Sani-Tech Silicone Tubing
Silicone tubing is one of the most widely used materials in the high purity process industry. As the industry shifts away from exclusively stainless fluid paths and introduces more and more single use options, silicone tubing shines for a couple of major reasons. First, and foremost, is safety. Silicone tubing does not contain plasticizers or other additives that can leach into product and cause toxicological issues. Second, silicone tubing is highly flexible, resistant to kinking and tearing, and a good choice for use with peristaltic pumps.
In order to create silicone tubing, or any polymer for that matter, we need monomers. Silicone tubing is made from silicone elastomer. The base silicone elastomers are the “monomers” of the soon to be polymer. In order to combine these monomer groups to form tubing which can be extruded into usable geometries, the tubing must be cured, or vulcanized, crosslinking the monomers into polymers. With silicones, there are two major cure systems, addition cure and free radical cure. From here, we get our two major flavors of silicone tubing: platinum and peroxide cured, whose differences will be the focus of this post.
Addition cure systems make use of vinyl-functional polymers and a catalyst, such as platinum, to jump start the reaction (most cure methods will use either an organo-metallic complex or a platinum salt). The chief advantage of addition cure systems is that there are no by-products which can leach from the final polymer. Cost of platinum catalyst, however, is higher than in other cure methods which is reflected in final product prices.
Free radical cure systems are some of the oldest cross linking chemistries around. Commonly called “high temperature vulcanization” or HTV, this cure system uses free radicals generated by organic peroxides. The oxygen-oxygen single bond in a peroxide is not very stable and degenerates rapidly at elevated temperatures, creating free radicals which initiate the crosslinking of silicone monomers. In this type of cure system, by-products generally remain in the form of volatile organic acids. In order to remove these by-products, a high temperature post-cure is necessary. Any material left in the tubing as the cure that is not an intricate part of the tubing can be a potential leachable or extractable.
So enough with the chemistry, what does this really mean for selecting a silicone tubing for a high purity process? Because peroxide cured systems utilize free radicals which are difficult to control, final product consistency generally isn’t as high with peroxide cured silicones as it is with platinum cured silicones. High temperature post-cures also affect opacity of peroxide cured tubing, making it sometimes cloudier than comparable platinum cured products. Peroxide cured tubing tends to be more opaque than platinum catalyzed products. Peroxide cured silicones also tend to have a tackier feel than silicones.
Other key tubing performance characteristics, such as tensile strength, elongation, and tear strength are slightly better with platinum cured products, but not overwhelmingly so. Peristaltic pump life is typically longer for peroxide cured tubing product than with platinum cured products of similar formulation, however special long pump life platinum cured silicone products are available.
The chief advantage to platinum cured silicone products are the extremely low levels of extractables and leachables. While post-cure methods are effective in reducing levels of siloxanes present in peroxide-cured methods, some organic acids remain. Because of ability to control platinum cured products, by-products are not an issue. This gives platinum cured products the reputation for being the highest purity products. All good silicone tubing products should have full characterization and validation profiles, including total organic carbon profiles and relative levels of extractables and leachables, to help aide in selection of tubing.
To conclude, there are two major types of silicone tubing available for use in the high purity process world. Both free radical (peroxide) and addition (platinum) cured mechanisms offer safe, durable, and effective means for fluid transport and handling. The two cure systems do, however, produce materials with different characteristics whose impact should be considered before tubing selection. Peroxide cured silicones, while less translucent, tend to have lower spallation tendencies and better pump life. Platinum cured products offer ultra-high purity characteristics with extremely low extractables and leachables. Contact Holland today for help with identifying the best tubing for your application.