Plastography

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Plastography

Basics

Plastography is the most recent scientific sub-discipline of materialography and serves to elucidate and provide a qualitative and quantitative description of the structure and morphology of plastics and their composites (see also: composite materials testing) using macroscopic and microscopic methods.

Plastography involves the investigation and representation of structural and morphological microstructural features, as well as the qualitative and quantitative description of the microstructure of pure, reinforced and filled plastics and composite plastics, using macroscopic and microscopic experimental investigation methods, including microstructural tomography and topographical methods for representing the surface of fractures (see also: fractography) [1–4].

This methodology is a specialised discipline of polymer materials technology that is used in quality assurance, failure analysis, and in the polymer testing and development of plastics. This encompasses the necessary preparation procedures, a wide variety of macroscopic and microscopic examination methods, including electron and digital microscopy, and high-resolution X-ray or ultrasound computed tomography for plastics. This also includes the analysis, evaluation and documentation of the test results to present the qualitative and quantitative structural and morphological characterisation, as well as the establishment of correlations between structure or morphology and properties (see: microscopic structure). This scientific discipline is an essential component of the Merseburg School and was significantly advanced by Trempler [5–11] and Michler [12–17].

According to [1], the term ‘microstructure’ is defined, independent of the material (metals, ceramics or polymers), as the description of the totality of those sub-volumes, each of which is, in a first approximation, homogeneous and isotropic (see: anisotropy) in terms of its composition and the spatial arrangement of its structural components with respect to a fixed reference frame (coordinate system). The microstructure itself is characterised by the type, shape, size, distribution and orientation of its associated microstructural components.

In order to visualise and evaluate the microstructure of a polymeric material, specialised preparation techniques are required, which may differ from those used for metallic and ceramic materials. These differences stem from the structural diversity of plastics (thermoplastics, thermosets and elastomers), their morphology, heterogeneity and anisotropy resulting from reinforcing and filler materials, as well as their thermal stress resistance. The viscoelasticity and varying degree of cross-linking of plastics, in conjunction with different fillers, also result in widely varying hardness, scratch resistance and abrasion resistance, whilst the condition of the test specimen may be further influenced by temperature and humidity. Here too, preparation errors generally lead to problems in assessing the material microstructure. Basic preparation techniques include specimen preparation—such as sawing or milling specimens from a component—embedding of objects, grinding and polishing, and etching the surface of the test specimen. Depending on the properties of the plastics or plastic composites, ductile or brittle material behaviour may be observed, resulting in different preparation techniques and conditions for these materials. In addition to cross-sections or thin sections, special preparation methods, such as thin sections, can also be used.

To visualise the prepared plastic microstructures, light or electron microscopy methods, X-ray diffraction and tomography are used for the qualitative and quantitative assessment and documentation of the microstructural states [1–3].

Specimen collection and preparation

Sample preparation encompasses all the steps required for the planned macroscopic and microscopic examinations of the microstructure, beginning with sampling – that is, the plastography-oriented cutting process from a plastic component or semi-finished product.

The informative value of macroscopic and microscopic analysis methods depends largely on the sampling and subsequent preparation, although often only limited quantities of sample are available, as is the case, for example, with damage analysis. The sample therefore represents only a subset and must be representative of the entire component in order to be interpreted successfully; consequently, the selection of the sample is crucial to success, as—unlike with metallic materials—the processing of plastics has a significant influence on the resulting microstructure or morphology [5, 15] (see: microscopic structure). Furthermore, as plastics exhibit greater structural diversity, coupled with variations in morphology, than metals, a clear definition of the intended investigation objective and the appropriate preparation methods for the specific polymeric material is required [5, 9] (Table 1).

**Table 1**: Selected preparation methods in plastic microscopy according to [5]
product	manufacture	materials	application
complete preparation	an intact sample from a component or semifinished product	all plastics	macroscopic examination for defect detection
grind	creating a grind, contrasting, ultra-milling	composites, hard and brittle polymers	reflected light microscopy, phase analysis of filled plastics
thick and thin-cut	grind to the required thickness, polish (ensuring the correct thickness is maintained)	composites, hard and brittle polymers	transmission microscopy: fibre distribution, distribution of spherulites
thin-cut	microtome section, cryosection	soft, transparent and translucent polymers	transmission microscopy, superstructures, flow lines, weld lines
fracture or surface	clean the surface without causing any damage	composite and filled polymers	reflection microscopy, electron microscopy
grain preparation	add the grains to a highly viscous immersion medium	amorphous and semi-crystalline polymers	particle size, distribution, morphology, roundness, surface of the particles
fibre embedding	sticking or embedding object slides	all types of fibre	reflected light microscopy, fibre data
film preparation	Levaporate the solution onto a microscope slide; allow the melt to solidify on the slide	solidified polymer melt or dried solution	transmission microscopy, optical data, porosity, particle size distribution

When taking samples from a component, a visual inspection of the object is essential in order to determine the sampling locations. In this context, visible damage, any flow lines, weld lines or discernible swirls, as well as colour differences (see also: colour and colour differences) are of particular significance [18, 19]. In line with damage analysis, these areas should be marked and documented (photographs) in a clear, distinct and smudge-proof manner, whilst the subsequent observation level and storage conditions (see: standard atmospheres) should already be determined at this stage.

The rough cutting at the defined positions can then be carried out using an angle grinder or jigsaw, and the sample preparation at the exact position is subsequently carried out using a band saw, table saw or water jet cutting. In any case, it is important to ensure that a low cutting and feed rate is selected and that sufficient cooling is provided to prevent thermal damage to the plastic, as well as changes in morphology or the formation of artefacts [18]. Furthermore, consideration should be given to whether the selected sample is still suitable for other analyses.

The resulting surface of the cut, which should be as flat and parallel as possible, often becomes the subsequent grinding surface; for this reason, attention must be paid to potential orientations, anisotropies and introduced defects right from the cutting stage (Figs. 1 and 2). Defects can primarily occur during the cutting process, embedding, and grinding and polishing; these manifest as microstructural changes due to excessively high grinding or cutting speeds, loosening of the microstructure and delamination of composite materials, or faulty solvents and etching agents, resulting in the destruction of the microstructure or morphology (Fig. 1). Localised heating can also lead to microstructural gradients or composite damage as a result of grinding (Fig. 2).

Fig. 1:

Preparation errors caused by (a) loosening of the fibres due to excessive polishing, (b) local melting during fine grinding with inadequate cooling, (c) melting due to the embedding resin being too hot, and (d) destruction of the specimen by the solvent

Fig. 2:

Preparation errors caused by (a) variations in spherulite size due to temperature gradients during grinding, (b) material separation in a PVC sample due to mechanical damage during sampling, (c) bonding defects caused by voids and bright foreign inclusions, and (d) impurities in the polyethylene matrix of a thin section (cut)

As the sections usually involve only small samples, these should be clamped (in section clamps) or embedded after deburring and cleaning to ensure optimal handling. After the drying process, the sample should only be handled with tweezers to avoid the transfer of grease and contaminants. In principle, cutting should be carried out gently to avoid undesirable microstructural changes resulting from heat generation, microcracking or delamination in composites (see also: multiple fracture UD tapes). Cutting by laser beam should not be used due to the thermal stress on the plastics. A scratch-free ground section for macro- and/or microscopic microstructure imaging should have a sharp-edged and flat surface without microcracks (see: crack) This should be representative of the local microstructure and should not show any manufacturing-related changes such as plastic deformation, chipping, scratch and grinding marks, or smearing or contamination (artificial microstructure).

Embedding specimens

The embedding of samples serves, on the one hand, to facilitate sample handling during grinding and polishing and, on the other hand, to support the edge zone and prevent manufacturing-related edge cracks; in the case of plastics, this is preferably carried out using the cold-embedding method [3, 5]. The embedding agents used should not react with the sample under investigation, the mould used or the subsequent etchant, and, due to their suitable viscosity, should not exhibit bubble or pore formation. Furthermore, these agents should exhibit similar grinding and polishing behaviour in terms of hardness and strength to the embedded sample [3]. Furthermore, to prevent microstructural changes, microcracking (see: fracture formation) and to ensure adequate adhesion at the edges, there must be no significant shrinkage during the cross-linking process. Moulds can be made from polyethylene (abbreviation: PE), polyvinyl chloride (abbreviation: PVC), silicone rubber or synthetic resins, although in some cases the mould needs to be greased.

The embedding agents used include, for example, cold-curing transparent or coloured multi-component systems (resin, hardener and accelerator) based on methyl methacrylate, polyester or epoxy resins. The samples to be examined can be positioned vertically or at a defined angle (angled section), and after mixing the powdered or liquid components, these are poured into the mould following homogenisation, whereupon an exothermic chemical reaction (polymerisation) takes place to cure and fix the sample. The thermal stress on the sample should not exceed 40 °C [5]. If the samples fixed in this way are required for subsequent examinations, care must be taken to ensure that they are stored in a manner appropriate to the sample and embedding medium, taking into account factors such as temperature, humidity, pressure and light exposure.

When using low-viscosity embedding agents such as epoxy, polyester or acrylic resins, the process is often carried out under vacuum; this allows the finest voids, pores and cracks to be consolidated and, when colouring, also provides contrast.

However, care must also be taken here to ensure that components of the embedding agent do not penetrate the sample, that no anisotropy or microstructural changes occur as a result of excessive temperature, and that no gaps or cracks are formed due to shrinkage effects (see: shrinkage test).

In some cases, conductive additives are also mixed into the embedding material to prevent electrostatic charging during subsequent electron microscopic examination.

Grinding and polishing specimens

When preparing cross-sections of polymeric materials, the embedded samples are usually ground and polished as well, although silicon carbide or diamond discs with a wide range of grit sizes are generally used for this purpose. In this process, interference layers and surface irregularities (roughness) are first removed by coarse or flat grinding, and the required surface finish is subsequently achieved by fine grinding. It should be noted, however, that this method of preparing sections is very time-consuming and requires the examiner to have sufficient experience.

As many plastics are relatively low in hardness and highly sensitive to temperature, grinding is carried out almost exclusively under running water; otherwise, abrasive grains would become embedded in the surface (Fig. 2c) or smearing would occur as a result of plastic deformation (Fig. 1b) [3, 5]. In any case, care must always be taken to ensure that the abrasive is adequately removed by the cooling medium. Due to the problems described above, the conventional grinding technique is tended to be used on hard, filled or reinforced plastics [5], although other problems may arise here due to the loosening and breaking out of the filler or reinforcing materials, thereby distorting the ground surface (Figs. 1a and 2b). Due to the specific properties of certain plastics, such as alcohol solubility or water absorption in polyamides (abbreviation: PA), surface components are dissolved or swelling occurs (see: water absorption), which cannot be remedied even by altering the cooling rate or spindle speed. In principle, the same equipment used in metallography or ceramography can be used for mechanical grinding (Fig. 3).

Fig. 3:

Wet grinding and polishing machines: (a) the EcoMet250 from Bühler, Esslingen, and (b) the Metcon Digiprep 301 from Schütz und Licht Prüftechnik GmbH, Langenfeld, both fitted with horizontal turntables

To improve material removal and control the formation of grinding marks, the sample (see: test piece) should be rotated by 90° after each grinding stage to ensure an optimal starting condition for the polishing process. Fine grinding (grit sizes 26 µm to 3 µm) serves to prepare the surface for the final polishing process and must also be carried out under cooling. Similarly to fine grinding, pre-polishing and fine polishing are also carried out under cooling or lubrication with water or, rarely, with glycerol, whereby grit sizes between 3 and 1 µm are used in conjunction with hard polishing cloths. Ultra-fine polishing with Al₂O₃ or diamond powder and soft cloths produces the final finish as a concluding treatment; however, this should still be cleaned with distilled water [5] and is then generally used for reflected-light microscopy in bright-field or dark-field mode.

If optical examinations of plastics are to be carried out using transmitted light microscopy, e.g. refraction index measurements, thin sections must be prepared; when done carefully, these are normally less damaged than those produced by the thin section method. In this case, the polished section surface is positioned on a glass slide using a transparent and isotropic mounting agent (EP resin, Canada balsam or wax), ensuring that no air is trapped. The sample fixed in this way is then processed under vacuum to the required thickness of the thick or thin section (7 to 10 µm), subsequently embedded and then examined under a light microscope [5].

Where the necessary equipment is available, the less time-consuming thin sectioning technique is recommended, particularly for soft plastics, which requires thicknesses of approx. 3 to 10 µm. To this end, using glass, metal or carbide blades, as well as diamond wedges, and employing microtomes or ultramicrotomes, extremely fine sections are prepared from a sample held in a holder – in some cases under microscopic guidance – and subsequently embedded, for example, in silicone oil as an immersion medium (Fig. 4). The cutting quality of the blades and the cutting forces generated by the holder are key factors in the usability of the thin sections, as otherwise inclusions may break out or plastic deformation may occur. The thin sections produced therefore depend on the type of blade and the hardness of the plastic, and are only embedded in the available embedding moulds if the result of the preparation is satisfactory [5]. In addition to this sectioning method, cryo-microtomy [4, 19] and ultra-milling are also significant in the production of thin sections for specific plastics or plastic composites [3, 4, 19].

Fig. 4:

Handelsübliche Geräte der Microtomie und Ultramicrotomie (a) Microtom RM 2245 und (b) Ultramicrotom EM UC7 der Fa. Leica Microsystems GmbH, Wetzlar

Etching

Before etching, a microscopic examination of the prepared section in its unetched state should be carried out at various magnifications, as macroscopic treatments and defects are already visible here despite the reflective surface. Depending on the quality of the surface condition, the following treatments and defects can be observed on the unetched surface:

Voids, pores and segregation effects,
Micro-voids and micro-cracks,
Delamination and edge chipping,
Mineral and polymer inclusions,
Phases in blends and composite plastics.

Optical contrast techniques, also known as optical etching, can be used to visualise the finest cracks and phase boundaries by means of dark-field illumination in reflected-light microscopy or the differential interference contrast method. Coating the sectioned surfaces with graphite and gold improves reflection on the surface of the plastics and should be used in microphotographic or scanning electron microscopy imaging techniques, whereby phase components, orientations (textures) or spherulites become partially visible. Etching is required to improve the visibility of these structural components; however, due to the structure of the polymers, only chemical etching methods can actually be used [3].

Various acids, acid mixtures or organic solvents are available as etching agents for plastics; these are summarised in [3]. In semi-crystalline plastics, the higher resistance of the crystalline phase results in greater attack on the amorphous regions, so that if the concentration of the etchant is too high, or the exposure time or temperature is too long, the morphology of the plastic is significantly changed. At the same time, a tempering effect may occur here, which also results in morphological changes due to stress relief (see: relaxation plastics). Appropriate test series must also be carried out to ensure that no stress cracking corrosion, crazing or microcracks with changes in the global and local stress state occur as a result of the etching.

Presentation and documentation of the microstructure

The fundamental aim of plastography is to characterise, both qualitatively and quantitatively, the structural components present in a cross-section in terms of their type, number, size, morphology and distribution across the surface [3–7], and to establish and explain the interactions between the microstructure and the properties of polymeric materials. In this sense, plastography is one of the most essential methods of polymer testing and serves specifically to illustrate the relationships between structure and morphology, manufacturing and processing technology, as well as the mechanical, thermal and technological performance of these materials [8–11, 20, 21]. The most important investigative methods for assessing and documenting the properties of the microstructure are based on optical testing methods, such as light or electron microscopy, microstructural tomography, as well as X-ray and electron diffraction, electron beam microanalysis and micro-CT methods.

A fundamental optical inspection method is light microscopy, which is carried out on plastic sections using reflected light microscopy in bright-field or dark-field mode (Fig. 5a) [3], and on thin sections or slices using transmitted light microscopy. For this purpose, bright-field, polarisation or phase-contrast microscopy can be employed, making use of the optical anisotropy of many plastics.

Fig. 5:

Reflected-light microscopes (a) Axio Scope A1 from Carl Zeiss Microscopy GmbH, Jena, and digital microscope (b) VHX 600F from Keyence Deutschland GmbH, Neu-Isenburg

Due to the directional dependence of the refraction index, specific colour contrasts can be used to detect both microstructural differences and residual stresses, for example in amorphous plastics.

For higher object resolutions, depending on the suitability of the test specimens, digital microscopy (Fig. 5b), transmission and scanning electron microscopy, atomic force microscopy, X-ray diffraction and electron beam analysis are also used. Using energy dispersive X-ray spectroscopy (EDX), the chemical composition of the structural components can also be analysed in the scanning electron microscope.

The visual documentation of the microscopic structure should be designed with a view to providing a clear and unambiguous representation of the results on the section surface; this is why sharp, high-resolution photographs are required. The equipment of modern microscopes usually allows for the recording of high-quality colour digital photographs, whereby, particularly in the case of zoomed images, a magnification scale and the date of record should always be included. In addition to digital photography, digital videography can also be used to produce visual structural documentation; in this case, the data is usually transmitted directly to a connected computer, where suitable software also enables quantitative structural analysis.

References

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