Distribution of water-soluble and surface-active low-molecular-weight species in acrylic latex films

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Abstract

Monodisperse core–shell latices were synthesized, differing in the acrylic acid (AA) content in the particle shell (1 or 4 wt%) and the Tg of the acrylic core (around −40 or 10 °C). In a first step, the drying mechanisms of the dialyzed latices were studied by confocal Raman spectroscopy. It was shown that, besides some unexpected features (briefly described in the article), drying occurred in a rather classical way, i.e., simultaneously from top to bottom and from edge to center. Then, the distributions of sulfate ion (SO4) (from sodium sulfate) and sodium dodecyl sulfate (SDS) in the dry latex films were established by confocal Raman spectroscopy and attenuated total reflectance (ATR). The two techniques were complementary. SO4 and SDS distributions were quite different, although presenting some common characteristics. In both cases, repartition of the low-molecular-weight species in the film was even less homogeneous when the AA content was lower and the particle core softer. However, SO4 showed enrichment at the film–substrate interface and depletion at the air side, whereas SDS showed concentration maxima at both interfaces. Interpretations stress the importance of desorption from the particle–water interface, transport by water, size effects, and diffusion.

Introduction

Matching the properties of solvent-borne polymeric films with water-borne systems for applications such as paints, adhesives, and all kinds of coatings is a major issue for industry. This requires further improvements in the understanding of film formation mechanisms from polymer colloids. A huge effort has already been devoted to this topic, as attested by recent review articles [1], [2], [3] containing hundreds of references. However, because of the diversity of the systems and complexity of the phenomena involved, a lot remains to be done. A good overview of the current state of the art and of the main open questions can be gained through the reading of a recent book on film formation in coatings [4]. In particular, there is a continuing effort to characterize and interpret data on the distribution of low-molecular-weight species, such as surfactants, in latex films. Started as early as 1936 [5], studies on this question can be found in the literature in the eighties [6], [7], a lot in the nineties [8], [9], [10], and still in the most recent times [11], [12], [13], to quote some representative examples.

There are several difficulties in the problem of the distribution of small molecules in latex films, especially when they are surface-active and/or soluble in water. The first problem is that distribution is closely related to drying mechanisms of polymer colloids, a very difficult topic in itself. Polymer colloids almost never dry homogeneously. The distribution of water in the system during drying is highly heterogeneous with drying fronts moving in directions either parallel or normal to the film surface, or, probably more commonly, in both direction at the same time, but not at the same rate. Furthermore, the factors controlling the drying mechanisms are still poorly understood [14].

Second, if convective transport of water soluble species by water occurs and plays a major role [12], diffusion, clearly, also takes place. The relative importance of diffusion versus convection during drying is completely unclear. After elimination of water, the only possible transport mechanism is diffusion. Evolution of the distribution in the dry state can be spectacular. It involves, in some cases, an unexpected diffusion of the small molecule through the polymer phase in which it is totally insoluble.

Before being transported by convection or diffusion, a molecule, like a surfactant, adsorbed at the particle–water interface has to be desorbed. When does desorption occur? And to what extent? These questions are related to the problem of coalescence mechanisms in emulsions, still under investigation [15].

The small molecule sometimes has a limited solubility in the polymer phase. This also influences its distribution. For instance, ethoxylated surfactants are known as partial plasticizers for acrylic polymers. The characterization of the compatibility of the low molecular weight species with the polymeric matrix is often difficult. A measurement of shifts in the glass transition temperature of the polymer due to the presence of the small molecule is sometimes used as a way to assess limited compatibility.

Foreign interfaces in contact with the latex or with the film, such as the film–substrate interface or the interfaces with a filler present in the system, have a profound effect on the distribution of surface active species. It has been shown by Evenson and Urban [16] that the nature of the substrate influences the migration of surfactants toward this interface. These authors claimed that interfacial tension was the key parameter. However, it is well established that, very often, there is far more surfactant present at the interface than needed to minimize the interfacial tension. Why is the surfactant still attracted or driven to the interface even when the interfacial tension is already minimal? This is another open question.

In the paper quoted above [16], Evanson and Urban also showed that the mechanical story of the film is also a parameter influencing surfactant distribution. This is a quite unexpected and troublesome additional difficulty, an issue never addressed again by any authors.

Finally, in this list of difficulties, one has to mention the problems related to experimental techniques. Despite of the continuous developments of improved techniques, there is still a lack of sensibility and resolution. If the surfaces of latex films (film–air and film–substrate interfaces) have been extensively examined by several means such as scanning electron microscopy [17], AFM [18], and spectroscopic techniques based on infrared reflection [7], little is known about the distribution inside of the films. Urban mode many efforts to apply photoacoustic spectroscopy to coatings, starting several years ago [19]. Interesting results were obtained (see, for example, Ref. [20]). However, this technique seems to suffer from severe drawbacks: it is not properly quantitative and depth profiling is only possible through analysis of layers of increasing thickness, over a thickness range limited to around 20 μm.

Under special circumstances [12], it was possible to use infrared microscopy to follow water, surfactant, and polymer all over the film thickness. This method is tedious and only adapted to very thick layers. On the other hand, Keddie and co-workers used magnetic resonance imaging (MRI) to study drying mechanisms of waterborne alkyd emulsions [21]. One should be able to adapt this technique in order to obtain data on the distribution of water and other species at the same time. The resolution, currently around 10 μm, should also be improved, if possible. Ideally, one aims at a technique giving information on the distribution of water, small molecules, and polymers at the same time during drying with the highest sensitivity and resolution, and quickly, in order to follow fast-drying systems.

In the work presented in this paper, we tried to address some of the problems mentioned above. First, latices were prepared with narrow particle size distributions and a core–shell structure with a high concentration of acrylic acid in the shell and a hydrophobic core of various stiffness (also acrylic in nature). These latices were extensively purified before addition of controlled amounts of low-molecular-weight species: either sodium sulfate or sodium dodecyl sulfate (SDS). One species, the sulfate ion, is small in size, highly water-soluble, and non-surface-active. It should try to remain in water as much as possible. The second one, SDS, is larger in size and hydrophilic, but its essential characteristic is its high surface activity. Distributions of these species in latex films were established using attenuated total reflectance (ATR), Fourier transform infrared spectroscopy (FT-IR), and confocal Raman spectroscopy. From the comparison of the distributions of these two species in latex films containing various amounts of acrylic acid and with various particle core stiffness, we expected to be able to draw interesting conclusions.

Section snippets

Latices

All reagents were from Merck (Darmstadt, Germany). The monomers butyl acrylate, methyl methacrylate, and acrylic acid were purified by filtration through basic alumina powder. The surfactant, sodium dodecyl sulfate (SDS) (purity over 99%), and the initiator, amonium persulfate ((NH4)2S2O8) (purity 99%), were used directly from the bottle. Distilled water was used throughout.

The latex syntheses were performed by a semicontinuous process, in a double-wall 1.5-l glass reactor, under a nitrogen

Drying

The concentration of water along the z-direction of a BuA 1 latex drying on a polyethylene substrate and its evolution with time was followed by confocal Raman spectroscopy (Fig. 2). There are two remarkable features in this figure. First, it shows that the concentration of water goes through a maximum at middle height in the film. There is less water on top and on bottom than in the middle of the film. The same effect was observed with the three other latices (the figures are not shown because

Conclusion

This work, once more, stresses the tremendous importance of drying mechanisms for understanding distributions of small molecules in latex films. As discussed above, one needs to know, and this is only one example, how long liquid water is present in the uppermost layers of the film during drying. Precise, high-resolution, and fast-acquired data are required. Currently, the two most interesting techniques, to our opinion, are magnetic resonance imaging and confocal Raman spectroscopy. Both have

Acknowledgements

We thank Rhodia for financial support. It is also a pleasure to thank Drs. M. Dorget, J.F. d'Allest, C. Bonnet-Gonnet, and B. Amram (Rhodia, Research Center Aubervilliers, France) for helpful discussions.

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