Quantitative method for evaluating fingernail induced mar damage of coatings

Quantitative method for evaluating fingernail induced mardamage of coatings

Jeff Anderson, Matthew Brown, Charles Kan,

Kumar Nanjundiah, Vivek Kalihari

� American Coatings Association & Oil and Colour Chemists’ Association 2013

Abstract A traditional method of coating qualification,known as fingernail mar resistance, historically has beenpracticed in industry where the back of a fingernail is‘‘flicked’’ across a coating surface. Any resulting per-manent mark from this qualitative analysis is deemed afailure. Lack of quantitative analysis and control overexperimental parameters in this type of test limits itsutility as an analytical test. Fundamental understandingof this specific type of mar damage is hindered, andresearch efforts to find unique solutions based onchanges in polymer structure are frustrated by thebinary response of the test. In an attempt to rectify thesituation, a high speed mar tester was developed tomimic and quantify the effects of the fingernail flick test.The work described in this report demonstrates theapplicability of this tester to produce similar damage tothat of the fingernail test. In addition, a quantitativemethod enabling differentiation of the degree of mardamage of different coatings is described. Finally, themethod is demonstrated by differentiating mar resis-tance of coatings that show similar qualitative results infingernail flick testing.

Keywords Mar resistance, Coatings, Fingernailscratch

Introduction

A traditional method of coating qualification, known asfingernail mar resistance, historically has been prac-ticed in industry where the back of a fingernail is‘‘flicked’’ across the coating surface. Any resulting

permanent mark from this qualitative analysis isdeemed as a failure. Coating formulators will oftenuse additives, such as polydimethyl siloxane, toincrease the surface slip of the coating, improving thistype of mar resistance. During the process of develop-ing a new polymer for clear wood coatings, this type offingernail mar resistance was identified as a require-ment of the base polymer (no additives) for a segmentof the industry. Correlation of fingernail mar resistanceto fundamental polymer properties was quickly frus-trated by the lack of quantification of this simple andprevalent test. Further complication ensued in that theresults of the fingernail mar test showed significant lackof consistency when administered by an individual;further correlation between individuals was alsodeemed as extremely inaccurate. Literature searchesproduced a variety of descriptions of scratch andmar1–4 as well as time intensive methods using analyt-ical tools, such as profilometers and atomic forcemicroscopy, to quantify the results.2,4–7 Other commonand less time-intensive methods utilize different typesof abrasive media to create multiple fine scratches ordrag a loop shaped stylus to impinge damage to acoating surface. Evaluations of these types of tests didnot show any significant correlation to fingernailinduced mar and tended to produce damage to thecoating that was visually different in character. Scan-ning electron micrograph (SEM) images of fingernailmar of an experimental polymer show predominantlyductile failure along with tearing of the surface, asshown in Fig. 1. Evaluations of individual tests showedthat the speed of the fingernail being flicked across thesurface of a coating can range between 0.4 and 2 m/s aswell as a normal impact force that ranges from 20 to300 g, depending on the person. As is well known inthe literature both normal force and velocity of anindenter (as well as bulk polymer properties, indentergeometry, etc.) can affect the scratch behavior ofpolymers.8,9 Lack of control of both velocity and

J. Anderson (&), M. Brown, C. Kan, K. Nanjundiah,V. KalihariDow Coating Materials, The Dow Chemical Company,Midland, MI, USAe-mail: [email protected]

J. Coat. Technol. Res., 10 (4) 579–588, 2013

DOI 10.1007/s11998-013-9478-2

579

normal force as well as a simple method for quantifyingthe outcome resulted in an effort to design a tool thatmarred the surface of a coating in a similar andconsistent fashion to fingernail mar as well as acorresponding method for quantifying the impingeddamage. The work described in this report demon-strates (i) the applicability of this tester to producesimilar damage to that of the fingernail test, (ii) aquantitative method to differentiate the degree of mardamage, and (iii) the response of different polymercoating formulations.

Experimental procedures

The high speed mar tester, shown pictorially in Fig. 2,was designed to visually mimic the damage caused byfingernail mar at calculated velocities ranging from 0.3to 3.4 m/s in a consistent manner by controlling thespeed and normal impact force. The basic principle of

the mar test is similar in theory to the pendulumscratch tester described by Liang et al. which relies ona fixed pendulum that results in scratching the sub-strate at a range of velocities.10 The unique barreldesign of the stylus holder11 (Fig. 3) allows for acontrollable and constant normal force to be applied tothe coating by using a replaceable internal spring. Theforce of the spring allows the stylus to remain incontact with the surface of the coating while travelingacross the surface of the coating for a distance ofapproximately 3 cm, similar to the effect of the ‘‘finger-nail flick’’.

Stylus

The stylus selected for this study is a steel loop hookwith a 150 mm diameter and 200 mm width wrappedwith a strip of copper approximately 50 mm wide. Thecopper wrap has the result of creating a mar that is

Fig. 1: SEM images of fingernail damage on experimental resin system showing ductile failure and surface tearing


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generally similar in appearance to one created by afingernail when viewed under magnification. A pictureof the stylus used is shown in Fig. 4.

Velocity of stylus

Releasing the pendulum arm from different heightscorrelates to different stylus, and therefore contact,velocities. The resultant velocity of the stylus can becontrolled in the range from 0.46 to 3.44 m/s (Fig. 5).

Coating selection and preparation

The coatings selected for this study are based on fourcommercially available wood coating products. Threeof the products are marketed as waterborne floorcoatings with the final product being a solventbornefurniture lacquer. General information pertaining tothese coatings is shown in Table 1.

Glass microscope slides were cleaned with isopropanolbefore applying the coatings. Coatings were applied usinga 50 RDS wire wound rod to consistently apply 5 wet mils

Fig. 2: High speed mar tester

Fig. 3: Design of stylus holder

Fig. 4: Copper wrapped stylus

Fig. 5: Variable release height, maximum height on the leftand minnimum height on the right

Table 1: Selected coatings

Coating Manufacturer description

CTG-1 Water-based commercial wood floor coatingCTG-2 Polyurethane dispersion for coating floors, contains

nanoaluminaCTG-3 Oxidative crosslinking polyurethane, satin finish

floor coatingCTG-4 Solventborne furniture lacquer


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of each coating. The thickness of each dried coating wasmeasured and found to be consistent to ±0.1 mil to eachother, with an average dry film thickness of 1.2 mils. Allcoatings were cured under ambient conditions for 1 weekbefore testing. Due to the oxidative crosslinking of CTG-3and the continued hardening of the solventborne lacquer(CTG-4), all coatings were tested exclusively at the 7 daymark.

Surface profile

Surface profiles were measured using a DekTak 150profilometer with a 12.5 lm diameter stylus using a1 mg force and a typical scan rate of 300 lm/s. Surface

profiles were only measured on a few of the samplecoatings to evaluate and compare mar damage createdby fingernail and by the high speed mar tester.

Image capture

Images of the mar damage were captured using adigital microscope, AM7013MFIT, from Dino-Lite.Images were taken at 2509 magnification for dataanalysis. Images were approximately 1.6 mm wide and1.2 in height.

Image analysis

All images are analyzed using photo software in abatch process to measure the ‘‘whitening’’ or visualdamage of the coating surface by extracting an averagegrayscale value. The grayscale values range from 0(black) to 255 (white), where higher grayscale valuesimply increased damage to the coating. The programwas set to analyze each image in a 9 9 12 array whichprovided a dataset of 108 points for each image.Undamaged sections of each coating were also mea-sured using the image analysis software to calculate anaverage ‘‘blank’’ value for each coating. This isnecessary as each coating has the potential to providediffering background values based on clarity, gloss, oradditives. Figure 6 shows a screen shot of the 9 9 12array used to measure the grayscale of an image.

Visual analysis

Each mar created by the instrumented tester wasvisually assessed by eye, without magnification, andgiven a rating as follows: M = mar, whitening of

Fig. 6: 9 3 12 Array for image analysis, image is 1.6 mm 31.2 mm, CTG-1

0–0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

100 200 300

Width (µµm)

Profilometer scan of fingernail mar

Hei

gh

t (µ

m)

400 500 600

Fig. 7: Profilometer scan of fingernail mar on a coating, CTG-1


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surface, easily visible damage; S = no whitening ofsurface, damage difficult to see.

This visual evaluation was used as a guide toqualitatively evaluate the method.

Dynamic friction

Dynamic friction was measured on a Symyx tack andfriction station, where a steel ball (3/8† diameter) wasdragged over a coating at a fixed velocity and under aconstant normal load and the lateral friction force wasmeasured. The normal load was plotted on the x-axis andthe corresponding dynamic friction force was plotted onthe y-axis. The dynamic coefficient of friction (DCOF)was given by the slope of the curve. The higher numberon the y-axis represents higher drag force due to therelative motion between the two bodies in contact.

Evaluation of fingernail mar

Fingernail mar resistance was evaluated for each of thecoatings used during the method validation phase of thestudy. Numerous attempts were made to damage thecoating in as consistent a manner as possible (velocityand force), by flicking the back of the fingernail acrossthe coating. Quantitative evaluations were undertakenusing the same method described previously to arrive atan average grayscale value for each coating.

Results and discussion

Surface profile of mar and scratch damage

Figure 7 shows the effect of mar from a fingernailwhere it appears that the surface is not being gouged

0

–0.5

0

0.5

1

1.5

2

2.5

1000 2000 3000

Mar width (µµm)

Profilometer scan: damage by fingernail

Hei

gh

t (µ

m)

4000 5000 6000

Finger - Base

Wiped - Base

Fig. 8: Profilometer scan: effect of wiping fingernail mar, CTG-1

2000

-1

-0.5

0

0.5

1

1.5

2

2200 2400 2600 2800 3000

Width (µµm)

Profilometer scan: damage by mar tester

Hei

gh

t (µ

m)

3200 3400 3600 3800 4000

Raw angstrome

After wipe

Fig. 9: Profilometer scan: effect of wiping mar created by instrumented mar tester, CTG-1


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but instead, there appears to be an increase in height ofthe surface profile caused by either a deposition ofmaterial on the surface of the coating or, hypotheti-cally, cavitation due to tensile stress. The cause of theapparent increase in height is left for future studies.

Figure 8 shows the effect of wiping a fingernail-marred coating with a soft cloth. Prior to wiping, theprofilometer detected three regions of damage causedby flicking the back of the fingernail across the coating,with large peaks up to approximately 2.0 lm. Afterwiping the material the maximum height averaged out

to approximately 0.7 lm at the same location. It ishypothesized that either material was removed fromthe damaged coating or that the coating is compress-ible and the action of wiping pressed the coating backdown. Additional analysis is needed to conclusivelydetermine the cause and is left for future study. Afterwiping, there appeared to be some narrow deforma-tions that penetrated below the surface of the coating.

Similar effects can be seen in Fig. 9 where the mardamage is created by the mar tester and the profilemeasured before and after wiping with a soft cloth. The

Profilometer scanArea under profilometer

curveImage at 250×

Av. (Grayscale

–blank)(a)

2.4 × 106 12.9

2.9 × 106 16.9

5.5 × 106 18.4

5.3 × 106 22.9

3.8 × 106 31.6

0 200 400 600 800

Width (µµm)

Width (µm)

Width (µm)

Width (µm)

Width (µm)

Hei

gh

t (µ

m)

Profilometer scan: 0.30 m/s





1000 1200 1400

0 200 400 600 800 1000 1200 1400

0 200 400 600 800 1000 1200 1400

0 200 400 600 800 1000 1200 1400

0–1

0

1

2

3

4

Hei

gh

t (µ

m)

–1

0

1

2

3

4

Hei

gh

t (µ

m)

–1

0

1

2

3

4

Hei

gh

t (µ

m)

–1

0

1

2

3

4

Hei

gh

t (µ

m)

43.5

32.5

21.5

10.5

0-0.5

–1

200 400 600 800 1000 1200 1400

(b)

(c)

(d)

(e)

Fig. 10: (a–e) Profilometer scans of marred areas. Image capture and average grayscale as a function of increasing velocity(CTG-1), images are 1.6 mm wide by 1.2 mm in height


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damage created by the tester shows a maximum peakat �1.5 lm before being wiped while the scan at thesame location shows a maximum of �0.3 lm after thepanel was wiped with a soft cloth.

Figures 10a–10e show the profiles, images, andaverage grayscale of mar damage of CTG-1 at differentvelocities. The area under the profilometer scans showsan increasing trend with velocity to a maximum at1.46 M/s followed by a drop in area at higher speeds,not allowing for an easily understandable correlationof the results. Conversely, it is readily apparent thatthere is a trend in increasing damage with velocity byexamining the images as well as the measured averagegrayscale values. It is hypothesized that the velocity ofthe stylus could correlate to higher contact tempera-ture between the stylus and the coating due to friction.Previous work has shown that the velocity of scratchindenter, as well as the depth of the scratch, can greatlyaffect the appearance and type of failure of theresultant damage.8 If we assume that the stylus is incontact with the surface of the coating and notimpinging to a significant depth, it could be theorizedthat as the contact temperature, between the stylus andthe coating, approaches the Tg of the coating theresultant damage could be the result of ductile failureat the surface. It could then be further hypothesizedthat as the temperature increased the depth of thetemperature effect would also increase, potentiallyresulting in increased depth of mar damage andtherefore increased refraction of light. Correlation toTg as well as a more fundamental understanding of theresultant damage could be explored in future studies.

Visual evaluation

Visual evaluation was carried out by examining 100insults of the mar tester for all coatings at a range ofvelocities. Rankings were given as either ‘‘m’’ (forvisible mar/scratch of the coating) or ‘‘s’’ (no visibledamage). These evaluations were compared against theaverage measured grayscale, with a distribution shownin Fig. 11. In this distribution the ‘‘s’’ values arehighlighted on the right with the corresponding, approx-imately bell-shaped distribution, grayscale values high-lighted on the left. Figure 12 shows the distribution ofthe average grayscale for only the data qualitativelydetermined as no visible damage (s). The line at 12.5represents the 95% confidence interval and indicatesthat damage corresponding to a grayscale value of <12.5should not be easily visible to the human eye.

Linear modeling of grayscale values by velocity

Figure 13 shows the relatively linear response for allfour coatings by fitting average grayscale vs velocity.The linear constants and R2 values are displayed inTable 2. Examining the slopes of the resultant linesshows that the waterborne products have a similarresponse with the rate of damage with increasedvelocity, while the solventborne coating has a flatterslope. Beyond temperature effects or bulk polymerproperties, it could be hypothesized that the rate of

m

s

m = mar, s = no mar

0

5

10

15

20

25

30

35

Average grayscale

Fig. 11: Representative distribution of visual evaluation vsaverage grayscale—all coatings

Average grayscale (s)

15

12.5

10

7.5

5

2.5

0

Fig. 12: Distribution of no visible damage (‘‘s’’)—allcoatings

00

5

10

15

20

25

30

35

CTG-1CTG-2CTG-3CTG-4

0.5 1 1.5

Velocity (m/s)

Velocity vs. average grayscale by coating

Ave

rag

e g

rays

cale

2 2.5 3

Fig. 13: Linear fit of each coating, average grayscale byvelocity for each coating


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increase of mar damage by velocity may be related todynamic friction or film formation characteristics suchas film morphology or surface roughness.

From the placement of the representative linear fitsin Fig. 13, it can be summarized that CTG-2 will havethe best mar resistance across the velocity range testedand CTG-1 the poorest. Due to the flatter calculatedslope of CTG-4, it is predicted to have mar resistancesimilar to CTG-1 at low velocities and to CTG-3 athigher velocities. It is reasonable to assume that at zerodrop height there should be zero damage to thecoating. Forcing the intercept to zero for these equa-tions destroys the linearity seen between 0.6 and2.44 m/s, indicating that the region between 0 and0.6 m/s may not have the same linearity seen at highervelocities. The intercepts shown in Table 2 are onlyused as a tool to calculate the linear regime between0.6 and 2.44 m/s. For coating systems that have similarslopes, such as CTG-1, CTG-2 and CTG-3, the inter-cept can be used as a rating to indicate relativeresistance to mar damage, with smaller values indicat-ing enhanced mar resistance.

Dynamic friction

As previously mentioned, it was hypothesized that thedynamic coefficient of friction (DCOF) for the water-borne coatings might be similar to each other with thesolventborne coating having a lower DCOF, allowingfor the different rate of grayscale increase between thetwo classes of coatings. The plot in Fig. 14 demon-strates that CTG-4 and CTG-2 show the smallestDCOF, 0.30 and 0.34, respectively. CTG-3 shows aDCOF � 0.42, whereas the CTG-1 coating exhibits thehighest DCOF � 0.52. Qualitatively, one can say thatthe CTG-4 coating will offer the least friction resis-tance, whereas the CTG-1 coating will offer themaximum friction resistance. This lack of correlationbetween DCOF and the mar tester may be due to thedifferences in velocity of the two test apparatus as wellas any difference in stylus material. DCOF testing atvelocities comparable to the mar tester, or potentiallyacross a temperature range at constant velocity, wouldneed to be performed in order to show whether there isany correlation between DCOF and the high speedmar tester.

Comparison to fingernail mar

Producing consistent mar profiles by flicking thefingernail across the coating proved to be problematic.It was discovered that different individuals can have adifferent effect on the same coating. In this currentstudy it was also determined that the same person cancause differing degrees of damage to the same coating.Figures 15a–15d show examples of the differentiationin damage that can be caused using the fingernail.Figures 15a and 15b are extreme examples of differentlevels of fingernail mar on CTG-2. Figures 15c and 15dshow similar results for CTG-3. Some of the differ-ences can be attributed to the width of the damage forCTG-2 but the same cannot necessarily be said aboutCTG-3. Even with the narrower mar damage inFig. 15a, there is an obvious visual difference in thecharacter of the mar.

A final dataset of four repeats was achieved thatcontained the smallest standard deviation, as shown inTable 3. The average trend of the coatings for finger-nail mar resistance is as follows:

CTG-2[CTG-4[CTG-1[CTG-3

With CTG-2 consistently offering the best resistanceand CTG-3 the worst.

Conclusions

It has been demonstrated that the high speed martester produces a surface profile similar to that of onecreated by flicking the back of a fingernail across thesurface of a dried coating. Furthermore, a method hasbeen demonstrated that quantifies this mar as an effectof velocity under a set of controlled conditions.

Table 2: Linear equation for each coating, grayscale byvelocity

Coating Intercept Slope R2

CTG-1 6.08 9.25 0.88CTG-2 �4.93 10.43 0.94CTG-3 0.06 9.66 0.92CTG-4 8.23 6.83 0.93

400

10

20

30

40

50

60

70

80

90 CTG-4

CTG-3

CTG-1

CTG-2

60 80 100

Normal force

Dynamic friction

Late

ral f

orce

120 140

Fig. 14: Dynamic friction, normal vs lateral force—allcoatings


586

Correlation to fingernail mar proved to be difficult asthe fingernail method did not offer the same level ofdifferentiation or reproducibility as that of the highspeed mar tester. The high speed mar tester does allowdifferentiation between coatings and is anticipated tobe a useful new tool for the development of newproducts requiring superior scratch/mar resistance.Future work is expected to focus on attempting toalign this methodology with fundamental polymerproperties such as Tg or modulus as well as examining

SEM images to gain a more in-depth understanding ofthe resultant damage.

Acknowledgments The authors would like to thankMichael Johnson, Scott Bis and Andrew Swartz fortheir support of this work, and Thomas Boomgaard,Ray Drumright and Jay Romick for valuablediscussions and insights.

References

1. Guevin, Paul, R, Jr, ‘‘Hardness.’’ In: Koleske, JV (ed.) Paintand Coatings Testing Manual. Fourteenth Edition of theGardner-Sward Handbook, Chapter 48. ASTM, West Cons-hohocken, PA, 1995

2. Lin, L, Blackman, GS, Matheson, RR, ‘‘A New Approach toCharacterize Scratch and Mar Resistance of AutomotiveCoatings.’’ Prog. Org. Coat., 40 85–91 (2000)

Table 3: Grayscale values of fingernail damage

Coating Average grayscale Standard deviation

CTG-3 26.9 9.5CTG-1 21.1 5.8CTG-4 14.0 4.2CTG-2 6.2 1.8

CTG-2, Grayscale = 6.7 CTG-2, Grayscale = 35.4

CTG-3, Grayscale = 12.8 CTG-3, Grayscale = 44.3

(a) (b)

(c) (d)

Fig. 15: Select examples of mar damage caused by fingernail for each coating—images are 1.6 mm wide by 1.2 mm inheight


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3. Osterhold, M, Wagner, G, ‘‘Methods for Characterizing theMar Resistance.’’ Prog. Org. Coat., 45 365–371 (2002)

4. Shen, WC, Jiang, B, Jones, FN, ‘‘Measurement of MarResistance and Study of Marring Mechanism of PolymericCoatings with Scanning Probe Microscope.’’ J. Coat. Tech-nol., 72 85–95 (2000)

5. Shen, W, ‘‘Characterization of Mar/Scratch Resistance ofPolymeric Coatings: Part I.’’ JCT CoatingsTech, 3 (3) 54(2006)

6. Shen, W, ‘‘Characterization of Mar/Scratch Resistance ofPolymeric Coatings: Part II.’’ JCT CoatingsTech, 3 (4) 44(2006)

7. Osterhold, M, ‘‘Characterizing Physical Properties of Coat-ing Surfaces—Mar Resistance and Surface Structure.’’ Prog.Colloid Polym. Sci., 132 41–47 (2006)

8. Bonne, M, et al., ‘‘Characterisation of Surface AbrasionPhenomena on Poly(methylmethacrylate) Surfaces.’’ Wear,254 (1–2) 55–64 (2003)

9. Gauthier, C, Lafaye, S, Schirrer, R, ‘‘Elastic Recovery of aScratch in a Polymeric Surface: Experiments and Analysis.’’Tribol. Int., 34 469–479 (2001)

10. Liang, YN, et al., ‘‘Some Developments for Single-PassPendulum Scratching.’’ Wear, 199 66–73 (1996)

11. Nanjundiah et al., US Patent 61/635321, 2012


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Quantitative method for evaluating fingernail induced mar damage of coatings