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Emerging Materials for EMS: Modifiable Silicone Foams Reduce Contamination
December 31, 1969 |Estimated reading time: 7 minutes
Silicone foams are highly adaptable products for potting, gasketing, and encapsulation. Michelle Velderrain, NuSil Technology, explains the benefits of silicone foams, and how their materials properties can be tailored for processability, chemical resistance, temperature cycling, vibration dampening, and other functions.
Silicone foams are not generally the first products that come to mind when sourcing material for gaskets, potting or encapsulating. However, silicone foam offers several benefits: increased fuel economy due to lighter loads, reworkability, and shock and vibration dampening. Silicone foams have versatile processing capabilities; they can be cured in place as potting and space fillers, or molded/cast into foam sheets for gaskets or rods. Silicones foams are crosslinked thermosets and have a low elastic modulus due to the siloxane polymeric structure. The soft, compliant quality of silicone foams provides an excellent seal when used as gaskets because the foam conforms to the topography of the surface interfaces while maintaining its general shape. Foams can also reduce premature failures by lowering shear forces against other adjacent surfaces, such as wire bonds.
Unlike polyurethanes, silicone materials maintain their elastomeric properties when exposed to extreme temperature conditions. Therefore, they suit a broad range of applications. They easily absorb stresses incurred from thermal cycling when used between materials with different coefficients of thermal expansion (CTE) and heated from 180° to 200°C for 12 months, or even up to 450°C for short periods, without any appreciable effect on physical properties. The typical glass transition temperature (Tg) of silicone is below -60°C, allowing silicone foams to maintain flexibility at lower temperatures than common polyurethane foams. Silicone is also electrically insulating with volume resistivity greater than 1011 ohm/cm.
R' groupChemical structureChemical/physical effectsMethylCH3-Standard refractive index = 1.40. Standard polymer.Trifluoropropyl Refractive index < 1.40. Hydrocarbon solvent resistance.Phenyl Refractive index > 1.43. Increased temperature stability. Decreased moisture permeability.Silicone Polymers
To understand how silicone foams can be tailored for use in a variety of applications, it helps to understand how silicone polymers can be modified and processed. Silicone polymer is an essential component of a silicone foam formulation, comprising 70–80% of the foam’s weight. Silicones can be modified to have different chemical and mechanical properties by using specific organic groups bonded onto the silicone chain. Dimethyl silicones, more commonly known as polydimethysiloxanes (PDMS), have been used for more than 50 years in a variety of applications ranging from aerospace to medical devices. There are two methyl (CH3- or Me) groups on the silicon atom, and the monomeric unit is expressed as -(Me2SiO)-. Other common organic groups that can be attached to the silicon atom are trifluoropropyl (-CH2CH2CF3) and phenyl groups (-C6H6), which are aromatic rings. These are commonly abbreviated F3 and Ph, respectively. F3 is used to make solvent-resistant polymers, and Ph is used for a variety of purposes. Adding a certain quantity of phenyl on the backbone of the silicone polymer, for example, reduces the Tg to below -115°C and brings thermal stability up to 250°C. Phenyl groups can also greatly reduce the permeability rates of moisture and select gases.
Foam viscosity. When considering viscosity in relation to foams, there is a trade off between processability and resiliency. The number of monomeric units can also be optimized to achieve a specific viscosity. The chain length of the silicone polymer is known as the degree of polymerization (DP), and the longer the polymer is, the higher the viscosity. Higher-viscosity polymers tend to have higher mechanical properties (more flexible and less brittle), but processing limitations. On the other hand, lower-viscosity polymers tend to produce more brittle foams, but are easy to process using various methods.
Fillers, cure, and blowing agents. Silicone foams can be designed and modified in a multitude of ways to improve specific properties. Fillers, blowing agent, and cure mechanism are critical factors to consider when formulating silicone foams. To explore these in great detail is beyond the scope of this article, so we will look at the common options available. A variety of reinforcing and semi-reinforcing fillers can reduce brittleness and increase vibration dampening capabilities, yet still maintain some viscosity control. There are also several techniques used to optimize foam density. Silicone foams are typically thermosets where the cure reaction forms strong covalent chemical bonds that make the foam elastomeric. This is referred to as crosslinking and is almost always directly related to curing the foam (form and its cure rate).
There are also several blowing agents, some utilizing the curing mechanism to initiate the blowing agent’s release. For open cell foams (bubbles can interconnect), the most common blowing agents are hydride (-MeHSiO)- and silanol (-SiOH), functional polymers reacting together to form water and hydrogen gas as part of the crosslinking mechanism. This can be used with tin-condensation and platinum-addition cure mechanisms.
The foam density ultimately produced can be affected by processing due to the gas produced while the foam is curing. In this scenario, the mixing time can affect final foam density. Over-mixing will allow gas to escape and not to cure within the silicone matrix, and this leads to a higher-density foam. Other blowing agents available are bicarbonate compounds typically used with platinum-cure silicones. These blowing agents produced from the thermal decomposition of the bicarbonate are mainly mixtures of water and CO2 that do not react until heat is applied. These do not participate in the crosslinking; here, foam density can be adjusted using time and temperature.
For closed cell or syntactic foams (cells not interconnecting), hollow spheres can be added to reduce density. Hollow spheres can be constructed of glass or plastics with various strength ratios, particle size, etc. Their use is dependent on the temperature, pressure, and mechanical performance needed from the foam. Syntactic foams can be spread and cured in place to form planar geometries without having to be cut to form flat surfaces.
Outgassing and Volatility
A major concern surrounding the use of silicones is the volatile component observed to outgas when silicones are exposed to high temperatures and low pressures (vacuum) for extended periods of time. These volatile species may contaminate sensitive surrounding surfaces and equipment, making adhesion or soldering difficult in an upstream process.Understanding what process generates the volatile species helps instruct us on how they can be removed. Most commercially available silicone polymers are made using the ring opening polymerization (ROP) technique, which reacts siloxane rings (cyclics) with chain terminators to form the linear polymer used in the foam. The polymerization reaction is thermodynamically controlled and is considered complete when concentrations of the final products remain relatively constant. The final molecular weight distribution between linear and cyclic molecules formed is based on the starting units of the polymer. PDMS is typically 88% linear polymers and the remaining 12% are cyclic species. Cyclic species do not typically crosslink into the cured matrix and can diffuse or evaporate out of the cured silicone over time. Silicone raw-materials suppliers can remove these cyclic species by various methods. End users can remove the un-reacted silicones from the cured foam by performing an extensive bake out. This has many potentially negative side effects such as contaminating bake-out ovens and lowering the mechanical properties of the foam. Using a low-outgassing foam avoids this additional, non-beneficial processing.
Recent developments and demands from industries such as aerospace, aircraft, electronics, and healthcare are to lower the un-reacted silicone levels to better manage risk and up reliability by managing outgassing from materials, Lowering the amount of un-reacted silicone also can eliminate the need for additional processing time and steps performed by the end user.
One such low-outgassing, space-grade foam meets outgassing standards as tested per ASTM E595: ≤1 % total mass loss (TMLs) and ≤0.1% collected volatile condensable material (CVCM). The low-contamination open cell foam, detailed in Table 1, cures in place into a tough durable foam. Recently, a low-density version was shown to have a vertical rebound resiliency (ASTM D2632) of 0 and compression set (ASTM D395, Method B) of 0%. The foam is platinum catalyzed and designed to cure at low temperatures or have an accelerated cure with the addition of heat. In this foam, hydrides and silanols are used as blowing agents. This design enables users to form the foam into gaskets, cure it in place to protect fragile electronic components from dirt and debris, and add it as protection from shock and vibration.
Listed in Table 1 are four examples of foams that have been modified for various specialized applications. Note that the fuel-resistant syntactic foam uses a fluoro-containing polymer with specific gravities of 1.2 or greater. In this case, hollow spheres are used to reduce the specific gravity while maintaining the solvent resistance.
Table 1.Silicone foam
Standard open cell foam
(R-2370)
Tough open cell foam
(SFM5-2350)
Fuel-resistant syntactic foam
(CF1-3710-2)
Low-contamination open cell foam
(CV-2391)
Application Standard space filling and damping High strength needed when high compression is appliedUsed when foam will have intermittent contact with fuels or solventsTough foam when recommended when for when low out gassing materials are required and eliminates post baking to achieve low volatilityAppearance (A/B)Light tan to dark brownBlack/off whiteOff white/dark grayWhite/translucentViscosity 4900 cP55000 cPHigh but extrudes with static mix tip3700 cPNon-confined foam density10 lbs/ft³25 lbs/ft³50 lbs/ft³18 lbs/ft³Conclusion
Foams can be modified to exhibit a myriad of properties for different application goals. The long-term reliability that a low-out-gassing foam offers offsets its process-intensive nature. The benefits of weight reduction, ability to rework, and protection that foams offer make them a good choice when considering gasket, potting, or encapsulant materials.
Michelle Velderrain, senior technical specialist, NuSil Technology.