Readily dispersible bentonite comprising bentonite particles having
adsorbed an alkyl-substituted silane compound represented by formula
(I) onto the surface thereof is disclosed: wherein R represents
an alkyl group having 1 to 18 carbon atoms; X represents a methoxy
group, an ethoxy group or a halogen atom; a and b each represents
1, 2 or 3, provided that (a+b) is 4; in an amount of not more than
1% by weight based on the bentonite. When poured into water and
stirred, the treated bentonite is rapidly dispersed while minimizing
formation of hardly dispersible lumps of powder to provide an aqueous
bentonite dispersion exhibiting satisfactory viscosity and water
What is claimed is:
1. Readily dispersible bentonite obtained by surface-treating bentonite
with at least one alkyl-substituted silane compound represented
by formula (I):
wherein R represents an alkyl group having 1 to 18 carbon atoms;
X represents a methoxy group, an ethoxy group or a halogen atom;
a and b each represents 1, 2 or 3, provided that (a+b) is 4;
in an amount of not more than 1% by weight based on the bentonite.
2. Readily dispersible bentonite as claimed in claim 1, wherein
said alkyl-substituted silane compound is selected from the group
consisting of methyltrimethoxysilane, trimethylchlorosilane, trimethylmethoxysilane,
hexyltrimethoxysilane, dodecyltriethoxysilane, and octadecyltriethoxysilane.
FIELD OF THE INVENTION
This invention relates to readily dispersible bentonite which can
rapidly be dispersed in water while retaining viscosity and water
impermeability inherent to bentonite.
BACKGROUND OF THE INVENTION
Bentonite is supplied on the market in a powdered form having a
particle size of not greater than 150 .mu.m, which is prepared by
coarsely crushing ore from a bentonite mine, which contains about
10 to 40 wt % of water, to a size of about 10 mm or less, drying
it in a rotary drier to reduce the water content to about 6 to 10
wt %, and pulverizing it in a roller mill or a hammer mill to the
A soil stabilizing fluid used in civil engineering works, such
as underground continuous wall construction or in-situ piling, and
a drilling fluid (mud) used in development of underground resource
energy, for example, drilling of an oil well, a geothermal well
or a spring well, is generally prepared by dispersing powdered bentonite
in fresh water in a concentration of 4 to 8 wt % by stirring it
in a stirring machine and, if desired, adding adequate amounts of
chemical substances, such as sodium carboxymethyl cellulose, polymers
and surface active agents, in order to prevent deterioration due
to contamination with cement or a brine such as seawater. On being
poured into water, powdered bentonite first forms lumps, which are
then disintegrated by stirring it a long time into a uniform dispersion.
In what follows, the single term "lumps" means undissolved
lumps of powder which are formed when the powder is poured into
water. Therefore, preparation of a soil stabilizing slurry or mud
has required great energy and time for stirring. The higher the
viscosity and water impermeability of bentonite due to the higher
bonding force with water, that is, the higher the quality of bentonite,
the greater the tendency to formation of lumps of powder which are
hard to disperse in water.
As the depth reached by recent ground drilling works has increased,
the demand for a stabilizing slurry or mud having high water impermeability
sufficient to withstand high underground water pressure has been
increasing. While bentonite of high quality meeting that demand
has been supplied, bentonite satisfying the requirements for easy
dispersion as well as high quality has been keenly sought.
Efforts have hitherto been made to satisfy the above requirements.
For example, a grinding method for minimizing formation of fine
particles of 75 .mu.m or smaller which are apt to form lumps was
suggested. Removal of fine particles by air classification was also
attempted. However, all the conventional attempts failed to achieve
Bentonite is an alkaline clay mainly comprising a clay mineral
montmorillonite and, unlike other clay minerals such as kaolinite
or pyrophyllite, has a great cation-exchange capacity of 40 to 120
milliequivalents per 100 g of clay. The exchangeable cations include
a sodium ion, a calcium ion, and a magnesium ion. In particular,
bentonite having an abundance of sodium ions has such high hydrophilic
properties that it absorbs a large quantity of water to increase
its own volume remarkably, that is, it exhibits high swellability
in water and is finally dispersed to colloidal particles and therefore
has been used as a fundamental element of a stabilizing slurry or
mud for drilling or boring works. Essential functions required for
the material of stabilizing slurries or mud are proper viscosity
and water impermeability.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide readily
dispersible bentonite which can be rapidly dispersed in water while
minimizing formation of lumps without substantial reduction in viscosity
and water impermeability inherent to bentonite.
That is, the present invention relates to readily (water) dispersible
bentonite obtained by surface treating bentonite by with adding
at least one alkyl-substituted silane compound represented by formula
wherein R represents an alkyl group having 1 to 18 carbon atoms;
X represents a methoxy group, an ethoxy group or a halogen atom;
a and b each represents 1, 2 or 3, provided that (a+b) is 4; in
an amount of not more than 1% by weight based on the bentonite.
Bentonite powder comprises solid bentonite particles with a water
content adsorbed on the surface thereof and air filling the gaps
among the particles, and the particles agglomerate to form porous
clay particles. The clay particles have a highly hydrophilic and
highly polar surface and therefore tend to gather. Thus, when bentonite
powder is poured into water, the individual bentonite particles
do not come into contact with water but gather to form clusters
with a large amount of air entrapped therein, and only the outermost
part of the clusters is first to be brought into contact with water.
Where the bentonite contains abundant sodium ions, the surface of
the bentonite cluster absorbs water owing to its strong hydrophilic
properties and immediately smells with water to form a continuous
water-impermeable gel film. Since the gel film is water impermeable
and also adhesive, the air entrapped in the inside of the cluster
cannot easily be displaced with outside water. It follows that the
bentonite clusters rise to the water surface as lumps in the initial
stage of stirring. If the lumps come into contact with each other
in a turbulent flow by stirring, they tend to grow into larger lumps.
In this stage, the lumps consist of white powder with its inside
remaining dry. In the meantime, displacement of inside air with
outside water through the gel film proceeds with time, and the water
having entered through the gel film swells inside bentonite particles
to gradually increase the transparency. At the same time, the lumps
are disintegrated from their outermost part and become thinner.
Finally, all lumps disappear to provide a uniform dispersion.
Accordingly, in order to accomplish the object of the present invention,
it would be an effective means to temporarily suppress formation
of a continuous water-impermeable gel film in the initial stage
of stirring, namely, when powdered bentonite is poured into water
and comes into contact with water. The inventors of the present
invention have found that existence of an adequate amount of a hydrophobic
substance on the surface of bentonite particles is desirable for
that effect. Such a hydrophobic substance as meets the object should
satisfy the following conditions; (1) to have no adverse influence
on the viscosity and water impermeability of bentonite, (2) to produce
a desired effect upon addition of a small amount, (3) to be moderately
priced so that the increase in production cost of bentonite may
be within an acceptable range, and (4) to be applied to the surface
of bentonite particles by a simple and easy method.
The means established by the present inventors is as follows. Coarsely
crushed and dried ore of bentonite obtained by a conventional process
for bentonite production is pulverized while sprinkling a proper
amount of at least one of the following alkyl-substituted silane
compounds, or powdered bentonite is mixed with a proper amount of
at least one of the alkyl-substituted silane compounds which is
added through sprinkling over the powdered bentonite, the compound
thereby adhering as a hydrophobic substance on the surface of bentonite
DETAILED DESCRIPTION OF THE INVENTION
The alkyl-substituted silane compounds which can be used in the
present invention are represented by formula (I):
wherein R represents an alkyl group having 1 to 18 carbon atoms
(more preferably 1 to 6 carbon atoms); X represents a methoxy group,
an ethoxy group or a halogen atom; a and b each represent 1, 2 or
3; and (a+b) is 4.
Typical examples of the alkyl-substituted silane compounds include
methyltrimethoxysilane [CH.sub.3 Si(OCH.sub.3).sub.3 ], trimethylchlorosilane
[(CH.sub.3).sub.3 SiCl], trimethylmethoxysilane [(CH.sub.3).sub.3
SiOCH.sub.3 ], hexyltrimethoxysilane [C.sub.6 H.sub.13 Si(OCH.sub.3).sub.3
], dodecyltriethoxysilane [(CH.sub.12 H.sub.25 Si(OC.sub.2 H.sub.5).sub.3
], and octadecyltriethoxysilane [C.sub.18 H.sub.37 Si(OC.sub.2 H.sub.5).sub.3
There is generally an optimum range of the amount of the alkyl-substituted
silane compound to be added to bentonite depending on the kind of
bentonite and the kind of the alkyl-substituted silane compound.
If the amount of the alkyl-substituted silane compound added is
lower than the respective optimum range, inhibition of the formation
of lumps is insufficient. If the amount exceeds the optimum range,
although the formation of lumps is inhibited, the bentonite clusters
exhibit water repellency and thereby poor dispersibility. The optimum
range for a combination of specific bentonite species and a specific
alkyl-substituted silane compound can be decided by preliminary
experimentation for evaluating the inhibitory effect on lump formation
as well as the viscosity and water impermeability of the resulting
dispersion. In many cases, the amount to be added for producing
a noticeable effect is from 0 to 1% by weight, preferably from 0.05
to 0.5% by weight, based on bentonite.
The water content of the bentonite to which the alkyl-substituted
silane compound is added may be in the same range as that of commonly
available bentonite products, i.e., in the range of from about 6
to 10% by weight. There is no need to dry or dehydrate bentonite
to a substantially water-free state as has been necessary in a conventional
surface treatment of inorganic silicate fillers with a silane coupling
agent. Neither is it necessary to limit the water content of bentonite
to be treated.
The alkyl-substituted silane compound added to bentonite powder
in an amount of not more than 1% by weight renders a part of the
surface of bentonite particles hydrophobic owing to its remarkable
water-repelling action attributed to the alkylsilyl group thereof.
As a result, when the thus treated bentonite is poured into water,
the surface of the bentonite particles is moderately prevented from
absorbing water to form a continuous water-impermeable gel film.
It follows that the clusters of bentonite particles achieve rapid
displacement of the internal air with the outside water while minimizing
formation and growth of lumps, and the clusters are crumbled and
dispersed in water at a considerably increased speed by the turbulent
flow of stirring.
The alkyl-substituted silane compound used in the present invention
is hydrolyzed on contact with water, thereby releasing the methoxy
or ethoxy group by dealcoholization, or halogen atom by dehalogenation
and, at the same time, providing a hydroxyl group on the silicon
atom (to form a silanol group). Since the resulting silanol group
is hydrophilic, it is adhered to the hydrophilic surface of the
water-containing bentonite particles.
As a technique relevant to the present invention, U.S. Pat. No.
5,292,908 discloses modified bentonite which is dispersed in water
to perform a function of adjusting rheology of an aqueous fluid,
which is obtained by adding an alkyltrialkoxysilane to bentonite
in such an amount that the resulting bentonite may not lose the
water dispersibility, usually in an amount of 1 to 10 parts by weight
per 100 parts by weight of bentonite, followed by treating in a
water-free atmosphere to add an alkylsilico group to a part of the
surface of bentonite particles. This technique is quite different
from the present invention; the modified bentonite of U.S. Pat.
No. 5,292,908 has an alkylsilico group permanently added to the
surface thereof through a covalent bond that is formed by condensation
(dealcoholation) between a hydroxyl group on the edge of montmorillonite
crystal grains and the alkyltrialkoxysilane. Therefore, the treatment
is accomplished under a water-free condition. On the other hand,
in the present invention, the alkyl-substituted silane compound
merely has to be physically adhered to the surface of water-containing
bentonite particles so that the surface of the bentonite particles
may temporarily exhibit hydrophobic properties to prevent formation
of lumps immediately after the bentonite particles are poured into
water and in the initial stage of stirring. Therefore, the silane
compound does not need to be permanently bound to the surface of
bentonite particles through a chemical bond, and the amount of the
silane compound to be added may be lower than the lower limit required
for the above-described modified bentonite. The viscosity and water
impermeability inherently possessed by bentonite are substantially
guaranteed in the presence of such an alkyl-substituted silane compound.
The present invention will now be illustrated in greater detail
with reference to examples and comparative examples, but it should
be understood that the present invention is not construed as being
limited thereto. Unless otherwise indicated, all the parts and percents
are by weight.