Silicone elastomers are widely used in the medical device industry. The remarkable physical properties of silicone — durable, flexible, low cost, structural versatility, resistance to bacterial growth, low extractables and easy sterilization —make it a desirable choice for medical grade tubing and components.
But versatility adds complexity. For example, mating complex molded silicone parts on to parts made of harder materials can be problematic for two reasons. First, while silicone lacks rigidity it will not generally expand or stretch without assistance. Secondly, silicone has a tacky surface, a high coefficient of friction, so sliding silicone tubing onto a fitting is difficult.
Options Used When Fitting Silicone Parts Together: There are two common methods of joining silicone tubing or parts: lubricating with alcohol or oil, or by elastomer swelling.
Isopropyl Alcohol (IPA): Ultra-pure isopropyl alcohol (IPA) can lubricate silicone for assembly. IPA is easy to obtain and is relatively inexpensive and it evaporates without residues. But IPA dries slowly, which can reduce throughput. In addition, if the assembly is not completely dry there may be problems with the device later on.
Silicone Oils: Another choice is to lubricate with silicone oils. This is logical but messy. The oil will stay on a surface indefinitely. Silicone oil makes housekeeping difficult because the oil will migrate through the factory. The oil also captures dirt from the environment. Lastly, medical grade silicone oils are expensive.
Silicone Swelling: A third choice is to use silicone swelling fluids. These are solvents that expand the elastomer predictably, like a sponge absorbing water. Swelling allows the tubing to easily slide over a fitting. Once the fluid evaporates the tubing returns to its original size, creating a secure, airtight seal. Because tubing dimensions may only need to expand by 1–2% for assembly, the entire swelling process can often be accomplished in less than a minute.
In use, one end of the silicone tube is dipped into the swelling fluid. Exposure time determines how much the tube swells. This can be predicted accurately to ensure that throughput is maximized.
Once the part is removed from the fluid, it immediately evaporates. As it evaporates, the part returns to its original size and state. There will be no structural or chemical change in the silicone tubing.
Typical Swelling Fluids: The two most common swelling agents are either hexane or engineered swelling fluids, such as Swellex™ silicone swelling agent.
Hexane is flammable and has a strong odor so proper equipment is essential. Hexane is an extremely aggressive solvent. Although effective at swelling silicone, hexane contains volatile organic compounds (VOCs) that contribute to ground-level smog. Hexane also may remove ink markings and may change the physical properties of the elastomer.
An environmentally-friendly option uses fluids such as Swellex which is engineered specifically for swelling silicone. This enables the fluid to swell the silicone while maintaining the physical properties of the part. Swellex evaporates quickly, is easy to handle, has a low global warming impact, and is VOC-exempt. Customers using Swellex brand fluids have found great success with the technology.
Silicone swelling is an efficient, low-cost and hassle-free method of joining silicone parts. Swelling with Swellex brand fluids provide design flexibility and an effective, efficient, and environmentally superior way to join silicone tubing or molded parts.