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omparison of different abrasion mechanisms on aesthetic propertiesf organic coatings
. Rossi ∗, F. Deflorian, E. Scrinziepartment of Materials Engineering and Industrial Technologies, University of Trento, Via Mesiano 77, 38100 Trento, Italy
r t i c l e i n f o
rticle history:eceived 24 June 2008eceived in revised form 10 March 2009ccepted 4 June 2009vailable online 21 June 2009
eywords:
a b s t r a c t
Organic coatings are the most commonly used system for protection from corrosion. In many applica-tions, the protective properties against corrosion are associated with several other properties, includingresistance to abrasion and good aesthetic appearance. This is particularly important for the automotiveand transport industry, building trade, domestic products and packaging.
The aim of this work is to compare two different test methodologies to characterize mar performancesof organic coatings. Taber test and Falling Abrasive test were chosen. In both tests sodium chloride and
aintbrasionrosionesthetic propertiesaber testalling Abrasive test
sucrose were used as abrasive agents in order to simulate a possible damage occurring in domestic envi-ronment. These abrasives produce a limited damage, due to the low hardness, that influences only theaesthetic aspect of the organic coating. The level of damage was evaluated through gloss measurements.The correlation between the changes of gloss and the damage was investigated using optical microscopyand environmental scanning electron microscopy.
The two tests showed a different morphology of the damage of the organic coating due to the differentbrasio
mechanism of damage: a
. Introduction
In several applications such in automotive and transport indus-ry, packaging, appliances, furniture, organic coatings need to show,n addition to good protection properties, a good resistance to abra-ion and a good aesthetic appearance. Mar is one kind of physicalamage that affects the appearance of coatings [1]. It is very impor-ant to understand this damage phenomenon, as well as to evaluateith accuracy the scratch resistance of the coating system. This type
f damage occurs within a few micrometers of the surfaces of theopcoats. The major contribution of mar is micro-scale scratchesaused by light abrasion. In this damage process there is the produc-ion of micro-scale scratches which change the pattern of reflectedight and reduce gloss. Considering this aspect to obtain usefulnformation about the mar phenomenon it is important to choosetest which produces minimal damage which could influence the
spect of the paint without modification of protective properties. Tovaluate the experimental data it is necessary to consider a property
irectly related to the aesthetic appearance [2,3].
The need to quantify the scratch and mar resistance has led tohe development of numerous test methods, some standardizedhrough groups as ASTM International, some standardized within
a particular company, and others that are ad hoc standards. Fieldsimulation tests have been developed based on service conditionsthat can cause scratching and marring of a coating that gener-ally involved either wet abrasion using abrasive slurries or dryabrasion using abrasive powders or papers [4–7]. In other studies[1,8–16], single-probe devices, including instrumented indentationand scratch systems and atomic force microscopes, have been usedto simulate single asperity contact, as opposed to multi-asperitycontact associated with the field simulation tests. In this case itis not easy to correlate the experimental data with the mar phe-nomenon. Although a good mapping of the surface state, using AFMtechnology or a roughness meter, is obtained, no direct correlationwith the aesthetic appearance is easily obtained. For this, the mea-surement of a characteristic that can be directly correlated with theoptical nature appears more interesting. The gloss measurement isthen the most suitable technique.
The aim of this work was to study the mar performance oforganic coatings using two different test methods with differentdamage mechanisms: abrasive wear and erosive wear [17]. In bothcases the damage was light in order to influence only aestheticalproperties. An epoxy-polyester powder resin was chosen because
this type of organic coating gives an excellent surface finish, withcomplex geometries too, and the lack of solvents in the applicationprocedure is an important aspect for the eco-design. Furthermorethis type of resin was investigated in other works and its propertieswere well known.
The standard ASTM D6037 (Standard Test Methods for Dry Abra-ion Mar Resistance of High Gloss Coatings) consider the use of theaber test apparatus to characterize the mar behavior of paint sys-em. The experimental set-up is based on the use of two abrasiverinders, which movement is activated by the rotational motion ofhe sample. The sample rotates at 60 turns per minute and fromhe interaction with the grinders the abrasive action is obtained. Toncrease the abrasive action it is possible to select different grindersnd to impose a load ranging from 250 to 1000 g. The damaged zoneonsists of a circular crown [18–20].
Some test methodologies are used to investigate the erosiveear of organic coatings. In some cases the abrasive particles are
ccelerated by using a flow of compressed air, however, it is alsoossible to let the abrasive fall under gravity [2]. One of the mostsed test is the methodology presented in the ASTM D968 standardStandard Test Methods for Abrasion Resistance of Organic Coat-ngs by Falling Abrasive). Two abrasion means are considered in thetandard: siliceous sand with round grains, called Ottawa sand, andilicon carbide abrasive with edge grains. In this work sodium chlo-ide and sucrose are used in order to create a limited damage thatnfluences only the aesthetic aspect of the sample surface. This kindf abrasive means was chosen because they have a limited hardnesshat do not cause a severe damage of the coatings; in addition, theyould easily found in domestic environment, producing the dam-ge of the coatings applied on appliances or furniture. In this wayt was possible to evaluate the influence of particle hardness on theevel of damage and changes of gloss; two different granulometriesre chosen to understand the influence of the grain size.
Estimation of the damage through measurements of loss of glossnd microstructural observations was carried out with the inten-ion to correlate the characteristics of the abrasives with the levelf damage.
. Experimental
Q-panels of carbon steel (SAE 1008/1010; 0.13 max C, 0.25–0.60n, dimensions 10 cm × 10 cm) were powder coated with an epoxy-
Fig. 1. ESEM micrographs of abrasive agents: (a) large size sodium chloride; (b)
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polyester resin. Prior to the coating application, the samples wereiron phosphodegreased. The organic coating was cured at 210 ◦Cfor 20 min. The mean coating thickness was 70 ± 10 �m, and it wasmeasured using a Phynix Surfix FN thickness gauge. For each sam-ple 10 measurements were made, from which the mean coatingthickness was obtained.
Instead of the abrasive grinders regularly used in Taber Abrasertest, a pair of wool felt wheels namely CS5 were employed togetherwith the abrasive agents. Sodium chloride and sucrose with twodifferent granulometries in anhydrous conditions were used. Thetest with NaCl with larger grains was not made because the grainswere displaced from the wear track, due to the rotational movementof the sample and did not abrade the coating. The experiments werecarried out with Taber Abraser 5131 using a load of 250 g. The testwas interrupted at intermediate steps (every 20 cycles) to allow themeasure of the gloss properties during all the abrasive process.
The experimental device (Elcometer 1700 Falling Sand Tester)for the Falling Abrasive test consists of a funnel at the upper partconnected to a guide tube (914 mm length, 19 mm internal diame-ter) by which the abrasive particles flow by gravity. The sample ismaintained at 45◦ respect to the guide tube, leaving gap of 25 mmfrom the sample to the end of the tube. The same abrasive agents ofthe Taber test were used. The gloss measurements were carried outafter a certain amount of abrasive was passed through the device,in increments of 500 g.
Morphological of the abrasive agents were investigated using anenvironmental scanning electron microscope (ESEM Philips XL30)in order to evaluate the grains size and the morphology of the abra-sive particles.
Gloss was measured with a Picogloss 20◦-60◦-85◦ mod. 503Erichsen glossmeter at 20◦, 60◦ and 85◦ angle. The measuring area ofthe glossmeter was: 10 mm × 10 mm at 20◦, 9 mm × 15 mm at 60◦,
5 mm × 38 mm at 85◦. For each sample 50 measurements on theabraded area were made, from which the mean gloss was obtained.
The worn surfaces were observed using optical microscope andESEM to highlight the morphology of the damage and correlate itto the changes of gloss.
small size sodium chloride; (c) large size sucrose; (d) small size sucrose.
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by the first surface and a fraction is transmitted. The amount of
ig. 2. Gloss/initial gloss (%) as a function of cycles, Taber test using NaCl (small size)ith an applied load of 250 g.
. Results and discussion
.1. Characterization of abrasives
NaCl crystals (Fig. 1a) had an average size of 2–3 mm; the grainsresented sharp edges. NaCl with a lower granulometry (Fig. 1b)resented smaller grains (hundreds of �m) with sharp edges;he range of grain dimensions was wider. The sucrose crystalsFig. 1c) had a very regular hexagonal shape, the range of grainimensions was narrow, the average size was around 1 mm. Therystals of sucrose in Fig. 1d showed a similar shape, were obvi-usly smaller (hundreds of �m), the range of grain dimensions wasarrow.
ig. 3. Gloss/initial gloss (%) as a function of cycles, Taber test using sucrose (smallize) with an applied load of 250 g.
Fig. 4. Gloss/initial gloss (%) as a function of cycles, Taber test using sucrose (largesize) with an applied load of 250 g.
3.2. Gloss
Samples presented the following initial gloss values: 20◦ glossbetween 39.0 and 42.2, 60◦ gloss between 74.1 and 78.4, and 85◦
gloss between 96.0 and 99.3. The percentage change of gloss inreference to the initial value was measured for all the samples.
3.2.1. Taber testConsidering Taber test with small size NaCl the loss of gloss was
gradual (Fig. 2): the gloss at 20◦ was equal to 85.6% of the initialvalue after 20 cycles, 68.9% after 100 cycles and 29.9% after 300cycles. The most sensitive measuring angle to the changes of glosswas 20◦. The illumination angle influences strongly the measure-ment. In case of coatings, a fraction of the incident light is reflected
reflected light will be greater with higher measurement angles.Obviously the value of gloss is influenced by the morphology ofthe surface and consequently a coating with a low roughness willpresent high gloss values. According to ASTM standard [23], high
Fig. 5. Comparison between particle size and decrease of gloss at 20◦ , Taber testwith sucrose.
S. Rossi et al. / Wear 267 (2009) 1574–1580 1577
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ig. 6. Comparison between particle hardness and decrease of gloss at 20◦ , Taberest with small size sucrose and sodium chloride.
loss surfaces (with 60◦ gloss >70, as in this work) are characterizedsing low measurement angles (20◦). Compared to the previousample, the sample abraded with small size sucrose showed aigher loss of gloss (Fig. 3). After 20 cycles the gloss was equal to7.9% of the initial value, after 100 cycles 22.9%, considering the 20◦
easurements. Both particles sizes resulted of the same order ofagnitude, nevertheless NaCl particles were smaller. Considering
he great difference observed in the decrease of gloss, it is possibleo affirm that the reason of this difference was probably the hard-ess of particles. The sucrose presented a higher hardness (680 MPaV) compared to sodium chloride (182 MPa HV).
Fig. 4 shows the loss of gloss of the sample abraded with largeize sucrose; after 20 cycles the 20◦ gloss was equal to 71.6% ofhe initial value, after 100 cycles 44.6%. The loss of gloss was lower
ompared to the previous sample abraded with small size sucrose:n increase of grain size caused a smaller reduction of aestheti-al properties: the bigger grains had the tendency to be displacedrom the wear track and so they produced few scratches on the sam-
ig. 7. Gloss/initial gloss (%) as a function of amount of abrasive agent, Falling Abra-ive test using NaCl (large size).
Fig. 8. Gloss/initial gloss (%) as a function of amount of abrasive agent, Falling Abra-sive test using NaCl (small size).
ple, the low density of defects was correlated to a greater value ofgloss.
Figs. 5 and 6 summarize the influence of the size and hardness ofthe abrasive particles respectively. Larger particles caused a lowerloss of gloss; considering the literature [18,20], bigger grains causea greater reduction of the protective properties from the corrosion,conversely they cause a lower loss of aesthetical properties; thisfact could be related to the morphology of the damage: larger grainsproduced few scratches although quite deep and the gloss measure-ments are more sensitive to the density of defects on the surface.An higher hardness of abrasive grains caused a larger reduction ofgloss.
3.2.2. Falling Abrasive testConsidering the Falling Abrasive test, the sample damaged by
using large size NaCl showed an important decrease of gloss (Fig. 7);after 500 g the 20◦ gloss was equal to 12.1% of the initial value, atthe end of the test (2500 g of abrasive) the loss was practically com-
Fig. 9. Gloss/initial gloss (%) as a function of amount of abrasive agent, Falling Abra-sive test using sucrose (large size).
1578 S. Rossi et al. / Wear 267 (2009) 1574–1580
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ig. 10. Gloss/initial gloss (%) as a function of amount of abrasive agent, Fallingbrasive test using sucrose (small size).
lete (the residual gloss was equal to 1.7%). The reduction of glossas done immediately, in the following steps the loss of brillianceecame more gradual. After 500 g of abrasive the damage caused arop of the gloss properties. As in the Taber test, 20◦ was the mostensitive angle to gloss variations, however, the values at 60◦ wereery close to those measured at 20◦, while 85◦ values were quiteifferent.
The loss of gloss was lower using small size NaCl (Fig. 8): after◦
00 g of abrasive the 20 gloss was equal to 32.4% of the initial value,
t the end of the test (after 2500 g) 8.8%. The smaller size of the abra-ive particles produced a lower reduction of the gloss properties. Inhis test the mechanism of damage was an erosion produced bybrasive grains that acquire kinetic energy by falling; smaller par-
ig. 12. ESEM micrographs of abraded sample surface using Taber test: (a) sample abradedycles); (c) sample abraded with large size sucrose (100 cycles).
Fig. 11. Comparison between gloss/initial gloss 20◦ (%) after 500 g of abrasive, FallingAbrasive test with sucrose and sodium chloride.
ticles with a lower weight had a minor energy and caused a lowerdamage of the sample surface.
This fact is in contrast with the results obtained by Taber testwhere a greater size of abrasive grains caused a lower loss of gloss.
The difference between the values of gloss measured at threedifferent geometries was accentuated; it seems to be more pro-nounced when damage is less severe.
Using large size sucrose the loss of gloss was important (Fig. 9):considering 20◦ measurements, the decrease was almost completeafter 500 g of abrasive (the residual gloss was equal to 3.9% of the
initial value). The values measured at 20◦ and 60◦ were very close.
The decrease of gloss was lower using small size sucrose (Fig. 10),confirming what was seen in the tests with sodium chloride. After500 g of abrasive the gloss at 20◦ was equal to 16.8% of the initial
with small size NaCl (300 cycles); (b) sample abraded with small size sucrose (100
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ig. 13. ESEM micrographs of abraded sample surface using Falling Abrasive test afgure); (b) sample abraded with small size NaCl; (c) sample abraded with large size
alue, at the end of the test 6%. The difference between the valuesf gloss measured at three different geometries increased.
Fig. 11 summarizes the influence of size and hardness of abrasivearticles in the Falling Abrasive test. It represents the gloss at 20◦
fter 500 g of abrasive using sodium chloride and sucrose with twoifferent sizes. As in the Taber test, 20◦ measurements were choseno compare the various tests because 20◦ is the most sensitive angle.arger particles with an higher hardness caused a greater reductionf gloss. The difference between the effect of small and large grainsn the gloss was more accentuated for the sodium chloride becausehe difference between the two granulometries was greater.
.3. Damage analysis
.3.1. Taber testFig. 12a shows the surface abraded with small size sodium chlo-
ide: the track was abraded homogeneously, the scratches hadarious lengths and width up to few tens of �m. The abrasive par-icles remained between the wheels and the sample producing anniform damage of the surface.
Fig. 12b shows the surface abraded with small size sucrose: visu-lly, the damage appeared clearly more severe than the previousest; the sucrose grains with a higher hardness produced deepercratches, the density of defects was higher too.
Fig. 12c shows the surface abraded with large size sucrose: theensity of defects and scratches was clearly lower, compared to therevious test with small size sucrose. In this test, given the size ofhe abrasive particles, they tended to be displaced from the wearrack. The lower reduction of gloss was related to the lower densityf scratches on the surface.
.3.2. Falling Abrasive test
Considering Falling Abrasive test, the morphology of the damage
ppeared clearly different from the damage obtained by using Taberest. The surface abraded with large size sodium chloride appearsomogeneously damaged (Fig. 13a), there were small craters ofifferent depths. In Taber test, the abrasive particles, dragged by
00 g: (a) sample abraded with large size NaCl (the damaged area is indicated in these; (d) sample abraded with small size sucrose.
wheels exercised their abrasive action creating scratches of a cer-tain length, grooves more or less deep; in the falling test the abrasiveparticles impacted the surface of the sample with an angle of 45◦
and the process of damage that occurred was an erosion of theorganic coating, with the formation of craters and not scratches.
The surface abraded with small size sodium chloride (Fig. 13b)presented a similar morphology of damage, the craters weresmaller. Increasing particles size the level of damage became lowerand the loss of gloss decreased.
The surface of the samples abraded with sucrose (Fig. 13c andd) appeared considerably damaged, the density of craters was high.The large size sucrose produced deeper craters (Fig. 13c), confirm-ing the role of grain size in this type of test.
4. Conclusions
In this work two methods for characterizing the mar resistancewere compared. Taber test and Falling Abrasive test were chosen.In both tests sodium chloride and sucrose were used in order tocreate a limited damage of the coatings that influenced only theaesthetical aspect.
The level of damage was evaluated with gloss measurements.The gloss measurement could quantify the level of aesthetic dam-age of the surface; 20◦ is the most sensitive angle to gloss changes,with this geometry different samples could be compared.
The two tests showed a different morphology of the damage ofthe organic coating due to the different mechanism of damage: inTaber test particles are dragged from the wheels creating scratchesof different depth and length; in the Falling Abrasive test a pro-cess of erosion created craters in the area of impact of abrasiveparticles.
The role of grain size and hardness of abrasive particles wasinvestigated. In the Taber test larger particles created few deepscratches with a minor reduction of gloss compared to tests withsmaller particles, this is because the gloss measurements are moresensitive to the density of defects than to their entity. Conversely
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n Falling Abrasive test larger particles caused greater damage withgreater loss of gloss.
Considering the hardness of abrasive particles, in both test, areater hardness had a negative effect on the aesthetical propertiesf the coating.
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