Designing with Silicone Materials
This article focuses on the nature of silicones and how this can affect process and the end device. For those readers with a limited knowledge of silicone materials, a brief review is necessary to discuss the variations in those products. The basis of virtually every silicone system is the silicone polymer, a repeating chain of silicon and oxygen molecules. For more information about silicone polymers click (http://www.nusil.com/whitepapers/index.aspx) Polymers can be supplied neat as fluids or can contain reactive species and reinforcing fillers to form greases, adhesives, gels, or elastomer systems. The most common commercial method of producing silicone polymers is via an acid or base catalyzed equilibration reaction of silicone cyclics and endblocking species. The reaction produces a distribution of polymer chain-lengths rather than one discrete polymer chain length. While further processing can narrow the distribution, the result is still variable. Polymer chain length can alter physical properties of adhesives, gels and elastomers – lower chain lengths can increase the system modulus by increasing the amount of crosslinker per unit area and lower the elongation of the system. Reactive species such as catalysts, crosslinkers and reinforcing fillers have some level of variation as well. The multiplication rule of probability statistics essentially rules out achieving the same result time after time when factoring in the variability of these systems. Statistical process control can at best reduce but not eliminate variability associated with these systems. Overall, engineers must realize that silicones can vary from lot to lot, this is evidenced by the allowable range of physical properties on most material certifications. What is the most relevant here is how the variability affects the process and the end device. In our example of a lower polymer chain length described above – two question immediately come to mind: Can the process to mix and dispense adequately handle the lower viscosity? How will the increase in modulus affect the final device? These questions can multiply rapidly as more key physical properties are considered. Other effects such as inter-property relationships must be considered as well (see previous Insight on this topic). Validation processes can go a long way to evaluate these materials. Good modeling programs and selecting product lots that have significant physical property differences within the certified range are just two ways to handle the variability associated with silicone systems. Visit www.nusil.com for even more information on this subject.