Application Report
Application report:

AR 272

Industry section:

Surface Treatment for Polymers

Author:TW, MJI

Date:

06/2014

Method:Keywords:

Mobile Surface Analyzer –MSA

Drop Shape Analyzer –DSA100contact angle, surface free energy, surface treatment, surface cleaning

Why test inks cannot tell the full truth about surface free energy
A comparative study between test inks and contact angle measurements in our laboratory
There are a range of methods for activating the surface when treating materials before coating, bonding or printing.
These include thermal or electrical methods, such as plasma, flame or corona treatment, and chemical treatment withoxidizing gases. Equally important are cleaning steps which remove hydrophobic substances from the surface. All thesemethods increase the surface free energy (SFE) and therefore improve wettability and adhesion.
Norms such as DIN 55660 for coating materials and DIN EN 828 for adhesive processes specify contact angle
measurement as the method for determining the SFE of surfaces and for checking an activating or cleaning pre-treatment
process. In addition, test inks, which are intended to reflect the SFE based on liquids with set surface tension (SFT), arealso frequently used.
According to the ink test method, complete wetting always occurs when the values of the SFE of the solid and the SFT ofthe liquid are equal. Many scientific authors have refuted this wetting theory and have shown that only an analysis of thepolar and dispersive interaction fractions of the SFE and the SFT provide a complete picture of the wetting process.
Contact angle measurements, which take these interactions into account, and ink tests should accordingly lead todifferent evaluations of surfaces and therefore also to different assessments of the quality of a pre-treatment process.
In the present study, we have compared SFE results from contact angle measurements with those from ink tests for 13
very different materials. We also carried out comparative measurements on three plasma-treated plastics. Both the small
number of consistent results and the large number of deviations can be conclusively explained when we take the effect ofpolar and dispersive interactions into account. It appears that the SFE result of an ink test must be called into question formany samples. In this article, we also point out some advantages of the contact angle method in measurement practice.

KRÜSS GmbH | Borsteler Chaussee 85 | 22453 Hamburg | Germany | www.kruss.de

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Background
Untreated plastics and materials with hydrophobic
contamination have a low SFE and low polarity. These
characteristics lead to poor wetting and adhesion,
particularly in contact with water-based varnishes andprinting inks as well as adhesives and coating substances.
A cleaning or activating pre-treatment process is therefore necessary for polymers and many other materials.
Established activation processes, such as plasma or flame
treatment, increase the SFE and therefore improve theprocessing characteristics. Contact angle measurement
and the ink test are two frequently used methods fordetermining the SFE and therefore for checking the needfor or the success of a pre-treatment process.

The test ink method
The ink test is based on the assumption that the SFE of
the solid is equal to the SFT of a liquid which just fully
wets the solid. Test inks consist of a series of liquid
mixtures, each with a set SFT, usually in increments of
2 mN/m. During the test, one of the inks is applied to thesample with a brush stroke. If the ink contracts, the ink
with the next lower SFT is used until the brush stroke
produces a stable film. This corresponds to complete
wetting of the sample. When the first stroke is stable, theSFT is increased in steps. The SFT of the test ink whichjust forms a film is equated to the SFE of the material.

The contact angle method
With a contact angle measurement, a drop of a pure test
liquid is dispensed onto the solid sample. The drop is
recorded using a camera, and the video image isevaluated.

Fig. 1: Evaluating the drop image to measure the contact
angle

The contact angle θ is measured at the point where thecontour line of the drop cuts the surface line. The betterthe wetting, the smaller this angle is; for complete
wetting, it is 0°.

Fig. 2: Variables affecting the formation of contact angle
according to Young's equation

Carrying out a contact angle measurement with several
characterized test liquids enables both the SFE and the
IFT to be calculated based on different models. Most ofthese models start from the polar and dispersive
fractions of the liquid SFT and the solid SFE.
The model according to Owens, Wendt, Rabel andKaelble (OWRK) (see bibliography) is frequently used in
the field of plastics analysis, and we therefore also use
this method for the present study.

Experimental part
Samples under investigation
To make the study as broad as possible, we haveinvestigated 13 materials including plastics, minerals andmetals. We also chose very different material qualities,
from almost ideally chemically homogenous surfaces andtechnical raw materials to finished commercial products.
With this selection we have covered a broad spectrum of
possible surface-analytical problems when comparing
methods.
SampleQuality

Designation

Acrylonitrile
butadiene styrene

Technical

ABS

Aluminum

Finished product(foil)

Aluminum

Glass

Finished product(microscope slide)

Glass

Mica

Chemicallyhomogenous monocrystal, freshly
cleavedMica

Polyamide 6(Akulon® K222-D)

Technical

PA6

Polyamide 6 + 3%
Cloisite® 30B

Technical

PA6 + 3% C30

Polydimethylsiloxane

Pure, synthesized

PDMS

Polyethylene

Finished product(film)PE

Polyethylene
terephthalate

Finished product(bottle)PET

According to Young’s equation, which is proven sinceover 200 years, there is a fundamental relationshipbetween the contact angle θ, the SFE of the solid σ s , theSFT of the liquid σ l and the interfacial tension (IFT) σ ls
between liquid and solid:σ σ = σ lσ + σ l coσθ(Equation 1)KRÜSS GmbH | Borsteler Chaussee 85 | 22453 Hamburg | Germany | www.kruss.de

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SampleQuality

Designation

Polypropylene

Technical

PP

PolytetrafluoroethyleneTechnical

PTFE

Polyvinyl chloride

Finished product(tablecloth)

PVC

Silicon wafer

Chemicallyhomogenous
monocrystal

Si

Tab. 1: Samples under investigation

The change in SFE during plasma treatment was also
investigated for PDMS, PVC and PET.

Conditions for plasma treatment
We used a Rheinhausen Plasma piezobrush® plasma pen
for plasma-treating PDMS, PVC and PET. The pen was
positioned above the DSA100's automatic sample table
at a distance of 8 mm from the sample. The axis speed
was set to 2 mm/sec. As soon as the treatment began,
the axis was moved over a distance of 40 mm and thenback again so that the total exposure time was 40 s.
Treatment with an exposure time of 60 s was also carriedout for PDMS and PVC; in this case, the axis moved thesample under the pen one more time.

Sample preparation
Before the measurements were carried out, all samples(except mica) were cleaned with a degreasing detergent
solution and then rinsed very thoroughly with hot andcold mains water and finally with distilled water.
Remaining water drops were subsequently removed withisopropanol. The samples were finally dried with clean
compressed air.

Test inks used
We carried out tests with two different series of test inks
for all samples:

Series A (yellow): Colored mixture of ethanol and
water



Series B (blue): Colored mixture of formamide andethylene glycol monomethyl ether (EGMM)

Fig. 3: Setup for the standardized plasma treatment of two
samples

The tests were carried out in accordance with thestandard ASTM D 2578-84.

Results and interpretation

Inks with SFT values in 2 mN/m increments in the range
between 30 and 56 mN/m were used for each test. SFE
values above 56 mN/m and below 30 mN/m could
therefore not be measured.

The following table contains the SFE results from thecontact angle measurements and the ink tests for thematerials which were not plasma-treated:

Measurements on untreated samples

SFE by OWRK from
SampleConditions for contact angle measurement
The contact angle measurements were carried out using
our Drop Shape Analyzer – DSA100. A softwarecontrolled dosing system for several liquids was used forthe dosing operation.
We chose the three standard test liquids water,
diiodomethane and ethylene glycol for determining theSFE. This choice covers the bandwidth from purely
dispersive to highly polar and is therefore particularly
suitable for determining polar and dispersive fractions. In
quality control, measurements are often carried out withthe two test liquids water and diiodomethane which
represent two extremes in respect to polarity.

contact angles[mN/m, pol. fract. in %]Test ink A[mN/m]

Test ink B[mN/m]

ABS

37.1 [16%]

34

34

Al

55.9 [46%]

≥56

≥56

Glass

64.6 [50%]

>56

>56

Mica

53.7 [24%]

»56

»56

PA 6

50.6 [19%]

≥56

≥56

PA 6+3% C30

52 [24%]

≥56

≥56

PDMS

21.6 [0%]

«30

«30

Static contact angles were measured (measurement atconstant drop volume). The drops were produced at theneedle of the dosing unit, carefully deposited on thesample and subsequently analyzed. There were no timedependent changes in the contact angle for any of thesamples.
KRÜSS GmbH | Borsteler Chaussee 85 | 22453 Hamburg | Germany | www.kruss.de

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SFE by OWRK from
Samplecontact angles[mN/m, pol. fract. in %]Test ink A[mN/m]

Test ink B[mN/m]

PE

32.2 [0%]

30

30

PET

44.2 [3%]

34

34

PP

29.6 [0%]

≤30

30

PTFE

16.1 [0%]

«30

«30

PVC

47.1 [3%]

30

32

Si

48.5 [39%]46

42

Tab. 2: Results of comparative measurements between
contact angles and ink tests for all untreated samples

In the case of the two very low-energy polymers, PDMS
and PTFE, the SFE values were less than 30 mN/m. In
principle, this measuring range cannot be measured withcommercially available test inks (starting from 28 or
30 mN/m). We were unable to draw any conclusion
regarding the correlation for glass, as the value of
64.6 mN/m measured by means of contact angles wasabove the range of the test inks used.
The results for the two samples, PE and PP, marked green
in the table corresponded well. With all other samples, inmany cases there were considerable differences between
the contact angle and test ink results, and also
sometimes differences between the test inks.

Interpretation based on interfacial tension
It can be seen that the two samples with goodcorrespondence have low energy and are non-polar,
while the deviations occur with higher-energy, polar
samples. This can be explained by the fact that the ink
test ignores a variable which is important for wetting,namely the IFT (see Fig. 2). According to the test ink
method, complete wetting occurs whenever the SFE isequal to the SFT of the liquid. This is the case when thevectors σ s and σ l in Fig. 2 are of equal length. The IFT σ ls
is not taken into account. In fact, the IFT can be greaterthan zero even when σ s and σ l are equal.

Fig. 4: Different wetting in spite of equality between SFE and
SFT. The smaller the contact angle, the smaller is the IFT.

Accordingly, incomplete wetting can occur even when
SFE and SFT are equal; in fact, this is very likely.
The IFT depends on the interactions between solid andliquid. In the absence of any attractive interactions
between liquid and solid the liquid tends to minimize its
interface with the solid, i.e. it avoids wetting the solid.
Accordingly, the IFT between solid and liquid is
maximized in the absence of interactions and decreases
with increasing interactions between solid and liquid.
Polar and dispersive contributions to SFE or SFT originate
from physically different interaction processes. As aresult, all common theories to determine the IFT take thepolar (index p) and dispersive (index d) contributions of
SFT and SFE into account:

σ total = σ p + σ d (Equation 2)

This is done such that the polar parts of the solids’ SFE
interact solely with polar parts of the liquids’ SFT anddispersive parts of the solids’ SFE interact solely with thedispersive parts of the liquids’ SFT.

The following diagrams use Young's equation to illustrate
the possibility of different wetting in cases where the SFE
and SFT are equal in each case.

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The following figure illustrates this behavior.

Fig. 5: Schematic diagram of the interactions for the contact
between phases (top: complete wetting; bottom: incomplete
wetting). IFT and work of adhesion were calculated
according to OWRKThe figure shows a liquid with a total SFT of 50 mN/mgetting into contact with a solid with an SFE of also
50 mN/m. Thus, following the idea of test inks, the liquidshould wet the solid completely.
The large hands represent the (often stronger) polar
parts and the small hands the (often weaker) dispersive
parts of SFE and SFT, respectively. The upper panel showsa perfect match between the polar and dispersive parts:
every polar hand of the solid grabs a polar hand of theliquid and every dispersive hand finds its dispersive
partner. The attracting interaction between solid andliquid is maximized and the interfacial tension vanishes.
In this case of a vanishing interfacial tension, the contact
angle becomes zero as illustrated in the lower panel ofFig. 4. However, in practice such a situation can indeed
almost exclusively be found if totally dispersive liquids
get in contact with totally dispersive solids.

The lower panel of the figure shows the case in which the
distribution into polar and dispersive parts of SFE andSFT are different. In this example, there is only one pair oflarge polar hands and small dispersive hands grabbing
each other, respectively. As a consequence, there is aremaining interfacial tension in the liquid-solid bond.
This remaining interfacial tension results in a nonvanishing contact angle as illustrated in the top andmiddle panel in Fig. 4. Despite SFE and SFT being equal,
the liquid does not fully wet the solid as expected when
following the test ink idea. This is exactly the case when atest ink is applied on a solid with a different degree ofpolarity.
In another study we have determined the polar anddispersive parts of the test inks used for the present
work. We found that the test inks with an SFT of
30 mN/m are almost completely non-polar and that thepolarity increases with the SFT value. Knowing that, it canbe understood why only for PP and PE the SFE values
detected with the test inks agree with that determinedwith contact angle measurements. For these samples, inaddition to SFE and OFS being equal, also the polarities
are the same (polar part of 0 mN/m). Thus, according to
the wetting theories, the IFT vanishes and the test inks
fully wet the solid, i.e. exhibit a contact angle of zero
degrees. For all other possible combinations of test inks
and solids in which SFE and SFT are equal, the polarities
are different so that the wetting test with the test inks
cannot give the solid’s SFE. Moreover, the polarities forthe blue and yellow test inks having the same SFT value
are different. This explains why for example for Si even
the SFEs determined with the two different test inks weredifferent.

KRÜSS GmbH | Borsteler Chaussee 85 | 22453 Hamburg | Germany | www.kruss.de

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Measurement on plasma-treated samples
The difference between contact angle measurements and test inks was most obvious in the case of investigations carriedout on plasma-treated samples. The following tables 3a), b) and c) contain the results for PDMS, PVC and PET before andafter treatment with different exposure times. We have documented the wetting patterns for the test inks.
a) PDMS
DurationSFE by OWRK
from contact angles

Test ink A

Test ink B

Total: < 30 mN/m

Total: < 30 mN/m

Total: < 30 mN/m

Total: < 30 mN/m

Total: 32 mN/m

Total: < 30 mN/m

Total: 21.6 mN/m0s

Polar: 0.0 mN/m
Dispersive:
21.6 mN/m
Total: 40.3 mN/m

40 s

Polar: 14.2 mN/m
Dispersive:
26.1 mN/m

Total: 67.9 mN/m
60 s

Polar: 42.3 mN/m
Dispersive:
25.5 mN/mb) PVC
DurationSFE by OWRK
from contact angles

Test ink A

Test ink B

Total: 30 mN/m

Total: 32 mN/m

Total: 34 mN/m

Total: 34 mN/m

Total: 36 mN/m

Total: 38 mN/m

Total: 47.1 mN/m0s

Polar: 1.3 mN/m
Dispersive: 45.7 mN/m

Total: 54.1 mN/m
40 s

Polar: 6.4 mN/m
Dispersive: 47.7 mN/m

Total: 61.7 mN/m
60 s

Polar: 14.25 mN/m
Dispersive:
47.44 mN/mKRÜSS GmbH | Borsteler Chaussee 85 | 22453 Hamburg | Germany | www.kruss.de

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DurationSFE by OWRK
from contact angles

Test ink A

Test ink B

Total: 34 mN/m

Total: 34 mN/m

Total: 54 mN/m

Total: 54 mN/m

Total: 47.2 mN/m0s

Polar: 1.6 mN/m
Dispersive: 45.6 mN/m

Total: 70.2 mN/m
40 s

Polar: 26.1 mN/m
Dispersive: 44.1 mN/mTab. 3 a-c: Results of comparative measurements between contact angles and ink tests
for three untreated and plasma-treated plastics

With methods such as plasma or corona treatment, polargroups are introduced into the surface structure due tooxidation. The change in SFE is therefore based
substantially on the increase in the polar fraction. Thiseffect of treatment is shown throughout in theevaluations of contact angle according to OWRK. Thevery significant increase in SFE due to plasma treatment
comes about almost entirely as a result of the increase inpolar fraction.
The test inks do not reflect the increase in SFE for PDMS
and PVC. With these two plastics, the test inks are to acertain extent blind to the effect of treatment.
In the case of PET, the effect of pre-treatment is visible toa greater extent in the ink test, but here too the values
lead to an under-estimation of the effect of plasma
treatment.

Advantages of contact angle measurement inpractice
As well as the reliable measurement of SFE in the overall
polarity spectrum of solid materials, there is also a seriesof other advantages compared with the test ink method.
For example, additional information can be gained fromthe contact angle measurement if the wetting liquid isalso characterized. Calculating the work of adhesion (seeFig. 5) and the IFT provides insight into the quality of aninterfacial contact which cannot be obtained from the ink
test. The work of adhesion describes the force with which
the two phases bond to one another. The IFT refers to
the inherent tension of an adhesive bond or coating.
When the IFT is high, unwanted detachment of thephases from one another occurs more easily, for example
due to the penetration of moisture.
Some test inks, in particular the range mixed inaccordance with ISO 8296, contain the toxic liquids
formamide and EGMM. As a result, the health of test
personnel can be endangered during routine quality
assurance. Contact angle measurements can be carriedout with harmless liquids or with liquids which are lesshazardous to health. In addition, significantly smaller
amounts of the substance are required for themeasurement than with the ink test.
Test inks on an ethanol basis, such as test ink A in ourstudy, are also available as less harmful substitutes. In
this case, however, the volatile alcohol content canpartially evaporate, so that the composition of the inkchanges if bottles are left open or are opened frequently.
For the same reason, the wetting behavior can change
when the ink is applied. This disadvantage is more
serious at higher temperatures.
A further advantage of the contact angle method is thatthe test media do not come into direct contact with thesample. As a result, the test liquids cannot be
contaminated by the sample. In the case of a test ink, thepen is brushed over the sample and, after the test, cancontaminate the whole ink supply in the bottle.
Many samples only offer a small area for carrying out awetting test. However, test inks require a relatively large
amount of space for applying several lines which must
not run into one another. On the other hand, the SFE canbe measured on a few square centimeters by means ofcontact angles.
Until the development of fast and manageable contact
angle measuring instruments, test inks offeredadvantages for testing surfaces directly on site. With our
Mobile Surface Analyzer - MSA, the measurement of SFE
is just as portable and even faster than an ink test.
In addition, with the ink test different interpretation of
the wetting behavior is possible so that the results
depend on the person carrying out the test. With the
MSA, the result is independent from subjective
assessment due to computerized automation of themeasurement and evaluation.

KRÜSS GmbH | Borsteler Chaussee 85 | 22453 Hamburg | Germany | www.kruss.de

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Summary
A comparative study of contact angle measurements andink tests on 13 materials identified major differences in
the SFE values in many cases. A good correspondence
was only obtained for two low-energy, non-polar plastics.
We were able to explain the differences with reference to
the OWRK wetting model. For good wetting, this requires
a compatibility of the phases with regard to the polarand dispersive infractions. This is not a prerequisite of the
ink test. It is highly probable that an ink, whose total SFT
is equal in value to the SFE of the solid, has a different
polarity and does not completely wet the sample.
In comparative measurements on three plasma-treatedplastics, the effect of treatment on two of the materials
could only be detected by means of the contact angle
and not by means of the ink tests. In the case of the third
plastic, the results of the contact angle evaluation weresignificantly higher than those of the ink tests.
As well as the systematic advantages of contact angle
measurement based on sophisticated theory, we havealso highlighted some benefits of the measuring method
in practice. These include the reduced health hazard due
to harmless substances and smaller quantities comparedwith the ink test.

The non-contact process compared with the ink test
avoids possible contamination of the liquids. The small
test area required for contact angle measurement andthe possibility of carrying out measurements at higher
temperatures are also beneficial. Former advantages of
the ink test, such as mobility and speed, cease to exist
thanks to the use of new, handy instruments such as the
Mobile Surface Analyzer – MSA. A further advantage ofthe MSA lies in the completely automated measurement
which is independent from an assessment by the user.

Bibliography
D. H. Kaelble, Dispersion-Polar Surface Tension
Properties of Organic Solids. In: J. Adhesion 2 (1970),
P. 66-81.
D. Owens; R. Wendt, Estimation of the Surface Free
Energy of Polymers. In: J. Appl. Polym. Sci 13 (1969),
P. 1741-1747.
W. Rabel, Einige Aspekte der Benetzungstheorie und ihre
Anwendung auf die Untersuchung und Veränderung der
Oberflächeneigenschaften von Polymeren. In: Farbe undLack 77, 10 (1971), P. 997-1005.

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