Hewlett Packard’s microwave-curable media coating

WO 2013/062510 A1
Inkjet recording medium, and method of using the same
Hewlett-Packard Development Company, L.P.

Over the years, much effort has been put into improving the image stability and gloss of photo prints.  Here, ink passes through a porous top coating which is subsequently made to be non-porous, so providing both image protection in the form of wet and dry rub resistance along with good levels of gloss and image distinctness.

The substrate can be any typical medium such as paper, olefin coated paper or polymer film.  It is first coated with an ink receiving coating that will absorb fluid from the ink.  This is a typical porous coating and primarily consists of a porous pigment and a binder.

Protection is given to the image by the particulate-based reactive coating applied above the porous coating.  This is in itself porous to the ink and so the ink will pass through it and be absorbed in the ink receiving coating.  After printing, barrier properties are created by exposure to microwave radiation which will cause cross-linking of some of the particles and coalescence of others.  This is shown in the figure in which ink (26) is shown absorbed in the ink absorbing layer (14), having almost entirely passed through the protective coating (16).

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The coating is applied as a latex in which both reactive and non reactive particles are dispersed.  Suitable reactive polymers include those with epoxy functionality, and those with fatty acid, alkoxy-silane, acetoacetoxy, hydroxyl, amine or carboxyl functional groups.  Self cross-linking polymers can be used or a cross-linking agent can be included.  The uncured particles (0.2 to 10 µm) should have good mechanical stability (Youngs Modulus 600 to 3000 MPa) at a temperature below 110C.

A polar microwave radiation cure promoter may also be included.  This acts to strengthen the dipole relaxation effect in which particles turn or rotate with some lag as the dipole direction changes at high frequency.  Heat is also generated which is helpful to both the curing and coalescence processes.  Suitable agents are calcium acetate monohydrate, calcium propionate and calcium propionate hydrate.  Exposure to microwaves also induces coalescence of the non reactive polymer particles which are generally hydrophobic polymers such as PTFE or hydrocarbon waxes.  It appears that much or all of the coalescence and cure processes will take place at temperatures exceeding 110C.

These media appear to be suitable for most types of aqueous, solvent or latex ink and can be applied using more or less any mode of ink jet.  Microwave exposure can be carried out either in-line or off-line at a frequency of 0.3 to 300 GHz.  Exposure time should be from around 10 seconds to 4 minutes, although the longer times would seem impractical.

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