Simulation of Vertical Surfactant Distributions in Drying Latex Films

Following our previous contribution ( Gromer, A. et al. Langmuir 2015 , 31 , 10983 - 10994 ) presenting a new simulation tool devoted to particle distributions in drying latex films, this Article describes the prediction of surfactant concentration profiles in the vertical direction during the complete film formation process. The simulation is inspired by cellular automata and equations by Routh and co-workers. It includes effects that were not considered before: surfactant convection by water and surfactant desorption upon particle deformation. It is based on five parameters describing the nature of the polymer/surfactant system and on film formation conditions. In particular, the viscoelastic properties of the polymer were taken into account through the λ̅ parameter introduced by Routh and Russel. Results show the importance of convection by water and the influence of the particular deformation mechanism on the final surfactant distribution. Excesses or depletions can be predicted either on the surface or on the substrate sides, in qualitative agreement with the numerous existing experimental studies. The complex interplay between parameters governing surfactant distributions makes the results unpredictable without the help of such a simulation tool. Therefore, it should be of interest to both industrial and academic scientists.

Characterizing the distribution of sodium alkyl sulfate surfactant homologues in water-based, acrylic pressure-sensitive adhesive films

The distributions of three sodium alkyl sulfate surfactants in dry adhesive films cast from water-based latexes were characterized using confocal Raman microscopy (CRM) and contact angle (CA) and tack measurements. Sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate (STS), and sodium octadecyl sulfate (SODS) were added to dialyzed commercial adhesive latex at various concentrations. Uneven distributions were found for all three surfactants along with a tendency to enrich film-air interfaces and, to a much lesser extent, film-glass interfaces. SDS demonstrated the greatest tendency to concentrate near film surfaces followed by STS and SODS. For all three surfactants, water CA values for dried films decreased sharply with increasing concentrations in the latex, but significant differences were observed, with SDS again having the greatest impact followed by STS and SODS. Tack of dried polymer films was also found to decrease with increasing latex surfactant levels, with SDS producing the sharpest drop as well as the lowest plateau values. Results indicate that interfacial enrichment by surfactants is detectable via both CRM and CA measurements, and this enrichment can significantly affect the performance of films. Finally, surface enrichment levels are qualitatively related to measures of the surfactants' affinity for aqueous solutions, as characterized by the logarithm of their 1-octanol-water distribution coefficients (K(ow)).

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.

Influence of selected surfactants on the tackiness of acrylic polymer films

Anti-tacking agents are always necessary in polymeric film coating formulations in order to prevent substrate agglomeration. The objective of this study was to investigate the abilities of certain nonionic surfactants in a group of sorbitan ester in reducing the tackiness of the films obtained from aqueous acrylic polymer dispersions (Eudragit), compared with those of talc and glyceryl monostearate (GMS). The results from the peel tests demonstrated that GMS, Span 60 and Span 40 could significantly reduce the tackiness of both Eudragit NE 30D and Eudragit RS 30D films. The mechanisms in reducing the film tackiness were investigated by analyzing the film compositions, using attenuated total internal reflectance infrared spectroscopy (ATR-IR) and optical microscopy. The storage modulus of the films was also examined. The results indicated that GMS, Span 60, and Span 40 could reduce the film tackiness by decreasing the polymer contents at the film surfaces, resulting in a notable reduction in the contact area of the polymers between the surfaces. The use of only 5% (w/w) of either GMS, Span 60 or Span 40 in the coating formulations is enough to prevent pellet agglomeration without adverse effects on film flexibility. The pellets coated with Eudragit RS 30D/RL 30D (9:1, w/w) did not exhibit any difference in the drug release profiles when either 100% (w/w) talc or 5% (w/w) GMS was used, whereas the formulations containing Span 60 or Span 40 gave a slightly faster release rate.

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

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 degrees 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.

An overview of polymer latex film formation and properties

The literature on polymer latex film formation has grown enormously in recent times--driven by the need to find alternatives for solvent-based systems with their adverse environmental impacts. Although greater insight has been shown by the use of modern instrumental techniques such as small angle neutron scattering, direct non-radiative energy transfer and atomic force microscopy, the actual mechanisms involved in deforming spherical particles into void-free films are still the subject of controversy and debate. Surfactant-free homopolymer model colloid latices, favoured in academic studies, together with latices containing surfactants whose redistribution can influence film properties, and the more complex copolymer, blended, core-shell and pigmented systems needed to satisfy a full range of film properties are all considered.