film formation of coatings studied by diffusing-wave spectroscopy
TRANSCRIPT
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Progress in Organic Coatings 61 (2008) 181–191
Film formation of coatings studied by diffusing-wave spectroscopy
A. Brun ∗, H. Dihang, L. BrunelFormulaction, 10 Impasse Borde Basse, 31240 L’Union, France
Received 1 June 2007; accepted 4 September 2007
bstract
The coating industry is facing nowadays a growth in environmental regulations, and imposed substitution of suspected toxic components orolvents and a constant need for improving performances. The new formulations undergo new film-formation processes, influencing the coatingerformance and appearance.
We have studied the film-formation process of several coating systems (water-based and solvent-based) using a new optical technology basedn diffusing-wave spectroscopy. This unique and simple technique allows a non-intrusive monitoring of the drying process on the appropriate
ubstrate (concrete, metal, plastic, wood, etc.). The kinetics of film formation, displayed in real-time, provide a new vision of the successive stepsf the mechanisms taking place (evaporation, packing, etc.) as well as accurate information on drying times such as open-time, for an in-depthharacterization of the film-formation process.2007 Elsevier B.V. All rights reserved.
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eywords: Diffusing-wave spectroscopy; Film formation; Drying; Water-based
. Introduction
Control and understanding of the film-formation processemain of great importance for coating manufacturers and rawaterial suppliers. They have to innovate constantly to provide
ew products of high performance to comply with the recenturopean Union environmental legislations: the European direc-
ive on the emission of volatile organic compounds (VOC), asell as the REACH legislation, active in the European Union
ince the beginning of 2007. REACH (Registration, Evaluationnd Authorisation of Chemicals) has the key central aim to pro-ect the human health and the environment from the risks arisingrom the use of chemicals. This legislation will have an impactn the availability and price of raw materials for coatings. Asconsequence, new raw materials have to be introduced in the
ormulations, with the possibility to modify the physical andhemical properties of the coating. To comply with the Euro-ean requirements of low VOC content (VOCs are emitted as
ases from certain solids and liquids), new formulations such asater-based latex or high solid solvent-based paints, UV-curabler solvent-free two-component coatings have already appeared∗ Corresponding author.E-mail addresses: [email protected] (A. Brun),
[email protected] (H. Dihang).
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300-9440/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2007.09.041
ngs; Latex; Open-time
n the market in the last decades. These new formulations areased on different chemistries and hence undergo different film-ormation processes compared to the classical solvent-basedormulas, leading to different final properties of the coating.
A wide range of characterization techniques have been usedn the last years for research on coating drying, curing and filmormation of latex [1–3]:
microscopy techniques [4]: optical or transmissionmicroscopy, atomic force microscopy (AFM), scanningand transmission electron microscopy [5], etc.spectroscopy scattering and optical techniques: transmissionspectro-photometry [3], FTIR [6], attenuated total reflec-tion infrared spectroscopy (ATR-IR) [7,8], confocal Ramanmicrospectroscopy [9], ellipsometry [4,10], small angle neu-tron scattering (SANS) [11], magnetic resonance profiling[12], fluorescence decay profiling [13], etc.thermal analysis [14]: differencial scanning calorimetry(DSC) or thermal mechanical analysis (TMA)physical testing methods [1,8,15]: rheology, dynamicmechanical analysis (DMA), electrical conductometry,
dielectric analysis (DEA), gravimetry (water loss), etc.For more routine analyses, the BK drying recorder is oftensed by paint manufacturers to extract characteristic drying
1 ganic Coatings 61 (2008) 181–191
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imes such as set-to-touch, tack-free, or dry-hard times [16,17]:ccording to a standard test method [16], the visual observationf the trace left by a needle drawn through the drying film atconstant speed gives the drying times. Each of these aboveentioned techniques provides very useful information on film
ormation and drying processes. However, most of them are notapable of measuring the build-up of properties under realis-ic conditions of solvent evaporation and/or on the appropriateubstrate.
We present here a new optical technique based on mul-ispeckle diffusing-wave spectroscopy (MS-DWS) [18–20] totudy film formation of coatings. Diffusing-wave spectroscopys an extension of classical dynamic light scattering (DLS orCS, photon correlation spectroscopy) [21] to concentrated andpaque media [22–25]. It has been developed several years agond has been used since then to study the dynamics of complexuids such as gels, colloidal suspensions, biocolloids, ceramiclurries, etc. The basic principle of dynamic light scattering is tolluminate a sample with a coherent beam of light (laser) and to
easure the temporal fluctuations in the resulting speckle pat-ern of scattered light. The temporal fluctuations of the scatteredight are directly related to the motion of scatterers (particulate
atters) inside the sample, which motion is directly related to theisco-elastic properties of the medium (see measurement prin-iple below). DWS theory establishes the relation between theight dynamics and scatterers dynamics for multiple scatteringamples, as described by Weitz and Pine [20]. DWS is used fornstance to study the microrheology of colloidal gels [25], mon-tor the gelation of milk [26,27] and starch suspensions [28,29],r the dynamics of bio-polymers in yoghurt and cheese [20,25].he use of single speckle DWS to study drying of paints has beenlready reported by Koper and co-workers [30]. A related tech-ique such as dynamic speckle interferometry [31–33] based onhe speckle contrast evaluation has been developed by Amalvyt al. and used to follow the drying of spray paints [34,35].ecently, Zhakarov and Scheffold reported on the observationf spatially and temporally heterogeneous dynamic propertiesf a drying colloidal film using a multispeckle DWS experiment36a]. Here we present our own multispeckle diffusing-wavepectroscopy technique and data processing to monitor film for-ation from various coating systems [37]. We have studied a
laboratory-made’ coating and commercially available water-ased and solvent-based architectural paints, and correlated thebtained film-formation kinetics either with gravimetric analy-is, BK drying recorder or standard manual tests. The drying pro-ess of a water-based paint on various substrates is also reported.
. Measurement principle
.1. The A.S.I.I. processing
We have developed an original multispeckle diffusing-wavepectroscopy (MS-DWS) technique using a simple and direct
rocessing of the backscattered light. This innovative process-ng, embedded since 2006 in a commercially available instru-ent (see Section 3) and patented, is named Adaptive Specklemaging Interferometry or A.S.I.I. [38,39]. The set-up of the
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Fig. 1. MS-DWS experimental set-up.
S-DWS experiment is presented in Fig. 1. A laser light illumi-ates the coating sample: if the sample contains scattering mate-ials (rough surface, latex particles, pigment particles, emulsionroplets, fibbers, etc.), the photons are scattered by these objectsearing a different refractive index than the bulk. The scatteredaser light is detected by a video camera without lens: in theseonditions, the camera monitor shows a peculiar image of gran-lar appearance called “speckle” image [22,23,40,41] (Fig. 1).he speckle image is composed of dark and bright areas (thepeckle spots) resulting respectively from destructive and con-tructive interferences of the backscattered waves. When theample undergoes time-dependent activity such as Brownianotion of particles inside a fluid or refractive index changes,
his activity causes temporal fluctuations in the backscatteredight and consequently changes of light intensity on the specklemage, i.e. intensity fluctuation of the speckle spots. A directelation exists between the motion of scatterers inside theample and the intensity fluctuations on the speckle image30,34–36]: variations of the scatterers’ motion induce varia-ions of the speed of intensity fluctuations on the speckle image.ast activity inside the sample generates fast changes of theackscattered light, and fast intensity fluctuations of the specklepots. Inversely a slow activity (slow motion of scatterers, slowhanges of refractive index, etc.) induces slow changes of theackscattered light, and slow intensity fluctuations of the specklepots.
In the case of a film-forming sample, the sample struc-ure changes with time due for instance to solvent evaporation,hemical reaction, particles diffusion, etc., depending on theechanism of film formation involved. The motion of scatterers
lows down as the film progressively forms due to an increasef the film coherence and viscosity: as a consequence, the speedf the intensity fluctuations on the speckle image decreases withime. The ‘speed of the speckle intensity fluctuations on thepeckle image’ will be referred in this paper as the ‘speckleate’. The speckle rate during a film-forming process can thuse measured and used to monitor structural changes in the dryingoating sample.
To plot the speckle rate as a function of time and get kinetics oflm formation, we have specifically developed the A.S.I.I. pro-
essing [38], a simple and direct processing of the backscatteredight, which allows an accurate and responsive quantification ofhe speed of the intensity fluctuations on the speckle image. The.S.I.I. processing works as a radar for the scatterers average
ganic Coatings 61 (2008) 181–191 183
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A. Brun et al. / Progress in Or
otion inside the sample. The speed of intensity fluctuationsan be characterized by the correlation time τ of the specklemages detected by the video camera: fast intensity fluctuationsf the speckle spots correspond to a short correlation time ofhe speckle images. Inversely, slow intensity fluctuations of thepeckle spots correspond to a long correlation time of the specklemages. The correlation time is computed from a set of specklemages acquired using the camera. The number of images ofhe set and the elapsed time between two consecutive imagesf the set are adjusted by tuning respectively the acquisitionuration and the acquisition frequency of the video camera. Therst image of the set is taken as the reference image. Then, the
nter-image distance is calculated for each image of the set withespect to the reference image: the inter-image distance betweenwo images is determined as the pixel to pixel difference of inten-ity between the images (Eq. (1)). The intensity I of one pixeloes from 0 (black) to 255 (white).
2 =
√√√√√x=dimx−1∑
x=0
y=dimy−1∑y=0
(I2(x, y) − I1(x, y))2 (1)
here d2: inter-image distance; dimx and dimy: number of pix-ls horizontally and vertically; (x, y): co-ordinates of the pixelnside the speckle image; I1(x, y): light intensity on the pixel ofoordinate (x, y) on the first image; I2(x, y): light intensity onhe pixel of coordinate (x, y) on the second image.
From the set of speckle images, the inter-image distanceetween each image and the reference one (first one) is plotteds a function of time to give the so-called correlation function2(t) (Fig. 2). The first point of the correlation function corre-ponds to the inter-image distance between the first image of theet and itself, this first image being taken as the reference image.hen the second point corresponds to the inter-image distanceetween the second image of the set and the reference image, thehird point to the inter-image distance between the third imagend reference image, etc. The correlation function has approxi-ately an exponential shape tending towards dmax, dmax being
he maximal inter-image distance corresponding to completelyncorrelated speckle images. From the correlation function the
haracteristic time τ is extracted: τ corresponds to the time decayf the exponential curve, i.e. to ca. 63% of dmax. The “correlationime” τ of the speckle images corresponds to the time duringhich the images are still significantly correlated to the firstFig. 2. Correlation function d2(t) extracted from a set of speckle images.
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ig. 3. Speckle rate as a function of time determined from the A.S.I.I. processingf speckle images acquired by MS-DWS.
mage of the set. For t > τ, the images of the set are significantlyifferent from the first image. When the correlation τ time haseen calculated from the set of images, the speckle rate (SR) isetermined as the inverse of the correlation time τ and expressedn Hertz (Hz). As a consequence, one plot on the kinetics oflm formation results from the processing of a set of speckle
mages (Fig. 3). As the film progressively forms, the scatter-rs motion generally decreases with time due to an increase inhe film viscosity and film coherence. The main strength of the.S.I.I. data processing is adaptivity: to ensure an accurate and
esponsive determination of the speckle rate whatever the speedariations of the intensity fluctuations, the number of specklemages of the set and the elapsed time between two consecu-ive images are optimised by continually tuning the acquisitionuration and the acquisition frequency of the video camera. As aonsequence, when fast changes occur inside the coating sampleevaporation at the beginning of a drying process for instance),short time between data point acquisition is needed because of
apidly changing speckle images (fast acquisition speed of theamera). Inversely, when slow changes occur inside the sampleend of a drying process or slow curing reaction for instance), aonger time between data point acquisition is sufficient to deter-
ine the speckle rate of slowly changing speckle images (slowcquisition speed of the camera). The benefits of this adaptiverocessing are numerous: real-time displayed kinetics, respon-iveness and accuracy, and measurement over a wide range ofpeckle rates, from 10 to 10−5 Hz as shown in the several exam-les of Section 4. More details on our MS-DWS technique and.S.I.I. processing are given in Ref. [39].
.2. Analysed zone
When the laser illuminates the sample, the photons are scat-ered by any material with a different optical index than the bulk,uch as particles, droplets, fibres, interfaces, etc. When analysingconcentrated and opaque media, photons penetrate into the
ample and are scattered several times before coming out ofhe sample. This phenomenon is called multiple-scattering. Thehotons backscattered by the sample as interfering waves com-ose the signal detected by the camera. Hence the analysed zones the one of the sample explored by the photons before coming
ut. The photons penetrate the sample more or less dependingn the number of scattering events on their pathway, i.e. depend-ng on the scattering properties of the sample: the concentration,article sizes, refractive indices and absorption properties. The
1 ganic Coatings 61 (2008) 181–191
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cattering properties of a product can be characterized by theingle parameter l* corresponding to the transport length of theight in the medium, also defined as the photon path after whichhe photon direction is totally randomised [20]. The penetrationepth of the light is therefore proportional to l*. Experimentsave been performed to evaluate the maximum depth for motionetection. These experiments consist in trying to detect a freshaint layer behind a completely dry layer of variable thickness.he result is that the instrument is still sensitive to motion takinglace at a depth of 30 times the transport length l* of the sample.ractically, the l* value for a paint goes from minimum 5 �m toore than 20 �m. The depth of analysis for paints goes hence
rom 150 to 600 �m depending on the composition of the paint.In case of a vertical non-uniformity of dynamics inside the
ample, for instance in case of skin formation (a surface layerrying faster than the inner part of the film), the signal detected isn average of the movement speed of scatterers in all the depths;he photons detected by the camera come from any depth. Thisverage is weighted: the scatterers close to the surface contributeore to the average than the scatterers located deeper in the sam-
le. A simple consideration about the statistics of the photonsrajectories indicates that the weight of averaging is inverselyroportional to the square of the depth considered for a depthigher than the transport length l*: scatterers contained in a layerwice deeper than another give four times less contribution to theignal. Consequently, when a skin forms on top of a sample, thekin contributes more than the fresh inner layer to the speckleate: the speckle rate detected is hence lowered compared to aample without skin formation. Work is currently in progress tollustrate this phenomenon.
As described previously, the speckle rate depends on thecatterers movements and in a minor extend on the number ofhotons scattering events. This number depends on the ratioetween the sample layer thickness and the optical transportength of the light l*. If the sample thickness is much higher than*, i.e. if the sample is opaque (thickness > 5 l* more or less), theumber of scattering events is a constant and the variations ofhe speckle rate are only due to the scatterers movements. In thease of less opaque layers (thickness < 5 l*) or products showingvarying transparency during film formation, the speckle rate
ariation might be due to both scatterers dynamics and trans-arency variations. The precise evaluation of the impact of l*ariation remains quite complex since it depends as well on thecattering properties of the substrate used. To identify whetherhe changes observed on the film-formation kinetics are due toptical or film structure variation, a simple measurement of thelm opacity variation versus time is performed simultaneously
o the speckle rate variation. In the following experiments, allaterials are opaque coatings showing a constant opacity amonglm formation.
. Experimental
.1. MS-DWS experiments
The MS-DWS measurements were carried out using theorus® Film Formation Analyser from Formulaction [42]
Baa2
Fig. 4. A.S.I.I./MS-DWS hardware set-up.
Fig. 4). The instrument is in a backscattering configuration:he camera sensor (CMOS) and the laser source are on the sameide with respect to the analysed sample and are gathered insingle measuring head. The measuring head is rigidly fixed
n a vertical mast, itself screwed to the aluminium base plate.he mechanics are designed to filter any external vibration ofmplitude more than 1 �m considering the relative position ofhe optical head with respect to the aluminium base plate. Theamera sensor does not bear any lens. A hole designed as a conef half angle (approximately 10◦) blocks a part of the parasiticight coming from outside. The rest of the parasitic light enter-ng through the optical axis is removed by an interference filterf bandwidth 10 nm placed between the hole and the camera.he laser source is a standard laser diode of power 0.9 mW andavelength 655 nm. The camera is a standard 320 × 240 pixels
ensor with a maximum frame rate of 30 images/s.The samples were applied manually on the appropriate sub-
trate. Just after draw down of the paint, the substrate was placednder the laser beam and the measurement was started. Theorus® instrument bears four measuring heads allowing four
imultaneous measurements of different samples.
.2. Materials
Commercial samples of white water-based and whiteolvent-based paints were used to perform the experimentsTable 1) as well as Sample B, a laboratory-made coatingased on an acrylic latex of chemical composition 10/89/1MA/BA/AA, Tg −30 ◦C and density 1.02 g/mL. The sampleformulation was mixed in a laboratory dispersion machine
Dispermat CV – manufacturer: VMA-GETZMANN GMBHermany – rotatory mixer speed 20,000 rpm) using the scheme
nd proportions indicated in Table 2.The measurements on commercial samples were carried out
t a relative humidity level of 50 ± 5%, and a temperature of3 ± 1 ◦C. The commercial paints 1–4 were applied manuallyn a glass panel using a Dr Blade type applicator (Erichsen60/60 04105), at a wet film thickness of 120 �m. Sample Bas spread manually using a stainless steel applicator (Doctor
lade type) giving a wet thickness of 200 �m. The coating waspplied on a glass panel and the measurements were carried outt a relative humidity level of 20 ± 5%, and a temperature of0 ± 1 ◦C.
A. Brun et al. / Progress in Organic Coatings 61 (2008) 181–191 185
Table 1Properties of commercial paint samples
Paint no. Description Polymersize (nm)
Pigmenttype
Pigmentsize (�m)
Paintcolour
VOC(g/l)
Solid(wt.%)
Solid(vol.%)
Density(kg/l)
Glass
1 WB styrene-acrylic polymers 100–150 TiO2 rutile 0.25 White <5 59 40 1.48 3 (85◦)2 25 ◦3 254 25
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sisamldTaobtained from the MS-DWS experiments (Fig. 5). To explain thedifferent variations observed on the kinetics, balance measure-ment of water loss have been performed on sample B in identicalconditions. The balance measurements of sample B are shown
Table 2Chemical composition of sample B
WB styrene-acrylic polymers 100–150 TiO2 rutile 0.WB pure acrylic 100–150 TiO2 rutile 0.SB alkyd – TiO2 rutile 0.
.3. Substrates
Substrates of different porosities and nature have been uti-ized and include glass panels, BK glass strips, plywood of
edium density and gypsum board unprimed dry wall.
.4. Balance (mass loss measurements)
The measurements of mass loss were performed on an ana-ytical balance coupled to a computer that recorded the mass asfunction of time using a specially written Labview program.he samples were measured in duplicate and the times averaged
o improve accuracy (Table 3). The measurements were carriedut at a relative humidity level of 20 ± 5%, and a temperaturef 20 ± 1 ◦C.
.5. Open-time measurement
The open-time was measured according to a proposed ASTMtandard test method [43]. The measurements were carried outt a relative humidity level of 50 ± 5%, and a temperature of3 ± 1 ◦C. The paints were applied on a glass panel using a Drlade type applicator (Erichsen 360/60 04105), at a wet film
hickness of 120 �m: the total area covered by the paint filmas 6 cm wide over 70 cm long (420 cm2). A mark was made
long the paint sample immediately after casting the film. Thepen-time was determined by brushing1 at regular time inter-als a virgin area of the sample (approximately 50 cm2), at arush pressure of 100–150 g. The brush was charged with someaint just once, before beginning the test. The open-time wasetermined by evaluating how long the brushing was effectivet removing the initial mark that was made.
.6. BK drying recorder experiments
The experiments were carried out using a standard BK dryingecorder from Mickle Laboratory Engineering. The measure-ents were carried out at a relative humidity level of 50 ± 5%,
nd a temperature of 23 ± 1 ◦C. The paints were applied on a
lass strip using a cubic Dr Blade type applicator, at a wet filmhickness of 75 �m. Paint 3 was analysed over 12 h, the timesiven in Table 5 result from the average of three identical tri-ls. Paint 4 was first analysed over 12 h and the experiment was1 The brush was moved five times in the direction of the width of the substratend five times in the direction of length of the substrate before visually assessinghe coating.
S
LONKAW
White <29 56 35 1.42 3 (85 )White <149 64 45 1.52 6 (85◦)White <400 68 42 1.36 3 (85◦)
epeated over 24 h for a higher resolution in determination ofhe drying times. Three different trials of each experiment wereerformed in order to average the times (Table 5) and improveccuracy.
The results obtained this way were judged according to theest method ASTM 5895 [16]:
End of stage 1 (T1): end of flow back behind the needle, theneedle starts to leave a track in the film revealing the glasssubstrate. This time is called ‘set-to-touch’ (STT).End of stage 2 (T2): end of the continuous track in the film,the needle starts to tear the film. This time is called ‘tack-freetime’ (TFT) or ‘touch-dry time’.End of stage 3 (T3): end of scratchy mark, the needle leavesa surface mark. This time is called ‘dry hard time’ (DHT).End of stage 4 (T4): end of the surface mark, the needle doesnot leave any trace on the film. This time is called ‘dry-throughtime’ (DTT).
. Results and discussion
.1. Correlation with balance measurements
The film-formation process of a water-based latex coating,ample B, was studied using our MS-DWS technique. The kinet-cs obtained by A.S.I.I. processing of the backscattered light arehown in Fig. 5 (A.S.I.I./MS-DWS kinetics). Two different tri-ls of the same experiment show the good reproducibility of theeasurement. In the case of sample B, the objects that scatter
ight inside the sample and generate the backscattered wavesetected by the camera (the scatterers), are mainly latex andiO2 particles (Table 2). Three distinct phases, marked as I, IInd III, are observed on the A.S.I.I. kinetics of film formation
ample B Weight (g)
atex (49.9 wt.%) 60.83rotan 1124 (solid) 0.87atrosol 250H4Br (solid) 0.10ronos 2190 (TiO2—solid) 21.42mmonia (25 wt.%) 0.02ater 10.88
186 A. Brun et al. / Progress in Organic Coatings 61 (2008) 181–191
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Table 3Water content (extracted from balance weight loss measurement) at differenttimes of the film formation observed on the A.S.I.I. kinetics (Fig. 5)
Sample B MS-DWS/A.S.I.I. time (min) Water content (wt.%)
T0 0 42–43T1 9 23–25T2 20 6–8T ′T
IhTosacfto
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ig. 5. (a) A.S.I.I. drying kinetics of sample B on glass substrate and (b) zoomn stage III from 20 to 100 min.
n Fig. 6. The mass loss measured with the balance was con-erted to water content as a function of time. The dry contentf sample B was measured by drying a known weight of paintnd resulted to be 57.13%, the wet content being thus 42.87%.he wet content was assumed to be the value at T = 0 for the bal-nce measurements, and the assumption was made that all waterad disappeared when the balance showed a constant value. Allxperiments were performed in duplicate in order to average the
imes and improve the accuracy. Times related to the differenthases of A.S.I.I. kinetics and the corresponding water-loss areeported in Table 3 and Fig. 5. The film-formation profile of sam-le B shows a film-forming process in three main steps I, II andig. 6. Sample B water loss as a function of time and A.S.I.I. kinetics (from theS-DWS experiment).
sowtsocdtbst(itbep(t
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II (Fig. 5). Over the first 9 min (phase I), the speckle rate remainsigh (>1 Hz) indicating fast motion of scatterers inside the film.he speckle rate decreases slowly of approximately one decadef Hz as the water content goes from 42–43 to 23–25 wt.%. Theolid content is lower than 77 wt.%, indicating that the scatterersre probably still well dispersed in bulk water. This first phaseorresponds then to a concentration stage, the water evaporatesrom the film surface, water molecules migrate from the liquid tohe atmosphere generating currents, hence fast Brownian motionf latex and pigment particles [22–25,30,34–36].
At the beginning of phase II, a clear change appears on theinetics, accelerations and decelerations of the speckle rate arebserved up to T2 = 20 min. During phase II, the water contentontinues to decrease from 23–25 to 6–8 wt.% The rate of evap-ration is constant compared to phase I, as shown by the balanceeasurement (Fig. 6). The concentration in solid increases up
o 93 wt.%. The turbulent signal and the high concentration inolid indicate that scatterers start to interfere in the motion ofach other; the particles start to rearrange and organize. Thehase II can be defined as a packing stage. Such noisy signal haseen observed as well on the MS-DWS experiments performedn a drying paint by Zakharov and Scheffold [36a]. It reflectsemporally dynamics heterogeneities, i.e. scatterers moving atifferent speeds in the analysed zone, strongly reminiscent ofhat of jammed colloids [36b].
At the beginning of phase III, the speckle rate decreasesharply (0.5 decade of Hz/min) indicating a drastic reductionf the average motion speed of scatterers inside the sample. Theater content in the film goes from 6–8 to 0 wt.%, assuming
hat all the water has escaped from the film when the balancehows a constant value at Tb = 33 min (Fig. 6). At the beginningf phase III, we can assume that the water content (6–8 wt.%)orresponds only to interstitial water remaining in the film. Therastic reduction of the scatterers mobility can be attributed tohe disappearance of the bulk water, close-packing of particleseing reached. During phase III, the A.S.I.I. kinetics profilehows a smoothly decreasing speckle rate up to 100 min, withwo changes in slope at T ′
3 = 21–22 min and T ′′3 = 42–43 min
Fig. 5b). The first change in slope at T ′3 occurs at the character-
stic time of the balance exponential curve Ta, the beginning ofhe falling rate period. The falling rate period observed on thealance measurements corresponds to a different rate of watervaporation as the water evaporates now through densely packed
articles [1,44]. The water content reaches 0 wt.% at Tb = 33 minFig. 6), ca. 10 min before the second change in slope at T ′′3 . Byesting with the finger, the sample appears to be dry-to-touch at
A. Brun et al. / Progress in Organic Coatings 61 (2008) 181–191 187
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bpe1toFoaAhawmatime [45]. The open-time can be defined as the time during whichthe polymer particles can be re-dispersed with the brush, it is thenrelated to the packing state of the particles and to the mobility ofthe particles inside the coating (viscosity). On the A.S.I.I. kinet-
Table 4Open-time and A.S.I.I./MS-DWS T1 of paints 1–3
Paint no. Open-time (ASTM) (min) T (A.S.I.I./MS-DWS) (min)
ig. 7. Different phases of the drying process observed on the A.S.I.I. kineticsf sample B.
ime T ′′3 . After T ′′
3 , the speckle rate slowly continues to decreaset a rate of −0.015 decades of Hz/min. The continuous reduc-ion of the scatterers’ motion after the total evaporation of waterndicates a continuous improvement of the film coherence. Thehase III observed on the A.S.I.I. kinetics of sample B can beefined as a consolidation stage. Film formation of latex coatingsave been intensively studied over the past 20 years, but the com-lete mechanism is still subject to interrogations and differentheories are proposed to attempt to explain the film-formation ofuch systems [45–47]. Current understanding of film formationrom latex coatings consists in three stages [1,3,5]: (i) the firsttage is described as an evaporation and ordering stage, i.e. par-icles immobilize by multiple contacts with one another whenolvent evaporates, (ii) the second stage consist in compactionr deformation of particles to fill the voids left by water evapo-ation, i.e. elimination of pore spaces by progressive flatteningnd rearrangements of particles, (iii) the third stage is a ‘coales-ence’ stage, i.e. fusion of particles due to interdiffusion of theolymer through particle-particle boundaries [1,47,48]. Work isurrently in progress to further investigate the phase III of thelm-formation process of sample B by means of confocal Ramanicroscopy. From now, the different phases of the A.S.I.I. kinet-
cs of sample B indicate that the motion of scatterers inside theample does not decrease continuously upon drying. Fig. 7 givesschematical representation and our interpretation of the differ-nt phases observed on the A.S.I.I. kinetics: (I) concentrationpon bulk water evaporation, (II) rearrangement and packingf particles, (III) consolidation by evaporation of interstitialater, deformation of particles and interdiffusion/coalescencerocesses between the latex particles.
.2. Correlation with open-time measurement
The current penetration of water-based paints in the Euro-ean market is 25% of the industrial paints and 70% of therchitectural paints, and it is continuously increasing thankso innovative chemistries of high performances. Despite their
umerous advantages such as easy water cleaning, low odour,ow environmental impact, one of the remaining problemselated to the use of water-based systems is the too short open-ime or ‘wet-edge time’ compared to the solvent-based systemsPPP
ig. 8. A.S.I.I. drying kinetics of paints 1–3 and arrows showing the correspond-ng open-times determined separately (zoom on first 80 min).
sed since then. In the literature [45,47], the open-time of a paints defined as the period of time during which a painter can makeorrections to the freshly applied wet paint film without leav-ng brush marks. Water-based decorative paints typically ‘close’round 5–10 min, while open-time is in the range of 30–40 minor decorative solvent-based paints. Improving the open-time ofater-based formulations is thus of great importance for paintanufacturers. To evaluate the open-time of a formulation, man-
al tests are performed that intend to reproduce the action of theainter [43,45]: one method consists of paint brushing a baseoat at regular time intervals, and by visual observation deter-ining when the brushing neither removes a mark previously
eft on the base coat (open-time) nor re-disperses the edges ofhe base coat (wet-edge time) [45].
The open-times of different commercially available water-ased paints 1–3 (Table 4) were determined according to aroposed ASTM standard test method [43] described in thexperimental Section 3.6. The film-formation processes of paints–3 were investigated by means of our MS-DWS technique andhe results of the standard manual open-time tests were reportedn the kinetics (indicated with arrows). Results are shown inig. 8. The three phases observed on the kinetics of the ‘lab-ratory made’ water-based sample B (cf. Section 4.1) appearlso on the kinetics of the commercial water-based paints 1–3.ccording to the manual tests, the three commercial samplesave different open-times (Table 4). The open-times of paints 1nd 2 are much shorter than the open-time of paint 3. The time athich the paint closes itself is known to be related to the criticalaximum packing of polymer particles and the related viscosity:low viscosity and a delayed close packing improve the open-
1
aint 1 9–10 11aint 2 8–9 9aint 3 21–22 19
188 A. Brun et al. / Progress in Organic Coatings 61 (2008) 181–191
istdmtwocottbpttwtttgsaki(tiVolmtkoittttd
Table 5Characteristic drying times of paints 3 and 4 determined using a BK dryingrecorder
BK time Paint 3 Paint 4
STT 14–17 min 26–29 minTFT1 (12 h) 1 h 48 min 1 h 14 min–1 h 17 minTFT2 (24 h) – 3 h 12 min–3 h 40 minDD
4
t(BtabficrMrtinbbTtwbUst(expectedly distributed along the A.S.I.I. kinetics, in both cases(paints 3 and 4). When a change in slope is observed on thekinetics, it indicates a change in the decrease rate of the scat-terers’ motion inside the sample. The changes in slope on the
Fig. 9. A.S.I.I. drying kinetics of paints 1–3 over 15 h.
cs obtained from the MS-DWS experiments we observe the firstharp decrease of speckle rate, i.e. the first sharp decrease of scat-erers’ motion, at time T1. T1 corresponds to the end of phase Iescribed Section 4.1, when the scatterers start to interfere in theotion of each other, giving accelerations and decelerations of
he speckle rate. The time T1 on the A.S.I.I. kinetics is comparedith the measured open-times in Table 4. A good correlation isbserved. The MS-DWS technique coupled to our A.S.I.I. pro-essing appears then as a useful tool to objectively evaluate thepen-time of a new formulation. The differences between T1 andhe manually measured open-time (ca. 20%) can be attributedo the brush mechanical effect and the additional paint carriedy the brush. To gain more open-time, different approaches areossible such as addition of organic solvent or water to controlhe rheology and delay the evaporation of the liquids, or addi-ion of hydrophilic compounds or surface active agents to keepater inside the film for a longer period of time [45]. The addi-
ion of water soluble organic solvents such as glycol derivativeso delay the open-time is efficient, however the solvent has alsoo evaporate out of the film after a period of time for the film toain its hardness. If some solvent is trapped in the film or is alow evaporating solvent, the film does not acquire its hardnessnd protective properties, it remains sticky. The film-formationinetics of paints 1–3 over 15 h are shown in Fig. 9. An interest-ng feature of paint 3 is the plateau at relatively high speckle rate4 × 10−3 Hz) observed from 50 min up to 5 h 30 min, comparedo the paints 1 and 2 that show a lower and continuously decreas-ng speckle rate over time. Paint 3 contains the higher level ofOC (Table 1): on one hand it has the longest open-time, andn the other hand the scatterers motion remains higher over aonger period of time. This indicates a slower hardness develop-
ent of the coating in the first 5–6 h of film-formation comparedo paints 1 and 2. The plateau of speckle rate observed on theinetics of paint 3 can be explained by the slow evaporationf an added organic solvent, such as monopropyleneglycol fornstance (higher VOC level). The addition of organic solventso water-based coatings enables to extend the open-time, hence
he easiness of paint application. However, the slow evapora-ion rate of such compounds leaves the film sticky for a longerime. This effect, on the open-time and film tackiness, is clearlyemonstrated by the A.S.I.I./MS-DWS experiment.Fi
HT Not visible 20 h 00 min–20 h 20 minTT 9 h >4 h
.3. Correlation with BK measurements
A technique widely used to determine characteristic dryingimes such as set-to-touch (STT), touch-dry or tack-free timeTFT), dry-hard time (DHT) or dry-trough time (DTT) is theK drying recorder [16,17]. The trace left by a needle drawn
hrough the drying film at a constant speed is analysed visuallynd the length of the different types of marks left on the filmy the needle gives the characteristic drying times [14]. Thelm-formation processes of a water-based and a solvent-basedommercial paints have been analysed using the BK dryingecorder in order to compare the results to the corresponding
S-DWS experiments. The results of the BK experiments areeported in Table 5. The time ranges given are obtained fromhree different trials. TFT1 was extracted from a 12 h BK exper-ment, and TFT2 from a 24 h BK experiment. It is important toote that the marks left by the BK needle on the drying film cane unclear and lead then to subjective interpretation. The water-ased paint 3 was analysed over 12 h using the BK recorder.he marks left on the paint did not allow the determination of
he dry-hard time (no tearing of the film by the needle, the traceent directly from a deep trace to a surface trace). The solvent-ased paint 4 was analysed first over 12 h and then up to 24 h.nfortunately the dry-through time could not be extracted as a
urface trace was still visible on the paint after 24 h. The BKimes were reported on the A.S.I.I. kinetics of paints 3 and 4Figs. 10 and 11). The drying times determined with the BK are
ig. 10. A.S.I.I. drying kinetics of the solvent-based paint 4 and the correspond-ng BK drying times (insert: zoom on first 120 min).
ganic Coatings 61 (2008) 181–191 189
kap
fimrsdctisoBrotsbhtWaarpthdi
fistpsacmcB
FB
d1fisstttitce
mscttfbok
A. Brun et al. / Progress in Or
inetics are hence consistent with changes in the film properties,nd can then be related to different phases of the film-formationrocess.
Paint 4 is an alkyd solvent-based system. Such systemslm form by solvent evaporation and entanglement of macro-olecules, polymer crosslink occurring by an autoxydation
eaction with the air oxygen [45]. The first sharp decrease ofpeckle rate occurs at T1 = 14 min and might correspond to theisappearance of the most volatile solvents that generate fasturrents upon evaporation and fast motion of scatterers. Theime T1 might correspond to the open-time of paint 4 as it isn the same range as the BK set-to-touch time. A change inlope is observed at time T2 = 1 h 45 min indicating a decreasef the film-formation rate. This time is in the same range as theK tack-free time (TFT1 and TFT2 have been determined from
espectively 12 and 24 h BK experiments). The change of slopen the kinetics at T2 might then correspond to the tack-free orouch-dry time of the sample. The touch-dry time is the mostubjective characteristic time of the film-formation process. Toe dry-to-touch, a paint film needs to reach a certain level ofardness and surface dryness that depends on the paint formula-ion, the type of film-formation process and . . . the user’s finger!
hen the paint becomes touch-dry, it means that the film reachesnew stage that corresponds to this new property, and can thenppear on a A.S.I.I./MS-DWS kinetics as a change in slope cor-esponding to the transition to a new motion rate of scatterers. Alateau is observed on the kinetics from T3 = 27 h, indicating thathe scatterers’ motion does not change significantly anymore; itas reach a stationary stage. This time can correspond to thery-through time of the paint 4 (DTT) as the BK experimentndicated that it is longer than 24 h.
The kinetics of the water-based paint 3, sampled at a wetlm thickness of 75 �m, is shown in Fig. 11, and the corre-ponding BK times are reported on the graph. It is noteworthyhat, as expected, the kinetics profiles of a 120 �m wet film ofaint 3 (Fig. 9) or a 75 �m wet film (Fig. 11) are identical inhape, but the complete film-formation is shorter in the case ofthinner film. The first decrease of speckle rate at T1 = 6 min,
orresponding to the first important decrease of the scatterers
otion, finds no correlation with a BK experiment event. T1an be related to the open-time as described in Section 4.2. TheK set-to-touch time is in the range of the second very sharp
ig. 11. A.S.I.I. drying kinetics of the water-based paint 3 and the correspondingK drying times (insert: zoom on first 120 min).
4
saofisieidsfsasi
Fig. 12. A.S.I.I. drying kinetics of paint 2 on different substrates.
ecrease of speckle rate (ca. 2 decades of Hz) occurring from0 to 20 min. After 20 min, the sample was set-to-touch as con-rmed by slightly touching it. The A.S.I.I. kinetics of paint 3hows a plateau between 20 min and 4 h, during which an organicolvent is probably slowly evaporating (see Section 4.2). Fingerest confirmed that the film remains tacky over this period ofime. The BK tack-free time (TFT) is also determined in theime range of this plateau. The BK dry-through time (DTT),ndicating the end of the film-formation process, is shorter thanhe time at which the curve reaches a plateau (ca. 13 h), indi-ating a constant and minimum motion of the scatterers and thend of film-formation.
The correlation between an intrusive mechanical measure-ent such as the BK and a non-intrusive optical measurement
uch as our MS-DWS technique may not be direct as theyorrespond to the measurement of different physical proper-ies, mechanical and optical. However, it remains interestingo compare the results of a well established technique for film-ormation analysis, such as the BK recorder, with the ones of arand new technique such as adaptive speckle imaging interfer-metry, in order to go further in the understanding of the obtainedinetics.
.4. Effect of the substrate nature
Paint 2 was applied at identical thicknesses on various sub-trates (glass, medium density plywood and gypsum dry wall)nd the film-formation process was monitored over time usingur A.S.I.I./MS-DWS technique. Different film-formation pro-les are observed depending on the nature and porosity of theubstrate (Fig. 12). The three phases of film-formation describedn Section 4.1 and Fig. 7 are observed on each kinetics, i.e. onach substrate. The effect of the substrate porosity is observedmmediately in the first minutes of the process: the speckle rateecreases faster when the paint is applied on the porous sub-trates (plywood and gypsum) compared to glass, indicating aaster immobilization of the scatterers by absorption inside the
ubstrate. It is noteworthy that the photons from the laser sourcere scattered by the coating as well as by the substrate: an opaqueubstrate constitutes a volume scattering material as long as its not made of metal (l* = 0). If the substrate is non-porous and
190 A. Brun et al. / Progress in Organic
Table 6Durations of the phases I and II of the A.S.I.I. drying kinetics of paint 2 ondifferent substrates
Substrate Phase I (T1) Phase II
Glass 9 min 23 minPG
rIttcacmitoocstTtTtkDacTto
5
SsfteDscrobdoiekco
imspnrid
A
‘ft
R
[
[
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[
[
[
[[
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[
lywood 5 min ca. 35 minypsum 1 min 30 s ca. 1 h
igid, as it is immobile its contribution to the signal is null.n the case of a rigid porous substrate, paint penetrates insidehe substrate by few hundreds of microns, modifying the scat-ering properties of the substrate with time as it dries. In thisase, the substrate as well as the paint inside the substrate andt the surface are scattering materials, changing with time andontributing to the signal and the speckle rate. Therefore, theotion of scatterers is detected as long as there is fresh paint
nside the pores of the substrate. The time T1, corresponding tohe elapsed time before particles start to interfere in the motionf each other and pack, giving accelerations and decelerationsf the speckle rate (Section 4.1), is reported in Table 6. T1 can beorrelated to the open-time (see Section 4.2). As expected, T1 ishorter in the case of porous substrates, as the paint penetrateshe substrate and disappears from the surface of the substrate.he phase II of the kinetics, corresponding to the packing of scat-
erers (Section 4.1), is longer on the porous substrates (Table 6).his can be explained by a slower evaporation rate of water
hrough the substrate after penetration, compared to glass. Theinetics show a longer phase II on gypsum than on plywood.ifferences between glass and porous substrates are observed
s well during the phase III: the decrease in speckle rate afterlose-packing is faster on glass than on gypsum or plywood.his experiment demonstrates the importance of a technique
hat enables to measure the film-forming properties of a paintn representative substrates.
. Conclusion
We have presented a new optical technology named Adaptivepeckle Imaging Interferometry (A.S.I.I.), based on multi-peckle diffusing-wave spectroscopy, to study film formationrom coatings. This technology monitors and displays in realime the motion of scatterers (particles, droplets, interfaces,tc.) inside the sample as a function of time. A.S.I.I./MS-WS measurements have been performed on water-based and
olvent-based coatings, on different substrates, and have beenompared with balance measurements of water-loss and BKecorder experiments. Three phases are systematically observedn the film-formation kinetics of water-based coatings and haveeen identified as concentration (I), packing (II) and consoli-ation (III) phases. The different changes in slope and shapebserved on the A.S.I.I. kinetics could be related to changesn the film structure by comparison with BK drying recorder
xperiments. Moreover, the characteristic time T1 on the A.S.I.I.inetics of water-based paints shows a good correlation with theorresponding manual determination of the commercial paintspen-times. The measurement is simple to perform: the paint[
Coatings 61 (2008) 181–191
s casted on a substrate, placed under the laser beam and theeasurement is started without any required calibration nor
et-up parameters to enter. This new MS-DWS technique cou-led to our A.S.I.I. processing appears then as an original andon-intrusive tool to investigate film-formation processes in rep-esentative conditions, provide a unique and complementarynformation compared to existing techniques and objectivelyetermine open-time, on appropriate substrates.
cknowledgements
We would like to thank Dr. Anders Larsson (Area ManagerCoatings and Concentrated Dispersions’ at Institute for Sur-ace Chemistry, Stockholm, Sweden) for his contribution to theheoretical and practical work described in this paper.
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