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Roger Dietr ich

PAINT ANALYSIS2nd Revised Edition

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Roger Dietrich

Paint AnalysisThe Handbook for Study and Practice

2nd Revised Edition

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Roger DietrichPaint Analysis, 2nd Revised EditionHanover: Vincentz Network, 2021European Coatings Library

© 2021 Vincentz Network GmbH & Co. KG, Hanover Vincentz Network GmbH & Co. KG, Plathnerstr. 4c, 30175 Hanover, GermanyThis work is copyrighted, including the individual contributions and figures.Any usage outside the strict limits of copyright law without the consent of the publisher is prohibited and punishable by law. This especially pertains to reproduction, translation, microfilming and the storage and processing in electronic systems.The information on formulations is based on testing performed to the best of our knowledge.

Please ask for our book catalogueVincentz Network, Plathnerstr. 4c, 30175 Hanover, GermanyT +49 511 9910-033, F +49 511 9910-029books@european-coatings.com, www.european-coatings.com

Layout: Vincentz Network, Hanover, GermanyPrinted by: Gutenberg Beuys Feindruckerei GmbH, Hanover, Germany

ISBN 3-7486-0425-4ISBN 978-3-7486-0425-9

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European Coatings Library

Roger Dietrich

Paint AnalysisThe Handbook for Study and Practice

2nd Revised Edition

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Foreword

I am really glad that you found this book and opened it, curious to find out what to expect. This is the second edition of the book which was first published in 2009 and I can promise a lot of new interesting insights into field analysis and laboratory work with respect to coatings. Compared to the first edition you will find new techniques, additional advanced preparation and sampling methods and a lot of more practical examples. My intention was and is to share my experiences that I have made over the last 30 years in our laboratory and in the field with respect to the anal-ysis of paints and coatings. I am a chemist and my approach to characterize a coating material, a raw material and coating failures is therefore primarily scientifically evidence based. I believe that is very important to have a profound insight into the basic, physical conditions and limitations of the analytical methods in order to comprehend their power and limitations. But I also have worked in the coating industry and know that is not always possible to investigate a problem extensively according to scientific rules. Therefore, I have tried in this second edition to shift the focus more to practical implications of analysing paints and coatings without neglecting the physical and methodical background. I added a section about sampling methods and representativeness be-cause I have made the experience that the implications of proper sampling on the investigation result are too often disregarded.

Of course, you know the routine measuring and testing methods that tell something about colour, gloss and other physical conditions of paint films and coating material. These techniques provide the information how a certain material behaves. But they fail when it comes to the ques-tion why a tested material exhibits a particular property.

This is where this book comes in. I present a toolbox of analytical methods which exist for decades but have not yet found widespread use in the coating industry. However, I would like to demonstrate that the potential of the featured methods is incredibly big and far from being ex-ploited.

The possible applications of the methods described here grow with the requirements of the samples to be examined. Almost every month a new question arises which can be answered with a process development based on the methods presented in this book.

Although I cannot deny my education as a chemist, the book is aimed at a broad group of users from the coatings industry. It is intended as an aid for the application engineer on site to start solving problems, but as well to provide the laboratory manager with suggestions for new ways of dealing with his tasks. I tried to build a bridge between the scientific view on the reality and the practical requirements on the site and in the applications laboratories.

I would like you to hold this book in your hands as a daily reference and tool, and to use it extensively to find out how you can – reveal the causes of coating defects, – investigate paint raw materials with respect to contaminations – or analyse lacquered products in terms of their properties

So, if after a short time this manual is lying on your desk or laboratory table with paint stains and marginal notes, I would be more satisfied. In the practical part of the manual I have, therefore, tried to structure and process the topics in such a way that you can draw immediate suggestions for action if, for example, paint craters occur when coating products or if you want to know wheth-er and how two paint batches differ. Of course, this is backed up with theoretical principles in a separate chapter. But the problem solutions described in the practical part should ideally enable

you, like a kind of “recipe”, to proceed directly to action with the book in your hand to tackle your daily challenges.

I do not see this book only as a handbook documenting the current state of the art, but I would like to use it to stimulate dialogues from which improvements of current procedures or perhaps even new developments can arise. I am therefore always open to suggestions and com-ments and hope to receive numerous feedbacks, which I will gladly take up and answer as soon as possible.

Münster, November 2020

Roger Dietrichdietrich@ofg-analytik.de

+49-(0)30-6780-6030 info@LUM-GmbH.de I www.LUM-GmbH.com

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Contents

ContentsPart I General information about paint analysis 111 The surface 112 Why paint analysis? 143 Relevance of modern analytical techniques to paint analysis 154 General considerations 165 Chemical mapping 19 5.1 Infrared microscopy mapping 20 5.2 TOF-SIMS imaging 21 5.3 SEM-EDS mapping 226�� �Depth�profiling 227 Instrumentation 24

Part II Coating failure analysis1 The bumpy road to knowledge 25 2 The analytical procedure 27 2.1 Inquiry 28 2.2 Inspection 29 2.2.1 Macroscopic inspection 29 2.2.2 Microscopic inspection 29 2.3 Informed guess 30 2.4 Instrument selection 32 2.5 Investigation 34 2.6 Interpretation 34 2.7 Iterance 37 2.8 Implementation 38 2.9 Incumbency 393 The power of sampling 40 3.1 The role of sampling in the analytical procedure 40 3.2 Random sampling and representativeness 40 3.3 Targeted sampling 45 3.4 Non-destructive sampling 45 3.4.1 Wipe sampling 45 3.4.2 Rinse sampling 46 3.4.3 Abrasive sampling 48 3.5 Typical sampling failures 49 3.5.1 Wrong sample collection 49 3.5.2 Non-representative sampling 50 3.5.3 Selecting the wrong amount of samples 50 3.5.4 Application of a wrong sampling procedure 51 3.5.5 Inappropriate sampling tools and containers 51 3.5.6 Insufficient storing and shipping of samples 52 3.6 Micro sampling 534 Paint failures and their analytical approach 54 4.1 Some considerations on failure reasons 54 4.1.1 Insufficient workpiece preparation 55 4.1.2 Handling failures 60

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Contents

4.1.3 Application conditions 60 4.1.4 Environment and climate 62 4.2 Investigation of adhesion failures 63 4.2.1 Delamination due to substrate contamination 66 4.2.2 Adhesion defects caused by migration processes 69 4.2.3 Delamination caused by moulding conditions 71 4.2.4 Delamination due to application faults 73 4.2.5 Delamination due to insufficient pretreatment 77 4.3 Paint cratering and “fisheyes” 78 4.3.1 Cratering caused by contamination of the paint material 82 4.3.2 Craters and pinholes caused by substrate contaminants 84 4.3.3 Craters caused by paint additive agglomeration 86 4.3.4 Cratering caused by the application conditions 88 4.4 Bubbles and blisters 90 4.5 Discolouration 93 4.6 Hazes and stains 97 4.7 Paint spots 102 4.8 Orange peel 108 4.9 References 109

Part III Quality control and process analysis 1111 Quality control of raw material 112 1.1 Binders 113 1.1.1 Identity check 114 1.1.2 Detection of trace contaminants 115 1.2 Solvents 117 1.3 Pigments and fillers 1212 Quality control of paint production 124 2.1 Analysis of filter residues 124 2.1.1 SEM/EDS analysis of filter residues 125 2.1.2 FT-IR analysis of filter residues 127 2.2 Analysis of fogging residues 130 2.3 Quality check of finished and semi-finished products 132 2.3.1 ATR-FT-IR screening 133 2.3.2 TOF-SIMS analysis 135 2.3.3 Headspace GC-MS analysis 135 2.4 Paint quality tests 1353 Field analysis 140 3.1 Process analysis of paint shops 141 3.2 Aerosol analysis 144 3.3 Operating test 145 3.4 Sampling of the painting air 147 3.5 Monitoring of pretreatment steps 149 3.6 Investigation of the degree of crosslinking in 2-pack paints 152 3.7 Investigation of paint additive migration 155 3.8 Marine and aircraft coating inspection 157 3.9 Handhelds and portables 158 3.10 References 160

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Contents

Part IV Methods of coating analysis 1611 Optical light microscopy 161 1.1 Extended focus imaging (EFI) 161 1.2 Differential interference contrast (DIC) 1642 Fluorescence microscopy 1643 Infrared spectroscopy 165 3.1 Physical background 165 3.2 Characteristic absorptions 166 3.3 Instrumentation 168 3.4 Sample preparation 169 3.5 Spectrum representation 170 3.6 Quantification 170 3.7 Data analysis and evaluation 172 3.7.1 Data processing 173 3.7.2 Use of databases 1744 Surface infrared spectroscopy 176 4.1 ATR-FT-IR spectroscopy 177 4.1.1 Physical background 179 4.1.2 Depth of penetration 179 4.1.3 Information depth 180 4.1.4 Effective path length 181 4.1.5 Quantification 181 4.1.6 Detection limit 182 4.1.7 Instrumentation 184 4.1.8 Sample preparation 187 4.2 Reflection infrared spectroscopy 188 4.2.1 Physical background 188 4.2.2 External reflection 190 4.2.3 Instrumentation 196 4.3 Diffuse reflection spectroscopy 196 4.3.1 Physical background 197 4.3.2 Penetration depth 199 4.3.3 Influences of variable parameters on the spectrum 199 4.3.4 Sample preparation 201 4.3.5 Instrumentation 202 4.3.6 Quantification 202 4.3.7 Optimization of the measuring parameters 203 4.3.8 Repeatability 2035 Infrared microscopy 204 5.1 Instrumentation 205 5.2 Sample preparation 207 5.3 Infrared microscopy, infrared transmission mode 207 5.4 Infrared microscopy, reflection mode 207 5.5 Infrared microscopy, ATR mode 210 5.6 Line-scan und mapping analyse 2136 Raman spectroscopy 215 6.1 Physical background 216 6.2 Instrumentation 217

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Contents

6.3 Advantages and limitations 218 6.4 Quantification 219 6.5 Applications 2197�� �Time-of-flight�secondary�ion�mass�spectrometry 222 7.1 Physical background 223 7.2 Instrumentation 225 7.3 Calibration and mass resolution 225 7.4 Sample preparation 226 7.5 Spectral evaluation 226 7.6 Imaging mode 229 7.7 Quantification 229 7.8 Summary 232 7.9 Applications 2328 Scanning electron microscopy 235 8.1 Physical background 235 8.2 Lateral resolution 238 8.3 Instrumentation 239 8.4 Sample condition 2399 Electron microanalysis 241 9.1 Physical background 242 9.2 Quantification 244 9.3 Detection limits 245 9.4 EDS-Imaging 245 9.5 Applications 24610 X-ray photoelectron spectroscopy 247 10.1 Physical background 247 10.2 Information depth 248 10.3 Lateral resolution 248 10.4 Information retrieval 249 10.5 Quantification 251 10.6 Instrumentation 252 10.7 Applications 253 10.8 Technical data 25411 GC-MS 254 11.1 Physical background of GC 255 11.2 Headspace 256 11.3 Data evaluation 258 11.4 Application 25912 Thin layer chromatography TLC-ATR-FT-IR 261 12.1 General principle 261 12.2 Separation procedure 261 12.3 Identification 262 12.4 Performance parameters of selected methods 263

13 References 264 Author/Acknowledgements 266 Index 268

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The surface

Part I General information about paint analysis

For the investigation of paints, semi-finished products, raw materials and finished coatings, a very extensive “toolbox” of the most diverse analytical methods is available today. This is sometimes a bit confusing for the non-analyst. For an effective use of the individual methods, it is necessary to have a very detailed knowledge of the possibilities and limitations of each method, the excitation modes and the information to be expected. The book that you just opened will try to shed some light on the thicket of methods. A great deal of attention is given to surface processes, as the ma-jority of questions arising in practice are related to surface phenomena in one way or another. But also the so-called “bulk analysis” plays an important role in this book. Classical test methods such as climate-testing, solar simulation etc. are deliberately omitted because these are test methods for a certain parameter but have a completely different objective and are not analysis techniques.

1 The surfaceThis 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

Roger Dietrich: Paint Analysis© Copyright 2021 by Vincentz Network, Hanover, Germany

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

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Part I General information about paint analysis

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 ana-lytical 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.

Definition of the term surfaceSo, 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.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 qual-ity 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

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

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The surface

corrosion during transport and storage, the paint will exhibit poor adhesion or craters after appli-cation. 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 pro-duce 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 reflec-tion spectroscopy, it is impossible to check the performance of the pretreatment process with respect to crystal morphology and percent-age of coverage. The author has worked on a lot of adhesion fail-ures 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 ma-terial 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 paint-ing process is started. – What is the chemical composi-

tion 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 ad-ditives have on paint adhesion?

Unfortunately, these questions of-ten can’t be answered by simple

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

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Part I General information about paint analysis

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 re-quirements simultaneously that are sometimes hard to match. This does not only create a de-mand 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 custom-er demands. In accordance with the requirements imposed on the polymer material, he adds ad-ditives to improve flame, light, impact or heat resistance. One parameter the supplier is not con-cerned 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 en-hance 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, dis-colouration, 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.

2 Why paint analysis?A few years ago, the author was talking to the CEO of a paint manufacturer who had different issues with costumers complaining about the performance of several paint types on a constant level. I asked him if he ever thought about using the powerful toolbox of analytical methods to improve his paint production. He answered: “We sell lacquer, we are not an analytical company!”

For him “selling” was the key word. For the quality of the incoming raw material he relied on the factory test certificate of the suppliers, with respect to the quality of the produced paint he banked on his processes.

But certificates provided by the supplier do not tell everything, what is essential to know to ensure a safe and high quality production. The best process documentation and compliance can-not guarantee a flawless production. In addition to that, the experience has shown that a lot of customer complaints in the field are not substantiated, because they based on application faults in the coating process. Without knowledge about the details of raw materials, the products and the field application a manufacturer is on a constant “blind flight”. Some paint producers have understood this and profit from a well-equipped laboratory and excellently educated lab staff. But for the commercial department a laboratory and analytical operations are very often simply “lost” costs without profit. On the long run however it pays off to understand the details of a production line. It is a competitive advantage to understand the details of a costumer`s paint shop to avoid unsubstantiated complaints and help him to improve his process. And it is good to know the raw

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Relevance of modern analytical techniques to paint analysis

materials in detail to save costs with respect to production failures caused by deficient raw mate-rial.

Often, when assigning an investigation task to an internal or external laboratory, high expec-tations are placed on the laboratory:– It should be done quickly (“Best by the day before yesterday”)– There must be no charge– One analysis should, if possible, comprehensively solve the entire possibly complex

production problem

It should not be surprising, that these wishes will not come true. Solving a production problem normally requires complex procedures, with analytical methods providing the necessary facts. But this process avoids expensive “try and error” actions and aimless activities. The prerequisite for a reliable solution based on measured data are:– A qualified problem analysis– caution during sampling– the selection of appropriate testing methods– specialist knowledge of the laboratory– experience in the evaluation of measurement data– accuracy in the interpretation and formulation of results – sound translation of the results into the process

This requires a very intensive exchange with the laboratory in order to effectively combine in-house knowledge and process know-how with the knowledge generated by the analysis. The di-viding line between the internal knowledge of the commissioning company and the external knowledge of the laboratory depends on the personnel structure of the company. A contract coater with a more artisan orientation will usually not know and need many details about paint chemistry, while an automotive supplier specializing in industrial series coating often has its own competence teams on site for all questions of paint chemistry. Both they can also benefit from instrumental analysis methods if these are used in the service and, if necessary, the knowledge of the laboratory service provider is taken into account. Roughly speaking, the less knowledge about the processes and materials in the commissioning company, the more the external knowledge of the service provider can and should be consulted. If this interaction works well and smoothly, in some cases even one analysis costing a few hundred Euro, can save five times as many costs for complaints or faulty batches.

3 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 product 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 tech-niques are perfect for checking the quality of a product. However, if a product is defective and the root cause has to be investigated, the standard techniques are not very helpful. For example, a monomolecular layer of a release agent on the surface can easily cause severe adhesion failure if