Effect of sintering on surface chemistry and surface energy of pigmented latex coatings

Drying in the absence of water (sintering) of pigmented coatings made of styrene–butadiene (SB) latex and kaolin clay at different levels of pigmentation was investigated. As found from X-ray photoelectron spectroscopy, sintered coatings showed a higher SB area percent on the surface than did latex with a high glass-transition temperature (T_g) and dried at room temperature. This was a result of latex spreading at the surface. Sintering the high-T_g coatings that were dried at room temperature caused a decrease in the surface energy. Drying in the presence of water (wet coalescing) was compared to drying in the absence of water (sintering). Even though sintered coatings were more porous and had higher gloss, no significant difference was found in the SB/clay ratio at the surface or in the surface energy above the critical pigment volume concentration (CPVC). However, at and below CPVC, the sintering process yielded a higher SB content at the surface and a lower surface energy than the wet-coalescing process. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 968–975, 2001     

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.     

Sintering of latex particles in pigmented coatings. II. Influence of the latex particle size

The light-scattering coefficient S of coatings formulated from a mixture of an inorganic pigment and polystyrene latex particles may be enhanced significantly when the dry coating is heated in order to cause sintering of the polystyrene spheres. The relative increase in S depends on the size and the amount of the latex particles and the type of the inorganic pigment. Of the latexes examined (0.5, 0.2, 0.1, and 0.03 μm in diameter) the 0.2μm latex is the most effective in blends with platelike delaminated clay. The coatings based on spherelike precipitated calcium carbonate are less responsive to the heat treatment. The gloss of the clay–latex coatings is not significantly altered by the heat treatment up to 20 pph of latex; the gloss of the calcium carbonate coatings generally decreases upon heating.     

New applications in studies of waterborne coatings by atomic force microscopy

Atomic force microscopy (AFM) is rapidly emerging as an important tool for coatings characterization. We report several new applications of AFM of particular value to the development of improved waterborne coatings systems. First, an AFM method was developed to quantitatively assess the extent of coalescence and film formation for latex films by measurement of particle number density of protruding (uncoalesced) particles in dried coatings. Second, the use of topographic imaging to evaluate environmental (temperature) effects on film formation was investigated for a waterborne latex system. Finally, specular gloss of waterborne epoxy coatings was studied by AFM and optical measurements, and topographic features analyzed using power spectral density calculations were found to correlate with optical gloss measurements. Mechanisms for gloss reduction over time (particularly in early pot life coatings) were elucidated in the studies. Further applications in coatings studies will be driven by the development of new modes of AFM (friction force, force modulation, and phase contrast) that can be used to map mechanical properties (friction, stiffness, and adhesion) while simultaneously imaging topography. Examples of the use of the phase contrast mode to identify chemically different domains in early pot life waterborne epoxy coatings are presented.     

The Segregation of Surfactant upon Film Formation of Latex Dispersions: an Investigation by Energy Filtering Transmission Electron Microscopy

We investigated the local distribution of emulsifier in dried films cast from latex dispersions. Energy filtering transmission electron microscopy (EFTEM) allowed the detection of heteroatom-containing domains in unstained samples by electron spectroscopic imaging and elemental mapping. If the samples were prepared at temperatures above the minimum film-forming temperature (MFT), separated domains of surfactant were observed in detachment replicas and ultrathin sections. If the preparation was carried out below MFT the emulsifier was retained at the particle surface and similar observations could not be made. It is suggested that the mobility of the emulsifier at the surface of the particles is increased upon deformation and coalescence in the process of film formation, and that this mobility is a prerequisite for its segregation. Both processes do not take place upon drying below MFT, owing to lower mobility. The correlation between the process of film formation and the segregation of surfactant is discussed for both a dispersion of poly(vinylacetate) and one of polyacrylate.     

Heterogeneous latices as binders in porous particle structures

Heterogeneous carboxylated styrene-butadiene (S/Bu) latices were prepared by a two-stage polymerization process, using three seeds of polystyrene with different molecular weights. The second-stage polymer was a copolymer with a fixed S/Bu-ratio of 1 and a methacrylic acid (MAA) content of either 1 or 10 wt %. It has been found that the morphology of the films made from these latices influenced the modulus in the rubbery region of these films. The heterogeneous latices were used as binders in porous structures based on micron-sized kaolin particles. Such structures are typically employed as paper coatings. Polyester substrates were coated with aqueous suspensions containing the kaolin particles and the heterogeneous latex. The coatings were dried at room temperature, which corresponds to the rubbery region of the latex films. It was found that a higher modulus (which is determined here by the morphology of the latex film) in the rubbery region of the films was associated with coating layers with higher porosity, greater light scattering ability, and higher coating gloss. This is interpreted as being the result of a retarded shrinkage of the coating layers during the drying of these structures due to the increase in modulus of the latex films. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 661–670, 1997     

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.     

细乳液聚合法制备PY-17/PE蜡/PS复合乳胶粒子

通过细乳液聚合法制备彩色墨粉用多组分复合乳胶粒子,即通过细乳液聚合法制备聚苯乙烯包覆有机颜料PY-17和聚乙烯蜡的三元复合乳胶粒子。通过透射电子显微镜、激光粒度仪、紫外可见分光光度计和热失重分析仪对制备的复合乳胶粒子形貌、粒度、热学和光学特性进行表征,着重研究了乳化剂种类及用量、颜料用量和聚乙烯蜡用量对复合乳胶粒子粒径和形貌的影响,并对复合乳胶粒子的热学及光学特性进行了表征。结果表明,使用复合乳化剂,颜料PY-17用量不大于单体量8%、聚乙烯蜡用量占单体量不大于10%时,制备的复合乳胶粒子粒径较小,分布窄,包覆较为完整;并且复合乳胶粒子的热学及光学特性发生了一定的改变。     

Crosslinking of latex polymers

The physicomechanical properties and processes of thermal crosslinking of latex acrylic polymer films containing functional groups of different chemical nature were studied. Improvement of properties characteristic of latex copolymers containing methylolamide groups is explained by orientated location of hydrophilic groups on the surface of latex particles during emulsion copolymerisation, that, in its turn, leads to a more ordered arrangement of macrochains. An optimum content of methylolamide groups as regards crosslinking processes was established and explained by steric location of these groups on the surfaces of latex particles. A similar optimum is not observed for crosslinking by watersoluble resin of latex copolymers containing groups of hydrophobic glycidylmethacrylate.     

The influence of substrate absorbency on coating surface chemistry

The composition of the top surface of a coating layer can influence its functional properties or subsequent processing steps. The effect of the substrate absorbency on the coating surface chemistry is reported. Different coating systems containing a kaolin clay pigment, fine or coarse precipitated calcium carbonates, and a common latex binder were examined. The influence of a soluble polymer added into the coating was characterized. The surface chemistry was measured with attenuated total internal reflectance (ATR) and X-ray photoelectron spectroscopy (XPS).

Absorbent substrates generate bulky coatings with high voids and low gloss. Rapid dewatering by the absorbent substrate pulls the small particles, like latex binder, away from the top layers causing a low latex concentration at the surface. On non-absorbent substrates, the addition of the soluble polymer generates coating layers with higher void volume, lower gloss, and lower latex concentrations at the coating surface. However, on absorbent substrates, polymer addition causes coatings with lower void volumes and higher gloss. In this case, the rapid dewatering and mobility of particles is reduced by the polymer, which helps to retain the small particles at the surface. As a result, latex concentration at the surface increases with polymer addition on absorbent substrates.     

Drying and collapse of hollow latex

Hollow latex particles are used as white pigments for paints and paper coatings. In the coating dispersion, each hollow particle is filled with water. As the coating dries, water vacates the latex, leaving an air-filled void sized to scatter light (~0.5 μm) within each particle. Examinations of dried coatings reveal that hollow particles can collapse, decreasing their light scattering efficiency. Cryogenic scanning electron microscopy (cryoSEM) was used to characterize the microstructure of coatings containing hollow latex during drying. Images suggest latex voids empty after air invades into the coating interstitial space and collapse occurs late in the drying process. The effects of temperature (10–60°C), humidity (20–80%), and binder concentration (0–30 wt%) on particle collapse were also studied through SEM of dried coating surfaces. High drying temperature, high humidity, and low binder concentrations promoted collapse. For hollow latex particles with porous shell walls, temperature and humidity had little effect, whereas binder increased collapse. From these results, a theoretical model is proposed. During drying, diffusion of water from the particle creates a vacuum inside the latex. The vacuum is either relieved by nucleation of a gas bubble from the dissolved air in the water-filled particle or it causes the particle to collapse by buckling.     

A hybrid polyacrylate/OMMT nanocomposite latex: Synthesis,characterization and its application as a coating binder

Hybrid latex particles have an increasing importance in industrial applications especially for high performance waterborne coatings. They provide combined properties of different phase compositions for the final polymer which can also be enhanced by additives such as nanoclays. The present study describes the preparation of a hybrid polyacrylate/OMMT nanocomposite latex via two stage in situ emulsion polymerization with a low emulsifier content (1 wt%) which is usually a challenge for the preparation of stable polymer/OMMT nanocomposite latexes. The obtained nanocomposite latex was stable and had a fine average size diameter of 151 nm with a very narrow size distribution. The copolymer films exhibited a well exfoliated structure observed by WAXD and TEM. Other polymer properties were investigated by FTIR, DSC, TGA, DMTA and rheological measurements. The results indicated that the addition of clay even in low amount (2 wt%) yielded significantly improved mechanical and thermal properties of the final polymer. In addition, the nanocomposite latex was also applied on leathers as coating binder in a finishing formulation and the results of the performance tests revealed substantially increased rubbing and heat resistance whereas a slight decrease was observed at water vapor permeability of the coated leathers.     

Calcium sulphate as pigment for improved functional properties of coated paper

In the last few years, interest and demand of high bright paper have forced paper manufacturers to think new ways to improve brightness and whiteness of coated paper. Pigment coating is widely used to enhance the optical properties such as brightness, whiteness, and gloss of paper and paperboard. These optical properties are the most important for end user and also determine the final cost of coated paper. Calcium sulphate has inherent better optical properties compared to other conventional pigments for example ground calcium carbonate, precipitated calcium carbonate and kaolin clay. The present study was carried out with an aim to synthesize calcium sulphate using waste procured from phosphoric acid industry and to study its impact on the rheology of coating color as well as optical properties of coated paper. Addition of calcium sulphate improved the water retention property of coating color which can be helpful for improving the machine runnability. The results also revealed that calcium sulphate can be used as a pigment to produce coated paper of high brightness and whiteness. The brightness and whiteness of the coated paper were improved 4 and 15 points, respectively by using 50 parts of calcium sulphate as a pigment replacing clay from the coating formulation. The surface strength in terms of IGT pick value of coated sheets was found significantly comparable using calcium sulphate as pigment. The print gloss results were observed analogous with matt grade coated paper.     

Relationship between surface chemistry and surface energy of different shape pigment blend coatings

The influence of pigment shapes and pigment blends on the surface energy was investigated and compared with the surface chemistry of pigmented latex coatings. The coatings were made of different volume ratios of two pigments: plate-like kaolin clay pigment and prismatic precipitated calcium carbonate (PCC) pigment. These were mixed together with carboxylated styrene–butadiene–acrylonitrile latex (SBA), and applied over nonabsorbent substrates as well as absorbent substrates. The composition of the surface of the coatings was investigated by X-ray photoelectron spectroscopy (XPS). Two approaches were used to estimate the total surface energy and the components of the coatings: a conventional approach—“the Kaelble approach”—and a more modern approach—“the van Oss approach.” Pigment blends with different shapes and increments caused a change in the surface chemistry and the surface energy of the latex coatings. As the prismatic PCC pigment particles increased in the kaolin/SBA coating system, the SBA latex content at the coating surface increased and the total surface energy of the coating decreased. This is valid for both nonabsorbent as well as absorbent substrates. It was found that there was a strong correlation between the surface energy and the surface composition. The surface energy of the coatings estimated by the Van Oss approach was always lower than that estimated by the Kaelble approach. Colloidal interactions between pigment–pigment and/or pigment–binder were thought to play an essential role in determining the final coating surface energy and its components. Changes in the surface latex content and the surface energy due to the different pigment blends investigated were found to fit straight-line equations.     

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.     

Facile preparation of nylon 6 nanocomposites based on clay reinforcement and core‐shell latex toughening: Morphology,properties, and impact fracture behavior

This work is focused on a facile route to prepare a new type of nylon 6‐based nanocomposites with both high fracture toughness and high strength. A series of nylon 6‐matrix blends were prepared via melting extrusion by compounding with poly (methyl methacrylate‐co‐butadiene‐co‐styrene) (MBS) or poly(methyl methacrylate‐co‐methylphenyl siloxane‐co‐styrene) (MSIS) latices as impact modifier and diglycidyl ether of bisphenol‐A (DGEBA) as compatibilizer. Layered organic clay was also incorporated into above nylon 6 blends for the reinforcement of materials. Morphology study suggests that the MBS or MSIS latex particles could achieve a mono‐dispersion in nylon 6 matrix with the aid of DGEBA, which improves the compatibilization and an interfacial adhesion between the matrix and the shell of MBS or MSIS. High impact toughness was also obtained but with a corresponding reduction in tensile strength and stiffness. A moderate amount of organic clay as reinforcing agent could gain a desirable balance between the strength, stiffness and toughness of the materials, and tensile strength and stiffness could achieve an improvement. This suggests that the combination of organic clay and core‐shell latex particles is a useful strategy to optimize and enhance the properties of nylon 6. Morphology observation indicates that the layered organic clay was completely exfoliated within nylon 6 matrix. It is found that the core‐shell latex particles and the clay platelets were dispersed individually in nylon 6 matrix, and no clay platelets were present in MBS or MSIS latex particles. So the presence of the clay in nylon 6 matrix does not disturb the latex particles to promote high fracture toughness via particle cavitation and subsequent matrix shear yielding, and therefore, provides maximum reinforcement to the polymer. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

原位聚合法制备高透光率韧性PMMA复合树脂的研究

采用原位聚合法制备PMMA/P(BA-St)/PMMA三层韧性有机玻璃复合树脂,分子设计方法的使用,保持了材料的透明性。考察了韧性粒子粒径、橡胶相组成以及橡胶含量对材料力学和光学性能的影响。借助透射电镜、扫描电镜和动态光散射方法对复合胶乳粒子以及所制材料的形态结构进行了表征。结果表明:橡胶相的折光指数对材料的透光率有明显影响,橡胶相玻璃化温度越低,越有利于增韧。     

Poly(vinyl alcohol) nanocomposite films: Thermooptical properties,morphology, and gas permeability

Poly(vinyl alcohol) (PVA)/saponite nanocomposites were prepared with various clay concentrations with the solution intercalation method. The intercalations of the polymer chains in the clay were examined with wide‐angle X‐ray diffraction and transmission electron microscopy. The variations of the dispersion, morphology, thermal properties, and gas permeability of the nanocomposites with clay concentrations in the range of 0–10 wt % were examined. Up to a 5 wt % clay loading, the clay particles were highly dispersed in the PVA matrix without any agglomeration of particles. However, some agglomerated structures formed in the polymer matrix above a 7 wt % clay concentration. The thermal stability of the hybrids increased linearly with increases in the clay loading up to 10 wt %. To measure the oxygen permeability and optical properties of the PVA hybrid films, the PVA hybrid solutions were coated onto both biaxially oriented polypropylene and poly(ethylene terephthalate) films, which were used as polymer substrates. The oxygen permeability values monotonically decreased with increases in the clay loading in the range of 0–10 wt %. The optical properties, such as the haze and gloss of the hybrid films when coated onto the matrix films, were nearly constant, that is, independent of the clay loading. These improvements arose because of the largely nanometer‐scale dispersion of the clay layers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 591–596, 2006     

Preparation and characterization by thermal analysis of magnetic latex particles

Emulsion polymerization of styrene and methyl methacrylate in the presence of a ferrofluid was briefly studied. Thermal properties of the resulted latex particles were investigated by TG–DTA analysis. Determination of the residue weight after the thermal analysis that indicated complete decomposition of the organic components was found to be a facile and practical method to determine the magnetite content in the latex particles. The method was applied to magnetic polystyrene latex particles prepared in the presence of various amounts of the ferrofluid. Analysis of the results suggested that the magnetic content in the latex particles is primarily determined by the weight ratio of the ferrofluid to monomer. © 1993 John Wiley & Sons, Inc.     

Effect of particle size distribution on stress development and microstructure of particulate coatings

The role of pigment particle size distribution on stress and microstructure development was studied for coatings prepared from aqueous suspensions of ground calcium carbonate (GCC) and latex binder. Stress development was monitored using a modified beam deflection technique under controlled environment. Microstructure was characterized by scanning electron microscopy (SEM) and cryogenic SEM. For coatings containing only GCC particles and no latex, a wide particle size distribution resulted in a significant particle size gradient in the cross-sectional microstructure and irregular stress development. With latex addition, uniform microstructures were observed in coatings with either wide or narrow GCC particle size distribution. GCC/latex coatings prepared using GCC with a wide particle size distribution developed a higher stress than those prepared using GCC with a similar average particle size but a narrow particle size distribution. The higher stress is related to the particle packing that results in smaller pore sizes and larger capillary pressures that drive compaction. In coatings prepared with the same GCC particles but different latex binders, the stress and cracking behavior of the coating depends on the latex properties.