Surface nonuniformities in latex paints due to evaporative mechanisms

A model is developed for predicting long‐wavelength nonuniformities in the thickness of drying latex paint films. The model includes the effects of temperature, latex particle volume fraction, surface surfactant density, bulk surfactant density, and several material and environmental factors. After the model is simplified by applying the lubrication approximation, equations for spatially independent base state solutions are derived. The base state solutions describe a drying latex paint film of uniform thickness. The equations for the base states are solved numerically and a linear stability analysis is conducted. This analysis indicates that evaporation, slow surfactant kinetics, low initial surface tension, substrate permeability, and high initial latex particle volume fractions destabilize the uniform film, while fast surfactant kinetics, high initial surface tension, and high viscosity are stabilizing. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1841–1858, 2018     

Drying of latex films and coatings: Reconsidering the fundamental mechanisms

The two existing theories describing drying of latex films or coatings are reconsidered. Subsequently, a novel mathematical drying model is presented, the simulations of which can match and explain experimental drying rate data of two previous investigations with latex films. In contrast to previous model studies, but in agreement with observations, simulations suggest that during the falling rate period of the drying process of a latex film, a porous skin of partly coalesced latex particles is indeed formed, which limits transport of water vapour from the receding air–liquid interphase to the surface of the film. The value of the effective diffusion coefficient of water vapour in the dry and partly coalesced layer (7 × 10⁻⁷ m²/s at 19–24 °C), the adjustable parameter of the model for the falling rate period, was found to be independent of initial wet film thickness (89–1322 μm), latex particle size (500–600 nm), initial polymer volume concentration (19–47 vol.%), and molecular weight of latex polymer (not quantified). Simulations also demonstrate that the transition from a constant to a falling drying rate in all cases takes place when the polymer volume concentration of the latex film is equal to that of hexagonal closest packed monodisperse spheres (74 vol.%). Consequently, the model has predictive properties and model inputs are only needed on the specific experimental (or field) conditions of interest. The effects on drying time of variations in relative humidity, wet film thickness, initial polymer volume concentration, and air flow velocity are simulated and analysed using the new model.     

Bimodal dispersions in coating applications

This paper describes a study of several model bimodal particle size distribution latex systems produced by blending large and small particle size anionically stabilised latices over a wide range of blend ratios. Minimum film forming temperature (MFT), drying rate, tensile and water uptake measurements were carried out. At a 80/20 weight ratio large/small particles a minimum was observed in the MFT and also in the extent of water absorption of latex films with short drying times, although for films dried for longer periods no such minimum in water absorption was observed. Drying profiles fit well with existing models, except for the 80/201/s blend which exhibits more complex drying behaviour. Low shear rate viscosity of selected blends was measured over a range of latex solids contents. The Theological data were fitted by the Krieger-Dougherty equation which was used to calculate the maximum volume packing fraction. An 80/20 blend (large/small) was found to exhibit a higher maximum volume fraction than that of either pure component of the blend, demonstrating the better packing achievable in blending. A theoretical treatment of the coalescence of bimodal particles is presented in an appendix to the paper.     

The onset of polymer diffusion in a drying acrylate latex: how water initially retards coalescence but ultimately enhances diffusion

The diffusion of polymer chains across the interface between distinct latex particles is the final step in latex film maturation. This step drives the transformation of a honeycomb of compacted latex particles bound by weak surface forces into a mechanically robust film. Knowledge of the onset of this diffusion process is limited. We have examined film formation in butyl acrylate-methyl methacrylate copolymer latex containing 1 wt% methacrylic acid. These films dry via a propagating drying front. We were able, via fluorescence resonance energy transfer measurements, to determine the extent of polymer interdiffusion at 23°C as a function of distance from the edge of the drying front for a series of partly wet latex films. Our apparatus allows us to arrest the latex drying process and to extract interdiffusion information from sub-millimeter regions of the drying film. We have tracked the latex drying process and subsequent polymer diffusion as a function of humidity. We find that adjacent to the drying front, increasing humidity initially delays the onset of interdiffusion, but once this initial barrier is overcome increasing humidity increases the rate of diffusion. This transition occurs within 1–2 mm of the drying front.

Stress development and film formation in multiphase composite latexes

Designed appropriately, multiphase soft-core/hard-shell latex particles can achieve film formation without the addition of a coalescing aid, while preserving sufficient film hardness. Achieving optimal performance in these materials requires an understanding of how particle morphology affects film formation and stress development in the film. In this study, soft-core/hard-shell latex particles with different shell ratios, core and shell glass transition temperatures (T _gs), and particle sizes (63–177 nm) were synthesized using a two-stage emulsion polymerization. The film formation behavior of the composite particles was investigated with cryogenic scanning electron microscopy, atomic force microscopy, and measurements of the minimum film formation temperature (MFFT). Results show that film formation was enhanced for particles with thinner hard shells, smaller particle size, and a smaller difference in T _g between the core and shell polymers. For example, the MFFT decreased and the particle deformation increased for particles with thinner shells and smaller particle sizes. Stress development during drying was characterized using a cantilever beam bending technique. A walled cantilever design was used to monitor stress development without the complication of a lateral drying front. The film formation behavior and stress development correlated well with practical paint properties like scrub resistance and gloss.     

Drying and cracking of soft latex coatings

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.     

Film formation from pigmented latex systems: Drying kinetics and bulk morphologies of ground calcium carbonate/functionalized poly(n‐butyl methacrylate‐co‐n‐butyl acrylate) blend films

The drying kinetics and bulk morphology of pigmented latex films obtained from poly(n‐butyl methacrylate‐co‐n‐butyl acrylate) latex particles functionalized with carboxyl groups and ground calcium carbonate blends were studied. Latex/pigment blends with higher carboxyl group coverage on the latex particle surfaces dried faster than films with few or no carboxyl groups present. The latex/pigment dispersions also dried faster when there was more stabilizer present in the blend system because of the hydrophilic nature of the stabilizer. The net effect of increasing the pigment volume concentration in the blend system was to shorten the drying time. The bulk morphologies of the freeze‐fractured surfaces of the pigmented latex films were studied with scanning electron microscopy. Scanning electron microscopy analysis showed that increased surface coverage of carboxyl groups on the latex particles in the latex/pigment blends resulted in the formation of smaller pigment aggregates with a more uniform size distribution in the blend films. In addition, the use of smaller latex particles in the blends reduced the ground calcium carbonate pigment aggregate size in the resulting films. Scanning electron microscopy analysis also showed that when the initial stabilizer coverage on the latex particles was equal to 18%, smaller aggregates of ground calcium carbonate were distributed within the copolymer matrix of the blend films in comparison with the cases for which the initial stabilizer coverage on the latex particles was 8 or 36%. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2267–2277, 2006     

Film formation from pigmented latex systems: Mechanical and surface properties of ground calcium carbonate/functionalized poly(n‐butyl methacrylate‐co‐n‐butyl acrylate) latex blend films

The mechanical and surface properties of films prepared from model latex/pigment blends were studied using tensile tests, surface gloss measurements, and atomic force microscopy. Functionalized poly(n‐butyl methacrylate‐co‐n‐butyl acrylate) [P(BMA/BA)] and ground calcium carbonate (GCC) were used as latex and extender pigment particles, respectively. The critical pigment volume concentration of this pigment/latex blend system was found to be between 50 and 60 vol % as determined by surface gloss measurement and tensile testing of the blend films. As the pigment volume concentration increased in the blends, the Young's modulus of the films increased. Nielsen's equations were found to fit the experimental data very well. When the surface coverage of carboxyl groups on the latex particles was increased, the yield strength and Young's modulus of the films both increased, indicating better adhesion at the interfaces between the GCC and latex particles. When the carboxyl groups were neutralized during the film formation process, regions with reduced chain mobility were formed. These regions acted as a filler to improve the modulus of the copolymer matrix and the modulus of the resulting films. The carboxyl groups on the latex particle surfaces increased the surface smoothness of the films as determined by surface gloss measurement. When the initial stabilizer coverage of the latex particles was increased, the mechanical strength of the resulting films increased. At the same time, rougher film surfaces also were observed because of the migration of the stabilizer to the surface during film formation. With smaller‐sized latex particles, the pigment/latex blends had higher yield strength and Young's modulus. Higher film formation temperatures strengthen the resulting films and also influence their surface morphology. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4550–4560, 2006     

Film formation of monodispersed polystyrene latex at high temperature

This report discusses the drying behavior of monodispersed polystyrene latex at elevating temperature with particular attention to the relationship between water evaporation rate and morphological evolution during the film formation process. At the first stage, water evaporation rate was less influenced by the skin film formed at the latex/air interface, which was consistent with Croll's model. During this stage, a drying front advanced from the top film toward the bulk dispersion. At the final stage of film formation, the water evaporation rate was less than that of the initial stage, and another drying front developed from the interior region outside the system. Two distinct boundaries corresponding to the opposite directions of the second drying front between completely dried region and wet region were found if the film was peeled off the container surface. Besides, some particular morphologies were found in the completely dried region, which was likely related to preferable coalescence among the particles induced by capillary force because of water evaporation. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1835–1840, 2001     

Packing effect on latex film formation: a photon transmission study

A photon transmission method has been used to study interdiffusion processes during film formation from hard latex particles. Films with different latex content were prepared separately by annealing poly(methyl methacrylate) (PMMA) particles above the glass transition temperature. The transmitted photon intensity from these films increases as the annealing temperature is increased. Monte Carlo simulations are performed for photon transmission through a rectangular lattice. The increase in the transmitted photon intensity (I_tr) is attributed to the latex content (film thickness) for the annealed film samples. It is observed that as the latex particles are packed (film thickness is increased) fewer voids or cracks are formed in the films. A negative absorbance coefficient has been measured above the 180 °C annealing temperature. Packing coefficients are obtained for films having various latex contents. © 2000 Society of Chemical Industry     

On the role of colloidal crystal-like domains in the film forming process of a carboxylated styrene-butadiene latex copolymer

Environmental scanning electron microscopy (ESEM) was employed to study the mechanism of film formation of a carboxylated styrene-butadiene latex copolymer with a glass transition temperature (T_g) of 6 °C. ESEM allows the investigation of wet samples in their native state which is required to study the drying process of latex dispersions. The film forming process was tracked by time-dependent ESEM monitoring of the latex particle morphology and by observing the different stages occurring during the drying process. The focus of our study was an analysis of the three-dimensional (3D) arrangement of the latex particles and a comparison of their appearance on the surface and in the center of the coalesced film. It was found that in the course of film formation, the latex particles arrange in domains which are similar to colloidal crystals. Such domains occur at the stage of dense particle packing. Particle coalescence appears to begin first in these domains before a continuous and homogeneous film is formed which then spreads across the entire substrate. The results suggest that for our carboxylated styrene-butadiene copolymer the current model known for the film forming mechanism which includes four main steps should be complemented by two additional ones, namely the arrangement of particles in crystal-like domains and the beginning of coalescence within these domains. This specific behavior only occurs for monodisperse latices.     

Turbidity study of the process of film formation of thin films of aqueous acrylic dispersions

This study was directed towards determining the factors that define the process of film formation of binder particles in drying aqueous dispersion coatings, based on acrylic polymers. The work described focuses on the infrastructure of drying and ageing thin films of acrylic latices.

In concentrated latices the binder particles are arranged in closely packed structures which cause colored light patterns, the so-called Bragg diffractions. The light waves move within the latex film, where the waves are scattered by the internal structure composed of the spheres and water voids. The pattern of light transmission reveals the internal structure of the latex film. From the change in interference during the drying process of a thin latex film, it is possible to follow the internal movement and deformation of polymer spheres (coalescence process). Further coalescence results in a transparent film. When this film is immersed in water, the remaining internal interfaces between the adhered binder particles swell, thus regenerating the interference pattern. It is expected that during ageing of the film, the proportion of internal interfaces will decrease with time, so that when the aged film is immersed in water the remaining internal interfaces will swell. The resulting interference pattern reveals the decrease in the interfaces between the deformed polymer particles in the dried latex film (auto-adhesion process).     

Correlation of hardness with MFFT and CPVC of latex/ceramic coatings

Hardness, porosity, and microstructure of film-forming in polyvinyl acetate/alumina coatings from aqueous suspensions were investigated with a minimum film formation temperature (MFFT) bar, Vickers hardness tester, and scanning electron microscopy (SEM). The hardness of opaque composite coatings (alumina:latex=1:2 by volume) increased abruptly at the MFFT of the latex, suggesting that the alumina particles did not change the latex film formation behavior and that the hardness measurement is an alterative to the optical criterion. Studies of coatings from latex particles that were smaller or larger than a common size of ceramic particles indicated that the composition of maximum hardness, called critical Vickers hardness concentration (CVHC), matched conventional critical pigment volume concentration (CPVC). More efficient polymer binding in the coatings from the smaller latex gave them higher peak hardness and CPVC. Department of Chemical Engineering & Materials Science, 421 Washington Ave., SE, Minneapolis, MN 55455-0132.     

Multiscale dynamic Monte Carlo/continuum model of drying and nonideal polycondensation in sol‐gel silica films

The process of forming sol‐gel silica thin films involves multiple length and time scales ranging from molecular to macroscopic, and it is challenging to fully model because the polymerization is nonideal. A multiscale model is described to link macroscopic flow and drying (controlled by process parameters) to film microstructure (which dictates the properties of the films). In this modeling strategy, dynamic Monte Carlo (DMC) polymerization simulations are coupled to a continuum model of drying. The entire DMC simulation is treated as a particle of sol whose position and composition are tracked using a diffusion/evaporation finite difference method. By simulating swarms of particles starting from different positions in the film, the multiscale model predicts different drying/gelation phenomena, and predicts the occurrence of gradients of concentration and gelation in the films which can lead to the formation of a gel skin near the top surface of the film. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Distribution of particles during solvent evaporation from films

We examine films, subject to evaporation, that contain particles. The volume fraction of particles increases as solvent is removed up to a maximum value of close packing. The governing equation for evolution of particle volume fraction is derived and numerical solutions obtained for various Peclet numbers. For large Peclet numbers an asymptotic solution gives the position and functional form of a front of close packed particles which passes through the film. For intermediate Peclet numbers the scaling for the spatial gradient in volume fraction is found to be Pe^1/2. This is different from conventional sedimentation, where convection rather than diffusion dominates.     

Morphological prediction and its application to the synthesis of polyacrylate/polysiloxane core/shell latex particles

A theoretical analysis and a morphological prediction of polyacrylate (PA)/polysiloxane (PSi) latex particles with core/shell morphologies were first conducted based on interfacial tensions and relative volumes of the two polymers in the latex system. The results indicated that the normal core/shell morphology particles (PSi/PA), with hydrophobic polysiloxane as the core and with hydrophilic polyacrylate as the shell, can be easily formed. Although the inverted core/shell morphology particles (PA/PSi) with polyacrylate as the core could not be formed in most cases, even if the fraction volume of polysiloxane was larger than 0.872, which is the smallest value of forming a PA/PSi particle, the PSi/PA particles were unavoidably formed simultaneously with PA/PSi particle formation. The synthesis of PA/PSi particles containing equal amounts of polyacrylate and polysiloxane was then carried out using seeded emulsion polymerization. Before the cyclosiloxane cationic polymerization, 3‐methacryloyloxypropyl trimethoxysilane (MATS) was introduced into the polyacrylate seed latex to form an intermediate layer and chemical bonds between the core and the shell polymers. The characterization by transmission electron microscopy (TEM) demonstrated that the perfect PA/PSi core/shell particle is successfully synthesized when both the core and the shell polymers are crosslinked. The experiments showed that both the hardness and water adsorption ratio characteristics of latex films of the PA/PSi particles are in good agreement with those of the polysiloxane film. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2251–2258, 2001     

Role of particle size on latex deformation during film formation

The ability of latex particles to deform and coalesce to form an integral film upon drying is an important property in many latex coating applications. Many theories have been proposed to account for the origin of the deformation forces. The capillary forces which depend inversely on particle size have been accepted as important for latex deformation and film formation. The minimum film forming temperature (MFFT) has been found to be a function of the particle size of latexes and has been used as evidence that the capillary forces are responsible for film formation. In this study, the deforming force at MFFT has been determined from the moduli of water-equilibrated latex polymers. No particle size dependence was observed. The magnitude of the deforming forces was at least an order of magnitude lower than that predicted by the capillary force theory. Electron microscopy of film formed below the MFFT, a condition that corresponds to early stage film formation, showed significant deformation, indicating that at the beginning of film formation, forces of magnitude predicted by the capillary force theory are present. However, the magnitude of the forces decreases rapidly as film formation progresses. The MFFT particle size dependency can be explained by the difference in the degree of water plasticization. Evidence that latexes of different particle size were plasticized by water to different extents was determined from the T_gs of the latex emulsions. Presented at the 76th Annual Meeting of the Federation of Societies for Coalings Technology, on October 15, 1998, in New Orleans, LA. Emusion Polymers Research 1604 Building, Midland, MI 48674.     

Effects of core-shell latex morphology on film forming behavior

A series of core-and-shell latex particles were made from methyl methacrylate/butyl acrylate copolymers. All latexes were almost monodisperse in particle size. The polymer hardness was varied by changing the methyl methacrylate/butyl acrylate ratio between the limits of 40/60 and 60/40 parts by weight. The minimum film temperatures(MFTs) of these particles were expected to vary with core and shell characteristics in the following order: soft/hard > medium/medium > hard/soft. In fact, this order was observed only if the shell thickness was greater than a certain minimum value that depends on the diameter of the core polymer. Thinner, softer shells on harder cores may require higher drying temperatures than thicker shells with the same composition because the former are required to deform more to produce void-free films.     

Formation of transparent conducting films based on core‐shell latices: Influence of the polypyrrole shell thickness

Transparent conducting latex films have been prepared from core‐shell latices. The latex particles have a poly(butyl methacrylate) (PBMA) core of about 700 nm and a very thin polypyrrole (PPy) shell. We have studied the film formation of latices with 1, 2, and 4 wt % PPy and compared this with the film formation of the pure PBMA latex. The film formation process was studied by transparency measurements, atomic force microscopy surface flattening, and transmission electron microscopy on ultrathin sections of films after various annealing times at 120°C. It is demonstrated that highly transparent (>90%) and antistatic films can be produced using these latices. The presence of a polypyrrole shell around the PBMA latex particle seriously hinders the deformation of the particles. The amount of polypyrrole, and thus the shell thickness, is the determining factor for the speed of film formation. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 900–909, 2001     

Effect of a coalescing aid on the earliest stages of polymer diffusion in poly(butyl acrylate-co-methyl methacrylate) latex films

In this article we use fluorescence resonance energy transfer (FRET) to investigate how a classic coalescing aid, such as 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (Texanol?) (TX), acts on the earliest stages of polymer diffusion as the latex film is still drying. In our approach, we temporarily arrest the drying process of a partially wet latex film by sealing it in an airtight chamber previously cooled to near the latex T_g. At these conditions, we are able to effectively stop the drying process and the polymer diffusion. FRET measurements at various locations on such a sample provide us information about the mechanism operating at the initial stages of polymer diffusion as the latex film is still drying. We complete our study with FRET measurements carried out at longer aging times on predried latex films. We analyze our diffusion data in terms of free volume theory and propose a mechanism that can account for the results obtained.