1

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

7

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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