Forming hidden patterns in porous substrates
The present invention concerns a method for manufacturing patterned porous substrates with hidden color patterns by forming hydrophobic patterns on a hydrophilic surface, wherein structural channels are formed as a pattern in a porous substrate using a hydrophobic printing solution lacking colorant, and wherein a colored area is applied on the rear surface of the porous substrate. Further, the present invention concerns said patterned porous substrate and a method for bringing said pattern into a visible state.
1. A method for manufacturing patterned porous substrates with hidden color patterns by forming hydrophobic patterns on a hydrophilic surface, said method comprising the steps of;
manufacturing structural channels in the form of a pattern in a porous substrate by flexo or gravure printing using a colorant-free hydrophobic printing solution on a front surface of the porous substrate, and
applying a visible colored area on a rear surface of the porous substrate wherein the visible colored area is not visible through the porous substrate before applying a marking liquid, wherein the porous substrate is essentially opaque when in a dry state and wherein the porous substrate becomes at least partially transparent when wetted.
2. The method according to claim 1 , further comprising the step of selecting the porous substrate from nitrocellulose sheets, cellulose-based papers, porous polymer sheets, and fabrics.
3. The method according to claim 1 , further comprising the step of optimizing the penetration of the printing solution into the substrate sheet with the aid of the printing-cylinder pressure, the number of printings, cell size of the printing roll, the solvent of the printing solution, and/or the viscosity of the printing solution.
4. The method according to claim 1 , further comprising the step of using a printing solution containing a polymer or an organic compound of C≧20, but lacking repeating units.
5. The method according to claim 1 , further comprising the step of using a printing solution containing one or more hydrophobic organic solvents.
6. The method according to claim 1 , further comprising the step of using a printing solution comprising polystyrene or alkyne ketene dimer as a dispersion as well as a solvent comprising toluene, xylene, or a mixture of these, the share of polystyrene in the printing solution being 2.5-40 weight-%, or an aqueous solvent.
7. The method according to claim 1 , further comprising the step of applying the coloured area on the rear side of the substrate as an area of uniform colour or sliding shades.
8. The method according to claim 1 , further comprising the step of applying the coloured area using flexo, gravure, offset, electrophotography or inkjet printing and a conventional printing ink.
9. The method according to claim 1 , further comprising the step of applying the coloured area on the rear side of the porous substrate using a printing solution containing one or more colorants for providing a visible coloured area, optionally giving different areas of the rear side of the substrate different colours.
10. The method according to claim 1 , wherein the hidden color patterns are capable of becoming visible through the porous substrate when the marking liquid is applied and becoming invisible when the marking liquid evaporates.