PUR has potential

The latest developments of film ethanol dissolvable polyurethane (PUR) resins have opened up new applications, especially in flexo printing. Their use in the manufacture of laminates is one example. In recent years, intensive research and development has focused on improving dispersion techniques. One limiting factor, low pigment concentration and colour strength, has been overcome, and stable pigment dispersions with high pigment concentrations are now available. Today PUR colour ink systems exhibiting excellent printing results in flexo printing are available.

PUR based colour inks can be universally applied on coex. OPP, pre-treated PET and OPA; and barrier films coated with SiOx and AlOx can be directly printed with these inks. PUR and PVB based white inks are available as backing whites. The print designs can be processed with both solvent-free and solvent-borne lamination adhesives. Boiling resistant and even retortable laminates such as PET/Alu/cPP can be manufactured when using suitable lamination adhesives.

Siegwerk’s latest PUR based inks open up new possibilities and markets in flexo printing, previously reserved to gravure printing.

Like all other inks and lacquers, printing inks need additional components beside the visible colouring agent (pigments) to ensure adhesion on the surface of the substrate. This is the purpose of the binder, which also coats the pigments and enables transfer of the ink from the printing unit to the substrate.

Binders generally help form a stable ink film exhibiting a resistant and more or less durable surface. On flexible packaging, the dried ink film often serves as contact layer: multilayer packages can be manufactured when combining different materials, and the ink is then sandwiched between two materials in order to form a laminate.

Polyurethanes are constantly increasing their share in ink formulation. The growth of flexo printing of high quality flexible packaging is highly correlated to the almost unlimited possibilities of the PUR synthesis. Properties of polyurethane resins can be varied in multiple ways by selecting specific intermediates. They can be tailor-made exactly to specific requirements.

In addition, polyurethanes used as cobinders in two-component systems contribute significantly to the outstanding resistance of the cured inks and lacquers.

Dr Gerd-Rüdiger Liewald is director development/technology flexible packaging at Siegwerk