10 Introduction Part I Introduction 1 Surface

Introduction10

Part I Introduction

1 Surface

This book deals with the application of modern techniques to paint analysis, with a special focus on surface analysis. If we pause to consider the word “surface”, we soon realise 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.

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 can’t 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.

Definition of surface

So let’s first define how we shall 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.

The surface plays a significant role in the physical and chemical properties of a material. Let’s look, for example, at a toll manufacturer who paints and coats coils and metal profiles.

The surface of the raw material might well look clean. However, the material has a long history before it has been delivered to this company to be painted or coated. Production, storage and transport of a coil, for exam-ple, afford much opportunity for numerous substances to be adsorbed onto the surface. This surface layer of, say, contami-nants may not be visible, but it

Figure I -1: AFM (atomic force microscope) image of a paint surface (60 x 60 µm)

Roger Dietrich: Paint Analysis

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ISBN: 978-3-86630-912-8

Dietrich engl 05 AM DT.indd 10 08.07.2009 14:09:54 Uhr

11Surface

is there nonetheless. 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 pre-vent 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. Unfor-tunately, 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.

The same is true of 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 pre-treatment 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

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 not only creates 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 sub-strate (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 matches the original 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 sup-plier 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.


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However, it has been shown in the past that additives present in polymers “designed” to enhance moulding processes, especially in the offering good release from injection moulds, can prove disastrous for the painting process. 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, discoloration, 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 we are going to describe in this book.

1.1 Relevance of modern analytical techniques to paint analysis

There are hundreds of techniques for analysing paints and coatings. They yield information about viscosity, gloss, haze, hardness, acid value, etc. In other words, they describe the prod-uct and its properties. They ensure that the desired level of quality is achieved. On the other hand, standard analytical tools often fall short when failures and production problems arise. The standard techniques are perfect for finding out the quality of a product. However, if a product is sub-quality and the question is asked as to why this happened, the standard tech-niques are not very helpful. For example, a monomolecular layer of a release agent on the sur-face can easily cause severe adhesion failure if the material is to be painted. The quantity of substance may be too low to be detected by standard techniques. Or, poor cleaning procedures in a paint shop might cause paint defects of a few microns in size. Before this problem can be solved, it is necessary to know what has caused the paint defect. The substance or inclusion particle causing this failure is too small to be characterised by standard techniques.

This is an analytical gap that can be closed by the surface analytical techniques described in this book. They will help to answer the question: Why does a product have unexpected properties and why do failures happen?

Typical topics in paint analysis are: • What do contaminants in paint layers or wet paint samples consist of?• What is the chemical composition of paint layers at a certain depth from the surface?• How are chemical bonds formed between paint components?• Why does a paint layer peel off a substrate and where does the delamination take place?• What is the reason for paint spots?

It should be mentioned that there is no all-embracing technique that can answer these ques-tions. In fact, there are many parameters which influence the decision as to which technique to employ for the analysis, including: • additional information about the appearance of the defect• preliminary sample investigation by optical light microscopy• chemical and physical properties of the coating • Desired detection limit

In other words, it takes an experienced user to find the best tool that can answer the ques-tions raised about the sample. These considerations will be discussed later in this book.

Surface


13General considerations

1.2 General considerations

Before we present the techniques that will be discussed in this book, it would be helpful to find a common basic principle to describe them. No matter whether we are talking about infrared spectroscopy or TOF-SIMS or SEM, the main principle consists in probing a sample with radiation.

The sample is essentially analysed by radiation that probes for specific properties and char-acteristics of the material. This radiation, which is called the primary radiation, can consist of electrons, ions, neutral particles and photons, such as infrared waves and X-rays. The primary radiation triggers a reaction specific to the sample that may take the form of the emission of electrons, ions or X-rays. This “reaction” by the sample is detected by an electronic system composed of an analyser and a detector. The result can be displayed as a spectrum on a com-puter or be printed on paper. The last step of the process is data evaluation by an experienced analyst. The evaluation must include • plausibility check• comparison with databases• interpretation with respect to the analytical problem

The nature of the interaction which occurs between the probing beam and the sample depends on the type, energy and angle of incidence of the probing radiation and, of course, the sample material.

Figure I-2: General concept of probing the surface of a sample with radiation

Figure I-3: Interaction between the primary radiation and the sample


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The primary radiation interacts with the sample in a specific way. Each type of sample reac-tion can be detected separately and analysed to reveal the chemical and physical composi-tion of the sample and its surface. The radiation emitted by the sample is called secondary radiation. Each type of primary radiation can produce a different type of secondary radiation. Probing with an electron beam, for example, may lead to the formation of:

• secondary electrons • X-rays • back-scattered electrons• fluorescence

Figure I-5: Result of sample excitation by primary radiation

Surface

Figure I-4: Components of primary radiation


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Figure I-6: Surface analysis techniques

Each type of radiation conveys different information about the sample that all adds up to a comprehensive understanding of the sample’s properties. Not only the primary radiation, but the secondary radiation emitted by the sample, too, can consist of electrons, ions, neutral par-ticles and photons that result from sample excitation or reflection of the primary radiation.

The latter is a consequence of diffraction and dispersion that change the energy, angle and intensity of the primary radiation in accordance with the topography, structure and chemical composition of the sample. The secondary radiation emanating from the sample is detected, analysed and displayed in the form of an angle-, energy- or mass-resolved spectrum, which contains information about the sample and its surface.

The various types of probing primary radiation and detected secondary radiation have spawned more than 50 different analytical techniques over the decades. Some of them are useful for solving practical problems and have made their way into routine work. Many of them, however, never passed the experimental stage and have very limited application to technical samples outside of academia.

In this book, we will cover those techniques which have proven to be very useful for routine work and can deliver data in a reasonable time and at reasonable cost.

The in Table I-1 (page 16) mentioned techniques yield different data about the sample. Each has its particular strengths and weaknesses. It is very important to appreciate this when try-ing to find the right combination for the given analytical problem. It is commonly said that one technique on its own is no use and so a combination is the best way of achieving the right results. The parameters to know about a technique are its

• information depth• detection limits• information content• suitability for technical problems

Some techniques, for example, allow only very limited sample sizes, which sometimes renders the technique useless for “real world samples”. Others require vacuum conditions, and that excludes liquid or volatile samples. Only a handful of techniques have proven useful for routine work. The limiting features are:

General considerations


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• measuring time per sample• suitability for technical samples• comprehensive databases of reference materials• sample preparation

Another important question is the sample area to be analysed. If, for example, a paint crater a few microns in diameter has to be analysed for possible surface contaminants capable of

causing cratering, the technique to use must allow for spot analy-sis. This means the investigation of a very small spot with a lateral resolution of a few microns.

If, on the other hand, it is the general surface quality of the sample which is of interest, a larger area measurement must be performed in order that a rep-resentative image of the surface composition may be obtained.

Analysis of the distribution of a specific substance over a certain

Surface

Table I-1: List of analytical techniques

Primary radiation

Secondary radiation

Technique Abbreviation Analysed area

electrons electrons auger electron spectroscopy

AES uppermost molecular layer

scanning electron microscopy

SEM sample surface down to a depth of a few microns

X-rays electron microanalysis

ESMAEDXWDX

sample surface down to a depth of a few microns

infrared infrared surface infrared spectroscopy

FT-IRATRIRRAS

sample surface down to a depth of a few microns

Infrared microscopy

IRM

X-rays electrons X-ray photoelectron

XPSESCA

sample surface down to a depth of a few nanometres

ions ions secondary ion mass spectroscopy

SIMSTOF-SIMS

uppermost molecular layer

Figure I-7: Measuring modes in surface analysis


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area calls for a scanning tech-nique that generates a chemi-cal map of the analysed area. On the assumption that not only the chemical composition of a surface area has to be analysed but also the depth distribution, a depth-profiling mode needs to be chosen.

That entails sputtering the sam-ple layer by layer and analysing the surfaces as they become exposed.

1.3 Instrumentation

Many of the analytical techniques we will describe here require vacuum conditions. Although they differ greatly in detail and in their chemical background, the instrumentation used fol-lows a general concept.

The instrumentation setup for all techniques consists of an excitation system (the primary system) that generates photons, electrons or ions. The primary beam is directed by a focusing system onto the sample surface and into the desired area. Some techniques have an additional sputtering system (e.g. an ion gun) that allows for subsequent sputtering of layers and thus for depth profiling. After interaction of the primary beam with the sample (surface), the excited secondary radiation is collected by a ray optics system which directs the secondary beam towards the analyser. The analyser separates the secondary radiation spectroscopically by energy, direction or mass. The detector records the separated or resolved signals and measures their intensity. The signals generated by it are displayed as a spectrum, which is a chart of intensity versus wavelength, mass, or energy.

Figure I-8: Schematic diagram of instrumentation

Instrumentation


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Part II Theory of analysis

1 Optical light microscopy

One of the main topics of this book is failure analysis. Years of experience of failure analysis show that a great deal of the serious damage is due to very small defects, particles, fibres, cracks or material defects. The first step in failure analysis is to find out what kind of prob-lem we are dealing, and in most cases the tool to use is (optical) light microscopy. This basic technique can be used to carry out a preliminary sample inspection to gain an overview of the problem. Light microscopy reveals initial, basic answers to such questions as

• What might be causing craters and spots in paint layers?• Where does paint delamination originate in a multi-layer system?• What does a residue in a raw material look like?

The basic theory of light microscopy (LM) has been described elsewhere and will not be repeated in this book. However, it is worth focusing on a special method of LM that was developed a few years ago and offers highly interesting possibilities with respect to material surfaces.

Figure II-1: Painted key panel showing paint adhesion failure after laser treatment; A= light microscopy image of the border between lasered symbol and paint, B= EFI-3D image of the same area

Theory of analysis



ISBN: 978-3-86630-912-8


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A main disadvantage of conventional light microscopy is the lack of depth of focus. At high resolution, rough material surfaces, such as those of structured polymers or metals, cannot be inspected both very sharply and at high resolution at the same time. Therefore, in the past, a scanning electron microscope had to be used to scan the surface topography of rough and structured samples, even for low-resolution purposes.

Thanks to the latest developments in digital cameras and software solutions, the depth of focus of light microscopy can be extended virtually by a so-called EFI option (Extended Focal Imaging). This is one module of the image-processing software “AnalySIS” developed by the company SIS/Olympus. It automatically takes pictures of several focal planes in rough objects, extracts only the sharp details of each focal plane and adds them together to produce an image of unlimited depth of focus.

Figure II-2: Light microscopy image of a paint crater (top) and calculated EFI-3D image of the same paint failure

Optical light microscopy


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It is thus an easy matter to obtain topographical images of extremely rough surfaces or paint failure without the help of scanning electron microscopy for magnifications of less than x1000. This additionally permits the layer thickness and topography to be measured.

2 Infrared spectroscopy

Infrared spectroscopy (IR) is an analytical tool that has been well known for decades but only lent itself to routine work with the application of Fourier transform to data process-ing. The basic principle of IR spectroscopy is the structural characterization of materials through the absorption of infrared radiation by inter-atomic bonds. A defined wavelength range is scanned with infrared light to yield a collection of absorption information which can be displayed as bands in an infrared spectrum. The spectrum is evaluated by compari-son with reference spectra and by examining the individual peaks to identify the various functional groups in the molecule or material, such as esters, hydrocarbons, acids, amines and the like.

2.1 Physical background

The probe used in IR spectros-copy is radiation from the infra-red region of the electromagnetic spectrum. This corresponds to energies between 0.001 and 1.6 eV. These photons excite char-acteristic vibrations of the inter-atomic bonds in a molecule. The energy needed to excite the vibrations is absorbed from the incident infrared radiation.

As the bonds between differ-ent atoms have distinct bond energies, it takes characteristic energies to excite them; this is known as the “chemical shift”. These energies correspond to certain wavelengths of the infra-red beam.

Figure II-4: Simplified model of an inter-atomic bond consisting of two masses m1 and m2 joined by a spring

Figure II-3: Basic principle of infrared spectroscopy

Infrared spectroscopy



ISBN: 978-3-86630-912-8


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10 Introduction Part I Introduction 1 Surface