Friction control and binding sealants for press-fit windows

Abstract

The present invention relates to friction control and binding sealants for pressed-in windows. An improved method of sealing a window into an opening in a body uses a lubricant comprising polymer particles suspended in a volatile, low viscosity, low surface tension carrier liquid. The carrier liquid is applied to one or both of the window and the sidewalls of the opening and the window is pressed into the opening so that the carrier liquid evaporates, leaving the polymer particles to fill the interstitial surface voids while enabling the sidewalls of the window to be in close mechanical contact with the sidewalls of the opening. Although having broader applications, the present disclosure is particularly useful for optical characterization techniques based on the Raman effect and fluorescent probes used in process monitoring and control.

Description

Technical Field

[0001] The present disclosure relates generally to optical measurement probes and, in particular, to probes constructed with windows suitable for on-line process control and other applications and, even more particularly, to improved techniques for incorporating such windows into a probe body or process vessel. Background Art

[0002] Optical characterization techniques based on the Raman effect and fluorescence have become important tools in process monitoring and control in industries involving hazardous materials, pharmaceuticals, etc. In such applications, a probe with a sealed window is typically introduced into the process flow to facilitate remote connection to analytical equipment, such as via optical fiber.

[0003] Current immersion window designs used by spectroscopic analysis systems rely on several technologies to seal the window to the probe body. The two most common categories are elastomeric seals, which include O-rings, gaskets, adhesives, etc., and metal seals. O-rings and other elastomeric seals have temperature, airtightness, and chemical compatibility issues.

[0004] Metal seals can be subdivided into soft solder, metal spring seals (e.g. C-rings), and crimps. For C-rings, standard metal C-rings can be used as seals. However, this type of seal has problems achieving a high level of hermeticity, has difficulty making the window flush with the probe body housing, and leaves many micro voids and dead volumes for sample material to entrap, making the technology unsuitable for life science applications due to contaminant transfer.

[0005] Generally, metal seals are the most desirable type, offering the potential for high temperature and pressure resistance, hermetic sealing, long life, and ruggedness. A bonding material, usually a metal alloy or low melting point glass-like material, is used to seal and hold the window in place by brazing, sintering, or fusing. Brazed window seals are common in the spectroscopy probe industry. Figure 1 As shown, such an arrangement comprises a window 102, such as a sapphire window, which is typically brazed to a surrounding probe body 104 using gold or a gold alloy 106. This technique has all the advantages of the metal seals listed above, with the addition of the fact that the edges of the window are sealed so that the surface is flush with the outside of the probe, without any special shaping of the window itself. However, this technique can generate undesirable thermal stresses between the window and the probe body 104, and the choice of material limits the application space due to corrosion resistance.

[0006] An alternative to brazing windows is Figure 2204. In this case, the window 202 is tapered and pressed into the probe body 204. The interface may or may not have a ductile metal such as gold to aid in sealing. This method is cheap and quick, but has the disadvantage of reducing the compression on the window as the probe body expands at high temperatures, potentially leading to seal failure.

[0007] The crimp window technique described in U.S. Pat. No. 6,831,739 and incorporated herein by reference uses a specially shaped window that is pressed directly into the probe body to form a permanent and airtight seal and corrosion resistance of the window and probe body materials. However, during the manufacturing process, this method has difficulties in achieving the correct combination of sealing and window retention without damaging the window or the probe body. The shape and surface finish of the window and probe body, as well as the pressing dynamics, must be very strictly controlled. In particular, in order to obtain an adequate seal, the window must be plastically deformed by sufficient holding force to cause the probe body housing to not fall out, but the pressing force must be low enough to prevent shear stresses on the window that may cause the window to break. In practice, it has proven difficult to meet all of these requirements simultaneously.

[0008] Therefore, there remains a need for further contributions in this area of ​​technology. Summary of the invention

[0009] According to one aspect of the present disclosure, a method of sealing a window into a body having an opening, the opening having sidewalls, the method comprising: providing a window having a front surface, a back surface, and sidewalls, applying a lubricant comprising polymer particles to at least the sidewalls of the window or the sidewalls of the opening, and pressing the window into the opening in the body to evaporate a carrier fluid, thereby leaving the polymer particles to fill the interstitial surface voids, while enabling close mechanical contact between the sidewalls of the window and the sidewalls of the opening. The sidewalls of the window are conformal to the sidewalls of the opening in the body, and the lubricant comprises polymer particles suspended in a volatile, low viscosity, low surface tension carrier fluid. In at least one embodiment, the window and the opening are circular.

[0010] In one embodiment, the size range of the polymer particles is 0.25 to 16 microns. In one embodiment, the nominal size of the polymer particles is 2 to 4 microns. In one embodiment, the carrier liquid is an alcohol or cyclohexane. In at least one embodiment, the carrier liquid is ethanol. In one embodiment, the lubricant has a mixing ratio of polymer to carrier liquid of 1:300 by weight. In another embodiment, the lubricant has a mixing ratio of polymer to carrier liquid of 1:200 by weight.

[0011] In another embodiment, the polymer is polytetrafluoroethylene (PTFE) particles and the lubricant is PTFE particles suspended in a volatile, low viscosity, low surface tension carrier fluid.

[0012] In another embodiment, the window is made of sapphire. In one embodiment, the front surface and back surface of the window are substantially parallel to each other. In another embodiment, the sidewalls of the window and the sidewalls of the aperture are tapered to complement each other. In yet another embodiment, the taper of the sidewalls of the window and the sidewalls of the aperture is in the range of 1 to 5 degrees. In one embodiment, the sidewalls of the window and the sidewalls of the aperture have a 3 degree locking taper. In one embodiment, the sidewalls of the window have a surface finish in the range of 1 to 3 microns. In one embodiment, the sidewalls of the aperture in the body have an average roughness of 4 to 8 microns RA.

[0013] In at least one embodiment, the body is composed of a metal or a metal alloy. In one embodiment, the body is part of an optical measurement probe. In another embodiment, the body is part of a processing vessel. In at least one embodiment, the force applied to the window during the pressing of the window into the opening in the body is at least 50% less than the maximum tensile stress on the window. In another embodiment, the force applied to the window during the pressing of the window into the opening in the body is at least 70% less than the maximum tensile stress on the window. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The described embodiments and other features, advantages, and disclosures contained herein, as well as the manner in which they are accomplished, will become apparent and the present disclosure will be better understood by referring to the following description of various embodiments of the present disclosure taken in conjunction with the accompanying drawings, in which:

[0015] Figure 1 A cross section is shown illustrating a prior art bonding technique in which a window, typically sapphire, is brazed to a surrounding probe body, typically with gold or a gold alloy;

[0016] Figure 2 A different prior art is shown in cross section, where a tapered window is pressed into the probe body;

[0017] Figure 3 is a diagram showing an improved crimping method according to the present disclosure, which adds a binding lubricant and a sealant for friction control; and

[0018] Figure 4 is a micrograph showing a fine PTFE powder applied to a Raman probe tip after evaporation of the alcohol carrier liquid leaving only the PTFE. DETAILED DESCRIPTION

[0019] The present disclosure relates to an improved method of sealing a window into a body having an opening having sidewalls. The present disclosure may be well suited for application in optical characterization techniques based on the Raman effect and fluorescence, wherein the sealed window is used in the wall of a probe body and a process vessel.

[0020] The disclosed method improves upon crimp window technology by including a lubricant to control the amount of friction experienced during the press-in process. Ideally, the lubricant would disappear completely after the process is complete, or a material would be used that is chemically and thermally compatible with the probe's intended application environment.

[0021] According to the present disclosure, there is provided a window having front and back surfaces and sidewalls, wherein the sidewalls of the window are conformal to the sidewalls of an opening in a body. A lubricant consisting of polymer particles suspended in a volatile, low viscosity, low surface tension carrier fluid is applied to one or both of the sidewalls of the window and the opening, and the window is pressed into the opening to evaporate the carrier fluid, leaving residual polymer particles behind to fill the interstitial surface voids while allowing the sidewalls of the window to be in close mechanical contact with the sidewalls of the opening. The window and the opening may be generally circular in peripheral shape.

[0022] However, if there is too much polymer, the window may essentially "float" on the polymer layer and may not be adequately retained or sealed. Figure 3 As shown in , the mating dimensions are controlled so that the window 302 achieves a close mechanical "window-to-housing" contact after being pressed into the body 304, wherein the residual polymer particles 306, 308 are at least partially, and in some embodiments substantially, pressed into and at least partially fill the interstitial surface voids in the window 302 and the body 304.

[0023] The lubricant can be a very dilute suspension of polymer micropowder in a volatile, low viscosity, low surface tension fluid that wets the surface, evenly distributes the polymer powder, and evaporates rapidly, leaving a sparse distribution of polymer particles. The surface finish of the housing and window allows the polymer to be smeared on the surface during the press-in process and then cold-formed (cold flow) into the voids of the microstructure, thereby enabling direct contact between the window and the housing, but at the same time squeezing and constraining the polymer, thereby filling the microscopic voids and producing an airtight seal that is resistant to high temperatures and chemicals. Therefore, the polymer particles (e.g., micropowders) can be any suitable polymer, including thermoplastics, thermosetting, elastomers, and semi-crystalline materials that can remain suspended in a carrier fluid and have suitable cold forming properties. In at least one embodiment, the particles can be graphite instead of a polymer.

[0024] The size of the particles is set to be small enough to fill the gap surface voids, micro cracks and gaps inside and between the window 302 and / or the body 304, and large enough to be used for suitable cold forming properties. In at least one embodiment, the size range of the polymer particles can be 0.25 to 16 microns (μm). In certain embodiments, the polymer particles can have a nominal or average size range of 2-4μm. The carrier liquid can be an alcohol, such as ethanol, or cyclohexane. The window can be a sapphire window, wherein the front surface and back surface of the window are substantially parallel to each other. The taper of the sidewalls of the window and the opening can be in the range of 1-5 degrees. In certain embodiments, the sidewalls of the window and the opening can have a locking taper of about 3 degrees. In at least one embodiment, the sidewalls of the window can have a surface finish in the range of 1-3μm, and the sidewalls of the opening in the body can have an arithmetic mean roughness (Ra) of 4-8μm Ra. The body can be composed of a metal or a metal alloy.

[0025] In at least one embodiment according to the present disclosure, the polymer is polytetrafluoroethylene (PTFE, e.g. ). In such embodiments, the size range of the PTFE particles can be 0.25 to 16 microns (μm). In certain embodiments, the nominal or average size range of the PTFE particles can be 2-4 μm. In other embodiments, the polymer can be linear polyethylene (PE), polypropylene (PP), polyamide (PA), polyamide 6,6 (PA66), polyester, polyolefin (such as poly alpha-olefin (PAO)), or another suitable polymer. In another embodiment, the polymer can be a thermoplastic elastomer having viscoelastic properties.

[0026] Applicants have conducted testing using at least one embodiment disclosed herein. The testing described herein shows that the diluted PTFE lubricant of the present disclosure is able to reduce the press-in force to less than 70% of the maximum tensile stress on the window (which is the most fragile component) while maintaining at least 50% of the maximum holding force. In one embodiment, the press-in force can be less than at least 50% of the maximum tensile stress on the window. The following test experiments provide details on the preparation and use of a friction reducing solution (i.e., lubricant) for assembling a crimped window in a probe body tip used in Raman spectroscopy applications. Two different window seal surface finishes, overlapping and non-overlapping, were tested.

[0027] Lubricant: PTFE powder, available from Micro Powders Inc., Terrytown, NY, USA, part number FLUO 300XF, with an average particle size range of 2-4 μm average, 16 μm maximum in an ethanol carrier liquid used to suspend the lubricant. Probe body tip seal surface area: 0.035 square inches; seal surface finish: 4 μm or better. Probe window seal surface area: 0.023 square inches. Probe window seal surface finish: overlapping windows – 1 μm Ra or better; non-overlapping windows – 1.5 μm Ra.

[0028] Procedure: Apply PTFE solution to the sealing surface of the probe body tip. For overlapping windows, the lubricant is a 1:300 mix by weight of PTFE to carrier fluid, and dispense 2 μL (0.00067 grams (g) PTFE) to 3 μL (0.0010 g PTFE) of lubricant solution. For non-lapping windows, the lubricant is a 1:200 mix by weight of PTFE to carrier fluid, and dispense 3.0 μL of lubricant solution (0.001507 g PTFE). Window Press Force: <370 lbs.

[0029] Figure 4 is a micrograph showing the lubricant PTFE after application to the probe body tip and after the ethanol carrier fluid evaporates, leaving only the PTFE particles pressed into the micro voids in the window and body.

[0030] Although various embodiments of the disclosed method have been described in considerable detail herein, these embodiments are provided by way of non-limiting examples of the disclosure described herein only. Therefore, it will be understood that various changes and modifications may be made, and equivalents may be substituted for the elements of the disclosure, without departing from the scope of the disclosure. The disclosure is not intended to be exhaustive or to limit the scope of the subject matter of the disclosure.

[0031] In addition, when describing representative embodiments, the present disclosure may have presented methods and / or processes in the form of a specific sequence of steps. However, insofar as the method or process does not rely on the steps in the specific order set forth herein, the method or process should not be limited to the specific sequence of steps described. Other sequences of steps may be possible and still remain within the scope of the present disclosure.

Claims

1. A method of sealing a window into a body having an opening, the opening having side walls, the method comprising: providing a window having a front surface, a back surface, and sidewalls, wherein the sidewalls of the window are conformal to sidewalls of an opening in the body; applying a lubricant for friction control to the side wall of the window or the side wall of the opening, wherein the lubricant consists of a suspension of micropowdered polymer particles suspended in a volatile, low viscosity, low surface tension carrier liquid, the suspension wetting the front and back surfaces, evenly distributing the polymer particles, and rapidly evaporating to leave a sparse distribution of polymer particles, wherein the carrier liquid is an alcohol or cyclohexane, The particles are sized small enough to fill interstitial surface voids, microscopic cracks within and between the windows and the body, and large enough for suitable cold forming properties; and pressing the window into the opening of the body such that the carrier fluid evaporates, leaving the polymer particles filling interstitial surface voids while enabling intimate mechanical contact between the sidewalls of the window and the sidewalls of the opening, wherein the lubricant is used to control the amount of friction experienced during the pressing process, and The surface finish of the body and the window allows the polymer to be smeared on the front surface and the back surface during the pressing process, and then cold formed into the voids in the gap surface, thereby enabling direct contact between the window and the body, but at the same time squeezing and constraining the polymer, thereby filling the gap surface voids and creating a hermetic seal that is resistant to high temperatures and chemicals.

2. The method of claim 1, wherein the polymer particles have a size range of 0.25 to 16 microns.

3. The method of claim 1 wherein the polymer particles have a nominal size of 2 to 4 microns.

4. The method of claim 1, wherein the front surface and the back surface of the window are substantially parallel to each other, and wherein the sidewall of the window and the sidewall of the opening are tapered to complement each other.

5. The method of claim 1, wherein the sidewalls of the window and the sidewalls of the opening have a taper in the range of 1 to 5 degrees.

6. The method of claim 1, wherein the sidewalls of the window have a surface finish in the range of 1 to 3 microns.

7. The method of claim 1 wherein the sidewalls of the openings in the body have an average roughness of 4 to 8 microns RA.

8. The method of claim 1, wherein the body is a portion of an optical measurement probe or a process vessel.

9. The method of claim 1, wherein the polymer of the polymer particles is polytetrafluoroethylene (PTFE).

10. The method of claim 1, wherein the lubricant has a mixing ratio of polymer particles to carrier liquid of 1:300 by weight.

11. The method of claim 1, wherein the lubricant has a mixing ratio of polymer particles to carrier liquid of 1:200 by weight.

12. The method of claim 1, wherein the polymer of the polymer particles is an elastomeric polymer.

13. The method of claim 1, wherein a force applied to the window during pressing the window into the opening in the body is at least 50% less than a maximum tensile stress on the window.

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