The relatively high coefficient of friction of silicone elastomers can limit the range of applications of these materials. A liquid silicone rubber top coat can be used to improve surface friction of silicone elastomer parts. It can be applied using conventional coating methods. The performance benefits it offers and potential product applications are described. (E. L. Klaassen-Momentive Performance Materials, formerly GE Bayer Silicones GmbH & Co KG, Leverkusen, Germany)
The need for reduced surface friction
One of the basic characteristics of silicone elastomers is relatively high coefficient of friction (COF). High friction is desired in applications where the silicone elastomer part needs to be well positioned, for example in closures or where excellent grip is required such as in dental tools. However, high friction can also be a limitation in applications where parts are sliding and soft touch is required or where dust collection, cleanability or stickiness are issues.
A number of techniques are available for reducing the surface friction of elastomeric parts such as surface texturing, lubricating fluids or vapour deposition coatings. However most of these have limitations in terms of application effectiveness, biocompatibility, mechanical performance, durability or processability.
Liquid silicone rubber (LSR) top coat is a two-component liquid silicone material that forms a colourless matt surface finish. It can be applied using standard coating processes to silicone elastomer substrates to provide reduced surface friction. It is particularly suitable for health-care applications because it provides potential biocompatibility, good adhesion to silicone elastomer substrates and it is sterilisable.
Coefficient of friction reduction and adhesion. When LSR top coat is applied on a silicone elastomer substrate, it can reduce the COF by more than 40%, while providing good adhesion to the substrate. As could be expected, the reduction of the COF versus other materials such as glass, aluminium or painted surfaces is much greater.
Abrasion resistance. The abrasion resistance of the LSR top coat is no better than that of a regular LSR. However, lower friction would reduce the forces applied to the surface and lead to improved overall abrasion performance. Figure 2 shows the abrasion resistance of LSR top coat applied to hot and cold surfaces.
Mechanical performance. The effect of a LSR top coat on mechanical performance was tested by performing standard mechanical tests on a coated LSR substrate. The LSR top coat does not appear to affect the hardness or the 100% modulus. Reduction of tensile strength and elongation was measured to be 22% and 16%, respectively.
Sterilisation performance. The LSR top coat does not appear to influence the steam sterilisation performance of silicone elastomers. Tensile strength and elongation do not seem to be significantly affected by sterilisation testing performed at 121°C for 1000 cycles. No peel off was observed during these tests.
Applying the coating
A LSR top coat can be applied on surfaces that are dust and grease free (but otherwise untreated) by spraying, brushing or dipping. It cures quickly at high temperatures (150-180°C) in a ventilated hot-air oven. If the substrate cannot withstand high curing temperatures, a temperature of 100-120°C is sufficient although the curing time would need to be extended to approximately 10-30 min. Experience shows that the amount of coating that is applied is typically 0.2-0.6 g/cm2.
The coating can also be applied to silicone tubing. To simulate inline extrusion, a LSR top coat was tested on vulcanised and unvulcanised surfaces made from peroxide curable and addition curable heat-cured elastomers. Applying the coating on unvulcanised tubes provided the same result as if it was applied as a postprocessing operation. However, precautions need to be taken to avoid deformation of the surface by the coating process itself, that is, spraying. When applied to vulcanised, but still hot tubing, the peroxide version showed adhesion issues as a result of the byproducts leaving the substrate, whereas the addition-cured tubing showed good adhesion.
The ability of LSR top coat to reduce the surface friction of silicone elastomer parts enables LSR, heat-cured and room temperature vulcanised elastomers to be employed in areas where silicones have not formerly been an option. Example applications are
- orthopaedics: reduced friction in skin contact applications
- tubing: tube handling and low dust pick up
- anaesthesia/respiratory devices: soft touch comfort for face masks and low dust pick up
- keypads: printed surface protection
- assembly: reduced stickiness of silicone parts.
The launch of this technology has inspired new applications of silicone elastomers. The coating material is being further developed to offer higher elongation to meet the needs of applications where the silicone is highly stretched. Figure 5 describes how elongation performance is tested and shows the first promising results of a LSR top coat with high elongation versus the original LSR top coat.
Egbert L. Klaassen is Industry Manager Consumer Goods and Healthcare at Momentive Performance Materials, formerly GE Bayer Silicones GmbH & Co KG, Building R20, Bayerwerk, D-51368 Leverkusen, Germany, tel. +49 173 657 7822
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