Note: Descriptions are shown in the official language in which they were submitted.
7~
Background of the Invention
The present invention relates to a method for
manufacturing a low density foamed body containing
rubber as a main constituent.
A conventional method for manufacturing foamed
rubber is known according to which a mixture containing
rubber,~a foaming agent, and a crosslinking agent is
fed into a mold, the mold is heated to foam the mixture,
and the produced foamed body is released from the mold.
However, with a foamed body manufactured according to
this method, an expansion ratio of only about 3 to 6
and a density of about 0.15 to 0.3 are obtainable. It
has thus been difficult with this method to either
increase the expansion ratio or to decrease the density.
When the foaming agent is used in a great amount to
raise the expansion ratio, the foaming pressure within
the mold rises excessively when the mold is heated.
Then, when the mold is opened, the foamed body immedi-
ately expands, explodes, and is scattered away.
Summary of the Invention~ ;~
It~is an object o the present invention to provide
a method for manufac~uring a low density rùbber~foamed
body.~ ~
It is another ob~ect of the present invention to
provide a method~for~manufacturing a rubber foamed body
which has a low density as well as a low shrinking
property.
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In order to achieve these and other objects, khere
is provided according to the present invention, a method
for manufacturing a low density rubber foamed body com-
prising the steps of forming a sheet or sheets from a
mixture of 100 parts by weight of natural rubber or a
synthetic rubber, 10 to 80 parts by weight of a foaming
agent, and 0.5 to 15 parts by weight of a crosslinking
agent; enclosing said sheet or sheets within a mold so
as substantially to fill the mold, and heating said mold
under pressure from the exterior to thereby foam said
~sheet or sheets; forcibly cooling said mold; and releas-
ing a foamed body from said mold. When 10 to 60 parts by
weight of a thermoplastic synthetic resin are further
added to the mixture in the method of the present inven-
tion described above, a rubber foamed body of low densityand low shrinking property is obtainable.
In accordance with the present invention, a large
amount of the foaming agent may be used while preventing
the drawbacks of the prior art technique such as explosion
or scattering of the foamed body when the mold is opened,
so that a low density rubber foamed body may be provided.
A lower shrinking property may be achieved by further
adding a thermoplastic synthetic resin to the raw material
mixture containing natural or the synthetic rubber, the
foaming agent and the crosslinking agent.
The low densit~ rubber foamed bodies obtained
according to the present inven-tion are preferably used for
various kinds of cushion materials, substitutes for springs,
buoys, automobile partsl heat insulators, impact absorbers,
and so on.
Detailed Description of the Preferred Embodiments
The raw material rubber to be used in this inven-
tion may be natural rubber or any kind o synthetic rubber.
Examples of such synthetic rubbers include acrylonitrile-
butadiene rubber (NBR), chIoroprene rubber (CR),isoprene rubber (IR), styrene-butadiene rubber (SBR),
and ethylene-propylenediene copolymer rubber (EPDM).
The foaming agent to be mixed with the rubber material
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may be any foaming agent which is used for manufacturing
conventional rubber foamed bodies and may, for example,
include azodicarbonamide, dinitrosopentamethylenetetramine,
p-toluenesulfonylhydrazine, azobisisobutyronitrile,
diazoaminobenzene, and toluenesulfonyl semicarbazide.
The foaming agent is used in the amount of 10 to 80
parts by weight based on 100 parts by weight of the raw
material rubber, although the preerred amount varies
depending upon the kind of natural or synthetic rubber
to be used as the main constituent and the density of
the desired foamed body. When this amount is smaller
than 10 parts by weight, a sufficiently highly foamed
body may not be obtained. When this amount exceeds 80
parts by weight, the foamed body may cause breakdown,
or a foamed body having adequate impact resilience may
not be obtained. The method also requires a crosslinking
agent in addition to the foaming agent. Preferable
crosslinking agents may include sulfur, zinc oxide,
and organic peroxides. Examples of organic peroxides
may include dicumylperoxide, 2,5-dimethyl-2,5
di(tert.-butyl)peroxide, 1,3-bis(tert.-butylperoxy-
isopropyl)benzene, m-octaldecylazidoformate, and
tert.-butylperoxycumene. Although the amount of the
crosslinking agent as mentioned above may differ
depending upon the raw material rubber used, the kind
and amount of foaming agent used and other conditions,
it is preferably within the range of 0.5 to 15 parts by
weight based on 100 parts of the raw material rubber.
In addition to the foaming agent and the crosslinking
agent, various other additives may optionally be added. `
For example, there may be added a filler such as carbon
black, clay, and calcium carbonate; a foaming auxiliary
such as urea, stearic acid, lauric acid, and salicylic
acid; a metal oxide such as zinc oxide; and a colorant.
An antioxidant may also be included. The thermoplastic
synthetic resin preferably used in the present invention
may include polyethylene, polypropylene, polystyrene,
.~..
.
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polyvinyl chloride, an ethyl~ne-vinyl acetate copolymer
resin, a polyamide resin and an acrylic resin. If the
amount of the thermoplastic synthetic resin does not
reach 10 parts by weight, satisfactory prevention of
shrinkage may not be achieved. If this amount exceeds
60 parts by weight, the rubbery resilience is degraded,
resulting in an unsatisfactory product.
The raw material mixture is well kneaded in a
kneader such as a roll or a mixer and is formed into a
sheet. For kneading the raw material, the components
other than the crosslinking agent are first well kneaded
with a roll at a surface temperature of 20 to 120C for
about 15 to 60 minutes. After the crosslinking agent
is added to the mixture, further kneading is performed
for 5 minutes to provide a final raw material rubber
sheet. Although the rubber sheet must have a thickness
such that it may be put in a mold, a plurality of such
rubber sheets may also be put in a single mold. There-
fore, the thickness of the rubber sheet need not be the
same as the size of the cavity of the mold.
The raw material rubber sheet thus obtained is
p aced in a mold, the lid of the mold is closed, and
the mold is pressed from the upper and lower sides.
The raw material rubber sheet is placed in the mold so
that the rubber sheet substantially fills the cavity of
the mold. The shape of the mold may oe arbitrary.
However, after the mold is heated under pressure for
foaming the sheet and the mold is opened, the foamed
body abruptly pops out of the mold while expanding in
volume. Therefore, the mold preferably does not have ~-
projections within the cavity and has an upwardly
flared opening. ~or pressing the mold from the upper
and lower sides thereof, a press machine may be used.
According to a further preferable method, the upper and
lower halves of the mold are prefixed to the press
machine, and the opening and closing of the mold is
performed by vertically displacing these upper and
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- 5 - ~ ~7~
lower halves. When placiny the raw material rubber sheet
which has been subjected to kneading into the mold, the
sheet need not be aged in advance. Thus, the sheet may
be placed directly in the mold where it is subjected to
heat and pressure according to the present invention.
According to the conventional method for manufacturing
the rubber foamed body, in order to foam the rubber sheet
which has been kneaded, aging for about 24 hours has been
required before the sheet may be heated under pressure as
an essential step of the method. However, this aging
step is not necessary according to the present invention.
Even when the raw material rubber sheet is subjected to
the foaming step immediately after kneading, phenomena
such as foam nonuniformity and breakdown do not occur,
and continuous production is possible. After the raw
material has been placed within the mold, the mold is
then heated under pressure for crosslinking and foaming.
Although the heating temperature is mainly determined by
the decomposition temperature of the foaming agent used,
it is usually 130 to 200C. The pressure exerted on the
mold is typically within a range of 100 to 200 kg/cm2.
After foaming has proceeded substantially, the mold is
cooled. The cooling temperature changes according to the
raw material rubber used, the kind and amount of the foam-
ing agent used, the kind and amount of the crosslinking
agant used, and other conditions. ~owever, typically it
is within the range of about 50 to 100C. The cooling
step utilizes forcible cooling rather than natural cooling
and may, for example, adopt the process of circulating
water through the jacket of the mold. Circulation of air
may alternatively be adopted for this purpose. As this
cooling is performed, the gas pressure generated within
the mold during the heating and foaming step is reduced
and the bonding between the polymer molecules is also
sufficiently maintained. The mold is then opened. Upon
opening the .....
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- 6 -
mold, the rubber foamed body pops out of tlle mold
without exploding, ~nd the volume of the foamed body
instantaneously increases more than about 15 times the
original volume. Thus, the low density rubber foamed
body of the present invention is obtained. The density
of the resultant rubber foamed body is reduced to about
0.12 or lower.
The present invention will be more clearly under-
stood from the ~ollowing description made with reference
to the examples.
Example 1
Acrylonitrile-butadiene
rubber (NBR) 100 parts by weight
Carbon black 20 parts by weight
Stearic acid 3.0 parts by weight
Process oil 10 parts by weight
Dioctylphthalate 10 parts by weight
Azodicarbonamide 25 parts by weight
Zinc oxide 5 parts by weight
Antioxidant (*) 1.5 parts by weight
Sulfur 0.5 part by weight
* 2,2,4-trimethyl-1,2-dihydroquinoline
The components above excluding sulfur were kneaded-
for S5 minutes with a roll having a surface temperature
of 30C. Sulfur was then added, and the mixture was
kneaded for 5 minutes to provide a rubber sheet 5 mm in
thickness. The sheet was cut into pieces of appropriate
size. Three such pieces were placed in a mol~ having a
cavity 150 mm in length, 230 mm in width, and 12 mm in
depth. After closing the mold with a lid, the mold was
pressed from the upper and lower sides thereof at a
pressure of 170 kg/cm2 with a press machine. Heated
vapor was introduced in the jacket of the mold to heat
the sides of the mold to 170C, and the mold was kept
at this temperature for 10 minutes. The heated vapor
in the jacket was removed; and water was then introduced.
After 5 minutes, the mold temperature was ~5C. The
,. ..
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~ 7 ~ 7~2~
mold was then opened, and the ~oamed body immediately
popped out of the mold, providing the Eoamed body of
the present invention. The density of this foamed body
was 0.031. The outer appearance of the foamed body
corresponded to that of the cavity of the mold, and no
cracks or explosions were observed. The cushion property
and resilience of the foamed body were excellent.
Example 2
Chloroprene rubber 100 parts by weight
Carbon black 20 parts by weight
Stearic acid 3.0 parts by weight
Dioc-tylphthalate 10 parts by weight
~zodicarbonamide 16 parts by weight
Zinc oxide 7 parts by weight
Antioxidant (*) 1.0 part by weight
Filler (Calcium carbonate)10 parts by weight
Magnesia 4.0 parts by weight
* N-phenyl-N'-isopropyl-p-phenylenediamine
A rubber sheet 5.5 mm in thickness was obtained
from the above components in a similar manner as in
Example 1, except that the kneading time was 40 minutes
and the roll temperature was 40C. This rubber sheet
- was cut into pieces of predetermined size. Three such
pieces were placed in a mold and the mold was heated
under pressure in a similar manner as in Example 1.
Ths heating temperature was 170C, the pressure was
170 kg/cm2, and the heating time was 8 minutes. The
mold was cooled in a simiIar manner as in Example 1.
The cooling time was 5 minutes and the temperature of
the mold after cooling was 85C.
The press machine was released and a low density
foamed body of 0.050 density was instantaneously obtained.
The outer appearance of the obtained foamed body was
the same as in Example 1. The foamed body had excellent
1mpact resilience.
.
,
.. .. . . .. .....
- 8 -
Example 3
Styrene-butadiene rubber (SBR~ 100 parts by weight
Carbon black40 parts by weight
Stearic acid3.0 parts ~y weight
Process oil20 parts by weight
Azodicarbonamide18 parts by weight
zinc oxide 5 parts by weight
Antioxidant (*)1.5 parts by weight
Sulfur 1.2 parts by weight
* N-phenyl-N'-isopropyl-p-phenylenediamine
A rubber sheet 5 mm in thickness was obtained from
the above components in a similar manner as in Example 1,
except that the kneading time was 30 minutes and the
roll temperature was 40C. The rubber sheet was cut
into pieces of predetermined size. Three such pieces
were placed in a mold and the mold was heated under
pressure in a similar manner as in Example 1. The
heating temperature was 150C, the pressure was
170 kg/cm2, and the heating time was 12 minutes. The
mold was cooled.in a similar manner as in Example 1.
The cooling time was 12 minutes, and the temperature of
the cooled mold was 50C.
When the press machine was released, a low density
foamed body of 0.044 density was instantaneously obtained.
The outer appearance and impact resilience were like
those of the foamed body obtained in Example 1.
Example 4
Isoprene rubber (IR)100 parts by weight
Carbon black40 parts by weight
Stearic acid3.0 parts by weight
Process oil. 20 parts by weight
Azodicarbonamide18 parts b~ weight
Zinc oxide 5 parts by weight
Antioxidant (*)1.5 parts by weight
Sulfur 1.2 parts by weight
* N-phenyl-N'-isopropyl-p-phenylenediamine
:
A rubber sheet 5 mm in thickness was obtained from
the above components in a similar manner as in Example 1,
except that the kneading time was 20 minutes and the
roll temperature was 50C. The rubber sheet was cut
into pieces of predetermined size. Three such pieces
were placed in a mold and the mold was heated under
pressure in a similar manner as in Example l. The
heating temperature was 160C, the pressure was
170 kg/cm2, and the heating time was 8 minutes. The
mold was then cooled in a similar manner as in Example 1,
except that the cooling time was 7 minutes and the
temperature of the cooled mold was 80C.
The press machine was then released, thus providing
a low density foamed body of 0.032 density. The obtained
foamed bod~ had an outer appearance and impact resilience
comparable to those of the foamed body obtained in
Example 1.
Example 5
Ethylene-propylenediene
copolymer rubber 100 parts by weight
Carbon black 15 parts by weight
Stearic acid 2.0 parts by weight
Process oil 10 parts by weight
Azodicarbonamide 25 parts by weight
Zinc oxide 5 parts by weight
Dicumylperoxide 5.3 parts by weight
A rubber sheet 5 mm in thickness was obtained from
the above components in a similar manner as in Example 1,
except that the kneading time was 30 minutes and the
roll temperature was 50C. The rubber sheet was cut
into pieces of predetermined size. Three such pieces
were placed in a mold and the mold was heated under
pressure in a similar manner as in Example 1. The
heating temperature was 165C, the pressure was
170 kg/cm2, and the heating time was 14 minutes. The
mold was then cooled in a similar manner as in Example l,
except that the cooling time was 3 minutes and the
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- 10 - ~ ~7~
temperature o~ the cooled mold was 100C.
The press machine was released and a low density
foamed body of 0.033 density was instantaneously obtained.
Example 6
Ethylene-propylenediene
copolymer rubber 100 parts by weight
Carbon black 15 parts by weight
Stearic acid 2.0 parts by weight
Process oil 15 parts by weight
Azodicarbonamide 35 parts by weight
Zinc oxide 3.0 parts by weight
Dicumylperoxide 5.3 parts by wei~ht
A rubber sheet 5 mm in thickness was obtained from
the above components in a similar manner as in Example 1,
except that the kneading time was 30 minutes and the
roll surface temperature was 50C. The rubber sheet
was cut into pieces o~ predetermined size. Three such
pieces were placed in a mold and the mold was cooled in
a similar manner an in Example 1, except that the
cooling time was 10 minutes and the temperature of the
cooled mold was 90C.
The press machine was released and a low density
foamed body of 0.02 density was instantaneously obtained~
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Rubber sheets 2 mm in thickness were obtained from
the above compositions by kneading at 50C for composi-
tion A and at 100C for composition B. The kneading
time was 25 minutes in each case. The two rubber
sheets were separately placed into molds having dimensions
of 25 cm x 25 cm x 1.5 cm and having an opening flaring
at an angle of 60C, to occupy substantially 95~ of the
cavity of each mold. The molds were heated and pressed
by press machines for 10 minutes at a heating temperature
of 170C and a pressure of 150 kg/cm2.
Water was circulated in the jackets of the molds
to drop the temperature of the molds to 85C and this
temperature was maintained for 6 minutes. The foamed
bodies were then released from the molds. The foamed
body of the composition A had a density of 0.048, and
the foamed body of the composition B had a density of
0.052; neither foamed body exhibited cracks or foam
nonuniformity. The shrinkage as measured over time was
as follows. For measuring the shrinkage, a mark 10 cm
in length was made on the surface of the foamed body
immediately after it was released from the mold, and
the shrinkage of this line was measured.
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Rubber sheets 2 mm in thickness were obtained from
the above compositions C and D by kneading the component
mixtures at 35C for 35 minutes for both of the composi-
tions. The rubber sheets were separately placed in
molds as used in Example 7 to fill substantially 95% of
the cavities of the molds. After heating the molds at
170C and at a pressure of 150 kg/cm2, cold water was
passed through the jackets of the molds to lower the
temperature to 85C. The molds were maintained at this
temperature for 6 minutes. The foamed bodies were
released from the molds. The foamed body of the compo- -
sition C had a density of 0.038 and the foamed body of
the composition D had a density of 0.04. The outer
appearance of the foamed bodies was the same as that of
the foamed bodies obtained in Example 7. The shrinkage
of the foamed bodies was measured in a similar manner
an in Example 7, and the obtained results are shown in
Table 2 below:
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- 15 - ~t7~6;~
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3 3 ~ ~ 3 3 3 3
Q rq R
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S~ ~ O ~ o
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o ~ oo U~
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U~ O ~el~ O O O O
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Rubber sheets ~ mm in thickness were obtained by
kneading the above components at 50C for composition E
and at 110C for composition F for 25 minutes each.
These rubber sheets were placed in molds as used in
Example 7 to occupy substantially 95% of the cavities
of these molds. The molds were then heated at 170C
and pressed at 150 kg/cm2 for 6 minutes. Cold water
was then passed through the jackets of the molds to
cool them to 85C. The molds were maintained at this
temperature for 6 minutes. The density of the obtained
product was 0.07 for the composition E and 0.073 for
the composition F. The outer appearance of the foamed
bodies was the same as that obtained in Example 7. The
shrinkage of the foamed bodies was measured in a similar
manner as in Example 7, and the obtained results are
- shown in-Table 3 below:
S ~ ,C
3 'a~ 3
3 3 R ~ 3 3
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S~ h Q. Q, h h 5-1
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D ~ ~ R ~, .,, .4 .4
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- 18 -
Rubber sheets 2 mm in thickness were obtained by
kneading the above components by a roll at 50C for
composition G and at 100C for composition H for 30
minutes each. These rubber sheets were separately
placed in molds as used in Example 7 to occupy substan-
tially 95% of the cavities of the molds. The molds
were heated and pressed at 170C and 150 kg/cm2 for 10
minutes by a press machine. Cold water was passed
through the jackets of the molds to cool the molds to
85C. The molds were kept at this temperature for 6
minutes. The foamed bodies were released from these
molds. The density of the foamed body of the composi-
tion G was 0.12 and the density of the foamed body of
the composition H was 0.12O The outer appearance of
these foamed bodies was the same as that obtained in
Example 7. The shrinkage of these foamed bodies was
measured in a similar manner as in Example 7, and the
obtained results are shown in table 4 below:
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