Drying Latex Films Research Articles

Thinning of drying latex films due to surfactant

Lateral non-uniformities in surfactant distribution in drying latex films induce surface tension gradients at the film surface and lead to film thinning through surfactant spreading. Here we investigate the influence of the surfactant driven to the air–water interface, during the early stages of latex film drying, on the film thinning process which could possibly lead to film rupture. A film height evolution equation is coupled with conservation equations for particles and surfactant, within the lubrication approximation, and solved numerically, to obtain the film height, particle volume fraction, and surfactant concentration profiles. Parametric analysis identifies the effect of drying rate, dispersion viscosity and initial particle volume fraction on film thinning and reveals the conditions under which films could rupture. The results from surface profilometry conform qualitatively to the model predictions.

Distribution of Surfactants in Latex Films: A Rutherford Backscattering Study

Uneven distribution of surfactant in dried latex films can affect the final film properties such as its water-resistance, gloss, and adhesiveness. Therefore, it is important to understand the driving force for surfactant transport during drying. In this paper, the accumulation of surfactant on the surface of poly(styrene-co-butyl acrylate) latex is studied using Rutherford Backscattering (RBS) and compared with results from a model that is based on the diffusive transport of particles and surfactant. Experimentally, a 30-50 nm thick surface layer, rich in surfactant, is seen and the concentration in the bulk of the film, obtained from RBS, agrees, at least qualitatively, with the model predictions for two of the surfactants tested.

Temperature Dependence of Crack Spacing in Drying Latex Films

Latex dispersions with different glass-transition temperatures have been dried in Petri dishes over a range of temperatures. The crack spacing is reported and is observed to increase as the temperature approaches the glass-transition temperature of the latex. The data for crack spacing over a range of temperatures is collapsed to a single curve by adapting a scaling for the crack spacing of hard particles. The increase in temperature requires an account to be made for particle deformation and, hence, an increase in particle volume fraction, as well as the temperature dependence of the evaporation rate. The importance of measuring the stress at which films crack is highlighted.

Film formation from monodisperse acrylic lattices: Part 3: Drying and ageing of acrylic latex films

The influence of drying/ageing on the structure and properties of acrylic latex films was investigated using turbidity measurements in combination with gravimetry, scanning electron microscopy (SEM), atomic force microscopy (AFM) and water vapour permeability. Ageing above the minimum film formation temperature (MFFT) leads to marked changes in a dried latex film, whereas, after ageing below the MFFT, changes are hardly found. Above the MFFT there is a continuous change in the film properties with time. This becomes obvious from the decreases in the regenerated interference minimum, water vapour permeability and corrugation height. The influence of ageing on the water absorption of the films is less straight forward. It was expected that films with a more compact structure would absorb less water. This is correct for short drying times only, from 0.5 to 3 h. Ageing of better-dried films, however, yields the opposite result: by increase of the ageing time from 3 to 150 h the water uptake increases. There are various reasons for this increase; they are discussed briefly.

Cracking in Drying Latex Films

Thin films of latex dispersions containing particles of high glass transition temperature generally crack while drying under ambient conditions. Experiments with particles of varying radii focused on conditions for which capillary stresses normal to the film deform the particles elastically and generate tensile stresses in the plane of the film. Irrespective of the particle size, the drying film contained, simultaneously, domains consisting of a fluid dispersion, a fully dried packing of deformed spheres, and a close packed array saturated with water. Interestingly, films cast from dispersions containing 95-nm sized particles developed tensile stresses and ultimately became transparent even in the absence of water, indicating that van der Waals forces can deform the particles. Employing the stress-strain relation for a drying latex film along with the well-known Griffith's energy balance concept, we calculate the critical stress at cracking and the accompanying crack spacing, in general agreement with the observed values.

A Model for Surfactant Distribution in Latex Coatings

The presence of surfactants in dried latex films can adversely affect the adhesive, water-resistant, and gloss properties, so investigating the surfactant distribution in latex coatings is of prime industrial relevance. Here we present a model that predicts the distribution of surfactant in a latex coating during the solvent evaporation stage. The conservation equation for surfactant during solvent evaporation is solved in the limit of infinite particle Peclet numbers, a dimensionless quantity giving the measure of relative magnitudes of evaporative to diffusive fluxes. A parametric analysis using the model reveals that the surfactant adsorption isotherm is the determining physical parameter. The model always predicts surfactant excesses at the top surface and either excess or depletion at the bottom surface depending on the isotherm. Uniform distributions are predicted for low surfactant Peclet numbers. Attenuated total reflection Fourier transform infrared spectroscopic probes on film surfaces conform to the behavior predicted by the model.

Surface Morphology of Drying Latex Films: Multiple Ring Formation

When a drop of aqueous solution containing suspended particles dries, particles accumulate at the contact line. Often the contact line undergoes a sequence of pinning−depinning events with particles accumulating whenever the contact line is pinned. These processes lead to ring formation, the details of which are examined here. In particular, we provide detailed measurements of ring formation as a function of the drop radius and the size of the suspended particles, and we report measurements of the time dependence of the essentially stochastic pinning−depinning cycles of the contact line motion.

The Film Formation of Polymer Particles in Drying Thin Films of Aqueous Acrylic LaticesII. Coalescence, Studied with Transmission Spectrophotometry

Abstract The aim of this study is to determine the factors that contribute to the process of film formation of binder particles in drying aqueous dispersion coatings, based on acrylic polymers. It is known that concentrated latices of uniform size show iridescent, colored light patterns. These colors are caused by interparticle interference, and they are only present when the latex particles are ordered in a regular structure. The interparticle interference can be characterized by measuring the transmission as a function of wavelength of the incident light. It appeared that the changes of the interparticle interference of a drying latex film can be related to changes in the interparticle distance and displacement. It was also found that the interparticle distance becomes “negative” upon coalescence of the latex particles. This means that from this point on, the change in interparticle interference is directly related to the indentation or deformation of the latex particles. It became clear that the coalescence process differs from deformation mechanisms accepted in the literature. It seems that the deformation of the particles follows a biaxial mechanism. This means that the particles deform only in one direction, perpendicular to the film surface.

The Film Formation of Polymer Particles in Drying Thin Films of Aqueous Acrylic Latices: II. Coalescence, Studied with Transmission Spectrophotometry

The aim of this study is to determine the factors that contribute to the process of film formation of binder particles in drying aqueous dispersion coatings, based on acrylic polymers. It is known that concentrated latices of uniform size show iridescent, colored light patterns. These colors are caused by interparticle interference, and they are only present when the latex particles are ordered in a regular structure. The interparticle interference can be characterized by measuring the transmission as a function of wavelength of the incident light. It appeared that the changes of the interparticle interference of a drying latex film can be related to changes in the interparticle distance and displacement. It was also found that the interparticle distance becomes “negative” upon coalescence of the latex particles. This means that from this point on, the change in interparticle interference is directly related to the indentation or deformation of the latex particles. It became clear that the coalescence process differs from deformation mechanisms accepted in the literature. It seems that the deformation of the particles follows a biaxial mechanism. This means that the particles deform only in one direction, perpendicular to the film surface.

Different types of water in the film formation process of latex dispersions as detected by solid-state nuclear magnetic resonance spectroscopy

The properties of polymer films prepared from latex dispersions are influenced by the drying or film formation process. In order to investigate this process, various systems of aqueous latex dispersions were dried until a specific solid content was reached. The samples investigated were based on vinyl acetate, vinyl acetate/ethylene and pure acrylics employing different surfactants and polyelectrolytes as stabilisers of the dispersions. The role of water in these partially dried films was investigated using 1H and 2H solid-state NMR spectroscopy. Different types of water could be distinguished in the spectra. The drying latex films were found to contain interfacial external water, water at ionic and nonionic groups at surfactants in the polymer/water interface and also water inside the swollen polymer. These different types of water were examined separately using various NMR techniques.

Self-opacifying aluminum phosphate particles for paint film pigmentation

Opaque particles of aluminum phosphate can be obtained in two ways: self-opacification within drying latex films and thermal treatment at 500–700°C (which was previously described). Self-opacification is easily achieved and allows for extensive replacement of TiO2 as a white pigment, with a gain (in an acrylic latex) in the optical properties of the pigmented film.

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).

The application of dielectric analysis to drying latex films

Measurements of real and imaginary capacitance (C′ andC″) have been made during the drying of a film-forming latex. In one experiment dielectric measurements at frequencies between 1 Hz and 100 kHz were made simultaneously with gravimetric measurements on a microbalance. It was found that both the rate of water evaporation and the a.c. conductance decrease sharply at high polymer volume fraction. These results are discussed qualitatively in terms of a model for the film-forming process. In another experimentC′ andC″ were recorded at 10 Hz along with automatic measurements of the build-up of the scratch resistance of the film. It was found that the mechanical response to film-formation appears significantly earlier than the dielectric response. This is also discussed qualitatively in terms of the model.

Studies on the γ‐irradiation of natural rubber latex

AbstractVulcanized latex was obtained by the γ‐irradiation of natural rubber latex. The optimum cure was attained with ca. 2 × 107 r, in the case of 40% latex. Owing to the acceleration action of the water present, rubber in latex crosslinked more easily than it did in previously dried latex film, but no appreciable difference was found between the crosslinking in latices of various water contents. The protein in the latex deteriorated under γ‐irradiation, but the properties of the latex were not damaged. The maximum tensile strength of the film obtained by drying this irradiated latex was larger than that of solid rubber irradiated in air; this may correspond to the fact that the irradiation, even when carried out in air, brought about no side reactions such as oxidation, etc., of rubber in latex. The aging behavior of the irradiated film was quite superior, the tensile strength being greater after aging than before.

Temperature Dependence of the Mechanical and Stress-Optical Behavior of Elastomers

Abstract The temperature functions of the elastic modulus K2 and of the stress-optical constant K1 or its reciprocal 1/K1 were investigated for several elastomers. In the case of a hypothetical rubber which we have called “ideal” rubber—in analogy to gases—theory requires a direct proportionality between K2 or 1/K1 and the absolute temperature. The temperature functions of K2 and 1/K1 which we found by experiments with “real” elastomers show characteristic negative and positive deviations Δa2 and Δa1 from “ideal” values. When we put these values of Δa2 and Δa1 into a coordinate system, we find a certain orderly arrangement of the different elastomers, which allows us to picture a relationship between molecular structure and the values of Δa2 and Δa1. This brings up the possibility of explaining the experiments with the help of already known molecular-physical concepts. Although other explanations are conceivable the attempt is made to develop the simplest and most obvious ideas. It is conjectured that negative values of Δa2 and Δa1 come about from a loosening of secondary valence bonds—in certain ways, like crystal bonds— between neighboring molecules. Negative Δa1 values were found only in the crystallizable elastomers. It is further conjectured that positive values of Δa2 and Δa1 may result from the liberation by heat, of blocked, bulky molecular segments. These molecular segments can then contribute to the entropy elasticity only at higher temperatures. Positive Δa2 and Δa1 values are found chiefly in strongly crosslinked elastomers. Brief attention is given to the physical processes which are responsible for the elongation—double refraction and the entropy-elasticity. From this, it seems that the stress-optical constant and its temperature function are connected with properties of the molecular chains and on their orientability and crystallizability. The elastic modulus and its temperature function are strongly affected by the structure of the network and the molecular cohesive forces. Worthwhile hints about crystallization tendency, polarity and degree of symmetry of the different systems are given by the Δa1 and Δa2 values in the above mentioned coordinate systems. Natural rubber was tested in different recipes. The results of milling, of sulfur and accelerator additions, of time and temperature of vulcanization, on the values of K2, 1/K1, Δa2 and Δa1 were all investigated. The values of 1/K1 are at their highest level for dried latex films (unvulcanized). Milling and vulcanization, particularly the use of rather long periods and high temperatures, lower the value of 1/K1. A drop in the value of 1/K1, which regularly appears with a reduction of the negative Δa1 value, is explained as a loosening of secondary valence molecular couplings. According to this, natural rubber in the latex state is most strongly associated. According to this explanation, stretching in the unvulcanized condition is sufficient to loosen the secondary valence molecular bonds. Milling and vulcanization also act to loosen the linkages. Secondary valence bonds which are loosened by warming, as a general rule, are reestablished by prolonged cooling. It is to be supposed that the secondary valence molecular bonds under consideration are limited to small regions, somewhat comparable to the ordering in liquids. With an increasing degree of vulcanization, the Δa2 values go through a maximum which perhaps coincides with the condition of optimum vulcanization. This is explained as a maximum of the entropy-elasticity. In the case of slightly milled natural rubber which is appropriately vulcanized, the value of Δa2 can become practically zero. The change of the elastic modulus with temperature then is “ideal.” Nevertheless, no “ideal” rubber exists here, for Δa1 is less than zero.

Preparation of Dried Latex Films

Abstract Various technical applications of latex call for the formation of a latex film and its drying by evaporation of the water in which the rubber is dispersed. The same problem is encountered also in the preparation of dried latex films, both uncompounded and compounded, for film testing for tensile, elongation, and other properties. Very frequently the preparation of such films is complicated by the development of surface cracks and ridges in the drying films, which mar the appearance of the films, detract from their strength, and make testing difficult and uncertain. These imperfections in the films generally arise from the formation of impervious surface skins on the drying latex. Such impervious skins lead to shrinkage and subsequent cracking of the films on the one hand, and to bubble formation on the other. The result is that the preparation of dried latex films by this method becomes a very slow process, which yields as a rule films of inferior appearance and quality. It was found in this laboratory that many of the difficulties encountered in the preparation of latex films by evaporation can be overcome by appropriate control of humidity in the initial drying stages, and the use of an agent which prevents formation of the impervious surface skins on the drying latex. The agent found to be most suitable with alkaline latexes for the latter purpose is carbon dioxide gas. By utilizing moisture and carbon dioxide in a definite technique, films can be prepared from various latexes which are smooth and uniform in thickness and possess improved appearance and properties. The purpose of this paper is to describe the technique developed for preparation of dried latex films by casting, and to give some results for tensile and elongation obtained on GR-S, Hycar, Neoprene, and natural latexes.

Preparation of Dried Latex Films

Abstract Various technical applications of latex call for the formation of a latex film and its drying by evaporation of the water in which the rubber is dispersed. The same problem is encountered also in the preparation of dried latex films, both uncompounded and compounded, for film testing for tensile, elongation, and other properties. Very frequently the preparation of such films is complicated by the development of surface cracks and ridges in the drying films, which mar the appearance of the films, detract from their strength, and make testing difficult and uncertain. These imperfections in the films generally arise from the formation of impervious surface skins on the drying latex. Such impervious skins lead to shrinkage and subsequent cracking of the films on the one hand, and to bubble formation on the other. The result is that the preparation of dried latex films by this method becomes a very slow process, which yields as a rule films of inferior appearance and quality. It was found in this labo...

Discoloration of Dried Latex Films

Abstract In the course of work on the application of latex to textiles, a staining effect was encountered which is the subject of this short note. Briefly stated, an uncompounded concentrated latex was applied to a cotton fabric; some months after manufacture, pink-brown stains developed. The appearance of the stain suggested iron, and both the circumstances and further facts indicated that light was a likely contributory factor. It was established that oxalic acid would not remove the stain, although this fact did not entirely rule out iron, since the rubber possibly prevented access of the oxalic acid. Furthermore, the stain was not removed by acetone, nor was it affected by a soap and soda scour, whereas if the fabric was treated with cobalt linoleate solution so as to oxidize the rubber, the stain was removed with the rubber in the course of subsequent scour. This suggested that the stain was associated with the rubber. An investigation was therefore commenced on the effect of light on the color of dried films of latex in the absence, and in the presence, of iron. The results obtained appear of some interest, and have revealed a phenomenon not previously recorded as far as the author is aware.

Sol and Gel in Hevea Latex and Crude. Rubber Influence of Oxidation on Gel-Sol Transformation

Abstract An apparatus and procedure to determine the amounts of sol and gel rubber in dried latex films and crude rubber have been described. It is shown that whereas plantation smoked sheet and crepe contain large and varying quantities of rubber soluble in petroleum ether, dried rubber films from latex are practically free from sol rubber when oxidation has been avoided. Latex increases in sol rubber content when subjected to various treatments permitting oxidation of the hydrocarbon. It appears from this work that the rubber in the form of latex is very susceptible to oxidation, which however involves only minute quantities of combined oxygen. When latex film rubber is subjected to creeping, it shows an increased content of sol rubber corresponding to that of crepe. A short milling of crepe was found to convert it completely into sol rubber. This confirms the gel-sol transformation mechanism of breakdown. Gel rubber from dried latex films is swelled to a maximum of 1500 to 7800 per cent but is not dissolved when placed in benzene, chloroform, carbon tetra-chloride or carbon disulfide and allowed to stand for several days. Gel rubber becomes soluble and therefore diffusible in these solvents as a result of oxidation. It was found that dried latex film and various types of crude rubber swelled to a maximum of 1000 to 2400 per cent in petroleum ether and that this swelling was not proportional to the sol content. This investigation strengthens the belief that sol rubber is an oxidation product of gel rubber. The formula for sol rubber is suggested as (C5H8)nO2, and a relationship between the molecular weight and the oxygen content is postulated on the basis that oxygen acts as a molecular splitting agent for both gel and sol.

Color Measurement and Its Application in the Rubber Industry

Abstract The object of this paper is to bring to the notice of rubber technologists the recent advances which have been made in the science of color measurement and its applications. As the demand for colored articles increases, it becomes necessary to use a more specific way of recording color than to say that an article is “rose pink” or “olive green.” The use of standard samples is cumbersome in regard to storage, and may be misleading in the case of rubber products, as they are liable to color changes. The increasing use of raw rubber in the form of latex has raised the question of the measurement of its “whiteness” as a guide to its quality. It has been suggested also that the transparency of thin dried latex films could be used for this purpose. Both these methods entail the measurement of the color of almost white objects. MacAdam has shown that it is difficult to judge the whiteness of articles by direct vision. He asked 30 observers, half of whom were women, to arrange white objects in order of increasing whiteness. The arrangements showed great divergence, and the observers were unable to repeat their observations. When these samples were classified by means of a suitable colorimeter, consistent and reproducible results were obtained by different observers. Visual observations are subject to the effect of many variables, and are therefore so misleading that it at once becomes apparent that such determinations can only be successfully carried out with a colorimeter used under carefully standardized conditions. Besides the measurement of the whiteness of latex such problems arise as the covering power of pigments, especially of white pigments, the use of dyestuffs in brightly colored articles, the transparency of rubber lacquers and varnishes, all of which require a measurement of color in some form or another.