An improved accelerated weathering protocol to anticipate Florida exposure behavior of coatings

An improved accelerated weathering protocol to anticipateFlorida exposure behavior of coatings

Mark Nichols, John Boisseau, Lynn Pattison, Don Campbell,

Jeff Quill, Jacob Zhang, Don Smith, Karen Henderson,

Jill Seebergh, Douglas Berry, Tony Misovski, Cindy Peters

� American Coatings Association & Oil and Colour Chemists’ Association 2013

Abstract A new accelerated weathering protocol hasbeen developed which closely replicates the perfor-mance of automotive and aerospace coating systemsexposed in South Florida. IR spectroscopy was used toverify that the chemical composition changes thatoccurred during accelerated weathering in devices witha glass filter that produced a high fidelity reproductionof sunlight’s UV spectrum matched those that occurredduring natural weathering. Gravimetric water absorp-tion measurements were used to tune the volume ofwater absorption during accelerated weathering tomatch that which occurred during natural weatheringin South Florida. The frequency of water exposure wasthen scaled to the appropriate UV dose. A variety ofcoating systems were used to verify the correlationbetween the physical failures observed in the acceler-ated weathering protocol and natural weathering inSouth Florida. The new accelerated weathering proto-col correctly reproduced gloss loss, delamination,cracking, blistering, and good performance in a variety

of diverse coating systems. For automotive basecoat/clearcoat paint systems, the new weathering protocolshows significant acceleration over both Florida andprevious accelerated weathering tests. For monocoataerospace systems, the new weathering protocolshowed less acceleration than for automotive coatings,but was still an improvement over previous acceleratedtests and was faster than Florida exposure.

Keywords Durability, Accelerated testing,Weathering, Automotive coatings, Aerospace coatings

Introduction

Coating scientists have pursued the means to anticipatethe long-term performance of coating systems since theearly 1900s via both natural and accelerated weatheringexperiments, as detailed by Martin.1 As the pace ofinnovation has accelerated, the need for accurate andfaster accelerated tests has grown accordingly. Formaterials that are often exposed outdoors, such ascoatings, the weathering resistance is a key, and manytimes dominant, characteristic determining the useful life.The need to quickly assess a material’s weatheringresistance has driven the construction of acceleratedweathering equipment capable of producing 20–100 timesthe intensity of natural sunlight in hopes of dramaticallyshortening the required testing time.2,3 To date, mostexperimental data do not support the ability to accelerateto such an extent and maintain proportional rates ofdegradation or the same degradation modes, particularlyfor complex multilayer coating systems that are formu-lated with stabilizer additives.4–6 However, the construc-tion of such devices illustrates the pervasive desire topossess a rapid and reliable accelerated weathering testfor coatings and other materials exposed outdoors.

A coating’s useful lifetime can be defined by adegradation in appearance (color or gloss), mechanical

M. Nichols (&), T. Misovski, C. PetersFord Research and Advanced Engineering, Dearborn, MI48121, USAe-mail: [email protected]

J. Boisseau, L. Pattison, D. CampbellBASF Coatings, Southfield, MI 48033, USA

J. QuillQ-Panel, Cleveland, OH 44145, USA

J. ZhangAtlas Material Testing Technology LLC, Chicago, IL 60613,USA

D. Smith, K. HendersonBayer MaterialScience, Pittsburgh, PA 15205, USA

J. Seebergh, D. BerryThe Boeing Company, Seattle, WA 98124, USA

J. Coat. Technol. Res.

DOI 10.1007/s11998-012-9467-x

failure (cracking or delamination), or by allowingfailure of the substrate through corrosion or biologicalattack. The coating has ceased to provide its intendedfunction when one of these failure modes occurs. Thisis similar to the lifetime metrics that are placed onother complex systems, such as the fatigue life of anaircraft structure or the hours until failure of acomputer hard drive. For each of these failure modes,key performance tests and/or models must be devel-oped to accurately anticipate the long-term durabilityof the system. To achieve similar advances in coatingsdurability, not only must the materials improve, butreliable test methods must be developed to assess theimprovements on a practical time scale. Materialsdevelopment controlled by the time scale of Floridaoutdoor exposure is not economical, as automotivecoating systems are required to perform for over10 years. This longevity is a goal for the next gener-ation of exterior aircraft coating systems as well.

For coatings exposed outdoors, chemical degrada-tion of the binder is the primary pathway by whichcoating performance is compromised.7,8 Researchersover the past 30 years have made significant progressin understanding the chemical origins of coatingdurability, with two primary mechanisms having beenshown to drive long-term degradation: photooxidationand hydrolysis.9–12 In many cases, photooxidation is thedominant mode of degradation. A variety of tech-niques have been developed to quantify both theamount and locus of photooxidation in coating sys-tems.13–19 As many coating systems have poor dura-bility without the addition of stabilizer additives, themechanisms of action of both hindered amine lightstabilizers (HALS) and ultraviolet light absorbers(UVA) have been thoroughly investigated.20–28 Theapplication of these techniques has led to the devel-opment of models to predict the lifetime of coatingsystems when certain fundamental chemical behaviorsare known. Bauer has detailed the application of UVAloss rate and UV transmission data to the lifetimeprediction of automotive coatings,29 where the use ofstatistical distributions for sun load, population, andparking frequency have allowed for the prediction ofnational or global failure rates of coatings.

Most efforts to anticipate the long-term durability ofcoatings have focused on correlating the performance inaccelerated weathering to performance after exposurein South Florida. This location has been shown to beharsher on paint systems than most places on Earth, withthe exception of a few equatorial regions where thehuman population density is relatively low.8 For com-mercial aircraft, significant exposure occurs at altitudesover 30,000 feet. While the intensity of UV radiation issignificantly higher, the wavelength distribution isessentially the same as that at the Earth’s surface.However, due to Arrhenius effects, the extremely lowsurface exposure temperatures (�30 to �55�C) maysuppress the rate of photooxidation during flight.29 InSouth Florida, the combination of high sun load, highhumidity, and high time of wetness drives rates of

photooxidation, hydrolysis, and surface erosion to neartheir maximum terrestrial rates. While the meteorolog-ical details of any given year will vary, South Floridaexposure is the de facto standard for coating weather-ability testing, and results from any accelerated testmethod must agree with those obtained from SouthFlorida exposure to be of practical significance.

An alternative approach to anticipating the long-termweathering performance of coating systems has beenforwarded by Martin and coworkers.1,30,31 This servicelife prediction (SLP) methodology relies on subjectingcoatings and coating components to exceptionally well-controlled exposures to quantify the mode and rate ofdegradation. The SLP approach rejects the notion that anatural environment can ever be sufficiently reproducedin an artificial weathering test because of the inherentvariability (day to day, year to year) of natural climates.Therefore, the goal of the SLP approach is not toreproduce Florida (or any other exposure site) behavior,but rather to understand the fundamental mechanismsof degradation, quantify the effect of exposure variables(temperature, wavelength, irradiance…) on the degra-dation mechanisms, and then mathematically calculatethe weathering response under any given exposureconditions. The SLP approach has to date yieldedexceptionally detailed studies into the fundamentalnature of several coatings’ degradation mechanismsand pathways.19,32 For nondurable coating systems, theapproach has shown promise by demonstrating bothreciprocity (degradation rate scales with dose) andadditivity (the rate is independent of radiation wave-length).33 However, in its current state, the SLPapproach as applied to coatings is incomplete, as it hasnot yet been verified for highly durable coating systemswhere the impact of nonpermanent stabilizer additivesplays a crucial role in the long-term performance of thecoating. From a practical standpoint, the database ofpotential coating constituents numbers in the tens ofthousands. Characterizing the response of each compo-nent or paint system to the full battery of tests requiredto model a coating’s long-term behavior is, at this time,not commercially feasible. In addition, the role of liquidwater has not been included, which is known to drivemany of the physical failures of commercial paintsystems.34 However, the continued extension of theSLP approach to address the current limitations dis-cussed above is certainly warranted.

While currently absent from the SLP approach,water is known to play a key role in the weathering ofcoatings. First, water has been shown to act as aplasticizer, decreasing both the Tg and modulus of acoating.35 Second, water is a necessary reactant in thehydrolysis of a coating binder. Third, water can disruptthe adhesion of coatings by competing for bondingsites at a coating/substrate interface.36 Finally, liquidwater will wash the surface of a coating leading to theerosion of loosely bound material. To date, no accel-erated weathering test has fully incorporated all of theeffects of water and reproduced the behavior of paintsystems exposed in sub-tropical climates.


Finally, as has been thoroughly demonstrated, thespectral power distribution (SPD) of the light source inan accelerated weathering device is critical to insuringthat the results from the accelerated test reproduce thechemical changes that occur during natural weathering.Previous researchers have shown that, until recently,all commercially available accelerated weathering lightsources distort degradation chemistry and thereforecould potentially provide either false positive or falsenegative results.37 The recent development of a lightsource that correctly reproduces sunlight, particularlyin the UV region of the spectrum, is a key hurdle thathas been overcome.38 Use of this light source isexpected to improve the correlation between artificialaccelerated testing and natural weathering.

In this article the development of a new acceleratedweathering protocol based on the temperature, radia-tion, and moisture data previously collected fromnatural outdoor exposures in Florida is detailed.34

The correlation of results from this new protocol alongwith results from natural weathering exposures ispresented for a variety of automotive and aerospacecoating systems. Both the chemical and physicalchanges that the paint systems undergo in naturaland accelerated testing are used to assess the utility ofthe weathering protocol.

Experimental

Materials

Automotive coating systems representing a variety ofcoating chemistries and layering scenarios wereapplied to 4 in. 9 12 in. steel panels and baked at theappropriate temperatures and times. Some coatingsystems were designed to intentionally demonstratecertain failure modes when exposed in South Florida.Other systems were formulated and applied to insureexcellent long-term durability. The details of thecoating systems including their layering, chemistry,color, expected Florida failure mode, and other detailsare provided in Table 1. For some of the experimentsusing automotive coating systems, a flexible polyester–urethane clearcoat (CC) was used. This polyester–urethane system has been previously shown to be very

sensitive to the SPD of the light source used inaccelerated weathering testing.37 The polyester–ure-thane CC was applied to silicon discs for transmissionIR spectroscopy experiments and cured for 30 min at130�C.

Aerospace coating systems representing three coat-ing chemistries and two layering scenarios wereapplied to 3 in. 9 6 in. 2024-T3 clad aluminum panelsand cured at the appropriate temperature and times.The monocoats (MC) and CC were polyurethane-based, solventborne, high solids systems designed tohave an initial 20� gloss value above 75. Each coatingsystem included a sol–gel conversion coating pretreat-ment and an epoxy primer. The target dry filmthickness was 0.1 lm for the conversion coating,15 lm for the primer, 80 lm for the topcoat, 40 lmfor the basecoat, and 40 lm for the CC. Details of thecoating systems are shown in Table 2.

Natural weathering

Panels from all the paint systems were sent to SouthFlorida for exposure using the standard 5�S unbackedexposure conditions. Automotive panels were returnedevery 6 months for evaluation and exposed for aminimum of 2 years. Aerospace panels were measuredfor gloss and color change every 3 months and exposedfor 24 months.

Accelerated weathering

Two types of accelerated weathering devices wereutilized—Atlas Ci4000 or Ci5000 Weather-Ometer�

with a rotating rack (Atlas Material Testing Technol-ogy LLC, Chicago, IL), and Q-Sun flat array (Q-Panel,Cleveland, OH). Both types of machines were fittedwith filter systems that most closely reproduce the SPDof sunlight. Atlas� machines were fitted with Right-LightTM inner and quartz outer filters. Q-Panelmachines were fitted with Daylight-F filters. A subsetof test panels was exposed in an Atlas Ci35 machinewhich ran the standard SAE J2527 accelerated weath-ering protocol using borosilicate inner and outer filters(boro/boro).39 The majority of paint panels were

Table 1: Details of automotive coating systems utilized in this study

System # CC/BC chemistry System layering Expected Florida failure mode Color

13 Acrylic melamine/SBBC CC/BC/e-coat Delamination off e-coat White25 Carbamate/SBBC CC/BC/e-coat Blistering, delamination off e-coat Blue97 2K Polyurethane/WBBC CC/BC/primer/e-coat Gloss loss, blistering, delamination of CC off BC Blue86 2K Polyurethane/WBBC CC/BC/primer/e-coat Positive control. No failure White103 2K Polyurethane/WBBC CC/BC/primer/e-coat Gloss loss, blistering, delamination of CC off BC Red150 Carbamate/SBBC CC/BC/primer/e-coat Positive control. No failure Black

SBBC solventborne basecoat, WBBC waterborne basecoat


exposed in the accelerated weathering devices runninga new test protocol.

The key features of the new test protocol comparedwith SAE J2527 were the inclusion of two long waterspray cycles at zero irradiance, one long cycle withhigher irradiance and no water spray, and a series ofshort wet/dry dark/light cycles. The water and irradi-ance cycles for both the new protocol and SAE J2527are shown graphically in Figs. 1 and 2 and tabularly inTables 3 and 4. Throughout this manuscript, the newtest protocol will simply be referred to as the ‘‘newprotocol.’’ The duration of the water sprays wasinvestigated using an iterative approach, where theobserved failure mechanisms were used as a guide forthe adjustments in water duration and frequency.

Paint samples were exposed for a minimum of3000 kJ (@340 nm) in the accelerated weatheringdevices and for a minimum of 21 months in Florida.After 3000 kJ (@340 nm) of accelerated weathering or2 years of Florida exposure, automotive samples were

checked for adhesion by using a razor to cut an Xscribe in the coating system. Tape (3M, Scotch 898)was then used to perform a standard paint adhesiontest after a 24-h water soak.

To examine machine-to-machine variation of waterdelivery, a total of six different Atlas Ci machines at sixdifferent physical locations were utilized. In addition,four different Q-Panel Q-Sun machines at threedifferent locations were utilized. All machines werecalibrated and maintained according to the manufac-turers’ guidelines. A demonstration of the machine-to-machine variation inherent in the test method was asecondary goal of this research, as this variation has animpact on the interpretation of data generated duringaccelerated weathering testing.

To insure that the same amount of water wasdelivered to all samples inside of each machine, a newwater measurement technique was utilized. A piece oflaboratory synthetic sponge, approximately 1 cm thick,was cut to fit in a standard sample holder. The sponge

0

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1

0 500 1000 1500 2000 2500 3000

Time (minutes)

IrradianceWater

Rad

iati

on

inte

nsi

ty (

W/m

2 @

340

nm

) o

r w

ater

sp

ray

on

/off

Fig. 1: Schematic of water and light cycles used for accelerated weathering protocol. Light lines indicate periods ofradiation, and dark lines indicate periods of water spray

Table 2: Details of aerospace coating systems utilized in this study

System # Chemistry System Layering Expected Failure Mode Color

1 Polyurethane A MC/Primer/Conv Significant gloss loss White3 Polyurethane B MC/Primer/Conv Moderate gloss loss White5 Polyurethane C CC/BC/Primer/Conv Minimal gloss loss White

For the MC, A and B represent two different commercially available systems qualified to the same material specifications

MC monocoat, CC clearcoat, BC basecoat, Conv conversion coating


was saturated with water, squeezed to remove themajority of the water, and weighed to the nearest 0.1 gon a digital balance. The sponge was then placed in theweathering device at a specific location. The spraycycle was run for 5 min and the sponge was removed

and weighed. The mass difference was taken as theamount of water that impacted that area of the weath-ering device during that specific interval. This measure-ment was repeated at various locations in the device. Forthe Atlas Weather-Ometer�, the measurement was

Rad

iati

on

inte

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ty (

W/m

2 @

340

nm

) o

r w

ater

on

/off

Water

UV

0

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0.75

1

0 500 1000 1500 2000 2500 3000

Time (minutes)

Fig. 2: Schematic of light and water cycles used in SAE J2527. Light lines indicate periods of radiation, and dark linesindicate periods of water spray

Table 3: Accelerated weathering test cycles for new protocol

Step# Waterspray

Irradiance (W/m2

@340 nm)Humidity

%Air temperature

(�C)Black panel temperature

(�C)Duration

(min)

1 On 0 95 40 40 3002 Off 0.4 50 40 50 603 Off 0.8 35 48 70 7804 Off 0 50 40 40 305 On 0 95 40 40 270Repeat below 89

6 On 0 95 40 40 307 Off 0.4 50 40 50 208 Off 0.8 55 48 70 1209 Off 0 40 40 40 10Return to Step 1

Table 4: Accelerated weathering test cycles for SAE J2527 protocol

Step# Waterspray

Irradiance (W/m2

@340 nm)Humidity

%Air temperature

(�C)Black Panel temperature

(�C)Duration

(min)

1 Off 0.55 50 47 70 402 On 0.55 95 47 70 203 Off 0.55 50 47 70 604 On 0 95 38 38 60


repeated for the top, middle, and bottom rack loca-tions. For the Q-Panel flat array machines, the mea-surement was repeated for the left, center, and rightpositions. Multiple measurements were taken at eachlocation and averaged. If significant differences existedbetween locations or between machines, the spraynozzles and water pressure were adjusted to achieve aminimum of 7.0 g of water at each location during a5 min spray.

PAS-FTIR spectroscopy

Photoacoustic Fourier transform infrared (PAS-FTIR)spectra were obtained using a Mattson Cygnus 100rapid scan FTIR system equipped with a water cooledsource, a variable source aperture at 50% and anMTEC 200 cell. A hyperbolic turning mirror was usedto bring the maximum intensity onto the PAS sample.A 65% carbon fill rubber plug, 1 cm in diameter, wasused as the reference. PAS samples were taken fromthe panels with an 11-mm punch and die set with thecentering dimple ground off. All of the spectra weretransformed using 4000 data points and one order ofzero filling to give a spectral resolution of 8 cm�1 and adigital resolution of 4 cm�1. The IR spectra were asampling of the top 8–12 lm of the coating. Data werecollected and processed using Mattson WinFirst�

software on a PC. IR spectra were acquired for allcoating systems as a function of weathering time.

Changes in the PAS IR spectra were quantifiedusing the method developed by Gerlock et al., whichtracks growth of the [–OH, –NH] region of the IRspectrum (�2200–3700 cm�1) vs exposure time. Thisgrowth was then normalized to the area of the [–CH]region (�2750–3200 cm�1). The change has beenshown to be representative of the amount of photoox-idation and hydrolysis products accumulated duringweathering.40 To compare the type of chemicalchanges that occur in the coatings during weathering,the IR spectra were compared using a previouslydeveloped comparison technique.37 Briefly, four peaksin the fingerprint region of the spectra were selectedand their heights measured. The height of the peakswas recorded as a function of weathering time. Thepeaks were then ratioed to each other, with the heightof peak a ratioed to the height of peak b and the heightof peak c ratioed to the height of peak d. These ratioswere then plotted against each other. In this manner,the changes in the IR spectra were quantitativelycompared. Weathering tests that produce the samechemical composition changes show the same slope ofthe data using this comparison technique. Furtherdetails on this technique are given elsewhere.31,37

Ultraviolet microspectroscopy

The distribution of UVA in some automotive paintsystems was quantified via ultraviolet microspectroscopy.

A full description of the technique has been previouslypublished.12 In brief, a 1 cm sample disk was firstpunched from each test panel for evaluation. Eachpunched disk was then bent until the paint systempopped free from the metallic substrate. The resultantpaint chip was then sandwiched between two pieces ofreference film and glued together using a two-part,solvent-free, high molecular weight polyurethaneadhesive (Ashland Chemical PliogripTM 7779/100).The solvent-free nature of the adhesive insured thatadditives did not migrate while the CC was beinganalyzed. The reference film used was an acrylic/melamine CC film with known UVA concentrationand known UV absorbance. The entire sandwich wasplaced in an RMC model MT990 rotary microtome anda 10-lm-thick cross section was cut from the sandwichusing a diamond blade. The 10-lm-thick cross sectionwas then mounted on a quartz microscope slide in asolution of 10% glycerin in water and a quartz coverslip was placed over the specimen. UV spectra werecollected using a Carl Zeiss, Inc., AxiotronTM MPM800 microscope/spectrometer system with programma-ble stage. A 5-lm wide 9 10-lm long sample aperturewas set and positioned so that the left 10 lm of theaperture lined up with the surface of the CC and the5 lm width of the aperture extended inward towardthe basecoat. A spectrum was collected from 250 to450 nm and the stage was programmed to move in5 lm increments from the surface of the CC toward thebasecoat layer of the sample, thus moving the apertureacross the entire thickness of the CC. UV spectra werecollected at each 5 lm step across the sample. Finally,the aperture was focused on the reference film and aUV spectrum was collected. Since the reference filmhas a known UVA concentration, it served as a way tomeasure actual cross-section thickness. All data werecorrected for thickness variations determined by themeasurement of absorbance in the reference film.

Gloss and color change

Twenty degree gloss values for all panels wereobtained with a BYK-Gardner microgloss meter(BYK-Gardner, Gerestried, Germany). The colorchange for some panels was measured using a X-RiteMA-68 Colorimeter (X-Rite, Michigan, USA). Colorwas evaluated using a D65 light source at 45� from thespecular reflection.

Visual evaluation

All panels were evaluated visually after each 6 monthincrement of natural weathering or 1000 kJ @340 nmof accelerated weathering. In the remainder of thisarticle, all dose values will be in reference to measure-ments at 340 nm. Photomicrographs were taken using adigital camera and with Image Pro Plus software(Media Cybernetics Inc, MD, USA).


Water sorption

Water absorption measurements were conducted witha Mettler AT20 balance, the details of which havepreviously been reported.41 The balance had a maxi-mum capacity of 22 g and an accuracy of ±0.002 mg.Data were taken via a computer connection to thebalance. The inherent stability of the balance wasmeasured prior to collecting data. Samples were cutfrom aluminum panels coated with the specific paintsystem of interest. Samples were typically 50 mm 950 mm in size. Before specimens were tested for waterabsorption, they were first dried in a vacuum oven.Each sample was then weighed and placed into theaccelerated weathering device with the water sprayactivated. Each sample was removed periodically,blotted dry with lab wipes, and weighed again. Thesamples were then placed back into the weatheringdevice for further water exposure. In this manner, thesample mass as a function of water exposure time wasrecorded. All water absorption experiments wereconducted during dark exposure in the weatheringdevices. A minimum of three replicates was used foreach system studied. The maximum difference betweensamples was less than 2% of the measured averagevalue. Some samples were also tested for water uptakeby soaking overnight in a container of 25�C water.Their mass was recorded before and after waterexposure.

Results

Water absorption

The absorption of water by paint system 25 whenexposed in a Ci4000 weathering device during anextended spray cycle is shown as a function of waterexposure time in Fig. 3. The initial absorption waslinear when plotted on a t0.5 scale, indicating standardFickian diffusion of water into the paint system.42 Thewater volume uptake of the same paint system afterexposures to both 16-h humidity (60�C, condensinghumidity) and after soaking overnight in 25�C water isshown in Fig. 4. Water saturation was reached afterboth of these exposures. Only a small fraction ofsaturation was achieved after typical wet cycle spraying(light and dark) during SAE J2527 in a Ci4000accelerated weathering chamber (Fig. 4). While wedid not directly measure the rate of desorption duringthis study, a previous study has shown that paintsystems dry rapidly outdoors once they becomewarmed up due to exposure to sun.34

Photochemistry

The UV portion of the SPD of the xenon arc lightsource with the newly developed filter combination is

shown in Fig. 5 on a log scale. For comparison, theSPD of Miami, FL sunlight and borosilicate/borosili-cate filtered xenon arc light are also shown. The newfilter combination correctly reproduced the cut-on ofradiation at �295 nm, while boro/boro filtered lightcontained some short wavelength UV not present interrestrial solar radiation. Unnaturally short radiationhas been previously shown to distort the resultsobtained during accelerated testing.37

The fingerprint region of the PAS-IR spectra of thepolyester–urethane CC is shown before and afteraccelerated weathering in Fig. 6. The ratio of thechanges in the four peaks (a–d) that occurred duringweathering is shown in Fig. 7 for the new protocol,Florida exposure, Arizona exposure, accelerated Fres-nel exposure, and SAE J2527 boro/boro exposure. Theslopes of the data obtained from Florida, Arizona,Fresnel, and the new protocol were very similar, whilethe slope of the data from the SAE J2527 boro/borotest was different. The distance from the origin in this

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Light/spray J2527

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16 h 60°C cond. humid.

Extended water soak 25°C

Fig. 4: Water absorption of paint system 25 after variouswater events including: 30 min spray with light on in SAEJ2527, 60 min spray with the light off in SAE J2527, 16 h of60�C condensing humidity, and 12-h soak in 25�C liquidwater

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)

Fig. 3: Water absorption of automotive paint system 25 asa function of exposure time during the spray cycle in anAtlas� Ci4000 weathering device. Water was sprayedcontinuously during the test and the specimen removedperiodically for weighing


type of plot is indicative of how much degradation (ortime) has occurred. The slope of the data is indicativeof the type of chemical changes that occurred duringweathering. Data sets with equivalent slopes indicatethat the type of chemistry that occurred during eachexposure was equivalent. Differences in slopes areindicative of differences in the balance of reactions thatoccur during the different exposures.37

To quantify the overall amount of photooxidation,as opposed to the balance of reactions that occurredduring weathering as illustrated in Fig. 7, the –OH,–NH region of the spectrum has been shown to bemore broadly applicable than a detailed examination

of the fingerprint region. This is particularly true whentrying to compare the extent of degradation in coatingsystems that differ in their basic composition, i.e. apolyurethane vs an acrylic/melamine CC. Therefore,the change in the [–OH, –NH] value as a function ofaccelerated weathering dose for four representativeautomotive coating systems is shown in Figs. 8, 9, 10,and 11 and is taken as representative of the amount ofdegradation in each coating system. We have chosen toomit the IR data for the aerospace coatings for brevityand because gloss loss measurements (detailed below)are typically a reliable gauge of MC degradation rate.

Each graph (Figs. 8, 9, 10, 11) contains data for thesame coating systems run in multiple acceleratedweathering devices in different laboratories to demon-strate the lab-to-lab and machine-to-machine variationinherent in the testing. Each graph contains data forthe new protocol, SAE J2527 (with boro/boro filters),and natural weathering in South Florida. South Floridadata was converted to dose by assuming an annual doseof 3080 kJ/m2 at 340 nm.43 In general, the amount ofdegradation in a coating system is directly proportionalto the (–OH, –NH/–CH) growth. Paint systems 13, 25,and 97 (Figs. 8, 9, 10) showed rapid photooxidationgrowth, which is a hallmark of poor long-term dura-bility. Paint system 86 displayed much slower photo-oxidation growth and from this analysis, would beexpected to have superior long-term weatherability.

While extreme care was taken to minimize thedifferences between machines (quantification of waterdelivery, same filter combinations, identical tempera-ture parameters), there is still measureable machine-to-machine variation in the IR data. This is likelyinherent in the design and control strategy of theweathering devices and should be accounted for when

65085010501250145016501850

Wavenumber (cm-1)

a

b

cd

New

Weathered

Fig. 6: IR spectra of a polyester–urethane CC before and after 3000 h of SAE J2527 accelerated weathering. Note change inrelative heights of lettered peaks

0.001

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1

250 270 290 310 330 350 370 390

Inte

nsi

ty (

W/m

2 )

Wavelength (nm)

Miami sunlight

New glass filter

Boro/boro

Fig. 5: SPD of Miami, FL sunlight, xenon arc light filteredby borosilicate inner and outer filters and xenon arc lightfiltered by Right LightTM inner and quartz outer filter. Notelog scale and high fidelity match of the UV cut-on betweensunlight and Right LightTM�


comparing the results of identical samples run indifferent machines/laboratories.

The UV spectra as a function of depth into theCC is shown in Fig. 12 for paint system 150 exposedto natural Florida weathering, accelerated weatheringin the new protocol, and no weathering. Theabsorption at any given wavelength is proportionalto the amount of UVA present at that location. Ascan be seen, the concentration of UVA is relativelyuniform for the nonweathered paint system. After

either natural or accelerated weathering, a gradientin UVA concentration developed, with the surfacebeing depleted of UVA relative to the bulk of theCC.

Physical failures of paint systems

Pictures of the five representative paint systems afteraccelerated weathering in the new protocol, SAE J2527

Δ (c/d )

Δ(a

/b)

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Arizona

New protocol

Florida Accelerated fresnel

SAE J2527 with boro/boro

Fig. 7: Change in the ratio of IR spectra peaks a/b vs c/d. The slope of the data is an indication of the balance ofdegradation reactions taking place during weathering. Equal slopes indicate the same balance of reactions. Data indicatesthe same balance of reactions in Florida, Arizona, accelerated Fresnel, and new protocol. Slightly different balance in SAEJ2527 boro/boro accelerated weathering

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Sample 13

New protocol J2527 boro/boro

Florida

Δ(–O

H, –

OH

)/–C

H

Fig. 8: Degradation, as measured by changes in the –OH, –NH portion of the IR spectrum, for coating system 13 as afunction of UV dose at 340 nm for Florida (triangles), SAE J2527 boro/boro (diamonds), and the new protocol (squares)


boro/boro, and Florida exposure are shown in Figs. 13,14, 15, 16, and 17.

System 13 (Fig. 13) failed the standard paint adhe-sion test (X-scribe after water soak) after 2 years ofFlorida exposure. This was seen as a slight loss ofadhesion of the basecoat from the e-coat. The speci-men tested for 3000 kJ/m2 in the new protocol alsodisplayed the same adhesion failure, while the speci-men tested in SAE J2527 did not fail the adhesion test.

This paint system was based on acrylic/melaminetopcoats that contained no UV light absorber toprotect the paint system from UV-induced degrada-tion, and was thus expected to undergo rapid photo-oxidation of the topcoat and underlying e-coat surface.

Paint system 25 (Fig. 14) failed by delamination ofthe basecoat from the electrocoat after less than2 years of Florida exposure. In addition to the delam-ination, the topcoat contained numerous microblisters.

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These blisters were most easily seen by high magnifi-cation optical microscopy, as shown in Fig. 15. Thespecimen testing in the new accelerated weatheringprotocol also failed an adhesion test after 3000 kJ/m2

and contained microblisters. The specimen exposed tothe J2527 protocol displayed less of an adhesion lossand did not contain any microblisters. System 25 wasbased on carbamate functional acrylic polymers, whichcan be quite durable when formulated properly. Thissystem was formulated with inferior UV stabilizationand pigmentation and thus, expected to fail prema-turely.

System 97 (Fig. 16) failed by delamination of the CCoff of the basecoat after less than 2 years of Floridaexposure. In addition, the CC is severely blistered. Thespecimen exposed to the new accelerated protocol alsoshowed delamination of the CC from the basecoat andblistering of the CC. The specimen exposed to SAEJ2527 showed neither the blistering nor delaminationfailure. This system lacked UV light absorbers in theCC, and was therefore expected to photooxidizerapidly and delaminate off of the basecoat.

Specimen 86 (Fig. 17) was a positive control; itperformed well in Florida and also performed well inboth accelerated weathering tests. It was formulatedfrom a highly durable polyurethane binder with theappropriate UV stabilizers and durable pigments.

System 103 (Fig. 18) failed by delamination of the CCfrom the basecoat in addition to blistering of the CC andloss of gloss. The same failure modes are observed in thespecimen exposed to the new protocol, while thespecimen exposed to J2527 only shows delaminationand gloss loss. Due to inferior UV stabilization, thissystem was expected to photooxidize rapidly and fail bydelamination at the CC/BC interface.

As previously mentioned, all the paint systems weretested in multiple machines and the pictures shown inthe manuscript are representative of the failure modesobserved in all machines. Excellent reproduction of thefailures was observed between machines. In addition,numerous other automotive paint systems have beentested under the same conditions as the panels detailedabove. Those systems that performed well in Floridaalso performed well in the new protocol. Those systemsthat failed by cracking also failed by cracking whentested with the new protocol. The cracking failures arenot always correctly reproduced by J2527 testing.Cracking failures are often difficult to capture photo-graphically, and have been omitted from this manu-script for that reason.

Aerospace coatings

The predominant failure mode for the aerospace MCsystems in Table 2 is gloss loss (arising from erosion ofthe binder resin and exposure of pigment44) ratherthan color shift or the delamination, blistering, andcracking failures expected for the automotive coatingsin Table 1. For the current study, the aerospacecoatings sample set was designed to test the newprotocol’s ability to correctly predict the occurrence ofgloss loss and the relative performance ranking ofsystems based on South Florida results. It should benoted that the outdoor exposure results reported hereare in qualitative agreement with expectations forperformance of these systems in a service environment,based on fleet surveys.

The change in 20º gloss as a function of dosage (kJ @340 nm) is reported for Sets 1, 3, and 5 in Figs. 19, 20,

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and 21. The South Florida data show the expectedservice performance for these coatings, where Set 1 hasthe worst gloss retention and Set 5 has the best glossretention. SAE J2527 provides little discriminationamong the coatings, where the MC (Set 1 and Set 3)perform identically, within the error of the data, and itis not until approximately 5000 kJ that the basecoat/CC system shows a slight advantage over the MC. Thenew protocol correctly ranks the gloss retention per-formance as compared with Florida, although the onsetof gloss loss is delayed and a high dosage (>4000 kJ) isrequired before the correct trend becomes apparent.The color change, as quantified by DE, was generallyless than 1.0 and did not show clear differentiationbetween the systems for exposure in Florida, the newtest protocol or SAE J2527. No delamination, blister-ing, cracking, or other defects were observed in any ofthe aerospace coating samples.

Discussion

The design of a credible accelerated weatheringprotocol requires a number of criteria to be met. First,to be useful in the development and qualification ofnew coatings, an accelerated weathering protocol mustprovide significant acceleration over the rate of naturalweathering. Second, the results obtained from theaccelerated test must reproduce the chemical changesthat occur during natural weathering. Unnatural chem-ical changes can drive false positive and false negativeresults and must be avoided.37 Finally, minimal sampleinvestigation should be required after the acceleratedweathering test. Most paint failures are easily observedby the naked eye (gloss loss, color change, cracking,and delamination). If an accelerated weathering testcannot reproduce these physical failures, the test hassignificantly less practical utility. Thus, paint systems

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Fig. 12: UV spectra as a function depth into the coating for paint system 150 before and after accelerated weathering.Spectra acquired via micro-UV spectroscopy on cross sections of CC. Note depletion of UVA at the surface of the CC afteraccelerated and Florida weathering


that crack after natural weathering exposure, forexample, should crack after the accelerated weatheringexposure and do so at a consistently scalable time. Inaddition, paint systems that perform well after naturalweathering exposure should also perform well afteraccelerated weathering exposure.

To date, no accelerated weathering tests haveachieved all of the above three goals. Acceleration

can easily be achieved by increasing temperature and/or irradiance. Reproduction of the correct degradationchemistry requires, at a minimum, the use of a lightsource with the same SPD as sunlight. To date, this hasonly been achieved using accelerated outdoor exposureusing Fresnel-type equipment. However, the physicalfailures observed after accelerated outdoor exposuredo not typically replicate those seen after natural

Florida exposure J2527 New protocol

Fig. 14: Appearance of paint coating system 25 after 24-h water soak and tape adhesion test. Panels previously exposed to:2 years’ Florida exposure, 3000 kJ SAE J2527 boro/boro exposure, and 3000 kJ new protocol exposure. Note completedelamination of basecoat off of electrocoat for Florida exposed panel, partial delamination for new protocol exposed panel,and slight delamination for SAE J2527 exposed panel


Fig. 13: Appearance of paint coating system 13 after 24-h water soak and tape adhesion test. Panels previously exposed to:2 years’ Florida exposure, 3000 kJ SAE J2527 boro/boro exposure, and 3000 kJ new protocol exposure. Note partialdelamination of basecoat off of electrocoat for Florida and new protocol. Holes in some panels are due to removal ofpunches for PAS IR spectroscopy


outdoor exposure. Previous study has suggested thatthis is due to the lack of water absorption by paintsystems exposed in that equipment.41 Most artificiallyaccelerated weathering tests do not reproduce thechemical changes observed after natural weatheringbecause of distortions in the degradation chemistrycaused by an incorrect SPD of the light source.Physical failures are often not correctly reproduced

because of the lack of water absorption comparedwith Florida exposure. The newly developed testprotocol reported here attempts to address all of theseissues.

It should be noted that the protocol described in thismanuscript is likely not the only combination oftemperature, moisture, time, and radiation that willcorrectly reproduce the behavior of materials exposed

Florida New protocol J2527

Fig. 15: Light micrograph of coating system 25. Panels previously exposed to: 2 years’ Florida exposure, 3000 kJ SAEJ2527 boro/boro exposure, and 3000 kJ new protocol exposure. Note presence of microblisters on Florida and new protocolpanels. Width of micrograph is 5 mm


Fig. 16: Appearance of paint coating system 97 after 24-h water soak and tape adhesion test. Panels previously exposed to:2 years’ Florida exposure, 3000 kJ SAE J2527 boro/boro exposure, and 3000 kJ new protocol exposure. Note completedelamination of CC off of basecoat and severe blistering for Florida exposed panel and new protocol exposed panel


in South Florida. Small changes in the duration,frequency, and intensity of the water and temperaturecycles will likely not significantly affect the test results.The protocol reported here is merely one reproducibleand practical manifestation of the test conditions thathave been found to provide a high fidelity prediction ofcoatings’ behavior in South Florida.

Water absorption

In South Florida, paint systems become covered withliquid water almost every night because of dewformation, and therefore are regularly saturated withwater.34 Upon heating, the samples dry out as the sunrises and warms the panels. This same behavior is


Fig. 17: Appearance of paint coating system 86 after 24-h water soak and tape adhesion test. Panels previously exposed to:2 years’ Florida exposure, 3000 kJ SAE J2527 boro/boro exposure, and 3000 kJ new protocol exposure. This coatingsystem performs well in all tests


Fig. 18: Appearance of paint coating system 103 after 24-h water soak and tape adhesion test. Panels previously exposedto: 2 years’ Florida exposure, 3000 kJ SAE J2527 boro/boro exposure, and 3000 kJ new protocol exposure. Note completedelamination of CC off of basecoat and severe blistering for Florida exposed panel and new protocol exposed panel. Onlydelamination shown in J2527 exposed panel


expected in the new protocol, where the samples arelargely saturated during the long water cycles andshould be essentially dehydrated during the long lightcycles because of their elevated temperature. Thiscontrasts significantly with the water absorption thatoccurs during most standard accelerated weatheringprotocols. For example, the 1-h water spray during thedark cycle of SAE J2527 only leads to approximately22% of the saturation water content in a typical paintsystem, and the spray during the light cycle only leadsto 11% of the saturation water content (Fig. 4). Failuremechanisms that are thought to depend strongly onwater content include blistering and delamination.

The newly developed protocol contains two rela-tively long water cycles. These long sprays allow thepanels to approach saturation. The effect of the longerspray cycles compared with the shorter cycles in SAE

J2527 is evident in Figs. 13, 14, 15, 16, and 18, wherethe accelerated and natural weathering results arecompared. For the paint system shown in Fig. 14, theexpected failure mode was catastrophic delaminationof the basecoat off of the electrocoat due to UV-induced degradation of the top of the electrocoat, asthis system was formulated without a primer layer.While both the specimens exposed in Florida and thespecimens exposed in the new protocol both showed anadhesion failure, the extent of delamination was muchgreater after Florida exposure than after exposure inthe new protocol. Even though the Florida and newprotocol panels both show similar amounts of chemicaldegradation (both show a D(–OH, –NH) value of �1.5at 6160 kJ for the Florida specimen and 3000 kJ for thenew protocol specimen), we speculate that smalldifferences may have allowed for a slightly differentamount of degradation in the Florida specimen.Imprecise time correlations are difficult to reconcilein weathering testing when samples are typically onlyexamined at regular intervals. In addition, small filmbuild differences between the panels will stronglyaffect this failure mode.

In addition to this delamination failure, the Floridaexposed samples also showed significant blistering ofthe basecoat prior to delamination. Only panelsexposed in the new protocol also displayed themicroblisters observed in Florida (Fig. 15). Theseblisters are potentially thought to be because of theinteraction of water with a hydrophobic portion of thecured basecoat film. This interaction causes blistersthat start in the basecoat layer and can both causestress at the basecoat/CC film interface and defects thatpropagate to the actual CC surface as well. The paintsystem shown in Fig. 16 failed by delamination of theCC off of the basecoat, which is observed in Floridaand the new protocol. Specimens tested in J2527 didnot show the blistering and showed inconsistent

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Fig. 21: Gloss loss of aerospace basecoat/CC system 5 asa function of UV dose at 340 nm for panels exposed inFlorida, SAE J2527 boro/boro, and the new protocol


reproduction of the delamination failure. Clearly,significant water penetration and absorption arerequired to disrupt the bonding between the CC andthe basecoat. The lower water absorption in SAEJ2527 is not sufficient to reproduce the failuresobserved after Florida exposure in this system. Thepaint system in Fig. 18 fails by delamination of the CCoff of the basecoat. Again, massive blistering beforedelamination is only observed in Florida and the newprotocol. These three examples demonstrate theimproved predictive capability of the new protocolcompared with SAE J2527 and the importance thatwater plays in the improved capability.

Spectral match to sunlight

To minimize both false positives and false negativesduring accelerated weathering testing, the SPD of thelight source in the accelerated weathering deviceshould match that of sunlight as closely as possible,particularly in the low wavelength UV portion of thespectrum. A good match between SPDs will insure thatthe photochemistry that occurs during acceleratedweathering matches that which occurs during naturalweathering. Most durable coatings absorb very littleUV light. However, even small amounts of absorptioncan compromise durability because of the free radicalnature of degradation in most coatings. If the absorp-tion bands are in an area of the UV spectrum wherethe SPDs of the accelerated device and sunlight arewell matched, the differences in photochemical degra-dation will be minimal. In cases for which the wave-length of absorption is in an area of the SPD where thematch is poor, significant discrepancies can occur in thetypes of photochemical degradation observed. Thislatter case exists for the flexible automotive polyester–urethane CC studied previously and in this research(Fig. 7). The absorption tail overlaps with the UV cut-on for boro/boro filtered xenon arc light. Thus,significant degradation is observed in SAE J2527boro/boro and other accelerated weathering testswhere the UV cut-on occurs at an unnaturally lowwavelength. Only when the solar cut-on is correctlyreproduced, such as in natural sunlight, Fresnel-typeexposures, or with the new filter set is the degradationchemistry correctly reproduced. This is shown in Fig. 7where the slopes of the curves are very close for thepolyester–urethane CC exposed in Florida, Arizona,Fresnel-type exposure, and with the new filters. Eventhe small difference between these slopes and the slopeobserved with boro/boro exposure are enough to showa false negative. This polyester–urethane CC hasexcellent Florida durability, but will crack prematurelybecause of significant photooxidation when tested in anaccelerated weathering device using boro/boro filters.37

The alternative to using a light source that matchesthe SPD of sunlight is to measure the absorption oraction spectrum of a coating or coating system andthen expose the coating to only that wavelength of light

to drive the correct photochemistry. This is bothdifficult and impractical. Often the absorbing speciesin a coating are present at very low concentrations.This precludes the accurate measurement of theirabsorption spectrum. In addition, for practical pur-poses, there are significant advantages to exposingmany different coatings at the same time in the sameweathering device. If accelerated weathering condi-tions have to be modified based on the absorptionspectrum of each coating, testing becomes significantlymore complex and costly.

Paint system 13 shown in Fig. 13 is an example of apaint system whose failure is strongly governed by theabsorption of UV light. In this system, the underlyingelectrocoat has a strong and broad absorption upthrough 400 nm. Thus, the SPD of the light source willnot strongly influence the degradation of the electro-coat. Instead, the lifetime is almost exclusively afunction of opacity of the basecoat and CC. Smalldifferences in film build can lead to significant differ-ences in failure time, and this is evidenced in Fig. 13where a small amount of adhesion loss is seen onsamples exposed in Florida and the new protocol. Inthe sample exposed in SAE J2527, the adhesion loss isnot observed, but the film build of the basecoat and CCare slightly higher leading to less UV dose on theelectrocoat. Thus, one must guard against broadconclusions without insuring that equivalent sampleshave been used for all tests.

Cyclic conditions

The details of the water and radiation cycles describedin this study are the result of an extensive iterativeapproach to designing a new accelerated weatheringprotocol. The final cycles described here are a hybridof two extreme approaches to cycle design. The manyiterative steps along the path to the protocol reportedon here are not detailed, as the iteration changes weremainly incremental. Changes were guided by theexpected failure modes and the subsequently observedfailure modes for a variety of paint systems, some ofwhich are reported on here. The process and rationaleis summarized below.

As shown previously, to correctly reproduce mostadhesion and blistering failures, deep water penetra-tion is required. In the absence of the water contentapproaching saturation, paint systems will not displaythe proper loss of adhesion or blistering in comparisonto the same paint system exposed in Florida. Thus, anaccelerated weathering protocol that contained verylong, dark spray cycles would be a very good approx-imation of what occurs in Florida. Unfortunately, thisstrategy is problematic for a number of reasons. First,the protocol would use excessive amounts of water ifthe spray cycle was to approach the twelve hour lengthof an actual time of wetness event in Florida. Deion-ized water is expensive, and this type of protocol wouldhave been fiscally difficult to implement. Second, by


devoting such long periods to dark spray cycles, theacceleration of the test would have suffered, as thechemical degradation to the paint system is drivenprimarily by photooxidation during the light cycles.

While recognizing the above difficulties with aprotocol that contains long, dark spray cycles, theapproach was still used as a starting point in thelaboratory. Results of a protocol that alternatedbetween 12-h spray cycles and 12-h light cycles (usingthe same filters with the correct SPD and an irradianceof 0.75 W/m2 @340 nm) did not produce the correctphysical failures in some paint systems. Those systemthat failed by blistering and delamination after Floridaexposure also failed by those same mechanisms afteraccelerated weathering with this cycle. However,systems that failed by cracking, either stress inducedor weathering induced after Florida exposure, did notfail appropriately when only long wet and dry cycleswere used in the accelerated weathering protocol.Relying exclusively on long wet cycles and long lightcycles apparently does not sufficiently fatigue samplesto cause cracking failures during accelerated weath-ering.

Using an extensive set of paint systems that showeddifferent modes of failure after Florida exposure, thecycles in the protocol were adjusted after each round oftesting. Insufficient adhesion or blistering failures ledto longer dark spray cycles. Insufficient cracking led tomore frequent short water cycles. After iterativelyadjusting the balance between long and short watercycles and long and short light cycles, the protocolreported on in this manuscript was shown to provide ahigh fidelity match to the behavior of paint systemexposed in South Florida.

The frequency of the temperature and wet/drycycles also scales appropriately with Florida exposure.On average, panels exposed in Florida receive 8.4 kJ(@340 nm) of radiation per day. These samples gothrough one wet/dry cold/hot cycle, which inducesfatigue stresses on the paint because of mismatches inmoisture absorption and the thermal expansion coef-ficient between the coating and the substrate. Thus,Florida exposure introduces 8.4 kJ of radiation perstress cycle. Exposure in J2527 results in approximately3.9 kJ/stress cycle, because of the short, frequent cyclesused in that protocol. The new protocol results in9.9 kJ/stress cycle, very similar to Florida exposure.This scaling also improves the correlation betweenFlorida and the new protocol for paint systems proneto cracking.

Acceleration of new test protocol

The main goal of any accelerated weathering test is toreduce the amount of time required to reliablyanticipate the long-term weathering performance ofa paint system. Thus, the rate at which degradation

occurs is one of the primary metrics by which theutility of an accelerated weathering protocol shouldbe judged. Previous researchers have shown thatmeasuring the growth of the [–OH, –NH] region ofa coating’s IR spectrum as a function of weatheringtime/dose is a sensitive means of quantifying acoating’s degradation.40 Concentrating on this regionof the IR spectrum avoids complications associatedwith other analysis techniques such as carbonylgrowth, which can change in a complex manner inurethane and other coatings.45 The use of a failure-based metric to quantify the degradation rate isdifficult, as some failures occur suddenly, with littlewarning, and are therefore difficult to monitor as afunction of time. For systems without a CC, gloss lossis a reasonably reliable method of assessing durabilityand the rate of degradation, as it is easily measurableas a function of exposure time and is one of the mostcommon failure modes for those coatings (see the‘‘Aerospace Coatings’’ section).

The rate of degradation for a subset of the coatingstested is shown in Figs. 8, 9, 10, and 11. On each plot thegrowth of the [–OH, –NH] region of the IR spectrum isplotted as a function of dose @340 nm. For example, inFig. 9 the rate of degradation in both the new protocoland SAE J2527 boro/boro is approximately the same ona dose basis. That is, the D[–OH, –NH] value of thecoating is approximately 1.0 after about 1700 kJ ofexposure in both tests. It should be noted, that the rate ofdegradation in Florida is less than half that of eitheraccelerated test (D[–OH, –NH] = 1.0 after �4300 kJ).In fact, on a dose basis there is little evidence to suggestthat the new protocol is any faster than the J2527 boro/boro. Specifically, the average amount of (–OH, –NH)growth for each coating system in all the weatheringdevices running the new protocol has been evaluated atboth 2000 and 3000 kJ. The values cannot be statisticallydiscriminated from the same coating systems exposed toJ2527 boro/boro.

However, as the irradiance and proportion of timespent in the light cycle is higher for the new protocol,the rate of degradation on a time basis is substantiallyfaster for the new protocol compared with SAE J2527.Averaging over all of the paint systems examined, thenew protocol reaches the same level of chemicaldegradation approximately 40% faster than the SAEJ2527 boro/boro test method. For this test protocol,acceleration over J2527 boro/boro is derived solelyfrom increased irradiance and a change in the propor-tion of time the samples are exposed to light.

The average time to a given amount of (–OH, –NH)growth can also be calculated for Florida exposure toderive the average acceleration factor for the newprotocol over Florida exposure. When this is done, theacceleration factor ranges from 16 for system 97 to 8for system 103. The universality of these accelerationfactors is unknown at this time and is undergoingfurther study.


Positive controls

While correctly anticipating paint systems’ correctphysical failures is critical for any trustworthy acceler-ated weathering protocol, false negatives must also beavoided. Thus, paint systems that perform well inFlorida and show little chemical degradation duringnatural exposure must also perform that way duringaccelerated weathering. The paint system shown inFig. 17 is an example of a positive control—it performsvery well in Florida and undergoes little chemicalchange as well in both Florida and accelerated weath-ering (Fig. 11). The system shows no cracking, blister-ing, or loss of adhesion in the new accelerated protocol.Numerous other positive control systems have alsoperformed as expected in the new protocol.

UVA longevity

HALS and UVA are added to all modern automotivepaint systems to improve the long-term weatherabil-ity.22 As weathering progresses, both of these additivesare typically consumed because of various photochem-ical and physical processes. This can be seen in Fig. 12,where the UV absorption spectra for paint system 150before and after weathering (natural and the newprotocol) are shown. The gradients developed in bothFlorida and after accelerated weathering were similar,the main difference being the somewhat shallowergradient and higher overall loss in the 4000 kJ accel-erated weathered sample compared with the 22-monthFlorida-exposed sample. The UVA was consumed atthe surface of the coating initially, and a gradient wasformed from the top surface toward the bulk of thecoating. The surface of the coating typically experi-ences a higher loss rate, not because of exudation andphysical evaporation of the UVA, but because of ahigher rate of photooxidation occurring at the surface.The higher rate of photooxidation is due to the UVA’sinability to protect the absolute surface of the coating.Attack of the UVA by free radicals is thought to be theprimary mechanism for UVA consumption.28 Thegradient is generated and then maintained as mostUVAs do not have sufficient mobility in weatheredcoatings to diffuse appreciable distances even over thetime scale of natural weathering.46

The loss rate of these additives during acceleratedweathering tests does not typically scale by the samefactor as the chemical degradation and the reasons forthis are not clear. For example, the coating whoseUVA data is shown in Fig. 12 is approximately twice asdegraded (as measured by [–OH, –NH] growth) after4000 kJ of accelerated weathering as is the 22 monthFL exposed sample. The rate of a coating’s degradationis a summation of both the photochemical degradationand degradation due to hydrolysis and thermal oxida-tion. The latter two processes do not largely affect thestabilizer additives, and thus are not as easily acceler-ated as photooxidation. Other reasons for the inability

to accelerate UVA consumption to the same extent aschemical degradation or physical failures remain spec-ulative.

Aerospace coatings

Unlike the automotive industry, aerospace OEMs andrepaint facilities use MC systems almost exclusively forexterior finishing, with basecoat–CC technology juststarting to be introduced into the worldwide fleet.47

The expected service life of the current generation ofaerospace MC systems is of the order of 3–5 years,depending on factors such as flight cycles, routes flownand color. The predominant failure mode for thesesystems is loss of gloss, where a 60� gloss value of 60units has been defined as a failure criterion by someairlines.48 The development of more durable exteriordecorative coating systems has been hindered, in part,by the inconsistent correlation between standardaccelerated weathering test methods and in-serviceperformance. Coatings that perform well in acceleratedtesting have been observed to fail prematurely inservice,49 while other coatings whose performance isdifferentiated in accelerated weathering have beenfound to perform equivalently in service.50 The datapresented in Figs. 19, 20, and 21 illustrate this point,where it can be seen that SAE J2527 cannot discrim-inate between the performance of MC and basecoat/CC systems, while the Florida data show significantdifferences. Indeed, it has been long established thatautomotive CC have superior gloss retention, andreports in the literature indicate that aerospace CC,though limited in use, do improve gloss retention ascompared with MC.51 The fact that the standardindustry accelerated test method cannot usefully pre-dict this outcome demonstrates the challenge offormulating, testing and implementing the next gener-ation of coatings with improved durability, and high-lights the pressing need for improved methodology.

Gloss loss is closely tied to the removal of materialfrom the surface of the coating, where the surface leftbehind tends to be rougher because of the presence ofpigment particles and the uneven removal of thebinder. This process should be highly dependent onthe details of the erosion processes that take place atthe very surface of the coating, and is likely to bedominated by the details of liquid water application.The new protocol, with its longer water cycles, doescorrectly predict the performance ranking of the threeaerospace coating systems, showing the anticipatedsuperior gloss retention for the basecoat/CC system.For Set 1 (Fig. 19), the gloss loss in Florida acceleratesearly in the exposure history, while the new protocoleventually mimics the behavior at a somewhat higherdose. Reasons for the delayed gloss response in thenew protocol are still under investigation and may beindicative of this particular coating system or may bemore broadly applicable to all MC systems. The otheraerospace coating systems (Figs. 20, 21) lose gloss


more gradually after outdoor exposure, and thisperformance is mimicked when the systems are testedusing the new protocol. Further experiments arecontinuing on these and other MC systems, both inthe comparison of the new protocol system withFlorida exposure and in the understanding of theeffects of high altitude exposure where the UVirradiance is more intense, but moisture levels andtemperatures are much lower.

We also note that the higher acceleration factorsobserved for the (–OH, –NH) growth of the automo-tive basecoat/CC system are not replicated for the glossloss of the aerospace systems. For these coatings, whilethe trend is reproduced, the acceleration over Floridaexposure is a more modest 3–4. As stated above, thisobservation is under continued investigation.

Conclusions

A new accelerated weathering test protocol has beendeveloped that correctly anticipates almost all failuremodes observed in South Florida exposed coatingsystems. In addition, the new protocol is 40% fasterthan the current standard, SAE J2527. The newprotocol’s improved performance is based on using alight source with an excellent match to terrestrialsunlight, particularly in the short wavelength UVregion of the spectrum. The use of the new lightsource drives the correct photochemistry in paintsystems, which enables the use of higher irradiance todrive acceleration. The correct physical failures areinduced by insuring that, during accelerated exposure,the paint systems are subjected to high amounts ofliquid water, thus enabling water absorption nearsaturation levels. This has been shown to be crucialto reproducing adhesion and blistering failures in paintsystems that experience those failure modes in Florida.The failure of paint systems that demonstrate crackingafter Florida exposure is also correctly reproduced bythe new protocol because of the inclusion of severalshort hot/cold dry/wet cycles in the new protocol,which correctly scales the stress cycle to the UV doseexperienced by a panel during a diurnal cycle inFlorida. Systems that demonstrate no failures afterFlorida exposure also perform well in the new testprotocol. This new test protocol offers improvedaccuracy and reduced testing time that will be bene-ficial to paint formulators and coatings users to reduceproduct development time and reduce the risk ofpremature failures of paint systems.

References

1. Martin, JW, ‘‘Repeatability and Reproducibility of FieldExposure Results.’’ In: Martin, JW, Bauer, DR (eds.) ServiceLife Prediction Methodologies and Metrologies, pp. 2–22.American Chemical Society, Washington, DC (2001)

2. Jorgenson, G, Bingham, C, Netter, J, Goggin, R, Lewan-dowski, A, ‘‘A Unique Facility for Ultra-Accelerated Nat-ural Sunlight Exposure Testing of Materials.’’ In: Bauer, D,Martin, W (eds.) Service Life Prediction of Organic Coatings,pp. 170–185. American Chemical Society, Washington, DC(1999)

3. Hardcastle, HK, Jorgensen, GJ, Bingham, CE, ‘‘Ultra-Accelerated Weathering System I: Design and FunctionalConsiderations.’’ In: Reichert, T (ed.) Natural and ArtificialAgeing of Polymers—4th European Weathering Symposium.Publication No. 11, Gesellschaft fur Umweltsimulation(2009)

4. White, K, Fischer, R, Ketola, W, ‘‘An Analysis of the Effectsof Irradiance on the Weathering of Polymeric Materials.’’ In:Martin, J, Ryntz, R, Chin, J, Dickie, R (eds.) Service LifePrediction of Polymeric Materials, pp. 71–82. Springer, NewYork (2009)

5. Scott, K, Hardcastle, HK, III, ‘‘A New Approach toCharacterizing Weathering Reciprocity in Xenon ArcWeathering Devices.’’ In: Martin, J, Ryntz, R, Chin, J,Dickie, R (eds.) Service Life Prediction of PolymericMaterials, pp. 83–92. Springer, New York. (2009)

6. Martin, J, Chin, J, Nguyen, T, ‘‘Reciprocity Law Experi-ments in Polymeric Photodegradation: A Critical Review.’’Prog. Org. Coat., 47 292–311 (2003)

7. Dickie, R, ‘‘Chemical Origins of Paint Performance.’’J. Coat. Technol., 66 29–37 (1994)

8. Bauer, D, ‘‘Perspectives on Weatherability Testing of Auto-motive Coatings.’’ J. Coat. Technol., 74 33–38 (2002)

9. Bauer, D, Gerlock, J, ‘‘Photo-Stabilization and Photo-Deg-radation in Organic Coatings Containing a Hindered AmineLight Stabilizer: Part III—Kinetics of Stabilization DuringFree Radical Oxidation.’’ Polym. Degrad. Stab., 14 97–112(1986)

10. Bauer, D, Mielewski, D, Gerlock, J, ‘‘PhotooxidationKinetics in Crosslinked Polymer Coatings.’’ Polym. Degrad.Stab., 38 57–67 (1992)

11. Bauer, D, Gerlock, J, Mielewski, D, ‘‘Photo-Stabilizationand Photo-Degradation in Organic Coatings Containing aHindered Amine Light Stabilizer: Part VII—HALS Effec-tiveness in Acrylic Melamine Coatings Having DifferentFree Radical Formation Rates.’’ Polym. Degrad. Stab., 369–15 (1992)

12. Smith, C, Gerlock, J, Carter, R, III, ‘‘Determination ofUltraviolet Light Absorber Longevity and Distribution inAutomotive Paint Systems Using Ultraviolet Micro-Spec-troscopy.’’ Polym. Degrad. Stab., 72 89–97 (2001)

13. Haacke, G, Longordo, E, Brinen, J, Andrawes, F, Campbell,B, ‘‘Chemisorption and Physical Absorption of Light Stabi-lizers on Pigment and Ultrafine Particles in Coatings.’’J. Coat. Technol., 71 87–94 (1999)

14. Wernstahl, K, ‘‘Service Life Prediction of AutomotiveCoatings, Correlating Infrared Measurements and GlossRetention.’’ Polym. Degrad. Stab., 54 57–65 (1996)

15. Van der Ven, LGJ, Leuverink, R, Hernderiks, H, vanOverbeek, R, ‘‘Durability Prediction of p-Urethane Clear-coats.’’ Prog. Org. Coat., 48 214–218 (2003)

16. Gerlock, JL, Mielewski, DF, Bauer, DR, ‘‘Photo-InitiationRate Behavior of Weathered Coatings: ESR NitroxideDecay Assay.’’ Polym. Degrad. Stab., 20 123–134 (1988)

17. Gerlock, JL, Prater, TJ, Kaberline, SL, de Vries, JE,‘‘Assessment of Photooxidation in Multi-Layer CoatingSystems by Time of Flight Secondary Ion Mass Spectrom-etry.’’ Polym. Degrad. Stab., 47 405–411 (1995)

18. Mielewski, DF, Bauer, DR, Gerlock, JL, ‘‘Determination ofHydroperoxide Concentrations in Crosslinked Polymer


Coatings Containing Hindered Amine Light Stabilizers.’’Polym. Degrad. Stab., 41 323–331 (1993)

19. Nguyen, T, Martin, J, Byrd, E, Embree, N, ‘‘RelatingLaboratory and Outdoor Exposure of Coatings II: Effectsof Relative Humidity on Photodegradation and the Appar-ent Quantum Yield of Acrylic Melamine Coatings.’’ J. Coat.Technol., 74 65–80 (2002)

20. Valet, A, Light Stabilizers for Paints. Vincetz Verlag,Hannover, 1997

21. Gerlock, JL, Kucherov, AV, Nichols, ME, ‘‘On the Com-bined Use of UV, HALS, Photooxidation and FractureEnergy Measurements to Anticipate the Long-Term Weath-ering Performance of Clearcoat/Basecoat Automotive PaintSystems.’’ J. Coat. Technol., 73 45–54 (2001)

22. Haacke, G, Andrews, FF, Campbell, BH, ‘‘Migration ofLight Stabilizers in Acrylic/Melamine Clearcoats.’’ J. Coat.Technol., 68 57–62 (1996)

23. Gerlock, JL, Kucherov, AV, Smith, CA, ‘‘Determination ofActive HALS in Automotive Paint Systems II: HALSDistribution in Weathered Clearcoat/Basecoat Paint Sys-tems.’’ Polym. Degrad. Stab., 73 201–210 (2001)

24. Nichols, ME, Gerlock, JL, ‘‘Rates of PhotooxidationInduced Crosslinking and Chain Scission in ThermosetPolymer Coatings II: Effect of Hindered Amine LightStabilizer and Ultraviolet Light Absorber Additives.’’Polym. Degrad. Stab., 69 197–207 (2000)

25. Gerlock, JL, Bauer, DR, Briggs, LM, ‘‘Photo-Stabilizationand Photo-Degradation in Organic Coatings Containing aHindered Amine Light Stabilizer: Part I—ESR Measure-ments of Nitroxide Concentration.’’ Polym. Degrad. Stab., 1453–71 (1986)

26. Gerlock, JL, Riley, T, Bauer, DR, ‘‘Photo-Stabilization andPhoto-Degradation in Organic Coatings Containing a Hin-dered Amine Light Stabilizer: Part II—Consumption ofHindered Amine.’’ Polym. Degrad. Stab., 14 73–84 (1986)

27. Pickett, JE, ‘‘Permanence of UV Absorbers in Plastics andCoatings.’’ In: Martin, JW, Bauer, DR (eds.) Service LifePrediction Methodologies and Metrologies, pp. 250–265.American Chemical Society, Washington, DC (2001)

28. DeBellis, AD, Iyengar, R, Kaprindis, NA, Rodebaugh, RK,Suhadolnik, J, ‘‘Computer Simulations of the Conforma-tional Preference of 3¢ Substituents in 2-(2¢-Hydroxyphenyl)Benzotriazole UV Absorbers: Correlation with UVA Photo-permanence in Coatings.’’ In: Martin, JW, Bauer, DR (eds.)Service Life Prediction Methodologies and Metrologies, pp.453–467. American Chemical Society, Washington, DC(2001)

29. Bauer, DR, ‘‘Predicting In-Service Weatherability of Auto-motive Coatings: A New Approach.’’ J. Coat. Technol., 6985–96 (1997)

30. Martin, JW, ‘‘A Systems Approach to the Service LifePrediction Problem for Coating Systems.’’ In: Bauer, DR,Martin, JW (eds.) Service Life Prediction of Organic Coat-ings, pp. 1–20. American Chemical Society, Washington, DC(1999)

31. Gu, X, Stanley, D, Byrd, WE, Dickens, B, Vaca-Trigo, I,Meeker, WQ, Nguyen, T, Chin, JW, Martin, JW, ‘‘LinkingAccelerated Laboratory Test with Outdoor PerformanceResults for a Model Epoxy Coating System.’’ In: Martin, JW,Ryntz, RA, Chin, JW, Dickie, RA (eds.) Service LifePrediction of Polymeric Materials, pp. 3–28. Springer, NewYork (2009)

32. Nguyen, T, Martin, J, Byrd, E, ‘‘Relating Laboratory andOutdoor Exposure of Coatings: IV. Mode and Mechanismfor Hydrolytic Degradation of Acrylic-Melamine Coatings

Exposed to Water Vapor in the Absence of Light.’’ J. Coat.Technol., 75 37–50 (2001)

33. Chin, J, Nguyen, T, Byrd, E, Martin, J, ‘‘Validation of theReciprocity Law for Coating Photodegradation.’’ J. Coat.Technol. Res., 2 499–508 (2005)

34. Henderson, K, Hunt, R, Boisseau, J, Pattison, L, ‘‘TheImportance of Water in the Weathering of AutomotiveCoatings.’’ CoatingsTech, 5 50–55 (2008)

35. Perera, DY, Vanden Eynde, D, ‘‘Moisture and TemperatureInduced Stresses (Hygrothermal Stresses) in Organic Coat-ings.’’ J. Coat. Technol., 59 55–63 (1987)

36. Dickie, RA, ‘‘Paint Adhesion, Corrosion Protection, andInterfacial Chemistry.’’ Prog. Org. Coat., 25 3–22 (1994)

37. Gerlock, JL, Peters, CA, Kucherov, AV, Misovski, T,Seubert, CM, Carter, RO, III, Nichols, ME, ‘‘TestingAccelerated Weathering Tests for Appropriate WeatheringChemistry: Ozone Filtered Xenon Arc.’’ J. Coat. Technol., 7535–45 (2003)

38. Nichols, ME, Boisseau, J, Pattison, L, Quill, J, Misovski, T,Peters, CA, Zhang, J, Henderson, K, Smith, D, Seebergh, JE,Berry, DH, ‘‘Accelerated Weathering Testing: A NewApproach to Anticipating Florida Exposure Results.’’ Proc.COSI Conf., Toronto (2011)

39. SAE Testing Standard J2527, ‘‘Performance Based Standardfor Accelerated Exposure of Automotive Exterior MaterialsUsing a Controlled Irradiance Xenon-Arc Apparatus.’’Society of Automotive Engineering (2004)

40. Gerlock, JL, Smith, CA, Cooper, VA, Dusbiber, TG, Weber,WH, ‘‘On the Use of Fourier Transform Infrared Spectros-copy and Ultraviolet Spectroscopy to Assess the WeatheringPerformance of Isolated Clearcoats from Different ChemicalFamilies.’’ Polym. Degrad. Stab., 62 225–234 (1998)

41. Misovski, T, Hardcastle, HK, Nichols, ME, ‘‘The Influenceof Water on the Weathering of Automotive Paint Systems.’’In: Martin, JW, Ryntz, RA, Chin, JW, Dickie, RA (eds.)Service Life Prediction of Polymeric Materials, pp. 295–308.Springer, New York (2009)

42. Van der Wel, GK, Adan, OCG, ‘‘Moisture in OrganicCoatings—A Review.’’ Prog. Org. Coat., 37 1–14 (1999)

43. Data courtesy, Atlas Weathering Services Group44. Braun, JH, Cobranchi, DP, ‘‘Durability and Gloss.’’ J. Coat.

Technol., 67 55–62 (1995)45. Bauer, DR, Gerlock, JL, Mielewski, DF, Paputa-Peck, MC,

Carter, RO, III, ‘‘Photodegradation and Photostabilizationof Urethane Cross-Linked Coatings.’’ Ind. Eng. Chem. Res.,30 2482–2487 (1991)

46. Peters, CA, Ellwood, KR, Srivastava, Y, Nichols, ME,Greenfield, ML, ‘‘Ultraviolet Light Absorber Mobility inCrosslinked Coatings: Experiments and Modeling.’’ Prog.Org. Coat., 58 272–281 (2007)

47. LeBlanc, L, ‘‘The New Basecoat-Clearcoat External PaintSystems for AIRBUS Applications.’’ Proc. 18th InternationalConference on Surface Treatments in Aero. and Space Ind.(SURFAIR) (2010)

48. Guseva, O, Brunner, S, Richner, P, ‘‘Service Life Predictionfor Aircraft Coatings.’’ Polym. Degrad. Stab., 82 1–13 (2003)

49. Blackford, RW, ‘‘Prediction of the Life of an AircraftExterior Coating in Service Flight.’’ Proc. 15th InternationalConference on Surface Treatments in Aero. and Space Ind.(SURFAIR), 10 (2004)

50. Varley, RJ, Simmonds, EK, Seebergh, JE, Berry, DH, ‘‘Inves-tigation of Factors Impacting the In-Service Degradation ofAerospace Coatings.’’ Prog. Org. Coat., 74 679–686 (2012)

51. Blackford, RW, ‘‘Improved Durability Airline PaintSchemes.’’ Mater. Sci. Monogr., 29 115–124 (1985)


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An improved accelerated weathering protocol to anticipate Florida exposure behavior of coatings

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