Comparison of Surfactant Distributions in Pressure-Sensitive Adhesive Films Dried from Dispersion under Lab-Scale and Industrial Drying Conditions

Film-forming latex dispersions are an important class of material systems for a variety of applications, for example, pressure-sensitive adhesives, which are used for the manufacturing of adhesive tapes and labels. The mechanisms occurring during drying have been under intense investigations in a number of literature works. Of special interest is the distribution of surfactants during the film formation. However, most of the studies are performed at experimental conditions very different from those usually encountered in industrial processes. This leaves the impact of the drying conditions and the resulting influence on the film properties unclear. In this work, two different 2-ethylhexyl-acrylate (EHA)-based adhesives with varying characteristics regarding glass transition temperature, surfactants, and particle size distribution were investigated on two different substrates. The drying conditions, defined by film temperature and mass transfer in the gas phase, were varied to emulate typical conditions encountered in the laboratory and industrial processes. Extreme conditions equivalent to air temperatures up to 250 °C in a belt dryer and drying rates of 12 g/(m(2)·s) were realized. The surfactant distributions were measured by means of 3D confocal Raman spectroscopy in the dry film. The surfactant distributions were found to differ significantly with drying conditions at moderate film temperatures. At elevated film temperatures the surfactant distributions are independent of the investigated gas side transport coefficients: the heat and mass transfer coefficient. Coating on substrates with significantly different surface energies has a large impact on surfactant concentration gradients, as the equilibrium between surface and bulk concentration changes. Dispersions with higher colloidal stability showed more homogeneous lateral surfactant distributions. These results indicate that the choice of the drying conditions, colloidal stability, and substrates is crucial to control the surfactant distribution. Results obtained under lab-scale drying conditions cannot be transferred directly to the industrial application. The results were similar for both tested adhesive material systems, despite their different properties. This indicates that other properties, such as the particle size distribution and glass transition temperature, have surprisingly little effect on the development of the surfactant distribution.

Coat formation of surface-active proteins on aqueous surfaces during drying

Segregation of the protein bovine serum albumin (BSA) and lactose in thin aqueous films during drying was investigated by examining the composition of the dried films using inverse micro Raman spectroscopy (IMRS) and X-ray photoelectron spectroscopy (XPS) sputter-depth profiling. The composition was uniform through the thickness of the dried films except within a 10nm region at the exposed surface where BSA had accumulated, most likely due to its surface activity. The thickness of the BSA layer was similar to the diameter of a BSA molecule, which suggests that a single monolayer of BSA adsorbed at the exposed surface. The BSA surface concentration of the dried films was constant over a wide range of BSA bulk concentrations, indicating that the aqueous surface became saturated with BSA during drying. The BSA surface layer of order 10nm was significantly thinner than the film thickness of order 10 μm, which implies that BSA formed a surface coating rather than a shell, and thus lent no structural rigidity to the film.