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Surface

From Encyclopedia of plastics testing
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Surface


General information

The surface of components or semi-finished products corresponds to a thin outer surface layer in the thickness range of approx. 1 nm to several µm, which is decisive for the optical properties, such as colour, reflectivity and gloss, and the mechanical surface properties, such as surface hardness and scratch resistance [1], see weblinks. The material properties on the surface differ from those of the voluminous solid due to the open interface with a medium such as air or water. Due to the lack of neighbouring lattice components or molecules in the interface, the binding forces (cohesion) create a force effect directed towards the interior, which is also referred to as surface tension and interfacial tension and which essentially accounts for the deviating properties of the surface layer in contrast to the bulk.

If technical material surfaces are to be functionally (e.g. conductive or insulating), optically (e.g. colour, reflection or gloss) or mechanically (e.g. hardness, scratch resistance or stress cracking resistance) refined or adapted, then pre-treatment is first required, which depends on the type of material. These surfaces have production-related roughness, processing grooves or waviness as well as coatings from processing aids, which can have a decisive influence on the composition of the thin surface layers, the phase interfaces and therefore also the surface quality.

Surface pre-treatment

Regardless of the materials to be treated, surface pre-treatment is generally carried out before surface treatment methods are used. This usually includes material-specific cleaning and degreasing as well as the removal of manufacturing and processing aids. In addition to improving the decorative and optical properties (gloss, reflection), the mechanical properties (surface hardness, abrasion and scratch resistance) and corrosion resistance should also be positively influenced to increase the service life of metallic components in the automotive industry or mechanical and plant engineering, for example. A frequently used method for metals, but also plastics or ceramics, is plasma treatment of the material surface, which, in addition to the cleaning effect, also increases the surface tension and can change the surface structure and chemical composition. In the case of polymeric materials, especially non-polar plastics such as polyethylene (abbreviation: PE) or polypropylene (abbreviation: PP) have poor conditions for the wetting and adhesion of surface layers, which is due to the comparatively low level of surface energy.

The reason for this is the lack of functional groups that are important for adhesion and wetting or the high surface quality, which means that the coating materials cannot enter into any chemical or physical interactions with the plastic surface [1]. The surface energy has a disperse and polar component that is specific to the material. The polar component, i.e. the permanent dipoles such as in polyvinyl chloride ( abbreviation: PVC), is decisive for wetting and adhesion, whereby ideally the polar component of the coating and the plastic surface should be approximately identical. The plastic surfaces of polypropylene or polyethylene can also be activated for coating by appropriate pre-treatment using primer application, corona treatment or flame treatment.

Different methods can be used to clean or pre-treat the surface of plastics, whereby soiling or processing aids must be treated in a material-specific manner.

Manual pre-treatment

The simplest method is particularly suitable for simple and straightforward component geometries, as undercuts, corners and recesses are difficult to reach even with liquid cleaning agents and therefore this method only offers low reproducibility and process reliability. The surfaces of the plastics can be cleaned and degreased using clean cloths soaked in alcohol, isopropanol, acetone or similar suitable solvents, for example. These solvents must not swell the surface or cause stress cracks (see: stress cracking resistance), but should, if possible, improve the wettability of the surface. Using ESCA (electron spectroscopy for chemical analysis) or TOF-SIMS analysis (time-of-flight secondary ion mass spectrometry) of the uncleaned and cleaned plastic component, the efficiency of the cleaning process can be verified [1, 2], see Weblinks.

Mechanical pre-treatment

Mechanical processes such as brushing, grinding, sanding, polishing or blasting (abrasive blasting) are used to clean elastomeric, thermoplastic and thermoset plastic surfaces and roughen them to increase the surface area, assuming that the surface has been degreased. Cleaning with different brushes or combined brush systems is the most advanced method, whereby both surface roughening and gentle polishing can be realised by means of rotational movement and different grain sizes of the abrasive medium. However, electrostatic charges caused by brushing or blasting must be removed afterwards, as otherwise new impurities may occur. Blasting (wet or dry) is carried out with different pressures and grain sizes of the blasting additive, in particular to roughen and enlarge the surface of polar plastics, which can improve the physical adhesion conditions. The change in surface quality can be verified using microscopic methods (light microscopy, scanning electron microscopy (SEM)) or AFM (atomic force microscopy), whereby the cross-cut method is also used as a technological test method [1, 2], see Weblinks.

Pre-treatment by ionisation

If components made of thermoplastics, thermosets or elastomers are brushed or blasted, electrostatic charges can occur as a result of charge separation, as with boundary layers between different materials, which can have a disruptive effect on subsequent production steps due to the attraction of dust particles. As cleaning with compressed air is not sufficient, the dust is removed by electrostatic discharge (earthing for metals or ionisation of the air for plastics) and bound by additional treatment methods (compressed air, extractors or brushes). With this method, non-polar plastics in particular can be discharged and made accessible for subsequent surface treatments such as painting, printing, laminating or galvanising [1, 3, 5].

Pre-treatment with dry ice

Cleaning with dry ice is similar to the abrasive or sandblasting process for metallic materials using high pressure, but dry ice (CO2) is used as the blasting medium instead of water and fine sand. The dry ice particles, which reach a temperature of -78.5 °C, hit the contaminated surface at almost the ultrasound velocity and cause localised cooling, resulting in embrittlement and reduced adhesion of the dirt particles due to different expansion coefficients. As a result of the high pressure and the resulting kinetic impact energy as well as the transition of the CO2 from a solid to a gaseous state, the dirt particles are blasted off the surface and carried away by the compressed air.

For plastics and fibre-reinforced plastics, as with glass or ceramics, the so-called snow blasting technique is used instead of a high-pressure process due to the abrasive effect. The dry ice particles are produced directly on site from the liquid CO2 and sprayed onto the surface as a mixture of CO2, CO2 snow particles and compressed air, whereby even finely structured, complex and high-gloss surfaces can be cleaned [1], see web links. In analogy to mechanical pre-treatment, the efficiency of the cleaning or pre-treatment can be assessed using the TOF-SIMS process.

Wet chemical pre-treatment

The wet-chemical treatment or cleaning (see weblinks), also known as RCA cleaning, published in 1970, is a very effective technique for cleaning components, whereby the Standard Clean 1 and 2 processes were originally developed for silicon wafers [6]. These cleaning baths, which contain a solution of ammonium hydroxide and hydrogen peroxide diluted with water or hydrochloric acid and hydrogen peroxide, can be used to remove particles and organic contaminants as well as metallic contamination at 75 to 85 °C. The cleaning process is also suitable for large components in the automotive industry. The multi-stage process, which is also suitable for large components in the automotive industry, combines the processes of washing, rinsing and drying (sometimes preceded by degreasing) in one or more process steps, whereby highly diluted solutions with partially adapted solvents are also used today, which are significantly more environmentally friendly. This process can be used to efficiently clean all thermoplastics and prepare them for finishing processes such as painting, printing, bonding, flocking or laminating (films), whereby the surface must sometimes be activated using special processes (vacuum or plasma treatment and flame treatment).

Particle counting systems can subsequently be used to assess the effectiveness of cleaning. On the other hand, analytical methods such as TOF-SIMS or ATR spectroscopy (infrared spectroscopy with attenuated total reflectance) also enable unknown soiling to be characterised and a more suitable cleaning agent to be selected. Technological test methods for paint adhesion, such as cross-cut, climate change test or salt spray test are also suitable for evaluating the effectiveness of the cleaning method.

See also

References

[1] Lake, M.: Oberflächentechnik in der Kunststoffverarbeitung – Vorbehandeln, Beschichten, Funktionalisieren und Kennzeichnen von Kunststoffoberflächen. Carl Hanser Verlag, München (2009) (ISBN 978-3-446-41849-3)
[2] Hofmann, H., Spindler, J.: Verfahren in der Beschichtungs- und Oberflächentechnik. Carl Hanser Verlag, München (2014), 3. Auflage (ISBN 978-3-446-44141-5)
[3] Müller, K.-P.: Praktische Oberflächentechnik, Vorbehandeln – Beschichten – Beschichtungsfehler – Umweltschutz (JOT-Fachbuch).Vieweg Verlag, Wiesbaden, 4. Auflage (2012) (ISBN 978-3-322-91548-1)
[4] Müller, K.-P.: Oberflächentechnik. Vieweg Verlag, Wiesbaden (1996) (ISBN 978-3-528-04953-9)
[5] Schulz, J., Holweger, W.: Wechselwirkung von Additiven mit Metalloberflächen. Expert Verlag, Renningen (2010) (ISBN 978-3-816-92921-5)
[6] Kern, W., Puotinen, D.: Cleaning Solutions Based on Hydrogen Peroxide for Use in Silicon Semiconductor Technology. In: RCA Review 187 (1970)

Weblinks

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