Note: Descriptions are shown in the official language in which they were submitted.
lZ~ (30
BACKGROUND OF THE INV~NTION
The present invention relates to a pre-expanded
particle of polyolefin suitable for foaming within a mold
which is able to close but unable to seal by heating with
a heating medium such as steam and a process for
preparing the same,
Hitherto, there have been proposed various
pre-expanded particles of polyolefin for foaming within a
mold. However, in case of molding in industrial scale
using a large size, multi-cavity mold, these pre-expanded
particles, especially non-crosslinked pre-expanded
particles have a number of drawbacks that the particles
are not well fused together~ that a surface of a molded
article is not flat and that appearance of a molded
article i8 not good, and thus, an adequate molded
article, as a matter of fact, cannot be obtained.
With respect to a process for preparing pre-
expanded particles of polyolefin for foaming within a
mold, a process including a step of heating the polyolefin
particles containing a volatile foaming agent with, for
example, steam is generally carried out. However, when
the polyolefin particles are polyethylene particles, it
is necessary to employ crosslinked polyethylene resin
because if polyethylene is not crosslinked, a melt
viscosity of the polyethylene resin is remarkably fell
down at a temperature about a melting point thereof and
it is very difficult to obtain pre-expanded particles
which are less shrinked and have a high pre-expansion
ratio. For preparing the crosslinked polyethylene resin,
high-pressure process, low density polyethylene is widely
used as a starting material because the high-pressure
process, low density polyethylene can be easily and well
crosslinked. A pre-expanded particle obtained from the
high-pressure process, low density polyethylene is
superior in pliability and bufferability or cushoning
property. However, the pre-expanded particle is inferior
in heat-resistance and wants for solidity. Therefore,
there can be used only the pre-expanded particle which
o
has a relatively low pre-expansion ratio. Further, ~rom
the viewpoint of an industrial production, it is required
to produce foamed articles constantly in a high yield
which have an excellent appearancel a high expansion
ratio and good mechanical properties by foaming process
employing pre-expanded particles of polyolefin.
The present inventors have studied in order to
eliminate the above-mentioned drawbacks which are
distinguished when the pre-expanded particles, especially
pre-expanded particles of non-crosslinked polymer are
employed for foaming within a mold, and we have reached
the following knowledge; that is, in case of industrially
producing a large size of foamed article, the
characteristic properties such as appearance and
mechanical strength of the obtained foamed article have a
great relation to the characteristic properties such as
viscoelasticity of polyolefin when melted, which is a
fundamental component of pre-expanded particles of
polyolefin used for foaming within a mold, and especially
polyolefin having a small change of melt tension due to
temperature variation is necessary particularly for
producing a large size of foamed article of excellent
appearance and good mechanical strength.
Consequently, by minimizing the change of melt
tension due to temperature variation, even if there is
partially generated a difference in temperature among the
locations of the pre-expanded particles filled in a mold
when the mold charged with the pre-expanded particles is
heated, a molded article which is uniform on the whole
having excellent properties can be obtained.
That means polyolefin employed as fundamental
component in a preparation of the pre-expanded particles
must have the same characteristic properties as described
above.
The difference in temperature among the
locations of the pre-expanded particles in a mold which
generates during heating, as a matter of course, becomes
large when a large size of foamed article is molded.
12~ 0
It is an object of the present invention to
provide a pre-expanded particle suitable for foaming
within a mold to produce a molded article having a higher
expansion ratio, and to provide a pre-expanded particle
prepared from non-crosslinked polymer without reducing a
pliability and a bufferability or a cushoning property
which are the characteristic properties of the
conventional pre-expanded particles employing high-
pressure process low density crosslin~ed polyethylene,
particularly worthwhile when applied in molding a large
size foamed article.
It is an another object of the present
invention to provide a process for preparing such
particles.
These and other objects of the present
invention will become apparent from the description
hereinafter.
SUMMARY OF THE INVENTION
In accordance with the present invention, there
is provided a pre-expanded particle of polyolefin having
a characteristic property that an inclination of melt
tension is less than 1,500.
In accordance with the present invention, there
is also provided a process for preparing pre-expanded
particles of polyolefin which comprises: dispersing
polyolefin particles and a volatile foaming agent into
water in the presence of a dispersing agent in an
autoclave, heating the resulting dispersion with stirring
up to a temperature in a following range:
(a melting point of the polyolefin -25)C
to (a melting point of the polyolefin +10) C,
thereby the polyolefin particles are impregnated with
the foaming agent and then, releasing the dispersion of
the polyolefin particles into an atmosphere having a
pressure lower than that in the autoclave; the polyolefin
having a characteristic property that an inclination of
melt tension is less than 1,500.
-- 5
A pre-expanded particle of the present
invention has a high pre-expansion ratio and is superior
in heat-resistance, mechanical strength, pliability and
cushoning property, and can be employed for foaming
within a mold to give a good foamed article.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a graph showing a relationship
between melt tension of non-crosslinked linear low
density polyethylene used in Example 1 and absolute
temperature.
DETAILED DESCRIPTION
According to the present invention, various
kinds of polyolefins, whether they are crosslinked or
not, can be employed. Examples of the polyolefin
are, for instance, low density polyethylene, medium
density polyethylene, high density polyethylene, linear
low density polyethylene, polypropylene, ethylene-
propylene copolymer, ethylene-butene-propylene
terpolymer, ethylene-vinyl acetate copolymer,
poly(butene-l) and the like. Particularly, low density
polyethylene, medium density polyethylene, linear low
density polyethylene and ethylene-propylene random
copolymer are preferably employed. Although in the
present invention, non-crosslinked polyolefin is
preferably used from an economical viewpoint and
practically usable for producing an excellent foamed
article, crosslinked polyolefin can also be employed.
The polyolefin employed in the present
invention has a characteristic property that an
inclination of slope line obtained on a graph when
logarithmic values of melt tension of the polyolefin
are plotted as ordinate and reciprocals of absolute
temperature at which the melt tension is measured are
plotted as abscissa is less than l,500. Hereinafter, the
above-defined inclination is referred to as "an
inclination of melt tension", and a meaning of the term
i~2;~1~0
of an inclination of melt tension described in Claims are
defined as same as above.
According to the descriptions in "Kobunshi Kako
Kogaku" (Japanese translation of "Polymer Processing", by
James M. McKelvey, published by JOHN WILEY & SONS, INC.,
New York, London, 1962), translated by Katsuhiko Itoh,
published by Maruzen Kabushiki Kaisha, p37, Arrhenius
stated that a relationship between log ~, wherein ~ is a
viscosity of the polymer, and reciprocal of absolute
temperature is proportional in a range of 100 degree
Fahrenheit.
An inclination of melt tension is, preferably
less than 1,000, more preferably less than 700. When an
inclination of melt tension is not less than 1,500, it is
difficult to control a pre-expansion ratio and cell size
during a preparation of pre-expanded particles. In that
case, even if pre-expanded particles of desired expanison
ratio and cell size are obtained, the resulting foamed
article is inferior in appearance and mechanical strength
because in general the pre-expanded particles are not
heated uniformly during foaming within a mold and they
are partl~ shrinked or varied in shape due to
overheating, or are not sufficiently expanded or fused
together due to underheating.
A reason why the pre-expanded particles are not
uniformly heated during foaming within a mold consists in
a fundamental structure of the used foaming apparatus and
this drawback cannot be eliminated substantially.
A foaming apparatus has a structure in which
steam passes through the packed pre-expanded particles in
a mold to obtain a foamed article and the steam flow more
and more hardly according as the pre-expanded particles
are expanded and fused together so as to be molded. The
foaming apparatus has also a mechanism that when a
pressure within a mold finally reaches to a
pre-determined pressure, steam flow is shut. In case of
employing a foaming apparatus for producing a larger size
of foamed article, the pre-expanded particles packed in a
O
mold are heated more ununiformly.
Melt tension of the polyolefin is roughly
decided at the time of preparing the polyolefin. The
polyolefin having a low melt tension can be prepared by a
number of processes such as that polymerization reaction
is carried out with a specified polymerization catalyst,
that a plurality of polymer resin having a different melt
tension are mixed together, that polymerization reaction
is carried out multi-stepwise to prepare a polymer resin
consisting of components having a different melt tension
and the obtained polymer resin is admixed and the like.
According to one embodiment of the present
invention, polyethylene or a copolymer of ethylene and
a~olefin of C4 to C20 having 0.1 to 50 g/10 min of melt
index measured according to JIS K 6760, 0.910 to 0.940
g/cm3 of a density and 110 to 130C of a melting point
is preferably employed.
When the melt index is less than 0.1 g/10 min,
it is difficult to prepare the pre-expanded particles
because a flowability of the polymer is not good during a
preparation of the pre-expanded particles. On the other
hand, when the melt index is more than 50 g/10 min, a
flowability of the polymer is very high, and the obtained
pre-expanded particles have low pre-expansion ratio and
tend to shrink.
When the density is less than 0.910 g/cm3, the
polymer resin is so soft that the obtained pre-expanded
particles are apt to shrink. On the other hand, when the
density is more than 0.940 g/cm3, the polymer behaves
like a high density polyethylene and a foaming procedure
is difficult because of a reason described below.
When the melting point is less than 110C, the
pre-expanded particles have insufficient heat-resistance.
On the other hand, when the melting point is more than
130c, the polymer behaves like a high density
polyethylene and a foaming is difficult.
Above-mentioned a-olefin having 4 to 20 carbon
atoms includes, for instance, l-butene, l-pentene,
100
l-hexene, 3,3-dimethyl-1-butene, 4-methyl-1-pentene,
4,4-dimethyl-1-pentene, l-octene, l-decene, l~dodecene,
l-tetradecene, l-octadecene and the like. The a-olefin
is employed alone or in admixture thereof.
~ content of a comonomer is varied according to
a ~ind of the used a-olefin and is generally about 3 to
about 12 % by weight in the obtained copolymer so as to
give a density of the copolymer in the range limited as
above.
Above-mentioned melting point is a peak
temperature of the endothermic curve obtained by raising
a temperature of a polymer sample at a rate of 20C/min
up to 200C to melt, cooling it at a rate of 20C/min
down to room temperature to crystallize and then,
re-heating it at a rate of 10C/min with a differential
scanning calorimeter (DSC).
Ethylene polymer used in the present invention
is preferable a non-crosslinked ethylene polymer.
However, ethylene polymer crosslinked by an organic
peroxide or electron beam irradiation can also be used
without any disadvantage.
According to another embodiment of the present
invention, the above-mentioned copolymer can contain one
or more polyolefin such as high-pressure process, low
density polyethylene, high density polyethylene,
polypropyrene or ethylenepropyrene copolymer in an mount
of less than 50 ~ by weight as long as the effects of the
present invention are not inhibited. Also, additives
such as an ultraviolet stabilizer, an antistatic agent, a
heat stabilizer, a fire retardant, a coloring agent and
inorganic powders can be added into the olefin polymer as
occasion demands.
A process for preparing the above-mentioned
pre-expanded particles is a second aspect of the present
invention.
In a pre-expanding process in which a mixture
of water and polymer resin particles impregnated with a
volatile foaming agent is released from a condition of
:~Z~ V
high temperature and high pressure to an atmosphere of
low pressure, a temperature of the polymer resin
particles must be maintained in a small temperature range
Therefore, a polymer having a small temperature range in
which the polymer has a suitable viscoelastisity for
pre-expansion, for instance, a high density polyethyler.e
can be employed in contrast with a conventional process
for pre-expanding the particles with steam. However, in
a foaming process in which a mold is charged with
pre-expanded particles and heated, when usually employed
ethylene polymer such as a high-pressure process, low
density polyethylene or a high density polyethylene is
used, foaming procedure is difficult because the heating
range to be applied during foaming is very small. The
particles are not fused together in case that a heating
temperature is low and the particles are shrinked in case
that the heating temperature is high.
It was found that olefin polymer used in the
present invention, especially polyethylene or a copolymer
of ethylene and a-olefin of C4 to C20 having 0.1 to
50 g/10 min of melt index, 0.910 to 0.940 g/cm3 of a
density and 110 to 130C of a melting point have a wide
range of heating temperature to be applied during
foaming, and that foaming procedure is easy to be carried
out. Though the reason has not yet been confirmed, it is
supposed to be the reason that a temperature range in
which the pre-expanded particles are crystallized is wide
as observed, for example, with DSC and that a temperature
range in which the pre-expanded particles are expanded to
a maximum extent in a mold i5 close to a temperature
range in which the pre-expanded particles are fused
together.
Contrary to that a conventional process has a
drawback that the pre-expanded particles are stuck to
each other, particularly when a mixture of the polyolefin
particles and water is released into an atmosphere of low
pressure through at least one orifice having a diameter
of 1.2 to 3 times a diameter of a sphere calculated from
O
-- 10
the volume of the polyolefin particle, that problem
is completely solved, i.e. all of the obtained pre-
expanded particles are separated piece ~y piece.
The words "a diameter of a sphere calculated
from the volume of the polyolefin particle" as used
herein means a diameter of a sphere having the same
volume as a volume of the polyolefin particle and
hereinafter, it is referred to as "a sphere diameter".
When a diameter of the orifice is too small,
the particles can not pass through the orifice. On the
other hand, when a diameter of the orifice is too large,
several particles are released simultaneously into an
atmosphere of low pressure and thus, the particles are
caused to be agglomerated and stuck to each other at a
time that they pass through the orifice or that they
expand thereafter, and blocking is occurred.
A shape of the orifice is generally a circle or
an ellipse, but an orifice of a polygon can be employed,
as occasion demands. An orifice size is determined from
a diameter of the used polyolefin particle. Because
a particle having a sphere diameter of about 0.5 to about
6 mm is generally used for a usual foaming within a mold,
the orifice has an area of about 0.3 to about 250 mm2
Consequently, a releasing speed is controled by changing
a number of the orifice. The orifice can be provided,
for instance, by inserting a pressure-resistant orifice
plate having at least one orifice behind a releasing
valve with a flange.
With respect to the third problem, i.e. a
defect that pre-expansion ratio of the pre-expanded
particles are not uniform, particularly when a
temperature and a pressure in a vehicle such as an
autoclave, preferably and a partial pressure of the
foaming agent in a gas phase in the vehicle are kept
constant at a time of releasing the aqueous dispersion of
the olefin polymer particles into an atmosphere of low
pressure, that defect is clearly eliminated.
The temperature varies preferably within 5
degree centigrade during releasing the dispersion. The
temperature can be easily controlled, for instance, by
using a usually employed autoclave having a jacket.
The pressure in an autoclave falls down
S according as a mixture of particles and water is
released because the upper space in an autoclave becomes
large, thereby the pre-expansion ratio of the released
particles was decreased. Thus, a pressure in an
autoclave is preferably kept constant during releasing
the aqueous dispersion of the particles.
When the releasing procedure is long continued,
introducing nitrogen gas or air under pressure into an
autoclave is not so effective to keep the pressure in an
autoclave constant and thus, pre-expansion ratio of the
pre-expanded particles are largely decreased. This is
because a content of a foaming agent is lowered owing to
a volatilization of the foaming agent from the particles
in proportion to a partial pressure of the foaming agent
in a gas phase in the autoclave falls down. Thus/ more
preferably a partial pressure of the foaming agent in a
gas phase in the autoclave, as well as a total pressure
in the autoclave, is kept constant during releasing
operation. This is carried out, for example, by
decreasing a volume of the upper space in the vehicle
according as the space increases to keep a volume of the
upper space constant, or by introducing a volatile
foaming agent as well as nitrogen gas or air into the
vehicle from outside according as the upper space
increases. However, above-mentioned methods are
effective only when the upper space in the vehicle is not
saturated with a vapour of the foaming agent and a
partial pressure of the foaming agent in the gas phase is
decreased during releasing $he dispersion. Conse~uently,
when the upper space is saturated with the vapour of the
foaming agent, for instance, when excess foaming agent is
present as liquid in the vehicle, the introduction of the
foaming agent is not necessary.
The latter method is prefer to the former
1()0
- 12
method, because the former method requires more
complicated pr~cess.
The latter method can be carried out, for
instance, by introducing the liquid volatile foaming
agent continuously or intermittently via a controlling
valve with adjusting the introduced amount to keep a
total pressure in the vehicle constant. Thus, the
pe-expanded particles, pre-expansion ratio of which is
nearly close to each other, can be obtained even when the
releasing operation is long continued.
The words "to keep a pressure constant"
described in the specification and Claims means to
control and maintain a pressure in a pressure range in
which the pre-expanded particles having pre-expansion
ratio of allowable variation are obtained.
According to the present invention, various
volatile foaming agents having a boiling point of -50 to
120C can be employed. Examples of such a foaming agent
are, for instance, an aliphatic hydrocarbon such as
propane, butane, pentane, hexane, heptane, cyclopentane
or cyclohexane; a halogenated hydrocarbon such as mono-
chloromethane, dichloromethane, monochloroethane,
trichloromonofluoromethane, dichlorodifluoromethane,
dichloromonofluoromethane, trichlorotrifluoroethane or
dichlorotetrafluoroethane and the like. The foaming
agent can be employed alone or in admixture thereof. A
used amount of the volatile foaming agent is decided
according to a kind thereof, a desired pre-expansion
ratio of the obtained pre-expanded particle, a ratio of a
volume of the polymer resin and a volume of the upper
space in the vehicle. In general, the foaming agent is
used in 5 to 50 parts by weight to 100 parts by weight of
oleifn polymer particles.
Dispersing agent is used in a little amount at
a time of dispersing the polyolefin particles into
water in the process of the present invention to inhibit a
flocculation of the particles during heating. Examples
of such a dispersing agent is, for example, a water-
100
- 13
soluble polymer such as polyvinylalcohol, methylcellulose
or N-polyvinylpyrrolidone; powders of a water-insoluble
inorganic compound such as calcium phosphate, magnesium
pyrophosphate, zinc carbonate, titanium oxide or aluminum
oxide and the like. The water-insoluble inorganic
compound is prefer to the water-soluble polymer because
the latter has a problem of environmental polution.
However, when the inorganic powders are used in much
amount, pre-expanded particles are not well fused
together at a time of foaming. So, a little amount of an
anionic surfactant such as sodium alkylbenzenesulfonate
including sodium dodecylbenzenesulfonate or the like,
sodium a-olefinsulfonate or sodium al~ylsulfonate is
preferably used with the inorganic powders to decrease
the used amount of the powders. In this case, the
powders of a water-insoluble inorganic compound are used
in 0.1 to 3 parts by weight and the anionic surfactant is
used in 0.001 to 0.5 part by weight to 100 parts by
weight of the polyolefin resin.
According to the present invention, aqueous
dispersion of the polyolefin particles are heated
into a temperature in a following range:
(a melting point of the polyolefin -25)C
to (a melting point of the polyolefin +10)C
preferably
(a melting point of the polyolefin -20)C
to (a melting point of the polyolefin + 5)C
The temperature can be varied within the the above-
mentioned temperature range according to a kind of used
volatile foaming agent and a desired pre-expansion ratio.
For example, as to an olefin polymer having a melting
point of 120C, heating temperature can be selected from
a range of 95 to 125 C. When a heating temperature is
less than the above-mentioned temperature range, an
expansion ratio is largely decreased. On the other hand,
when a heating temperature is more than the temperature
range defined above, the cells in the obtained pre-
~ expanded particles are collapsed.
0
_ 14
The thus o~tained pre-expanded particle of
polyolefin of the present invention can be employed in a
foaming process, for instance, a process that the
obtained pre-expanded particles are, just as they are,
or after aging or drying for a suitable time, or after
giving them additional expansion ability, filled in a
mold, and the pre-expanded particles are heated with a
heating medium such as steam up to about 1~5 to about
130C for about 3 sec to about 2 min to give a molded
article.
A method for further giving the pre-expanded
particles additional expansion ability can be carried
out, for instance, by introducing a gas such as nitrogen
gas or air under pressure into the cells in the particles
to elevate a pressure in the cells, or by pressing the
pre-expanded particles with a compressed air to elevate a
pressure in the cells. It is also effective to compress
a mold charged with the pre-expanded particles made
expansible or not before foaming.
The pre-expanded particles of polyolefin of the
present invention can also be prepared by other
conventional process.
The pre-expanded particles of polyolefin of the
present invention are not stuck together at all and have
uniform expansion ratio, and can be industrially employed
for a conventional foaming process with ease because they
- have a wide range of heating temperature. The pre-
expanded particles of the present invention can give a
large size foamed article having a flat surface and a
good mechanical strength. A foamed article produced from
the pre-expanded particles of the present invention has
an excellent appearance and a uniform density in which
the pre-expanded particles are well fused together. The
foamed article has a good cushoning property as similar
to a conventional molded article of high-pressure process,
low density crosslinked polyethylene, an excellent heat-
resistance and a toughness, and are useful for wrapping
material, insulating material, a container or a fender of
- 15
cars.
The present invention is ~ore particularly
described and explained by means of the following
Examples, in which all parts are parts by weight unless
otherwise noted. It is to be understood that the present
invention is not limited to the Examples and various
changes and modifications may be made in the invention
without departing from the spirit and scope thereof.
Example 1
A 100 ~ autoclave equipped with a stirrer was
charged with 100 parts (25 kg) of non-crosslinked,
linear, low-density polyethylene (hereinafter, referred
to as "L-LDPE") particles containing 0.01 part of talc,
and having an inclination of melt tension of about 585,
a melting point measured by DSC method of 117C, a
density of 0.920 g/cm3, MI of 0.8 g/10 min and a sphere
diameter of 2 mm, 0.5 part of basic powdered calcium
phosphate and 0.006 part of sodium dodecylbenzene-
sulfonate as dispersing agent and 300 parts of water.After stirring the mixture, to the dispersion thus
obtained was added 45 parts of dichlorodifluoromethane
with stirring and the temperature of the dispersion was
raised to 112C. A pressure in the autoclave at that
time was 26 kg/cm2-G.
By opening a releasing valve provided in a
lower part of the autoclave, the aqueous dispersion of
L-LDPE particles was released into an atmosphere of
normal pressure through a round orifice having 4 mm in a
diameter in an orifice plate provided behind the valve to
give pre-expanded particles.
The obtained pre-expanded particles had an
average pre-expansion ratio of 25 times and an average
cell size of 230 ~m. The results are summarized in
Table 1.
The pre-expanded particles were dried at 60 C
for 24 hrs and subjected to foaming within a mold using
SC-lOB foaming macnine (commercially available from Toyo
~2~
- 16
Kikai Kinzoku Kabushiki Kaisha) under a condition as
shown in Table 2 to give a foamed article.
With respect to the obtained foamed article,
characteristic properties shown in Table 3 were evaluated
as described below.
Rate of shrinkage:
After aging, a foamed article was left at room
temperature for 24 hrs. A real volume of the article was
measured by doping it into water. The obtained real
volume and a volume of the used mold were inserted into a
following equation to give a volume of article.
A real volume of the
foamed article
Rate of shrinkage = X 100 (~)
A volume of the used mold
cavity
_ sibility:
A foamed article was broken at several
portions and appearance of the exposed surfaces were
observed. A proportion of particles which were broken
themselves not at their fused surfaces was counted.
~: The proportion is over 70 %.
O : The proportion is over 50 %, less than 70 %.
X : The proportion is over 30 %, less than 50 %.
Surface property:
A surface of the obtained foamed article was
observed.
O : The surface is almost smooth and flat.
/~: Fused bounds are remarakable.
X : Fused bounds are very remarkable and an article is
not commercially valuable.
Sink:
A molded article was judged from the result of
a rate of shrinkage and a shrinkage in a demension
thereof.
~: Volume of article is over 90 % and a shrinkage
is less than 5 %.
: Volume of article is over 90 % and a shrinkage is
PO
- 17
5 % to 10 %.
: Volume of article is over 80 ~, less than 90 ~.
X : Volume of article is less than 80 %.
Total jud~ement:
A molded article was judged from the result of
surface property and sink.
~: Surface property and sink are both ~ , or ~ and
0-
O: Surface property and sink are both O .
A: At least one of surface property and sink is ~.
: At least one of surface property and sink is X .
An inclination of n~elt tension of the used
L-LDPE was obtained by measuring melt tension under a
condition o~ a scan speed of 1.52 m/min, an air gap o~
25 cm and an extruding speed of 0.25 g/min, at 180 +
0.4C, 200 + 0.4C and 220 + 0.4C, respectively using a
melt tension tester commercially available from Toyo
Seiki Seisakusho, Ltd. The obtained graph is shown in
Fugure 1.
Examples 2 to 3
The procedures of Example 1 were repeated except
that L-LDPE having an inclination of melt tension of
about 900 (Example 2) and L-LDPE having an inclination of
melt tension of about 1,400 (Example 3) were employed
instead of L-LDPE having an inclination of melt tension
of about 585.
The characteristic properties of the obtained
pre-expanded particles and the foamed article are
summarized in Table 1 and Table 3, respectively.
Comparative Examples 1 and 2
The procedures of Example 1 were repeated
except that L-LDPE having an inclination of melt tension
of about 1,700 and non-crosslinked low density
polyethylene (hereinafter, referred to as "LDPE") having
an inclination of melt tension of about 3,000 were
- 18
employed instead of L-LDPE having an inclination of melt
tension of about 585.
The characteristic properties of the obtained
pre-expanded particles and the foamed article are
summarized in Table 1 and Table 3, respectively.
~2;~
-- 19
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-- 20
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- 25
With respect to Comparative Example 2, as seen
in Table 1, a pressure in the autoclave is lower because
a melting point of used LDPE is low and the pre-expanded
particles are shrinked when steam of 0.7 kg/cm2-G is
employed.
In Table 2, with respect to a mold size, S
represents a single cavity mold of 200 mm X 200 mm X 50 mm
in cube, and L represents a single cavity mold o~
400 mm X 900 mm X 50 mm in cube. In Table 2, with
respect to addition of internal pressure of pre-exapaned
particle, Yes represents that the pre-expanded particles
were subjected to a compressing treatment by air before
foaming under a condition of 60C and 10 kg/cm2 G for 1
to 2 hrs to give an internal pressure of the pre-expanded
particle of 1.4 to 1.8 atm, and No represents that the
pre-expanded particles were not subjected to such a
compressing treatment, i.e. an internal pressure of the
pre-expanded paticle before foaming was atmospheric
pressure.
As shown in Table 1, pre-expanded particles
of Example 1 to 3 have similar characteristic properties
without being so influenced by an inclination of melt
tension, because a control of heating is precisely
realized during a preparation of the pre-expanded
particles.
As clearly understood from Table 2 and 3, when
the pre-expanded particles having an inclination of melt
tension less than 1,400 are employed for foaming within
a mold, a foaming condition, a restriction of which
is relaxed, can be employed and foamed articles can be
constantly obtained.
On the other hand, when the pre-expanded
particles having an inclination of melt tension is over
1,700 are employed, though a small foamed article is
obtained, a large size of foamed articl~ having a good
appearance, in which the pre-expanded particles are well
fused together, cannot be obtained.
o
- 26
Example 4
A 1000 Q autoclave equipped with a stirrer was
charged with 100 parts (225 kg) of particles of a
copolymer of ethylene and 4-methyl-1-pentene having a
melting point of 120C, a density o~ 0.920 g/cm3, MI of
2.1 g/10 min and a sphere diameter of about 2 mm, 0.5
part of basic powdered calcium phosphate and 0.006 part
of sodium dodecylbenzenesulfonate as dispersing agent and
300 parts of water. After stirring the mixture, to the
dispersion thus obtained was added 30 parts of dichloro-
difluoromethane with stirring, and the temperature of the
dispersion was raised to 117C. A pressure in the
autoclave at that time was 27 kg/cm2~G.
By opening a releasing valve provided at a
lower part of the autoclave, the aqueous dispersion of
the copolymer particles was released into an atmosphere
of normal pressure through a round orifice having 4 mm in
a diameter in an orifice plate provided behind the valve
to give pre-expanded particles. The releasing operation
was carried out for half an hour while a pressure in the
autoclave was kept constant at 27 kg/cm G by introducing
liquefied dichlorodifluoromethane under pressure into
the autoclave with controlling the introduced amount
through an injection valve.
The obtained pre-expanded particles were not
stuck to each other and have average pre-expansion ratio
of 26.7. Almost all the pre-expansion ratio were in a
range of 24 to 28. Decrease of the pre-expansion ratio
was not observed during the latter half of the releasing
operation and the pre-expansion ratio of the obtained
pre-expanded particles were all nearly close to each
other.
After drying the pre-expanded particles at 60C
for 24 hrs, the particles were pressed with compressed
35 air of 60 C and 20 kg/cm G for 2 hrs to immerse air into
the cells of the pre-expanded particles. A mold of 290
mm x 270 mm x 50 mm was charged with thus treated
~ J
particles and the mold was heated with steam of 1.5
o
- 27
kg/cm G for 15 sec to give a foamed article. The
obtained foamed article was an excellent article having a
flat surface and a density of 0.024 g/cm3, in which the
pre-expanded particles were well fused together.
Characteristic properties of the foamed article
and a molded article of high-pressure process, low
density, crosslinked polyethylene (cor~mercially available
from Kanegafuchi ICagaku Kogyo Kabushiki Kaisha under a
col~nercial name of "EPERAN") are shown in Table 4.
As clearly understood frorn Table 4, a foamed
article haviny 38 of expanison ratio of the present
invention has a solidity and a cushoning property of the
same rank as a molded article of crosslinked polyethylene
having 27 of expansion ratio.
Table 4
_ _ _, _ _
~ . A molded article
A foamed artlcle of crosslinked
of Example 4 polyethylene
Density (g/cm3) 0.024 0.034
Expansion-ratio 38 27
Solidity (kg/cmZ)* 0.7 0.7
Minimum static
cushoning 3 4 3 6
coefficient
(C min)
*: measured according to JIS K 6767
Comparative Example 3
In a 4 Q autoclave equipped with a stirrer 100
parts (900 g) of high-pressure process, low density
polyethylene particles having particle size of about 2 mm
(Experimental No. 1) or high density polyethylene
particles having particle size of about 2 mm
(Experimental No. 2) shown in Table 5, 0.5 part of basic
powdered calcium phosphate and 0.006 part of sodium
dodecylbenzenesulfonate as a dispersing agent were
aispersed into 300 parts of water.
* Trade Mark
V
~ 28
After 30 parts of dichlorodifluoromethane was
added to the obtained dispersion with stirring, the
temperature of the dispersion was raised to the
temperature shown in Table 5. While maintaining a
pressure in the autoclave at that time by introducing
nitrogen gas under pressure into the autoclave, the
aqueous dispersion was released into an atmosphere of
normal pressure through a round orifice having 4 mm in
diameter provided in an orifice plate.
The pre-expansion ratio o~ thus obtained pre-
expanded particles of the low density polyethylene and
the high density polyethylene were shown in Table 5.
The foaming procedure of Example 4 was repeated
except that the obtained low density and high density
polyethylene particles were used. As a result, good
foamed articles could not be produced as shown in .~able
5.
Table 5
High-presSure
process, High density
low density polyethylene
polyethylene
. .
Meling point ( C) 109 131
Density (g/cm ) 0.922 0.950
MI (g/10 min) 1.5 2.5
Heating temperature
(C) 103 125
Pressure in the 2 22 5 26 2
autoclave (kg/cm C)
Pre~expansion ratio 30.0 26.1
Any foamed article could not be
produced, since the packed pre-
35 Molding property expanded particles were
immediately shrinked when the
particles were heated with steam.
. _
- 29
Comparative Example 4
The procedure of Example 1 was repeated except
that the aqueous dispersion was released through a
releasing valve having 25 mm in inner diameter instead of
the orifice.
The obtained pre-expa~ded particles included
many agglomerated particles which had 2 to 20 pre-
expanded particles stuck to each other. Those pre-
expanded particles could not be used for the further
molding process.
Comparative Example 5
The procedure of Example 4 was repeated except
that, at the time of releasing the dispersion, compressed
nitrogen gas was introduced instead of liquefied
dichlorodifluoromethane. As a result, the pre-expansion
ratio of the obtained pre-expanded particles was changed
from 26.7 to 15.3 during the releasing procedure, i.e. 30
mins.
Example 5
The procedure of Comparative Example 3 was
repeated except that the copolymer particles of ethylene
and the monomers as shown in Table 6 having a sphere
diameter of about 2 mm to produce foamed articles. The
obtained foamed articles were not shrinked at all and had
an excellent molding property, i.e. the pre-expanded
partices were completely fused together. The results are
shown in Table 6.
-- 30
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