Application Report
Application report:

AR280

Industry section:

AutomotiveAuthor:

Daniel FreseDate:

10/2016

Mobile Surface Analyzer – MSA

Method:Keywords:

Surface modification, flame treatment, fiber-reinforced polymer, surface free energy

Optimizing flame treatment of polymer surfaces
Mobile contact angle measurements on glass fiber reinforced composite materials forautomobile interiors
Polymer-based composite materials are increasingly being used in automobile construction to reduce weight. As a rule, itis necessary to treat the surface of these materials to achieve stable adhesion and coating. Using body trim parts for theautomobile interior as an example, we show how flame treatment can be accurately followed using mobile contact angle
measurements.
One of our customers observed an unacceptably high reject rate after applying a decorative film – the film became
increasingly detached at certain points. Test inks showed no local differences. Within a few hours, various parameters of
the flame treatment process on site were varied and the surface free energy was determined at different points. The SFE
was calculated based on contact angle measurements at the treated surface with the help of the Mobile Surface Analyzer– MSA. This showed that one side of the workpieces was systematically underactivated, which was presumably due to anincreased distance between flame and composite surface.

Background
Polymer and composite materials in theautomobile industry

Fig. 1: Automobile interior. Basic components of a dashboard cover as seen here were measured as part of thisinvestigation.

New composite materials based on polymers areincreasingly being used in the automobile sector as analternative to materials such as steel, aluminum andglass. The main reason for this is a drastic reduction inweight and consequently an accompanying reduction infuel consumption and CO 2 emissions. In order to satisfy
the high quality and safety standards in the automobileindustry, such materials must be reliably coated andstably bonded. But this is exactly where problems such as
unstable adhesive bonds or detachment of paints andfilms arise in practice.

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Surface free energy and its contribution toadhesion behavior
The reason for such problems is the typically low surface
free energy (SFE) of polymers, which impedes thespreading of liquid coatings and a physical-chemicalbond between solid and liquid. The surface of polymer
materials must therefore first be modified, thusincreasing the SFE. In this way, physical effects which
determine the adhesion behavior of coatings andadhesives (e.g. spreading coefficient, work of adhesion,interfacial tension) are optimized. Available techniques
for modifying polymer surfaces include flame treatment,
plasma, corona or chemical treatment. A change in theSFE after treatment can be quantified by contact angle
measurements of at least two liquids using the Owens,Wendt, Rabel and Kaelble (OWRK) model [1].

deposited on the surface of the composite part and thedrop shape was subsequently analyzed. Measurementson the untreated and flame-treated PP are shown inFig. 2 by way of example.

Flame treatment of glass fiber reinforced
polypropylene
A supplier of glass fiber reinforced polypropylene parts(GFR-PP) for automobile interiors contacted us andreported problems with the adhesion of decorative filmon dashboard covers. The covers were modified bymeans of flame treatment before applying the film.
According to subsequent determination of the SFE withtest inks, the effect of the treatment was apparently
satisfactory. However, in fact, the decorative film became
detached at certain points in an unacceptable number of
the activated parts. It was impossible to detect any
differences between these problem areas and otherflame-treated areas using test inks.

Contact angle measurement as a valid test
methodOur answer to the supplier’s unusable experiments withtest inks was to measure the contact angle. While test
inks provide no information on polar and disperse
fractions of the SFE, contact angle measurements enable
these to be differentiated and therefore concrete physicalparameters, such as the adhesion energy or theinterfacial tension, to be calculated. These can therefore
be specifically optimized, thus enabling maximum
adhesion between adhesive or coating and surface to beachieved. A comparison of test inks and contact angle
measurements is available in the form of an application
report [2] and is described in more detail in [3].

Fig. 2: Contact angle measurements of water (left) anddiiodomethane (DIM; right) on untreated (A) and flametreated (B) polypropylene. SFE values calculated inaccordance with OWRK.

The SFE values were calculated in accordance with OWRK
using the parameters listed in Table 1.
Tab. 1: Surface tension (σ) with polar (p) and disperse (d)fractions of the test liquids used.
Test liquiddp

σ [mN/m]

σ [mN/m]

σ [mN/m]

Water

72.8

21.8

51.0

Diiodomethane

50.8

50.8

0.0

Screening of the flame-treatment parameters was carriedout based on contact angle measurements. For thispurpose, three dashboard covers were in each casetreated with the same parameters (gas flow, gas/air ratio,etc.). The SFE with polar and disperse fractions was thendetermined at eight different spots on each sample(Fig. 3) and the next three samples were then flametreated and measured under different conditions. In total,
24 parts were flame-treated under eight different
conditions and more than 380 contact angle
measurements were carried out with the mobile
instrument and subsequently evaluated on site in only
about two hours.

Experimental part
Contact angle measurements were carried out on site at
the customer’s premises in a production hall using thefully automatic Mobile Surface Analyzer – MSA. The
measurements were non-destructive. With the MSA, it is
not necessary to cut up the large, non-flat parts or totransfer the samples to a test laboratory. During themeasurement, two test liquids (water as the polar anddiiodomethane as the non-polar, disperse liquid) wereFig. 3: Schematic representation of a sample dashboard
cover. In each case, measurements were carried out at eightspots, which are marked accordingly here.

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Results
The SFE values with polar and disperse fractions
determined on the basis of the contact angle
measurements for all eight areas investigated are shownby way of example in Fig. 4 for sample 11.

This finding provides a good explanation for thecustomer’s observation that a subsequently applied
decorative film exhibited adhesion problems, particularlyin this area. One solution for the inadequate modificationof the right-hand side of the sample is to adjust thedistance between flame and sample in this area.
Furthermore, an optimum for the modified flametreatment parameters can be quickly found by looking at
the SFE values of the differently treated parts. In eachcase, three samples were flame treated under the sameconditions. Samples No. 10 to 12 exhibit the greatest
increase in SFE, in particular the polar fractions, as aresult of which the parameters used here signify theoptimum conditions for treatment with good subsequentadhesion.

SummaryFig. 4: SFE values with polar and disperse fractions forsample 11.

While the regions 1-4 have comparably high SFE values
and at the same time, in particular, high polar fractions
due to the flame treatment, the values for the regions 58 are considerably lower. Accordingly, the left-hand sideof the sample is well activated and provides adequate
interaction for a stable bond in a subsequent coatingprocess. On the other hand, the right-hand side with thespots 5-8 exhibits only a small change in SFE values
compared with an untreated surface (cf. Fig. 2).
A glance at the complete overview of the calculated SFE
values for all samples and measuring regions
substantiates this finding (Fig. 5). One side of the flametreated parts (spots 5-8) was evidently systematicallyunderactivated.

The treatment of polymer composite materials is
indispensable for stable coating. However, while test inks
or measurements with only one test liquid are unable toprovide information on the polarity of the surface andresults are highly user-dependent, mobile measurementsof the surface free energy with the MSA offer a reliable,
quantitative measurement of the surface modification.
This was demonstrated by way of example with the flame
treatment of dashboard covers made from glass fiber
reinforced polypropylene. Mobile contact angle
measurements showed an unequal activation along thesurface. Screening of eight different flame-treatment
settings enabled the parameters to be quickly optimized.
A two-hour, non-destructive measurement directly onsite enabled the rejection rate to be reduced quickly andefficiently.

Bibliography[1]D. K. Owens, R. C. Wendt, “Estimation of the Surface
Free Energy of Polymers”, J Appl Polym Sci 13, 1741
(1969).

[2]

T. Willers, M. Jin, AR 272 - “Why test inks cannot tell
the whole truth about surface free energy” (2014),
www.kruss.de[3]

K. L. Mittal, “Advances in contact angle, wettability
and adhesion”, Vol. 2, Wiley-Scrivener, ISBN 9781119116981 (2015).

You can find many more interesting Application Reports
on our website underhttps://www.kruss.de/services/educationtheory/literature/application-reports/

Fig. 5: SFE of all eight investigated spots on all 24
investigated samples.
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