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BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a granular stabilizer for
chlorine-containing polymers and a process for the preparation thereof.
More particularly, the present invention relates to a granular stabilizer
for chlorine-containing polymers, which is excellent in the dispersibility
in a resin, the uniformity of the composition, the resistance to dusting
or scattering, the flowability and the blocking resistance even though it
is formed by granulating a hardly fusible or infusible stabilizer powder
with use of a very small amount of a binder or dispersion medium, and a
process for the preparation of this granular stabilizer.
(2) Description of the Prior Art
A stabilizer such as tribasic lead sulfate is incorporated into a
chlorine-containing polymer such as a vinyl chloride resin so as to
improve the thermal stability thereof. In this case, a hardly fusible or
infusible stabilizer should be as fine as possible in view of the desire
for dispersibility in a resin, however fine powder involves a problem of
scattering. Accordingly, the hardly fusible or infusible stabilizer is
widely handled in the form of granules.
For example, Japanese Patent Publication No. 3554/66 proposes a process for
the preparation of a lubricating stabilizer for a vinyl chloride resin
which comprises adding a high-melting-point powdery stabilizer, infusible
at the processing temperature for a vinyl chloride resin or a lower
temperature, to a melt of a metal soap which is to be incorporated as a
lubricating stabilizer at the step of processing a vinyl chloride resin,
mixing them homogeneously to form a solid solution, and shaping the solid
solution into granules or small masses. According to this process, the
problem of scattering is overcome, and this process is prominently
advantageous in that a so-called one-package stabilizer having a uniform
composition can be provided. In order to granulate the powdery stabilizer
by using the metal soap melt as the binder, it is necessary to use the
metal soap in such a large amount as at least 1/3 part by weight of the
powdery stabilizer. Accordingly, if this granular one-package stabilizer
is incorporated in a vinyl chloride resin in an amount sufficient to
attain an intended stabilizing effect, the amount of the metal soap, such
as lead stearate, becomes excessive and, at the step of processing the
vinyl chloride resin, precipitation of the metal soap is often caused.
Therefore, according to the granulation process using a binder which is
softened at the resin processing temperature, it is difficult to provide a
granular one-package stabilizer in which the mixing ratio of a powdery
stabilizer is increased.
A granular one-package stabilizer containing a powdery stabilizer at a
relatively high mixing ratio has also been known. For example, in Japanese
Patent Publication No. 47138/78, we have proposed a composition comprising
a lead pigment, such as tribasic lead sulfate, affined with a liquid
plasticizer, such as dioctyl phthalate, in a specific amount relative to
the critical liquid absorption of the pigment. This composition has a
reduced powder-scattering property and a flowability suitable for handling
and is advantageous in that the mixing ratio of the powdery additive in
the composition can be increased. However, this composition is defective
in that the shape and size of particles of the final composition are not
uniform, the mechanical strength of the particles is ordinarily low, and
blocking is caused more or less.
Furthermore, Japanese Patent Publication No. 36932/79 discloses a granular
stabilizer comprising cores containing a lubricant as a continuous phase
and a powdery stabilizer as a dispersed phase and shells formed on the
surfaces of the cores, which shells are composed of a powdery stabilizer
and a plasticizer in an amount smaller than the critical liquid
absorption. This granular stabilizer is advantageous over the
first-mentioned granular stabilizer in that the powdery stabilizer can be
incorporated at a higher mixing ratio. However, this granular stabilizer
is still insufficient in that the blocking tendency cannot completely be
eliminated and the dispersibility of the powdery stabilizer into resins is
low.
Moreover, Japanese Patent Publication No. 21819/72 discloses a process in
which several stabilizers, at least one of which is a fusible substance,
are sinter-granulated without melting said fusible substance by using a
powder blender provided with a rotor rotated at a high speed. If this
sintering granulation is carried out, a homogeneous granulation product
cannot be obtained unless the fusible substance is used in a considerable
amount.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide a
granular stabilizer which is excellent in the dispersibility in a resin,
the uniformity of the composition and the resistance to dusting or
scattering, the flowability and the blocking resistance, though it is
formed by granulating a stabilizer powder with use of a very small amount
of a solid binder or dispersion medium, and a process for the preparation
of this granular stabilizer.
Another object of the present invention is to provide a process in which a
granular stabilizer can be prepared in a high yield while preventing
segregation of the components or heterogeneous granulation even with use
of a very small amount of a solid binder medium.
Still another object of the present invention is to provide a process in
which the above-mentioned granular stabilizer can be prepared with a good
operation adaptability at a high productivity.
In accordance with one aspect of the present invention, there is provided a
granular stabilizer for chlorine-containing polymers, which is formed by
granulating a hardly fusible or infusible powdery stabilizer for
chlorine-containing polymers or a mixture of said powdery stabilizer with
a powdery stabilizer aid through an organic solid binder or dispersion
medium, wherein respective primary particles of said powdery stabilizer or
said powdery mixture are surface-treated with the organic solid binder or
dispersion medium in an amount of 2 to 15 parts by weight per 100 parts by
weight of said powdery stabilizer or said powdery mixture, which is
smaller than the critical liquid absorption of said powder, and said
primary particles are granulated into particles having an average particle
size of 0.1 to 2 mm by fusion bonding of the surface treatment layers of
the organic solid binder or dispersion medium.
In accordance with another aspect of the present invention, there is
provided a process for the preparation of a granular stabilizer for
chlorine-containing polymers, which comprises the steps of mixing a hardly
fusible or infusible powdery stabilizer for chlorine-containing polymers,
or a mixture of said powdery stabilizer with a powdery stabilizer aid,
under pulverizing conditions with an organic solid binder or dispersion
medium in an amount of 2 to 15 parts by weight per 100 parts by weight of
said powdery stabilizer or said powdery mixture, which is smaller than the
critical liquid absorption of said powder, to surface-treat respective
primary particles of said powder with said organic solid binder or
dispersion medium, and granulating the surface-treated powder into
particles having an average particle size of 0.1 to 2 mm at a temperature
higher than the melting point of the organic solid binder or dispersion
medium.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a microscope photograph (600 magnifications) illustrating the
particle structure of tribasic lead sulfate used in Example 1.
FIG. 2 is a microscope photograph (600 magnifications) illustrating the
particle structure of a product obtained by subjecting tribasic lead
sulfate shown in FIG. 1 to a pulverizing treatment without addition of an
organic solid binder or dispersion medium.
FIG. 3 is a microscope photograph (600 magnifications) illustrating a
surface-treated powder (Example 1) obtained by subjecting tribasic lead
sulfate shown in FIG. 1 to a pulverizing treatment in the presence of lead
stearate.
FIG. 4 is a microscope photograph (60 magnifications) illustrating the
particle structure of the granular stabilizer obtained in Example 1.
FIG. 5 is a microscope photograph (60 magnifications) illustrating the
surface dispersion state in a vinyl chloride resin sheet in which the
granular stabilizer of Example 1 is incorporated.
FIG. 6 is a microscope photograph (60 magnifications) illustrating the
surface dispersion state in a vinyl chloride resin sheet in which the
stabilizer of Comparative Example 1C-1 is incorporated.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
One of important features of the present invention is based on the novel
finding that when a hardly fusible or infusible powdery stabilizer for
chlorine-containing polymers or a mixture of said powdery stabilizer and a
powdery stabilizer aid (often referred to as "powdery stabilizer"
hereinafter) is mixed under pulverizing conditions with an organic solid
binder or dispersion medium melting at the processing temperature for
chlorine-containing polymers (often referred to as "binder medium"
hereinafter), respective primary particles of said powder are effectively
surface-treated with the binder medium even if the amount of the binder
medium is 2 to 15 parts by weight, especially 4 to 10 parts by weight, per
100 parts by weight of the powdery stabilizer and is smaller than the
critical liquid absorption of the stabilizer powder.
According to any of the conventional techniques of granulating stabilizers,
secondary particles of a powdery stabilizer are integrated with the
granulation product through a binder medium acting also as the dispersion
medium while retaining the form of secondary particles. In contrast, in
the granular stabilizer of the present invention, the powdery stabilizer
is disintegrated into primary particles and the respective primary
particles are surface-treated with the binder medium. This surface
treatment is different from the so-called surface treatment of a metal and
the treatment of surfaces of particles is meant. Accordingly, fine
covering or coating of the binder medium on the surfaces of respective
primary particles is caused, but the surface-treated product as a whole is
still in the powdery state and is different from a granular product.
In the present invention, by surface-treating respective primary particles
of the powdery stabilizer with the binder medium, even in this powdery
state the surface-treated stabilizer is improved over the untreated
powdery stabilizer in its surface characteristics and various powder
characteristics.
For example, as illustrated in examples given hereinafter, if the surface
treatment is carried out by mixing tribasic lead sulfate with a wax under
pulverizing conditions, the surface-treated powder becomes water-repellent
and the surface characteristics are greatly changed. Furthermore, the
surface-treated powder has an angle of repose much smaller than the
untreated powder and its flowability is highly improved. Moreover, by this
surface treatment, the dispersibility is prominently increased when it is
incorporated into a chlorine-containing polymer. It is deemed that this
effect is attained because strong aggregation of primary particles of the
powdery stabilizer is prevented by the surface treatment covering
described above, the surface characteristics of the powdery stabilizer are
improved and the binder medium present in the form of the surface
treatment covering promotes the dispersion of the powdery stabilizer in
the form of primary particles.
Furthermore, if the powdery stabilizer is mixed with the binder medium
under pulverizing conditions to surface-treat the powdery stabilizer in
the form of primary particles with the binder medium, the composition of
the powdery stabilizer and the binder medium is very homogeneous even
microscopically, and segregation is completely prevented during
transportation, storage or granulation. The composition of the final
granular stabilizer is kept homogeneous in respective particles, among
particles and among production lots.
Another characteristic feature of the present invention is that the powdery
stabilizer is surface-treated with the binder medium in an amount smaller
than the critical liquid absorption and granulation is effected with a
very small amount of this binder medium used for the surface treatment.
The critical liquid absorption referred to in the instant specification and
appended claims is determined according to the method for determining the
oil absorption except that the critical liquid absorption is determined on
a specific combination of the powdery stabilizer and binder medium used
and the measurement is carried out in the state where the binder medium is
in the form of a molten liquid. More specifically, the binder medium is
added little by little to 100 g of the powdery stabilizer and the state of
the mixture is observed while the mixture is kneaded at a temperature
higher than the melting point of the binder medium. The point at which the
particles of the powdery stabilizer are formed into one mass from the
disintegrated state is found and the amount (g) of the binder medium added
to this point is defined as the critical liquid absorption.
This critical liquid absorption has close relations to the characteristics
of the powdery stabilizer used, for example, the particle size, particle
structure, particle size distribution and bulkiness, and the melt specific
gravity of the binder medium. A powdery stabilizer having a large bulk has
a large critical liquid absorption, and as the melt specific gravity of
the binder medium is large, the critical liquid absorption tends to
increase.
In view of the foregoing, in the instant specification and appended claims,
the value obtained by multiplying an oil absorption (ml/100 g), measured
by using linseed oil, by the melt specific gravity (g/ml) of a binder
medium used is adopted as the critical liquid absorption.
In order to surface-treat the powdery stabilizer in the form of primary
particles with the binder medium, it is surprisingly important that the
binder medium should be used in an amount of 2 to 15 parts by weight,
especially 4 to 10 parts by weight, per 100 parts by weight of the powdery
stabilizer, which is smaller than the critical liquid absorption. Namely,
if the binder medium is used in an amount larger than 15 parts by weight
or larger than the critical liquid absorption, granulation is readily
caused, even at the pulverizing and mixing step, and it becomes difficult
to effect the surface treatment while disintegrating the powdery
stabilizer into primary particles. If the amount of the binder medium is
too small and below the above-mentioned range, however efficiently the
pulverizing mixing may be carried out, it is difficult to uniformly
surface-treat the respective primary particles, and also the granulation
becomes difficult.
According to the conventional common sense, in order to perform the
granulation by using a solid binder medium and melting this solid binder
medium, it will be considered necessary to use the solid binder medium in
an amount exceeding the critical liquid absorption. However, according to
the present invention, only by surface-treating the powdery stabilizer in
the form of primary particles with a solid binder medium in an amount
smaller than the critical liquid absorption, preferably in an amount
corresponding to 15 to 80%, especially 35 to 65%, of the critical liquid
absorption, does it becomes possible to obtain a granular product having a
homogeneous composition without causing scattering or blocking,
Furthermore, since the amount of the binder medium added such as a wax is
reduced, such troubles as blooming and precipitation can be effectively
eliminated and a resin composition in which the granular stabilizer is
incorporated is prevented from being excessively lubricative.
In the present invention, since the solid binder medium is used in an
amount smaller than the above-mentioned critical liquid absorption for the
powdery stabilizer, pulverizing mixing of these components and subsequent
granulation are very important.
For example, if a mixture containing the powdery stabilizer and the solid
binder medium in an amount smaller than the critical liquid absorption is
directly subjected to rolling granulation at a temperature higher than the
melting point of the binder medium, extreme segregation is caused between
the powdery stabilizer and the powdery binder medium, and, in the binder
medium-rich portion, aggregation of particles of the binder medium and
growth of these particles are readily caused. Accordingly, by this
granulation method, there are formed a small amount of the binder
medium-rich granular product and a large amount of the powdery stabilizer
containing a very small amount of the binder medium, each having a
composition quite different from the starting recipe. Accordingly, the
composition of the obtained granular product is extremely heterogeneous
and extremely different from the starting recipe, and the granulation
efficiency is extremely insufficient.
In contrast, if pulverizing mixing of the powdery stabilizer with the
powdery binder medium is carried out prior to granulation according to the
present invention, at the subsequent rolling granulation under shearing,
segregation is prevented between the powdery stabilizer and the powdery
binder medium and a granular product having the same composition as the
starting recipe and being excellent in the flowability, the dispersibility
and the resistance to scattering can be obtained at a high efficiency.
Stabilizers and stabilizer aids which satisfy the abovementioned
requirements may be selected from those customarily used in this field and
may be used in the present invention. Preferred examples will now be
described though the stabilizers and stabilizer aids that can be used in
the present invention are not limited to those exemplified below.
I. Stabilizers
1. Lead salts, especially basic lead salts represented by the following
formula:
nPbO.PbX.sub.2/m (1)
wherein n is a number of from 0.5 to 5, especially from 1 to 4,
X stands for an inorganic acidic oxide anion (especially an anion of an
oxyacid of phosphorus, sulfur or carbon) or an organic anion (especially a
carboxylic acid anion),
and m stands for the valency of the anion X,
such as mono- to tetra-basic lead sulfates, dibasic lead sulfite, dibasic
lead phosphite, white lead, basic lead phosphate, basic lead acetate,
dibasic lead stearate, basic lead maleate, dibasic lead phthalate and
basic lead tartrate.
Lead silicate having a composition represented by the following formula:
PbO.pSiO.sub.2 (2)
wherein p is a number of from 0.1 to 10, especially from 0.5 to 5,
and lead oxides such as litharge, massicot, red lead and minium.
2. Hydroxides, basic salts and silicates of alkaline earth metals and zinc:
Hydroxides such as calcium hydroxide, magnesium hydroxide, zinc hydroxides
and strontium hydroxides.
Basic salts having a composition represented by the following formula
(oxide base):
MO.qMX.sub.2/m (3)
wherein M stands for an alkaline earth metal or zinc,
X and m are as defined in the formula (1),
and q is a number of from 0.1 to 10, especially from 0.5 to 5,
such as basic calcium carbonate, basic magnesium carbonate, basic calcium
stearate, basic zinc stearate and basic magnesium palmitate.
Silicates having a composition represented by the following formula (oxide
base):
MO.kSiO.sub.2 (4)
wherein M stands for an alkaline earth metal or zinc,
and k is a number of from 0.1 to 10, especially from 0.5 to 5.
Aluminosilicates, for example, synthetic zeolites such as zeolite A,
zeolite X and zeolite Y, and partially or completely neutralized products
and metal ion-exchanged products thereof (for example, calcium, magnesium
or zinc ion-exchanged products).
3. Others:
Sodium sulfite, sodium thiosulfate, disodium phosphate, calcium sulfite and
calcium monohydrogen-phosphate.
II. Stabilizer Aids
As the stabilizer aid, there can be used inorganic stabilizers such as
calcium carbonate, magnesium carbonate, aluminum hydroxide and
hydrotalcite. Furthermore, the following organic stabilizers and
stabilizer aids can be used.
1. Polyhydric alcohols:
Monopentaerythritol, dipentaerythritol, other polypentaerythritols,
mannitol, sorbitol, glucose, fructose, trimethylol propane, polyethylene
glycol having a molecular weight of, for example, 200 to 9,500,
polypropylene glycol having a molecular weight of, for example, at least
1,000, polyoxyethylene-polyoxypropylene block polymers having a molecular
weight of, for example, 1,900 to 9,000, and adducts of ethylene oxide or
propylene oxide to a polyhydric alcohol such as glycerin, pentaerythritol,
sorbitol or the like.
2. Phenols:
Alkyl gallates such as propyl, octyl and dodecyl gallates, guaiac fat,
p-methoxyphenol, alkylated phenol, styrenated phenol, styrenated cresol,
butylstyrenated cresol, phenylphenol, methyl salicylate, phenyl
salicylate, p-tert-butylphenyl salicylate, methyl p-hydroxybenzoate,
propyl p-hydroxybenzoate, hydroquinone monobenzyl ether, butylated
hydroxytoluene, 2,5-di-tert-anyl hydroquinone,
4-hydroxymethyl-2,6-di-tert-butylphenol, 2,6-di-tert-p-phenylphenol,
4,6-dinonyl-o-cresol, butylated hydroxyanisole, 2,2'-bis(4-hydroxy)propane
(bisphenol A), polybutylated 4,4'-isopropylidene diphenol,
2,2'-methylene-bis(4-methyl-6-tert-butylphenol),
2,2'-methylene-bis(4-ethyl-6-tert-butylphenol),
2,2'-methylene-bis(4-methyl-6-tert-octylphenol),
2,2'-methylene-bis(4-methyl-6-nonylphenol),
2,2-methylene-bis[6-(1-methylcyclohexyl)-p-cresol], a mixture of
2,2'-methylene-bis(4-methyl-6-nonylphenol) and
2,6-bis(2-hydroxy-3-nonyl-5-methylbenzyl)p-pcresol,
4,4'-butylidene-bis(6-tert-butyl-m-cresol),
tris(2-methyl-4'-hydroxy-5-tert-butylphenyl)butane,
tetrakis[methylene-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]meth
ane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene,
n-octadecyl-.beta.-(4'-hydroxy-3',5'-di-tert-butylphenyl)propionate,
N-n-alkyl-N'-(carboxymethyl)-N,N'-trimethylenediglycine,
2-n-octyl-thio-4,6-di-(4'-hydroxy-3',5'-di-tert-butyl)phenoxy-1,3,5-triazi
ne, thio-bis[methylbutylphenol], 4,4'-thio-bis[6-tert-butyl-m-cresol],
2,4,5-trihydroxybutyrophenone, 2-hydroxy-4-methoxybenzophenone,
2-hydroxy-4-n-octoxybenzophenone, 2,2'-dihydroxy-4-methoxybenzophenone,
2-hydroxy-4-methoxy-2'-carboxybenzophenone,
2-(2'-hydroxy-5'-methylphenyl)benzotriazole, 2-(2'-hydroxy-3'
-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole and
2-(2'-hydroxy-3',5'-di-tert-butylphenyl)-5-chlorobenzotriazole.
3. Nitrogen Compounds:
Ammonium higher fatty acid salts, ammonium citrate, urea,
N,N-diphenylthiourea, hexamethylene tetraamine, 2-phenylindole, esters of
.beta.-aminocrotonic acid with such alcohols as 1,3-butane diol,
1,4-butane diol, 1,2-dipropylene glycol, thiodiethylene glycol and linear
saturated alcohols, N,N'-diphenylethylenediamine,
N,N'-disalicylal-propylene-diamine and N,N'-di-o-toluyl-ethylene-diamine.
4. Phosphorus Compounds
Tris(nonylphenyl)phosphite,
2-tert-butyl-.alpha.-(3-tert-butyl-4-hydroxyphenyl)-p-cumenyl-bis(p-nonylp
henyl)phosphite, hydrogenated 4,4'-isopropylidene-diphenyl phosphate and
tris(nonylphenyl)phosphite-formaldehyde condensate.
5. Sulfur Compounds
Thiodipropionic acid, diethyl thiodipropionate, dilauryl thiodipropionate,
dimyristyl thiodipropionate, distearyl thiodipropionate, laurylstearyl
thiodipropionate and stearyl-(3,5-dimethyl-4-hydroxybenzyl)thioglycolate
phenothiazine.
The mixing ratio of the stabilizer to the stabilizer aid can be changed
within a broad range, but ordinarily, the stabilizer aid may be used in an
amount of 0.1 to 100 parts by weight per 100 parts by weight of the
stabilizer.
Lubricants and lubricating stabilizers satisfying the above-mentioned
requirements are used as the organic solid binder or dispersion medium.
Furthermore, they may be used in combination with a chlorine-containing
polymer or modifier resin. Suitable examples are described below.
I. Lubricating Stabilizers
High fatty acid salts (especially salts of saturated fatty acids having 8
to 22 carbon atoms), resin acid salts and montanic acid salts of various
metals, for example, metals of the group II of the Periodic Table such as
calcium, zinc, cadmium and magnesium, and aluminum and lead.
For example, there can be mentioned lead stearate, lead laurate, lead
palmitate, lead montanate, lead abietate, calcium montanate, calcium
stearate, magnesium stearate, zinc stearate and cadmium stearate.
II. Lubricants
1. Aliphatic Hydrocarbon Type Lubricants
Synthetic paraffin, petroleum wax, petrolatum, polyethylene wax and
.alpha.-olefin wax.
2. Fatty Acids
Higher fatty acids such as fatty acids derived from animal and vegetable
oils and fats and hydrogenated products thereof, which have 12 to 22
carbon atoms.
3. Amides and Amines of Higher Fatty Acids
Oleylpalmitamide, 2-stearomidoethyl stearate, ethylene-bisfatty acid
amides, N,N'-oleoylstearylethylene diamine,
N,N'-bis(2-hydroxyethyl)alkyl(C.sub.12 -C.sub.18)amides,
N,N'-bis(hydroxyethyl)lauroamide, oleic acid reacted with N-alkyl(C.sub.16
-C.sub.18)trimethylene diamine, fatty acid diethanol amines and
di-(hydroxydiethyl)-diethylene-triamine monostearate ester of distearic
acid.
4. Monohydric Alcohol and Polyhydric Alcohol Esters of Fatty Acids
n-Butyl stearate, hydrogenated methyl resinate, di-n-butyl sebacade,
dioctyl sebacate, pentaerythritol tetrastearate, sorbitol fatty acid
esters, polyethylene glycol fatty acid esters, polyethylene glycol
monostearate, polyethylene glycol dilaurate, polyethylene glycol
mono-oleate, polyethylene glycol dioleate, polyethylene glycol coconut
fatty acid diester, polyethylene glycol tall oil fatty acid diester,
ethane diol montanic acid ester, 1,3-butane diol montanic acid diester,
diethylene glycol distearate, and propylene glycol fatty acid diesters.
5. Triglycerides and Waxes
Hydrogenated edible oils and fats, hydrogenated cotton seed oil,
hydrogenated linseed oil, palm oil, glycerin 12-hydroxystearate,
hydrogenated fish oils, beef tallow, spermaceti wax, montan wax, carnauba
wax, beeswax, haze wax, monohydric aliphatic alcohol-aliphatic saturated
acid esters such as hardened sperm oil lauryl stearate and stearyl
stearate, and lanolin.
6. Alkali Metal Salts of Higher Fatty Acids
Sodium soaps.
7. Others
Propylene glycol alginate and dialkylketone.
A resin modifier such as a copolymer of vinyl chloride and/or vinylidene
chloride with at least one comonomer selected from mono-olefins such as
ethylene, propylene and styrene, diolefins such as butadiene,
ethylenically unsaturated acids such as acrylic acid, methacrylic acid and
maleic acid, esters, amides and anhydrides thereof, vinyl esters such as
vinyl acetate, and acrylonitrile may be added to a chlorine-containing
polymer together with the granular stabilizer of the present invention.
Mixing of the powdery stabilizer with the binder medium under pulverizing
conditions can be performed according to various methods. For example, in
the case of wet pulverizing mixing, the powdery stabilizer and the binder
medium are mixed under pulverization in the presence of a solvent, and in
the case of dry pulverizing mixing, the powdery stabilizer is dry-blended
with the powdery binder medium in the absence of a solvent or the like
under pulverization.
In each case, it is important that pulverization should be effected so that
the powdery stabilizer is disintegrated into primary particles. For this
purpose, there may be used a crusher, a sand grinder mill, an attriter, a
high-speed shearing stirrer, a Tokyo atomizer, a Nara type pulverizer, a
disc shaking mill, a vibrating ball mill and a rotary ball mill. Of
course, two or more of these pulverizing means may be used in combination.
In these pulverizing mixers, the powdery stabilizer is disintegrated into
primary particles and simultaneously, the primary particles are
surface-treated with the binder medium.
In the case of the former wet pulverizing mixing treatment, the binder
medium is dissolved or dispersed in a solvent, the powdery stabilizer is
dispersed in the resulting liquid to form a slurry, the slurry is supplied
into a pulverizing mixer as mentioned above, pulverizing mixing is
sufficiently carried out, and the solvent is distilled off under mixing or
stirring conditions to obtain a surface-treated powder.
In order to prevent aggregation of the stabilizer particles, it is
preferred that a non-polar solvent be used as the solvent, and a solvent
capable of partially or completely dissolving the binder medium therein is
especially preferred. Accordingly, or aromatic solvents such as benzene,
toluene and xylene, aliphatic solvents such as n-hexane, n-heptane butane
and cyclohexane may be used, though the present invention is not limited
to these specific examples. The solvent may be used in an amount of 15 to
150 parts by weight per 100 parts by weight of the powdery stabilizer.
In the latter dry pulverizing mixing treatment, metered amounts of the
powdery stabilizer and the powdery binder medium are supplied into a
pulverizing mixer as mentioned above, pulverizing mixing is carried out at
a temperature lower than the melting point of the binder medium, and the
resulting surface-treated powder is taken out from the mixer.
The degree of the pulverizing mixing depends on the intensity of
pulverization and it is difficult to generally define the degree of the
pulverizing mixing. However, since the degree of the surface treatment can
be determined by testing the surface characteristics or the particle
characteristics such as the dispersibility and flowability as pointed out
hereinbefore, the treatment degree may be determined according to the kind
of mixer used based on the results of these tests.
For example, when tribasic lead sulfate is mixed with lead stearate under
pulverization, it is confirmed that with advance of the surface treatment
of particles, (1) the particle size distribution is shifted toward the
smaller particle size side, (2) the average particle size is reduced, (3)
the shape of particles is changed to a spherical shape from a needle-like
shape, (4) the water repellency is increased, (5) the angle of repose is
decreased, (6) the dispersibility in a vinyl chloride resin is prominently
improved and (7) the X-ray diffraction intensity is hardly changed.
The above-mentioned wet pulverizing mixing is advantageous in that complete
covering of the primary particles with the binder medium is easily
accomplished. However, this mixing method is disadvantageous in that the
cost is increased by use of the solvent and removal of the solvent is
necessary after mixing, and aggregation of the primary particles tends to
occur when heating is carried out for removal of the solvent. In contrast,
the dry pulverizing mixing is very advantageous because the treatment is
very simple, though covering of primary particles tends to be incomplete.
In the present invention, the surface-treated powder is granulated at a
temperature higher than the melting point of the organic solid binder or
dispersion medium to obtain particles having an average particle size of
0.1 to 2 mm.
Various known granulation means may be adopted. However, since only a very
small amount of the binder medium is contained in the surface-treated
powder, the granulation product may preferably be obtained by the rolling
granulation method. Not only an ordinary rolling granulator but also a
mixer type granulator such as a Henschel mixer or a super mixer may be
used for the rolling granulation. In the former apparatus, the machine
wall is moved and in the latter apparatus, the powder is moved. However,
both the apparatuses are in agreement with each other in the point where
granulation is accomplished by the relative movement between the machine
wall and the powder.
It is very important that this rolling granulation should be carried out at
a temperature higher than the melting point of the binder medium. If this
temperature is lower than the melting point of the binder medium,
granulation is not accomplished at all, or the conversion to the
granulation product is extremely low even if granulation is accomplished
to some extent.
In the granulation according to the present invention, there is no
difference of the composition between the granulation product and the
ungranulated powder. Accordingly, since the granulation product having a
predetermined particle size, which is formed at the granulating step, is
recovered by sieving while the ungranulated powder is recycled to the
granulator, all of the surface-treated powder can be finally recovered in
the form of the granulation product. This is one of the prominent
advantages attained in the present invention.
Of course, in the present invention, the granulation is not limited to the
above-mentioned rolling granulation. For example, the granulation product
can be prepared by kneading and extruding the surface-treated powder at a
temperature higher than the melting point of the binder medium by a
pelletizer or the like. Moreover, the intended granulation product can be
obtained according to the fluidized bed granulation method or the like.
As is apparent from the foregoing description, in the granular stabilizer
of the present invention, the primary particles of the powdery stabilizer
are surface-treated with the organic solid binder or dispersion medium,
and therefore, the granular stabilizer of the present invention has
various advantages not attained in the conventional granular stabilizers.
The granular stabilizer of the present invention may be incorporated into
vinyl chloride resins, various vinyl chloride copolymers, chlorinated
polyethylene, chlorinated polypropylene and chlorinated vinyl chloride
resins in amounts of 1 to 10 parts by weight per 100 parts by weight of
the chlorine-containing polymer to improve the thermal stability.
The present invention will now be described in detail with reference to the
following Examples that by no means limit the scope of the invention.
Incidentally, in these Examples, all of "parts" and "%" are by weight
unless otherwise indicated.
EXAMPLE 1
A granular stabilizer was prepared by selecting and using commercially
available powdery tribasic lead sulfate ("STABINEX Tc" supplied by
Mizusawa Kagaku Kogyo) as the powdery stabilizer.
The physical properties of the powdery tribasic lead sulfate
(3PbO.PbSO.sub.4.H.sub.2 O) used were as follows.
PbO content: 88.6%
SO.sub.3 content: 7.88%
H.sub.2 O content: 1.78%
Fatty acid content: 1.54%
Bulk specific gravity: 2.6 cc/g
Average crystal size: 8.7.mu.
Critical liquid absorption: 10.3 g/100 g
Specific gravity: 7.1
Refractive index: 2.1
Water content (after drying at 110.degree. C.): 0.2%
Commercially available powdery lead stearate ("STABINEX NC18" supplied by
Mizusawa Kagaku Kogyo) was selected and used as the binder medium. The
physical properties of this binder medium were as follows.
PbO content: 29.8%
Fatty acid content: 70.2%
Bulk specific gravity: 1.25 cc/g
Specific gravity: 1.32
Melting point: 108.degree. C.
A dispersing stirrer customarily used in the field of paints (attriter
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