Hybrid Coatings Increase Hardness of Plastic and Allow Properties to be Tailored

Mechanical behaviour of hybrid SiO₂-PMMA coatings measured by nanoindentation

Increasing the surface hardness of many materials opens them up for use in a wide variety of new applications. These new hybrid materials could be used in areas like anticorrosion coatings for metals, scratch and abrasion resistant coatings for plastics, antistatic films plus colour decorative coatings for glasses and plastics.

The sol-gel method of materials preparation is a suitable technique that can be used to obtain composite materials with reinforced properties and produce hybrid coatings. It is capable of producing materials that combine the complementary properties of the inorganic and organic materials.

Engineering plastics such as polymethylmetacrylate or PMMA are distinct candidate materials that can benefit from reinforcement with inorganic materials. Silica (SiO₂), is a suitable reinforcing material that was used in this investigation.

In this paper researchers from Universidad Autonoma de Sinaloa, Fuente de Poseidon y Prol and Centro de Investigacion y Estudios Avanzados del Instituto Politécnico Nacional, J.L. Almaral-Sanchez, M. Lopez-Gomez, R. Ramirez-Bon, J. Munoz-Saldana, prepared hybrid SiO₂-PMMA coatings using the sol-gel method and measured coating hardness. They applied two depth sensing indentation (DSI) systems, each one using indenters with different geometry and size, to analyze the mechanical behaviour of hybrid coatings of SiO₂-PMMA with different molar ratios. Applying an indentation model, the absolute hardness of the coatings was evaluated using both systems as a function of the PMMA content of the hybrid films.

From the indentation measurements the researchers found that the mechanical properties of the hybrid coatings were improved in comparison with those of conventional acrylics. Hardness of the hybrid coatings was from three times to more than one order of magnitude higher than the acrylic hardness, depending on the molar ratio. An extra hardening phenomenon was measured at the surface of the coatings, which is probably due to an enrichment of silica at this region. Finally they found that hardness of the coatings decreases monotonically with the PMMA content. These results allow hybrid coatings to be made with mechanical properties tailored to the individual application.

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