UV Curable Silicone Elastomer Principles
Silicone elastomer vulcanization technology can lead to new opportunities in a wide range of applications, such as medical devices, that combine a silicone rubber with thermally sensitive materials.
Extended cure times for thick parts and temperature sensitive pharmaceutical additives are two challenges for silicone molding and extrusion that can be overcome with new low-temperature UV curing technology.
Silicone Elastomer Cured with UV Light
Silicone elastomers are being used in many different industries. They have a unique combination of properties which includes high heat resistance, physiological inertness, and excellent electrical properties. They are typically cured by peroxide initiated free radical polymerization of methylvinyl polysiloxanes, and alternatively, by hydrosilylation using silicon hydride cross linking agents. Both of these curing mechanisms require elevated temperatures to obtain an effective degree of cure of a silicone elastomer. For example, when tubing is processed by extrusion, the heated vulcanization tunnel uses air with a temperature between 800°F and 1200°F. Because silicone rubber has low thermal conductivity, a lot of heat is required to reach the center of thick wall profiles. This leads to high energy usage during vulcanization, generating a substantial waste of heat. For that reason, a new technology is being used: Ultraviolet (UV) curing.
This cure technology is a photochemical process where UV radiation is used only to initiate the catalytic curing reaction, thereby reducing energy consumption. The cure system is based on a platinum catalyzed addition which uses a photosensitive catalyst to absorb the UV radiation. It typically results in products with high purity and low levels of extractables. The cross linking reaction promotes rapid cure at relatively low temperatures while eliminating the use of highly reactive peroxides. The formulation must be relatively clear, but can accommodate bright, vivid transparent colors, as long as the pigments are compatible with the platinum catalyst. Liquid Silicone Rubbers (LSRs) are two-part formulations that contain a UV-activated platinum catalyst and can be cured with common UV lamps that emit light in the wavelength range of 315 to 430 nm. UV curing of LSR requires transparent molds or cavity inserts. For extrusion, the UV light is provided externally using a modified lamp fixture. The reaction occurs at temperatures below 140°F without substantial heat input and the degree of cure from skin to core of the products is uniform which allows high speeds of production while reducing the production costs.
There are many advantages of the UV-cure process. One such advantage is that there is no need for a water-cooled injection barrel or cold deck on the mold; this can reduce tooling costs. Since mold filling is accomplished at room temperature, there is virtually no risk of scorching. Low-temperature curing also means no pressure buildup in the mold and lower post-mold shrinkage.
Additionally, productivity increases almost 3 times, the consumption of energy is lower, the quality is improved because the porosity is reduced or eliminated, and the system enables innovative product design.
This curing system is perfect for thick parts and temperature sensitive pharmaceutical additives. In the latter, some drugs are mixed into the silicone elastomer formulation for extended-release applications. For example, three steroidal drugs and one antibiotic type drug were tested in a 64 Shore A LSR, and it was demonstrated that the drugs did not inhibit cure at room temperature, and NMR testing showed that the UV process did not degrade the drug.
We encourage you to visit our website www.SIMTEC-Silicone.com to learn more about our industry-specific applications and benefits of LSR!