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Exploring the Reliability Limits for Silicone Adhesives
Exploring the Reliability Limits for Silicone Adhesives
Paper determines "life expectancies" as well as an extrapolated estimate of the Underwriter's Laboratories' "continuous use" temperature rating.
Materials Tech

Authored By:
Carlos Montemayor
Dow Corning Corporation
Midland, MI USA
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Summary
The thermal stability of silicone polymers, fluids and resins has been well documented and studied extensively. The high temperature performance of silicone adhesives and sealants used for electronics applications has only moderately been investigated. This report documents the effects of very high temperature exposures to electronics-grade silicone adhesives and sealants for such properties as tensile strength, elongation, tensile modulus, weight loss, shrinkage, durometer, and lap shear adhesion.

The goal of the work is to determine application "life expectancies" of the products as well as an extrapolated estimate of the Underwriter's Laboratories' "continuous use" temperature rating, the highest temperature at which a product is expected to lose no more than 50% of its original value for whatever key property degrades the fastest Four different formulations of silicone adhesives and sealants were evaluated for high temperature stability.

For these products, elongation was found to be the fastest degrading property among those tested. The data was found to fit a power curve of exposure temperature vs. time to reach a 50% loss of initial tensile strength and elongation to an R-squared value of 0.99 and to a linear fit in an Arrhenius plot to the same very strong fit. These plots could be used to closely estimate the effects of heat aging on the material over a wide range of temperatures.
Conclusions
When silicone adhesives/sealants are exposed to temperatures above 200°C, and in some cases above 150°C (depending on the silicone formulation), some chemical degradation starts impacting some of the physical properties of these products. This chemical degradation is promoted by a variety of processes, including oxidation, siloxane rearrangement and hydrolysis.

The property that shows the highest and fastest degree of degradation is the elongation. Elongation was used then to predict the temperature limits exposure for the adhesives/sealants evaluated under the scope of this paper, following the guidelines offered by the UL Standard 746B10. In this way, the limits indicated here were determined by the time required for the adhesive/sealant to lose no more than 50% of its original elongation when exposed to different temperatures.

It is important to remark that for some silicone adhesives/sealants with initial elongation values in the range of 400 or 500%, losing 50% of its original value leave them with an elongation in the range of 200 or 250%. A material retaining 250% elongation is still highly elastomeric and this property may still be more than adequate for a great number of applications.

Therefore, the data reported in this paper should be used only as a guideline to establish general temperature exposure limits. Any specific application will require extensive testing and evaluations to determine the durability and maximum temperature exposure for the silicone, taking into consideration operating conditions and environmental factors.
Initially Published in the IPC Proceedings
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