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.     

Particle size and shell composition as effective parameters on MFFT for acrylic core–shell particles prepared via seeded emulsion polymerization

In this study, acrylic-based copolymer particles were prepared with core–shell morphology and the effect of T_g of the shell, particle size and their bimodal size distribution on minimum film formation temperature (MFFT) were investigated. The main goal was to optimize conditions to obtain latexes with low MFFT and appropriate mechanical properties. These will develop the applicability of such water-borne binders as paints and coatings at ambient conditions. A series of latexes with core–shell morphology with variable T_g of the shell from −56 to 30 °C were prepared and the MFFT, hardness and thermal behavior of the obtained films were studied. Then a series of latexes with particle sizes ranging from 46 to 960 nm were prepared and the effect of particle size on MFFT was studied too. By inducing the formation of secondary nucleation during emulsion polymerization, latexes with bimodal size distribution were obtained and the effect of presence of such particles on film formation was investigated. Results indicate that latexes with appropriate composition and bimodal particle size distribution lead to optimized performance in both mechanical and film formation properties as a proper choice for water-borne coatings.     

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.     

Correlation between water permeability of latex-modified concrete (LMC) and water diffusion coefficient of latex film

Cement-based materials are generally known as weak materials in flexure and tension in comparison with compression. Polymers are used in cement-based materials to improve their flexural and tensile behaviors. The composite is called as polymer-modified concrete/mortar. Furthermore, polymers decrease permeability of water into cementitious matrices. Polymers are usually used as admixtures in concretes in form of latexes. Latexes are water-based polymers, which are consistent with water-based concrete matrices. On this basis, these kinds of products are called latex-modified concretes (LMCs). However, it has been found that chemical composition, particle size distribution, molecular weight, physical/mechanical properties of latexes affect performance of modified concretes. In this investigation, six latexes in three categories (acrylic, SBR and polyvinyl acetate) were used as concrete admixtures. They were characterized for chemical composition (by FTIR analysis), minimum film formation temperature, pH, glass transition temperature (T _g), particle size and particle size distribution to evaluate the effect of each property on LMC performance. Due to the formation of latex film in the microcracks and pores of concrete microstructure, it was suggested that diffusion of water into films controls permeability of whole concrete structures. On this basis, the diffusion coefficient of the latex films subjected to water was measured using a new method (continuous FTIR analysis). Capillary water absorption test was performed on concrete specimens to verify validity of the suggestion. It was found that there is a correlation between capillary water absorption of LMCs and water diffusion coefficient of latex films.     

An ESEM investigation of latex film formation in cement pore solution

Environmental scanning electron microscopy (ESEM) and complementary methods were employed to study the time dependent film formation of a latex dispersion in water and cement pore solution. First, a model carboxylated styrene/n-butyl acrylate latex dispersion possessing a minimum film forming temperature (MFFT) of 18 °C was synthesized in aqueous media via emulsion polymerization. Its film forming property was at a temperature of 40 °C, studied under an ESEM. The analysis revealed that upon removal of water, film formation occurs as a result of particle packing, particle deformation and finally particle coalescence. Film formation is significantly retarded when the latex dispersion is present in cement pore solution. This effect can be ascribed to adsorption of Ca²⁺ ions onto the surface of the anionic latex particles and to interfacial secondary phases. This layer of adsorbed Ca²⁺ ions hinders interdiffusion of the macromolecules and subsequent film formation of the latex polymer.     

聚丙烯酸酯/铜酞菁蓝复合乳胶膜中颜料颗粒的分散状态

引言功能颗粒存在下,水性胶乳在织物、纸张、皮革等目标基质表面的焙烘成膜是众多生产领域中普遍存在着的一个固化成型过程。通常认为功能颗粒在乳胶膜内初级粒子形式的均匀稳定分散可赋予涂层最佳的功能性和黏结牢度[1]。但目前常见的、基     

Crosslinking systems and film properties for surfactant-free latexes based on anhydride-containing polymers

In this paper, we report novel crosslinking systems for surfactant-free artificial latexes based on anhydride-containing polymers. Surfactant-free latexes with average particle diameters of about 150 nm and a ζ-potential of −70 mV have been successfully obtained from anhydride-containing polymers with various T_gs and polarities, including poly(octadecene-alt-maleic anhydride) (POMA) and maleinized polybutadiene (PBDMA). When adipic dihydrazide (ADH), a water-soluble crosslinker, was added to these latexes, no differences in particle size or ζ-potential were found; the presence of ADH did not affect the latex stability. In contrast, when 1,6-diaminohexane (DAH) was added to these latexes, it was found to interact with the polymer particles, indicated by a decrease in absolute ζ-potential for the latex particles and even gelation in the case of POMA. From ¹H NMR and LC-MS studies, it has been shown that no free DAH was present after being added to the latex, while free, unreacted ADH was present in aqueous phase upon its addition to the latex. Kinetic studies revealed that irreversible imide formation between anhydride and ADH took place at temperatures of 90 °C and above. In comparison, DAH only formed imides with the copolymers at significantly higher curing temperatures, i.e. >130 °C. Furthermore, the film formation of these latexes was studied; for the different copolymer latexes, curing at temperatures above the T_g of the respective copolymers led to homogeneous film formation. These systems based on surfactant-free latexes crosslinked with ADH have displayed promising properties for future coating applications.     

Effects of surface functional groups on film formation and tensile strength development in reactive latex systems

The effect of surface functional groups on chain interdiffusion at the interfacial zone of reactive latexes was investigated. A series of latexes with different surface functionalities was prepared by batch or shot-growth emulsion polymerizations. The particle surface properties were varied by changing the number density of functional groups. The rate of tensile strength development decreases with the increasing number density of surface functional groups, which indicated that inter-particle crosslinking restricted the interdiffusion of polymer chains during film formation and annealing. The average diffusion coefficient D of polymer chains across particle interfaces was obtained from dynamic mechanical analysis and compared well with the results from the rate of tensile strength development. The magnitude of D of the reactive latex film was lower than that of the homopolymer film. The lower chain mobility of reactive latexes prevented mechanical strength development.     

Submicron particle size and polymerization excess surfactant analysis by capillary hydrodynamic fractionation (CHDF)

The particle size distribution of a blend of two polymer latex monodisperse standards, 86 and 238 nm, has been measured by capillary hydrodynamic fractionation (CHDF). The particle size distributions obtained agreed well with the expected particle diameters. The relative amounts of the two standards in the blends were accurately measured at different blend ratios. The particle size distribution of a parenteral lipid sample was successfully measured. A soluble species was fractionated from latex standards smaller than 100nm. This soluble species appears to be excess surfactant used in the preparation of small size monodisperse latexes. The amount of excess surfactant increases as the particle diameter of the standard decreases.     

Synthesis and properties of branched organosilicon-acrylate copolymer latexes

Summary Stable organosilicon-acrylate copolymer latexes with high silicon content were prepared by seeded semibatch emulsion polymerization of butyl acrylate (BA), methyl methacrylate (MMA) with a novel branched organosilicon monomer 3-methacryloxypropyl tris(trimethylsiloxy) silane (MPTS). Monomer conversion, evolution of the particle size and its distribution were monitored by dynamic light scattering. The effects of MPTS on the polymerization kinetics, the nucleation mechanism and properties of latex were investigated. The results indicated that, in addition to micellar nucleation, a coagulative nucleation step was also observed as a result of the addition of the organosilicon monomer, accordingly, the particle number of the silicon-acrylate latexes increased, the average particle diameter decreased and the polymerization rate accordingly increased compared to those of the acrylate latexes without organosilicon monomer. Moreover, the particle size distribution presented bimodal curves, which indicated that there were large particles formed at an early stage. However, the particle size distribution curves became monomodal at the later stage, and the final latex shows a narrow particle size distribution. It was found that the properties of latex and latex film were obviously influenced by MPTS content. With increasing MPTS content, latex film glass transition temperature and water absorption ratio decreased, the degradation temperature and water contact angle were increased. Hence, the resulting latex films containing MPTS showed lower glass transition temperature and excellent water-resistance, which probably due to the incorporation of the bulky branched hydrophobic group of MPTS into the copolymer chains.     

Facile fabrication and modification of polyacrylate/silica nanocomposite latexes prepared by silica sol and silane coupling agent

Polyacrylate/silica nanocomposite latexes were prepared by silica sol and facilely modified with a silane coupling agent γ-methacryloxypropyltrimethoxysilane (KH-570) aimed at reinforcing the interaction between silica nanoparticles and latex particles in a convenient way. The effects of silica sol and KH-570 amounts on the performance of latexes and films are discussed. Particle size and morphology tests demonstrated that the silica nanoparticles could form hydrogen bond interactions between latex particles, and thus influence the rheological properties of latexes. Tensile measurements and SEM photographs showed the reinforcing and toughening roles of silica nanoparticles. SEM images also indicated that the addition of silica sol increased the roughness of films, which resulted in the increase of hydrophilic silanol groups on the film surface and the decrease of water resistance of latex films. The latex films retained good adhesion force, flexibility, and impact resistance even when the silica sol content was as much as 25%. TGA data revealed that the incorporation of silica sol enhanced the thermal stability of the films. After introducing KH-570, the particle size increased with the increase of the amount of KH-570. Moreover, the addition of KH-570 improved the water resistance and maintained other properties of the latex films appropriately.     

Effect of vinylidene chloride content on film-formation property of vinylidene chloride-methyl acrylate copolymer latex

Minimum film-formation temperature (MFFT) of vinylidene chloride (VDC)-methyl acrylate (MA) copolymer latexes prepared by batch emulsion polymerization with various compositions from 20 to 97 wt % of VDC were measured. For latexes with VDC content below 90 wt %, MFFT was similar to polymer T_g. As VDC content increased beyond 90 wt %, the MFFT curve plotted against VDC content rose sharply, in contrast with the T_g curve that descended smoothly. Measurements of infrared absorption of latexes in the dispersed state, and X-ray diffraction and infrared absorption of lyophilized polymers were conducted on 40 : 60, 80 : 20, and 95 : 5 VDC-MA specimens. These observations indicated that only 95 : 5 VDC-MA specimens were highly crystalline. It was therefore believed that film-formation property of latex with high VDC content was significantly affected by polymer crystallinity of particles in the dispersed state. Morphology and oxygen gas transmission rate of heat-treated and non-heat-treated coatings of 95 : 5 VDC-MA latex were investigated. Heat treatment of coatings beyond the temperature at which crystalline polymer began to melt induced effective particle coalescence, resulting in reduced oxygen gas transmission rate. This supported our belief that film-formation property of VDC-MA latex with high VDC content was significantly affected by polymer crystallinity. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 565–572, 1998     

Determination of the influence of the polymer structure and particle size on the film formation process of polymers by atomic force microscopy

The particle size and morphology of a synthetic polymer latex were shown to influence the film formation behavior. Theoretical models predict that small particles coalesce more easily than large colloids do.The influence of particle size and morphology of differently structured lattices on the film-formation process was investigated by atomic force microscopy (AFM). Sequences of AFM images were acquired over a certain temperature range or at room temperature as a function of time. From the resulting images the average particle diameter of the latex particles in the surface layer was determined as a function of the time or temperature. The resulting curves could be compared to observe differences in the film formation kinetics of the different lattices. These AFM studies confirmed that the film formation behavior is influenced by the particle size and particle morphology, but that the core/shell ratio of core-shell particles has no significant influence on the film formation kinetics.     

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.     

Effect of storage on film-formation property of vinylidene chloride-acrylonitrile-methyl acrylate terpolymer latex

Latex prepared from 91 : 5 : 4 wt % vinylidene chloride-acrylonitrile-methyl acrylate monomer mixture by seeded semicontinuous emulsion polymerization was investigated for a change in minimum film-formation temperature (MFFT) during storage, with focus placed on polymer crystallinity in the dispersed state. MFFT rose from 20°C to 32°C, with storage at 20°C for 49 weeks. Infrared absorption of fresh and stored latexes in the dispersed state indicated an increase in absorbance at 1048 cm⁻¹, which is characteristic of a crystalline vinylidene chloride polymer, that correlated with the MFFT rise with storage. This suggested that the MFFT rise with storage was caused by increasing crystallinity of the polymer in the dispersed state. X-ray wide-angle diffraction and infrared absorption of powder polymers obtained by lyophilization of fresh and stored latexes also indicated increasing crystallinity with latex storage. Oxygen gas permeabilities of films coated with fresh and stored latexes were measured. Latex stored for long periods exhibited poor barrier property, indicating that such latex is unsuitable as an industrial barrier coating material for films and papers. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 573–579, 1998     

聚硅氧烷/聚丙烯酸酯共聚乳液的合成与表征

合成了含氢聚甲基硅氧烷／聚丙烯酸酯共聚乳液。研究了聚合工艺，配组成及操作方式对聚合稳定性，动力学，乳液的粒径分布和产物性能的影响。采用部分预乳化单体滴加法（工艺A）的部分纯单体滴加法（工艺B）两种工艺，聚硅氧烷采用高速预乳化滴加进料方式。用COULTER LS粒径仪和Nicolet傅立叶变换红外光谱仪分别测定共取乳液的粒径分布和产物的结构。研究结果表明，采用A，B两种工艺均能得到平均粒径为0.101-0.103μm的单峰窄分布共聚乳液，并能有效地将含氢聚硅氧烷引入了到共取物大分子中。含氢聚硅氧烷的引入量为3％（质量）时，共聚物涂膜具有柔软，滑爽和强度高等优点。     

Latex film formation: atomic force microscopy and theoretical results

We have critically examined the kinetics of latex film formation using an atomic force microscope to obtain corrugation height data as a function of time, temperature, molecular weight, particle size, etc. The results show that the film forming process obeys viscoelastic time/ temperature superposition principles, thus indicating a direct relationship between the kinetics of film formation and rheological properties. Film formation kinetics are examined under ‘wet’ and ‘dry’ conditions, with film formation occurring almost ten-times faster under wet conditions than dry. This proves for the latex system examined that capillary pressure from the water meniscus is the dominant driving stress for film formation. Past theories for latex film formation are reviewed, and problems and deficiencies are noted. A new theory for film formation from a dry latex system is presented, which is based on the use of the Boltzmann Superposition Principle to relate the changing stress and strain fields as the latex particles deform. The predictions of theory and the experimental data show excellent agreement over nearly four decades of time.     

Influence of ingredients in water-based polyurethane–acrylic hybrid latexes on latex properties

Polyurethane–acrylic (PU–AC) hybrid latexes were prepared. Main monomers for PU preparation were isophorone diisocyanate, DMPA (dimethylol propanic acid) and polypropylene oxides (PPO) of different molecular weights. Acrylic monomers included butyl acrylate, methyl methacrylate and a crosslinker, trihydroxymethyl propane triacrylates (TMPTA). Several important ingredients in PU–AC latex preparation, such as surfactants, initiator, DMPA and PU/AC ratio, etc., were varied, and their effects on latex properties studied. Compared with surfactant free latexes, a sharp increase in particle size was observed in latexes done with 0.1% of surfactant regardless of the nature of the surfactants used (anionic, nonionic and anionic with long chain of amphiphilic alkylphenyl polyethoxylate). Further increase in surfactant content, however, led to latexes with smaller particle size and narrower particle size distribution when compared between latexes prepared using a same surfactant. When amount of the oil soluble initiator, azobisisobutyronitrile, was increased, AC monomers conversion was increased. It is interesting to observe that PPO with long propylene oxides brought about larger particle size combined with broader size distribution and less charge on particle surface; whereas lower DMPA levels led to latexes also of larger size combined with broader size distribution but more charges on particle surface. AC monomer crosslinker, TMPTA, contributed to reduce particle size, narrower size distribution and lower particle surface charges. By increasing AC amount in PU–AC latex, latex particle size significantly increased accompanied by a remarkable increase in particle surface charges. Mechanisms of particle formation and of DMPA stabilization were discussed in order to understand the experimental results.     

Role of van der Waals force in latex film formation

Capillary pressure force and direct surface tension force are known to be sufficient though probably not necessary to drive the compaction stage of latex film formation. There is abundant evidence that van der Waals force can draw particles together progressively more around the perimeters of interparticle contacts, but their role in compaction remains unanswered. With the powerful technique of high-resolution cryogenic scanning electron microscopy (cryo-SEM), together with fast-freezing and freeze-drying followed by controlled annealing at temperatures below and around the nominal glass transition temperature, we have documented the role of van der Waals force in film formation in the water-free condition, i.e., with capillary pressure and surface tension forces absent. Results of imaging the freeze-dried and annealed coatings are fully consistent with the hypothesis that van der Waals force alone can compact a latex coating. The rate at which particles flatten and thus the coating compacts by annealing increases, as expected, with temperature and time. The results of rewetting tests of coatings annealed at various temperatures demonstrate that compacted coating is not necessarily coalesced, and that even full compaction of solid particles can be elastic, hence reversible, rather than viscoelastic or viscoplastic. Some of the results suggest that soluble ionic surfactant and oligomeric and grafted polymeric stabilizers at particle surfaces, collapse to undetectable dimensions during freeze-drying, yet reswell to detectable size during rewetting, if they have not dissolved into polymer particles during annealing. Presented at 2006 FutureCoat! Conference, sponsored by Federation of Societies for Coatings Technology, November 1–3, 2006 in New Orleans, LA.