In-depth analyses - European Coatings · between them is essential for highly scratch-resistant...

Quelle/Publication: European Coatings Journal

Ausgabe/Issue: 05/2010

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In-depth analyses

Several advanced coatings tests are discussed. Highlysophisticated measurement equipment can now offerdetailed insights into the structure and behaviourof paints and coatings. New test procedures alsoprovide more accurate assessments of long-termcoating performance by simulating field conditions andusing sensitive equipment to detect the first stages ofdegradation.

Detailed studies of coating structure and performance arenow possibleUlrich Christ*Rolf Nothhelfer-RichterMatthias WannerThadeus SchauerPaints, coatings and their raw materials have gainedin economic importance over recent decades. Thedevelopment of measuring and testing strategies has beenessential to this success, as the focus of the coatingsindustry has shifted from colour and protective properties tocoatings with specific functionalities. Today, measurementtechniques for paints and coatings are categorised asfollows:Reliable and simple measurement techniques forcomparative quality control testing. These are often single-point measurements performed at room temperature,disregarding the different environmental conditions found inpractice.Multi-point measurements covering a wide range oftemperature, humidity and other conditions found inpractical use can be carried out with modified stan-dardequipment.A better understanding of structure-property relationshipsand specific insights into molecular aspects and interfacialphenomena offer a basis for more precise testing withsophisticated equipment.Accelerated short-term tests allow the prediction of long-term coatings performance.Below, some examples of recently developed methods arepresented to illustrate scientific progress in this field.

Speed and relevance of corrosion testing enhancedThe introduction of new substrates, pre-treatments andcoatings requires a shared corrosion test for multi-metalapplications, such as aluminium, steel and zinc substratesfor the automotive industry and its suppliers.The VDA 621-415 cyclic laboratory test, developed morethan 20 years ago, served as the standard corrosion test forautomotive coatings and substrates, but has now reachedits limits. A working group comprising members of steel,automotive and automotive sub-supplier industries made acritical analysis of its deficits and designed a new three-cycle laboratory cyclic corrosion test (new VDA test, VDA621-415 B) (Figure 1).Round-robin tests confirmed its high reproducibility. Onestriking result is that coating delamination on galvanisedsteel is ranked in the correct ratio to steel and in accordance

with practical experience. This test is also faster, taking sixweeks to produce the same degree of delamination that theold test produced in ten. Filiform corrosion on aluminium andsteel can also be included in the ranking [1, 2].

Tests of protective properties, old and newThe protective properties of coatings are widely and reliablycharacterised by long-term or field exposure tests such asimmersion in sea water or atmospheric exposure. However,short-term laboratory tests are required to reduce costs andtime delays.These tests involve, for example, immersion in wateror electrolyte solutions (NaCl, Na2SO4, or a dilutesolution of ammonium sulfate and sodium chloride termed"Harrison Solution" [3]) as well as water condensation orcorrosion chambers [4]. These tests are still relatively time-consuming, often correlate poorly with practical results [5],and their evaluation is subjective rather than quantitative.By contrast, the FPL test is based on the forced build-up ofinternal stress in the coating under controlled conditions. Aquantitative evaluation of the loss of protection is obtainedvia Electrochemical Impedance Spectroscopy (EIS) and/orScanning Kelvin Probe (SKP).

Coating stresses can be varied as requiredThe loading concept of the new accelerated test consistsbasically of thermal cycling combined with electrolyteexposure [6] implemented by the set-up shown in Figure2. The sample is located in an open box with a coolingplate controlled by the thermostat. The IR and UV lampsabove the box irradiate the samples uniformly. Since thesample surface is heated by this irradiation while the bottomis cooled, a vertical temperature gradient results.The control unit then induces temperature changes,typically sinusoidal variations in the temperature rangebetween 10 °C and 70 °C. The temperature of paintedsamples within the electrolyte compartments is measuredcontinuously by a thermocouple.Thermal cycling creates internal stress which increases withtime of loading (Figure 3). The Tg also tends to increaseduring temperature cycling, clearly indicating a reductionin elasticity and increasing rigidity. This may lead to thebuild-up of pores, fissures, cracks and coating delamination,followed by formation of blisters and corrosion of thesubstrate.Parameters which influence coating degradation are thetemperature gradient, temperature changes (sinusoidal,triangle or square-wave function of time, and the frequencyof changes), electrolyte and any UV irradiation. With allthese parameters, both conventional laboratory tests andfield trials can be simulated and the test duration can beeffectively shortened.

How early stages of coating degradation are detectedEIS is highly sensitive to even the early stages of coatingdegradation. The analysis is usually performed by fittingraw data to spectra calculated for equivalent circuits,



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which represent in the intact coating, pores, delamination,corrosion and water uptake (Figures 4 and 5).Derived numerical values of the specific resistiveand capacitive components of the impedance provideinformation about water uptake, pore formation, coatingdelamination and substrate corrosion. However, theimpedance data are integral values over the surface, andcannot resolve localised surface phenomena. SKP may beused for this purpose. The surface of a coated sample isscanned and expressed in terms of the work function ofelectrons which indicates the energy required to extract anelectron from the substrate (Figure 6).The correlation between the work function and theelectrochemical potential allows this potential to be usedto evaluate the electrochemical state of the metal surfaceas well as the mechanism of the metal protection withcoatings [7]. The high lateral resolution allows local surfacephenomena to be evaluated, such as development of pits,cracks and coating delamination [8]. Time-resolved SKPdata provide additional information about kinetic aspects ofthese phenomena.The SKP scan in Figure 7 represents the work function fora steel substrate with an epoxy coating, tested after 70 h ofthermal cycling under 1.0 Harrison solution. The artificiallydamaged area (a hole of 0.5 mm diameter) shows largervalues of the work function, indicating oxidation of the metalsurface due to corrosion. In contrast, the area around thisshows significantly reduced values of the work function,indicating active metal corrosion and the extent of coatingdelamination.

Coating permeability can be measured accuratelyThe oxygen and water vapour permeabilities of coatingsaffect their protective properties. Several standards exist(e.g. DIN EN ISO 7783-2, DIN EN ISO 1062-3, DIN53380-3, ASTM D 3985-05, ASTM F 1249), which defineprocedures for determining the oxygen and water vapourpermeabilities of films or foils. However, these non-automated procedures are subject to experimental flawsand need several trials to rule out statistical errors.Much better reproducibility is provided by automated watervapour and oxygen permeability testers, which detect thegradient of the water vapour or oxygen concentrationbetween a reference and a measuring chamber separatedby the film to be tested. Under defined conditions, the watervapour or oxygen permeability of the film can be determinedin units of g/(m2 24 h) (water vapour permeability) or ml/(m2 24 h) (oxygen permeability). When stable free films arenot available, coatings can be applied on highly permeablesupports.

Scratch resistance studied in detailScratch-resistant coatings are currently the state of theart for painting of cars, wood products and other metallicand plastic substrates [9-11]. The principles of scratchresistance and the optimal analytical test procedures are,however, still not fully known. In particular, the role ofnanoparticles appears controversial [12-16]. Research onautomotive clearcoats has shown that careful matching ofthe binder matrix to nanoparticles with chemical crosslinkingbetween them is essential for highly scratch-resistantcoatings [17].The nano-scratch tester (NST) provides detailedinformation about the generation of a single scratch [18, 19]

and is regarded as the most scientific method. During thescratch process, data of the tangential Ft and normal forceFn as well as the depth are recorded. The penetration depthdp of the indenter and the permanently remaining depth ofthe scratch (dr) are calculated.Three multi-scratch methods are also common in theautomotive coating industry: "Crockmeter", "Rota-HubScratch-Tester" and "Amtec-Kistler" laboratory car wash[18-20]. The samples are ranked according to the residualgloss obtained from gloss measurements before and afterscratching. Reflow properties are evaluated by re-testingafter one or two hours’ storage at 60 °C or 80 °C.

Surface analysis is even possible on the nanoscaleNanoparticles and nanostructures are also used to controlpaint rheology, to provide easy to clean surfaces, to protectagainst microbiological attack and to improve corrosionprotection. Highly sophisticated measurement tools tovisualise or even manipulate the nanoscale are essential tounderstand their effects.One powerful tool is SEM combined with a focused ionbeam (FIB). The field emission cathode with a resolutionbelow 2 nm provides visualisation of nanoparticles andnanostructures. The FIB is able to mill cross-sectionsand even produce TEM-Lamella. These thin films enablehigh resolution EDX-analysis because the volume of theexcited photons is limited to the lamella (about 100 nmthick), whereas the typical scattering volume of organic bulkmaterial has a radius of about 1 to 2 µm. This technique isapplied, for example, to identify inorganic nanoparticles orto detect ultra-thin pretreatment layers.

Detailed evaluation of Tg has practical benefitsDynamic Mechanical Analysis (DMA) is a common me-thodto determine the Tg of coatings. In most cases free filmsof the coatings are measured in tensile mode. During atemperature sweep the storage modulus E’, loss modulusE’’ and the loss angle tan d are recorded. The transition fromthe glassy to rubberlike state is indicated by a pronounceddrop of E’ combined with a maximum of the tan d curve.The plateau of E’ in the rubbery state is also an indicator ofcrosslinking density.The deduction of structure-property-relationships requiresfrequency scans to be made at a series of differenttemperatures, allowing the application of the Boltzmanntime-temperature superposition principle. Shifting thesweeps along the frequency axis yields an activationdiagram according to the Williams-Landel-Ferry (WLF) orVogel-Fulcher equation.This procedure allows system behaviour to be predicted atvery high frequencies (which occur at impacts, stone chipsetc.) as well as at very low frequencies (extrapolating tomonths or years).In recent research projects, the stone-chip resistance ofcoatings on plastic substrates was evaluated. The glasstransition temperature in respect to stone chipping wasfound to be about 20 °C to 30 °C above the Tg measuredat 1 Hz. Thus, coatings and substrates can show brittlebehaviour above their Tg.In the case of coated plastic parts, the substrate determinesthe level of damage due to stone chipping [17, 21].Substrates showing a low Tg or two-phase materialswith one low Tg component were plastically deformed,whereas high Tg materials showed brittle failure. The



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applied coatings yielded to the deformation without crackingor delamination (Figure 8).

Test method development continuesA key aim of method development is to deduce practicalproperties from physical and chemical characteristics andevaluate structure-property relationships. Another aim is toreduce testing time and enhance the prediction of life cycleperformance.

REFERENCES[1] Luckeneder G., Stellnberger K. H., AktuelleEntwicklungen in der Laborkorrosionsprüfung mitWechselklimata für die Automobilindustrie, ÖGUSTechnical Meeting, 25 Sept 2008, Vienna, Austria. [2]Stellnberger K. H., Geisler S., JOT, 2009, Vol 49 (3)pp 32ff. [3] Harrison J. B., Jnl. Oil & Col. Chem.Assoc., 1979, Vol 67 pp 18ff. [4] Appleman B. R., Jnl.Coat. Technol., 1990, Vol 62, 787, pp 57ff. [5] SkerryB. S., Alavi A., Lindgren K. I., Jnl. Coat. Technol., 1988,Vol 60, 765 pp 97ff. [6] Fernandez Gonzalez M. et al,Device and procedure for accelerated corrosion testing,Ger. Offen. DE 102004027792 A1 20060105, 2006, 12 pp.[7] Rohwerder M., Stratmann M., Proc. Electrochem. Soc.,2000, pp 302ff. [8] Yee S., Oriani R. A., Stratmann M., Jnl.Electrochem. Soc., 1991, Vol. 138 pp 55ff. [9] Osterhold M.et al, Macromol. Symposia 2002, Vol 187, pp 823-834ff. [10]Meier-Westhues U. et al, Internat. Waterborne, High-Solidsand Powder Coatings Symposium, New Orleans, 2002,pp 277ff. [11] Hilt M., 10th DFO Automotive Conference,Munich, 2003, pp 20-29. [12] Flosbach C., Macromol.Symposia, 2002, Vol. 187, pp 503-513. [13] Roscher Ch.,7th Nürnberg Congress, 2003, Vol. II, pp 417-426. [14]Poppe A., Westhoff E., Stuebbe W., 10th DFO AutomotiveConference, Munich, 2003, pp12-19. [15] van Wijk F. G.H. et al, 8th Nürnberg Congress, 2005, Vol. II, pp9-16.[16] Michael G., Mertsch R., 9th Nürnberg Congress, 2007,Paper XXV, 5. [17] Nothhelfer-Richter R., Joos-Müller B.,Phänomen Farbe, 2006, (04/05) pp. 34-35. [18] Nothhelfer-Richter R., Klinke E., Eisenbach C. D., Farbe u. Lack, 2005,Vol 111 (12), pp 42-46. [19] Klinke E. et al, Farbe u. Lack,2002, Vol. 108 (2), pp 52-61. [20] Osterhold M., WagnerG., Prog. in Org. Coatings, 2002, Vol. 45, pp 365-371. [21]Nothhelfer-Richter R., Kunz G., Eisenbach C. D., StuttgarterKunststoff-Kolloquium, 2005.

Results at a glanceNew test methods can provide detailed insights intothe structure-property relationships of coatings and moremeaningful analyses of their degradation. Unlike older tests,some procedures simulate the field conditions coatings areexposed to. Several such methods used at FraunhoferIPA, department "Coatings and Pigments" are described.The new VDA corrosion test allows coatings on differentmetal substrates to be tested, with enhanced correlationto practice. The FPL rapid test applies controlled cyclicalstresses to coatings, with the early stages of degradationassessed by EIS and SKP procedures. Water vapour andoxygen permeation tests evaluate coating barrier propertiesrapidly. Sophisticated tests are required to evaluate theperformance of nanoparticles. These range from the nano-scratch test to dual-beam SEM. DMA measurements in thefrequency domain identify critical properties related to Tg,from impact resistance to long-term performance.

* Corresponding Author:Dr. Ulrich ChristFraunhofer IPA Coatingsand PigmentsT +49 711 687 [email protected]



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Figure 7: Example of an SKP scan on a coated steel substrate, where the redcircle indicates an artificially damaged coating zone



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Figure 8: Activation diagram for different plastic substrates; the curves separatethe brittle (glassy) state (top right) from the rubbery state (bottom left)



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Figure 1: Test procedure for new laboratory cyclic corrosion test VDA 621-415B



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Figure 2: Schematic arrangement for building up internal stress in coatings bythermal cycling combined with exposure to an electrolyte



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Figure 3: Plot of internal stress as a function of temperature for a coatedsample after one thermal cycle (black) and 50 cycles (red); Tg is found from theintersection of the tangents from each end of a curve



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Figure 4: Schematic arrangement for EIS measurements



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Figure 5: Two typical equivalent circuits for data analysis of EIS



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Figure 6: Schematic set-up for SKP analysis

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