Using the dyne test to evaluate the cleanliness of metals

USING THE DYNE TEST TO EVALUATE THE CLEANLINESS OF METALS

This report details a method of using surface tension test fluids, formulated in accord with ASTM Std. D2578⁽¹⁾, to evaluate the cleanliness of metal surfaces. It uses solutions of 2-ethoxyethanol and formamide to determine the wettability of metal surfaces (for formulations of 58 dynes/cm and above, reagent-grade water replaces the 2-ethoxyethanol). Because contaminants are of lower surface energy than the underlying metal surface, the wettability of the surface, as measured in dynes/cm, is a good proxy for the degree of cleanliness of the material. Additional background information on this application of the dyne test is shown here.

To ensure replicability of this test, sample selection and handling and test technique must be standardized. Ambient, sample, and test solution 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. Temperature control is most critical, as the relationships between temperature and surface energy of different metals vary considerably: For example, one study⁽²⁾ shows that the surface energy of chrome plated steel remains essentially unchanged (42 dynes/cm) at 40°C compared to 160°C. By contrast, aluminum foil drops from 45 dynes/cm to 38 dynes/cm over the same temperature change.

Inspection methodology is also critical, and the following points need to be considered:

Cleanliness testing of metals should always be done with bottled test solutions, applied with cotton applicator swabs. We recommend dropper bottles, which can apply a standardized amount of test fluid to each swab. The smallest amount of test solution that can practically be applied is ideal: surface energy is a two-dimensional property. Applying too thick a film of test fluid can cause gravitational spreading, which will result in higher dyne level readings.
Do not touch or in any way contaminate the surface to be tested.
Do not use contaminated test fluid; dispose of it immediately and use a fresh batch.
Dyne pens should not be used for cleanliness testing: Even ACCU DYNE TEST™ Marker Pens will end up with contaminated tips and produce errant results if used on oily or silicone-contaminated surfaces.
A fresh, unused swab must be used for each application of test fluid.
Each test must be performed on a previously untested part of the sample.
The test should always start from a low dyne level — one that you expect should fully wet the surface. As long as that dyne solution stays wetted out, and does not shrink in and form beads within two seconds, the test should be repeated at the next higher dyne level (if the test fluid beads immediately, drop down at least four dynes/cm, and start the test over).
Always keep bottles securely capped when not in use. Evaporation of the two test fluid constituents will not be equal; the 2-ethoxyethanol will evaporate preferentially, increasing the surface tension of the test fluid.
If the part that is tested must be retained for subsequent processing (as in testing expensive aerospace components, e.g.), the test area should be cleaned with isopropyl alcohol immediately, then sent back through the cleaning process for a second pass. We also recommend re-testing this location vis a vis a contiguous location on the same part to ensure that the dyne test did not permanently alter the surface.
For large-scale test programs, one master trainer should directly train all testers to ensure uniformity of technique.

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. For further information, refer to the product SDS or call Diversified Enterprises at 800-833-4644 or (603) 543-0038.

1.0 Materials/Equipment

1.1 ACCU DYNE TEST™ Surface Tension Test Fluids
1.2 Cotton Applicator Swabs
1.3 Subject Material
1.4 Clean Test Area

2.0 Method

2.1 Sample appropriately from the cleaning line or lot of incoming goods. Be sure to pull good specimens; surface aberrations cause poor results. Do not touch the surface.
2.2 Place the sample on a clean, level surface. Orient the sample to assure that the area to be tested is horizontal.
2.3 Record ambient temperature and relative humidity. If sample temperature differs from ambient, allow it to stabilize. If this is not possible, a correlation study of temperature vs. dyne level reading should be performed to allow accurate adjustment of raw data to standardized conditions.
2.4 Determination of Wetting
2.4.1 Choose a dyne level test fluid which you believe is slightly lower than that of the subject material.
2.4.2 Wet the very tip of a new swab with just enough of this test fluid to coat a small area on the test sample.
2.4.3 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 - especially at low dyne levels (which are predominantly 2-ethoxyethanol, a very effective solvent), excessive application pressure will scrub the surface, and mix any residual oils into the surface tension test fluid.
2.4.4 Note how long it takes from the time the solution is applied until it starts to bead 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 necessarily 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 at least four dynes/cm instead, and re-start the test.
2.4.5 Use a new swab. Never re-use a swab, even in the same dyne level fluid. Use the test fluid level indicated by step 2.4.4.
2.4.6 Repeat steps 2.4.2 through 2.4.5 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 surface, evaluating a fresh location for each test. Record your result.

3.0 Reporting Results

3.1 If desired, more than one location per sample can be tested. Record individual test results by location on the sample.
3.2 If the results as recorded 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.

We estimate that an experienced inspector can replicate results on this test to within better than +/-1.0 dyne/cm; multi-operator replicability in the same lab is usually within about +/-1.5 dynes/cm. Between labs, replicability should be within about +/-2.0 dynes/cm. If precision is critical and must be quantified, a designed experiment can be used to estimate actual variability. Please note: We have not performed inter-laboratory or cross-inspector comparisons to verify these estimates.

⁽¹⁾ Annual Book of ASTM Standards, Wetting Tension of Polyethylene and Polypropylene Films.⁽²⁾ "Effect of temperature on the surface energy of solids - sometimes it does matter (Kruss Application Note AN250e)," Chris Rulison, Augustine Scientific, December 2005.

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.