A surface that is easy to wet is necessary when printing and coating in order to achieve good adhesion and appearance results. “Wetting” is the ability of a liquid to spread, contacting a solid surface. All liquids have a surface tension and all solids have a surface energy. Both are measured as a force applied over an area, expressed as a dyne level.
When printing and/or coating it is important to consider surface energy and surface tension. Both have a direct effect on printing and/or coating on any substrate, especially non-porous, low dyne plastics. Without good wetting and leveling, surface defects may appear as crawling, cratering, pinholes, or orange peel. Adhesion will also be an issue.
In order to obtain good wetting, lay and adhesion, the surface tension of an ink or coating must be equal to or less than the surface energy of the substrate. A contact angle of zero degrees indicates perfect wetting. One way to think of it is that optimal wetting is necessary to provide maximum surface contact. Printing success depends on the attraction and acceptance of inks, and coatings by the substrate through good wetting and bonding, promoting adhesion.
Nature always moves toward the lowest energy state in an effort to lower the total energy of a system. Nature will always move to cover a high energy surface with a lower energy surface.
Common examples may be seen all around us. For example, gasoline has a surface tension of 20 dynes/cm², while water has a surface tension of 73 dynes/cm². Just one drop of gasoline placed on a large surface of water will spread until the gasoline is only one molecule thick. Here nature has covered as much of the higher energy surface as possible with a lower energy surface.
An example of the relationship between surface tension and surface energy may be seen when water droplets form on a waxed auto paint finish. In this case, the 73 dynes/cm² (surface tension of water) causes spherical water beads to form on the lower 23 dynes/cm² waxed surface. The forces present cause each water droplet to shape itself into a sphere, indicating non-wetting with each sphere having the smallest surface area possible for the volume of the single drop. A perfect sphere would only make point contact and not cover any of the 23 dynes/cm² surface. However, gravity causes the water droplet to flatten slightly, countering surface tension which is acting to hold the sphere perfectly round.
Considering an example where the auto paint finish is oxidized and unwaxed, the water droplet would spread out working to wet the surface. “Wetting out” happens because the surface energy of the oxidized surface would be higher than the surface tension of water. Once again, nature's effort to move toward a lower energy state by covering a higher energy surface with a lower energy surface is seen.
A substrate with relatively high surface energy is easy to coat. A liquid with relatively lower surface energy wets easily. Particularly good wetting is obtained when the liquid has a substantially lower surface energy than the substrate. Substrates with low surface energy are difficult to coat. The dyne level of a material is called its surface energy. Substrates such as paper and paper board generally are not a concern due to porosity, roughness and more than adequate bond sites.
However, paper coatings, laminations and treatments can alter these good conditions. Non-porous plastic substrates with their low dyne values present a challenge, as they are difficult to wet and coat.
Substrate / Dynes
- Clay Coated Paper 100
- Aluminum Foil 45
- Polyethylene Terephthalate 42
- Polyester 41-44
- Polyvinyl Chloride 35
- Polystyrene 33
- Polyethylene 30-31
- Polypropylene 29-31
- Polyvinyl Fluoride 28
The surface energy of substrates can be changed (raised) by the use of primers, or surface treatments to increase roughness. Treatments include flame, corona, plasma, and chemical modifiers that will oxidize a substrate's surface. Plastic substrates, as noted above, typically have low surface energy levels. The greater the distance between the surface tension of an ink or coating and the surface energy of a substrate, the better adhesion will be. For example, if a plastic is treated to 45 dynes or higher, then an ink of 32 dynes will produce better wetting and adhesion than if a plastic is treated to only 38 dynes.
Ideally, the substrate should be about 10 dynes higher than that of the inks or coating being applied. Surfactants can be used within limits to reduce the surface tension of inks and coatings.
The surface energy of non-porous solids can be tested by a number of means. Testing is done to pre-check that a substrate has a dyne level that will accept the inks and/or coating to be used. Dyne solutions with varying surface
tensions are used to determine wettability. When a solution wets a surface, the solution has a dyne level lower than the substrates; if the solution beads up, its’ dyne level is greater than that of the substrate indicating non-wetting.
Water contact angle measurement is an expensive test method that is used mainly in laboratory settings. In this test method the angle of contact that a drop of dyne solution makes with a substrate surface is measured.
Understanding of the relationship between the surface tension of inks and coatings and the surface energy of substrates Is critical to printing and coatings success. Cork has coating experts ready to help on your next project. Find out how we can optimize your operation with the best coatings on the market.