GB1573925A

GB1573925A - Synthetic tetra-silicic mica and a process for producing same

Info

Publication number: GB1573925A
Application number: GB1129676A
Authority: GB
Original assignee: DAIMON N
Current assignee: DAIMON N
Priority date: 1977-03-15
Filing date: 1977-03-15
Publication date: 1980-08-28

Description

(54) SYNTHETIC TETRA-SILICIC MICA AND A PROCESS FOR PRODUCING SAME (71) We, NOBUTOSHI DAIMON a Japanese national of No. 195112, Aza Kanda, Mikuriya, Kawanakajima-cho, City of Nagano, Nagano Prefecture, Japan, and KUNIO KITAJIMA, a Japanese national of 1700--99-5, Wakasato, City of Nagano,.

Nagano Prefecture, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a process of producing improved synthetic tetra-silicic mica which can be cleft into ultra-fine particles by hydration and to a water sol of this improved mica.

Synthetic mica is a layered structure material, which is strong in the direction parallel to the layer, but which is weakly bonded between layers. Accordingly, pulverized mica naturally takes the form of flake-like particles.

Mica particles pulverized by mechanical means have a thickness of more than 200 A at the smallest. It is impossible to obtain mica particles having a thickness of less than 200 A by mechanical means, and it is difficult to form a stable suspension such as a sol or gel using such coarse particles.

It is known that certain mica can be cleft into small particles to some extent by hydration by introducing water between the crystal layers. However, in order to obtain ultra-fine particles having a thickness of less than 50 A, conventional mica must then be heated to 300500 C, thereby vaporizing the water between the layers and further cleaving the particles.

Flake-like ultra-fine mica particles having a thickness of less than 50 A have the following unique properties: (a) They form a stable and uniform sol in water (even in an amount of less than 1 weight %). A 7-10 weight Vn sol of these flake-like ultra-fine particles can be easily molded into a film by spreading the sol on a substrate and drying. The flake-like ultrafine mica particles provide a strong cohesive force, and they are self-bonded to each other by the action of Van der Waals molecular cohesion force. The particles overlap in parallel and form a film having a high tensile strength.

(b) These ultra-fine mica particles have negatively charged oxygen atoms coordinated on the surface of the particles and between layers. Due to this activity, the mica reacts and bonds with organic materials.

Using these unique properties, the flakelike ultra-fine mica particles of this invention can be formed into electrically insulating film, heat-resistant sheet and a composite with synthetic resin. These products have excellent heat-resistance and electrically insulating properties. They can also be used in combination with mineral fibers such as glass, silica, alumina, or silicates to prepare a non-flammable sheet.

The non-flammable sheet thus prepared is highly flexible. The flexibility is due to the fact that the ultra-fine particles of mica of this invention are flake-like particles uniformly cleft to molecular size, and the fact that the flakes are reformed into a product by uniform over-lapping.

In addition to the above uses, this synthetic mica can be used as a base for various paints and as a starting material for the preparation of non-flammable building materials.

A typical example of the conventional micas which are known to be swellable by hydration is Na-taeniolite, expressed by the formula NaMg2Li(Si4O,0)F2.

Na-taeniolite can be synthesized, and has been heretofore used to prepare reformed mica products. However, conventional Nataeniolite stops swelling in water at the tetra-hydrate stage, i.e.

NaMg2Li(Si4O,O)F24 H2O, and hydration does not proceed any further. This is a "limited" swelling type of hydration. In order to obtain the desirable finer particles which provide an effective cohesive force, the tetra-hydrate particles must be subjected to heating at 3(X5500C.

Moreover, since a single heating step does not always provide satisfactory fine particles, it is necessary to repeat the hydration step and the heating step several times in order to complete the cleavage and to obtain satisfactory ultra-fine particles suitable for practical use. The need for the heating procedure makes conventional Nataeniolite less suitable for industrial use.

Accordingly, an object of this invention is to provide a synthetic mica which can be swollen to the desired ultra-fine particles by hydration only without any heating step.

We have found as a result of a study of the hydration or swelling mechanism of mica that mica having a satisfactorily hydratable structure should have the following characteristics: (a) the bonding between layers should be weak; (b) the hydration energy of ions between layers should be high; and (c) the ionic radius of ions between layers should be small and the valency of ions between layers should be as low as possible.

Tetra-silicic mica having the formula, NaOss 08Mg2.8~27(Si4O,O)F2 produced in accordance with the present invention satisfies the above conditions. Moreover, as can be seen from the above formula, the ions between layers, i.e. Na+, are deficient.

That is, 2 to 4 molecules of Na between layers are missing per 10 molecules of mica.

Consequently, the bonding between layers is quite unstable or weak in comparison with regular tetra-silicic mica wherein one cation per one molecule of mica is regularly coordinated between layers. Regular tetrasilicic mica has the formula, NaMg25(Si4O10)F2, while the tetra-silicic mica of the present invention having improved hydratability has the formula, NaO,6 0 aMg2 -2 7(si4oro)F2 wherein Mg2+ ions in the octahedron layer are increased and Na+ ions between layers are decreased in proportion to the increase of the Mg2+ ions.

The improved tetra-silicic mica of this invention can be easily swollen and cleft into flake-like ultra-fine particles which cohere well to each other, due, it is thought to Van der Waals molecular cohesion force, and these particles can be easily formed into a film and various other products. The swelling or cleavage of the mica of this invention can be efficiently done by a single hydration reaction without any heating procedure as in conventional mica. This is a "free swelling" type of hydration, and it is only a very small part (less than 3 Vn) of the particles that remain in the tri-hydrate stage. All other mica particles are completely swollen and cleft into flake-like ultra-fine particles having a thickness of less than 50 A and an average diameter in the flat plane of the flake ranging between 500 A and 3000 A.

The mica of this invention may be swollen and cleft into ultra-fine particles in 10 to 100 eg 50 to 100 times the amount by weight of water while stirring, thereby forming a uniform suspension of the particles. The swelling time can be shortened by subjecting the mica in an atmosphere containing water vapour before contacting with water.

Preferably the sol is diluted to a concentration of not more than 10% by weight of solids based on the total weight of the suspension, the suspension agitated sufficiently to keep the synthetic tetra silicic mica particles in suspension whilst any unreacted materials or glassy solids are allowed to settle out, the suspension is separated from the settled out materials, the suspension shaped and dehydrated, the shaped suspension optionally being compressed. Preferably the sodium ions in the mica particles are cation exchanged either in the suspension or after shaping of the suspension, by contacting the particles with a salt solution containing nonhydratable cations.

The water sol of the ultra-fine mica particles of this invention may be shaped and cohered e.g., to sheet form by dehydration or molded into a product by shaping and dehydration, and optionally, compression. The suspension may be shaped by being blended with fibres or particles of minerals or organic resins and then formed in a sheet. The presence of hydratable ions i.e., Na+, in the product would make the material liable to rehydration and swelling. Such hydratable ions are preferably replaced by nonhydratable ions at the suspension stage or after being molded into a product, thereby making the product non-hydratable.This ion-exchange can be conveniently carried out by contacting the suspension or the product in a salt solution containing K+, Ba2+, Awl3+ Pb2+, Ag+ or Mg2+, for example a l80% by weight electrolyte solution such as KCOOH, KNO3, Ba(NO3)2, AlCI3, Pb(NO3)2, or AgNO3 at from room temperature to 600 C.

The starting materials used in producing the mica of this invention are sodium fluoride (NaF), sodium silicofluoride (Na2SiF6), magnesium fluoride (MgF2), magnesia (MgO) and silica (SiO2).

These materials are mixed in a mole ratio of (a) 1-1.2 NaF: 0.5-1.2 MgF2: 2 MgO: 4 SiO2 or (b) 0.5-0.8 Na2SiF6: 0.5-1.2 MgF2: 2 MgO: 4 SiO2. The mixture is then melted at a temperature of 1300"C--1500"C, preferably 14000 C.

The melting procedure can be conducted by employing any of the well-kown internal heating techniques, external heating techniques using an alkali-resistant crucible made of platinum, SiC, graphite, beryllium oxide, or alumina, and electrical heating techniques using an electric conductor crucible made of platinum, or graphite or a furnace with conductive electrodes.

According to the internal heating techniques, the mixture of starting materials is filled around a pair of carbon electrodes connected to a carbonaceous electric heating element, and is melted by applying an electric current thereto. After the carbonaceous electric heating element is totally consumed by oxidation, the melted starting materials, which can be considered as a resistive element, take the place of the heating element. Thus, melting is continued using the melt as the heating element and the sintered layer which forms around the container as a crucible.

The melt is then cooled to obtain a crystal.

The present invention is further illustrated by the following Examples.

Example 1 120 kg of a mixture of starting materials was prepared in the following weight ratio corresponding to a mole ratio of NaF: MgF2: 2 MgO: SiO2.

sodium fluoride (NaF) 10% magnesium fluoride (MgF2) 14% magnesia (MgO) 18% silica (SiO2) 58% The above prepared mixture was then charged into a furnace of 50 cm (length) x 50 cm (width) x 100 cm (depth) lined with chamotte brick, and was melted at 1400"C for 4 hours by means of an internal heating technique. The melt was then cooled by allowing it to stand until 23 kg of a crystal was obtained. The crystal lump thus obtained was allowed to stand in an atmosphere of water vapor for 1 hour to swell the crystal. The swollen crystal was then immersed with stirring in a water tank containing 1,000 liters of water for 3 hours, thus forming a sol of flake-like ultra-fine particles of the crystal. During this procedure, 0.95 kg of impurities such as unreacted materials and glassy materials settled on the bottom of the tank.

Almost all of the flake-like ultra-fine particles thus prepared had a thickness of less than 50 A while the average diameter in the flat plane ranged between 500 A and 3000 A. Thus, the swelling of the crystal was "free-swelling", and the cleavage of the crystal was rapidly and completely carried out by hydration. According to flame spectrochemical analysis, it was proved that this crystal was Na ion (between layers)deficient type tetra-silicic mica having the composition of Na0.,Mg2.65(Si4O10)F2.

The sol was dehydrated to sheet form.

Example 2 120 kg of a mixture of starting materials was prepared in the following weight ratio corresponding to a mole ratio of 1/2 Na2SiF6: 1/2 MgF2: 2 MgO: 4 SiO2.

sodium silicofluoride (Na2SiF6) 21% magnesium fluoride (MgF2) 7% magnesia (MgO) 18% silica (SiO2) 54% The mixture was melted at 14000C for 4 hours in the same manner as in Example 1, and the melt was then cooled by allowing it to stand until 20.5 kg of a crystal was obtained. The crystal lump thus obtained was allowed to stand in the atmosphere of water vapor for 1 hour to swell the crystal.

The swollen crystal was then immersed with stirring in a water tank containing 1,000 liters of water for 3 hours, thus forming a sol of flake-like ultra-fine particles of the crystal. During this procedure 1.2 kg of impurities such as unreacted materials and glassy materials settled on the bottom of the tank.

Almost all of the flake-like ultra-fine particles thus prepared had a thickness of less than 50 A while the average diameter in the flat plane ranged from 500 A to 3000 A.

Thus, the swelling of the crystal was "freeswelling", and the cleavage of the crystal was rapidly and completely carried out by hydration. According to flame spectrochemical analysis, it was proved that this crystal was Na ion (between layers)deficient type tetra-silicic mica having the composition of NaO,6Mg2.7(Si4o1o)F2 The sol was dehydrated to sheet form.

WHAT WE CLAIM IS: 1. Tetra-silicic mica having the formula Na00.aMg2,2.7(Si4O10)F2.

2. A process for producing tetra-silicic mica having the formula, NaO,6~0 8Mg2 6-2 7(si4o1o)F2

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

2 MgO: 4 SiO2. The mixture is then melted at a temperature of 1300"C--1500"C, preferably 14000 C.

The melting procedure can be conducted by employing any of the well-kown internal heating techniques, external heating techniques using an alkali-resistant crucible made of platinum, SiC, graphite, beryllium oxide, or alumina, and electrical heating techniques using an electric conductor crucible made of platinum, or graphite or a furnace with conductive electrodes.

According to the internal heating techniques, the mixture of starting materials is filled around a pair of carbon electrodes connected to a carbonaceous electric heating element, and is melted by applying an electric current thereto. After the carbonaceous electric heating element is totally consumed by oxidation, the melted starting materials, which can be considered as a resistive element, take the place of the heating element. Thus, melting is continued using the melt as the heating element and the sintered layer which forms around the container as a crucible.

The melt is then cooled to obtain a crystal.

The present invention is further illustrated by the following Examples.

Example 1

120 kg of a mixture of starting materials was prepared in the following weight ratio corresponding to a mole ratio of NaF: MgF2: 2 MgO: SiO2.

sodium fluoride (NaF) 10% magnesium fluoride (MgF2) 14% magnesia (MgO) 18% silica (SiO2) 58% The above prepared mixture was then charged into a furnace of 50 cm (length) x 50 cm (width) x 100 cm (depth) lined with chamotte brick, and was melted at 1400"C for 4 hours by means of an internal heating technique. The melt was then cooled by allowing it to stand until 23 kg of a crystal was obtained. The crystal lump thus obtained was allowed to stand in an atmosphere of water vapor for 1 hour to swell the crystal. The swollen crystal was then immersed with stirring in a water tank containing 1,000 liters of water for 3 hours, thus forming a sol of flake-like ultra-fine particles of the crystal. During this procedure, 0.95 kg of impurities such as unreacted materials and glassy materials settled on the bottom of the tank.

Almost all of the flake-like ultra-fine particles thus prepared had a thickness of less than 50 A while the average diameter in the flat plane ranged between 500 A and 3000 A. Thus, the swelling of the crystal was "free-swelling", and the cleavage of the crystal was rapidly and completely carried out by hydration. According to flame spectrochemical analysis, it was proved that this crystal was Na ion (between layers)deficient type tetra-silicic mica having the composition of Na0.,Mg2.65(Si4O10)F2.

The sol was dehydrated to sheet form.

Example 2

120 kg of a mixture of starting materials was prepared in the following weight ratio corresponding to a mole ratio of 1/2 Na2SiF6: 1/2 MgF2: 2 MgO: 4 SiO2.

sodium silicofluoride (Na2SiF6) 21% magnesium fluoride (MgF2) 7% magnesia (MgO) 18% silica (SiO2) 54% The mixture was melted at 14000C for 4 hours in the same manner as in Example 1, and the melt was then cooled by allowing it to stand until 20.5 kg of a crystal was obtained. The crystal lump thus obtained was allowed to stand in the atmosphere of water vapor for 1 hour to swell the crystal.

The swollen crystal was then immersed with stirring in a water tank containing 1,000 liters of water for 3 hours, thus forming a sol of flake-like ultra-fine particles of the crystal. During this procedure 1.2 kg of impurities such as unreacted materials and glassy materials settled on the bottom of the tank.

Almost all of the flake-like ultra-fine particles thus prepared had a thickness of less than 50 A while the average diameter in the flat plane ranged from 500 A to 3000 A.

Thus, the swelling of the crystal was "freeswelling", and the cleavage of the crystal was rapidly and completely carried out by hydration. According to flame spectrochemical analysis, it was proved that this crystal was Na ion (between layers)deficient type tetra-silicic mica having the composition of NaO,6Mg2.7(Si4o1o)F2 The sol was dehydrated to sheet form.

WHAT WE CLAIM IS: 1. Tetra-silicic mica having the formula Na00.aMg2,2.7(Si4O10)F2.
2. A process for producing tetra-silicic mica having the formula, NaO,6~0 8Mg2 6-2 7(si4o1o)F2

which comprises mixing components selected from sodium fluoride (NaF), sodium silicofluoride (Na2SiF6), magnesium fluoride (MgF2), magnesia (MgO) and silica (SiO2) in a mole ratio of (a) 1-1.2 NaF: 0.5-1.2 MgF2: 2 MgO: 4 SiO2 or (b) 0.50.8 Na2SiF6: 0.5-1.2 MgF2: 2 MgO: 4 SiO2, melting the mixture at a temperature of 1300"C to 1500"C and cooling the melt to obtain crystals.
3. A process as claimed in Claim 2 substantially as specifically described herein with reference to Example 1 or Example 2.
4. A synthetic tetra silicic mica whenever made by a method as claimed in Claim 2 or Claim 3.
5. An aqueous sol of flake-like ultra-fine particles made from tetra-silicic mica as claimed in Claim 1 or Claim 4 wherein most of the flake-like particles dispersed in water have a thickness of less than 50 A and an average diameter in the flat plane of between 500 A and 3000 A.
6. A sol as claimed in Claim 5 substantially as specifically described herein with reference to Example 1 or Example 2.
7. A method of making a sol as claimed in Claim 5 from a solid as claimed in Claim 1 or Claim 4 which comprises exposing a mass of the solid to an atmosphere containing water vapour until it has broken into small particles and then mixing these small particles with water in an amount 10 to 100 times the weight of the original mass of synthetic tetra silicic mica.
8. A method of making a sol as claimed in Claim 7 substantially as specifically described herein with reference to Example 1 or Example 2.
9. A sol whenever made by a method as claimed in Claim 7 or Claim 8.
10. A method of making a product having good electrical resistance which comprises diluting a sol as claimed in Claim 5, 6 or 9 to a concentration of not more than 10% by weight of solids based on the total weight of the suspension, agitating the suspension sufficiently to keep the synthetic tetra silicic mica particles in suspension whilst any unreacted materials or glassy solids settle out, separating the suspension from said settled out materials, shaping the suspension and dehydrating the shaped suspension.
Il. A method as claimed in Claim 10 including cation exchanging the sodium ions in the mica particles, either in the suspension or after shaping of the suspension, by contacting the particles with a salt solution containing non hydratable cations, namely cations which do not function to cleave the mica into small particles by hydration.
12. A method as claimed in Claim 10 in which the nonhydratable cations are potassium, barium, aluminium, lead, silver, or magnesium ions.
13. A method as claimed in Claim 10 or Claim 11 in which the suspension is shaped by being blended with fibres or particles of minerals or organic resins and then formed into a sheet.
14. A method as claimed in Claim 10 substantially as specifically described herein with reference to Example 1 or Example 2.
15. A product whenever made by a method as claimed in any one of claims 10 to 13.