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Using ACCU DYNE TEST™ Marker Pens to Measure Surface Energy

This report describes how ACCU DYNE TEST™ Marker Pens can be used to measure the surface energy of films and other non-absorptive substrates. This method parallels ASTM Std. D25781 and ISO 8296, which cover the testing of polyethylene (PE) and polypropylene (PP) films via the application of solutions of 2-ethoxyethanol and formamide (reagent grade de-ionized water replaces the 2-ethoxyethanol for formulations above 57 dynes/cm). The fluids used in ACCU DYNE TEST™ Marker Pens are formulated as specified in ASTM D2578 (and ISO 82962; the critical difference between this test and the standard technique is the manner in which the fluids are applied to the test sample).

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.

ACCU DYNE TEST™ Marker Pens perform well on most non-absorptive materials. 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. D6183 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.

Stabilization of substrate and test markers to ambient conditions is critical; therefore, ambient, sample, and test marker temperatures, as well as relative humidity, must all be controlled: For industrial testing, we recommend testing within a maximum range of 15°C (59°F) and 30°C (86°F), and 35% to 70% relative humidity. Ideally, the range would be 20°C to 25°C and 40% to 60% RH.

Inspection methodology is especially important. Have one trainer instruct all testers to minimize variability. Static charges, which can be significant on corona treated films, can affect dyne testing results. As needed, a de-ionizer should be used to neutralize any static charge in the test area. Finally, the elapsed time between extrusion or coating to test (or from test to printing, etc.) must be controlled: the surface energy of treated films can drop dramatically over a very short period of time just after treatment.

IMPORTANT SAFETY NOTICE: The fluids contained in ACCU DYNE TEST™ Marker Pens are considered hazardous materials. Avoid contact with skin. Use with adequate ventilation. Avoid contact with eyes. Pregnant women should not perform this test. For further information, refer to product SDS or call Diversified Enterprises at 800-833-4644 or (603)543-0038.

For the results of this test to be meaningful, the following five 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 or outdated ACCU DYNE TEST™ Marker Pens.

3) Never retest the same location on a sample; move along the sample, or pull a new one.

4) Store and use ACCU DYNE TEST™ Marker Pens at room temperature.

5) Always store ACCU DYNE TEST™ Marker Pens upright, with the tips facing upwards.

TEST PROCEDURE

1.0 Materials/Equipment

1.1 ACCU DYNE TEST™ Marker Pens

1.2 Subject Material

1.3 Clean Level Test Area

1.4 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. Keep track of the orientation of the sample as related to web direction, etc.

2.2 Place the sample on a clean, level surface. 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 Test at least three points across the sample; 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 an ACCU DYNE TEST™ Marker Pen of a dyne level you believe is several dynes/cm lower than that of the test sample.

2.5.2 Press applicator tip firmly down on subject material until the tip is saturated with ink.

2.5.3 Use a light touch to draw the pen across the test sample in two or three parallel passes. Disregard the first pass(es); to flush any contamination from the tip, and to ensure that the test fluid layer is thin enough for accurate measurement, evaluate only the last pass.

2.5.4 If the last ink swath remains wetted out on the test sample for three seconds or more, repeat steps 2.5.2 and 2.5.3 with the next higher dyne level marker. If the last ink swath beads up, tears apart, or shrinks into a thin line within one second or less, repeat steps 2.5.2 and 2.5.3 with the next lower dyne level marker. If the ink swath holds for one to three seconds before losing its integrity, the dyne level of the marker closely matches that of the sample.

This is a relatively accurate surface energy measurement technique; used in standard 2 dyne/cm increments, ACCU DYNE TEST™ Marker Pens can generally produce results with a precision of +/- 2.0 dynes/cm. Repeated use of ACCU DYNE TEST™ Marker Pens should enable testers to estimate surface energy to within +/- 1.0 dyne/cm.

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. Periodic variations along the web may relate to non-concentricity of the treater roll. 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.

To investigate discrepancies between obtained and expected results, a more precise measurement method should be considered; application of ACCU DYNE TEST™ surface tension test fluids with cotton swabs (per ASTM D2578) or by use of a drawdown rod is recommended (test method here). Alternatively, if results are suspect, replicate the test with a set of unused markers. This test has proven itself on a wide variety of substrates; it is, however, theoretically more prone to contamination than some other techniques. For this reason, even in the absence of unexpected results, you should establish a quality assurance plan which calls for regular audits during the phase-in stage of ACCU DYNE TEST™ Marker Pen use. The effect of all changeovers from one substrate to another should be monitored especially closely. Slip and other additives tend to bloom to the surface of extruded sheets and films; transferring surface-active additives from one material to another can have a profound effect on surface energy measurement. In general, once you demonstrate that a switch from substrate A to substrate B has no effect, it is safe to assume that future changeovers from A to B will act similarly. Thus, the frequency of backup audits decreases over time.

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.

2International Organization for Standardization, Plastics - Film and Sheeting - Determination of Wetting Tension.

3Annual Book of ASTM Standards, Conditioning Plastics and Insulating Materials for Testing.

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 by held responsible for any losses or damages incurred by any party using this report.