Film Testing

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Film testing

General

Various experimental methods of polymer testing and diagnostics can be used to evaluate the mechanical and fracture mechanical properties of plastic films. In addition to the basic tensile test according to ISO 527-3 and, as a special feature, peel tests, the methods described in more detail below are used to obtain information, in particular on the toughness properties of the materials under investigation.

Conventional impact and notched tensile impact test

The aim of the conventional impact tensile test according to ISO 8256 ‘Plastics – Determination of impact strength’ is to examine the behaviour of test specimens under relatively high impact velocity and to evaluate the toughness or brittleness (see: fracture types) of films.

In principle, the tensile impact test (using unnotched test specimens) and notched tensile impact test (using notched test specimens) are particularly suitable for testing materials for which impact and notched impact tests according to Charpy (3-point bending arrangement) are unsuitable due to the nature of the test specimens (thickness, flexibility). Very thin test specimens, e.g. those made of film or very flexible test specimens (elastomers), can therefore be subjected to impact stress (see also: impact loading plastics and impact loading free-falling dart test) and their toughness properties can thus be evaluated under impacting loading conditions.

The test is carried out in the impact and notched tensile impact test at a relatively high test speed with the aid of specially designed pendulum impact tester. The method is suitable for test specimens made from moulding compounds, semi-finished products or moulded parts and is used, among other things, for production and quality control. With the conventional impact and notched tensile impact test, it is also possible to determine the mechanical anisotropy behaviour by taking test specimens from test plates or components in different directions and testing them.

For conducting such experiments, Polymer Service GmbH Merseburg has pendulum impact testers (Zwick HIT5.5P and HIT25P as well as Ceast Resil Impactor Junior) with the additional equipment required for impact tensile tests, such as pendulum hammers of various masses and clamping devices. The dimensions of the test specimens for determining the notched impact tensile strength a_tN are: Length L = 80 mm, width W = 10 mm, notch depth a of the notches on both sides 2 mm each. The test specimens for determining the impact tensile strength a_tU are preferably shoulder bars with a length L = 80 mm, length of the parallel part of the web l₀ = 10 mm, web width 10 mm and shoulder width 15 mm.

To perform impact and notched tensile impact tests, the test specimens are fixed within the test device by means of a fixed clamping device on one side and a cross-head on the other (see Fig. 1). After the pendulum hammer is released from its swing position, the test specimens are loaded in the longitudinal direction until fracture. The result of the experiment is the corrected impact energy E_c, which is used to determine the conventional notched tensile-impact strength a_tN or impact tensile strength a_tU.

Fig. 1:

Schematic representation of the test setup for the impact and notched tensile impact test with a fork-shaped pendulum hammer

Instrumented tensile impact test

The instrumented tensile impact test (ITIT) is performed with the aim of determining the fracture mechanics characteristics of plastics in accordance with the MPK procedure ‘Testing of plastics – Determination of crack resistance behaviour using the instrumented tensile-impact test’ (accredited standard of the MPK testing laboratory – Mechanical testing of plastics).

The instrumentation of pendulum tensile impact testers, i.e. the attachment of strain gauges or a piezoelectric force transducer to record the load–time signal as a basis for calculating the load–extension diagram, provides additional information for evaluating the toughness properties. It is possible to define and evaluate different energy contributions to the total deformation and to determine measured variables such as maximum load F_max and the associated deformation variable l_max. In the context of a material comparison or material optimisation, the measured variables provide important information for interpreting the determined material values of crack toughness.

For the tests, PSM GmbH Merseburg has access to the instrumented pendulum impact tester Resil Impactor Junior from Ceast (see image). This enables tests to be carried out to determine fracture mechanical characteristics on test specimens with metal blade notches on both sides (DENT-specimen) in various energy and hammer speed ranges. The test specimens have dimensions of L = 64–80 mm, W = 10 mm and a total notch depth of a = 2 mm.

As a result of the instrumented tensile impact test, fracture mechanical toughness parameters are calculated on the basis of the evaluated load–extension diagrams. These toughness parameters are preferably J_d values (see also: J-integral evaluation methods (overview)), which quantify the resistance of the material under investigation to the propagation of an unstable crack. One advantage of these material parameters compared to the notch impact toughness a_tN determined in the conventional notched tensile impact test is, for example, their particular sensitivity to structure.

Instrumented puncture impact test

The instrumented puncture impact test according to ISO 7765-2 is used to determine the multiaxial impact behaviour of films. Falling weights with appropriate equipment for measuring the load–time signal are used for the experimental implementation. A selection of impact bodies and a temperature control chamber are available for the Fractovis device from Ceast with electronics from Coesfeld, which is available from PSM GmbH. This means that investigations can be carried out on the influence of temperatures from -70 to 150 °C on the impact behaviour under multiaxial stress (see: impact loading free-falling dart test).

The instrumented puncture test is a metrological extension of the conventional puncture test and is used when a load–deformation diagram or measured variables from this diagram are required for material characterisation. The load–deformation diagrams are recorded by the instrumentation of the impact test body.

During the test, a film or a thin, plate-shaped test specimen (square, 80 mm x 80 mm, or round, diameter 80 mm) is pierced perpendicularly to its surface by the impact test body at a practically constant velocity (standard: 4.4 m/s), while the load–deformation diagram is recorded. This diagram can then be used to evaluate the material behaviour under impact stress. For example, the shape of the diagram (see Fig. 2) can be used to distinguish between brittle, tough or very tough fractures (see: fracture types).

Fig. 2:

Schematic representation of different types of load–deformation diagrams from the instrumented puncture impact test: brittle material with almost linear-elastic deformation behaviour and unstable crack propagation during puncture (a); ductile material with elastic-plastic deformation behaviour and stable crack growth during puncture (b) and highly ductile materials with predominantly stable crack growth (c): characteristic measured variables: F_M – maximum load, s_M – deformation at maximum load, W_M – energy up to maximum load, W_T – total puncture energy

In the diagrams in the Fig. 2, F_F represents the damage load, s_F represents the deformation at the damage load, and W_F represents the damage energy.

Comparisons between individual materials can only be made using this method if the test specimen production, test specimen dimensions, surface quality and test conditions (see also: surface testing technology) are comparable. The thickness of the film or test specimen plays a particularly important role here. In addition to the qualitative evaluation of the load–deformation diagrams and the fracture appearance of the test specimens, the results of the puncture impact test can also include the mean values of the following measured variables:

maximum load F_M,
deformation at maximum load s_M,
energy up to maximum load W_M.

Tear test for films

A tried-and-tested method for characterising the tear behaviour of films is the tear test to determine the tear resistance T_s under quasi-static test conditions. The testing machines to be used for this test are universal testing machines.

To determine the tear resistance, trapezoidal specimens can be used in accordance with DIN 53363 (see Fig. 3), which have a central incision from which the tear process is initiated by the notch stress. The length L of these trapezoidal test specimens on the long side is 120 mm, and the width W is 50 mm.

Fig. 3:

Trapezoidal specimen for determining the tear resistance of polymeric films

During the loading of the test specimen, the load–extension curve is recorded (see Figure 4 below for an example). The tear resistance T_S is calculated using the following equation:

T_{s}={\frac {F_{M}}{B}}

with

F_M		maximum load or median load and
B		specimen thickness

Fig. 4:

Typical load–extension diagram for a polyamide (abbreviation: PA) film (a) and clamped trapezoidal specimen (b)

Depending on the material behaviour, either the maximum load (as shown in the figure) or the median load from the load–time or load–extension diagram is used for F_M. However, the material value determined using this test method only allows relative comparisons between different materials. In the case of plastics, it is particularly dependent on:

the material quality and the treatment condition,
the preferred direction resulting from the processing process for thermoplastics,
the duration of vulcanisation for elastomers, and
the test temperature and the deformation velocity.

The determination of the tear resistance of elastomers is explained under tear test.

References

Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22)

Reincke, K., Grellmann, W.: Verfahren zur Charakterisierung der mechanischen Eigenschaften von Folien und Elastomeren. Kautsch. Gummi Kunstst. 63 (2010) 203–208

MPK-Procedure MPK-ITIT (2012-06): Testing of Plastics – Instrumented Tensile-Impact Test – Procedure for Determining the Crack Resistance Behaviour Using the Instrumented Tensile-Impact Test

ISO 527-3 (2019-02): Plastics – Determination of Tensile Properties – Part 3: Test Conditions for Films and Sheets

ISO 7765-2 (2025-05): Plastic Film and Sheeting – Determination of Impact Resistance by the Free-falling Dart Method – Part 2: Instrumented Puncture Test (Replacement for DIN 53373 (1970-09): Testing of Plastic Films – Impact Penetration Test with Electronic Data Recording (withdrawn))

ISO 8256 (2023-11): Plastics – Determination of Tensile-impact Strength

DIN 53363 (2003-10): Testing of Plastic Films – Tear Test Using Trapezoidal Test Specimen with Incision (withdrawn); replaced by DIN EN 17679 (2022-08)

Reincke, K., Grellmann, W.: Verfahren zur Charakterisierung der mechanischen Eigenschaften von Folien und Elastomeren. In: Grellmann, W. (Eds.): Neue Entwicklungen in der Werkstoffprüfung – Herausforderung an die Kennwertermittlung. Tagung "Werkstoffprüfung 2011", December 1st and 2nd, 2011, Berlin, Proceedings pp. 185–192 (ISBN 978-3-9814516-1-0; see AMK-Library under A 13)

Reincke, K., Grellmann, W.: Approaches to Characterise the Mechanical Properties of Films and Elastomers. In: Grellmann, W., Langer, B. (Eds.): Deformation and Fracture Behaviour of Polymer Materials. Springer Series im Materials Science 247, Springer Berlin Heidelberg (2017) 257–270 (ISBN 978-3-319-41877-3; e-Book: ISBN 978-3-319-41879-7; see AMK-Library under A 19)

Nentwig, J.: Kunststoff-Folien – Herstellung, Eigenschaften, Anwendung. Carl Hanser, Munich Vienna (2006) 3rd Edition (ISBN 978-3-446-40390-1; see AMK-Library under G 7-1)

Reincke, K.: Testing of Polymeric Films. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022); 3rd Edition, pp. 643-678 (ISBN 978-1-56990-806-8; e-Book ISBN 978-1-56990-807-5; ePub ISBN 978-1-56990-802-2; see AMK-Library under A 22)