Wetting Tension and Contact Angle
All of the following terms are used in the polymer films industry to represent the relative receptivity of a film surface to the addition of inks, coatings, adhesives and extruded polymers.
- Surface energy is a solid surface characteristic associated with the molecular forces of its interaction with another material. Surface energy is the true film characteristic that we want to measure, but it can't be measured directly. So, we deduce this property by measuring one of two substitute properties: wetting tension or contact angle. Both of these measurements involve observing the behavior of liquids placed on the film's surface.
- Surface tension is the force that exists between a liquid and the atmosphere it is in. For example, in atmospheric air, a drop of water will bead up on some solid surfaces. It is the surface tension existing between the water and the air that allows this to occur. The drop of water can spread, or wet-out, on another solid surface if the new surface has molecular forces (surface energy) high enough to overcome the water/air surface tension and draw the water flat onto it.
- Wetting tension is the maximum liquid surface tension that will spread, rather than bead up, on the film surface. It is a measurable property that estimates a film's surface energy. ASTM D 2578 is a procedure for determining wetting tension by applying different test solutions of increasing surface tensions until one is found that just spreads (wets) the film surface. Units are dynes/cm.
- Treatment level refers to "how much" or "how well" the polymer film surface was treated in the film-making process. It is most commonly quantified with a wetting tension value in units of dynes/cm. Therefore, you will also hear it called "dyne level."
NOTE: Treatment level, dyne level, surface energy, surface tension, and wetting tension are all terms frequently used interchangeably. As you can see from the definitions, this is not precisely correct. Wetting tension, which is the term ASTM uses, is the correct term to describe the value determined by the test solutions procedure.
- Contact angle is a measurement of the behavior of pure water in contact with the film surface. (Other liquids of known surface tension could be used, but water is most commonly used.) The ASTM D 5946 procedure requires a precise volume droplet of pure water be placed on the film surface. The surface energy of the film controls whether the droplet tends to stand up or flatten out. This is quantified by measuring the contact angle of the droplet with the surface. A higher energy ("higher treatment") film will cause the droplet to be flatter and closer to the surface, which results in a smaller contact angle value (8). Units are degrees, or the result can be reported as (unitless) cosine 8. ExxonMobil uses a dynamic contact angle (DCA) tester to evaluate the same property. Our test is substantially different than the ASTM standard and may not produce comparable results.
Relevance to converting performance
The surface energy of a film is critical to achieving good wet-out and adhesion of inks, coatings, and adhesives. The film's surface energy must be higher than the surface tension of the solution being laid down in order to get good wetting. The molecular forces that allow good wetting also contribute to adhesion. Water-based systems have higher surface tensions than solvent-based systems and, therefore, require a higher minimum level of film surface energy to perform well.
|Polytetrafluoroethylene (PTFE, Teflon®)||19|
|Table 13: Wetting tensions (as an estimate of surface energy) of common untreated polymer surfaces|
In extrusion lamination or extrusion coating processes, a molten extrudate (not a liquid solution) is being applied to the film surface. Wet-out is not an issue, but an adequate surface energy is required for good bond strengths.
Some polymer films, most prominently, polyethylene and polypropylene, have inherently low surface energies, and these surfaces need to be modified to be "converter-processable." The easiest and most common modification is called "treatment" ExxonMobil uses two in-line treatment processes to increase the energy, polarity, and processability of film surfaces.
- Corona treatment involves exposing a moving film surface to a plasma discharge created in the air gap between a grounded electrode and a high-voltage electrode. The plasma is purple glowing ionized air that reacts chemically with the film surface. Oxidation reactions occur, which add polar functional groups and increase surface energy.
- Flame treatment exposes a moving film surface to a gas-fired flame at a high enough temperature to create a plasma of free oxygen and nitrogen atoms, electrons, and ions. The plasma reacts chemically with the film surface, which adds polar functional groups and increases surface energy.
Depending on the product design, a treated OPP film will have a wetting tension between 35 and 55 dynes/cm. This whole range of values works well in most flexpack converting applications. Challenging situations, like some water-based applications, may require films in the higher end of the range. For example, the treated high-energy surfaces of LBW and CSR-2 with wetting tensions of 50 to 55 dynes/cm provide exceptional wet-out and adhesion in challenging applications.
What affects the "treatment level" of OPP films
For many reasons, treatment level can be a controversial and complicated topic. Treatment level really refers to the surface energy of a treated film, which can only be estimated by tests that quantify the behavior of specific liquids on the surface. The following is a list of the primary factors affecting treatment level.
- Measurement: The method of using dyne solutions to measure wetting tension is notoriously prone to a high variation of results. A few years ago, the Flexible Packaging Association (FPA) performed a round-robin evaluation and found that wetting tension measurements of identical materials, made in different laboratories, varied as much as 11 to 15 dynes/cm. Many factors contribute to this high variation. Bottom-line, the wetting tension test is an unreliable predictor of performance. Therefore, ExxonMobil has invested in contact angle testers, which provide more reliable data.
- Time: The effect of treatment can decay with time. Very old films may not convert well, but ExxonMobil films are designed to have a good performance shelf-life of one year.
- Treatment process: Many factors like voltage, residence time, and temperature affect the degree of surface energy modification.
- Polymer reactivity: Polymers vary in their reactivity to corona or flame treatment. With a more reactive polymer, it is easier to achieve the chemical changes that increase surface energy. Polypropylene is more challenging to treat than many other materials.
- Additives: Most resins need one or more additives to produce films with maximized performance in a particular application. Slip and anti-static additives, in particular, can either cause artificially high or low wetting tension values, or interfere with wet-out and adhesion in the converting process.
CAUTION: Treatment pens or dyne pens are available from several manufacturers. They are magic marker-type pens that contain liquid rated at a certain surface tension. The pen tip is swiped onto a film surface, and if the liquid does not bead up, then surface is said to be treated to a dyne/cm-Ievel equal to or greater than that pen rating. If it does bead up, then the treatment level is said to be less than the pen rating. THESE RESULTS DO NOT CORRELATE WELL WITH STANDARD TEST PROCEDURE VALUES. Pen solutions can be contaminated with each film swipe. It is appropriate to use treatment pens to distinguish a treated surface from an untreated surface, but not to assign a specific wetting tension value.
There are two tests used to estimate the surface energy of a film: wetting tension and contact angle. Each has its advantages and disadvantages, which are described in Table 14. For over five years, ExxonMobil has been expanding its use of the contact angle test as a process control measurement. Because of the improved data quality, ExxonMobil will continue to do so.
ExxonMobil procedure #417 uses a Cahn Model DCA-312 or DCA-315 to measure dynamic contact angle (DCA). Unlike the ASTM contact angle test that applies a single water droplet, our Cahn test immerses a prepared film sample into a beaker of distilled water at a constant rate of speed, while a sensitive balance records the wetting force. The Cahn DCA software contains well established theoretical calculations, which convert this measurement into advancing and receding contact angle values. ExxonMobil has standardized on recording the cosine of the receding contact angle value.
|Test Attributes||Wetting Tension
|Test results variation||Poor||Excellent|
|Cost of test equipment||Low||High|
|Speed and simplicity||Very good||Moderate|
|Table 14: Comparison of basic test attributes|
A more common, but less reliable, test is wetting tension, as described in ASTM D 2578. This method uses standard wetting tension solutions consisting of varying ratios of formamide in cellosolve to create different surface tensions. A test solution is selected that is rated less than the anticipated film value. Using a new cotton-tipped applicator, the solution is applied in a zigzag pattern over about one square inch of film. If the applied solution holds together for more than two seconds, rather than breaking up into droplets, then the test is repeated with the next higher surface tension solution. If the solution holds together for less than two seconds, the test is repeated with a lower surface tension solution. This scenario is reiterated until you can determine the solution that comes nearest to wetting the film surface for exactly two seconds. This is the film's wetting tension. A new cotton applicator must be used each time to prevent contamination of the solutions. A new section of film must be tested with each iteration.
Dyne test: Dyne test is a commonly used name for the wetting tension test, because it uses solutions that are rated at different dynes/cm.
Wet-out: Wet-out is the spreading of a liquid on a film, rather that it beading up. It is important that inks, coatings, and adhesives wet-out, or spread, on a film's surface for proper appearance and adhesion.