Boron-free glass composition and filtration media

Details Boron-free glass composition and filtration media Boron-free glass composition and filtration media

1999-08-03

2001-08-21

Marcheschi, Michael (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S057000, C501S068000, C501S069000, C501S072000, C501S035000, C501S038000, C055S527000, C055S522000, C428S357000, C442S180000, C065S376000, C065S465000

Reexamination Certificate

active

06277777

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to glass compositions with unique combinations of properties which are especially useful for high-efficiency fiberglass clean-room filters, especially filters for microelectronic clean-rooms where boron contamination on electronic integrated circuit chips must be avoided. Such filters are herein referred to as High Efficiency Particle Air or “HEPA” filters. The glass compositions of the present invention are substantially free of boron in the sense that no boron oxide (B
2
O
3
) is intentionally added to the glass compositions as an ingredient and that boron is present only in insignificant or trace amounts.
BACKGROUND OF THE INVENTION
Fine diameter fiberglass products intended for end-use service as HEPA clean-room filters have been known for many years, e.g. the fiberglass products marketed by Johns Manville International Inc. under the trademark MICROFIBER. These HEPA clean-room filters are used by the medical, pharmaceutical, and microelectronics industry in settings where ultra-pure air is required. Recently, a particular problem has come to light in the microelectronics industry as the circuits on chips have evolved toward nano-scale. Boron vapors from glass fiber HEPA filters have been implicated as a source of defects found in such micro-electronic components. As a result, it has become important to invent low boron glass formulations to better serve this specialty market. As used herein, the terms “low boron” and “substantially boron-free” mean that no boron is intentionally added to the glass-making formulation. Some natural glass-making raw materials contain trace amounts of boron oxide (B
2
O
3
) as a low-level contaminant, but the intent in low-boron glasses is to hold the boron level as low as practically feasible.
HEPA filtration products are typically made by processing very fine glass fibers through specialty paper-making equipment to make a blanket of fiber glass media known as fiberglass “paper”. This equipment uses a wilt process wherein an aqueous solution known in the industry as “white water” is maintained under specific conditions which are necessary to achieve the required dispersion of the fibers therein. In particular, white water is often controlled to a low pH condition of about 2.5. This condition results in good fiber dispersion as well as an acceptable bond between the fibers thereby giving the resulting paper adequate strength. The fibers must have appropriate and balanced levels of acid attack resistance to give a level of bonding sufficient to form a suitable paper structure. Too little acid attack will result in weak papers from inadequate bonding. Too much acid attack will result in weak fibers from excessive dissolution. In addition, the papers will become weaker over time, unless they have adequate resistance to humidity attack. In particular, HEPA filters require the fiberglass paper to be pleated to get high filter surface areas in a small volume. If the paper is brittle or weak after storage, cracking occurs during pleating, thus ruining the filter.
Conventional boron-containing glass is an industry standard for manufacturing microfibers suitable for HEPA filters. For example, JM-475 glass manufactured by Johns Manville International Inc. contains significant levels of B
2
O
3
(above 10 weight percent), thus making this glass composition undesirable for use in filters intended for semiconductor clean-rooms. Such glass has a viscosity sufficiently low to achieve a reasonable glass fiber production rate and allow the use of high temperature super-alloy as bushing or pot materials instead of using very expensive precious metal alloy (Pt—Rh alloy). It has also been found that, during the flame-attenuation fiberization process used to make the preferred fine diameter glass fibers, a highly durable “shell” is formed on the surface of the glass fiber as a result of the volatizing of alkali oxide (R
2
O) and B
2
O
3
from the fiber surface, most likely in the form of alkali metaborate. The formation of this durable shell is believed to be one of the main reasons that boron-containing glass has the desired combination of properties, such as a relatively low High Temperature Viscosity “HTV” for fiberization, defined as the temperature at which glass viscosity is 1000 poises, appropriate chemical durability against acid attack for the paper-making process, and excellent resistance against humidity aging, which are required for HEPA filtration media.
Unfortunately, it has been found that no such durable shell is formed on the surface of low-boron glass fibers during the flame-attenuation fiberizing process. Without the advantage of the formation of this durable shell, low-boron glass must possess high durability against both acid and moisture attack as its bulk property. This can be accomplished by significantly raising the SiO
2
and Al
2
O
3
level in the glass formulation, which unfortunately results in much higher glass viscosity. Low-boron glass formulations that are presently commercially available have significantly higher viscosity than is desired for cost effective glass fiber manufacturing. Typically, the HTV for these formulations is greater than 2190° F. (1199° C.). In addition, their humidity resistance is typically somewhat poorer than is the case for conventional boron-containing HEPA filtration glass fibers like JM-475 glass.
Since boron has long been considered an air and water pollutant which is released from the glass composition during the glass-making process, considerable efforts have been made in the past to provide substantially boron-free glass compositions for making glass fibers. Although boron-free fiberizable glass compositions are described in the prior art, these compositions are not ideal for making fibers suitable for HEPA filters. For example, the boron-free glass compositions described in Erickson et al U.S. Pat. Nos. 3,847,626; 3,847,627; 3,876,481 and 4,026,715; Clark-Monks U.S. Pat. No. 3,929,497, Neely U.S. Pat. No. 4,199,364; Sproull U.S. Pat, No. 4,542,105 and Eastes et al U.S. Pat. No. 5,789,329 contain more than optimal amounts of Al
2
O
3
, which is partially soluble in the acid white water used in specialty paper-making equipment. Certain boron-free glass compositions described by Von Wranau et al U.S. Pat. No. 3,095,311 have lower amounts of Al
2
O
3
, but contain less than optimal levels of SiO
2
, which adversely affect the durability of the fibers against the acid and humidity attack to which they are exposed when used as an HEPA filter media. Glass compositions available commercially from Evanite and Fibron (Laushar) have high silica, low-boron formulations, but have liquidus and high temperature viscosities that are not suitable for efficiently producing the fine glass fibers required for use in HEPA filters.
The achievement of very low boron glass formulations that result in glass fibers with both appropriate resistance to acid attack and adequate humidity resistance is quite challenging. This is especially true when it is also required that the glasses have appropriate values for viscosity and liquidus temperatures to meet the needs of commercial fine diameter glass fiber manufacturing. Accordingly, improved boron-free glass compositions suitable for efficiently making fibers for use in HEPA filters are greatly desired.
SUMMARY OF THE INVENTION
In general, the glass compositions of the present invention, and the fibers made therefrom, comprise 60-70% SiO
2
, 1-7% Al
2
O
3
, 8-19% Na
2
O, 0-6% K
2
O (wherein the sum of Na
2
O and K
2
O is less than 20%), 0-8% Li
2
O, 3-10% CaO, 0-10% MgO, 0-4% BaO, 0-8% ZnO, 0-4% TiO
2
and 0-2% F
2
, all expressed as mole percent of the glass composition. In a preferred embodiment, the glass composition and fibers comprise 62-68% SiO
2
, 2-6% Al
2
O
3
, 10-16% Na
2
O, 0-6% K
2
O (wherein the sum of Na
2
O and K
2
O is less than 18%), 0-6% Li
2
O, 3-10% CaO, 0-8% MgO, 0-3% ( BaO, 2-6% ZnO, TiO
2
and 0-2% F
2
.
The glass compositions of the present invention are particularly adapted to provide glass fiber media whi

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