Modern polycarbonate identity documents have been approved in many countries due to the material’s exceptional mechanical and optical properties and fraud resistance. The manufacturers of identity documents endeavour to secure the card body, but in most situations it is the personalised data that are the object of attacks and manipulations. In this article, Arkadiusz Karpinski and Agata Wojciechowska describe the personalisation of polycarbonate documents using a proprietary, commercially unavailable digital solution developed by Polish Security Printing Works (PWPW). This method is based on printing with an invisible to human eye, machine readable and near-infrared absorbing ink which provides a unique individual security feature for each polycarbonate document.
Development of personalisation techniques for polycarbonate documents

The personalisation of polycarbonate documents is commonly achieved by means of laser techniques, resulting in a greyscale contrasted image, card perfo­ration or tactile elements. Such post-personalisation methods are very flexible since the manufacturing and personalisation process can be separated from each other. However, these methods have a significant drawback related to the existence of blank documents, which is always a concern. What is more, laser personalisation techniques have been on the market in the document sector for over two decades, but they are available in the graphic and marking sector as well. Nowadays, engraving devices are accessible at reasonably low prices and can be a powerful tool in the hands of counterfeiters. Although it is not easy to erase the personalised data, as they are marked on all transparent layers on the already issued document, there is always a risk of alternation of those elements.

Colour personalisation
In recent years, considerable efforts have been made to develop the technology of colour portraits for poly­carbonate documents. Colour personalisation can take place in two ways: a pre-personalisation method using inkjet technology and a post-personalisation method based on selective laser bleaching of colourants or selective darkening of a patterned background. Inkjet personalisation has been successfully applied in poly­carbonate documents, for example by PWPW (PCP®) and Bundesdruckerei (POLYCORE®), while Oberthur Technologies and Gemalto offer ready-to-use technology for colour laser personalisation.[1] The benefits of inkjet technology include a satisfactory colour gamut and natural colour reproduction. The specification of the manufacturing process eliminates the existence of blank cards. However, this technology still suffers from the limitation to its centralised personalisation process.
PWPW started researching polycarbonate inkjet printing in 2005. Taking their interest in colour personalisation one step further, the company concentrated on inkjet printing of security elements which previously were available only for analogue printing. In other words, the challenge was to print variable data using materials that had never been adapted for personalisation techniques. The combination of commercially unavailable materials and the complexity of inkjet technology provides a demanding barrier for counterfeiting.

Digital invisible near-infrared ink
The near-infrared (NIR) absorbing elements are key security features present on modern banknotes and documents. Standard verification systems for bank­notes are set up to cut off the visible range of light and to read out only near-infrared elements. Nowadays, the near-infrared absorbing inks are available for intaglio, offset and each type of analogue printing techniques in several colours excluding white.
PWPW took the challenge to develop an infrared absorbing ink, invisible to the human eye, for printing variable data onto polycarbonate products with a resolu­tion of up to 1440 dpi. The idea of invisible infrared personalisation was related to providing an individual security element for each blank document at the manufacturing stage. That solution eliminates the existence of blank-equivalent documents which are delivered to personalisation centres for a final laser engraving. The registration of each manu­factured blank document is highly important for export products.

Spectral requirements of invisible near-infrared personalisation
The readability of near-infrared personalisation depends significantly on the settings of the optical filters and the illumination source, as well as on properties of the absorption of infrared compound. In order to match the commercial verification systems, the ink has been designed to absorb strongly between 700‑1,000 nm, ensuring good contrast and fast recognition by the standard CCD and CMOS optical sensors, filters and illumination sources. On the other hand, the absorption profile in the visible spectrum of light (400-700 nm) is low in order to ensure lack of detection by the human eye. To eliminate the interaction of the laser engraving beam with printed matter, the near-infrared ink is not absorbing at 1,064 nm, which is a characteristic wavelength for neodymium based lasers.

Manufacturing process of documents with NIR inkjet personalisation
The manufacturing process of polycarbonate documents is based on the assembly and lamination of a few thin sheets of polycarbonate material under high pressure and temperature. The inkjet printing is performed onto one of the internal layers of the card structure. After the lamination process the personal data are enclosed inside the monolithic card body, protected by external layers.
Such a card structure ensures that every attempt of manipulating printed data will leave visible marks on the surface. The specificity of the process means that from the outset the card contains hidden invisible near-infrared information which is unique for each blank card. At the end of the process the card can be personalised by conventional laser engraving.

Time markers
As inkjet printing is a digital technique, any type of variable data can be printed, including portraits (resulting in a ghost image), OCR fonts and 1-D and 2-D barcodes.
PWPW has introduced the concept of near-infrared time-markers which is offered to decentralised personalisation centres. The time marker is an OCR number which is assigned to each blank card during the manufacturing stage process. During quality control the time marker is read by the NIR optical system and stored in the database. With this solution the per­sonalisation centre can easily verify the document’s manufacturing and shipment date.
Finally, during the laser engraving step the time marker is assigned to the card holder’s data by a machine readable device and stored in a database at the personalisation centre. The authentication process is implemented by comparing the visible data to the data from the confidential database, resulting in a binary answer: match or no match.
The near-infrared time marker is an easy track-and-trace solution which eliminates the problem of blank cards. Additionally, the invisibility of this feature causes no impact on the graphic design of the document and can be applied to any project.

Advanced chemistry behind the novel inkjet ink
The inkjet printing process requires ink optimisation for each type of print head in a different manner. Require­ments include: chemical and physical properties of the components, a pigment size of approx. 0.1 µm with uniform size distribution and long‑term colloidal stability. The chemistry needs to be optimised for the polycarbonate substrate in order to provide proper ink wetting, dot gain, drying, polymer swelling and ink adhesion. Finally, the high temperatures which are involved in the processing of polycarbonate sheets require thermal stability for each component of the inkjet ink. The special binding agents are used to avoid ink cracking under gradients of pressure and temperature during the lamination process, selected to provide a permanent bonding between single poly­carbonate sheets and to provide resistance against delamination. Due to the fact that polycarbonate documents are issued for 10 years of usage, all printed elements are tested under accelerated ageing con­ditions to guarantee the same properties over the entire lifetime of the docu­ments. Because of the above-mentioned conditions the development of new inks is a time-consuming process which requires sophisticated equipment, but the policy of self-manufacturing the inks increases the security level of personalised elements.

Conclusion
The invisible to human eye and near-infrared active personalisation based on the inkjet printing onto a single sheet of polycarbonate before the lamination process allows for printing any variable data with a resolution of up to 1440 dpi. Consequently, it eliminates the existence of blank cards and gives an individual character to each document at the manufacturing stage process. Furthermore, it provides the possibility for machine readability of printed elements by standard devices used as verification tools for banknotes and identity documents. It might be perceived as the first track-and-trace technology for polycarbonate docu­ments which does not impact their graphic design. The chemistry of the ink provides long thermal stability over 10 years of usage without any risk of fading. The self-manufacturing of the novel near-infrared ink using the complex chemistry ensures that the card manu­facturer will be one step ahead of would‑be forgers.

Reference
1 Muth, O. (2012). Secure colour personalisation: New inkjet process makes pre‑personalisation of polycarbonate docu­ments possible. Keesing Journal of Documents & Identity, Vol. 39, pp. 23‑27.