determination of metal adhesion strength of metallised films by … · 2018-10-08 · 1...
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Determination of Metal Adhesion Strength of Metallised Films by Peel Tests
Esra Kucukpinar1, Marius Jesdinszki1, Norbert Rodler1, Carolin Struller1, Klaus Noller1, David
Blondin2, Valerio Cassio2 and Horst-Christian Langowski1 1 Fraunhofer Institute for Process Engineering and Packaging, Giggenhauser Strasse 35, 85354
Freising, Germany 2 MET-LUX Vacuum Metallizing, B.P. 28, Site du P.E.D., L-4801 Rodange, Luxembourg
Abstract
One major quality indicator of metallized films is the metal adhesion strength between the inorganic
layer and the substrate. The weaknesses of ethylene-acrylic acid (EAA)-peel test for better adhering
Aluminum layers are shown. A mechanically stronger film than EAA is required for more accurate
measurements. There is still a need for further research on the improvement of available adhesion
tests relevant to packaging and packaging materials. The packaging industry would then be able to
prove superior adhesion and optimize their quality control.
Metallized (Vacuum Web Coated) Polymer Films for Packaging
The metallized (vacuum web coated) polymer films are an important and steadily growing sector in
the packaging industry. The worldwide metallized film production reached 2.4 x 1010 m2 / year and
about 85% of the metallized films produced are used in flexible packaging [1]. The metallized films
used for packaging applications are mostly used in laminate structures, which consist of a heat
sealable polymeric top layer (e.g. poly(ethylene)), laminated to the metallic layer using a suitable
adhesive (Figure 1). The metallization provides the polymeric substrate film several special
functionalities at a higher productivity with low-cost production such as: 1) Oxygen and water
vapor barrier, leading to a longer shelf life of the packed product, 2) Light barrier, 3) Decorative
characteristics.
Figure 1. Metallized film laminates for packaging or encapsulation
Sealable top layer (e.g. PE)
Adhesive to metal layer
Deposited metal layer (e.g. Aluminum)
Substrate film (e.g. PET)
2
One of the performance indicators of such metallized films is the adhesion strength between the
metallized layer and the polymeric substrate film [2]. Delamination between the layers in a
metalized film laminate structure might cause functional problems such as the reduction of the
barrier performance. Therefore, in most cases, metal adhesion problems can be the main reason for
disagreements between the customer and supplier in the process chain for production and usage of
metallized films and laminates [3]. The metallized film producers and their customers need a
reliable and an applicable method for the assessment of Aluminum metal adhesion before the
subsequent conversion processes.
Peel Test Procedures for Metal Adhesion
The peel test is widely used for the assessment of Aluminum metal adhesion on the polymeric
substrate, mainly for the quantitative evaluation of the metal adhesion. In most cases, ethylene
acrylic acid copolymer (EAA) film is heat sealed onto the metal layer and peeled off under defined
conditions using a tensile testing device [4].
Figure 2. Schematic representation of the 180o peel adhesion test [2] (edited)
Peel-off angle: 180o
Peel distance
Double-sided adhesive tape
Al-plate (1 mm thick)
Substrate film (e.g. PET)
Laminated-film: EAA (used in peel tests)
.
Metal layer
Peel direction
Fixed in the bottom clamp
3
Figure 2 is a schematic representation of the 180o EAA peel adhesion test. It has to be stressed here
that the peel strength measured by the peel test method is a combination of the basic adhesion and
other additional factors [5, 6]. These other factors such as intrinsic stresses, the elastic and plastic
deformations, the metallic layer width and thickness, the peel angle and rate, the failure mode
during the peel test, etc. contribute to the final measured peel forces [5, 7, 8]. In addition, EAA film
thickness and its thermal sealing conditions onto the metal layer have also a clear effect on the
results. Therefore, the forces measured by peel tests are only comparable, when everything else is
equal [9-11].
Weaknesses of EAA-peel Test
One of the important weaknesses of EAA-peel test is that the heat laminated EAA film has a low
cohesive strength. When a tensile force is applied on EAA, the measured tensile strength is low in
both the elastic and the plastic regions. This low strength of EAA becomes a major problem,
especially for the cases where the adhesion strength of the metal layer to the polymeric substrate is
high. In such cases, the EAA film starts to overstretch during the peel test, and therefore the
measured force is determined by the stretching action of the EAA.
The weaknesses of the EAA-peel test have been recently published by our group [2]. In this work,
we used three different metallised PET films with metal adhesion strengths at different levels (i.e.
low adhesion strength, medium adhesion strength, and high adhesion strength). Figure 3, 4 and 5 is
for the sample with the lowest, medium and highest metal adhesion strength, respectively. The
figures show the force recorded during the EAA-peel test versus measurement distance curves
obtained for some representative test specimens of each sample.
Even if it is possible to compare the metal adhesion strengths of the three samples qualitatively
(Sample A < Sample B << Sample-C) in this example, it is not possible to have a quantitative
comparison by using the EAA-peel test results. Due to the low yield strength of the EAA-film,
when the adhesion strength of the metal layer to the polymeric substrate is very high (as in the case
shown in Figure 5), the measured forces during an EAA-peel test is a superposition of the plastic
deformation of the EAA film as well as the delamination. The yield strength of EAA- film is low,
which makes it very vulnerable to plastic deformation.
4
0.0
0.5
1.0
1.5
2.0
0 20 40 60 80
Measurement distance (mm)
Fo
rce
(N/1
5 m
m)
A representative test specimen
Average of 10 specimens
Peak-1Peak-2
Peak-3
Peak-4
Figure 3. Sample A with a low metal adhesion strength (0.7 N/15 mm) – Successful EAA-peel test [2] (edited)
Figure 4. Sample B with a medium metal adhesion strength (partial delamination of Aluminum) – Problematic EAA-peel test [2] (edited)
Peak-2
Peak-3
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 20 40 60 80 100 120 140 160
Measurement distance (mm)
Fo
rce
(N/1
5 m
m)
Representative test specimen-1
Representative test specimen-2
Peak-4
(a)
„0.7“
(b) Representative test specimen
Peak-1
Peel distance: ≈ 30 mm
„3“
Peel distance: ≈ 41 mm
5
0
1
2
3
4
5
6
0 20 40 60 80 100 120 140 160
Measurement distance (mm)
Fo
rce
(N/1
5 m
m)
EAA-film (tensile force)
Representative specimens (peel test)
Figure 5. Sample C with a high metal adhesion strength (no delamination of Aluminum, only EAA-film stretching) – Problematic EAA-peel test [2] (edited).
Alternative Peel Test Aproaches
In this work further improvements of the EAA-peel test have been presented by following two
different approaches: (1) Instead of the soft EAA-film, heat sealable polymeric films with higher
mechanical strength were used. (2) Instead of the soft EAA-film, a sealing film was laminated on
top of the metallized layer with an adhesive. In approach (2), the laminated structure is similar to
the end product.
Approach (1): Amorphous, thermally sealable films such as amorphous polyamide (aPA) or
amorphous poly(ethylene terephthalate) (aPET), which are mechanically stronger than the EAA
film would be an alternative to the EAA-film in such kind of peel tests. As an example, Figure 6
shows the average forces measured by peeling the aPET-film as a function of the measurement
distance for the Samples A, B and C. For all the samples, full delamination of Aluminum was
achieved without any stretching, when aPET or aPA were used instead of the EAA-film. This
enables a quantitative comparison between the metal adhesion strengths of the three different
metallized PET samples. The differences in the force-strain curves of EAA, aPA and aPET films
(a)
Peel distance: ≈ 30 mm
Specimen-1
„4“ ? „5“ ?
Specimen-2
6
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 20 40 60 80
Measurement distance (mm)
Fo
rce
(N/1
5 m
m)
Average
Typical range
Laminate-A (III)
Laminate-B (III)
Laminate-C (III)
are shown in Figure 7. The measured yield stress of aPA film corresponds to a force of 17 N/15
mm, which is higher than that of the EAA-film (3.4 N/15 mm)
(a) (b)
Figure 6. (a) Metal adhesion strength of Samples A, B and C using aPET-film instead of EAA-film. Full delamination of Aluminum with a correct sequence (A < B < C), (b) Representative test specimens for Samples A, B, and C after the peel tests. [2] (edited).
Figure 7. Force measured as a function of tensile strain for EAA, aPA and aPET [2] (edited)
0
10
20
30
40
50
60
70
80
90
0 20 40 60 80 100 120 140
Tensile Strain (%)
Fo
rce
(N/1
5 m
m)
aPET
aPA
EAA
Representative test specimens
Peel distance: ≈ 42-44 mm
Sample C
Sample B
Sample A
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Approach (2): In the packaging industry, the metallized PET films are laminated to a heat sealable
film, which is usually a polyethylene (PE) film, via an adhesive-lamination process. In this laminate
structure, the metal adhesion strength can be assessed in a similar way as in the case of the EAA-
peel test by peeling the PE-film from the laminate in a tensile tester. We have shown that laminate
structures similar to those produced by an industrial scale lamination (pilot-lamination) process can
be produced by means of a bench scale lamination process (semi-automatic coating unit) for quality
control. The average peel forces measured with an adhesive laminated PE-film in comparison to
EAA-peel test measurements are shown in Figure 8 (a). The peel test results obtained with the pilot-
laminated samples and bench-laminated samples are in very good agreement. We have observed no-
stretching of the PE film during the peel tests, which makes a quantitative comparison possible
between Sample A and B. There was again no Al-delamination in Sample-C. This approach could
be a useful method for both metallizers and converters of the packaging industry, e.g. to understand
whether the adhesive lamination process has any consequence on the metal adhesion strength of the
metallized films. One prerequisite is the right selection of the lamination adhesive.
(a) (b)
Figure 8. (a) Peel forces measured with adhesive-laminated PE-LD film in comparison to EAA-peel tests, comparison between bench and pilot-lamination samples (b) Test specimens after thee peel tests.
Summary and Conclusions
There is still a lack of test methodologies for the measurement of metal adhesion on polymeric
substrates. Current measurement standards are largely incomplete. The available EAA-peel tests are
not always applicable by the metallizing companies due to the overstretching of the EAA-film
during the peel test. Due to the elongation of the EAA-film, a metallized film may give higher peel
forces during an EAA-peel test, but much weaker metal adhesion forces after its adhesive
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lamination to a sealing film. In most of the cases, the metallizing companies are not in a position to
measure the adhesion forces quantitatively, especially when the adhesion strength is high (above 3
N/15 mm). It has been shown in this work that mechanically stronger heat sealable films can be
more promising; however absolute values of the peel forces obtained in different peel tests cannot
be directly compared.
Peel test results of laminates are influenced by several factors, which are usually not recorded
during the measurements such as peel distance, peel-off angle, and thickness, elastic, viscoeleastic
properties of the peeled material, its sealing procedure and the peel-off angle. The peel test results
do not show any correlation to the practical load conditions. A reliable and applicable methodology
is still required for the quality control of the individual production steps along the process chain of a
metallized film laminate structure, where polymeric film producers, metallizers and converters are
involved. There is still a considerable amount of work to be done about development of adhesion
test methods relevant to packaging and packaging materials.
Acknowledgements
The authors thank Mr. Wolfgang Busch from the Materials Development Department of the
Fraunhofer Institute for Process Engineering and Packaging (IVV) for the laminations performed in
this work and the schematic figures in this paper, Ms. Verena Jost and Ms. Birgit Hillebrand for the
peel tests and preparation of the test specimens, Dr. Cornelia Stramm and Dr. Klaus Noller for the
useful discussions in the course of this study.
References
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