Ultraviolet radiation is used to control and decrease the level of adhesion.
Seals are used to facilitate the joining of two items, usually temporarily. At some point in the future, it is expected that the items will need to be separated. This innovation enables control of the adhesive properties of silicone-based elastomers. The innovation may also be effective on elastomers other than the silicone- based ones. A technique has been discovered that decreases the level of adhesion of silicone-based elastomers to negligible levels. The new technique causes less damage to the material compared to alternative adhesion mitigation techniques.
Silicone-based elastomers are the only class of “rubber-like” materials that currently meet NASA’s needs for various seal applications. However, silicone-based elastomers have natural inherent adhesive properties. This stickiness can be helpful, but it can frequently cause problems as well, such as when trying to get items apart.
In the past, seal adhesion was not always adequately addressed, and has caused in-flight failures where seals were actually pulled from their grooves, preventing subsequent spacecraft docking until the seal was physically removed from the flange via an extravehicular activity (EVA). The primary method used in the past to lower elastomer seal adhesion has been the application of some type of lubricant or grease to the surface of the seal. A newer method uses ultraviolet (UV) radiation — a mixture of UV wavelengths in the range of near ultraviolet (NUV) and vacuum ultraviolet (VUV) wavelengths. UV radiation also causes damage to the seal, with different wavelengths causing different levels of damage.
Low-wavelength VUV radiation attenuates rapidly; it is absorbed quickly and does not penetrate deeply into solids or gases. VUV is absorbed by air, thus does not reach the surface of Earth. Seals exposed to near-VUV radiation achieve the desired level of adhesion reduction without raising the seal leakage level. The radiation likely breaks weaker atomic bonds on long polymer molecules near the surface, which can then cross-link with other molecules, thereby absorbing the weaker bonds and preventing adhesive bonds.
The novel feature of the innovation is that it uses near-VUV wavelength radiation to control and decrease the level of adhesion of silicone-based elastomers without significantly damaging the elastomer. It is expected that the innovation can be implemented using handheld radiation sources, thereby enabling the technique to be used on odd-shaped and very large parts.
The innovation has the potential to use off-the-shelf radiation sources in air, thus circumventing the need for a vacuum chamber. Exposures could be done, for example, using a radiation “oven” through which a conveyor belt passes.
This work was done by Henry C. de Groh III, Bernadette J. Puleo, and Deborah L. Waters of Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steven Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. LEW-18948-1