A Strong Bond – Without Any Pretreatment

Molded plastic parts very commonly consist of two components: one hard, and the other soft and elastic. The hard component for such hard/soft combinations is usually a rigid thermoplastic and provides the strength. By contrast, the soft component functions as a seal, absorbs vibrations or sounds or improves the tactile properties of the molded article.

Product designers tend to choose a polycarbonate if the hard component is not only expected to exhibit high impact strength, heat resistance and transparency, but should also lend itself to medical products. For the soft component, they are increasingly turning to silicone elastomers, especially where the property set offered by silicones is needed: chemical and biological inertness, flexibility at low temperatures and resistance to heat and aging.

Good adhesion is essential for creating hard/soft combinations that function reliably and for manufacturing such articles cost-effectively on a large scale. “A particular challenge as regards adhesion is the polycarbonate/silicone pairing,” says Dr. Florian Liesener, head of an applications laboratory for liquid silicone rubber at WACKER in Burghausen. “With conventional silicones, the only way to achieve reliable adhesion is to either coat the surface of the polycarbonate with a primer or activate it with a plasma treatment.” For the parts manufacturer, this represents an additional, costly step, he adds.

The new line of ELASTOSIL^® LR 3078 silicone rubber products developed by WACKER provides strong adhesion to polycarbonate with no need for treating the thermoplastic substrate ahead of time. “These self-adhesive silicones are capable of being used on fully automated two-part injection-molding machines,” says Dagmar Rische, marketing manager in WACKER’s Engineering Silicones department. “This capability stems from a novel, patented self-adhesive technology that has been specially developed for the polycarbonate/silicone combination.”

As a result of this technology, which is deployed in the ELASTOSIL^® LR 3078 series, the adhesion builds up rapidly – reaching high levels even as the part is still inside the mold. However, the silicone does not stick to the metal mold and the multicomponent part is easy to demold. No secondary finishing is needed and the parts can be processed further straight away. The hardness values of these product grades cover the range (20 to 70 Shore A) specified by parts designers for the soft component of the polycarbonate/silicone combination.

“The lab tests and injection-molding trials also show that the ELASTOSIL^® LR 3078 range scores points for ease of processing,” marketing manager Rische is keen to stress. The liquid silicone rubbers cure very quickly and the injection-molding cycle times are short, even at relatively low processing temperatures. “This means that converters can adapt their processes to the limited heat-deflection temperature of the hard component and still achieve high productivity,” she says, further emphasizing her point.

Another plus point for ELASTOSIL^® LR 3078 when it comes to injection molding is that the silicone rubber formulations have a very low tendency to form deposits in the mold. When these build up, the molds have to be dismantled and cleaned. The use of ELASTOSIL^® LR 3078 more or less rules out such production interruptions altogether, with the result that fully automated processes are possible.

ELASTOSIL^® LR 3078 can also be used to produce intricate designs and will reproduce the finest of details with great accuracy. The new product series thus goes some way toward meeting the trend to miniaturize parts.

Given the limited heat-deflection temperature of the thermoplastic hard component, polycarbonate/silicone combinations are not postcured before use, i.e. they are not heat-treated, as the polycarbonate would not withstand the thermal stress. “We have formulated our materials in the ELASTOSIL^® LR 3078 series in such a way that they already possess very good mechanical properties without the need for postcuring,” underscores Dr. Liesener, a WACKER developer.

Thus, the new silicone grades are notable for having low compression set values in their non-postcured state. Elastomers that have low compression set feature high elastic recovery. Non-postcured ELASTOSIL^® LR 3078 grades have compression set values far below 20 percent, as determined by 22-hour compression at 125 °C. These are very low values. Their significance is that sealing elements made from the new silicones will function reliably for long periods of time.

Cured rubber made from ELASTOSIL^® LR 3078 is biocompatible, as demonstrated by various tests meeting ISO 10993 and United States Pharmacopeia (USP) Class VI standards. The materials have been tested in accordance with ISO 10993 for their cytotoxicity, pyrogenicity and sensitizing properties. And their acute systemic toxicity, intracutaneous toxicity, and short-term implantation have been assessed under USP Chapter <88> Class VI. Plus, the new self-adhesive technology does not contain any bisphenol A structures at all. WACKER’s decision to dispense with this class of substance is helping to boost the occupational safety aspects of the production and processing of the silicone rubbers while also serving to protect consumers.

What is more, polycarbonate/silicone combinations made with ELASTOSIL^® LR 3078 and the right polycarbonate grades can be steam-sterilized, because the mechanical properties of silicone remain virtually unchanged even after 100 sterilization cycles involving exposure to steam heated to 134 °C.

They also continue to adhere well to Apec^® 1745 after 100 cycles (5 minutes at 134 °C). Thus, ELASTOSIL^® LR 3078/40, for instance, only exhibits a slight reduction in peel-force values after more than ten sterilization cycles. And, although the failure mode switches to adhesive failure once 50 cycles have been exceeded, the peel forces after 50 and 100 cycles, respectively, are still high, at 10 newtons per millimeter.

“A chemical bond created by ELASTOSIL^® LR 3078 to polycarbonate offers considerably more design freedom than traditional mechanical joining techniques involving interlocking openings and undercuts,” says laboratory manager Dr. Liesener. In addition, the chemical bond prevents gaps from arising between the hard and soft components, where dirt can collect, or bacteria and mildew can form colonies.