Start ing Off wi th a Clean Slate Using dyne liquids is one of the easiest and most cost- effective means of assessing surface cleanliness. By Anselm Kuhn, Metal Finishing Information Services Ltd.

A s any expert will confirm, poor cleaning is the most important single cause of defective finish- ing. In the case of paint- and powder-coated

surfaces, the main result is poor adhesion. For plated surfaces, blistering and spotting are commonly found. So how does the metal finisher ensure that surfaces are properly cleaned, prior to plating or coating?

Over the years, a wide range of cleanliness testing methods have been developed. A search of the on- line database "Surface Finishing Abstracts" using search terms "cleanliness" + "testing "1 reveals that new instrumental methods continue to be reported. A fairly comprehensive overview of such cleanliness and testing methods was published a few years ago in Metal Finishing. 2

In spite of this, one suspects there are many fin- ishers who use nothing more sophisticated than the simple water-break test 2 and are oblivious to the fact tha t this rudimentary "yes-no" test is fallible under some circumstances. "Cleanliness" is far from easy to define. Increasingly, though, "contamina- tion," which is the opposite, is divided into particu- late contamination and associat- ed with more or less continuous films of oils or greases.

The use of dyne liquids, dis- 0 cussed here, deals exclusively 2.5 with the latter. In the approach 10.5

19.0 discussed in this article, "surface 26.5 energy," or "wetting tension" (% 35.0 or (~c), are taken as proxies for 42.5 cleanliness. Figure 1 shows an 48.5 idealized drop sitting on a solid 54.0 surface. The drop height and 59.0 diameter are shown as "d" and 63.5

67.5 "h," while the contact angle 0 is 71.5 also shown. 74.7

The surface tension of a liquid 78.0 can be visualized as the ease 80.3 with which it wets a solid sur- 83.0 face. Liquids with low surface 87,0

90,7 tension wet any surface more easily than those with higher 93.7

96.5 surface tension. In the same way, 99.0 a solid surface can be character- ized in terms of its surface ener-

gy. The relationship between these parameters is expressed by Young's equation, which can be applied to a droplet of liquid formed on a solid surface:

COS 0 -- (O S -- GSL)/(~ L (1) where 0 is the so-called contact angle, % is the sur- face energy of the solid on which the liquid droplet or film is applied, %L is the interfacial energy of the solid-liquid interface, and (~L is the surface energy or surface tension of the liquid. When a liquid "wets" a surface, 0 is very small, approaching zero. In non-wet- ting cases, 0 will be large, perhaps as much as 110% In Equation (1), the term %L cannot be directly deter- mined. This difficulty is overcome defining a new

Figure 1: Parameters characterizing a sessil drop.

Formamlde Ethyl Cellosolve 100.0 97.5 89.5 81,0 73.5 65.0 57.5 51.5 46.0 41.0 36,5 32.5 28,5 25,3 22.0 19,7 17.0 13,0 9.3 6.3 3.5 1,0

We#in(] Tension dynes/cm 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 48 50 52 54 56

**Originally from ASTM D 2578 and widely reproduced on websites quoted in Reference 9.

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.... ~ : 46 L~ ~ /~ i~: :~: : ~ / 4 8

i ~i:::;=!i!: i;5o¸ 52 54 RR

m 22 24 26 28 30 32 34 36 38 40 4 2

Weight % Water 0.00

20.11 35.31 51.54 60.89 67.98 72.57 76.61 79.04

R 9 R9

93,40 94.62

Weight%Ethanol 100.00 79.89 64.69 4&46 39.11 32,02 27.43 23.39 20.96 17.48 15.12 13.01 10.92 9.26 :

8 .02 ..... 6.60 5,38 4.37 3.37 2,57

:i:! :: : 1,78 .... :%1 1.18

:~ : : : : : ....... :0.60

0.00

parameter, °C, known as the criti- cal surface energy for the solid.

(a s - a s L ) = a c ( 2 ) Equation (2) is substituted into

Eq. (1) to give an expression where 0 can be directly meas- ured while a L is the surface ten- sion of the dyne liquid, which is given by the supplier or is known for liquids of known composition (see Tables I-III).

Values of a s for metals or plas- tics in their perfectly clean state are known. The extent to which the measured value of a s or a C is less than the "clean" value indi- cates the extent to which the sur- face is contaminated by oils or similar films.

DYNE LIQUIDS FOR MEASURING SURFACE ENERGY AND AS TROUBLESHOOTING TOOL The family of techniques described here share one common feature-- employ a series of test liquids that provide a range of specified surface tensions. In the U.S., these are known as "dyne liquids." The German term is "test tinten" (test- ing inks). These liquids are applied

to the surface to be tested, where their wetting or nonwetting behav- ior is observed.

The behavior of each liquid can be described as a "go/no-go" test. But by determining the point in the series of progressively higher surface tension liquids where the transition occurs, a quantitative result can be obtained.

The use of such a test method was originally applied to meas- urements on plastic films or foils where some form of surface treat- ment was usually necessary if paint or printing inks were to adhere properly. The approach is equally valid for use in metalliz- ing of plastics, or for gauging the cleanliness of metal surfaces, or even those with a conversion coat- ing. Hansen 7 cites a number of case studies where measurement of surface energy proved to be a valuable troubleshooting tool.

The concept is incorporated in several standards, which will be considered in detail below. These include ASTM D 2578 (the latest version being 2578-04a), ISO 8296-97, and DIN 53 364. There is also a Japanese Standard, JIS Circle 019 on reader information card

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May 2005 73

Ethylene glycol Monoethyl ether Formamide Methanol 2-Propanol n-He×ane n-Heptane n-Octane

m

30.0 (23%) :58,0 (23%) 22.6 {23%)

121.4 (23%) 17.9 (25%)

: :::~ :t9,8 (25%) .... 21.1 (25°C)

ISO 8296:1987 (E) ISO 8296:1987 (E) ISO 8296:1987 (E)

* Data from "Organic Solvent; Physical Properties & Methods 0f Purification," J.A. Riddick & W.B. Bunger, 3 'd Edition (1970), Wiley & Sons. Table otherwise adapted from Ref 6.

K6768. The lat ter was recently revised and is now substantial ly equivalent to the ISO and ASTM Standards. The German DIN Standard is similar, though with significant differences. Finally, there are Nordtest POLY 1766 and Method AFCO-C. The TAPPI Test Method T698 is also similar.

In all these methods, the dyne liquids are applied by painting, swabbing, or as liquid droplets to the surface. However, as will be seen, each method gives somewhat different results, and wetting tension values obtained should always record the method used to obtain them. Surface wetting tension values should not be compared if they were obtained using different methods.

Some of these s tandards (such as ASTM) prescribe the chemical

-- composition of the test liquids to be used, while others (such as the Nordtest) leave the choice open to

. the user. The test liquids may be * a single pure species, a mixture of * two liquids, or a solution of a solid

in a liquid, thereby covering a range of surface tension values. These are applied, in a prescribed

manner, to the surface to be tested. Those with the highest surface tension will break

up into individual droplets, while those with lower surface tension will spread out or "wet" the surface As one works through the series of liquids, the point at which the "no-spread"-"spread" t rans i t ion is observed. The ASTM method is sometimes known as the "Union Carbide Test," probably because this company is one source of the test liquid components cited in the standard.

D Y N E L I O U I D C O M P O S I T I O N S The ASTM standard prescribes a series of test liquid

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. . . . . . . . . . . . i 2 J i _ . . . . _ f - ~ - i "

,~L ......... . & "

• ,[ ~ •

Figure 2: Alternative means of dyne liquid application. Top: Desi~ln after Baker, adjustable for film thicknesses of 30, 60, 90, 190 Apm, 5-cm stripe or wide. Center: Spiral dyne liquid appli- cator. Stainless wire sprial wound around stainless steel core. It allows film thicknesses over a wide range (4 to 500 Apm thick- ness). Bottom: Cube-shaped dyne liquid applicator with adjustable sluice gate for film thickness control.

mixtures made up of formamide and "Cellosolve," which is the Union Carbide trade name for ethylene

glycol monoethyl ether. Table I lists the composi- tions and their wet t ing tension values. The da ta relate to an ambient temperature of 23°C _+ 2°C and a relative humidity of 50% _+ 5%. The mixture has limited shelf life and formamide is somewhat toxic.

Some sources suggest t ha t p regnan t women should not be exposed to such liquids. Cellosolve is flammable and mixtures should be treated as such.

The point is made that the two components of this (and indeed other) mixtures will evaporate at differ- ent rates, when exposed to atmosphere. The result of this would be a change in composition and surface tension value. Storage in a refrigerator has been rec- ommended. In order to improve visibility, a dye may be added to the mixtures. DuPont Victoria Pure Blue BO at a maximum concentration of 0.03% is sug- gested in the ASTM Standard.

The German DIN Standard uses the same series of liquids, which are described as the "blue series" or "Series A," covering the range 30 to 58 dynes, but also offers a "red series" (Series B) based on methanol- water mixtures, which cover a range of 23 to 72 dynes.

Other Test Series commercially available 9 include water- ethanol (25 to 72 dyne), formamide-water (58 to 72), alka- ne series (16, 20, 22, 24, 25, 27, 28, 45) and aqueous salt solutions (72 to 82). Tables I to III set out surface tension values of a range of organic liquids or mixtures of liquids.

Because some of these standards were originally conceived for testing plastic surfaces, the range of surface tensions covered extends to only 58 dynes/cm. As the use of the test methods was

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extended to metal and other surfaces, addit ional ranges of test liquids were developed, as noted above. Liquid formulations (see Table I) are avail- able in steps of 1 dyne/cm units. Liquid-filled felt- tipped pens (see below) are usual ly available in steps of 2 dyne/cm ranges.

HOUSEKEEPING Gerstenberg s is one of several authors who suggest storage of test liquids in a refrigerator when not in use is prudent. He notes that on removal from the refrigerator, there is no need for temperature equili- bration, since the temperature coefficient of surface tension is only 0.5mN/m per 10 (though for which liq- uid series is not stated). He describes the use of test liquids contained in bottles, each with its own appli- cator brush mounted in the screw-top lid. He empha- sizes the danger of progressive contamination of the test liquid, as the brush is used, and then returned to the bottle, inevitably contaminated by whatever was present on the surface last tested.

He and others suggest tha t bottles containing dyne liquids should be discarded after a few months, depending on how much they have been used, or for how long they were exposed to air.

LIQUID APPLICATION & PROCEDURE The dyne liquids are applied to the test surface, either to form a film, or as individual droplets.

FILM DEPOSITION METHODS The ASTM standard suggests the use of a cotton bud applicator, of the sort available from most pharma- cies. However, the point has been made s tha t an adhesive is used to bind the cotton wool to the wood- en or plastic holder. Since the adhesive may vary depending on the manufacturer and may dissolve in the Cellosolve, this could introduce an element of unforeseen variability. Gerstenberg notes tha t cot- ton buds may have a moisture content of 5 to 8%, which could dilute the test liquid.

Several sources caution against contamination of the test liquid, such as by re-use of a cotton bud (expressly forbidden in some standards) first dipped into one test liquid, then into another one. Hartwig et. al 5 suggest using a spatula to apply a film 10- to 15-mm thick. One assumes this is a misprint for 0.1- to 0.15-mm thick. DIN 53 364 suggests use of a small brush to apply the liquids, painting a stripe some 10 cm long. Gerstenberg s suggests use of proprietary applicators.

Some of these are shown in Figure 2. Technically speaking, this will invalidate any result from being expressed in terms of the ASTM and DIN standards. Likewise, in some circumstances, a painted stripe

less than 10 cm long may be used. Both the above standards require a reasonably large, fiat, and hori- zontal surface.

DROPLET DEPOSIT AND OTHER METHODS The Nordtest method (Nordtest Poly 176), now adopt- ed as a standard) e , differs in tha t it uses a micropipette to apply a single drop of the test liquid, which is then observed. In contrast with the two stan- dards above, it can be used to test very small areas, even when these are steeply inclined or vertical. Otherwise, however, the conditions (temperature, rel- ative humidity, etc.) are more or less identical with those specified in the ASTM standard, although it offers the user complete freedom with respect to dyne liquids used.

The AFCO method, like the Nordtest , involves droplets being deposited onto the surface. In theory, both methods do not incur the problem of test liquid contamination result ing from re-immersion of the brush or immersion of a cotton bud, which may include an adhesive. In this method, the surface to be tested, which has to be flat, is inclined at an angle of 40 ° to 60 ° and a droplet of constant value is deposited from a constant height.

The droplet then runs down the inclined surface under gravity. The test criterion is whether or not it leaves a liquid track behind it. The li terature notes one or two other means of applying the dyne liquids for testing, though these are not based on any stan- dard. One is the application of three droplets of three values of dyne liquid onto a fiat surface, which is then tilted. Whether each droplet "runs" or not indicates the surface energy.

Another application method is to use a set of liq- uid-filled pens. These are available from various commercial sources. They resemble highl ight ing pens, with a chisel-shaped felt tip. In the case of the DIN standard, a stripe of liquid approximately 10 cm long is applied with a brush. A color photo of the test, used on a printed circuit board, is shown in Reference 3.

A last method noted by Gerstenberg s is complete immersion of the object to be tested in the dyne liq- uid and its withdrawal , observing the wet t ing behavior. This might be seen as a more sophisticat- ed form of the widely-used water-break test.

OTHER METHOD DETAILS Whatever method is used, the minimum amount of test liquid is transferred to the surface (use of excess liquid can adversely affect the test result). The liq- uid is spread over an area of approximately one square inch (6.5 cm 2) in the case of the ASTM

76 www.metalfinishing.com

method, and the time taken for a continuous film to break up into droplets is noted. If the film remains coherent for more than two seconds, the tes t is aban- doned and the next higher sur- face tension liquid should be used. A fresh cotton bud applica- tor should be used for each appli- cation, even if using the same liquid as before.

The observation of the wet ted film behavior should be made in the center, not at the periphery, where a degree of contract ion due to solvent evaporat ion is acceptable. Break-up of the liq- uid film within two seconds is taken to indicate "non-wetting." Excess liquid applied to the sur- face may be another cause of peripheral shrinkage. By repeat- ing the tes t with the different liquids, one can select the liquid that most closely meets the crite- ria above.

The surface tension of this liq- uid, in dynes/cm, is then equiva- lent to the so-called wett ing ten- sion of the surface. In the case of the Nordtes t , the droplet is observed for two seconds to determine whether it spreads or remains roughly spherical.

RESULTS & O T H E R C O M M E N T S What is measured by this family of tes t methods is the "wett ing tension" of the surface, expressed in dynes/cm or nM/m. Though one might work through the series of tes t l iquids in ei ther direction, the ASTM s tandard recommends one should work from lower to higher surface ten- sion liquids.

ASTM and other s t andards recommend (though one fears this is rare ly done in practice) tha t the surface tension of the chosen liquid be verified using an instrumental method for sur- face tension measurement .

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These are described in most physical chemistry text- books. But one should also mention an excellent CD- ROM produced by the AESF 4 in the form of a PowerPoint tutorial , which describes the var ious techniques in considerable detail.

ACCURACY & METHOD COMPARISONS How accurate are these methods? How reproducible are they? For the DIN Standard, Gerstenberg sug- gests that after a little practice, results can be repro- duced to _+ 0.5nM/m. He identifies, though without fur ther detail, the action of the brush on the test surface as the main cause of inaccuracy. The ASTM standard includes the results of an interlaboratory test for three polypropylene films, each pretreated with a different power density.

The Repeatabi l i ty S tandard Deviat ion SR lies between 0.32 and 0.74. The Reproducibility Stan- dard Deviation SR ranges (in the same ranking) from 0.87 to 1.97. There are horror stories of cases where discrepancies of 10 dynes/cm or more have been found. However, comments in the Nordtest doc- ument suggest that a given operator should be able to completely repeat results, that is to say, report the

transition for the same liquid in the series. Using two different operators can increase the error.

METHOD COMPARISONS It should be noted that wetting, though it is widely regarded as a static, equilibrium parameter, is in prac- tice a clynamic, where hysteresis is observed between surface tension values measured in advancing or retreating states of the liquid. This is where the various methods described above do really differ.

In the Nordtest , the drop formed on the surface can only advance, not retreat . By contrast, where the dyne liquid is painted or swabbed onto the sur- face, it is unlikely to advance, but may well break up into droplets, i.e. retreat.

Secondly, the implicit basis of these methods assumes a homogeneous surface. In practice, the much larger test area of the DIN standard as com- pared with the ASTM method, renders it more sus- ceptible to surface inhomogeneities. Gerstenberg notes that values obtained with the ASTM method are usually lower than those from the DIN method.

How do the three main methods compare? For ease of use, the ASTM method, using inexpensive

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78 www.meta l f in ish ing .corn

and disposable cotton buds, scores well. At the other extreme, the Nordtes t involves filling the micropipette with one liquid and then, after observ- ing the drop behavior, cleaning and drying the pipette before refilling with a different test liquid. Against this, the Nord tes t is the most t ight ly defined with the smallest scope for operator effects.

Practical experience has shown that, after cleaning, there can be local variations of cleaning quality over small distances. This is especially true when the sur- face to be cleaned consists of more than one material, such as mixed metals or metals and plastics. The Nordtest is ideal for exploring such differences, since is gives results for areas of a few square millimeters. Little has been actually published regarding the use of dyne liquids for cleanliness measurement. Hansen 7 has reported on the Nordtest for this.

AVAILABILITY OF STANDARDS & TEST MOULDS All s tandards can be obtained from national stan- dards agencies. However, the Nordtest Standard is available for free on the Internet 6 while a number of Web sites 9 present what is a reasonably full precis of the ASTM standard. Users can decide for them- selves whether to prepare their own dyne liquids or purchase these from suppliers. 9

OTHER COMMENTS & DEVELOPMENTS It is hard to see what might supersede this family of low-cost, easy-to-use tests for front-line evaluation of surface energies and cleanliness. Strictly speak- ing, these methods cannot be considered to be non- destructive, in tha t they contaminate the surface tes ted and, in some cases, actual ly react with it (PVC and PUR react with the ASTM test liquids).

There is no fundamenta l reason, however, for clinging onto the original formamide-Cellosolve liq- uids. Indeed, several p ropr ie ta ry suppliers s ta te their dyne liquids to be formamide-free. The liquids shown in Tables II and III are, likewise, so. With thousands of organic liquids available, it should not be difficult to find single liquids or mixtures having low vapor pressure and are easy to use. The so- called "inert liquids," f luor inated or mixed halo- genated al iphatic hydrocarbons, come to mind. Equally, one could conceive of semi-au tomated methods of applying droplets of the test liquids, per- haps using the sampling and dispensing technolo- gies widely used in pharmaceutical laboratories.

CONCLUSION Measu remen t of surface wet t ing tensions using dyne liquids is unques t ionably one of the easiest and most cost-effective means of assessing surface

cleanliness. For process control or troubleshooting, it is more than sufficiently accurate and reproducible.

In considering the basic principles of these tech- niques, it is true there are some aspects that might cause concern, and these have been noted. However, two points should be noted. First, because one can indicate a possible weakness in a method is not to say this actually manifests i tself in practice. Second, almost all of the possible reservations noted earlier apply equally to all the other wetting test methods for cleanliness, such as the sessile drop.

A much expanded version of this is available from Finishing Publications Ltd. (finpubs~compuserve.com).

ACKNOWLEDGEMENTS Thanks are due to John Durkee, Charles M. Hansen, Herr Boss of Messrs Ahlbrandt (www.ahlbrandt.de), and K. Gersternberg of Tigres, for their helpful comments.

REFERENCES 1. "Surface Finishing Abstracts" available on-line at

www.surfacequery.com. 2. Kuhn, A.T., Metal Finishing, 91(9):25-31; 1993. 3. Anon, Metalloberflaeche, 58(11):38-39; 2004. 4. CD-ROM, AESF, Orlando, FL; www.aesf.org. 5. Hartwig, A. et al., Oberflaeche-JOT, 11:90-92: 1993. 6. Nordtest Poly 176, www.nordicinformation.net/

nordtesti~ler/poly176.pdf. 7. Hansen, C.M., "New Nordtest Method Shows

Contamination on Surfaces," Pigment & Resin Technology, 27(5):304-307; 1998.

8. Gerstenberg, K., JOT, (3)32-34; 1995. 9. Commercial sources of dyne liquids with the most inform-

ative Web sites include Accudyne (www.accudynetest.com); Tigres (www.tigres.de);Ahlbrandt (www.ahlbrandt.de); and Sherman (www.shermantreaters.co.uk). A Web search for "dyne liquids" will produce others.

Metal Finishing Information Services Ltd., Stevenage, Hertfordshire, England, U.K., (e-mail) finpubs@compuserve.com Elf

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