Mapping the route from wet to dry

New test methods reveal details of waterborne nanocomposite drying

NAPOLEON Film Formation Team 1*

Film formation in emulsions is a complicated, multi-stage process that is a fundamental requirement for the creation of a binder in coatings. Good film formation ensures that the loss of the separate emulsion particles yields desirable properties, such as high barrier resistance to water and solvents, durability, scratch resistance, transparency and gloss.

Film formation involves three processes which are inter-related and can overlap in time. Firstly, evaporation of water leads to an increase in the solids content and particle packing. Secondly, the spherical particles are deformed under the action of interfacial forces or capillary pressure so that the particles fill up all available space. Finally, interdiffusion across the particle boundaries erases the particle identity so that the film is homogeneous.

The use of nanocomposite particles opens up new possibilities for the formation of films in which two distinctly different phases, such as a silicate and an acrylic, are mixed at the nano-scale. Control of the film formation process is then essential.
In a nanocomposite coating, there are several potential complicating factors to consider, such as the arrangement of the two phases, phase separation, and the possibility of the blocking of particle interdiffusion by an inorganic phase.
Experimental studies of the film formation process present challenges because it is a dynamic process. In order to observe film formation as it happens, experimental techniques should be non-invasive, fast and able to maintain the emulsion in its wet state. This latter requirement restricts the analytical techniques to operating temperatures above 0 °C and does not allow the use of vacuum.
Here, some of the techniques developed within the EC Framework 6 Integrated Project, "NAPOLEON" for the study of drying the first stage of the film formation process are considered.
The NAPOLEON ("NAnostructured waterborne POLymEr films with OutstaNding properties") project brought together seven companies and 13 universities and research centres, as listed in Table 1, to develop a technology platform to create waterborne nanostructured films for applications in coatings, adhesives, cosmetics and additives for textiles.

The measurement of stresses is of paramount importance in solving the problems of film buckling and cracking. There is a need for a good understanding of how stresses are generated, how they relax, and how the development of stress interrelates with the drying speed, the movement of the drying front, the deformability of the particles and their internal structure. In this project, stress measurements were performed by two separate novel instruments.
In a beam-bending experiment [1] the emulsion film is deposited on a flexible metal sheet, which has a mirror fixed to its underside. A laser beam deflected from this mirror hits a position-sensitive detector, which supplies a DC output voltage that is proportional to the deflection of the beam. As the substrate laterally constrains the shrinking of the film, an in-plane stress develops if the films contracts during the film formation process. This stress bends the substrate upwards [2].
This is a simple and convenient way to measure film stresses. Its limitation is that lateral variations in drying are not identified. Spatially-resolved techniques therefore give a more detailed picture. The development of both tensile and dilational stress during drying can be mapped in the plane of the film using the new technique of membrane bending [3].
The sample is deposited on a flexible membrane which warps under the influence of surface stress. Using the back of the membrane as a mirror, one can infer the stress distribution in detail from the distortion of the membrane (Figure 1a). An array of gridlines is imaged across the membrane. The apparent displacement from the original rectangular grid is proportional to the slope of the membrane at the respective point, which is proportional to the gradient of the surface stress, Ds(x,y).
Typically, there is a stress concentration at the edge of a film, but the stress is low elsewhere. Figure 1b/c shows data obtained during the film formation of a clay/acrylic nanocomposite emulsion. The stress pattern is rather complicated. There are concentrations of stress, which lead to cracks at later stages in the drying process. Using this instrument, the crack patterns as visualised in optical images can be correlated with the stress distribution.

Photon correlation spectroscopy (PCS) is a classic tool to investigate the dynamics of colloidal media [4]. In sufficiently dilute solution, the diffusion coefficient and the hydrodynamic radius of particles can be determined. For strongly interacting systems, such as emulsions, the concentration fluctuations have to be interpreted as modes of the collective dynamics.
The relaxation time of these modes diverges if the sample goes through a glass transition (also termed "gel point"). Within this project, the technique has been developed so that it can be applied to drying colloidal films.
The instrument was designed to be simple and flexible [5]. It consists of a laser, the sample, an optical fibre to pick up the scattered light, a photomultiplier, and some electronic equipment producing the autocorrelation function (see Figure 2). The sample can be investigated from the top or bottom. Looking from the top or from the bottom, one investigates the filmair interface or the filmsubstrate interface, respectively.
Multiple scattering (indicated by turbidity) introduces a complication. In the initial stages, the film is completely turbid and the experiment occurs in the "diffusing wave spectroscopy" (DWS) limit [6]. In DWS experiments, the value of the scattering angle is not critical, and therefore there is no need to control or measure the angle with high precision.

A development of NMR profiling known as GARField provides information non-invasively on the water concentration in a drying film both as a function of depth and as a function of time. It is then possible to determine whether drying is uniform with depth or whether there is "skin formation" (particle packing and coalescence in a surface layer lying above a wet layer).
Recent experiments have shown that the water concentration gradient agrees with the predictions of a drying model [7]. Also, where there is mobility in a polymer phase (such as an oligomer or an uncrosslinked resin), then an NMR signal is detected from that phase in addition to the water phase. Variations of molecular mobility with depth in a film, such as those resulting from crosslinking, can then be determined.
The technique works through the application of a gradient in the magnetic field across the sample to encode spatial position through the dependence of the resonant frequency on the local magnetic field strength [8].
There is a trade-off between the time of acquisition and the level of noise in a profile. Longer data acquisition times lead to less noisy profiles, but are less effective in probing dynamic systems. Depending on the choice of parameters, it is possible to achieve pixel resolutions as low as 6 µm in one dimension i.e. along the normal to the substrate, which is sufficient for the study of most coatings.
A strong NMR signal is obtained from mobile molecules such as water, whereas no signal is obtained from a polymer near its Tg. By always choosing the same NMR parameters, direct comparisons of profiles for different samples are possible.

Figure 3a shows representative GARField profiles obtained from a clay/acrylic emulsion. This drying pattern is what is desirable and "normal". The film thickness decreases at a constant rate as the water evaporates and the concentration of water is uniform as a function of depth from the surface.
In strong contrast, in an emulsion with excess surfactant concentration (Figure 3b), the film thickness increases over time in the later stages of drying. A linear concentration gradient develops from the top to bottom of the film, with less water at the film surface. This unusual behaviour is tentatively explained by the lateral flow of water and particles to the film centre. A surfactant concentration gradient develops laterally across the film and drives flow.
The PCS technique has been recently extended by combining it with NMR profiling, so that both the particle dynamics and the water distribution within a drying film can be determined simultaneously.

The high vacuum required by conventional scanning electron microscopy (SEM) has always prevented the direct study of wet samples and hence the dynamic study of film formation. Environmental SEM (ESEM) is a development that allows hydrated samples to be observed, though under relatively low pressures.
The first reported work on the film-forming process in an environmental electron microscope were performed in reflection mode in an ESEM [9, 10]. This method faces several restrictions: only the top layer of the sample surface is revealed, and the smallest particles imaged are around 300-600 nm.
Within the current project, emulsions have been imaged from the wet state through to the dry state, using wet scanning transmission electron microscopy (STEM) performed in an ESEM [11]. This new technique allows in-situ observations of emulsion film formation, with especially good resolution and volume information.
The specially-designed electron gun column and electron detector of an environmental scanning electron microscope enable the sample chamber pressure and temperature ranges to be set to allow the observation of wet samples in their natural state by keeping the water vapour pressure above the saturated value.
The liquid state of water can be maintained in a chamber at 2 °C and 5.3 Torr vapour pressure. Decreasing the pressure or increasing the temperature controls the evaporation of the sample and enables particles to deform and film formation to occur.
In an experiment, a droplet of emulsion dispersion is placed on a carbon TEM (transmission electron microscopy) grid through which the electron beam passes. It is essential to use a diluted emulsion suspension (ca. 0.05 % solids content) in order to obtain a sample thickness that provides enough transmitted electrons to form an image.
The backscattered electron detector (BSED), which is usually located above the sample in reflection mode, is fixed under this grid in order to produce a transmission image (bright or dark field). In annular dark field imaging, the BSED is centred on the electron beam axis, so that electrons from the direct beam are not collected.
This configuration ensures an image contrast sensitive to variations of mass and thickness. The thicker the material and the heavier the elements, the brighter is the image.

A sequence of images during the film-forming process of an acrylic emulsion has been obtained using wet-STEM within the ESEM. Figure 4a shows the emulsion after water evaporation. The temperature in the microscopy examination is below the minimum film formation temperature (MFFT) of the emulsion. Thus, the particles are in contact but not deformed.Figure 4b shows the morphology of the sample after a rise of temperature above the MFFT. The particles are no longer distinguishable; film formation has occurred. Bright interfaces (attributed to surfactant layers) are still visible. The observation of the same grid after a few days at room temperature shows that the bright interfaces are converted into bright lumps throughout the film, which means that the surfactant segregates while interdiffusion of the particles occurs.

Recent modelling carried out within the project has identified instabilities that lead to an island-like distribution of surfactant laterally across a film surface [12]. The vertical distribution is affected by the drying conditions and the adsorption isotherm of the surfactant, according to recent models and experiments [13].
Confocal Raman Microscopy (CRM) can reveal the distribution of surfactant both laterally and vertically in dry films. Sodium dodecyl sulfate (SDS) is Raman active and can be distinguished from acrylic copolymers. Hence, it has been used as a model surfactant.
At the film/substrate interfaces, two problems lead to a decrease in precision. First, the interface is not optically visible because the refractive indexes of the polymer and glass are too similar. Second, the depth resolution gets worse as the focus moves deeper within the sample [14]. In this case, FTIR attenuated total reflectance spectroscopy can be used to obtain complementary information near the interfaces.
In conclusion, the myriad of techniques that has been developed within the past few years hold great promise for providing insight into the film formation of emulsion and waterborne nanocomposite particles. The combined use of techniques is particularly powerful.

Funding is gratefully acknowledged from the EC Framework 6 project, NAPOLEON (under contract IP 011844-2). The nanocomposite emulsions were kindly provided by NAPOLEON partners at the University of the Basque Country, Spain, at BASF, Ludwigshafen, Germany, and at LCCP, CNRS, Villeurbanne, France.

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