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Drying and cracking of soft latex coatings.

Abstract The minimum film formation temperature (MFFT) is the minimum drying temperature needed for a latex coating to coalesce into an optically clear, dense crack-free film. To better understand the interplay of forces near this critical temperature, cryogenic scanning electron microscopy (cryoSEM) was used to track the latex particle deformation and water migration in coatings dried at temperatures just above and below the MFFT. Although the latex particles completely coalesced at both temperatures by the end of the drying process, it was discovered that particle deformation during the early drying stages was drastically different. Below the MFFT, cracks initiated just as menisci began to recede into the packing of consolidated particles, whereas above the MFFT, partial particle deformation occurred before menisci entered the coating and cracks were not observed. The spacing between cracks measured in coatings dried at varying temperatures decreased with decreasing drying temperature near the MFFT, whereas it was independent of temperature below a critical temperature. Finally, the addition of small amounts of silica aggregates was found to lessen the cracking of latex coatings near the MFFT without adversely affecting their optical clarity.

Keywords Latex film formation, Cracking, Cryogenic scanning electron microscopy, Stress, Minimum film formation temperature

Introduction and motivation

Latex-based coating formulations are environmentally friendly substitutes for many traditionally solvent borne polymeric coatings. Paints, varnishes, floor coatings, and reflective roof coatings are now often latex-based as environmental restrictions are being placed on the coatings industry. (1), (2) Latex is a complex dispersion of polymer colloids in water along with additives such as ceramic particles to modify the color or hardness of the coating, dispersants, defoamers, and rheology modifiers. (3) When such a dispersion is applied to a substrate and dried, the latex particles deform during the drying process to create a homogeneous, nonporous coating. (4-6)

A conflict exists in formulating latex dispersions: the latex particle modulus must be sufficiently low so that the particles deform during drying, yet sufficiently high so that the tack resistance, dirt pickup resistance, and protective nature of the final dried coating are not compromised. (7) Many current formulations contain plasticizers that reduce the modulus of the latex during film formation, yet evaporate after the coating is dried to increase the long-term modulus of the final coating. (8), (9) The most common plasticizers are volatile organic compounds (VOCs) and as such are subject to increasingly strict environmental restrictions against their use. (10) Understanding the factors that influence particle coalescence is valuable for designing a latex coating without the use of plasticizers.

Studies have shown that depending on the latex particle modulus and size, particle deformation can occur at various stages of film formation. Particles initially are well separated in the dispersion, but evaporation of the water brings them into contact. Further evaporation causes the tops of the latex particles to break though the air--water interface, creating curved water menisci between particles (see Fig. 1). The low capillary pressure under these curved menisci can deform soft particles during this "wet" drying stage. (11) If the particles do not deform, further evaporation causes air to enter even farther into the pore space, beginning the "moist" drying stage. In this stage, water exists as pendular rings that hang between the contact points of the particles. (12) Eckersley and Rudin (13) postulated that air--water surface tension and the negative pressure imposed by the pendular ring can deform the latex at this stage. Finally, when all of the water is gone, Gong showed

that van der Waals forces can be sufficient to continue to deform particles. (14), (15) Many of these microstructural changes have been directly visualized using cryogenic scanning electron microscopy (cryoSEM), (14), (16), (17) as well as environmental scanning electron microscopy (18), (19) and atomic force microscopy. (12), (20)

[FIGURE 1 OMITTED]

Several researchers have attempted to predict a critical latex shear modulus ([G.sub.c]) for particle deformation to occur in the wet stage. (11), (21-25) One of the simplest was derived by Brown, (11) who balanced the force to compress two Hertzian spheres with the estimated lowest capillary pressure that can be supported by a stable meniscus between three latex particles (12.9[gamma] / R) to obtain:

[G.sub.c] = 35[gamma] / R (1)

where [gamma] is the air--water surface tension and R is the particle radius. Mason (26) corrected Brown's derivation by realizing that the capillary pressure and the elastic response do not act over the same areas to find:

[G.sub.c] = 266[gamma] / R (2)

The capillary pressure that deforms the latex particles also causes stress within a drying coating. Numerous researchers have used various techniques to characterize the developing stress in a latex coating. (27-34) Some researchers have measured a mild compressive stress in the coating early in the drying process, followed by a transition to a tensile stress with further drying. The tensile stress, which is normally attributed to capillary forces due to water menisci, either plateaus or decreases over time due to water evaporation or polymer relaxation. The origin of the initial compressive stress is disputed. Pekurovsky and Scriven (35) propose that it is due to the air--water surface tension pushing the particle packing toward the substrate. Konig et al. (32) disagree, claiming that it is the result of a springback of compressed particles after stress due to solvent flow is released.

When the particles in the coating do not coalesce, the drying stresses can cause cracking in the film. (36-38) In general, cracking occurs when the energy required to create two new surfaces is less than the elastic energy that would be released in opening the crack. (39) Since the structure, properties, and thickness of a coating change during drying, predicting whether or not a coating will crack is complicated. Drying both creates capillary stress and also drives the particle coalescence, which increases a coating's resistance to cracking. Hence, drying conditions along with the latex particle properties are important to the stress development and cracking.

Although studies have addressed film formation and stress development in the two limits of particle modulus: coalescing and noncoalescing, VOC restrictions are now necessitating a more detailed understanding of the factors that influence the boundary between these two regimes. This boundary, termed the "minimum film formation temperature" (MFFT), is the lowest possible temperature that a coating, can he dried to produce a coalesced, crack-free film. (4) In this research, an acrylic latex with a modulus that highly depends on temperature was dried both just above and just below its MFFT. The extent of particle coalescence, behavior of the water, and existence of cracks at various drying stages were examined using cryoSEM. Furthermore, when small amounts of silica aggregates were added to the dispersion, the cracking was reduced noticeably. This is the first known study of a latex film that cracks despite particle coalescence.

Methods and materials

Materials

Acrylic latex Rhoplex El 2000 (100% acrylic, pH = 9, Dow Chemical, Midland, MI) was used in this study. The glass transition temperature ([T.sub.g]) of this latex was measured by differential scanning calorimetry to be 5[degrees]C. Static and receding contact angles measured for a 1 mm high droplet of distilled water on a dry latex film were found to be 85[degrees] and 26[degrees], respectively. The latex polymer shear modulus was measured as a function of temperature on a dry latex film by an RSA rheometer tensile test using a 1 Hz strain rate, a 1 x [10.sup.-4] total strain, and a 3[degrees]C temperature ramp between 0 and 20[degrees]C. The shear modulus data are included in the supporting material.

In addition, Aerodisp W7520 fumed silica (Evonik, Essen, Germany) was used as an additive. Aerodisp silica had a primary particle size of 12 nm and an average aggregate size of 100 nm. Aerodisp silica is predispersed in water. Adding only the liquid phase of Aerodisp W7520 did not affect the behavior of the latex, so any changes were determined to be due to the silica only. Materials properties of both suspensions are summarized in Table 1.
Table 1: Materials properties

               Solids       Particle        PH    [T.sub.g]
                (wt%)  diameter (nm)

El 2000 Latex    46.5            140   9.0-9.6  5[degrees]C

Aerodisp           20         100 nm  9.5-10.5           --

W7520 Silica              aggregales


Cryogenic scanning electron microscopy

CryoSEM was used to characterize the developing microstructure as the latex coatings dried. Coatings were deposited on a 5 x 7 mm silicon chip or a 7 x 20 mm glass coverslip and allowed to dry under 1 L/min of cool or room temperature nitrogen flow. Dry coatings were approximately 0.1 mm thick. After the desired amount of drying, samples were then plunge frozen into liquid ethane at -196[degrees]C. The samples were kept under a bath of liquid nitrogen as they were loaded into a cryoSEM sample holder. To expose a coating cross section, the substrate was fractured. Samples were then transferred to an Emitech K-1250 cryo preparation chamber where they were sputtered with a 2 nm layer of platinum. The samples were finally imaged in a Hitachi S4700 FESEM at -160[degrees]C. Ice was not intentionally sublimed from the samples, yet limited sublimation was likely unavoidable during sample coating and transfer to the microscope. The extent of this sublimation is not expected to affect the interpretation of the images.

Top surface cryoSEM samples were similarly prepared, with the exception that the samples were not fractured before imaging. Top surface images were also prepared at the Technion Institute in Haifa. Israel, on a Zeiss Ultra Plus FESEM. These coatings Were frozen after drying at room temperature without air flow: however, the drying behavior of these samples was found to be similar to room temperature drying with air flow.

MFFT and crack spacing measurements

To characterize the cracking tendency of the films at various conditions, coatings were cast on a minimum film formation temperature bar (MFFT90, Rhopoint), which dries samples on a temperature gradient (0.6[degrees]C/cm) under 3 L/min of dry blowing air. The MFFT bar was covered with a thin polyethylene terephthalate substrate to aid in the removal of the coating. Coatings were applied in one inch wide strips and dried to a final film thickness of 0.1 mm. The average spacing between cracks was then measured to give a quantitative estimate of the extent of cracking. The use of crack spacing to describe cracking tendency was inspired by the case of a brittle film on a ductile substrate, where the spacing has been shown to be directly proportional to the fracture strength, (40) and has been used by others to describe the extent of cracking. (41) Quantitative models for predicting the crack spacing in particulate coatings based on drying and formulation parameters is an active area of research. (21), (23), (36), (42), (43) In general, the spacing between parallel cracks is determined by a balance between drying stresses and stress recovery in the coating, with greater stresses promoting less spacing between the cracks.

Contact angle measurements

The static and dynamic contact angles of distilled water on a dried coating were measured at room temperature using the Kruss Drop Shape Analysis System DSA-10 (Kruss, Hamburg, Germany). An image of the liquid drop was taken and analyzed using Kruss Drop Shape Analysis software to find the contact angle.

Results

Macroscopic appearance

Thick latex coatings (~0.1 mm) were dried on an MFFT bar to demonstrate the macroscopic effect of temperature on cracking behavior. Drying initiated at the edges of the coatings and proceeded inward toward the center. The samples dried below temperatures of 17[degrees]C formed parallel cracks that were perpendicular to the drying front and went all of the way through to the substrate (Fig. 2). MFFT is assigned based on optical clarity, (4) which in this case is compromised by cracking rather than incomplete coalescences and light scattering by residual pores. Hence, the MFFT of the latex in this research is 17[degrees]C. Room temperature SEM investigations showed that particles were coalesced in coatings dried at all of the temperatures, regardless of the presence of cracks or the presence of silica. These images were taken within 24 h of the start of the drying process.

[FIGURE 2 OMITTED]

Figure 3 displays the spacing between parallel cracks with respect to temperature for coatings with and without silica dried on the MFFT bar. Here, the spacing between cracks is a qualitative measure of the amount of cracking, with larger spacings indicating less severe damage. There are three distinct regimes observed for the crack spacing in the pure latex coatings. At low temperatures (T < 9.5[degrees]C), the spacing is independent of temperature. At intermediate temperatures, the spacing increases with temperature, and at high temperatures above the MFFT (17[degrees]C), no cracking is observed. The same pattern is also seen in coatings containing silica, but the crack spacing in the first regime is slightly greater than that of the pure latex coating, whereas in the second regime silica has less of an effect. Silica also slightly lowers the MFFT to 16[degrees]C.

[FIGURE 3 OMITTED]

Microstructure development

CryoSEM was used to probe the microstructure development of the latex coating above (22 [+ or -] 2[degrees]C) and below (10 [+ or -] 1[degrees]C) the MFFT, or minimum crack temperature, to determine the role of microstructure in influencing crack behavior. Both top surface and cross-section images were taken.

Figures 4 and 5 show the top surface and cross section, respectively, of a latex coating dried under 22[degrees]C nitrogen (i.e., above the MFFT). Evaporation concentrates the particles and brings them into contact with one another. The thinning water layer covering the particles becomes unstable and dewets from the latex (Fig. 4a). The static contact angle for water on dry latex bulk films was measured to be 85[degrees], confirming the appearance of the water in Fig. 4a that the water initially forms droplets on top of the particle packing that are concave toward the film. In this state, the capillary pressure within the coating is greater than atmospheric, which would create a small compressive stress in the coating, as illustrated in Fig. 6. This effect would be another explanation for the source of the mild compressive stresses that were detected during drying of other particulate dispersions (30), (32) and in the drying of this latex (44) (Supporting material).

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Figure 4a also shows some areas where the water has started to invade into the particle packing and the water curvature is less pronounced. These states are stable, since the receding contact angle, which is the angle that water would be expected to take if the contact line were moving due to evaporation, was measured to be about 26[degrees]. Also in Fig. 4a it is clear that the latex--latex contact points that are not obscured by water have already begun to flatten against each other.

Figure 4b shows the degree to which the particle contact points have flattened at the time the water invades into the particle packing. Some small droplets cling between particles, but in general the pore space has completely closed. Figure 4c shows the gradual flattening of the coating top surface and the reduction of particle--particle boundaries. Not shown is the final, completely coalesced coating that develops after a few hours of aging. This state appears as a featureless, flat surface.

Cross-sectional images (Fig. 5) corroborate this story. Initially, particles are mobile, suspended in the water with space in between them (Fig. 5a). Figure 5b shows that the particle contact points well below the coating top surface flatten during the wet stage of drying. Frozen water appears as sharp shapes filling the space between particles. The fracture during the sample preparation of Fig. 5b created pullouts, or plastic deformation of particles, indicating good adhesion at particle contacts. (45) Water appears to fill the shrinking pore space; no pendular rings were detected to indicate air invasion. The last image, Fig. 5c, shows a packing of highly deformed particles with almost no pore space. Pullouts bridge multiple particles, signifying that polymer interdiffusion has begun.45 Therefore, during drying at temperatures above the MFFT the particles deform in the wet drying stage (Fig. 1).

Figure 7 shows top surface cryoSEM images of a coating drying under 10[degrees]C nitrogen flow (i.e., below the MFFT). In Fig. 7a, as in Fig. 4a, the thinning layer of water on top of the latex particles has become unstable, and the top surface latex particles are exposed to the air. Another, earlier view of this drying stage is shown in Fig. 8. Figure 7a also shows much less flattening at particle--particle contact points in early stages of drying as compared to drying at the higher temperature.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

When drying under 10[degrees]C nitrogen flow, cracking occurs in a state between Figs. 7a and 7b; Fig. 9 shows a top surface image of the latex packing very near a propagating crack tip. The coating is still clearly in the wet stage, with curved menisci just beginning to develop between particles. Capillary forces due to these curved menisci act to close the pore space, deforming the particles and causing the coating to shrink. This shrinkage is constrained by the adherence of the coating to the substrate, causing stress. This stress leads to cracking likely because particle--particle adhesion is weak due to low amounts of particle deformation. Others have also hypothesized from weight loss measurements, (38), (46), (47) and shown in lower resolution images (36), (48), (49) that cracks form at the start of the wet stage.

[FIGURE 9 OMITTED]

After cracks develop, the coating continues to dry. Figure 7b shows a fully developed wet stage with water menisci that are concave toward the air. As shown in Fig. 7c, particle flattening, occurs by the end of the moist drying stage, under the larger capillary forces that exist during this stage. (35), (50) The pore space does not close to the extent noted in the warmer drying condition. In Fig. 7d, the coating is completely dry, yet deformation has clearly not proceeded to the extent seen in Fig. 4. Both of these images were taken after approximately 10 min of drying.

Cross-sectional images of coatings dried at 10[degrees]C show similar results as the top surface images. Figures 10b-10e were taken of the fracture surface of natural cracks. Because these cracks occurred while the coating was drying and not after the sample was frozen, there are no pullouts. The cracks propagate during the wet drying stage before air invades into the bulk of the coating (Fig. 10b). In contrast to drying in warm temperatures, pendular rings can be seen in Fig. 9c, signifying that the pore space remains open enough for air to invade into the particle packing. These pendular rings are rounded, distinguishing them from the sharper shapes noted previously in Fig. 5b. In this moist stage, the particles flatten slightly until all of the water disappears (Fig. 10d). Finally, particle deformation continues over time after the water is removed in an aging process. Over 24 h elapsed between Figs. 10d and 10e. It appears that van der Waals forces are responsible for this final deformation; cryoSEM images do not show evidence for curved polymer vapor menisci, which would be necessary for viscous sintering. (51)

[FIGURE 10 OMITTED]

CryoSEM images of drying coatings containing silica aggregates (4 wt% dwb) show that silica does not influence the particle coalescence or the evaporation rate in any way. Silica aggregates were also found to uniformly distribute through the coating thickness. A cryoSEM image of a silica aggregate in a drying latex coating is provided in the supporting material.

From the cryoSEM images, we conclude that the latex particles deform almost completely in the wet stage in coatings dried at 22[degrees]C and deform partially in the moist stage in coatings dried at 10[degrees]C. From these observations and rheometry measurements on the dry acrylic film, the critical shear modulus ([G.sub.c]) for deformation in the wet stage is between G(22[degrees]C) = 24 MPa and G(10[degrees]C) = 200 MPa for this latex. Here, it is assumed that the latex particles are not plasticized by water, which would lower the polymer modulus in the wet coating, and cause us to overestimate [G.sub.c]. Since no weight change was measured for a piece of dried coating after soaking in water, plasticization was not considered to be a significant factor.

Discussion

It is commonly known that latex coatings that are dried at temperatures below the [T.sub.g] of the latex are prone to cracking, whereas latexes that are dried above the latex [T.sub.g] are crack resistant. (43), (52), (53) The latex in this study, however, cracks at drying temperatures above its glass transition temperature (5[degrees]C). SEM and cryoSEM images of the final, dried coatings confirm that complete coalescence occurred at these temperatures. Although a final film may be completely coalesced, it is the extent of particle deformation when tensile stresses develop in the film that matters. For example, we documented that the particles deform after fractures develop in coatings dried at 10[degrees]C.

From cryoSEM images, we conclude that cracking occurs during the wet stage of film formation, when drying causes the water at the top surface to be concave to the air. It is the extent of coalescence during this drying stage that is relevant to cracking. Using Brown and Mason's elastic particle force balance (equation 2), a critical modulus, [G.sub.c], was calculated for particles to coalesce in the wet drying stage, assuming a coating liquid phase surface tension of 50 dyn/cm. (54) The predicted critical modulus is 190 MPa. Rheological measurements on the dry acrylic film (Fig. 11) show that the shear modulus of the particles is 190 MPa at a temperature of 10.3[degrees]C, which is very close to the boundary (T = 9.5[degrees]C) between regime I and II in the crack spacing measurements (Fig. 3).

[FIGURE 11 OMITTED]

Using this [G.sub.c] prediction, an explanation can be proposed for the dependence of crack spacing on temperature. In regime I, when the particle modulus is higher than [G.sub.c] particles do not coalesce at all in the wet stage and the coating at this stage has very little mechanical strength to resist tensile forces. Lowering the temperature even further (and hence raising the shear modulus even more) has little effect on the cracking that is observed. However, in regime II, it is predicted that particles have started to coalesce at the beginning of the wet stage. The extent of particle--particle flattening depends on the particle modulus, which decreases with increasing temperature. Above 17[degrees]C, the particle deformation in the wet stage has increased sufficiently that the drying coating can withstand the tensile stress. CryoSEM images of drying at 22[degrees]C demonstrate that particle--particle flattening does not have to be complete during the wet stage to resist cracking.

Griffith crack theory states that the fracture toughness of a coating that cracks under tensile stress increases with its modulus and strain energy release rate. (39) In a partially coalescing latex coating, decreasing the modulus of the latex particles increases the extent to which the particles deform and, consequently, the strain energy release rate of the coating. Decreasing the particle modulus has a complex effect on the coating modulus, since although the particles become softer the coating porosity decreases. The total of all of these effects defines the MFFT, above which the coating does not crack. Further study is necessary for accurate predictions of the MFFT based on particle modulus; however, this study suggests that partial particle deformation may be an important contributing factor to coating strength.

The addition of small amounts of silica was shown to not affect the coalescence of the latex through cryoSEM. Yet, silica increases the crack spacing and mildly decreases the MFFT, possibly by its effects on the coating modulus. Since silica does not affect the clarity of the final product, it may be an interesting zero VOC option to explore for reducing the extent of cracking.

Conclusions

As the permissible amount of coalescing aids in latex-based products is reduced, it is critical to understand microstructure development near the minimum film formation temperature. Here, it was demonstrated that a coating may fracture during drying, even if the drying temperature is above the particle glass transition temperature.

CryoSEM experiments show a clear picture of film formation above and below the MFFT. First, evaporation concentrates the particles and the thinning film of water at the free surface becomes unstable and dewets from the solids packing. This process, which produces an air--water interface curvature that is concave toward the coating, is a possible explanation for positive capillary stresses that have been observed in the initial drying of particulate coatings. Further drying causes this interface to be concave toward the air, beginning the wet drying stage. At room temperature, the particles deform significantly under capillary stress. At 10[degrees]C, smaller amounts of deformation were observed, and the relatively weak coating was subject to cracking. After further evaporation, the particles continued to deform. Al room temperature, the pore space completely closed during the wet drying stage, whereas at 10[degrees]C van der Waals forces closed the pore space in a dry coating over a period of 24 h. Therefore, although a latex coating may coalesce completely, it is the amount of deformation that is present at the wet stage that is relevant to cracking.

The spacing between cracks, a measure of the severity of the cracking, was independent of drying temperature under cold conditions and increased with temperature just below the MFFT. The critical temperature between regimes was predicted by Mason as the lowest temperature for particle coalescence in the wet drying stage. It was postulated that partial particle coalescence during the wet drying stage can explain the increase of crack spacing with temperature near the MFFT. To accurately predict whether a coating will crack under given drying conditions, this suggests that the two limits of coalescing and noncoalescing may be insufficient. Rather, partial particle coalescence may affect the fracture toughness in a significant way.

The inclusion of small amounts of silica aggregates also increased the crack spacing of the film and also reduced the MFFT. Since many commercial latex coatings often contain ceramic particles, this study shows that the concentration and perhaps the aggregate dimensions of such materials are important parameters for controlling the product fracture toughness without introducing VOCs.

Acknowledgments This research was supported by the University of Minnesota Industrial Partnership for Research in Interfacial and Materials Engineering (IPRIME) and Evonik Industries. C. C. R. gratefully acknowledges a Doctoral Dissertation Fellowship sponsored by the University of Minnesota graduate school. CryoSEM was performed with the help of Chris Frethem at the University of Minnesota Characterization Facility, which receives partial support from NSF through the MRSEC program. CryoSEM images were also obtained at the Technion--Israel Institute of Technology under the knowledgeable direction of Prof. Yeshayahu Talmon. Financial support from the University of Minnesota graduate school made this travel possible. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

[c] ACA and OCCA 2012

Electronic supplementary material The online version of this article (doi:10.1007/s11998-012-9425-7) contains supplementary material, which is available to authorized users.

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C. C. Roberts ([??])

Sandia National Laboratory, P.O. Box 5800, Albuquerque, NM 87185, USA

e-mail: ccrober@sandia.gov

L. F. Francis

Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave, SE, Minneapolis, MN 55455, USA

DOI 10.1007/s11998-012-9425-7
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Author:Roberts, Christine C.; Francis, Lorraine F.
Publication:JCT Research
Date:Jul 1, 2013
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