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Description  |
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FIELD OF THE INVENTION
This invention relates to a process for producing expanded particles of
styrene-acrylonitrile-butadiene copolymer which are useful as cushionings
for bumper of automobiles and molding materials for various containers.
BACKGROUND OF THE INVENTION
Molded articles made of expanded polystyrene foamed articles are widely
employed, especially as heat insulating materials, packaging materials or
cushioning materials, but have a disadvantage of low heat resistance,
e.g., about 70.degree. to 80.degree. C. at the highest.
The problem of low heat resistance can be solved by using polypropylene
foamed articles or styrene-acrylonitrile-butadiene copolymer (ABS resin)
foamed articles.
Expanded polypropylene particles can be prepared by a process comprising
dispersing polypropylene particles in water in a closed vessel, feeding a
blowing agent to the vessel, heating the dispersion to a temperature of
from the softening point of the polypropylene particles up to a
temperature higher than the melting point of the polypropylene particles
by 20.degree. C. while keeping the inner pressure at the vapor pressure of
the blowing agent or higher, and opening an outlet provided below a liquid
level of the closed vessel to thereby release an aqueous dispersion
containing polypropylene particles impregnated with the blowing agent into
an atmosphere having a pressure lower than the inner pressure of the
closed vessel. For details, reference can be made to it, e.g., in Japanese
Laid-Open Patent Application Nos. 12035/82, 25336/82, 90027/82, 195131/82,
1732/83, 23834/83, 25334/83, 33435/83, 55231/83, 76229/83, 76231/83 to
76234/83 and 87027/83. According to this process, expanded polypropylene
particles having a bulk density of from 0.026 to 0.06 g/cm.sup.3 can be
obtained. It is known that the above process is also applicable to
production of polyethylene particles or crosslinked polyethylene particles
as described in the above-cited publications.
The expanded polypropylene particles can be molded into a core of bumper or
a container by incorporating air or nitrogen gas in the particles to
impart secondary expandability, charging the expandable particles in a
cavity of mold having steam vents, and heating the particles with steam of
from 1.5 to 6 kg/cm.sup.2 G in pressure to expand and fuse with each
other, followed by cooling. The molded articles of expanded polypropylene
particles can also be obtained by compressing the expanded polypropylene
particles with pressurized nitrogen gas or air, charging the compressed
particles into a cavity of mold, and heating the particles to fuse with
each other, followed by cooling.
The above-mentioned process for producing highly expanded polypropylene
particles cannot be applied to production of expanded ABS resin particles
because a non-crystalline ABS resin having high heat resistance and high
bending strength behaves differently from crystalline polypropylene.
In the production of expanded polypropylene particles, the temperature for
heating the aqueous dispersion of polypropylene particles is usually
controlled between a range of from a temperature lower than the melting
point of polypropylene by 10.degree. C. to a temperature higher than the
melting point of polypropylene by 5.degree. C. in order to prevent fusion
among polypropylene particles. It is considered in the art that
impregnation of a volatile blowing agent into the resin particles is
effected at non-crystalline portions or voids formed by shrinkage
accompanying crystallization but not at crystalline portions. It appears
that the polypropylene particles in the aqueous dispersion are plasticized
by the presence of the volatile blowing agent to have a decreased apparent
melting point.
Therefore, it is anticipated that a volatile blowing agent must be
sufficiently impregnated into the non-crystalline ABS resin particles at
around a glass transition point (Tg) that corresponds to a melting point
of crystalline resins. Nevertheless, the volatile blowing agent can hardly
be impregnated into center of the ABS resin particles in an aqueous
dispersion system for some unknown reasons, failing to obtain expanded
particles.
SUMMARY OF THE INVENTION
Accordingly, one object of this invention is to provide a process for
producing expanded styrene-acrylonitrile-butadiene copolymer (ABS resin)
beads or particles having a high degree of expansion.
The expanded ABS resin particles obtained by the process of the present
invention have a degree (ratio) of expansion of from 10 to 50. The process
of the present invention is epoch-making seeing that the expanded ABS
resin particles having a degree of expansion of 5 at the most are
conventionally obtained by injection expansion molding or extrusion
expansion molding.
The present invention relates to a process for producing expanded particles
of a styrene-acrylonitrile-butadiene copolymer which comprises dispersing
styrene-acrylonitrile-butadiene copolymer particles in water in a closed
vessel, feeding a volatile blowing agent to the aqueous dispersion in the
closed vessel, heating the aqueous dispersion to a temperature higher than
a glass transition point (Tg) of the copolymer particles by at least
20.degree. C. to impregnate the volatile blowing agent into the copolymer
particles, and opening one end of the closed vessel to release the aqueous
dispersion containing the expandable copolymer particles into a low
pressure zone having a pressure lower than the inner pressure of the
closed vessel.
DETAILED DESCRIPTION OF THE INVENTION
The styrene-acrylonitrile-butadiene copolymer particles which can be used
in the present invention include those obtained by blending a binary
styrene-acrylonitrile copolymer (usually called AS resin or SAN) and a
butadiene-acrylonitrile rubber and those obtained by dissolving
polybutadiene in a mixed monomer of styrene and acrylonitrile to effect
suspension polymerization.
These styrene-acrylonitrile-butadiene copolymer resins may be used either
individually or in combination among themselves or in combination with
other polymers, e.g., polystyrene, high-impact polystyrene, a
styrene-methyl methacrylate copolymer, an
.alpha.-methylstyrene-styrene-acrylonitrile copolymer, a nitrile rubber, a
styrene-butadiene rubber, etc.
The resin particles may contain additives, such as inorganic fillers (e.g.,
talc, clay, calcium carbonate, titanium oxide, zeolite, etc.),
antioxidants, antistatic agents, ultraviolet ray absorbents, carbon black,
lubricants (e.g., aluminum stearate, zinc stearate, aluminum
p-tertiary-butylbenzoate, etc.), flame-retardants (e.g.,
2,3-dimethyl-2,3-diphenylbutane, tris(dibromopropyl)phosphate,
pentabromodiphenyl ether, tetrabromobutane, dibromoethylbenzole,
1,2,5,6,9,10-hexabromocyclodecane, etc.), plasticizers, and the like in a
total amount of from 0.05 to 5% by weight based on the resinous
components.
The expanded resin particles have a particle size of from 2 to 8 mm, and a
bulk density of from 10 to 100 g/liter, and preferably from 10 to 70
g/liter. They may have a form of either beads or particles.
Although highly expanded particles cannot be obtained from polystyrene or
SAN particles, such can be obtained from ABS resin particles if ample time
is devoted to impregnation of a blowing agent presumably because the ABS
resin particles contain a rubber component, e.g., a
butadiene-acrylonitrile rubber, polybutadiene, etc.
The volatile blowing agent which can be used in the present invention
includes organic compounds have a boiling point of 80.degree. C. or less,
such as aliphatic hydrocarbons, e.g., propane, butane, pentane, hexane,
heptane, etc., and halogenated hydrocarbons, e.g., trichlorofluoromethane,
dichlorodifluoromethane, dichlorotetrafluoroethane, methyl chloride, ethyl
chloride, methylene chloride, etc., either individually or in combinations
of two or more thereof.
The amount of the blowing agent to be fed varies depending on the kind of
the blowing agent and the desired degree of expansion, and usually ranges
from 10 to 50 parts by weight per 100 parts by weight of the resin
particles.
A dispersing agent to be used as an aid for dispersing the resin particles
in water includes inorganic suspending agent, e.g., aluminum oxide,
titanium oxide, calcium carbonate, basic magnesium carbonate, calcium
tertiary phosphate, etc.; water-soluble high polymeric protective
colloids, e.g., polyvinyl alcohol, methylcarboxy cellulose,
N-polyvinylpyrrolidone, etc.; and anionic surface active agents, e.g.,
sodium dodecylbenzenesulfonate, sodium alkanesulfonates, sodium
alkylsulfates, sodium olefin sulfates, acylmethyltaurines, sodium
dialkylsulfosuccinates, etc. Of these, a combination of calcium tertiary
phosphate having a particle size of from 0.01 to 0.8 .mu.m and, as a
suspension aid, sodium dodecylbenzenesulfonate is preferred. Such fine
calcium tertiary phosphate can be obtained by reacting 1 mol of calcium
hydroxide with 0.60 to 0.67 mol of phosphoric acid in water.
The amount of water to be used as a dispersing medium ranges from 150 to
1,000 parts by weight, and preferably from 200 to 500 parts by weight, per
100 parts by weight of ABS resin particles. If it is less than 150 parts
by weight, blocking among the resin particles readily takes place during
heating and pressing. If it exceeds 1,000 parts by weight, productivity of
the expanded ABS resin particles would be uneconomically reduced.
To the aqueous dispersion of ABS resin particles is then fed a gaseous or
liquid blowing agent. The aqueous dispersion is then heated to a
temperature higher than the glass transition point (Tg) of the ABS resin
particles at least by 20.degree. C., and preferably a temperature within a
range of from (Tg+25.degree. C.) to (Tg+55.degree. C.). By the heating,
the pressure within the vessel elevates to impregnate the blowing agent
into the ABS resin particles.
In order to assure complete impregnation of the volatile blowing agent into
the ABS resin particles, it is desirable that the aqueous dispersion
heated to a temperature higher than Tg by at least 20.degree. C. be
retained at that temperature for a period of from 0.5 to 5 hours. The
retention time varies depending on the particle size of the starting ABS
resin and the heating temperature. In cases where it is intended to
impregnate the volatile blowing agent midway between the surface and the
center of the resin particles to thereby obtain resin particles whose
shell portion is expanded with their core (center) portion remaining
non-expanded, the retention time can be selected from the range of from 5
minutes to 2 hours. Since the particle size and weight of the starting
resin are not standardized, it is most likely that the expanded resin
particles obtained by heating for a period of from 1 to 2 hours comprise
those particles which are wholly expanded and those particles which are
expanded only in their shell (surface) portion.
Either before or after the addition of a blowing agent to the closed
vessel, an inorganic gas, e.g., nitrogen, helium, argon, air, etc., is
preferably introduced into the closed vessel to elevate the inner pressure
as taught in Japanese Laid-Open Patent Application No. 55231/83 and
Japanese Patent Application No. 156056/83 (corresponding to Japanese
Laid-Open Patent Application No. 49039/85). The introduction of the
inorganic gas may be effected either before or after the heating of the
aqueous dispersion. The introduction of the inorganic gas into the closed
vessel facilitates impregnation of the blowing agent into the resin
particles and the subsequent release of the dispersion into a lower
pressure zone (open air) to obtain expanded resin particles having a fine
cellular structure.
The glass transition point (Tg) of the resin can be determined as follows.
A resin sample is heated up to 200.degree. C. at a rate of 10.degree.
C./min by means of a differential scanning calorimeter (DSC), and the
point of inflection accompanying glass transition is first-order
differentiated. The temperature of the resulting peak is taken as Tg.
During the heating, the aqueous dispersion in the closed vessel is stirred
in order to prevent blocking of the softened resin particles. After the
heating, the resin particles are released together with water from an
outlet (e.g., a slit, a nozzle, etc.) provided at the lower portion of the
closed vessel into a lower pressure zone, usually having atmospheric
pressure to thereby obtain expanded resin particles having a bulk density
of from 10 to 100 g/liter.
The thus obtained expanded ABS resin particles are dried at room
temperature for one day to remove water. The molding of the resulting
expanded ABS resin particles can be carried out by, for example, charging
them into a cavity of mold and heating them with a heating medium, such as
steam, at a temperature of from 105.degree. to 130.degree. C. for a period
of from about 5 seconds to about 1 minute.
The resulting foamed molded articles of ABS resin show excellent fusion
among expanded particles and high mechanical strength. The expanded
particles and the foamed molded articles have a ratio of closed cells as
high as 80% or more, with fine cells having a diameter of from 20 to 200
.mu.m.
The present invention will now be illustrated in greater detail by way of
the following examples, but it should be understood that the present
invention is not limited thereto. In these examples, all the parts and
percents are given by weight unless otherwise indicated.
EXAMPLE 1
In an autoclave were charged 100 parts of ABS particles having a particle
size of from 2 to 3 mm ("ABS-10"; trade name produced by Japan Synthetic
Rubber Co., Ltd.; Tg: 104.7.degree. C.), 250 parts of water, 1.0 part of
calcium tertiary phosphate having a particle size of from 0.3 to 0.5
.mu.m, and 0.007 part of sodium dodecylbenzenesulfonate (packing: 62
vol%). Nitrogen gas was introduced into the autoclave while stirring until
the inner pressure reached 5 kg/cm.sup.2 G. After stopping the nitrogen
feed, 18 parts of butane was fed in the closed vessel. The mixture was
heated up to 140.degree. C. over a period of 1 hour and kept at that
temperature for 1 hour. The inner pressure of the autoclave was 28
kg/cm.sup.2 G.
A valve of a discharge nozzle at the bottom of the autoclave was opened to
release the dispersion into open air within about 2 seconds to effect
expansion. The inner pressure of the autoclave at the time when the
release of the dispersion completed was about 9 kg/cm.sup.2 G. During the
release, the inner temperature of the autoclave was maintained at
140.degree. C.
The resulting expanded ABS resin particles had a bulk density of about 37
g/liter and a particle size of about 6 mm. The particles had expanded
throughout the shell and the core with foamed cells of from 20 to 100
.mu.m in diameter. The expanded particles were free of any blocking.
After the expanded particles were allowed to stand at 40.degree. C. for 1
day to remove water, they were charged into a cavity of mold having steam
vents. Steam of 0.7 kg/cm.sup.2 G was introduced therein through the vents
to fuse the expanded particles to each other. The molded article was
cooled with water for 30 minutes and then allowed to cool for 60 seconds,
and removed from the mold to obtain foamed molded article made of the
expanded ABS resin particles, having a density of about 37 g/liter, a
length of 300 mm, a width of 100 mm, and a thickness of 50 mm.
The resulting foamed molded article was evaluated for appearance,
compressive strength, and heat resistance according to the following test
methods. The results obtained are shown in Table 1.
(1) Appearance:
Excellent: Smooth surface with satisfactory gloss.
Good: Smooth surface with gloss.
Poor: Smooth surface with slightly poor gloss.
Very poor: Rough surface.
(2) Compressive Strength:
Measured on a specimen measuring 50 mm+50 mm.times.25 mm according to JIS
K6767. Calculated from a stress at 50% compression.
(3) Heat Resistance:
A specimen measuring 80 mm.times.80 mm.times.50 mm was heated at 80.degree.
C. for 24 hours. After allowing to cool at 20.degree. C. for 24 hours, a
percentage of dimensional shrinkage by heating was calculated through
equation:
##EQU1##
EXAMPLE 2 AND COMPARATIVE EXAMPLES 1 AND 2
Expanded resin particles and foamed molded articles were prepared in the
same manner as in Example 1, except for replacing ABS-10 with "ABS-35"
(trade name: "JSR-ABS"; ABS particles produced by Japan Synthetic Rubber
Co., Ltd.; Tg: 99.4.degree. C.) (Example 2), "SAN-A" (trade name:
"Sanlex"; SAN particles produced by Mitsubishi Monsant Chemical Co., Ltd.;
Tg: 103.3.degree. C.) (Comparative Example 1) or "LB-B" (trade name:
"Denka Styrol"; polystyrene beads produced by Electrochemical Industry
Co., Ltd.; Tg: 103.3.degree. C.) (Comparative Example 2) and changing the
heating temperature in Example 2 to 135.degree. C. The physical properties
of the resulting expanded particles and foamed molded particles are shown
in Table 1.
TABLE 1
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Comparative
Comparative
Example 1
Example 2
Example 1
Example 2
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Resin (Tg: .degree.C.)
ABS (104.7)
ABS (99.4)
SAN (103.3)
Polystyrene (103.3)
Heating:
Temperature (.degree.C.)
140 135 140 140
Time (hour) 1 1 1 1
Expanded Particles:
Bulk Density (g/l)
37.2 33.0 Not expanded
155.5*
Particle Size (mm)
ca. 6 ca. 6 -- --
Cell Diameter (.mu.m)
100-200
80-170 -- --
Foamed Molded Article:
Appearance Excellent
Excellent
Compressive Strength
3.1 2.7
(kg/cm.sup.2)
Heat Resistance (%)
Less than
1.5
1.0
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Note:
*Serious shrinkage was observed.
EXAMPLE 3
Expanded ABS resin particles were prepared in the same manner as in Example
1, except for changing the retention time of heating at 140.degree. C. to
15, 30 or 45 minutes.
The bulk density of the resulting expanded particles was measured, and the
state of expansion was observed on a cross section of the expanded
particle. The results obtained are shown in Table 2 below.
TABLE 2
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Heating Bulk
Time Density State of Expansion
(min) (g/l) Shell Portion
Core Portion
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15 61.2 Expanded Not expanded
30 36.9 Expanded Not expanded
45 35.0 Expanded Expanded
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EXAMPLES 4 AND 5 AND COMPARATIVE EXAMPLES 3 AND 5
Expanded ABS resin particles were prepared in the same manner as in Example
1, except for changing the heating temperature as shown in Table 3.
The physical properties of the resulting expanded particles are shown in
Table 3.
TABLE 3
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Heating
Bulk State of Expansion
Temperature
Density
Shell Core
Example No.
(.degree.C.)
(g/l) Portion
Portion
Blocking
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Comparative
95 Not expanded
Not expanded
None
Example 3
Comparative
110 Not expanded
Not expanded
None
Example 4
Comparative
115 Not expanded
Not expanded
None
Example 5
Example 4
130 77.6 Expanded
Not expanded
None
Example 5
150 27.4 Expanded
Expanded
Observed
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While the invention has been described in detail and with reference to
specific embodiments thereof, it will be apparent to one skilled in the
art that various changes and modifications can be made therein without
departing from the spirit and scope thereof.
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