U.S. patent application number 11/529209 was filed with the patent office on 2007-02-01 for composite foamed polypropylene resin molding and method of producing same.
This patent application is currently assigned to JSP Corporation. Invention is credited to Keiichi Hashimoto, Akinobu Hira, Hidehiro Sasaki.
Application Number | 20070026218 11/529209 |
Document ID | / |
Family ID | 28035507 |
Filed Date | 2007-02-01 |
United States Patent
Application |
20070026218 |
Kind Code |
A1 |
Hira; Akinobu ; et
al. |
February 1, 2007 |
Composite foamed polypropylene resin molding and method of
producing same
Abstract
A composite foamed polypropylene resin molding including a
plurality of sections which are fuse-bonded to each other, at least
two of which differ from each other in color, apparent density,
composition and/or mechanical strengths, each of which is formed
from expanded polypropylene resin beads, and each of which shows a
high temperature endothermic peak in a DSC curve thereof. At least
one of the sections satisfies conditions (d) to (f) at the same
time: (d) to be formed from specific expanded polypropylene resin
beads of a base resin having a tensile modulus of at least 1,200
MPa, (e) to have an apparent density D2 of 10-500 g/L, and (f) to
have such a high temperature endothermic peak with a calorific of
E2 J/g, wherein D2 and E2 have the relationship
20-0.020.times.D2.ltoreq.E2.ltoreq.65-0.100.times.D2. The composite
molding may be prepared by filling expanded polypropylene resin
beads in each of a plurality of contiguous spaces defined in a mold
cavity and heating the expanded beads to fuse-bond respective
expanded beads together into a unitary body. At least one of the
spaces is filled with the specific expanded polypropylene resin
beads.
Inventors: |
Hira; Akinobu; (Kanuma-shi,
JP) ; Hashimoto; Keiichi; (Utsunomiya-shi, JP)
; Sasaki; Hidehiro; (Tochigi-ken, JP) |
Correspondence
Address: |
BACON & THOMAS, PLLC
625 SLATERS LANE
FOURTH FLOOR
ALEXANDRIA
VA
22314
US
|
Assignee: |
JSP Corporation
|
Family ID: |
28035507 |
Appl. No.: |
11/529209 |
Filed: |
September 29, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10502934 |
Jul 29, 2004 |
|
|
|
PCT/JP03/03318 |
Mar 19, 2003 |
|
|
|
11529209 |
Sep 29, 2006 |
|
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|
Current U.S.
Class: |
428/313.3 ;
428/218; 428/316.6 |
Current CPC
Class: |
Y10T 428/24992 20150115;
Y10T 428/249971 20150401; B29C 44/0469 20130101; B29C 44/445
20130101; Y10T 428/249977 20150401; B29C 44/3461 20130101; Y10T
428/249953 20150401; Y10T 428/249981 20150401 |
Class at
Publication: |
428/313.3 ;
428/316.6; 428/218 |
International
Class: |
B32B 7/02 20060101
B32B007/02; B32B 3/00 20060101 B32B003/00; B32B 3/26 20060101
B32B003/26 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 19, 2003 |
JP |
2002-77383 |
Claims
1. A composite foamed polypropylene resin molding comprising a
plurality of sections which are fuse-bonded to each other and at
least two of which differ from each other in apparent density,
wherein each of said sections is formed from expanded beads of a
base resin including a polypropylene resin, wherein each of said
sections shows a high temperature endothermic peak, in a DSC curve
thereof, in addition to an intrinsic endothermic peak located at a
lower temperature side of said high temperature peak, wherein at
least one of said sections satisfies the following conditions (a)
to (c) at the same time: (a) that section is formed from specific
expanded polypropylene resin beads of a base resin having a tensile
modulus of at least 1,200 Mpa, (b) that section has an apparent
density D2 g/L which is not smaller than 10 g/L but not greater
than 500 g/L, and (c) the high temperature endothermic peak of that
section has a calorific value of E2 J/g, wherein D2 and E2 have the
following relationship
20-0.020.times.D2.ltoreq.E2.ltoreq.65-0.100.times.D2.
2. A composite foamed molding as claimed in claim 1, wherein each
of said specific expanded beads has a surface region and an inside
region, wherein each of said surface and inside regions shows a
high temperature endothermic peak, in a DSC curve thereof, in
addition to an intrinsic endothermic peak located at a lower
temperature side of said high temperature peak, wherein said high
temperature endothermic peaks of said surface region and said
inside region have calorific values of Hs and Hi, respectively, and
wherein Hs and Hi have the following relationship:
HS<0.86.times.Hi.
3. A composite foamed molding as claimed in claim 1, wherein one of
two adjacent sections has a relatively high apparent density which
is higher than that of the other one of said adjacent two sections
and which is in the range of 30-450 g/L, with the other section
having a relatively low apparent density in the range of 15-90
g/L.
4. A composite foamed molding as claimed in claim 3, wherein the
relatively high apparent density is 1.2-25 times as high as that of
the relatively low apparent density.
5. A composite foamed molding as claimed in claim 1, in the form of
a shock absorber.
6. A composite foamed molding as claimed in claim 5, wherein said
shock absorber is a core of an automobile bumper.
Description
TECHNICAL FIELD
[0001] This invention relates to a composite foamed polypropylene
resin molding and to a method of producing same.
BACKGROUND ART
[0002] A polypropylene resin is now increasingly utilized in
various fields because of excellent mechanical strengths, heat
resistance, chemical resistance, machinability, cost balance and
recyclability thereof. Foamed moldings of a base resin including a
polypropylene resin (hereinafter referred to simply as "PP
moldings" or "polypropylene resin moldings"), which retain the
above excellent properties and which have excellent additional
characteristics such as cushioning property and heat insulating
properties, are thus utilized for various applications as packaging
materials, construction materials, heat insulation materials, etc.
In particular, PP moldings obtained by heating expanded beads of a
base resin including a polypropylene resin (hereinafter referred to
as "expanded PP beads" or "expanded polypropylene resin beads") in
a mold are now used as bumper cores and door pats of automobiles
because of their good shock absorbing properties and
moldability.
[0003] Thus, a need for light weight and high rigidity PP moldings
is increasing in this field. In one structure of such PP moldings,
a dual density molding is known which has a relatively high density
section and a relatively low density section. Because of the
presence of the low density section, the dual density molding has a
reduced weight as a whole as compared with a structure in which no
such a low density section is present and is advantageously
utilized as a high functional bumper core in which offset collision
and pedestrian protection are taken into account. One typical
dual-density molding has a center low density section which is
sandwiched between a pair of high density sections. In the
production of such a dual-density molding, high density expanded PP
beads and low density expanded PP beads are filled in predetermined
spaces of a mold cavity and are heated to fuse-bond the expanded PP
beads into a unitary structure, as disclosed in U.S. Pat. No.
5,164,257, Japanese Laid-Open Patent Publications No. H11-334501
and 2001-150471 and Japanese Utility Model Examined Publication
S62-22352. The thus obtained foamed molding is then cooled and
taken out of the mold.
[0004] Such a dual density PP molding is, however, apt to expand to
a size greater than the mold cavity, when the molding is not
sufficiently cooled after the fuse-bonding of expanded PP beads has
been completed. The PP molding is also apt to shrink to a size
smaller than the mold cavity, when the molding is excessively
cooled after the fuse-bonding of expanded PP beads has been
completed. Thus, depending upon the degree of cooling, the dual
density PP molding expands or shrinks. In this case, since a
relatively low density section is more quickly cooled than a
relatively high density section, the low density section is more
likely to shrink, when the cooling of the molding is carried out
evenly. Since expansion is less desirable than shrinkage, the
cooling is generally carried out while preventing expansion of the
high density section. Thus, the shrinkage of the low density
section is generally unavoidable in the case of production the dual
density PP molding unless specifically controlled cooling
conditions are adopted.
DISCLOSURE OF THE INVENTION
[0005] The present invention has been made in view of the problems
of the conventional methods.
[0006] In accordance with the present invention, there is provided
a method of producing a composite foamed polypropylene resin
molding, comprising:
[0007] providing a mold having a mold cavity including a plurality
of contiguous spaces;
[0008] filling expanded beads of a base resin including a
polypropylene resin in each of said spaces; and
[0009] heating said expanded beads in each of said spaces to
fuse-bond respective expanded beads together into a unitary
body;
[0010] wherein each of said expanded beads shows a high temperature
endothermic peak, in a DSC curve thereof, in addition to an
intrinsic endothermic peak located at a lower temperature side of
said high temperature peak, and
[0011] wherein those expanded beads which are filled in at least
one of said spaces are specific expanded beads which satisfy the
following conditions (a) to (c) at the same time:
(a) said specific expanded beads are formed of a base resin having
a tensile modulus of at least 1,200 MPa,
(b) the high temperature endothermic peak of said specific expanded
beads has an apparent density D1 g/L which is not smaller than 10
g/L but not greater than 700 g/L, and
(c) the high temperature endothermic peak of said specific expanded
beads has such an area that corresponds to a calorific of E1 J/g,
wherein D1 and E1 have the following relationship
20-0.014.times.D2.ltoreq.E1.ltoreq.65-0.072.times.D1.
[0012] In another aspect, the present invention provides a
composite foamed polypropylene resin molding comprising a plurality
of sections which are fuse-bonded to each other and at least two of
which differ from each other in at least one characteristic
selected from color, apparent density, composition and mechanical
strengths,
[0013] wherein each of said sections is formed from expanded beads
of a base resin including a polypropylene resin,
[0014] wherein each of said sections shows a high temperature
endothermic peak, in a DSC curve thereof, in addition to an
intrinsic endothermic peak located at a lower temperature side of
said high temperature peak,
[0015] wherein at least one of said sections satisfies the
following conditions (d) to (f) at the same time:
(d) that section is formed from specific expanded polypropylene
resin beads of a base resin having a tensile modulus of at least
1,200 MPa,
(e) that section has an apparent density D2 g/L which is not
smaller than 10 g/L but not greater than 500 g/L, and
(f) the high temperature endothermic peak of that section has such
an area that corresponds to a calorific of E2 J/g, wherein D2 and
E2 have the following relationship
20-0.020.times.D2.ltoreq.E2.ltoreq.65-0.100.times.D2.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The present invention will now be described in more detail
below with reference to the accompanying drawings, in which:
[0017] FIG. 1 is an initial DSC curve of expanded polypropylene
resin beads;
[0018] FIG. 2 is a second time DSC curve of polypropylene resin
particles which have not yet been subjected to surface modification
and which have been once subjected to DSC measurement;
[0019] FIG. 3 is a sectional view schematically illustrating one
embodiment of a composite foamed polypropylene resin molding
according to the present invention;
[0020] FIG. 4 is a sectional view schematically illustrating
another embodiment of a composite foamed polypropylene resin
molding according to the present invention;
[0021] FIG. 5 is a sectional view schematically illustrating a
further embodiment of a composite foamed polypropylene resin
molding according to the present invention; and
[0022] FIG. 6 is a sectional view schematically illustrating a
further embodiment of a composite foamed polypropylene resin
molding according to the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0023] A composite PP molding of the present invention comprises at
least two sections which are fuse-bonded to each other and which
differ from each other in at least one characteristic selected from
color, density, composition and mechanical strengths. Each of the
sections is made from expanded PP beads of a base resin including a
polypropylene resin. The expanded PP beads may be obtained by
expanding and foaming base resin particles using a blowing
agent.
[0024] The term "polypropylene resin" as used herein refers to (1)
polypropylene homopolymer, (2) a copolymer of propylene and one or
more comonomers having a propylene content of at least 70 mole %,
preferably at least 80 mole %, a mixture of two or more of the
copolymers (2), or a mixture of the homopolymer (1) and the
copolymer (2). The copolymer may be, for example,
ethylene-propylene random copolymers, ethylene-propylene block
copolymers, propylene-butene random copolymers or
ethylene-propylene-butene random copolymers.
[0025] If desired, the base resin may contain one or more
additional resins or one or more elastomers. The amount of the
additional resin or elastomer in the base resin is preferably no
more than 35 parts by weight, more preferably no more than 20 parts
by weight, still more preferably no more than 10 parts by weight,
most preferably no more than 5 parts by weight, per 100 parts by
weight of the polypropylene resin. Examples of the additional
resins include polyethylene resins such as high density
polyethylenes, medium density polyethylenes, low density
polyethylenes, linear low density polyethylenes, linear very low
density polyethylenes, ethylene-vinyl acetate copolymers,
ethylene-acrylic acid copolymers, ethylene-methacrylic copolymers;
and polystyrene resins such as polystyrene and styrene-maleic
anhydride copolymers. Examples of elastomers include
ethylene-propylene rubber, ethylene-1-butene rubber,
propylene-1-butene rubber, styrene-butadiene rubber, isoprene
rubber, neoprene rubber, nitrile rubber, styrene-butadiene block
copolymers and hydrogenated products of the above rubbers and
copolymers.
[0026] The base resin may also be blended with one or more
additives such as an antioxidant, a UV absorbing agent, a foam
controlling agent, an antistatic agent, a fire retardant, a
metal-deactivator, a pigment, a nucleus agent, a filler, a
stabilizer, a reinforcing material and a lubricant. The foam
controlling agent may be, for example, an inorganic powder such as
zinc borate, talc, calcium carbonate, borax or aluminum hydroxide.
The additive or additives are generally used in an amount of 20
parts by weight or less, preferably 5 parts by weight or less, per
100 parts by weight of the base resin. These additives may be
incorporated into the expanded PP beads during the fabrication of
raw material non-expanded PP beads by kneading the base resin
together with the additives. The kneaded mixture is generally
extruded through a die into strands, which are then cut into
pellets to obtain the raw material non-expanded PP beads. The
non-expanded PP beads are thereafter expanded as will be described
in detail hereinafter.
[0027] It is important that at least one of the sections of the
composite PP molding should be formed from specific expanded beads
of a high modulus base resin which includes a polypropylene resin
and which has a tensile modulus of at least 1,200 MPa, since
otherwise it is difficult to prevent shrinkage and expansion of the
PP molding. The tensile modulus of the high modulus base resin is
preferably at least 1,250 MPa, more preferably at least 1300 MPa.
The upper limit of the tensile modulus is generally 2,500 MPa,
though a base resin having a tensile modulus of more than 2,500 MPa
may be used for the purpose of the present invention.
[0028] The high tensile modulus base resin may be obtained by
using, for example, a high modulus polypropylene resin having a
tensile modulus of at least 1,200 MPa, preferably at least 1,250
MPa, more preferably at least 1300 MPa. The upper limit of the
tensile modulus of the high modulus polypropylene resin is
generally 2,500 MPa, though a high modulus polypropylene resin
having a tensile modulus of more than 2,500 MPa may be used for the
purpose of the present invention. Such a high tensile modulus of
the polypropylene resin may be obtained by using a
homopolypropylene or a propylene copolymer having a high propylene
monomer unit content (preferably at least 99% by weight). The
tensile modulus of the high modulus polypropylene resin is
preferably at least 1,250 MPa, more preferably at least 1300 MPa.
The upper limit of the tensile modulus is generally 2,500 MPa,
though a polypropylene resin having a tensile modulus of more than
2,500 MPa may be used for the purpose of the present invention.
[0029] The "tensile modulus" as used herein is measured according
to the method disclosed in Japanese Industrial Standard JIS
K7161-1994 using Type 1A test sample (directly prepared by
injection molding) at a test speed of 1 mm/min.
[0030] When the high modulus polypropylene resin is used as a base
resin for the specific expanded PP beads in conjunction with one or
more additional resins, one or more elastomers or one or more
additives, the amount thereof should be such that the tensile
modulus of the base resin composition should not decrease below
1,200 MPa and should be preferably at least 1,250 MPa, more
preferably at least 1,300 MPa.
[0031] The high modulus polypropylene resin for the specific
expanded PP beads preferably has a melting point of at least
145.degree. C., more preferably at least 155.degree. C., still more
preferably at least 160.degree. C., for reasons of high heat
resistance and high compression strength of the PP molding. The
melting point of the high modulus polypropylene resin is generally
170.degree. C. or less.
[0032] The high modulus polypropylene resin preferably has a
tensile yield point of at least 31 MPa, more preferably at least 32
MPa, for reasons of high compression strength of the PP molding.
The tensile yield point of the high modulus polypropylene resin is
generally 45 MPa or less. The high modulus polypropylene resin also
preferably has a tensile breaking elongation of at least 20%, more
preferably at least 100%, most preferably 200-1000%, for reasons of
prevention of breakage of cells during the fabrication of expanded
PP beads and during the fabrication of PP moldings. The tensile
yield point and tensile breaking elongation are measured in
accordance with the method of Japanese Industrial Standard JIS
K6758-1981.
[0033] It is further preferred that the high modulus polypropylene
resin for the specific expanded PP beads have molecular
distribution Mw/Mn of at least 4.4, more preferably 4.5-10, for
reasons of capability of using low temperature steam for heating
the expanded PP beads in a mold for the fabrication of a PP
molding. The weight average molecular weight Mw and the number
average molecular weight Mn are measured by gel permeation
chromatography (GPC) using polystyrene as standard under the
following conditions:
GPC device: Waters 150C
Column: Toso GMHHR-H(S)HT
Detector: RI detector for liquid chromatogram
Solvent: 1,2,4-trichlorobenzene
Temperature: 145.degree. C.
Elution rate: 1.0 mL/min
Sample concentration: 2.2 mg/mL
Sample injection amount: 160 .mu.L
Calibration curve: Universal Calibration
Analysis program: HT-GPC (Ver. 1.0)
[0034] For reasons of strengths of PP moldings and capability of
using a low temperature steam in the fabrication of PP moldings,
the high modulus polypropylene resin preferably has a melt flow
rate (MFR) of 1-100 g/10 min, more preferably 10-70 g/10 min. The
MFR herein is as measured in accordance with the Japanese
Industrial Standard JIS K7210-1976, Test Condition 14.
[0035] The high modulus polypropylene resin is commercially
available and may be suitably produced by, for example, a slurry or
bulk polymerization process or by a multi-polymerization process
including a slurry or bulk polymerization method (e.g. a
multi-stage polymerization process including liquid phase
polymerization and bulk polymerization) such that the polymer
obtained (inclusive of a product obtained after the removal of
atactic components) has an isotactic index (content of boiling
n-heptane insoluble matters) of at least 85% by weight, isotactic
(mmmm) pentads, as determined by .sup.13C-NMR analysis, of
85-97.5%, a weight average molecular weight of at least 200,000
(preferably 200,000) and a number average molecular weight of at
least 20,000 (preferably 20,000). In this case, by selecting
polymerization or copolymerization conditions so as to provide a
propylene component content of at least 99%, the desired high
modulus polypropylene resin may be easily obtained. Polypropylene
resins obtained by a slurry or bulk polymerization process or by a
multi-polymerization process including a slurry or bulk
polymerization method are more suited for use as the base resin in
the present invention than those obtained by other polymerization
processes are. Both metallocene catalyst and Ziegler-Natta catalyst
may be suitably used for the production of the high modulus
polypropylene resin, though the Ziegler-Natta catalyst is more
preferred in the case of the slurry or bulk polymerization process
or the multi-polymerization process.
[0036] The resin particles used as a raw material for the
production expanded PP beads (inclusive of the specific expanded PP
beads) may be obtained by any suitable known method. For example,
the above-described base resin containing the high modulus
polypropylene resin, which is generally in the form of pellets,
and, if desired, one or more additives are charged, mixed and
kneaded in an extruder. The kneaded mass is then extruded through a
die into strands and cut to obtain the resin particles. The resin
particles are then expanded using a blowing agent to obtain
expanded PP beads.
[0037] It is preferred that the strands be quenched immediately
after having been extruded for reasons that the succeeding surface
modification with an organic peroxide, which will be described
hereinafter, may be efficiently performed. The quenching may be
carried out by introducing the strands in water at 50.degree. C. or
less, preferably 40.degree. C. or less, more preferably 30.degree.
C. or less. The cooled strands are taken out of the water and cut
into particles each having a length/diameter ratio of 0.5-2.0,
preferably 0.8-1.3, and a mean weight of 0.1-20 mg, preferably
0.2-10 mg. The mean weight is an average of 200 arbitrarily
selected particles.
[0038] It is preferred that the resin particles used for the
production of expanded PP beads, especially the specific expanded
PP beads, be previously subjected to surface modification with an
organic peroxide. The expanded PP beads obtained from such surface
modified resin particles have excellent fuse-bonding properties and
give a high rigidity PP molding in a mold using steam at a
relatively low temperature.
[0039] In performing the surface modification, the resin particles
are dispersed in a dispersing medium containing an organic peroxide
to obtain a dispersion. Any dispersing medium may be used as long
as it can disperse the resin particles therein without
substantially dissolving components of the particles. Examples of
the dispersing medium include water, ethylene glycol, glycerin,
methanol, ethanol or a mixture of them. An aqueous dispersion
medium, such as ion-exchanged water containing an alcohol may be
suitably used. The dispersion is heated at a temperature lower than
the melting point of the base resin but sufficient to decompose the
organic peroxide, thereby obtaining surface-modified resin
particles. The surface-modified resin particles are then expanded
using a blowing agent to obtain expanded PP beads.
[0040] Any organic peroxide may be used for the purpose of the
present invention as long as it decomposes when heated at a
temperature lower than the melting point of the base resin.
[0041] Illustrative of suitable organic peroxides are shown below:
[0042] Isobutylperoxide [50.degree. C./85.degree. C.] [0043] Cumyl
peroxy neodecanoate [55.degree. C./94.degree. C.] [0044]
.alpha.,.alpha.'-Bis(neodecanoylperoxy)diisopropylbenzene
[54.degree. C./82.degree. C.] [0045] di-n-Propyl peroxydicarbonate
[58.degree. C./94.degree. C.], [0046] Diisopropyl peroxydicarbonate
[56.degree. C./88.degree. C.], [0047] 1-Cyclohexyl-1-methylethyl
peroxy neodecanoate [59.degree. C./94.degree. C.] [0048]
1,1,3,3-Tetramethylbutyl peroxy neodecanoate [58.degree.
C./92.degree. C.] [0049] Bis(4-t-butylcyclohexyl) peroxydicarbonate
[58.degree. C./92.degree. C.] [0050] Di-2-ethoxyethyl
peroxydicarbonate [59.degree. C./92.degree. C.], [0051]
Di(2-ethylhexylperoxy)dicarbonate [59.degree. C./91.degree. C.],
[0052] t-Hexyl peroxy neodecanoate [63.degree. C./101.degree. C.],
[0053] Dimethoxybutyl peroxydicarbonate [64.degree. C./102.degree.
C.], [0054] Di(3-methyl-3-methoxybutylperoxy)dicarbonate
[65.degree. C./103.degree. C.], [0055] t-Butyl peroxy neodecanoate
[65.degree. C./104.degree. C.], [0056] 2,4-Dichlorobenzoyl peroxide
[74.degree. C./119.degree. C.], [0057] t-Hexyl peroxy pivalate
[71.degree. C./109.degree. C.], [0058] t-Butyl peroxy pivalate
[73.degree. C./110.degree. C.], [0059] 3,5,5-Trimethylhexanoyl
peroxide [77.degree. C./113.degree. C.], [0060] Octanoyl peroxide
[80.degree. C./117.degree. C.], [0061] Lauroyl peroxide [80.degree.
C./116.degree. C.], [0062] Stearoyl peroxide [80.degree.
C./117.degree. C.] [0063] 1,1,3,3-Tetramethylbutyl peroxy
2-ethylhexanoate [84.degree. C./124.degree. C.]; [0064] Succinic
peroxide [87.degree. C./132.degree. C.] [0065]
2,5-Dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane [83.degree.
C./119.degree. C.) [0066] 1-Cyclohexyl-1-methylethyl peroxy
2-ethylhexanoate [90.degree. C./138.degree. C.], [0067] t-Hexyl
peroxy 2-ethylhexanoate [90.degree. C./133.degree. C.], [0068]
t-Butyl peroxy 2-ethylhexanoate [92.degree. C./134.degree. C.],
[0069] m-Toluoyl benzoyl peroxide [92.degree. C./131.degree. C.],
[0070] Benzoyl peroxide [92.degree. C./130.degree. C.], [0071]
t-Butyl peroxy isobutylate [96.degree. C./136.degree. C.], [0072]
1,1-Bis(t-butylperoxy)-2-methylcyclohexane [102.degree.
C./142.degree. C.] [0073]
1,1-Bis(t-hexylperoxy)-3,3,5-trimethylcyclohexane [106.degree.
C./147.degree. C.] [0074]
1,1-Bis(t-butylperoxy)-3,3,5-trimethylcyclohexane [109.degree.
C./149.degree. C.], [0075] 1,1-Bis(t-hexylperoxy)cyclohexane
(107.degree. C./149.degree. C.], [0076]
1,1-Bis(t-butylperoxy)cyclohexane [111.degree. C./154.degree. C.],
[0077] 2,2-Bis(4,4-dibutylperoxycyclohexyl)propane [114.degree.
C./154.degree. C.] [0078] 1,1-Bis(t-butylperoxy)cyclododecane
[114.degree. C./153.degree. C.], [0079] t-Hexyl peroxy isopropyl
monocarbonate [115.degree. C./155.degree. C.] [0080] t-Butyl peroxy
maleic acid [119.degree. C./168.degree. C.] [0081] t-Butyl peroxy
3,5,5-trimethylhexanoate [119.degree. C./166.degree. C.] [0082]
t-Butyl peroxy laurate [118.degree. C./159.degree. C.], [0083]
2,5-Dimethyl-2,5-di(m-toluoylperoxy)hexane [117.degree.
C./156.degree. C.] [0084] t-Butyl peroxy isopropyl monocarbonate
[118.degree. C./159.degree. C.] [0085] t-Butyl peroxy 2-ethylhexyl
monocarbonate [119.degree. C./161.degree. C.], [0086] t-Hexyl
peroxy benzoate [119.degree. C./160.degree. C.], and [0087]
2,5-Dimethyl-2,5-di(benzoylperoxy)hexane [119.degree.
C./158.degree. C.].
[0088] These organic peroxides may be used alone or in combination.
The amount of the organic peroxide in the dispersion is generally
0.01-10 parts by weight, preferably 0.05-5 parts by weight, more
preferably 0.1-3 parts by weight, per 100 parts by weight of the
resin particles.
[0089] In the dispersion obtained by dispersing the resin particles
in a dispersing medium containing an organic peroxide, it is
preferred that the weight ratio of the resin particles to the
dispersing medium be 1.3:1 or less, more preferably 1.2:1 or less,
much more preferably 1.1:1 or less, most preferably 1:1 or less,
for reasons of uniformly treating the particles with the organic
peroxide. Namely, when the weight ratio of the resin particles to
the dispersing medium is excessively high, a difficulty might be
caused in uniformly treating the surfaces of the resin particles.
Thus, a part of the resin particles which excessively undergo the
surface modification tend to for an aggregate in the dispersion so
that the discharge of the dispersion from the vessel at the time of
the expansion is not smoothly carried out. From the standpoint of
economy, the weight ratio of the resin particles to the dispersing
medium is desirably at least 0.6:1, more preferably at least
0.7:1.
[0090] In the present invention, the organic peroxide is heated at
a temperature lower than the melting point of the base resin but
sufficient to substantially decompose the organic peroxide. It is
preferred that 1 Hr half life temperature Th (the temperature at
which the amount of the organic peroxide decreases to half when the
peroxide is heated at that temperature for 1 hour) of the organic
peroxide be not higher than the Vicat softening point of the base
resin. The "Vicat softening point" in the present specification is
in accordance with Japanese Industrial Standard JIS K 6747-1981.
When the 1 Hr half life temperature Th is higher than the Vicat
softening point of the base resin, it is difficult to substantially
decompose the organic peroxide at a temperature lower than the
melting point of the base resin. When the decomposition of the
organic peroxide is carried out at a temperature not lower than the
melting point of the base resin, the decomposed organic peroxide
will attack not only the surfaces of the resin particles but also
inside regions thereof, so that expanded PP beads obtained cannot
give a desired PP molding.
[0091] Thus, it is preferred that the 1 Hr half life temperature Th
be lower by at least 20.degree. C., more preferably by at least
30.degree. C., than the Vicat softening point of the base resin. It
is also preferred that the 1 Hr half life temperature Th be in the
range of 40-100.degree. C., more preferably 50-90.degree. C., for
reasons of easiness of handling.
[0092] The organic peroxide in the dispersion is desirably
substantially decomposed at a temperature not higher than, more
preferably lower by at least 20.degree. C. than, most preferably
lower by at least 30.degree. C. than, the Vicat softening point of
the base resin. Further, the organic peroxide in the dispersion is
desirably substantially decomposed at a temperature not lower than
the glass transition point of the base resin, more preferably at a
temperature in the range of 40-100.degree. C., most preferably
50-90.degree. C., for reasons of easiness in handling of the
peroxide.
[0093] It is further preferred that the decomposition of the
organic peroxide be performed by maintaining the organic peroxide
at a temperature in the range of (Tn-30.degree. C.) to
(Tn+30.degree. C.) for at least 10 minutes, where Tn is 1 min half
life temperature of the organic peroxide (the temperature at which
the amount of the organic peroxide decreases to half when the
peroxide is heated at that temperature for 1 minute) for reasons of
decomposition efficiency. When the decomposition is carried out at
a temperature lower than (Tn-30.degree. C.), a long time is
required for completing the decomposition. Too high a decomposition
temperature in excess of (Tn+30.degree. C.) might adversely affect
the uniformity of surface treatment. From the standpoint of process
cost and efficiency, the heat treatment at a temperature of
(Tn-30.degree. C.) to (Tn+30.degree. C.) is desired to be performed
for 60 minutes or shorter. Preferably, the dispersion of the resin
particles in the organic peroxide-containing liquid medium is
prepared at such a temperature that the peroxide is prevented from
decomposing and, then, the temperature is increased continuously or
stepwise so that the peroxide is maintained at a temperature range
of (Tn-30.degree. C.) to (Tn+30.degree. C.) for at least 10
minutes. In this case, it is preferred that the peroxide be
maintained at a constant temperature of (Tn-5.degree. C.) to
(Tn+5.degree. C.) for at least 5 minutes.
[0094] The "glass transition point" as used herein is measured in
accordance with JIS K7121-1987 and is calculated from the midpoint
of a heat flux. The "glass transition point is measured after the
sample has been heat treated under specified conditions".
[0095] The term "substantially decompose" as used herein means that
the active oxygen content of the peroxide is reduced to less than
50% of the original value. Preferably, the peroxide is decomposed
so that the active oxygen content thereof be reduced to 30% or
less, more preferably 20% or less, most preferably 5% or less of
the original value.
[0096] The "1 hour half life temperature Th" and "1 min half life
temperature Tn" of the organic peroxide are measured as follows. A
sample peroxide is dissolved in a suitable solvent inert to
radicals, such as benzene or mineral spirit, to obtain a solution
having a peroxide concentration of 0.1 mol/L or 0.05 mol/L. This is
placed in a glass tube whose inside space has been substituted by
nitrogen. The glass tube is sealed and immersed in a constant
temperature bath maintained at a predetermined temperature for a
given period (1 minute or 1 hour) to permit the peroxide to
decompose. The change in concentration of the organic peroxide with
the time is measured. Under the above reaction conditions, since
the decomposition reaction of the organic peroxide can be regarded
as being a first-order reaction, the following equations can be
formed: dx/dt=k(a-x) ln[a/(a-x)]=kt wherein x denotes a
concentration of the organic peroxide, a denotes the initial
concentration of the organic peroxide, k denotes the decomposition
rate constant, and t denotes a time. Since the half-life period
t.sub.1/2 is a time required for reducing the concentration of the
organic peroxide to half by decomposition (x=a/2), the following
relationship is obtained: kt.sub.1/2=ln2. From the above
measurement of the change in concentration of the organic peroxide
with the time (t), relationship between the time (t) and
ln[a/(a-x)] is plotted to give a straight line. The gradient
represents the constant (k) Thus, the half life t.sub.1/2 is
calculated from the above equation. The 1 Hr half life temperature
and 1 min half life temperature of an organic peroxide are the
temperatures at which t.sub.1/2 of the organic peroxide are 1 hour
and 1 minute, respectively.
[0097] The surface-modified resin particles are then foamed and
expanded to obtain expanded PP beads using a blowing agent.
Preferably, the expansion step is carried out by a conventional
dispersion method in which the resin particles are dispersed in a
dispersing medium in a closed vessel in the presence of a blowing
agent and heated to impregnate the resin particles with the blowing
agent. While being maintained under a pressurized condition and at
a temperature sufficient to expand the resin particles, the
dispersion is discharged from the vessel to an atmosphere of a
pressure lower than the pressure in the vessel, thereby obtaining
expanded PP beads.
[0098] While the surface modification of the resin particles with
the organic peroxide and the subsequent expansion of the
surface-modified resin particles may be carried out in separate
vessels, it is preferred that the expansion step be carried out by
the dispersion method and that the expansion step be carried out in
the same vessel for reasons of efficiency. Namely, the surface
modification the resin particles and expansion of the
surface-modified resin particles may be carried out by simply
conducting the dispersion method after addition of a predetermined
amount of the organic peroxide in the dispersion.
[0099] In performing the expansion, it is preferred that the weight
ratio of the surface-modified resin particles to the dispersing
medium be 0.5:1 or less, preferably 0.1:1 to 0.5:1, for reasons of
prevention of melt adhesion of the surface-modified resin particles
in the dispersion. Thus, when the surface modification of the resin
particles is carried out in a vessel with the ratio of the resin
particles to the dispersing medium being maintained in a range of
0.6:1 to 1.3:1, and when the expansion is performed in the same
vessel, a fresh dispersing medium is added to the vessel before
subjecting the dispersion to the expansion step.
[0100] In the present invention, the polypropylene resin, the high
modulus polypropylene resin, the base resin, the resin particles,
the surface-modified resin particles, expanded PP beads and PP
molding are preferably substantially non-crosslinked. The term
"substantially non-crosslinked" as used herein is as defined below.
Sample resin is immersed in boiling xylene (100 ml xylene per 1 g
sample resin) and the mixture is refluxed for 8 hours. The mixture
is then immediately filtered through a 74 .mu.m wire net (specified
in Japanese Industrial Standard JIS Z8801-1966). The dry weight of
the xylene-insoluble matters left on the wire net is measured. A
crosslinking degree P (%) is calculated from the formula:
P(%)=(M/L).times.100 wherein M represents the weight (g) of the
xylene-insoluble matters and L represents the weight (g) of the
sample. "Substantially non-crosslinked" means that the crosslinking
degree P is 10% or less.
[0101] In the present invention, the crosslinking degree P of the
base resin, the resin particles, the surface-treated (or surface
modified) resin particles, expanded PP beads and PP molding is
preferably 5% or less, more preferably 3% or less, most preferably
1% or less. In general, the surface treatment does not result in an
increase of the crosslinking degree P.
[0102] The surface-modified resin particles, expanded PP beads
obtained therefrom and PP molding obtained from the beads may
contain 100-8000 ppm by weight of an alcohol having a molecular
weight of 50 or more and produced by the decomposition of the
organic peroxide. For example, p-t-butylcyclohexanol may be present
in the expanded PP beads, when
bis(4-t-butylcyclohexyl)peroxydicarbonate is used as the organic
peroxide. i-Propanol, s-butanol, 3-methoxybutanol,
2-ethylhexylbutanol or t-butanol may be detected, when the
corresponding peroxide is used.
[0103] To prevent melt-adhesion of the surface-treated resin
particles with each other during the expansion step, it is
desirable to add to the dispersing medium a dispersing agent which
is finely divided organic or inorganic solids. For reasons of
easiness of handling, the use of an inorganic powder is preferred.
Illustrative of suitable dispersing agents are natural or synthetic
clay minerals (such as kaolin, mica, pyrope and clay), alumina,
titania, basic magnesium carbonate, basic zinc carbonate, calcium
carbonate and iron oxide. The dispersing agent is generally used in
an amount of 0.001-5 parts by weight per 100 parts by weight of the
resin particles.
[0104] To improve the dispersing efficiency of the dispersing
agent, namely to reduce the amount of the dispersing agent while
retaining its function to prevent melt-adhesion of the
surface-treated particles, a dispersion enhancing agent may be
preferably added to the dispersing medium. The dispersion enhancing
agent is an inorganic compound capable of being dissolved in water
in an amount of at least 1 mg in 100 ml of water at 40.degree. C.
and of providing divalent or trivalent anion or cation. Examples of
the dispersion enhancing agents include magnesium chloride,
magnesium nitrate, magnesium sulfate, aluminum chloride, aluminum
nitrate, aluminum sulfate, ferric chloride, ferric sulfate and
ferric nitrate. The dispersion enhancing agent is generally used in
an amount of 0.0001-1 part by weight per 100 parts by weight of the
resin particles.
[0105] The blowing agent may be an organic physical blowing agent
or an inorganic physical blowing agent. Examples of the organic
physical blowing agents include aliphatic hydrocarbons such as
propane, butane, pentane, hexane and heptane, alicyclic
hydrocarbons such as cyclobutane and cyclohexane, and halogenated
hydrocarbons such as chlorofluoromethane, trifluoromethane,
1,2-difluoroethane, 1,2,2,2-tetrafluoroethane, methylchloride,
ethylchloride and methylenechloride. Examples of inorganic physical
blowing agents include air, nitrogen, carbon dioxide, oxygen, argon
and water. These organic and inorganic blowing agents may be used
singly or as a mixture of two or more. For reasons of stability
(uniformity) of apparent density of expanded PP beads, low costs
and freedom of environmental problem, the use of air or nitrogen is
preferred. Water as the blowing agent may be that used in
dispersing the surface-modified resin particles in the dispersing
medium.
[0106] The amount of the blowing agent may be suitably determined
according to the kind of the blowing agent, expansion temperature
and apparent density of the expanded PP beads to be produced. When
nitrogen is used as the blowing agent and when water is used as the
dispersing medium, for example, the amount of nitrogen is
preferably such that the pressure within the closed vessel in a
stable state immediately before the initiation of the expansion,
namely the pressure (gauge pressure) in the upper space in the
closed vessel, is in the range of 0.6-8 MPa(G). In general, the
pressure in the upper space in the closed vessel is desirably
increased as the apparent density of the expanded PP beads to be
obtained is reduced.
[0107] In a method of producing a composite foamed polypropylene
resin molding according to the present invention, expanded PP beads
are filled in a mold cavity including a plurality of contiguous
spaces. The expanded PP beads in each space are then heated to
fuse-bond respective expanded resin beads together into a unitary
body. The expanded PP beads used should show a high temperature
endothermic peak, in a DSC curve thereof, in addition to an
intrinsic endothermic peak located at a lower temperature side of
the high temperature peak.
[0108] The expanded PP beads filled in at least one of the spaces
should be specific expanded PP beads which satisfy the following
conditions (a) to (c) at the same time:
(a) the specific expanded beads are formed of a base resin having a
tensile modulus of at least 1,200 MPa,
(b) the high temperature endothermic peak of the specific expanded
beads has an apparent density D1 g/L which is not smaller than 10
g/L but not greater than 700 g/L, and
(c) the high temperature endothermic peak of the specific expanded
beads has such an area that corresponds to a calorific of E1 J/g,
wherein D1 and E1 have the following relationship
20-0.014.times.D2.ltoreq.E1.ltoreq.65-0.072.times.D1.
[0109] When the apparent density of the specific expanded PP beads
is less than 10 g/L, the open cell content is so high that it is
difficult to mold the expanded PP beads. When the specific expanded
PP beads have an apparent density greater than 700 g/L, it is
difficult to obtain expansion forces sufficient to fill the
interstices between expanded PP beads filled in the space. The
apparent density of the specific expanded PP beads is preferably 20
to 200 g/L, more preferably 30 to 150 g/L.
[0110] When the high temperature endothermic peak of the specific
expanded PP beads have a calorific value of E1 below
[20-0.014.times.D1] J/g, the shrinkage of the PP molding is
significant. When the calorific value E1 of the high temperature
peak of the specific expanded PP beads exceeds [65-0.072.times.D1]
J/g, it is difficult to obtain expansion forces sufficient to fill
the interstices between expanded PP beads filled in the space.
[0111] The apparent density (g/L) is obtained by dividing the
weight W (g) of the expanded PP beads by the volume V (L) of the
apparent volume thereof (density=W/V) The apparent volume is
measured as follows:
[0112] In a measuring cylinder, about 5 g of expanded PP beads are
allowed to stand at 23.degree. C. for 48 hours in the atmosphere
and thereafter immersed in 100 ml water contained in a graduation
cylinder at 23.degree. C. From the increment of the volume, the
apparent volume can be determined.
[0113] In general, a dual density PP molding is apt to expand to a
size greater than the mold cavity, when the molding is not
sufficiently cooled after the fuse-bonding of expanded PP beads has
been completed. The PP molding is also apt to shrink to a size
smaller than the mold cavity, when the molding is excessively
cooled after the fuse-bonding of expanded PP beads has been
completed. Thus, depending upon the degree of cooling, the dual
density PP molding expands or shrinks. In this case, since a
relatively low density section is more quickly cooled than a
relatively high density section, the low density section is more
likely to shrink, when the cooling of the molding is carried out
evenly. Since expansion is less desirable than shrinkage, the
cooling is generally carried out while preventing expansion of the
high density section. Thus, the shrinkage of the low density
section has been hitherto unavoidable in the case of production of
known dual density PP molding unless specifically controlled
cooling conditions are adopted. When the degree of shrinkage caused
during cooling is relatively small, the shape may return during a
succeeding aging stage which is generally carried out at
50-100.degree. C. for 24 hours. When the shrinkage is significant,
however, it is impossible to restore the shape.
[0114] By using the specific expanded beads formed of the
above-described high modulus base resin, on the other hand, a
molding produced therefrom does not expand even when the molding is
not sufficiently cooled after the completion of the molding
process. Further, the molding does not shrink even when it is
excessively cooled after the completion of the molding. Therefore,
a composite PP molding of the present invention having at least two
sections, which are fuse-bonded to each other, which differ from
each other in at least one characteristic selected from color,
density, composition and mechanical strengths and at least one of
which is formed from the specific expanded beads, has good quality,
is free of shrinkage or expansion and does not cause breakage at an
interface between the sections. Namely, even when the composite PP
molding has one or more sections which are not formed from the
specific expanded beads, shrinkage can be avoided when cooling is
carried out so as to avoid shrinkage of those sections, as long as
the composite PP molding has at least one section formed from the
specific expanded beads. Especially when each of the different
sections is formed from the specific expanded beads, foamed PP
moldings free of expansion and shrinkage may be easily obtained
even when the cooling time is shortened.
[0115] The calorific value of E1 [J/g] of high temperature
endothermic peak of the specific expanded PP beads is preferably
10-60%, more preferably 20-50%, based on a total calorific value of
the high temperature endothermic peak and the intrinsic peak. The
term "calorific value" of the high temperature endothermic peak and
the intrinsic peak is intended to refer to heat of fusion in an
absolute value.
[0116] The DSC curve herein is as obtained by the differential
scanning calorimetric analysis wherein a sample (2-10 mg of
expanded PP beads) is heated from room temperature (10-40.degree.
C.) to 220.degree. C. in an atmosphere of nitrogen at a rate of
10.degree. C./min. FIG. 1 shows an example of a DSC curve having an
intrinsic endothermic peak P1 at a peak temperature T1 and a high
temperature endothermic peak P2 at a peak temperature T2. The area
of a peak corresponds to the heat of fusion thereof.
[0117] The area of the high temperature peak P2 is determined as
follows. In the DSC curve (first DSC curve) C having two
endothermic peaks P1 and P2 at temperatures T1 and T2,
respectively, as shown in FIG. 1, a straight line A extending
between the point Z1 in the curve at 80.degree. C. and the point Z2
in the curve at a melt completion temperature Tmc is drawn. The
melt completion temperature Tmc is represented by a point at which
the high temperature peak P2 ends and meets the base line on a high
temperature side. Next, a line B which is parallel with the
ordinate and which passes a point Bc between the peaks P1 and P2 is
drawn. The line B crosses the line A at a point BA. The position of
the point BC is such that the length between the point BA and the
point Bc is minimum. The area of the high temperature peak P2 is
the shaded area defined by the line A, line B and the DSC curve
C.
[0118] A total of the heat of fusion of the high temperature peak
P2 and the heat of fusion of the intrinsic peak P1 corresponds to
an area defined by the line A and the DSC curve.
[0119] When expanded PP beads having a weight per bead of less than
2 mg are measured for the intrinsic peak P1 and high temperature
peak P2 using a differential scanning calorimeter, two or more
beads are sampled for the measurement such that the total weight of
the sample is in the range of 2-10 mg. When expanded PP beads to be
measured have a weight per bead of 2-10 mg, one bead is sampled for
the DSC measurement. When expanded PP beads to be measured have a
weight per bead of more than 10 mg, one of the beads is cut into
two or more pieces and one of the pieces having a weight of 2-10 mg
is sampled for the DSC measurement. In this case, an expanded PP
bead having a weight W and an outer peripheral surface area of S is
preferably cut into n number of pieces so that cut pieces have
nearly equal weight of W/n and have a surface portion which is
derived from the outer peripheral surface of the bead and which has
an area of nearly S/n. For example, when the expanded PP beads to
be measured have a weight per bead of 18 mg, one of the beads is
cut along a plane bisecting the bead and one of the cut pieces is
used for measurement. In the present specification, except
otherwise noted, the term "heat of fusion of the high temperature
peak of expanded PP bead(s)" is intended to refer to the heat of
fusion as measured in the above-described method, and should be
discriminated from "heat of fusion of the high temperature peak of
a surface region or an inside region of an expanded PP bead" which
will be described hereinafter.
[0120] The above-described high temperature peak P2 is present in
the DSC curve measured first. Once the expanded PP beads have
completely melted, the high temperature peak P2 no longer appears.
Thus, when the sample after the first DSC measurement is cooled to
about 40.degree.-50.degree. C. and is measured again for a DSC
curve by heating to 220.degree. C. in an atmosphere of nitrogen at
a rate of 10.degree. C./min, the second DSC curve does not show
such a high temperature peak but contains an endothermic peak
attributed to the melting of the base resin, like a DSC curve shown
in FIG. 2.
[0121] In the present specification and claims, the term "melting
point of the base resin" is intended to refer to that measured by
DSC analysis of base resin particles which have not yet been
subjected to surface modification treatment with an organic
peroxide. Namely, "melting point of the base resin" is measured by
the differential scanning calorimetric analysis wherein a sample
(2-4 mg of resin particles of the base resin) is heated from room
temperature (10-40.degree. C.) to 220.degree. C. in an atmosphere
of nitrogen at a rate of 10.degree. C./min. The sample is then
cooled to room temperature (10-40.degree. C.) and is measured again
for a DSC curve by heating to 220.degree. C. in an atmosphere of
nitrogen at a rate of 10.degree. C./min to obtain a second DSC
curve as shown in FIG. 2. The temperature Tm of the endothermic
peak P3 at 130-170.degree. C. in the second DSC curve as shown in
FIG. 2 is inherent to the polypropylene resin and represents the
"melting point of the base resin". Two or more endothermic peaks
might be observed in the second DSC curve, when, for example, the
resin particles are composed of two or more different polypropylene
resins. In this case, the melting point Tm is the peak temperature
of that peak which has the greatest peak height among those peaks.
When there are a plurality of peaks having the same greatest peak
height, then the melting point Tm is the highest peak temperature
among those peaks. The term "peak height" herein refers to the
length S between the top of the peak P3 and a point Q at which a
line parallel with the ordinate and passing through the top of the
peak P3 crosses the base line B.sub.L. In FIG. 2, the temperature
Te at which the endothermic peak P3 ends and meets the base line
B.sub.L refers to the "melt completion temperature of the base
resin".
[0122] The high temperature peak P2 of expanded PP beads generally
appears at a temperature T2 ranging from (Tm+5.degree. C.) to
(Tm+15.degree. C.), more generally ranging from (Tm+6.degree. C.)
to (Tm+14.degree. C.). The endothermic peak P1 of expanded PP beads
generally appears at a temperature T1 ranging from (Tm-5.degree.
C.) to (Tm+5.degree. C.), more generally ranging from (Tm-4.degree.
C.) to (Tm+4.degree. C.). The endothermic peak in the second DSC
measurement of expanded PP beads generally corresponds to that in
the second DSC curve of the precursor base resin particles and
generally appears at a temperature ranging from (Tm-2.degree. C.)
to (Tm+2.degree. C.).
[0123] As described above, it is preferred that the expanded PP
beads have such a crystal structure that a high temperature peak is
present in a first DSC curve thereof in addition to an intrinsic
peak. A difference between the melting point of the polypropylene
resin and expansion temperature has a great influence upon the heat
of fusion (peak area) of the high temperature peak.
[0124] The heat of fusion of the high temperature peak of the
expanded PP beads is a factor for determining the minimum
temperature of steam which provides a saturated steam pressure
required for melt-bonding the beads to each other. In general, when
the same base resin is used, the smaller the heat of fusion of the
high temperature peak, the lower becomes the minimum temperature.
Further, the higher the expansion temperature, the smaller becomes
the heat of fusion of the high temperature peak.
[0125] When expanded PP beads having a small heat of fusion of the
high temperature peak are used, the mechanical properties of the
resulting PP molding are relatively low, though the minimum
temperature required for melt-bonding the beads can be low. On the
other hand, when expanded PP beads having a large heat of fusion of
the high temperature peak are used, the mechanical properties of
the resulting PP molding are relatively high. In this case,
however, since the minimum temperature required for melt-bonding
the beads is high, it is necessary to use high pressure steam for
the production of PP moldings. Thus, the most preferred expanded PP
beads would be such that the heat of fusion of the high temperature
peak thereof is large but the minimum temperature required for
melt-bonding the beads is low. The expanded PP beads obtained from
the surface-modified resin are such ideal expanded PP beads. Such
expanded PP beads can give a high rigidity PP molding without using
a high temperature steam.
[0126] The expanded PP beads providing a DSC curve having such a
high temperature peak can be suitably produced by maintaining the
dispersion containing the surface-modified resin particles in a
vessel at a first fixed temperature between a temperature lower by
20.degree. C. than the melting point of the base resin
(Tm-20.degree. C.) and a temperature lower than the melt completion
point of the base resin (Te) for a period of time of preferably
10-60 min, preferably 15-60 min and then discharging the dispersion
from the vessel after increasing the temperature of the dispersion
to a second fixed temperature between a temperature lower by
15.degree. C. than the melting point of the base resin
(Tm-15.degree. C.) and a temperature higher by 10.degree. C. than
the melt completion point of the base resin (Te+10.degree. C.) or,
if necessary, after maintaining the dispersion at the second fixed
temperature for a period of time of 10-60 min.
[0127] The area of the high temperature peak mainly depends upon
the above first fixed temperature at which the dispersion is
maintained before expansion treatment, the time for which the
dispersion is maintained at the first fixed temperature, the above
second fixed temperature, the time for which the dispersion is
maintained at the second fixed temperature, the heating rate at
which the dispersion is heated to the first fixed temperature and
the heating rate at which the dispersion is heated from the first
fixed temperature to the second fixed temperature. The area of the
high temperature peak increases with an increase of the retention
time at the first and second fixed temperatures. The heating rate
(average heating rate from the commencement of heating until the
fixed temperature is reached) in each of the heating stage up to
the first fixed temperature and the succeeding heating stage from
the first fixed temperature to the second fixed temperature is
generally 0.5-5.degree. C. per minute. Suitable conditions for the
preparation of expanded PP beads having desired heat of fusion of
the high temperature peak can be determined by preliminary
experiments on the basis of the above points.
[0128] The above temperature ranges for the formation of the high
temperature peak and for the expansion of the resin particles are
suitably adopted in the case where an inorganic physical blowing
agent is used. When an organic physical blowing agent is used, the
suitable temperature ranges will shift toward low temperature side
and vary with the kind and amount of the organic physical blowing
agent.
[0129] The expanded PP beads (inclusive of the specific expanded PP
beads) obtained from the surface-modified resin particles
preferably have at least one of the following characteristics.
[0130] A surface region of the expanded PP bead preferably has a
melting point (Tms) lower than the melting point (Tmi) of an inside
region thereof (Tms<Tmi). The difference between the melting
point (Tmi-Tms) is preferably at least 0.05.degree. C., more
preferably at least 0.1.degree. C., most preferably at least
0.3.degree. C. The melting point Tms is determined as follows. A
surface region of the expanded PP bead is cut and about 2-4 mg of
such cut samples are collected. The sample is subjected to DSC
analysis in the same manner as described previously with regard to
the measurement of the melting point Tm. The peak temperature of a
peak corresponding to the endothermic peak P3 in the second DSC
curve represents the melting point Tms. The melting point Tmi is
also measured in the same manner as above except that inside region
of the bead is cut and collected.
[0131] In the case of the expanded PP bead having a high
temperature endothermic peak in a DSC curve thereof, the heat of
fusion Hs of the high temperature endothermic peak of the surface
region of the bead is preferably smaller than the heat of fusion Hi
of the high temperature endothermic peak of the inside region of
the bead such that the following relationship is established:
Hs<0.86.times.Hi for reasons that the expanded PP beads can be
molded at a lower temperature as compared with surface unmodified
expanded PP beads. Such an effect increases with a decrease of Hs.
Thus, the Hs and Hi of the expanded PP bead preferably have the
following relationship: Hs<0.83.times.Hi, more preferably
Hs<0.80.times.Hi,
[0132] still more preferably Hs<0.75.times.Hi,
[0133] yet still more preferably Hs<0.70.times.Hi,
[0134] most preferably Hs<0.60.times.Hi.
Preferably, Hs is not smaller than 0.25.times.Hi
(Hs.gtoreq.0.25.times.Hi).
[0135] It is also preferred that Hs is in the range of 1.7-60 J/g,
more preferably 2-50 J/g, still more preferably 3-45 J/g, most
preferably 4-40 J/g, for reasons of availability of a low molding
temperature
[0136] The surface region and inside region of an expanded PP bead
are sampled by cutting the bead with a knife or a microtome. The
surface region or regions are sliced off the bead at any arbitral
position or positions to a thickness of 200 .mu.m or less such that
the outer surface of the bead provides one of the both sides of
each of the sliced surface regions. Thus, the other side of each of
the sliced surface regions does not contain that part of the PP
bead which was present at a depth of more than 200 .mu.m before
cutting. The depth herein is in the direction from the outer
surface of the bead to the center of gravity thereof. When the
sliced surface region or regions contain that part of the PP bead
which was present at a depth of more than 200 .mu.m, precise data
cannot be obtained. When the amount of the surface region or
regions sampled from the bead is less than 2 mg, one or more
additional beads are cut to collect 2-4 mg of the sample.
[0137] The inside region is obtained by removing all of the surface
region of the bead up to the depth of 200 .mu.m in the direction
from the outer surface of the bead to the center of gravity
thereof. When the size of the bead is so small that no inside
region is obtainable after removal of surface region of the 200
.mu.m thick, then the inside region is obtained by removing all of
the surface region of the bead up to the depth of 100 .mu.m in the
direction from the outer surface of the bead to the center of
gravity thereof. When the size of the bead is so small that no
inside region is obtainable after removal of surface region of the
100 .mu.m thick, then the inside region is obtained by removing all
of the surface region of the bead up to the depth of 50 .mu.m in
the direction from the outer surface of the bead to the center of
gravity thereof.
[0138] When the amount of the inside region obtained from one bead
is less than 2 mg, one or more additional beads are used to collect
2-4 mg of the sample. The thus collected samples are measured for
the melting point and heat of fusion of the high temperature peak
according to the method described above.
[0139] The expanded PP bead preferably has an MFR value which is
not smaller than that of the resin particles before the surface
modification with the organic peroxide and which is in the range of
0.5-150 g/10 min, more preferably 1-100 g/10 min, most preferably
10-80 g/10 min. It is also preferred that the MFR value of the
expanded PP bead be at least 1.2 times, more preferably at least
1.5 times, most preferably 1.8-3.5 times, that of the resin
particles prior to the surface modification.
[0140] For measuring the MFR, the expanded PP beads are pressed at
200.degree. C. using a heat press into a sheet having a thickness
of 0.1-1 mm. Pellets or columns are prepared from the sheet to
obtain a sample. The sample is measured for MFR in accordance with
the Japanese Industrial Standard JIS K7210-1976, Test Condition 14.
In the measurement of MFR, air bubbles must be removed from the
sample. If necessary, heat press treatment should be repeated up to
three times in total to obtain bubble-free sheet.
[0141] The expanded PP bead preferably has a surface region having
a greater oxygen content per unit weight than that of the inside
region. When the organic peroxide used for the surface modification
of the resin particles is of a type which generates oxygen radicals
upon being decomposed, part of the oxygen radicals are bound to
surfaces of the particles. The analysis, using an infrared
spectrometer equipped with the attenuated total reflectance (ATR
analysis), of a surface of a PP molding obtained from expanded PP
beads of the present invention shows a stronger absorption at a
wavelength of near 1033 cm.sup.-1 than that of a PP molding
obtained from conventional expanded PP beads. Thus, the ratio of
the peak height at 1033 cm.sup.-1 to the peak height at 1166
cm.sup.-1 in the case of the PP molding of the present invention is
greater than that of the conventional molding. Further, the
analysis using an energy dispersion spectroscope (EDS) shows that a
surface of the expanded PP bead according to the present invention
has an oxygen to carbon molar ratio (O/C molar ratio) is 0.2
whereas an inside of the bead has an O/C molar ratio of 0.1.
Further, a surface of the conventional expanded PP bead has O/C
molar ratio of 0.09. Such an oxygen-added surface of the expanded
PP bead is considered to enhance steam permeability thereof. The
preferred O/C ratio is at least 0.15.
[0142] The minimum temperature required for melt-bonding the
surface-modified expanded PP beads is effectively lowered as a
result of a reduction of the heat of fusion of the high temperature
peak of the surface region of the expanded PP beads and/or as a
result of a reduction of the melt initiation temperature of the
surfaces of the expanded PP beads.
[0143] The expanded PP beads obtained by the above process are aged
in the atmosphere. If desired, the PP beads may be treated to
increase the pressure inside of the cells thereof and, thereafter,
heated with steam or hot air to improve the expansion ratio
thereof.
[0144] A PP molding may be suitably obtained by a batch-type
molding method in which expanded PP beads (if necessary, after
being treated to increase the pressure inside of the cells thereof)
are filled in a mold adapted to be heated and cooled and to be
opened and closed. After closing the mold, saturated steam is fed
to the mold to heat and fuse-bond the beads together. The mold is
then cooled and opened to take a PP molding out of the mold. A
number of molding machines are commercially available. They are
generally designed to have a pressure resistance of 0.41 MPa(G) or
0.45 MPa(G). Thus, the above method is generally carried out using
steam having a pressure of 0.45 MPa(G) or less, more preferably
0.41 MPa(G) or less.
[0145] The above-mentioned treatment of the expanded PP beads to
increase the pressure inside of the cells thereof may be carried
out by allowing the beads to stand for a suitable period of time in
a closed vessel to which a pressurized gas has been fed. Any gas
containing an inorganic gas as a major ingredient may be used for
the pressure increasing treatment as long as it is in the form of
gas under conditions where the expanded beads are treated. Examples
of the inorganic gas include nitrogen, oxygen, air, carbon dioxide
and argon. Nitrogen or air is suitably used for reasons of costs
and freedom of environmental problems.
[0146] Described below will be a specific method of increasing the
inside pressure of the cells using air and a method of measuring
the thus increased inside pressure in the cells.
[0147] Expanded PP beads are placed in a closed vessel into which
pressurized air is fed. The beads are allowed to stand in the
vessel for a certain period of time (generally several hours) while
maintaining the pressure inside the vessel at 0.98-9.8 MPa(G) so
that the inside pressure of the cells increases. The thus treated
expanded PP beads are placed in a mold for the production of a PP
foam molding. The inside pressure of the cells Pi (MPa(G)) as used
herein is defined as follows: Pi=Wi.times.R.times.Te/(M.times.V)
wherein Wi is an amount of air increased (g), R is the gas constant
and is 0.0083 (MPaL/(Kmol), Te is an ambient temperature and is
296K, M is the molecular weight of air and is 28.8 (g/mol), and V
is the volume (liter) of the air in the expanded beads.
[0148] The amount of air increased Wi (g) is measured as
follows.
[0149] A quantity of expanded beads whose cells have been just
pressurized with air in the vessel are taken out of the vessel and
collected in a polyethylene film bag having a size of 70
mm.times.100 and provided with a multiplicity of perforations each
having a size preventing the passage of the beads. The beads in the
bag are placed, within 60 seconds after the take-out, on a weighing
device provided in a thermostatic space maintained at 23.degree. C.
and 50% relative humidity under ambient pressure. The weight Ua (g)
of the beads is measured just 120 seconds after the expanded beads
have been taken out from the vessel. The expanded beads are then
allowed to stand for 48 hours in the space at 23.degree. C. and 50%
relative humidity under ambient pressure. The air in the cells of
the expanded beads gradually permeates through the cell walls and
escapes from the beads. Therefore, the weight of the beads
decreases with the lapse of time. However, an equilibrium has been
established and the weight decrease no longer occurs after lapse of
the 48 hours period. Thus, the weight of the expanded beads Ub (g)
is measured in the same space after the lapse of the 48 hours
period. Of course, the weight of the polyethylene bag is also
measured and taken in consideration. The measurement of the weight
should be carried out precisely to the fourth decimal place (0.0001
g). The balance between the weights Ua and Ub represents the amount
of gas increased (Wi=Ua-Ub).
[0150] The volume of the air in the expanded PP beads V (L) is
defined as follows. V(L)=Va-Vb wherein Va is the apparent volume of
the expanded PP beads, and Vb is the volume of the base resin of
the beads and is obtained by dividing the weight of the beads Ub
(g) by the density of the base resin (g/L).
[0151] The apparent volume Va (L) of the expanded PP beads is
measured as follows. The expanded PP beads which have been
subjected to the measurement of the weight Ub as described above,
are immersed in 100 ml of water at 23.degree. C. contained in a
graduated measuring cylinder. From the volume increment, apparent
volume Va (L) of the beads is determined. The quantity of the
above-described expanded beads sampled and collected in the bag is
such that Ub and Va fall within the ranges of 0.5 to 10 g and 50 to
90 cm.sup.3, respectively.
[0152] The inside pressure Pi of the cells of the expanded PP beads
is preferably 0.98 MPa(G) or less, more preferably 0.69 MPa(G) or
less, still more preferably 0.49 MPa(G) or less, most preferably
0.1 MPa(G) or less, for reasons of suitable foaming power while
permitting a heating medium (saturated steam) to penetrate into the
central region of the molding, thereby ensuring fuse bonding of the
expanded PP beads into a unitary structure.
[0153] FIG. 3 schematically depicts one embodiment of a composite
PP molding 6 of the present invention. The molding 6 has three,
first through third sections 1, 2 and 3. Designated as 4 and 5 are
interfaces between the first and second sections 1 and 2 and
between the second and third sections 2 and 3, respectively. Each
of the first through third sections shows a high temperature
endothermic peak, in a DSC curve thereof, in addition to an
intrinsic endothermic peak located at a lower temperature side of
the high temperature peak.
[0154] In the illustrated embodiment, the first through third
sections 1, 2 and 3 have apparent densities of D2.sub.1, D2.sub.2
and D2.sub.3, respectively, which satisfy the following conditions:
D2.sub.2>D2.sub.1 and D2.sub.2>D2.sub.3. The apparent
densities D2.sub.1 and D2.sub.3 of the first and third sections may
be the same or different. Preferably, the apparent density D2.sub.2
is 1.2-25 times, more preferably 1.2-20 times, as high as apparent
densities D2.sub.1 and D2.sub.3. The molding 6 as a whole has a low
weight because of the presence of the low density sections 1 and 3
and yet exhibits high mechanical strengths because of the presence
of the high density section 2.
[0155] It is important that at least one of the first through third
sections 1-3 should meet the following conditions (d) to (f) at the
same time:
(d) that section is formed from specific expanded polypropylene
resin beads of a base resin having a tensile modulus of at least
1,200 MPa,
[0156] (e) that section has an apparent density D2 g/L which is not
smaller than 10 g/L but not greater than 500 g/L
(10.ltoreq.D2.ltoreq.500), and
[0157] (f) the high temperature endothermic peak of that section
has such an area that corresponds to a calorific of E2 J/g, wherein
D2 and E2 have the following relationship
20-0.020.times.D2.ltoreq.E2.ltoreq.65-0.100.times.D2. The
conditions (e) and (f) are preferably as follows:
25-0.020.times.D2.ltoreq.E2.ltoreq.55-0.100.times.D2 (e')
15.ltoreq.D2.ltoreq.450 (f').
[0158] Preferably, each of the first through third sections 1-3
meets the above requirements (d) to (f).
[0159] An apparent density below 10 g/L will result in considerable
reduction of mechanical strengths, while too high an apparent
density above 500 g/L fails to contribute to a reduction of weight
of the molding. In the embodiment shown in FIG. 3, the apparent
density D2.sub.2 of the second section 2 is preferably 30-450 g/L,
while the apparent densities D2.sub.1 and D2.sub.3 of the first and
third sections 1 and 3 are each less than D2.sub.2 and are each
preferably 15-90 g/L.
[0160] When E2 is less than (20-0.020.times.D2), shrinkage of that
section might be caused when cooling is excessively carried out. On
the other hand, when E2 is greater than (65-0.100.times.D2), that
section might not have sufficiently high bonding strength between
the cells.
[0161] The term "apparent density" of the PP molding as used herein
is as specified in JIS K7222-1999. The volume of a PP molding used
for the calculation of the apparent density is determined from the
external dimensions thereof. When the external shape of the molding
is so complicated that the volume thereof is difficult to be
determined, then the volume thereof is measured by immersing the
molding in water and is given as a volume of water replaced by the
molding. To measure the apparent density of a given section of the
PP molding, that section is cut out along each interface between
adjacent sections. The cut section is then measured for the
apparent density in the same manner as that for the above PP
molding.
[0162] FIG. 4 schematically illustrate a second embodiment of a
composite PP molding of the present invention composed of two
different sections 1 and 2 fuse bonded to each other at an
interface 4. At least one of (preferably each of) the first and
second sections 1 and 2 meets the above requirements (d) to (f) at
the same time.
[0163] FIG. 5 depicts a third embodiment of a composite PP molding
21 of the present invention having five, first through fifth
sections 11-15 which are fuse bonded at interfaces 16-19. The first
through fifth sections 11-15 have apparent densities of D2.sub.11,
D2.sub.12, D2.sub.13, D2.sub.14 and D2.sub.15, respectively, which
satisfy the following conditions:
D2.sub.31>D2.sub.12>D2.sub.11 and
D2.sub.13>D2.sub.14>D2.sub.15 At least one of (preferably
each of) the first through fifth sections 11-15 meets the above
requirements (d) to (f) at the same time.
[0164] The first through third embodiments shown in FIGS. 3 through
5, in which adjacent two sections are directly fuse-bonded to each
other, may be suitably prepared using a molding method disclosed
in, for example, Japanese Laid-Open Patent Publications No.
H.sub.11-334501, No. 2000-16205, No. 2001-63496, No. 2001-150471
and No. 2002-172642, Japanese Examined Patent Publication No.
S62-22352 and U.S. Pat. No. 5,164,257, entire disclosure of which
is hereby incorporated by reference herein.
[0165] If desired, two sections are fuse-bonded to each other
through an insert interposed therebetween. One such an embodiment
is shown in FIG. 6. Designated as 22 and 23 are first and second
sections which are fuse-bonded to each other through an insert 24
interposed therebetween to form a composite PP molding 25. At least
one of (preferably each of) the first and second sections 22 and 23
meets the above requirements (d) to (f) at the same time.
[0166] In molding expanded PP beads to obtain the composite PP
molding 25, the insert 24 is placed in a mold cavity to partition
the mold cavity into two, contiguous first and second spaces. Each
of the spaces is then filled with required expanded PP beads. Thus,
the insert 24 serves as a partition between the first and second
spaces. The beads are then heated to fuse-bond respective expanded
resin beads in each space together to obtain the composite PP
molding 25 having the insert 24 sandwiched between the first and
second sections 22 and 23.
[0167] The fuse-bonding of the first and second sections 22 and 23
through the insert 24 is not always sufficiently high when the
affinity between the insert and the sections 22 and 23 is not high.
In such a case, it is preferred that the insert 24 be provided with
one or more perforations 27, because part of the first and second
sections 22 and 23 can be directly fuse-bonded to each other
through the perforations 27 to enhance the bonding strength
therebetween.
[0168] The insert 24 may be a sheet, net or plate of an any desired
material such as metal, glass, ceramic or plastic. The thickness of
the insert 24 is not specifically limited but is generally 1-10 mm,
preferably 2-8 mm. The perforations 27 formed in the insert 24 may
be in the form of, for example, holes or slits. The size of the
perforation 27 is such that the expanded particles to be placed on
at least one of the both sides of the insert 24 are unable to pass
therethrough. Generally, the area of the perforation 27 is in the
range of 0.2S-0.9S, preferably 0.5S-0.8S, where S is a central
sectional area of the smallest expanded particle used. The size and
number of the perforations 27 are such that the adjacent two
sections 22 and 23 can be directly fuse-bonded to each other
through the perforations 27 to provide sufficient bonding strength
therebetween. Generally, a total area of the perforations 27 is
25-90%, preferably 50-80%, of an area of the insert 24.
[0169] The composite PP molding of the present invention may be
also produced by a continuous method in which expanded PP beads (if
necessary, after being treated to increase the pressure inside of
the cells thereof) are fed to a path which is defined between a
pair of belts continuously running in the same direction and which
has a heating zone and a cooling zone. During the passage through
the heating zone, the expanded PP beads are heated with saturated
steam and fuse-bonded to each other. The resulting molding is
cooled in the cooling zone, discharged from the path and cut to a
desired length. The above continuous method is disclosed in, for
example, Japanese Laid-Open Patent Publications No. H09-104026, No.
JP-A-H09-104027 and No. JP-A-H10-180888, the disclosure of which is
hereby incorporated by reference herein.
[0170] A surface layer, such as a reinforcing layer or a decorative
layer) may be integrally provided on a surface of the above
composite PP molding. A method of producing such a composite
article is disclosed in, for example, U.S. Pat. No. 5,928,776, No.
6096417, No. 6033770 and No. 5474841, European Patent No. 477476,
International Publications No. WO98/34770 and No. WO98/00287 and
Japanese Patent No. 3092227, the disclosure of which is hereby
incorporated by reference herein.
[0171] An insertion material (together with or without using the
above-described insert 24) may be integrated with the above PP
molding such that at least part of the insertion material is
embedded therein. A method of producing such a composite article is
disclosed in, for example, U.S. Pat. No. 6,033,770 and No.
5,474,841, Japanese Laid-Open Patent Publications No. S59-127714
and Japanese Patent No. 3092227, the disclosure of which is hereby
incorporated by reference herein.
[0172] When the composite PP molding has a first section made from
the above-described specific PP beads and a second section provided
adjacent to the first section and made from expanded PP beads which
do not meet with one or more of the above-described conditions
(a)-(c), it is preferred that the weights and calorific values of
the high temperature endothermic peaks of the first and second
sections have the following condition:
[(C1.times.d1)+(C2.times.d2)]/(d1+d2)>22 [J/g] wherein C1 and C2
represent the calorific values [J/g] of the first and second
sections, respectively, and d1 and d2 represent the weights of the
first and second sections, respectively.
[0173] The PP molding of the present invention preferably has an
open cell content (according to ASTM-D2856-70, Procedure C) of 40%
or less, more preferably 30% or less, most preferably 25% or less,
for reasons of high mechanical strengths.
[0174] The composite PP molding may be suitably used as a shock
absorber such as an automobile bumper core, a pat for an automobile
door, a helmet core or a container.
[0175] The following examples will further illustrate the present
invention. Parts are by weight.
PREPARATION EXAMPLES 1-4, 8 And 9
Preparation of Expanded PP Beads:
[0176] 100 Parts of polypropylene homopolymer (base resin) having a
melting point, MFR and a tensile strength indicated in Table 1 were
blended with 0.05 part of zinc borate powder (cell controlling
agent) and the blend was kneaded in an extruder and extruded into
strands. The strands were immediately introduced in water at
25.degree. C. for quenching. The cooled strands were taken out from
the water and then cut into particles each having a length/diameter
ratio of about 1.0 and a mean weight of 2 mg.
[0177] In a 400 liter autoclave, 100 kg of the above resin
particles were charged together with 120 kg of ion-exchanged water
at 25.degree. C. (dispersing medium; weight ratio of the resin
particles to the dispersing medium: 0.83), 0.002 kg of sodium
dodecylbenzenesulfonate (surfactant), 0.4 kg of kaolin powder
(dispersing agent), 0.013 kg of aluminum sulfate powder (dispersion
enhancing agent), and 0.32 kg of
bis(4-t-butyl-cyclohexyl)peroxydicarbonate (organic peroxide). The
mixture in the autoclave was heated to 90.degree. C. at an average
heating rate of 5.degree. C./min with stirring and maintained at
that temperature for 10 minutes. Then, 100 kg of ion-exchanged
water and carbon dioxide (blowing agent) were fed to the autoclave
under pressure until the inside pressure thereof was stabilized at
0.49 MPa(G). The dispersion in the autoclave was then stirred,
heated to a temperature lower by 5.degree. C. than the expansion
temperature shown in Table 1 at an average heating rate of
4.degree. C./min. Thereafter, the temperature was raised with
stirring to a temperature lower by 1.degree. C. than the expansion
temperature at an average heating rate of 0.16.degree. C./min.
Subsequently, a high pressure carbon dioxide gas (blowing agent)
was charged in the autoclave until the inside pressure shown in
Table 1 was reached. The temperature was raised to the expansion
temperature at an average heating rate of 0.029.degree. C./min.
Then, one end of the autoclave was then opened to discharge the
dispersion to the atmosphere to obtain expanded PP beads. The
discharge was carried out while feeding carbon dioxide gas such
that the pressure within the autoclave was maintained at a pressure
equal to the pressure in the autoclave immediately before the
commencement of the discharge. The expanded PP beads were washed,
centrifuged and allowed to stand in the atmosphere at 23.degree. C.
for 24 hours for aging, thereby obtaining Beads Nos. 1-4, 8 and 9.
The Beads Nos. 1-4, 8 and 9 were then measured for heat of fusion
of a high temperature peak of thereof (one bead as a whole), heat
of fusion of high temperature peaks of surface and inside regions
thereof and apparent density thereof. The results are summarized in
Table 1. The Beads Nos. 1-4, 8 and 9 were found to be substantially
non-crosslinked (the boiling xylene insoluble content was 0).
PREPARATION EXAMPLES 5-7
Preparation of Expanded PP Beads:
[0178] Using polypropylene homopolymer (base resin) having a
melting point, MFR and a tensile strength indicated in Table 1,
resin particles were prepared in the same manner as that of the
above Preparation Examples.
[0179] In a 400 liter autoclave, 100 kg of the thus obtained resin
particles were charged together with 220 kg of ion-exchanged water
(dispersing medium; weight ratio of the resin particles to the
dispersing medium: 0.45), 0.005 kg of sodium
dodecylbenzenesulfonate (surfactant), 0.3 kg of kaolin powder
(dispersing agent), and 0.01 kg of aluminum sulfate powder
(dispersion enhancing agent). Then, carbon dioxide (blowing agent)
was fed to the autoclave under pressure until the inside pressure
thereof was stabilized at 0.49 MPa(G). The dispersion in the
autoclave was then stirred, heated to a temperature lower by
5.degree. C. than the expansion temperature shown in Table 1 at an
average heating rate of 4.degree. C./min. Thereafter, the
temperature was raised with stirring to a temperature lower by
1.degree. C. than the expansion temperature at an average heating
rate of 0.16.degree. C./min. Subsequently, a high pressure carbon
dioxide gas (blowing agent) was charged in the autoclave until the
inside pressure shown in Table 1 was reached. The temperature was
raised to the expansion temperature at an average heating rate of
0.029.degree. C./min. Then, one end of the autoclave was then
opened to discharge the dispersion to the atmosphere to obtain
expanded PP beads. The discharge was carried out while feeding
carbon dioxide gas such that the pressure within the autoclave was
maintained at a pressure equal to the pressure in the autoclave
immediately before the commencement of the discharge. The expanded
PP beads were washed, centrifuged and allowed to stand in the
atmosphere for 24 hours for aging, thereby obtaining Beads Nos.
5-7. The Beads Nos. 5-7 were then measured for heat of fusion of a
high temperature peak of thereof (one bead as a whole), heat of
fusion of high temperature peaks of surface and inside regions
thereof and apparent density thereof. The results are summarized in
Table 1. The Beads Nos. 5-7 were found to be substantially
non-crosslinked (the boiling xylene insoluble content was 0).
TABLE-US-00001 TABLE 1 Preparation Example No. 1 2 3 4 5 6 7 8 9
Base Resin: Melting point (.degree. C.) 161 161 161 161 161 144 145
164.5 164 MFR (g/10 min) 21 21 21 21 21 7 7 7 7 Tensile modulus
(MPa) 1440 1440 1440 1440 1440 1120 1120 1350 1350 Expansion
Conditions: Temperature (.degree. C.) 165.7 166.0 167.4 167.0 166.0
153 157 166 171.5 Pressure stabilized (MPaG) 2.16 2.16 1.18 1.18
2.16 2.54 0.78 2.55 1.57 Expanded PP Beads: Surface treatment yes
yes yes yes no no no yes yes Apparent density D1 (g/L) 42 66 69 88
35 37 143 99 43 Calorific value of high temperature endothermic
peak Whole (J/g) 37.3 36.8 30.3 35.8 26 12.7 7.7 59 13.1 Surface
region (J/g) 21.0 20.3 17.5 21.0 25.0 11.5 7.0 25.0 9.8 Inside
region (J/g) 43.0 39.1 35.0 40.5 26.5 13.0 8.0 67.0 15.0 (20 -
0.014 .times. 19.4-62.0 19.1-60.2 19.0-60.0 18.8-58.7 19.5-62.5
19.5-62.3 18.0-54.7 18.6-57.9 19.4-61.9 D1) - (65 - 0.072 .times.
D1)(J/g) Beads No. No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8
No. 9
EXAMPLES 1-5 And COMPARATIVE EXAMPLES 1-5
[0180] Using the thus obtained expanded PP beads, composite PP
moldings of a shape as shown in FIG. 3 were produced with a molding
device which had a male mold and a female mold adapted to be
displaced relative to each other. When the two molds were located
in a fully closed position, a mold cavity having a size of 700 mm
(length).times.200 mm (width).times.50 mm (thickness) was defined
therebetween, with the distance between the opposing inside walls
of the molds providing the thickness (50 mm) of a molding produced
in the mold cavity. Two stainless steel partition plates were
disposed in the mold cavity at positions corresponding to the
interfaces 4 and 5 so that the mold cavity was divided into three
spaces aligning in series along the lengthwise direction of a PP
molding to be produced and having lengths of 150 mm (corresponding
to the length of the first section 1), 400 mm (corresponding to the
length of the second section 2) and 150 mm (corresponding to the
length of the first section 3), respectively. The molds were first
positioned such that a gap of about 10 mm (the distance between the
opposing inside walls of the molds was about 60 mm). Expanded PP
beads shown in Tables 2 and 3 were fed to respective spaces of the
mold cavity in such a combination that high density beads were
filled in the center space (corresponding to the length of the
second section 2) and low density beads were filled in both end
spaces (corresponding to the length of the first and third sections
1 and 3) and, thereafter, the partition plates were removed from
the mold cavity.
[0181] The molds were then closed. Steam was fed into the mold
cavity to substitute for air. Then, steam at a pressure of 0.8
MPa(G) was fed from a male mold side to the mold cavity until a
pressure lower by 0.04 MPa(G) than a predetermined molding pressure
shown in Tables 2 and 3 was reached (1st heating step). Next, steam
at a pressure of 0.8 MPa(G) was fed from a female mold side to the
mold cavity until a pressure lower by 0.02 MPa(G) than the
predetermined molding pressure was reached (2nd heating step).
Finally, steam was fed from the both male and female sides to the
mold cavity until the predetermined molding pressure was reached
and, thereafter, the mold cavity was maintained at that temperature
for 20 seconds (3rd heating step). Then, the molds were cooled with
water until a surface pressure on the molding of 0.059 MPa(G) was
reached. The molding was taken out of the mold cavity, dried at
60.degree. C. and allowed to stand in a chamber at 23.degree. C.
and a relative humidity of 50% for 24 hours. In the case of
Comparative Examples 1-3, the cooling of the molds were started as
soon as the predetermined molding pressure was reached, without the
maintenance for the 20 seconds. The time required from the
commencement of the 1st heating step until the molding pressure was
reached was measured, from which a pressure increasing rate was
calculated.
[0182] The above-mentioned predetermined molding pressure was the
minimum steam pressure P.sub.min (MPa(G)) required for properly
fuse-bonding the beads to each other and determined by repeatedly
producing moldings at various saturated steam pressures increasing
from 0.15 MPa(G) to 0.55 MPa(G) at an interval of 0.01 MPa(G).
Thus, at a pressure (P.sub.min-0.01 MPa), the beads were incapable
of properly fuse-bond together.
[0183] The DSC analysis for the measurement of the physical
properties of the polypropylene resin and the expanded PP beads was
carried out using Shimadzu Heat Flux Differential Scanning
Calorimeter DSC-50 (manufactured by SHIMADZU corporation).
[0184] In determining the minimum steam pressure P.sub.min required
for properly fuse-bonding the beads to each other, whether or not
the beads were properly bonded to each other was evaluated as
follows:
[0185] A cut with a depth of 10 mm is formed on one of the two
largest sides (700 mm.times.200 mm) of a sample of PP molding along
a bisecting line (L1, L2 and L3) of each of the first through third
sections 1, 2 and 3 perpendicular to the longitudinal direction
thereof. The sample is then broken into halves along each of the
cut lines L1, L2 and L3 by bending. The interface along which the
halves have been separated is observed to count a total number C1
of the beads present on the interface and the number C2 of the
beads having destroyed cells. When the ratio C2/C1 is at least 0.5
in each of the first through third sections 1, 2 and 3, the sample
is regarded as having properly fuse-bonded beads. The ratio C2/C1
increases with an increase of the steam pressure. The minimum steam
pressure P.sub.min is a pressure at which the ratio C2/C1 is at
least 0.5 in each of the first through third sections 1, 2 and 3.
At a pressure of (P.sub.min-0.01 MPa), however, the ratio C2/C1 is
lower than 0.5 in at least one of the first through third sections
1, 2 and 3 and the beads are incapable of properly fuse-bond
together. The number C1 is a total of the beads having no destroyed
cells and the beads having destroyed cells (C2).
[0186] The minimum steam pressure P.sub.min is shown in Tables 2
and 3. In the case of Comparative Example 4, a molding pressure of
0.44 MPa(G) which is the maximum withstand pressure of the molding
device was used. Even at such a high pressure, it was impossible to
properly fuse-bond the beads to each other. TABLE-US-00002 TABLE 2
Example No. 1 2 3 4 5 Expanded High PP Beads Density Beads Beads
No. No. 3 No. 4 No. 3 No. 3 No. 7 Inside 0 0 0 0 0 pressure (MPa)
Low Density Beads Beads No. No. 1 No. 1 No. 2 No. 5 No. 1 Inside 0
0 0 0 0 pressure (MPa) Molding Molding 0.41 0.41 0.41 0.43 0.41
Conditions Pressure (MPaG) Pressing 0.041 0.046 0.037 0.039 0.046
Rate (MPaG/sec) Maintenance 20 20 20 20 20 Time (sec) Cooling 40 40
40 50 200 Time (sec)
[0187] TABLE-US-00003 TABLE 3 Comparative Example No. Comp. Comp.
Comp. Comp. Comp. 1 2 3 4 5 Expanded High PP Beads Density Beads
Beads No. No. 7 No. 7 No. 7 No. 8 No. 7 Inside 0 0 0 0 0 pressure
(MPa) Low Density Beads Beads No. No. 6 No. 6 No. 6 No. 6 No. 9
Inside 0 0 0 0 0 pressure (MPa) Molding Molding 0.33 0.33 0.33 0.44
0.41 Conditions Pressure (MPaG) Pressing 0.013 0.013 0.013 0.059
0.050 Rate (MPaG/sec) Maintenance 0 0 0 20 20 Time (sec) Cooling 80
140 200 40 200 Time (sec)
Each of the composite PP moldings thus obtained was measured for
fuse-bonding efficiency, surface appearance and dimensional
stability, weight, compression strength, apparent density (D2), and
calorific value (heat of fusion) of the high temperature
endothermic peak of DSC curve thereof to give the results shown in
Tables 4 and 5.
[0188] The use-bonding efficiency was evaluated in terms of whether
the ratio C2/C1 was at least 0.5 or not in all the sections, when
the molding pressure of 0.44 MPa(G) was adopted as follows: [0189]
Good: C2/C1 is at least 0.5 [0190] No good: C2/C1 is less than 0.5.
[0191] The surface appearance was evaluated with naked eyes as
follows: [0192] Good: Top surface is smooth and has good appearance
[0193] No good: Significant depressions and protrusions are
present.
[0194] To evaluate the dimensional stability, the thickness of the
composite PP molding at the center of each of the first through
third sections 1, 2 and 3 was measured after the dried molding was
allowed to stand in a chamber at 23.degree. C. and a relative
humidity of 50% for 24 hours. The dimensional stability was
evaluated according to the following ratings: [0195] Excellent:
49.0 mm.ltoreq.thickness.ltoreq.50.0 mm [0196] Good: 48.0
mm.ltoreq.thickness<49.0 mm or [0197] 50.0
mm<thickness.ltoreq.51.0 mm [0198] Fair: 47.0
mm.ltoreq.thickness<48.0 mm or [0199] 51.0
mm<thickness.ltoreq.52.0 mm [0200] No good: thickness<47.0 mm
or [0201] thickness>52.0 mm
[0202] The compression strength was measured as follows. The dried
molding, after having been allowed to stand in a chamber at
23.degree. C. and a relative humidity of 50% for 14 days, was cut
without leaving any outer surfaces thereof to obtain a sample
having a size of 50 mm.times.50 mm.times.25 mm. The sample was
subjected to compression test in accordance with Japanese
Industrial Standard JIS Z0234-1976, A method. Thus, the sample was
compressed at 23.degree. C. at a loading rate of 10 mm/min until a
strain of 55% was reached to obtain a stress-strain curve. The
stress at 50% strain represents the compression strength.
TABLE-US-00004 TABLE 4 Example No. 1 2 3 4 5 High Density Section:
Fuse-bonding efficiency Good Good Good Good Good Surface appearance
Good Good Good Good Good Dimensional stability Excellent Excellent
Excellent Excellent Good Weight (g) 235 313 237 192 501 Compression
strength (kPa) 1154 1964 1154 863 3134 Apparent density D2 (g/L) 84
111 84 66 190 Calorific value E2 (J/g) 31.2 36.6 31.3 31.5 8.0 (20
- 0.02 .times. D2) - (65 - 0.100 .times. D2) 18.3-56.6 17.9-53.9
18.3-56.6 18.7-58.4 16.2-46.0 (J/g) Low Density Section:
Fuse-bonding efficiency Good Good Good Good Good Surface appearance
Good Good Good Good Good Dimensional stability Excellent Excellent
Excellent Excellent Excellent Weight (g) Compression strength (kPa)
191 192 249 148 180 Apparent density D2 (g/L) 598 598 863 340 598
Calorific value E2 (J/g) 51 51 66 38 51 (20 - 0.02 .times. D2) -
(65 - 0.100 .times. D2) 38.0 37.9 37.4 27.5 38.0 (J/g) 19.0-59.9
19.0-59.9 18.7-58.4 19.2-61.2 19.0-59.9 Composite PP Molding as a
whole Weight (g) 426 505 486 340 681 Apparent density (g/L) 65 77
74 52 103
[0203] TABLE-US-00005 TABLE 5 Comparative Example No. Comp. 1 Comp.
2 Comp. 3 Comp. 4 Comp. 5 High Density Section: Fuse-bonding
efficiency Good Good Good No good Good Surface appearance Good Good
Good No good Good Dimensional stability No good Good Good Good Good
Weight (g) 537 547 553 338 540 Compression strength (kPa) 3134 3241
3295 1020 3140 Apparent density D2 (g/L) 190 194 196 120 191
Calorific value E2 (J/g) 7.9 8.0 7.9 67.1 8.0 (20 - 0.02 .times.
D2) - (65 - 0.100 .times. D2) 16.2-46.0 16.1-45.6 16.1-45.4
17.6-53.0 16.2-45.9 (J/g) Low Density Section: Fuse-bonding
efficiency Good Good Good Good Good Surface appearance Good Good
Good Good Good Dimensional stability Good Fair Fair No good No good
Weight (g) 170 173 178 175 192 Compression strength (kPa) 395 406
417 410 375 Apparent density D2 (g/L) 45 46 47 46 51 Calorific
value E2 (J/g) 12.9 12.8 12.9 12.8 15.2 (20 - 0.02 .times. D2) -
(65 - 0.100 .times. D2) 19.1-60.5 19.1-60.4 19.1-60.3 19.1-60.4
19.0-59.9 (J/g) Composite PP Molding as a whole Weight (g) 707 720
731 513 429 Apparent density (g/L) 164 167 170 78 65
[0204] From the results shown in Table 4, it is seen that the
composite PP moldings obtained in Examples 1-4, in which the
specific expanded PP beads (Beads Nos. 1-5) are used in each of the
high and low density sections, show excellent dimensional
stability.
[0205] In the case of Example 5 in which the specific expanded PP
beads (Beads No. 1) are used in the low density section and a low
tensile modulus polypropylene is used in the expanded PP beads
(Beads No. 7) for the high density sections, good dimensional
stability is obtained in the high density sections while retaining
excellent dimensional stability in the low density section.
[0206] Comparative Examples 1-3 use expanded beads (Beads Nos. 6
and 7) of a low tensile modulus polypropylene in each of the high
and low density sections. The molding conditions of Comparative
Examples 1-3 are the same except for the cooling time. When the
cooling time is short (Comparative Example 1), the dimensional
stability of the high density section is poor. While an increase of
the cooling time can improve the dimensional stability of the high
density section (Comparative Examples 2 and 3), the dimensional
stability of the low density section is adversely affected by such
an increased cooling time. Thus, comparison of Examples 1-5 with
Comparative Examples 1-3 shows that, unless the specific expanded
PP beads are used in at least one of the low and high density
sections, it is difficult to produce a composite PP molding having
good dimensional stability, even when the cooling time is
controlled.
[0207] In Comparative Example 4, the expanded PP beads (Beads No.
8) used for the high density sections have excessively high
calorific value E1 of the high temperature endothermic peak and are
not the specific expanded PP beads, although the base resin thereof
has a high tensile modulus. The low density section is formed from
Beads No. 6 which are not the specific expanded PP beads, either.
The high density sections of the composite PP molding of
Comparative Example 4 has poor fuse-bonding efficiency and poor
surface appearance, in spite of the fact that a high molding
pressure (maximum withstand pressure of the molding device used) is
employed. The low density section of the composite PP molding of
Comparative Example 4 has poor dimensional stability. In
Comparative Example 5, the expanded PP beads (Beads No. 9) used for
the low density section have excessively low calorific value E1 of
the high temperature endothermic peak and are not the specific
expanded PP beads. The high density section is formed from Beads
No. 7 which have a low tensile modulus and which are not the
specific expanded PP beads. The high density sections have good
dimensional stability, good fuse-bonding efficiency and good
surface appearance, since the cooling is carried out under
conditions suited for the high density sections. However, the low
density section has poor stability.
* * * * *