U.S. patent application number 09/832849 was filed with the patent office on 2002-04-11 for process for producing silica particles suitable for use as filler for paper.
This patent application is currently assigned to Oji Paper Co., Ltd.. Invention is credited to Kitao, Osamu, Matsuda, Masashi, Okada, Hitoshi, Wada, Motohide, Watanabe, Masasuke.
Application Number | 20020040773 09/832849 |
Document ID | / |
Family ID | 27297411 |
Filed Date | 2002-04-11 |
United States Patent
Application |
20020040773 |
Kind Code |
A1 |
Matsuda, Masashi ; et
al. |
April 11, 2002 |
Process for producing silica particles suitable for use as filler
for paper
Abstract
The present invention relates to a process for easily and
efficiently producing silica particles having a narrow particle
size distribution and a high porosity from inexpensive starting
materials such as sodium silicate. The silica particles of the
present invention can be obtained in the form of a slurry
containing them by (1) forming a slurry by mixing first particles
difficultly soluble in an alkali and soluble in an acid, with an
aqueous alkali silicate solution to form a first slurry containing
the first particles, (2) neutralizing the first slurry with a
mineral acid to prepare a second slurry containing second particles
wherein silica is deposited on the first particles, and (3) adding
a mineral acid to the second slurry to dissolve the first particles
from the second particles, to prepare a third slurry containing
silica particles. When the silica particles of the present
invention are used as a filler even in a small amount for paper
making, the resultant papers have excellent brightness, opacity,
opacity-after-printing, etc.
Inventors: |
Matsuda, Masashi;
(Yokohama-shi, JP) ; Watanabe, Masasuke;
(Kawasaki-shi, JP) ; Okada, Hitoshi; (Setagaya-ku,
JP) ; Wada, Motohide; (Shinjuku-ku, JP) ;
Kitao, Osamu; (Yokohama-shi, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
Oji Paper Co., Ltd.
Tokyo
JP
|
Family ID: |
27297411 |
Appl. No.: |
09/832849 |
Filed: |
April 12, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09832849 |
Apr 12, 2001 |
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09266575 |
Mar 11, 1999 |
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6264907 |
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Current U.S.
Class: |
162/181.6 ;
423/325; 423/331; 423/333 |
Current CPC
Class: |
C01P 2004/61 20130101;
D21H 21/52 20130101; C01B 33/187 20130101; C01P 2006/60 20130101;
C01P 2004/52 20130101; C01P 2006/10 20130101; C01B 33/18 20130101;
D21H 17/68 20130101; C01P 2006/12 20130101; C01P 2006/19 20130101;
C01P 2006/16 20130101; C01P 2006/14 20130101 |
Class at
Publication: |
162/181.6 ;
423/325; 423/331; 423/333 |
International
Class: |
D21H 011/00; D21H
017/68; C01B 033/32; C01B 033/24; C01B 033/12 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 12, 1998 |
JP |
10-061166 |
Apr 16, 1998 |
JP |
10-121636 |
Apr 24, 1998 |
JP |
10-129697 |
Claims
What is claimed is:
1. A process for producing silica particles; comprising the steps
of: (1) mixing an aqueous alkali silicate solution with first
particles difficultly soluble in an alkali and soluble in an acid,
so as to form a first slurry containing said first particles; (2)
neutralizing said first slurry with a mineral acid to prepare a
second slurry containing second particles wherein silica is
deposited on said first particles; and (3) adding a mineral acid to
said second slurry to dissolve said first particles and form a
third slurry containing silica particles.
2. The process of claim 1, wherein said first particles are made of
one member selected from the group consisting of metals, metal
salts, metal oxides, metal hydroxides and organic materials.
3. The process of claim 2, wherein a metal is selected from the
group consisting of K, Rb, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni and
Zn.
4. The process of claim 2, wherein said first particles are made of
one member selected from the group consisting of calcium carbonate,
barium carbonate, magnesium carbonate, nickel carbonate, potassium
manganate, magnesium oxide, zinc oxide, calcium oxide, manganese
oxide, magnesium hydroxide, calcium hydroxide and manganese
hydroxide.
5. The process of claim 1, wherein the amount of said first
particles is 5 to 120% by weight based on the solid (in terms of
silica) in said aqueous alkali silicate solution.
6. The process of claim 1, wherein the amount of said first
particles is 10 to 60% by weight based on the solid (in terms of
silica) in said aqueous alkali silicate solution.
7. The process of claim 1, wherein said first particles have an
average particle diameter of 0.01 to 10 .mu.m.
8. The process of claim 1, wherein said first particles have an
average particle diameter of 0.1 to 5 .mu.m.
9. The process of claim 1, wherein said third slurry in step (3)
has a pH of 2 to 6.5.
10. The process of claim 1, further including, after step (1),
heating said first slurry to a temperature in the range of
70.degree. C. to the boiling point of said slurry.
11. The process of claim 10, wherein, in step (2), 10 to 50% of
said mineral acid necessitated for neutralizing said alkali
silicate solution is added to said alkali silicate solution at 20
to 60.degree. C., then the resultant slurry is heated to a
temperature of 70.degree. C. or higher, and the balance of said
mineral acid necessitated for the neutralization is added.
12. The process of claim 1, wherein said third slurry is ground
and/or classified by a wet method, after step (3).
13. A filler-containing paper containing said silica particles
obtained by the process of claim 1 as the filler.
14. Silica particles having a cumulative volume of 4.0 to 6.0 cc/g
for pores having a diameter of 10.sup.5 .ANG. or less, that of at
least 2.0 cc/g for pores having a diameter of 6,000 to
8.times.10.sup.4 .ANG., that of at least 1.0 cc/g for pores having
a diameter of 200 to 2,000 .ANG., an oil absorption of 300 to 500
ml/100 g, and a bulk specific gravity of 0.1 g/ml or less.
15. The silica particles of claim 14, having a specific surface
area of 30 to 200 m.sup.2/g.
16. A filler-containing paper containing the silica particles of
claim 14 as the filler.
17. A process for producing a filler-containing paper, comprising
adding a slurry of silica particles to a pulp slurry and using the
resultant slurry for making a sheet of paper, said silica particles
having an average diameter of 5 to 30.mu.m as determined by laser
method and a standard deviation of 0.10 to 0.25 in respect of a
particle volume distribution to a particle diameter (.mu.m)
represented by logarithm.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to porous silica particles
particularly suitable for use as a filler for papers, a process for
producing them and the use thereof for producing filler-containing
papers.
TECHNICAL BACKGROUND OF THE INVENTION
[0002] Papers used for printing or writing usually contain, as
fillers, inorganic particles of silica, hydrous silicic acid, talc,
calcium carbonate, clay, kaolin, titanium dioxide, etc. and organic
particles of urea/formaldehyde polymers or the like for improving
the optical properties such as opacity and brightness, smoothness,
touch, printability, writing suitability, etc.
[0003] Papers containing the above-described fillers are produced
by adding the fillers and other assistants usually used for making
paper, to paper pulp dispersed in water, forming a wet paper from
the obtained stuff with a Fourdrinier paper making machine,
twin-wire paper-making machine or the like and drying it.
[0004] Recently, the thickness of the paper sheets tends to be
reduced to reduce the basis weight thereof. However, particularly
when printing paper sheets are reduced in weight, the opacity of
the printed paper (hereinafter referred to as
"opacity-after-printing") is reduced to cause a problem that the
printed letters on the reverse side of the paper sheets are seen
through the paper from the surface of the paper.
[0005] Various fillers are usually added to the papers for the
purpose of improving the opacity (including the
opacity-after-printing) of the papers.
[0006] Although investigations have been conducted for the purpose
of developing inorganic and organic fillers for improving the
opacity, inexpensive fillers having an excellent effect of
improving the opacity have not yet been developed. Further, since
the tendency to the further reduction in weight is recently
increasing, the development of a filler having a higher power of
improving the opacity than that of ordinary fillers is eagerly
demanded.
[0007] Among the fillers currently used for improving the opacity,
titanium dioxide has only a low power of inhibiting the penetration
of inks, while it is capable of improving the opacity of white
papers. Therefore, the improvement in opacity-after-printing is
impossible when titanium dioxide is used. Further, under such
conditions that the maximum light scattering capacity can be
exhibited, the retention of titanium oxide in the paper is
uneconomically very low.
[0008] Although organic urea/formalin resins have effects of
improving both opacity-after-printing and opacity of white paper
(opacity before printing), each absolute effect is
insufficient.
[0009] Hydrous silicic acid is less expensive than the other
fillers and it is relatively effective in imparting the
opacity-after-printing to a paper by inhibiting the penetration of
inks when it is added to a pulp and the paper is made therefrom.
However, its effects including that of improving the opacity of
white paper have not yet reached the expected level.
[0010] As for hydrous silicic acid, it is known that the oil
absorption, which is an index of the capacity of preventing ink
from the penetration and greatly contributes to the improvement in
the opacity-after-printing, is increased in proportion to the
specific surface area of hydrous silicic acid and can be controlled
by changing the hydrous silicic acid synthesis conditions.
[0011] However, when the specific surface area of hydrous silicic
acid is increased to a level higher than that of the ordinary one
and the paper is dried under the same conditions as the
paper-making conditions, the filler itself is shrunk to reduce the
oil absorption and, as a result, the improvement in the
opacity-after-printing is reduced.
[0012] Amorphous silica particles disclosed in Japanese Patent No.
2,604,316 have a high oil absorption and not so large specific
surface area. However, the effect thereof on the opacity obtained
by adding these particles to paper is not significantly different
from that by adding hydrous silicic acid commonly used at
present.
[0013] Japanese Patent Unexamined Published Application
(hereinafter referred to as "J. P. KOKAI") No. Hei 5-301707
discloses hydrous silicic acid of the formula: SiO.sub.2.nH.sub.2O
(wherein n is a positive integer) wherein the cumulative volume is
1.9 to 4.0 cc/g for pores having a pore radius in the range of
5.times.10.sup.4 .ANG. or less, that is at least 0.5 cc/g for pores
having a pore radius in the range of 3,000 to 4.times.10.sup.4
.ANG., and that is at least 0.6 cc/g for pores having a pore radius
in the range of 100 to 1000 .ANG.. However, the absolute volume for
the pores is small because the cumulative volume of pores having a
pore radius of 5.times.10.sup.4 .ANG. or smaller is up to 4.0 cc/g.
In addition, the cumulative volume of the pores having a pore
radius of 3,000 to 4.times.10.sup.4 .ANG. and capable of absorbing
the pigment in the ink and also that of the pores having a pore
radius of 100 to 1,000 .ANG. and capable of absorbing the vehicle
in the ink are yet below the necessary levels.
[0014] Japanese Patent No. 2,710,529 discloses a hydrous silicic
acid filler for paper making, which is fine, amorphous hydrous
silicic acid obtained by the neutralization reaction of an aqueous
sodium silicate solution in the absence of alkali metal salts, and
which contains at least amorphous magnesium silicate as a fine,
amorphous metal compound. However, when the amorphous metal
compound content is increased for improving the opacity, the oil
absorption is reduced to impair the power of inhibiting the
penetration of the ink and, as a result, the improvement in the
opacity-after-printing is unsatisfactory.
[0015] The diameters of primary particles usually and widely used
are very small. Although the particle diameters are relatively
uniform, they are not in the form of the primary particles, but
they form aggregates as secondary particles in most cases, and the
particle diameter distribution is usually wide when the particles
are used. In addition, even when the average particle size is
almost equal, the state of the distribution is different. It is
known that when particles of a small diameter are thoroughly
dispersed in a paper, the contribution of the particles on the
optical properties of the paper is greater than that of particles
of a large diameter or a small diameter retained in the paper with
a reagent such as a retaining improver.
[0016] However, the particle size of the filler used ranges in a
wide range as described above. When such a filler of various
particle sizes is added to a paper-making pulp and a paper is made
therefrom, the retaining rate (or retention) of the particles of
small sizes is usually and seriously low, though it varies
depending on the paper-making machine. For improving the optical
properties, the addition rate of the filler must be increased.
However, the improvement in the optical properties by merely
increasing the addition rate is limited because when the addition
rate of the filler is increased, the strength of the paper is
lowered. On the other hand, although the particles of a large size
are retained in the paper, they also have problems that the
specific surface area of them is small and the contribution of them
to the optical properties is only slight. Under these
circumstances, it is desired to easily obtain a filler having a
uniform particle size.
SUMMARY OF THE INVENTION
[0017] Therefore, the object of the present invention is to provide
a process for easily and efficiently producing silica particles
having a high porosity from inexpensive sodium silicate or the
like.
[0018] Another object of the invention is to provide silica
particles capable of imparting excellent brightness, opacity,
opacity-after-printing, etc. to a paper obtained by using such
particles as a filler in the paper making.
[0019] Still another object of the invention is to provide a filler
comprising silica particles having an opacity superior to that of
another filler when they are used in the same amount.
DETAILED DESCRIPTION OF THE INVENTION
[0020] After intensive investigations made for the purpose of
attaining the above-described object, the inventors have found that
silica particles with a high porosity can be obtained by depositing
silica from an aqueous alkali silicate solution on first particles
difficultly soluble in an alkali and soluble in an acid to obtain a
slurry containing second particles wherein silica is deposited on
the first particles and then dissolving the first particles from
the second particles with a mineral acid. The present invention has
been completed on the basis of this finding. The inventors have
also found that the opacity can be improved by narrowing the range
of the particle size distribution even when the average particle
size is on the same level or, in particular, by using silica
particles having a very uniform size and a particle size
distribution very close to a single peak. The present invention has
been completed on the basis of these findings.
[0021] The detailed description will be made on the present
invention.
[0022] The silica particles of the present invention can be
obtained in the form of a slurry thereof by the following
steps:
[0023] (1) mixing first particles difficultly soluble in alkalis
and soluble in acids with an aqueous alkali silicate solution to
form a first slurry containing first particles;
[0024] (2) neutralizing the first slurry with a mineral acid to
prepare a second slurry containing second particles wherein silica
is deposited on the first particles; and
[0025] (3) adding a mineral acid to the second slurry particles to
dissolve the first particles from the second particles, to prepare
a third slurry containing silica particles.
[0026] The silica particles of the present invention have a volume
of 4.0 to 6.0 cc/g, preferably 4.0 to 5.5 cc/g, for pores having a
diameter of 10.sup.5 .ANG. or less, and that of 2.0 cc/g or more
for pores having a diameter in the range of 6,000 to
8.times.10.sup.4 .ANG., and that of 1.0 cc/g or more for pores
having a diameter in the range of 200 to 2,000 .ANG..
[0027] When the volume is less than 4.0 cc/g for the pores having a
diameter of 10.sup.5 .ANG. or less, the oil absorption is small and
the desired opacity-after-printing cannot be easily imparted to the
paper. On the contrary, the silica particles having a volume of
larger than 6.0 cc/g for the pores having a diameter of 10.sup.5
.ANG. or less cannot be easily produced. As for the pores having a
diameter in the range of 6,000 to 8.times.10.sup.4 .ANG., when the
volume of them is less than 2.0 cc/g, a paper-making filler having
a high ink absorption cannot be easily obtained because the
absorption of a pigment component in the ink, absorbable in the
pores having diameters of this range, is poor. When the volume of
the pores having a diameter in the range of 200 to 2,000 .ANG. is
below 1.0 cc/g, the absorption of the vehicle, among the ink
components, is insufficient for obtaining the paper-making filler
having excellent ink absorption and effect of preventing
strike-through of ink to the reverse side of the paper (hereinafter
referred to as "non-strike through effect"). For obtaining a paper
having a high ink absorption, the volume of pores having a diameter
in the range of 200 to 2000 .ANG. is desirably 1.0 to 2.0 cc/g.
[0028] The pore volume was determined with a mercury porosimeter
(type: Poreosizer-9320; a product of Micro Meritics). As for the
volume for pores having a diameter of 10.sup.5 .ANG. or less, that
for pores having a diameter of 12 to 10.sup.5 .ANG. was
determined.
[0029] The particle size distributions of the first particles and
silica particles were determined with a laser diffraction particle
size distribution determination device (type: SALD-2000 J; a
product of Shimadzu Corporation). This method will be referred to
as "laser method" hereinafter.
[0030] The oil absorption of the silica particles of the present
invention is 300 to 500 ml/100 g, preferably 350 to 500 ml/100 g.
When the oil absorption is below 300 ml/100 g, the
opacity-after-printing cannot be easily imparted to the paper, and
the silica particles having an oil absorption of above 500 ml/100 g
cannot be easily produced. The oil absorption was determined
according to JIS K 5101.
[0031] The bulk specific gravity of the silica particles of the
present invention is 0.1 g/ml or below, preferably 0.09 g/ml or
less (the lower limit is usually 0.06 g/ml). It is supposed that
when the bulk specific gravity of the silica particles is as low as
0.1 g/ml, the volume of these particles in a paper will be large
when they are added to the paper to make a large contribution to
the ink absorption. When the bulk specific gravity is above 0.1
g/ml, such an effect is difficultly exhibited. The bulk specific
gravity was determined according to JIS K 5101.
[0032] The average particle diameter as determined by the laser
method is 5 to 30 .mu.m, preferably 8 to 25 .mu.m, and the standard
deviation of the particle volume distribution to the particle
diameter (.mu.m) represented by the logarithm is in the range of
0.1 to 0.25, preferably 0.1 to 0.2.
[0033] When the amount of the silica particles in the paper is
fixed, the smaller the average particle diameter, the higher the
opacity. However, when the average silica particle diameter is
lower than 5 .mu.m, a large amount of the particles is necessitated
because the retention of the added silica particles in the paper is
seriously lowered. Although an inorganic or organic retention aid
can be added for the purpose of increasing the retention, the
amount thereof is limited because a large amount thereof impairs
the formation of the paper. On the contrary, when the particle
diameter is larger than 30 .mu.m, the number of the particles
contained in the paper is reduced to also reduce the
light-scattering effect of the silica particles and thereby to
reduce the opacity of the paper, though the retention thereof in
the paper is extremely high.
[0034] Another characteristic of the silica particles used in the
present invention is that the particle diameter distribution can be
controlled in such a range that the standard deviation of the
particle volume distribution to the particle diameter (.mu.m)
represented by logarithm is in the range of 0.1 to 0.25, preferably
0.1 to 0.2. Since the silica particles are in the form of
aggregates of single particles as a secondary particles, as
described above, they are actually used in the form of a mixture of
the aggregates of the particles having small and large particle
diameters. Therefore, when they are used as they are, the particles
of small diameters are difficultly retained in the water and, on
the contrary, although the particles of large diameters are
retained in the paper, they do not effectively contribute to the
opacity of the paper. When the standard deviation is higher than
0.25, the amount of the particles having small diameters and those
having large diameters are increased, and the function of them as
the filler is insufficient.
[0035] The silica particles having an average particle diameter of
5 to 30 .mu.m as determined by the laser method and a standard
deviation of 0.1 to 0.25 for the particle volume distribution to
the particle diameter represented by the logarithm are obtained by,
if necessary, subjecting the silica particle-containing slurry
obtained as described above to a dry or wet pulverization and then
classifying the particles into two groups with a vibrating screen
or the like.
[0036] The characteristic values of the first particles (in case
they are determined) and silica particles are those obtained by
filtering the slurry containing these particles, washing the
particles with water, drying them with a dryer at 105.degree. C.
and determining them by the above-described methods.
[0037] The detailed description will be made on a suitable method
of producing the silica particles of the present invention.
[0038] The aqueous alkali silicate solution used in step (1) of the
present invention is not particularly limited, and is preferably an
aqueous sodium silicate solution or aqueous potassium silicate
solution. As for the molar concentration of the alkali silicates in
the aqueous solution, that of sodium silicate is selected from a
molar ratio of SiO.sub.2/Na.sub.2O in the range of 2.0 to 3.4.
[0039] The first particles to be added to the aqueous alkali
silicate solution in the present invention are those difficultly
soluble in an alkali and soluble in an acid. The expression
"difficultly soluble in an alkali" herein indicates that the first
particles are not soluble in an aqueous alkali solution of pH 9 or
above in a short time, namely in 120 minutes.
[0040] The materials for the first particles are not particularly
limited so far as the diameter of the particles can be
controlled.
[0041] The first particles include those of metals, metal salts,
metal oxides, metal hydroxides and organic materials. Metals
include those metals belonging to Groups 1A to 7A, 8, 1B and 2B of
the Periodic Table, such as K, Rb, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni,
Zn, etc. Metal salts include those salts of the metals such as
carbonates and manganates, for example, calcium carbonate, barium
carbonate, magnesium carbonate, nickel carbonate, potassium
manganate, etc. Metal oxides include the oxides of these metals
such as magnesium oxide, zinc oxide, calcium oxide, manganese
oxide, etc. Metal hydroxides include those hydroxides of the metals
such as magnesium hydroxide, calcium hydroxide, manganese
hydroxide, etc. These particles may be used singly or as a mixture
of two or more of them.
[0042] The first particles are mixed with the aqueous alkali
silicate solution to obtain a first slurry containing the first
particles. The amount of these first particles in the first slurry
is usually 5 to 120% by weight, preferably 10 to 60% by weight,
based on the solid (in terms of silica) in the aqueous alkali
silicate solution.
[0043] Various mineral acids are usable for neutralizing the first
slurry and thereby to precipitate silica on the first particles.
These mineral acids are usable in the form of a mixture of two or
more of them. Examples of the mineral acids include hydrochloric
acid, sulfuric acid and nitric acid. Sulfuric acid is suitable for
use as the mineral acid because it is easily available on the
market at a relatively low cost. The concentration of the mineral
acid which is not particularly limited is usually 10 to 30% by
weight.
[0044] The diameter of the first particles is controlled as desired
before they are mixed with the aqueous alkali silicate solution.
The average particle diameter is in the range of 0.01 to 10 .mu.m,
preferably 0.1 to 5 .mu.m. When the average particle diameter is
larger than 10 .mu.m, the quantity of the first particles to be
contained in the second particles wherein silica is deposited on
the first particles is reduced and, therefore, the particles added
are wasted. On the contrary, first particles smaller than 0.01
.mu.m in diameter are economically disadvantageous because the
production thereof necessitates a high cost and much labor but the
effect thereof in improving the brightness and opacity is
insufficient. Further, by making the size of the first particles
uniform, the particle size distribution of the obtained silica
particles of the present invention can be made uniform and to have
a single peak.
[0045] Various mineral acids are usable for dissolving the first
particles in the second particles wherein silica is deposited on
the first particles. Mineral acids usable in the present invention
are those reactive with the first particles to form salts which are
easily removable by washing with water or the like. The mineral
acids are usable either alone or in the form of a mixture of two or
more of them. The concentration of the mineral acids which is not
particularly limited is usually 10 to 30% by weight.
[0046] The first particles are usually added to the aqueous alkali
silicate solution under stirring the solution, or the aqueous
alkali silicate solution may be added to an aqueous slurry of these
first particles.
[0047] The first particles are added to the aqueous alkali silicate
solution in a period ranging from before the addition of the
mineral acid for the neutralization to the precipitation of silica
by the addition of the mineral acid in case the mineral acid is
added only once or two or more times as will be described below.
Namely, the order and number of times of the addition of the first
particles and the mineral acid for the neutralization are not
particularly limited so far as they are added prior to the complete
neutralization of the alkali silicate. The addition may be
conducted at once, intermittently in small portions or
continuously.
[0048] The amount of the first particles is preferably 5 to 120% by
weight, more preferably 10 to 60% by weight, based on the solid
content (in terms of silica) in the aqueous alkali silicate
solution. The amount of these first particles in this range is
desirable from the viewpoint of the suitableness as the
paper-making filler. When the amount of these first particles is
smaller than 5% by weight, the obtained silica filler cannot impart
the desired brightness and opacity to the paper. On the contrary,
even when the amount of these first particles exceeds 120% by
weight, the excellent brightness and opacity are no more improved.
In such a case, a larger amount of the mineral acid is necessitated
for dissolving the first particles, after the completion of the
precipitation by neutralization to increase the production cost of
the silica filler economically disadvantageously.
[0049] When the mineral acid for neutralizing the aqueous alkali
silicate solution is added at once in the step (2) of the present
invention, the temperature of the first slurry is 60.degree. C. or
higher and not above the boiling point of the slurry, preferably
70.degree. C. or higher and not above the boiling point thereof, to
form the second particles wherein silica is deposited on the first
particles. The mineral acid may be added either at once or
continuously.
[0050] Although the boiling point of the first slurry is usually
equal to the boiling point of the aqueous alkali silicate solution,
it may vary depending on ions dissolved therein, the pressure of
the system, etc. In practice, the boiling point herein indicates
the temperature at which the liquid in the slurry boils. Usually,
the boiling point is in the range of 95 to 105.degree. C.
[0051] When the mineral acid for neutralizing the aqueous alkali
silicate solution is added dividedly in two or more portions, 10 to
50%, preferably 20 to 40%, of the total amount thereof necessitated
for neutralizing the aqueous alkali silicate solution is added
first at a slurry temperature of 20 to 60.degree. C., preferably 30
to 60.degree. C. Then, the temperature is elevated to a range of
70.degree. C. to the boiling point of the slurry to conduct the
aging if necessary. In this step, the above-described amount of the
mineral acid can be added at once or continuously to the
slurry.
[0052] Then, the aqueous alkali silicate solution is heated to a
temperature in the range of 70.degree. C. to the boiling point of
the slurry, preferably 85.degree. C. to the boiling point of the
slurry in a short period of time such as 10 to 30 minutes, and
aged, if necessary. Thereafter, the second portion of the mineral
acid is added at once or continuously to neutralize the aqueous
alkali silicate solution and, further aging is conducted if
necessary.
[0053] In the step (3) of the present invention, the mineral acid
is further added to the second slurry containing the second
particles, obtained as described above, to dissolve the first
particles from the second particles. The mineral acid used for the
dissolution can be added at once, in portions or continuously. The
pH of the second slurry containing the second particles is
controlled at 2 to 6.5, preferably 4 to 6.
[0054] The temperature at which the first particles in the second
particles are to be dissolved is not particularly limited. Namely,
the dissolution can be conducted after lowering the temperature to,
for example, 20.degree. C. or without lowering the temperature.
[0055] The amount of the mineral acid to be added in this step is
such that the whole first particles are dissolved therein.
[0056] In the present invention, an electrolytic substance such as
sodium sulfate can be previously added so that the viscosity of the
slurry is kept low and stable when the formation and aging of the
second particles are accelerated. The term "aging" herein indicates
that the slurry is stirred at a predetermined temperature in the
range of, for example, 60.degree. C. to the boiling point of the
slurry for a predetermined time such as 10 to 180 minutes.
[0057] The particle size and distribution were determined with the
particle size distribution determination device (Type: SALD-2000 J;
a product of Shimazu Corporation). There was no peak due to the
first particles in the second particles and no difference was found
between the second particles and the silica particles after the
dissolution. It was also found that the pore volume after the
dissolution was larger than that before the dissolution. From this
fact, it is supposed that the second particles contain the first
particles therein in the step of forming the second particles, and
that by dissolving the first particles with the mineral acid, the
porosity of the particles is increased.
[0058] Namely, supposedly, silica is deposited on the surfaces of
the first particles so that silica surrounds the first particles,
at least partially or substantially completely. The silica layer
can be apparently continuously deposited or, alternatively, the
fine primary particles can be aggregated together to form secondary
particles which form the silica layer. Further, after the
dissolution of the first particles, a part of these first particles
may possibly remain and is adsorbed on the porous silica
surfaces.
[0059] In the present invention, the silica particles are obtained
in the form of a slurry thereof, and well-known means and equipment
are usable without any change for the transportation and storage of
them. If necessary, the silica particles obtained by the present
invention may be subjected to the wet grinding and/or wet
classification before they are added to the papers. The means for
the wet pulverization include well-known continuous homomixer,
colloid mill, disc refiner, sand grinder, ball mill, rod mill, etc.
When the silica particles are to be classified after the grinding,
they are classified by wet method with a classifying machine such
as a well-known vibrating screen to remove coarse particles larger
than 70 .mu.m. The silica particles obtained after the
above-described treatment has an average particle diameter in the
range of 5 to 30 .mu.m, preferably 6 to 25 .mu.m and a standard
deviation of the particle volume distribution to the particle
diameter represented by the logarithm of the particle diameter
(.mu.m) in the range of 0.1 to 0.25, preferably 0.1 to 0.2. When
the silica particles thus having a narrow particle size
distribution and large pores therein are used as a paper-making
filler, an excellent opacity-after-printing can be obtained even
when the paper sheets are thin and the amount of the filler is not
so large.
[0060] As a matter of course, the wet grinding and wet
classification are not always necessary when the diameter of the
obtained particles are in this range.
[0061] The silica particles of the present invention have a
specific surface area of 30 to 200 m.sup.2/g, preferably 60 to 180
m.sup.2/g as determined by the mercury porosimetry. When the
specific surface area is smaller than 30 m.sup.2/g, it is difficult
to obtain an oil absorption of 300 ml/100 g or more. On the
contrary, when the specific surface area exceeds 200 m.sup.2/g, the
properties of the silica particles become like those of a gel, the
shrinkage by drying is increased and the oil absorption is inclined
to be low.
[0062] Papers obtained by incorporating the silica particles of the
present invention as a filler into a pulp material and making the
papers from the resultant mixture have a high opacity, particularly
a high opacity-after-printing. A reason therefor is considered to
be that since the pore volume in the silica particles is increased
to increase the oil absorption, the capacity of inhibiting the ink
from the penetration into the paper after the printing is
increased.
[0063] The silica particles of the present invention are usable as
a filler to be dispersed in pulp fibers used as a starting material
for paper in any of acid paper making method, neutral paper making
method or alkaline paper making method, or as a pigment for surface
coating agents.
[0064] The silica particles in the form of the slurry produced by
the above-described process can be mixed with a starting material
for paper and the obtained mixture can be used for making a paper
with a wet paper-making machine; or the silica particles produced
by the above-described process can be dried and kept in the form of
a powder to be dispersed again in water and mixed with the starting
material for paper.
[0065] The amount of the silica particles used as the filler varies
depending on the desired ash content of the paper and is usually 1
to 30% by weight, preferably 1 to 20% by weight.
[0066] The pulps used for preparing papers containing the silica
particles of the present invention as the filler are known,
ordinary paper-making pulps. They include chemical pulps such as
sulfite pulps, craft pulps and soda pulps; wood pulps such as
semichemical pulps and mechanical pulps; and non-wood pulps such as
paper mulbery, paper bush (Edgeworhia papyrifera) and hemp. These
pulps may be either unbleached pulps or bleached pulps, and either
unbeaten pulps or beaten pulps. They may be used either alone or in
the form of a mixture of two or more of them.
[0067] The silica filler-containing paper of the present invention
may contain other fillers than the silica particles of the present
invention and also other additives usually used for the paper
making such as a sizing agent, defoaming agent, slime-controlling
agent, dye, coloring pigment, fluorescent dye, dry strength
additive, wet strength additive, drainage aid and retention aid, if
necessary.
[0068] The surfaces of the papers containing the silica filler of
the present invention can be coated with a starch, polyvinyl
alcohol, polyacrylamide, surface sizing agent, etc.
[0069] The wet paper making machine used in the present invention
is suitably selected from well-known, commercial-scale paper making
machines such as a cylinder paper machine, inclined former,
Fourdrinier machine and twin-wire paper machine depending on the
purpose.
[0070] As described above, the silica particles of the present
invention have a high oil absorption because the pore volume in
each particle was increased while the specific surface area thereof
was kept so that a serious shrinkage is not caused when the silica
particles in the form of a slurry are directly dried, and when the
particles are used as the filler in the paper making, a paper
having an excellent opacity-after-printing can be obtained.
EXAMPLES
[0071] The following Examples will further illustrate the present
invention, which by no means limit the scope of the present
invention. In the Examples, percentages are given by weight.
Example 1
[0072] 240 g of commercially available JIS No. 3 aqueous sodium
silicate solution (a product of Tokuyama, solid concentration: 30%)
was diluted with pure water to 1,000 g. The silica (silicon
dioxide) concentration was 72 g/kg. The diluted solution was fed
into a two-liter stainless steel beaker, and 17.9 g of anhydrous
sodium sulfate was added thereto at 50.degree. C. Then, 180 g of an
aqueous magnesium hydroxide dispersion (#200, a product of
Konoshima Kagaku Kogyo, solid concentration: 8%) having an average
particle diameter controlled at 0.5 .mu.m with a sand grinder was
added as the first particles difficultly soluble in an alkali and
soluble in an acid. 54 g (30% based on the whole amount of acid
necessitated for neutralizing sodium silicate) of sulfuric acid
(concentration: 20%) was continuously added over a period of 12
minutes under stirring with Three-One motor.
[0073] After the completion of the addition of sulfuric acid, the
temperature was elevated to 90.degree. C. under stirring in a
period of 25 minutes. The stirring was conducted at that
temperature for 10 minutes to conduct the aging. Then, 126 g of
sulfuric acid was continuously added for 23 minutes and the aging
was conducted for additional 20 minutes. 110 g of sulfuric acid was
continuously added for 15 minutes to dissolve magnesium hydroxide.
The pH of the slurry in this step was 5.2.
[0074] The slurry containing the reaction product was passed
through a 200-mesh sieve to remove the residue. The average
particle diameter of the obtained silica particles was 21.3 .mu.m
as determined by the above-described laser method. The slurry
passed through the sieve was filtered through a Buchner funnel to
obtain silica particles in the form of a cake. A part of the cake
was dried at 105.degree. C. overnight, and the oil absorption,
specific surface area, pore volume and bulk specific gravity of the
particles were determined. The balance was dispersed again in water
and stirred to obtain a slurry thereof having a concentration of
8%. This slurry was used as the paper-making filler slurry in the
following step.
[0075] 25 g (absolute dry weight) of a mixed pulp comprising 15% of
semi-bleached soft wood kraft pulp, 34% of a thermomechanical pulp
(TMP), 11% of mechanical pulp (GP) and 40% of deinked pulp (DIP)
obtained from waste newspapers was dispersed in tap water and the
dispersion was diluted to a volume of 2 liters to obtain a 1.25%
slurry. The filler slurry obtained as described above was added to
this slurry in such amounts that the filler content would be 3%
based on the absolute dry weight of the pulp. After stirring for 2
minutes, 1%, based on the absolute dry weight of the pulp, of
aluminum sulfate [Al.sub.2(SO.sub.4).sub.3.18H.sub.2O]was added and
the resultant mixture was stirred for 2 minutes. The whole mixture
was diluted to 12.5 liters and thoroughly stirred. A paper having
an absolute dry weight of 40 g/m.sup.2 was made with a square
sheeting machine (a product of Tozai Seiki) and dried.
[0076] The moisture of the hand-made sheet was controlled in a room
having a relative humidity of 65% at 20.degree. C. and the sheet
was passed through an experimental machine calender (a product of
Kumagai Riki Kogyo) under a linear pressure of 40 kg/cm twice to
control the smoothness. Then, the paper quality tests for
determining the ISO brightness and opacity and the printing tests
were conducted by the following methods to evaluate the paper
sheet:
[0077] (1) Brightness: The brightness of the paper was determined
according to JIS P 8148 (ISO 2470).
[0078] (2) Opacity of white paper: The opacity of white paper was
determined according to J. TAPPI 53 (ISO 2471).
[0079] (3) Opacity after printing: The solid printing was conducted
with an offset ink for newspapers and an RI printing tester, and
the opacity-after-printing Y (%) was defined by the following
formula (1):
Y(%)=A/B.times.100
[0080] wherein A represents the reflectance of the reverse side of
a paper after printing, and B represents the reflectance of the
reverse side of the paper before printing.
[0081] (4) Retention of silica particles in paper:
[0082] The ash content (A1) of a paper free of silica particles and
the ash content (A2) of a paper containing silica particles were
determined according to JIS P 8128, and the yield was calculated
according to the formula:
[(A2)-(A1)]/(addition rate of silica particles in sheet-making
step)
Example 2
[0083] 600 g of a 12% aqueous solution of magnesium hydroxide
(controlled at 0.5 .mu.m) was added to 240 g of an aqueous solution
of No. 3 sodium silicate, and the resultant mixture was diluted
with pure water to a volume of 1,000 g. 27 g of sulfuric acid and
then 153 g thereof were added thereto to conduct the reaction.
Then, the reaction and treatment were conducted in the same manner
as that of Example 1 except that the amount of sulfuric acid used
for dissolving magnesium hydroxide was altered to 550 g. The
obtained slurry containing the silica particles was evaluated in
the same manner as that of Example 1.
[0084] After the completion of the reaction, the pH of the slurry
was 4.2, and the average diameter of the obtained particles was
15.6 .mu.m.
Example 3
[0085] The reaction and treatment were conducted in the same manner
as those of Example 2 except that the amount of the aqueous
magnesium hydroxide solution to be added to the aqueous sodium
silicate solution was altered to 240 g, the amounts of sulfuric
acid used in the first and the second steps were altered to 63 g
and 117 g, respectively, and the amount of sulfuric acid used for
dissolving magnesium hydroxide was altered to 220 g. The obtained
slurry containing the silica particles was evaluated in the same
manner as that of Example 1.
[0086] After the completion of the reaction, the pH of the slurry
was 4.4, and the average diameter of the obtained particles was
19.7 .mu.m.
Example 4
[0087] The reaction and treatment were conducted in the same manner
as those of Example 1 except that the particle diameter of
magnesium hydroxide was altered to 0.1 .mu.m. The obtained slurry
containing the silica particles was evaluated in the same manner as
that of Example 1.
[0088] After the completion of the reaction, the pH of the slurry
was 5.1, and the average diameter of the obtained particles was
18.4 .mu.m.
Example 5
[0089] The reaction and treatment were conducted in the same manner
as those of Example 1 except that the particle diameter of
magnesium hydroxide was altered to 1 .mu.m. The obtained slurry
containing the silica particles was evaluated in the same manner as
that of Example 1. After the completion of the reaction, the pH of
the slurry was 5.0, and the average diameter of the obtained
particles was 23.3 .mu.m.
Comparative Example 1
[0090] The reaction and treatment were conducted in the same manner
as those of Example 1 except that sulfuric acid for dissolving
magnesium hydroxide was not added. The obtained slurry containing
the filler was evaluated in the same manner as that of Example
1.
[0091] After the completion of the reaction, the pH of the slurry
was 9.3, and the average diameter of the obtained particles was
20.8 .mu.m.
Comparative Example 2
[0092] The reaction and treatment were conducted in the same manner
as those of Example 1 except that magnesium hydroxide was not
added, the amount of sulfuric acid added at the first time was 72 g
and sulfuric acid for dissolving magnesium hydroxide was not added.
The obtained slurry containing the filler was evaluated in the same
manner as that of Example 1.
[0093] After the completion of the reaction, the pH of the slurry
was 4.3, and the average diameter of the obtained particles was
19.9 .mu.m.
Referential Example 1
[0094] For comparison, hand-made sheets were made in the same
manner as that of Example 1 except that the filler was not used,
and the products were evaluated.
[0095] The results of the Examples, Comparative Examples and
Referential Examples are shown in following Table 1.
1 TABLE 1 Specific Oil surface Pore Opacity (%) absorption area
volume Brightness White After ml/100 g m.sup.2/g cc/g % paper
printing Ex.1 400 97 5.2 51.6 90.1 86.4 Ex.2 350 83 4.3 51.5 90.3
86 Ex.3 450 125 5.5 51.8 90.4 86.6 Ex.4 400 111 4.9 51.5 90.1 86.4
Ex.5 370 104 4.6 51.7 90.1 86.2 Comp. 230 79 3 52 90.6 84.7 Ex.1
Comp. 250 160 3.2 51.1 88.9 84.9 Ex.1 Ref. -- -- -- 50.6 88.5 82.3
Ex.1
[0096] It is apparent from Table 1 that the silica particles
obtained by the present invention are capable of imparting high
brightness and opacity, particularly an extremely excellent
opacity-after-printing, to the papers containing them (Examples 1
to 5). On the contrary, when the particles difficultly soluble in
alkalis and soluble in acids are not used at all (Comparative
Example 2) or when no filler is used (Referential Example 1), the
brightness, opacity of white paper and opacity-after-printing are
poor disadvantageously.
[0097] On the other hand, when the particles difficultly soluble in
alkalis and soluble in acids are not dissolved (Comparative Example
1), the opacity-after-printing cannot be sufficiently improved
because the pore volume in the silica particles is small, though
the brightness and opacity of white paper can be improved by the
effect of these particles difficultly soluble or insoluble in
alkalis and soluble in acids and included in the silica
particles.
Example 6
[0098] 240 g of a commercially available JIS No. 3 aqueous sodium
silicate solution (a product of Tokuyama, solid concentration: 30%)
was diluted with pure water to 820 g. The silica (silicon dioxide)
concentration was 72 g/kg. The diluted solution was fed into a
two-liter stainless steel beaker, and 17.9 g of anhydrous sodium
sulfate and 180 g of an aqueous magnesium hydroxide dispersion
(#200, a product of Kamishima Kagaku Kogyo, solid concentration:
8%) having an average particle diameter controlled at 0.5 .mu.m
with a sand grinder were added at 50.degree. C. 63 g (35% based on
the whole amount of acid necessitated for neutralizing sodium
silicate) of sulfuric acid (concentration: 20%) was continuously
added over a period of 13 minutes under stirring with Three-One
motor. After the completion of the addition of sulfuric acid, the
temperature was elevated to 90.degree. C. under stirring during 25
minutes. The stirring was conducted at that temperature for 20
minutes to conduct the aging. Then, 117 g of sulfuric acid was
continuously added for 23 minutes and the aging was conducted for
additional 20 minutes. 110 g of sulfuric acid was continuously
added for 15 minutes to dissolve magnesium hydroxide. The pH of the
slurry in this step was 4.9.
[0099] The slurry containing the reaction product was passed
through a 200-mesh sieve to remove the residue. The average
particle diameter of the obtained silica particles was 21.3 .mu.m
as determined by the above-described laser method. The filler
slurry passed through the sieve was filtered through a Buchner
funnel to obtain the filler in the form of a cake. Apart of the
cake was dried at 105.degree. C. overnight, and the oil absorption,
specific surface area, pore volume and bulk specific gravity of the
product were determined. The balance was dispersed again in water
and stirred to obtain a slurry thereof having a concentration
controlled at 8%.
[0100] 25 g (absolute dry weight) of a mixed pulp comprising 15% of
bleached coniferous wood kraft pulp, 34% of a thermomechanical pulp
(TMP), 11% of mechanical pulp (GP) and 40% of deinked pulp (DIP)
obtained from waste newspaper was dispersed in tap water and the
dispersion was diluted to a volume of two liters to obtain a 1.25%
slurry. The filler slurry obtained as described above was added to
this slurry in such amounts that the filler content would be 3%
based on the absolute dry weight of the pulp. After stirring for 2
minutes, 1%, based on the absolute dry weight of the pulp, of
aluminum sulfate [Al.sub.2(SO.sub.4).sub.3.18H.sub.2O] was added,
and the resultant mixture was stirred for 2 minutes and then
diluted to 12.5 liters. After the thorough stirring, a paper having
an absolute dry weight of 40 g/m.sup.2 was made with a square
sheeting machine and dried.
[0101] The moisture of the hand-made sheet was controlled in a room
having a relative humidity of 65% at 20.degree. C. and the sheet
was passed through an experimental machine calender under a linear
pressure of 40 kg/cm twice to control the smoothness. Then, the
printing tests were conducted by the above-described methods to
evaluate the opacity-after-printing.
Example 7
[0102] The reactions and treatments were conducted in the same
manner as that of Example 1 except that 300 g of an aqueous
magnesium hydroxide solution (concentration: 12%) having a particle
diameter controlled at 0.5 .mu.m was added to 240 g of an aqueous
solution of No. 3 sodium silicate, that the resultant mixture was
further diluted to 1,000 g with pure water, that 45 g of sulfuric
acid was added first and then 135 g thereof was added, and that the
amount of sulfuric acid used for dissolving magnesium hydroxide was
275 g. The obtained filler slurry was evaluated in the same manner
as that of Example 1.
[0103] After the completion of the reaction, the pH of the slurry
was 4.5 and the average diameter of the obtained particles was 15.6
.mu.m.
Example 8
[0104] The reactions and treatments were conducted in the same
manner as that of Example 6 except that the amount of the aqueous
magnesium hydroxide solution to be added to the aqueous solution of
sodium silicate was altered to 180 g, that 54 g of sulfuric acid
was added first and then 126 g thereof was added, and that the
amount of sulfuric acid used for dissolving magnesium hydroxide was
165 g. The obtained filler slurry was evaluated in the same manner
as that of Example 6.
[0105] After the completion of the reaction, the pH of the slurry
was 5.3 and the average diameter of the obtained particles was 19.7
.mu.m.
Comparative Example 3
[0106] The reactions and treatments were conducted in the same
manner as that of Example 6 except that magnesium hydroxide was not
added, the amount of sulfuric acid added first was 72 g, and that
sulfuric acid for dissolving magnesium hydroxide was not added. The
obtained filler slurry was evaluated in the same manner as that of
Example 6.
[0107] After the completion of the reaction, the pH of the slurry
was 4.3 and the average diameter of the obtained particles was 19.9
.mu.m.
2 TABLE 2 Spe- Oil cific Absorp- Bulk Pore volume cc/g Opacity sur-
tion specific 6000.about. after face ml/ gravity 8 .times. 10.sup.4
200.about. printing area 100 g g/ml .ltoreq.10.sup.5 .ANG. .ANG.
1000 .ANG. % m.sup.2/g Ex.6 450 0.076 5.2 3.26 1.52 86.4 115 Ex.7
350 0.082 4.9 2.33 1.71 86.2 135 Ex.8 430 0.066 5.5 3.64 1.31 86.6
106 Comp. 250 0.115 3.3 1.53 1.56 84.9 164 Ex.3
[0108] It is apparent from Table 2 that the pore diameters of most
of the silica particles obtained by the present invention are
within the predetermined range and, therefore, a paper having a
remarkably high opacity-after-printing can be obtained by using
these particles (Examples 6 to 8).
[0109] On the other hand, when the volume of the pores having
diameters within the predetermined range is small (Comparative
Example 3), the opacity-after-printing is poor unfavorably.
Example 9
[0110] Preparation of Silica Particles
[0111] 480 g of a commercially available aqueous solution of JIS
No. 3 sodium silicate (a product of Tokuyama, solid concentration:
30%) was diluted with water to a volume of 2,000 g. Silicon dioxide
(silica) concentration was controlled at 72 g/kg. They were fed
into a 5-liter stainless steel beaker. 36 g of anhydrous sodium
sulfate was added thereto. The temperature of the aqueous solution
was adjusted to 50.degree. C., and then 350 g of an aqueous
dispersion (solid concentration; 8%) of magnesium hydroxide (a
product of Kamishima Kagaku Kogyo; #200) having an average particle
diameter controlled at 0.5 .mu.m with a sand grinder was added
thereto. 108 g (30% based on the whole amount of sulfuric acid
necessitated for neutralizing sodium silicate) of sulfuric acid
(20%) was continuously added for a period of 12 minutes under
stirring. After the completion of the addition of sulfuric acid,
the temperature was elevated to 90.degree. C. under stirring in a
period of 25 minutes and then the aging was conducted at that
temperature for 10 minutes. The remaining sulfuric acid (252 g,
concentration: 20%) was continuously added for 23 minutes. Then,
the aging was conducted for 20 minutes at that temperature.
[0112] 220 g of sulfuric acid (concentration: 20%) was continuously
added thereto for 15 minutes to dissolve magnesium hydroxide. The
pH of the slurry was 5.2.
[0113] The obtained slurry was filtered, the filter cake was washed
and the average particle diameter and the standard deviation of the
silica particles thus obtained were determined to obtain 20.8 .mu.m
and 0.18, respectively. A paper-making slurry having a solid
concentration of 8% was prepared from the silica particles thus
obtained.
[0114] Method of making paper
[0115] 3% (in terms of the solid) (based on the absolute dry weight
of mixed pulp) of the silica particle slurry was added to a mixed
pulp slurry (pulp concentration: 1.2%) comprising 15% of bleached
coniferous wood kraft pulp, 35% of a thermomechanical pulp, 10% of
groundwood pulp and 40% of deinked pulp obtained from waste
newspapers. After stirring for 2 minutes, 1% of aluminum sulfate
was added thereto and the obtained mixture was stirred for 2
minutes. The slurry thus obtained was diluted to a solid
concentration of 0.5% and used as a stock.
[0116] Sheets having an air-dried basis weight of 43 g/m.sup.2 were
prepared from the stock with an experimental square sheeting
machine (a product of Tozai Seiki). After cooling, the moisture of
the sheets was controlled in a room having a relative humidity of
65% at 20.degree. C. for 24 hours and the sheets were passed
through an experimental machine calender (a product of Kumagai Riki
Kogyo) under a linear pressure of 40 kg/cm twice to control the
smoothness.
Example 10
[0117] Silica particles were prepared in the same manner as that of
Example 9 except that the amounts of the aqueous magnesium
hydroxide dispersion and sulfuric acid (concentration: 20%) for
dissolving magnesium hydroxide were altered to 180 and 110 g,
respectively, and paper sheets were made therefrom. The average
particle diameter and standard deviation of the obtained silica
particles were 26.8 .mu.m and 0.24, respectively. The obtained
paper sheets were evaluated in the same manner as that of Example
9.
Example 11
[0118] Silica particles were prepared in the same manner as that of
Example 9 except that 72 g (20% based on the whole amount) of
sulfuric acid was used first and then 288 g thereof was used, and
paper sheets were made therefrom. The average particle diameter and
standard deviation of the obtained silica particles were 10.2 .mu.m
and 0.14, respectively. The obtained paper sheets were evaluated in
the same manner as that of Example 9.
Example 12
[0119] The silica particle slurry (solid concentration: 8%)
obtained in Example 9 was treated with a sand grinder (SL-1/2G; a
product of AIMEX) to obtain silica particles having an average
particle diameter of 12.2 .mu.m. The silica particles had a
standard deviation of 0.19. The average particle diameter was
changed, but the standard deviation were not so different from each
other.
[0120] The same procedure as that of Example 9 was repeated except
that the silica particles thus obtained were used as the
filler.
Example 13
[0121] Hydrous silicic acid was prepared in the same manner as that
of Example 9 except that the amounts of the aqueous magnesium
hydroxide dispersion and sulfuric acid (concentration: 20%) for
dissolving magnesium hydroxide were altered to 600 g and 380 g,
respectively, and paper sheets were prepared by using the product.
The average particle diameter and standard deviation of the
obtained hydrous silicic acid were 15.8 .mu.m and 0.203,
respectively. The obtained paper sheets were evaluated in the same
manner as in Example 9.
Comparative Example 4
[0122] Preparation of Silica Particles
[0123] 480 g of a commercially available aqueous solution of JIS
No. 3 sodium silicate (a product of Tokuyama, solid concentration:
30%) was diluted with water to a volume of 2,000 g. Silicon dioxide
(silica) concentration was controlled at 72 g/kg. They were fed
into a 5-liter stainless steel beaker. 36 g of anhydrous sodium
sulfate was added thereto. The temperature of the aqueous solution
was adjusted to 50.degree. C. 144 g (40% based on the whole amount
of sulfuric acid necessitated for neutralizing sodium silicate) of
sulfuric acid (20%) was continuously added over a period of 12
minutes under stirring. After the completion of the addition of
sulfuric acid, the temperature was elevated to 90.degree. C. under
stirring for a period of 25 minutes and then the aging was
conducted at that temperature for 10 minutes. The remaining
sulfuric acid (216 g, concentration: 20%) was continuously added
for 23 minutes. Then, the aging was conducted for 20 minutes at
that temperature. The pH of the slurry was 5.2 (slurry A).
[0124] The slurry A was classified with a 200-mesh sieve. The
residue (remaining rate: 22%) on the sieve was ground with a sand
grinder and mixed with the particles which had passed through the
sieve. The obtained mixture was filtered and the filter cake was
washed. The average particle diameter and standard deviation of the
obtained silica particles were determined by the laser method to be
25.8 .mu.m and 0.33, respectively. A paper-making slurry having a
solid concentration of 8% was prepared from the silica
particles.
[0125] Paper sheets were prepared in the same manner as that of
Example 9 except that the silica particles obtained as described
above were used as the filler. The obtained paper sheets were
evaluated in the same manner as in Example 9.
Comparative Example 5
[0126] The silica particle slurry (solid concentration: 8%)
obtained in Comparative Example 4 was treated with a sand grinder
(SL-1/2G; a product of AIMEX) to obtain silica particles having an
average particle diameter of 11.8 .mu.m. The silica particles had a
standard deviation of 0.35. The average particle diameter was
reduced by the grinding, and the range of the distribution was
slightly widened. The same procedure as that of Example 9 was
repeated except that the silica particles thus obtained were used
as the filler. The obtained paper sheets were evaluated in the same
manner as that of Example 9.
Referential Example 2
[0127] Paper sheets were made in the same manner as that of Example
9 except that the filler was not used. The obtained paper sheets
were evaluated in the same manner as that of Example 9. The results
were employed as the standards of the evaluation of those obtained
in Examples and Comparative Examples.
[0128] The results of the Examples and Referential Example 2 are
shown in Table 3.
3 TABLE 3 Average Opacity particle Standard Retention Bright- after
diameter deviation of filler ness Opacity printing .mu.m % % %* %*
%* Ex. 9 20.8 0.18 40.1 +1.4 +2.0 +3.7 Ex. 10 26.8 0.24 43.6 +1.4
+2.0 +3.3 Ex. 11 10.2 0.14 32.8 +1.5 +1.8 +2.8 Ex. 12 12.2 0.19
33.4 +1.5 +1.9 +2.9 Ex. 13 15.8 0.20 35.8 +1.5 +2.0 +3.0 Comp. 25.8
0.33 35.2 +1.4 +2.0 +2.4 Ex. 4 Comp. 11.8 0.35 26.7 +1.5 +1.7 +2.0
Ex. 5 Ref. -- -- -- standard standard standard Ex.2 *An increase as
compared with Referential Example 2
[0129] It is apparent from Table 3 that by narrowing the particle
size distribution of the silica particles, the retention of the
silica particles in the paper is improved, and the paper sheets
excellent in the opacity, particularly opacity-after-printing, can
be obtained (Examples 9 to 13). On the other hand, when the
characteristic value (standard deviation) of the silica particles
is not within the range of the present invention (Comparative
Examples 4 and 5), even though the particle diameter levels are the
same, the retention of the silica particles in the paper is lower
and the degree of the increase in the opacity-after-printing is
lower as compared with those obtained when the particle size
distribution is narrow, while the effect of improving the opacity
is obtained to some extent.
Effect of the Invention
[0130] As described above, silica particles having a narrow
particle size distribution and a high porosity can be easily and
efficiently produced from inexpensive starting materials such as
sodium silicate according to the present invention. When these
silica particles are used as a filler in the paper making, paper
sheets having excellent brightness, opacity and
opacity-after-printing can be obtained.
* * * * *