Increase in bonding strength of décor ink using primer

WO 2014/084787 A1
Method of producing a building panel using digital printing
Ceraloc Innovation AB

In recent times, there has been a move towards digital decoration of architectural parts and panels. Radiation cure inks lend themselves well to this sort of application as they can be cocured with other primer and protective layers. Conversely, aqueous inks give problems in terms of inter-layer adhesion and image quality. However, environmental considerations make the use of aqueous inks an attractive proposition.

The application addressed in this instance is that of Luxury Vinyl Tiles and Planks (LVT) in which a thermoplastic décor layer, often PVC-based, is laminated to a core which may be a thermoplastic or a wood based material. A pattern such as a wood or natural stone effect is printed on the carrier, conventionally by rotogravure or direct printing, and a further protective coating bonded to it by a heating process to finish the décor film. At the lamination stage a texture such as a wood grain can be embossed. Neither of these printing methods lends itself well to alterations in the design, nor is a certain amount of repetition of the pattern avoidable.

The problems of print quality and bonding strength are addressed by applying and drying a primer to the thermoplastic foil, prior to printing with aqueous pigment-based inks. The primer principally consists of a concentrated metal salt solution, generally with the addition of a surfactant as a wetting agent. The deposited salt crystals act to fix the ink in place by causing agglomeration of the pigment in the ink dispersion on contact with the surface so that the ink is unable to spread unduly. The salt solution must have a concentration that is at least 50, and preferably 90% of the saturation concentration; and to obtain uniform wetting of the surface, it should have a contact angle with the carrier that is ideally less than 35 degrees. Addition of a wetting agent will ensure that this is the case. The salt can alternatively be applied as a dry powder, but if so, the crystals must be distributed homogeneously over the surface and adhere to it. To achieve this, the carrier surface can be electrically charged such that it attracts the salt powder, or the surface can be plasticised or porous so that the salt particles adhere to, or are absorbed by it.

May-14aA structure for a LVT to which a décor film has been laminated is illustrated. The carrier (3) is shown laminated to the core, which in this case is formed with shaped ends to allow an interlocking structure to be constructed. The image or pattern (10) is printed on the core and coated with a protective coating to which, in this case, an additional coating (5) is applied. This may be a UV cured material.