JPH0959039A - Glass bead having high refractive index - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a glass bead having high whiteness (transparency) and high refractive index by preventing coloring of a glass bead even when mixed with colored impurities. SOLUTION: This glass bead is composed of, by mass, 30-55% TiO2 , 20-55% BaO, 0-17% ZnO, 0-6% CaO, 1-15% (ZnO+CaO) and 2-50ppm cobalt oxide. Preferably, the based has >=1.85 refractive index.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to high-refractive-index glass beads for retroreflection, and more particularly to glass beads in which coloring of the glass beads is eliminated.

[0002]

2. Description of the Related Art Since glass beads used for a reflective material such as a retroreflective sheet are required to have high brightness, glass having a high refractive index is used. Generally, such a glass bead composition includes TiO ₂ or B which is a high refractive index oxide.
TiO ₂ —BaO based glass containing aO as a main component is used. However, glass containing TiO ₂ , BaO, etc. at an extremely high ratio is apt to devitrify and is not suitable for production on an industrial scale. Therefore, glass beads actually mass-produced have a refractive index of 1.85 to 85. It is usually about 2.20.

The refractive index of retroreflective materials, mainly glass beads used for retroreflective sheets, is 2.23 to 2.30.
It is said that it is desirable that the glass beads have a refractive index of 2.20 or less.
According to the method described in Japanese Patent Publication No. 57-28375 and Japanese Patent Publication No. 57-28375, means for increasing the refractive index by heat treatment is used.

According to JP-B-48-17844, glass beads containing at least 25% by weight of a glass-forming component capable of crystallizing are heat-treated under predetermined conditions.
The refractive index can be increased by changes that are believed to be due to nucleation or transition of structural units that lack the formation of crystals of sufficient size to cause light scattering and opacification.

[0005]

The above-mentioned refractive index is 1.85.
In the glass beads of about 2.20, the glass beads may appear yellowish due to impurities in the glass raw material, impurities mixed in during the manufacturing process such as the raw material mixing step and the melting step. Further, if the TiO ₂ content is high, the color itself may appear yellowish.

By performing the above heat treatment, T
In the iO ₂ -BaO system glass, fine barium titanium oxide crystals, which are high refractive index substances, are precipitated in the glass beads,
It has been found that this increases the refractive index of the glass beads. However, depending on the heat treatment conditions,
Although the refractive index is increased, it often happens that the glass beads are colored yellow.

In any case, the yellowing of the glass beads causes a problem in terms of whiteness such that the sheet looks yellowish when used for a reflective material, particularly a white retroreflective sheet, and the yellowed glass beads are Must be treated as defective.

When the cause of this is investigated, if iron or other coloring impurities are present in the glass constituting the glass beads, the barium titanium oxide crystals are colored in the glass or the heat-treated one, and the entire glass beads are It turned out to be colored yellow. Such a problem of coloring due to impurities can be technically possible by strict control so as to prevent mixing of an impurity component having a coloring property. However, in the actual manufacturing site, managing the mixing of impurities such as iron so that coloring is not reliably seen requires a cost increase due to the upgrading of raw material grades and non-ferrous / clean manufacturing equipment, It will be impossible to supply glass beads at the price required in the market.

Therefore, in the present invention, in the high-refractive-index glass beads for retroreflection, the coloring of the glass beads is prevented even when a certain amount of coloring impurities are mixed, and the high-refractive index of high whiteness (transparency) is high. Aims to provide rate glass beads.

[0010]

In order to achieve the above object, the present invention improves the color tone by adding a trace amount of cobalt oxide to the glass composition. That is, according to the present invention, TiO _{2 is} 30 to 60% and BaO is 20 to 55% by mass.
%, ZnO 0-17%, CaO 0-6%, ZnO +
CaO 1-15%, cobalt oxide 2-50ppm
It has a composition consisting of

Since the present invention is a high-refractive-index glass bead for retroreflection, the refractive index is 1.85 or more and the particle size is 2 or more.
It is 5 to 150 μm.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The glass beads of the present invention can be manufactured as follows. First, raw materials are mixed so as to have the above composition, and the mixture is uniformly mixed, and then housed in a platinum crucible and placed in an electric furnace at 1200
Melted at 1500 ° C. for about 30 minutes. For mass production, a melting furnace made of electroformed brick is used. Next, this melt is granulated with water and further pulverized with a ball mill to form fine particles, which are put into the flame of a gas burner at 1400 to 2000 ° C. to float, and the spherical shape by surface tension is taken out and cooled. And obtain glass beads. The obtained glass beads are sieved to collect 25 to 150 μm.

Next, the action of each component constituting the present invention and the reason for limiting the composition range as described above will be described.

TiO ₂ is a component that most significantly contributes to increasing the refractive index of glass. Therefore, it is preferable to contain a large amount in order to enhance the retroreflectivity of the glass beads, but if it exceeds 60%, the melting temperature of the glass rises, the devitrification becomes high, and the color tone becomes yellow, so 60% is recommended. The upper limit was set. On the other hand, if it is less than 30%, the refractive index of 1.85 or more, which should be retained as high refractive index glass beads, cannot be obtained.
It is preferably 33 to 55%.

BaO is added in combination with TiO ₂ to lower the melting temperature of glass and to form crystals of barium titanium oxide when heat treatment is performed after bead molding. If the content is less than 20%, there is no effect of improving the meltability, and vitrification becomes difficult, and 5
If it exceeds 5%, the refractive index tends to decrease and vitrification becomes difficult. It is preferably in the range of 30 to 50%.

ZnO is more gradual than BaO, but has the effect of lowering the melting temperature, and the reduction in refractive index by addition is smaller than that of BaO. However, if added in excess of 15%, the devitrification tendency becomes stronger, so the content is limited to 15%. Preferably it is in the range of 5 to 10%.

CaO has the effect of remarkably improving the devitrification property of glass if the amount of addition is appropriate, but if it exceeds 6%, the refractive index decreases. It is preferably up to 4%.

Cobalt oxide is known as a coloring agent for glass that exhibits a blue color when added, but it has the effect of making the glass that is yellowed by adding an appropriate amount colorless due to the complementary color relationship. However, if the addition amount is less than 2 ppm, the effect cannot be obtained, and if it exceeds 50 ppm, the coloring by the cobalt oxide becomes more remarkable and the glass beads look bluish. It is preferably added in the range of 5 to 30 ppm.

In addition to the above, ZrO ₂ may be used in place of ZnO, and in this case also, the upper limit is 15%, and ZnO and ZrO ₂ are used.
_{Even when 2} is used in combination, the upper limit is preferably up to 15%. Further, SrO may be added in the range of up to 6% instead of CaO, and when CaO and SrO are used in combination, it does not matter if the total amount of both is up to 5%. In addition to these, for example, one or both of B ₂ O ₃ and SiO ₂ within a range of up to 2%, Al ₂ O ₃ ,
Any one kind or two or more kinds of Li ₂ O, Na ₂ O, K ₂ O and MgO can be added within a range of up to 3%.
The addition of these components mainly improves the meltability and the devitrification property, but the addition of these components in excess of their respective upper limits affects the refractive index, so it is desirable to keep the above upper limits.

One of the major causes of the glass beads appearing colored yellow is iron contained in the raw material as an impurity or mixed in from the manufacturing process. However, most of the coloring impurities such as iron are contained. When the above cobalt oxide is added to glass which is originally not colored, the glass naturally looks blue. However, a slight blue color that is not clearly recognized as a blue color makes the glass beads look transparent, so that a very small amount of cobalt oxide near the above lower limit can be added. As the coloring source of the glass beads, there are impurities other than iron and impurities due to TiO ₂ as described above, so it cannot be said that this is the case. However, when converted to iron, the Fe ₂ O ₃ content is 1
The effect of the present invention is obtained within the range of about 00 to 600 ppm. When Fe ₂ O ₃ is less than 100 ppm, coloring due to iron is hardly recognized, and the addition of cobalt oxide increases the tendency to blue. This is because when the content of Fe ₂ O ₃ exceeds 600 ppm, even if cobalt oxide is added, the glass only darkens and no improvement effect can be obtained. .

Regarding the particle size of the glass beads, considering the reflection characteristics of the retroreflective material, if the particle diameter is less than 25 μm, the desired reflection characteristics cannot be ensured, and if it exceeds 150 μm, the sheet is formed into a reflection sheet. The flexural strength of is decreased, and sufficient flexibility cannot be obtained. The thickness is more preferably 45 to 75 μm for a retroreflective sheet.

As for the refractive index, as described above, 1.85 is necessary for the retroreflective material, and when the reflective sheet is used, the reflection brightness is 1.9 to 2.3, which is a good result.

[0023]

EXAMPLES Examples of the present invention will be described below. Table 1 shows examples of the present invention. In Table 1, No. 1 to No. 12 are examples of the present invention, and No. 13 to No. 15 are comparative examples. The composition in the table is shown by mass percentage, and the refractive index is the value obtained by the Becke line method.

Raw materials were prepared so as to have the composition shown in Table 1, and glass beads having a particle size of 25 to 150 μm were obtained according to the above-mentioned production method. With respect to the obtained glass beads, the color tone was visually evaluated, and the results are shown in Table 1.
In the examples of the present invention, except for Nos. 7, 9, and 12, no color could be visually sensed. The samples of Nos. 7, 9, and 12 seemed to have a slight yellowish tint, but there was no problem in practical use. On the other hand, in the samples of Nos. 13 and 14, as a result of adding cobalt oxide in excess of 50 ppm, the color tone became bluish despite the high TiO ₂ content. Further, the sample of No. 15 is a glass having almost the same composition as that of the example and no cobalt oxide was added, but it was clearly colored yellow.

In addition, the glass beads described above are used in Japanese Patent Publication No. 57-283.
According to the method described in Japanese Patent Laid-Open No. 75-75, each was placed in a refractory container, housed in an electric furnace, heated to 700 to 780 ° C., held for 5 hours, and then further heated to 780 ° C. and held for 10 hours. After that, it was naturally cooled to increase the refractive index by about 0.05 from the value in Table 1. The color tone of the glass beads whose refractive index was increased by the heat treatment in this manner was evaluated in the same manner as above. Although some of the color densities had some changes, the basic coloring tendency was the same as in Table 1, and the samples of Comparative Examples Nos. 13 to 15 were visually confirmed to be colored.

[0026]

[Table 1]

[Table 1]

As described above, according to the present invention, the coloring caused by the inevitable impurities and TiO ₂ which is the main component, which occur in the industrially produced high-refractive-index glass beads, is substantially eliminated, and it is colorless and transparent. Glass beads can be obtained. Moreover, since the decolorizing effect is exhibited even in a glass containing a coloring impurity such as iron, it is possible to manufacture using conventional manufacturing equipment without using a high-purity raw material, which causes a cost increase. In addition, it is possible to provide glass beads having higher whiteness than ever before.

Claims (3)

[Claims] 1. TiO ₂ 30 to 60% by weight, Ba
O 20-55%, ZnO 0-17%, CaO 0-
6%, ZnO + CaO 1-15%, cobalt oxide
High refractive index glass beads having a composition of 2 to 50 ppm. 2. The high refractive index glass beads according to claim 1, which has a refractive index of 1.85 or more. 3. The particle size is 25 to 150 μm.
Alternatively, the high refractive index glass beads according to item 2.