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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a red phosphorus flame retardant and a
nonflammable resinous composition containing the same. In particular, the
present invention is directed to a red phosphorus having a special surface
configuration which has been produced by a special process and a
nonflammable resinous composition containing the red phosphorus which
composition is greatly improved in its moisture-resistance,
corrosion-resistance and heat resistance. Further, the present invention
is directed to the provision of a nonflammable resinous composition which
can be easily and safely handled and is highly stable.
2. Description of the Prior Art
Since red phosphorus is useful as a flame retardant for synthetic resins,
it has been heretofore used in thermosetting resins and thermoplastic
resins to provide various nonflammable resinous compositions which have
been extensively utilized in a variety of applications, such as electronic
components or parts, electric articles, machines, automobiles, buildings,
etc.
However, when red phosphorus is used as it is, the following problems have
been encountered because of its lability and sensitivity to heat, friction
and shock. Namely,
red phosphorus presents a danger in handling, storing and mixing with
resins;
formation of poisonous phosphine gas and oxidation products is caused due
to the reaction of red phosphorus with moisture in the air, thereby
polluting the working environment and impairing the physical and
electrical properties of resinous compositions; and
there are difficulties in preparing a nonflammable composition due to the
lack of compatibility with synthetic resins.
For these reasons, various ways of stabilizing the red phosphorus flame
retardant with various organic or inorganic substances have been tried in
order to overcome the foregoing problems but they have been entirely
successful. Accordingly, the use of the red phosphorus flame retardant is
restricted to certain fields and it has been difficult to satisfy the
requirements for high qualities.
Presently, the red phosphorus flame retardant has been extensively used as
a flame retardant for thermosetting resin, particularly epoxy resin, and
has been mainly used in insulating cast resinous compositions for use in
electronic components for high voltage applications.
However, in recent years, with an increasing trend toward miniaturization
and high-voltage application in electric or electronic articles,
increasing demand is being directed to electrically insulating materials
with a high performance. For such a demand, the requirements for the
physical properties of the red phosphorus flame retardant have become more
critical and, thus, red phosphorus flame retardants heretofore available
can not fully meet the requirements. In other words, the electronic parts
or components using, as an insulator, the nonflammable resinous
composition containing the conventional red phosphorus flame retardants
are subjected to degradation of insulation and corrosion at metallic
portions due to deterioration of the used resin with the passing of time,
and thereby their properties are impaired. In such circumstances, it has
been pointed out that the known nonflammable articles lack durability and
lability. Such a lack is considered to be caused mainly due to
deterioration of the red phosphorus flame retardant and, thus, improvement
for this has been required. The deterioration of the red phosphorus flame
retardant has been considered to be due to the formation of phosphine and
corrosive oxidation products resulting from the reaction of the red
phosphorus with a small amount of moisture and, as a method of stabilizing
the known red phosphorus flame retardants, their powders are coated with
various substances so as to be screened from the contact with moisture.
However, actually, such a known method itself has limitations and, thus,
can not meet the requirements for resinous materials intended to use in
high performance electronic components in which an extremely high
resistance to moisture and corrosion is required.
As an alternative method to render the insulating cast resin nonflammable
for the high voltage applications, organic halide flame retardants have
been practically used either singly or in combination with antimony
trioxide in some cases, because they have good moisture resistance and
corrosion resistance as compared to the foregoing red phosphorus. However,
these known halide flame retardants, in addition to the inherent
disadvantage that they evolve a large quantity of poisonous gases when
burning cause serious deterioration of the electrical properties of the
resins because of the use of them is required in large amounts. Further,
since the halide flame retardants are expensive, the production cost is
increased.
In contrast to this, red phosphorus is considered as a hopeful flame
retardant material meeting the requirements, such as safety and
minimization of environmental pollution, because evolution of poisonous
gases and smoking when burning are slight as compared with the organic
halides. Further, since it exhibits a very high flame-retarding ability in
a small amount, the use of it not only reduces detrimental effects on the
physical properties of the resins, but also is advantageous from the point
of cost. Under such circumstances, there is a growing demand for
improvements in the heat resistance and moisture resistance of flame
retardants of red phosphorus and more stabilized red phosphorus flame
retardants are awaited.
Thermoplastic resins have been extensively used in various fields, such as
electric articles, machines, automobiles and buildings, because of their
superior physical and chemical properties. Generally, thermoplastic resins
are subjected to mixing and molding operations at relatively high
temperatures in comparison with thermosetting resins and, thus, red
phosphorus flame retardant has not so often been used in the resins
because of the lack of thermal stability. As other known flame retardants,
organic halides, organic phosphorus compounds, antimony trioxide, etc.,
have been used practically either singly or combinations thereof in
thermoplastic resins. However, these known flame retardants have, for
example, the disadvantages that they present problems in safety and
stability or cause serious deterioration of the physical properties of the
resins. Recently, with an increasing demand for much higher quality in all
industrial fields, the requirements for thermoplastic resins have also
become more strict. For example, with respect to nonflammability
contemplated by the present invention, with increasing public demand for
safety, a further higher technique has been required not only for
obtaining a higher burning resistance but also for securing safety in
working and burning and stability. However, most of these retardants can
not meet such a requirement. For example, thermoplastic resins are
subjected to forming operations at relatively high temperatures and,
during such a high temperature operation, the organic halide flame
retardant yields corrosive thermal decomposition products or hydrolysis
products, thereby damaging the metal mold. Further, after molding,
bleed-out occurs at the surfaces of the resulting molded articles and the
surface appearance and the electrical properties of the articles are
impaired. Further, the organic halide flame retardant should be added in
large amounts to impart an enough burning resistance to the resulting
products but such a large amount of addition not only adversely affects
the mechanical properties, such as tensile strength, folding endurance or
impact resistance, but also results in increased production cost. In
recent years, as the most serious problems associated with the use of
organic halide flame retardants in thermoplastic resins, particular
attention has been given to the problems caused by a large amount of smoke
or toxic gas generated when burning. With an increasing demand for safety
from burning in the use of synthetic resins, the additives like organic
halides, which may cause evolution of a large quantity of gas pollutants
when burning, have been gradually limited from the viewpoints of personal
safety and maintenance of equipments or tools. Antimony trioxide has been
usually employed as a flame-retarding assistant for, the organic halide
flame retardants, but it not only exhibits detrimental effects on the
physical properties of the used resins, particularly with regard to the
reduction of tensile strength and impact resistance, but also presents
problems or troubles in ensuring the safety of working environments
because of its toxicity. Further, it has known that most organic
phosphorus compounds themselves act as a plasticizer and, therefore, cause
an unfavorable reduction in the heat-resistance and mechanical properties
of resins. Also, the organic phosphorus compounds increase the water
absorbing property of the nonflammable resinous article, thereby leading
to an unfavorable deformation of the article.
In contrast to this, red phosphorus exhibits a very high flame-retarding
ability in a small amount and evolution of poisonous gases and smoking are
slight as compared to the halide type flame retardant. Therefore, red
phosphorus is considered as a hopeful flame retardant material which is
safe from burning and minimizes environmental pollution problems. Under
such circumstances, the foregoing methods of stabilizing red phosphorus
powder by coating have been tried to improve the heat resistance of the
red phosphorus flame retardant used in thermoplastic resins, but they have
not been successful. Therefore, there is a growing demand for a red
phosphorus flame retardant which is stable and safe in working and
burning.
In response to such a demand, the present inventors have made many studies
on the foregoing problems, such as moisture resistance, corrosion
resistance and heat resistance of red phosphorus as a flame retardant, and
consider that there are limitations in the conventional method for surface
treating red phosphorus powder. On the base of such consideration, the
inventors have carefully studied the properties in question from a
different angle and, as a result, found that the red phosphorus powder
obtained from a novel process different from any prior art have a special
configuration and are entirely different in their surface states and
physical properties from those obtained from the prior art. The novel red
phosphorus has a very high stability and may be employed as a flame
retardant as it is. However, such a novel type of red phosphorus has been
found to be considerably stabilized by a surface modifying treatment and,
thereby, be very useful as a flame retardant for resin compositions. The
present inventions have been arrived based on the above findings wherein
the above problems with respect to moisture resistance, corrosion
resistance and heat resistance can be overcome.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a flame
retardant of red phosphorus and a nonflammable resinous composition
containing the same in which the flame retardant is provided in a special
surface, configuration and thereby its properties, particularly with
respect to moisture resistance, corrosion resistance and thermal stability
are greatly improved.
Another object of the present invention is to make it possible to work or
handle with ease and in safety.
A further object of the present invention to provide red phosphorus coated
with resin and/or hydroxide, such as aluminum hydroxide and/or zinc
hydroxide.
According to the present invention, there is directly provided a flame
retardant material of red phosphorus powder in the form of spherical fine
particles free of pulverized angular face and aggregate thereof by a
conversion process of yellow phosphorus without requiring pulverizing
process.
In a further feature of the present invention, the red phosphorus may be
coated with thermosetting resin and/or hydroxide, such as aluminum
hydroxide and/or zinc hydroxide.
In a still further feature, a nonflammable resinous composition eliminating
the foregoing troubles or problems heretofore experienced can be obtained
by adding the red phosphorus as a flame retardant to synthetic resins,
i.e., thermosetting resin or thermoplastic resin. As the thermosetting
resin, epoxy resins can be used and the thermoplastic resin may be at
least one selected from the group consisting of polyamide, polyester,
polyether, polycarbonate, polystyrene, polyurethane and polyacrylate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Red phosphorus has been usually produced by heat treating yellow phosphorus
over a period of several days in a reactor and the red phosphorus
resulting from such a known process has been obtained as a solidly
coagulated cake-like lump of high density. When red phosphorus is used as
a flame retardant in synthetic resins, it should be in a fine powder form
and, thus, a pulverizing step is indispensable for the conventional red
phosphorus obtained as a lump.
In contrast to this, according to the present invention, there is directly
obtained red phosphorus in a fine powder form by a novel conversion
process, without requiring a pulverization step and the thus obtained red
phosphorus is a light amorphous powder having a small bulk density, in
comparison with the conventional pulverized powder of red phosphorus.
Although such light, amorphous red phosphorus itself is highly stable, a
very high stability can be obtained by coating a thermosetting resin
and/or hydroxide, such as aluminum hydroxide and/or zinc hydroxide, and
the reactivity of the coated red phosphorus to moisture is almost
negligibly small in comparison with the reactivity of known pulverized red
phosphorus similarly coated. When the coated phosphorus of the present
invention is incorporated as a flame retardant into synthetic resin, the
resulting nonflammable resinous composition is outstandingly improved in
moisture resistance and corrosion resistance as compared to any known
nonflammable compound and, with respect to these properties, is well
comparable with a resinous composition not containing a flame retardant.
Further, since the coated red phosphorus of the present invention has a
high ignition point, it can be safely incorporated into thermoplastic
resin without accompanying evolution of phosphine gas.
It is considered that the unusual stability of the flame retardant of the
present invention is ascribable to the surface state of the red phosphorus
which is quite different from the surface state of the pulverized red
phosphorus in the prior art. More specifically, pulverized powder obtained
by pulverizing a strongly coagulated lump, as in the prior art, is made up
of particles having a complicated polyhedral configuration consisting of
acute ridge lines and sharp-edged angular facets. In contrast to this,
since the particles of the present invention are not subjected to
pulverization, such ridge lines and facets are rarely found. It has been
confirmed by means of an electron microscope that the invention red
phosphorus powder is made up of spherical fine particles having a
naturally occurring continuous surface and aggregate thereof. In the
specification, the red phosphorus of the present invention is referred to
as "spherical red phosphorus" in the sense of a red phosphorus having a
spherical surface.
In the known pulverized red phosphorus, since the pulverizing step produces
many active sites on the surface of particle and makes it labile, moisture
and oxygen tend to adhere onto the sites and thereby phosphine and
oxidation products result from disproportionation and burning occurs. On
the other hand, such active sites are rarely found in the spherical red
phosphorus particles which have not been subjected to a pulverization
process and their surface state is very stable. Therefore, it can be
considered that adsorption of oxygen and moisture and disproportionation
do not occur in the spherical red phosphorus and the red phosphorus itself
is considerably stabilized. Further, with respect to coating of red
phosphorus powder with thermosetting resin or aluminum hydroxide, etc., it
is difficult to coat uniformly the pulverized powder due to its surface
state and some portions of the unstable faces tend to be left uncoated. In
contrast to this, the spherical red phosphorus can be uniformly and wholly
coated and, it is considered that such a difference in uniformity of the
coating leads to a definitive difference in stability over known
pulverized powder.
Since the spherical red phosphorus itself has such a very highly stable
surface, it exhibits abilities which are by no means inferior to any
conventional coated flame retardant obtained from the pulverized red
phosphorus, even when it is employed as a flame retardant without any
coating treatment, in applications in which the required levels for
moisture resistance and corrosion resistance are not so high, or the
operation temperatures, for example, in mixing with resin or molding, are
relatively low. However, for applications such as electronic parts, in
which high levels of moisture-resistance and corrosion resistance are
required, or for use in resins with a high molding temperature, it is
desired to coat the spherical red phosphorus with a thermosetting resin or
hydroxide, such as aluminum hydroxide, and thereby most of the possible
problems which may be caused by the addition of the red phosphorus flame
retardant will be eliminated.
This coating not only provides almost perfect red phosphorus in moisture
resistance and corrosion resistance properties, but also favorably
increases the compatibility with resins used in the preparation of a
nonflammable composition, thereby facilitating processing operations.
As a further advantage of the coated red phosphorus of the present
invention it has no detrimental effect on the inherent properties of the
used resin. It has been known that when the conventional pulverized red
phosphorus is added as a flame retardant to a resinous composition, the
tensile strength, flexural strength and electrical properties of the resin
are adversely affected. However, such deterious effects on those physical
properties are hardly detected on addition of the spherical red phosphorus
of the present invention to the resinous composition. The deterioration of
the physical properties of the resin associated with the addition of the
pulverized red phosphorus is considered to be caused by the surface state
of the particles having angular pulverized faces and the degradation
products. In contrast to this, the spherical red phosphorus powder is not
only chemically stable, but also has an advantageous shape causing no
deterioration of the physical properties of the resin.
As set forth above, the nonflammable resinous composition according to the
present invention can be safely handled and is highly stabilized by using
the spherical red phosphorus as a flame retardant, without losing the
advantages of the red phosphorus flame retardant.
The spherical red phosphorus according to the present invention can be
produced by the following method.
In a sealed container filled with an inert gas, yellow phosphorus is heated
to a temperature near its boiling temperature to initiate the conversion
reaction to red phosphorus, and when the resulting nuclei of red
phosphorus are grown to the desired particle size, the conversion reaction
is discontinued. After removing unconverted yellow phosphorus, the
spherical red phosphorus is obtained in a fine powder form having a small
bulk density, without requiring any pulverizing process. The conversion
ratio and the particle size of the red phosphorus can be arbitrarily
adjusted by controlling the time and temperature of the conversion
process. As preferable conditions of the production of the red phosphorus
contemplated by the resent invention, the reaction temperature is in the
range of 250.degree. to 600.degree. C. and the conversion is 70% or less.
When the reaction temperature is less than 250.degree. C., the conversion
rate is slow and is impractical. On the other hand, since a temperature
exceeding 600.degree. C. makes it difficult to control of the conversion,
the resulting products are not uniform in their properties and can not
satisfy the requirements for the surface shape purposed by the present
invention. When the conversion is more than 70%, the resulting red
phosphorus becomes a lump and needs a pulverizing step for use as a flame
retardant. This pulverizing step makes it impossible to achieve the
objects of the present invention. Usually, the longer the reaction time
and the higher the reaction temperature, the greater the conversion and
the larger the particle size become. For example, conversion at
280.degree. C. for four hours provides a conversion of 40% and an average
particle size of 50 .mu.m. The particle size distribution of the red
phosphorus thus obtained is in a very narrow range and extremely uniform
as compared to the ordinary pulverized powder. Therefore, even in case
where the invention red phosphorus has the same average particle size as
that of the pulverized one, it has a higher porosity, and, thereby, it can
be obtained as a light powder having a small bulk density. In the
nonflammable composition of the present invention, the particle size of
the red phosphorus may be 200 .mu.m or less, and, more preferably, it is
100 .mu.m or less in view of influence on the physical properties of the
resulting resinous composition and the appearance quality of the molded
articles.
In the present invention, when the foregoing spherical red phosphorus is
desired to be coated with hydroxide, an aqueous solution of water-soluble
salts of aluminum or zinc, for example, aluminum sulfate, aluminum
chloride, zinc sulfate or zinc chloride, is added to an aqueous suspension
of the red phosphorus powder and is allowed to be adsorbed onto the powder
in the form of aluminum hydroxide or zinc hydroxide resulted from
neutralization by sodium hydroxide or double decomposition by addition of
ammonium bicarbonate. In this coating, if necessary, the foregoing water
soluble salts may be used in combination thereof to form aluminum
hydroxide and zinc hydroxide on the red phosphorus powder.
In practicing this coating process, it is preferred that the amount of the
red phosphorus in the aqueous suspension be in the range of 10 to 100
parts by weight with respect to 100 parts by weight of water and the
concentration of the water soluble salt of aluminum or zinc in the aqueous
solution be in the range of 5 to 30% by weight. The coating amount of the
hydroxide is preferably from 0.3 to 30 parts by weight with respect to 100
parts by weight of the red phosphorus and, thereby, a superior red
phosphorus flame retardant can be obtained. However, this invention is not
limited only to those.
In the present invention, when the spherical red phosphorus is required to
be coated with thermosetting resin, any raw material of the resin and its
initial condensate may be used as long as they can readily cause
polymerization in the red phosphorus aqueous suspension or the initial
condensate can be emulsified in the suspension, and are allowed to
uniformly deposit onto the surface of the red phosphorus powder, thereby
forming a coating of the thermosetting resin. Usually, the coating
material is selected from various types of materials, such as
phenol-formaldehyde system, urea-formaldehyde system,
melamine-formaldehyde system, furfuryl alcohol-formaldehyde system,
aniline-formaldehyde system and polyhydric alcohol-polybasic acid system
and, among them, for example, the materials of furfuryl
alcohol-formaldehyde system, aniline-formaldehyde system and polyhydric
alcohol-polybasic acid system are desirably added to the aqueous red
phosphorus suspension after preparing their initial condensation products,
because the polymerization of these materials is difficult in the presence
of a large quantity of water.
Although the conditions of coating the red phosphorus with the resin are
varied somewhat depending the kind of the used resin, the resin-forming
raw material or the initial condensate thereof is added in an amount of 1
to 35 parts by weight with respect to 100 parts by weight of the red
phosphorus to an aqueous suspension containing the red phosphorus in an
amount of 10 to 100 parts by weight with respect to 100 parts by weight of
water. In the case of using the resin-forming raw material, the material
is stirred at temperatures of 40.degree. to 100.degree. C. for a period of
time of one to three hours, and, in the case of using the initial
condensate previously prepared, the condensate is stirred at temperatures
of 60.degree. to 100.degree. C. for a period of time of one to two hours.
In this step, a polymerization catalyst and a filler, such as aluminum
hydroxide, magnesium hydroxide or titanium hydroxide, may be coexistent in
the mixture. Addition of the filler increases the mechanical strength of
the resin coating and, at the same time, has an effect of covering the
purple color characteristic of red phosphorus, thereby making contribution
to a further expanded use of the red phosphorus of the present invention.
The filler is preferably added in amounts of 1 to 35 parts by weight with
respect to 100 parts by weight of the red phosphorus. The intended
reaction product is removed, washed with water and is dried at
temperatures of 130.degree. to 140.degree. C. to complete the
polymerization reaction. After such procedures, there can be obtained the
invention red phosphorus flame retardant having a very high level of
stability combined with a very high resistance to moisture and corrosion.
As a further method, when aluminum hydroxide and/or zinc hydroxide is
adsorbed onto the red phosphorus powder prior to coating with the
thermosetting resin, the red phosphorus is further improved in its
moisture resistance, corrosion resistance and stability and the resinous
composition which is rendered nonflammable by the red phosphorus thus
coated is not affected by the addition of the red phosphorus over a long
period of time. The pretreatment with aluminum hydroxide and zinc
hydroxide is performed in an aqueous suspension containing 100 parts by
weight of water and 5 to 100 parts by weight of the red phosphorus by
forming aluminum hydroxide or zinc hydroxide by the neutralization of a
water soluble compound, such as sulfate or chloride of aluminum or zinc,
with caustic alkali or double decomposition with ammonium bicarbonate and
then causing adsorption of the thus formed hydroxide onto the red
phosphorus powder. The aluminum salt or zinc salt is added in amounts
required to yield 0.1 to 30 parts by weight of the hydroxide with respect
to 100 parts by weight of the red phosphorus.
As shown in Examples below, the red phosphorus flame retardant of the
present invention exhibits an extremely high resistance to moisture and
corrosion and is extremely highly stable. Further, this flame retardant
has a high ignition temperature and hardly causes the problems of
phosphine and corrosive oxidation products, which are considered to be
produced due to adsorption of oxygen and moisture. As a result, the red
phosphorus may be safely incorporated into resins to be cast at high
temperatures and resinous compositions containing it can be stored stably
over a long period in the presence of moisture or at high temperatures,
without deterioration. Such advantageous properties make the red
phosphorus highly valuable and useful in nonflammable resinous
compositions.
For example, since the red phosphorus flame retardant according to the
present invention is free from the deterioration problems of resinous
compositions due to the deterioration of red phosphorus flame retardant,
it is desirable as a flame retardant for thermosetting resins used in high
voltage electronic parts in which a high degree of stability is required.
Therefore, one feature of the present invention resides in the provision of
a nonflammable thermosetting resin composition containing the highly
stable red phosphorus set forth above, the resinous composition comprising
100 parts by weight of epoxy resin as a thermosetting resin, 5 to 40 parts
by weight of the red phosphorus flame retardant, 5 to 150 parts by weight
of aluminum hydroxide as a filler or a flame-retarding assistant, 20 to 90
parts by weight of acid anhydride hardener and an appropriate amount of a
hardening promoter. In the present invention, the term "epoxy resin" is
intended to mean epoxide of aromatic-, alicyclic- or aliphatic-type having
one or more epoxy groups in their molecules and, epoxy resin which is
liquid at room temperature is particularly preferable for insulating cast
resin compositions for electronic parts. For example, bisphenol A
diglycidyl ether, bisphenol F diglycidyl ether, and polyglycidyl ester of
polycarboxylic acid (e.g. phthalic acid or terephthalic acid), are
suitable for practical use.
An excess use of aluminum hydroxide leads to an unfavorable increase in the
viscosity of the resinous composition and thereby will present
difficulties in the casting operation. On the other hand, an insufficient
use of aluminum hydroxide can not provide a sufficient effect as a flame
retarding assistant. Therefore, aluminum hydroxide is preferably employed
in the range of 5 to 150 parts by weight with respect to 100 parts by
weight of epoxy resin.
The amount of the red phosphorus flame retardant is preferably in the range
of 5 to 40 parts by weight with respect to 100 parts by weight of epoxy
resin, taking into account the flame retarding effect and the influence on
the viscosity of the resin.
As the hardener, an acid anhydride is most preferable and known anhydrides,
such as phthalic anhydride, tetrahydrophthalic anhydride, succinic
anhydride, etc., are widely useful. As the hardening promoter, imidazole
derivatives of 2-phenylimidazole, 2-ethyl-4-methylimidazole, etc., are
preferable from the view-point of ease of operations.
The red phosphorus of the present invention may be also used with
thermoplastic resin and is particularly useful in the so-called
engineering plastic compositions for structural materials and functional
parts of electric articles or machines which are used under relatively
severe conditions and the present invention is directed to a resinous
composition for such applications.
The thermoplastic resin to be rendered nonflammable by the present
invention may be selected from the group consisting of polyamide,
polyester, polyether, polycarbonate, polystyrene, polyurethane and
polyacrylate. In addition to the red phosphorus flame retardant,
appropriate additives known in the art, such as filler, stabilizer,
plasticizer, colorant, glass fiber or lubricant may be added, if
necessary. The red phosphorus flame retardant is preferably added in an
amount of 0.1 to 30 parts by weight with respect to 100 parts by weight of
the thermoplastic resin. When the amount is less than 0.1 part by weight,
a sufficient flame retarding effect can not be expected. However, an
excess use exceeding 30 parts by weight adversely affects the physical
properties of the resin component.
In the nonflammable compositions of the present invention, known flame
retardants may be employed in combination of the flame retardant of the
present invention if necessary.
The present invention will now be described in detail with reference to the
following Examples.
EXAMPLE 1
Preparation of Spherical Red Phosphorus
500 g of yellow phosphorus was placed in the stainless vessel filled with
nitrogen gas, sealed and heated at 270.degree. C. for four hours to
convert it to red phosphorus. Unconverted yellow phosphorus was removed
and there was obtained 211 g of spherical red phosphorus in flowable
spherical powder having an average particle size of 50 .mu.m and a bulk
density of 0.86 g/cm.sup.3. The spherical red phosphorus thus obtained was
employed in the following Examples.
EXAMPLE 2
500 g of the spherical red phosphorus was suspended in 800 ml of water and
then 300 ml of a 10% aqueous solution of aluminum sulfate was added. After
100 ml of a 5% aqueous solution of sodium hydroxide was added dropwise
while fully stirring, the suspension was heated to 50.degree. C. and was
kept at the temperature for 30 minutes. The resultant suspension was
filtered, washed with water and dried at 120.degree. C. The yield of the
resultant coated red phosphorus | | |
