Figure 1. Comparison of total mass lost during the outgassing of two silicone materials.

Silicone is an outstanding material for flexible heaters because of its high temperature resilience and elastomeric mechanical properties. How- ever, dealing with the outgassing of silicone is an area of concern, especially with sensitive applications. Even in areas where contamination is not a major issue, the presence of outgassing is an olfactory nuisance.

The aerospace, biomedical and semiconductor industries all use flexible silicone heaters where outgassing can be problematic. Recent progress has occurred in understanding and reducing silicone outgassing. Al- though outgassing appears to be a simple phenomenon, it occurs due to a number of factors. For example, outgassing depends on the amount and type of filler and catalyst used. Other factors, including the cure history of the silicone, degree of cross-linking, operating temperature, and whether the silicone is encapsulated or open to the atmosphere, also are important. In order to understand how to reduce outgassing, one first should examine the sources of outgassing and the mechanisms involved.

There are two primary sources of outgassing species:

  • Low molecular weight (LMW) siloxanes that are present in the uncured silicone.
  • LMW siloxanes that are the products of the degradation of the material at elevated temperatures.

Both of these sources are difficult to avoid for most silicone manufacturers. LMW siloxanes are present in silicone formulations because they are needed to facilitate the processing and mixing of pigments and other essential components. Degradation of the polymer, which is constantly producing LMW siloxanes, proceeds at a very slow rate at lower temperatures but has an appreciable rate at higher temperatures, which are typical of many applications.1 While it may not be possible to eliminate all outgassing species, by understanding the mechanisms involved, some silicone manufacturers have been successful in reducing outgassing by almost an order of magnitude.

Here is an example of an LMW cyclic siloxane that will outgas. Cyclics are formed during depolymerization or reversion reactions.

Outgassing occurs in two fundamental steps. First, LMW siloxanes diffuse to the surface of the elastomer. Second, at the surface of the elastomer, desorption of the siloxanes occurs as they are released into the surrounding environment. In conditions where the concentration of LMW siloxanes in the air near the surface of the elastomer is low or negligible, desorption occurs quickly and is not a rate-limiting step. This is the case in applications where the silicone is not encapsulated and there is sufficient airflow.

The diffusion step is more important in determining the rate of outgassing. A great deal of research into this subject has been driven by power cable designers who use silicone insulation and are interested in understanding the unique ability of silicone to recover its hydrophobicity after exposure to corona discharges and pollution. This recovery is accomplished by the diffusion of low molecular weight siloxanes toward the surface of the insulation, which acts to hydrophobically seal the surface.

In these studies, diffusion of LMW siloxane in silicones has been modeled as a function of the square root of time.2 Empirical studies tend to confirm this model.

Understanding the sources and mechanisms of outgassing has led to the development of low outgassing silicones by allowing researchers to attack both the sources of outgassing as well as optimizing processes used to diffuse out LMW siloxanes cost effectively. Newer formulations using unique chemistries and processing methods have reduced outgassing by nearly one-third. Figure 1 shows an example of two materials and the results obtained from developmental work in this area. Outgassing normally is measured by the total mass lost (%TML) at a specific temperature, pressure and over a given period of time. (Typically, 250oF [121oC], 10-5 atm, and 24 hr, respectively, are used.) As illustrated, outgassing can range from 4.5 percent mass lost with traditional silicones to less than 1 percent with newer technologies.

For decades, silicone and silicone composites have been a major insulation platform for high temperature flexible heaters. While silicone offers numerous performance advantages, outgassing concerns have limited its use for some applications. Significant research has been done to identify the sources and mechanisms of silicone outgassing, which has resulted in ways to significantly reduce the content of low molecular weight siloxane species and processing methods to reduce the evolution of outgassing while in service. These advances now allow for broader use of silicone in sensitive applications where outgassing concerns have previously limited its use.