JPH0411498B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a sheet material of inorganic fine particles or inorganic short fibers by a casting method using microfibrillated cellulose as a binder.

Inorganic particles or fibers such as silica, alumina, titanium oxide, and potassium titanate are generally
These materials have characteristics such as being nonflammable, heat resistant, chemical resistant, and have high rigidity, and when molded into sheets, they can be used for a variety of purposes.

Methods for forming these small-sized inorganic materials, ie, fine particles and short fibers, into sheet shapes include paper-making methods, casting methods, and extrusion methods. The paper making method is generally a highly productive method, but
In particular, if the particles or fibers are small in size and have a smooth surface, it is difficult to obtain a sheet with a 100% yield. Furthermore, the extrusion method requires the intervention of a large amount of thermoplastic material, making it difficult to produce sheets with a high inorganic content. On the other hand, the casting method has a 100% inorganic yield and produces a sheet with a high inorganic content. This method also
Suitable for forming thin uniform sheets (films).

In order to form inorganic fine particles or short fibers into a sheet by the casting method, the inorganic material is usually mixed with an aqueous solution of a water-soluble polymer such as carboxymethyl cellulose or polyvinyl alcohol to form a slurry, and then the inorganic material is made into a slurry. It is cast on top, dried and made into a sheet. That is, it is a sheet using a water-soluble polymer as a binder. The sheets produced in this way have too dense a structure as a whole and are unsuitable for impregnation, unless the inorganic material itself is porous.
Additionally, the sheet generally becomes hygroscopic, which can be a problem depending on the application.

Furthermore, some water-soluble polymers have large shrinkage during drying, resulting in partial internal stress, such as a difference between the periphery and the center, resulting in sheets that are prone to deformation and cracking. Regarding the use of substances other than water, such as organic solvents, inorganic substances generally have poor compatibility with organic solvents and have poor dispersibility in slurry, making it difficult to obtain good sheets and increasing costs.

The inventors previously discovered that microfibrillated cellulose (hereinafter abbreviated as MFC) is an excellent binder and dispersant when forming sheets using inorganic particle papermaking methods, and filed a patent application. However, we have now discovered that this MFC is also useful as a binder in the casting method.

That is, the present invention relates to a method for producing an inorganic sheet material, which comprises casting an aqueous slurry containing inorganic fine particles or inorganic short fibers and MFC as essential components onto a flat substrate and drying the slurry.

The MFC used in the present invention is obtained by beating cellulose raw materials such as bulbs and linters into a viscous state in water using the method disclosed in JP-A-56-100801. It is defined as "microfibrous cellulose". MFC is
Cellulose fibers are finely divided into aggregates of many fibrils with diameters of several μm or less. The length of fibrils varies somewhat depending on the raw material and manufacturing conditions, but those obtained from wood pulp are on the order of several hundred μm. MFC is raw material 6
It has a water retention capacity of about twice as much, and when dispersed in water at a concentration of cellulose solid content of 0.4% or more, it forms a stable suspension. When this suspension is cast and dried, a translucent film is obtained.

The inorganic fine particles or inorganic short fibers used in the present invention include oxides, hydroxides, sulfides, chlorides, or free metals such as aluminum, magnesium, iron, titanium, and nickel, as well as silicon carbide, titanium carbide, and nitride. These include silicon, potassium titanate, mica, vermiculite, rock wool, glass fiber, and shirasu balloon, and include needle-like, fibrous, columnar, plate-like, spherical, and amorphous shapes.

When an aqueous suspension of the above inorganic material and MFC is mixed, cast on a horizontal substrate, and dried, a sheet with self-shape retention is obtained. MFC can be formed into a sheet as an inorganic binder with an amount of up to five times its solid content. Therefore, it is suitable for producing sheets for baking with a low organic content. MFC
When used as a binder, a sheet with little shrinkage during drying and excellent impregnability and processability can be obtained.

It is also possible to improve sheet strength by adding other types of binders to the slurry containing inorganic fine particles and MFC. MFC has particularly good affinity with water-soluble polymers, and by using water-soluble polymers together as a second binder, sheets of various densities can be manufactured.

The sheet material obtained by the method of the present invention can be used as a base material for impregnation, a base material for lamination, a sheet for firing, etc., and can be used as a separator, a speaker diaphragm, a printed circuit board, a capacitor, a surface heating element, etc. It can be used for manufacturing.

The present invention will be explained below with reference to Examples.

Example 1 2 parts by weight of aluminum hydroxide (Al ₂ O ₃ .3H ₂ O C-301 manufactured by Sumitomo Chemical) with an average particle size of 1.0 μm was added to 100 parts by weight of MFC suspension with a solid content concentration of 2% and mixed. Stirring resulted in an aqueous slurry. Place this on a polyester sheet placed on a flat surface.
It was cast using an applicator with a clearance of 1 mm and air-dried to obtain a uniform sheet with a thickness of 36 μm. The shrinkage rate in the plane direction during the drying process was 0%.

The above sheet was fired in an electric furnace at 1300°C for 3 hours to produce an alumina sheet with self-shape retention. The shrinkage rate in the planar direction before and after firing was approximately 14%.

Comparative Example 1 A sheet was obtained by the casting method in the same manner as in Example 1, except that an aqueous carboxymethyl cellulose solution with a solid content concentration of 2% was used in place of the MFC suspension in Example 1.

The shrinkage rate in the drying process is approximately 4 in the casting direction.
%, and about 7% in the direction perpendicular to the casting direction. The thickness of the dried sheet was approximately 22 μm, but cracks had appeared in various places. A part without cracks was taken and fired at 1300°C for 3 hours to obtain an alumina sheet. The shrinkage rate before and after firing was 34%.

Comparative Example 2 A sheet was obtained by the casting method in the same manner as in Example 1, except that an aqueous polyvinyl alcohol solution having a solid content concentration of 2% was used in place of the MFC suspension in Example 1. The thickness of the dry sheet was 27 μm. Similarly, it was fired at 1300°C to obtain an alumina sheet. The shrinkage rate in the drying process is 0%, and the shrinkage rate in the baking process is 25%.
It was hot.

Claims (1)

[Claims] 1. A method for producing an inorganic sheet material, which comprises casting an aqueous slurry containing inorganic fine particles or short inorganic fibers and microfibrillated cellulose as essential components onto a flat substrate and drying the slurry.