Innovative Dissolvable and Degradable Materials Optimize Oil and Gas Extraction

For some applications, a critical component of selecting a seal material is a phenomenon known as “outgassing”. However, even within the elastomer community, outgassing is not something that is commonly considered. Which begs the questions: what is outgassing and why is it important? Outgassing is usually most relevant in vacuum applications, where the vacuum causes the elastomer to release constituent material. The constituent material could include water vapor, plasticizers, oils, byproducts of the cure reaction, or other additives used in the seal material. Outgassing becomes a problem if a thin film of those chemicals condenses and is deposited on nearby surfaces. Such a film poses major challenges in highly sensitive applications, such as optics or electronics, where cleanliness is of utmost importance. A seal material with low outgassing  is essential because it shows the seal material does not emit volatile constituents under vacuum conditions.

Weight loss of compounds in vacuum

Outgassing is most often characterized by weight loss of the seal material. The ASTM test method E595 is one way to quantify outgassing by measuring Total Mass Loss (TML %), Collected Volatile Condensable Materials (CVCM %) and a reported value for Water Vapor Regain (WVR %).  Measurements are taken following a 24 hour exposure to vacuum of 5x10-5 torr at a temperature of 257°F.

Taken together, these three parameters tell a complete story. The TML is reported as the percent of the specimen’s initial weight that is lost during the test; under standard criteria, the result must be less than 1.00% mass loss. Obviously, minimizing TML is a good thing, but it is not the only important factor. Collected volatile condensable material (CVCM) is the amount of outgassed matter from a specimen that condenses onto a collector during the maintained time and temperature. CVCM is of particular concern because any material that readily condenses in the test is likely to condense on and contaminate nearby surfaces during use. To pass the standard CVCM requirement, the amount collected relative to the initial mass of the specimen must be less than 0.10%. The final measurement, WVR, is the mass of the water vapor absorbed by the specimen after a 24-hour stabilization at 23°C in a 50% relative humidity atmosphere. There is seldom a pass/fail limit for WVR; instead this result is merely reported. In many applications, the small amount of water vapor lost by a seal may not be of concern, particularly if the application already includes a means of controlling moisture. Further, any WVR is presumed to be equal to the portion of original TML that was water vapor. The difference between TML and WVR is therefore presumed to be volatile organic material that has evaporated out of the material (only some of which condenses in the CVCM test), so minimizing the difference between TML and WVR is also of considerable importance.

To illustrate, we can look at the most recent outgassing data completed on a few popular low temperature fluorocarbon materials. Table 1 contains the results from a 3rd party laboratory to measure the outgassing properties of VM125-75 and VX065-75. Both had undetectable amounts of CVCM and very small differences between TML and WVR.  VX065-75 in particular displayed remarkably little outgassing as well as a low WVR.

table { font-family: arial, sans-serif; border-collapse: collapse; width: 100%; } td, th { border: 1px solid #dddddd; text-align: center; padding: 8px; } tr:nth-child(even) { background-color: #ffb91d; }   Limit VM125 VX065 Total Mass Loss (TML, %) 1.00 % max 0.48 0.15 Collected Volatile Condensable Material (CVCM, %) 0.10% max <0.01 <0.01 Water Vapor Regain (WVR, %) Report 0.39 0.17

There are a few additional resources detailing seal materials that are known for having low weight loss. The O-Ring Handbook ORD 5700, Table 3-19 (page 65 of the pdf), has a few legacy materials with weight loss percent after a two-week exposure to 1 x 10-6 torr vacuum level, at room temperature. Additionally, non-Parker resources such as the NASA website contain an interesting summary of a much broader range of materials.  

For more information on low outgassing seal materials, please contact a Parker Application Engineer at OESmailbox@Parker.com or 859-335-5101.

 

This article was contributed by Dorothy Kern, applications engineering manager, Parker O-Ring & Engineered Seals Division.

 

 

 

 

 

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When it comes to semiconductor fabrication processes, reducing the cost of ownership is a multi-faceted goal approached from a variety of angles. Tool engineers and equipment technicians take pride in their ability to identify factors that limit tool uptime. One constant headache they face is the mechanical failure of seals in dynamic environments. This can lead to premature downtime or reduced preventative maintenance (PM) intervals, both of which lead to higher cost of ownership. Fortunately, tool owners have begun to implement seal designs better suited for these dynamic environments: Parker EZ-Lok is a proven solution.

 

Spiral failure

 One of the more extreme forms of mechanical failure to be prevented is twisting and spiraling of an O-ring during operation. This occurs with O-rings in dovetail glands where one of the sealing surfaces is a door that opens and closes against the seal. The combination of stiction to the door and stretch in the gland causes the O-ring to roll and twist repeatedly with each cycle, resulting in permanent cyclic deformation. This means that a seal profile with a flat contact surface is vital for this type of dynamic function.

  Other designs

The basic D-profile is the fundamental simple shape that serves as the basis of the EZ-Lok solution. The flat portion of the “D” holds the seal in place and prevents rolling, while the opposite, round contact surface focuses the sealing force and helps keep volume requirements at a minimum. These geometric features make for sound sealing function while preventing the drastic spiral damage seen so often in the industry.

 

 

 

 

 

 

 

A standard D-ring is still more limited by volume requirements than traditional seals like O-rings. In addition, a D-ring’s sharp corners can become difficult to install past the top groove radii if the seal is made much wider than the groove opening. On the other hand, a seal made any narrower would be easily removed without intention, such as that induced by stiction to the door. These reasons are why the basic D-profile alone is not the answer to these failure modes.

 

The Solution

The solution to these dilemmas is a unique D-shaped profile with a geometry that lends itself to the spacial constrictions of dovetail glands, prevents rolling, and locks into place: the Parker EZ-Lok seal. These seals are designed with special retention ribs placed with precise frequency around the seal circumference that allows for smooth installation and keeps the seal retained in the gland. This design also removes any tendency to stretch the seal during installation, which is often seen with more conventional seals.

A common question often asked by our customers is the reason why flow rate is reported on datasheets of liquid-dispensed thermal interface materials instead of viscosity. And it’s a fair question; viscosity is a fundamental property of fluids such as thermally conductive pastes. But measuring viscosity, however, is more complicated than meets the eye.

Parker Chomerics THERM-A-GAP Thermally Conductive GELs belong to a class of fluids referred to as “thixotropic.” In case you’re not familiar, thixotropy is a time-dependent decrease in viscosity as shear stress is applied.

This shear stress may be introduced during mixing, pumping, or dispensing of the product. The extent of the viscosity decrease depends of the duration and magnitude of the agitation, and viscosity recovers gradually over time after the stress is removed. The act of measuring viscosity introduces shear stress, so gathering repeatable data requires carefully controlling the duration of the test and the relaxation time between trials.

 

 

 

 

 

 

 

 

 

 

 

 

To complicate the measurement further, viscosity is strongly influenced by sample temperature. A warmer dispensable thermal paste flows more quickly than one at room temperature, a characteristic that is particularly relevant in thermal interface materials.

Therefore, a viscosity measurement is only accurate for a given shear stress, duration, and temperature. The resulting value of viscosity is difficult to generalize, so it is more convenient to use flow rate, as it is a measurement that is more representative of actual end-use conditions.

Automated dispensing is one of the primary advantages of dispensable thermal pastes, such as Parker Chomerics THERM-A-GAP GELs. Publishing flow rate data provides a framework with which to compare different gels based on their dispensability.

Additionally, measuring flow rate instead of viscosity controls the critical parameters described above, such as:

• Shear stress, which is controlled by dispensing from a specified container at a constant pressure
• Test duration
• Ambient temperature

Measuring flow rate is repeatable and more representative of actual dispensing situations than a viscosity value.

So, the next time you’re reading a datasheet, you’ll be armed with the knowledge that flow rate is a more accurate representation of real-world conditions.

 

 

 

 

 

 

 

This blog post was contributed by Jon Appert, R&D engineer, Chomerics Division.

 

 

 

 

Related content:

The Difference Between Thermal Conductivity and Thermal Impedance

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