To the Editor:
I understand that there has been some interest in the CCI results on the stability of parylene. To clarify the issue, I submit the following summary of our work.
So far we have carried out three studies--and, as so often is the case, things started out beautifully clear and subsequently became more and more complex. Our studies were:
(i) Thermal aging of parylene C and parylene N at temperatures between 150° and 200° Celsius. This study also included an investigation of the effect of adding an antioxidant on retardation of degradation.
(ii) Thermal aging of parylene C and parylene N between 55° and 150° Celsius (the lower temperature study).
(iii) The degradation of parylene C and parylene N by UV-filtered fluorescent lighting.
Three papers which describe the results of experimental work are cited below--but perhaps the following summary will be more immediately useful.
What we found is that at higher temperatures (150° to 200°) both parylene C and N exhibit excellent Arrhenius behavior and the "lifetimes" predicted are very long. Furthermore, our results agreed very precisely with those of two earlier studies.
At lower temperatures it appears that the linearity of the Arrhenius plots breaks down and parylene's longevity deviates from that predicted by the earlier higher temperature work. Our results indicate a much lower stability than we had originally predicted. There are problems with this lower temperature work which introduce an element of uncertainty.
The chemistry of deterioration seems to alter at lower temperatures. We have followed aging rates by measuring the height of the carbonyl peak in the infrared spectrum. There are some questions about the reliability of doing this. In the earlier study we correlated the carbonyl peak with two separate physical tests--tensile strength and fold failure. (Correlation was excellent.) However, at lower temperatures the shape of the carbonyl peak alters, perhaps indicating the formation of a different product balance, and the correlation may not be valid. And no correlation of the changes in physical properties with chemical properties has been attempted at lower temperatures, although it does appear that the parylene films aged at lower temperatures have indeed deteriorated physically.
As for the light aging study, we have found that parylene is sensitive to light even with reduced UV levels, and we have suggested maximum exposure times which the material can withstand.
We have received a number of comments, particularly from industry, to the effect that parylene cannot be as unstable as we predict. Some very old samples have been examined which show few signs of degradation; however, we have no proper baseline data for the older samples. Hence the results at this stage are inconclusive. We are now trying to obtain some funding to continue our study of the low temperature aging behavior with a study of the decline of physical properties.
We have continued to use parylene for conservation, but only on the clear understanding that it is a technique of last resort. We continue to believe that it has the capability to save objects which would otherwise be lost.David Grattan, Manager, Conservation Processes and Materials Research
1. D.W. Grattan and M. Bilz, "The Thermal Aging of Parylene and the Effect of Antioxidant." Studies in Conservation 36 (1991) 44-52.
2. Malcolm Bilz and David Grattan, "The Development of an Apparatus for Studying the Effect of Light Exposure on Museum Materials." ICOM-CC Proceedings 1993, Working Group 16 (Resins), p. 559-565.
3. M. Bilz and D.W. Grattan, "The Aging of Parylene: Difficulties with the Arrhenius Approach." ICOM-CC Proceedings 1996, Working Group B16 (Resins), p. 925-928.