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46 SMT Magazine • June 2014 feaTure eXPLORIng HIgH-TeMPeRATuRe ReLIABILITy LIMITS FOR SILICOn ADHeSIveS continues adhesion did indeed deteriorate with high-tem- perature exposure, it did not change as fast as elongation. Since elongation was the fastest degrading property, this was used to generate a predictive plot of time to lose 50% of the original value. Using the same procedure and method as indicated for the adhesive 1 (one-part, self-lev- eling, heat cure adhesive), the estimated high- est temperature exposure for the other two ad- hesives/sealants evaluated here is shown below (Table 5). Conclusions When silicone adhesives/sealants are ex- posed to temperatures above 200°C, and in some cases above 150°C (depending on the sili- cone formulation), some chemical degradation starts impacting some of the physical properties of these products. This chemical degradation is promoted by a variety of processes, includ- ing oxidation, siloxane rearrangement and hy- drolysis. The property that shows the highest and fastest degree of degradation is the elonga- tion. Elongation was used then to predict the temperature limits exposure for the adhesives/ sealants evaluated under the scope of this pa- per, following the guidelines offered by the UL Standard 746B10. In this way, the limits indicated here were determined by the time required for the adhe- sive/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 ini- tial 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% elonga- tion is still highly elastomeric and this proper- ty 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 tem - perature exposure for the silicone, taking into consideration operating conditions and envi- ronmental factors. SMT Carlos Montemayor is an applications engineer with Dow Corning. Table 5: Time to lose 50% of original elongation for silicon adhesives.