Adoption of water-based inks for flexible packaging has been growing in recent years, driven by changing industry requirements and supported by improvements in the performance of water-based alternatives.
Let’s look at the background of packaging and ink applications, to include the historical challenge in utilizing water-based inks on non-porous substrates, as well as evolving technology that is making further conversion to water-based more possible.
Flexible packaging, such as bags or pouches, is used in many types of applications, including dry, wet, frozen and microwaveable foods. According to industry consultants Smithers, the market for flexible packaging inks in North America is growing approximately 3 percent per year. Growth in the flexible packaging market is attributed to easier transportation with lightweight materials, smaller package sizes for snacking and consumer friendly packaging.
Substrates & Sustainability
A few examples of substrates used in flexible packaging applications include low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyethylene terephthalate (PET), polypropylene (PP), oriented polypropylene (OPP) and biaxially oriented polypropylene (BOPP). These substrates have surfaces and surface energy properties that are very different from traditional paper and paper board substrates. Solvent-based inks have often been used on flexible packaging applications and traditional paper and paper board applications generally use water-based inks.
The growing use of water-based inks for flexible packaging has been supported by growing external pressure on the printing industry from regulatory and societal factors. Governments and global organizations are taking increasingly strict measures in response to the growing demand for more sustainable products and the need to balance environmental concerns with economic and societal interests.
Conversion of solvent- to water-based printing technologies represents an important step forward in terms of sustainability, innovation and business differentiation. Furthermore, as manpower is at a premium, there is also a focus on the working conditions of press operators. Some printers and converters are therefore choosing to adopt water-based inks as a more VOC- and odor-friendly approach. There is also an ongoing focus to reduce the handling of volatile or hazardous components that are more present in solvent-based inks to improve worker quality of life, job satisfaction and employee retention.
Dispersion & Letdown
Water-based inks are a formulation of pigment dispersion and letdown or extender. The pigment dispersion is generally composed of resins to provide efficient and effective dispersion of pigment to allow for uniform ink color. The letdown is typically composed of emulsions which bring the ink to the surface of the substrate and provide physical properties to the ink, such as water resistance, leveling and adhesion.
Emulsions also help maintain a uniform viscosity at different shear conditions, which is advantageous in print processes. Additives such as defoamers, wetting agents and waxes are added to enhance the performance properties of the finished ink.
Water-based acrylic resins have a long and interesting history in providing this performance. Initially developed for floor care applications, they were later discovered to be excellent materials for water-based printing. Water-based inks were initially applied to paper and paper board substrates. Regulations on emissions from traditional solvent-based inks left packaging manufacturers with the need to implement additional emissions controls or transition to lower emissions inks. Water-based inks reduced VOC content, combined with the natural tendency to absorb into paper substrates, provided effective solutions for packaging manufacturers trying to reduce VOC emissions and looking to avoid expensive capital projects for additional VOC controls.
These inks also improve conditions for people working in and around ink manufacturing environments, who are less exposed to VOCs. Water-based inks can be used on paper substrates in applications ranging from economical carbon black cardboard packaging to high-quality preprint display applications seen in stores.
Adhesion & Wettability
Inks for flexible packaging can be used on the surface of the package or reverse printed to an inner layer of the package in a lamination application. Inks for surface and lamination prints each have their own criteria to consider. For example, surface print applications are concerned with the resistance properties of the inks and lamination applications are concerned more with how the ink will affect bond strength of the package layers.
Inks for flexible packaging applications are typically dominated by solvent-based inks. Difficulties with ink’s adhesion, wetting and drying on low surface energy, non-absorbent flexible substrates have contributed to a historical preference for solvent-based inks over water-based in the market. Solvents help polymers attach to lower-energy substrates, lower the surface tension of the ink to provide better wettability and evaporate very quickly.
Advancements in water-based inks have led to improved adhesion and wetting, which allow the ink to be used more frequently in flexible packaging applications. Porous paper substrates allow the ink to penetrate the substrate and adhere very well. Substrates used for flexible packaging are generally non-porous, which present challenges for adhesion. Flexible packaging substrates generally have low surface energy, which makes it more difficult for high surface tension, traditional water-based inks to spread consistently on the surface of the substrate.
An option for improving the wettability of an ink on a substrate is to increase that substrate’s surface energy via corona treatment. Water-based inks use letdown polymers with lower glass transition temperature (Tg) to provide effective wetting and adhesion. Lower Tg acrylic polymers have lower surface tension and tend to wet substrates more effectively than traditional higher Tg acrylic polymers.
Wetting agents can also be added to ink formulation to improve an ink’s ability to wet a substrate. Water-based inks also have the additional challenge of slower drying speeds in comparison to solvent-based inks. Solvents have a lower boiling point and lower heat capacity than water—less energy is needed to evaporate the solvent.
Further developments in components and formulations for water-based inks have enabled their broader use in surface print applications, which require resistance properties such as water, chemical, heat and rub resistance. Water and chemical resistance in these applications have been traditionally improved by using external crosslinkers.
Covalent links between the polymer chains improve the adhesion and cohesion—ink sticking to itself—to prevent the ink from being absorbed or removed by water or chemicals. There are several potential drawbacks to external crosslinkers, depending on their chemistry. These include limited pot life, EHS and regulatory issues, and the added complexity of press-side additions.
Another option to improve properties is the use of metal-based crosslinking agents. Adding metal cross-linking agents to the formulation can improve the heat resistance of the ink in surface print applications. These metals form hydrogen bonds with acid functional groups of the polymers and improve the resistance of the ink. These ionic bonds are readily reversable, so they typically do not improve chemical resistance.
Self-crosslinking polymers can also be used as the main binder in water-based surface print inks to improve resistance. Recent developments in self-crosslinking polymers also offer an excellent balance of resistance properties and resolubility. These self-crosslinking polymers are stable and do not have pot life limitations, EHS, regulatory or complexity concerns, compared to polymers that require press-side additions of crosslinking agents. Rub resistance is improved with the addition of a wax to the formulation, providing more flexibility to the coating to prevent rub damage.
Extrusion & Lamination
Water-based inks have also been further adopted in lamination ink applications, which do not require the resistance properties of surface inks because they are between two layers of non-porous flexible substrate protecting them from water, chemicals and abrasion. These inks do, however, require good cohesion and adhesion when used in extrusion or adhesive lamination.
Water-based polyurethane dispersions (PUDs) have excellent substrate adhesion and cohesion, approaching the performance of solvent-based polyurethane inks. Early lamination inks, based on conventional PUD technology developed for coatings applications, often provide good bond strength performance but exhibit poor printability and resolubility, and limited compatibility with various pigment concentrates when used as ink binders.
Recent developments provide a balance of both bond strength and printability, even in demanding applications like extrusion lamination or retort packaging. In addition, acrylic-polyurethane hybrids offer the improved adhesion and cohesion of polyurethanes combined with the improved printability and resolubility of acrylic polymers. They also show compatibility with a broad range of pigment concentrates, additives and co-binders used in conventional water-based printing inks.
These developments in resin technology, combined with improvements in the drying capabilities of printing equipment, have positioned water-based inks as a viable alternative to solvent borne inks on non-porous substrates used in flexible packaging. Though they have been more widely adopted in paper and porous substrate printing applications in the past, these formulations are now seeing impressive growth into flexible printing applications.
Through this further adoption of water-based inks, printers can meet regulatory, societal and sustainability goals without compromising performance. This mutual benefit is sure to support further expansion of water-based inks as the industry looks to harness the growing flexible packaging market.
About the Author
When he’s not creating chemistry with customers, Josh is focused on furthering innovative solutions for flexible packaging and heat-seal lacquer applications. His technical expertise began with his bachelor’s degree in chemical engineering from the University of Michigan and has grown over the last six years as he has gained diverse experience in chemical production and end-use applications.