Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2001-10-25
2003-11-04
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S409000, C065S411000, C065S439000, C065S477000
Reexamination Certificate
active
06640588
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the making of microporous structures with micro-channels that are particularly useful as containment devices.
BACKGROUND OF THE INVENTION
In capillary electrochromatography (CEC) and related analytical methods, there are currently no repeatable techniques or materials which ensure that the medium inside the capillary does not leak. There is a need, especially in the separation sciences, for fluid-permeable containment devices to retain fluids or packings or to filter particles from flowing streams of gas or liquid Common containment devices for this purpose include fiberglass packings, screens, and bonded particles, typically referred to as frits.
There are many different methods of making frits but most techniques employ the consolidation of small particles by sintering or melting compressed particles of a known size together. In one typical method, an appropriate material is ground up into small pieces and screened for a selected size range of particles. The particles are then compressed together in a mold and heated. The objective is to apply sufficient heat to fuse the particles together but not to melt the particles. Generally, the heating cycle is moderately complex in that there is a temperature profile of several different temperatures and times that give the best results After heating, the mold is removed and the resultant maze of material is then further processed by machining to trim the edges and/or welded or glued to an appropriate substrate. Another approach uses filaments of a given diameter and length that are randomly arranged, compressed, and fused together. The materials that are common to frit manufacture are both metals and plastics
The present art in this type of frit manufacture is to a) size the particles or strands, b) determine the amount of compression needed in the mold, and c) determine the temperature cycle to produce restrictive paths that have a nominal diameter of distribution. This means that the void space within these matrices have a consistent cross-sectional shape. The greater the void space relative to the total cross-sectional volume, the better the frit. The narrower the pore diameter distribution, the better the frit, and the more consistent the path length of the pores, the better the frit. The technology seems to be based more on statistics than on rigorous mechanical design since it is impossible to position millions of particles or filaments exactly in some geometrical pattern. Chaos theory appears to be the primary means of predicting performance.
Screens provide a containment device that serves as an alternate to frits but screens generally have a lower limit of performance based on the size of the wire or filament used. This lower limit is probably in the range of 25 to 125 &mgr;m and is established by the tensile strength of the filament and its ductility which is needed in the weaving process (over and under lapping in the loom). The pores or holes created by a screen using very small wire approaches the diameter of the wire and the open area of the screen for small holes is less than 50%. However, screens offer low back pressure compared to frits since screens are planar in construction and frits derive their functionality from thickness.
Neither the frit nor the screen offers an ideal structure for the containment of a packing or for providing a particle filter in applications that require small hole or pore sizes, particularly for a packed capillary column as used in either liquid chromatography (LC) or capillary electrophoresis (CE). The frit, because of the convoluted route of the pore including paths that contain lateral translations, has high back pressure. While a screen has low back pressure, the screen has a lower limit on “pore” size.
It is an object of this invention to provide a containment structure that reduces the pack pressure relative to a frit structure while not limiting the minimum size of the micro-passages to those associated with a screen.
It is a broader object of this invention to provide a microporous structure having low pressure drop and pore diameters of 5 &mgr;m or less.
SUMMARY OF THE INVENTION
This invention accomplishes these objectives with a microporous structure that defines a plurality of singular micro-passages that extend along the axial length of the structure. The plurality of singular micro-passages provide low restriction flow paths for the fluid through the structure. Each singular micro-passage can provide a direct flow path through the structure with a length that equals that of the structure and is free of obstructions The walls that define the micro-passages have a relatively straight configuration that permits extension of wall length without limiting the minimum diameter of the walls or disproportionately increasing pressure through the passages. Increasing the wall length increases the overall strength of the structure to pressure imposed by the fluid or packing.
The structure achieves tight dimensionality of the micro-passages which contributes to the dramatic reduction in back pressure that has been achieved. This dramatic reduction in back pressure is achieved even though the thickness of the structure may exceed that of a conventional compressed particle frit.
The structure is readily manufactured using a modified form of the technique common to glass drawing. The method involves the fabrication of a number of tubes in “macro” to provide a preform which is then drawn down into an extended length of the structure. Lateral slicing of the extended structure supplies wafers of the structure in desired lengths for particular applications. The technology employed to fabricate the structure is similar to the drawing of polyimide coated fused silica or optical waveguide fibers but with distinctly different drawing conditions. A key difference is the use of a low temperature in the drawing furnace compared to either capillary tube drawing or optical waveguide drawing. In drawing the structure of this invention, the furnace is heated to a temperature that is just sufficient to draw the preform. The viscosity of the preform is kept high, near the low end of the softening temperature of the glass, resulting in a tractive force being required to draw the filament from the preform. This is counter to the typical process of drawing optical waveguide where the preform will “drop” due to gravity alone.
The advantage of the formed structure is very high porosity (open space to structure), very low back pressure because the “pore” path is straight with no obstructions and statistically the “pore” diameter falls within a very narrow range. Furthermore, there is no lower range on “pore” diameter. Theoretically, the “pore” diameter can approach the size of the molecules that make up the glass. For example, if fused silica were employed, the “pore” diameter could approach a few nanometers. Overall, this manufacturing process is suitable for manufacturing structures with multiple micro-pores, also referred to as multicapillaries that can have outer diameters of from several millimeters to less than 100 &mgr;m. Internal bore diameters of the micro-passages provide pores that can range in size from several hundred micrometers to less than 10 nm. The “pore” field, i.e. the area defined by the outer diameter of the micro-passages, can have diameters of several millimeters to less than 10 nm. The number of pores can range from 7 to several thousand and possibly as high as 100,000. This approach has been found to be very flexible—meaning that the outer size can be varied over a large range, the bores can be varied over an even larger range, and the number of bores within a structure does not seem to have an upper limit.
The preform assembly is typically in the form of a tube that defines the outer circular wall and that retains the internal micro-passage area of the pore field. In most applications, it is preferred that the regularly recurring shape of the capillary cross-section also define capillaries of the same size. The regularly recurring shape o
Hoffmann John
Paschall James C.
Tolomei John G.
UOP LLC
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