Munich — Low-temperature cure liquid silicone rubber is "redefining boundaries of LSR technology," according to Patrick Beyer, silicon rubber product development specialist at Wiesbaden, Germany-based Dow Silicones Deutschland GmbH.
Beyer, who spoke at the recent Silicone Elastomers World Summit in Munich, was referring to Dow's new LTC 9400 series. He confirmed that the technology is the same as for the new fast and low-temperature-cure QP1-33XX series of medical-quality LSR launched at Compamed 2017 two weeks earlier.
While the Compamed presentation referred to curing well below 140° C, Beyer was more specific, talking about "fast curing at temperatures down to 90° C." There are also energy-efficient benefits with LTC, as its cure rate at 120° C is similar to that of standard LSR at 170° C.
He pointed out that fast cure of thick section moldings can be obtained when LTC is used at higher temperatures. Aside from widening potential co-molding material options to polycarbonate, polypropylene and even polyethylene, with use of adhesion promoters, lower-temperature cure benefits integration of thermally sensitive medicines and electronic components into moldings, Beyer said.
Beyer said conventional LSR begins to cure by hydrosilyation at room temperature by bonding silicon hydrides to olefinic double bonds. But "as this is too fast for injection molding, with limited pot life and pre-cure in the mold, it needs to be controlled by, for example, inhibitors to allow for stable molding," ensuring high cure activation starts above 100° C. After mixing LTC 9400 series A and B components, the LSR has at least 72 hours pot life.
More enhanced flow is important for moving the activation threshold below 100° C, Beyer said, showing a curve 50 percent lower viscosity than standard LSR and slightly lower than Dow's Xiameter RBL-9200 high shear-thinning, low viscosity grade, which Dow introduced in 2002.
Referring to benchmark tests, Beyer stressed more than twice the injection speed with LTC over standard LSR, due to lower viscosity, which also means cavities can be filled quickly before onset of cure.
In an LSR molding demonstration by Engel Austria GmbH at the October 2017 Fakuma fair, LTC 9400-50 with an acceleration agent achieved a 35-second cycle time in an Elmet 16-cavity mold for the 198-gram shot weight, Beyer said.
Other LSR-related developments at the Silicone Elastomers World Summit included: