Designing A Silicone System To Reduce Permeability To Moisture
Authors: Julie Harber and Michelle Velderrain Company: NuSil Technology, LLC Date Published: 10/14/2012
Abstract: Among other characteristics, silicone’s broad operating temperature, thermal stability, and low glass transition point, or Tg , make it a viable candidate for use in encapsulating electronic components. For this and other electronics applications, it is important that encapsulants have low water vapor transmission rates (WVTRs) because water can cause fogging, corrosion of electronic components and, in some cases, side reactions that produce unwanted chemicals such as ammonia1. Conversely, in healthcare applications in which water plays a different role, very permeable materials are needed so that moisture can reach the surrounding tissue, as is the case for wound care dressings, external prosthetic devices, and contact lenses. Silicones are inherently highly permeable due to their large free volume, yet this is also what enables them to be designed as protective barriers for electronic systems. Changes can be made to the mechanical and physical properties of a silicone by adjusting the polymer backbone chemistry and crosslink density, as well as by adding filler. In particular, filler strongly affects how permeable a formulation is to water. Water vapor transmission rates were recently quantified for silicone samples containing silica, silver, or boron nitride filler. Each of these fillers produced a decrease in the respective sample’s permeability to water compared to a non-filled sample, showing that fillers can optimize silicone as a barrier to moisture for near-hermetic packages and other applications.
silicone, moisture permeability, water vapor transmission rate (WVTR), filler, R-group