New LSRs for injection molding.

Some fairly dramatic new developments in liquid silicone rubbers were announced at K by Momentive Performance Materials and Wacker Chemie. At the top of the list is the first commercial family of fully fluorinated silicone liquid (FFSL) elastomers (pictured). Up to now, there have been partially fluorinated silicone liquid (FSL) grades, which are resistant to hot diesel fuel and hot oils. The new FFSL types add the full chemical resistance of fluorosilicone heat-cured silicone rubber (FVMQ) gums. For example, FFSL is resistant to gasoline, unlike FSL. At the same time, FFSL offers the faster cure of platinum-cured LSR versus peroxide cure of FVMQ.

The new products range from 30 to 70 Shore A hardness and include grades for automotive o-rings, seals, gaskets, and connectors. One grade is said to be the first oil-bleeding fluorosilicone (40 Shore A), aimed at wire harnesses and electrical connectors that could be exposed to fuels, oils, blow-by gas, or hot air.

Also new is a family of LSRs said to offer the lowest available compression set. The first commercial grade in this family, Silopren LSR 3596/30, is a self-lubricating, 30A product for automotive underhood connector seals and cable assemblies. It contains 5% of a silicone fluid, which migrates to the surface over time. Without post-cure, compression set (22 hr at 175 C) is said to be only 10%.

Wacker also introduced oil-bleeding LSRs for automotive connector seals. The Elastosil LR 384x series are enhanced versions of existing products. They are said to be much easier to process, thanks to a wide processing window that is tolerant of process variations. They also boast much higher tensile and tear strengths than earlier oil-bleeding grades, which gives them high cut resistance and less danger of damage during installation of sharp-edged connectors. These grades come with a range of oil contents and Shore hardnesses.

Wacker also came out with an unusual alternative to oil-bleeding products for automotive connector seals--LSRs that form low-friction surfaces immediately after molding but are oil-free. Thus parts do not get covered with an oily film that attracts dust, nor do they leave an oil film on other surfaces with which they come in contact. Elastosil LR 3065 products also boast high tensile strength and extremely low compression set.

Other novelties include a uv-curable LSR from Momentive for large, thick, clear or tinted parts, such as electrical switch gear. The parts must be molded in a tool made of glass or clear polymer to allow passage of uv light. With this material, a 500-g, 80-mm-thick part cures in 40 sec instead of the usual 20 min. Such a "cold-curing" LSR could possibly be overmolded onto less heat-resistant thermoplastics, though adhesion might be a problem, since bonding is usually heat-activated.

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In addition, Momentive has a hard (90A), high-clarity, high-refractive-index LSR developed in Japan for molding complex-shaped lenses. The firm is also working on a soft, high-clarity grade for LEDs.