Direct thickness measurement of doctor-bladed liquid film on gravure roll surface

We provide experimental evidence that the liquid film remaining on a doctor-bladed gravure roll surface significantly influences the liquid transfer from the engraved roll surface to a moving substrate. The local free surface profiles of doctor-bladed liquid films were directly measured on both tri-helical grooves and lands (the nongrooved areas between grooves) at different roll speeds and blade thicknesses. The liquid film covered the roll surface with a finite thickness on the lands below the critical capillary number. An increase in the capillary number led to a transition from a fully filled to a starved configuration, in which the liquid barely remained on the lands and partly filled the grooves with a concave surface profile. The liquid transfer ratio monotonically decreased with a decrease in the liquid thickness on the gravure roll surface, obeying a single curve for different blade thicknesses. Furthermore, the decrease in liquid thickness promoted a flow instability transition from unstable dripping though a stable state with a uniform surface to a cascade.This paper was presented at the 17th International Coating Science and Technology Symposium, September 7-10, 2014, in San Diego, CA (USA)

Adsorption-mediated nonlinearity of critical cracking thickness in drying nanoparticle–polymer suspensions

We examine the critical cracking thicknesses of as-dried colloidal TiO2 suspensions containing water-soluble polymeric additives, above which cracks spontaneously propagate. In contrast to common observations, the critical cracking thickness increases in a nonlinear, stepwise manner as increasing the concentration of the polymeric additive in the suspension. The critical thickness diverges when the polymer content increases above a certain threshold. We demonstrate that the first and second critical cracking thickness increments are attributable to the onset of polymer adsorption and a subsequent transition in the conformations of the adsorbing polymer chains, respectively. The critical thickness profiles with respect to normalized polymer adsorption amounts at different particle diameters converged into a single master curve. These results illustrate nontrivial evaporation processes in which the various roles of polymeric additives delay cracking into thicker films

Drying paths of phase-separating solution coatings exposed to humidity

Drying process paths, i.e., time-dependent trajectories of solution compositions on a phase diagram, were examined for coatings of a water-immiscible acrylic monomer dissolved in ethanol and exposed to humidity. The absorption of water vapor quenched the solution into a thermodynamic nonequilibrium state and promoted a spontaneous phase separation when the water content in the solution exceeded a critical value. A simultaneous mass-loss and heat-flux measurement technique was proposed and adapted for phase-separating coatings to determine the evaporation rates of two volatile components with different latent heats of vaporization. The measured drying paths agreed with those predicted from a drying model that takes into account the gas-diffusion-limited evaporation and composition-dependent activities. The numerical predictions also revealed that the pseudo-azeotrope determined the critical drying conditions, under which any drying paths on the phase diagram did not cross the two-phase region, and thus the drying solutions remained stable

Latex Migration in Battery Slurries during Drying

We used real-time fluorescence microscopy to investigate the migration of latex particles in drying battery slurries. The time evolution of the fluorescence signals revealed that the migration of the latex particles was suppressed above the entanglement concentration of carboxymethyl cellulose (CMC), while it was significantly enhanced when CMC fully covered the surfaces of the graphite particles. In particular, a two-step migration was observed when the graphite particles flocculated by depletion attraction at high CMC/graphite mass ratios. The transient states of the nonadsorbing CMC and graphite particles in a medium were discussed, and the uses of this novel measurement technique to monitor the complex drying processes of films were demonstrated

Latex Migration in Battery Slurries during Drying

We used real-time fluorescence microscopy to investigate the migration of latex particles in drying battery slurries. The time evolution of the fluorescence signals revealed that the migration of the latex particles was suppressed above the entanglement concentration of carboxymethyl cellulose (CMC), while it was significantly enhanced when CMC fully covered the surfaces of the graphite particles. In particular, a two-step migration was observed when the graphite particles flocculated by depletion attraction at high CMC/graphite mass ratios. The transient states of the nonadsorbing CMC and graphite particles in a medium were discussed, and the uses of this novel measurement technique to monitor the complex drying processes of films were demonstrated

Flow Pattern Transition of Fine Cohesive Powders in a Gas-Solid Fluidized Bed under Mechanical Vibrating Conditions

AbstractIn this study, the fluidization characteristics for fine cohesive powders under mechanical vibrating conditions were experimentally investigated. In a case of handling of fine cohesive powders with fluidized bed, stable gas channels and cracks were easily formed in the bed due to the strong cohesiveness acting among particles. A mechanical vibration is one of the methods to improve the fluidity of fine cohesive powders. In this study, we focused on the transition of the dominant bed flow pattern induced by mechanical bed vibrating conditions. The powders used in this study were different sizes (0.7 – 20μm) and materials (silica, alumina, polymer). All powders belonged to group-C in the Geldart classification. The bed expansion ratio was lower in the relatively smaller vibration strengths regardless of the gas velocity, however, it steeply increased with larger a certain vibration strength. Form the visual observation of the bed, the bubble formation appeared with larger than such a critical vibration strength. This implied that the dominant bed fluidization pattern was transitioned from channeling behavior (group-C) to bubbling behavior (group-A or –AC) by the addition of the mechanical vibration

Drying-induced surface roughening of polymeric coating under periodic air blowing

We carried out numerical simulations of drying-induced surface roughening of a moving liquid layer under periodic air blowing conditions. Using spatially non-uniform Biot number profiles along the coating surface, the solutions of coupled diffusion/heat transfer equations reveal a periodic growth and relaxation in concentration gradients in the vicinity of the liquid–gas interface. The resultant interfacial stress variations promote particular asymmetric surface topographies that grow and level in a sequential manner. The unique surface roughness is identified by considering time scales required for the web motion, the stress-induced surface evolution, and the pressure-driven leveling. Predicted drying maps represent the surface roughening behavior as regions in parameter space. © 2009 American Institute of Chemical Engineers AIChE J, 200

Measuring the drying rate of liquid film coatings using heat-flux method

We propose a simple method of determining solvent drying rates from heat flux measurements across thin liquid films. The theory is based on quasi-steady conductive heat transport through coatings, combined with simultaneous heat and mass transfer in the gas phase. The measured evaporation rates well reproduce conventional gravimetric measurements with an uncertainty of less than 5%. Drying experiments also revealed that the proposed method is robust in systems with high levels of fluctuation and thus provides an alternative tool for monitoring drying kinetics in forced air flows

Drying-induced reduction in electrical resistivity of carbon black-polyamideimide nanocomposite films

We provide evidence that the electrical resistivity of carbon black (CB)/polyamideimide (PAI) composites are tunable by drying below the glass transition temperature of the polymer matrix. Solution casting experiments of N-methyl-2-pyrrolidone (NMP)/CB/PAI systems reveal that with increasing drying temperature, the measured volume resistivity reduces by four orders of magnitude in an intermediate range of particle volume fractions, which is between 12% and 15%. However, no temperature-dependence was observed for particle-free polymeric films and nanocomposites with high particle volume fractions, which is above 23%. The measured values obey a single master curve, independent of the primary size and surface pH of the nanoparticles. This suggests that the desorption of PAI molecules from the nanoparticle surface enhances the evolution of conductive pathway formation in the course of drying

Two-step migration of particles in evaporating bimodal suspension films at high Peclet numbers

During the drying of bimodal colloidal suspensions containing particles of various sizes, smaller particles preferentially migrate to the top surface under particular drying conditions, leading to undesirable drying defects in batteries and in other coating applications. Despite extensive previous studies, the migration mechanism is far from being understood because few in situ observations are available to support the hypotheses. To remedy this, we use real-time photoluminescence (PL) microscopy to investigate the migration of small fluorescent latex particles co-dispersing with large nonemissive latex particles. Comparing the measured PL intensity with that predicted by a model allows us to determine the quantity of small particles near the evaporating surface. The results reveal that the fluorescent particles segregate in two steps: The primary segregation occurs early in the evaporation stage, whereas the secondary stepwise migration occurs when the air–liquid interface invades the particle consolidation layer. The latter migration is attributed to the flow-induced motion of small particles that move through interstitial spaces between large particles