Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2000-11-02
2003-12-02
Sample, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S066000, C501S035000, C055S527000
Reexamination Certificate
active
06656861
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to glass compositions which are uniquely applicable to the preparation of ultrafine fibers for filtration and separation applications. Fiber made from the glasses disclosed exhibit the necessary properties of moisture resistance, chemical resistance and strength, as well as excellent biosolubility.
2. Description of the Related Art
Glass forming compositions suitable for fiberization are typically restricted by their melt and end properties to conform to process specifications and product performance criteria. For example, in both rotary and flame attenuation processes, only certain values for high temperature viscosity (T at 10
3
poise) and liquidus are acceptable. Moreover, such compositions must demonstrate adequate physical properties such as tensile strength and moisture resistance when formed into fibers by these processes. In addition, more recently, it has become increasingly more important that these fibers degrade at sufficiently high rates in the body, such that they pose little to no potential risk to humans if inhaled and can at least be demonstrated to have limited biological effects to laboratory animals when tested.
A good glass fiber forming composition should also have good “runability”—the ability to be easily fiberized into long fibers of small diameter with good production rates and little or no shot. While there are many factors involved in this, not all of which have been clearly identified, it is believed that surface tension and lack of tendency for the melt to phase separate play key roles. In specific, it is desirable for a glass composition to have as low a surface tension as possible at fiberization temperatures (keeping in mind the other factors above), such that the work done in forming a unit area of surface is kept to a minimum.
All of these factors are especially important in the production of ultrafine fibers used to produce specialty papers and other media for air and liquid filtration applications. For example, 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. Such applications place some significant demands on the glass in terms of both fiberization and end properties. Specific requirements can be as follows.
The glass must be capable of being melted and fiberized at temperatures low enough for the capability of the equipment and to attain reasonable (economic) production rates. This requires that the HTV value of the glass (temperature when the melt viscosity is 1000 poise) be less than 2025° F. (1107° C.) and the surface tension of the glass at 1642° F. (900° C.) be less than 315 dynes/cm.
The glass must not crystallize or devitrify in the melters, pots, bushings or anywhere in the system used to melt, contain, transport, or fiberize the glass. Crystallization impairs flow of melt to fiberization orifices. To prevent devitrification the liquidus temperature of the glass must preferably be at least 350° F. (194° C.) below the HTV.
The glass must not corrode or have adverse reactions with metal parts or refractories used to contain the melt or the newly-forming fiber. Also, the glass must be capable of being drawn into ultrafine diameters (as low as 0.2 &mgr;m) without breakage into short lengths.
The glass must not produce excessive volatilization during melting or fiberizing. Volatilization leads to compositional variations, increased corrosion of refractories, increased emissions, and, when volatiles condense, to unacceptable dust levels in the product.
The glass must provide strength to the fiber—suitable to form fibers capable of being wet or dry (air) processed into papers or felts and meet all of the requirements (tensile, elongation) required for the paper product. Because of its high specific surface area, the glass must also have sufficient chemical durability, particularly with regard to ambient atmospheric moisture, so that little to no deterioration in fiber strength occurs with time during packaging, shipping, and storage prior to use in a papermaking process. Loss in fiber strength correlates with increase in fiber surface area as measured by BET methods (using krypton). After exposure of fibers to 122° F. (50° C.) for 72 hours, the change in surface area should be less than 10%. Glass must also be sufficiently durable and resistant to moisture attack after contact with whitewaters or other media used in the papermaking process, so that little to no deterioration occurs with time after the fiber is in a paper product.
Because the fiber is of very fine diameter and capable of being respired, it must also degrade in the body at a rate that is sufficiently rapid so as not to induce respiratory diseases, especially chronic diseases such as emphysema or cancer. Measured in vitro dissolution rates for the fiber in simulated physiological saline (k
dis
) must be greater than 100 ng/cm
2
hr. Such biosolubility performance is difficult to achieve when balancing biodissolution against properties such as sufficient strength for ultrafine fibers. It is particularly difficult to achieve with regard to flame attenuated fibers.
In preparing flame attenuated fibers, the glass fibers are exposed to temperatures much higher than in a rotary process. The higher temperatures cause a loss of the more volatile compounds of the glass composition from the outside of the fibers, resulting in a “shell” which has a different composition than the fiber interior. As a result, the biosolubility of glass fibers prepared from pot and marble or other flame attenuated fiberglass is not the same as that derived from the rotary process. As glass fibers must necessarily dissolve from the fiber ends or the cylindrical exterior, a more highly resistant shell will dramatically impede the dissolution rate. Fibers having such a shell, which are flame attenuated, are also prepared by the rod method or direct melt method. These latter methods involve conveying raw materials, in any form, to an orifice or bushing to form primaries, which are then flame attenuated, as in a pot and marble method. While flame attenuated fibers exhibit excellent chemical and moisture resistance due to this core/sheath structure, biosolubility of the fibers can be a problem.
Glass fibers must also be leachable in paper making media such as acid whitewaters such that hydrolytic bonds can form between leached fiber surfaces when the paper is dried. Such bonds provide strength and a structural integrity in the final product. However, too great a leach rate can leave the fiber with a porous surface structure which is too susceptible to moisture attack after the paper is formed. For a one hour residence in simulated whitewater (H2SO4 pH 2.5) at 77° F. (25° C.), total leach rates should be at least 0.2 &mgr;g/cm
2
hr, but less than 0.7 &mgr;g/cm
2
hr.
Glass fibers must also show good performance in handsheets, both in initial tensile strength and in loss in tensile strength over time. This is evaluated by determining load to failure at a gauge length of 4″ (10.2 cm) of mechanically formed handsheets. For handsheets made of fiber with a 1 &mgr;m mean diameter, initial tensile strengths should be at least 1.8 lbs. with no statistically significant loss in tensile strength after aging at 95° F. (35° C.) and 95% relative humidity for up to 168 hrs.
The glass fibers must also show good performance in doubly folded handsheets, both in initial tensile strength and in loss in tensile strength over time. This is evaluated by determining load to failure at a gauge length of 4″ (10.2 cm) for the handsheets whose properties are defined above. For handsheets made of fiber with a 1 &mgr;m mean diameter, initial folded tensile strengths should be at least 0.7 lbs. Tensile strength after agin
Bauer Jon Frederick
Harding Foster Laverne
Russell, III Harry Hand
Bolden Elizabeth
Burns Doane Swecker & Mathis L.L.P.
Johns Manville International Inc.
Sample David
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