Silicone elastomers are high-performance thermoset materials broadly used in diverse industries, including automotive, aerospace, electronics, consumer goods, and health care. They are recognized for their unique combination of properties, including high heat resistance, physiological inertness, and excellent electrical properties. One of the most desirable features of silicones is their ability to remain flexible and relatively unchanged over a wide range of temperatures, e.g., from –50 °C to 200 °C.
Silicone elastomers are typically cured by peroxide initiated free radical polymerization of methylvinyl polysiloxanes, and alternatively, by hydrosilylation using silicon hydride crosslinking agents. Both of these curing mechanisms require elevated temperatures to obtain an effective degree of cure of the silicone elastomer. Silicone elastomers are fabricated using most conventional rubber processing techniques. In particular, extrusion is a process that provides continuous profile of a specific cross-section (such as tubing), which is then vulcanized by passing through a heated air tunnel typically at temperatures of 800 °F to 1200 °F. In order to activate the catalyst and achieve full cure, the extrudate itself achieves a temperature of 400 °F to 450 °F.
Beating the Heat
Because silicone rubber has low thermal conductivity, extensive heating is required to reach the center of thick wall profiles. This leads to high energy usage during vulcanization and generation of substantial waste heat. In comparison, ultraviolet (UV) curing is a photochemical process where UV radiation is used only to initiate the catalytic curing reaction, thereby reducing energy consumption. Since silicone is a UV transparent material, only the photosensitive catalysts absorb the UV radiation, so there is little production of waste heat.
Momentive Performance Materials Inc. (Albany, NY) recently developed an extrusion process utilizing patent-pending UV curing technology. This approach employs a photosensitive platinum catalyst, which, upon exposure to UV light, initiates the hydrosilylation reaction needed to cure the silicone rubber. The reaction proceeds vigorously at temperatures below 140 °F without substantial heat input. Traditional silicone elastomers can be readily modified to be compatible with this UV curing system, but Momentive also introduced the Addisil* UV 60 EX elastomer to specifically utilize the UV curing process.
The process was demonstrated in a pilot scale extrusion trial at a Momentive Performance Materials laboratory in Waterford, NY. Silicone compound containing a UV active catalyst was extruded through a tape die at room temperature and then passed through a UV chamber with a residence time of 0.5–2.0 seconds (Fig. 1). As displayed above in Table 1, the UV cured silicone elastomer offered properties comparable to those of a similarly prepared thermally cured elastomer.
The UV cure technology may allow energy savings and increased extrusion speed while maintaining the classic properties of silicone elastomers. As the cure is initiated through UV radiation, the degree of cure from skin to the core of the three-dimensional product is uniform, which allows high-speed extrusion of thick wall tubing and profile free from common defects such as internal porosity. The UV exposure time for the cure can be as short as 0.5 seconds, and a 10-inch length UV lamp will allow extrusion of silicone tubing at three to five times the rate of existing thermal curing systems. Since the cure occurs at relatively low temperature (under 140 °F), this new technology opens up the possibility of co-extrusion of silicones with other temperature-sensitive materials such as polyolefin and thermoplastic elastomers. Such co-extrusion is not possible with traditional thermally cured silicone elastomers.
Processing Advantages of UV Curing
Specialty Silicone Fabricators (SSF) (Paso Robles, CA) successfully trialed Momentive UV materials by extruding single and multi-lumen tubing with various wall thicknesses. SSF holds patents related to their Geo-Trans®* process technology, which employs computer aided sequencing that allows extrusion tooling, including the die and mandrel, to be manipulated such that cross-sectional geometries can be changed in fractions of a second within a short section of tubing. An example is shown in Fig. 2.
This Geo-Trans tube provided an important opportunity to evaluate the Momentive material and the UV process. In addition to its complex and variable cross-section, the tubing construct included pigmented areas as well as a barium sulfate stripe.
SSF extrusion engineers identified several processing advantages of the UV system that resulted in reduced production cost:
Increased throughput. SSF engineers were able to completely vulcanize the tube roughly 3× faster than with traditional thermal curing. Dimensional stability was excellent. In another example, relatively simple single lumen tubing was vulcanized at 100 feet per minute. Process optimization is expected to further improve throughput.
Reduced footprint of production equipment. An extruder, as shown in the schematic above (Fig. 1), is required to produce silicone tubing. Key extruder components include a feedbox, barrel, screw, and tooling. The silicone raw material is pumped via screw motion through the tooling producing the desired shape. The tubing, still uncured at this stage, is then pulled through a heating tunnel, typically referred to as an HAV, or hot air vulcanizer, that can be up to 20 feet in length. The UV process replaces the large HAV with a small chamber housing the UV light source. A puller is used to take up material, which has been passed through a light chamber. SSF process and facility engineers estimate that the UV extrusion process reduces the equipment footprint by up to 80%.