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13. November 2019

Low temperature cure of liquid silicone rubber

Liquid silicone rubber (LSR) is a high-performance elastomer used in many markets and applications. New application trends require enhanced design flexibility, process efficiency and co-molding using combinations of different material types. In co-molding applications, the cure temperature of LSR is inherently limited by the thermal stability of the overmolded counterpart, and conventional high process temperatures often cannot be used. A new generation of low temperature cure (LTC) LSR materials that cure between 90 °C and 120 °C can overcome those co-molding challenges and enable new designs and applications.

Simulation of heat transfer in LSR spheres of varying radii

Simulation of heat transfer in LSR spheres of varying radii

Low temperature cure of liquid silicone rubber
P. Beyer, N. Gerard, H. P. Wolf
RFP Rubber Fibres Plastics 03 2019

Liquid silicone rubber (LSR) is known for its fast vulcanization and high productivity, which result from a platinum catalyzed addition cure reaction (hydrosilylation). To maximize the cure rates, LSR is typically cured at high temperatures of 160 °C up to 220 °C, enabling heating times in the range of seconds. In co-molding applications, LSR can provide reliable sealing function, moisture protection, soft elastomeric elements in hard-soft composite materials, or encapsulation of sensitive components. Examples include overmolding of electrical devices used in consumer electronics, automotive electrification and automation, or the encapsulation of functional additives and actives in the field of consumer and hygiene applications. In those cases, the application of conventional high cure temperatures would lead to thermal decomposition of the encapsulated component during heat-cure.

Another important trend is the overmolding of thermoplastic substrates by LSR to produce hard-soft composites. Here the cure temperature is limited by the softening temperatures of the thermoplastic substrate. Depending on the type of substrate, the use of conventional (i.e., high temperature curing) LSR technology would lead to deformation of the plastic during molding, or to unacceptably long curing times and inefficient processes when cured at reduced temperatures. A new class of low temperature cure (LTC) LSR is presented for use in a wide range of process temperatures.

The temperature activation of LSR is a function of many factors, including amount and type of platinum catalyst and inhibitor. Kinetic studies allowed us to derive key structure-property relations, and enabled development of the new low temperature cure (LTC) LSR series. In this new generation of LSR materials the temperature activation threshold is shifted to below 100 °C, with an onset temperature of about 85 °C. This shift in activation temperature significantly increases the reactivity in the targeted low temperature range 100°C to 120 °C. To further enhance reactivity at low temperatures, a complementary product – the Silastic LTC Acceleration Additive – has been developed. The additive has low viscosity and can be added at 1 – 3 wt% during the process through the third-stream injector.

Low temperature cure is a novel technology platform for LSR pioneered by Dow Silicones, enabling a step-change reduction in curing temperatures and resulting process cycle times. It enables new design options by allowing co-molding of LSR onto thermosensitive substrates and components in consumer, electronics and automotive applications. At conventional high temperatures Silastic LTC LSR allows for a fast bulk activation, resulting in enhanced efficiency and quality. A complementary additive approach allows manufacturers to further maximize reactivity and to lower the application range to temperatures as low as 90 °C.

Silastic LTC LSR technology is considered a key trend in 2K applications, where self-adhesive LTC grades can open new performance levels in the co-molding of low melting plastics, such as polycarbonates or polyolefins. For high melting engineering plastics widely used in automotive and consumer applications, this new class of materials can enable increased robustness and process efficiency by allowing for fast bulk vulcanization and reduced sensitivity to interfacial temperature gradients inherent to 2K co-molding applications. These developments are covered in subsequent extensions of the Silastic LTC LSR product portfolio.

Citation:
P. Beyer, N. Gerard, H. P. Wolf, RFP Rubber Fibres Plastics, 03 2019, 168-172

https://www.gupta-verlag.com/magazines/rfp-rubber-fibres-plastics-international/03-2019

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