Determination Of The Mechanical Durability Of Organic Coil Coatings
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Orgaanisten maalipinnoitteiden mekaanisen kestävyyden määrittäminen
The goal of the thesis was to measure mechanical durability of a variety of organic coatings. The present measurement methods used at SSAB, Pencil Hardness and resistance to scratching, were found insufficient and alternative measurement methods were studied. Multiple standardized and a few novel scratch and wear methods were implemented on the coatings. The measurement methods were chosen or modified to fit measuring the properties of two layered organic coating. The chosen coatings for the thesis represent eight different coating types and from each coating type multiple colours were studied to decrease the scatter on results. The effect of surface structure on durability was studied between smooth, wrinkled and particle structured coatings. The surface structure is partially linked on gloss level of the coating and different gloss levels were also compared. Effects of the coating colour were also studied, because in different colours the amount and quality of pigments differs. The coatings were chosen on the bases, that the same colours were represented in most of the coatings and so, that from the same coating type low gloss and high gloss were available. With the test methods mostly critical levels were measured. The critical levels are when the first scratch is visible (Lc1), when the coating is scratched down to primer (Lc2), when the first pinholes through both organic coating layers are visible (Lc3) and when the whole two layered coating is worn off (Lc4). In wear testing, also visual changes in the coating were evaluated in scale of 0 to 5 and volume loss of coating was estimated from mass changes during the test. In many cases the critical levels were only applicable measure of wear and the evaluation was mainly done visually. Twelve different scratching tests were implemented in the thesis. Three of the methods were measuring Lc1 level, two of the methods measured Lc2 level, five of the methods measured Lc3 level and one method measured level Lc4. In addition, scratch hardness was measured from the width of the scratch implemented by Bruker Tribolab UMT-3 device. In principle, all four Lc levels are possible to measure from a progressive load scratch and it was studied among multiple constant load scratches. Six different wear devices were used in the thesis and sixteen different methods of determining the durability were studied. With one device, it was typically possible to estimate the amount of wear after certain amount of time and study the amount of time needed to wear the samples until certain Lc level. The wear tests were chosen to make it possible to study abrasive wear, fatigue wear, adhesive wear and erosive wear separately. Also, a novel method especially designed for measuring the wear event inside coils was studied. To help the comparison of test results and test methods, all the results were scaled mathematically in the range of 0 to 5. With the uniform scale, it was possible to determine average results for the durability of the coatings and estimate systematic and statistical differences between methods and average results. Correlation coefficient between measurement methods and average results were used to help the comparison of different test methods. The average result from the coatings was estimated to describe the mechanical durability of coatings so well, that the statistical analysis could be used for ranking the measurement methods as well. In the tests, polyurethane based coatings were most durable, polyvinylidene based coatings were slightly less durable and polyester based coatings had the lowest durability. The gloss level was noticed to influence on the durability of the coatings, favouring higher gloss products. Also, the colour of the coating was noted to have some influence on the durability of the coating, and black coloured coatings were more durable in most coating types. The cause for the difference might have been either the amount or the quality of the pigments. In black coatings, there is less pigment by the volume and thus there is more binder to be worn. Also, the used pigment, carbon black, may improve wear properties of polymer-based materials. The thickness of the coatings had some impact on the mechanical durability of the coatings, but for example thin low gloss polyurethane coatings were approximately on the same level of durability as thicker polyester based coatings. It was determined, that the best way to estimate mechanical durability of organic coatings is to measure separately scratch resistance and wear resistance of the coating. The scratch measurement device must have enough accuracy in the adjustment of load, stabile load and stable scratching angle. Examples of suitable scratching devices are Braive Instrument Multifunction Scratcher, Bruker UMT-3 Tribolab and Erichsen Scratch Hardness tester 413. In wear testing, multiple different test methods can be used and at least the tested abrasive and erosive methods gave rather uniform results. In determination of mechanical wear resistance of coatings, for example Bruker UMT Tribolab rotary wear, Taber Rotary Platform Abrasion Tester and Solid Particle Erosion tester were found to be suitable.