Rubber Journal Asia

New breed of LSRs

Included in the new developments in liquid silicone rubbers (LSRs) are grades targeted at replacing engineering plastics in automotive and electronic applications and fully fluorinated grades with higher resistance to oil, as well as highly transparent offerings that look more like plastics than rubber.

Highly transparent range
Recent innovations in optical systems, light guide applications and mobile phones have increasingly seen LSRs beginning to replace materials such as transparent PC and PMMA, due to the unique processing and productivity advantages of the material. 

One such new development comes from Momentive Performance Materials’s LSR7000 series, which combines 98% transparency with the productivity advantages of an addition-cured, platinum-catalysed LSR. It is for this reason that the 7000 series features outstanding optical characteristics and increased design flexibility, says the company, adding that customers are thus able to reduce manufacturing and assembly cycle times for complex parts. The products have a Shore A hardness of 60-80. 

“By offering high transparency, high elasticity and easy processing, the LSR7000 series may help designers and manufacturers to effectively address many of the toughest design challenges they face,” said Yang Nuo, Pacific Electronics Marketing Manager. 

In China, Momentive has cooperated with Sunrex, a leading input device manufacturer, to apply the series to light guide film for commercial production with further new applications targeted for 3D light guide films.

Another example of this materials use comes from German company Kostal, a technology specialist for automotive electronics and mechatronics, that has used it in its camera driver assistance system. The material used to couple the compact front view camera to the windscreen of a car is the LSR7005, which is being launched by Momentive as part of its new  highly transparent LSR7000 series. The low hardness of the material (5 Shore A) allows a form fitting, tolerance compensating connection of the sensor to the windscreen, regardless of the radius and low mounting forces. In addition, the new LSR does not adhere to the glass even at high temperatures and after long periods of time. This means the complete module can be used again when the windscreen is replaced.

New breed of FFSLs 
Partially fluorinated silicone liquid (FSL) grades, which are resistant to hot diesel fuel and oils, have been available for some time now. Used in applications requiring jet/automotive fuels, solvents and/or engine oils contact, fluorosilicones (FSRs) are increasingly finding use in a number of high-volume automotive applications. As more fabricators manufacture products using FSRs, process optimisation and part yield become more important but still difficult issues.

At the K show, Momentive went a step further with its FSL range by introducing fully fluorinated silicone liquid elastomers (FFSLs). These combine the full chemical resistance of fluorosilicone heat-cured silicone rubber (FVMQ), thus making FFSL resistant to oil, unlike FSL. At the same time, FFSL offers the faster cure of platinum-cured LSR compared to the peroxide-cured FVMQ. The new products range from 30 to 70 Shore A hardness and include grades for automotive O rings, seals, gaskets and connectors. 

A 40 Shore A grade is said to be the first oil bleeding FSR, within the new FFSL family, aimed at SR wire harnesses and electrical connectors in automotives 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 under-the-hood use in automotive 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 hours at 175°C) is said to be only 10%.

Another American introduction comes from Dow Corning that has been offering FSL solutions with trifluoropropylmethyl-dimethyl co-polymer LSR
grades. Since these solutions were not fully satisfactory for some harsh applications, Dow Corning united the FSR and LSR technology with new 100% fluorinated liquid silicone technology that can be processed on standard liquid injection moulding machines.

Also introduced last year, available in the Silastic range are high Shore A hardness, self lubricating and self adhering grades as well as special grades for applications in the automotive, aviation and electrical industries. Dow Corning says the new product range allows the production of small, intricate parts that are able to withstand contact with aggressive fuels and oils while performing reliably under a range of extreme temperatures. The difference, the company says, between FSL and the new products are the wide temperature range and good mechanical proper-ties like tensile and tear strengths.

More on self lubricating grades
Last year, at the K show, Wacker Silicones introduced self-lubricating product grades, the novel Elastosil LR 3800 line, with a compete range of oil contents and Shore hardnesses. Besides being more resistant to injection moulding, the new grades can also be moulded and demoulded more easily. Additionally, novel tear-resistant grades improve puncture resistance. Where the 3800 line’s mechanical properties are still not enough, Wacker offers a new self-lubricating platinum-catalysed Elastosil R plus 4800 solid silicones, which have been designed for cable bushing applications.

It also has the LR 3065 grade, which has a greatly reduced coefficient of friction, allowing for easy sliding and thus is better suited to automotive assembly. Another grade, the plus 4305/90, likewise boasts low friction and looks more like plastic than rubber. Said to be the hardest silicone elastomer known to date, it can be used to manufacture rigid parts that previously needed non-silicone support materials. What is more, this silicone rubber is highly transparent.