Material movement: How to reduce friction with silicone elastomers
Brian Reilly, business development director, biomaterials for NuSil, highlights the options for reducing friction when using silicone elastomers.
Silicone has a long and proven history of use with medical devices and can provide many benefits, from flexibility to cushioning. However, silicones can present challenges due to an inherent surface tack. The surface of cured silicone elastomers is sometimes characterised by a high coefficient of friction (CoF), some degree of tackiness and a tendency for blocking (sticking to itself by virtue of chemical affinity). These characteristics may need to be addressed in applications that require a moulded or extruded silicone component to move or slide with minimal friction.
As medical device designers account for friction, one solution is self-lubricating liquid silicone rubber (LSR), which eliminates an extra step to apply lubrication. Another solution is the use of a low CoF silicone coating applied to the surface and then cured to chemically bond the coating to the substrate for a non-tacky surface.
Self-lubricating LSRs
The traditional solution to these friction challenges requires medical device manufacturers to apply a silicone lubricant onto the moulded part as a separate manufacturing step, however, self-lubricating LSRs are now available. The lubricity is built into the silicone elastomer system which yields a lubricious surface on the moulded component. The elastomer system is formulated with a proprietary additive that elutes out over time after vulcanisation. The performance of many medical devices depends upon minimising friction at the interfaces between various components, and self-lubricating LSRs provide the basis on which to produce effective results.
Potential uses for self-lubricating silicone elastomers include balloons, valves, stoppers, O-rings, silicone devices with moving or sliding parts and parts that require assembly.
Low CoF silicone coatings
Other methods to overcome the high degree of surface tack and blocking with silicone elastomers include cured silicone coatings. When applied to the surface of the part and cured to chemically bond to the substrate, the coating mimics the mechanical properties of the underlying substrate. The outcome is a “dry” lubricity that reduces the CoF on the surface of silicone parts and eliminates concerns about migration, leaking or rubbing off that are commonly associated with traditional lubricants.
Coating options differ in how they are designed to cure e.g heat cure (HTV) or room temperature vulcanisation (RTV).
With approximately the consistency of water, the coatings can be applied by dipping, but spraying is recommended. It achieves a smooth finish that results in more than a 50% decrease in the CoF when compared to a non-coated silicone.
NuSil technology experts recently conducted a study to evaluate the static and kinetic CoF of coated vs. non-coated silicone surfaces. The force it takes to initiate movement between the silicone rubber and a steel panel (static CoF) was measured, as well as the force needed to keep the silicone rubber moving against a steel panel (kinetic CoF). Both a 70 shore A durometer liquid silicone rubber and 50 shore A durometer high-consistency rubber were tested according to ASTM D 1894 at ambient conditions. During the test, samples were placed on a stainless-steel panel with a mirror finish. On average, the coated samples demonstrated up to 74% reduction in static CoF, and up to 59% reduction in kinetic CoF depending on the silicone rubber substrate.
Both lubrication options provide advantages for the healthcare industry. Low CoF coatings are ideal, not only for their performance relative to friction reduction and regulatory concerns, but also because they achieve critical performance goals with negligible impact on the mechanical properties of the silicone substrates they coat. Therefore, a silicone device that must bend, twist, elongate, etc., can handle this movement, coated or uncoated, without cracking, flaking or peeling. Self-lubricating silicone elastomers may be chosen to reduce the number of processing steps required; the elastomer can also be specified with the physical properties and level of lubrication needed for the application.