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`In Short
`• Synthetic dyes are transferred from one plastic onto another using heat
`• Photographic images can now be printed on three dimensional plastic objects
`The growth of plastics in our lives has been mirrored by a desire to decorate this material,
`whether for simple aesthetics, to incorporate brand logos, or for security reasons. However,
`possibly the main reason for wanting to print on plastics lies in the emergence of digital
`photography. So how's it done? Morven McAlpine
`For the past two decades, the dye diffusion thermal transfer (D2T2) method has been used to print
`photographic quality images onto plastic substrates. Images are produced using heat which
`promotes the diffusion of dyes from a dye donor ribbon directly into the plastic substrate. The dyes
`can penetrate into several plastics, including polyesters and polyvinylchloride (PVC).
`
`The dye donor ribbon (see Fig 1) consists of a thin layer of polyethylene terephthalate (PET) coated
`on one side with repeating panels of the three dye formulations - yellow, magenta and cyan - mixed
`in a solvent. The solvent is removed during the coating process, leaving a kinetically stable yet
`thermo-dynamically unstable coating of dye in a polymer binder, such as PVAA (poly(vinyl aceto
`acetal-co-vinyl alcohol-co-vinyl acetate, 1).
`
`Fig 1 Cross section of a D2T2 ribbon
`
`This polymer binder has good affinity for the dye molecules and a glass transition temperature (Tg)
`in the region of 80-120 ºC, which gives good colour delivery during printing and stability on storage.
`(The Tg of a polymer is the point at which the polymer chains have sufficient energy to move freely
`and become more liquid-like.)
`On the opposite side of the ribbon is another polymer coating, the
`back-coat. This is typically a highly cross-linked urethane-acrylate-
`based polymer, with added lubricants and fillers. The back-coat aids
`the transfer of the ribbon across the thermal print head during the
`printing process.
`There are many commercially available 'diffusion' dyes, but for use in
`D2T2 printing they should have a high melting point to reduce the risk
`of plasticisation of the dye-sheet polymer binder; high optical strength to give good colour intensity at
`low concentrations; and good affinity/solubility for the binder polymers. They should also be stable to
`light to reduce the possibility of fading. Examples of suitable yellow, magenta and cyan dyes are (2),
`(3) and (4) respectively.
`The 'receiver paper' or plastic substrate is made up of special polymer
`coatings which receive the dyes from the ribbon during thermal
`transfer and can separate from the dye ribbon after transfer is
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`complete. On the one hand the receiver coating must allow the dye
`molecules to move into the polymer matrix at certain temperatures (ie
`the temperature at the ribbon/receiver interface during printing) but,
`on the other hand, act as a barrier to further movement at lower
`temperatures.
`Dye molecules are large and therefore
`diffusion is enhanced if the free volume
`within the receiver polymer is large.
`However, a high free volume can often
`result in a low Tg, and the receiver
`polymer should have a high Tg to
`prevent the dye molecules from
`migrating once transfer is complete.
`
`To achieve a balance between these two opposing properties, the
`receiver layer is made up of a cross-linked polymer. However, too
`high a level of cross-linking will result in low optical densities when
`printing, and too low a level of cross-linking may result in dye
`migration after printing, ie the image will lose definition over time as
`the dyes diffuse laterally through the polymer matrix. The cross-
`linking of the receiver polymer also reduces the release force from the
`dye sheet when printing. This, in combination with a release agent, prevents the dye-sheet and the
`receiver layer from sticking during the high temperature printing process.
`Other layers within this 'paper' can include a whitening layer (normally consisting of a mixture of
`polyurethanes, polyacrylics and polyvinyl alcohols plus titanium dioxide) and a voided polypropylene
`layer to give some thermal insulation. A cross section of a typical receiver paper is shown in Fig 2.
`
`Fig 2 Cross section of D2T2 'receiver paper'
`
`Inside a D2T2 printer a microprocessor breaks down the digital image into the three component
`colours - yellow, magenta and cyan. The thermal print head comprises numerous pixels, which are
`in contact with the dye donor ribbon during printing. The thermally stable, highly lubricated coating
`(back-coat) on the dye ribbon can slide over the heated pixels without sticking or damaging the PET
`ribbon. As the back-coat is heated the dyes contained in the coatings on the opposite side of the
`ribbon leave the coating and diffuse into the receiver paper, which is in contact with the ribbon
`during the heating process. The printing process is shown in Fig 3.
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`Fig 3 D2T2 printing process
`
`The colours needed to build up the full-colour image are printed separately in the order of yellow,
`magenta then cyan. After each of the three coloured dyes has been transferred, the dye ribbon and
`receiver paper separate from each other, leaving the spent dye ribbon and the imaged receiver
`sheet. When all three colours have been printed a full-colour image is produced. The image can
`then be protected by thermally transferring another polymer on top of the printed image.
`Using the D2T2 process, 256 shades of each component colour can be achieved. Each component
`colour can be blended with the other two component colours, giving a huge colour gamut (16.7
`million colours) and thus continuous tone images are possible. Photographic quality images can be
`produced within 15-60 seconds.
`
`D2T2 printing is not limited to the production of digital photographs on plastics but can also be used
`to print directly onto PVC cards - ID cards, plastic driving licences, visitors' passes, loyalty cards etc.
`However, other methods have been developed specifically for different applications, such as security
`features. These include: laser engraving; wax thermal transfer; and indirect D2T2 transfer.
`Laser engraving
`This involves the controlled burning of a plastic substrate using a laser. A monochrome image of the
`charred and non-charred areas of plastic is produced. Since the plastic card has been physically
`altered and the image cannot be easily removed, this provides a secure feature within the card.
`Plastics such as polycarbonates are most suited to this method because sharp, dark images can be
`produced with the laser. The downside of this method is that polymers best suited for laser
`engraving are often not receptive to the dye diffusion process so a combination of a laser engraved
`image and a full-colour image is difficult to achieve.
`Wax thermal transfer
`This process is used to transfer a single colour (typically black) onto a substrate, for example to
`incorporate barcodes and text. The whole coloured layer is transferred from the donor ribbon to the
`substrate, and there is no controlled diffusion of dyes from one polymer matrix into another. Thus
`wax thermal transfer does not offer the continuous tone images of D2T2 nor the high security of a
`burned image within the card offered by laser engraving. As such it is considered to be a low-tech
`printing method. However, wax transfer ribbons are cheap and require low print energy, and are
`therefore used widely to print labels.
`Indirect transfer
`Indirect D2T2 transfer produces full-colour images on substrates, such as polycarbonate, which are
`not receptive to diffusible dyes. An image is printed onto a clear polymer coating on an intermediate
`ribbon using direct D2T2 printing. This coating with the image is then transferred from the
`intermediate ribbon onto the final substrate, eg a polycarbonate card.
`High-quality images can be produced on a range of different substrates, and surface irregularities on
`the substrate are not an issue. In contrast, direct D2T2 printing requires flat, regular surfaces
`because the thermal print head, dye donor ribbon and receiver substrate must all be in contact
`during printing. However, in the indirect method, the image is transferred into a polymer layer on
`another ribbon and then this entire polymer layer is transferred onto the receiver substrate. This
`allows full-colour images to be transferred onto irregular cards such as those containing chips. And
`because the dyes diffuse into the intermediate material there is no need for the final substrate to be
`receptive to the dyes. This can be important if a combination of colour images and laser engraved
`images are required on the final product because currently the best materials for laser engraving are
`not receptive to standard D2T2 dyes. Indirect transfer does have its disadvantages, however. The
`use of the intermediate material and transfer step makes this process more expensive and slower
`than direct D2T2, both of which restrict its use.
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`While printing a photographic image onto a card is one way of adding a security feature, there are
`other ways of doing this. For example, the image can be printed with an ultraviolet (uv) fluorescent
`dye that cannot be seen under visible light, but is visible under uv light.
`The use of fluorescent images as security features can be improved by using three different uv
`fluorescing dyes. If red, green and blue fluorescent dyes are used to print the inverse of an image in
`the same manner as the yellow, magenta and cyan dyes are used to print a colour image, then a
`full-colour fluorescent image can be produced. This can be used to produce easily identifiable ID
`images that are only visible under uv light. An example of how this can be used as a security feature
`is shown in Fig 4. The cards on the left are viewed under visible light whereas the cards on the right
`are viewed under a uv light. Under visible light both cards look genuine. However, when viewed
`under uv light the top card is seen to be a fake whereas the bottom card is genuine.
`
`Fig 4 Example of how full-colour fluorescent images can be used as a
`security feature
`
`Another method uses optically variable pigments (OVP). These pigments change colour when
`viewed at different angles, see Fig 5. They can be incorporated into a polymer layer and a mass
`transfer image can be printed. A mass transfer image is one produced when both the polymer and
`pigment transfer onto the substrate when heated (as opposed to dyes diffusing out of the polymer
`matrix and into the receiver material).
`
`Fig 5 Optically variable pigments as an overt security feature on an ID card
`
`Mass transfer images are of much lower quality than dye diffusion images (dot pattern as opposed
`to continuous tone), though they do allow the use of dyes and pigments that would not otherwise
`transfer from the ribbon to the substrate ie dyes/pigments that will not diffuse out of one polymer
`matrix into another.
`Recently card manufacturers have been moving away from using printed security features on cards
`to non-visual techniques, such as RFID (radio frequency identification) and chip and pin. The future
`of card security features is more likely to be what's in the card as opposed to what's on it.
`
`Digital photography and printing on cards are now both well-established industries producing
`millions of images a year. However, the ability to print on plastics doesn't end there. There are some
`novel printing techniques emerging from this industry, including the ability to print photographic
`images onto three dimensional objects.
`One method, developed by ICI Imagedata, uses a combination of heat and vacuum to transfer a pre-
`printed image. A recyclable polyester sheet is coated with specially designed polymer coatings that
`are receptive to ink-jet inks containing sublimable colourants. This sheet is pre-printed with an image
`by using aqueous ink-jet technology.
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`The pre-printed sheet and object to be decorated are then placed in an adapted vacuum forming
`press and a combination of heat and vacuum is applied to mould the pre-printed film around the
`object. Heat is then applied and the colourants within the sheet diffuse into the three dimensional
`object. The film can then be removed, leaving an object with a perfect image wrapped around every
`contour. Any digital image, such as a full-colour design or photographic image, can be transferred
`onto highly complex shapes with excellent image quality. The image can also be printed around 90°
`angles.
`Some materials, such as PVC and a range of polyesters, can be directly decorated by this method,
`whereas, others such as metal and glass require a receptive coating to be applied first to the object.
`The receptive coating is sprayed onto the three dimensional object and is thermally cured, after
`which it can accept the dyes from the pre-printed donor sheet during the transfer process. This
`method can be used to decorate plastics, metals, wood, ceramic, stone and composite materials
`and is being exploited in consumer electronics, sports goods, and interior and exterior trims.
`With novel imaging techniques emerging and constant improvements in the existing technology
`printing on plastics will hopefully remain an active area of research and innovation for many years to
`come.
`Dr Morven McAlpine is a senior development chemist at ICI Imagedata, Brantham, Manningtree,
`Essex CO11 1NL.
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