Determining levels of moisture permeation through Kalrez and Chemraz O-ring material.

PDF Version

Problem Statement

As the manufacturers of corrosive resistant gas cells we are often called upon to design a system for the detection of moisture in various gases. Most of our requests are to detect moisture in the low ppb levels. This can be a rather difficult task to accomplish when the surrounding environment has high levels of moisture (humidity). There are several steps that we take to provide our customers with the lowest possible detection limit for moisture, because as little as <10ppb of permeation through the O-rings can be a big issue. Some of these steps include the use of CIC Photonics permeation box (secondary level of purging) surrounding the system, the use of 1/8” SS electropolished tubing, drying down the system for at least one or two days prior to beginning analysis, and using an O-ring material that has little moisture permeation. In order to recommend using an O-ring material that has little moisture permeation, we needed to compare the materials for ourselves. Therefore we conducted a moisture permeation comparison test for Kalrez 4079 against Chemraz 585.

IRGAS Applicability

To compare the two O-ring materials we used an IRGAS LPA system that consisted of a 9.6m Ranger- EN long path gas cell, and a Bomem WorkIR spectrometer. During the tests the outside temperature and humidity were monitored, in addition to the testing room being kept at room temperature +/- 1°. A constant flow rate of 2 slpm of purified compressed dry air (moisture <2ppb) was used for each test run for both O-ring materials. The first test we ran was using the Kalrez O-rings. In this test all of the 9 O-rings used in a Ranger-EN gas cell were Kalrez material. During the second test the two O-rings with the largest areas in the gas cell were replaced with Chemraz material. Each Oring material was tested at temperature intervals of 40, 60, 80, and 100°C. In addition to the moisture permeation, the outgassing of HF was also monitored between the two O-ring materials. Along with the experimental data, provided by testing, in the data graphs there is theoretical data for a Ranger-EN that would have all 9 O-rings replaced with Chemraz. The way that theoretical concentration for all Chemraz O-rings in the Ranger- EN was determined as follows:

Where:

K = Kalrez experimental concentration e

A = Total area of all 9 O-rings T

x = Concentration / area of Kalrez

A = Area of Kalrez O-rings K

K = Contribution Kalrez O-rings make to T experimental concentrations

C = Chemraz experimental concentration e

A = Area of Chemraz O-rings C

C = Chemraz theoretical concentration T

Conclusions

The data analyzed for this experiment were collected without the use of a permeation box surrounding the IRGAS system, thus allowing the moisture in the atmosphere to permeate into the gas cell. In the event that an IRGAS system were being used to measure the lowest possible concentration for moisture we recommend the use of a permeation box for any analyses being conducted under 60°C. From the graphs included in this application note it can be seen that in the case of moisture permeation and HF outgassing, the Ranger-EN that had the two largest O-rings replaced with Chemraz O-rings is permeating less moisture and outgassing less HF. In the instance that all of the O-rings in the Ranger-EN had been replaced with Chemraz O-rings, it can be deduced from the theoretical data that the amount of moisture permeating and HF outgassing would be reduced even further.

Kalrez is well known for its superior performance in the corrosive gas applications because of its high chemical resistiveness. For this reason we have been using Kalrez for our corrosive and toxic application for years. However, for this particular application it is critical to be able to measure the lowest moisture concentration possible, which lead us to select the Chemraz 585 O-ring material over the Kalrez 4079 O-ring material. This basis was solely made on the premise of moisture permeation, not chemical resistiveness. Seemingly, Chemraz 585 for liquid applications is comparable to that of PTFE. This chemical resistiveness only applies to liquid applications and not to gas applications. There are some exceptions in which Chemraz 585 should not be used such as for molten or gaseous alkali metals (potassium, sodium, etc..). For a complete list of the chemical resistiveness of the Chemraz for liquid applications please visit http://www.gtweed.com/852571F1006FB63A/chemraz

In regards to the chemical resistiveness of the Chemraz 585 for gas applications it appears that most common gases such as HCl, HBr, SiH4, and NH3, have a rating* of excellent from Green Tweed. However there are some of the gases like BCl3, NF3, and BF3 who's ratings drop to a rating of good. Prior to beginning tests with the Chemraz 585 material it is suggested to check with Green Tweed to ensure the safety of using this material in certain applications.

An excellent rating means that it is suggested for both dynamic and static uses. A rating of good means that its use in static conditions is usually satisfactory and that it may work for dynamic situations. A good rating also indicates that the resistance of the material is good as well.

Based on the permeation and offgassing concentrations in conjunction with the chemical resistiveness of Chemraz it appears that depending on the gas matrix of the application and the desired limit of detection needed for certain applications will depend on which O-ring material to utilize.

If it is necessary to reach low levels of moisture and small amounts of HF outgassing affect the application then Chemraz 585 O-rings are the better option, assuming that it is compatible with the gas matrix. In the case where low levels of moisture and a little HF outgassing are not a concern then the best option would be the use of the Kalrez 4079 material. Which O-ring material to use is case dependent and to determine which one is the best option for a certain application it best to contact us or the O-ring material manufacturer.