Moisture outgassing from siloxane elastomers containing surface-treated-silica fillers

Abstract

The outgassing kinetics from siloxane elastomers is dominated by moisture desorption from the reinforcing silica filler and can be detrimental in moisture-sensitive applications. In this study, a custom 3D printable siloxane rubber (LL50) was analyzed in three different states: after a high temperature vacuum heat treatment, limited re-exposure to moisture after vacuum heat treatment, and in the as-received condition. The outgassing kinetics were extracted using isoconversional and iterative regression analyses. Moisture release by physisorbed and chemisorbed water from the samples have activation energies in the range of 50 kJ/mol (physisorbed type) to 220 kJ/mol (chemisorbed type). Overall, moisture outgassing from LL50 was 10 times lower than that from traditionally prepared siloxane rubbers. The vastly diminished moisture content in LL50 is attributed to the existence of a finite low level of silanol groups that remain on the fumed silica surface even after hydrophobic treatment.

General H2O Outgassing Model Applicable to Silica-Filled Silicones

“…From our analysis of multiple formulations (see Figures 3a, 4a,d,g, and previous literature 6,16 ), a sum of six desorption peaks or less generally seems to fit the data (discussed in the next section). These sets of kinetic parameters correspond to a total of six H 2 O desorption peaks and are used to deconvolute the outgassing profile from room temperature to ∼850 K. In some, fewer or more than six peaks have been reported.…”

“…were obtained from previously published literature. 6,11,12,16,20 Samples of roughly 1 cm 2 area by 0.1 cm thickness were used for accurate determination of outgassing kinetic parameters by the isoconversional analysis from TPD experiments. 6,16 More details on material composition are provided in Supporting Information S3.…”

“…Water desorption from the silica surface is due to physisorbed (reaction order n = 1) and chemisorbed moisture (reaction order n = 2). The general expression in terms of extent of conversion, α (α = 0 to 1), can be written as 6,16,23 i k j j j y…”

“…Mechanism of Water Outgassing from Silica. Because silica particles are primarily responsible for the moisture uptake and outgassing from the silica-filled silicones, a silica filler, functionalized Aerosil R8200 16,40 (the silica surface is specifically modified by an intensive hydrophobization treatment with trimethylsilyl groups), was chosen to develop the kinetic reference parameters. Experimental data (Figure 3a) clearly shows two distinct outgassing regions with a significant overlap between them.…”

“…6,15−17 In addition, it has been established that the loss of moisture from silica surfaces affects the segmental mobility of polymer chains in silicones and may lead to material stiffening. 16,18,19 Thus, accurate moisture outgassing kinetics are essential for estimating the lifetime of these materials.…”