U.S. patent application number 15/101437 was filed with the patent office on 2016-10-20 for foamed sheet.
This patent application is currently assigned to NITTO DENKO CORPORATION. The applicant listed for this patent is NITTO DENKO CORPORATION. Invention is credited to Kohei DOI, Kazumichi KATO, Tadao TAKAHASHI, Hideyuki TOKUYAMA.
Application Number | 20160303822 15/101437 |
Document ID | / |
Family ID | 55581137 |
Filed Date | 2016-10-20 |
United States Patent
Application |
20160303822 |
Kind Code |
A1 |
KATO; Kazumichi ; et
al. |
October 20, 2016 |
FOAMED SHEET
Abstract
The foamed sheet includes a foamed body having an average cell
diameter of 10 to 200 .mu.m, and a compression set at 80.degree. C.
of not more than 80% and an impact absorption change rate of not
more than .+-.20% defined by: Impact absorption change rate
(%)={(an impact absorption rate b after high-temperature
compression-an initial impact absorption rate a)/the initial impact
absorption rate a}.times.100, where the impact absorption rate a is
an impact absorption rate (%) of a test piece A and the impact
absorption rate b (%) after high-temperature compression is an
impact absorption rate (%) acquired by storing the test piece A at
80.degree. C. for 72 hours in the state of being compressed by 60%
with respect to the initial thickness of the test piece A,
thereafter releasing the compression state, and thereafter
conducting measurement after a lapse of 24 hours at 23.degree.
C.
Inventors: |
KATO; Kazumichi;
(Ibaraki-shi, JP) ; DOI; Kohei; (Ibaraki-shi,
JP) ; TOKUYAMA; Hideyuki; (Ibaraki-shi, JP) ;
TAKAHASHI; Tadao; (Ibaraki-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NITTO DENKO CORPORATION |
Ibaraki-shi |
|
JP |
|
|
Assignee: |
NITTO DENKO CORPORATION
Ibaraki-shi, Osaka
JP
|
Family ID: |
55581137 |
Appl. No.: |
15/101437 |
Filed: |
September 18, 2015 |
PCT Filed: |
September 18, 2015 |
PCT NO: |
PCT/JP2015/076729 |
371 Date: |
June 3, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C09J 2400/24 20130101;
B32B 2309/02 20130101; C09J 2301/312 20200801; G02F 2201/503
20130101; B32B 2266/0278 20130101; C08J 5/18 20130101; B32B 2309/04
20130101; C08J 2205/05 20130101; C09J 2433/006 20130101; B32B
2405/00 20130101; B32B 2307/306 20130101; B32B 2266/025 20130101;
C08J 9/12 20130101; C09J 7/26 20180101; C09J 201/00 20130101; C09J
2203/318 20130101; C09J 2203/326 20130101; C08J 2205/044 20130101;
C08J 2333/08 20130101; B32B 27/065 20130101; B32B 2307/308
20130101; C09J 2301/414 20200801; C08J 2333/06 20130101; C08J
2433/08 20130101; C08J 9/0061 20130101; C09J 2423/046 20130101;
B32B 5/18 20130101; C08J 9/365 20130101; B32B 2266/08 20130101;
C08J 9/30 20130101; C09J 2301/302 20200801; B32B 27/283 20130101;
B32B 2307/558 20130101; C08J 2207/00 20130101; C08J 2205/06
20130101; C09J 2433/00 20130101; C08J 2201/026 20130101; B32B
2305/022 20130101; B32B 2457/20 20130101; C09J 2475/006 20130101;
C08J 2491/00 20130101 |
International
Class: |
B32B 5/18 20060101
B32B005/18; C08J 9/00 20060101 C08J009/00; C09J 7/02 20060101
C09J007/02; C08J 5/18 20060101 C08J005/18; C08J 9/12 20060101
C08J009/12 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 24, 2014 |
JP |
2014-193576 |
Sep 24, 2014 |
JP |
2014-193577 |
Sep 24, 2014 |
JP |
2014-193578 |
Claims
1. A foamed sheet, comprising a foamed body having an average cell
diameter of 10 to 200 .mu.m, and having a compression set at
80.degree. C. of not more than 80% and an impact absorption change
rate of not more than .+-.20% as defined by the following: an
impact absorption change rate (%)={(an impact absorption rate b
after high-temperature compression-an initial impact absorption
rate a)/the initial impact absorption rate a}.times.100, wherein
the initial impact absorption rate a: an impact absorption rate (%)
of a test piece A; the impact absorption rate b (%) after
high-temperature compression: an impact absorption rate (%)
acquired by storing the test piece A at 80.degree. C. for 72 hours
in the state of being compressed by 60% with respect to an initial
thickness of the test piece A, thereafter releasing the compression
state, and thereafter conducting measurement after a lapse of 24
hours at 23.degree. C.; and the impact absorption rate: a value
defined by the following expression in an impact absorption test (a
weight of an impactor: 28 g, a swing-up angle: 40.degree.)
(23.degree. C.) using a pendulum impact tester: an impact
absorption rate (%)={(F.sub.0-F.sub.1)/F.sub.0}.times.100 wherein
F.sub.0 is an impact force when the impactor is made to collide
with a support plate alone; and F.sub.1 is an impact force when the
impactor is made to collide with the support plate of a structural
body composed of the support plate and the test piece A.
2. The foamed sheet according to claim 1, wherein the foamed sheet
has a thickness of 30 to 1,000 .mu.m; and the foamed body has an
apparent density of 0.2 to 0.7 g/cm.sup.3.
3. The foamed sheet according to claim 1, wherein the foamed body
has a peak top of a loss tangent (tan .delta.) in the range of not
less than -30.degree. C. and not more than 30.degree. C., the loss
tangent (tan .delta.) being a ratio of a loss elastic modulus to a
storage elastic modulus at an angular frequency of 1 rad/sec in a
dynamic viscoelasticity measurement.
4. The foamed sheet according to claim 1, wherein the foamed body
is formed of at least one resin material selected from the group
consisting of acrylic polymers, rubbers, urethanic polymers and
ethylene-vinyl acetate copolymers.
5. The foamed sheet according to claim 1, wherein the foamed body
is formed through step A of mechanically foaming an emulsion resin
composition.
6. The foamed sheet according to claim 5, wherein the foamed body
is formed further through step B of coating a base material with
the mechanically foamed emulsion resin composition followed by
drying.
7. The foamed sheet according to claim 6, wherein step B comprises
preliminary drying step B1 of drying the bubble-containing emulsion
resin composition applied on the base material at not less than
50.degree. C. and less than 125.degree. C., and regular drying step
B2 of thereafter further drying the resultant at not less than
125.degree. C. and not more than 200.degree. C.
8. The foamed sheet according to claim 1, wherein the foamed sheet
has a compression set at 80.degree. C. of not more than 50%.
9. The foamed sheet according to claim 8, wherein the foamed sheet
has the compression set at 80.degree. C. of not more than 25%.
10. The foamed sheet according to claim 2, wherein the foamed sheet
has a thickness of 40 to 500 .mu.m.
11. The foamed sheet according to claim 10, wherein the foamed
sheet has a thickness of 50 to 300 .mu.m.
12. The foamed sheet according to claim 2, wherein the foamed body
has an apparent density of 0.21 to 0.6 g/cm.sup.3.
13. The foamed sheet according to claim 12, wherein the foamed body
has an apparent density of 0.22 to 0.5 g/cm.sup.3.
14. The foamed sheet according to claim 1, wherein the foamed sheet
has a pressure-sensitive adhesive layer on one face or both faces
of the foamed body.
15. The foamed sheet according to claim 1, being used as an impact
absorption sheet for an electric or electronic device.
16. An electric or electronic device, using a foamed sheet
according to claim 1.
17. An electric or electronic device comprising a display member, a
housing, and the foamed sheet according to claim 1 between the
housing of the electric or electronic device and the display
member.
Description
TECHNICAL FIELD
[0001] The present invention relates to a foamed sheet which, even
if having a very small thickness, is excellent in impact absorption
and excellent in heat resistance, and to an electric or electronic
device using the foamed sheet.
BACKGROUND ART
[0002] There are conventionally used foamed materials when optical
members such as image display members fixed on image display
apparatuses such as liquid crystal displays, electroluminescence
displays and plasma displays, display members installed on
so-called "cellular phones," "smartphones" and "personal digital
assistants," cameras and lenses are fixed on predetermined sites
(for example, housings). As such foamed materials, there have been
used compression-molded high-density fine-cell urethanic foamed
bodies having a closed cell structure and low-density urethane and
besides, polyethylenic foamed bodies having closed cells and an
expansion ratio of about 30, and the like. Specifically, there are
used, for example, a gasket (see Patent Literature 1) composed of a
polyurethanic foamed body having an apparent density of 0.3 to 0.5
g/cm.sup.3, and a sealing material for electric or electronic
devices (see Patent Literature 2) composed of a foamed structural
body having an average cell diameter of 1 to 500 .mu.m.
[0003] In recent years, along with the thickness reduction of
electronic devices such as PCs (personal computers), tablet PCs,
PDAs (personal digital assistants) and cellular phones, impact
absorption sheets have been used on panel rear faces for the
prevention of breakage of liquid crystal panels, organic EL panels
and the like. Then, the thickness reduction is demanded also on the
impact absorption sheets. In the case where conventional foamed
materials are used as such impact absorption sheets, however, the
sheets cannot exhibit sufficient impact absorption.
[0004] Further along with the function enhancement of electronic
devices, in heat generating bodies such as electronic components,
the amount of heat generation becomes large. When conventional
foamed materials are used for electric or electronic devices having
such heat generating bodies exhibiting a large amount of heat
generation, a problem which arises is that the performance reduces
due to heat accumulated in the interiors and the electric or
electronic devices are broken due to impact when being dropped or
otherwise.
CITATION LIST
Patent Literature
Patent Literature 1: Japanese Patent Laid-Open No. 2001-100216
Patent Literature 2: Japanese Patent Laid-Open No. 2002-309198
SUMMARY OF INVENTION
Technical Problem
[0005] Therefore, an object of the present invention is to provide
a foamed sheet which, even if having a very small thickness,
exhibits excellent impact absorption and is excellent in heat
resistance in which the performance does not reduce even under high
temperatures.
[0006] Another object of the present invention is to provide an
electric or electronic device which, even if being down sized and
reduced in thickness, and having a heat generating body exhibiting
a large amount of heat generation, is hardly broken due to impact
in dropping.
Solution to Problem
[0007] As a result of exhaustive studies in order to achieve the
above objects, the present inventors have found that: a foamed
sheet, comprising a foamed body having a specific average cell
diameter and having a compression set at 80.degree. C. of not more
than 80% and a small difference between the impact absorption after
the high-temperature (80.degree. C.) compression and the initial
impact absorption, even if the foamed sheet has a very small
thickness, exhibits excellent impact absorption and is excellent in
heat resistance as well; and an electric or electronic device
installed with the foamed sheet, even if having a heat generating
body exhibiting a large amount of heat generation, is not broken by
impact in dropping. The present invention has been completed by
carrying out further studies based on these findings.
[0008] That is, the present invention provides a foamed sheet
comprising a foamed body having an average cell diameter of 10 to
200 .mu.m, and having a compression set at 80.degree. C. of not
more than 80% and an impact absorption change rate of not more than
.+-.20% as defined by the following.
Impact absorption change rate (%)={(an impact absorption rate b
after high-temperature compression-an initial impact absorption
rate a)/the initial impact absorption rate a}.times.100
[0009] The initial impact absorption rate a: an impact absorption
rate (%) of a test piece A
[0010] The impact absorption rate b (%) after high-temperature
compression: an impact absorption rate (%) acquired by storing the
test piece A at 80.degree. C. for 72 hours in the state of being
compressed by 60% with respect to the initial thickness of the test
piece A, thereafter releasing the compression state, and thereafter
conducting measurement after a lapse of 24 hours at 23.degree.
C.
[0011] The impact absorption rate: a value defined by the following
expression in an impact absorption test (the weight of an impactor:
28 g, the swing-up angle: 40.degree.) (23.degree. C.) using a
pendulum impact tester.
[0012] Impact absorption rate
(%)={(F.sub.0-F.sub.1)/F.sub.0}.times.100 wherein F.sub.0 is an
impact force when the impactor is made to collide with a support
plate alone; and F.sub.1 is an impact force when the impactor is
made to collide with the support plate of a structural body
composed of the support plate and the test piece A.
[0013] In the foamed sheet, it is preferable that the thickness is
30 to 1,000 .mu.m, and the apparent density of the foamed body is
0.2 to 0.7 g/cm.sup.3.
[0014] The foamed body preferably has a peak top of a loss tangent
(tan .delta.) in the range of not less than -30.degree. C. and not
more than 30.degree. C., the loss tangent (tan .delta.) being a
ratio of a loss elastic modulus to a storage elastic modulus at an
angular frequency of 1 rad/sec in a dynamic viscoelasticity
measurement.
[0015] The foamed body can be formed of at least one resin material
selected from the group consisting of acrylic polymers, rubbers,
urethanic polymers and ethylene-vinyl acetate copolymers.
[0016] The foamed body may be formed through step A of mechanically
foaming an emulsion resin composition. Further the foamed body may
be formed further through step B of coating a base material with
the mechanically foamed emulsion resin composition followed by
drying. Step B may comprise preliminary drying step B1 of drying
the bubble-containing emulsion resin composition applied on the
base material at not less than 50.degree. C. and less than
125.degree. C., and regular drying step B2 of thereafter further
drying the resultant at not less than 125.degree. C. and not more
than 200.degree. C.
[0017] In the foamed sheet, the compression set at 80.degree. C. is
preferably not more than 50%, and more preferably not more than
25%.
[0018] In the foamed sheet, the thickness is preferably 40 to 500
.mu.m, and more preferably 50 to 300 .mu.m.
[0019] The apparent density of the foamed sheet is preferably 0.21
to 0.6 g/cm.sup.3, and more preferably 0.22 to 0.5 g/cm.sup.3.
[0020] The foamed sheet may have a pressure-sensitive adhesive
layer on one face or both faces of the foamed body.
[0021] The foamed sheet may be one to be used as an impact
absorption sheet for electric or electronic devices.
[0022] The present invention also provides an electric or
electronic device using the foamed sheet. The electric or
electronic device includes one having a display member, and having
a structure in which the foamed sheet is interposed between a
housing of the electric or electronic device and the display
member.
Advantageous Effects of Invention
[0023] The foamed sheet according to the present invention, since
it comprises a foamed body having a specific average cell diameter
and has a compression set at 80.degree. C. of not more than 80% and
an impact absorption change rate of as low as not more than
.+-.20%, even if having a very small thickness, exhibits excellent
impact absorption and is excellent in heat resistance as well.
Hence, even in the case where the foamed sheet is used for electric
or electronic devices having heat generating bodies exhibiting a
large amount of heat generation, and the like, the performance as
an impact absorption sheet does not reduce and a high reliability
can be attained.
BRIEF DESCRIPTION OF DRAWINGS
[0024] FIG. 1 is a schematic constitution view of a pendulum impact
tester (impact testing apparatus).
[0025] FIG. 2 is a view illustrating a schematic constitution of a
holding member of the pendulum impact tester (impact testing
apparatus).
DESCRIPTION OF EMBODIMENTS
[0026] The foamed sheet according to the present invention
comprises a foamed body having an average cell diameter of 10 to
200 .mu.m. The lower limit of the average cell diameter of the
foamed body is preferably 15 .mu.m, and more preferably 20 .mu.m;
and the upper limit is preferably 150 .mu.m, more preferably 130
.mu.m, and still more preferably 100 .mu.m. Since the average cell
diameter is not less than 10 .mu.m, the foamed sheet exhibits
excellent impact absorption. Further since the average cell
diameter is not more than 200 .mu.m, the foamed sheet is excellent
in compression recovery as well. Here, the maximum cell diameter of
the foamed body is, for example, 40 to 800 .mu.m, and the lower
limit thereof is preferably 60 .mu.m, and more preferably 80 .mu.m;
and the upper limit is preferably 400 .mu.m, and more preferably
220 .mu.m. Further the minimum cell diameter of the foamed body is,
for example, 5 to 70 .mu.m, and the lower limit thereof is
preferably 8 .mu.m, and more preferably 10 .mu.m; and the upper
limit is preferably 60 .mu.m, and more preferably 50 .mu.m.
[0027] In the foamed sheet according to the present invention, the
compression set at 80.degree. C. is not more than 80%, preferably
not more than 50%, more preferably not more than 25%, and
especially preferably not more than 10%.
[0028] A compression set test at 80.degree. C. can be carried out
according to the provision of JIS K6262. The compression set (%) is
determined by the following expression.
CS={(t0-t1)/(t0-t2)}.times.100
[0029] CS: a compression set (%)
[0030] t0: an original thickness (mm) of a test piece
[0031] t1: a thickness (mm) of the test piece 30 min after the test
piece is removed from a compression apparatus
[0032] t2: a thickness (mm) of the test piece in the state of being
under a compressive strain
[0033] Here in the present invention, the compression set is a
value when the test piece is compressed by 60%.
[0034] In the foamed sheet according to the present invention, the
impact absorption change rate defined by the following is not more
than .+-.20%, preferably not more than .+-.15%, and still more
preferably not more than .+-.5%.
Impact absorption change rate (%)={(an impact absorption rate b
after high-temperature compression-an initial impact absorption
rate a)/the initial impact absorption rate a}.times.100
[0035] The initial impact absorption rate a: an impact absorption
rate (%) of a test piece A
[0036] The impact absorption rate b (%) after high-temperature
compression: an impact absorption rate (%) acquired by storing the
test piece A at 80.degree. C. for 72 hours in the state of being
compressed by 60% with respect to the initial thickness of the test
piece A, thereafter releasing the compression state, and thereafter
conducting measurement after a lapse of 24 hours at 23.degree.
C.
[0037] The impact absorption rate: a value defined by the following
expression in an impact absorption test (the weight of an impactor:
28 g, the swing-up angle: 40.degree.) (23.degree. C.) using a
pendulum impact tester.
[0038] Impact absorption rate
(%)={(F.sub.0-F.sub.1)/F.sub.0}.times.100 wherein F.sub.0 is an
impact force when the impactor is made to collide with a support
plate alone; and F.sub.1 is an impact force when the impactor is
made to collide with the support plate of a structural body
composed of the support plate and the test piece A.
[0039] Since the foamed sheet according to the present invention
has a low compression set at 80.degree. C., and a low rate of the
impact absorption change rate, even if the foamed sheet is
compressed at a high temperature, cells hardly collapse and
excellent thickness recovery is attained; and even in the case
where the foamed sheet is subjected to an impact at a high
temperature, high impact absorption is exhibited similarly as at
normal temperature. Hence, also in the case where the foamed sheet
is used for electric or electronic devices having heat generating
bodies exhibiting a large amount of heat generation, and the like,
even if an impact is applied on the electric or electronic devices
in dropping or otherwise, breakage of the devices can be
prevented.
[0040] The schematic constitution of a pendulum impact tester
(impact testing apparatus) will be described by way of FIG. 1 and
FIG. 2. As illustrated in FIG. 1 and FIG. 2, an impact testing
apparatus 1 (pendulum tester 1) is constituted of a holding member
3 as a holding means to hold a test piece 2 (foamed sheet 2) by an
arbitrary holding force, an impact applying member 4 to apply an
impact stress on the test piece 2, a pressure sensor 5 as an impact
force detecting means to detect an impact force on the test piece 2
by the impact applying member 4, and the like. Further the holding
member 3 to hold the test piece 2 by an arbitrary holding force is
constituted of a fixing jig 11, and a pressing jig 12 facing the
fixing jig 11 and being slidable so that the test piece 2 is
interposed and held between the fixing jig 11 and the pressing jig
12. Further the pressing jig 12 is provided with a pressure
adjusting means 16. Further the impact applying member 4 to apply
an impact force on the test piece 2 held by the holding member 3 is
constituted of a support rod 23 (shaft 23) whose one end 22 is
rotatably supported on a support column 20 and whose other end side
has an impactor 24, and an arm 21 to lift and hold the impactor 24
at a predetermined angle. Here since a steel ball is used as the
impactor 24, by providing an electromagnet 25 on one end of the
arm, the impactor 24 is enabled to be unifiedly lifted at the
predetermined angle. Further the pressure sensor 5 to detect an
impact force acting on the test piece 2 by the impact applying
member 4 is provided on the face side of the fixing jig 11 opposite
to the face thereof contacting the test piece.
[0041] The impactor 24 is a steel ball (iron ball). Further the
angle (swing-up angle a in FIG. 1) at which the impactor 24 is
lifted by the arm 21 is 40.degree.. The weight of the steel ball
(iron ball) is 28 g.
[0042] As illustrated in FIG. 2, the test piece 2 (foamed sheet 2)
is interposed between the fixing jig 11 and the pressing jig 12
through a support plate 28 constituted of a highly elastic plate
material such as a resinous plate material (acryl plate,
polycarbonate plate or the like) or a metal plate material.
[0043] The impact absorption is calculated by the expression
described before by using the above impact testing apparatus, and
determining an impact force F.sub.0 measured by closely fixing the
fixing jig 11 and the support plate 28 on each other and then
making the impactor 24 collide with the support plate 28, and an
impact force F.sub.1 measured by inserting and closely fixing the
test piece 2 between the fixing jig 11 and the support plate 28 and
then making the impactor 24 collide with the support plate 28.
Here, the impact testing apparatus is a similar apparatus as used
in Example 1 of Japanese Patent Laid-Open No. 2006-47277.
[0044] The foamed sheet according to the present invention has
excellent impact absorption while having a small thickness. The
impact absorption rate (the weight of the impactor: 28 g, the
swing-up angle: 40.degree.) is usually 5 to 70%, and the lower
limit is preferably 10%, more preferably 20%, and still more
preferably 28%; and the upper limit is preferably 60%.
[0045] The thickness of the foamed sheet according to the present
invention is not especially limited, but is, for example, 30 to
1,000 .mu.m, and the lower limit thereof is more preferably 40
.mu.m, and still more preferably 50 .mu.m; and the upper limit is
more preferably 500 .mu.m, still more preferably 300 .mu.m, and
especially preferably 200 .mu.m. When the thickness of the foamed
sheet is not less than 30 .mu.m, cells can be incorporated
uniformly, and better impact absorption can be exhibited. Further
by making the thickness of the foamed sheet to be not more than
1,000 .mu.m, the foamed sheet can easily conform to fine
clearances. The foamed sheet according to the present invention,
even if having as small a thickness as 30 to 1,000 .mu.m, is
excellent in impact absorption.
[0046] In the present invention, from the viewpoint of the impact
absorption, the ratio of the average cell diameter (.mu.m) to the
thickness (.mu.m) of the foamed sheet (the former/the latter) is
preferably in the range of 0.1 to 0.9. The lower limit of the ratio
of the average cell diameter (.mu.m) to the thickness (.mu.m) of
the foamed sheet is preferably 0.2, and more preferably 0.3; and
the upper limit is preferably 0.85, and more preferably 0.8.
[0047] The apparent density of the foamed body constituting the
foamed sheet according to the present invention is not especially
limited, but is preferably 0.2 to 0.7 g/cm.sup.3. The lower limit
thereof is preferably 0.21 g/cm.sup.3, and more preferably 0.22
g/cm.sup.3; and the upper limit is preferably 0.6 g/cm.sup.3, more
preferably 0.5 g/cm.sup.3, and especially preferably 0.4
g/cm.sup.3. When the apparent density of the foamed body is not
less than 0.2 g/cm.sup.3, a high strength can be maintained; and
when it is not more than 0.7 g/cm.sup.3, higher impact absorption
is exhibited. Further when the apparent density of the foamed body
is in the range of 0.2 to 0.4 g/cm.sup.3, much higher impact
absorption is exhibited.
[0048] In the case where the thickness of the foamed sheet is large
in some degree, the impact absorption can be regulated by selection
of the average cell diameter, the apparent density and the like; in
the case where the thickness of the foamed sheet is very small (for
example, the thickness is 30 to 500 .mu.m), however, the impact
cannot sufficiently be absorbed only by regulation of these
properties, in some cases. This is because in the case where the
thickness of the foamed sheet is very small, cells in the foamed
body instantly collapse by an impact and the impact buffering
function by cells disappears. From such a viewpoint, it is
preferable that the peak top of the loss tangent (tan .delta.),
which is a ratio of a loss elastic modulus to a storage elastic
modulus at an angular frequency of 1 rad/sec in a dynamic
viscoelasticity measurement of the foamed body, is present in the
range of not less than -30.degree. C. and not more than 30.degree.
C. In such a way, even after cells collapse, the constituting
material of the foamed body exhibits more a function of buffering
impacts.
[0049] The lower limit of the temperature range where the peak top
of the loss tangent is present is more preferably -25.degree. C.,
still more preferably -20.degree. C., and especially preferably
-10.degree. C.; and the upper limit is more preferably 20.degree.
C., and still more preferably 10.degree. C. In the case of
materials having not less than two peak tops of the loss tangent,
at least one of the peak tops is desirably in the above range. When
the peak temperature is not less than -30.degree. C., better
compression recovery is exhibited. Further when the peak
temperature is not more than 30.degree. C., higher flexibility is
exhibited and better impact absorption is exhibited.
[0050] It is preferable that the peak top intensity (maximum value)
of the loss tangent (tan .delta.) in the range of not less than
-30.degree. C. and not more than 30.degree. C. is higher from the
viewpoint of the impact absorption, and the peak top intensity is,
for example, not less than 0.2, and preferably not less than 0.3.
The upper limit value of the peak top intensity (maximum value) is,
for example, 2.0.
[0051] The peak temperature of the loss tangent (tan .delta.)
contributes to the impact absorption of the foamed body in such a
manner, in many cases. Although the reason is not completely clear
why when the peak top of the loss tangent (tan .delta.), which is a
ratio of a loss elastic modulus to a storage elastic modulus at an
angular frequency of 1 rad/sec in a dynamic viscoelasticity
measurement of the foamed body, is present in the range of not less
than -30.degree. C. and not more than 30.degree. C., the impact
absorption of the foamed sheet becomes high, it is presumably due
to that the peak of the loss tangent (tan .delta.) is present in
frequencies corresponding to those of impacts. That is, it is
presumed that since the range of not less than -30.degree. C. and
not more than 30.degree. C. of the loss tangent (tan .delta.) is
reduced to a range of frequencies corresponding to dropping impacts
of a structural material based on the temperature-time conversion
rule in viscoelasticity measurements, foamed sheets having a peak
temperature of the loss tangent (tan .delta.) in the range of not
less than -30.degree. C. and not more than 30.degree. C. exhibit
higher impact absorption. Further the storage elastic modulus is a
resilient force to an impact energy applied on the foamed sheet;
and when the storage elastic modulus is high, the impact is
repulsed as it is. By contrast, the loss elastic modulus is a
physical property which converts an impact energy applied on the
foamed sheet to heat; since a higher loss elastic modulus causes an
impact energy to be converted to more heat, the impact is absorbed
and the deformation is reduced. It is presumed from this that
foamed sheets which convert impacts to more heat and exhibit a
lower resilient force, that is, exhibit a higher loss tangent (tan
.delta.), which is a ratio of a loss elastic modulus to a storage
elastic modulus, exhibit a higher impact absorption rate.
[0052] With respect to the foamed body constituting the foamed
sheet according to the present invention, the composition, the cell
structure and the like thereof are not especially limited as long
as the foamed body has the above properties. The cell structure may
be any of an open cell structure, a closed cell structure and a
semi-open semi-closed cell structure. From the viewpoint of the
impact absorption, an open cell structure and a semi-open
semi-closed cell structure are preferable.
[0053] The foamed body can be constituted of a resin composition
containing a resin material (polymer). Here, it is preferable that
the peak top of the loss tangent (tan .delta.), which is a ratio of
a loss elastic modulus to a storage elastic modulus at an angular
frequency of 1 rad/sec in a dynamic viscoelasticity measurement of
the resin composition in an unfoamed state [the resin composition
(solid material) in the case of not being foamed], is in the range
of not less than -30.degree. C. and not more than 30.degree. C. The
lower limit of the temperature range where the peak top of the loss
tangent is present is more preferably -25.degree. C., still more
preferably -20.degree. C., and especially preferably -10.degree.
C.; and the upper limit is more preferably 20.degree. C., and still
more preferably 10.degree. C. In the case of materials having not
less than two peak tops of the loss tangent, at least one of the
peak tops is desirably in the above range. A higher peak top
intensity of the loss tangent (tan .delta.) in the range of not
less than -30.degree. C. and not more than 30.degree. C. of the
resin composition (solid material) is preferable from the viewpoint
of the impact absorption, wherein the value of the peak top
intensity corresponds to a value obtained by dividing a peak top
intensity of the loss tangent (tan .delta.) in the range of not
less than -30.degree. C. and less than 30.degree. C. of the foamed
body by an apparent density (g/cm.sup.3) of the foamed body. The
peak top intensity of the loss tangent (tan .delta.) in the range
of not less than -30.degree. C. and not more than 30.degree. C. of
the resin composition (solid material) is preferably not less than
0.9 (g/cm.sup.3).sup.-1, and the upper limit is, for example, about
3 (g/cm.sup.3).sup.-1.
[0054] A resin material (polymer) constituting the foamed body is
not especially limited, and publicly or commonly known resin
materials constituting foamed bodies can be used. Examples of the
resin material include acrylic polymers, rubbers, urethanic
polymers and ethylene-vinyl acetate copolymers. Among these, from
the viewpoint of the impact absorption, acrylic polymers, rubbers
and urethanic polymers are preferable. The resin material (polymer)
constituting the foamed body may be of a single kind or of not less
than two kinds.
[0055] In order to make the peak top of the loss tangent (tan
.delta.), which is a ratio of a loss elastic modulus to a storage
elastic modulus at an angular frequency of 1 rad/sec in a dynamic
viscoelasticity measurement of the foamed body, to be in the range
of not less than -30.degree. C. and not more than 30.degree. C., Tg
of the resin material (polymer) can be made to be an index or an
indication. The resin material (polymer) can be selected, for
example, from resin materials (polymers) having a Tg in the range
of not less than -50.degree. C. and less than 50.degree. C. (the
lower limit is preferably -40.degree. C., and more preferably
-30.degree. C.; and the upper limit is preferably 40.degree. C.,
and more preferably 30.degree. C.)
[0056] The acrylic polymer is preferably one formed of, as
essential monomer components, a monomer having a Tg of its
homopolymer of not less than -10.degree. C. and a monomer having a
Tg of its homopolymer of less than -10.degree. C. By using such an
acrylic polymer and regulating the amount ratios of the former
monomer and the latter monomer, there can comparatively easily be
obtained a foamed body having a peak top of the loss tangent (tan
.delta.), which is a ratio of a loss elastic modulus to a storage
elastic modulus at an angular frequency of 1 rad/sec in a dynamic
viscoelasticity measurement, of not less than -30.degree. C. and
not more than 30.degree. C.
[0057] Here, a "glass transition temperature (Tg) when a
homopolymer is formed" (simply referred to as "Tg of a homopolymer"
in some cases) in the present invention means a "glass transition
temperature (Tg) of a homopolymer of the corresponding monomer";
and numerical values are specifically cited in "Polymer Handbook"
(3rd edition, John Wiley & Sons, Inc., 1987). Here, Tgs of
homopolymers of monomers which are not described in the above
literature are values acquired, for example, by the following
measurement method (see Japanese Patent Laid-Open No. 2007-51271).
That is, 100 parts by weight of a monomer, 0.2 parts by weight of
2,2'-azobisisobutyronitrile, and 200 parts by weight of ethyl
acetate as a polymerization solvent are charged in a reaction
vessel equipped with a thermometer, a stirrer, a nitrogen
introducing tube and a refluxing cooling tube, and stirred for 1
hour under the introduction of nitrogen gas. After oxygen in the
polymerization system is removed in such a way, the system is
heated up to 63.degree. C. and allowed to react for 10 hours. Then,
the system is cooled to room temperature to thereby obtain a
homopolymer solution having a solid content concentration of 33% by
weight. Then, the homopolymer solution is cast and applied on a
separator, and dried to thereby fabricate a test sample (sheet-like
homopolymer) having a thickness of about 2 mm. Then, the test
sample is punched out into a disc of 7.9 mm in diameter, and
interposed between parallel plates; and the viscoelasticity is
measured by using a viscoelasticity tester (ARES, manufactured by
Rheometric Scientific, Inc.) and in a temperature region of -70 to
150.degree. C. at a temperature-rise rate of 5.degree. C./min in a
shearing mode under a shearing strain of 1 Hz in frequency, and the
peak top temperature in tan .delta. is defined as Tg of the
homopolymer. Here, also Tg of the resin material (polymer) can be
measured by this method.
[0058] In a monomer having a Tg of its homopolymer of not less than
-10.degree. C., the Tg is, for example, -10.degree. C. to
250.degree. C., preferably 10 to 230.degree. C., and more
preferably 50 to 200.degree. C.
[0059] Examples of the monomer having a Tg of its homopolymer of
not less than -10.degree. C. include (meth)acrylonitrile; amide
group-containing monomers such as (meth) acrylamide and
N-hydroxyethyl(meth)acrylamide; (meth)acrylic acid; alkyl
(meth)acrylates having a Tg of their homopolymer of not less than
-10.degree. C., such as methyl methacrylate and ethyl methacrylate;
isobornyl (meth)acrylate; heterocycle-containing vinyl monomers
such as N-vinyl-2-pyrrolidone; and hydroxyl group-containing
monomers such as 2-hydroxyethyl methacrylate. These can be used
singly or in combinations of not less than two. Among these,
(meth)acrylonitrile (particularly acrylonitrile) is especially
preferable. When (meth)acrylonitrile (particularly acrylonitrile)
is used as a monomer having a Tg of its homopolymer of not less
than -10.degree. C., probably because of the strong intermolecular
interaction, the peak top intensity of the loss tangent (tan
.delta.) of the foamed body can be made high.
[0060] In the monomer having a Tg of its homopolymer of less than
-10.degree. C., the Tg is, for example, not less than -70.degree.
C. and less than -10.degree. C., preferably -70.degree. C. to
-12.degree. C., and more preferably -65.degree. C. to -15.degree.
C.
[0061] Examples of the monomer having a Tg of its homopolymer of
less than -10.degree. C. include alkyl (meth)acrylates having a Tg
of their homopolymer of less than -10.degree. C., such as ethyl
acrylate, butyl acrylate and 2-ethylhexyl acrylate. These can be
used singly or in combinations of not less than two. Among these,
C.sub.2-8 alkyl acrylates are especially preferable.
[0062] The content of a monomer having a Tg of its homopolymer of
not less than -10.degree. C. with respect to the whole monomer
components (total amount of monomer components) forming the acrylic
polymer is, for example, 2 to 30% by weight, and the lower limit is
preferably 3% by weight, and more preferably 4% by weight; and the
upper limit is preferably 25% by weight, and more preferably 20% by
weight. The content of a monomer having a Tg of its homopolymer of
less than -10.degree. C. with respect to the whole monomer
components (total amount of monomer components) forming the acrylic
polymer is, for example, 70 to 98% by weight, and the lower limit
is preferably 75% by weight, and more preferably 80% by weight; and
the upper limit is preferably 97% by weight, and more preferably
96% by weight.
[0063] Here, when the monomer forming the acrylic polymer contains
a nitrogen atom-containing copolymerizable monomer, and when an
emulsion resin composition is subjected to shearing by a mechanical
stirring or the like to be thereby caused to foam, the viscosity of
the composition decreases and it becomes easy for a large number of
bubbles to be entrapped in the emulsion; and thereafter, when the
emulsion resin composition containing bubbles is applied on a base
material and dried in its standing-still state, since the
composition becomes easily aggregated and the viscosity increases,
and the bubbles are held in the composition and it becomes
difficult for the bubbles to diffuse outside, a foamed body
excellent in the foamed property can be obtained.
[0064] Examples of the nitrogen atom-containing copolymerizable
monomer (nitrogen atom-containing monomer) include cyano
group-containing monomers such as (meth)acrylonitrile; lactam
ring-containing monomers such as N-vinyl-2-pyrrolidone; and amide
group-containing monomers such as (meth)acrylamide,
N-hydroxyethyl(meth)acrylamide, N-methylolacrylamide,
N,N-dimethylacrylamide, N,N-diethylacrylamide and
diacetoneacrylamide. Among these, preferable are cyano
group-containing monomers such as acrylonitrile, and lactam
ring-containing monomers such as N-vinyl-2-pyrrolidone. The
nitrogen atom-containing monomers can be used singly or in
combinations of not less than two.
[0065] In an acrylic polymer having a structural unit originated
from such a nitrogen atom-containing monomer, the content of the
structural unit originated from the nitrogen atom-containing
monomer is, with respect to the whole structural units constituting
the acrylic polymer, preferably 2 to 30% by weight, and the lower
limit thereof is more preferably 3% by weight, and still more
preferably 4% by weight; and the upper limit thereof is more
preferably 25% by weight, and still more preferably 20% by
weight.
[0066] Further in an acrylic polymer having a structural unit
originated from such a nitrogen atom-containing monomer, in
addition to the structural unit originated from the nitrogen
atom-containing monomer, a structural unit originated from a
C.sub.2-18 alkyl acrylate (particularly a C.sub.2-8 alkyl acrylate)
is preferably contained. The C.sub.2-18 alkyl acrylates can be used
singly or in combinations of not less than two. In such an acrylic
polymer, the content of the structural unit originated from a
C.sub.2-18 alkyl acrylate (particularly a C.sub.2-8 alkyl acrylate)
is, with respect to the whole structural units constituting the
acrylic polymer, preferably 70 to 98% by weight, and the lower
limit thereof is more preferably 75% by weight, and still more
preferably 80% by weight; and the upper limit thereof is more
preferably 97% by weight, and still more preferably 96% by
weight.
[0067] The rubber may be either of natural rubber and synthetic
rubber. Examples of the rubber include nitrile rubber (NBR), methyl
methacrylate-butadiene rubber (MBR), styrene-butadiene rubber
(SBR), acrylic rubber (ACM, ANM), urethane rubber (AU) and silicone
rubber. Among these, preferable are nitrile rubber (NBR), methyl
methacrylate-butadiene rubber (MBR) and silicone rubber.
[0068] Examples of the urethanic polymer include
polycarbonate-based polyurethane, polyester-based polyurethane and
polyether-based polyurethane.
[0069] As the ethylene-vinyl acetate copolymer, publicly or
commonly known ethylene-vinyl acetate copolymers can be used.
[0070] The foamed body constituting the foamed sheet, in addition
to the resin material (polymer), may contain, as required, a
surfactant, a crosslinking agent, a thickener, a rust preventive, a
silicone-based compound and other additives.
[0071] For example, for the micronization of the cell diameter and
the stabilization of cells foamed, an optional surfactant may be
contained. The surfactant is not especially limited, and there may
be used any of an anionic surfactant, a cationic surfactant, a
nonionic surfactant, an amphoteric surfactant and the like; but
from the viewpoint of the micronization of the cell diameter and
the stabilization of cells foamed, an anionic surfactant is
preferable, and a fatty acid ammonium-based surfactant,
particularly ammonium stearate or the like, is more preferable. The
surfactants may be used singly or in combinations of not less than
two. Further dissimilar surfactants may be used concurrently, and
for example, an anionic surfactant and a nonionic surfactant, or an
anionic surfactant and an amphoteric surfactant may be used
concurrently.
[0072] The amount [solid content (nonvolatile content)] of the
surfactant to be added is, with respect to 100 parts by weight of
the resin material (polymer) [solid content (nonvolatile content)],
for example, 0 to 10 parts by weight, and the lower limit is
preferably 0.5 parts by weight; and the upper limit is preferably 8
parts by weight.
[0073] Further in order to improve the strength, heat resistance
and moisture resistance of the foamed body, an optional
crosslinking agent may be contained. The crosslinking agent is not
especially limited, and either of an oil-soluble one and a
water-soluble one may be used. Examples of the crosslinking agent
include epoxy-based, oxazoline-based, isocyanate-based,
carbodiimide-based, melamine-based and metal oxide-based ones.
Among these, oxazoline-based crosslinking agents are
preferable.
[0074] The amount [solid content (nonvolatile content)] of the
crosslinking agent to be added is, with respect to 100 parts by
weight of the resin material (polymer) [solid content (nonvolatile
content)], for example, 0 to 10 parts by weight, and the lower
limit is preferably 0.01 parts by weight, and more preferably 0.1
parts by weight; and the upper limit is preferably 9 parts by
weight, and more preferably 8 parts by weight.
[0075] Further for the stabilization of cells foamed and the
improvement of the film formability, an optional thickener may be
contained. The thickener is not especially limited, and includes
acrylic acid-based, urethanic and polyvinyl alcoholic ones. Among
these, polyacrylic acid-based thickeners and urethanic thickeners
are preferable.
[0076] The amount [solid content (nonvolatile content)] of the
thickener to be added is, with respect to 100 parts by weight of
the resin material (polymer) [solid content (nonvolatile content)],
for example, 0 to 10 parts by weight, and the lower limit is
preferably 0.1 parts by weight; and the upper limit is preferably 5
parts by weight.
[0077] Further in order to prevent the corrosion of metal members
adjacent to the foamed sheet, an optional rust preventive may be
contained. The rust preventive is preferably an azole
ring-containing compound. When an azole ring-containing compound is
used, both the corrosion prevention of metals and the close
adhesion with objects can be met simultaneously in high levels.
[0078] The azole ring-containing compound suffices as long as being
a compound having 5-membered ring containing not less than one
nitrogen atom in the ring, and examples include compounds having a
diazole (imidazole, pyrazole) ring, a triazole ring, a tetrazole
ring, an oxazole ring, an isoxazole ring, a thiazole ring or an
isothiazole ring. These rings may be condensed with an aromatic
ring such as a benzene ring to thereby form condensed rings.
Examples of compounds having such a condensed ring include
compounds having a benzimidazole ring, a benzopyrazole ring, a
benzotriazol ring, a benzoxazole ring, a benzisoxazole ring, a
benzothiazole ring or a benzisothiazole ring.
[0079] The azole ring and the condensed rings (benzotriazole ring,
benzothiazole ring and the like) may each have a substituent.
Examples of the substituent include alkyl groups having 1 to 6
carbon atoms (preferably having 1 to 3 carbon atoms) such as a
methyl group, an ethyl group, a propyl group, an isopropyl group
and a butyl group; alkoxy groups having 1 to 12 carbon atoms
(preferably having 1 to 3 carbon atoms) such as a methoxy group, an
ethoxy group, an isopropyloxy group and a butoxy group; aryl groups
having 6 to 10 carbon atoms such as a phenyl group, a tolyl group
and a naphthyl group; an amino group; (mono- or di-) C.sub.1-10
alkylamino groups such as a methylamino group and a dimethylamino
group; amino-C.sub.1-6 alkyl groups such as an aminomethyl group
and 2-aminoethyl group; mono- or di-(C.sub.1-10
alkyl)amino-C.sub.1-6 alkyl groups such as an
N,N-diethylaminomethyl group and an
N,N-bis(2-ethylhexyl)aminomethyl group; a mercapto group;
alkoxycarbonyl groups having 1 to 6 carbon atoms such as a
methoxycarbonyl group and an ethoxycarbonyl group; a carboxyl
group; carboxy-C.sub.1-6 alkyl groups such as a carboxymethyl
group; carboxy-C.sub.1-6 alkylthio groups such as
2-carboxyethylthio group; N,N-bis(hydroxy-C.sub.1-4
alkyl)amino-C.sub.1-4 alkyl groups such as an
N,N-bis(hydroxymethyl)aminomethyl group; and a sulfo group. Further
the azole ring-containing compound may form a salt such as a sodium
salt or a potassium salt.
[0080] From the viewpoint of the rust preventive action on metals,
preferable are compounds in which an azole ring forms a condensed
ring with an aromatic ring such as a benzene ring; and among these,
especially preferable are benzotriazole-based compounds (compounds
having a benzotriazole ring) and benzothiazole-based compounds
(compounds having a benzothiazole ring).
[0081] Examples of the benzotriazole-based compounds include
1,2,3-benzotriazole, methylbenzotriazole, carboxybenzotriazole,
carboxymethylbenzotriazole,
1-[N,N-bis(2-ethylhexyl)aminomethyl]benzotriazole,
1-[N,N-bis(2-ethylhexyl)aminomethyl]methylbenzotriazole,
2,2'-[[(methyl-1H-benzotriazol-1-yl)methyl]imino]bisethanol, and
sodium salts thereof.
[0082] Examples of the benzothiazole-based compounds include
2-mercaptobenzothiazole, 3-(2-(benzothiazolyl)thio)propionic acid,
and sodium salts thereof.
[0083] The azole ring-containing compounds may be used singly or in
combinations of not less than two.
[0084] The amount [solid content (nonvolatile content)] of the rust
preventive (for example, the azole ring-containing compound) [solid
content (nonvolatile content)] to be added suffices as long as
being in the range of not impairing the close adhesion with objects
and the intrinsic property of the foamed body, and is, for example,
with respect to 100 parts by weight of the resin material (polymer)
[solid content (nonvolatile content)], for example, preferably 0.2
to 5 parts by weight. The lower limit thereof is more preferably
0.3 parts by weight, and still more preferably 0.4 parts by weight;
and the upper limit thereof is more preferably 3 parts by weight,
and still more preferably 2 parts by weight.
[0085] Further in order to improve the recovery and the recovery
speed of the thickness of the foamed sheet after being compressed,
a silicone-based compound may be added. Further for the same
purpose, a silicone-modified polymer (for example, a
silicone-modified acrylic polymer, a silicone-modified urethanic
polymer) may be used as at least a part of the resin material
(polymer). These can be used singly or in combinations of not less
than two.
[0086] As the silicone-based compound, preferable are
silicone-based compounds having not more than 2,000 siloxane bonds.
Examples of the silicone-based compounds include silicone oils,
modified silicone oils and silicone resins.
[0087] Examples of the silicone oils (straight silicone oils)
include dimethyl silicone oils and methyl phenyl silicone oils.
[0088] Examples of the modified silicone oils include
polyether-modified silicone oils (polyether-modified dimethyl
silicone oils and the like), alkyl-modified silicone oils
(alkyl-modified dimethyl silicone oils and the like),
aralkyl-modified silicone oils (aralkyl-modified dimethyl silicone
oils and the like), higher fatty acid ester-modified silicone oils
(higher fatty acid ester-modified dimethyl silicone oils and the
like) and fluoroalkyl-modified silicone oils (fluoroalkyl-modified
dimethyl silicone oils and the like).
[0089] Among these, polyether-modified silicones are preferable.
Examples of commercially available products of the
polyether-modified silicone oils include straight chain-type ones
such as "PEG 11 Methyl Ether Dimethicone," "PEG/PPG-20/22 Butyl
Ether Dimethicone," "PEG-9 Methyl Ether Dimethicone," "PEG-32
Methyl Ether Dimethicone," "PEG-9 Dimethicone," "PEG-3 Dimethicone"
and "PEG-10 Dimethicone"; and branched chain-type ones such as
"PEG-9 Polydimethylsiloxyethyl Dimethicone" and "Lauryl PEG-9
Polydimethylsiloxyethyl Dimethicone" (which are all manufactured by
Shin-Etsu Silicone).
[0090] The silicone resins include straight silicone resins and
modified silicone resins. Examples of the straight silicone resins
include methyl silicone resins and methyl phenyl silicone resins.
Examples of the modified silicone resins include alkyd-modified
silicone resins, epoxy-modified silicone resins, acryl-modified
silicone resins and polyester-modified silicone resins.
[0091] The total content of the silicone-based compound and the
silicone chain moiety present in the silicone-modified polymer in
the foamed body is, with respect to 100 parts by weight of the
resin material (polymer) in the foamed body, for example, 0.01 to 5
parts by weight in terms of nonvolatile content (in terms of solid
content). The lower limit of the total content is preferably 0.05
parts by weight, and more preferably 0.1 parts by weight; and the
upper limit is preferably 4 parts by weight, and more preferably 3
parts by weight. In the case where the total content of the
silicone component and the silicone chain moiety in the foamed body
is in the above range, the recovery and the recovery speed after
compression can be improved without impairing the properties as the
foamed body.
[0092] Further the total content of the silicone-based compound and
the silicone chain moiety present in the silicone-modified polymer
in the foamed body is, for example, 0.01 to 5% by weight in terms
of nonvolatile content (in terms of solid content). The lower limit
of the total content is preferably 0.05% by weight, and more
preferably 0.1% by weight; and the upper limit is preferably 4% by
weight, and more preferably 3% by weight. In the case where the
total content of the silicone component and the silicone chain
moiety in the foamed body is in the above range, the recovery and
the recovery speed after compression can be improved without
impairing the properties as the foamed body.
[0093] The foamed body constituting the foamed sheet may contain
optional other suitable components in the range of not impairing
the impact absorption. Such other components may be contained in
one kind thereof alone, or may be contained in not less than two
kinds thereof. Examples of the other components include polymer
components other than the above, softening agents, antioxidants,
antiaging agents, gelling agents, curing agents, plasticizers,
filling agents, reinforcing agents, foaming agents (sodium
bicarbonate and the like), microcapsules (thermally expandable
microballs and the like), flame retardants, light stabilizers,
ultraviolet absorbents, coloring agents (pigments, dyes and the
like), pH regulators, solvents (organic solvents),
thermopolymerization initiators and photopolymerization initiators.
The amounts [solid contents (nonvolatile contents)] of these
components to be added suffices as long as being in the range of
not impairing the close adhesion with objects and the intrinsic
property of the foamed body, and are each, for example, with
respect to 100 parts by weight of the resin material (polymer)
[solid content (nonvolatile content)], preferably in the range of,
for example, 0.2 to 60 parts by weight. The amount [solid content
(nonvolatile content)] of the foaming agent (sodium bicarbonate or
the like) to be added is, with respect to 100 parts by weight of
the resin material (polymer) [solid content (nonvolatile content)],
more preferably 0.5 to 20 parts by weight. The amount [solid
content (nonvolatile content)] of the microcapsule (thermally
expandable microball or the like) to be added is, with respect to
100 parts by weight of the resin material (polymer) [solid content
(nonvolatile content)], more preferably 0.2 to 10 parts by weight.
The amount [solid content (nonvolatile content)] of the filling
agent to be added is, with respect to 100 parts by weight of the
resin material (polymer) [solid content (nonvolatile content)],
more preferably 0.3 to 50 parts by weight.
[0094] Examples of the filling agents include silica, clay (mica,
talc, smectite and the like), alumina, aluminum hydroxide,
hydroxides of alkaline earth metals (magnesium hydroxide and the
like), carbonate salts of alkaline earth metals (calcium carbonate
and the like), titania, zinc oxide, tin oxide, zeolite, graphite,
carbon black, carbon nanotubes, inorganic fibers (carbon fibers,
glass fibers, potassium titanate fibers and the like), organic
fibers, metal powders (silver, copper, and the like) and waxes
(polyethylene wax, polypropylene wax, and the like). Further as the
filling agent, there can also be added piezoelectric particles
(titanium oxide, barium titanate and the like), electroconductive
particles (electroconductive carbon black, electroconductive
titanium oxide, tin oxide and the like), thermoconductive particles
(boron nitride and the like), organic fillers (silicone powder,
polyethylene powder, polypropylene powder and the like) and the
like. In the case of using silica as the filling agent, the amount
thereof to be added is, with respect to 100 parts by weight of the
thermoplastic resin [solid content (nonvolatile content)],
especially preferably in the range of 0.5 to 40 parts by weight.
Further in the case of using clay such as mica as the filling
agent, the amount thereof to be added is, with respect to 100 parts
by weight of the thermoplastic resin [solid content (nonvolatile
content)], especially preferably in the range of 0.3 to 10 parts by
weight. Further in the case of using a piezoelectric particle as
the filling agent, the amount thereof to be added is, with respect
to 100 parts by weight of the thermoplastic resin [solid content
(nonvolatile content)], especially preferably in the range of 5 to
40 parts by weight. Further in the case of using an
electroconductive particle as the filling agent, the amount thereof
to be added is, with respect to 100 parts by weight of the
thermoplastic resin [solid content (nonvolatile content)],
especially preferably in the range of 5 to 40 parts by weight.
Further when a piezoelectric particle and an electroconductive
particle are used in combination as the filling agent, the amount
of charge to be generated can be regulated by the pressure. In this
case, the ratio of the piezoelectric particle to the
electroconductive particle, for example the former/the latter
(weight ratio) is preferably 10/90 to 90/10, preferably 20/80 to
80/20, and still more preferably 30/70 to 70/30.
[0095] The foamed sheet according to the present invention can be
produced by subjecting a resin composition containing the resin
material (polymer) constituting the foamed body to expansion
molding. As a foaming method (method of forming cells), there can
be employed methods usually used for expansion molding, including
physical methods and chemical methods. The physical methods
generally involve dispersing a gas component such as air or
nitrogen in a polymer solution, and forming bubbles by mechanical
mixing. The chemical methods are ones in which cells are formed by
a gas generated by thermal decomposition of a foaming agent added
to a polymer base, to thereby obtain foamed bodies. From the
viewpoint of environmental problems and the like, the physical
methods are preferable. Cells to be formed by the physical methods
are open cells in many cases.
[0096] As the resin composition containing the resin material
(polymer) to be subjected to expansion molding, there may be used a
resin solution in which the resin material is dissolved in a
solvent, but from the viewpoint of the foamability, an emulsion
containing the resin material is preferably used. Not less than two
emulsions may be blended and used as the emulsion.
[0097] It is preferable from the viewpoint of the film formability
that the solid content concentration of the emulsion is higher. The
solid content concentration of the emulsion is preferably not less
than 30% by weight, more preferably not less than 40% by weight,
and still more preferably not less than 50% by weight.
[0098] In the present invention, a method is preferable in which a
foamed body is fabricated through a step (step A) of mechanically
foaming an emulsion resin composition to thereby foam the emulsion
resin composition. A foaming apparatus is not especially limited,
and examples thereof include apparatuses such as a high-speed
shearing system, a vibration system, and a discharge system of a
pressurized gas. Among these, from the viewpoint of the
micronization of the cell diameter and the fabrication of a large
volume, a high-speed shearing system is preferable.
[0099] Bubbles foamed by mechanical stirring are ones of gas
entrapped in the emulsion. The gas is not especially limited as
long as being inactive to the emulsion, and includes air, nitrogen
and carbon dioxide. Among these, from the viewpoint of the
economical efficiency, air is preferable.
[0100] By subjecting the emulsion resin composition foamed by the
above method to a step (step B) of coating a base material with the
emulsion resin composition followed by drying, the foamed sheet
according to the present invention can be obtained. The base
material is not especially limited, but examples thereof include
release-treated plastic films (release-treated polyethylene
terephthalate films and the like), plastic films (polyethylene
terephthalate films and the like) and thermoconductive layers. In
the case of coating by using a thermoconductive layer as the base
material, the close adhesion of a foamed body layer with the
thermoconductive layer can be improved and the efficiency of a
drying step in the fabrication of the foamed body layer can also be
improved.
[0101] As a coating method and a drying method in step B, usual
methods can be employed. It is preferable that step B comprises
preliminary drying step B1 of drying the bubble-containing emulsion
resin composition applied on the base material at not less than
50.degree. C. and less than 125.degree. C., and regular drying step
B2 of thereafter further drying the resultant at not less than
125.degree. C. and not more than 200.degree. C.
[0102] By providing preliminary drying step B1 and regular drying
step B2, the coalescence of bubbles and the burst of bubbles due to
a rapid temperature rise can be prevented. Particularly in the
foamed sheet having a small thickness, since bubbles coalesce or
burst in a rapid rise of the temperature, the significance of the
provision of preliminary drying step B1 is large. The temperature
in preliminary drying step B1 is preferably not less than
50.degree. C. and not more than 100.degree. C. The time of
preliminary drying step B1 is, for example, 0.5 min to 30 min, and
preferably 1 min to 15 min. Further the temperature in regular
drying step B2 is preferably not less than 130.degree. C. and not
more than 180.degree. C., and more preferably not less than
130.degree. C. and not more than 160.degree. C. The time of regular
drying step B2 is, for example, 0.5 min to 30 min, and preferably 1
min to 15 min.
[0103] In the present invention, the average cell diameter, the
maximum cell diameter and the minimum cell diameter of the foamed
body can be controlled by regulation of the kind and the amount of
the surfactant, and regulation of the stirring rate and the
stirring time in the mechanical stirring.
[0104] Further the apparent density of the foamed body can be
controlled by regulation of the amount of the gas component
entrapped in the emulsion resin composition in the mechanical
stirring.
[0105] Further the value of the compression set at 80.degree. C.
and the value of the impact absorption change rate can be
controlled, for example, by regulation of the degree of
crosslinking and Tg of the resin material (polymer) constituting
the foamed body. More specifically, for example, by regulation of
the amount of the crosslinking agent to be added, and regulation of
the proportion accounted for by the monomer having a Tg of its
homopolymer of not less than -10.degree. C. in the whole monomer
components forming the resin material (polymer), the value of the
compression set at 80.degree. C. and the value of the impact
absorption change rate can be controlled in predetermined ranges.
By increasing the amount of the crosslinking agent to be added, and
increasing the proportion accounted for by the monomer having a Tg
of its homopolymer of not less than -10.degree. C. in the whole
monomer components forming the resin material (polymer), the value
of the compression set at 80.degree. C. and the value of the impact
absorption change rate can be made low.
[0106] The foamed sheet according to the present invention may have
a pressure-sensitive adhesive layer on one face or on both faces of
the foamed body. A pressure-sensitive adhesive constituting the
pressure-sensitive adhesive layer is not especially limited, and
may be any of an acrylic pressure-sensitive adhesive, a
rubber-based pressure-sensitive adhesive, a silicone-based
pressure-sensitive adhesive and the like. Further in the case of
providing the pressure-sensitive adhesive layer, a release liner to
protect the pressure-sensitive adhesive layer until its usage may
be laminated on its face. Here, in the case where the foamed body
constituting the foamed sheet according to the present invention
exhibits slight tackiness, members and the like can be fixed even
without providing the pressure-sensitive adhesive layer.
[0107] The foamed sheet according to the present invention may be
distributed to markets as a wound body (roll-like material) wound
in a rolled form.
[0108] As described above, the foamed sheet according to the
present invention, even if having a small thickness, is excellent
in impact absorption. Further the foamed sheet is excellent in heat
resistance, and even if being subjected to compressions or impacts
under high temperatures (for example, about 80.degree. C.), holds a
force to recover its original shape (thickness). The foamed sheet
according to the present invention has, for example, an 80.degree.
C. stress retention rate of not less than 68% as defined by the
following. Here, in conventional foamed sheets, the 80.degree. C.
stress retention rate is usually low, and the stress relaxes at
80.degree. C. and the recovering force decays.
<The 80.degree. C. Stress Retention Rate>
[0109] A test piece (foamed sheet) is held in an atmosphere of
80.degree. C. for 30 min; thereafter, by using a tensile tester,
the test piece is set at 80.degree. C. at an interchuck distance of
40 mm, stretched by 50% at a tensile rate of 500 mm/min, and
thereafter held for 120 sec; a maximum load and a load after the
120 sec are measured, and the 80.degree. C. stress retention rate
is determined by the following expression.
80.degree. C. stress retention rate (%)=[a load(N) after the 120
sec/a maximum load(N)].times.100
[0110] Thus, the foamed sheet according to the present invention,
since even if having a small thickness, being excellent in impact
absorption and moreover excellent in heat resistance, is high in
installation (adhesion) reliability even at high temperatures; and
for example, in electric or electronic devices, the foamed sheet is
useful as a member, particularly an impact absorption sheet, for
electric or electronic devices to be used when various types of
members or components (for example, optical members) are attached
(installed) on predetermined sites (for example, housings).
[0111] Examples of optical members attachable (installable) by
utilizing the foamed sheet according to the present invention
include image display members (particularly small-size image
display members) installed on image display apparatuses such as
liquid crystal displays, electroluminescence displays and plasma
displays, display members, such as touch panels, installed on
mobile communication apparatuses such as so-called "cellular
phones," "smartphones" and "personal digital assistants," cameras
and lenses (particularly small-size cameras and lenses).
[0112] The electric or electronic device according to the present
invention uses the foamed sheet according to the present invention.
Such an electric or electronic device includes, for example, an
electric or electronic device having a display member, and having a
structure in which the foamed sheet is interposed between a housing
of the electric or electronic device and the display member.
Examples of the electric or electronic device include mobile
communication apparatuses such as so-called "cellular phones,"
"smartphones" and "personal digital assistants."
EXAMPLES
[0113] Hereinafter, the present invention will be described in more
detail by way of Examples, but the present invention is not limited
to these Examples. Here, unless otherwise mentioned, "%"
representing a content means % by weight. Here, the numbers of
parts (parts by weight) blended are all values in terms of solid
content (nonvolatile content).
Example 1
[0114] 100 parts by weight of an acryl emulsion solution (the
amount of the solid content: 55%, an ethyl acrylate-butyl
acrylate-acrylonitrile copolymer (45:48:7 in weight ratio)), 1 part
by weight of a silicone-based compound A (a dimethyl silicone oil,
the number-average molecular weight Mn: 7.16.times.10.sup.3, the
weight-average molecular weight Mw: 1.71.times.10.sup.4, the amount
of the solid content (nonvolatile content): 100%), 3 parts by
weight of a fatty acid ammonium-based surfactant (a water
dispersion of ammonium stearate, the amount of the solid content:
33%), 2.0 parts by weight of an oxazoline-based crosslinking agent
("Epocros WS-500," manufactured by Nippon Shokubai Co., Ltd., the
amount of the solid content: 39%), 1 part by weight of a
benzotriazole sodium salt (the solid content: 40%) (a rust
preventive), and 0.8 parts by weight of a polyacrylic acid-based
thickener (an ethyl acrylate-acrylic acid copolymer (acrylic acid:
20% by weight), the amount of the solid content: 28.7%) were
stirred and mixed by a Disper ("Robomix," manufactured by Primix
Corp.) and thereby foamed. The foamed composition was applied on a
release-treated PET (polyethylene terephthalate) film (the
thickness: 38 .mu.m, the trade name: "MRF#38," manufactured by
Mitsubishi Plastics, Inc.), and dried at 70.degree. C. for 4.5 min
and 140.degree. C. for 4.5 min to thereby obtain a foamed body
(foamed sheet) of an open cell structure having a thickness of 100
.mu.m, an apparent density of 0.34 g/cm.sup.3, a cell porosity of
65.7%, a maximum cell diameter of 72.5 .mu.m, a minimum cell
diameter of 28.5 .mu.m and an average cell diameter of 45
.mu.m.
Example 2
[0115] 100 parts by weight of an acryl emulsion solution (the
amount of the solid content: 55%, an ethyl acrylate-butyl
acrylate-acrylonitrile copolymer (45:48:7 in weight ratio)), 1 part
by weight of a silicone-based compound A (a dimethyl silicone oil,
the number-average molecular weight Mn: 7.16.times.10.sup.3, the
weight-average molecular weight Mw: 1.71.times.10.sup.4, the amount
of the solid content (nonvolatile content): 100%), 3 parts by
weight of a fatty acid ammonium-based surfactant (a water
dispersion of ammonium stearate, the amount of the solid content:
33%), 0.35 parts by weight of an oxazoline-based crosslinking agent
("Epocros WS-500," manufactured by Nippon Shokubai Co., Ltd., the
amount of the solid content: 39%), 1 part by weight of a
benzotriazole sodium salt (the solid content: 40%) (a rust
preventive), and 0.8 parts by weight of a polyacrylic acid-based
thickener (an ethyl acrylate-acrylic acid copolymer (acrylic acid:
20% by weight), the amount of the solid content: 28.7%) were
stirred and mixed by a Disper ("Robomix," manufactured by Primix
Corp.) and thereby foamed. The foamed composition was applied on a
release-treated PET (polyethylene terephthalate) film (the
thickness: 38 .mu.m, the trade name: "MRF#38," manufactured by
Mitsubishi Plastics, Inc.), and dried at 70.degree. C. for 4.5 min
and 140.degree. C. for 4.5 min to thereby obtain a foamed body
(foamed sheet) of an open cell structure having a thickness of 100
.mu.m, an apparent density of 0.45 g/cm.sup.3, a cell porosity of
54.5%, a maximum cell diameter of 87.5 .mu.m, a minimum cell
diameter of 48.5 .mu.m and an average cell diameter of 65
.mu.m.
Example 3
[0116] 100 parts by weight of an acryl emulsion solution (the
amount of the solid content: 55%, an ethyl acrylate-butyl
acrylate-acrylonitrile copolymer (45:48:7 in weight ratio)), 1 part
by weight of a silicone-based compound A (a dimethyl silicone oil,
the number-average molecular weight Mn: 7.16.times.10.sup.3, the
weight-average molecular weight Mw: 1.71.times.10.sup.4, the amount
of the solid content (nonvolatile content): 100%), 3 parts by
weight of a fatty acid ammonium-based surfactant (a water
dispersion of ammonium stearate, the amount of the solid content:
33%), 0.35 parts by weight of an oxazoline-based crosslinking agent
("Epocros WS-500," manufactured by Nippon Shokubai Co., Ltd., the
amount of the solid content: 39%), 1 part by weight of a
benzotriazole sodium salt (the solid content: 40%) (a rust
preventive), and 0.8 parts by weight of a polyacrylic acid-based
thickener (an ethyl acrylate-acrylic acid copolymer (acrylic acid:
20% by weight), the amount of the solid content: 28.7%) were
stirred and mixed by a Disper ("Robomix," manufactured by Primix
Corp.) and thereby foamed. The foamed composition was applied on a
release-treated PET (polyethylene terephthalate) film (the
thickness: 38 .mu.m, the trade name: "MRF#38," manufactured by
Mitsubishi Plastics, Inc.), and dried at 70.degree. C. for 4.5 min
and 140.degree. C. for 4.5 min to thereby obtain a foamed body
(foamed sheet) of an open cell structure having a thickness of 120
.mu.m, an apparent density of 0.26 g/cm.sup.3, a cell porosity of
73.7%, a maximum cell diameter of 57.5 .mu.m, a minimum cell
diameter of 15.3 .mu.m and an average cell diameter of 30
.mu.m.
Example 4
[0117] 100 parts by weight of an acryl emulsion solution (the
amount of the solid content: 55%, an ethyl acrylate-butyl
acrylate-acrylonitrile copolymer (45:48:7 in weight ratio)), 1 part
by weight of a silicone-based compound A (a dimethyl silicone oil,
the number-average molecular weight Mn: 7.16.times.10.sup.3, the
weight-average molecular weight Mw: 1.71.times.10.sup.4, the amount
of the solid content (nonvolatile content): 100%), 3 parts by
weight of a fatty acid ammonium-based surfactant (a water
dispersion of ammonium stearate, the amount of the solid content:
33%), 1 part by weight of a benzotriazole sodium salt (the solid
content: 40%) (a rust preventive), and 0.8 parts by weight of a
polyacrylic acid-based thickener (an ethyl acrylate-acrylic acid
copolymer (acrylic acid: 20% by weight), the amount of the solid
content: 28.7%) were stirred and mixed by a Disper ("Robomix,"
manufactured by Primix Corp.) and thereby foamed. The foamed
composition was applied on a release-treated PET (polyethylene
terephthalate) film (the thickness: 38 .mu.m, the trade name:
"MRF#38," manufactured by Mitsubishi Plastics, Inc.), and dried at
70.degree. C. for 4.5 min and 140.degree. C. for 4.5 min to thereby
obtain a foamed body (foamed sheet) of an open cell structure
having a thickness of 130 .mu.m, an apparent density of 0.37
g/cm.sup.3, a cell porosity of 62.6%, a maximum cell diameter of
82.5 .mu.m, a minimum cell diameter of 43.5 .mu.m and an average
cell diameter of 60 .mu.m.
Comparative Example 1
[0118] 45 parts by weight of a polypropylene [the melt flow rate
(MFR): 0.35 g/10 min], 55 parts by weight of a mixture (MFR
(230.degree. C.): 6 g/10 min, JIS A-hardness: 79.degree., 30 parts
by mass of a softening agent was blended in 100 parts by mass of a
polyolefinic elastomer) of the polyolefinic elastomer and the
softening agent (paraffinic extender oil), 10 parts by weight of
magnesium hydroxide, 10 parts by weight of a carbon (the trade
name: "Asahi #35," manufactured by Asahi Carbon Co., Ltd.), 1 part
by weight of stearic monoglyceride, and 1.5 parts by weight of a
fatty acid amide (lauric acid bisamide) were kneaded in a
twin-screw kneader, manufactured by The Japan Steel Works, Ltd.
(JSW), at a temperature of 200.degree. C., then extruded in a
strand form, and water cooled and then formed into a pellet form.
The pellet was charged in a single-screw extruder, manufactured by
The Japan Steel Works, Ltd.; and a carbon dioxide gas was injected
in the atmosphere of 220.degree. C. at a pressure of 13 (after the
injection, 12) MPa. The carbon dioxide gas was injected in a
proportion of 5.6% by weight with respect to the total amount of
the pellet. After the carbon dioxide gas was fully saturated, the
resultant was cooled to a temperature suitable for being foamed,
and thereafter extruded in a cylindrical form through a die; the
resultant was passed through between a mandrel to cool the inner
side face of the foamed body and an air ring for cooling the foamed
body to cool the outside face of the cylindrical foamed body
extruded from the ring die of the extruder; and a part of the
diameter was cut and the cylindrical foamed body was unfolded into
a sheet form to thereby obtain a long-size foamed body original
sheet. In the long-size foamed body original sheet, the average
cell diameter was 55 .mu.m, and the apparent density was 0.041
g/cm.sup.3.
[0119] The long-size foamed body original sheet was cut (subjected
to a slitting work) into a predetermined width; and by using a
continuous slicing apparatus (slicing line), high-density layers on
faces were separated away one by one to thereby obtain a resin
foamed body.
[0120] By passing the resin foamed body through in the above
continuous treatment apparatus in which the temperature of the
induction heating roll was set at 160.degree. C. and the gap was
set at 0.20 mm, one face thereof was subjected to a melting
treatment by the heat; and the resultant was subjected to a
slitting work, and thereafter taken up to thereby obtain a wound
body. Here, the taking-up speed was made to be 20 m/min.
[0121] Then, by rewinding the wound body, and passing the rewound
body through in the above continuous treatment apparatus in which
the temperature of the induction heating roll was set at
160.degree. C. and the gap was set at 0.10 mm, a face thereof
(untreated face) not having been subjected to a melting treatment
was subjected to a melting treatment by the heat; and the resultant
was subjected to a slitting work, and thereafter taken up to
thereby obtain a foamed body (foamed sheet) whose both faces had
been subjected to the heat melting treatment and which had an open
cell structure having a thickness of 100 .mu.m, an apparent density
of 0.12 g/cm.sup.3, a cell porosity of 88%, a maximum cell diameter
of 90 .mu.m, a minimum cell diameter of 30 .mu.m and an average
cell diameter of 60 .mu.m.
<Evaluations>
[0122] The foamed bodies (foamed sheets) obtained in the Examples
and the Comparative Example were evaluated for the following. The
results are shown in Table 1 and Table 2. In Table 1, there is
shown the number of parts (parts by weight) [in terms of solid
content (nonvolatile content)] of each component blended in each
Example and Comparative Example. "Em" indicates an emulsion.
(The Average Cell Diameter)
[0123] The average cell diameter (.mu.m) was determined by taking
and image analyzing an enlarged image of a foamed body
cross-section by a low-vacuum scanning electron microscope
("S-3400N type scanning electron microscope," manufactured by
Hitachi High-Tech Science Systems Corp.). Here, the number of cells
analyzed was about 10 to 20. Similarly, the minimum cell diameter
(.mu.m) and the maximum cell diameter (.mu.m) of the foamed sheet
were determined.
(The Apparent Density)
[0124] A foamed body (foamed sheet) is punched out with a punching
knife of 100 mm.times.100 mm, and the size of the punched-out
sample is measured. Further the thickness is measured by a 1/100
dial gage having a diameter (.phi.) of its measuring terminal of 20
mm. The volume of the foamed body was calculated from these
values.
[0125] Then, the weight of the foamed body is measured by an even
balance whose minimum division is not less than 0.01 g. The
apparent density (g/cm.sup.3) of the foamed body was calculated
from these values.
(The Dynamic Viscoelasticity)
[0126] A temperature-dispersion test was carried out at an angular
frequency of 1 rad/sec in a film tensile measurement mode of a
viscoelasticity measuring apparatus ("ARES2KFRTN1-FCO,"
manufactured by TA Instruments Japan Inc.). There was measured the
temperature (.degree. C.) and intensity (maximum value) of the peak
top of the loss tangent (tan .delta.), which was a ratio of the
loss elastic modulus E'' to the storage elastic modulus E' at this
time.
[0127] In the column of "tan .delta. Temperature" of Table 2, the
temperatures (.degree. C.) of peak tops of loss tangents (tan
.delta.) of foamed bodies are indicated; and in the column of "tan
.delta. Maximum Value," intensities (maximum values) of the peak
tops are indicated.
(The Compression Set Test)
[0128] The foamed sheets (sample size: 30 mm.times.30 mm) obtained
in the Examples and the Comparative Example were used as test
pieces. By using the test piece, the compression set test was
carried out at 80.degree. C. (according to the provision of JIS
K6262). More specifically, the test piece was compressed (until the
thickness of the compressed test piece became a thickness of 40% of
its original thickness) in an atmosphere of 80.degree. C., held in
this state for 24 hours, thereafter released from the compressed
state, and left as it was at 23.degree. C. for 30 min; and the
thickness of the test piece was measured at 23.degree. C. Then, the
compression set (%) at 80.degree. C. was determined by the
following expression.
CS={(t0-t1)/(t0-t2)}.times.100
[0129] CS: a compression set (%)
[0130] t0: an original thickness (mm) of a test piece
[0131] t1: a thickness (mm) of the test piece at 30 min after the
test piece is removed from a compression apparatus
[0132] t2: a thickness (mm) of the test piece in the state of being
under a compressive strain
(The Impact Absorption Change Rate)
[0133] For the foamed sheets (sample size: 20 mm.times.20 mm) (test
pieces A) obtained in the Examples and the Comparative Example, by
using the above-mentioned pendulum impact tester (impact testing
apparatus) (see FIG. 1 and FIG. 2), an impact absorption test was
carried out under the conditions of 23.degree. C., a weight of an
impactor of 28 g, and a swing-up angle of 40.degree.. The impact
absorption rate acquired at this time is defined as an initial
impact absorption rate a.
[0134] Then, the test piece A was stored at 80.degree. C. for 72
hours in the state of being compressed by 60% with respect to the
initial thickness of the test piece A, and thereafter, the
compression state was released; and thereafter, as in the above, an
impact absorption test was carried out under the conditions of
23.degree. C., a weight of an impactor of 28 g, and a swing-up
angle of 40.degree. after a lapse of 24 hours at 23.degree. C. The
impact absorption rate acquired at this time is defined as an
impact absorption rate b after the high-temperature
compression.
[0135] Then, the impact absorption change rate (%) was determined
by the following expression.
Impact absorption change rate (%)={(the impact absorption rate b
after high-temperature compression-the initial impact absorption
rate a)/the initial impact absorption rate a}.times.100
[0136] Here, the impact absorption rate is a value defined by the
following expression.
Impact absorption rate
(%)={(F.sub.0-F.sub.1)/F.sub.0}.times.100
wherein F.sub.0 is an impact force when the impactor is made to
collide with a support plate alone; and F.sub.1 is an impact force
when the impactor is made to collide with the support plate of a
structural body composed of the support plate and the test piece
A.
[0137] (The 80.degree. C. Stress Retention Rate)
[0138] The foamed sheets [the shape and size of the samples:
dumbbell No. 1 (see JIS K6251)] obtained in the Examples and the
Comparative Example were each held in an atmosphere of 80.degree.
C. for 30 min, thereafter, by using a tensile tester, set at
80.degree. C. at an interchuck distance of 40 mm, stretched by 50%
at a tensile speed of 500 mm/min, and thereafter held for 120 sec;
and the maximum load and the load after the 120 sec were measured
and the 80.degree. C. stress retention rate was determined by the
following expression.
80.degree. C. stress retention rate (%)=[a load (N) after 120 sec/a
maximum load (N)].times.100
TABLE-US-00001 TABLE 1 Silicone- Surfactant Em based (Foaming
Thickener Rust the Compound Agent) Crosslinking the Preventive
number the number the number Agent number of the number of parts of
parts of parts the number of parts of parts blended blended blended
parts blended blended blended Example 1 100 1 3 2 0.8 1 2 100 1 3
0.35 0.8 1 3 100 1 3 0.35 0.8 1 4 100 1 3 0.35 0.8 1 Comparative 1
-- -- -- -- -- -- Example
TABLE-US-00002 TABLE 2 Impact Absorption Initial Rate b after
Impact 80.degree. C. Average tan.delta. Impact High- Absorption
Stress Cell Apparent tan.delta. Maximum Absorption Temperature
Change Compression Retention Thickness Diameter Density Temperature
Value Rate a Compression Rate Set Rate (.mu.m) (.mu.m) (g/cm.sup.3)
(.degree. C.) (--) (%) (%) (%) (%) (%) Example 1 100 45 0.34 -3
0.37 33 32 -3 0 74 2 100 65 0.45 -3 0.38 30 30 0 0 76 3 120 30 0.26
-2 0.42 35.4 31.2 -12 14 73 4 130 60 0.37 -3 0.37 34.8 31.3 -10 13
73 Comparative 1 100 60 0.12 0 0.21 26 13 -50 92 64 Example
INDUSTRIAL APPLICABILITY
[0139] The foamed sheet according to the present invention, since
even if having a small thickness, being excellent in impact
absorption and moreover excellent in heat resistance, is high in
installation (adhesion) reliability even at high temperatures; and
for example, in electric or electronic devices, the foamed sheet is
useful as a member, particularly an impact absorption sheet, for
electric or electronic devices to be used when various types of
members or components (for example, optical members) are attached
(installed) on predetermined sites (for example, housings).
Examples of optical members attachable (installable) by utilizing
the foamed sheet according to the present invention include image
display members (particularly small-size image display members)
installed on image display apparatuses such as liquid crystal
displays, electroluminescence displays and plasma displays, display
members, such as touch panels, installed on mobile communication
apparatuses such as so-called "cellular phones," "smartphones" and
"personal digital assistants," cameras and lenses (particularly
small-size cameras and lenses). The electric or electronic device
according to the present invention uses the foamed sheet according
to the present invention. Such an electric or electronic device
includes, for example, an electric or electronic device having a
display member, and having a structure in which the foamed sheet is
interposed between a housing of the electric or electronic device
and the display member. Examples of the electric or electronic
device include mobile communication apparatuses such as so-called
"cellular phones," "smartphones" and "personal digital
assistants."
REFERENCE SIGNS LIST
[0140] 1 PENDULUM IMPACT TESTER (IMPACT TESTING APPARATUS) [0141] 2
TEST PIECE (FOAMED SHEET) [0142] 3 HOLDING MEMBER [0143] 4 IMPACT
APPLYING MEMBER [0144] 5 PRESSURE SENSOR [0145] 11 FIXING JIG
[0146] 12 PRESSING JIG [0147] 16 PRESSURE ADJUSTING MEANS [0148] 20
SUPPORT COLUMN [0149] 21 ARM [0150] 22 ONE END OF SUPPORT ROD
(SHAFT) [0151] 23 SUPPORT ROD (SHAFT) [0152] 24 IMPACTOR [0153] 25
ELECTROMAGNET [0154] 28 SUPPORT PLATE [0155] a SWING-UP ANGLE
* * * * *