The plastic packaging materials used for food products and daily commodities are printed with brightly colored product names and images. These packages are made of polyethylene or polypropylene films (flexible packaging films), and over 90% of them are printed using gravure printing.

Gravure printing is a form of indented plate printing. The image is engraved onto a metal roller. Ink is then applied to the recessed areas, and when the roller is pressed against the surface of the film, the ink is transferred to the film to create the characters and patterns. The color density can be controlled by adjusting the indentation depth, thereby making detailed printing possible. However, specially prepared rollers are required for each printed image; therefore, gravure printing is only profitable when used for high-volume printing of the same image and is thus unsuitable for low-volume printing. Hence, the companies that use gravure printing should either reduce their small-volume orders or print excess volume and retain it as inventory, which leads to increased waste.

Oil-based inks that dry quickly and produce high-quality images are used for printing on flexible packaging films. These oil-based inks contain pigments, resins, organic solvents, and other additives that emit large volumes of VOCs into the atmosphere, necessitating good ventilation in the workplace. VOCs is a collective term used for organic compounds that evaporate easily, such as toluene and xylene, and more than 200 VOCs have been identified thus far. These VOCs are transformed into air pollutants called photochemical oxidants by the action of UV rays. High concentrations of photochemical oxidants cause photochemical smog, which negatively impacts the human body, causing eye irritation and headaches as well as physiological damage to agricultural crops. Legal regulation of VOCs began in 2004 with the promulgation of the Revised Air Pollution Control Act. This law requires printers that use oil-based inks to implement measures against VOCs.

Kao Corporation proposed inkjet printing using water-based ink to print only the required volume of the packaging film. Inkjet printing is widely used in household inkjet printers. In this printing method, a minute volume of ink is sprayed directly onto the paper, and the image is formed by controlling the volume and density of the ink. The images were printed based on digital signals sent from a computer. This precludes the creation of special printing plates, thus making it suitable for high-mix, low-volume printing.

Dye ink and pigment ink were used in inkjet printing; however, Kao Corporation selected the water-based pigment ink. This was because they already possessed the related know-how, and water-based ink emits lower VOCs.

For existing water-based pigment inks, the ink is absorbed into the paper to a certain extent and dries, resulting in stable colors and images. However, the ink cannot be absorbed by film materials, and the drying time is long, resulting in the running and mixing of the ink. Hence, a good ink setting and clear images cannot be achieved via this method.

To obtain clear images on the film surface, the ink must not mix or run. Therefore, the ink should dry quickly on the film surface. To overcome this issue, the composition of the ink was adjusted to hasten drying, and the ink concentration was increased. Consequently, the ink droplets were smaller, and the ink dried faster.

Even though a high concentration of the ink dried quickly and the image was clear, the ink droplets were small, the color density was insufficient, and the image quality was poor with the appearance of streaks. As mentioned previously, tiny ink droplets are sprayed through small nozzles in inkjet printing. If the ink dots sprayed onto the film are not evenly spread during printing, the image quality decreases.

Hence, the research team attempted to evenly spread the ink droplets while maintaining a small droplet size. In water-based pigment ink, the insoluble pigment is dispersed in water. Compared to the oil-based ink used in gravure printing, the pigment surface of water-based ink has a low affinity for the solvent, making it extremely difficult to disperse it. Pigments are particulates (secondary particles) composed of nanosized primary particles. Separating the particulates into primary particles improves the image quality.

However, as the particles became smaller, the surface area increased, resulting in stronger intermolecular forces that induced particle agglomeration. This leads to unstable pigment dispersion and decreased image quality.

To disperse the pigment particles, the technology for laundry detergent, one of the main products manufactured by the Kao Corporation, was used. The surfactants in detergents adhere to oil and create an emulsion that disperses the oil in water. Surfactants contain both a hydrophilic portion with an affinity for water and a hydrophobic portion with an affinity for oil. The hydrophobic portion of the surfactant binds to the oil, forming a small sphere around it. The hydrophilic portion of the surfactant faces outward and disperses in the water.

This technology was used to disperse nanosized pigment particles in water. Similar to the surfactant, the polymer dispersant binds to the surface of the pigment (secondary particles). Upon application of energy, the pigment breaks down into nanosized primary particles. The hydrophilic end of the polymer dispersant then faces outward and disperses in the water.

Furthermore, the pigment particles should not agglomerate in water for uniform dispersion. Hence, the structure of polymer dispersants is altered to create mutual repulsion. This could be achieved by introducing a functional group with an electrical charge, as molecules with the same charge repel each other. In addition, steric repulsion between functional groups was used to ensure dispersion. The substituted polymer dispersant adsorbs to the surface of the pigment and encapsulates its particles. The outward-facing repulsion-inducing groups ensured that the pigment capsules were stably dispersed in water.

This technology, the newly developed polymer dispersant and dispersion process involving the encapsulation of the pigment in the polymer dispersant, was named “Nano-pigment dispersion technology.”

The application of this nano pigment dispersion technology for printing on plastic film is shown in the figure bellow. The ink dots spread much further than those of the existing ink. In addition, without the nano-pigment dispersion technology, pigment particles were collected at the edge of the dot, resulting in ring-shaped coloring. Hence, even coloring was achieved using th

Another factor that influences ink-dot mixing is the difference in the surface tension of the ink. During the printing process, each color is printed individually, leading to a short time lag until the next color is printed. This staggered ink spraying leads to differences in the surface tension of the ink. When ink dots with different surface tensions are located next to each other, the dot with the lower surface tension attempts to cover the dot with a higher surface tension, resulting in the mixing of the ink dots.

However, this novel ink manufacturing process involves the dispersion of pigment in water, followed by an adjustment in the final ink composition. The addition of surfactants increases the permeability and drying speed of the ink and controls the surface tension of adjacent dots. Therefore, the ink composition, including the type and amount of polymer dispersant, influences and controls the physical properties of the ink, thus preventing the mixing of adjacent dots.

The nanosized dispersion of the pigment particles and the precise control of the physical properties of the ink enabled the development of a water-based pigment ink for inkjet printing that fulfilled the objectives of GSC.

LUNAJET^®, a water-based pigment ink for inkjet printing that enabled printing on flexible packaging films, was launched on the market in 2016. This water-based ink prints clear characters and images on flexible packaging films. Unlike gravure printing that requires specially prepared plates, which require time and money, inkjet printing with this water-based ink is suitable for high-mix, small-volume printing. This method precludes large inventories and the consequent waste.

The lower VOC emissions from printing is revolutionary progress. The flammable oil-based inks used for gravure printing must be handled with care as dangerous goods and require ventilation and explosion-proof equipment. Similar measures are not required for water-based inks, leading to an improved work environment. In addition, inkjet printing with water-based ink reduces VOC emissions by 75% compared with gravure printing with oil-based ink, thus precluding the use of VOC treatment.

Kao Corporation conducted a lifecycle assessment (LCA) to evaluate the environmental impact and burden throughout the product lifecycle, from the procurement of raw materials, design, and manufacturing to transportation, use, and disposal. Efforts were made to use environmentally friendly products and technology. The environmental assessment of low-volume inkjet printing with water-based ink revealed a significantly reduced environmental impact, CO₂ emissions, and VOC emissions compared to those from gravure printing with oil-based ink.

The newly developed water-based ink for inkjet printing has received significant attention. In addition to the GSC award, it received the nnovation Award at the 2017 National Commendation for Invention. It has been adopted for printing snack packages and is a highly regarded original Kao product.

Nevertheless, gravure printing remains the leading technique in flexible packaging film printing. The development team will continue working to improve this new technology and promote its widespread use.