Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
1999-09-14
2002-03-05
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C423S337000
Reexamination Certificate
active
06352679
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ultrafine particle silicon dioxide which can be obtained by decomposing a volatile silicon compound at a high temperature under a flame. The ultrafine particle silicon dioxide according to the present invention is useful as a filler for rubbers, resins and the like such as thermosetting silicone rubber or liquid silicone rubber in which transparency and mechanical strength are required, rubber in which especially high transparency is required, and an unsaturated polyester resin for gel coating or an epoxy resin in which high thixotropic properties are required along with transparency.
2. Prior Art
It is known that non-porous silica may be produced by processing a fumed silica dispersed in water with heat at a temperature of 1200° C. or less. However, the thus produced non-porous silica, namely, silica particles, have number average of primary particle diameters of about 3 to 1000 microns and a BET specific surface area of about 1 m
2
/g or less. It is difficult to obtain finer silica powders than the above-described silica particles.
As a process for producing fine silica, powders, a process for producing the silica powders having a ratio of BET/CTAB specific surface areas of 0.8 to 1.1 is known (Japanese Patent Application Laid Open No. 172815/1995). However, the process is a so-called wet process using alkali silicate as a raw material. Therefore, a product silica is called as precipitated silica which inevitably contains a trace amount of alkali metal originated in the raw material and shows different characteristics in density of free silanol groups and the like from those of fumed silica so that it is difficult to obtain the similar effects as those of fumed silica when the product silica is employed as a filler or the like for the above-described resin compositions.
Contrary to the aforementioned wet process, a process called as a dry process has long been implemented in which the fine silicon dioxide powders can be produced by hydrolyzing silicon halide under a flame. This process is known advantageous in that a gas which burns while forming water, for example, a mixture composed of hydrogen or methane and oxygen or air is uniformly mixed with silicon halide to be then hydrolyzed under a flame (Japanese Patent Application Publication No. 3359/1961). The ultrafine particle silicon dioxide produced by the above process has a BET specific surface area of about 50 m
2
/g to 600 m
2
/g.
It is generally considered that, when the BET specific surface area is in the range of about 300 m
2
/g or less, this value represents the geometrical size of a particle, while, when the BET specific surface area exceeds the above range, a value of the BET specific surface area does not strictly represent the particle size. In other words, in the range where the BET specific surface area exceeds 300 m
2
/g, the BET specific surface area appears larger due to the effect of pores present at the surface of a primary particle to generate a problem such that the accurate size of the primary particle can not be obtained from the BET specific surface area.
Thus, in the range where the BET specific surface area exceeds 300 m
2
/g, expected characteristics as the ultrafine particle silicon dioxide can not be obtained. That is to say, even if a conventional fine silica has a BET specific surface area over 300 m
2
/g, the actual particle size thereof is almost the same as that of the fine silica whose BET specific surface area is 300 m
2
/g so that, when the ultrafine particle silicon dioxide power is used as a filler for applications which require mechanical strength, high transparency or high thixotropy, a desired effect can not be obtained in some cases by means of a material design based on the particle size of the conventional BET specific surface area.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to solve the above-described problem of the conventional ultrafine particle silicon dioxide and to provide an ultrafine particle silicon dioxide that is most appropriate as a filler of which excellent mechanical strength and transparency as well as high thixotropy are required.
Namely, the present invention relates to an ultrafine particle silicon dioxide which comprises silicon dioxide powders in the range of ultrafine particles having a BET specific surface area of 350 m
2
/g or more and has a ratio of BET/CTAB specific surface areas of 0.6 to 1.1, a number average of primary particle diameters of 1 to 20 nm, void volume measured with a mercury porosimeter of 0.1 to 1.0 ml/g and linseed oil absorption of 350 to 600 ml/100 g.
The present invention includes the ultrafine particle silicon dioxide which comprises a BET specific surface area of 350 to 600 m
2
/g, CTAB specific surface area of 400 m
2
/g or more, a ratio of BET/CTAB specific surface areas of 0.75 to 1.0, a number average of primary particle diameters of 1 to 6 nm and void volume measured with a mercury porosimeter of 0.1 to 0.7 ml/g.
The present invention also relates to a process for producing an ultrafine particle silicon dioxide comprising the steps of:
mixing a volatile silicon compound with a mixed gas including an inflammable gas and oxygen to be a mixture; and
decomposing said volatile silicon compound at temperatures as high as 1000 to 2100° C. under a flame generated by burning said mixture, wherein mole equivalents of oxygen and hydrogen in said mixed gas are controlled to be 2.5 to 3.5 and 1.5 to 3.5 respectively against 1 mole equivalent of said volatile silicon compound.
DISCLOSURE OF THE INVENTION
Silicon dioxide (silica) of the present invention comprises ultrafine particle silicon dioxide which has a BET specific surface area of 350 m
2
/g or more, a ratio of BET/CTAB specific surface areas of 0.6 to 1.1, a number average of primary particle diameters of 1 to 20 nm, void volume measured with a mercury porosimeter of 0.1 to 1.0 ml/g and linseed oil absorption of 350 to 600 ml/100 g. The term “ultrafine particle” as used herein refers to a particle having a BET specific surface area of 300 m
2
/g or more.
In such ultrafine particle silica, the BET and CTAB specific surface areas show a positive interrelation therebetween in the range where the BET specific surface area is 100 to 300 m
2
/g, whereas the BET and CTAB specific surface areas does not show a linear relation therebetween in the range where the BET specific surface area is over 300 m
2
/g.
A CTAB method refers to a method which determines a specific surface area by measuring the amount of CTAB (cetyltrimethylammoniun bromide) absorbed by silica particles, while a BET method refers to a method which determines a specific surface area by measuring a nitrogen gas absorbed by silica particles. In the BET method, nitrogen has a small molecular weight so that nitrogen is absorbed in pores present at the surface of the particle, while CTAB has a large molecular weight so that CTAB is not absorbed in pores present at the surface of the particle. Therefore, in the range where the BET specific surface area is 300 m
2
/g or more, it is considered that interrelation between the BET and the CTAB specific surface areas is lost due to the fact that the BET specific surface area is increased with the formation of pores at the surface of the primary particle in the above-described range of the BET specific surface area while the CTAB specific surface area is not affected.
The present invention obtains the ratio of BET and CTAB specific surface areas to control the thus obtained ratio to stay in a specified range. Through controlling the ratio to be in a specified range, it became possible to obtain an accurate particle diameter by eliminating the influence of pores formed at the surface of the primary particle.
Thus, silicon dioxide according to the present invention is controlled in the range of a BET specific surface area of 350 m
2
/g or more, preferably 350 to 600 m
2
/g, a ratio of BET/CTAB specific surface areas of 0.6 to 1.1, and preferably 0.75 to 1.0. At t
Konno Kazuhisa
Shibasaki Takeyoshi
Shirono Hirokuni
Griffin Steven P.
Ildebrando Christina
LaPointe Dennis G.
Mason & Associates, PA
Nippon Aerosil Co., Ltd.
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