Category: Questions and Answers

Question:

I’ve been told that some polymers cannot be evaluated with the dyne test, and I’m especially concerned about any solubility of the plastic in the test ink. Can you give any guidelines on when the ink may interact with the surface, and explain how to tell if this is a problem?

Answer:

A simple rule of thumb is that if a solvent causes any melting of the polymer surface, or swelling of the polymer bulk, there is a degree of solubility present. Most published data on solubility look at how resistant the solid is to the solvent over a long period of time – this is important for prediction of durability and suitability for most commercial applications. But whether or not the material interacts over a period of hours or days or if it interacts only at elevated temperatures is not really relevant to the dyne test, which is based on the solid—liquid interaction over a time frame of only a couple seconds, hopefully in a standard laboratory environment.

The specific answer to your question is that it really comes down to discretion: Is there evidence of solubility within several seconds, or does it take minutes or longer to see any swelling or melting of the surface? My personal feeling is that if the surface remains unaltered for at least about 10 – 15 seconds (a scuff test or similar would demonstrate this empirically) you are probably OK, though I would consider the results to be somewhat tentative. In other words, if there is some evidence of solubility within, say a quarter to a half minute, and there is any other reason to believe the dyne test results may be inaccurate, that would be a concern. If the solubility is apparent more quickly, I would not trust the results at all.

You question has piqued my curiosity, so I’ve done a more scientific look at the issue, using Hansen Solubility Parameters (HSP) as the basis for the investigation. In HSP terms, the affinity between two molecules is considered as a 3 dimensional distance, defined as⁽¹⁾

Ra² = 4(δ_D1 – δ_D2)² + (δ_P1 – δ_P2)² + (δ_H1 – δ_H2)²    (1)

where in this case δ_D1, δ_P1, and δ_H1 are the dispersion, polar, and hydrogen HSP components, which define the location of the molecule in “Hansen space” for the polymer and δ_D2, δ_P2, and δ_H2 are the HSP components for the solvent. The Relative Energy Difference (RED) is defined as Ra/Ro, where Ro is the radius of the sphere of influence (interaction radius) for the polymer. Any solvent which lies less than the RO away from the centerpoint of the polymer’s sphere will allow solubility of the polymer in that solvent; thus, an RED of less than 1.0 suggests that some degree of solubility will occur. The Ro of different polymers varies quite a lot; the smaller it is, the more precisely the solvent’s solubility parameters must match those of the polymer for solubility to occur. A higher molecular weight version of a given polymer will have a smaller sphere of influence, and be less readily soluted; the same holds true for a crystalline, as opposed to amorphous, molecular structure⁽²⁾.

Based on this conceptually simple (though not so easy to wrap one’s head around) model, it is a straightforward process to calculate whether or not a polymer will be soluble in a solvent, including those used in the formulation of dyne solutions. Table I shows the Hansen parameters for the three constituents of the test fluids⁽¹⁾. Table II shows the Hansen parameters for the formulations of all the even levels from 30 through 72 dynes/cm, as determined by their volumetric percentages. Table III shows the critical surface energy and Hansen parameters for a variety of polymers.

TABLE I
Solvent	Dispersion	Polar	Hydrogen
2-ethoxyethanol	16.2	9.2	14.3
formamide	17.2	26.2	19.0
water	15.6	16.0	42.3

TABLE II(a)
Dyne Level	% 2-ethoxyethanol	% formamide	%water	Dispersion	Polar	Hydrogen
30	100.0	0	0	16.2	9.2	14.3
32	89.5	10.5	0	16.3	11.0	14.8
34	73.5	26.5	0	16.5	13.7	15.5
36	57.5	42.5	0	16.6	16.4	16.3
38	46.0	54.0	0	16.7	18.4	16.8
40	36.5	63.5	0	16.8	20.0	17.3
42	28.5	71.5	0	16.9	21.4	17.7
44	22.0	78.0	0	17.0	22.5	18.0
46	17.2	82.8	0	17.0	23.3	18.2
48	13.0	87.0	0	17.1	24.0	18.4
50	9.3	90.7	0	17.1	24.6	18.6
52	6.3	93.7	0	17.1	25.1	18.7
54	3.5	96.5	0	17.2	25.6	18.8
56	1.0	99.0	0	17.2	26.0	19.0
58	0	81.2	18.8	16.9	24.3	23.4
60	0	65.0	35.0	16.6	22.6	27.2
62	0	47.0	53.0	16.4	20.8	31.3
64	0	30.6	69.4	16.1	19.1	35.2
66	0	18.2	81.8	15.9	17.9	38.1
68	0	8.6	91.4	15.7	16.9	40.3
70	0	3.6	96.4	15.7	16.4	41.5
72	0	0	100.0	15.6	16.0	42.3

(a) Formulation data for dyne levels 30 through 56 is from ASTM Std. D2578⁽³⁾. Values for dyne levels 58 through 72 are not specified by ASTM – the values are from actual ACCU DYNE TEST formulations.

Table III(a)
Polymer Name	CST(b)	Dispersion Parameter(c)	Polar Parameter(c)	Hydrogen  Parameter(c)	Ro(d)
Acrylonitrile butadiene styrene	38.5	17.0	5.7	6.8	9.4
Cellophane	45.4(e)	16.1	18.5	14.5	7.3
Cellulose acetate	37.5	17.1	13.1	9.4	10.6
Cellulose acetate butyrate	34.0(e)	17.2	13.8	2.8	12.6
Epoxies	44.5	19.2	10.9	9.6	11.1
Ethyl cellulose	30.3	19.0	5.6	4.9	7.9
Fluorinated ethylene polypropylene	19.1	19.0	4.0	3.0	4.0
Nitrocellulose	42.7(e)	16.2	14.1	9.5	10.7
Nylon 6	43.9	17.0	3.4	10.6	5.1
Nylon 6,6	42.2	17.2	9.9	16.5	4.4
Nylon 11	35.6	17.0	3.4	10.6	5.1
Nylon 12	37.1	18.5	8.1	9.1	6.3
Poly a-methyl styrene(f)	38.7(e)	18.5	2.4	2.4	–(g)
Poly iso-butyl methacrylate	30.9(e)	18.3	4.9	6.3	9.4
Poly n-butyl methacrylate	29.8	16.0	6.2	6.6	9.5
Polyacrylonitrile	47.0	21.7	14.1	9.1	10.9
Polybutadiene	29.3	17.5	2.3	3.4	6.6
Polycarbonate	44.0	19.6	8.8	5.7	10.2
Polychlorotrifluoroethylene	30.8	15.6	2.5	4.7	5.8
Polyether sulfone	47.0(e)	18.3	8.2	6.4	6.3
Polyetherimide	–(g)	17.7	6.0	6.4	4.8
Polyethyl methacrylate	32.8	17.5	8.5	4.7	9.6
Polyethylene	31.6	16.8	3.8	3.8	6.6
Polyethylene oxide(h)	43.0	17.0	10.0	5.0	–(g)
Polyethylene terephthalate	39.0	18.7	6.3	6.7	6.5
Polyisobutylene	27.0	16.4	1.7	4.7	7.9
Polyisoprene	32.3(e)	18.1	2.4	2.3	10.3
Polylactic acid(i)	38.0(j)	18.5	8.0	7.0	–(g)
Polymethacrylonitrile	39.0	17.2	14.4	7.6	3.8
Polymethyl methacrylate	37.5	17.9	10.1	5.4	11.0
Polyoxymethylene(h)	37.0	17.2	9.2	9.8	–(g)
Polyphenylene oxide	47.0	17.9	3.1	8.5	8.6
Polyphenylene sulfide	38.0	18.7	5.3	3.7	6.7
Polypropylene	30.5	17.7	2.9	1.2	6.2
Polypropylene glycol(h)	32.0	16.5	9.0	7.0	–(g)
Polystyrene	34.0	22.8	5.8	4.3	12.7
Polysulfone	42.1	18.5	8.5	7.0	9.4
Polytetrafluoroethylene	19.4	16.2	1.8	3.4	3.9
Polyurethane	37.5(e)	18.8	10.0	8.2	9.8
Polyvinyl acetate	35.3	20.9	11.3	9.7	13.7
Polyvinyl alcohol	37.0	14.7	14.1	14.9	10.5
Polyvinyl butyral	28.0	19.1	9.5	12.2	10.0
Polyvinyl chloride	37.9	16.8	8.9	6.1	8.5
Polyvinyl pyrrolidone(h)	46.0(e)	17.5	8.0	15.0	–(g)
Polyvinyl fluoride(f)	32.7	17.4	13.7	11.3	–(g)
Polyvinylidene chloride	40.2	19.0	9.6	9.0	5.8
Polyvinylidene fluoride	31.6	17.0	12.1	10.2	4.1

(a) Critical surface tension data is from column 3 of Diversified Enterprises’ Polymer Table 1⁽⁴⁾, except as otherwise noted. Hansen Solubility parameters and Ro data are the average of results cited by Hansen⁽¹⁾, except as otherwise noted.
(b) Critical surface tension in mJ/m² (equivalent to dynes/cm), generally determined by the Zisman method (regression of the cosine of the contact angle), or by the wetting tension method, using solutions of 2-ethoxyethanol and formamide, per ASTM Std. D-2578⁽³⁾.
(c) MPa^1/2 (equivalent to joules/cubic centimeter; 2.0455 x (cal/cc)^1/2) @ 20-25^oC.
(d) Interaction radius (sphere of influence); units also MPa^1/2.
(e) No critical surface tension data available; surface energy data are cited, mostly from contact angle measurements; from column 4 of Diversified Enterprises Polymer Table 1⁽⁴⁾, except as otherwise noted.
(f) Solubility parameter data from Yu, et al⁽⁵⁾.
(g) No data available.
(h) Solubility parameter data from Abbott⁽⁶⁾.
(i) Solubility parameter data from Abbott⁽⁷⁾.
(j) Critical surface tension data from Jacobsen, et al⁽⁸⁾;

Using the data from Tables II and III and applying formula 1, the Relative Energy Difference between any polymer whose HSP values are known and any dyne level test fluid can be readily calculated. Fortunately, the RED works out to greater than 1.0 for most pairs when considered at the critical surface tension of each polymer.

The most notable exception is polyvinyl alcohol, which has a surface energy of about 36 to 40 dynes/cm. In that range, RED vs. dyne solutions is only about 0.44 to 0.73. Cellophane is another material that could prove difficult to dyne test: its surface energy is about 42 to 48 dynes/cm, with a corresponding RED of 0.63 to 0.96. Of particular note with this material is that the RED is only 0.36 to 0.48 at 38 and 40 dynes/cm, meaning that some degree of surface alteration if testing at these dyne levels is quite likely, even within the brief 2 to 3 second time frame required for the test.

Other polymers that could possibly present problems include polyvinyl acetate (dyne level range of 32 – 36 with REDs of 0.77 to 0.87), poly n-butyl methacrylate (dyne level of about 30, with an RED of 0.87), and nitrocellulose (dyne level range of about 38 to 45, with REDs of 0.80 to 1.15). Given the brief contact time required for dyne testing, it seems unlikely any of these would have a degree of solubility that would have a practical impact, but the possibility needs to be considered.

It is not possible to determine the RED values for the polymers listed that do not have data on their interaction radii Ro. However, of the group only polyoxomethylene and polypropylene oxide appear to have any real potential for solubility with the dyne solutions, as the calculated RA values are 7.4 and 9.8 at 34 and 36 dynes/cm, respectively for the former, and 8.1 at 32 dynes/cm for the latter.

It is interesting that polymethyl methacrylate (acrylic) is not among the polymers cited, as it has always been an axiom that it is a polymer which is especially prone to solubility issues in dyne testing. In fact, according to the data presented by Hansen, the RED for this material works out to 1.17 to 1.42 over a range of 36 to 40 dynes/cm, which reflects its normal critical surface tension range. Possibly some polymerization methods result in significantly different HSPs from the mean, or perhaps some materials which are assumed to have an acrylic surface have actually been coated or otherwise modified. Additionally, if it is an acrylic with unusually low molecular weight, its RO will increase, which could then include dyne solutions within its sphere of influence (as well as making it amenable to a broader range of coatings, inks and adhesives).

Determining potential solubility issues for treated polymer surfaces presents a layer of uncertainty, as corona, flame, or atmospheric plasma all increase the polar component (and likely to a lesser degree the hydrogen bonding component) of HSPs, as well as the polar component of surface energy. There is little information available on this – the only data I could find was for UHMW polyethylene fibers treated by atmospheric plasma. The HSP data presented by Kusano, et al⁽⁹⁾ was δ_D 16.5, δ_P 15.3, and δ_H 8.2 hydrogen after plasma treatment. Their baseline (untreated) data was δ_D 18.0, δ_P 1.2, and δ_H 1.4. (By comparison, the average HSP from studies cited by Hansen⁽¹⁾ for untreated PE was δ_D 16.8, δ_P 3.8, and δ_H 3.8. Also of interest is that the Kusano states an Ro of 4.3, compared to 6.6 from the Hansen data; this is likely due to the high molecular weight of the fibers).

Unfortunately, no contact angle measurements or critical surface tension data was reported for the treated fibers in this study, so it is impossible to match these HSP data to that of the dyne solutions that would be used to test the treated material. Additionally, the changes in HSP for UHMW polyethylene likely differ – perhaps substantially – from what would be true for other polymers. To make things even more nebulous, each treatment method (corona, flame, plasma, UV bombardment, etc.) will render a different blend of reactive sites on the surface; treatment dwell time will also affect this; and treatment under non-atmospheric gases in plasma systems can result in dramatically different changes in surface chemistry.

Nevertheless, it seems safe to say that the likelihood of solubility affecting dyne testing results for treated surfaces is probably higher than it is for unmodified polymers: under most scenarios, the major effect of surface treatment is to increase the polar component of surface energy, and the results from Kusano suggest that this would also be broadly true (along with a significant increase in the hydrogen bonding parameter) of its impact on HSPs. A perusal of Tables II and III shows that the dispersion components of the dyne solution HSPs generally match those of the polymers rather closely, so differences or similarities in the polar and hydrogen parameters are the primary drivers of soluble vs. insoluble interactions.

As a rule of thumb, those two components are higher for the dyne solutions than they are for the polymers. Whereas the rate of increase of dyne solution polar parameter is rather steep from 30 to about 38 dynes/cm, the slope decreases progressively as you reach higher dyne levels. The same trend holds true for the hydrogen parameter, at least until you reach the dyne levels (58 and above) that include water as a constituent.

The question boils down to whether or not the polar (and hydrogen) parameter increase is roughly linear as a function of increased dyne level in treated polymers. Only a designed study could answer this question, and it would be a rather daunting challenge, given all the variables involved, including polymer type, treatment method, treatment intensity, etc. This would make for an excellent PhD thesis!

In the absence of actual data, I am willing to go out on a limb on this – a little bit anyway. My guess is that the increase in polar + hydrogen parameters will, in fact, be relatively linear with respect to the amount of surface treatment imparted, at least within a normal treatment range and for treatments that utilize atmospheric gases (that leaves out more sophisticated plasma treatment options).

If this is true, then there is in fact a greater likelihood that solubility issues may affect the results obtained from dyne testing of treated polymers, especially in the range of 42 to 56 dynes/cm. And, as the polar and hydrogen parameters do both continue to climb, albeit at a reduced rate, this may be the explanation for why spurious wetting is sometimes observed when testing is mistakenly continued at levels above the critical surface tension (as indicated by a beading time of 2 seconds): That extra boost in these solubility parameter components may be the critical difference between insolubility and solubility of the polymer in the dyne solution. The latter scenario would lead to artificially easy wetting.

References:

(1) C.M. Hansen, Hansen Solubility Parameters: A User’s Handbook, 2nd Ed., CRC Press, Jul 2007. 

(2) S. Abbott, “The HSP sphere,” https://www.hansen-solubility.com/HSP-science/sphere.php.

(3) no author cited, “ASTM D2578-17: Standard test method for wetting tension of polyethylene and polypropylene films,” ASTM, 2017.

(4) R.E. Smith, “Critical surface tension, surface free energy, contact angles with water, and Hansen Solubility Parameters for various polymers,” Diversified Enterprises, http://www.accudynetest.com/polytable_01.html.

(5) W. Yu and W, Hou, “Correlations of surface free energy and solubility parameters for solid substances,” J. Colloid and Interface Science, 544, 8-13.

(6) S. Abbott, “Make your own designer solutions,” https://www.hansen-solubility.com/HSP-science/solvent-blends.php.

(7) S. Abbott, “HSP examples: poly lactic acid (PLA),” https://www.hansen-solubility.com/HSP-examples/pla.php.

(8) J. Jacobsen, M. Keif, X. Rong, J. Singh, and K. Vorst, “Flexography printing performance of PLA film,“ https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1036&context=it_fac.

(9) Y. Kusano, S. Teodoru, and C.M. Hansen, “The physical and chemical properties of plasma treated ultra-high-molecular-weight polyethylene fibers,” Surface and Coatings Technology, 205, 2793-2798.

 

Question: I know corona treatment does not create a permanent effect. I’d appreciate any comments you might have on treatment loss, including what causes it and what can be done to minimize it.

Answer: Thanks for the question. Treatment loss is an issue that can have a serious impact on printers, converters, and other material processors, and it can result in disagreements between converters and substrate suppliers. As such, it is critical to have a firm understanding of the phenomenon, and how you can best deal with it.

This post focuses on polymers, specifically films, but some of the comments – especially those relating to contaminants and environmental factors – also apply to metals, composites, and other materials.  Also, while the topic is corona treatment, the factors discussed will also affect the durability of flame or plasma treatment, albeit to a lesser effect.

A variety of factors contribute to treatment loss. The story starts even before the polymer is extruded: Most film feedstock includes slip agents to improve handling characteristics of the extruded film. The concentration of these, as well as their formulation, have major ramifications downstream with regard to treatment loss. Traditional slip agents such as erucamide will by design bloom to the surface, displacing the oxidized layer imparted by corona treatment. High loading of these agents can wreak havoc on treated films. Alternate slip formulations are available (discussed here) which help to minimize this problem.

A second factor related to the compounded resin is the tendency of short chain molecules – oligomers – to migrate to the surface as well. Residual oligomer content will vary depending on the rate at which the material is polymerized (faster polymerization = cheaper material = more oligomers = more treat loss problems). This is the reason why some processors find a strong inverse correlation between film cost and treatment loss. Finally, anti-oxidants and processing aids such as calcium stearate can migrate to the surface over time as well. The latter can also build up on takeup rolls and transfer to the freshly extruded film’s surface directly.

The extrusion process, when surface treatment should ideally be initially applied, is the stage at which the fastest and most dramatic drop in surface energy occurs, though it is hard to quantify, as treatment testing is rarely done prior to takeup at the end of the extrusion line. But the fact remains that the freshly formed polymer matrix is susceptible to changes in molecular orientation – notably at the surface – due to thermodynamic forces, and contact with each metal roll as the film is drawn off tends to neutralize the treatment effect. Higher initial treatment levels result in faster treat loss, and it should be noted that different polymer materials have inherently different rates of change.

Allowing static forces to form between extrusion and finished roll winding can result in film handling difficulties, while attracting airborne contaminants to the film surface, which will further degrade the treatment. Another very common source of contaminants and degraded short-chain polymer molecules comes from the blending of reclaimed material into the feedstock.

Once the film is wound, surface changes continue, especially if the film has been treated on one side only. Contact between the treated and untreated surfaces promotes transfer of energized compounds on the treated surface to the untreated side. A tighter wind increases the intimacy of the interface between the surfaces, and can accelerate this process. And, of course, the presence of slip agents and an excess of oligomers exacerbates the problem.

When processing the extruded film, bump (boost) treating is strongly recommended. This has two beneficial effects: It burns off contaminants and the shortest chain oligomers, and it imparts a fresh layer of treatment on the surface of the polymer. The latter effect is especially helpful, as blooming of additives does not usually occur uniformly – often aggregates of short-chain molecules will form low energy “pools” on the surface.

It is important that the bump treater be placed as close to the print, coating, or lamination station as possible. As is true during extrusion, every roller contact causes treat loss, as well as the potential to pick up contamination from the roller surfaces. Static control is also important at this stage, as these charges will attract airborne contaminants, and may affect liquid flowout. Humidity control is also not to be overlooked: High humidity can carry airborne contaminants, and low humidity promotes static charges.

The preceding discussion presents a litany of reasons for why treatment loss will be a reality for anyone processing corona treated films. So, what can be done to offset this inevitable phenomenon?

Whether you are working with purchased film or are extruding it yourself for future use, it is important to keep storage conditions constant, preferably at 60°-70° Fahrenheit and at 45-60% RH. Higher temperatures will greatly accelerate treatment loss, and low temperatures at high humidity can result in condensation. Temperature and/or humidity cycling will amplify any adverse effects. It is best to keep storage time as constant as possible, and avoid allowing stock to age excessively. If feasible, the relationship among storage time, temperature, and humidity should be determined for each material you process.

For purchased film, always test its dyne level when the material is received. Generate a database that includes the supplier’s surface energy measurements (and ensure that those are generated in a standardized and replicable manner!), your measurements from incoming QC, and what is measured when the rolls go to the converting or printing operation. When possible, it is advisable to work with the supplier to control and monitor storage conditions and duration. Be aware of seasonal differences that affect the rollstock during transport. As noted earlier, inline treating is always strongly recommended, and this is even more true when processing film that was not under your control from the get go.

If you are extruding your own film and storing it in-house prior to printing or converting, all of the above (with the exception of the comments regarding transport) hold true. You might also want to look into reducing the treatment level at extrusion, and increasing the power applied when you bump treat; as noted above, higher treat levels at extrusion are prone to faster treat loss. There will be an optimum balance between the two steps, and this will certainly vary from material to material, and perhaps from process to process.

The immediate effect of treat loss is widely known: poor adhesion and flowout of the ink, coating, or adhesive. This is easily and readily identified in any well-monitored process. But longer term effects can also manifest. Microscopic contaminants introduced after inline treatment but before the print or coating station may not show as defects immediately, but they can create a less than perfect bond between the coating and the film. Gaps or inconsistencies in the interface between the substrate and ink, coating or adhesive act like magnets for any additive or oligomer that has not already bloomed to the surface; if these low molecular weight compounds find their way to those gaps, adhesive failure can occur days, weeks, or even months after the finished product passes final inspection.

One final comment, though it does not relate to treatment loss, is worth noting: Beware of back treatment from your inline bump treater, as it can result in blocking of your wound rolls, and can also cause serious problems if the film is destined for heat sealing at some point downstream.

Question: Currently, we take a Q-tip, dunk it quickly in the bottle of test fluid, then cover a small area of the sample surface and wait a couple of seconds to determine if it is a pass or fail. We believe this may be slightly inaccurate due to an inconsistent amount of solution being applied to the Q-tip. Is this a valid concern?

Answer: Your concern is legitimate. The more test fluid that is absorbed by the cotton applicator tip, the thicker the fluid film will tend to be when applied unless extreme care is taken to barely touch the swab to the surface when applying the test fluid. A thicker film will, due to gravitational forces, push itself outward, resulting in a higher (and less accurate) dyne level reading.

The best way to meter the amount of fluid applied is to use a dropper bottle to apply the fluid to the swab, and specify a given number of drops to apply. For plastic film testing, we usually limit the volume to 4 or 5 drops. Your best method will depend on how large an area you choose to test, as well as empirical feedback from the tester as to which volume is easiest to read with the best replicability.

One tip that may be helpful: I am most able to control the transfer of fluid from the swab to the surface if I hold the swab almost flat, and parallel to the surface, instead of handling it like a pencil or pen. Also, it is very important to use a light touch; bearing down with the swab could scrub off surface contaminants that affect the material’s actual surface energy. And, of course, always use a fresh swab for each test, even if it is at the same dyne level.

This technique suggests purchasing small bottles of test fluid, which will increase your cost per fluid volume, but on the other hand you should save quite a bit of fluid, so you may find there’s actually no extra cost in the long run. But keep in mind the purpose of the test is to produce accurate and repeatable results – the value that accomplishing this objective offers will pay dividends in improved product quality and consistency.

Question: We extrude and corona treat high-slip PE blown film. In a recent evaluation, the film went from a COF of .420 hot off the line to .188 five days later. The treat level went from over 50 to below 36. This seems like an excessive amount of treat loss. Any comments or suggestions?

Answer: That is an unusual amount of treatment loss, but on the other hand, trying to treat freshly blown PE to 50 dynes/cm in one step is pushing things – usually the film extruder will treat to about 38 to 44 dynes/cm, and the printer/converter will then bump treat as needed. The higher the treat level, the faster the treat loss, especially on high-slip films.

The degree to which the COF (and dyne level) dropped in five days suggests that you are using a traditional low molecular weight amide slip agent, such as erucamide or oleamide. These are intended to migrate quickly to the surface, especially on treated film, where the surface polarity attracts the slip molecules. Secondary amides such as oleyl palmitamide have approximately twice the molecular weight, and subsequently bloom less aggressively. They are also amenable to higher process temperatures without degradation, and tend to offer somewhat more stable results from corona treatment.

As a rule of thumb, films extruded from resin blends containing these compounds should condition for at least a day or two to allow the migration to take its course, with a corresponding COF reduction. After this conditioning period, blooming and COF reduction will continue, but at a much slower rate.

There is an alternative to these compounds: non-migratory slip agents which have a molecular weight 30 to 50 times that of the traditional amide formulations. These compounds are too massive to bloom to the surface, so their effect on COF is more or less immediate, and stable. As such, they will also have a vastly reduced effect on corona treatment loss. Another advantage is that they are stable at higher processing temperatures than are the amide-based slip agents.

Based on this information, my suggestion is twofold: Consider treating to a slightly lower dyne level initially, and investigate the feasibility of non-migratory slip agents, or at least of the secondary amide formulations.

Question: We have determined that a dyne level result of about 42 to 44 qualifies an aluminum surface for our bonding operations. We get this dyne level pretty consistently from our cleaning line, but recently we had a case where the surface wetted out all the way to 50 dynes/cm. It’s hard to imagine that the cleaning process was actually providing that clean of a surface. Any ideas on this?

Answer: For most aluminum alloys, a reading of 42 to 44 dynes/cm reflects a relatively clean surface, essentially free of oils or other problematic contaminants. So, your historical experience that this result predicts good adhesion is not surprising.

As to how you could end up with a reading considerably higher for no obvious reason, I have a few thoughts. First, is it possible the test fluids became contaminated from exposure to oils or other surface contaminants? If you had previously tested samples that failed at the usual dyne levels (in other words, were not clean), then re-used the same test markers – or somehow re-introduced contaminants into bottles of test fluid – their surface tension would be affected. Specifically, it would decrease due to incorporation of the low surface tension contaminant into the reagent. This would cause “higher” readings.

Second, did you perform the test starting at lower dyne levels, which wetted for considerably more than two seconds, then worked upwards until you determined the dyne level which started to bead up in just a couple of seconds? Based on your usual results, I would recommend starting at about 34 to 36 dynes/cm to ensure full wetting at the outset of the test. Finding the transition point from wetting to beading is important: if you start the test at too high a dyne level, spurious wetting can occur, which will invalidate results. This is why we stress that the test is finished as soon as you find the dyne level which beads up in 2 seconds or less. If this is the case, it suggests that your parts actually have a low surface energy, and will likely prove difficult to bond.

Finally, if your cleaning process uses solvents or surfactants (detergents), it is possible that the rinse cycle was ineffective. This would leave a film of low surface tension solvent or surfactant on the part, which could become solubilized in the dyne solutions, dropping their surface tension as soon as they contact the surface. This is the same effect as the first possibility noted above, but from a different cause. The way to test for this possibility would be to hand-rinse the parts and re-test. If the results return to your expected values, that would be an indication that the cleaning solution is not being entirely rinsed from the parts. In that case, the first thing I would look at is the purity of the rinse water. Distilled water has a surface tension of 72 dynes/cm at 20° C – if your rinse water has a lower surface tension, it is contaminated.

Question: We have a customer that uses your test markers to measure the surface energy of aluminum before bonding it to a composite. They find the test works great on a smooth surface, but on rougher surfaces they are unsure of results, and don’t think they are accurate. Is there a rule of thumb on how smooth a surface needs to be for surface energy testing?

Answer: I don’t know that you could really come up with a “rule” as far as roughness goes, but there is an obvious problem with textured surfaces: the test fluid will tend to settle into the “valleys” so wetting vs. beading will become more and more difficult to gauge as the roughness increases. This will be as true for plastics, composites, glass, and other materials as it is for metals. It’s important to keep in mind that the dyne test is based on the behavior of a retreating liquid/solid contact line. When this line is on anything other than a horizontal plane, gravity comes into play, either aiding or abetting the retreat of the liquid.

One thing is certain – the testing of this material should be done with bottled test fluids, applied as lightly as possible (in terms of both amount of fluid used and pressure applied) with a cotton swab. Test markers really will not be controllable enough, and will tend to flood the valleys with test fluid. Also, as testing of metals is usually performed to evaluate surface cleanliness, test markers are not a good option, as surface contaminants can affect results (for details on this, please see our discussion here).

Using a strong magnifier will be helpful – I’d look for signs that the fluid is creeping away from even the valleys, and tending to aggregate in micro-puddles rather than coating the entire low area. This discernment may be easiest at the perimeter of the test area. Also, while it may be rather interpretive rather than an absolute indicator, if the high spots on the surface retain a thin film of test fluid, that is a strong suggestion that wetting has been achieved.

Question: We convert silicone-coated PET, and test it to make sure we are working with the coated side. At present we’re testing with 30 dyne and 50 dyne inks. Any comments on this, and do you have a test procedure for this application?

Answer: First, the test procedure, which covers virtually all polymers, is available at https://www.accudynetest.com/qctest.html.

Untreated, uncoated PET generally has a surface energy of about 43 dynes/cm. As such, if the PET surface has not been modified, I would expect the 50 dyne/cm test fluid to bead more or less instantaneously when applied to the material. The 30 dyne/cm test fluid would wet out for a long period of time – permanently as likely as not.

For silicone-coated PET, it is obvious the 50 dyne/cm test fluid should bead instantly, as silicone compounds have surface energies ranging in the 20s. In most cases the 30 dyne/cm test fluid would do the same, but some formulations may result in a surface slightly higher than 30 dynes/cm, so some short-lived wetting may be observed.

I would suggest testing at 36 dynes/cm. Print-primed PET has a surface energy of about 38 dynes/cm; virgin PET is at 43, and surface-treated PET should be at 48 dynes/cm or higher. All these surfaces should show wetting for at least 4 seconds, and perhaps even permanently, when tested at 36 dynes/cm. The presence of any silicone compound would cause immediate beading at this dyne level.

Question: We use your wire-wound draw down bars here, and have a couple of questions on their care, and how to evaluate their condition. First, please let us know the recommended care and cleaning instructions. Would blasting with baking soda be safe and effective? Second, what is the best way to determine their overall condition?

Answer: Thanks for the question. The most important factor in keeping Mayer rods clean is to use an appropriate solvent immediately after every use. For non-aggressive, low viscosity fluids, simply wiping with a solvent-wetted soft, lint-free cloth may be all that is needed. In other cases, solvent immersion in an ultrasonic cleaning tank, along with brisk scrubbing with a very fine bristle brass brush, may be required. Please keep in mind that this abrasive method may cause burrs on the wire, which will affect subsequent coating performance.

It is important that the final rinse or wipe be done with a liquid such as water or high purity isopropyl alcohol, which will not leave a residue on the surface.

Once cleaned, it is imperative that metering rods be thoroughly dried before re-use, or the residual solvent may interact with the wet film coating when next used. Please note that compressed air – a fast, non-contact drying technique – generally contains trace amounts of oil, and should not be employed.

I don’t necessarily recommend blasting them with baking soda – or any other media – as even with stainless steel there is a potential for some degree of etching, which one would expect would be most pronounced at the “high” spots on the wire winding. This would reduce the area between the windings, with a concommitant reduction of coating thickness. However, if other methods have proven unsatisfactory, I suppose it would be worth a try, especially if the coating formulation is acidic. I would certainly want to check the resulting wet film thickness and finish after trying this! Other than those potential effects, the worst that is likely to happen is that the wire loses its weld or simply breaks and unwinds. If you try this, start with the largest wire sizes first.

As to evaluation of the rod’s condition, precise measurement of the wet film thickness applied would be most important . Evaluating the surface finish quality of the dried (or cured) coating would also be important. If you have a good microscope, that would be an effective inspection tool in this case.

Finally, as the cost of replacement rods is relatively modest, if there is any doubt about quality, it is probably best to simply replace the rods, rather than putting too much time and effort into trying to maintain them forever.

Question: I used your test markers to test some film extruded from a masterbatch we are developing, and the results seemed inconclusive. It was hard to tell if the ink was beading or maintaining its integrity. Any comments?

Answer: If the test marker results seem inconclusive, it sounds like the surface energy of the film varies from spot to spot. There are several possible reasons for this.

First, if there are spots of contamination on the surface (fingerprints, transfer of contaminants from a process roll, contact of the sample with a foreign object, etc.), the contaminated spots will definitely have a lower surface energy, which would cause the swath of test ink you applied to show inconsistency in wetting vs. beading.

In the case of a polymer blend, or a polymer modified with multiple additives, it is possible that the blend is not being adequately dispersed in the extruder barrel, causing the various components of the formulation to segregate in the melt. In this case, concentrations of lower surface energy additives – or of lower surface energy polymers in a blend – will naturally bloom to the surface, creating “puddles” of low energy scattered across the film. Blends that include additives or modifiers with incompatible solubility coefficients could have the same problem: Even if they blend in the extruder barrel, they may disassociate before setting in the final film structure.

If the film is surface treated, it is possible that the treatment itself is contributing to the variation in dyne level. This case usually manifests in patterns that can be traced back to the mechanics and geometry of the treater.

To properly investigate any of these causes, it would be best to perform the dyne test using the drawdown method, which allows you to evaluate a relatively large area of film surface all in one pass. Patterns of variation will appear, which will be a valuable clue in determining the root cause of the problem. Also, by testing over a variety of dyne levels, you may be able to determine what might be called surface energy topography: just as an example, you may find that some randomly dispersed areas are testing at only about 32 dynes/cm, whereas the majority of the surface holds at 38. That might suggest blooming of low surface energy partials at those locations. And, in the case of film which has not been surface treated, it may provide a clue to which constituent is not being properly dispersed.

Even by using the simple method of applying the test fluids with cotton swabs, some patterns are easily recognizable. For example, spreading 38 dyne/cm test fluid over and around a fingerprint on a 44 dyne/cm surface will show the print as clearly as a forensics lab photo.

Question: I know that ideally dyne testing should be done under standard laboratory conditions, but as we test in our shop, which is not air conditioned, that is not possible for us – we sometimes reach temperatures as high as 40° Celsius, at high humidity. How do we deal with this?

Answer: Ideally, we do recommend testing at about 20° to 25° Celsius, with humidity in the range of 40% to 60% RH, Some variation from these conditions is not likely to affect results meaningfully, as the surface tension of liquids and the surface energy of solids are similarly affected by temperature changes, as shown in the following table. But with more seriously elevated temperatures, caution should be employed.

Material	Surface Tension(a)	Change per °C
2-ethoxyethanol	28.8	-0.13
40 dyne/cm test fluid	40.0	-0.14
Formamide	57.0	-0.15
Water	72.7	-0.21
Nylon 6-6	42.2	-0.065
PC	44.0	-0.060
PE	31.6	-0.057
PET	39.0	-0.065
PMMA	37.5	-0.076
PP	30.5	-0.058
PS	34.0	-0.072
PTFE	19.4	-0.058

Surface tension, and change per degree Celsius are shown in dynes/cm (equivalent to mJ/m²).

^(a) Critical surface tension in dynes/cm at 20° to 25° C, generally determined by the Zisman method (regression of the cosine of the contact angle), or by the wetting tension method, using solutions of 2-ethoxyethanol and formamide per ASTM Std. D-2578. A more complete list of polymers is available here, and a more complete list of liquids is available here.

The largest numeric effect would be seen when testing at very high treat levels, where the test solutions are formulated from formamide and water. In the most extreme case – polyethylene treated to be water-wettable at 72 dynes/cm – the net drift works out to 0.153 dyne/cm per degree Celsius. This translates to an effect on test results of roughly -3 dynes/cm at 40° C, as the substrate surface energy would be reduced by only 1.14 dynes/cm, whereas the test solution (100% reagent grade water) would be reduced by 4.2 dynes/cm. It can be argued that even in this extreme case, an error of 3 dynes/cm at a treat level of around 70 dynes/cm would not be critical in most instances. But even at a more typical treat level of 40 dynes/cm, the net drift is still 0.083 dyne/cm per degree Celsius, resulting in an effect of about 1.7 dynes/cm, which could be significant.

So much for the hard data. Please note that this analysis assumes that both the test solution and the substrate have been stabilized at the elevated ambient temperature. If this is not ensured, then one component or the other will obviously be affected more by the temperature change, and test results could deviate from those that would be obtained under laboratory conditions in a less predictable fashion.

However, elevated temperatures pose their own problems. Surface tension test fluids will tend to degrade faster at elevated temperatures, and, while open, bottles will be far more prone to preferential evaporation of 2-ethoxyethanol, which has a higher evaporation rate. When testing with ACCU DYNE TEST^TM Marker Pens, the test fluid may pass somewhat more readily through the spring-loaded valve tip – this can have an effect due to a tendency towards gravitational wetting when an excess of test fluid is applied. Also, the rate of evaporation of the test fluids once applied to the surface will increase, so the two second timeframe on which the test is based could come into question. Finally, solubility parameters are affected by temperature as well, so the chemical affinity of the fluids to the surface may be changed.

With regard to the substrate, the rate of crosslinking may be affected, which could have an impact on the surface energy when tested vs. its level over time. Finally, elevated temperature (as well as humidity) levels will tend to accelerate treat loss of the substrate. The mobility of surface-blooming additives will be enhanced, and transfer of the treatment from the treated to untreated side of a film once it is wound will also be more pronounced. This will not only drop the dyne level, but can also lead to blocking of the film when it is paid off from the roll during further processing operations. The latter effects are perhaps the best arguments for limiting product exposure to extreme environments to whatever extent is possible.

As to RH, it is best to avoid excessive humidity, as it can cause higher variability in test results. Also, if there is any moisture on the surface, it will absorb into the test solution, changing its surface tension and invalidating the test. Nylons especially may be prone to this, as they absorb water vapor far mote readily than most other polymers.

Given the range of physicochemical effects which can have an impact on the accuracy of test results produced at elevated temperature and or humidity, if product end-use requirements are rigorous, such as in the automotive industry and for many medical applications, it would be prudent to set up an experimental study to correlate dyne testing results obtained in the shop vs. those obtained under laboratory conditions. You can randomly split your samples into two groups. On one set, test at the machine as usual. For the other set, remove to the laboratory, allow the material to stabilize under that environment, then perform the same dyne test (with a separate set of test fluids or test markers which are to be kept in the lab!). Results can be compared readily. If there is a meaningful difference, I would also suggest looking closely at which set of results best predicts end-use performance or adherence to customer specs. Considering all the factors discussed above, you might even find that the results from the elevated temperature testing are the better predictor of product suitability. I wouldn’t want to bet against it a priori, even though such an outcome does seem rather counter-intuitive.