Microstructure development in drying latex coatings

Progress in Organic Coatings 52 (2005) 46–62

Microstructure development in drying latex coatings Yue Ma∗ , H.T. Davis, L.E. Scriven Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA Received 11 April 2003; accepted 20 July 2004

Abstract The technical details of high-resolution cryo-scanning electron microscopy (cryo-SEM) in studying latex formation are presented. This technique was used to visualize the microstructure development during drying of monodisperse and bimodal latex coatings. The glass transition temperature of the monodisperse latexes ranged from 100 to −11 ◦ C. The film formation process of these latexes, after they were coated onto silicon substrates, was followed. The micrographs showed particle ordering, formation of consolidation fronts, air invasion during drying, particles deformation, film coalescence and skinning phenomena, illuminating important physics that govern the drying process of latex coatings. The bimodal systems consisted of latex blends of large, hard and small, soft particles. Cryo-SEM revealed how porous, permeable structures were created by drying these latex blends. The porous structures were modulated by drying rate, the volume ratio and size ratio of the two kinds of particles. Binder migration phenomenon was observed at higher drying rate. In addition to microstructure investigation, permeability and strength in tension of the porous films were separately measured, illustrating the basic trade-off between integrity and strength on one hand, porosity and permeability on the other. © 2004 Published by Elsevier B.V. Keywords: Monodisperse latexes; Microstructure development; Cryo-SEM

1. Introduction Latex film formation refers to a dynamic process that transforms a deposited layer of stably suspended colloidal polymer particles into a continuous, mechanically coherent coating or film as it dries, usually in air. Current understanding of film formation consists of three stages: (i) consolidation, i.e. particle immobilization by multiple contacts with one another as solvent evaporates; (ii) compaction, i.e. elimination of pore space by progressive flattening of consolidated particles and by local rearrangement of particles – usually quite minor; (iii) coalescence, i.e. development of tensile strength and continuous polymer phase by inter-particle diffusion of polymer. As a drying coating transforms from stage (i) to stage (ii), the air-solvent menisci may recede into the pore space in ∗ Corresponding author. Present address: Logic Technology Development, Intel Corporation, 5200 N.E. Elam Young Parkway, M/S RA3-301, Hillsboro, OR 97124, USA. Tel.: +1 612 625 4088. E-mail address: [email protected] (Y. Ma).

0300-9440/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.porgcoat.2004.07.023

the packing of polymer particles created by consolidation, followed by air that creates moist zones where liquid persists only in pendular rings around inter-particle contacts and perhaps in the smallest interstices or pore bodies [1]. Either capillary force or van der Waals force, or both, flatten polymer particles against one another and thereby shrink their interstices; ultimately all of the solvent may evaporate except that trapped in isolated pore bodies of an almost fully compacted coating [2]. In stage (iii), the interfaces between flattened particles disappear as polymer molecules interdiffuse across them in the process of coalescence by which the coating, or film, acquires permanent mechanical integrity [3]. While conceptually film formation can be divided into stages, the whole sequence of events of microstructure evolution is continuous. Moreover, the entire process may not be traversed. The extent to which it is realized depends on the properties of the polymer, the types of additives in the initial dispersion, the conditions of drying, and the circumstances of any aging. A boon to understanding the process

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of latex film formation and its relation to coating formulation is the ability to witness the microstructure evolution, throughout the coating thickness, from polymer particles suspended in liquid, to a fully coalesced solid with mechanical strength. Latex dispersions and coatings built from them are difficult to visualize because they are incapable of surviving in high vacuum and are sensitive to electron beam and mechanical damage. Rapid thermal motion of fine polymer particles and other additives in the water medium further complicates the structural characterization. Cryo-SEM has overcome these difficulties and proven to be a powerful means of studying microstructure in latex systems [4–7]. The technique consists of rapid cryogenic immobilization of hydrated samples at successive times of drying to arrest the structure without appreciable change in arrangement or composition; fracturing to expose internal cross-sections; and subliming small amounts of frozen liquid remaining in the structure to produce topographical contrast. With time-sectioning sample preparation and the use of cold-stages on field-emission scanning electron microscopes (FESEM), the resolution of this technique has advanced to an unprecedented level: 3–4 nm, across the thickness of drying latex coatings [8]. Fig. 1 shows a high-resolution cryo-SEM image of a frozen acrylic latex dispersion; the fine features on the fractured surface of a single latex particle are clearly shown. With such resolution, it is now possible to establish a more complete map of microstructure evolution by probing the consolidation, compaction, and coalescence processes with this technique as latexes dry.

Fig. 1. A high-resolution cryo-SEM image of an acrylic latex particle that was fractured through image courtesy of Dr. E. Sutanto.

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Two model systems were chosen for this purpose: [1] monodisperse latexes of various “hardness”, or “softness”, and [2] bimodal latexes of large, hard and small, soft particles. The “hardness”, or “softness”, of the latex particles is indexed by how much the polymer’s glass transition temperature (Tg ) falls above or below the drying temperature. Usually the film formation tends to proceed by stages that are sequential when the particles are “harder” but may overlap when they are “softer”. When a latex is too hard, the ultimate of film formation is not possible unless a coalescing aid or an anneal process is used. The film formation process starting from a soft latex is well-recognized, qualitatively, to be different from the processes that start with hard latexes. Presumably the forces required to flatten the polymer particles at a given rate are less or much less, depending on the relation between Tg and elastic, plastic, and viscoelastic properties of the polymer. In some instances, the particles may flatten and begin compacting, or even coalescing if the latex stabilization is insufficient, before they have consolidated fully, thereby creating a barrier, or ‘skin’, to water and water vapor movement, slowing down or halting the film formation process. The monodisperse model latexes cover a wide range of Tg to examine these phenomena under normal drying conditions. Latex blends of both hard and soft components have shown interesting film forming properties [9]. While most published reports on latex blends focused on creating continuous coatings, free of porosity or “voids”, with tailored mechanical, structural, and optical characteristics, in the absence coalescing aids [10], these systems can also be used to create porous, permeable coatings. The basic idea is that the hard component in the system, whose modulus is too high to deform during drying, remains as hard spheres and leads to a porous packing when drying is complete; whereas the soft component deforms and fuses during drying to bind or glue the hard spheres together so that the packing has mechanical strength. Ideally, as water evaporates the small, soft particles are carried to the pendular rings of liquid around the contacts between consolidating large, hard particles, and there they deform, adhere and bind the large, hard particles together. If the volume fraction of the small soft particles in the blend is less than the void fraction in the packing of the large hard particles, pores must survive at the end of drying; thus by varying the volume fraction of the two types of particles, the porosity of the coatings can be controlled. The size ratio of the two types of particles can also affect the final structure of the coating because the mobility of particles with difference sizes differs during drying. Latex blends with a bimodal particle size distribution serve as good model systems to explore the general features of these composite latex systems and to understand the mechanism of their film formation, especially the development of porous structure. To establish a structure–property relationship, the diffusive permeability and mechanical strength of the final porous coatings were also investigated.

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Fig. 2. SEM (a) and cryo-TEM (b) images of dried and freshly prepared PS latex (in thin vitrified ice matrix). The white arrow in (b) indicates a TEM microgrid.

2. Materials and methods 2.1. Model latexes Monodisperse polystyrene (PS) and polymethyl methacrylate-co-n-butyl acrylate (PMMA-co-PBA) latexes were synthesized by surfactant-free emulsion polymerization in a batch reaction scheme [11]. The particle size of latexes was measured with a Coulter LS Particle Size Analyzer (Beckman Coulter, Inc., Fullerton, CA). Fig. 2 shows SEM and cryo-transmission electron microscopy (cryo-TEM) micrographs of dried and freshly synthesized PS latex, respectively. The Tg s of the latexes were measured with a Pyris 1 Differential Scanning Calorimeter (DSC)

(Perkin-Elmer, Inc., Wellesley, MA). The optical minimum film formation temperature (MFFT) of the PMMA-co-PBA latexes was measured with a RhopointTM temperature gradient bar (Rhopoint Instrumentation, Ltd., Surrey, England), model MFFT-60. Table 1 shows the properties of a series of six synthesized latexes. They all have a polydispersity index value (p) smaller than or equal to 1.06. The stabilization is achieved electrostatically through potassium persulfate (initiator) remnants on surface of the polymer particles. Latex blends were constructed with two of the three latexes in Table 2: Ropaque HP1055 with JP1225 or with JP1332, all gifts from Rohm and Haas company (Spring House, PA). HP1055 latex particles have a hollow morphology. JP1225 and JP1232 are solid binder particles.

Table 1 Tg , particle size, and size distribution of PMMA-co-PBA latexes synthesized at 70 ◦ C; Dn is the number averaged diameter, and Dv the volume averaged diameter of a particle Latex

PS

co-00

co-01

co-02

co-03

co-04

co-05

MMA:BA (100 parts) Solid content (%) Dn (nm) Dv (nm) P Tg (K) MFFT (K)

– 8.6 509 525 1.03 373 –

100:0 18.3 5095 533 1.06 378 –

62.8:37.2 19.3 511 537 1.05 328 297

55:45 19.2 542 559 1.03 307 293

48.4:51.6 19.6 523 548 1.05 292 280

40.2:59.8 19.3 526 548 1.04 281 –

30:70 20.0 536 557 1.04 262 –

Table 2 Properties of three model latexes; the ratios of diameters of the two binder particles JP1225 and JP1332 to HP1055 particles are approximately 0.094 and 0.188, respectively Latexes

Ropaque HP1055

JP1225

JP1332

Mean particle size (nm) Tg (◦ C) Monomer unit Particle morphology Dispersion pH Solid content (%)

850 (d) 100 Styrene Hollow 10 19.5

80 (0.094d) –50 Styrene/acrylic Solid 2.6 19

160 (0.188d) –50 Styrene/acrylic Solid 2.6 20

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2.2. Coating preparation 2.2.1. Monodisperse latexes Latexes were coated on 5 mm × 8 mm silicon substrates with a wire-wound rod (R.D. Specialities, Webster, NY) and dried at 21–23 ◦ C and 30–40% relative humidity. 2.2.2. Bimodal latexes Bimodal latex blend dispersions were prepared by combining either JP1225 or JP1332 with HP1055 in five volume proportions, denoted by Vsml /Vlrg : 0.15, 0.20, 0.25, 0.30, and 0.35. The latex blend was then coated onto either Rapitone P1-4 photographic paper (Agfa, Germany), for permeability and strength measurements, or onto 5 mm × 8 mm silicon substrates, for microstructure observation, all with a wirewound rod. For each volume blend ratio, two coatings were prepared by the same coating method but dried under two different conditions: at 4 ◦ C and at room temperature. The relative humidity of both drying environments was controlled at around 40%. 2.3. Microstructure observation 2.3.1. Dispersions A drop of a dispersion, from either monodisperse or bimodal latexes, was sandwiched between two freezing planchette (Type A, Ted Pella, Redding, CA). The assemblage was loaded in a Bal-Tec HPM 010 high-pressure freezing machine (Bal-Tec AG, Balzers, Liechtenstein), and the dispersion was frozen within 8 ms at 2100 bar. The frozen sandwich was longitudinally fractured in liquid nitrogen, and one side of the fractured sample was mounted in a Gatan 626 cryo-transfer stage (Gatan, Pleasanton, CA) at −195.8 ◦ C. The mounted sample was transferred into a precooled Balzers MED 010 sputtering coater (Balzers Union, Balzers, Liechtenstein) against a counter flow of dry nitrogen gas. Ice in the frozen specimen was partially sublimed away at −96 ◦ C and 2 × 10−9 bar. A 2 nm thick layer of platinum was sputtered on the specimen surface. The specimen was then transferred to and examined in a Hitachi S900 FESEM. The sample was kept at −160 to −170 ◦ C during imaging. Cryo-transmission electron microscopy (cryo-TEM) was also used for imaging PS latex dispersion, and the procedure was described elsewhere [11]. 2.3.2. Microstructure development of coatings during drying Coating microstructure evolution was followed by timesectioning cryo-SEM. The drying coatings were fast-frozen at different times after the dispersions were coated onto silicon wafer substrates by plunging them into liquid ethane at its freezing temperature. The frozen samples were transferred to an Emitech K-1250 cryo-system, where they were fractured cross-sectionally. The fractured surfaces were then sputtered with a layer of platinum 2–3 nm thick at −120 ◦ C. A Hitachi

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S4700 FESEM was used to examine the samples. All samples were kept at a temperature around −160 ◦ C during imaging. 2.3.3. Dried coatings The top surface and fracture surfaces of the coatings were imaged on either Hitachi S800 or S900 FESEM by standard room temperature SEM technique. 2.4. Permeability measurement The diffusive permeabilities of the films prepared from the latex blends were measured in a half-cell diffusion apparatus [12]. The latex coating was removed by hand-delaminating it from the photographic paper in water. The thickness of the detached film was measured with a Federal Products model 691B-R2 M foot-padded micrometer (Providence, RI). The sample film was then installed between the two half-cells. The entire assemblage was immersed in a water bath at a constant temperature of 30 ◦ C. KNO3 , in 5 M aqueous solution, was used as the tracer diffusant. Diffusive permeability (Deff /Dself ) of KNO3 through the film was determined by using Nightingale’s equation (12). 2.5. Mechanical strength measurement Films prepared from the blends were dried and cut into 2.0 cm × 3.2 mm strips. The sample films were mounted on the sample stage of a Perkin-Elmer DMA 7e dynamical mechanical analyzer (Wellesley, MA), and tensile tests were conducted. Both elongation and tensile stress of the film samples at their breaking points were recorded.

3. Results and discussion 3.1. Image interpretation and artifacts Cryo-SEM generally includes five steps: rapid cryoimmobilization, freeze-fracture, sublimation, cryo-transfer, and imaging, as illustrated in Fig. 3. To correctly interpret the results from cryo-SEM, the facts and artifacts associate with latex systems during these steps must be understood and separated. Major artifacts observed during these steps are summarized. 3.1.1. Freezing artifacts The ideal case of cryo-immobilization is vitrification – converting water, or a solvent, to vitreous state – because crystallization of liquid introduces unwanted structure and rearranges components in the sample. Vitreous ice can be obtained by extremely fast cooling of water, at least 105 ◦ C/s. Thus it requires samples to have dimensions on the order of several hundreds nanometers or less and high surface to volume ratios for sufficient heat transfer when the samples are in contact with a cryogen. For large, water-rich samples, satisfactory cryo-immobilization occurs only in the region near

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by plunge freezing in liquid ethane; (b) 300 ␮m thick sample of the same dispersion by high-pressure freezing, followed by freeze-fracture. The stringy artifacts of minor constituents excluded from growing ice crystals into grain boundaries in the plunge-frozen sample (Fig. 4a) are absent in the highpressure frozen sample (Fig. 4b). The clumpy small particles in the plunge-frozen sample were pushed into groups by the growing ice crystals; they distribute as individual particles in high-pressure frozen sample. For drying coatings that contain high solid contents, or cryo-protectants in the solvent, plunge-freezing also produces satisfactory freezing results.

Fig. 3. Procedures required for cryo-SEM sample preparation after cryoimmobilization of a latex dispersion.

their surfaces with conventional freezing methods. To reduce the damage from cryo-immobilization when preparing latex dispersion samples, a high-pressure freezing technique (at 2100 bar) was used. The high-pressure lowers the limiting temperature of supercooling of water to 183 K and elevates its viscosity by 1500-fold, delaying the nucleation and slowing the growth of ice crystals. High-pressure freezing can produce frozen samples with negligible freezing artifacts up to 500 ␮m thick [13]. The advantage in freezing quality of highpressure freezing is clearly shown in Fig. 4: (a) 30 ␮m thick bimodal acrylic latex dispersion sample cryo-immobilized

3.1.2. Freeze-fracture artifacts The major artifact in this step is the change in particle shape, as shown in Fig. 4b – visible features on the fracture surface of particles are the elongated protuberances. These must have been drawn from the other parts of particles embedded in the now absent fracture surface, of which they were originally a part. In some cases the elongated portion accounts for the entirety of the complementary part of the particle [14]. The fracture surface of a monodisperse PMMA-co-PBA latex dispersion sample is shown in Fig. 5. Because the fracture surface is not flat, that the arrays of particles are ordered can be confirmed only by careful examination of the relative positions of the pull-out features and the imprints left behind by their parted neighbors. The “pull-out” feature in freezefracture is a rather surprising phenomenon because the polymer particles were fractured at temperatures far below their glass transition temperatures. The most likely cause for the plastic deformation to occur at such low temperatures is that the submicroscopic size of the particles and the high surface area of individual particles affect their molecular organization so that they more readily deform plastically below Tg , that is, in the fine particles the brittle–ductile transition is shifted to

Fig. 4. Comparison between plunge-frozen (a) and high-pressure frozen (b) samples of bimodal acrylic latex dispersion containing 300 and 70 nm latex particles. White arrows in (a) indicate the stringy freezing artifacts.

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Fig. 7. Freeze-fracture of latex particles in wet and moist states. Fig. 5. Fracture surface of PMMA-co-PBA dispersion. The white arrows indicate features that are fracture steps.

lower temperature than in their bulk phase. van der Sanden et al. reported such a shift in thin films of polystyrene: in films less than 1 ␮m thick, polystyrene, whose Tg is 100 ◦ C, becomes ductile at room temperature [15]. Tg reduction was also observed in free standing polystyrene films as their thickness reached 0.1 um or below [16]. The reduction in Tg was found to be molecular weight-dependent – more dramatic when the molecular weight is less. In this case, the authors suggested that the Tg reduction was due to chain confinement effect. A critical dimension also seems to exist in latex systems. Fig. 6 shows freeze-fractured samples of dispersions containing the same latex particles and pigment particles (titanium oxide). In Fig. 6a, the latex particles distributed as individual particles bound by the ice matrix; in Fig. 6b, the latex particles flocculated and fused before cryo-immobilization, existing as small solid clusters of several particles. The long pull-out extensions, indicated by the white arrows in the non-flocculated sample, are absent in the flocculated and fused sample, im-

plying that the dimension of the particles (dimension of a single particle versus dimension of several particles) plays an important role in material properties. The elongation must initially have been anchored in the absent fracture face by adhesion either to the ice or to the polymer particles there; in either case the connection soon broke. When the coating is in the wet stage before it is frozen and freeze-fractured, fracture often propagates through polymer particles, and the parting fracture faces deform the particles by drawing them. When the coating is in the moist stage before freeze fracturing, the particles are held only by residual water at their contacts, and fracture likely propagates through the frozen residual water at particle contacts, presumably points weakest in tension in the frozen sample. These situations are illustrated in Fig. 7. The final morphology of the fracture surface depends mostly on the water distribution and the degree of polymer particle interdiffusion, if the polymer particles are in contact with each other, in wet or moist samples. When wet regions, moist regions, and coalesced regions co-exist, the morphology of fractured surface can be

Fig. 6. Freeze-fracture patterns of well-dispersed (a) and flocculated and fused (b) dispersion samples. White arrows in (a) indicate pull-outs, and black arrows in (b) indicate titanium oxide particles.

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Fig. 8. Effect of sublimation on the structure of a frozen polystyrene dispersion: (a) 5 min at −110 ◦ C, (b) 10 min at −96 ◦ C, both at 2 × l0− 9 bar.

complex and requires careful interpretations to sort out the consequences from the freeze-fracture step. The particle deformation can further complicate the interpretation of particle arrangement from the images. For example, in Fig. 5, areas that appear random at the first glance are actually ordered after careful examination of the roots of distorted pull-out features. 3.1.3. Sublimation artifacts In many cases, sublimation is needed to remove enough frozen solvent in the sample to produce topographical contrast for imaging. Over-sublimation is to be avoided if the feature of interest is a disperse phase because removing the continuous water phase causes the disperse phase to collapse, altering the original structure and distribution. This effect is illustrated in Fig. 8, which shows cryo-SEM samples of the same 10 wt.% monodisperse polystyrene latex suspension sublimed under different conditions. The sublimation of the sample in Fig. 8a was just sufficient to produce the topographical contrast between the edges of the particles and the surrounding ice matrix. The sublimation of the sample in Fig. 8b was overdone so that most of the ice matrix was lost, and the dispersed particles collapsed and possibly rearranged. In this case, however, over-sublimation helps make clear what the bright features are in Fig. 8a – they are the long pull-out tips shown in Fig. 8b. 3.1.4. Cryo-transfer artifacts There are several transfer steps from where the sample is frozen to the cold-stage in the SEM. Because of the extremely low temperature of cryo-samples, ice can easily condense from moisture and damage the sample surface. The condensed ice takes many forms, usually stone-like or dendritic, sometimes droplet-shaped in the size of microns. The ice condensation during cryo-transfer can be largely reduced

by protecting the samples in various ways: in cold enclosure, in vacuum, or in the vapor of liquid nitrogen. 3.1.5. Imaging artifacts The most frequently encountered operation-related artifact in imaging latex systems by cryo-SEM is electron beam radiation damage. Electron beam damage can cause latex particles to change shape, surface morphology, and relative positions. Due to the presence of ice, which is an excellent source of free radicals when exposed to ionizing electrons, radiation damage can be much more severe than in its absence. The use of high electron dose in imaging such samples is always to be avoided, and sometimes resolution has to be compromised. 3.2. Film formation in monodisperse latex systems 3.2.1. Stage I – consolidation An edge consolidation front advancing laterally from perimeter inward can be easily identified visually as the moving boundary at which the turbidity of the suspension is replaced by a more translucent, slightly cloudy appearance that signals a transition from the strongly light-scattering suspension of latex spheres to the weaker scattering wet packing of consolidated spheres. Away from the perimeter of a coating, the inwardly propagating front bends over to become a descending consolidation front whose downward propagation produces a more gradual progression of appearance in time, from turbid to translucent [17]. Fig. 9a and b show the cryo-SEM images that clearly identify an abrupt front of consolidation advancing inward from the earliest drying stage of a PS coating. The lateral consolidation starts at the very edge of the coating, where latex particles protrude above the air–water interface as soon as enough water evaporates there that the particles come in contact with the substrate. Around the protruding particles the

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Fig. 9. Formation and progression of lateral (a and b) and top-down (c–f) consolidation fronts (7). The fracture steps between the suspension and consolidated material appear as vertical bright stripes in a and b.

menisci are concave toward the air [18]. The surface tension force on the particles and the capillary-pressure difference that accompanies curvature of the menisci between the particles act locally to draw particles together and consolidate them. Moreover, because capillary-pressure makes the pressure low beneath the curved menisci, it acts regionally to suck suspension from the thicker region farther away from the edge, as illustrated in Fig. 10. In the case of PS latex used in this experiment, the final curvature of water between the latex particles was indeed concave towards air, as shown in the cryo-TEM image in Fig. 10. When the suspension arrives

at the front of consolidated particles, the water is drawn between them, and the particles that arrived with it lodge in the consolidation front. In this way the front advances just as filter cake grows in a filtration process. Away from edges, water also evaporates through the air–suspension interface, more or less uniformly [19]. As it does so, the particles that were suspended are stranded at and then beneath the surface, causing the concentration of particles to rise there. The Brownian motion of particles drives them to diffuse back toward the bulk of the suspension. If the rate of convection caused by evaporation is high

Fig. 10. (a) Menisci between particles at coating’s edge, and (b) cryo-TEM image of menisci between PS particles.

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enough, particles become concentrated at the surface, and driven by random thermal fluctuations they overcome the repulsive forces from the double layer to collide. As a result, they begin to clump into clusters. Once the free particles join the close-packed cluster, van der Waals force and the surface tension force in the menisci between the crowding particles hold the particles on the free surface. In the close-packed surface array of particles, the menisci become increasingly curved when evaporation removes water faster than replacement of water by overcoming the viscous resistance, maintaining a constant drying rate. As the water is drawn upward between the already clustered particles at the surface, the particles born by the water lodge in a consolidation front that is parallel to the surface and advances downward into the coating, as demonstrated by the sequence of cryo-SEM images in Fig. 9c–f, cross-sections of PS coatings dried for 1 (c), 2 (d), 6 (e), and 60 min (f). The images show successive layers of highly ordered PS particles forming at the propagating consolidation front. The high degree of ordering suggests that the rate of arrival of PS particles at the front is low enough to leave them sufficient time to rearrange by Brownian motion and/or particle–particle rotation into ordered close packing before the next arriving particles lock them into a permanent arrangement. The downward compressive force progressively diminishes through the consolidated layers of particles until it reaches the liquid suspension region where the pressure is atmospheric. For a hard latex such as PS, evidently this compressive force was not high enough to deform the particles in the consolidated region at the drying rate in the experiment. Fig. 11 shows the high-resolution cryo-SEM image that reveals the shape of the PS particles in the consolidated structure in the same coating dried for 8 min. Fig. 11a shows the top view of the fracture surface, where the fracture happened to cut a plane of polymer particles unevenly, alternately de-

forming icebound particles and plucking out their neighbors, whose imprints remain on the ice matrix. The bright spots in the image arise from high secondary electron emission on the plastically deformed parts of particles, as viewed from an angle in Fig. 11b. 3.2.2. Stage II – compaction When a consolidated layer of hard latex particles becomes thick enough that the resistance to liquid flow exceeds the pressure difference that can be produced by curvature of the menisci between particles in the topmost layer, one of the menisci jumps. The maximum meniscus curvature that can develop is that allowed by the largest pore throat in the layer (i.e. the “entry curvature”), and it is from that throat a meniscus jumps through the pore body beneath and lodges in the entries to one or more pore throats beneath that, followed by invading air. What is seen at the macroscopic level is a consolidation front, at which the transition is from a milky to a translucent appearance, followed by a dry-out front, at which the transition is from a translucent to an opaque, white appearance. The new appearance is caused by intense lightscattering of particle/air interfaces in the moist packing of consolidated spheres. By moist is meant the situation in which most of the pore space is occupied by air that has invaded behind the menisci, but pendular rings of residual water around the sphere contacts are plentiful. As the consolidation front is propagating inward laterally to the center of a coating, air invasion begins from the very edge. This edge dry-out front is followed by another away from the coating edge, just like the top-down consolidation front. Fig. 12a–e show the sequence of cryo-SEM images that capture the transition from a wet, consolidated coating to an air-invaded moist coating, at both the edge (a) and the middle of the coating (b–e). Apparently water still occupied some pore space at the very edge of the coating, which were only a few layers of particles thick, and

Fig. 11. Top (a) and side (b) views of a fracture surface of PS latex coating dried for 8 min.

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Fig. 12. Fracture surface of PS-01 latex coating dried for (a) 8 min and 45 s at the edge, (b) in the middle, (c) 1 min after (b), (d) 20 s after (c), and (e) higher magnification of (d).

much of the pore space further from the edge was full of water, indicating that the air invasion had just begun at 8 min of drying. In some regions along the coating marked by the white arrows, air fingers have penetrated through several layers of consolidated particles all the way down to the substrate. Near the middle of the coating, air invasion proceeds by the same mechanism as in the case of edge dry-out. At 8 min and 45 s of drying, the top layers toward the middle of the coating remained wet as shown in Fig. 12b. Moreover, the regular pattern of pull-outs in the fracture surface tells eloquently that the particles there were highly ordered. One minute later air had broken into the coating and occupied pore space several layers of particles beneath the surface, as shown in Fig. 12c. The ragged front of invading air had not penetrated more than five layers deep. The particle arrangement thereafter can no longer be interpreted from the micrographs due to the random distribution of water as these samples were frozen and the particle deformation at different degrees during fracture. However, the ordering is expected to reach 10–30 layers under the experimental conditions, as in the finally dried coating shown in Fig. 9f. About 20 s later, air invasion reached several tens of layers below the coating surface, and at this stage, the co-existing wet zones and air pockets scattered about are plainly visible in Fig. 12d. At higher magnification Fig. 12e shows a wet cluster and the large air pocket around it. In the fracture surface of the wet cluster there are both pull-outs and imprints (dimples) of parted particles. In the air-invaded region, the ice of residual water that once held several particles together is marked by white arrows – frozen pendular rings – where particles were plucked out by the other fracture face. Other than the surface layer, the originally ordered consolidated coating in Fig. 12b, after further drying, appears

to be random in Fig. 12c. This is due to the fracture effect mentioned earlier – the random distribution of wet clusters of particles and air pockets complicates the morphology of the fracture surface. The particles all remained spherical or nearly spherical at this point of drying, and they did not flatten noticeably at the end of drying under room temperature owing to the high modulus. In coatings of softer latexes, after the consolidation and dry-out fronts have passed, the coating with time becomes relatively clear and transparent, progressing inward from the coating’s edge. This transition marks the shrinking of the airfilled pore space as the latex particles flatten against one another so that the interstices between them grow smaller and less able to scatter light. If the modulus of particles is low enough, as is often indicated by their minimum film formation temperature (MFFT) being much lower than the drying temperature, the consolidated coating can compact before air invades appreciably. In that case no dry-out front can develop because it is overtaken by the more diffuse “pore-shrinking” front [20]. One mechanism by which particles flatten is downward compression as a result of the concave menisci at the drying surface until the pore space there is closed off by the flattening. The other possible mechanism that deforms particles is wet sintering [21]. Fig. 13 shows the time-sequence of cryo-SEM images of coatings from co-03 latex (with a Tg of 19 ◦ C and a MFFT of 7 ◦ C) dried for 3 (a), 6 (b), 8 (c), and 15 min (d). In Fig. 13a, the particles, embedded in an ice matrix, appear to be highly ordered and undeformed. The fracture cut through the particles consisting of nanoscopic domains, whose formation was discussed elsewhere [17]. After drying had proceeded further, to 6 min, the cryo-SEM image of fracture surfaces in Fig. 13b

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Fig. 13. A time-sequence of cryo-SEM images of coatings from co-03 latex dried for 3 (a), 6 (b), 8 (c), and 15 min (d).

shows that the particles had partially flattened against each other in the ordered array. Note the “cubic” to “hexagonal” packing transition shown in these images is an artifact due to the different crystal planes of particles that the fracture has gone through. The small features visible in many of the interstices of particles (marked by arrows) are ice-frozen water that was trapped there by the deformed latex particles. In Fig. 13c, most of the remaining water was no longer visible in the packing of completely deformed particles. This suggests that water was probably not really “trapped”; the passages for it in the coating must have been connected, and under downward compression force particles continue to deform, squeezing any remaining water out. The action of downward compression is clear in Fig. 13c, which shows fracture surfaces near the coating’s surface at 8 min of drying. The particles were squashed in one direction and bulging in another. In Fig. 13d, the same coating was frozen and fractured at 15 min of drying, and it was warmed back up to room temperature before imaging. Those parts of particles that were relieved from the constraints imposed by the once neighboring particles that left with the other fracture face appear to have rebounded to spherical shape. Those particles with straight sides in between the partially free particles are still deformed. These observations imply that the deformation was elastic. All the particles maintained their identities after fracturing, implying that no polymer interdiffusion had taken place across particle boundaries.

3.2.3. Stage III – coalescence If the mobility of the polymer chains is high enough, the polymer chains can interdiffuse through the flattened particle boundaries even as the particles flatten against each other more. Insofar as the Tg of the polymer is relevant, it must be low enough to allow sufficiently high interdiffusion rates of polymer molecules across the contact areas between them that the particles weld, or fuse. co-05 latex with an even lower Tg of −11 ◦ C was selected as a model system for studying coalesence during drying. Fig. 14 shows cryo-SEM images of the co-05 coating dried for 8 min. Fig. 14a was taken at the very edge of the coating. The particles remain spherical on the air side of the coating. Below the surface, however, the particles appear totally coalesced: no individual particle can be identified. Fig. 14b is an overview of the coating cross-section, about 15 ␮m thick, and about 100–200 ␮m away from the edge. Fig. 14c and d are high magnification images of regions in Fig. 14b. The particles maintained their spherical shape on the air side of the coating but completely coalesced just below the coating surface. In Fig. 14d, particles have deformed fully, but are only slightly fused, because in the fracture the faces of the polyhedra are somewhat blurred. These small pull-out features are likely to be fracture effects as a result of drawing partially fused particle contacts. Further away from the edge, the fracture pattern just below the coating’s surface appears to be more ordered, as show in

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Fig. 14. Fracture surfaces of co-05 latex coating dried 7 min: (a) at the very edge, (b) an overview near the edge, (c) and (d) high magnification images of regions as indicated in (b).

Fig. 15a, and a front is clearly visible at higher magnification in Fig. 15b, showing the different contrast. Fig. 15c–f are high magnification images of regions marked in Fig. 15a. In Fig. 15c, the ordered array of particles must have begun fusing, because their boundaries are heavily blurred, presumably owing to a high degree of polymer interdiffusion across particle boundaries. Fig. 15d shows ordered particles that have fully compacted. The boundaries of the deformed particles are identifiable as the six sides on the fracture surface. Below the coalesced region shown in Fig. 15c, the compaction and coalescence of particles were mixed. Most particles appear fully compacted and partially coalesced, as suggested by the deformed shape and pull-out features. Particles in Fig. 15f were uncon-solidated, therefore undeformed, and distributed randomly in the ice matrix, indicating that the consolidation stage was never reached in this region after the coating was dried for 8 min. The full compaction of particles above this region must have formed a masstransfer barrier that prevented water from escaping out of the coating by evaporation, known as “skinning”. The location of the region in Fig. 15b is likely a joint where an edge coalescing front, progressing inward, and a top-down “skinning” front meet. In this region, further particle compaction can only be driven by the wet-sintering phenomenon [21].

3.3. Porous film from bimodal latex blend systems 3.3.1. Microstructure of dried coatings Fig. 16a shows the cryo-SEM image of a wet dispersion of JP1225/HP1055 blend with Vsml /Vlrg = 0.15. Evidently the fracture has broken open many HP1055 particles, revealing their hollow structure. Small JP1225 particles are distributed evenly around large HP1055 particles and show no sign of aggregation. Fig. 16b shows the SEM image of top surfaces of a dried coating from the same JP1225/HP1055 blend at room temperature. The top surface is highly porous, and the HP1055 particles are connected by fused JP1225 particles at their contacts, which is clearly shown in the higher resolution image in Fig. 16c. As Vsml /Vlrg rose to 0.25 and then to 0.35, the coating’s surface became less porous, as shown in Fig. 16d and e, respectively. Fused JP1225 particles occupied most of the porous space in the coating at Vsml /Vlrg = 0.35. The image of the fractured surface in Fig. 16f shows the fused JP1225 material was highly deformable. Fig. 17 show fracture surfaces just beneath the coating’s top surface for JP1225/HP1055 blends with Vsml /Vlrg = 0.15 (a), 0.25 (b), and 0.35 (c), all dried at room temperature. The fracture surface also became less porous as Vsml /Vlrg rose. The number of fractured HP1055 particles served as a good indication of how much binder material fused around the HP1055 particles

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Fig. 15. Fracture surfaces of co-05 latex coating dried for 8 min: (a) an overview; (b)–(f) higher magnification images of regions as indicated in (a).

– the more the HP1055 particles fractured, the stronger they were held together with their neighbors so that fracture cut them through, instead of separating them at contacts. 3.3.2. Porosity, permeability, and structure evolution By using digital imaging analysis of multiple SEM images of the porous coatings, the surface porosity of the prepared films can be determined [17]. The JP1225A porosity series in Fig. 18 shows the surface porosity of JP1225/HP1055 blend with Vsml /Vlrg = 0.15, 0.20, 0.25, 0.30, and 0.35. The porosity falls with increasing Vsml /Vlrg . Permeability also falls with increasing Vsml /Vlrg for JP1225A (JP1225/HP1055 blend dried at room temperature), JP1225B (JP1225/HP1055 blend dried at 4 ◦ C), JP1332A (JP1335/HP1055 blend dried at room temperature), and JP1332B (JP1332/HP1055 blend dried at 4 ◦ C) series. Two general trends are observed in Fig. 18: [1] permeability is higher if the binder size is smaller, and [2] permeability is higher if the drying rate is higher. The larger the binder particles are, the more likely they are, after they

fuse, to block the pore throats in the packing of HP1055 particles, thereby reducing the number of connected passages through which ions can diffuse and in that way lowering the film permeability [22]. Presumably fast drying generates a steeper concentration gradient of particles – high near the drying surface and low near the substrate. When the convection relative to the evaporating surface is faster, a packing of low density could arise in which the HP1055 particles can rearrange when air does invade. The rearrangement could redeploy the space between the HP1055 particles into larger pockets filled with air. In the case of slow drying, the concentration of the small, soft JP1225 or JP1232 particles is more uniform throughout the coating thickness because they are more mobile and so diffuse more rapidly, whereas convection relative to the evaporating surface is slower. When the drying rate is low enough, air does not invade the coating before the HP1055 particles are fully consolidated. Thus the larger particles cannot rearrange appreciably to produce large air pockets. The paths for ions’ diffusion through the film is

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Fig. 16. Dispersion of JP1225/HP1055 blend with Vsml /Vlrg = 0.15 (a) and dried coatings from JP1225/HP1055 blends with Vsml /Vlrg = 0.15 (b and c), 0.25 (d), and 0.35 (e and f).

longer when the pores are smaller, and hence the measured permeability is lower. The evolution of the structure during the drying of sample HR1225A Vsml /Vlrg = 0.15 was investigated by means of time-sectioning cryo-SEM to confirm the postulated mechanism of drying rate modulation of permeability. Fig. 19a and b show cryo-SEM images at lower and higher magnifications, of both top surfaces and fracture surfaces of a coating dried for 3 min. Evidently among the larger particles, the menisci between the air and suspension became concave by 3 min so that the pressure beneath the menisci was lowered. At six minutes of drying, as shown in the images in Fig. 19c and d,

the menisci curved in even further. Sooner or later the menisci must have curved enough to reach throats in the pore space and become unstable, thereupon jumping to new locations, followed by invading air – Haines jumps. At 8 min of drying, as shown in the images in Fig. 19e and f, air invaded the coating, the Haines jumps ahead of the air swept water, small JP1225 particles, and a few unconsolidated large HP1055 particles into wet, locally consolidated clusters. The fingers of invading air left behind air-filled pockets with a volume equivalent to the volume of several HP1055 particles. These pockets of air eventually evolved into large pores when the coating was completely dried.

Fig. 17. Fracture surfaces of dried coatings from JP1225/HP1055 blends with Vsml /Vlrg = 0.15 (a), 0.25 (b) and 0.35 (c).

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Fig. 18. Permeability and porosity of dried coatings.

3.3.3. Binder migration The bimodal latex blends in this study resemble simple “binder-pigment” systems: the smaller soft particles are the binder and the larger hard particles are the surrogate pigment. Fig. 20a–e are cryo-SEM images of fracture surfaces across a coating of HR1225A (Vsml /Vlrg = 0.15) at 3 min of drying. Fig. 20a is an overview of the fracture surface of the coating. The areas marked b and c are the locations of the higher magnification images in Fig. 20b and c. Near the surface of the coating, JP1225 particles are concentrated and occupy most of the pore space between the consolidated HP1055 particles. Near the substrate, the concentration of scattered JP1225 particles is much lower. Plainly the binder particles have been stranded at the evaporating surface as water departed and have had no time to diffuse by Brownian motion back down their concentration gradient. Fig. 20d and e show fracture surfaces near the top and bottom of the coating dried

Fig. 19. Fractured surfaces of JP1225A with Vsml /Vlrg = 0.15 dried for 3 min (a and b), 6 min (c and d), and 8 min (e and f). Images were taken near the edge of the coating.

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Fig. 20. Fracture surfaces of JP1225A (Vsml /Vlrg = 0.15) dried for 3 min (a–c) and 12 min (d and e).

for 12 min. Most of the HP1055 particles in Fig. 20d are broken, whereas nearly none are in Fig. 20e. This indicates that the concentration of JP1225 particles, now fused, was much higher near the top surface. High concentration of JP1225 particles was also observed at the coating edges. The circular features, likely residues of fused JP1225 particles, on the surface of HP1055 particles shown in Fig. 20e are broken contacts, where they were bonded to the HP1055 particles on the now parted surface after the coating was fractured, indicating fewer JP1225 particles near the bottom of the coating. The small soft particles have “migrated” from the originally uniformly dispersed blends though the drying process. To reduced the steep concentration gradient of JP1225 particles along the thickness of the coating, a slowly dry rate should be used. Slower drying rate should make the final coatings less permeable but with a more uniform pore size distribution at an optimized Vsml /Vlrg [17].

Fig. 21. Breaking strength and the elongation at break vs. volume ratio of JP1225A samples.

4. Conclusion 3.3.4. Mechanical strength The breaking strength and the elongation at break of free standing JP1225A samples were measured, and the results are shown in Fig. 21. The “error” bars represent standard deviations of four repeated measurements. Both breaking strength and the elongation at break rose as Vsml /Vlrg rose. This is expected because more small, soft particles provide stronger binding until all pore space in the packing of HP1055 particles is filled by them, thereafter a continuous phase is formed by the small soft component. Before this continues phase is formed, samples with lower Vsml /Vlrg should exhibit less elongation as the films were composed by more hard and brittle particles. The location of the breakage was largely random along the film, and the new edges generated were macroscopically straight, perpendicular to the film length.

Cryo-SEM was used to follow microstructure development during drying of both simple monodisperse and bimodal latex coatings. With refinement in the technique and careful image interpretation, cryo-SEM a powerful tool in understanding mechanisms of latex film formation. It provided visual access to the coating structures and component distribution at different stages of latex film formation, allowing one to investigate the effects of key parameters such as polymer glass transition temperature, dispersion composition, and coating drying rate on the drying process. With its unique capabilities, this technique can be extended to studying morphology of colloids, distribution of components in dispersions, and development of structures in complex coating systems.

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Acknowledgement We thank Drs. E. Sutanto, Y. Chen, Professors S.L. Erlandsen, M.C. Flickinger, Mr. C. Frethem, and Mr. B.J. Wiley at the University of Minnesota for various contribution to this research. We thank Dr. M. Gebhart from Rohm and Haas Company for providing bimodal latexes used in this research.

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