FLAT DIES | MACHINERY
Nano-layering can enhance the properties of film and sheet structures beyond those achieved by simple co-extrusion, says Olivier Catherine of Cloeren
Multi-layered approach boosts physical properties
Co-extrusion is widely used in the film and sheet industry, combining the properties of several polymers in a multi-layer structure. In conventional co-extrusion, the number of layers in the final product is typically between two and 17. A varia- tion of this, called nano-layer co-extrusion, can create structures with hundreds or even thousands of layers – each with micron- or sub-micron individual thicknesses. Recent research shows that structures with a high number of nanometre-scale layers have remarkable properties, driven by interfacial or confinement phenomena.
Natural effect This can be seen in nature. Natural materials often exhibit complex, yet elegant structures that are adapted to a specific function. An example is nacre – a natural nanocomposite found in the inner shell of some molluscs such as the pearl oyster[1
]. Nacre is made of Aragonite (CaCO3
www.filmandsheet.com ) platelets with typical
thicknesses of 400nm, assembled together with biopolymers of typical thickness of 10nm. While Aragonite has very ordinary properties, those of nacre are exceptional – despite it being 95% Aragonite. In particular, nacre’s toughness is about 3,000
times that of pure Aragonite. This huge gain in mechanical properties is a result of the unique nano-layered structure. Another consequence of the structure is the
pearl oyster’s ‘rainbow’ colours. Researchers have attributed these properties to
several complex factors: n Scaling effects – such as the effect of thickness reduction; n Multi- scale effects – the effect of several mecha- nisms at several different scales; and, n Interfacial effects – such as adhesion and slippage. This is a perfect illustration of the potential
synergies that can develop in a multi-layered nano-scale composite structure.
� April 2018 | FILM & SHEET EXTRUSION 21
Main image: Nano-disper- sion effects are responsible for the colour – and strength – of pearl oyster shells
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