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1The surface

Figure I.1: Solvent droplets on a brushed, hydrophobic steel surface

This book deals with the application of modern techniques to paint analysis, with a special focus on surface analysis. If one thinks about the word “surface”, it quickly becomes clear what a relative and vague term it is. To a painter “surface” does not mean the same as it does to a surface chemist. To a painter, the surface represents that part of an object which is usually presented to the outside world and can be touched and observed directly, see Figure I.1. A painter considers a brushed visually clean steel surface. However, it can also be defined as the boundary layer between a solid or liquid material and a surrounding liquid or gaseous phase.

A surface physicist would probably refer to it as a phase interface. Alternatively, it could be defined as the area of a solid or liquid thing at which the bulk physical and chemical properties change instantly, a so-called property boundary.

A surface chemist, however, is talking about the uppermost molecular layers of a material when he uses the word surface. This is an area that cannot be observed without the help of analytical techniques. In fact, the uppermost layers of an object often determine the quality and behaviour of the material as far as (paint) adhesion is concerned. The uppermost layer of the steel surface is not directly visible without the help of machines. Probably the steel surface exhibits a chemical surface modification which explains the hydrophobic behaviour visible in Figure I.1. Or there is a thin layer of contaminations that produces a hydrophobic property!? – To define this is the task of surface analysis.

3.1.1.1Definition of the term surface

So, let’s first define how to use the word surface in this book. A surface is a boundary layer which separates a substrate from the surrounding environment (air, liquid). It is typically 1 nm to 1 μm thick. In contrast, a “thin layer” is defined as being 1 μm to 10 μm thick.

Figure I.2: AFM (atomic force microscope) image of a paint surface (60x60 μm)

The surface plays a significant role in the physical and chemical properties of a material. A contract painter, for example, who paints and coats coils and metal profiles has to rely on the surface quality of the goods he is going to coat. The surface of the raw material that he receives might well look clean. However, the material has a long history before it has been delivered to this company to be painted or coated. During production, storage and transport of a coil, for example, numerous substances may have been adsorbed onto the surface. This surface layer contaminants may not be visible, but it always exists! And sometimes even traces of contaminants can seriously impair the adhesion of a coating to a surface.

When it comes to processing of the coil, the chemical composition of the outermost molecular layer plays a significant role. If the coil has been coated with a protective layer of oils to prevent corrosion during transport and storage, the paint will exhibit poor adhesion or craters after application. Even a monomolecular layer of some of these oils can have deleterious effects on coating procedures.

As these ultra-thin layers are invisible, the unfortunate manufacturer is in fact “blind” as far as the surface quality of his coils is concerned. In most cases, therefore, he will decide to install a cleaning process before applying the coating. But he will do so without knowing if it is necessary and, even worse, without knowing what to remove from the surface. Unfortunately, there is no “magic” process for eliminating all the various kinds of contaminants. His efforts might well produce a surface quality worse than before, due to the presence of oil residues and traces of cleaning chemicals, such as surfactants.

Some goods require a pretreatment like e.g. phosphating before coated. The quality of the deposited anti-corrosion phosphate conversion layer is significantly depending on the parameters of the process. Only well crystallized zinc or iron phosphate guarantees a perfect adhesion of the subsequently applied coating. Without analytical methods like e.g. SEM or infrared external reflection spectroscopy, it is impossible to check the performance of the pretreatment process with respect to crystal morphology and percentage of coverage. The author has worked on a lot of adhesion failures issues during the last twenty years which have been caused by poor conversion layer application, although on impulse the painter has been blamed.

Another focus is on the coating material itself. As the paint and the painted substrate have to be a chemical match if good adhesion is to be obtained, a few questions need to be asked before the painting process is started.

 What is the chemical composition of the substrate surface?

 Which pretreatment can be used to improve paint adhesion and what effect will it have?

 How do the paint ingredients influence the surface of the material that has to be painted?

 What influence do the paint additives have on paint adhesion?

Unfortunately, these questions often can’t be answered by simple tests or classical chemical analysis, because they require an ability to analyse tiny amounts of substances that have high surface sensitivity . Only the surface analysis techniques described in this book can answer these questions

Figure I.3: SEM Images of different zinc phosphate conversion layer morphology

A growing field of application for modern surface analytical techniques is not only paint application but also paint production. Modern high-performance paints have to fulfil many requirements simultaneously that are sometimes hard to match. This does not only create a demand for characterisation of the raw materials and products. The chemical interaction of paint compounds and the reaction between each compound and the ingredients of the substrate (e.g. a polymer) are also key parameters.

If, for example, a moulded polymer part has to be coated, it is not just the polymer which is of interest. The manufacturer or supplier of the raw material compounds the polymer to customer demands. In accordance with the requirements imposed on the polymer material, he adds additives to improve flame, light, impact or heat resistance. One parameter the supplier is not concerned about is the paintability of the product made from the granules which he supplies. That is a process which the polymer supplier does not see.

However, it has been shown in the past that additives present in polymers “designed” to enhance moulding processes, e.g. offering for example good release from injection moulds, exhibit poor properties with respect to surface finishing by painting. Most of the additives incorporated into a polymer migrate to the surface, driven by temperature, humidity, time or solvents. This sometimes leads to unpredictable results, such as paint adhesion failure, chemical reactions, discolouration, and wetting failure. Many manufacturers of paint for automotive interior parts have therefore discovered, that it is essential not only to know their own paint manufacturing process, but also to learn something about the polymers which have to be painted. This is a task that can easily be fulfilled by the techniques which are described in this book.