Printing on film creates unique challenges. Films commonly used in package printing are polyolefins (polyethylene and polypropylene), polyamide and polyester. All of these materials have similar characteristics that affect the printing process and the physical characteristics of the final printed article.
- Non-porous: Their surfaces do not absorb solvents or any component of the printed ink system
- Extremely smooth surfaces: Ink components do not have nooks and crannies to fill in, reducing the mechanical adhesion
- Very low surface energy: Typically, non-treated polyolefins have surface energies less than 30 dynes/cm, making it very difficult for inks to wet out the surface of films
- Very flexible: When being printed, compared to paper and board substrates
All of these unique properties make formulating inks on film very challenging and impact the printing process in various ways.
The Challenges of Film Wetting
Films that are commonly printed have two characteristics that make wetting them a unique challenge. First, they are non-absorbent. Ink and its components will not soak into film substrates and have a tendency to sit up on the surface. Second, the films often have very low surface energies. Solvent-borne inks generally have surface energies in the 36-38 dynes/cm range. If the film has surface energy below this level, the inks will not lay smoothly and evenly on the surface. The appearance of the printed ink will be pinholed, or fish-eyed (see Figure 1). The pinholes and fish-eyes are the result of the ink pulling in on itself. In order to have defect-free print quality, the film and ink need to have similar surface energies.
Most film printers are employing some type of inline surface treatment. The most common is corona treatment, but other options include flame and plasma. The purpose of the treatment process is to raise the surface energy of the film by generating oxygen-containing moieties on the surface. This treatment also cleans the surface of the substrate of waxes, processing aids and other chemicals that impact the print quality of the ink.
Formulators also employ special techniques to help lower an ink’s surface energy, which assists in printing, and can use appropriate solvent and solvent blends to lower an ink’s surface tension. For example, water-borne formulations have very high surface energy, as water has a surface tension of 72 dynes/cm. In order to overcome high surface energy, the addition of some low-boiling alcohol (often ethanol or isopropyl alcohol) can be utilized when printing on film. Formulators may also use resins with low surface tensions that will wet the surface of films. Resins with low glass transition temperatures, often referred to as “soft” resins, will wet film surfaces better than those that have high glass transition temperatures, often referred to as “hard” resins.
Finally, ink formulations can be improved for film printing by adding special additives that lower the surface tension of the inks. These additives can be used in both solvent- and water-borne inks, and are often referred to as “wetting agents.” The materials act as surfactants to modify the surface energy of the ink. Again, the target of these formulation adjustments is to reduce the surface energy of the ink to match that of the film.
Drying Ink on Film
As mentioned earlier, films will not absorb any of the solvents from the printing ink. Therefore, all of the drying of inks printed on film must occur from the surface, which can be a limiting factor. When printing on paper or board stocks, a significant amount of the solvent is absorbed by the stock itself. Films do not give this luxury. Therefore, when formulating inks for film printing, the drying process must be considered. The solvents chosen must evaporate rapidly enough to allow the inks to fully dry at press speeds.
Drying capacity is often a limiting factor in printing on film. However, recent improvements in drying capacity allow for very high speeds when printing on film. In water-borne inks, the addition of small amounts of a low-boiling alcohol often increases the evaporation of water, due to the fact that alcohol and water produce a low-boiling azeotrope. The mixture of water and alcohol evaporates faster than water alone. Again, all of the solvent must evaporate from the surface of the ink. Therefore, the inks must be formulated so they do not form a “skin” on the surface of the ink. “Skinning” will trap additional solvents in the bulk of the printed ink film below the surface. The inks need to dry fully to make sure the maximum amount of solvent evaporates. Trapped solvent is detrimental to adhesion, physical properties and odor, and could affect subsequent converting processes like lamination.
Mechanical Adhesion & Chemical Adhesion
There are two components to adhesion—mechanical and chemical. Mechanical adhesion is produced when ink fills voids and crevices on the surface of the printed substrate. The ink flows into the voids and creates adhesion to the substrate. When printing on very rough surfaces, like paper and board stocks, mechanical adhesion is generated readily. There are substantial crevices that can be filled by the ink components to create adhesion. However, the very smooth surface of films does not allow for large amounts of mechanical adhesion. The level of mechanical adhesion is greatly reduced when moving from porous to non-porous stocks.
Chemical adhesion can be a reaction between the chemical moieties on the surface of the substrate and those in the ink. However, most inks utilized on films are not reactive. Therefore, the chemical adhesion referred to here is based on the weak interaction of polar groups of the surface of the substrate with polar groups in the ink. This is called van der Waals forces. These forces can be compared to small, weak magnets attracting one another between the substrate and the ink.
Surface wetting of the inks (as discussed herein) will also play an important role in adhesion in addition to appearance. If the inks do not fully wet out the substrate, there can be gaps between the dry ink film and the substrate (see Figure 2). These gaps decrease the amount of mechanical adhesion; therefore, wetting of the substrate plays a role. Inks must be able to fully wet the surface of the film in order to achieve maximum adhesion. Again, it is more difficult to wet films than paper and board stocks due to the low surface energies, so attention must be paid to the surface energies of both the printed ink and the stock. Surface treatment and low surface energy ink components are necessary to maximize the limited mechanical adhesion afforded by films. The ink must flow and fill the small surface crevices of the film in order to bond to it. Additionally, inks for film are formulated with “soft” resins that allow them to flow and fill the surface imperfections of the film. The goal is to get the maximum surface contact between the ink and the film.
All three types of surface treatment for film—corona, plasma and flame—create oxygen-containing groups on their surfaces. As mentioned earlier, the treatment process in air oxidizes the surface of the film. This converts the inert surface into polar groups. The presence of polar groups allows for the van der Waals forces to occur to give chemical adhesion. To take advantage of the treated surface, inks must also contain polar groups. The surface of the film gets polarized by treatment in air. Most inks for printing on films contain amines, amides, acids or ester groups. These groups deliver the polarity needed to give some chemical adhesion.
It is important to note that apparent adhesion is the sum of both the mechanical and chemical adhesion components. It is possible to achieve excellent adhesion, if inks take advantage of both the mechanical and chemical adhesion that is available.
When films are creased for gussets and seals or converted into the finished package, the ink must not crack and flake off of the film, but rather inks must be flexible and withstand physical abuse. Given this, inks must be formulated with flexibility to match that of the film being printed.
As mentioned above, film inks are often formulated with low glass transition temperature resins to assist in both wetting and flow to achieve good print quality and mechanical adhesion. Low glass transition temperature resins by their nature are soft and flexible at room temperature and are commonly employed when printing on films. Higher glass transition resins may have unique properties that make them necessary for use in printing inks, such as heat resistance and scuff resistance, among others. These more resistant resins can be used when printing on films, but they need to be formulated in combination with materials that can allow them to be flexible. Polyamides, polyurethanes and low glass temperature acrylics are all quite common in printing on films. Nitrocellulose-based inks are also common, but the nitrocellulose resins are plasticized in some way to make them flexible enough to adhere and flex with the substrates.
Film Properties & Film Challenges
A wide variety of films or plastics are printed every day by the flexographic process. These films include polyolefins, like polyethylene and polypropylene, polyamide and polyester. These substrates are characterized by their low surface energy, smooth surfaces and flexibility. These characteristics give them certain advantages over paper and board. For instance, films have very high gloss and are able to be made into non-rigid structures.
And these characteristics also pose unique challenges to printers. The low surface energy must be overcome through treatment, and ink formulators must match the low surface energies to get good quality print and adhesion. The smooth surface reduces the amount of mechanical adhesion that can be obtained, requiring inks to be formulated to achieve higher levels of chemical adhesion. Inks must also be formulated to meet the flexibility of the substrates to ensure they do not flake off during flexing of the finished product.
As long as both the printer and the ink formulator keep these properties and challenges in mind, excellent print quality and physical properties can be achieved on all types of flexible films.
About the Author: Scot Pedersen is the regional director of development at Siegwerk. He received his bachelor’s degree in chemistry in 1991 from Central University of Iowa in Pella, IA. He completed his Doctorate of Philosophy in 1996 at the University of Iowa in fluorinated organic polymer chemistry.
In 1998, he joined Color Converting Industries (now Siegwerk) in the R&D lab. He has worked on the development of solvent-borne, water-borne and radiation-curable inks and coatings. He was responsible for the development of all polyurethane lamination inks. He now manages the process development, technical development and Center for Printing Excellence labs at Siegwerk in the U.S. He works closely with the technical groups of raw material suppliers in developing new raw materials for inks.