Glass fiber production method

Abstract

Provided is a method for producing glass fiber, capable of stably performing the spinning of glass fibers without mixing of red crystals in glass fibers. When glass fibers are formed by discharging, from a nozzle tip, a molten glass obtained by melting glass raw materials mixed so as to give a glass composition including, when melted, in relation to the total amount thereof, SiO.sub.2 in a range from 57.0 to 62.0% by mass, Al.sub.2O.sub.3 in a range from 15.0 to 20.0% by mass, MgO in a range from 7.5 to 12.0% by mass, and CaO in a range from 9.0 to 16.5% by mass, and having a total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO of 98.0% by mass or more, the glass composition includes B.sub.2O.sub.3, Li.sub.2O, or B.sub.2O.sub.3 and Li.sub.2O as an additive or additives capable of suppressing the generation of red crystals.

Claims

1. A method for producing glass fiber, when glass fibers are formed by discharging, from a nozzle tip, a molten glass obtained by melting glass raw materials mixed so as to give a glass composition comprising, when melted, in relation to the total amount thereof, SiO.sub.2 in a range from 57.0 to 58.8% by mass, Al.sub.2O.sub.3 in a range from 18.0 to 20.0% by mass, MgO in a range from 7.5 to 12.0% by mass, and CaO in a range from 9.0 to 12.5% by mass, and having a total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO of 98.0% by mass or more, the glass composition comprises B.sub.2O.sub.3, or a combination of B.sub.2O.sub.3 and Li.sub.2O, as an additive or additives capable of suppressing generation of red crystals, wherein a ratio of the product between a content of the additive or additives capable of suppressing generation of red crystals (% by mass) and the content (% by mass) of CaO to the content (% by mass) of Al.sub.2O.sub.3 is in a range from 0.25 to 1.00.

2. The method for producing glass fiber according to claim 1, wherein the glass fibers each have a modified cross sectional shape in which a ratio (major axis/minor axis) of the major axis to the minor axis of the cross sectional shape falls within a range from 2.0 to 6.0.

3. The method for producing glass fiber according to claim 1, wherein the glass fibers each have a modified cross sectional shape in which a fiber diameter defined as a diameter when a cross-sectional area is converted to a perfect circle, falls within a range from 10 to 30 μm.

4. The method for producing glass fiber according to claim 1, wherein the glass fibers have perfect circular cross sections, and fiber diameters falling within a range of 3 μm or more to less than 10 μm.

5. The method for producing glass fiber according to claim 1, wherein the glass raw materials mixed so as to give the glass composition are melted by using a melting furnace in which a portion in contact with the molten glass is formed of bricks containing chromium oxide.

6. The method for producing glass fiber according to claim 1, wherein the glass composition comprises B.sub.2O.sub.3 as the additive capable of suppressing the generation of red crystals, in a content of 0.5 to 1.5% by mass in relation to the total amount of the glass composition.

7. The method for producing glass fiber according to claim 1, wherein the glass composition comprises B.sub.2O.sub.3 and Li.sub.2O as the additives capable of suppressing the generation of red crystals, in a content of 0.5 to 1.5% by mass in relation to the total amount of the glass composition.

8. The method for producing glass fiber according to claim 1, wherein in the glass composition, an 1000-poise temperature thereof is a temperature falling within a range from 1300 to 1370° C., a liquid phase temperature thereof is a temperature falling within a range from 1200 to 1270° C., and a working temperature range thereof is 50° C. or higher.

9. The method for producing glass fiber according to claim 1, wherein the glass fibers each have a modified cross sectional shape in which a ratio (major axis/minor axis) of the major axis to the minor axis of the cross sectional shape falls within a range from 2.0 to 6.0, and a fiber diameter defined as a diameter when a cross-sectional area is converted to a perfect circle, falls within a range from 10 to 30 μm.

10. The method for producing glass fiber according to claim 1, wherein the glass composition comprises CaO in a range of 10.3 to 12.5% by mass.

11. The method for producing glass fiber according to claim 1, wherein the glass composition comprises MgO in a range of 8.8 to 12.0% by mass.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 1.

(2) FIG. 2 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 2.

(3) FIG. 3 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 3.

(4) FIG. 4 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 4.

(5) FIG. 5 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 5.

(6) FIG. 6 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 6.

(7) FIG. 7 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 7.

(8) FIG. 8 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 8.

(9) FIG. 9 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 9.

(10) FIG. 10 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 10.

(11) FIG. 11 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 11.

(12) FIG. 12 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 12.

(13) FIG. 13 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 13.

(14) FIG. 14 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Example 14.

(15) FIG. 15 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Comparative Example 1.

(16) FIG. 16 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Comparative Example 2.

(17) FIG. 17 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Comparative Example 3.

(18) FIG. 18 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Comparative Example 4.

(19) FIG. 19 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Comparative Example 5.

(20) FIG. 20 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Comparative Example 6.

(21) FIG. 21 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition of Comparative Example 7.

DESCRIPTION OF EMBODIMENTS

(22) Hereinafter, the embodiments of the present invention are described in more detail.

(23) In the method for producing glass fiber of the present embodiment, first, there is obtained a glass batch prepared by mixing the glass raw materials so as to give, when the resulting mixture is melted, a glass composition having a composition including, in relation to the total amount thereof, SiO.sub.2 in a range from 57.0 to 62.0% by mass, Al.sub.2O.sub.3 in a range from 15.0 to 20.0% by mass, MgO in a range from 7.5 to 12.0% by mass, CaO in a range from 9.0 to 16.5% by mass, and a component capable of suppressing the generation of the red crystals in a range from 0.5 to 1.5% by mass, and having a total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO of 98.0% by mass or more. As the additive (hereinafter, abbreviated as the red crystal suppressing additive) capable of suppressing the generation of the red crystals, B.sub.2O.sub.3 and Li.sub.2O can be used each alone, or a mixture composed of B.sub.2O.sub.3 and Li.sub.2O can be used.

(24) In the glass composition having the foregoing composition, the 1000-poise temperature is a temperature falling within a range from 1300 to 1370° C., and the liquid phase temperature is a temperature falling within a range from 1200 to 1270° C.

(25) Next, the glass batch is supplied to a melting furnace, and melted in a temperature region equal to or higher than the 1000-poise temperature, specifically at a temperature falling within a temperature range from 1450 to 1550° C. The molten glass melted at the foregoing temperature is discharged from the nozzle tips of a bushing controlled at a predetermined temperature, taken up at a high speed and thus cooled while being stretched, and thus solidified to form glass fibers.

(26) The foregoing melting furnace is a large-scale furnace having a monthly production capacity of a few hundred tons or more, the heating in the furnace is performed by, for example, indirect flame heating using gas burners, and chromium oxide bricks, excellent in glass corrosion resistance at high temperatures, are used in the portion brought into contact with the molten glass.

(27) As the foregoing nozzle tips, for example, when glass fibers having modified cross-sections such as flat shapes are produced, it is possible to use nozzle tips formed in the nozzle plate on the bottom of the bushing, with the openings (orifice holes) each having an opening size such that the ratio (major axis/minor axis) of the major axis to the minor axis falls within a range from 2 to 10, and the major axis is 1.0 to 10.0 mm and the minor axis is 0.5 to 2.0 mm, wherein the nozzle tips have cooling devices such as notches or protrusions to rapidly cool the molten glass having passed through the openings.

(28) As the foregoing nozzle tips, for example, when glass fibers having perfect circular cross sections and having a fiber diameter of 3 μm or more and less than 10 μm are produced, it is possible to use nozzle tips each provided with a circular opening having an opening diameter of 0.5 to 1.5 mm.

(29) When the glass fibers having the modified cross sections such as the flat shapes are produced, the controlled temperature of the bushing is 1260 to 1370° C. When the controlled temperature of the bushing is lower than 1260° C., the viscosity of the molten glass is extremely high, and in addition, the controlled temperature approaches the liquid phase temperature; consequently, the crystals originating from the glass (devitrification) tend to be precipitated, the discharge from the nozzle tips is made difficult and the production of the glass fibers themselves is made difficult. When the controlled temperature of the bushing exceeds 1370° C., the viscosity of the molten glass is lowered, the surface tension is allowed to act easily, and hence it is impossible to produce glass fibers having modified cross sections such as flat shapes.

(30) When glass fibers having perfect circular cross sections and having a fiber diameter of 3 μm or more and less than 10 μm are produced, the controlled temperature of the bushing is 1300 to 1470° C. When the controlled temperature of the bushing is lower than 1300° C., the viscosity of molten glass is high, accordingly the discharge from thin nozzle tips is difficult and the production of the glass fibers themselves is made difficult. When the controlled temperature of the bushing exceeds 1470° C., the molten glass discharged from the nozzle tips turns into droplets but not into fibers.

(31) In the production method of the present embodiment, by producing glass fibers as described above, the spinning can be stably performed without mixing of the red crystals in glass fibers having modified cross sections such as the flat shapes, or glass fibers having perfect circular cross sections and having fiber diameters of 3 μm or more and less than 10 μm.

(32) The glass fibers having modified cross sections can be made to each have the ratio (major axis/minor axis) of the major axis to the minor axis of the cross sectional shape falling within a range from 2.0 to 6.0, or made to each have the fiber diameter defined as the diameter of the perfect circle having the same area as the actual cross-sectional area of the fiber falling within a range from 10 to 30 μm. In each of the glass fibers having modified cross sections, the ratio of the major axis to the minor axis of the cross sectional shape may fall within a range from 2.0 to 6.0, and the fiber diameter defined as the diameter of the perfect circle having the same areas as the actual cross-sectional area of the fiber may fall within a range from 10 to 30 μm.

(33) Hereinafter, Examples and Comparative Examples of the present invention are presented.

EXAMPLES

Example 1

(34) In present Example, first, there was obtained a glass batch prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 59.3% by mass, Al.sub.2O.sub.3 in a content of 19.0% by mass, MgO in a content of 10.0% by mass, CaO in a content of 11.0% by mass, B.sub.2O.sub.3 in a content of 0.5% by mass, and Na.sub.2O, K.sub.2O and Fe.sub.2O.sub.3 as the other components in a content of 0.2% by mass. In the glass composition of present Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 99.3% by mass. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 1.

(35) Next, the glass batch was placed in a platinum crucible, and maintained in an electric furnace at a temperature of 1550° C. for 6 hours to be melted under stirring to yield a uniform molten glass. Next, the obtained molten glass was allowed to flow out on a carbon plate to prepare glass cullet. In this case, the 1000-poise temperature and the liquid phase temperature of the molten glass were measured, and the working temperature range (ΔT) was calculated.

(36) The glass cullet was melted in a platinum crucible by using a high temperature electric furnace equipped with a rotational viscometer (manufactured by Shibaura System Co., Ltd.), and the 1000-poise temperature was determined by measuring the temperature corresponding to the rotational viscosity of 1000 poises by continuously measuring the viscosity of the molten glass while the melting temperature was being varied by using a Brookfield rotation type viscometer.

(37) The liquid phase temperature was determined by the following procedure. First, the glass cullet was crushed, 40 g of the resulting glass particles having particle sizes of 0.5 to 1.5 mm were placed in a 180×20×15 mm platinum boat and heated for 8 hours or more in a tubular electric furnace having a temperature gradient of 1000 to 1400° C., then the glass particles were taken out from the tubular electric furnace and observed with a polarization microscope, and the positions from which the crystals (devitrification) originating from the glass started to be precipitated were specified. The temperature inside the tubular electric furnace was measured by using a type B thermocouple, and the temperature of the position at which the precipitation started was determined to be taken as the liquid phase temperature.

(38) The difference between the 1000-poise temperature and the liquid phase temperature measured by the foregoing methods was calculated as the working temperature range (ΔT).

(39) Next, the obtained glass cullet was placed in a small cylindrical platinum bushing having a circular nozzle tip on the bottom of the vessel, and melted by heating to a predetermined temperature; the molten glass discharged from the nozzle tip was taken up at a predetermined speed and thus cooled and solidified while being stretched, and thus glass fibers having perfect circular cross sections and having a fiber diameter of 13 μm were obtained.

(40) Next, a string of fiber (monofilament) between a nozzle tip and the winder was sampled, a string of fiber free from degradation due to contact or friction was used as a sample, and the fiber strength and the fiber modulus of elasticity of the glass fibers obtained in present Example were measured.

(41) The fiber strength was obtained as follows: a monofilament free from scratches, degradations and the like due to contact, friction and the like was allowed to adhere to a sheet of predetermined backing paper with a hole of 25 mm in diameter in the center thereof to prepare a specimen, the specimen was set to the grips of a tensile tester (manufactured by Orientec Co., Ltd.), the ends of the sheet of backing paper were cut off, then a tensile test was performed at a crosshead speed of 5 mm/min, and the fiber strength was calculated from the maximum load value at break and the fiber cross-sectional area. The fiber cross-sectional area was calculated from the fiber diameter obtained by observing the monofilament with a scanning electron microscope (trade name: S-3400, manufactured by Hitachi, Ltd.). Those specimens undergoing filament cast-off or filament crease during the measurement were excluded, and the average value of the fiber strength values of the 30 specimens was taken as the measurement value of the fiber strength.

(42) The fiber modulus of elasticity was obtained as follows: the monofilament was allowed to adhere to a sheet of predetermined backing paper with a hole of 50 mm in diameter in the center thereof to prepare a specimen, the specimen was set to the grips of the foregoing tensile tester, the ends of the sheet of backing paper were cut off, then a tensile test was performed at a crosshead speed of 5 mm/min, and the fiber modulus of elasticity was calculated from the initial strength variation value and the corresponding rate of elongation. Those specimens undergoing filament cast-off during the measurement were excluded, and the average value of the fiber modulus of elasticity values of the 15 specimens was taken as the measurement value of the fiber modulus of elasticity. The results thus obtained are shown in Table 1.

(43) Next, in present Example, the relationship between the glass composition and the red crystal suppressing component and the red crystals was verified by reproducing the situation leading to the generation of the rarely-occurring red crystals in the production of glass fibers.

(44) In present Example, Cr.sub.2O.sub.3 was added to the glass composition in order to reproduce the situation leading to the generation of the red crystals, the addition amount of Cr.sub.2O.sub.3 was based on the maximum concentration of Cr.sub.2O.sub.3 included in the glass lump staying in the glass melting furnace in which the portion in contact with the molten glass is formed of the chromium oxide bricks. In the glass lump, the Cr.sub.2O.sub.3 eluted over a long period of time from the chromium oxide bricks was condensed, and accordingly the concentration of the Cr.sub.2O.sub.3 capable of being included in the molten glass to be formed into fibers after passing through the melting furnace in a short time does not exceed the maximum concentration of Cr.sub.2O.sub.3 in the glass lump.

(45) Accordingly, next, a glass batch was prepared so as to contain chromium oxide (Cr.sub.2O.sub.3) in a content of 0.10% by mass in relation to the total amount of the glass composition of present Example. Next, the chromium oxide-containing glass batch was placed in a platinum crucible, and maintained in an electric furnace at a temperature of 1550° C. for 6 hours to be melted under stirring to yield a uniform molten glass. Next, the obtained molten glass was allowed to flow out on a carbon plate to prepare glass cullet.

(46) In a 60×30×15 mm platinum boat, 40 g of the obtained glass cullet was placed, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. lower than the controlled temperature of the bushing, and maintained at 1250° C. for 12 hours. Next, the glass was removed from the platinum boat, the interface portion on the platinum surface with the glass was observed by using a laser microscope (trade name: Laser Scanning Microscope LEXT OLS, manufactured by Olympus Corp.) at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 1.

(47) It is to be noted that when the number of the crystals of 10 μm or more was 5 or less in the view field (1.30×1.05 mm) at the microscope magnification of 200, the precipitation of the red crystals was determined not to occur.

Example 2

(48) In present Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 58.8% by mass and B.sub.2O.sub.3 in a content of 1.0% by mass. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 1.

(49) Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Example was used, and glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used. Next, the following quantities were measured in exactly the same manner as in Example 1: the 1000-poise temperature, the liquid phase temperature and the working temperature range of the glass composition of present Example, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Example. The results thus obtained are shown in Table 1.

(50) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 2.

Example 3

(51) In present Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 58.3% by mass and B.sub.2O.sub.3 in a content of 1.5% by mass. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 1.

(52) Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Example was used, and glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used. Next, the following quantities were measured in exactly the same manner as in Example 1: the 1000-poise temperature, the liquid phase temperature and the working temperature range of the glass composition of present Example, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Example. The results thus obtained are shown in Table 1.

(53) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 3.

Example 4

(54) In present Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 59.2% by mass and Li.sub.2O in a content of 0.6% by mass, but absolutely not including B.sub.2O.sub.3. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 1.

(55) Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Example was used, and glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used. Next, the following quantities were measured in exactly the same manner as in Example 1: the 1000-poise temperature, the liquid phase temperature and the working temperature range of the glass composition of present Example, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Example. The results thus obtained are shown in Table 1.

(56) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 4.

Example 5

(57) In present Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 58.8% by mass and Li.sub.2O in a content of 1.0% by mass, but absolutely not including B.sub.2O.sub.3. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 1.

(58) Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Example was used, and glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used. Next, the following quantities were measured in exactly the same manner as in Example 1: the 1000-poise temperature, the liquid phase temperature and the working temperature range of the glass composition of present Example, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Example. The results thus obtained are shown in Table 1.

(59) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 5.

Example 6

(60) In present Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 58.9% by mass, B.sub.2O.sub.3 in a content of 0.6% by mass and Li.sub.2O in a content of 0.3% by mass. The glass composition of present Example includes, in relation to the total amount thereof, B.sub.2O.sub.3 and Li.sub.2O as a mixture of these in a content of 0.9% by mass. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 1.

(61) Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Example was used, and glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used. Next, the following quantities were measured in exactly the same manner as in Example 1: the 1000-poise temperature, the liquid phase temperature and the working temperature range of the glass composition of present Example, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Example. The results thus obtained are shown in Table 1.

(62) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 6.

Example 7

(63) In present Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 59.6% by mass, Al.sub.2O.sub.3 in a content of 18.2% by mass, MgO in a content of 9.2% by mass, CaO in a content of 11.8% by mass, B.sub.2O.sub.3 in a content of 0.9% by mass, Li.sub.2O in a content of 0.1% by mass, and Na.sub.2O, K.sub.2O and Fe.sub.2O.sub.3 as the other components in a content of 0.2% by mass. The glass composition of present Example includes, in relation to the total amount thereof, B.sub.2O.sub.3 and Li.sub.2O as a mixture of these in a content of 1.0% by mass. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 1.

(64) Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Example was used, and glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used. Next, the following quantities were measured in exactly the same manner as in Example 1: the 1000-poise temperature, the liquid phase temperature and the working temperature range of the glass composition of present Example, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Example. The results thus obtained are shown in Table 1.

(65) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 7.

Example 8

(66) In present Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 58.8% by mass, Al.sub.2O.sub.3 in a content of 19.5% by mass, MgO in a content of 9.0% by mass, CaO in a content of 12.0% by mass, B.sub.2O.sub.3 in a content of 0.5% by mass, and Na.sub.2O, K.sub.2O and Fe.sub.2O.sub.3 as the other components in a content of 0.2% by mass. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 2.

(67) Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Example was used, and glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used. Next, the following quantities were measured in exactly the same manner as in Example 1: the 1000-poise temperature, the liquid phase temperature and the working temperature range of the glass composition of present Example, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Example. The results thus obtained are shown in Table 2.

(68) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 8.

Example 9

(69) In present Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 58.8% by mass, Al.sub.2O.sub.3 in a content of 19.0% by mass, MgO in a content of 9.0% by mass, CaO in a content of 12.0% by mass, B.sub.2O.sub.3 in a content of 1.0% by mass, and Na.sub.2O, K.sub.2O and Fe.sub.2O.sub.3 as the other components in a content of 0.2% by mass. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 2.

(70) Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Example was used, and glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used. Next, the following quantities were measured in exactly the same manner as in Example 1: the 1000-poise temperature, the liquid phase temperature and the working temperature range of the glass composition of present Example, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Example. The results thus obtained are shown in Table 2.

(71) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 9.

Example 10

(72) In present Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 58.8% by mass, Al.sub.2O.sub.3 in a content of 18.5% by mass, MgO in a content of 9.0% by mass, CaO in a content of 12.0% by mass, B.sub.2O.sub.3 in a content of 1.5% by mass, and Na.sub.2O, K.sub.2O and Fe.sub.2O.sub.3 as the other components in a content of 0.2% by mass. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 2.

(73) Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Example was used, and glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used. Next, the following quantities were measured in exactly the same manner as in Example 1: the 1000-poise temperature, the liquid phase temperature and the working temperature range of the glass composition of present Example, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Example. The results thus obtained are shown in Table 2.

(74) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 10.

Example 11

(75) In present Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 62.0% by mass, Al.sub.2O.sub.3 in a content of 16.0% by mass, MgO in a content of 8.8% by mass, CaO in a content of 12.0% by mass, B.sub.2O.sub.3 in a content of 1.0% by mass, and Na.sub.2O, K.sub.2O and Fe.sub.2O.sub.3 as the other components in a content of 0.2% by mass. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 2.

(76) Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Example was used, and glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used. Next, the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Example were measured in exactly the same manner as in Example 1. The results thus obtained are shown in Table 2.

(77) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 11.

Example 12

(78) In present Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 59.0% by mass, Al.sub.2O.sub.3 in a content of 18.0% by mass, MgO in a content of 8.0% by mass, CaO in a content of 13.8% by mass, B.sub.2O.sub.3 in a content of 1.0% by mass, and Na.sub.2O, K.sub.2O and Fe.sub.2O.sub.3 as the other components in a content of 0.2% by mass. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 2.

(79) Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Example was used, and glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used. Next, in exactly the same manner as in Example 1, the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Example were measured. The results thus obtained are shown in Table 2.

(80) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 12.

Example 13

(81) In present Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 58.3% by mass, Al.sub.2O.sub.3 in a content of 18.0% by mass, MgO in a content of 9.0% by mass, CaO in a content of 13.5% by mass, B.sub.2O.sub.3 in a content of 1.0% by mass, and Na.sub.2O, K.sub.2O and Fe.sub.2O.sub.3 as the other components in a content of 0.2% by mass. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 2.

(82) Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Example was used, and glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used. Next, in exactly the same manner as in Example 1, the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Example were measured. The results thus obtained are shown in Table 2.

(83) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 13.

Example 14

(84) In present Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 59.8% by mass, Al.sub.2O.sub.3 in a content of 20.0% by mass, MgO in a content of 8.0% by mass, CaO in a content of 11.0% by mass, B.sub.2O.sub.3 in a content of 1.0% by mass, and Na.sub.2O, K.sub.2O and Fe.sub.2O.sub.3 as the other components in a content of 0.2% by mass. The composition of the glass composition obtained by melting the glass batch of present Example is shown in Table 2.

(85) Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Example was used, and glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used. Next, in exactly the same manner as in Example 1, the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Example were measured. The results thus obtained are shown in Table 2.

(86) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 14.

Comparative Example 1

(87) In present Comparative Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 59.8% by mass, but absolutely not including B.sub.2O.sub.3. The composition of the glass composition obtained by melting the glass batch of present Comparative Example is shown in Table 3.

(88) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 15.

Comparative Example 2

(89) In present Comparative Example, a glass batch was obtained in exactly the same manner as in Example 2 except that Na.sub.2O was used in place of B.sub.2O.sub.3. The composition of the glass composition obtained by melting the glass batch of present Comparative Example is shown in Table 3.

(90) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 16.

Comparative Example 3

(91) In present Comparative Example, a glass batch was obtained in exactly the same manner as in Example 2 except that K.sub.2O was used in place of B.sub.2O.sub.3. The composition of the glass composition obtained by melting the glass batch of present Comparative Example is shown in Table 3.

(92) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 17.

Comparative Example 4

(93) In present Comparative Example, a glass batch was obtained in exactly the same manner as in Example 2 except that SrO was used in place of B.sub.2O.sub.3. The composition of the glass composition obtained by melting the glass batch of present Comparative Example is shown in Table 3.

(94) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 18.

Comparative Example 5

(95) In present Comparative Example, a glass batch was obtained in exactly the same manner as in Example 2 except that Y.sub.2O.sub.3 was used in place of B.sub.2O.sub.3. The composition of the glass composition obtained by melting the glass batch of present Comparative Example is shown in Table 3.

(96) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 19.

Comparative Example 6

(97) In present Comparative Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 61.0% by mass, Al.sub.2O.sub.3 in a content of 20.0% by mass, MgO in a content of 12.0% by mass, CaO in a content of 5.8% by mass, B.sub.2O.sub.3 in a content of 1.0% by mass, and Na.sub.2O, K.sub.2O and Fe.sub.2O.sub.3 as the other components in a content of 0.2% by mass. The composition of the glass composition obtained by melting the glass batch of present Comparative Example is shown in Table 3.

(98) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 20.

Comparative Example 7

(99) In present Comparative Example, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the glass raw materials so as to give, when the resulting mixture was melted, a melt (molten glass) of a glass composition including, in relation to the total amount thereof, SiO.sub.2 in a content of 57.1% by mass, Al.sub.2O.sub.3 in a content of 20.7% by mass, MgO in a content of 12.0% by mass, CaO in a content of 9.0% by mass, B.sub.2O.sub.3 in a content of 1.0% by mass, and Na.sub.2O, K.sub.2O and Fe.sub.2O.sub.3 as the other components in a content of 0.2% by mass. The composition of the glass composition obtained by melting the glass batch of present Comparative Example is shown in Table 3.

(100) Next, a chromium oxide-containing glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and in exactly the same manner as in Example 1, the glass cullet was placed in a platinum boat, melted in an electric furnace at 1550° C. for 2 hours, then decreased in temperature to 1250° C. and maintained at 1250° C. for 12 hours. Next, in exactly the same manner as in Example 1, the interface portion on the platinum surface with the glass was observed by using a laser microscope at a magnification of 200, and thus the occurrence or nonoccurrence of the red crystals was examined. The results thus obtained are shown in FIG. 21.

(101) TABLE-US-00001 TABLE 1 Examples 1 2 3 4 5 6 7 SiO.sub.2 (% by mass) 59.3 58.8 58.3 59.2 58.8 58.9 59.6 Al.sub.2O.sub.3 (% by mass) 19.0 19.0 19.0 19.0 19.0 19.0 18.2 MgO (% by mass) 10.0 10.0 10.0 10.0 10.0 10.0 9.2 CaO (% by mass) 11.0 11.0 11.0 11.0 11.0 11.0 11.8 B.sub.2O.sub.3 (% by mass) 0.5 1.0 1.5 — — 0.6 0.9 Li.sub.2O (% by mass) — — — 0.6 1.0 0.3 0.1 Others (% by mass) 0.2 0.2 0.2 0.2 0.2 0.2 0.2 (R × CaO)/Al.sub.2O.sub.3 0.29 0.58 0.87 0.35 0.58 0.52 0.65 Li.sub.2O/B.sub.2O.sub.3 0.0 0.0 0.0 — — 0.5 1.0 1000-Poise temperature (° C.) 1338 1331 1321 — — — — Liquid phase temperature (° C.) 1229 1221 1215 — — — — Working range temperature (° C.) 109 110 106 — — — — Fiber strength (GPa) 4.3 4.2 4.1 4.1 4.1 4.2 4.3 Fiber modulus of elasticity (GPa) 85 84 83 84 83 84 83 In the table, “R” represents “the red crystal suppressing component,” and “(R × CaO)/Al.sub.2O.sub.3” represents “(R (% by mass) × CaO (% by mass))/Al.sub.2O.sub.3 (% by mass).”

(102) TABLE-US-00002 TABLE 2 Examples 8 9 10 11 12 13 14 SiO.sub.2 (% by mass) 58.8 58.8 58.8 62.0 59.0 58.3 59.8 Al.sub.2O.sub.3 (% by mass) 19.5 19.0 18.5 16.0 18.0 18.0 20.0 MgO (% by mass) 9.0 9.0 9.0 8.8 8.0 9.0 8.0 CaO (% by mass) 12.0 12.0 12.0 12.0 13.8 13.5 11.0 B.sub.2O.sub.3 (% by mass) 0.5 1.0 1.5 1.0 1.0 1.0 1.0 Li.sub.2O (% by mass) 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Others (% by mass) 0.2 0.2 0.2 0.2 0.2 0.2 0.2 (R × CaO)/Al.sub.2O.sub.3 0.31 0.63 0.97 0.75 0.77 0.75 0.55 Li.sub.2O/B.sub.2O.sub.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1000-Poise temperature 1334 1333 1325 — — 1317 1365 (° C.) Liquid phase 1241 1223 1207 — — 1220 1253 temperature (° C.) Working range 93 110 118 — — 97 112 temperature (° C.) Fiber strength (GPa) 4.2 4.3 4.0 4.3 4.1 4.1 4.0 Fiber modulus of 84 84 83 81 82 82 82 elasticity (GPa) In the table, “R” represents “the red crystal suppressing component,” and “(R × CaO)/Al.sub.2O.sub.3” represents “(R (% by mass) × CaO (% by mass))/Al.sub.2O.sub.3 (% by mass).”

(103) TABLE-US-00003 TABLE 3 Comparative Examples 1 2 3 4 5 6 7 SiO.sub.2 (% by mass) 59.8 58.8 58.8 58.8 58.8 61.0 57.1 Al.sub.2O.sub.3 (% by mass) 19.0 19.0 19.0 19.0 19.0 20.0 20.7 MgO (% by mass) 10.0 10.0 10.0 10.0 10.0 12.0 12.0 CaO (% by mass) 11.0 11.0 11.0 11.0 11.0 5.8 9.0 Na.sub.2O (% by mass) — 1.0 — — — — — B.sub.2O.sub.3 (% by mass) — — — — — 1.0 1.0 K.sub.2O (% by mass) — — 1.0 — — — — SrO (% by mass) — — — 1.0 — — — Y.sub.2O.sub.3 (% by mass) — — — — 1.0 — — Others (% by mass) 0.2 0.2 0.2 0.2 0.2 0.2 0.2

(104) As can be seen from FIGS. 15 to 19 and Table 3, in the case where B.sub.2O.sub.3 and Li.sub.2O are absolutely not included (Comparative Example 1) or in the case where in place of B.sub.2O.sub.3 or Li.sub.2O, any one of Na.sub.2O.sub.3, K.sub.2O, SrO and Y.sub.2O.sub.3 is included (Comparative Examples 2 to 5), microparticles (red crystals) are generated in the interface portion on the platinum surface with the glass.

(105) In contrast, as can be seen from FIGS. 1 to 14 and Tables 1 and 2, in the case where either of B.sub.2O.sub.3 and Li.sub.2O, or a mixture composed of B.sub.2O.sub.3 and Li.sub.2O is included in a content of 0.5 to 1.5% by mass of the total amount of the glass composition (Examples 1 to 7), microparticles (red crystals) are not generated at all in the interface portion on the platinum surface with the glass.

(106) However, as can be seen from FIGS. 20 and 21 and Table 3, in the case where B.sub.2O.sub.3 is included in a content of 1.0% by mass of the total amount of the glass composition, but the content of CaO is less than 9.0% by mass (Comparative Example 6) or the content of Al.sub.2O.sub.3 exceeds 20.0% by mass (Comparative Example 7), microparticles are generated in the interface portion on the platinum surface with the glass, and the generation of red crystals cannot be suppressed.

(107) Accordingly, it is apparent that according to the production method of the present invention, by using the glass compositions of Examples 1 to 14, the spinning of glass fibers can be stably performed without mixing of red crystals in glass fibers.

(108) As can be seen from Tables 1 and 2, according to the production method of the present invention, it is possible to obtain glass fibers having a sufficient fiber modulus of elasticity of 80 GPa or more and a sufficient fiber strength of 4.0 GPa or more.

Example 15

(109) In present Example, the glass batch obtained in Example 1 was melted in a large size furnace in which the portion brought into contact with the molten glass was formed of chromium oxide bricks, and the obtained molten glass was discharged from the nozzle tips of a bushing regulated at a temperature of 1300° C. The nozzle tips have oval-shaped orifice holes.

(110) Consequently, there were obtained glass fibers having an elongated oval-shaped cross section and having a modified cross section in which the ratio (major axis/minor axis) of the major axis to the minor axis of the cross sectional shape was 4.

(111) The obtained glass fibers having a modified cross section did not include red crystals, and the spinning of the glass fibers was able to be performed without occurrence of spun fiber breakage over 8 hours or more.

Example 16

(112) In present Example, the glass batch obtained in Example 1 was melted in a large size furnace in which the portion brought into contact with the molten glass was formed of chromium oxide bricks, and the obtained molten glass was discharged from the nozzle tips of a bushing regulated at a temperature of 1350° C. The nozzle tips have circle-shaped orifice holes of 1 mm in diameter.

(113) Consequently, there were obtained glass fibers having a circular cross sectional shape and having a fiber diameter of 5 μm.

(114) The obtained glass fibers having a circular cross sectional shape and a fiber diameter of 5 μm did not include red crystals, and the spinning of the glass fibers was able to be performed without occurrence of spun fiber breakage over 8 hours or more.