ADDITIVES | SURFACE MODIFICATION
Right: Appli- ance control panels place high demands on surface finish, accord- ing to Dow Performance Silicones
ways to improve the surface aesthetics of plastics and composites used in countless applications, Paulo claims, adding that the additive tech- nologies also help maintain these attributes over the long term. Silicone additives are also well known for their high perfor- mance at low loadings, which minimises any negative impact on mechanical properties. Scratch marks are the result
e
of two phenomena: a ductile failure that leaves an indenta- tion; and a brittle failure that leaves a visible white mark. These marks can be especially noticeable in talc-filled plastics where scratching exposes the white talc filler. Silicone additives can prevent or minimise scratching and marring of polymer surfaces in applications ranging from electronic device covers to appliance handles, buttons and control panels, Paulo says. Dow Performance Silicones has designed silicone additives that migrate to the surface of the resin, self-level and anchor themselves to the plastic matrix during the melt phase of processing. This action decreases the surface coefficient of friction (COF) of the material. Because the silicone molecules are soft, they absorb some of the stress and, as such, reduce the brittle failure.
oticeable in Improving haptics
Silicone-based additives can do more than just prevent scratches, Paulo says. As an added benefit, they can improve surface haptics by contributing a soft, silky feel. They also exhibit lower exudation than some alternative products used to impart slip properties, such as organic waxes, which can create
Dow Performance Silicones’ additives can help prevent dulling and scratching of polymers such as PC in portable devices
a greasy film that acts as a dust magnet. Silicone technologies remain anchored in the resin matrix after migrating to the surface and this property, together with their good slip performance, makes them a better choice for improving surface quality in numerous applications, he claims. Weathering resistance is also a plus. Just as many polymers are sensitive to UV light, additives used to enhance properties and processing can also exhibit instability under UV exposure. In particular, organic
a g m re m su to pe be su ap
a ar ad al x
waxes such as erucamide are highly sensitive to sunlight. Traditional talc-filled PP compounds containing organic slip additives can develop undesirable stickiness or tackiness after just a few months of UV exposure. In contrast, silicone-based slip additives are UV stable. Paulo cites a Kalahari- type test protocol on the surface of a talc-filled PP copolymer, in which a Dow siloxane additive, HMB-0221, showed no exudation, delivering higher UV stability than both erucamide and oleamide additives.
es such as erucam Some polymers absorb moisture and many
inorganic fillers used in compounds are also highly hydrophilic. Because silicone is highly hydropho- bic, silicone-based additives can prevent moisture uptake. Not only do they inhibit water absorption, they can also improve the dispersion and de-ag- glomeration of inorganic fillers.
Automotive effects Another silicone specialist, Wacker, says that auto interior parts moulded in talc-filled polypropylene containing its Genioplast Pellet S silicone additive maintain an optimal surface appearance even after long and intensive use. Parts such as instrument panel components, glove compartment covers, door panels, pillar trims, and centre console housings are typically hard with matt-effect surfaces that provide a velvety feel, pleasing appearance, and avoid light reflections that might irritate or dazzle the driver, says Michael Geck, Applications Engineer for Wacker’s plastic additives segment. However, the more matted such surfaces are, the more susceptible they are to scratches and abra- sion. Even blunt objects can mar them. Geck describes Genioplast Pellet S as a highly
effective silicone additive that has no undesired side effects and remains active throughout the
54 COMPOUNDING WORLD | November 2017
www.compoundingworld.com
PHOTO: DOW/SHUTTERSTOCK
PHOTO: DOW/SHUTTERSTOCK
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