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European Coatings Journal
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A closer look at low gloss powder coatingsAFM measurements clarify the modification effect ofincompatible polyester-β-hydroxyalkylamide resin blends.For the defined matting of TGIC-free polyester β-hydroxyalkylamide powder coatings, dry blends offormulations based on resins that are incompatible due totheir different reactivity have been shown to have a highpotential. To clarify the effects that govern the glossreduction, the correlation of coatings performace data withAtomic Force Microscopy results can be extremely helpful.Louis T. Germinario, Damiano Beccaria, Andrea Capra, ImirBejko.Many outdoor applications in powder coatings specify a lowgloss finish. Coating systems based upon polyester resinsand beta-Hydroxyalkylamide as the curative play anincreasingly important role in this arena. Dry blending of twocoatings of different reactivity has been a standard methodfor achieving low gloss in powder coatings made with TGIC.However, the substitution of TGIC with β-Hydroxyalkylamide(β-HAA) has required the design of a new generation ofpolyesters. As we have shown in the preceding paper [1],new formulations based on dry blends of powder coatingsmade with six different polyester resins with different ratiosof β-HAA polyester-curing agent provide coatings withdifferent levels of gloss, chemical and mechanicalproperties. For such blends, a strategy was found, whichallows consistent, well defined target gloss values to beformulated. This study has shown that the relative differencein acid values of two resins is a key parameter and can beused as a measure to control the final gloss values of theformulated coating.In an effort to better understand the structure-propertyrelationships found for the blends, Atomic Force Microscopywas used to characterize the surface roughness and surfacemechanical properties of these systems.
Experimental detailsTypical properties of the six resins used for the primarypowder coatings are reviewed in Table 1. Only white coatingformulations were used for the AFM studies reportedhere.These formulations based on the six resins are given inTable 2. They were prepared by pre-grinding the resins,curing agents and additives, together. Formulations werethen made in an "APV MP-30" extruder, followed by grindingin a strand mill and classified using a 106 µ sieve. The resulting coatings were dry blended in variouscombinations, always at a 50/50 weight percent ratio. Theblended coatings were sprayed on "QD-36" panels using a60KV corona spray gun. All coatings used for the AFManalyses were cured at 200°C for 10 minutes andcharacterized using the following ASTM test methods:- Method D 523 for testing the gloss, using the Byk Gardner"Micro-Tri-Gloss".- Method D 2794 for testing direct and reverse impact values
Measuring surface stiffness via AFMFor the AFM measurements, the cured blended powdercoatings were analyzed directly on the panels. In order toaccurately characterize the coating's surface morphologyand topography, all surface images were obtained with acommercial atomic force microscope, "Dimension seriesD3000" AFM (Digital Instruments, Santa Barbara, CA).Probes used for analysis are from Nanodevices, "Multi 75",with spring constants of 5 N/m, resonant frequencies of 75kHz and a tip radius below 10nm. AFM was used tomeasure both the topological surface roughness and the
surface stiffness using the force modulation imaging mode.The latter is a very useful imaging mode, which identifiesand maps differences in surface stiffness and also providesthe surface topography. Force modulation is a contactimaging mode, where the probe's movement perpendicularto the sample surface (in the z-direction) is modulated oroscillated. As the tip is scanned over a surface, under thesame applied force, a stiff area on the sample will deformless than a softer area. Thus, stiffer areas will resist vertical(z) oscillations of the tip and lead to greater tip deflectionand consequently appear as higher image brightness.Image data were recorded as both height and forcemodulation modes, which were recorded simultaneously.Surface roughness analysis was also performed on filmsurfaces by determining the root-mean-square (RMS)surface roughness. This is based on the following equation:RMS = [y1
2 + y22 + y3
2 + ... + yN2]1/2 / N
where y is the height value of each image point and N is thetotal number of points for a given image.To improve the sampling statistics, surface roughnessmeasurements were collected from 10 areas on eachsample, each area 100 by 100 microns wide and randomlychosen on each panel.
beta-HAA content affects the roughnessThe left-side image in Figure 1 is a top view or height imagethat contains topographic information from the resin 1/resin4 white blend, while the image on the right is a forcemodulation image. Overall, this system contains 3.75% β-Hydroxyalkylamide in the binder. The average RMSroughness for this system is 0.44 microns, while the averageRMS force modulation value is 1.86 nm.Height images can also be displayed as 3-D images, wherethe white peaks are more easily recognizable from thedarker, low-lying "valleys". Figure 2 provides an example ofsuch a 3-D view of the resin1/resin 4 white coating. Thisview clearly displays the gradual undulations that arecharacteristic for the surface topography of this formulation.Upon increasing the concentration of β-HAA to 6.75% (resin3/resin 6 white coating, Figure 3), the morphology issignificantly changed. An increase in peak heights andsurface roughness is found. The average RMS roughnessincreases to 0.72 microns, while the average RMS forcemodulation value also increases to 6.13 nm.These data provide some direct evidence for aconcentration effect of the curative on the coating's surfaceroughness and stiffness. As the amount of β-HAA isincreased, the surface RMS roughness increasessignificantly, as does the average surface stiffness. The datasupport the hypothesis that higher β-Hydroxyalkylamidelevels, and higher crosslinking densities, lead to an increasein both surface roughness and stiffness. Figure 4 provides a3-D view of the surface topography of the resin 3/resin 6white coating. The presence of both 'stiff' and 'soft' domains,a few tens of microns in diameter, that are visible in theforce modulation images, suggests that the film leveling isminimized due to a lack of mixing of the individual resincomponents. In contrast, the resin 1/resin 4 white coatingsystems, that contains 3.75% β-HAA (Figure 1), shows noevidence of separate domains in the force modulationimage, suggesting an intimate mixing of the resincomponents.
Roughness is the cause for gloss reductionTable 3 summarizes the test results of all white blends,
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including the gloss, impact, and the RMS roughness andaverage surface stiffness values calculated from height andforce modulation images, respectively.The correlation analyses for these measurements are givenin Table 4, expressed as the linear correlation cofficients R2.Clearly, a high negative correlation is found between RMSroughness and gloss (R2 = -0.95). This high correlation isencouraging, because it provides direct evidence for asufficiently rigorous sampling protocol used for calculation ofaverage RMS surface roughness. This relationship betweensurface roughness and gloss is also explicable from theAFM data, because the average size of the surfaceimperfections and irregularities, measured from the RMSroughness calculations, is equal to or larger than theincident wavelength, supporting the hypothesis that surfaceimperfections or irregularities lead to a diffuse scattering ofthe incident light and thus to a loss in brilliance and gloss.A reasonably good negative correlation is also foundbetween gloss, or RMS roughness, and the average RMSforce modulation, implying that an increase in roughness isassociated with an increase in the stiffness of the coating.
Higher functionality - lower gloss - higher stiffnessThere is also a good correlation (R2 = 0.87) between theRMS roughness and β-HAA content in the blend (Table 4).This supports the view that a higher crosslinking density in acoating may lead to increased surface roughness, possiblydue to a restriction in resin flow and lack of leveling duringfilm formation.There is also a reasonably high correlation (R2 = 0.82)between the average RMS Force Modulation and the β-HAAcontent. As previously mentioned, height images containsurface topography information, while force modulationimages differentiate soft from stiff polymer segments. Ahigher average value of force modulation intensity is a clearindicator of a stiffer coating. This relationship is furthervalidated by the good correlation observed betweenAverage RMS Force Modulation and the Direct (R2 = -0.77)and Reverse (R2 = -0.81) Impact (Table 4). β-Hydroxyalkylamide-polyester systems are a growing forcein powder coatings for exterior applications. Since thesesystems cannot be catalyzed, the attainment of highreproducibility for low-gloss coatings is more challengingthan for their TGIC counterparts.
ConclusionIn studying gloss in various dry-blended polyester systems,using both black and white coatings, it was determined thatthe relative difference in acid value (AV) of resins was themost useful predictor of gloss [1]. This relationship gives theformulator some options for achieving a specific gloss value,at a specified price and performance. For instance, for theformulations listed in Table 2 cured for 10 minutes at 200°C,the following formula holds:Gloss 60° = 82 - 0.6 ∆rel(AV) where: ∆rel(AV) = 100 abs(AVresin 1- AVresin 2) / Average (AVresin 1, AVresin 2)This relationship has a correlation coefficient that is veryclose to minus one, and can be used to predict the level ofgloss, for a two-resins blended system, by judicious choiceof resins and their and acid values. These results furthersupport the significance of tight acid value control in a lowgloss system.Scanning probe microscopy and statistical tools have linkednanostructure and chemical composition with macroscopicproperties such as gloss and impact strength.The high correlation between the macroscopic gloss and themicroscopic surface roughness provide supporting evidencefor the ability of Scanning Probe Microscopy, and AtomicForce Microscopy in particular, to measure surface
properties that are relevant to a coating's performance.
AcknowledgementThe authors wish to thank Penny J.E. James, William T.Sade, Gregory C. Alexander, Donato di Lorenzo, KeithMiddleton, Wayne T Riddick, the Sant'Albano Center ofExcellence and all other people who contributed to thedevelopment of this project.
REFERENCES[1] D. Beccaria, A. Capra, I. Bejko, L. T. Germinario; ECJ7-8/2003, p. 21.
The authors:-> Louis T. Germinario, Ph.D., is Senior Research Associatein Eastman's Physical Chemistry Research Laboratory inKingsport, Tennessee.-> Dr. Damiano Beccaria is Manager of the Eastman Centerof Excellence Sant'Albano Stura Technical Lab in Italy,which is in charge of all Eastman Coatings (liquid andpowder) polyester chemistry for the EMEA geographicalarea.-> Andrea Capra is Supervisor of the R&D Laboratory forliquid and powder polyesters chemistry at Sant'Albano Sturaand responsible for the site Process Support Team.-> Dr. Imir Bejko is supervisor of the powder coatingapplication laboratory at Sant'Albano Stura synthetic resinsplant.
Powder Coating Training CourseOrganized by the Paint Research Association (PRA), a newtraining course, Powder Coating Technology, is to be heldon 4-5 November 2003. It provides an insight into theformulation and manufacture of coating powders, boththermoplastic and thermosetting.The course includes a half-day practical session at a nearbypowder coating installation where there will be anopportunity to apply coating powders, understandapplication parameters, and test coatings in productionconditions.Contact:Elisabeth Brown,The Paint Research Association,Tel. +44 (20) 86 14 48 15,[email protected]
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Figure 1: Height image (left) and force modulation image (right) of resin 1/resin 4 whitecoatings. Average (RMS) roughness is 0.44 microns, average (RMS) force modulation
is 1.86 nm..
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Figure 2: 3D-Topography of resin 1/resin 4 white coatings.
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Figure 3: Height image (left) and force modulation image (right) of resin 3/resin 6 whitecoatings. Average (RMS) roughness is 0.72 microns, average (RMS) force modulation
is 6.13 nm..
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Figure 4: 3D-Topography of resin 3/resin 6 white coatings..
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