GLASS COMPOSITION FOR GLASS FIBER

Abstract

Provided is a glass composition for glass fiber allowing spinning to be stably performed without mixing of red foreign substances into glass fibers. The glass composition for glass fiber includes, in relation to the total amount thereof, SiO.sub.2 in a content falling within a range from 57.0 to 60.0% by mass, Al.sub.2O.sub.3 in a content falling within a range from 17.5 to 20.0% by mass, MgO in a content falling within a range from 8.5 to 12.0% by mass, CaO in a content falling within a range from 10.0 to 13.0% by mass and B.sub.2O.sub.3 in a content falling within a range from 0.5 to 1.5% by mass, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO being 98.0% by mass or more.

Claims

1. A glass composition for glass fiber comprising, in relation to the total amount thereof, SiO.sub.2 in a content falling within a range from 57.0 to 60.0% by mass, Al.sub.2O.sub.3 in a content falling within a range from 17.5 to 20.0% by mass, MgO in a content falling within a range from 8.5 to 12.0% by mass, CaO in a content falling within a range from 10.0 to 13.0% by mass and B.sub.2O.sub.3 in a content falling within a range from 0.5 to 1.5% by mass, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO being 98.0% by mass or more.

2. The glass composition for glass fiber according to claim 1, comprising, in relation to the total amount thereof, SiO.sub.2 in a content falling within a range from 57.5 to 59.5% by mass, Al.sub.2O.sub.3 in a content falling within a range from 18.0 to 19.5% by mass, MgO in a content falling within a range from 8.8 to 11.5% by mass and CaO in a content falling within a range from 10.3 to 12.5% by mass.

3. The glass composition for glass fiber according to claim 1, comprising, in relation to the total amount thereof, SiO.sub.2 in a content falling within a range from 58.0 to 59.3% by mass, Al.sub.2O.sub.3 in a content falling within a range from 18.2 to 19.0% by mass, MgO in a content falling within a range from 9.0 to 11.0% by mass and CaO in a content falling within a range from 10.5 to 12.0% by mass.

4. The glass composition for glass fiber according to claim 1, comprising, in relation to the total amount thereof, SiO.sub.2 in a content falling within a range from 58.0 to 59.3% by mass, Al.sub.2O.sub.3 in a content falling within a range from 18.2 to 19.0% by mass, MgO in a content falling within a range from 9.0 to 11.0% by mass and CaO in a content falling within a range from 10.5 to 11.9% by mass.

5. The glass composition for glass fiber according to claim 1, wherein the ratio (CaO (% by mass)/Al.sub.2O.sub.3 (% by mass)) of the content of CaO (% by mass) to the content of Al.sub.2O.sub.3 (% by mass) falls within a range from 0.50 to 0.72, and the ratio ((B.sub.2O.sub.3 (% by mass)×CaO (% by mass))/Al.sub.2O.sub.3 (% by mass)) of the product of the content of B.sub.2O.sub.3 (% by mass) and the content (% by mass) of CaO to the content (% by mass) of Al.sub.2O.sub.3 falls within a range from 0.22 to 1.00.

6. The glass composition for glass fiber according to claim 1, wherein the glass fibers produced from the glass composition for glass fiber comprises Cr.sub.2O.sub.3 in a content falling within a range from 0.001 to 0.010% by mass.

7. The glass composition for glass fiber according to claim 1, wherein the fiber modulus of elasticity of the glass fibers produced from the glass composition for glass fiber is 83 GPa or more.

8. The glass composition for glass fiber according to claim 1, wherein the linear expansion coefficient of the glass fibers produced from the glass composition for glass fiber in a temperature range of 50 to 200° C. is 4.2 ppm/K or less.

9. The glass composition for glass fiber according to claim 1, wherein the fiber strength of the glass fibers produced from the glass composition for glass fiber is 4.0 GPa or more.

10. Glass fibers formed from a glass composition for glass fiber comprising, in relation to the total amount thereof, SiO.sub.2 in a content falling within a range from 57.0 to 60.0% by mass, Al.sub.2O.sub.3 in a content falling within a range from 17.5 to 20.0% by mass, MgO in a content falling within a range from 8.5 to 12.0% by mass, CaO in a content falling within a range from 10.0 to 13.0% by mass and B.sub.2O.sub.3 in a content falling within a range from 0.5 to 1.5% by mass, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO being 98.0% by mass or more, wherein the glass fibers have non-circular cross sections each having 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 and 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.

11. Glass fibers formed from a glass composition for glass fiber comprising, in relation to the total amount thereof, SiO.sub.2 in a content falling within a range from 57.0 to 60.0% by mass, Al.sub.2O.sub.3 in a content falling within a range from 17.5 to 20.0% by mass, MgO in a content falling within a range from 8.5 to 12.0% by mass, CaO in a content falling within a range from 10.0 to 13.0% by mass and B.sub.2O.sub.3 in a content falling within a range from 0.5 to 1.5% by mass, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO being 98.0% by mass or more, wherein the glass fibers have perfect circular cross sections and fiber diameters falling within a range of 3 μm or more and less than 10 μm.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0045] FIG. 1 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Example 1.

[0046] FIG. 2 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Example 2.

[0047] FIG. 3 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Example 3.

[0048] FIG. 4 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Comparative Example 1.

[0049] FIG. 5 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Comparative Example 2.

[0050] FIG. 6 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Comparative Example 3.

[0051] FIG. 7 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Example 4.

[0052] FIG. 8 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Example 5.

[0053] FIG. 9 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Example 6.

[0054] FIG. 10 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Example 7.

[0055] FIG. 11 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Comparative Example 4.

[0056] FIG. 12 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Example 8.

[0057] FIG. 13 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Example 9.

[0058] FIG. 14 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Example 10.

[0059] FIG. 15 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Comparative Example 5.

[0060] FIG. 16 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Comparative Example 6.

[0061] FIG. 17 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Comparative Example 7.

[0062] FIG. 18 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Comparative Example 8.

[0063] FIG. 19 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Comparative Example 9.

[0064] FIG. 20 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Comparative Example 10.

[0065] FIG. 21 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Comparative Example 11.

[0066] FIG. 22 is a laser microscope photograph showing the occurrence or nonoccurrence of the red crystals in the glass obtained from the glass composition for glass fiber of Comparative Example 12.

DESCRIPTION OF EMBODIMENTS

[0067] Hereinafter, the embodiments of the present invention are described in more detail.

[0068] The glass composition for glass fiber of present Embodiment includes, in relation to the total amount thereof, SiO.sub.2 in a content falling within a range from 57.0 to 60.0% by mass, Al.sub.2O.sub.3 in a content falling within a range from 17.5 to 20.0% by mass, MgO in a content falling within a range from 8.5 to 12.0% by mass, CaO in a content falling within a range from 10.0 to 13.0% by mass and B.sub.2O.sub.3 in a content falling within a range from 0.5 to 1.5% by mass, and the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.0% by mass or more.

[0069] The glass composition for glass fiber includes B.sub.2O.sub.3 as an additive capable of suppressing the generation of the red crystals, in a content falling within the foregoing range, accordingly can prevent the mixing of the red crystals into the obtained glass fibers, and allows the production of glass fibers to be performed stably. The glass composition for glass fiber includes SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO in the foregoing contents in relation to the total amount of the glass composition for glass fiber, and accordingly even when B.sub.2O.sub.3 is added in a content falling within the foregoing range, it is possible to produce glass fibers having a higher fiber strength and a higher fiber modulus of elasticity than the general-purpose glass.

[0070] The glass composition for glass fiber can be obtained by melting the glass raw materials (glass batch) mixed so as to give the foregoing composition.

[0071] In the glass composition for glass fiber having the foregoing composition, the 1000-poise temperature is a temperature falling within a range from 1300 to 1350° C., and the liquidus temperature is a temperature falling within a range from 1200 to 1250° C. In this case, the working temperature range, a temperature range between the 1000-poise temperature and the liquidus temperature, is 50° C. or higher, and accordingly a stable spinning is made possible, and such a working temperature range is suitable for a large-scale production of glass fibers, having a monthly production capacity of a few hundred tons or more. In the glass composition for glass fiber having the foregoing composition, the working temperature range is, for example, a range from 100 to 130° C., preferably a range from 102 to 125° C. and more preferably a range from 105 to 122° C.

[0072] In the production of glass fibers from the glass composition for glass fiber, the glass raw materials mixed as described above are 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.

[0073] 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 abrasion resistance at high temperatures, are used in the portion brought into contact with the molten glass.

[0074] As the foregoing nozzle tips, for example, when glass fibers having non-circular cross sections such as flat shaped glass fibers 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 of the major axis to the minor axis (major axis/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.

[0075] 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.

[0076] When the glass fibers having non-circular cross sections such as flat shaped glass fibers are produced, the controlled temperature of the bushing is 1260 to 1350° 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 liquidus temperature; consequently, the crystals originating from the glass (devitrification) tend to be precipitated. Therefore, 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 1350° C., the viscosity of the molten glass is lowered, and the surface tension is allowed to act easily. Hence it is impossible to produce glass fibers having non-circular cross sections such as flat shaped glass fibers.

[0077] 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 1450° 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 1450° C., the molten glass discharged from the nozzle tips turns into droplets but not into fibers.

[0078] According to the glass composition for glass fiber of present Embodiment, by producing glass fibers as described above, the spinning can be stably performed without mixing of the red crystals into the glass fibers. Hereinafter, Examples and Comparative Examples of the present invention are presented.

EXAMPLES

Example 1

[0079] In present Example, first, a glass batch was prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition (molten glass) for glass fiber including, in relation to the total amount thereof, SiO2 in a content of 59.3% by mass, Al2O3 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, B2O3 in a content of 0.5% by mass, and Na2O, K2O and Fe2O3 as the other components in a content of 0.2% by mass. In the glass composition for glass fiber 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 for glass fiber of present Example is shown in Table 1.

[0080] 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 liquidus temperature of the molten glass were measured, and the working temperature range (ΔT) was calculated.

[0081] 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.

[0082] The liquidus 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 vessel with boat-like shape 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 crystals started to be precipitated was determined to be taken as the liquidus temperature.

[0083] The difference between the 1000-poise temperature and the liquidus temperature measured by the foregoing methods was calculated as the working temperature range (ΔT). The results thus obtained are shown in Table 2.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] The linear expansion coefficient was measured as follows. First, the glass cullet was melted, and then the melt of the glass cullet was cooled to prepare a glass bulk material; the glass bulk material was heated at a distortion-removing temperature (660 to 750° C.) for 2 hours in order to remove the distortion of the glass bulk material, and cooled to room temperature (20 to 25° C.) over 8 hours; then from the glass bulk material, a 4×4×20 mm specimen was prepared. Next, the specimen was heated at a temperature increase rate of 10° C./min, the elongation magnitude was measured within a temperature range from 50 to 200° C. by using a thermal mechanical analyzer (manufactured by Hitachi High-Tech Science Corp.), and from the elongation magnitude, the linear expansion coefficient was calculated.

[0089] The fiber strength, the fiber modulus of elasticity and the linear expansion coefficient of the glass fiber obtained in present Example are shown in Table 2.

[0090] Next, in present Example, the relationship between the glass composition for glass fiber and the red crystals was verified by reproducing the situation leading to the generation of the infrequently-occurring red crystals in the production of glass fibers.

[0091] In present Example, Cr.sub.2O.sub.3 was added to the glass composition for glass fiber 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.

[0092] 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 for glass fiber 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.

[0093] In a 60×30×15 mm platinum vessel with boat-like shape, 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 vessel with boat-like shape, 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 and Table 2.

[0094] 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

[0095] In present Example, first, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) 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. In the glass composition for glass fiber of present Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.8% by mass. The composition of the glass composition for glass fiber of present Example is shown in Table 1.

[0096] 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 liquidus temperature, the working temperature range and the linear expansion coefficient of the glass composition for glass fiber 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.

[0097] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 2.

Example 3

[0098] In present Example, first, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) 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. In the glass composition for glass fiber of present Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.3% by mass. The composition of the glass composition for glass fiber of present Example is shown in Table 1.

[0099] 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 liquidus temperature, the working temperature range and the linear expansion coefficient of the glass composition for glass fiber 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.

[0100] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 2.

Comparative Example 1

[0101] In present Comparative Example, first, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) including, in relation to the total amount thereof, SiO.sub.2 in a content of 59.8% by mass and absolutely not including B.sub.2O.sub.3. In the glass composition for glass fiber of present Comparative Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 99.8% by mass. The composition of the glass composition for glass fiber of present Comparative Example is shown in Table 1.

[0102] Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Comparative 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 liquidus temperature, the working temperature range and the linear expansion coefficient of the glass composition for glass fiber of present Comparative Example, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Comparative Example. The results thus obtained are shown in Table 2.

[0103] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 2.

Comparative Example 2

[0104] In present Comparative Example, first, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) including, in relation to the total amount thereof, SiO.sub.2 in a content of 59.5% by mass and B.sub.2O.sub.3 in a content of 0.3% by mass. In the glass composition for glass fiber of present Comparative Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 99.5% by mass. The composition of the glass composition for glass fiber of present Comparative Example is shown in Table 1.

[0105] Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Comparative 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 liquidus temperature, the working temperature range and the linear expansion coefficient of the glass composition for glass fiber of present Comparative Example, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Comparative Example. The results thus obtained are shown in Table 2.

[0106] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 2.

Comparative Example 3

[0107] In present Comparative Example, first, a glass batch was obtained in exactly the same manner as in Example 1 except that the glass batch was prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) including, in relation to the total amount thereof, SiO.sub.2 in a content of 57.8% by mass and B.sub.2O.sub.3 in a content of 2.0% by mass. In the glass composition for glass fiber of present Comparative Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 97.8% by mass. The composition of the glass composition for glass fiber of present Comparative Example is shown in Table 1.

[0108] Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Comparative 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 liquidus temperature, the working temperature range and the linear expansion coefficient of the glass composition for glass fiber of present Comparative Example, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Comparative Example. The results thus obtained are shown in Table 2.

[0109] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 2.

TABLE-US-00001 TABLE 1 Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 1 Example 2 Example 3 SiO.sub.2 (% by mass) 59.3 58.8 58.3 59.8 59.5 57.8 Al.sub.2O.sub.3 (% by mass) 19.0 19.0 19.0 19.0 19.0 19.0 MgO (% by mass) 10.0 10.0 10.0 10.0 10.0 10.0 CaO (% by mass) 11.0 11.0 11.0 11.0 11.0 11.0 B.sub.2O.sub.3 (% by mass) 0.5 1.0 1.5 — 0.3 2.0 Others (% by mass) 0.2 0.2 0.2 0.2 0.2 0.2 CaO/Al.sub.2O.sub.3 0.58 0.58 0.58 0.58 0.58 0.58 (B.sub.2O.sub.3 × CaO)/Al.sub.2O.sub.3 0.29 0.58 0.87 0 0.17 1.16
In the table, “CaO/Al.sub.2O.sub.3” represents “CaO (% by mass)/Al.sub.2O.sub.3 (% by mass).” “(B.sub.2O.sub.3×CaO)/Al.sub.2O.sub.3” represents “(B.sub.2O.sub.3 (% by mass)×Cao (% by mass))/Al.sub.2O.sub.3 (% by mass).”

TABLE-US-00002 TABLE 2 Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 1 Example 2 Example 3 1000-Poise temperature (° C.) 1338 1331 1321 1346 1342 1318 Liquidus temperature (° C.) 1229 1221 1215 1236 1233 1207 Working temperature range (° C.) 109 110 106 110 109 111 Fiber strength (GPa) 4.3 4.2 4.1 4.4 4.3 3.9 Fiber modulus of elasticity (GPa) 85 84 83 86 85 82 Linear expansion coefficient 4.0 4.0 4.0 4.0 4.0 4.1 (ppm/K) Occurrence/nonoccurrence of Not Not Not Observed Observed Not red crystals observed observed observed observed

[0110] As can be seen from FIGS. 4 and 5 and Table 1, when the content of B.sub.2O.sub.3 is less than 0.5% by mass of the total amount of the glass composition for glass fiber (Comparative Examples 1 and 2), microparticles (red crystals) are generated in the glass structure of the glass cullet.

[0111] In contrast, as can be seen from FIGS. 1 to 3 and Table 1, when the content of B.sub.2O.sub.3 falls within a range from 0.5 to 1.5% by mass of the total amount of the glass composition for glass fiber (Examples 1 to 3), microparticles (red crystals) are not generated at all in the glass structure of the glass cullet.

[0112] As can be seen from FIG. 6 and Table 1, when the content of B.sub.2O.sub.3 exceeds 1.5% by mass of the total amount of the glass composition for glass fiber (Comparative Example 3), microparticles (red crystals) are not generated in the glass structure of the glass cullet, but it is impossible to obtain a sufficient fiber modulus of elasticity of 83 GPa or more and a sufficient fiber strength of 4.0 GPa or more.

[0113] Moreover, as is obvious from Tables 1 and 2, according to the glass compositions for glass fiber in each of Examples 1 to 3 in which in the composition of the glass composition for glass fiber of Comparative Example 1, 0.5 to 1.5% by mass of B.sub.2O.sub.3 is contained and the content of SiO.sub.2 is reduced by the content of B.sub.2O.sub.3, it is possible to obtain, in the glass fiber, a fiber strength and a fiber modulus of elasticity equivalent to the fiber strength and the fiber modulus of elasticity of Comparative Example 1.

Example 4

[0114] In present Example, first, there was obtained a glass batch prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) 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. In the glass composition for glass fiber 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 for glass fiber of present Example is shown in Table 3.

[0115] 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 3.

[0116] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape 1, 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 and Table 3.

Example 5

[0117] In present Example, first, a glass batch was obtained in exactly the same manner as in Example 4 except that the glass batch was prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) including, in relation to the total amount thereof, Al.sub.2O.sub.3 in a content of 19.0% by mass and B.sub.2O.sub.3 in a content of 1.0% by mass. In the glass composition for glass fiber of present Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.8% by mass. The composition of the glass composition for glass fiber of present Example is shown in Table 3.

[0118] 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 3.

[0119] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape 1, 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 and Table 3.

Example 6

[0120] In present Example, first, a glass batch was obtained in exactly the same manner as in Example 4 except that the glass batch was prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) including, in relation to the total amount thereof, Al.sub.2O.sub.3 in a content of 18.5% by mass and B.sub.2O.sub.3 in a content of 1.5% by mass. In the glass composition for glass fiber of present Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.3% by mass. The composition of the glass composition for glass fiber of present Example is shown in Table 3.

[0121] 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 3.

[0122] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 3.

Example 7

[0123] In present Example, first, a glass batch was obtained in exactly the same manner as in Example 5 except that the glass batch was prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) including, in relation to the total amount thereof, Al.sub.2O.sub.3 in a content of 18.2% by mass, MgO in a content of 10.0% by mass and CaO in a content of 11.8% by mass. In the glass composition for glass fiber of present Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.8% by mass. The composition of the glass composition for glass fiber of present Example is shown in Table 3.

[0124] 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 3.

[0125] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 3.

Comparative Example 4

[0126] In present Comparative Example, first, a glass batch was obtained in exactly the same manner as in Example 4 except that the glass batch was prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) including, in relation to the total amount thereof, Al.sub.2O.sub.3 in a content of 20.0% by mass and absolutely not including B.sub.2O.sub.3. In the glass composition for glass fiber of present Comparative Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 99.8% by mass. The composition of the glass composition for glass fiber of present Comparative Example is shown in Table 3.

[0127] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape 1, 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 and Table 3.

TABLE-US-00003 TABLE 3 Comparative Example 4 Example 5 Example 6 Example 7 Example 4 SiO.sub.2 (% by mass) 58.8 58.8 58.8 58.8 58.8 Al.sub.2O.sub.3 (% by mass) 19.5 19.0 18.5 18.2 20.0 MgO (% by mass) 9.0 9.0 9.0 10.0 9.0 CaO (% by mass) 12.0 12.0 12.0 11.8 12.0 B.sub.2O.sub.3 (% by mass) 0.5 1.0 1.5 1.0 — Others (% by mass) 0.2 0.2 0.2 0.2 0.2 CaO/Al.sub.2O.sub.3 0.62 0.63 0.65 0.65 0.60 (B.sub.2O.sub.3 × CaO)/Al.sub.2O.sub.3 0.31 0.63 0.97 0.65 0 Fiber strength (GPa) 4.2 4.3 4.0 4.2 — Fiber modulus of elasticity 84 84 83 83 — (GPa) Occurrence/nonoccurrence Not observed Not observed Not observed Not observed Observed of red crystals In the table, “CaO/Al.sub.2O.sub.3” represents “CaO (% by mass)/Al.sub.2O.sub.3 (% by mass).” “(B.sub.2O.sub.3 × CaO)/Al.sub.2O.sub.3” represents “(B.sub.2O.sub.3 (% by mass) × CaO (% by mass))/Al.sub.2O.sub.3 (% by mass).”

[0128] As can be seen from FIG. 11 and Table 3, in the case of Comparative Example 4 absolutely not including B.sub.2O.sub.3, microparticles (red crystals) are generated in the glass structure of the glass cullet.

[0129] In contrast, as is obvious from FIGS. 7 to 10 and Table 3, it is possible to get rid of the precipitation of the red crystals, according to the glass compositions for glass fiber of Examples 4 to 6 in which in the composition of the glass composition for glass fiber of Comparative Example 4, 0.5 to 1.5% by mass of B.sub.2O.sub.3 is included and the content of Al.sub.2O.sub.3 is reduced by the content of B.sub.2O.sub.3, and according to the glass composition for glass fiber of Example 7 in which 1.0% by mass of B.sub.2O.sub.3 is included, the content of Al.sub.2O.sub.3 and the content of CaO are reduced and the content of MgO is increased.

Example 8

[0130] In present Example, first, there was obtained a glass batch prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) 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 19.0% by mass, MgO in a content of 12.0% by mass, CaO in a content of 10.0% 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 for glass fiber 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 for glass fiber of present Example is shown in Table 4.

[0131] 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 4.

[0132] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 4.

Example 9

[0133] In present Example, first, a glass batch was obtained in exactly the same manner as in Example 8 except that the glass batch was prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) including, in relation to the total amount thereof, Al.sub.2O.sub.3 in a content of 18.7% by mass, MgO in a content of 11.8% by mass and B.sub.2O.sub.3 in a content of 1.0% by mass. In the glass composition for glass fiber of present Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.8% by mass. The composition of the glass composition for glass fiber of present Example is shown in Table 4.

[0134] 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 4.

[0135] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape 1, 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 and Table 4.

Example 10

[0136] In present Example, first, a glass batch was obtained in exactly the same manner as in Example 8 except that the glass batch was prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) including, in relation to the total amount thereof, Al.sub.2O.sub.3 in a content of 18.5% by mass, MgO in a content of 11.5% by mass and B.sub.2O.sub.3 in a content of 1.5% by mass. In the glass composition for glass fiber of present Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.3% by mass. The composition of the glass composition for glass fiber of present Example is shown in Table 4.

[0137] 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 4.

[0138] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 4.

Comparative Example 5

[0139] In present Comparative Example, first, a glass batch was obtained in exactly the same manner as in Example 8 except that the glass batch was prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) including, in relation to the total amount thereof, SiO.sub.2 in a content of 58.8% by mass and absolutely not including B.sub.2O.sub.3. In the glass composition for glass fiber of present Comparative Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 99.8% by mass. The composition of the glass composition for glass fiber of present Comparative Example is shown in Table 4.

[0140] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape 1, 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 and Table 4.

TABLE-US-00004 TABLE 4 Compar- Exam- Exam- Exam- ative ple 8 ple 9 ple 10 Example 5 SiO.sub.2 (% by mass) 58.3 58.3 58.3 58.8 Al.sub.2O.sub.3 (% by mass) 19.0 18.7 18.5 19.0 MgO (% by mass) 12.0 11.8 11.5 12.0 CaO (% by mass) 10.0 10.0 10.0 10.0 B.sub.2O.sub.3 (% by mass) 0.5 1.0 1.5 — Others (% by mass) 0.2 0.2 0.2 0.2 CaO/Al.sub.2O.sub.3 0.53 0.53 0.54 0.53 (B.sub.2O.sub.3 × CaO)/Al.sub.2O.sub.3 0.26 0.53 0.81 0 Fiber strength (GPa) 4.1 4.2 4.0 — Fiber modulus of 84 84 84 — elasticity (GPa) Occurrence/ Not Not Not Observed nonoccurrence observed observed observed of red crystals In the table, “CaO/Al.sub.2O.sub.3” represents “CaO (% by mass)/Al.sub.2O.sub.3 (% by mass).” “(B.sub.2O.sub.3 × CaO)/Al.sub.2O.sub.3” represents “(B.sub.2O.sub.3 (% by mass) × CaO (% by mass))/Al.sub.2O.sub.3 (% by mass).”

[0141] As can be seen from FIG. 15 and Table 4, in the case of Comparative Example 5 absolutely not including B.sub.2O.sub.3, microparticles (red crystals) are generated in the glass structure of the glass cullet.

[0142] In contrast, as is obvious from FIGS. 12 to 14 and Table 4, it is possible to get rid of the precipitation of the red crystals, according to the glass compositions for glass fiber of Examples 8 to 10 in which in the composition of the glass composition for glass fiber of Comparative Example 5, 0.5 to 1.5% by mass of B.sub.2O.sub.3 is included and only the content of SiO.sub.2 or the contents of SiO.sub.2, Al.sub.2O.sub.3, MgO are reduced by the content of B.sub.2O.sub.3.

Comparative Example 6

[0143] In present Comparative Example, first, there was obtained a glass batch prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) including, in relation to the total amount thereof, SiO.sub.2 in a content of 57.0% by mass, Al.sub.2O.sub.3 in a content of 22.0% by mass, MgO in a content of 9.8% by mass, CaO in a content of 10.0% 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. In the glass composition for glass fiber of present Comparative Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.8% by mass. The composition of the glass composition for glass fiber of present Comparative Example is shown in Table 5.

[0144] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 5.

Comparative Example 7

[0145] In present Comparative Example, first, there was obtained a glass batch prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) 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 20.0% by mass, MgO in a content of 12.0% by mass, CaO in a content of 7.8% 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. In the glass composition for glass fiber of present Comparative Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.8% by mass. The composition of the glass composition for glass fiber of present Comparative Example is shown in Table 5.

[0146] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 5.

Comparative Example 8

[0147] In present Comparative Example, first, there was obtained a glass batch prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) including, in relation to the total amount thereof, SiO.sub.2 in a content of 57.5% by mass, Al.sub.2O.sub.3 in a content of 22.0% by mass, MgO in a content of 11.8% by mass, CaO in a content of 7.5% 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. In the glass composition for glass fiber of present Comparative Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.8% by mass. The composition of the glass composition for glass fiber of present Comparative Example is shown in Table 5.

[0148] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 5.

TABLE-US-00005 TABLE 5 Comparative Comparative Comparative Example 6 Example 7 Example 8 SiO.sub.2 (% by mass) 57.0 59.0 57.5 Al.sub.2O.sub.3 (% by mass) 22.0 20.0 22.0 MgO (% by mass) 9.8 12.0 11.8 CaO (% by mass) 10.0 7.8 7.5 B.sub.2O.sub.3 (% by mass) 1.0 1.0 1.0 Others (% by mass) 0.2 0.2 0.2 CaO/Al.sub.2O.sub.3 0.45 0.39 0.34 (B.sub.2O.sub.3 × CaO)/Al.sub.2O.sub.3 0.45 0.39 0.34 Occurrence/nonoccurrence Observed Observed Observed of red crystals In the table, “CaO/Al.sub.2O.sub.3” represents “CaO (% by mass)/Al.sub.2O.sub.3 (% by mass).” “(B.sub.2O.sub.3 × CaO)/Al.sub.2O.sub.3” represents “(B.sub.2O.sub.3 (% by mass) × CaO (% by mass))/Al.sub.2O.sub.3 (% by mass).”

[0149] As is obvious from FIGS. 16 to 18 and Table 5, it is impossible to prevent the precipitation of the red crystals, even when the content of B.sub.2O.sub.3 in relation to the total amount of the glass composition for glass fiber falls within a range from 0.5 to 1.5% by mass, in the case of Comparative Example 6 in which the content of Al.sub.2O.sub.3 in relation to the total amount of the glass composition for glass fiber exceeds 20.0% by mass, in the case of Comparative Example 7 in which the content of CaO in relation to the total amount of the glass composition for glass fiber is less than 10.0% by mass, and in the case of Comparative Example 8 in which the content of Al.sub.2O.sub.3 in relation to the total amount of the glass composition for glass fiber exceeds 20.0% by mass and the content of CaO in relation to the total amount of the glass composition for glass fiber is less than 10.0% by mass.

Comparative Example 9

[0150] In present Comparative Example, first, there was obtained a glass batch prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) 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 16.0% by mass, MgO in a content of 10.5% by mass, CaO in a content of 12.5% 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. In the glass composition for glass fiber of present Comparative Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.8% by mass. The composition of the glass composition for glass fiber of present Comparative Example is shown in Table 6.

[0151] Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Comparative Example was used, and the 1000-poise temperature, the liquidus temperature and the working temperature range of the glass composition for glass fiber of present Comparative Example were measured. Glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Comparative Example were measured in exactly the same manner as in Example 1. The results thus obtained are shown in Table 6.

[0152] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 6.

Comparative Example 10

[0153] In present Example, first, there was obtained a glass batch prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) 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. In the glass composition for glass fiber of present Comparative Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.8% by mass. The composition of the glass composition for glass fiber of present Comparative Example is shown in Table 6.

[0154] Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Comparative Example was used, and the 1000-poise temperature, the liquidus temperature and the working temperature range of the glass composition for glass fiber of present Comparative Example were measured. Glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Comparative Example were measured in exactly the same manner as in Example 1. The results thus obtained are shown in Table 6.

[0155] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 6.

Comparative Example 11

[0156] In present Comparative Example, first, there was obtained a glass batch prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) 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. In the glass composition for glass fiber of present Comparative Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.8% by mass. The composition of the glass composition for glass fiber of present Comparative Example is shown in Table 6.

[0157] Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Comparative Example was used, and the 1000-poise temperature, the liquidus temperature and the working temperature range of the glass composition for glass fiber of present Comparative Example were measured. Glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) obtained in present Comparative Example were measured in exactly the same manner as in Example 1. The results thus obtained are shown in Table 6.

[0158] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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 and Table 6.

Comparative Example 12

[0159] In present Comparative Example, first, there was obtained a glass batch prepared by mixing the respective glass raw materials so as to give, when the resulting mixture was melted, a melt of a glass composition for glass fiber (molten glass) including, in relation to the total amount thereof, SiO.sub.2 in a content of 57.0% by mass, Al.sub.2O.sub.3 in a content of 18.0% by mass, MgO in a content of 13.0% by mass, CaO in a content of 10.8% 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. In the glass composition for glass fiber of present Comparative Example, the total content of SiO.sub.2, Al.sub.2O.sub.3, MgO and CaO is 98.8% by mass. The composition of the glass composition for glass fiber of present Comparative Example is shown in Table 6.

[0160] Next, glass cullet was prepared in exactly the same manner as in Example 1 except that the glass batch of present Comparative Example was used, and the 1000-poise temperature, the liquidus temperature and the working temperature range of the glass composition for glass fiber of present Comparative Example were measured. Glass fibers were spun in exactly the same manner as in Example 1 except that the glass cullet was used; in present Comparative Example, the content of MgO exceeded 12.0% by mass, and accordingly, the 1000-poise temperature and the liquidus temperature were close to each other, and the working temperature range was narrowed. Consequently, in present Comparative Example, the spinning of glass fibers was not able to be stably performed, and the fiber strength and the fiber modulus of elasticity of the glass fibers (monofilaments) were not able to be measured. The results thus obtained are shown in Table 6.

[0161] Next, a chromium oxide-containing glass batch was prepared in exactly the same manner as in Example 1 except that the glass batch obtained in present Comparative Example was used, and glass cullet was prepared in exactly the same manner as in Example 1 except that the chromium oxide-containing glass batch was used. Next, in exactly the same manner as in Example 1, the obtained glass cullet was placed in a platinum vessel with boat-like shape, 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. 22 and Table 6.

TABLE-US-00006 TABLE 6 Compar- Compar- Compar- Compar- ative ative ative ative Exam- Exam- Exam- Exam- ple 9 ple 10 ple 11 ple 12 SiO.sub.2 (% by mass) 59.8 58.3 59.8 57.0 Al.sub.2O.sub.3 (% by mass) 16.0 18.0 20.0 18.0 MgO (% by mass) 10.5 9.0 8.0 13.0 CaO (% by mass) 12.5 13.5 11.0 10.8 B.sub.2O.sub.3 (% by mass) 1.0 1.0 1.0 1.0 Others (% by mass) 0.2 0.2 0.2 0.2 CaO/Al.sub.2O.sub.3 0.78 0.75 0.55 0.60 (B.sub.2O.sub.3 × CaO)/Al.sub.2O.sub.3 0.78 0.75 0.55 0.60 1000-Poise temperature 1314 1317 1365 1273 (° C.) Liquidus temperature 1226 1220 1253 1242 (° C.) Working temperature 88 97 112 31 range (° C.) Fiber strength (GPa) 3.8 4.1 4.0 — Fiber modulus of 81 82 82 — elasticity (GPa) Occurrence/ Not Not Not Not nonoccurrence observed observed observed observed of red crystals In the table, “CaO/Al.sub.2O.sub.3” represents “CaO (% by mass)/Al.sub.2O.sub.3 (% by mass).” “(B.sub.2O.sub.3 × CaO)/Al.sub.2O.sub.3” represents “(B.sub.2O.sub.3 (% by mass) × CaO (% by mass))/Al.sub.2O.sub.3 (% by mass).”

[0162] As is obvious from FIGS. 19 to 22 and Table 6, microparticles (red crystals) are not generated in the glass structure of the glass cullet, but it is impossible to obtain a sufficient fiber modulus of elasticity of 83 GPa or more, in the case of Comparative Example 9 in which the content of Al.sub.2O.sub.3 in relation to the total amount of the glass composition for glass fiber is less than 17.5% by mass, in the case of Comparative Example 10 in which the content of CaO in relation to the total amount of the glass composition for glass fiber exceeds 13.0% by mass, and in the case of Comparative Example 11 in which the content of MgO in relation to the total amount of the glass composition for glass fiber is less than 8.5% by mass,

[0163] In addition, in the case of Comparative Example 12 in which the content of MgO exceeds 12.0% by mass, the 1000-poise temperature and the liquidus temperature are close to each other and the working temperature range is as narrow as lower than 50° C., and accordingly, it is difficult to stably perform the spinning of glass fibers, and this case is not suitable for the production of glass fibers.