SUBSTRATE SURFACE ENERGY TESTING
This report details a method of determining the surface energy of plastic films and other non-absorptive substrates. This method complies with ASTM Std. D25781. It uses solutions of ethyl Cellosolve* and formamide to determine the wetting characteristics of the subject material. In general, the ability of a substrate to anchor inks, coatings, or adhesives is directly related to its surface energy. If the substrate surface energy does not significantly exceed the surface tension of the fluid which is to cover it, wetting will be impeded and a poor bond will result. Thus, for most solvent based printing, plastics need to be treated to 36 to 40 dynes/cm; water based inks usually require 40 to 44 dynes/cm; some laminating and coating applications require surface energies of 50 dynes/cm or more. Clearly, surface energy must be assessed before printing, coating, or laminating is attempted.
This paper is specifically directed at extruded film and extrusion coated paperboard. However, the technique should be transferable to many other materials as well. It is critical that the test fluid does not alter the surface properties of the substrate. For example, if the test fluid permeates a fibre substrate (such as paper) and causes swelling, results will indicate unrealistically easy wetting. A chemical reaction between the test fluid and the substrate invalidates results altogether.
To ensure replicability of this test, material preparation and test technique must be standardized. ASTM Std. D6182 documents suggested conditioning methods. Unfortunately, this standard is untenable for treated film testing; conditioning times range from 24 to 96 hours. Such rigorous controls may be of value for R & D, but for normal QC testing, much shorter conditioning times should be used. Standardization of ambient, substrate, and test solution temperatures is critical, as is inspection methodology. Have one trainer instruct all testers to minimize variability. Relative humidity should not be excessive; higher RH tends to increase data variability. Finally, the elapsed time between extrusion or coating to test (or from test to printing, etc.) must be controlled.
IMPORTANT SAFETY NOTICE:
The constituents of these solutions are classified as hazardous materials. Avoid contact with skin. Perform tests with adequate ventilation. Pregnant women should not perform this test. Avoid contact with eyes; safety goggles are recommended. Otherwise, toxic effects should be negligible at the low exposure levels required for this test. For further information, refer to MSDS or call Diversified Enterprises at 800-833-4644 or (603) 543-0038.
For the results of this test to be meaningful, the following four points are absolutely essential and must be followed:
1) Do not touch or in any way contaminate the surface to be tested. Dirty surfaces lose their wettability.
2) Do not use contaminated test fluid; dispose of it immediately and use a fresh batch.
3) Never retest the same location on a sample; move along the sample, or pull a new one.
4) Store and use all test fluids at room temperature.
1.0 Materials/Equipment
1.1 Surface Tension Test Fluids
1.2 Cotton Applicator Swabs
1.3 Subject Material
1.4 Clean Level Test Area
1.5 Thermometer and Hygrometer
2.0 Method
2.1 Pull test sample. Be sure to pull a good specimen; surface aberrations cause poor results. For extruded film, one entire web cross-section should suffice. Do not touch the surface.
2.2 Place the sample on a clean, level surface. Maintain orientation of the sample, so traceability of left vs. right is possible. If necessary, anchor the edges to avoid curling or other deformation.
2.3 Record ambient temperature and relative humidity. If sample temperature differs from ambient, allow it to stabilize.
2.4 A minimum of three sample points should be tested; 1/4, 1/2, and 3/4 across the film section. It is good practice to test the outer edges as well. For non-film materials, test locations must be determined in-house.
2.5 Determination of Wetting
2.5.1 Choose a dyne level test fluid which you believe is slightly lower than that of the subject material.
2.5.2 Wet the very tip of a new swab with just enough of this test fluid to coat one square inch of the test sample.
2.5.3 Use as little application pressure as possible to spread the solution evenly over one square inch of the test sample. Be sure all testers use the same technique in this step.
2.5.4 Note how long it takes from the time the solution is applied until it beads up. This assessment should be based on the reaction of the fluid in the center of the liquid film; the reaction on the perimeter is not a reliable indicator of wetting. If beading does not occur within two seconds, go to the next higher level test fluid. If beading occurs in less than two seconds, drop down by two to four dynes/cm instead.
2.5.5 Use a new swab. Never re-use a swab, even in the same dyne level fluid. Use the test fluid level indicated by step 2.5.4.
2.5.6 Repeat steps 2.5.2 through 2.5.5 until you determine the level which comes closest to wetting the surface for exactly two seconds. This is the material's surface energy in dynes/cm. Remember to keep moving along the film sample, testing untouched material. Record your result.
2.5.7 Repeat steps 2.5.1 through 2.5.6 for all test locations.
3.0 Reporting Results
3.1 Record individual test results by location on the sample. Calculate the average and range of wetting levels.
3.2 If the results in step 3.1 do not fall within control limits, or if they differ significantly from historic values, it is suggested that two more samples be pulled, and the procedure repeated. Often outlying results are due to surface contamination or poor sample quality.
Typically, a good inspector can replicate results on this test to within 0.5 dynes/cm; multi-operator replicability is usually within about 1.0 dyne/cm. If precision is critical and must be quantified, a designed experiment can be used to estimate actual variability.
Watch for differential treat levels over the cross-section of your extrudate. Repitition of the same trend over more than two consecutive tests is unlikely to be random. Rather, it can indicate mechanical problems in your treater, coater, etc., or systematic variations in the film you are running. This feedback can be of great value in arresting process problems before they become excessive.
Remember that aging affects surface characteristics, notably on corona treated films. If the constraints of your process preclude good standardization of test timing, designed experimentation should be used to measure the effect of aging on your substrates. Results from such a study can be used to plan product flow and inventory management, as well as being of use during discrepant material disposition. Substrate suppliers should set specs conservatively to counter treatment loss.
Finally, a few qualifying words. Surface energy is critically important to many converting operations. Unfortunately, it is not the sole determinant of product suitability. Other factors, such as surface topography, coating rheology, and chemical incompatibility, must also be considered. This is why broad-based communications with vendors and customers is so important. But at least by systematically measuring substrate surface energy, you will have a sound starting point from which to resolve other problems which may arise.
1Annual Book of ASTM Standards, Wetting Tension of Polyethylene and Polypropylene Films.
2Annual Book of ASTM Standards, Conditioning Plastics and Insulating Materials for Testing.
*ethyl Cellosolve is a registered trademark of Union Carbide Corporation for ethylene glycol monoethyl ether (2-ethoxyethanol).
This report is provided by Diversified Enterprises, 101 Mulberry St., Suite 2N, Claremont, NH 03743. It is intended to provide quality control information for converters and others who need to measure surface energy. We believe all information contained herein is accurate. Diversified Enterprises will not, however, under any circumstances be held responsible for any losses or damages incurred by any party using this report.