December 31, 2021
Printing ink systems for flexible packaging have undergone major developments over the past decades – now is the time for a review. What is the status quo and what can we expect in the future? Do the ink systems currently on the market meet the technical and regulatory requirements of a packaging market undergoing a dynamic technological development?
Established binder systems
For the most part, today’s established systems use the solvent-based binders nitrocellulose (NC), polyvinyl butyral (PVB) and polyvinyl chloride (PVC), each with its own specific set of properties.
The main area of application for NC-based inks is on mono films and simple laminates, printed in gravure or flexo. For surface printing they exhibit excellent printability combined with high heatseal and water resistance. On the downside, however, adhesion to difficult types of film is limited and they offer a lower level of lamination bond strength in combination with solvent-based adhesives. Nor are they suitable for pasteurisation or sterilisation. In order to at least improve lamination bond strength and suitability for pasteurisation, binder systems based on a combination of nitrocellulose and polyurethane have been used. These, however, have limitations in surface printing.
PVB and PVC-based inks can be used for high-quality laminates. These two ink types differ with respect to their properties and limitations. PVB-based inks exhibit very good adhesion to a wide variety of films. They are highly suitable for printing on polyolefin and metallised films. However, because the maximum pigment concentration in PVB systems is limited, their use in fine-screen flexo is restricted. PVC-based inks are particularly suitable for printing on PET and PA films and they are highly resistant in sterilisation processes. By contrast, they are quite unsuitable for use on polyolefin films and for flexographic printing in general.
The market for laminated packaging materials
The laminates market can be divided into three categories: high, medium and low performance. Packaging for liquids that need to be sterilised or pasteurised and often contain quite aggressive components is particularly demanding. This applies to pharmaceutical packaging, for example, or to barrier laminates such as PET-SiOx/OPA/CPP or PET/Alu/CPP. The medium performance category is for dry, moist or liquid materials that do not require pasteurisation or sterilisation and covers PET/Alu/PE or OPA/Alu/PE laminates. Dry filling goods such as snacks or pasta create comparatively low demands on packaging performance. OPP/OPP, PET/PE, Paper/PE or OPP/PE laminates are typical examples of this category.
There is no single binder system fulfilling all requirements. At the high-performance end of the laminates scale, adequate bond strength can only be achieved through the use of PVC and, with certain limitations, PVB systems.
One key disadvantage of conventional PVB and PVC systems is the fact that they exhibit limited compatibility with the flexo process or even cannot be used for flexo at all. On the other hand, NC/Polyurethane systems offer stable print quality on gravure and flexo presses, but they can only be used in low or medium performance applications.