U.S. patent application number 10/362170 was filed with the patent office on 2003-09-04 for expandable thermoplastic resin molded product, method of producing expandable thermoplastic resin molded product and thermoplastic resin foam.
Invention is credited to Miyazaki, Kenji, Nakamura, Masanori.
Application Number | 20030166733 10/362170 |
Document ID | / |
Family ID | 26598498 |
Filed Date | 2003-09-04 |
United States Patent
Application |
20030166733 |
Kind Code |
A1 |
Miyazaki, Kenji ; et
al. |
September 4, 2003 |
Expandable thermoplastic resin molded product, method of producing
expandable thermoplastic resin molded product and thermoplastic
resin foam
Abstract
A thermoplastic resin foam, which is high in thickness-direction
compressive strength and low in flexural modulus of elasticity, can
be set pliably along a curved applicable portion such as body's
outer surface, and can effectively absorb an impact force imparted
from the outside; and an expandable thermoplastic resin molded
product providing the above foam. An expandable thermoplastic resin
molded product comprises an expandable thermoplastic resin
sheet-form material of expandable thermoplastic resin having a
sheet body and many projections formed scattered on at least one
surface of the sheet body, and an elastic thermoplastic material
layer laminated on at least one surface of the expandable
thermoplastic resin sheet-form material so as to be spread over the
outer surfaces of the projections; and a thermoplastic resin foam
obtained by foaming the expandable thermoplastic resin molded
product.
Inventors: |
Miyazaki, Kenji;
(Hasuda-city, JP) ; Nakamura, Masanori;
(Hasuda-city, JP) |
Correspondence
Address: |
Law Offices of Townsend & Banta
1225 Eye Street N W
Suite 500
Washington
DC
20005
US
|
Family ID: |
26598498 |
Appl. No.: |
10/362170 |
Filed: |
February 21, 2003 |
PCT Filed: |
August 17, 2001 |
PCT NO: |
PCT/JP01/07074 |
Current U.S.
Class: |
516/115 |
Current CPC
Class: |
B32B 2305/024 20130101;
B32B 37/0053 20130101; B32B 37/08 20130101; B32B 5/18 20130101;
Y10T 428/249987 20150401; Y10T 428/249981 20150401; Y10T 428/24612
20150115; A63B 71/08 20130101; A41D 31/28 20190201; B32B 37/06
20130101; Y10T 428/249976 20150401; Y10T 428/24998 20150401; Y10T
428/249989 20150401; Y10T 428/249953 20150401; B29L 2031/768
20130101; A41D 31/285 20190201; B32B 2038/0076 20130101; B32B
2398/20 20130101; B32B 2305/72 20130101; A41D 13/0156 20130101;
B29C 51/14 20130101; B32B 2250/22 20130101; B32B 2038/0088
20130101; B29C 51/225 20130101; B32B 3/26 20130101; B29C 44/30
20130101; B29C 43/222 20130101; B29C 44/332 20161101; B29C 44/06
20130101; B32B 5/32 20130101 |
Class at
Publication: |
516/115 |
International
Class: |
C09K 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 25, 2000 |
JP |
2000-255927 |
Apr 26, 2001 |
JP |
2001-129646 |
Claims
1. An expandable thermoplastic resin product including: a
sheet-like structure of expandable thermoplastic resin having a
base sheet and a plurality of raised portions provided in a
distributed manner on at least one surface of said base sheet; and
a layer of elastic thermoplastic material placed on at least one
surface of said sheet-like structure of expandable thermoplastic
resin such that it covers outer surfaces of said raised
portions.
2. The expandable thermoplastic resin product as recited in claim
1, wherein said sheet-like structure of expandable thermoplastic
resin contains a high crosslinked thermoplastic resin composition
and a low crosslinked thermoplastic resin composition, which are
little compatible with each other, and a heat decomposable blowing
agent.
3. The expandable thermoplastic resin product as recited in claim 1
or 2, wherein said elastic thermoplastic material has a flexural
modulus in the range of 5 MPa-1,000 MPa.
4. The expandable thermoplastic resin product as recited in any one
of claims 1 -3, wherein said elastic thermoplastic material
comprises a thermoplastic resin having an elastic property.
5. The expandable thermoplastic resin product as recited in any one
of claims 1-4, wherein said plurality of raised portions are
provided in a distributed manner on one surface of the base
sheet.
6. A method for manufacture of an expandable thermoplastic resin
product comprising the steps of: superimposing an elastic
thermoplastic material or the like on a surface of an expandable
thermoplastic resin sheet to provide a laminate sheet; providing a
pair of couterrotating shaping rolls spaced from each other by a
clearance smaller in dimension than a thickness of said laminate
sheet, a plurality of recesses being provided in a distributed
manner on a periphery of at least one of said rolls; introducing
the laminate sheet while its surface is in a molten state into said
clearance between said pair of shaping rolls and then withdrawing
the laminate sheet from the pair of rolls so that the laminate
sheet is shaped to define, on at least one surface thereof,
projections corresponding to the recesses defined on the periphery
of the shaping roll; and cooling an expandable thermoplastic resin
product having said projections.
7. The method for manufacture of an expandable thermoplastic resin
product as recited in claim 6, wherein said laminate sheet is
introduced into the clearance between said pair of shaping rolls
such that a layer of said elastic thermoplastic material is brought
into contact with said plurality of recesses.
8. A thermoplastic resin foam including: a foam sheet composed of a
thermoplastic resin and having a relative low expansion ratio; a
plurality of basic raised portions integrally provided on at least
one surface of said foam sheet in a distributed manner and
comprising a thermoplastic resin foam having a relatively high
expansion ratio; a low foamed layer having a relatively low
expansion ratio and disposed to cover areas of the basic raised
portions that exclude those in contact with the foam sheet so that
the low foamed layer and the basic raised portions together
constitute raised portions; and a layer of elastic thermoplastic
material disposed to cover the at least one surface of the foam
sheet and the raised portions and also fill spaces between the
raised portions.
9. The thermoplastic resin foam as recited in claim 8, wherein said
layer of elastic thermoplastic material has a foam structure.
10. The thermoplastic resin foam as recited in claim 8 or 9,
wherein said layer of elastic thermoplastic material has a flexural
modulus of 5 MPa-1,000 MPa.
11. The thermoplastic resin foam as recited in any one of claims
8-10, wherein said elastic thermoplastic material comprises a
thermoplastic resin produced via crosslinking of the thermoplastic
resin which constitutes said foam sheet.
12. The thermoplastic resin foam as recited in claim 11, wherein
said crosslinking of the thermoplastic resin is achieved by a
method using a silane-grafted polymer.
13. A thermoplastic resin foam including: a foam sheet-having a
relatively low expansion ratio and composed of a thermoplastic
resin; a plurality of basic raised portions integrally provided on
at least one surface of said foam sheet in a distributed manner and
comprising a thermoplastic resin foam having a relatively high
expansion ratio; a low foamed layer having a relatively low
expansion ratio and disposed to cover areas of the basic raised
portions that exclude those in contact with the foam sheet so that
the low foamed layer and the basic raised portions together
constitute raised portions; and a second foam layer disposed to
cover the at least one surface of the foam sheet and said plurality
of raised portions and also fill spaces between the raised
portions.
Description
TECHNICAL FIELD
[0001] The present invention relates to an expandable thermoplastic
resin product, a method for manufacture thereof and a thermoplastic
resin foam, more particularly to an expandable thermoplastic resin
product useful in obtaining a thermoplastic resin foam which
exhibits superior compressive strength in its thickness direction
and has a suitable application on a protector or the like, a method
for manufacture thereof, and the preceding thermoplastic resin
foam.
BACKGROUND ART
[0002] Thermoplastic resin foams are lightweight, exhibit superior
heat resistance and flexibility and are readily heat-processable
into shapes. Such thermoplastic resin foams have thus achieved wide
use as various cushioning, packaging and heat-insulating
materials.
[0003] In the sporting area, various protectors have been proposed
which serve to protect a part of a human body from a high impact
force. These types of protectors generally incorporate a layer of
thermoplastic resin foam as a shock absorber. If a thermoplastic
resin foam is to be used as a shock absorber, it must have the
ability to absorb a large part of energy when compressed in its
thickness direction. This accordingly requires that the
thermoplastic resin foam should have a high compressive strength in
its thickness direction. Also because a human body has a contoured
surface, it is desired that the thermoplastic resin foam layer is
flexible enough to readily conform to the contours of the human
body, i.e., has a low flexural modulus.
[0004] Conventionally, methods have been proposed for manufacture
of a foam which has the increased compressive strength only in its
thickness direction. For example, Japanese Patent Laying-Open No.
Hei 9-150431 discloses a method in which foaming of an expandable
thermoplastic resin sheet is effected after it has been laminated
with a second sheet having a sufficient strength to suppress planar
expansion of the expandable thermoplastic resin sheet. Accordingly,
the second sheet acts to suppress planar expansion of the
expandable thermoplastic resin sheet while being expanded. Hence,
as foaming proceeds, the expandable thermoplastic resin sheet
expands nearly one-dimensionally in its thickness direction, so
that foam cells are shaped into spindles extending in the thickness
direction.
[0005] The above-described prior method successfully increases the
compressive strength in the thickness direction but fails to
increase the compressive strength in the planar direction. The
resulting thermoplastic resin foam is expected to have a high
compressive modulus but a low flexural modulus. However, the
presence of the second sheet which acts to suppress planar
expansion of the expandable thermoplastic resin sheet actually
increases a flexural modulus of the laminate. Consequently, its
flexibility has been insufficient.
[0006] In Japanese Patent Laying-Open No. Hei 10-44178, a method is
disclosed for allowing an expandable thermoplastic resin sheet to
undergo pseudo-one-dimensional expansion in its thickness direction
by devising its shape. However, the method described in this prior
reference results in the formation of high-density fusion bond
interfaces which extend in the thickness direction of a resulting
foam. These interfaces increase a flexural modulus of the foam,
making its flexibility insufficient. That is, this prior art failed
to reduce a flexural modulus of a resulting foam while increasing
its compressive strength across the thickness.
[0007] In Japanese Patent Laying-Open No. Hei 5-208421, a method is
disclosed for manufacturing a composite foam product by integrating
a non-foamable layer with a foamable layer. This prior art
contemplates to obtain the composite foam product using clean
materials which maintain high fluidity even in such conditions that
a pressure and a shear force are little exerted thereon. For this
purpose, this reference discloses a method in which the
non-foamable layer consisting of thermoplastic elastomer powder,
together with the foamable layer consisting of resin powder of
ethylene-vinyl acetate copolymer and a heat decomposable foaming
agent, are subjected to powder molding so that they are foam
processed.
[0008] However, the manufacturing method described in this prior
reference simply suggests an easy way to manufacture the composite
foam having a complex shape without a residual strain. That is, the
resulting composite foam has a foam layer which is different from
that produced via one-dimensional expansion of the foamable layer
in its thickness direction. Accordingly, a preferential and
sufficient increase in compressive strength of the composite foam
in its thickness direction does not result.
DISCLOSURE OF THE INVENTION
[0009] In view of the current state of the above-described prior
art, it is an object of the present invention to provide a
thermoplastic resin foam which has a high compressive strength in
its thickness direction and a low flexural modulus so that it can
readily conform to a contoured application site, such as an outer
surface of a human body, and effectively absorb an impact force, if
applied from outside, along its thickness; an expandable
thermoplastic resin product which enables us to obtain the
thermoplastic resin foam; and a method for manufacture of such an
expandable thermoplastic resin product.
[0010] In accordance with a broad aspect of the present invention,
an expandable thermoplastic resin product is provided which
includes a sheet-like structure of expandable thermoplastic resin
and a layer of elastic thermoplastic material. The sheet-like
structure has a base sheet and a plurality of raised portions
provided in a distributed manner over at least one surface of the
base sheet. The layer of elastic thermoplastic material is placed
on at least one surface of the sheet-like structure of expandable
thermoplastic resin so that the layer covers outer surfaces of the
raised portions.
[0011] In a particular aspect of the expandable thermoplastic resin
product in accordance with the present invention, the expandable
thermoplastic resin product contains a high crosslinked
thermoplastic resin composition and a low crosslinked thermoplastic
resin composition, which are little compatible with each other, and
a heat decomposable blowing agent.
[0012] In another particular aspect of the expandable thermoplastic
resin product in accordance with the present invention, the elastic
thermoplastic material preferably has a flexural modulus in the
range of 5 MPa-1,000 MPa, preferably in the range of 5 MPa-500
MPa.
[0013] In a further particular aspect of the expandable
thermoplastic resin product in accordance with the present
invention, a thermoplastic resin having an elastic property
constitutes the elastic thermoplastic material.
[0014] In a further particular aspect of the expandable
thermoplastic resin product in accordance with the present
invention, the plurality of raised portions are provided in a
distributed manner on one surface of the sheet-like structure of
expandable thermoplastic resin.
[0015] In accordance with another broad aspect of the present
invention, a method for manufacture of an expandable thermoplastic
resin product is provided. The method comprises the steps of
superimposing a layer of elastic thermoplastic material on one
surface of a sheet of expandable thermoplastic resin to prepare a
laminate sheet; providing a pair of counterrotating shaping rolls
spaced apart by a clearance smaller in dimension than the thickness
of the laminate sheet, with at least one of the shaping rolls
having a plurality of recesses in a distributed manner on its
peripheral surface; introducing the laminate sheet while its
surface is in a softened condition into the clearance between the
pair of shaping rolls and then withdrawing the laminate sheet from
the pair of shaping rolls so that the laminate sheet is shaped to
define, on at least one side thereof, projections corresponding to
the recesses defined on the peripheral surface of the shaping roll;
and cooling an expandable thermoplastic resin product having the
projections. These projections correspond to the preceding raised
portions.
[0016] In a particular aspect of the method for manufacture of an
expandable thermoplastic resin product, in accordance with the
present invention, the laminate sheet is introduced into the
clearance between the pair of shaping rolls such that the layer of
elastic thermoplastic material is brought into contact with the
recesses. In this case, each projection comprises the raised
portion and a portion of the elastic thermoplastic material layer
that covers the raised portion.
[0017] The thermoplastic resin foam in accordance with the present
invention includes a sheet of thermoplastic resin foam having a
relative low expansion ratio; a plurality of basic raised portions
integrally provided on at least one surface of the thermoplastic
resin foam sheet in a distributed manner and comprising a
thermoplastic resin foam having a relatively high expansion ratio;
a low foamed layer having a relatively low expansion ratio and
disposed to cover areas of the basic raised portions that exclude
those in contact with the thermoplastic resin foam sheet so that
the low foamed layer and the basic raised portions together
constitute raised portions; and a layer of elastic thermoplastic
material disposed to cover the at least one surface of the foam
sheet and the raised portions and also fill spaces between the
raised portions.
[0018] In a particular aspect of the thermoplastic resin foam in
accordance with the present invention, the layer of elastic
thermoplastic material has a foam structure.
[0019] In another particular aspect of the thermoplastic resin foam
in accordance with the present invention, the layer of elastic
thermoplastic material has a flexural modulus of 5 MPa-1,000
MPa.
[0020] In a further particular aspect of the thermoplastic resin
foam in accordance with the present invention, the layer of elastic
thermoplastic material is composed of a thermoplastic resin
produced via crosslinking of the thermoplastic resin which
constitutes the foam sheet.
[0021] In a further particular aspect of the thermoplastic resin
foam in accordance with the present invention, crosslinking of the
thermoplastic resin is achieved by a method using a silane-grafted
polymer.
[0022] In the thermoplastic resin foam in accordance with the
present invention, the raised portions preferably have a
cylindrical shape. Preferably, such raised portions are arranged in
a zigzag fashion.
[0023] In a further broad aspect of the thermoplastic resin foam in
accordance with the present invention, the thermoplastic resin foam
is provided which includes a sheet of thermoplastic resin foam
having a relative low expansion ratio; a plurality of basic raised
portions integrally provided on at least one surface of the
thermoplastic resin foam sheet in a distributed manner and
comprising a thermoplastic resin foam having a relatively high
expansion ratio; a low foamed layer having a relatively low
expansion ratio and disposed to cover areas of the basic raised
portions that exclude those in contact with the sheet of
thermoplastic resin foam so that the low foamed layer and the basic
raised portions together constitute raised portions; and a second
foam layer disposed to cover the raised portions and also fill
spaces between the raised portions. Preferably, the second foam
layer comprises the preceding layer of elastic thermoplastic
material.
[0024] In accordance with a further broad aspect of the present
invention, a method for manufacture of the thermoplastic resin foam
of the present invention is provided. This method involves applying
heat to the expandable thermoplastic resin product constructed in
accordance with the present invention to thereby initiate expansion
thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a perspective view which shows an appearance of an
expandable thermoplastic resin product embodiment of the present
invention with a portion being cut away to reveal its section;
[0026] FIG. 2 is a schematic plan view of an expandable
thermoplastic resin product embodiment of the present
invention;
[0027] FIG. 3 is a schematic plan view which explains an altered
expandable thermoplastic resin product embodiment of the present
invention;
[0028] FIG. 4 is a schematic constitutional view which explains an
apparatus for use in the manufacture of the expandable
thermoplastic resin product of the present invention;
[0029] FIG. 5 is a schematic constitutional view which explains
manufacture of an expandable thermoplastic resin product with a
laminate sheet in the process of being shaped to define raised
portions, in accordance with one embodiment of the present
invention;
[0030] FIG. 6 is a partially cut-away sectional view which shows a
thermoplastic resin foam obtained in one embodiment of the present
invention; and
[0031] FIG. 7 is a schematic plan view which explains an altered
thermoplastic resin foam embodiment of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0032] The details of the expandable thermoplastic resin product,
the method for manufacture thereof and resulting thermoplastic
resin foam are below described with reference to the drawings.
[0033] (Thermoplastic resin for use in a sheet-like structure of
expandable thermoplastic resin)
[0034] The type of the thermoplastic resin which constitutes the
sheet-like structure of expandable thermoplastic resin is not
particularly specified, so long as it is expandable. Examples of
such thermoplastic resins include olefinic resins, olefinic
copolymers such as an ethylene-vinyl acetate resin, polyvinyl
chloride, hydrogenated polyvinyl chloride, ABS resin, polystyrene,
polycarbonate, polyamide, polyvinylidene fluoride, polyphenylene
sulfide, polysulfon, polyether ether ketone and their copolymers.
Examples of olefinic resins include low-density polyethylene,
high-density polyethylene and linear low-density polyethylene
("polyethylene" is hereinafter intended to encompass low-density
polyethylene, high-density polyethylene, linear low-density
polyethylene and their mixtures); random polypropylene,
homopolypropylene and block polypropylene ("polypropylene" is
hereinafter intended to encompass random polypropylene,
homopolypropylene, block polypropylene and their mixtures); and the
like. The above-listed thermoplastic resins may be used alone or in
combination.
[0035] The use of olefinic resins, such as polyethylene and
polypropylene, alone or in combination, is preferred for their
ability to improve surface smoothness. Particularly preferred are
high-density polyethylene, homopolypropylene and mixtures
containing at least one of these reins, which can accomplish
simultaneous improvement of surface smoothness and compressive
strength.
[0036] As stated above, the sheet-like structure of expandable
thermoplastic resin has a plurality of raised portions provided in
a distributed manner on at least one surface of the base sheet
which constitutes a main part of the sheet-like structure. The
thermoplastic resin for use in the base sheet may be either
identical to or different in type from that for use in the raised
portions. In view of expandability and adhesion, they are
preferably similar in type to each other.
[0037] Preferably, the expandable thermoplastic resin for use in
the sheet-like structure of expandable thermoplastic resin
comprises a mixture of a high crosslinked thermoplastic resin
composition and a low crosslinked thermoplastic resin composition
which are little compatible with each other (as recited in claim
2). This increases expansion stability and more likely results in
obtaining a product having a higher expansion ratio.
[0038] The high and low crosslinked thermoplastic resin
compositions, as used herein, are relative expressions used
properly depending upon the relative level of crosslinking. Out of
two crosslinked thermoplastic resin compositions, one composition
comprised chiefly of a thermoplastic resin having a relatively
higher level of crosslinking is referred to as the high crosslinked
thermoplastic resin composition (a) and the other composition as
the low crosslinked thermoplastic resin composition (b). The low
crosslinked thermoplastic resin composition (b) can be a
noncrosslinked thermoplastic resin composition.
[0039] The low crosslinked thermoplastic resin composition (b), if
little compatible with the high crosslinked thermoplastic resin
composition (a), readily flows during expansion. This reduces a
possibility that the resulting thermoplastic resin foam may show
short shot and results in the formation of regular and well-ordered
fusion bond interfaces between the foam sheet, raised portions and
elastic thermoplastic material.
[0040] A difference in solubility parameter between respective
thermoplastic resin components of the high crosslinked
thermoplastic resin composition (a) and the low crosslinked
thermoplastic resin composition (b) is preferably within the range
of 0.1-2, more preferably within the range of 0.2-1.5. This
combination enables fine dispersion of those resin components. If
the solubility parameter difference is greater than 2.0, the high
crosslinked thermoplastic resin composition (a) and the low
crosslinked thermoplastic resin composition (b) are dispersed very
coarsely to result in the reduced expansion ratio of the resulting
foam. If the solubility parameter difference is less than 0.1,
these two types of thermoplastic resin components become more
compatible with each other to result in the difficulty to achieve
independent formation of a portion consisting of the high
crosslinked thermoplastic resin composition (a) and a portion
consisting of the low crosslinked thermoplastic resin composition
(b).
[0041] The solubility parameter refers to a value calculated from
the following equation:
.sigma.=.rho..epsilon.Fi/M
[0042] where, .rho. is a density of a thermoplastic resin
component; M is a molecular weight of a monomer which constitutes
the resin component; and Fi is a molar attraction constant of a
constituent group of the monomer.
[0043] A difference in melt index (MI) between thermoplastic resins
for use in the high crosslinked thermoplastic resin composition (a)
and the low crosslinked thermoplastic resin composition (b) is
preferably 5-13 g/10 minutes, more preferably 7-11 g/10 minutes. If
the MI difference is larger than the specified range, the high and
low crosslinked thermoplastic resin composition (a) and (b) may be
dispersed very coarsely to occasionally result in the reduced
expansion ratio of the resulting foam. On the other hand, if it
becomes smaller than the specified range, these two types of
thermoplastic resins become more compatible with each other to
result in the difficulty to achieve independent formation of a
portion consisting of the high crosslinked thermoplastic resin
composition (a) and a portion consisting of the low crosslinked
thermoplastic resin composition (b).
[0044] The MI refers to a value determined according to JIS K
7210.
[0045] The high crosslinked thermoplastic resin composition (a) and
the low crosslinked thermoplastic resin composition (b) are
preferably mixed at a ratio by weight of 2:8-8:2, more preferably
4:6-6:4, further preferably 5:5. The above-specified mixing
proportion permits fine dispersion of the high and low crosslinked
thermoplastic resin compositions (a) and (b) and results in
obtaining a thermoplastic resin foam with a high expansion ratio
and superior surface smoothness.
[0046] If the crosslinking level of the high crosslinked
thermoplastic resin composition (a) is excessively high, an
expansion ratio of a resulting thermoplastic resin foam may be
lowered. On the other hand, if it is excessively low, cell breakage
may occur during expansion to result in the failure to obtain
uniform cells. Accordingly, the high crosslinked thermoplastic
resin composition (a) preferably has a gel fraction, indicative of
the level of crosslinking, of 5-60% by weight, more preferably
10-30% by weight.
[0047] If the crosslinking level of the low crosslinked
thermoplastic resin composition (b) is high, the flowability and
surface smoothness of a resulting thermoplastic resin foam may be
reduced. Hence, the low crosslinked thermoplastic resin composition
(b) preferably has a gel fraction, as indicative of the level of
crosslinking, of up to 5% by weight, more preferably up to 3% by
weight. The gel fraction, as used herein, refers to a percentage of
a weight of a crosslinked thermoplastic resin component remained
after immersion in xylene at 120.degree. C. for 24 hours relative
to a weight of the crosslinked thermoplastic resin component prior
to immersion in xylene.
[0048] The mixture of the high crosslinked thermoplastic resin
composition (a) and the low crosslinked thermoplastic resin
compositions (b) can be prepared by mixing two types of
thermoplastic resins and crosslinking the high crosslinking
thermoplastic resin composition (a) preferentially relative to the
low crosslinking thermoplastic resin composition (b). Applicable
techniques include, for example, (1) a technique which uses a
crosslinking agent capable of preferential crosslinking of the high
crosslinking thermoplastic resin composition (a) relative to the
low crosslinking thermoplastic resin composition (b) ; (2) a
technique which comprises a first step wherein the high
crosslinking thermoplastic resin composition (a) is mixed with a
crosslinkable resin (c) to provide a mixture which is subsequently
crosslinked and a second step wherein the resultant is mixed with
the low crosslinked thermoplastic resin composition (b); (3) a
technique which comprises mixing a crosslinkable resin (c) with the
high and low crosslinking thermoplastic resin compositions (a) and
(b) to provide a mixture which is subsequently crosslinked.
[0049] The technique (3) is most preferred among them since it
results in the formation of small and uniform particle size
portions of the high and low crosslinked thermoplastic resin
compositions (a) and (b), is easier to achieve preferential
crosslinking of the high crosslinking thermoplastic resin
composition (a), and enables easy preparation of the thermoplastic
resin.
[0050] The crosslinkable resin (c) refers to a thermoplastic resin
which is crosslinkable and has approximately the same MI as the
high crosslinking thermoplastic resin composition (a), and can be
illustrated by thermoplastic resins having an unsaturated group
such as vinyl, allyl or propenyl, or a hydroxyl, carboxyl, epoxy,
amino, silanol or silanoate group.
[0051] Examples of crosslinkable resins (c) include maleic acid
modified thermoplastic resins, silane modified thermoplastic resins
and the like. The silane modified thermoplastic resins are
preferred for their ability to readily crosslink with the high
crosslinking thermoplastic resin composition (a) either exclusively
or preferentially relative to the low crosslinking thermoplastic
resin composition (b) and to readily crosslink subsequent to the
mixing.
[0052] Specific examples of silane modified thermoplastic resins
include silane modified polyethylene, silane modified polypropylene
and the like. The silane modified thermoplastic resins can be
obtained via graft modification of thermoplastic resins using an
unsaturated silane compound.
[0053] If the difference in melt index between the silane modified
thermoplastic resin and the high crosslinking thermoplastic resin
composition (a) is larger, it becomes more difficult for the silane
modified thermoplastic resin to crosslink with the high
crosslinking thermoplastic resin composition (a) either exclusively
or preferentially over the low crosslinking thermoplastic resin
composition (b). Hence, the melt index difference is preferably up
to 10 g/10 minutes, more preferably up to 6 g/10 minutes.
[0054] The unsaturated silane compound refers to a compound
represented by the general formula R.sup.1SiR.sup.2.sub.mY.sub.3-m,
wherein m is 0, 1 or 2.
[0055] In the formula, R.sup.1 represents an organic functional
group. Examples of organic functional groups include alkenyl groups
such as vinyl, allyl, propenyl and cyclohexenyl; glycidyl; amino;
methacryl; and halogenated alkyl groups such as .gamma.-chloroethyl
and .gamma.-bromoethyl.
[0056] In the formula, R.sup.2 represents an aliphatic saturated
hydrocarbon group or aromatic hydrocarbon group and may be methyl,
ethyl, propyl, decyl or phenyl, for example. In the formula, Y
represents a hydrolyzable organic functional group. Examples of
such groups include methoxy, ethoxy, formyloxy, acetoxy and
propionoxyarylamino, for example. When m is 0 or 1, Y's may be
identical to or different from each other.
[0057] For the purpose of accelerating a crosslinking reaction, the
compound represented by the general formula
CH.sub.2.dbd.CHSi(OA).sub.3 may preferably be used as the
unsaturated silane compound. In this formula, A is an aliphatic
saturated hydrocarbon group containing preferably 1-8, more
preferably 1-4 carbon atoms.
[0058] Examples of preferred unsaturated silane compounds as
represented by CH.sub.2.dbd.CHSi(OA).sub.3 include
vinyltrimethoxysilane and vinyltriethoxysilane.
[0059] The technique used to produce the silane-grafted polymer is
not particularly specified and can be any technique generally known
in the art. One exemplary technique involves reacting polyethylene
with the preceding unsaturated silane compound represented by
R.sup.1SiR.sup.2.sub.mY.sub.3-m and an organic peroxide to obtain
silane-modified polyethylene.
[0060] For the above silane-grafted polymers having a silyl group,
Y, if methoxy, is hydrolyzed when contacted with water to form a
hydroxyl group. The hydroxyl groups in different molecules react
with each other to form Si-O-Si linkages, so that the
silane-grafted polymers are crosslinked to each other.
[0061] The above-described water treatment technique includes a
steam exposure technique, as well as a water immersion technique.
In the case where such a treatment is carried out at a temperature
of higher than 100.degree. C., it may be performed under pressure.
In the treatment, if a temperature of water or steam is lowered, a
crosslinking reaction rate decreases. On the other hand, if it is
excessively raised, the expandable thermoplastic resin product is
caused to melt. Accordingly, the temperature of water or steam may
be suitably chosen depending upon the type of the thermoplastic
resin used. It is preferably in the range of 80-120.degree. C.
Also, the shortened water treatment period possibly prevents the
crosslinking reaction from going to completion. Thus, the water
treatment period is preferably in the range of 0.5-12 hours.
[0062] A technique used to mix the silane-grafted polymer with the
thermoplastic resin is not particularly specified, so long as it
provides a uniform mixture thereof. For example, a technique may be
utilized in which the thermoplastic resin, together with the
silane-grafted polymer, are fed into a single-or twin-screw
extruder where they are melt mixed. In accordance with other
applicable techniques, they are melt mixed using a roll, a kneader
or the like.
[0063] If the silane-grafted polymer is added in an excessively
large amount, excessive crosslinking may occur to result in the
reduced expansion ratio of the resulting thermoplastic resin foam.
On the other hand, an excessively small amount thereof causes
breakage of cells to result in the difficulty to obtain uniform
foam cells. Accordingly, the silane-grafted polymer is preferably
added in the amount of 5-50 parts by weight, more preferably 10-35
parts by weight, based on 100 parts by weight of the thermoplastic
resin.
[0064] When silane crosslinking is carried out by using the
silane-grafted polymer, a suitable silane crosslinking catalyst may
be used, if necessary. The type of the silane crosslinking catalyst
is not particularly specified, so long as it promotes a
crosslinking reaction between the silane-grafted polymers. Examples
of such catalysts include dibutyltin diacetate, dibutyltin
dilaurate, dioctyltin dilaurate, stannous octoate, stannous oleate,
lead octoate, zinc 2-ethylhexoate, cobalt octoate, lead
naphthenate, zinc caprylate, and zinc stearate.
[0065] The higher loading of the silane crosslinking catalyst
reduces an expansion ratio of the resulting thermoplastic resin
foam. On the other hand, the lower loading thereof slows down the
crosslinking reaction and thus prolongs a period required for the
water treatment. Accordingly, the silane crosslinking catalyst is
preferably added in the amount of 0.01-0.1 parts by weight, based
on 100 parts by weight of the thermoplastic resin.
[0066] Another technique is below described which utilizes a
peroxide to crosslink the thermoplastic resin.
[0067] The type of the peroxide for use in the present technique is
not particularly specified. Examples of peroxides include dibutyl
peroxide, dicumyl peroxide, tert-butylcumyl peroxide, di-isopropyl
peroxide and the like. The use of dicumyl peroxide and
tert-butylcumyl peroxide is preferred because their decomposition
temperatures are closer to a melting point of the thermoplastic
resin. Particularly preferred is dicumyl peroxide.
[0068] If the peroxide is added in an excessively large amount, a
decomposition reaction of the thermoplastic resin may be allowed to
proceed favorably, resulting in the formation of colored
thermoplastic resin foam. On the other hand, if the peroxide is
added in an excessively small amount, insufficient crosslinking of
the thermoplastic resin may result. Hence, the peroxide is
preferably added in the amount of 0.5-5 parts by weight, more
preferably of 1-3 parts by weight, based on 100 parts by weight of
the thermoplastic resin.
[0069] A further technique is now described which utilizes electron
beam exposure to crosslink the thermoplastic resin.
[0070] The higher doses of irradiation lead to excessive
crosslinking which causes reduction in expansion ratio of a
resulting foam. The lower doses of irradiation cause breakage of
foam cells to result in the failure to obtain uniform foam cells.
Accordingly, a suitable dosage may be preferably in the range of
1-20 Mrads. The dosage of 3-10 Mrads is particularly preferred. Any
technique can be employed which exposes the thermoplastic resin to
an ionizing radiation. An exemplary technique involves passing the
thermoplastic resin between a pair of opposing electron beam
generators for exposing an electron beam to the thermoplastic
resin.
[0071] The blowing agent incorporated in the sheet-like structure
of expandable thermoplastic resin is preferably heat decomposable.
The type of the heat decomposable blowing agent is not particularly
specified, so long as it exhibits a decomposition temperature
higher than a melting temperature of the thermoplastic resin used.
Examples of heat decomposable blowing agents include inorganic heat
decomposable blowing agents such as sodium bicarbonate, ammonium
carbonate, ammonium bicarbonate, azido compounds and sodium
borohydride; azodi-carbonamide, azobisformamide,
azobisisobutyronitrile, barium azodicarboxylate, diazoaminobenzene,
N, N'-dinitrosopenta-methylenetetramine, p-toluenesulfonyl
hydrazide, P, P'-oxybis (benzene sulfonyl hydrazide), trihydrazine
triazine and the like. The use of azodicarbonamide is preferred
because its decomposition temperature and rate are readily
controllable and because it has a higher level of hygienic quality
and generates a larger volume of gases. If the heat decomposable
blowing agent is added in an excessively larger amount, bubble
breakage may occur to result in the failure to form uniform cells.
On the other hand, if added in an excessively smaller amount,
insufficient expansion may result. Accordingly, the heat
decomposable blowing agent is preferably incorporated in the amount
of 1-25 parts by weight, based on 100 parts by weight of the
thermoplastic resin.
[0072] When necessary, the thermoplastic resin for use in the
sheet-like structure of expandable thermoplastic resin may contain
a reinforcing material such as a glass, carbon or polyester short
fiber; and/or a filler such as calcium carbonate, aluminum
hydroxide or glass powder in order to improve strength thereof.
[0073] (Elastic thermoplastic material)
[0074] The elastic thermoplastic material for use in the expandable
thermoplastic resin product of the present invention preferably
shows a wide range of elastic deformation when it is placed under a
compressive, bending, tensile or other stress. Specifically, the
elastic thermoplastic material refers to a material which shows
elastic deformation in the 10-100% range within deformation stain.
Examples of such materials include thermoplastic elastomers such as
olefinic elastomers, polyester elastomers and styrenic elastomers;
ethylene-vinyl acetate copolymer (EVA); thermoplastic polyurethane;
soft polyvinyl chloride; and the like. Since the above-listed
materials are thermoplastic and accordingly melt-fabricable, they
are readily laminated with the sheet-like structure of expandable
thermoplastic resin into a unitary form.
[0075] The elastic thermoplastic material generally has a flexural
modulus in the range of 5 MPa -1,000 MPa, preferably in the range
of 5 MPa -500 MPa.
[0076] If its flexural modulus exceeds 1,000 MPa, the resulting
thermoplastic resin foam may also have a high flexural modulus. If
below 5 MPa, the resulting thermoplastic resin foam may exhibit
poor durability against friction.
[0077] Examples of elastic thermoplastic materials include olefinic
thermoplastic elastomers, styrenic thermoplastic elastomers,
polyester thermoplastic elastomers and amide thermoplastic
elastomers.
[0078] Preferably, the elastic thermoplastic material is
expandable. The elastic thermoplastic material, if expanded, shows
a lower flexural modulus than its original flexural modulus. This
is more favorable in accomplishing the purpose of the present
invention.
[0079] Various elastic thermoplastic materials such as those listed
above can be used in combination with a blowing agent to constitute
such an expandable elastic thermoplastic material. Examples of
useful blowing agents include the above-listed heat decomposable
blowing agents. The loading of the blowing agent may be determined
depending upon the expansion ratio of the elastic thermoplastic
material used. If the loading of the blowing agent is excessively
high, a compressive strength of a resulting thermoplastic resin
foam may become low. On the other hand, if excessively low, its
flexural modulus reducing effect may become small. Accordingly, the
blowing agent is preferably loaded in the amount of 0.3 parts-10
parts, more preferably 0.5 parts-5 parts, based on 100 parts by
weight of the elastic thermoplastic material.
[0080] When necessary, the elastic thermoplastic material may be
crosslinked as by the preceding crosslinking techniques.
[0081] The ratio by amount of the elastic thermoplastic material to
the raised portions may be suitably varied depending upon the
diameter of the raised portions. The excessively high ratio lowers
a compressive strength of a thermoplastic resin foam obtained via
expansion of the expandable thermoplastic resin product. On the
other hand, the excessively low ratio results in the failure to
accomplish the object of the present invention, i.e., to achieve
reduction of a flexural modulus. Hence, where the plurality of
raised portions are integrally provided on the base sheet, the
elastic thermoplastic material constitutes a diameter portion that
preferably accounts for 10-60%, more preferably30-50%, of a
diameter of a projection which consists of each raised portion and
a covering elastic material portion.
[0082] (Configuration of an expandable thermoplastic resin
product)
[0083] The expandable thermoplastic resin product of the present
invention is composed of the aforesaid expandable thermoplastic
resin and includes a sheet-like structure of expandable
thermoplastic resin which has a plurality of raised portions
provided on at least one surface thereof in a distributed manner.
Also, a layer of the elastic thermoplastic material is placed on
the at least one surface of the sheet-like structure of expandable
thermoplastic resin so that the layer of elastic thermoplastic
material covers outer surfaces of the raised portions but leaves
spaces between the raised portions. One embodiment of the
expandable thermoplastic resin product is below described with
reference to FIG. 1.
[0084] An expandable thermoplastic resin product 1 has a plurality
of raised portions 2 provided on one surface of a base sheet 3 in a
distributed manner. An elastic thermoplastic material 4 is further
placed on the base sheet 3 in such a way to cover outer surfaces of
the raised portions 2.
[0085] In the expandable thermoplastic resin product 1, those
raised portions 2 covered with the elastic thermoplastic material 4
are generally uniformly arranged in a lattice-like pattern, as
shown in FIGS. 1 and 2. The shape of the raised portions 2 is not
particularly specified and may be a hexahedron, cylinder or sphere,
for example. In considerations of uniform expansion of the raised
portions, the cylindrical shape is preferred, as shown in FIGS. 1
and 2.
[0086] If the raised portion 2 is cylindrical, its diameter may be
varied depending on the thickness and expansion ratio of the
contemplated foam and is not particularly specified. If the
diameter is excessively large, expansion becomes slow. On the other
hand, if the diameter is excessively small, the cylindrical raised
portions become more likely to melt and undergo shape change when
exposed to heat during expansion. This disturbs
pseudo-one-dimensional expansion thereof to result in the increased
variations of thickness and weight precisions and the reduced
surface smoothness of the resulting foam. Accordingly, the raised
portions 2, if cylindrical, preferably have diameters of 1-30 mm,
more preferably 2-20 mm, after they have been covered with the
elastic thermoplastic material.
[0087] The distance between neighboring raised portions 2, 2 is
varied depending on the expansion ratio, thickness and the like of
the foam contemplated and is not particularly specified. If the
distance is excessively long, short shot may occur when the raised
portions are expanded. On the other hand, if the distance is
excessively short, the large expansion in the lateral and
longitudinal directions may occur due to the limited expansion
area. Accordingly, the center distance between neighboring raised
portions 2, 2 is preferably 2-50 mm, more preferably 3-30 mm.
[0088] If the final foam is to show improved thickness and weight
precisions, high surface smoothness and uniform expansion ratio,
the plurality of raised portions 2 must be arranged generally
uniformly on a plane of the base sheet 3. The particular
arrangement thereof is not specified. They may be arranged in a
lattice pattern as shown in FIGS. 1 and 2, or alternatively, in a
zigzag pattern as shown in FIG. 3. If arranged in a lattice
pattern, the individual raised portions 2 when expanded define
tetragonal prisms in the final foam, so that the foam exhibits the
increased compressive strength.
[0089] If arranged in a zigzag pattern, the individual raised
portions 2 when expanded define hexagonal prisms, so that a
honeycomb-like structure is formed in the resulting foam. This
structure increases surface smoothness and further improves
compressive strength of the resulting foam. It is accordingly
preferred that the raised portions are arranged in a zigzag
pattern.
[0090] In the sheet-like structure of expandable thermoplastic
resin, the thickness of the base sheet 3 is varied depending on the
expansion ratio, thickness and the like of the foam contemplated
and is not particularly specified. If the thickness is excessively
large, the base sheet when expanded may dislocate the raised
portions to result in the large expansion in the lateral and
longitudinal directions. If the thickness is excessively small, the
base sheet may fail to retain the raised portions. Accordingly, the
base sheet 3 preferably has a thickness of 0.05-3 mm, preferably
0.1-2 mm.
[0091] The ratio by amount of the elastic thermoplastic material
for use in the present invention to the sheet-like structure of
expandable thermoplastic resin may be suitably varied depending
upon the diameter of the raised portions. If the ratio is
excessively high, short shot may occur when the sheet-like
structure of expandable thermoplastic resin expands. On the other
hand, the excessively low ratio results in the failure to
accomplish the purpose of this invention, i.e., to obtain a foam
having a low flexural modulus. For the above reasons, the elastic
thermoplastic material is proportioned to constitute a diameter
portion which is preferably 15-75%, more preferably 35-60% of a
diameter of the raised portions covered with the elastic
thermoplastic material 4.
[0092] In the case where the elastic thermoplastic material 4 is
placed to lie over the opposite surface of the base sheet 3 of
expandable thermoplastic resin that does not carry the raised
portions 2, the thickness of the elastic thermoplastic material may
be varied depending on the sum in height of the expandable
thermoplastic resin base sheet and the raised portion. If the
thickness of the elastic thermoplastic material relative to the sum
in height thereof is excessively large, a thermoplastic resin foam
obtained via expansion of the expandable thermoplastic resin
product shows a reduction in compressive strength. On the other
hand, if it is excessively small, it becomes difficult to obtain a
low flexural modulus foam.
[0093] Any technique by which the elastic thermoplastic material
and sheet-like structure of expandable thermoplastic resin can be
laminated into an integral form may be utilized. Examples of such
lamination techniques include thermal bonding, co-injection,
co-extrusion and the like.
[0094] In a particular aspect of the present invention, a laminate
sheet 8 having an elastic thermoplastic material provided on at
least one surface of an expandable thermoplastic resin sheet, each
in a molten state, is introduced between a pair of shaping rolls 7
and 7A with a clearance smaller in dimension than the thickness of
the laminate sheet, as shown in FIG. 5. At least one of the pair of
shaping rolls 7 and 7A has a plurality of recesses 71 on its
periphery. The laminate sheet 8 is withdrawn from between the pair
of shaping rolls 7 and 7A and subsequently cooled. As a result, an
expandable thermoplastic resin product is obtained which has a
plurality of projections corresponding in location to the recesses
71.
[0095] In the above process, the expandable thermoplastic resin
sheet in a molten state is prepared by extruding an expandable
thermoplastic resin while in a molten state from a sheet die. The
elastic thermoplastic material in a molten state may be laminated
therewith by any technique known in the art. For example, an
elastic thermoplastic material is supplied into a kneading extruder
where it is melt kneaded. An expandable thermoplastic resin is
supplied into another kneading extruder where it is melt kneaded.
The individual melt components are combined by extrusion through a
three layer sheet die 6 as shown in FIG. 6 to provide the laminate
sheet. This technique enables continuous manufacture of such a
laminate sheet. In the case where the elastic thermoplastic
material is placed on one surface of the expandable thermoplastic
resin sheet, an extruder B may be used alone to supply the elastic
thermoplastic material. In the case where the elastic thermoplastic
material is placed on both surfaces of the expandable thermoplastic
resin sheet, extruders B and C may be used in combination to supply
the elastic thermoplastic material.
[0096] The technique used to melt knead the expandable
thermoplastic resin is not particularly specified. Melt kneading
thereof can be achieved by a single- or twin-screw kneading
extruder or by a pair of counterrotating rolls, for example. The
use of a twin-screw extruder is preferred for its higher kneading
ability.
[0097] Preferably, the plurality of recesses 71 on the periphery of
the shaping roll 7 are arranged in a generally uniform fashion. The
generally uniform arrangement improves weight and thickness
precisions of a resulting expandable thermoplastic resin product.
It is particularly preferred that they are arranged in a zigzag
fashion. This zigzag arrangement adds to uniformity and results in
the formation of a honeycomb structure in a resulting foam and the
further improvement in compressive strength of the resulting
foam.
[0098] The shape of the recesses 71 on the periphery of the shaping
roll 7 is chosen depending on the shape of the raised portions and
may be a hexahedron, cylinder or sphere, for example. The
cylindrical shape is most preferred since it eases removal of the
cooled laminate sheet from the shaping roll.
[0099] In the case where the recesses 71 on the periphery of the
shaping roll 7 have a cylindrical shape, a diameter of the recess
71 must be varied depending on the contemplated shape of the
thermoplastic resin product and is not particularly specified. If
the recess diameter is excessively large, removal of the cooled
laminated sheet from the shaped roll is made difficult to result in
the breakage of the base sheet of the sheet-like structure of
expandable thermoplastic resin. On the other hand, if it is
excessively small, the raised portions may be broken when the
cooled laminate sheet is removed from the shaping roll.
Accordingly, the recesses 71 preferably have diameters of 1-30 mm,
particularly preferably 2-20 mm.
[0100] In the case where the recesses 71 on the periphery of the
shaping roll 7 have a cylindrical shape, a depth of the recess 71
must be varied depending on the contemplated shape of the
thermoplastic resin product and is not particularly specified. If
the recess depth is excessively large, removal of the cooled
laminated sheet from the shaped roll is made difficult to result in
the breakage of the base sheet of the sheet-like structure of
expandable thermoplastic resin. On the other hand, the provision of
excessively shallow recesses results in the failure to manufacture
an expandable thermoplastic resin product capable of
pseudo-one-dimensional expansion. In the light of the foregoing,
those recesses preferably have depths of 1-30 mm, more preferably
2-20 mm.
[0101] The clearance a defined between the pair of shaping rolls 7
and 7A must be smaller in dimension than the thickness of the
laminate sheet 8 in a molten state. The clearance must be varied,
within the specified range, depending on the contemplated shape of
the resulting expandable thermoplastic resin product and is not
particularly specified. The excessively wide clearance makes it
difficult to manufacture an expandable thermoplastic resin product
capable of pseudo-one-dimensional expansion. On the other hand, the
excessively narrow clearance increases a tendency of the base sheet
of the sheet-like structure of expandable thermoplastic resin to
break upon removal of the cooled laminated sheet from the shaped
roll. Accordingly, the clearance a is maintained to generally
measure 0.05-3 mm, more preferably 0.1-2 mm.
[0102] The laminate sheet 8 may be partly pressed into the recesses
71 of the shaping roll 7 in the following manner. The pair of
shaping rolls 7 and 7A, if the clearance a defined therebetween is
maintained unvaried, exerts a pressure on the laminate sheet 8 to
thereby impart a shape on a sheet surface. The technique for
cooling the pressed and shaped laminate sheet 8 is not particularly
specified, so long as it can reduce its temperature to a melting
point of the expandable thermoplastic resin or below. For example,
a cooling water may be circulated within the shaping rolls 7 and
7A.
[0103] A specific procedure for manufacture of the expandable
thermoplastic resin product 1 is below described with reference to
FIGS. 4 and 5.
[0104] A resin composition which contains constituents of an
expandable thermoplastic resin product, is supplied into the
extruder A of a three layer sheet extrusion device, as shown in
FIG. 4, where it is melt kneaded at a temperature of not below a
melt temperature of a thermoplastic resin but below a decomposition
temperature of a blowing agent. Concurrently, the preceding elastic
thermoplastic material is supplied into either one or both of the
extruders B and C where it is kneaded at or over a melt temperature
of the elastic thermoplastic material. Respective melt components
are passed through a three layer sheet extrusion die 6 having the
structure shown in FIG. 4 to provide a laminate sheet having a two-
or three-layer structure. As shown in FIG. 5, the laminate sheet 6
is shaped and cooled when it is passed between the pair of
counterrotating shaping rolls 7 and 7A with the constant clearance
a and the recesses. As a result, an expandable thermoplastic resin
product 1 is obtained which has a plurality of raised portions
corresponding in shape to those recesses and projecting from a
surface of the base sheet 3 of expandable thermoplastic resin, as
shown in FIG. 1.
[0105] Preferably, the laminate sheet is introduced between the
rolls 7 and 7A such that the elastic thermoplastic material in a
molten state is brought into contact with the shaping roll 7 having
the plurality of recesses generally uniformly provided on its
periphery.
[0106] As described earlier, the elastic thermoplastic material,
when subjected to a compression, bending, tensile or other form of
stress, shows elastic deformation over a wider range relative to
the thermoplastic resin. Accordingly, if the laminate sheet 8 is
introduced such that the elastic thermoplastic material is brought
into contact with the shaping roll 7, the laminate sheet, when
withdrawn from between the rolls 7 and 7A, extends to a larger
degree at locations where the elastic thermoplastic material
exists. This prevents breakage of the base sheet covered with the
elastic thermoplastic material.
[0107] As a consequence, a higher-speed manufacture of the
expandable thermoplastic resin product is enabled.
[0108] (Thermoplastic Resin Foam)
[0109] The thermoplastic resin foam in accordance with the present
invention includes a foam sheet having a relatively low expansion
ratio and composed of a thermoplastic resin; a plurality of basic
raised portions integrally provided on at least one surface of the
foam sheet in a distributed manner and comprising a foam having a
relatively high expansion ratio; a low foamed layer having a
relatively low expansion ratio and disposed to cover areas of the
basic raised portions that exclude those in contact with the foam
sheet so that the low foamed layer and the basic raised portion
together constitute raised portions; and a layer of elastic
thermoplastic material disposed to cover the at least one surface
of the foam sheet and the raised portions and also fill spaces
between the raised portions.
[0110] Expansion of the expandable thermoplastic resin product 1
shown in FIG. 1 results in manufacture of the thermoplastic resin
foam as shown in FIG. 6. The base sheet 3 shown in FIG. 1 is
converted to a foam sheet 9. Expansion of the raised portions 2
shown in FIG. 1 results in the formation of high foamed, basic
raised portions 10a and a low foamed layer 10b which cover areas of
the basic raised portions 10a that exclude those in contact with
the foam sheet 9. When the sheet-like structure of expandable
thermoplastic resin is thermally expanded, a surface layer of the
raised portions 2 is expanded to a lower degree relative to the
rest. This is why expansion of the raised portions results in the
formation of the basic raised portions 10a and the low foamed layer
10b which covers outer surfaces of the basic raised portions 10a.
Further, the elastic thermoplastic material 4 covers outer surfaces
of the raised portions 10 which consist of the basic raised
portions 10a and the low foamed layer 10b. In this case, the
elastic thermoplastic material 4 not only covers the raised
portions but also fills spaces between the raised portions 10
resulting from expansion of the raised portions 2 (FIG. 1).
Accordingly, the resulting thermoplastic resin foam is provided in
the form of a sheet or plate having planar top and bottom
surfaces.
[0111] The thermoplastic resin foam generally takes a sheet or
plate form. The elastic thermoplastic material 4, if expanded,
reduces a flexural modulus of a resulting thermoplastic resin foam.
Such an expansion ratio is preferably 4-20, more preferably 4-10.
If the expansion ratio is lower, a thermoplastic resin foam is
obtained having a higher compressive strength. Such an expansion
ratio is preferably 1-3, more preferably 1-2. The expansion ratio
of 1 means the absence of expansion.
[0112] If the thickness of the elastic thermoplastic material 4 is
large, a weight reduction of the thermoplastic resin foam may not
be accomplished. On the other hand, if it is small, a reduction in
flexural modulus of the thermoplastic resin foam may not be
achieved. Accordingly, it is preferably in the range of 50-300
.mu.m, more preferably in the range of 100-1,000 .mu.m. The elastic
thermoplastic material may be either uniform or varied in
thickness.
[0113] The lower expansion ratio of the basic raised portions 10a
increases a flexural modulus of the thermoplastic resin foam 11 to
result in the failure to accomplish the purpose of the present
invention. In contrast, the higher expansion ratio thereof results
in obtaining the thermoplastic resin foam having a lower flexural
modulus. Accordingly, the expansion ratio of the basic raised
portions may preferably be in the range of 2-30, more preferably
3-20.
[0114] If the height of the raised portions 10 is excessively
large, a flexural modulus of the resulting thermoplastic resin foam
11 may become too high. If it is excessively small, a weight
reduction of the thermoplastic resin foam 11 may not be achieved.
Accordingly, it is preferably 3-50 mm, more preferably 5-30 mm.
[0115] The raised portions 10 may be either uniform or varied in
size. If the expansion ratio of the foam sheet 9 is low, a flexural
modulus of the resulting thermoplastic resin foam may become too
high. If it is high, a compressive strength of the thermoplastic
resin foam decreases. Accordingly, the expansion ratio of the foam
sheet is preferably 1.1-10, more preferably 2-8. The larger
thickness of the foam sheet 9 increases a flexural modulus of the
thermoplastic resin foam. On the other hand, the smaller thickness
of the foam sheet 9 reduces a surface strength of the thermoplastic
resin foam. Accordingly, the thickness of the foam sheet is
preferably 0.1-5 mm, more preferably 0.3-3 mm, further preferably
0.5-2 mm.
[0116] The foam sheet 9 may be either uniform or varied in
thickness.
[0117] In order that the variations of thickness precision and
compressive strength of the thermoplastic resin foam are made
small, it is preferred that the raised portions 10 are generally
uniformly arranged in a plane extending in the direction of a cross
section of the thermoplastic resin foam. The raised portions 10 may
be generally uniformly arranged in accordance with various
patterns. For example, they may be arranged in a lattice pattern,
as described earlier, or alternatively, in a zigzag pattern, as
shown in FIG. 7.
[0118] When arranged in a lattice pattern, the raised portions 10
have a shape of a tetragonal prism and increase a compressive
strength of the thermoplastic resin foam. When arranged in a zigzag
pattern, the raised portions 10 have a shape of a hexagonal prism
and together constitute a honeycomb structure in the thermoplastic
resin foam. Such a thermoplastic resin foam having a honeycomb
structure exhibits a particularly excellent compressive
strength.
[0119] The preceding thermoplastic resin foam can be obtained by a
manufacturing method which comprises thermally expanding the
expandable thermoplastic resin product 1 at a temperature equal to
or higher than a decomposition temperature of the blowing agent and
then cooling the expanded product.
[0120] In the thermally expanding step, the expandable
thermoplastic resin product 1 may be heated to a temperature equal
to or higher than a decomposition temperature of a heat
decomposable blowing agent incorporated in the raised portions 2.
Heating may be achieved, for example, by using an electric heater,
a far-infrared radiating heater or a heating unit which circulates
therein a heating medium such as hot oil or air. Various techniques
can be utilized to effect cooling. A technique which can cool the
expanded product to a temperature equal to or lower than a
softening point of a resin constituting the foam 11 may be
utilized. For example, cooling can be achieved by using a cooling
unit which circulates therein a coolant such as cool water or
air.
[0121] In a further broad aspect of the present invention, a
thermoplastic resin foam is provided which uses a second foam layer
in the place of the elastic thermoplastic material 4. In this case,
the second foam layer comprises a thermoplastic resin foam. The
second foam layer, if having an elastic property in addition to
comprising the thermoplastic resin foam, then becomes identical to
the elastic thermoplastic material 4. However, the second foam
layer does not necessarily comprise the elastic thermoplastic
material.
[0122] The present invention is below described in more detail by
referring to non-limiting examples and comparative examples.
EXAMPLE 1
[0123] A composition containing 100 parts by weight of
thermoplastic resins mixed at the proportion specified in Table 1,
1 part by weight of a silane crosslinking catalyst master batch
(product of Mitsubishi Chemical Corp., product name "PZ10S") and 5
parts by weight of an azodicarbonamide heat decomposable blowing
agent (product of Otsuka Chemical Co., Ltd., product name "UNIFOAM
AZ SO-20", decomposition temperature of 201.degree. C.) was
supplied into the twin screw extruder A shown in FIG. 5. A styrenic
elastomer (product of Kuraray Co., Ltd., product name "SEPTON
2043", flexural modulus of 10 MPa) was supplied into the twin screw
extruder B shown in FIG. 5.
[0124] The twin screw extruders A and B both have a screw diameter
of 44 mm and respectively have L/D ratios of 35 and 28. The
above-specified composition and the styrenic elastomer were melt
kneaded at 180.degree. C. in the twin screw extruders A and B,
respectively, introduced into the two layer sheet extrusion die 6,
and then extruded from its head having a face of 500 mm and a lip
opening of 1.0 mm into the form of a two layer sheet. Each of the
styrenic elastomer and thermoplastic resin layers measured 0.6 mm
thick and a total thickness of the obtained sheet was 1.2 mm.
[0125] The two layer sheet obtained was introduced between a 250 mm
diameter and 500 mm long roll 7 having recesses 71 arranged in a
zigzag pattern on its outer surface, as shown in FIG. 3, and a roll
7A having no recesses (roll clearance of 0.2 mm) such that the
styrenic elastomer layer of the two layer sheet was brought into
contact with the roll 7 having the recesses 71, then cooled while
shaped and finally removed from the rolls 7 and 7A to provide a
thermoplastic resin product. The thermoplastic resin product was
subsequently immersed in 99.degree. C. water for 2 hours so that a
crosslinking reaction was caused to proceed, and then dried to
obtain an expandable thermoplastic resin product 1. In the
expandable thermoplastic resin product 1, the raised portions
(inclusive of the elastomer covering layer) arranged in a zigzag
pattern were found as being in the form of cylinders having a
height of 5 mm and a diameter of 4 mm and arranged at intervals of
12.1 mm. The base sheet measured 0.4 mm thick.
[0126] The expandable thermoplastic resin product 1 obtained in
accordance with the above procedure was interposed between two
sheets of ethylene fluoride, heated for 10 minutes by a heat press
controlled at 230 centigrade, and then cooled for 10 minutes by a
cold press controlled at 20.degree. C. to obtain the aimed
thermoplastic resin foam 11. The such-obtained thermoplastic resin
foam 11 measured 6.6 mm thick.
[0127] A 50.times.50.times.6.6 mm piece for a compression test and
a 200.times.25.times.6.6 mm piece for a flexural test were cut out
from the thermoplastic resin foam 11. These test pieces were
subjected to compression and flexural tests in accordance with JIS
K 7220 and JIS K 7221, respectively, to measure a 25% compressive
strength and a flexural modulus. The results are given in Table
2.
[0128] In Table 2, the results for expansion ratios measured in
accordance with JIS K 6767 are also given.
1 TABLE 1 Example Comp. Ex. 1 2 3 4 5 6 1 2 Homopolypropylene 80 80
80 80 30 80 80 80 Silane-Grafted Homopolypropylene 20 20 20 20 20
20 20 20 High-Density -- -- -- -- 50 -- -- -- Polyethylene (wt. %)
Homopolypropylene: Product Name "MA3" (Product of Nippon Polychem
Co., Ltd., MFR = 10 g/10 min., Flexural Modulus of 1350 MPa)
Silane-Grafted Homopolypropylene: Product Name "XPM800HM" (Product
of Mitsubishi Chemical Corp., MFR = 16 g/10 min.) High-Density
Polyethylene: Product Name "HJ340" (Product of Nippon Polychem Co.,
Ltd., MFR = 1.5 g/10 min.)
[0129]
2 TABLE 2 Example Comp. Ex. 1 2 3 4 5 6 1 2 25% Com- 0.60 0.58 0.61
0.52 0.65 0.59 0.60 0.65 pressive Strength (MPa) Flexural 18.3 14.5
21.5 12.2 15.5 19.7 80.5 120.2 Modulus (MPa) Expansion 6.0 6.0 6.0
10.0 6.0 6.0 10.0 6.0 Ratio (cc/g)
EXAMPLE 2
[0130] A composition containing 100 parts by weight of
thermoplastic resins at the proportion specified in Table 1, 1 part
by weight of a silane crosslinking catalyst master batch (product
of Mitsubishi Chemical Corp., product name "PZ10S") and 5 parts by
weight of an azodicarbonamide heat decomposable blowing agent
(product of Otsuka Chemical Co., Ltd., product name "UNIFOAM AZ
SO-20", decomposition temperature of 201.degree. C.) was supplied
into the twin screw extruder A shown in FIG. 5. A styrenic
elastomer (product of Kuraray Co., Ltd., product name "SEPTON
2043", flexural modulus of 10 MPa) was supplied into the twin screw
extruders B and C shown in FIG. 5.
[0131] The twin screw extruders A , B and C all have a screw
diameter of 44 mm. The twin screw extruder A has an L/D ratio of
35. The twin screw extruders B and C both have an L/D ratio of 28.
The above-specified composition and the styrenic elastomer were
melt kneaded at 180.degree. C. in the twin screw extruder A and the
twin screw extruders B and C, respectively, introduced into a three
layer sheet extrusion die and extruded from its head having a face
of 500 mm and a lip opening of 1.0 mm into the form of a three
layer sheet. For the sheet obtained, a top styrenic elastomer
layer, a middle thermoplastic resin layer and a bottom styrenic
elastomer layer measured 0.3 mm, 0.6 mm and 0.3 mm, respectively.
Subsequently, the procedure of Example 1 was followed to
manufacture the expandable thermoplastic resin product and the
thermoplastic resin foam.
EXAMPLE 3
[0132] The material to be extruded from the twin screw extruder B
was changed to an ethylene-vinyl acetate copolymer (product of
Mitsubishi Chemical Corp., product name "NOVATEC LV660": vinyl
acetate content of 28%, MFR=6.0 g/10 minutes, flexural modulus of
100 MPa). Otherwise, the procedure of Example 1 was followed.
EXAMPLE 4
[0133] The material to be extruded from the twin screw extruder B
was changed to a composition containing 100 parts by weight of a
resin composition containing 90 wt. % of an ethylene-vinyl acetate
copolymer (product of Mitsubishi Chemical Corp., product name
"NOVATEC LV660": vinyl acetate content of 28%, MFR=6.0 g/10
minutes, flexural modulus of 100 MPa) and 10 wt. % of a
silane-grafted ethylene-vinyl acetate copolymer (product of
Mitsubishi Chemical Corp., product name "LINKRON XVF750N", flexural
modulus of 40 MPa), 0.5 parts by weight of a silane crosslinking
catalyst master batch (product of Mitsubishi Chemical Corp.,
product name "VZ10") and 5 parts by weight of an azodicarbonamide
heat decomposable blowing agent (product of Otsuka Chemical Co.,
Ltd., product name "UNIFOAM AZ SO-20", decomposition temperature of
201.degree. C.). Otherwise, the procedure of Example 1 was
followed.
EXAMPLE 5
[0134] The proportion of the thermoplastic resins to be extruded
from the twin screw extruder A was changed to that specified in
Table 1. Otherwise, the procedure of Example 1 was followed.
EXAMPLE 6
[0135] In obtaining the expandable thermoplastic resin product in
Example 1, the two layer sheet was introduced such that the
thermoplastic resin layer was located to contact with the roll 7
having the recesses. Otherwise, the procedure of Example 1 was
followed. The obtained expandable thermoplastic resin product was
found to include raised portions covered with the elastomer and
arranged in a zigzag pattern. Such elastomer covered raised
portions were observed to exist in the form of cylinders having a
height of 5.3 mm and a diameter of 4 mm and arranged at intervals
of 12.1 mm, and accordingly be almost comparable in configuration
to those of the expandable thermoplastic resin product obtained in
Example 1. However, undesirable results, such as partial breakage
of some raised portions, occurred.
COMPARATIVE EXAMPLE 1
[0136] A composition containing 100 parts by weight of
thermoplastic resins at the proportion specified in Table 1, 1 part
by weight of a silane crosslinking catalyst master batch (product
of Mitsubishi Chemical Corp., product name "PZ10S") and 5 parts by
weight of an azodicarbonamide heat decomposable blowing agent
(product of Otsuka Chemical Co., Ltd., product name "UNIFOAM AZ
SO-20", decomposition temperature of 201.degree. C.) was supplied
into the twin screw extruder A shown in FIG. 5. No material was
supplied into the twin screw extruders B and C. A single layer
sheet from the die was introduced between the rolls 7 and 7A to
provide an expandable thermoplastic resin foam composed of those
thermoplastic resins. Otherwise, the procedure of Example 1 was
followed.
COMPARATIVE EXAMPLE 2
[0137] The material to be extruded from the twin screw extruder B
was changed to a polyamide-based nylon 66 (product of Ube
Industries, Ltd., product name "UBE NYLON 2015B", flexural modulus
of 2,900 MPa). Otherwise, the procedure of Example 1 was
followed.
EXAMPLE 7
[0138] As the material for constituting the sheet-like structure of
expandable thermoplastic resin, a mixture containing 100 parts by
weight of thermoplastic resins at the proportion specified in Table
3, 1 part by weight of a silane crosslinking catalyst master batch
(product of Mitsubishi Chemical Corp., product name "PZ10S") and 5
parts by weight of an azodicarbonamide heat decomposable blowing
agent (product of Otsuka Chemical Co., Ltd., product name "UNIFOAM
AZ SO-20", decomposition temperature of 210.degree. C.) was
supplied into a corotating twin screw extruder (PCM 30,
manufactured by Ikegai Tekko Co., Ltd.) where it was melt kneaded
at 190.degree. C.
[0139] As the material for constituting the second foam layer, a
mixture containing 100 parts by weight of a resin composition
containing 90% by weight of an ethylene-vinyl acetate copolymer
(product of Mitsubishi Chemical Corp., product name: NOVATEC LV660,
vinyl acetate content of 28 weight %, MFR of 6.0 g/10 minutes,
flexural modulus of 100 MPa) and 10% by weight of a silane-grafted
ethylene-vinyl acetate copolymer (product of Mitsubishi Chemical
Corp., product name: LINKRON XVF750N, flexural modulus of 40 MPa),
0.5 parts by weight of a silane crosslinking catalyst master batch
(product of Mitsubishi Chemical Corp., product name: VZ10) and 5
parts by weight of an azodicarbonamide heat decomposable blowing
agent (product of Otsuka Chemical Co., Ltd., product name: UNIFOAM
AZ SO-20, decomposition temperature of 210.degree. C.) was supplied
into a corotating twin screw extruder (PCM 30, manufactured by
Ikegai Tekko Co., Ltd.) where it was melt kneaded at 190.degree.
C.
[0140] The melt resins from those two extruders were introduced
into a two layer sheet extrusion die and extruded from its head
having a face of 500 mm and a lip opening of 1.0 mm into the form
of a two layer sheet. Each of the sheets for the sheet-like
structure of expandable thermoplastic resin and the second foam
sheet measured 0.6 mm thick and a total thickness of the obtained
two layer sheet was 1.2 mm.
[0141] Subsequently, the two layer sheet was introduced between a
roll 7 having recesses and a roll 7A such that the sheet for the
second foam layer was brought into contact with the roll 7 having
the recesses, cooled while shaped and then removed from the rolls 7
and 7A to provide a laminate sheet. This laminate sheet was
subsequently immersed in 99.degree. C. water for 2 hours so that a
crosslinking reaction was caused to proceed, and then dried to
obtain an expandable resin product. The expandable resin product
was found to include raised portions in the form of cylinders
having a height of 5 mm and a diameter of 4 mm and arranged in a
zigzag pattern at intervals of 12.1 mm, and a sheet portion which
measured 0.4 mm thick.
[0142] The expandable resin product was processed in the same
manner as in Example 1 to obtain a layered thermoplastic resin
foam. The layered thermoplastic resin foam measured 6.6 mm thick.
Test pieces were cut out from this foam to measure 25% compressive
strength, flexural modulus and expansion ratio in the same
procedure as in Example 1. The results are given in Table 4.
EXAMPLE 8
[0143] In Example 7, the ethylene-vinyl acetate copolymer used as
the material for constituting the second foam layer was replaced by
low-density polyethylene having a flexural modulus of 500 MPa.
Otherwise, the procedure of Example 7 was followed to obtain a
layered resin foam and measure 25% compressive strength, flexural
modulus and expansion ratio of the foam. The results are given in
Table 4.
EXAMPLE 9
[0144] In Example 7, the ethylene-vinyl acetate copolymer used as
the material for constituting the second foam layer was replaced by
linear low-density polyethylene having a flexural modulus of 700
MPa. Otherwise, the procedure of Example 7 was followed to obtain a
layered resin foam and measure 25% compressive strength, flexural
modulus and expansion ratio of the foam. The results are given in
Table 4.
COMPARATIVE EXAMPLE 3
[0145] As the material for constituting the sheet-like structure of
expandable thermoplastic resin, a mixture containing 100 parts by
weight of thermoplastic resins at the proportion specified in Table
3, 1 part by weight of a silane crosslinking catalyst master batch
(product of Mitsubishi Chemical Corp., product name: PZ10S) and 5
parts by weight of azodicarbonamide (product of Otsuka Chemical
Co., Ltd., product name "UNIFOAM AZ SO-20", decomposition
temperature of 210.degree. C.) as a heat decomposable blowing agent
was supplied into a corotating twin screw extruder (PCM 30,
manufactured by Ikegai Tekko Co., Ltd.) where it was melt kneaded
at 190.degree. C.
[0146] The melt resin from the extruder was introduced into a
single layer sheet extrusion die and extruded from its head having
a face of 500 mm and a lip opening of 1.0 mm into the form of a
single layer sheet. The resulting sheet measured 1.2 mm thick.
[0147] The single layer sheet was subsequently introduced between
the preceding rolls 7 and 7A (roll clearance of 0.2 mm), cooled
while shaped and then removed from those rolls to provide a shaped
sheet. This shaped sheet was subsequently immersed in 99.degree. C.
water for 2 hours so that a crosslinking reaction was caused to
proceed, and then dried to obtain an expandable resin product. The
expandable resin product was found to include raised portions in
the form of cylinders having a height of 5 mm and a diameter of 4
mm and arranged in a zigzag pattern at intervals of 12.1 mm. A
basic sheet portion thereof measured 0.4 mm thick.
[0148] The expandable resin product made in accordance with the
above procedure was processed in the same manner as in Example 1 to
obtain a thermoplastic resin foam. The thermoplastic resin foam
measured 6.5 mm thick. Test pieces were cut out from this foam to
measure 25% compressive strength, flexural modulus and expansion
ratio of the foam. The results are given in Table 4.
3 TABLE 3 Type of Resin Resin Grade wt. % Homopolypropylene MA3,
Product of Nippon 30 Polychem Co., Ltd. Silane-Grafted XPM800HM,
Product of 20 Homopolypropylene Mitsubishi Chemical Corp.
High-Density HJ340, Product of Nippon 50 Polyethylene Polychem Co.,
Ltd.
[0149]
4 TABLE 4 Comp. Example Ex. 7 8 9 3 Ex- Thermoplastic -- Resins
Resins Resins Resins pand- Resin Components Speci- Speci- Speci-
Speci- able of the Sheet-Like fied in fied in fied in tied in Resin
Structure of Table 3 Table 3 Table 3 Table 3 Expandable
Thermoplastic Resin Thermoplastic -- EVA/ Low- Linear -- Resin
Component Si-EVA Density Low- of the Second PE Density Foam Layer
PE Resin Flexural MPa 40 500 700 -- Modulus of the Second Foam
Layer Lay- 25% Compressive MPa 0.65 0.63 0.64 0.62 ered Strength
Foam Flexural Modulus MPa 12.5 14.8 18.3 40.6 Mean Expansion -- 8.0
7.9 7.5 7.3 Ratio
[0150] Also, a 50 mm.times.50 mm rectangular plate having a
thickness of 6.6 mm was cut out from the foam obtained to prepare a
sample. The sample was compressed such that a strain rate (a change
in thickness of the sample per unit time/an original thickness of
the sample) was maintained at a value of 20 to measure an
absorption energy versus compressive strength. The energy
absorption increases as the volume displaced by the compression
increases, i.e., as a compressive stress increases. In this
measurement, the energy absorption when the compressive stress
reached 10 kgf/cm.sup.2 was measured using a universal testing
machine (marketed under the product name "TENSILON" and
manufactured by A & D Co., Ltd.) and recorded as the energy
absorption for each sample. The results are given in the following
Table 5.
5 TABLE 5 Example Comp. Ex. 1 2 3 4 5 6 7 8 9 1 2 3 Absorp- 0.30
0.34 0.26 0.40 0.35 0.23 0.35 0.32 0.29 0.21 0.15 0.18 tion Energy
(kgf .multidot. cm/cm.sup.3)
EFFECTS OF THE INVENTION
[0151] The expandable thermoplastic resin product in accordance
with the present invention has an elastic thermoplastic material
layer superimposed on a sheet-like structure of expandable
thermoplastic resin. Accordingly, when expansion of the expandable
thermoplastic resin product is effected, the base sheet of the
sheet-like structure of expandable thermoplastic resin is converted
into the foam sheet having a relatively low expansion ratio, the
raised portions of the sheet-like structure of expandable
thermoplastic resin are converted into the basic raised portions
having a high expansion ratio and located on a surface of the foam
sheet and the low foamed layer which covers outer surfaces of the
areas of the basic raised portions that exclude those in contact
with the foam sheet, and the elastic thermoplastic material is
converted into a such form that covers outer surfaces of the raised
portions. As a result, the thermoplastic resin foam in accordance
with the present invention can be obtained.
[0152] The thermoplastic resin foam in accordance with the present
invention exhibits a high compressive strength upon application
thereto of a compressive stress, since the elastic thermoplastic
material covers the outer surfaces of the raised portions
consisting of the high foamed basic raised portions and the low
foamed layer. Also, it exhibits a low elastic modulus when a
flexural stress is applied thereto since such a stress is
concentrated at the highly flexible elastic thermoplastic material
which covers the basic raised portions having a high expansion
ratio. Therefore, thermoplastic resin foams suitable for use as
various cushioning an damping materials can be provided. Because of
the high compressive strength in a thickness direction and the low
flexural modulus, as stated above, the thermoplastic resin foam
readily conforms to a contoured surface and thus can be suitably
used as shock absorbers such as a protector.
[0153] When the elastic thermoplastic material is a foam,
particularly when a thermoplastic elastomer having a flexural
modulus in the range of 5 MPa-1,000 MPa, the thermoplastic resin
foams can be obtained which has an outer surface covered with a
flexible material and thus has imparted thereto a further reduced
flexural modulus. Therefore, thermoplastic resin foams can be
provided which constitute further improved shock absorbing,
cushioning and damping materials.
* * * * *