WUERZBURG, Germany—Oliver Franssen, global marketing director elastomers and transportation at Momentive Performance Materials in Leverkusen, Germany, presented a paper at the recent SKZ Silicone Elastomers conference about a new material, Silopren LSR 3366/50.
Franssen's paper, given with support from Momentive lead scientist Oliver Safarowsky noted that the 3 percent oil-content self-lubricating LSR has comparable mechanical properties as the established 2 percent oil-content LSR 3286/50, but it represents the most heat-resistant self-lubricating grade from Momentive.
The material does not rely on heat stabilizers or abrasive fillers, and customer trials have already resulted in positive feedback, Franssen said.
Automotive mat or frame seals for multicontact connectors, single wire seals and insulation are important application area for such LSR materials, especially with high temperature extremes in engine compartments.
"A cable harness can have as many as 200 connectors over a length of 3,000 meters," Franssen said.
Temperatures rise through downsizing with new vehicle designs, and there are many variants of the "same" connector, Franssen said. Electromobility raises challenges with high-voltage systems and reliability becomes vital with autonomous driving, he added.
He illustrated higher heat performance in ISO 815 (Method A) compression set tests (after post-curing for four hours at 200° C), for 22, 144 and 1,000 hours at 175° C. Tests in the past have been usually limited to 24 or 168 hours, seeking compression set below 40 percent.
LSR 3366/50 showed better compression set values of respectively 6 percent, 12 percent and 24 percent, compared with 10 percent, 23 percent and 62 percent for LSR 3286/50.
Better values of 15 percent, 29 percent and 38 percent were also obtained with LSR 3360/50 in the Volkswagen PV3307 compression set test over 22, 168 and 504 hours, compared with 31 percent, 54 percent and 58 percent with LSR 3286/50.
Compared with T5 class of -40 to 175° C under the 2013 Revision 6 of the SAE USCAR-2 standard, Franssen said of LSR 3360/50: "The limit has been shifted up now by more than 25° C, from 175° C to 200° C."
Franssen said lubrication or friction are not directly measurable. They are better characterized by oil sweating rate weight loss than oil percentage or friction coefficient measurement—and that hardness, type of LSR polymer, degree of cross-linking and the oil used all play important roles in achieving the required degree of lubrication.
Reducing cure time
Having talked at earlier conferences about low-temperature cure Silastic LTC 4000 series cutting cure time, Patrick Beyer, R&D scientist at Dow Silicones Deutschland, showed how dosing 1-3 percent of a Dow Silicones reaction acceleration additive via the third stream coloring dosing system reduces cure time even more.
For example, trials at 90° C showed that although LTC 9400 cures within 425 seconds on its own, 1 percent additive results in around 250 seconds and 3 percent in 125 seconds cure time. This is key to extending the range of temperature-sensitive thermoplastics and components that can be technically and efficiently overmolded with LSR.
Dow Silicones has shown how PC can be overmolded with LTC, with surface activation of the PC thermoplastic to promote adhesion. But it is clear from the LTC 9400 work that two-component molding solutions can now be extended with the reaction acceleration additive to thermoplastics with even lower heat resistance, such as surface-activated PP and PE.
Acceleration was less pronounced when curing at 110-150° C. But it reduces curing sensitivity to temperature gradients, Beyer said, so LTC used at higher temperatures compensates for LSR low thermal conductivity. This results in fast deep-section cure in thicker electrical connector seals and gasket moldings in LTC 9402/50, a 2 percent oil content self-lubricating grade.
Efficient metering, dosing
A joint presentation by Michael Kleinebrahm, application technology manager at Neuwied, Germany-based injection molding machinery producer Dr. Boy & Co. KG, and Klaus Germeshausen, founder and manager of mixing and dosing equipment producer, EMT Dosiertechnik in Werdohl, Germany, focused on efficient metering and dosing (EMT) equipment for small LSR quantities.
Germeshausen said using small cartridges instead of drums enables a compact mixing pump design that can be mounted on an injection molding machine close to the plasticizing screw. The mixing chamber and static mixer are integrated within a single block, directly connected to the molding machine. The system works using the molding machine control system.
Short, small cross-section feeding lines mean less LSR material in the system and there is no need for a shut-off needle valve on the mixing pump, as access to the screw can be hydraulically closed off once each dosing cycle has been completed.
Germeshausen said these are ideal conditions to ensure high dosing accuracy, also as there are fewer parts in contact with the LSR, which remains held in the dosing/metering system at constant reproducible pressure after each dosing cycle, so that a pressure regulator is no longer needed. It also means easier cleaning.
EMT displayed the compact basis version of the EMT micro metering/dosing equipment at the conference alongside the larger drum-fed up to 100-gram shot weight servo-pneumatic LSR 100pv system.
Using additives to improve properties
As head of innovation management for polymer interfaces and performance, Kathrin Lehmann of Essen, Germany-based Evonik Nutrition & Care GmbH spoke about extending silicone elastomer applications with additives, suggesting 2-6 percent Tegosil Heatban additive in LSR with the aim to increase heat resistance, minimize heat aging and have minimal influence on mechanical properties.
In work with Dortmund, Germany-headquartered LSR processor M+S Formteile GmbH, seal profiles cut out from plates compression molded in Wacker Elastosil LR 2003 containing 4 percent Heatban 400 and colored with two different red color additives at 2 weight percent addition, were post-cured for four hours at 200° C, then aged for 10 days at 250° C and 300° C. There were significant differences between the two red colorants in mechanical properties, but most impressive was Heatban 400 preventing complete destruction that occurred without its use at 300° C.
Similarly, good mechanical properties and red color were well maintained in an LSR automotive gasket application at 250° C, with samples at least "surviving" at 300° C, while without Heatban 400, the same parts lost mechanical properties and color at 250° C and were destroyed at 300° C. Despite promising results, Lehmann says potential is greater in Asia than in Europe, as there are only a few European independent LSR compounders.
New cold runner system
Stefan Eimeke, CEO at Frankenberg, Germany-based Ewikon Heisskanalsysteme GmbH talked about introduction in June 2018 of Coolshot water-cooled cold runner systems for LSR molding. Eimeke explained how Coolshot benefits from Ewikon hot runner experience by featuring an electric step motor driven valve gate system and flexible spiral tubular cartridges on the optional coil-heated separator plate with temperature control for each cavity, ensuring even mold plate heating for homogeneous LSR cross-linking.
The valve gate system precisely positions valve pins in 0.01-millimeter steps for balanced cavity filling, without laborious manual valve pin length adjustment. Valve pin positions are permanently monitored and automatically adjusted with encoders. The entire system is controlled via a touch screen panel with remote access.
A cavity plate mechanism option insulates the separator plate from cold runner nozzles, preventing heat transfer into the nozzle gate area and cold runners when not molding. Eimeke said absence of material hang-ups in manifold dead spots and ease of dismantling and opening for cleaning result in fast color changeovers. Eimeke stressed, "We didn't want Coolshot to be a 'me too' development, but one with its own special features."