The name of HARMAN technology Ltd may be new, but we have a long history in the field of photography. We have recently introduced a range of HARMAN PHOTO ink jet printing media.
- Receiving Layers
- Swelling Polymer
During recent years there has been considerable growth in the use of ink jet printers for producing photographs, either as output from digital cameras or from scans of conventional photographic materials. Ink jet printers work by jetting very small droplets of ink on to a suitable receiving material to form the image. Such images are normally disappointing when printed on plain paper because they lack density, vibrancy, and sharpness, and therefore special coated papers are recommended for the purpose. Such materials comprise one or more ink-receptive recording layers coated on a supporting substrate such as resin-coated (RC) paper or baryta-coated paper that give the look, weight, and feel of a traditional photograph.
The electron microscope image above shows a view of a typical system: the thin ink-receiving coating and thicker substrate are visible. In general, two types of coating are in use in such image receiving systems: so-called swelling polymer types and so-called porous types.
HARMAN's coating technology allows us to coat two or more layers simultaneously, and we can also control the layer formulations for optimum overall properties. The microscopic cross-section on the left shows a two-layer swelling-polymer type ink jet assembly coated on resin-coated (RC) paper base: the fibrous paper substrate is at the bottom, and the thick dark layer is a polyethylene resin coating that is filled with titanium dioxide white pigment. There are two ink receiving layers: to maximise the brightness and detail of the image it is important that the colorant from the ink be retained at or near the surface after printing, and therefore there is a thin upper layer to trap the dyes and a thicker lower layer to absorb the water-based fluid components of the ink.
The receiving layer of a swelling-polymer system comprises a continuous coating of one or more water-accepting hydrophilic polymers, together with other addenda. It is easy to prepare highly glossy ink jet coatings of this type, but they are rather slow to absorb the ink because absorption depends on the hydrophilic polymer swelling when it is printed.
Typically, an ink jet ink comprises a water-based carrier liquid and at least one colorant (a dye or a pigment), together with other components. A dye is a colorant that is soluble in the carrier liquid whereas a pigment is insoluble and supplied as very small particles.
The microscopic views here show cross-sections through a two-layer swelling-polymer type ink receiving assembly. The individual layers are not very clear on the view of the unprinted material: the thicker lower layer absorbs the ink solvent and the thin surface layer traps the dyes. The second view shows a cross-section after printing with a pigmented ink: the very fine particles of pigment are clearly held on the surface of the material. The final view shows the same material after being printed with dyed inks. In this case the colorant penetrates into the assembly and is retained near the surface by the dye trapping layer for maximum colour and image brightness.
The ink receiving layer of a porous type recording system comprises very fine particles of one or more colourless fillers or pigments such as silica or alumina, a polymeric binder to hold the particles together, and other addenda. The ink is taken into the interstitial volume between the particles. For many applications this type is preferred because prints on such materials are quickly dry to the touch after printing as the liquid phase of the ink absorbs very rapidly into the open pores on the surface of the material.
The Scanning Electron Microscope view of the surface of a porous ink jet layer clearly shows the pigment particles (in this case silica) and the open pores between them. The particles are approximately 1 - 10 microns (µm, micrometres, or millionths of a metre) in size. Particles of this size scatter light and therefore the material has a bright white matt surface. HARMAN PHOTO matt inkjet materials have an attractive smooth surface finish.
The second SEM image shows the surface of a porous inkjet material which has very much smaller particles (alumina in this case) in the size range of approximately 10 - 20 nm (nanometres). These particles are smaller than the wavelength of light and thus do not scatter light like the larger particles do; the coating offers a smooth, sharp, and robust glossy finish.
For full details of the HARMAN PHOTO range of inkjet media, and availability, visit our dedicated website www.harman-inkjet.com .
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