When we step into a printing pressroom it’s common to observe a hum of productive activity. Presses are running, printing is taking place, substrate is moving, and pressmen are professionally doing their job. But problems can occasionally appear, seemingly out of nowhere, to surprise. One of these can involve surface energy.
What is the most significant aspect of any printing process? Since printing is the process of applying inks to a substrate to communicate with images and text, it follows that the inks must adhere with fidelity to the substrate. It is only then that the intended printed product communication occurs.
The major components of printing, aside from printing presses and pressmen printing skills, are inks and substrates. Considering substrates, some substrates are easy to print on and some are challenging.
What makes substrates easy to print on? The rules guiding this are pretty simple. Transfer of an ink onto a substrate depends on the surface energy of the material delivering the ink, the surface tension of the ink, and the surface energy of the substrate receiving the ink.
As stated in the previous TECHTALK, “SURFACE ENERGY & PRINTING/COATING SUCCESS”, two basic rules apply:
- Substrate wetting must be optimized to achieve good lay and adhesion.
- Optimized wetting occurs when the surface tension of an ink or coating is equal to or less than the surface energy of the substrate.
The substrate must have a surface energy higher than that of the ink/coating, with forces of attraction great enough to promote good transfer and spreading, which facilitates good adhesion. The greater the gap between the surface tension of ink/coating and the surface energy of the substrate, the better adhesion will be.
Commonly used paper and paper board substrates are typically not an issue because they exhibit a high surface energy, i.e. clay-coated paper 100 dynes. However, non-porous plastic substrates with low surface energy values (28-44 dynes) are a problem as they resist wetting and coating because their dyne values can be lower than those of inks and coatings.
Typically, conventional lithographic inks have dyne values of 30-32 dynes, and UV inks 34-35 dynes. Flexographic solvent inks have dyne values of 28-30 dynes, water inks 31-35 dynes, and UV inks 35-38 dynes.
Digital printing uses electronic files in print production on two types of equipment, direct imaging presses (DI), and digital color printers. The DI press is based on an offset printing method and may use solvent, water, or UV inks. Digital color printers use electrophotography, inkjet, or xerography to develop images with toners, dry inks, or dyes. Inkjet printers use inks while laser printers use dry polyester powder toners. Several properties of a substrate affect the printing result. These are electrical properties, moisture & surface properties, (surface energy and roughness.)
Untreated commonly used polymer films have non-porous surfaces that are chemically inert with low surface energy. This makes them unreceptive to wetting and bonding with printing inks, coatings, adhesives, and other substrates.
The surface energy of these may be raised by coating with primers, or by using surface treatments such as flame, corona, plasma, and chemical modifiers. Generally, basic plastic substrate manufacturers treat their products to raise dyne levels so that they will wet well and accept inks, coatings and adhesives.
Corona and plasma treatments effectively oxidize the surface of plastic substrates, foils and paper, increasing surface energy. Treating is best done at the time of extrusion in-line before converting. Film not treated at this time will not print or coat acceptably. Unfortunately, whatever level of treatment is achieved there will be a drop off over time as plasticizers and other processing lubricants bloom to the surface, lowering surface energy.
The result is that films can become unprintable and/or uncoatable in a short period of days or weeks. It’s necessary that films received by the printer be printed in a timely fashion to avoid this problem. Some films can be re-treated in-line with printing, but some plastics notably polyolefins are difficult to re-treat.
In order to guarantee consistent film quality, films should be treated twice, once when produced and again before converting (printing/coating). Treating in-line only cannot replace treatment done during film production.
It is strongly recommended that non-porous plastic substrates be tested before being used. Testing pre-press is inexpensive insurance against expensive printing/coating surprises on press.
The measurement of wetting tension, or surface energy, of substrates is done regularly and inexpensively using “dyne liquid” solutions. These are available as a series of proportional mixtures of ethyl Cellosolve® and formamide, each with a given wetting tension covering a 30-56 dynes/cm range.
Testing is easily done using dyne pens by simply drawing a pen across a substrate. If the solution wets the surface the solution has a dyne level lower than that of the substrate. If the solution beads up, not wetting, then the solution’s dyne level is greater than that of the substrate. Repeated testing using adjacent dyne numbered pens can zero in on the substrate’s dyne level.
Printers/pressmen need to have an educated awareness of the surface energy of the substrates that they are to print/coat on. if a pressman encounters an adhesion issue, surface energy must be suspect.
Remember:
- A substrate with a relatively high surface energy is easy to coat.
- A liquid with a relatively lower surface energy will wet easily.
- Particularly good wetting occurs when the liquid has a substantially lower surface energy than the substrate
Understanding of the relationship between the surface tension of inks and coatings, and the surface energy of substrates Is critical to printing and coating success.
Cork continues to advance its cost effective, environmentally responsible GREEN coating technology, utilizing sustainable renewable (bio) resource materials content.
