WO 2014/120149 A1
Hewlett-packard Development Company L.P.
Wallpaper and other wall covering papers are used both for decorative and display purposes. One function of the paper will be to conceal cracks and other imperfections in the underlying surface, so it is important that it should have a high degree of opacity. This is achieved by using a laminated structure in which an adhesive is tinted with a dark-coloured opaque pigment or dye and is sandwiched between two sheets of paper. In fact, several layers can be laminated in this way if necessary. The laminated structure is shown in the diagram.
At the core of the laminate is an opacity enhancing adhesive layer (110) that can be from 2-50 μm in thickness. On either side of this is a paper layer (104, 108). This part of the structure forms the basic substrate, which will have 150 to 400 μm thickness and a weight of 100-500 gsm. On the top or outer surface of the paper adjacent to paper 104 is a printable image receiving layer (106) while a glue layer (118) may be positioned on the reverse surface adjacent to paper 108.
The paper layers are composed of a mixture of wood pulp and synthetic fibres and may also contain fillers, which in themselves will impart a certain degree of opacity. The paper will be dimensionally stable with a machine direction to cross direction tensile stiffness that is less than 2.5 and with hydroexpansibility of less than 1%.
The light blocking adhesive layer will contain up to 3% of a light absorbing pigment or dye for which the lightness (L*) will be around 30, giving a dark colour. So light will consequently be blocked from passing through the paper, giving an opacity defined as i(x)/io ≤ 0.05 where i(x) is the light intensity at distance x from the surface of the substrate where the light intensity is io. Any black or dark coloured pigment or dye is suitable, for example direct black dyes such as Pergasol black BTB or carbon black pigments such as irgalite black 2BL. The adhesive itself, which can be cross-linkable for improved hardness, may be an aqueous polymer emulsion such as polybutadiene or styrene-butadiene or it may be a solution polymer in which case the solvent can be aqueous or organic.
Finally, the ink receiving layer is coated on the surface of the substrate and comprises pigments and a polymeric binder. Provision is also made for a latex ink film-forming agent, such as a citrate of a glycol polymer.
An exemplary paper showed superior physical properties (tear and tensile strength, hydroexpansion and taber stiffness) in comparison to HP PVC free wallpaper.