Silicone elastomers can be classified into two primary subgroups: high-viscosity compounds that require conventional rubber mixing and fabrication processes, such as heat-cured elastomers (HCEs); and liquid silicone rubber (LSR). Silicone HCEs have traditionally been used in the fabrication of rubber components exposed to wide temperature fluctuation and harsh environments, making them a top candidate for many sealing applications. The trend in the marketplace for molded silicone rubber parts clearly favors the increased use of injectable LSR due to its ease of processability, design versatility, excellent quality, and lower system costs. A new class of highly fluorinated LSRs provides added fuel, oil, and solvent resistance for the most demanding mobility applications.
Conventional HCE silicone rubber is usually supplied in bulk, uncatalyzed block form directly from the silicone manufacturer. It is then converted to colored, catalyzed, and preformed stock using either two roll mills or sigma blade mixers. Typical molding time for injection-molded HCE is 1.5 to 5 min dependent on part geometry, and flash removal is usually necessary as a secondary operation to produce the final part. HCE can also be fabricated by conventional compression molding and transfer molding, the latter particularly effective for producing high-volume, highly detailed parts such as connectors, seals, and grommets.
In contrast, LSR—such as Momentive Performance Materials’ LIM liquid silicone rubbers—requires no material preparation and offers an efficient process for delivering high-quality, highly detailed parts with a minimum of human intervention. LSR can be supplied as a two-component system with a platinum catalyst in the A component and a silicone hydride crosslinker in the B component, which also contains an inhibitor to afford reasonable mixed pot life.
The material is supplied in either 5-gal (19-L) pails or 55-gal (208-L) drums that are pumped directly through a static mixer in a 1:1 ratio into the metering valve of the injection press. Precisely controlled injection into a multi-cavity heated mold in conjunction with cold runner block and valve gate technology can result in ready-to-package parts with little or no scrap. Vulcanization in the temperature range of 150 to 200°C (302 to 392°F) is rapid, providing finished, flashless parts in 10 to 30 s and yielding exceptional productivity for the custom molder.
Today, LSR is available in a wide durometer range from 3 to 80 Shore A and can be further modified to provide enhanced functionality such as self-lubrication for automotive connectors, self-bonding for overmolding and two-shot assemblies, and high heat resistance for proximity to exhaust manifolds.
One major development that has remained elusive until recently, however, was the creation of a highly fluorinated LSR having fuel- and solvent-resistant properties comparable to those of fluorinated HCE and flow properties equivalent to standard LSR. Through advancements in product and process technology, Momentive Performance Materials has now succeeded in developing a family of fully fluorinated LSR compounds (FFSL) that fulfill both of these requirements.
These materials are currently available in the 30 to 70 Shore A durometer range and are expected to play a major role in providing cost-effective solutions to demanding applications in the ground-vehicle and aerospace industries.
Gail Riley, Transportation Marketing Manager Americas–RTV, and Mel Toub, Application Development Manager, Elastomers, both of Momentive Performance Materials, wrote this article for SAE Off-Highway Engineering.