JPH0139979B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention provides sufficient strength and hardness required for cover glasses for watches such as mobile watches and table clocks.
The present invention also relates to a cover glass for wristwatches that supplies plate-shaped raw materials that are advantageous as a cost reduction method. Conventionally, as a cover glass for a wristwatch, a borosilicate glass that is generally used as an optical glass, such as a glass called BK-7, has been used. However, since the original purpose of optical glasses such as BK-7 is optical, they focus on optical properties such as refractive index and dispersion, and they also contain boric acid (B ₂ O ₃ ). Since it contains more than 10%, it has excellent strength, but it has a high viscosity when forming the raw material, so it is supplied as a raw material for menko-shaped cover glass with a thickness of about 3 mm and a diameter depending on the purpose. was. Therefore, in order to process it as a regular cover glass, the thickness of the finished cover glass must be 0.8~1.
Because it is as thin as mm, it takes a lot of man-hours to finish it by grinding from the raw material size mentioned above. In addition, as mentioned above, since it is an optical glass, care is taken in raw materials and processing methods to maintain optical properties that are not necessarily necessary for a cover glass for wristwatches, resulting in high raw material costs. . On the other hand, recently, in order to eliminate the cost disadvantages due to the shape of the raw material of borosilicate glass, soda lime glass type glass, commonly known as blue plate or white plate, has come into use. However, when it comes to sheet materials, after cutting raw material close to the thickness of the finished cover glass or having the same size into an external shape that is close to the finished shape, such as a rectangle, square, or circle, the outer diameter is ground and the surface is polished if necessary. Because it can be completed by polishing and then chemically strengthening, it has a significant cost reduction effect compared to conventional borosilicate glass. However, regarding the blue plate,
As the name suggests, the trace amount of iron (Fe ₂ O ₃ ) in the glass gives it a bluish-green appearance, so it could not be used as glass for luxury goods or ordinary dress watches. On the other hand, white plates have a plate-like shape and are colorless and transparent, so there are no problems with their appearance and they can also be expected to reduce costs. Because it is inferior to borosilicate glasses and similar soda-lime glasses, it is necessary to increase the plate thickness for practical use, which goes against the recent trend toward thinner glass, and has the disadvantage of a narrow range of application. In view of the above-mentioned drawbacks, the present invention provides a plate-shaped raw material that has sufficient strength required for a watch cover glass, is colorless and transparent, and is advantageous as a cost reduction method. That is, the composition of the glass according to the present invention is:
All in terms of weight ratio: SiO ₂ 61.9-62.9 Na ₂ O 12.6-14.8 K ₂ O 1.4-7.1 CaO 5.3-8.5 ZnO 1.3-4.8 MgO 3.0-4.5 B ₂ O ₃ 1.2-3.4 A ₂ O ₃ 1.4-4.6 TiO ₂ , As ₂ O ₃ , Sb ₂ O ₃ , SO ₃ , ZrO ₂ , P ₂ O ₅ individually 0.8% or less, combined 1.89% or less, Fe ₂ O ₃ 0.01%
Consists of the following and unavoidable impurities. Next, a compressive stress layer is formed on the glass surface by bringing the glass having the above composition into contact with a potassium molten salt. In the present invention, the compressive stress layer increases with the strengthening treatment time, but at the same time, stress relaxation at the treatment temperature also progresses, and the surface compressive stress value, which is a measure of strength, becomes low. Even if the stress layer is too shallow, a slight scratch will cause the reinforcing layer to pass through and reach the base glass, making it more likely to break. Therefore, the optimal stress layer depth is 25 to 80 μm, particularly 30 to 50 μm. As the potassium molten salt, carbonates, bicarbonates, sulfates, bisulfates, nitrates, chlorides, or mixtures thereof can be used, but potassium nitrate molten salt is most suitable because it is easy to manage. However, metal ions such as copper, neodymium, aluminum, and strontium may be included in this. Next, the reason for limiting each component of the glass composition according to the present invention will be described. SiO ₂ is the basic substance that forms glass,
Necessary as a mesh structure formation material. In order to provide sufficient strength for a watch cover glass, ion exchange strengthening is performed through contact with molten potassium salt, but the strain generated during ion exchange does not relax, but instead becomes compressive stress. In order to be retained, the glass mesh structure must be strong. When SiO ₂ is less than 61.9%, the mesh structure becomes weak, compressive stress caused by ion exchange strengthening is relaxed, and sufficient strength cannot be obtained. When SiO ₂ exceeds 62.9%, the amount of alkali, which is responsible for strengthening ion exchange, becomes relatively small, and the amount of ion exchange decreases. Moreover, the general glass melting temperature and viscosity for obtaining the plate-shaped raw material, which is the aim of the present invention, cannot be obtained, making processing into a plate shape difficult. Na ₂ O has an important effect on the ion exchange effect and significantly lowers the formability in raw material processing, that is, the melting point and viscosity, making it possible to process the material into a plate shape. The higher the Na ₂ O content, the higher the efficiency of ion exchange strengthening, but if it exceeds 14.8, the number of non-crosslinked oxygen ions in the glass structure will increase, leading to a decrease in glass hardness and chemical durability, as well as weakening of the glass structure. Glass with high strength cannot be obtained.
As mentioned above, Na ₂ O significantly lowers the softening point and improves formability, making it easier to process into plate materials. The Na ₂ O content is required to be 12.6% or more in order to fully exhibit workability into plate materials and ion exchange effects. K ₂ O is added to prevent devitrification during the production of glass raw materials, and K ₂ O also has the effect of deepening the stress layer due to ion exchange strengthening. 1.4
The effect becomes apparent from the addition of 7.1%, and the effect increases as the amount added increases, but since the K ₂ O raw material is expensive, addition of up to 7.1% is sufficient. CaO is extremely effective in improving the mechanical properties of glass. Furthermore, the addition of Na ₂ O has the effect of compensating for the chemical durability that is slightly lowered. These effects are
Appears at 5.3%. However, CaO also has the effect of stopping the movement of alkali ions, so in order to enhance the effect of strengthening ion exchange, the upper limit is 8.5%. Due to the synergistic effect with CaO mentioned above, MgO
and MgO in almost the same number of moles (weight ratio CaO:MgO
≒2:1), the viscosity is the lowest, making it a useful element for plate processing. As for the addition range, the above
From the relationship with the amount of CaO added, there will be no practical problem if it is set to 3.0 to 4.5%. ZnO is a compound that improves strength, from 1.3 to
Addition in the range of 4.8% is sufficient. B ₂ O ₃ has the effect of lowering viscosity, and its effect appears from 1.2%, and the viscosity decreases as the amount added increases, but when it is added over 10%, the viscosity increases, which is seen in borosilicate glasses. This makes processing into a plate shape difficult, so it is limited to 3.4% or less. A ₂ O ₃ has the effect of preventing devitrification phenomenon, and
It is an element that significantly improves the mechanical properties of glass and is useful for preventing stress relaxation during ion exchange strengthening treatment. The effect appears from 1.4% and increases as the amount added increases, but on the other hand, it increases the melting point, so it is limited to 4.6% or less. Next, TiO ₂ , As ₂ O ₃ , Sb ₂ O ₃ , SO ₃ , ZrO ₂ ,
P ₂ O ₅ are all mesh structure forming elements,
By increasing the amount added, the mechanical properties of the glass can be improved, but due to the effect of adding B ₂ O ₃ and A ₂ O ₃ , which are also mesh structure forming elements in the present invention, when added alone, it is 0.8% or less, and when combined In case of addition
1.89% or less is sufficient. Regarding Fe ₂ O ₃ , as can be seen from the fact that it is a cause of coloring of blue plate glass, it is necessary to suppress it to 0.01% or less in order to obtain the colorless and transparent glass that is the aim of the present invention. Next, the present invention will be explained in further detail based on Examples. Example 1 In Table 1, A is conventional borosilicate glass,
B indicates a conventional soda-lime glass, and C indicates a glass according to the invention. Cutting, grinding, and polishing each glass with the components shown in Table 1 to form a watch cover glass with a diameter of 3 cm and a thickness of 1 mm.The glass was immersed in 100% potassium nitrate molten salt at 400°C for 5 hours to obtain the effect of ion exchange. In order to confirm this, the compressive stress value and stress layer depth on the glass surface were measured non-destructively using a glass surface stress meter FMS-1000 manufactured by Toshiba Glass. In addition, regarding the breaking strength, which is a guideline for practical design, using a compression testing machine, a load was applied at a loading rate of 2 mm per minute using a simple support on the periphery and a center load method, and the load at the time of glass breakage was measured. As shown in Table 1, glass C according to the present invention has a plate-like raw material shape, and is significantly superior to conventional glasses A and B in terms of surface stress value, stress layer depth, and fracture strength. . It is also colorless and transparent, and the light transmittance was measured using a spectrophotometer.
It was good at 92%. Example 2 Next, in Table 2, D is a conventional borosilicate glass, which has superior strength characteristics than A in Example 1, and is therefore used as special high-strength glass for reinforced waterproofing. be. Table 2 E and F show glasses according to the present invention, and these three types of glasses were produced in the same manner as in Example 1 and comparatively tested. As a result, as shown in Table 2, glasses E and F according to the present invention both exhibited significantly superior performance to glass D. Glasses E and F were colorless and transparent, and exhibited a light transmittance of 93%. As described above, according to the present invention, by mixing predetermined components in predetermined amounts, it has excellent mechanical strength as a glass material, is inexpensive, and has excellent toughening properties. We can supply plate-shaped raw materials for colorless and transparent watch cover glasses, and those that have undergone strengthening treatment are
It has sufficient strength as a watch cover glass.

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Claims (1)

[Claims] 1 SiO ₂ 61.9-62.9% by weight,
_Na2O12.6 ~14.8%, _K2O1.4 ~7.1%, CaO5.3~
8.5%, ZnO1.3~4.8%, MgO3.0~4.5%,
_{B2O3 1.2-3.4} %, _{A2O3 1.4-4.6} %, _TiO2 ,
As ₂ O ₃ , Sb ₂ O ₃ , SO ₃ , ZrO ₂ , P ₂ O ₅ alone at 0.8%
The following has a glass composition consisting of 1.89% or less of composite addition, 0.01% or less of Fe ₂ O ₃ and unavoidable impurities,
A cover glass for a watch, characterized in that a compressive stress layer is formed on the glass surface by contacting it with a potassium molten salt.