In a "low is the new high" presentation, Patrick Beyer of Dow Silicones showed advantages of new low-viscosity, low-compression set, low-temperature cure and low-volatility LSR grades.
Low viscosity Silastic RBL-9200-50 retains low viscosity prior to injection into mold cavities, enabling lower injection pressure and higher injection speeds, with benefits of a faster cycle time, a wider process window and improved consistency.
Dow reduced compression set in LSR by suppressing thermo-oxidative decomposition and reactive group presence in cured LSR that react with hydrosilation under compression, time and at elevated temperature, to form new cross-links and permanent deformation. The result is improved sealing performance under harsh conditions and elimination of post-cure treatment.
Beyer suggested these benefits become important as automotive under-the-hood temperatures continue to rise with engine downsizing, also to meet new technological requirements arising with shifts to electric drive vehicles and autonomous driving.
Silastic NPC 9300-xx series low volatile nonpost-cure grades typically have 0.25 percent certified volatile content, not influenced between 120° C press cure temperatures right up to in-mold curing of injection moldings at up to 200° C.
Dow Silastic LTC low-temperature cure grades address significantly longer cure times at reduced temperatures, which are required when overmolding LSR to lower-temperature thermoplastics and sensitive components.
While cure time hardly differs vs. standard LSR at 150° C, Beyer showed it being 60 percent faster at 100° C, yet with a long 72 hours pot life. He showed successful molding of a LTC grade to Dowlex 2035, a 97.8° C Vicat softening temperature polyethylene. As LTC grades have a low degree of temperature gradient sensitivity, they can be used at higher temperatures to obtain fast deep-section cure in thicker moldings.
Reviewing new self-bonding LSR grades, Clemens Trumm, global application development center manager at Momentive Performance Materials GmbH in Leverkusen, Germany, said peel strength against other substrates should be at a lower level and rising more slowly during part demolding. But this requirement works against the need for relatively high bond strength needed to demold highly complex parts.
Momentive proposes UV-curing LSR grades for bonding to polyolefins. They do not require post-cure treatment, but at least part of mold cavities need a transparent PMMA window.
Silopren LSR 2730/2740/2750 self-bonding grades have been available since at least 2014.
Trumm implied the potential to eliminate plasma treatment, used to raise surface tensile strength from 45 mN/m to 85 mN/m on a polycarbonate lens overmolded by polybutylene terephthalate and then LSR. The process limits between the LSR and polycarbonate in a multicomponent rain-and-light sensor molded since 2010 by Esslingen, Germany-based molder and mold maker Weber-Formenbau GmbH & Co. KG.
Momentive illustrated polycarbonate bonding by molding a cell phone holder at NPE2018 in Orlando, Fla.
The LSR 27x9/47x9 grades provide low temperature cure, do not need post-cure heat treatment and have "targeted adhesion," so that while bonding well to PC and copolyester, the materials do not adhere during molding to metal mold and cold runner metal surfaces.
Trumm said work now focuses on soft/hard combinations with new BPA-free resins, adhesion to UL94 V-0 flame retardancy resins and low temperature cure with longer pot life.