At the Asian High Security Printing Conference Sean Purdy of PPG Industries introduced a new optically variable security film, which is being commercialised in partnership with Centro Grafico, with a launch expected in 2012. The first films will be targeted for security laminates for ID documents, but PPG also sees banknotes and other security documents as a future market.

PPG, headquartered in Pittsburgh, PA, is a giant manufacturer of paints, coatings, chemicals, optical products, glass, fiber glass and specialty materials with sales in 2009 of $12 bn. Centro Grafico is the Italian producer of holographic base film and security laminates (see HN Vol 21 No 7). The new film is named Legitica^TM and it is based on crystalline colloidal array technology, which is a proprietary, opal-like structure that can be structured to form highly detailed, colour-changing images. A CCA is 'a three-dimensional ordered array of mono-dispersed colloidal particles, (which) can self-assemble into ordered arrays (crystalline structures) having lattice spacings that are comparable to the wavelength UV, visible or IR light' according to PPG's published patent application (US 2009/0317623).

SEM micrograph of a Legitica film showing the matrix array

For Legitica, this CCA is dispersed in an ordered manner through the carrier polymer film which then constructively interferes with light at the determined wavelength. This wavelength depends on the interplane spacing of the crystals, the angle of incidence and the refractive index, while the reflectance intensity depends on the refractive index and the number of layers in the film. So for the CCA to produce a bright, recognisable image the particles must be dispersed in a specified periodic array, otherwise the light is simply scattered to give a hazy white material. Determining and creating this periodic array is part of PPG's proprietary processes.

Legitica film showing the colour change when viewed at an angle

In Legitica, these controlled arrays can be specified to deliver, high resolution images in multiple colours which can change with angle of view, on opaque or transparent films. Further, the film can be personalised or serialised and through the use of layering effects it can have hidden images and full-colour images. It can also be made to react to specified activators, such as water or other liquids.

Full-colour Legitica formed from red, green and blue films

The fundamental Legitica effect is a colour shift as the angle of view changes. Several colour combinations are possible, including orange/green, green/blue, dark-to light-blue. Some of these effects are achieved using a single film with the CCA layered within, others result from combining several films. Full colour images, for example, are made by combining red, blue and green Legitica films - yet the result is bright and clear (as shown). 

A 6mm wide Legitica serialised stripe

The film can be produced in clear laminates, opaque laminates or in thread form, as might be used in banknotes. Purdy showed samples of serialised stripes 3mm wide, clearly aimed at banknotes. He also mentioned that the reactive film could be used in applications such as product security, in particular for pharmaceuticals, to demonstrate the legitimacy of the product.

Legitica overlay on an ID card showing the transparency for reading personal data

But ID documents are the first market to be pursued by the company, which is why it is working with Centro Grafico which has much experience in the production of optically-variable overlay security films. Paul Samuels, Business Development Director at Centro, told Holography News® that they were excited to be working with PPG to bring Legitica to the ID laminates field because they see it having great potential.