Rate-Limiting Steps in Film Formation of Acrylic Latices as Elucidated with Ellipsometry and Environmental Scanning Electron Microscopy

Abstract

Our data indicate that evaporation is the rate-limiting step in film formation when the temperature of a latex is about 20 Κ or more above its glass transition temperature (T g ). When a latex is nearer to its T g , the rate-limiting step in film formation is deformation of the latex particles, possibly by viscous flow of the polymer driven by the reduction in surface energy. In this latter case, there is evidence that a drying front first creates air voids. Subsequently, a coalescence front moves inward from the periphery in the plane of the film. Evaporation rates are retarded in a latex that is well-above its T g , probably as a result of the reduced surface area of water, caused by extensive particle deformation. We studied the kinetics of film formation in an acrylic latex using ellipsometry and environmental-SEM, techniques which allow in situ observation of wet and partially-wet latices. We fit our data to a model describing the coalescence of voids by viscous flow.We have employed methodology (similar to what we used previously (7)) to determine the effect of temperature on the kinetics of film formation. We correlate our experimental findings with the kinetics of the passage of drying and coalescence fronts in the plane of the film and normal to its surface.In our previous publication (7), we studied the kinetics of film formation of acrylic latices, paying close attention to the effects of the glass transition temperature (T g ). Using a combination of ellipsometry and environmental scanning electron microscopy (ESEM), we identified the four conventional stages of film formation. We found that

Determining the compacting of latex films upon drying by interference measurements — an approach for the investigation of film formation

“…Further film formation may then proceed through a dry sintering mechanism driven by the polymer surface tension [11,12] or not at all if particles are too rigid. If the particles do not deform fast enough they will dewet.…”

“…Light-turbidimetry [10,24], interferometry [25] and evanescent dynamic light scattering [26] have been used to determine particle spacings and mobility in drying films. ESEM [11,20] and AFM [27 -29] have been applied to characterize deformation and coalescence of particles at the surface of films. Some authors observed the rupture of hydrophilic membranes and segregation of surfactant and concluded that they play an important role in the process of film formation [3,30].…”

“…In homogeneously drying ones, the solids content increases equally throughout the whole film [1 -3]. Both lateral and vertical heterogeneous drying have been considered as the same phenomenon [11]. Besides lateral, some authors describe vertically heterogeneous drying, i. e. skin formation at the surface [5].…”

Determining the compacting of latex films upon drying by interference measurements — an approach for the investigation of film formation

“…If the particles do not deform fast enough they will dewet. Further film formation may then proceed through a dry sintering mechanism driven by the polymer surface tension [11,12] or not at all if particles are too rigid. In the cases where particle deformation is not the rate-limiting process, particle deformation proceeds along with water evaporation and is hence driven by capillary effects and by polymer -water interfacial tension [13 -16].…”

“…Such a skin may be a porous, dry layer [9] or wet [10]. Both lateral and vertical heterogeneous drying have been considered as the same phenomenon [11]. Scheme 1.…”