing at
least 50 mol % of lactic acid monomer component units. Examples of
the polylactic acid resin include (a) a homopolymer of lactic acid,
(b) a copolymer of lactic acid with one or more aliphatic
hydroxycarboxylic acids other than lactic acid, (c) a copolymer of
lactic acid with an aliphatic polyhydric alcohol and an aliphatic
polycarboxylic acid, (d) a copolymer of lactic acid with an
aliphatic polyhydric alcohol, (e) a copolymer of lactic acid with
an aliphatic polycarboxylic acid, and (f) a mixture of two or more
of (a)-(e) above.
[0023] The term "lactic acid" as used herein is intended to refer
to L-lactic acid, D-lactic acid, DL-lactic acid, a cyclic dimer
thereof (i.e. L-lactide, D-lactide or DL-lactide) or a mixture
thereof.
[0024] The polylactic acid resin preferably has an endothermic
calorific value (heat of fusion) .DELTA.H.sub.endo:Material, as
measured by heat flux scanning differential calorimetry, of at
least 10 J/g, more preferably at least 20 J/g, most preferably at
least 30 J/g. The upper limit of the endothermic calorific value is
not specifically limited but is generally about 60 J/g. The
polylactic acid resin having an endothermic calorific value
.DELTA.H.sub.endo:Material of at least 10 J/g may be, for example,
a crystalline polylactic acid resin or a mixture of crystalline and
non-crystalline polylactic acid resins.
[0025] The term "crystalline polylactic acid resin" as used herein
is intended to refer to a polylactic acid resin having
.DELTA.H.sub.endo:Material of more than 2 J/g. The crystalline
polylactic acid resin generally has .DELTA.H.sub.endo:Material of
20-65 J/g. The term "non-crystalline polylactic acid resin" as used
herein is intended to refer to a polylactic acid resin having
.DELTA.H.sub.endo:Material of 2 J/g or less, or a polylactic acid
resin which shows no endothermic peak in heat flux scanning
differential calorimetry.
[0026] The term "endothermic calorific value
.DELTA.H.sub.endo:Material" as used herein is intended to refer to
heat of fusion as determined from DSC curve of heat flux
differential scanning calorimetry in accordance with JIS
K7122-1987. The DSC measuretnent is carried out as follows. A
polylactic acid resin sample (about 1 to 4 mg) is charged in a pan
of a differential scanning calorimeter and heated to a temperature
higher by about 30.degree. C. than the temperature at which the
endothermic peak meets the base line to melt the sample. After
maintaining the sample at that temperature for 10 minutes, the
sample is cooled to 125.degree. C. at a cooling rate of 2.degree.
C./minute. The sample is maintained at 125.degree. C. for 120
minutes and then cooled to 40.degree. C. at a cooling rate of
2.degree. C./minute. After the above-described pretreatment, the
DSC curve is measured while heating the sample again at 2.degree.
C./minute to a temperature higher by about 30.degree. C. than the
temperature at which the endothermic peak ends.
[0027] FIG. 1 depicts an example of such a DSC curve. In FIG. 1,
there are dual endothermic peaks which begin from a point "a" where
the DSC curve begins separating from a low temperature-side base
line BL1 and which terminates at a point "b" where the DSC curve
returns to a high temperature-side base line BL2. The endothermic
calorific value .DELTA.H.sub.endo:Material is an integration of the
endothermic peak area, namely the area defined by a line passing
the points "a" and "b" and the endothermic curve. The DSC device
should be preferably operated so that each of the base lines BL1
and BL2 is straight as shown in FIG. 1. When the base line or lines
are inevitably curved, the points "a" and "b" are determined as
follows. For example, when the base lines BL1 and BL2 are curved as
shown in FIG. 2, the curved base line BL1 is extended to the high
temperature side with the radius of the curvature of the base line
being maintained. The point at which the DSC curve begins
separating from the curved base line BL1 is the point "a".
Similarly, the point "b" is a point where the DSC curve returns to
a curved base line BL2 on the high temperature-side.
[0028] The above-described pretreatment is carried out for the
purpose of crystallizing the polylactic acid resin sample as much
as possible. Thus, the endothermic calorific value
.DELTA.H.sub.endo:Material obtained represents a calorific value of
the completely or nearly completely crystallized polylactic acid
resin. Further, the above-described DSC measurement employs a
heating rate of 2.degree. C./minute. The heating rate of 2.degree.
C./minute is suitable for separating the endothermic peak and
exothermic peak in the DSC curve and for obtaining accurate
endothermic calorific value .DELTA.H.sub.endo:Material.
[0029] As described above, the base resin from which the foamed
sheet of the present invention is composed contains at least 50% by
weight of a polylactic acid resin. Thus, the base resin may be
composed only of the polylactic acid resin or composed of a mixture
of the polylactic acid resin with an additional resin. The amount
of the polylactic acid resin in the mixture is at least 50% by
weight, preferably at least 70% by weight, more preferably at least
90% by weight, based on a total weight of the polylactic acid resin
and the additional resin.
[0030] Examples of the additional resin include a polyethylene
resin, a polypropylene resin, a polystyrene resin and a polyester
resin. The use of a biodegradable polyester resin containing at
least 35 mol % of aliphatic ester component units, such as a
polycondensation product of a hydroxyacid other than lactic acid, a
ring open polymerization product of a lactone (e.g.
polycaprolactone), a polycondensation product of an aliphatic
polyhydric alcohol with an aliphatic polycarboxylic acid (e.g.
polybutylene succinate, polybutylene adipate, polybutylene
succinate adipate and polybutylene adipate/terephthalate) is
preferred.
[0031] The polylactic acid resin may be prepared by any suitable
known method such as a method in which lactic acid or a mixture of
lactic acid and aliphatic hydroxycarboxylic acid is subjected to a
dehydration polycondenation (disclosed, for example, in U.S. Pat.
No. 5,310,865); a method in which a cyclic dimer of lactic acid
(namely lactide) is subjected to ring-open polymerization
(disclosed, for example, in U.S. Pat. No. 2,758,987); a method in
which a cyclic dimer of an aliphatic hydroxycarboxylic acid (e.g.
lactide or glycolide) and .epsilon.-caprolactone are subjected to
ring-open polymerization in the presence of a catalyst (disclosed,
for example, in U.S. Pat. No. 4,057,537); a method in which lactic
acid and a mixture of an aliphatic dihydric alcohol and an
aliphatic dibasic acid are subjected to dehydration
polycondensation (disclosed, for example, in U.S. Pat. No.
5,428,126); a method in which a lactic acid polymer and a polymer
of an aliphatic dihydric alcohol and an aliphatic dibasic acid are
subjected to condensation in an organic solvent (disclosed, for
example, in EP-A-0712880); and a method in which lactic acid is
subjected to dehydration polycondensation in the presence of a
catalyst, with a step of polymerization in a solid phase being
involved during the course of the polycondensation. The above
methods may be performed in the presence of a minor amount of an
aliphatic polyhydric alcohol (e.g. glycerin), an aliphatic
polybasic acid (e.g. butanetetracarboxylic acid) or polyhydric
alcohol (e.g. polysaccharide) to obtain a copolymer.
[0032] The foamed sheet of the present invention may be prepared as
follows. The base resin and an additive such as a cell controlling
agent are heated and kneaded in an extruder. A physical blowing
agent is then fed under a pressure to the extruder and the mixture
is further kneaded. The kneaded mass is then extruded at a suitable
temperature through a die, such as a T-die or a circular die,
attached to the extruder in the form of a flat or tubular sheet so
that the extrudate foams and expands. The extrudate immediately
after being extruded is generally rapidly cooled by air or mist
spray. A tubular extrudate from the circular die may be hauled and
slid over a mandrel of a cooling device. The cooled tubular
extrudate is cut in the extrusion direction and opened to obtain a
foamed sheet. For reasons of obtaining a foamed sheet with a
uniform thickness and a suitable apparent density, the use of a
circular die is desirable.
[0033] When a non-crystalline polylactic acid resin is extruded and
foamed in the customarily employed manner, a foamed sheet of the
non-crystalline polylactic acid resin having a low apparent density
may be obtained. Such a foamed sheet, which is excluded from the
scope of the present invention, shows good thermoformability.
However, since the rigidity of the foamed sheet abruptly reduced
when heated above the glass transition point thereof, the
thermoformed product cannot retain its shape and is ill-suited for
practical use. Namely, the heat resistance of the foamed sheet of a
non-crystalline polylactic acid resin is unsatisfactory, though the
thermoformability thereof is good. In contrast, when a polylactic
acid resin having an endothermic calorific value
.DELTA.H.sub.endo:Material of at least 10 J/g is used as a base
resin of the foamed sheet of the present invention, the foamed
sheet shows good thermoformability and gives a thermoformed product
having good heat resistance.
[0034] When a polylactic acid resin having an endothermic calorific
value .DELTA.H.sub.endo:Material of at least 10 J/g is used as a
base resin, a difficulty may arise in obtaining a foamed sheet
having an apparent density of 63 to 630 kg/m.sup.3 and a thickness
of 0.5 to 7 mm because the viscoelasticity of the molten, foamable
composition containing the base resin is not easily adequately
adjusted to a suitable range. For reasons of obtaining sufficient
melt tension during extrusion foaming and of avoiding such a
difficulty, it is preferred that the base resin used have a melt
tension at 190.degree. C. of at least 3 cN, more preferably at
least 5 cN, still more preferably at least 8 cN, most preferably at
least 10 cN. The upper limit of the melt tension is generally about
40 cN.
[0035] Due to thermal hysteresis and shearing force, the melt
tension of the base resin tends to reduce when the base resin is
subjected to extrusion foaming. Since a great reduction of the melt
tension results in a deterioration of the excellent properties of
the foamed sheet, excessive heating or shearing force should not be
applied to the base resin during extrusion foaming. The foamed
sheet of the present invention preferably has a melt tension at
190.degree. C. of at least 2 cN, more preferably at least 3 cN,
particularly preferably at least 5 cN. The upper limit of the melt
tension is generally about 40 cN.
[0036] The melt tension may be measured using Melt Tension Tester
II (manufactured by Toyo Seiki Selsaku-Sho, Ltd.) having a cylinder
and a nozzle with an orifice diameter of 2.095 mm and a length of 8
mm. The cylinder and orifice are set at a temperature of
190.degree. C. A specimen (base resin or ground foamed sheet) is
charged into a cylinder and held therein for 5 minutes. The melt is
then extruded in the form of a string under conditions including a
resin temperature of 190.degree. C. and a piston speed of 10
mm/minute from the orifice. The extruded resin string is put on a
tension-detecting pulley having a diameter of 45 mm and is taken up
on a roller having a diameter of 50 mm while increasing the take-up
speed at a rate of about 5 rpm/sec (take-up acceleration of the
resin string: 1.3.times.10.sup.-2 m/sec.sup.2). During the
extrusion, care should be taken to avoid intrusion of bubbles in
the string.
[0037] The take-up speed is increased until the string put on the
pulley breaks. The take-up speed R (rpm) when the string breaks is
measured. Then, the string is taken up at a constant speed of
0.7.times.R (rpm) while measuring the melt tension of the string
over time using a detector connected to the is tension-detecting
pulley. The results are plotted on a chart with the measured melt
tension as ordinate and the time as abscissa to obtain a graph as
shown in FIG. 3. The melt tension of the specimen herein is the
median value (X) of the amplitudes in the stable portion of the
graph in FIG. 3. An abnormal amplitude which might be appear in the
graph on rare occasion should be ignored in obtaining the median
value. In the above procedures, when the resin string does not
break up to the take-up speed of 500 rpm, then the melt tension is
measured at a take-up speed of 500 rpm rather than 0.7.times.R
(rpm).
[0038] It is also preferred that the base resin for the formation
of the foamed sheet have a melt flow rate (MFR) of 0.1 to 10 g/10
min, more preferably 0.1 to 5 g/10 min, still more preferably 0.3
to 3 g/10 min, for reasons good extrusion moldability and good
thermoformability of the foamed sheet obtained. As used herein, the
melt flow rate is as measured in accordance with JIS K7210-1976,
Test Method A, at a temperature of 190.degree. C. and a load of
21.2 N.
[0039] A polylactic acid resin having a melt tension of at least 3
cN and MFR of 0.1 to 10 g/10 min may be suitably obtained by, for
example, a method in which a raw material polylactic acid resin
having a melt tension of less than 3 cN (excluding 0 cN) and MFR of
2 to 12 g/10 min is reacted with an organic peroxide to slightly
crosslinking the resin (the gel fraction is substantially 0) to
obtain a modified polylactic acid resin, or a method in which the
raw material polylactic acid resin is reacted with a agent for
increasing the molecular weight thereof, such as an isocyanate, an
epoxy compound, a metal complex, a polyvalent carboxylic acid or a
mixture thereof, to obtain a modified polylactic acid resin.
[0040] The organic peroxide used for modifying a polylactic acid
resin preferably has a 1 min half life temperature Th (the
temperature at which the amount of the active oxygen of the organic
peroxide decreases to half when the peroxide is heated at that
temperature for 1 minute) which is higher than the melting point Mp
of the polylactic acid resin minus 10.degree. C. (Th>Mp
10.degree. C.). When the 1 minute half life temperature Th is lower
by 10.degree. C. or more than the melting point Mp (Th.ltoreq.Mp
10.degree. C.), the organic peroxide will decompose and react
before the organic peroxide is uniformly mixed with the polylactic
acid resin during the heating and kneading and, therefore, the
resin cannot be uniformly modified. Additionally, since the organic
peroxide needs to be used in an increased amount, crosslinking
tends to proceeds excessively in the succeeding extrusion foaming
step so that the gel fraction of the foamed sheet undesirably
increases. On the other hand, when Th is considerably higher than
Mp, it is necessary to carry out the modification at a high
temperature. This may cause degradation of the molecular weight of
the polylactic acid resin and deterioration of the properties of
the foamed sheet. For this reason, Th is desirably not higher than
Mp plus 20.degree. C. (Th.ltoreq.Mp+20.degree. C.).
[0041] As used herein, the term "melting point" of the polylactic
acid resin is intended to refer to a temperature of the apex of the
endothermic peak in a DSC curve obtained by heat flux differential
scanning calorimetry in accordance with WS K7121-1987. More
specifically, a test piece is heat treated under the condition
specified in "3. Condition Adjustment (2)" (a cooling rate of
10.degree. C./min is used). The condition-adjusted test piece is
then subjected to DSC analysis at a heating rate 10.degree. C./min
to obtain an endothermic peak. When two or more endothermic peaks
are present, the temperature of the peak having the largest area
represents the melting point.
[0042] Examples of suitable organic peroxides are shown below
together with 1 min half life temperature indicated in the
brackets: isobutylperoxide [85.degree. C.], cumyl peroxy
neodecanoate [94.degree. C.],
.alpha.,.alpha.-bis(neodecanoylperoxy)diisopropylbenzene
[82.degree. C.], di-n-propyl peroxydicarbonate [94.degree. C.],
diisopropyl peroxydicarbonate [88.degree. C.],
1-cyclohexyl-1-methylethyl peroxy neodecanoate [94.degree. C.],
1,1,3,3-tetramethylbutyl peroxy neodecanoate [92.degree. C.],
bis(4-t-butylcyclohexyl) peroxydicarbonate [92.degree. C.],
di-2-ethoxyethyl peroxydicarbonate [92.degree. C.],
di(2-ethylhexylperoxy)dicarbonate [91.degree. C.], t-hexyl peroxy
neodecanoate [101.degree. C.], dimethoxybutyl peroxydicarbonate
[102.degree. C.], di(3-methyl-3-methoxybutylperoxy)dicarbonate
[103.degree. C.], t-butyl peroxy neodecanoate [104.degree. C.],
2,4-dichlorobenzoyl peroxide [119.degree. C.], t-hexyl peroxy
pivalate [109.degree. C.], t-butyl peroxy pivalate [110.degree.
C.], 3,5,5-trimethylhexanoyl peroxide [113.degree. C.], octanoyl
peroxide [117.degree. C.], lauroyl peroxide [116.degree. C.],
stearoyl peroxide [117.degree. C.], 1,1,3,3-tetramethylbutyl peroxy
2-ethylhexanoate [124.degree. C.], succinic peroxide [132.degree.
C.], 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane [119.degree.
C.], 1-cyclohexl-1-methylethyl peroxy 2-ethylhexanoate [138.degree.
C.], t-hexyl peroxy 2-ethylhexanoate [133.degree. C.], t-butyl
peroxy 2-ethylhexanoate [134.degree. C.], m-toluoyl benzoyl
peroxide [131 .degree. C.], benzoyl peroxide [130.degree. C.],
t-butyl peroxy isobutylate [136.degree. C.],
1,1-bis(t-butylperoxy)-2-methylcyclohexane [142.degree. C.],
1,1-bis(t-hexylperoxy)-3,3,5-trimethylcyclohexane [147.degree. C.],
1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane [149.degree. C.],
1,1-bis(t-hexylperoxy)cyclohexane [149.degree. C.],
1,1-bis(t-butylperoxy)cyclohexane [154.degree. C.],
2,2-bis(4,4-dibutylperoxycyclohexyl)propane [154.degree. C.],
1,1-bis(t-butylperoxy)cyclododecane [153.degree. C.], t-hexyl
peroxy isopropyl monocarbonate [155.degree. C.], t-butyl peroxy
maleic acid [168.degree. C.], t-butyl peroxy
3,5,5-trimethylhexanoate [166.degree. C.], t-butyl peroxy laurate
[159.degree. C.], 2,5-dimethyl-2,5-di(m-toluoylperoxy)hexane
[156.degree. C.], t-butyl peroxy isopropyl monocarbonate
[159.degree. C.], t-butyl peroxy 2-ethylhexyl monocarbonate
[161.degree. C.], t-hexyl peroxy benzoate [160.degree. C.],
2,5-dimethyl-2,5-di(benzoylperoxy)hexane [158.degree. C.] and
dicumyl peroxide [175.degree. C.]. Above all, dicumyl peroxide is
particularly preferably used. The organic peroxides may be used
singly or in combination with two or more thereof. The organic
peroxide or peroxides are generally used in an amount of 0.3 to 0.7
part by weight, preferably 0.4 to 0.6 part by weight, per 100 parts
by weight of the polylactic acid resin.
[0043] The 1 min half life temperature of the organic peroxide is
measured as follows. A sample peroxide is dissolved in a suitable
solvent inert to radicals, such as benzene or mineral spirit, to
obtain a solution having a peroxide concentration of 0.1 mol/L.
This is placed in a glass tube whose inside space has been
substituted by nitrogen. The glass tube is sealed and immersed in a
constant temperature bath maintained at a predetermined temperature
for 1 minute to permit the peroxide to decompose. The above
procedures are repeated at various temperatures to determine the
half life temperature.
[0044] The polylactic acid resin thus modified has a gel fraction
of substantially zero %. The term "gel fraction" as used herein is
as measured by the following method. Sample resin (weight W1; about
1 g) is immersed in 100 ml of chloroform contained in a 150 ml
flask and the mixture is refluxed for 10 hours in boiling
chloroform at about 61.degree. C. The mixture is then immediately
filtered through a 200 mesh wire net using a vacuum filtration
device. The solids on the wire net are dried in an oven at
20.degree. C. for 8 hours under a reduced pressure of 30-40 Torr.
The dry weight (W2) of the chloroform-insoluble matters left on the
wire net is measured. A gel fraction (% by weight) is calculated
from the formula:
Gel fraction (%)=(W2/W1).times.100
The term "gel fraction of substantially zero %" as used herein is
intended to refer that the gel fraction is 2% or less. The gel
fraction of the modified polylactic acid is preferably 0.5% by
weight or less.
[0045] The blowing agent used for the production of a foamed sheet
having a low apparent density may be an organic physical blowing
agent or an inorganic physical blowing agent. Examples of the
organic physical blowing agents include aliphatic hydrocarbons such
as propane, n-butane, isobutane, n-pentane, isopentane and hexane;
and halogenated aliphatic hydrocarbons such as methyl chloride and
ethyl chloride. Examples of inorganic physical blowing agents
include air, nitrogen and carbon dioxide. Above all, n-butane,
isobutane and carbon dioxide are preferred. A chemical blowing
agent may be also used, if desired. However, for the production of
a foamed sheet having a low apparent density, the use of a physical
blowing agent or a mixture of a physical blowing agent and a
chemical blowing agent is preferred.
[0046] The foamed sheet of the present invention may contain an
inorganic cell controlling agent such as talc or silica, or an
organic cell controlling agent such as calcium stearate. One or
more additives such as a colorant and an anti-oxidant may also be
incorporated into the foamed sheet.
[0047] The foamed sheet of the present invention has an apparent
density of 63 to 630 kg/m.sup.3, preferably 84 to 504 kg/m.sup.3.
Too low an apparent density below 63 kg/m.sup.3 is undesirable
because the thermoformability of the foamed sheet is so poor that
it is impossible to obtain a thermoformed product having a shape
conforming to the shape of the mold. Further, the thermoformed
product obtained has a low mechanical strength. When the apparent
density is higher than 630 kg/m.sup.3, the heat insulating
property, cushioning property and lightness in weight of the foamed
sheet are unsatisfactory.
[0048] As used herein, the "apparent density of the foamed sheet"
is measured as follows. From the foamed sheet, a square specimen
having a length of 10 mm, a width of 10 mm and a thickness equal to
that of the foamed sheet is cut and measured for its weight. The
apparent density is obtained by dividing the weight of the specimen
by the volume thereof.
[0049] The foamed sheet has a thickness of 0.5 to 7 mm, preferably
0.5 to 5 mm, more preferably 0.7 to 3 mm. A thickness of the foamed
sheet smaller than 0.5 mm is disadvantageous because the mechanical
strength of a thermoformed product obtained from the foamed sheet
is low. Too large a thickness in excess of 7 mm will deteriorate
the thermoformability of the foamed sheet so that a thermoformed
product obtained has not uniform thickness.
[0050] As used herein, the "thickness of the foamed sheet" is
measured as follows. The thickness of the foamed sheet is measured
at every 10 mm interval throughout the width thereof (in the
direction perpendicular to the extrusion direction). The thickness
of the foamed sheet is an arithmetic mean of the measured
thicknesses.
[0051] It is preferred that the foamed sheet have the following
cell geometry:
[0052] 0.05 mm<Z<0.8 mm
[0053] 0.2<Z/X<0.8
[0054] 0.2<Z/Y<0.65
[0055] wherein X is an average cell diameter (mm) in the extrusion
direction (machine direction) Y is an average cell diameter (mm) in
the transverse direction and Z is an average cell diameter (mm) in
the thickness direction, because the mechanical strength,
thermoformability, flexibility and appearance of the foamed sheet
are excellent and because the thermoformed product obtained
therefrom has excellent mechanical strength. More preferably, Z is
0.1 to 0.5 mm, Z/X is 0.3 to 0.7 and Z/Y is 0.25 to 0.60.
[0056] As used herein the "average cell diameters X, Y and Z" of a
foamed sheet are measured as follows.
[0057] To determine X and Z, the foamed sheet is cut in the
extrusion direction and the cross-section is photographed using a
microscope. On the photograph, which is diagrammatically shown in
FIG. 4(a), a pair of straight lines L1 and L2 are drawn in parallel
with the lines S1 and S2 indicating both surfaces of the sheet
having a thickness T. The lines L1 and L2 extend at positions
spaced apart a distance 0.1T from the surface lines S1 and S 2.
Using a caliper, the diameters in the extrusion direction (x.sub.1,
x.sub.2, x.sub.3, . . . x.sub.n) and in the thickness direction
(z.sub.1, z.sub.2, z.sub.3, z.sub.n) of all the cells 2 present in
the center region (having a thickness 0.8T) defined between the
lines L1 and L2 over the length 5T (5 times the thickness of the
sheet) are measured. In this case, the cells 2 a present on the
Lines L1 and L2 are excluded. The arithmetic mean of the measured
diameters (x.sub.1, x.sub.2, x.sub.3, . . . x.sub.n) represents the
average cell diameter X in the extrusion direction and the
arithmetic mean of the measured diameters (z.sub.1, Z.sub.2,
Z.sub.3, . . . , Z.sub.n) represents the average cell diameter Z in
the thickness direction.
[0058] To determine Y, the foamed sheet is cut in the transverse
direction and the cross-section is photographed using a microscope.
On the photograph, which is diagrammatically shown in FIG. 4(b), a
pair of straight lines L1 and L2 are drawn in the same manner as
above. The diameters in the transverse direction (y.sub.l, y.sub.2,
y.sub.3, . . . , y.sub.n) of all the cells 2 present between the
lines L1 and L2 over the length 5T are measured. The arithmetic
mean of the measured diameters (y.sub.1, y.sub.2, y.sub.3, y.sub.n)
represents the average cell diameter Y in the transverse
direction.
[0059] The average cell diameters may be controlled by using an
organic or inorganic cell controlling agent such as talc or sodium
hydrogen carbonate in an amount of 0.1 to 3 parts by weight per 100
parts by weight of the base resin or by controlling the pressure in
the die at the time of extrusion molding. In particular, by
increasing the amount of a cell controlling agent within such a
range that permits the preparation of a foamed sheet having good
appearance and desired apparent density and thickness, the cell
size can be made small. The cell size can be also made small by
increasing the pressure in the die. The ratio Z/X can be controlled
by controlling the hauling speed of the foamed sheet immediately
after the extrusion foaming. The ratio Z/Y can be controlled by
controlling the spreading ratio in the transverse direction of the
foamed sheet immediately after the extrusion foaming.
[0060] The foamed sheet of the present invention preferably has a
closed cell content of 50 to 100%, more preferably 70 to 100%, and
most preferably 80 to 100%, since the foamed sheet has high
mechanical strength and exhibits good thermoformability and gives a
thermoformed product (foam molding) having high mechanical
strength, molding reproducibility and good appearance.
[0061] The closed cell content of the foamed sheet is obtained
according to Procedure C of ASTM D-2856-70 (reapproved 1976) as
follows. The true volume Vx of a specimen of the foamed sheet is
measured using Air Comparison Pycnometer Type-930 manufactured by
Toshiba Beckmann Inc. The foamed sheet is cut to have a size of 25
mm.times.25 mm and a thickness equal to that of the foamed sheet.
The cutouts are stacked in such a number that the stack has an
apparent volume of as near 15 cm.sup.3 as possible to obtain the
specimen. The closed cell content S (%) is calculated by the
following formula:
S(%)=(Vx-W/p).times.100/(Va-W/p)
wherein
[0062] Vx represents the true volume (cm.sup.3) of the specimen
measured by the above method, which corresponds to a sum of a
volume of the resin constituting the specimen and a total volume of
all the closed cells in the specimen;
[0063] Va represents the apparent volume (cm.sup.3) of the specimen
used for the measurement, which is calculated from the outer
dimension thereof;
[0064] W represents the weight (g) of the specimen used for the
measurement; and
[0065] p represents the density (g/cm.sup.3) of the base resin
constituting the specimen.
[0066] It is important that the foamed sheet of the present
invention should have such characteristics in heat flux scanning
differential calorimetry (DSC) at a heating rate of 2.degree.
C./min that a difference .DELTA.H.sub.x
(.DELTA.H.sub.x=.DELTA.H.sub.endo:2-.DELTA.H.sub.exo:2) between an
endothermic calorific value .DELTA.H.sub.endo:2 and an exothermic
calorific value .DELTA.H.sub.exo:2 is less than 40 J/g and that the
endothermic calorific value .DELTA.H.sub.endo:2 is at least 10 J/g.
Namely, the foamed sheet should have a controlled crystallinity so
that it has good thermoformability and ability to improve heat
resistance and gives a thermoformed product (foam molding) having
excellent heat resistance. The crystallinity of the foamed sheet
may be controlled by rapidly cooling the extruded and foamed sheet
immediately after extrusion with air or mist.
[0067] The exothermic peak is attributed to a heat of
crystallization, i.e. a heat generated from the sample foamed sheet
as a result of the crystallization thereof during the course of
heating at a rate of 2.degree. C./minute in the DSC measurement.
The exothermic calorific value .DELTA.H.sub.exo:2 is the calorific
value of the exothermic peak. The greater is the exothermic
calorific value .DELTA.H.sub.exo:2, the lower is the degree of
crystallization of the sample. The endothermic peak is attributed
to a heat of fusion, i.e. a heat absorbed by the sample as a result
of the fusion of the crystals thereof during the course of heating
at a rate of 2.degree. C./minute in the DSC measurement. The
endothermic calorific value .DELTA.H.sub.endo:2 is the calorific
value of the endothermic peak. The greater the endothermic
calorific value .DELTA.H.sub.endo:2, the greater is the
crystallizability of the foamed sheet (therefore, the higher are
the rigidity and the ability to improve heat resistance of a foam
molding obtained from the foamed sheet). The difference
.DELTA.H.sub.x(.DELTA.H.sub.x=.DELTA.H.sub.endo:2-.DELTA.H.sub.exo:2)
represents a calorific value required for fusing the crystals
originally contained in the foamed sheet sample before the DSC
measurement. The smaller the difference .DELTA.H.sub.x is, the
lower is the degree of crystallization of the foamed sheet before
the DSC measurement. Thus, a foamed sheet having .DELTA.H.sub.x of
less than 40J/g (including 0 J/g) has not a high degree of
crystallinity and is excellent in thermoformability. A foamed sheet
having .DELTA.H.sub.endo:2 of at least 10 J/g can exhibit excellent
rigidity and heat resistance when treated, for example, by heat
treatment, so as to have an increased degree of crystallinity. When
the difference .DELTA.H.sub.x is 40 J/g or greater, the
thermoformability of the foamed sheet is not satisfactory. In
particular, it is difficult to thermoform such a foamed sheet into
an open-topped foamed receptacle having a large draw ratio of S2/S1
(where S1 represents an area of the top opening thereof and S2
represents an inside surface area thereof) of 1.5 or more,
particularly 1.8 or more.
[0068] The difference .DELTA.H.sub.x is preferably 1 to less than
20 J/g, more preferably 2 to 18 J/g, from the standpoint of very
excellent deep drawability of the foamed sheet. The the difference
.DELTA.H.sub.x is preferably 20 to less than 40 J/g, more
preferably 20 to 35 J/g, from the standpoint of improved efficiency
of a heat treatment which is conducted for the purpose of improving
the heat resistance of the thermoformed product, though the deep
drawability is reduced a little.
[0069] The endothermic calorific value .DELTA.H.sub.endo:2 of the
foamed sheet should be at least 10 J/g, since otherwise
satisfactory rigidity and heat resistance cannot be obtained even
when the foamed sheet is heat treated for increasing the degree of
crystallinity. The .DELTA.H.sub.endo:2 is preferably at least 20
J/g, more preferably at least 25 J/g, most preferably at least 30
J/g. The upper limit of .DELTA.H.sub.endo:2 is not specifically
limited but is generally about 60 J/g. The exothermic calorific
value .DELTA.H.sub.exo:2 of the foamed sheet can be 0 J/g.
[0070] As described previously, the exothermic peak is attributed
to a heat generated from the sample foamed sheet as a result of the
crystallization thereof during the course of heating at a rate of
2.degree. C./minute in the DSC measurement. The greater is the
.DELTA.H.sub.exo:2 of the exothermic peak, the lower is the degree
of crystallization of the sample. From the standpoint of improved
heat resistance of the foam molding attained during or after the
thermoforming, .DELTA.H.sub.exo:2 is desired to be high. Thus, the
foamed sheet preferably has .DELTA.H.sub.exo:2 of at least 3 J/g,
more preferably at least 5 J/g, still more preferably at least 15
J/g, most preferably at least 20 J/g. The upper limit of
.DELTA.H.sub.exo:2 is not specifically limited but is generally
about 60 J/g.
[0071] The terms "endothermic calorific value .DELTA.H.sub.endo:2"
and "exothermic calorific value .DELTA.H.sub.exo:2" as used herein
are determined from DSC curve of heat flux differential scanning
calorimetry in accordance with JIS K7122-1987. The DSC measurement
is carried out as follows. A sample (about 1 to 4 mg) of a foamed
sheet is charged in a pan of a differential scanning calorimeter.
Without performing any pretreatment for the adjustment of the
conditions of the sample, the DSC curve of the sample is measured
while heating the sample at 2.degree. C./minute to a temperature
higher by about 30.degree. C. than the temperature at which the
endothermic peak meets the base line to melt the sample. FIG. 5
depicts an example of such a DSC curve. In FIG. 5, there is an
exothermic peak which begin from a point "c" where the DSC curve
begins separating from a low temperature-side base line BL1 and
which terminate at a point "d" where the DSC curve returns to a
high temperature-side base line BL2. The exothermic calorific value
.DELTA.H.sub.exo:2 is an integrafion of the exothermic peak area,
namely the area defined by a line passing the points "c" and "d"
and the exothermic curve. There is also an endothermic peak which
begin from a point "e" where the DSC curve begins separating from a
low temperature-side base line BL3 and which terminate at a point T
where the DSC curve returns to a high temperature-side base line
BL4. The endothermic calorific value .DELTA.H.sub.endo:2 is an
integration of the endothermic peak area, namely the area defined
by a line passing the points "e" and T and the endothermic curve.
The DSC device should be preferably operated so that each of the
base lines BL1 through BL4 is straight as shown in FIG. 5. When the
base line or lines are inevitably curved, the points "c" to "f" are
determined in the same manner as described with reference to FIG.
2. For example, when the base lines BL1 and BL2 are curved, the
curved base lines BL1 and BL2 are extended to the high temperature
side and low temperature side, respectively, with the radius of the
curvature of the base lines being maintained. The point at which
the DSC curve begins separating from the curved base line BL1 is
the point "c". The point "d" is a point where the DSC curve returns
to a curved base line BL2 on the high temperature-side. Similarly,
the points "e" and "f" are points where the DSC curve begins
separating from the curved base line BL3 and where the DSC curver
returns to a curved base line BL4, respectively.
[0072] When the exothermic and endothermic peaks are not separated
from each other as shown in FIG. 6, the points "c" and "f" are
first determined in the same manner as that in FIG. 5. Then, the
intersection between a line passing the points "c" and "f" and the
DSC curve is assigned as point "d(e)". The exothermic calorific
value .DELTA.H.sub.exo:2 is an area defined by a line passing the
points "c" and "d(e)" and the exothermic curve, while the
endothermic calorific value .DELTA.H.sub.endo:2 is an area defined
by a line passing the points "d(e)" and "f" and the endothermic
curve.
[0073] When there are two or more exothermic and/or endothermic
peaks, the exothermic calorific value .DELTA.H.sub.exo:2 and/or the
endothermic calorific value .DELTA.H.sub.endo:2 are each a total of
the area of respective peaks. For example, when there are two,
first and second exothermic peaks A and B and one endothermic peak
C and when the peaks B and C are continuous, as shown in FIG. 7,
the exothermic calorific value .DELTA.H.sub.exo:2 is a sum of an
area defined by a line passing through points "c" and "d" and the
first exothermic peak A and an area defined by a line passing
through points "g" and "e" and the second exothermic peak B. In
this case, the points "c" and "d" are similar to those in FIG. 5,
while the points "g" and "e" are similar to the points "c" and
"d(e)" in FIG. 6. Thus, the point "e" in FIG. 7 is an intersection
between a line passing the points "g" and "f" and the DSC curve.
The endothermic calorific value .DELTA.H.sub.endo:2 is an area
defined by the line passing the points "e" and "f" and the
endothermic peak C.
[0074] In the above-described heat flux differential scanning
calorimetry, a heating rate of 2.degree. C./minute has been found
to be suitable for obtaining a DSC curve in which endothermic and
exothermic peaks are independently present and for measuring
precise endothermic and exothermic calorific values
.DELTA.H.sub.endo:2 and .DELTA.H.sub.exo:2.
[0075] It is preferred that the foamed sheet of the present
invention have a heat sag of at least 10 mm for reasons of improved
thermoformability. As used herein, the "heat sag" is as measured in
accordance with JIS K7195-1993. More specifically, from a foamed
sheet, a specimen having a length of 125 mm, a width of 10 mm and a
thickness of 3 mm is cut out such that at least one side of the
specimen is the surface of the foamed sheet. When the thickness of
the foamed sheet is less than 3 mm, the thickness of the foamed
sheet is the thickness of the specimen. The specimen is fixed to a
specimen holder with the surface of the foamed sheet facing upward.
The test temperature is 75.degree. C.
[0076] It is also preferred that the foamed sheet have an
exothermic calorific value .DELTA.H.sub.exo:10, as measured by heat
flux differential scanning calorimetry at a cooling rate of
10.degree. C./min, of at least 20 J/g, since the crystallization of
the foamed sheet can proceed within a short time by a heat
treatment. Thus, a foam molding having an excellent heat resistance
may be obtained by thermoforming the foamed sheet, followed by the
heat treatment. The upper limit of .DELTA.H.sub.exo:10 is
preferably about 60 J/g, since premature crystallization of the
foamed sheet during the extrusion foaming stage can be prevented
and since the foamed sheet can be thermoformed with good
thermoformability and with a high draw ratio.
[0077] Particularly, when the .DELTA.H.sub.exo:10 of the foamed
sheet is in the range of 20 to 45 J/g, the foamed sheet exhibits a
suitable crystallization speed so that the thermoforming of the
foamed sheet can be carried out without difficulty even at a high
draw ratio and the thermoformed product obtained therefrom can be
imparted with a high degree of crystallization and an improved heat
resistance by heat treatment. Thus, the .DELTA.H.sub.exo:10 is more
preferably 20 to 45 J/g, still more preferably 25 to 40 J/g, most
preferably 30 to 38 J/g.
[0078] The DSC analysis at a cooling rate of 2 .degree. C./min
cannot properly determine whether or not a given foamed sheet has a
crystallization speed suitable both for thermoforming and for heat
treating a thermoformed product obtained from the foamed sheet,
since crystallization can proceed during the DSC measurement even
when the foamed sheet has a low crystallization speed.
[0079] The "exothermic calorific value .DELTA.H.sub.exo:10" as used
herein is a heat of crystallization determined from DSC curve of
heat flux differential scanning calorimetry in accordance with JIS
K7122-1987. The DSC measurement is carried out in nearly the same
manner as that for the above-described measurement of
.DELTA.H.sub.endo:2 and is as follows. A sample (about 1 to 4 mg)
of a foamed sheet is charged in a pan of a differential scanning
calorimeter. The sample is heated to a temperature higher by about
30.degree. C. than the temperature at which the endothermic peak
meets the base line to melt the sample and maintained at that
temperature for 10 minutes. Thereafter, the sample is measured for
a DSC curve while cooling the sample at a cooling rate of 10
.degree. C./min. The exothermic calorific value .DELTA.H.sub.exo:10
is an integration of the exothermic peak area, namely the area
defined by a line, which passes the point where the DSC curve
begins separating from a high temperature-side base line and the
point where DSC curve returns to a low temperature-side base line,
and the endothermic curve. The DSC device should be preferably
operated so that each of the base lines is straight. When the base
line or lines are inevitably curved, the two points are determined
in the same manner as described previously.
[0080] It is further preferred that the foamed sheet have a half
crystallization time (half crystallization time) of 2 to 200
seconds, more preferably 10 to 150 seconds, most preferably 20 to
120 seconds, at 110.degree. C., since the degree of crystallization
of a thermoformed product obtained from the foamed sheet can be
increased by a short time heat treatment and since premature
crystallization of the foamed sheet during the extrusion foaming
stage can be prevented so that the foamed sheet can be thermoformed
with good thermoformability and with a high draw ratio.
[0081] The "half crystallization time" as used herein is as
measured using a crystallization speed analyzer (Model MK-801
manufactured by Kotaki Shoji Co., Ltd. (currently Shimadzu Science
West Corporation.)). A foam sheet is defoamed into a film having a
thickness of 0.1.+-.0.02 mm. Alternatively, a similar film is
prepared in such a manner that no foam is contained. The film is
cut into a square of a 15 mm.times.15 mm size to obtain a sample.
The sample is held by a cover glass for a microscope, heated to
200.degree. C. and then placed in a crystallization bath maintained
at 110.degree. C. An indicated voltage of 3 V is selected for
setting the brightness of the lamp for a light source. The
crystallization speed analyzer utilizes a relationship between
crystallization and birefringence. Namely, since the birefringence
increases as the crystallization proceeds, the change of the
quantity of the light transmitted from the sample is monitored by a
photoelectric detector. FIG. 8 shows an example of a chart obtained
by the analyzer, in which the voltage outputted from the
photoelectric detector is plotted as a function of crystallization
time until a constant voltage "A" is reached. The point T at which
the voltage is A/2 represents the half-crystallization time.
[0082] The foamed sheet having .DELTA.H.sub.exo:10 of at least 20
J/g and/or the half crystallization of 2 to 200 seconds may be
obtained by incorporating thereinto an inorganic nucleating agent
such as talc, silica, zeolite, kaolin, montmorillonite, bentonite,
clay, magnesium carbonate, aluminum oxide or calcium sulfate, in an
amount of 0.05 to 10 parts by weight, preferably 0.1 to B parts by
weight, particularly preferably 0.1 to less than 4 parts by weight,
per 100 parts by weight of the base resin. The nucleating agent is
preferably a silicate such as talc or a layered silicate such as
montomorillonite. The crystallization speed may be further improved
by forming a nanocomposite in which nanoparticles of the nucleating
agent are dispersed in the base resin.
[0083] If desired, the foamed sheet of the present invention may be
formed into a composite sheet in which a thermoplastic resin layer
is provided on one or both sides thereof by bonding with an
adhesive, by fusion bonding, by coextrusion, by extrusion
lamination or any other suitable laminating method. The
thermoplastic resin may be, for example, a polyethylene resin, a
polypropylene resin, a polyester resin, a polystyrene resin or a
polyamide resin. Particularly preferred thermoplastic resin is a
biodegradable resin such as a polyester resin containing at least
35 mol % of aliphatic ester monomer units.
[0084] The foamed sheet of the present invention is suitably used
as a raw material sheet for the production of foam moldings. Thus,
the foamed sheet is thermoformed by any suitable known method, such
as by vacuum molding, air pressure forming, matched molding or plug
assist molding, into open-topped foam receptacles such as trays,
cups, mugs, bowls and saucers. The thermoforming temperature is
generally 40 to 120.degree. C.
[0085] In one preferred embodiment, an open-topped foam receptacle
having a draw ratio of S2/S1 where S1 represents an area of the top
opening thereof and S2 represents an inside surface area thereof is
thermoformed using a foamed sheet having such .DELTA.H.sub.x
(.DELTA.H.sub.x=.DELTA.H.sub.endo:2-.DELTA.H.sub.exo:2) that
satisfies the following equation:
S2/S1 .ltoreq.-0.08.DELTA.H.sub.x+4.2.
Namely, depending on the draw ratio of a desired foam molding, the
crystallinity of the foamed sheet used as a raw material is
determined so as to meet the above condition. By using such a
foamed sheet, it is possible to produce foam moldings with good
thermoformability within a short molding cycle. When the foamed
sheet used does not meet the above condition, the foamed sheet
fails to be sufficiently drawn so that the wall thickness of the
receptacle becomes non-uniform and cracks are apt to form. More
preferably, the foamed sheet satisfies the following equation:
S2/S1 .ltoreq.-0.07.DELTA.H.sub.x+3.8.
[0086] The .DELTA.H.sub.x may be adjusted by controlling the
composition of the base resin and by controlling the extrusion
molding conditions. More particularly, .DELTA.H.sub.exo:2 can be
increased by rapidly cooling the extrudate immediately after the
extrusion with blowing air or mist, while .DELTA.H.sub.endo:2 may
be increased by increasing the proportion of crystalline polylactic
acid resin in the base resin.
[0087] The draw ratio S2/S1 is generally 1.1 to 4.2, preferably 1.1
to 3.4.
[0088] The foam molding obtained by thermoforming the foamed sheet
is desirably subjected to a heat treatment. Preferably, the foam
molding is heated at 80 to 130.degree. C., more preferably 90 to
120.degree. C., for 10 to 60 seconds, so that the crystallization
effectively proceeds to improve the heat resistance of the foam
molding while preventing reduction of the mechanical strength and
deformation of the foam molding.
[0089] The heat treatment may be also preferably performed by aging
the foam molding at a temperature higher than the glass transition
temperature of the polylactic acid resin but not causing
deformation thereof, preferably at a temperature of 60 to
80.degree. C., for 6 to 36 hours, so that the crystallization
effectively proceeds to improve the heat resistance of the foam
molding.
[0090] The "glass transition point" as used herein is measured in
accordance with JIS K7121-1987 and is calculated from the midpoint
of a heat flux of a DSC curve obtained by heat flux differential
scanning calorimetry at a heating rate of 10.degree. C./minute. The
"glass transition point is measured after the sample has been heat
treated under specified conditions" as described in JIS K7121-1987,
Paragraph 3, Control of conditions of sample (3). Namely, a sample
is placed in a pan of a DSC device and heated to 200.degree. C. at
a heating rate of 10.degree. C./minute and maintained at
200.degree. C. for 10 minutes. The melted sample is then cooled to
0.degree. C. at a cooling rate of 10.degree. C./minute. The
resulting sample is then subjected to the DSC measurement.
[0091] It is preferred that the foam molding of the present
invention have a difference
(.DELTA.H.sub.endo:Mold-.DELTA.H.sub.exo:Mold) between an
endothermic calorific value .DELTA.H.sub.endo:Mold and an
exothermic calorific value .DELTA.H.sub.exo:Mold, as measured by
heat flux differential scanning calorimetry at a heating rate of
2.degree. C./min, is not less than 10 J/g, preferably not less than
15 J/g, more preferably not less than 20 J/g, most preferably not
less than 25 J/g, for reasons of high degree of crystallization and
excellent rigidity and heat resistance. The difference
(.DELTA.H.sub.endo:Mold-.DELTA.H.sub.exo:Mold) represents a heat
required for fusing the crystals contained in the foam molding
before the DSC measurement. Thus, the greater the difference, the
higher is the degree of crystallization of the foam molding. The
upper limit of the difference
(.DELTA.H.sub.endo:Mold-.DELTA.H.sub.exo:Mold) is not specifically
limited but is generally about 60 J/g. The .DELTA.H.sub.exo:Mold of
the foam molding can be zero.
[0092] The .DELTA.H.sub.endo:Mold and .DELTA.H.sub.exo:Mold of the
foam molding may be measured in accordance with JIS K7122-1987.
Except that a sample (1-4 mg) is cut out from the foam molding, the
DSC measurement is carried out in the same manner as that for the
above-described measurement of endothermic calorific value
.DELTA.H.sub.endo:2 and exothermic calorific value
.DELTA.H.sub.exo:2 of the foamed sheet.
[0093] The foam molding according to the present invention, which
is biodegradable in nature, may be suitably used as packaging
receptacles such as food receptacles (e.g. lunch trays, noodle
bowls, fruit and vegetable containers, etc.) and cushion
receptacles for various articles such as electrical appliances and
precision instruments.
[0094] The following examples and comparative examples will further
illustrate the present invention. Parts and percentages are by
weight except otherwise specifically noted.
[0095] Resins A to E used in Examples and Comparative Examples as
raw material resins for foamed sheets are as shown in Table 1
below. Resins A to D were prepared as follows. To a two-axis
extruder, 100 parts of crystalline polylactic acid resin (Trade
name: H-100, manufactured by Mitsui Chemical Corporation, density:
1,260 kg/m.sup.3, endothermic calorific value
.DELTA.H.sub.endo:Materiail49 J/g) and dicumyl peroxide (DCP) in an
amount shown in Table 1 were fed. The mixture was heated to melt
the resin and kneaded. The melt was adjusted to a temperature of
215.degree. C. and extruded in the form of strands. The strands
were immersed into water at about 25.degree. C. and cut into
pellets, thereby obtaining Resins A to D. The crystalline
polylactic acid resin H-100 was used Resin E as such. The melt
tensions and half crystallization times of Resins A to E are shown
in Table 1.
TABLE-US-00001 TABLE 1 Raw material resin Resin A Resin B Resin C
Resin D Resin E Polylactic acid H-100 H-100 H-100 H-100 H-100 resin
Peroxide Kind DCP DCP DCP DCP -- Amount 0.4 0.35 0.45 0.6 --
(parts) Melt tension (cN) 16 13 20 27 0.4 Half crystallization 109
122 97 73 1097 time (sec)
EXAMPLE 1
[0096] Two, first and second extruders having inside diameters of
90 mm and 120 mm were connected in tandem and used for the
preparation of a foamed sheet. Thus, Resin A and the cell
controlling agent shown in Table 2 are fed to the first extruder in
the amounts shown in Table 2 and heated and kneaded to obtain a
melt. The melt was in the first extruder was kneaded with the
blowing agent shown in Table 2 in the amount shown in Table 2. The
resulting kneaded mass was then fed to the second extruder and
cooled therein to 171.degree. C. and extruded through a circular
die having a diameter of 110 mm and a lip clearance of 0.5 mm to
obtain a tubular extrudate. The tubular extrudate was hauled, while
being cooled, and longitudinally (in the extrusion direction) cut
and opened to obtain a foamed sheet. The cooling of the tubular
extrudate was carried out by blowing air on the inside surface of
the tubular extrudate immediately after extrusion at a rate of 0.4
m.sup.3/min (23.degree. C., 1 atm) while blowing air around the
outside thereof at a rate of 0.9 m.sup.3/min (23.degree. C., 1 atm)
and by sliding the tubular extrudate over a mandrel (adjusted to
5.degree. C. and having a diameter of 333 mm) of a cooling
device.
EXAMPLE 2
[0097] A foamed sheet was prepared in the same manner as described
in Example 1 except that the kind and amount of the blowing agent
and the amount of the cell controlling agent were changed as shown
in Table 2 and that the kneaded mass was cooled to 172.degree. C.
in the second extruder and extruded through a circular die having a
diameter of 135 mm and a lip clearance of 0.5 mm.
EXAMPLE 3
[0098] A foamed sheet was prepared in the same manner as described
in Example 1 except that Resin C was used in place of Resin A, that
the kind and amount of the blowing agent were changed as shown in
Table 2 and that the kneaded mass was cooled to 167.degree. C. in
the second extruder and extruded through a circular die having a
diameter of 90 mm and a lip clearance of 0.5 mm.
EXAMPLE 4
[0099] A foamed sheet was prepared in the same manner as described
in Example 1 except that Resin E was used in place of Resin A, that
the kinds and amounts of the blowing agent and the cell controlling
agent were changed as shown in Table 2 and that the kneaded mass
was cooled to 183.degree. C. in the second extruder and extruded
through a circular die having a diameter of 135 mm and a lip
clearance of 0.5 mm.
EXAMPLE 5
[0100] A foamed sheet was prepared in the same manner as described
in Example 1 except that Resin B was used in place of Resin A, that
the kind and amount of the blowing agent were changed as shown in
Table 2 and that the kneaded mass was cooled to 180.degree. C. in
the second extruder and extruded through a circular die having a
diameter of 135 mm and a lip clearance of 0.5 mm.
EXAMPLE 6
[0101] A foamed sheet was prepared in the same manner as described
in Example 1 except that Resin E was used in place of Resin A, that
0.4 part of DCP per 100 parts of Resin E was fed together with
Resin E to the first extruder, that the kind and amount of the
blowing agent and the amount of the cell controlling agent were
changed as shown in Table 2 and that the kneaded mass was cooled to
170.degree. C. in the second extruder and extruded through a
circular die having a diameter of 135 mm and a lip clearance of 0.5
mm.
EXAMPLE 7
[0102] A foamed sheet was prepared in the same manner as described
in Example 1 except that the kind and amount of the blowing agent
and the amount of the cell controlling agent were changed as shown
in Table 2 and that the kneaded mass was cooled to 170.degree. C.
in the second extruder and extruded through a circular die having a
diameter of 135 mm and a lip clearance of 0.5 mm.
EXAMPLE 8
[0103] A foamed sheet was prepared in the same manner as described
in Example 1 except that Resin D was used in place of Resin A, that
the kind and amount of the blowing agent and the amount of the cell
controlling agent were changed as shown in Table 2 and that the
kneaded mass was cooled to 170.degree. C. in the second extruder
and extruded through a circular die having a diameter of 135 mm and
a lip clearance of 0.5 mm.
EXAMPLE 9
[0104] A foamed sheet was prepared in the same manner as described
in Example 1 except that the amount of the blowing agent was
changed as shown in Table 3 and that the kneaded mass was cooled to
174.degree. C. in the second extruder and extruded through a
circular die having a diameter of 135 mm and a lip clearance of 0.5
mm.
EXAMPLE 10
[0105] A foamed sheet was prepared in the same manner as described
in Example 1 except that Resin C was used in place of Resin A, that
the kind and amount of the blowing agent and the amount of the cell
controlling agent were changed as shown in Table 3 and that the
kneaded mass was cooled to 167.degree. C. in the second extruder
and extruded through a circular die having a diameter of 90 mm and
a lip clearance of 0.5 mm.
EXAMPLE 11
[0106] A foamed sheet was prepared in the same manner as described
in Example 1 except that Resin C was used in place of Resin A, that
the kind and amount of the blowing agent and the amount of the cell
controlling agent were changed as shown in Table 3 and that the
kneaded mass was cooled to 181.degree. C. in the second extruder
and extruded through a circular die having a diameter of 135 mm and
a lip clearance of 0.5 mm.
[0107] Each of the foamed sheets obtained in Examples 1-11 was
measured or tested for apparent density, thickness, closed cell
content, cell geometry (Z, Z/X and Z/Y), exothermic calorific value
at a heating rate of 2.degree. C./min (.DELTA.H.sub.exo:2),
endothermic calorific value at a heating rate of 2.degree. C./min
(.DELTA.H.sub.endo:2), difference (.DELTA.H.sub.x) between
.DELTA.H.sub.exo:2 and .DELTA.H.sub.endo:2
(.DELTA.H.sub.x=.DELTA.H.sub.endo:2-.DELTA.H.sub.exo:2), exothermic
calorific value (.DELTA.H.sub.exo:10) at a cooling rate of
10.degree. C./min, half crystallization time, melt tension, heat
sag, moldability, appearance and ability to improve heat
resistance. The results are shown in Table 2 and Table 3.
[0108] In the measurement the average cell diameters X (in the
extrusion direction) and Z (in the thickness direction), the foamed
sheet was cut along the centerline in the extrusion direction
thereof so as to divide the sheet in equal halves.
[0109] The DSC analysis for the measurement of the exothermic and
endothermic calorific value was carried out using Shimadzu Heat
Flux Differential Scanning Calorimeter DSC-50 (manufactured by
SHIMADZU corporation) and a data analyzing software (Partial Area
Analyzing Program version 1.52 for Shimadzu Thermoanalysis Work
Station TA-60WS).
TABLE-US-00002 TABLE 2 Example No. 1 2 3 4 5 6 7 8 Raw material
resin Resin A Resin A Resin C Resin E Resin B Resin E Resin A Resin
D Kind and proportion of n-butane 35 70 -- 30 -- 70 70 70 blowing
agent isobutane 65 30 100 70 -- 30 30 30 components (mol %)
CO.sub.2 -- -- -- -- 100 -- -- -- Amount of blowing agent (%) 2.3
2.5 4.0 1.0 0.8 2.5 2.5 2.5 Cell controlling agent talc talc talc
citric talc talc talc talc acid Amount of cell controlling agent
0.1 3.0 0.1 0.5 0.1 2.0 1.2 3.0 (part per 100 parts of the raw
material resin) Apparent density (kg/m.sup.3) 180 250 97 573 420
250 200 263 Thickness (mm) 1.4 1.3 2.0 0.6 1.0 1.3 1.5 1.3 Closed
cell content (%) 87 82 85 15 85 84 87 88 Z (mm) 0.50 0.15 0.16 0.20
0.10 0.20 0.47 0.18 Z/X 0.63 0.41 0.69 0.58 0.30 0.50 0.60 0.50 Z/Y
0.58 0.39 0.53 0.43 0.26 0.43 0.55 0.45 .DELTA.H.sub.exo:2 (J/g)
36.0 29.2 36.0 33.2 36.3 27.3 27.4 17.8 .DELTA.H.sub.endo:2 (J/g)
39.0 37.0 39.5 33.9 38.7 34.5 34.3 33.2 .DELTA.H.sub.x (J/g) 3.0
7.8 3.5 0.7 2.4 7.2 6.9 15.4 .DELTA.H.sub.exo:10 (J/g) 33.0 36.9
33.0 1.0 33.0 37.0 33.4 34.0 Half crystallization time (sec) 109 69
97 1097 120 72 79 60 Melt tension (cN) 16 16 20 0.4 13 15 16 27
Heat sag (mm) over 30 25 over 30 over 30 over 30 25 25 15 Draw
ratio (S1/S2) 2.29 2.29 2.29 2.29 2.29 2.29 2.29 2.29 -0.08 .times.
.DELTA.H.sub.x + 4.2 3.96 3.58 3.92 4.14 4.01 3.62 3.65 2.96
Moldability A A A A A A A A Appearance A A A A A A A A Ability to
improve heat resistance B B B C B B B B
TABLE-US-00003 TABLE 3 Example No. 9 10 11 Raw material resin Resin
A Resin C Resin C Kind and proportion n-butane 35 -- -- of blowing
agent isobutane 65 100 -- components (mol %) CO.sub.2 -- -- 100
Amount of blowing agent (%) 1.8 4.5 0.6 Cell controlling agent talc
talc talc Amount of cell controlling agent (part 0.1 0.05 0.50 per
100 parts of the raw material resin) Apparent density (kg/m.sup.3)
252 105 573 Thickness (mm) 2.0 8 1.0 Closed cell content (%) 85 86
20 Z (mm) 0.20 2.5 0.03 Z/X 0.97 0.72 0.75 Z/Y 0.95 0.66 0.67
.DELTA.H.sub.exo:2 (J/g) 36.1 34.7 35.9 .DELTA.H.sub.endo:2 (J/g)
39.1 36.4 37.9 .DELTA.H.sub.x (J/g) 3.0 1.7 2.0 .DELTA.H.sub.exo:10
(J/g) 33.0 33.0 33.0 Half crystallization time (sec) 107 140 130
Melt tension (cN) 16 12 12 Heat sag (mm) over 30 over 30 over 30
Draw ratio (S1/S2) 2.29 2.29 2.29 -0.08 .times. .DELTA.H.sub.x +
4.2 3.96 4.06 4.04 Moldability B A B Appearance A B A Ability to
improve heat resistance B B B
[0110] The moldability, appearance and ability to improve heat
resistance shown in Tables 2 and 3 are evaluated as follows.
Moldability:
[0111] A foamed sheet is subjected to a thermoforming test using a
vacuum forming machine (Model FKS manufacture by Asano Laboratories
Co., Ltd.). All four side edges of the foamed sheet are clamped at
all its four side edges and both surfaces thereof are heated with a
heater to 40.degree. C. Then the foamed sheet is shaped in a mold
into a cup in the shape of an inverted circular truncated cone
having a top diameter of 125 mm, a bottom diameter of 110 mm and a
depth of 50 mm (draw ratio: 2.29). From the state of the molded cup
obtained, the moldability is evaluated according to the following
ratings:
[0112] A: The cup has uniform thickness and has no cracks in the
interior and exterior surfaces thereof.
[0113] B: The cup has slight variation in its thickness but has no
cracks in the interior and exterior surfaces thereof.
[0114] C: Cracks are formed in the interior and/or exterior surface
thereof.
Appearance:
[0115] A foamed sheet is observed with naked eyes. The appearance
is evaluated according to the following ratings:
[0116] A: The sheet has uniform surface gloss.
[0117] B: Cells are noticeably seen on the surface of the
sheet.
Ability to improve heat resistance:
[0118] From the exothermic calorific value .DELTA.H.sub.exo:10,
endothermic calorific value .DELTA.H.sub.endo:2 and difference
.DELTA.H.sub.x
(.DELTA.H.sub.x=.DELTA.H.sub.endo:2-.DELTA.H.sub.exo:2), evaluation
is made according to the following ratings:
[0119] A: .DELTA.H.sub.exo:10.gtoreq.20 J/g,
.DELTA.H.sub.endo:2.gtoreq.20 J/g and 40 J/g
>.DELTA.H.sub.x.gtoreq.20 J/g.
[0120] B: .DELTA.H.sub.exo:10.gtoreq.20 J/g,
.DELTA.H.sub.endo:2.gtoreq.20 J/g and .DELTA.H.sub.x<20 J/g.
[0121] C: .DELTA.H.sub.exo:10 <20 J/g,
.DELTA.H.sub.endo:2.gtoreq.20 J/g and .DELTA.H.sub.x<20 J/g.
[0122] D: .DELTA.H.sub.exo:10<20 J/g, .DELTA.H.sub.endo:2<20
J/g and .DELTA.H.sub.x<20 J/g.
Example 12
[0123] The foamed sheet obtained in Example 1 was thermoformed
using a vacuum forming machine (Model FKS manufacture by Asano
Laboratories Co., Ltd.). All four side edges of the foamed sheet
were clamped at all its four side edges and both surfaces thereof
were heated with a heater to 40.degree. C. Then the foamed sheet
was shaped in a mold into a cup in the shape of an inverted
circular truncated cone having a top diameter of 125 mm, a bottom
diameter of 110 mm and a depth of 50 mm (draw ratio: 2.29). The
resulting molded cup was then heat treated by being held at
90.degree. C. in the mold for 30 seconds.
Example 13
[0124] Example 12 was repeated in the same manner as described
except that the foamed sheet obtained in Example 2 was used and
that the heat treatment was performed at 110.degree. C. for 30
seconds.
Example 14
[0125] Example 12 was repeated in the same manner as described
except that the foamed sheet obtained in Example 6 was used.
Example 15
[0126] Example 12 was repeated in the same manner as described
except that the foamed sheet obtained in Example 8 was used and
that the heat treatment was performed at 90.degree. C. for 15
seconds.
Example 16
[0127] Example 12 was repeated in the same manner as described
except that the foamed sheet obtained in Example 8 was used.
Comparative Example 1
[0128] Example 12 was repeated in the same manner as described
except no heat treatment was performed.
[0129] Each of the molded cups obtained in Examples 12-16 and
Comparative Example 1 was measured or tested for exothermic
calorific value .DELTA.H.sub.exo:Mold, endothermic calorific value
.DELTA.H.sub.endo:Mold, difference
(.DELTA.H.sub.endo:Mold-.DELTA.H.sub.exo:Mold) and heat resistance.
The results are shown in Table 4.
TABLE-US-00004 TABLE 4 Comparative Example No. Example No. 12 13 14
15 16 1 Foamed sheet Example 1 Example 2 Example 6 Example 8
Example 8 Example 1 Molded product cup cup cup cup cup cup
.DELTA.H.sub.exo:Mold (J/g) 20.7 0 7.0 11.7 1.2 29.8
.DELTA.H.sub.endo:Mold (J/g) 41.9 42.9 35.2 33.8 33.8 34.8
.DELTA.H.sub.endo:Mold - 21.2 42.9 28.2 22.1 32.6 5.0
.DELTA.H.sub.exo:Mold (J/g) Heat treatment 90.degree. C.
110.degree. C. 90.degree. C. 90.degree. C. 90.degree. C. --
temperature Heat treatment 30 sec 30 sec 30 sec 15 sec 30 sec --
time Heat resistance B A A B A C
[0130] The heat resistance in Table 4 was tested by heating a
molded cup specimen at 70.degree. C. and 90.degree. C. in an oven
for 5 minutes. Whether or not deformation of the specimen occurred
was determined. The heat resistance was evaluated according to the
following ratings:
[0131] A: No deformation by heating at 90.degree. C.
[0132] B: No deformation by heating at 70.degree. C. but
significant deformation by heating at 90.degree. C.
[0133] C: Significant deformation by heating at 70.degree. C.
Example 17
[0134] A foamed sheet was prepared in the same manner as described
in Example 1 except that Resin D was used in place of Resin A and
that the kind and amount of the blowing agent and the amount of the
cell controlling agent were changed as shown in Table 5. The foamed
sheet thus obtained was thermoformed using a vacuum forming machine
(Model FKS manufacture by Asano Laboratories Co., Ltd.). All four
side edges of the foamed sheet were clamped at all its four side
edges and both surfaces thereof were heated with a heater to
70.degree. C. The thus obtained foamed sheet was shaped in a mold
into a cup in the shape of an inverted circular truncated cone
having a top diameter of 165 mm, a bottom diameter of 110 mm and a
depth of 50 mm (draw ratio: 1.60). The resulting molded cup was
then heat treated by being held at 110.degree. C. in the mold for
15 seconds.
[0135] When the above foamed sheet was subjected to deep drawing by
shaping in a mold adapted to form a cup in the shape of an inverted
circular truncated cone having a top diameter of 130 mm, a bottom
diameter of 90 mm and a depth of 105 mm (draw ratio: 3.26), it was
not possible to obtain such a cup.
Example 18
[0136] A foamed sheet was obtained in the same manner as that in
Example 17 except that the kinds of the blowing agent and the
amount of the cell controlling agent were changed as shown in Table
5 and that flow rate of air blown around the outside of the tubular
extrudate immediately after the extrusion was at a rate of 1.2
m.sup.3/min (23.degree. C., 1 atm). The thus obtained foamed sheet
was shaped in a mold into a cup in the shape of an inverted
circular truncated cone having a top diameter of 130 mm, a bottom
diameter of 100 mm and a depth of 70 mm (draw ratio: 2.54). The
resulting molded cup was then heat treated by being held at
110.degree. C. in the mold for 30 seconds.
[0137] When the above foamed sheet was subjected to deep drawing by
shaping in a mold adapted to form a cup in the shape of an inverted
circular truncated cone having a top diameter of 130 mm, a bottom
diameter of 90 mm and a depth of 105 mm (draw ratio: 3.26), it was
not possible to obtain such a cup.
Example 19
[0138] A foamed sheet was obtained in the same manner as that in
Example 17 except that Resin A was used in place of Resin D, that
the amount of the cell controlling agent was changed as shown in
Table 5 and that the kneaded mass was cooled to 169.degree. C. in
the second extruder and extruded through a circular die having a
diameter of 90 mm and a lip clearance of 0.5 mm. The thus obtained
foamed sheet was shaped in a mold into a cup in the shape of an
inverted circular truncated cone having a top diameter of 130 mm, a
bottom diameter of 90 mm and a depth of 105 mm (draw ratio: 3.26).
The resulting molded cup was then heat treated by being held at
110.degree. C. in the mold for 30 seconds.
Example 20
[0139] A foamed sheet was obtained in the same manner as that in
Example 17 except that the amount of the blowing agent and the
amount of the cell controlling agent were changed as shown in Table
5, that the kneaded mass was cooled to 174.degree. C. in the second
extruder and extruded through a circular die having a diameter of
135 mm and a lip clearance of 0.5 mm and that the flow rate of air
blown around the outside of the tubular extrudate immediately after
the extrusion was at a rate of 0.6 m.sup.3/min (23.degree. C., 1
atm). The thus obtained foamed sheet was shaped in a mold into a
cup in the shape of an inverted circular truncated cone having a
top diameter of 180 mm, a bottom diameter of 130 mm and a depth of
25 mm (draw ratio: 1.20). The resulting molded cup was then heat
treated in the same manner as described in Example 17.
[0140] When the above foamed sheet was subjected to deep drawing by
shaping in a mold adapted to form a cup in the shape of an inverted
circular truncated cone having a top diameter of 125 mm, a bottom
diameter of 110 mm and a depth of 50 mm (draw ratio: 2.30), it was
not possible to obtain such a cup.
Example 21
[0141] A foamed sheet was obtained in the same manner as that in
Example 17 except that Resin E was used in place of Resin D, that
0.4 part of DCP per 100 parts of Resin E was fed together with
Resin E to the first extruder, that the kind and the amount of the
blowing agent and the amount of the cell controlling agent were
changed as shown in Table 5 and that the kneaded mass was cooled to
175.degree. C. in the second extruder and extruded through a
circular die having a diameter of 135 mm and a lip clearance of 0.5
mm. The thus obtained foamed sheet was shaped in a mold into a cup
in the shape of an inverted circular truncated cone having a top
diameter of 130 mm, a bottom diameter of 100 mm and a depth of 70
mm (draw ratio: 2.54). The resulting molded cup was then heat
treated by being held at 110.degree. C. in the mold for 60
seconds.
Example 22
[0142] A foamed sheet was obtained in the same manner as that in
Example 17 except that the kind and the amount of the blowing agent
and the amount of the cell controlling agent were changed as shown
in Table 5 and that the flow rate of air blown around the outside
of the tubular extrudate immediately after the extrusion was at a
rate of 0.6 m.sup.3/min (23.degree. C., 1 atm). Then, in the same
manner as described in Example 17, the foamed sheet was shaped into
a cup and the cup was heat treated.
Comparative Example 2
[0143] In this example, a mixed resin composed of 25% of Resin D
and 75% of Resin F was used as a raw material resin. Resin F is a
non-crystalline polylactic acid resin (trade name: H-280,
manufactured by Mitsui Chemical Corporation) having a melt tension
of 1.6 cN. The mixed resin had a melt tension of 9 cN. A foamed
sheet was obtained in the same manner as that in Example 17 except
that the mixed resin was used in place of Resin D and that the
amount of the cell controlling agent was changed as shown in Table
5. The thus obtained foamed sheet was shaped in a mold into a cup
in the shape of an inverted circular truncated cone having a top
diameter of 130 mm, a bottom diameter of 90 mm and a depth of 105
mm (draw ratio: 3.26). The resulting molded cup was then heat
treated by being held at 110.degree. C. in the mold for 600
seconds.
[0144] Each of the foamed sheets obtained in Examples 17-22 and
Comparative Example 2 was measured or tested for apparent density,
thickness, closed cell content, cell geometry (Z, Z/X and Z/Y),
exothermic calorific value at a heating rate of 2.degree. C./min
(.DELTA.H.sub.exo:2), endothermic calorific value at a heating rate
of 2.degree. C./min (.DELTA.H.sub.endo:2), difference
(.DELTA.H.sub.x) between .DELTA.H.sub.exo:2 and
.DELTA.H.sub.endo:2(.DELTA.H.sub.x=.DELTA.H.sub.endo:2-.DELTA.H.sub.exo:2-
), exothermic calorific value (.DELTA.H.sub.exo:10) at a cooling
rate of 10.degree. C./min, melt tension of foam molding,
moldability, appearance, ability to improve heat resistance, and
heat resistance. The results are shown in Table 5.
TABLE-US-00005 TABLE 5 Comparative Example No. Example No. 17 18 19
20 21 22 2 Raw material resin Resin D Resin D Resin A Resin D Resin
E Resin D Resins D&F Kind and proportion of n-butane 70 -- 70
70 -- 35 70 blowing agent isobutane 30 100 30 30 -- 65 30
components (mol %) CO.sub.2 -- -- -- -- 100 -- -- Amount of blowing
agent (%) 2.5 2.5 2.5 1.2 0.8 2.2 2.5 Cell controlling agent talc
talc talc talc talc talc talc Amount of cell controlling agent 4.5
5.0 1.2 5.0 0.1 10 3.0 (part per 100 parts of the raw material
resin) Apparent density (kg/m.sup.3) 263 263 200 504 420 263 263
Thickness (mm) 1.5 1.5 1.5 0.8 1.0 1.4 1.3 Closed cell content (%)
88 87 87 85 85 28 88 Z (mm) 0.12 0.10 0.47 0.07 0.10 0.07 0.18 Z/X
0.50 0.40 0.60 0.40 0.30 0.40 0.41 Z/Y 0.45 0.40 0.55 0.40 0.26
0.38 0.39 .DELTA.H.sub.exo:2 (J/g) 11.8 19.2 27.4 5.0 36.3 5.5 8.5
.DELTA.H.sub.endo:2 (J/g) 33.2 34.9 34.3 34.8 38.7 31.8 9.0
.DELTA.H.sub.x (J/g) 21.4 15.7 6.9 29.8 2.4 26.3 0.5
.DELTA.H.sub.exo:10 (J/g) 32.3 34.2 33.4 33.9 33.0 31.5 8.2 Melt
tension of foam molding (cN) 9 9 7 9 6 9 3 -0.08 .times.
.DELTA.H.sub.x + 4.2 2.49 2.94 3.65 1.82 4.01 2.10 4.16 Draw ratio
(S1/S2) 1.60 2.54 3.26 1.20 2.54 1.60 3.26 Moldability A A A A A A
A Heat treatment temperature (.degree. C.) 110 110 110 110 110 110
110 conditions time (sec) 15 30 30 15 60 15 600 Appearance A A A A
A A A Ability to improve heat resistance A B B A B A D Heat
resistance of foamed cup A A A A A A C
[0145] The moldability in Table 5 was evaluated according to the
following ratings:
[0146] A: The cup has uniform thickness and has no cracks in the
interior and exterior surfaces thereof.
[0147] B: The cup has slight variation in its thickness but has no
cracks in the interior and exterior surfaces thereof.
[0148] C: Cracks are formed in the interior and/or exterior surface
thereof.
[0149] The heat resistance of cup in Table 5 was tested in the same
manner as that in Table 4. The appearance and ability to improve
heat resistance were tested in the same manner as that in Table
2.
[0150] The invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The present embodiments are therefore to be considered in
all respects as illustrative and not restrictive, the scope of the
invention being indicated by the appended claims rather than by the
foregoing description, and all the changes which come within the
meaning and range of equivalency of the claims are therefore
intended to be embraced therein.
* * * * *