A Hard Character - Wacker Chemie AG

A Hard Character

The hardness and rigidity of thermoplastics and thermosets has so far been unrivaled by cured liquid silicone rubber products. Now, with ELASTOSIL® LR 3003/90, WACKER is launching a new grade. With a hardness of 90, measured in Shore A, it enables dimensionally stable silicone parts to be produced cost-effectively in an injection-molding process.

Panel made of ELASTOSIL® LR 3003 silicone: with a hardness in the Shore D range, this test piece lies between an elastomer and a thermoplastic and remains dimensionally stable. In contrast, this test panel of standard silicone rubber reveals clear mechanical deformation under its dead load.

Liquid silicone rubbers can be processed in a fully automated injection-molding step, thereby enabling silicone articles to be industrially produced in a cost-effective manner. Processors achieve short cycle times during production, ideally with no need for secondary finishing – even for the smallest, high-precision and geometrically challenging finished products. The processing method can be highly automated and requires minimum personnel.

However, the cured rubber products of typical liquid silicone rubbers have one drawback when it comes to specific applications for which dimensional stability is important. “Even the hardest commercially available liquid silicone rubbers have not yet come anywhere near the rigidity of thermoplastics or thermosets,” explains Dr. Thomas Frese, who is responsible for the liquid silicone rubber product line at WACKER’s Technical Marketing department. The hardness values of cured liquid silicone rubbers are typically in the Shore A range.

Tensile test with ELASTOSIL® LR 3003 products: following curing, ELASTOSIL® LR 3003/90 possesses extremely high rigidity, which can be derived from the slope of the stress-strain curve (see graph on left).

WACKER has now expanded its acclaimed ELASTOSIL® LR 3003 liquid silicone rubber product range to include a grade whose cured rubber products have exceptional dimensional stability and a hardness that can be measured in the Shore D range. In terms of its properties, the grade is close to the transition zone between an elastomer and a thermoplastic. “Parts produced from this liquid silicone reach a dimensional stability that would previously have been inconceivable for liquid silicone elastomers,” explains Dr. Wolfgang Schattenmann, head of the Rubber Solutions business team at WACKER SILICONES. Like all liquid silicone rubbers, the new grade is also cured via a platinum-catalyzed addition reaction, whereby the silicone polymers react with the crosslinker to form a three-dimensional network. The closer meshed the network and the more reinforcing filler the rubber contains, the harder and more rigid the resulting elastomer is after curing.