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Description  |
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This invention relates to a process for producing a highly foamed
styrene/maleic anhydride copolymer resin foam.
In producing a thermoplastic resin foam say by the extrusion molding
technique, the usual process for obtaining the desired shaped article
comprises extruding a blowing agent-containing melt-plasticized resinous
composition from a pressurized state into an atmosphere of normal pressure
thereby effecting the expansion of the resinous composition by its change
in pressure.
Polystyrene and polyethylene are well known as being typical all-purpose
thermoplastic resins for use in the foregoing process. Especially in the
case of polystyrene, since the dependence on temperature of its melt
viscosity is small and thus low-temperature extrusion is possible, not
only the choice of the blowing agent is simplified, but also a viscosity
suitable for foaming can be readily obtained. It is hence possible to form
highly foamed products relatively easily by the use of a low-boiling
blowing agent. Usable as this blowing agent are the inert gases such as
carbon dioxide, nitrogen and helium, saturated aliphatic hydrocarbons of
up to 8 carbon atoms such as methane, ethane, propane, butane and pentane,
the halogenated hydrocarbons such as methylene chloride and Freon,
saturated alicyclic hydrocarbons such as cyclohexane ethylcyclopentane,
aromatic hydrocarbons such as benzene and xylene, ketones such as acetone
and methyl ethyl ketone, and petroleum ethers, etc.
In using a low-boiling blowing agent, a costly apparatus for metering the
high-pressure gas to be introduced into the extruder from a cylinder is
required. Further, care must be exercised to ensure against such hazards
as explosions and poisoning resulting from the leakage of the blowing
agent gas.
On the other hand, there are available inorganic and organic chemical
blowing agents, which do not require the use of costly apparatuses and in
which there is practically no need to consider such hazards as explosions
and poisoning. Inorganic chemical b-owing agents that are usually used
include, for example, sodium hydrogencarbonate, ammonium carbonate,
ammonium hydrogencarbonate, sodium borohydride and light metals (e.g. Mg,
Zn and Al). On the other hand, the usually used organic chemical blowing
agents include, for example, azodicarbonamide, azobisformamide,
azobisisobutyronitrile, diazoaminobenzene,
N,N'-dinitrosopentamethylenetetramine,
N,N'-dimethyl-N,N'-dinitroterephthalamide, benzenesulfonyl hydrazide,
p-toluenesulfonyl hydrazide and p,p'-oxybisbenzenesulfonyl hydrazide.
However, in the method which uses these chemical blowing agents the
blowing agent forms cells by the separation and association of the gas in
solution in the resin, with the consequence that there is no quenching and
hardening of the membranes of the cells. Hence, the permeability of the
gas evolved is great, and thus a highly foamed product cannot be readily
obtained.
Our extensive styrene/maleic with the view of obtaining a highly foamed
product free of the aforementioned drawbacks of the prior art led to the
discovery that by using a dry blended resinous composition consisting of a
styrele/maleic anhydride copolymer resin and as a blowing agent at least
one compound selected from the group consisting of a carboxylate of a
metal of group Ia of the periodic table of elements, a carbonate of a
metal of group Ia, a carboxylate of a metal of group IIa and a carbonate
of a metal of group IIa and foaming this composition by say extrusion
molding, it was possible to obtain a highly foamed product having an
expansion ratio of at least 8 times, an expansion ratio impossible of
attainment by the use of conventional chemical blowing agents.
Specifically, in accordance with the present invention, 0.1-10% by weight,
preferably 0.5-5% by weight, of the aforesaid blowing agent that has been
rendered into a powdered state in advance and 99.9-90% by weight,
preferably 99.5-95% by weight, of a styrene/maleic anhydride copolymer
resin optionally added a small quantity of a mineral oil are dry blended,
after which the blend is heat-melted and foamed say by extrusion molding
or injection molding to give a foamed product. It is thus possible to
obtain a highly foamed product by the blowing gas that evolves as a result
of the reaction between the styrene/maleic anhydride copolymer resin and
the blowing agent.
Now, as regards the processes for producing the foams of the aforementioned
thermoplastic resins inclusive of the styrene/maleic anhydride copolymer
resin, in the case of say the usually employed extrusion molding method,
there is (1) the so-called two-stage method which comprises first
preparing a foamable resin by impregnating the resin with a low-boiling
blowing agent such as the aforementioned freon, butane or pentane and
thereafter feeding the so prepared foamable resin to the extruder where it
is foamed to yield the intended foamed product or (2) the so-called
one-stage method in which the foamed product is obtained either by
introducing a low-boiling blowing agent such as described above into a
melt-plasticized resin and extruding the blend while kneading it under
pressure or by blending with the resin a chemical blowing agent that
evolves either nitrogen or carbon dioxide, such as the aforementioned
azodicarbonamide or sodium hydrogencarbonate, and feeding this blend to
the extruder for extrusion therefrom.
In the case of the two-stage method described above, it is necessary that
the foamable resin be prepared into foamable particles of uniform particle
size before feeding it to the extruder. It is hence a disadvantage from
the standpoint of the total cost involved because of the additional
investment in the required equipment. On the other hand, in the case of
the direct blowing agent injection system such as the one-stage method
described hereinabove, this also has is shortcomings. Not only is there
required a costly equipment such as an apparatus for injecting the blowing
agent, but also there is the need to exercise special care to ensure
against the hazards of explosion or poisoning due to leakage of the gas
that is to become the blowing agent. Above all, in the case where the
method that uses the aforementioned chemical blowing gents is adopted, the
expansion ratio is at most about 3 times. There was thus the drawback that
highly foamed products could not be obtained.
After having discovered the basic method of obtaining a highly foamed
product by thc use of a specified compound as the blowing agent, as
hereinbefore described, we furthered our research concerning embodiments
thereof. As a result thereof, we found a much more advantageous process, a
process by which all of the shortcomings of the conventional methods could
be eliminated.
There is thus provided in accordance with this invention a process that is
operated in the following manner. The aforesaid blowing agent is blended
with a styrene/maleic anhydride copolymer resin in a
hereinbefore-indicated proportion. The blend thus obtained is fed to an
extruder where it is uniformly melt-blended and extruded from a pellet die
of the extruder at such a temperature as to obtain an expansion ratio of
not greater than 2.0 times, and preferably not greater than 1.5 times. The
resulting preexpanded pellets are then fed to a usual foaming apparatus to
yield the intended highly foamed product.
The term "styrene/maleic anhydride copolymer resin", as used herein,
denotes a copolymer resin that is obtained by heat-polymerizing monomeric
styrene with maleic anhydride in the presence of a chain transfer agent
and a radical generator. Such a styrene/maleic anhydride copolymer resin
(SMA resin) can be specifically produced by say a procedure comprising
adding to a ketonic solvent such as acetone or methyl isobutyl ketone a
monomeric mixture of styrene and maleic anhydride in a mole ratio of
1.4-49, preferably 4.6-17.0, with a known radical generator and a chain
transfer agent, followed by carrying out the heat-polymerization reaction
at 60.degree.-180.degree. C., preferably 75.degree.-140.degree. C., and
thereafter precipitating the resulting polymer using say a poor solvent
such as petroleum benzene or methanol. The SMA resin may be one that has
been granulated by means of an extruder after having been added an
antioxidant, as required.
Suitably used as this SMA resin is one having a weight average molecular
weight (Mw) of preferably 100,000-40O,OOO, and more preferably
150,000-300,000 as measured by gel permeation chromatography, i.e. GPC
based on a polystyrene standard method.
The metals of group Ia are typically lithyium, sodium and potassium, while
the metals of group IIa are magnesium, calcium and strontium. Typical
examples of the carboxylic acids are aliphatic monocarboxylic acids such
as formic acid, acetic acid, propionic acid, butyric acid, caprylic acid,
capric acid, lauric acid, stearic acid and myristic acid; aliphatic
saturated or unsaturated dicarboxylic acids such as oxalic acid, malonic
acid, succinic acid, glutaric acid, maleic acid, fumaric acid and
glutaconic acid; hydroxymonocarboxylic acids such as glycollic acid,
lactic acid, ethylenelactic acid and glyceric acid; hydroxypolycarboxylic
acids such as malic acid, tartaric acid and citric acid; and aromatic
carboxylic acids such as benzoic acid and terephthalic acid. Typical
carbonates are say sodium carbonate potassium carbonate and sodium
hydrogencarbonate.
While this sodium hydrogencarbonate strictly speaking is a
hydrogencarbonate, it forms as a result of thermal decomposition sodium
carbonate in the following manner.
2NaHCO.sub.3 .fwdarw.Na.sub.2 CO.sub.3 +CO.sub.2 +H.sub.2 O
Hence, in the process of this invention it is considered to be a substance
having effects equal to a carbonate of an alkali metal.
Further, since sodium hydrogencarbonate evolves carbon dioxide on thermal
decomposition, it is possible to consider it to be one of the so-called
chemical blowing agents. However, this carbon dioxide does not possess any
substantial effect in obtaining the highly foamed product of aforesaid SMA
resin.
The carboxylates and carbonates are used as blowing agents in this
invention either independently of each other or in combination. Of these
blowing agents, those containing water of crystallization are especially
preferred.
Further, it is preferred to add to the SMA resin as a wetting agent a
mineral oil such as CARNATION (a product of Witco Chemical Corp. U.S.A.)
or as a lubricant a metallic soap such as zinc stearate in practicing the
process of this invention. Such nucleating agents as talc, calcium
silicate and kaolin can also be added, as required. These nucleating
agents should preferably be of the finest possible particle size for
achieving a uniform mixture with the blowing agent. Usually preferred is a
powder having particles of diameter of the order of 0.1-10 microns. These
additives should be added to the SMA resin prior to the addition of the
blowing agent.
In practicing the process of this invention, it is preferred that an
auxiliary foaming agent be added. As such an auxiliary foaming agent,
mention can be made of the inorganic salts having water of crystallization
whose dehydration temperature is relatively close to the extrusion die
temperature, such as CaSO.sub.4.2H.sub.2 O (dehydration temperature
163.degree. C.), Na.sub.2 SO.sub.4.9 1OH.sub.2 O (dehydration temperature
100.degree. C.), and CaHPO.sub.4.2H.sub.2 O.
These auxiliary foaming agent serve to slow down the reaction between the
blowing agent and the SMA resin to a still greater degree and thus cause
the reaction-evolved carbon dioxide to remain in the resin for a maximum
period of time.
In carrying out the process of this invention, the setting of the expansion
ratio of the pre-expanded pellets and the choice of the extrusion die to
be used in the secondary foaming operation are important factors, since
these factors greatly influence the expansion of the final foamed product.
In the first place, the extrusion temperature for obtaining the
pre-expanded pellets should preferably be low for restraining the
pre-expansion as much as possible. A temperature in the neighborhood of
130.degree.-140.degree. C. is usually used. On the other hand, the
pre-expansion ratio, as previously indicated, should preferably be not
greater than 2 times.
The foaming method by means of an extrusion die for carrying out the
secondary foaming include such methods as the T-die extrusion molding
method, the circular die extrusion incision method or circular die
extrusion press-adhesion method, or the fish tail die extrusion molding
method. Any of these known methods can be used in carrying out the process
of this invention. The intended final expansion ratio should preferably
range from 5 to 20 times.
The SMA resin foam thus obtained can be utilized for such applications as
insulating boards, various lagging materials and building materials such
as panels. Above all, it is expected to find use as a structural material
possessing superior heat resistance.
The following examples and comparative examples will serve to illustrate
the present invention more specifically. Unless otherwise specified, the
parts and percentages used in the examples are on weight basis.
EXAMPLE 1
A styrene/maleic anhydride copolymer resin (990 g) having a melt flow rate
(MFR) of 1.2 g/10 min (JIS Method K7210-1976, load: 5 kg, temperature:
200.degree. C.) whose surface was coated with a small quantity of a
mineral oil, and 10 g of powdered sodium acetate (Wako 1st grade, produced
by Wako Jyunyaku Kogyo Co., Ltd., Japan) were thoroughly blended. The
resulting blend was molded by extruding it through an extruder having a
cylinder of 26-mm diameter [manufactured by Sanjo Seiki Co., Ltd., Japan,
length/diameter ratio (L/D)=14, compression ratio 2.2] under the
conditions of die temperature: 125.degree. C., C.sub.1 : 225.degree. C.,
C.sub.2 : 180.degree. C., and screw rotation speed: 40 rpm.
The foam thus molded by extrusion had a satisfactory skin layer at its
surface and was one whose interior was made up of minute cells 0.1-0.3 mm
diameter. The expansion ratio of this foam was 9.8 times, this being
calculated as follows:
##EQU1##
(This expansion ratio will apply equally in the following examples.)
EXAMPLE 2
Example 1 was repeated but using a styrene/maleic anhydride copolymer resin
having an MFR of 0.41 g/10 min (maleic anhydride content 15%). The
extrusion-molding was otherwise carried out as in Example 1.
The foam thus extrusion molded had a satisfactory skin layer at its surface
and its interior was made up of minute cells 0.1-0.3 mm diameter. Its
expansion ratio was 14 times.
EXAMPLE 3
The extrusion-molding operation was carried out as in Example 1, except
that 970 g of a styrene/maleic anhydride copolymer resin having an MFR of
0.41 g/10 min and as the blowing agent 30 g of sodium oxalate (Wako 1st
grade, produced by Wako Jyunyaku Kogyo Co., Ltd.) were used.
The thus extrusion-molded foam had a satisfactory skin layer at its
surface, while its interior was made up of fine cells 0.2-0.5 mm diameter.
The expansion ratio was 9.2 times.
EXAMPES 4
Example 3 was repeated but changing the amount used of the styrene/maleic
anhydride copolymer resin to 990 g and using as blowing agent 10 g of
sodium lactate instead of sodium oxalate. The resulting extrusion-molded
foam had a good skin layer at its surface, and its interior was made up of
fine cells 0.2-0.5 mm diameter. Its expansion ratio was 8.5 times.
EXAMPLE 5
The extrusion-molding was carried out by operating as in Example 3 but
using instead of sodium oxalate an equal amount of sodium tartrate (Wako
1st grade, a product of Wako Jyunyaku Kogyo Co., Ltd.). The resulting foam
had a good skin layer at its surface, and its interior was made up of
minute cells 0.2-0.5 mm diameter. The expansion ratio of this foam was 9.0
times.
EXAMPLE 6
The experiment was operated as in Example 4, except that instead of sodium
lactate an equal amount of sodium benzoate (Wako 1st grade, a product of
Wako Jyunyaku Kogyo Co., Ltd.) was used. The resulting foam had a
satisfactory skin layer at its surface, and its interior was made up of
0.1-0.4 mm diameter fine cells. The expansion ratio of this foam was 8.3
times.
EXAMPLE 7
Example 4 was repeated but using instead of sodium lactate an equal amount
of magnesium acetate (Wako 1st grade, Wako Jyunyaku Kogyo Co., Ltd.). The
resulting foam had a good skin layer at its surface, while its interior
was made up of minute cells 0.2-0.5 mm diameter. The expansion ratio of
the foam was 13 times.
EXAMPLE 8
The experiment was operated as in Example 3 but using instead of sodium
oxalate an equal amount of calcium oxalate. The resulting foam had a
satisfactory skin layer at its surface, and its interior was made up of
fine cells 0.2-0.5 mm diameter. The expansion ratio of this foam was 9.2
times.
COMPARATIVE EXAMPLE 1
The extrusion-molding operation was carried out as in Example 1, except
that the blowing agent used was changed to lead acetate (Wako 1st grade, a
product of Wako Jyunyaku Kogyo Co., Ltd.). The extrusion-molded foam had a
satisfactory skin layer at its surface, but its interior was made up of
relatively large cells, and the expansion ratio was 3.2 times.
COMPARATIVE EXAMPLE 2
The extrusion-molding operation was carried out as in Example 1, except
that the resin used was changed to DIC STYRENE XC-510 having an MFR of 1.6
g/10 min (a styrene resin produced by Dainippon Ink & Chemicals, Inc.,
Japan). The extrusion-molded foam had a rough surface, and its interior
foaming was irregular. Moreover, its expansion ratio was 1.5 times.
EXAMPLE 9
The extrusion-molding operation was carried out as in Example 1 but using
instead of sodium acetate an equal amount of sodium carbonate (Wako 1st
grade, a product of Wako Jyunyaku Kog yo Co., Ltd.). The resulting foam
had a good skin layer at its surface, and its interior was made up of
0.2-0.5 mm diameter fine cells. The expansion ratio of this foam was 10
times.
EXAMPLE 10
Example 2 was repeated but using instead of sodium acetate an equal amount
of sodium carbonate (the same as that used above). The foam thus obtained
had a satisfactory skin layer at its surface, and its interior was made up
of fine cells 0.1-0.3 mm diameter. The expansion ratio was 13 times.
EXAMPLE 11
The extrusion-molding operation was carried out as in Example 1, except
that the styrene/maleic anhydride copolymer resin used was one having an
MFR of 0.41 g/10 min, which was used in an amount of 970 g, and the
blowing agent was changed to 30 g magnesium carbonate (Wako 1st grade, a
product of Wako Jyunyaku Kogyo Co., Ltd.).
The extrusion-molded foam had a satisfactory skin layer at its surface, and
its interior was made up of fine cells 0.2-0.5 mm diameter. Its expansion
ratio was 11 times.
EXAMPLE 12
Example 11 was repeated but using instead of magnesium carbonate an equal
amount of sodium hydrogencarbonate. The foam thus obtained had a
satisfactory skin layer at its surface, while its interior was made up of
0.2-0.5 mm diameter cells. The expansion ratio of this foam was 15 times.
EXAMPLE 13
The extrusion-molding operation was carried out as in Example 1, except
that the molding machine used was changed to a 1-ounce in-line screw type
injection molding machine (manufactured by Sanjo Seiki Co., Ltd., Japan),
and sodium acetate was changed to an equal amount of sodium carbonate. The
expansion ratio of the resulting foam was 2.3 times. The molding
temperature conditions in this case were: C.sub.1 =225.degree. C., C.sub.2
=200.degree. C., and nozzle=180.degree. C.
EXAMPLE 14
Example 13 was repeated but using instead of sodium carbonate an actual
amount of sodium hydrogencarbonate. The expansion ratio of the resulting
foam was 2.6 times.
EXAMPLE 15
A styrene/maleic anhydride copolymer resin (970 g) having an MFR of 0.41
g/10 min whose surface was coated with a small quantity of oil was
thoroughly blended with 15 g of powdered sodium carbonate and 15 g of
sodium acetate, after which the blend was extruded through a 26-mm
diameter extruder.
The resulting foam had a good skin at its surface, and its interior was
made up of 0.2-0.5 mm diameter minute cells. The expansion ratio of the
form was 14 times.
COMPARATIVE EXAMPLE 3
The extrusion molding was carried out by operating as in Example 9, except
that instead of sodium carbonate an egual amount of an iron carbonate
(Wako 1st grade, a product of Wako Jyunyaku Kogyo Co., Ltd.) was used.
The extrusion-molded foam had a satisfactory skin layer at its surface, but
its interior was made up of relatively large cells, and its expansion
ratio was 1.7 times.
COMPARATIVE EXAMPLE 4
The extrusion-molding operation was carried out as in Example 9, except
that the resin used was changed to DIC STYRENE XC-510 whose MFR was 1.6
g/10 min.
The surface of the extrusion-molded foam was rough, and its interior
foaming was irregular. Its expansion ratio was 1.5 times.
COMPARATIVE EXAMPLE 5
The extrusion molding was carried out as in Example 12, except that the
resin used was changed to DIC STYRENE XC-510. The surface of the resulting
foam was rough, and its interior foaming was also irregular. Its expansion
ratio was 3 times.
COMPARATIVE EXAMPLE 6
The extrusion molding was carried out as in Example 13 but using as the
resin DIC STYRENE XC-510. Scarcely any foaming took place in the product,
the expansion ratio being only 1.1 times.
COMPARATIVE EXAMPLE 7
The extrusion molding was carried out as in Example 14, except that the
resin used was changed to DIC STYRENE XC-510. There was scarcely any
foaming in the resulting product, the expansion ratio being only 1.5
times.
EXAMPLES 16-19 AND COMPARATIVE EXAMPLES 8-10
One hundred parts each of Dylark #232 and Dylark #332 (SMA resins produced
by ARCO/Polymers Inc., U.S.A.) and DIC STYRENE CR-4500 (polystyrene
produced by Dainippon Ink & Chemicals, Inc.), after having coated their
surface with the spreading agents indicated in Table 1, were mixed with
the alkali metal salts, lubricants, nucleating agents and auxiliary
foaming agents indicated in said table in the amounts shown therein. The
several mixtures were then melt-blended, after which the several blends
were extruded through a 26 mm diameter extruder (manufactured by Sanjo
Seiki Co., Ltd., Japan; L/D=14, compression ratio 2.2 and having two 3 mm
diameter rod dies). The extrudate was pelleted by cutting it at a point
2-3 mm from the outlet of the die followed by quenching the so formed
pellets. Pre-expanded pellets whose foaming has been restrained were thus
obtained. The pelleting conditions were as follows:
______________________________________
C.sub.1 C.sub.2 Die
______________________________________
Temperature 225.degree. C.
150.degree. C.
130.degree. C.
Screw rotation speed
40 r.p.m.
Quenching conditions
Water-cooled
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The several pellets thus obtained were then severally foamed using a
circular die extruder (manufactured by Union Plastics Co., Ltd., Japan, 50
mm diameter, L/D=24, compression ratio 36; die 23 mm (outside
diameter/17.9 mm (inside diameter)). There was thus formed tubular foams.
In this case the distance between the die and sizer was set at 500-1000
mm, and the following extrusion conditions were employed.
______________________________________
C.sub.1
C.sub.2 C.sub.3 Flange
D.sub.1
D.sub.2
______________________________________
Temperature
210.degree. C.
170.degree. C.
150.degree. C.
165.degree. C.
155.degree. C.
155.degree. C.
Screw 50 r.p.m.
rotation speed
Die pressure
350-430 kg/cm.sup.2
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As is apparent from Table 1, the products of this invention (Examples
16-19) showed far greater expansion ratios than the product of Comparative
Example 8 (two-stage extrusion product of polystyrene). The expansion
ratios of the invention products also exceeded those of the products of
Comparative Examples 9 and 10 (commercially available foams).
TABLE 1
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Foaming compound Expansion
DIC Sodium
Sodium Nucle-
Auxiliary
ratio
Expansion
Dylark Dylark
Styrene
Sodium
car-
hydrogen-
Lubri-
Wetting
ating
foaming
prefoamed
ratio of
#232 #332
CR-4500
acetate
bonate
carbonate
cant
agent
agent
agent
pellets
foam
__________________________________________________________________________
Example 16
100 1.0 0.5 0.05 1.5 0.3 2.0 15.0
Example 17
100 1.0 0.5 0.05 1.5 0.3 2.0 18.0
Example 18
100 1.0 0.5 0.05 1.5 0.3 2.0 14.0
Example 19
100 1.0 0.5 0.05 1.5 0.3 2.0 16.0
Compar- 100 1.0 0.5 0.05 1.5 0.3 1.2 1.5
ative
example 8
Compar-
Commercial product (polystyrene-type closed cellular
--ticle
8.5
ative which used low-boiling foaming agent)
example 9
Compar-
Commercial product (polystyrene-type closed cellular
--ticle
10.0
ative which used low-boiling foaming agent)
example 10
__________________________________________________________________________
Lubricant: zinc stearate
Wetting agent: CARNATION
Nucleating agent: CROWN TALC (P2, particle diameter 8.5 microns, produced
by Matsumura Sangyo Co., Ltd., Japan)
Auxiliary foaming agent: CaSO.sub.4.2H.sub.2 O
EXAMPLES 20-24
The experiments were carried out by operating as in Examples 16-19 and
Comparative Examples 8-10, except that the foaming compounds were changed
as shown in Table 2. As shown in Table 2, products of high expansion
ratios of the order of 11-15 were obtained in all of the examples.
TABLE 2
__________________________________________________________________________
Expansion
Cal- Lub-
Wet-
Nucle-
Auxiliary
ratio
Expansion
Dylark Dylark
cium
Sodium
Sodium
Sodium
Magnesium
ri-
ting
ating
foaming
prefoamed
ratio of
#232 #332
oxalate
lactate
tartrate
benzoate
carbonate
cant
agent
agent
agent
pellets
foam
__________________________________________________________________________
Exam- 1.0 1.5
0.1 3.0 0.3 2.0 14.0
ple 20
Exam-
100 1.0 1.5
0.1 3.0 0.3 2.0 13.0
ple 21
Exam- 100 1.0 1.5
0.1 3.0 0.3 1.4 11.0
ple 22
Exam- 100 1.0 1.5
0.1 3.0 0.3 1.3 12.0
ple 23
Exam- 100 1.0 1.5
0.1 3.0 0.3 1.5 14.0
ple 24
__________________________________________________________________________
Nucleating agent: HAKUENKA CCR (particle diameter 0.08 micron, produced b
Shiraishi Calcium Co., Ltd., Japan)
Auxiliary foaming agent: CaHPO.sub.4.2H.sub.2 O
The wetting agent and lubricant were the same as those indicated
hereinbefore.
* * * * *
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