A team of scientists from ITMOUniversity in Saint Petersburg has developed a new printing method that is expected to reduce the cost and time needed to create the so-called rainbow holograms that are sometimes used for security purposes.
The team, led by Alexander Vinogradov, senior research associate at the International Laboratory of Solution Chemistry of Advanced Materials and Technologies (SCAMT) in ITMO University, has developed a colorless ink made of nanocrystalline titania, which can be loaded into an inkjet printer and then deposited on special microembossed paper, which results in unique patterned images.
This ink makes it possible to print custom holographic images on transparent film in minutes, as opposed to traditional methods that take days.
"The conventional way of preparing a hologram is incredibly time-consuming and consists of several stages. First, one needs to create a master hologram, which is usually laser recorded on a thin layer of photosensitive polymer. The polymer is then dried and wash-out to get rid of unexposed parts," says Aleksandr Yakovlev, first author of the study and researcher at the SCAMT laboratory. "The resulting stencil is then transferred to a metallic matrix, which eventually serves to emboss holographic microrelief on the surface of a transparent polymer film."
According to the team, the entire process of obtaining a holographic image can take up to several days and a set of rigid requirements must be met, including temperature control and vibration.
"Printing separate holographic images in a quick and effective manner is a challenge that, until now, has been unresolved," says Yakovlev.
The new nanocrystalline ink can be applied with a simple inkjet printer on a microembossed surface, which is then covered by varnish. The holographic image is seen only in the areas where the protective ink was deposited.
"The peculiarity of our ink lies in its high refractive index in all visible range of light," says Vinogradov. "The use of nanocrystalline ink forms a layer with high refractive index that helps preserve the rainbow holographic effect after the varnish or a polymer layer is applied on top."
Rainbow holograms are commonly used to protect against forgery of credit cards, money, and documents.
The new technique opens the possibility of obtaining holograms in almost any size, as well as simplifies and lessens the cost of the process.
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