Month: November 2018

Question: I am conducting an environmental review on ACCU DYNE TEST Marker Pens, and the SDS does not have the information I need for approval. I need to verify that “all the components of the product are either listed or exempt from listing under the TSCA section 8(b) chemical inventory list.” Your SDS lists the TSCA inventory but references only the TSCA section 5(a) (significant new use) for one ingredient. Can you please provide the requested information?

Answer: All constituents of ACCU DYNE TEST^TM Marker Pens and surface tension test fluids (both use the same formulations) are listed in the TSCA inventory. For full information on 2-ethoxyethanol (CAS 110-80-5), please see https://www.federalregister.gov/documents/2005/11/29/05-23421/2-ethoxyethanol-2-ethoxyethanol-acetate-2-methoxyethanol-and-2-methoxyethanol-acetate-significant.

Question: We purchase 4 ounce bottles of dyne solution at 72 dynes/cm. Can you tell me how you determine the expiration date? We don’t always use up the container before its shelf life is over.

Answer: Given that your purchase is of 72 dyne/cm surface tension test fluid, containing only reagent-grade water and Methyl Violet dye, this is an interesting question. The typical dyne solutions containing 2-ethoxyethanol and formamide (mixed per ASTM Std. D2578) definitely have a finite shelf life, as the constituents will react with one another over time, eventually changing their wettability regardless of whether they have been used or not. I do not believe it has ever been determined whether the dye acts as a catalyst, whether it is the cause, or whether the change happens with dye or without. At any rate, historically, suppliers of these test fluids have assigned a shelf life of approximately 3 to 4 months from date of manufacture. We are confident enough with our reagent grade materials to offer a shelf life of 5 months.

No data is available on the aging of water/Methyl Violet mixes. My best guess is that any degradation would be slower than with the binary mix dyne levels, but I am not willing to bet on that and change the expiration dates for this single constituent dyne level (the same can be said for 30 dyne/cm test fluid, which contains only 2-ethoxyethanol and Methyl violet dye, or 57 dyne/cm test fluid, which contains only formamide and Methyl violet dye).

A final concern pertaining to the stability of the 72 dyne/cm test fluid regards potential leaching of compounds from the packaging bottle (HDPE for narrow and wide mouth bottles; LDPE for dropper bottles) into the test fluid. A good deal of information on this phenomenon is available from the literature, but most of it pertains specifically to health effects, and most of the studies are not limited to additive-free polymer formulations. One review⁽¹⁾ presents a litany of mostly solvents and short-chain molecules that have been identified leaching from HDPE water pipes – if these do, in fact, leach from unmodified HDPE, they would generally reduce the surface tension of water over time. Another study⁽²⁾ demonstrates changes in LDPE over time when exposed to pure water. It seems reasonable to assume that eventually there will be some effect on the test fluid due to polymer leaching.

You could run a study comparing results from fresh test fluids vs. those that are aged, though this could be a challenge, as any number of variables may affect the aging process (average storage temperature and RH, degree of temperature and humidity cycling, exposure to light, etc.). Realistically, the best strategy would be to purchase one or two ounce bottles, which will offer full usage before there are any problems related to aging.

References:

1) no author cited, https://plasticpipesleach.org/wp-content/uploads/2018/01/White-Paper_A-Review-of-Chemical-Substances-Shown-to-Leach-from-Common-Drinking-Water-Piping-Materials.

2) S. Massey, A. Adnot, A. Rjeb, and D. Roy, “Action of water in the degradation of low-density polyethylene studied by X-ray photoelectron spectroscopy,” eXPRESS Polymer Letters, 1, No.8 (2007) 506–511.

Question: We test plastics at incoming inspection that will later be processed in a dry room at <1% RH, and your test procedure sets limits of 30% to 70% RH. What impact would you expect this to have on the validity of the dyne readings?

Answer: The only effect that I know of regarding humidity is that unusually high levels tend to increase the variability of test results. I suspect this is due to condensate on the sample surface. If that is, in fact, the mechanism, then extremely low humidity should not significantly affect readings for most materials. There is one notable exception to this – the nylon family of polymers (polyamides), which are quite hydrophilic in the presence of water vapor. If your testing includes these materials, I would strongly recommend that you do a controlled study, as discussed below. There is little question that a significant change in adsorbed water vapor will have an impact on surface energy test results.

You should keep in mind that evaporation rate comes into play with the dyne test, as discussed here, and extremely low humidity levels may affect the rate of test fluid evaporation. The question is whether the constituents of the test fluids evaporate preferentially in a moisture-starved, compared to a moisture-rich, environment. One would intuitively assume so, but both 2-ethoxyethanol and formamide are miscible in water, so it is not out of the question that they would be drawn more readily to the gas phase in the presence of water vapor. Dyne levels 58 and higher contain water in their formulations, so there is no question that extremely low humidity would to some degree alter the evaporation effect on these formulations.

The evaporation of 2-ethoxyethanol and formamide under varying humidity conditions is an interesting question from the perspective of thermodynamic theory (and if any readers are experts on such matters, I would love to have some feedback on this!), but it probably does not amount to much in terms of real-world test results. The only warning I would have is to pay strict attention to the two second timeframe specified by the test, as that does relate to evaporation, as well as to de-wetting behavior.

In summary, the only way to be sure of the effect would be to test identical sets of samples at both 1% and 40% to 60% RH, and compare the results. In light of the comments pertaining to nylon noted above, if that is a polymer you are testing, I do suggest doing this comparison. But, even if you find a significant humidity effect, as long as the environment in the incoming inspection area is kept constant, results derived there should still provide good predictive information regarding the behavior of the material in the dry room.

Another consideration is that the time elapsed from testing at incoming inspection to introduction to the dry room, as well as the time elapsed from introduction to the dry room to when the materials are processed, should all be controlled as closely as possible. The surface characteristics of plastics – especially those that have been corona treated – change over time and under varying environmental conditions. So, controlling these variables will be important in making the most of the dyne testing results in the context of your overall quality program.

With these considerations in mind, the dyne level number you come up with may not exactly match the material’s surface energy at the time it is processed at low humidity, but it should still offer data which will be effective in helping predict its adhesion and wetting characteristics.

Question: Why is two to three seconds the criterion of determination of a materials’ wetting level? Wouldn’t permanent wetting be a better criterion?

Answer: Actually, ASTM Std. D2578 and ISO 8296 both specify 2 seconds as the timeframe for evaluation, but we usually suggest 2 to 3 seconds, as most testers seem more comfortable when a brief range is specified, rather than a single instant in time. The timeframe is partly historical artifact, and partly due to the basis of the test, which derives from the behavior of retreating contact angles.

As the test fluid is applied to the surface, it is spread over a given area – usually about a square inch (about 6 or 7 square cm) either in a line or as a block. The results of the test are based on how (and if) the fluid film reticulates (shrinks) into individual beads, and this is obviously a process that takes a finite amount of time to achieve a balance.

I believe the 2 second timeframe was originally established to balance the effect of evaporation (the lower surface tension component of the test fluids evaporates more readily) and the effect of de-wetting per se. In other words, if you wait longer, evaporation will start to have more of an effect, which induces greater de-wetting. The idea is to evaluate at the time that surface forces per se are most important to the interaction of the test fluid and the substrate. When the test was first developed some 60 years ago, the 2 second timeframe was established as a way to standardize interpretation and meet this goal. Based solely on empirical evidence, it appears to be an effective specification, as no serious alternatives have been suggested.

Like most questions regarding surface energy testing, whereas the question may be simple, often the answers are not so much so!