Vacuum Web Coating

The film that caught my eye is one aimed at the security market.  The idea is not new but has been around for 20 years.  Pilots flying over the Amazon rainforest noticed flashes of blue or green within the forest which for some time remained unexplained. What caused these flashes were Morph moths which are quite large moths that have a very vivid colour to their wings.  The colour is particularly vivid at only one angle.  The wing structure was investigated and the shape of the ‘hairs’ that make up the wings allowed the alternation of keratin and air which gives an optical interference effect which is tuned to a single colour.

Other animals replace the air with another solid material but with a different refractive index than the keratin. In this way fishes produce the silvery effect from keratin that is essentially transparent and some beetles produce their green or iridescent carapace.  Many optical coatings are made up of alternating refractive index materials. There are a group of security coatings that change colour with viewing angle and so are secure against colour photocopying.  This can be achieved by as few as three layers of inorganic materials but can also be achieved by co-extruding polymers. The polymers have refractive indices that are similar to each other.  There is a small reflection at each interface and so to achieve the same vivid optical effect requires many more layers. By using extrusion dies that divide and layer polymers it is possible to produce co-extrusions with more than 100 layers that show iridescence or colour shifting properties.  This layering process not only divides and layers the film but also keeps reducing the film and so even with over layers the total thickness of the film was still only 12 microns thick. Originally although this worked as a process it was not perfect as the uniformity of the layers could not be maintained accurately enough and so the colour varied. Some years later and 3M developed the process further to make large area optical filters with controlled polarisation by this co-extrusion layering technology.

Now a research group at the University of Sheffield claim to have produced an alternative method of mixing polymers to better replicate what is seen in nature and produce uniform colours.  Using block copolymers that naturally separate into lamella forms enables them to develop into a structure that can be thought of as an organic form of opal.  The materials and process suggest that this could be a lower cost method of producing some of these colour shifting security devices than the current options that often require vacuum deposited coatings.  Certainly this looks to be an interesting development for some new polymeric films for the future.