Dietrich Paint Analysis

2nd Revised Edition
2. Auflage 2021
ISBN: 978-3-7486-0434-1
Verlag: Vincentz Network
Format: EPUB
Kopierschutz: 0 - No protection

E-Book, Englisch, 275 Seiten

The market demands modern, high-performance, flawless paints that possess specified properties. Where deviations from set points occur, the cause must be investigated and the error must be remedied. What "standard methods" don't disclose is why a particular coating either meets or fails to meet a requirement. Thus the author presents modern analytical techniques and their applications in the coatings industry that answer further complex questions. The information in this book can be used for performing failure analysis, production control and quality control, and also meet the requirements of modern high-level quality management. An excellent combination of theory and practice for formulators, paint engineers and applied technologists seeking a sound basic introduction to instrumental paint analysis and concrete answers to everyday problems.

Autoren/Hrsg.

Dietrich, Roger

Leseproben

2The analytical procedure
It is a common misbelief that the process of investigation of coating failures starts when the samples arrive in the laboratory. In fact, the process sequence starts earlier on the site of production. The whole process flow of failure analysis comprises: the investigation goal the design of experiment the sampling procedure the storage of samples the transport to the laboratory the first sample inspection sample preparation selection of the appropriate measurement technique sample measurement data evaluation data interpretation report perception of the results Figure II.2: The risk to fail in the analytical process (arbitrary units)   Each step has its challenges and can ruin the whole outcome with respect to the final goal of removing the root cause of the fault. But what is the best path to a sustained solution of production failures? A close look at each step one realizes the obstacles that can appear. It will be demonstrated how internal production knowledge, low cost technical aids, laboratory analyses, expert knowledge and last but not least common sense generate a base on which a reliable solution can grow Figure II.3: Coating failure analysis proceeding after sampling   Normally the trouble starts with suddenly and unexpectedly appearing failure parts. Of course, this requires immediate action in order to find the cause and define measures to eliminate the issue. But how? Quite often the author has experienced a procedure like this when called to help with these issues: let us have a meeting let us look for someone guilty let us have a meeting again put pressure on the employees let us have a meeting again ask for different plans to solve the problem then panic All these “actions” are driven by the understandable desire to get rid of the issue as soon as possible to make sure that the costumer can be served without any delay. The call for a systematic investigation of the root causes is often unheard facing pressure from costumers and financial accountants. Under the pressure of doing “something” plans and measures often suffer from a severe lack of reliable facts but are based on assumptions, rumours or feelings. Nevertheless, sometimes by chance the problems disappear, but nobody can say why. Moreover, nobody can guarantee that it will not pop up again and there is no solution for the future. But what is the value of a removal of the issue instead of a solid solution? For sure, the systematic approach is slower and sometimes does not deliver “results” within a short period of time, but at the end it is more reliable and thus cheaper. Of course, a good plan of what to do is a good start: Figure II.3 shows the favourable path through the whole investigation process. The necessary tool are analytic methods to gather facts and place the latter in opposition to assumptions. But the analytical option is “only” delivering reliable data which are necessary to circle the source of the failure and the area of the production line, where it can be located. From that end to the real solution there is more to do. 2.1Inquiry
Let us assume a paint crater issue appearing in an automotive supplier plant. Someone sampled a failed bezel and presented it. To solve the issue, you would want to know as much as possible about the circumstances of this sudden production failure. Figure II.4: Visible light microscopy picture of a paint crater   A good method is an interview of all the people involved in the project. The questions to be answered with respect to this case are: Where does the sampled part come from? (directly from your process, somewhere from the supply chain, from the customer) What is the history of this sample? (which does not mean which way parts of this kind typically go, but what the sample in your hand has “experienced”) How many parts are affected? (percentage of the lot/production) When did the issue appear? When has it been realized? Are there still good parts which have been produced in the same process? Are there any changes of production parameters that can be correlated to the failure? Are there any unusual circumstances around the production line that might have influenced the issue? (e.g. construction works, repair, cleaning procedures) Are there any (proven) correlations to certain lots, production shifts, material lots, production tools and so on? Please note: You can never enquire too much but always too little. Be curious and vigilant during the whole data acquisition process. Each piece of information can be very precious, when it comes to the data evaluation of the analyses. The history of a sample also includes the exact circumstances of the sampling. Interviews of the people on the site that deal with that kind of product every day and know the machines very well are a versatile tool. 2.2Inspection
2.2.1Macroscopic inspection
The next step of the analytical process is a personal, visual inspection of the samples. The human visual sense is very sensitive and sometimes realizes details that are hard to measure by machines. It is sometimes quite frustrating to admit that e.g. discolourations are clearly visible but often cannot be detected even by the most sensitive instruments. On the other hand, there are people who call themselves experts and believe that they can classify a failure type just by macroscopic visual inspection. This must be doubted because e.g. a paint bubble of 20 µm diameter in a primer layer of a multicoat system may appear as a crater or a speck in the topcoat upon visual inspection and this is impossible to distinguish without instrumental assistance. But the watchful eye can find out very important details about the samples which can be precious, when it comes to the evaluation of the analytical data: Do the samples comply with the targeted goal of the analyses and the design of experiment? How does the failure appear? Are the failure spots (e.g. paint specks) randomly distributed or located in specific areas? Are there any optical differences between failed parts and sound parts? Does the sample exhibit further unusual features which might hint at application failures? How many failure spots are noticeable? Are all visible failure spots similar or are there different failure types on the same part? Of course, this list is not comprehensive. Each individual issue requires a customized set of questions. It is good practice that the person who plans the final analytical approach (see Part II Chapter 2.3 “Design of experiment”) should have seen and thoroughly inspected the samples himself before. Please note that touching, wiping, rinsing or other influences on the samples must be avoided during this first inspection, because it can ruin the target area for the analysis. 2.2.2Microscopic inspection
Figure II.5: Microscopic inspection by a computer microscope   For the first inspection it is very helpful to use a simple mobile microscope (see Figure II.5). These low-cost computer microscopes are easy to transport, easy to use and often deliver surprisingly good pictures that help to improve the sample documentation and support the design of experiment for the next more sophisticated analytical steps. Especially, it is essential when it comes to field investigations at a costumer site or in a production plant. The main task for this inspection is to distinguish easy-to-solve issues from more complex problems. If (for example) a paint chip is sampled from a steel object (machine, steel structure) and the backside of this paint chip exhibits microscopic traces of steel dust or if there are suspicious features of the surface underneath the detached paint like grinding or scratching marks that might hint at a sample pretreatment or drying residues that can point at insufficient cleaning. This information can narrow the scope of the analyses which have to be planned. A paint failure in a multilayer coating system can appear as a bubble of the clear coat upon first microscopic inspection but, in fact, is caused by voids or cracks of the base-material. This first tool can help to gather more information about the failure but should not be overestimated on the other hand. If e.g. a paint chip disbonds from a clear primer layer because the primer has not been cured sufficiently, the...