Würzburg, Germany — Thomas Frese, manager of technical marketing at Burghausen, Germany-based Wacker Chemie AG, focused on low-volatile LSR developments in a recent presentation at the SKZ Silicone Elastomers Conference in Würzburg.
The silicone materials producer company had first mentioned its capability in this area at K 2016. Wacker offers its transparent slight blue tinge Elastosil LR 5040 series in six Shore hardnesses of 30-70A. Its 45A hardness material is widely used in sensitive applications such as baby care, food and medical devices.
LR 5040 series materials comply with European regulations requiring maximum 0.5 weight percent volatile content and also fulfill challenging limits for migrating and extractable substances, which can be achieved without the inconvenience and cost of post-cure heat treatment.
Frese showed conventional non-post-cured LSR as having around 1-2 weight percent volatile content, of which 0.7 percent Dx siloxanes and 0.4 weight percent other volatile substances, while LSR 5040 series moldings come out at around 0.2-0.3 weight percent volatiles. He gave extractable content of Dx siloxanes in total of 7,000 parts per million and less than 500 parts per million for respectively conventional LSR and LR 5040.
He revealed that an upgrade of all LSR products produced at Wacker's facility in Burghausen, Germany, (Elastosil LR 3xxx and LR 6xxx, as well as medical/pharma Silpuran 6xxx) to low Dx siloxane content, with more than 90 percent reduction now achieved compared to 2017. For technical LSR products such as the oil-bleeding Elastosil LR 384x, for example, the Toyota standard of maximum 350 parts per million content of D4-D8 siloxanes can be met by a wide range of LSR grades, without post-curing after molding.
The upgraded products also include the third-generation high transparency Lumisil LR 7601 grade for optical lenses. Frese stated that no European-made LSR products now exceed 20 parts per million D4, D5 and D6 siloxane volatile content.
The result, Frese said, is that "post-curing time can be reduced in some cases and certain requirements met without any post curing at all." And if LSR molders still want or need to post-cure, the risk of explosion is lower, Frese added. As mechanical, physical and chemical properties remain unchanged, existing product approvals and certification remain valid. Injection molding and mold release characteristics, as well as mold shrinkage, have not changed, although shrinkage after post-cure is slightly lower than before.
It means, as Frese explained, "most Dx siloxanes that could earlier only be removed by post-curing are now already removed beforehand by Wacker." This now offers LSR processors the prospect of supplying non-post-cured LSR moldings to the automotive industry that comply with the industry's requirements for low volatile organic content emission in order to protect electrical contacts and to avoid fogging.
Other new product technologies from Wacker for Elastosil LSR-brand products include very low long-term compression set LR 3834, very low viscosity LR 63xx, reduced coefficient of friction LR 306x, extremely high flexible module and hardness of the LR 3003/03-90 range (introduced at K 2016), hot-water-resistant LR 3020 and LR 3271, high heat LR 3093 and low temperature cure LR 3004 FC.
In a review of solid silicone rubber (HTV), Wacker silicone polymers technical marketing manager for rubber solutions Christian Anger showed overlapping areas such as oil-bleeding silicone rubber automotive connectors, where both LSR (for example, Elastosil LR 3856/20) and HTV (Elastosil R plus 4806/20) materials are used, as many properties are similar. But LSR 19 percent compression set after 22 hours at 175° C compares favorably with 13-30 percent for the HTV. On the other hand, HTV has much higher cut resistance.
Anger also defined areas where HTV and LSR do not overlap, with magnetizable, highly flame retardant and low temperature flexible compounds for HTV, while the exclusive domain for LSR is transparency, low hardness (down to below Shore 10A) and low volatile materials.