Using ATR Spectroscopy to Distinguish Release Agents on O-Rings

ATR Spectroscopy can be used for distinguishing release agents on O-rings. During the O-ring manufacturing process, O-ring molds are coated with release agents or the release agents are imbedded in the mold so as to enable O-ring removal from the mold. The material used for the O-ring determines the release agent’s composition. The ability to detect the release agent is important for both the manufacturer and the user.

During the manufacturing process, the O-ring may be stuck in the molds. An O-ring user may be considering the O-ring to be chemically inert, and any residual release agent on it may cause critical contamination. Hence, both the manufacturer and the user must know about the release agent’s composition.

Infrared spectroscopy is typically used for detecting release agents that are on O-rings. However, numerous problems may be faced during their detection due to the formation of a very thin layer on the ring’s surface. Furthermore, O-rings are mostly filled with carbon and are quite small in size.

Hence, a surface sampling method that is suitable for a small sampling area is required. Single reflection ATR is a suitable technique and this applications note describes the manner in which ATR can be used for distinguishing the release agents from the material used for the O-ring.

Experimental Procedure

The experiment analyzed a reddish brown Buna O-ring, a black Viton^®1 O-ring, and another black O-ring, whose origin was not known. The black Viton^®1 O-ring and the other black O-ring had black outer layers, while the Buna O-ring had a whitish surface coating. Their exterior surface measurements were obtained in “as is” condition, while a small slice of each O-ring was used for examining its interior. The Harrick MVP-Pro™ and a commercial FT-IR spectrometer were used in combination for the examination.

Figure 1. The MVP-Pro.

The MVPPro™ is an ATR accessory that is equipped with a 3mm diameter hemispherical Si crystal that is fixed at a 45º incident angle. Low-angle rays strike the ATR crystal. These rays are not desired, and hence, a 40º mask is used along with the MVP-Pro™ for blocking some of these rays. With a resolution of 8cm^-1, the complete infrared spectra were collected over the mid-infrared region, and the signal was averaged over 32 scans.

Results and Discussion

Figure 2. ATR spectra of a Viton O-ring. Outside surface: Black; inside: Blue.

The two ATR spectra of a Viton O-ring demonstrate the characteristics of a typical poly(aliphatic) fluoride that is expected to be exhibited by this fluoroelastomer. Both the spectra are very similar, which show that the O-ring has a very thin release agent coating. At around 800cm^-1 and 1020cm^-1, marginal light spectral differences can be observed.

Figure 3. The difference between the inner and outer surfaces of the Viton^® ORing.

When the two spectra results shown in Picture 3 are subtracted, the result shows that it belongs to silicone oil. Silicones are commonly used as release agents.

Figure 4. ATR Spectra of a Buna O-Ring. Outside Surface: Black; Inside: Blue.

The Picture 4. shows that both the spectra possess a curving baseline. This indicates a high refractive index for the sampled areas. A possible cause may be due to the release agent coating thickness and the amount of carbon-filling that the substrate is made up of.

The ATR spectrum that is on the outer side of the O-ring also samples the O-ring material as the coating is quite thin. However, at around 3515cm^-1, 1532cm^-1, 1456cm^-1, and 1404cm^-1, distinct bands are observed. The release agents are the reason for these distinct bands. Furthermore, the inner O-ring material’s spectrum does not have these bands.

Figure 5. ATR Spectra of a Black O-Ring. Outside Surface: Black; Inside: Blue.

The ATR Spectra in Picture 5 is from an O-ring whose origin is known. The O-ring substrate material and the release agent can be differentiated clearly. The spectrum of the O-ring’s inner side is in the form of a curving baseline, which shows that the O-ring is filled with carbon. The coating’s spectrum does not exhibit any significant amount of the baseline effect, which demonstrates that the coating is not very thin when compared to the coating on the Buna O-ring.

Conclusion

The infrared spectra shown in this article demonstrate that Si micro- ATR methods and the MVPPro™ can be used for differentiating between the bulk O-ring material and the O-ring release agent. Further work is being carried out to find out ways to further mitigate carbon- filling’s effect on O-ring spectra.

This information has been sourced, reviewed and adapted from materials provided by Harrick Scientific Products, Inc.

For more information on this source, please visit Harrick Scientific Products, Inc.