Plastic Films & Varnishes – Surface Technology

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Plastic films & varnishes – Surface technology

Requirements for film products

Plastic films are products having extremely different applications: flexible or rigid packaging, decorative films for the automotive or furniture industry, films in the construction sector, agricultural films, etc. This results in very different requirements for the respective film products. In addition to certain mechanical properties, barrier properties, ageing and UV resistance, sterilisability, food safety or defined surface properties may be required. The necessary level of properties of the film products is basically determined by the selection of raw materials, by the composition of the material (polymer, fillers and reinforcing materials, other additives) and by the structure and morphology to be adjusted in the manufacturing process. In the case of multilayer films, the layer structure also plays a decisive role, especially with regard to the mechanical properties. For the packaging of food or medical products, the printability of the selected films is a further important aspect.

Films are often semi-finished products that are used for the manufacture of final products. An example of this are decorative films for plastic window profiles, with which almost every consumer wish regarding the appearance of a window can be realized. These films are applied to the window profile in a lamination process. Multilayer packaging films have also undergone a manufacturing process in which the various functional layers (e.g. strength-delivering layer, barrier layer, sealing layer) are firmly bonded together. They may then have to be printed, cut to size and sealed to produce a packaging such as a stand-up pouch or a bag made of it. The cutting and sealing steps are often carried out directly by the manufacturer of the packaged goods. The sealing properties in combination with the sealing parameters (see also: sealed seam) are of great importance for the production or closing of packaging. In order to achieve a defined opening ability of the packaging, the film properties and the sealing parameters must be carefully matched [1] (see also: testing plastic packaging).

Characterisation of film surfaces

The surface characteristics of consumer goods and packaging are becoming increasingly important, which is consequently then associated with increasing research tasks in this area [2–7]. For this reason, surface modification also of films plays an important role in current research activities and product development, like for coated as well as micro- or nanostructured surfaces. Since the quality of any material development and optimisation is always co-determined by the available material testing methods, there is also an increasing need for innovative methods for the surface characterisation of polymer films. As it has already been mentioned, these semi-finished products/products are often multi-layered systems with different tasks in the system. The outer layer in packaging films represents the visual and haptic interface of a package to the consumer and, moreover, often assumes the function of the strength carrier. Furthermore, many coated polymer films are used to realise desired or individual designs of consumer goods like furniture, windows or cars. For this reason, there is a high need to quantitatively evaluate the properties of the outer layers or their adhesive strength (see: instrumented adhesion test) and/or detachment behaviour.

Status of international standardisation for scratch testing

In Table 1, the definitions and technical options of testing according to the various standards and the scratch-testing equipment instrumented surface tester „ISRT“ are summarised. As can be seen, the standards are limited. Beside very simple methods like the cross-cut test e. g. according to ISO 2409 [8], a number of standards with respect to the surface characteristics in the sense of scratch resistance exist, like ISO 19252 [9], ASTM D 7027 [10], or ISO 1518 [11].

**Table 1**: Characteristics of the existing standards for the recording scratch test on plastics and paints
		ISO 19252 [9]	ASTM D 7027 [10]	ISO 1518 [11]	IKP
Scribe mode	constant normal load	x	x	x	x
	linearly increasing load	x	x	x	x
	constant intendation depth				x
	linearly increasing intendation depth				x
	intendation depth after test				x
Value range	scribe length (mm) *	≥ 100	100	40 – 100	≤ 100
	speed of scribing (mm/s) *	1 – 200	100	10 – 40	≤ 3.33
	load range (N) *	1 – 50	2 – 50	1 – 20	0.1 – 50
Measured variables	tangential force	x	x		x
	intendation depth	x	x		x
	elastic and plastic intendation depth				x
	width of scratching grooves				x
	scribing path (time)	x	x		x
Material parameters	scratch hardness and other hardness values like plough hardness				x
	coefficient of friction **		x		x
	recovery				x
	critical normal stresses ***	x	x	x	x
	fracture mechanics parameters				x

* depends partly on the selected scribe mode

** like apparent coefficient of friction

*** at damage initiation, change in scratch mechanism or layer separation

Modern methods for instrumented scratch testing

Due to the increasing demands placed on the surfaces of plastic products, it is also necessary to develop or adapt meaningful test methods, which in turn requires the development of appropriate testing equipment for the quantitative characterisation of surface and adhesion properties. One example is the equipment for performing an instrumented scratch test (IKP), which was developed by Coesfeld GmbH, Dortmund (Germany)] (see last column in Table 1 and Figure 1a). Figure 1b shows the measuring principle. The surface to be tested is subjected to a preset load, and then a relative movement parallel to the surface (= scratching/scoring) is performed over a specific, preset length. The test system opens up extended possibilities, such as load ramps to determine the peel force between layers, or fracture mechanical surface characterisation (see also: fracture mechanical testing).

Fig. 1:

Instrumented Scratch resistance tester (ISRT) for testing the surface properties of films (a) and schematic representation of the measurement principle (b)

An example of the optical characterisation of scratches introduced into the surface is shown in Figure 2. By selecting different normal loads, the scratches created in the surface vary in size. The scratches can be quantitatively analysed using the adapted chromatic-confocal distance measurement system of the instrumented test equipment, e.g. in terms of their width or depth or in relation to the extent of the material pushed out (see Figure 2a). Figure 2b shows a light microscope image (see: plastography) of two defined scratches in a transparent film in a top view. The load levels during the test were different in both cases, resulting in different scratch sizes. In this case, the scratch width was determined.

Fig. 2:

Example of surface profiles of a thermoplastic film with 2 scratches (a) and top view to 2 exemplary scratches in a transparent film (b) [12]

The option to record the whole scribing process in form of a complete load–path diagram allows for a comprehensive material characterization and thus detailed knowledge as the basis for a better understanding of the material behaviour.

Application example for instrumented scratch testing

As an example of such experimental investigations, Figure 3 shows load-path diagrams from an instrumented scratch test on coated thermoplastic films. The polymer films examined were polyethylene terephthalate (abbreviation: PET) and polyethylene (abbreviation: PE), each coated with the same type of gelatin [12, 13]. The aim of the experiment was to characterise the separation behaviour as a function of the polymer substrate. The evaluation of the load-path diagrams for the two test specimens shows that the adhesion of the gelatine layer to the PE substrate is significantly lower than to the PET substrate. The failure point at which the gelatine top layer detaches is marked by a red dot in the diagrams.

Fig. 3:

Tangential load–tangential path diagram from an ISRT measurement at gelatine-coated PET film (a) and PE film (b); detachment point marked by red dot [12]

References

[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; see AMK-Library under A 22)
[2]	Reincke, K., Grellmann, W.: Verfahren zur Charakterisierung der mechanischen Eigenschaften von Folien und Elastomeren. Kautsch. Gummi Kunstst. 63 (2010) 203–208
[3]	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", 1. und 2. Dezember 2011, Berlin, Proceedings pp. 185–192 (ISBN 978-3-9814516-1-0; see AMK-Library under A 13)
[4]	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 Verlag 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)
[5]	Rybnicek, J., Lach, R., Dominguez, S. R., Tondl, D., Valek, R., Grellmann, W.: Kratzfestigkeit von PA6-Nanokompositen. GAK – Gummi Fasern Kunststoffe 65 (2012) 775–783
[6]	Rybnicek, J., Lach, R., Schöne, J., Tondl, D., Domínguez, S. R., Valek, V., Grellmann, W.: Microstructure-Related Scratch Resistance and Indentation Creep Behavior of PA6 and PA6 Nanocomposites. Materials Science. Key Engineering Materials Vols. 592-593; https://doi.org/10.4028/www.scientific.net%2FKEM.592-593.586
[7]	Nase, M.: Charakterisierung von polymeren Peelsystemen durch Anwendung neuartiger Methoden der experimentellen Bruchmechanik. Habilitation, Otto-von Guericke-Universität Magdeburg, Shaker Verlag 2022 (ISBN 978-3-8440-8635-5; see AMK-Library under B 2-3) (Contents as pdf)
[8]	ISO 2409 (2020-08): Paints and Varnishes – Cross-Cut Test
[9]	ISO/DIS 19252 (2024): Plastics – Determination of Scratch Properties (Draft)
[10]	ASTM D 7027 (20209: Standard Test Method for Evaluation of Scratch Resistance of Polymeric Coatings and Plastics Using an Instrumented Scratch Machine
[11]	ISO 1518: Paints and Varnishes – Determination of Scratch Resistance – Part 1 (2023-02): Constant-loading Method Part 2 (2019-06): Variable-loading Method
[12]	Lach, R.: Entwicklung funktionaler Polymerfolien und Polymerbeschichtungen unter Verwendung von Lignin-Zwischenprodukten für innovative Anwendungen (LignoFol), Teilprojekt G: Mechanische Einsatzbewertung. Schlussbericht BioEcomomy-Cluster, Projekt Nr. 3.4, Förderkennzeichen: 031A572G (Teilprojekt), Merseburg (2018) online version on tib.eu
[13]	Lach, R., Richter, S., Heilmann, A., Grellmann, W.: Recording microindentation and adhesion tests to analyse the depth-dependent mechanical and adhesive properties of multilayer polymer films as shown for gelatine-coated polyethylene terephthalate and polyethylene films. Journal of Plastic Film and Sheeting 37 (2021) 53–69; https://doi.org/10.1177/8756087920942810