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Effect of shear strain in coating on the particle packing of gelled-clay particle dispersions during drying.

Abstract Particle aggregates formed in a dispersing medium may be destroyed depending on the shear history applied in the coating process. Yielding behavior of gelled-clay particle dispersion can be interpreted as the destruction of particle network structure with the increase in shear strain. In the present study, gelled-clay particle dispersion was coated at different shear strain to control the initial status of particle aggregation of the drying process. The packing behavior of clay particles in the coated film was investigated from the viewpoint of void fraction in the film by the simultaneous measurements of weight loss and thickness decrease. Additionally, the change of surface roughness was researched based on the scattering pattern analysis. As a result, it was found that dispersed particles started to be packed tightly in the constant drying rate period, and that void fraction and surface structure of particle-packed layer scarcely changed in the latter part of the falling drying rate period. In contrast, if clay particle aggregates remained at the beginning of drying, a loosely packed particle layer was formed without changing surface structure in the constant drying rate period. However, the surface roughness was increased continuously in the falling drying rate period in spite of keeping constant film thickness, probably because void in the particle layer was collapsed.

Keywords Yielding behavior, Particle aggregates, Void formation, Surface roughness

Introduction

Structure control of the film in the coating and drying processes of particle dispersion has been investigated in many studies, (1-3) but many problems in the improvement of performance of final products still remain unsolved. In the case of less concentrated particle dispersions containing a sufficient amount of soluble polymer, final products will be the polymer film containing dispersed particles, such as antireflective films (4) and transparent insulating films. (5) The interaction between particles and polymer should be controlled carefully, because uniform particle distribution is indispensable for the improvement of the performance. If particles are sufficiently dispersed, the dispersion can be apparently treated as a bulk material in the coating process as well as a polymer solution. In other words, the particles will be suitably dispersed not only in the polymer solution but also in the final film.

In contrast, porous particle-packed film, such as a battery electrode, is usually manufactured by coating and drying of highly concentrated particle dispersions. (6-8) Highly concentrated particle dispersion has the advantages of the reduction in the amount of usage of organic solvent and the saving of energy to evaporate dispersing solvent. The particle dispersion for producing porous particle layers usually contains small amounts of polymer to fill partially the void spaces between particles. In the drying process of such particle dispersions, the aggregate structure of particles in the dispersions significantly affects the final structure of the particle-packed layer. Since the status of particle aggregation is affected by various factors including material composition, dispersion procedure, and shear history in coating, the structural control of final films must be considered in the overall process from particle dispersion to drying the particle dispersion.

In view of the effect of shear history on the internal structure of particle dispersion, its rheological behavior provides useful information. It is well known that highly concentrated particle dispersions show complex rheological behavior, such as shear thinning, yielding, and sometimes viscoelasticity. (9) The decrease of dispersion viscosity with the increase in shear rate, known as shear thinning, is interpreted as the destruction of network structure of aggregated particles under a shear flow. The network structure is also the origin of viscoelasticity, showing constant storage modulus as a response of low frequency strain oscillation. (10) Although the destruction and formation of the network structure equilibrate under a very slow shear flow, the structure will be destroyed with the application of sufficiently large shear stress, called yield stress. As for the coating and drying processes, the initial internal structure of the particle dispersion film in the drying process is largely determined by the shear rate and/or shear strain in the coating process. In the polymer melt, we demonstrated that the number of particles composing aggregates is controlled by the applied shear strain. (11) Similarly, we have elucidated the packing process of latex particles in the drying of latex-coated film which is affected significantly by the shear strain applied in the coating. (12) If fragile aggregates generated in a less viscous dispersing medium are destroyed with a small shear strain, the shear strain rather than shear rate turns to be a crucial factor.

On the contrary, the drying process of the coated film of particle dispersion has been investigated from various points of view. Drying rate is the most important factor, which is calculated from weight loss, (13) surface temperature change, (14) or thickness decrease. (12,15) Although the decreasing rate of weight can be predicted from surface temperature and thermal properties, (14) the thickness becomes constant when dispersing medium is still evaporating from pores generated in the drying film of particle dispersions. Therefore, the packing density or void fraction of the particle layer can be estimated quantitatively with the combination of film thickness and weight. Additionally, surface morphology of the drying film of particle dispersion has been studied intensively for the purpose of clarifying the mechanism of particle packing behavior. Scattering is a common technique to characterize a time-averaged structure of concentrated materials. Various newly developed technologies have then been contributed to obtain structural information with highly precise spatial and time resolutions. Small angle X-ray scattering was modified to vertical type for the application to dispersion of settling particles. (16) Structural information related to various length scales is simultaneously obtainable by the interference of scattering lights, which is observed as a Speckle pattern. (17) The correlation of Speckle images with a short time difference was recently reported to provide structural information with a higher time resolution. (18) Such a highly sophisticated scattering analysis is not required in the present system because of the long drying time and rough surface. Considering the technique to measure surface roughness using a laser beam scattering, a projected image of scattering light can be used to clarify the mechanism of the drying process.

In this article, we deal with a gelled-clay particle dispersion as a coating material because clay particles formed network structure and its strength is adjustable by composition. Firstly, we investigate the effect of shear strain on the status of particle network structure in order to control the initial condition of drying process. The formation of void in the coated film of the particle dispersion during drying is then evaluated by the simultaneous measurements of weight loss and thickness decrease. We also perform scattering pattern analysis for characterizing the change of surface structure. Finally, we discuss the relationship between the status of particle network before drying and the process of particle packing during drying.

Materials and experimental methods

Materials

The particle dispersion investigated in the present study consisted of clay particles (pro-organic bentonite particles), activator (polyether-modified silicone), and silicone oil (cyclopentasiloxane). The clay particle is produced by replacing the interlayer inorganic cation of hectorite with quaternary ammonium salt. The activator is a modified silicone fluid, which is produced by the introduction of ethylene oxide into common dimethyl silicone oil. The silicone oil used is volatile because its main structure is cyclopentane. The clay particles are frequently used as an organogelator together with the activator in order to control rheological properties of cosmetics because the particles form loosely connected networks (19-21) due to the hydrophobic interaction between ethylene oxide and clay particle. Plate-like shaped clay particles have equivalent diameters of approximately 0.5 [micro]m, and formed aggregates having the size of 5 [micro]m in the dispersing medium.

Clay particles were added gradually over a couple of minutes into the mixture of activator and silicone oil during mixing using a high shear homogenizer. The dispersion was initially less viscous, but gradually showed viscoelasticity. In order to obtain reproducibility in the rheological properties, the dispersion was stored over a night. Less concentrated particle dispersion showed too-small viscosity, indicating no significant formation of particle network, whereas much highly concentrated dispersion was difficult to be spread over a substrate. We have thus determined the particle concentration as 6 wt%. Next, we have changed the concentration of activator under the constant particle concentration, and investigated the behavior of linear viscoelastic regime. As a result, it turned out that stable aggregated structure is not obtainable at an activator concentration less than 4 wt%, and particle aggregates become fragile at a higher concentration than 6 wt%. Consequently, we have determined the composition of clay particle dispersion; that is 6 wt% of clay particles, 6 wt% of activator, and 88 wt% of silicone oil. Since the specific gravities of the clay particle, silicone oil, and activator are 1.5-1.8, 0.958, and 1, the initial volume fractions of clay particles and activator were 3.5 and 6 vol%.

The rheological behavior of the clay particle dispersion described above was measured using a stress-controlled rheometer (MCR-301, Anton Paar) with a cone-plate system. Figure 1 shows the results of frequency and strain sweep tests of the dispersion. A frequency sweep test indicates viscoelastic response of materials against a small strain oscillation of various frequencies. Storage modulus was sufficiently larger than loss modulus and roughly constant regardless of frequency. This is a characteristic behavior of gel-like materials.

In contrast, the strain sweep test is usually used to clarify a strain range, where the internal structure is held. In other words, constant storage modulus at a strain smaller than 0.1 shows that the network structure of particles remains, while dramatic decrease in storage modulus at a larger strain means the structure is destroyed.

Experimental setup

The experimental setup for coating, drying, and evaluating the drying process is shown in Fig. 2. In the coating experiment, a moving stage, a heating plate, and a doctor blade are used. A glass plate is fixed onto the heating plate controlled at 40[degrees]C. By moving the heating plate using the electrically controlled moving stage, the doctor blade slides over the glass plate. Particle dispersion placed on the glass plate and in front of the doctor blade is thus spread over the plate as shown in Figs. 2a and 2b. In the present study, we have used two doctor blades having different coating length of 2 and 20 mm and a constant coating gap of 100 [micro]m. As a result, different shear strains of 20 and 200 can be applied to the particle dispersion in the coating process. The shear rate applied was constant at 100 [s.sup.-1] because the moving stage was operated at the speed of 10 mm/s.

We have so far applied the thickness decrease measurement using a laser displacement sensor (LT-9010, Keyence) to the drying process of suspension coating. (12,16) We showed that this measurement is useful for the evaluation of drying rate as well as packing behavior of micro- and nanometer particles. Although the surface temperature was measured similar to our previous studies, the clay dispersion showed roughly constant temperature, probably due to the slow drying rate and small latent heat of evaporation of silicone oil (57.8 kJ/kg). We have thus performed weight loss measurement instead. Due to the limitation of an electronic balance (resolution 0.1 mg, capacity 210 g), the clay particle dispersion was coated over a small glass plate (50 mm x 70 mm). The assembly of the glass plate and the small heated plate was placed on the electronic balance, which is placed below the displacement sensor. Thickness decrease was measured using the laser displacement sensor with the spatial resolution of 10 nm. Weight loss measurement was performed during drying as well. In this experiment, the initial weight of coated dispersion can be calculated from the initial composition and weight loss, because total loss of weight corresponds to the initial content of volatile silicone oil. The initial film thickness is also calculated using the dispersion density and coated area. Then, we obtain actual changes of thickness and weight of the coating.

We have also carried out simultaneous evaluations of thickness change and surface roughness. In this experiment, the dispersion was coated over a large glass plate (100 mm x 200 mm). Since the surface profile of coated dispersion is obtainable using the electrically controlled moving stage, the thickness of the coated dispersion after drying is calculated by subtracting surface profiles of the glass plate and the dried dispersion coating. Actual thickness during drying is then obtained from the thickness decrease and the final thickness at the measurement point. The surface roughness was evaluated by the analysis of a projected image of a laser beam scattered at the surface of coated dispersion. (22,23) A red semiconductor laser beam is irradiated onto the coated dispersion at the angle of 45[degrees], and a scattered ray is projected on a white paper aligned normally to the glass substrate. The projected scattered image was recorded using a USB camera. The results of scattered pattern change and the analytical procedure will be explained later. Additionally, with the help of the precise controlled moving stage, thickness measurement and scattering analysis have been carried out at exactly the same position in the coated dispersion.

All experiments were carried out in the chamber where temperature and humidity-controlled air (25[degrees]C and 50% RH) were supplied.

Results and discussion

Determination of shear strain necessary for structure destruction

Since the time scale of shear application in a coating process is on the order of 1 s or less, the destruction of the internal structure of particle dispersion is not evaluated accurately by a usual viscosity measurement under a constant shear rate, which requires at least 10 s for measurement. Then, we have measured shear stress while shear rate is increased exponentially from 0.001 to 1000 [s.sup.-1] over 150, 300, or 600 s.

Figure 3 shows the stress response of the clay particle dispersion with an exponential increase in shear strain. In a smaller strain region smaller than 0.01, the shear stress increases in proportion to strain, which is an elastic or a solid-like response of gel-like material. In a larger shear strain region smaller than 10, the shear stress deviates from the elastic response and then shows notable decrease. This suggests the first yielding behavior or the start of internal structure deformation. In this region, it is considered that the destruction and formation of particle networks takes place at the same time. When the shear strain is increased further, the shear stress shows a second gradual decrease, corresponding to the complete destruction of the network structure of particles. Since the shear strain for the second stress decrease scarcely changed depending on the shear application times, the destruction behavior of the network structure is basically determined by the applied shear strain; that is, the structure is held at the strain of 20 and is completely destroyed at the strain of 200.

Effect of activator concentration on drying rate

In advance to the drying experiment, we measured the drying rate of the mixture of silicone oil and activator at 40[degrees]C, and studied the effect on the drying rate of silicone oil of activator concentration, which is increased as drying proceeds. The mixtures having different activator concentrations are poured into a shallow bath on the glass plate heated at 40[degrees]C, and the vertical position of the liquid surface of the mixture was measured continuously. Thickness decreases were calculated and plotted as functions of time in Fig. 4. As a result, the thickness was decreased at a constant rate even though activator concentration was increased accompanied with the evaporation of silicone oil. Similarly, the decreasing rate was not affected by the initial composition of mixtures. Therefore, it is found that silicone oil will evaporate at a constant rate with no effect of activator.

Thickness decrease and weight loss

Gravimetric analysis is a popular measurement for characterizing the drying process of coatings and used as reference data for discussing the validity of new experimental approaches. (24-26) In the case of the drying process of coated films of highly aggregated or concentrated particle dispersions, the film thickness usually becomes constant even though solvent is still in evaporation. In that stage, it is considered that void is generating in the film. In order to analyze the quantitatively void fraction change, we have carried out simultaneous measurements of thickness decrease and weight loss. It is expected that packing density of clay particles is calculated from the thickness and weight of the film measured at the same time.

The results of simultaneous measurements of weight loss and film thickness are shown in Fig. 5. As described above, the thickness decrease (Fig. 5a) takes roughly constant value after 40 min, though the weight loss continues decreasing until 80 min (Fig. 5b). Constant weight loss after 80 min indicates the end of evaporation and corresponds to the amount of evaporated silicone oil. Then, the initial values of weight and thickness of the coated clay particle dispersion on the small glass plate can be estimated. Since the weight and composition of the film at any time is predictable, we can calculate equivalent thickness of nonporous film. Finally, the time variation of actual film thickness and void-less film thickness are shown in Fig. 5c. Therefore, initial film thickness was identical and denoted by [[delta].sub.0]. Both film thicknesses show good agreement and were decreased at a constant slope, corresponding to a constant drying rate period. In this stage, clay particles are completely filled by the mixture of activator and silicone oil, and the silicone oil is evaporated from the mixture. After a while, actual film thickness continues decreasing at a constant drying rate and shortly attains constant value, although the decreasing rate of void-less film thickness became gradually small. The change in the slope of void-less film thickness indicates the transition from a constant drying rate period to a falling drying rate period.

Figure 6 shows the effect of shear strain applied in the coating process on the decreasing behavior of coated film thicknesses of clay particle dispersion. In the early stage of drying, the decreasing rate in the constant drying rate period was almost the same regardless of the shear strain applied. This result is reasonable because clay particles are sufficiently separated from each other and have no effect on the evaporation of silicone oil from the coated film. Therefore, the slope in Fig. 6 is roughly same as that in Fig. 4. However, the constant drying rate period lasted long only in the case of larger shear strain applied where the clay particle networks are completely destroyed. Additionally, two significant differences can be seen in the following falling drying rate period. One is the actual film thickness at the end of drying, and the other is the variation of the slope of void-less film thickness. Since the initial volume fraction of nonvolatile components is roughly 10 vol%, the volume of clay particle dispersion must be decreased by 90 vol% at the end of drying. Although the shrinking ratio of void-less film thickness was roughly 10%, the actual film was shrunk by roughly 80 vol%. The result clearly shows the formation of a porous clay particle layer. Therefore, clay particles, which are well dispersed under the larger shear strain, formed a tightly packed layer leading to a smaller shrinking ratio. In contrast, when insufficient strain was applied in the coating process, the network structure of clay particles turned into a loosely packed particle layer containing a large amount of silicone oil at the end of the constant drying rate period. As a result, in order to remove the remaining silicone oil, the falling drying rate period lasted a long time.

Scattering pattern analysis

A laser beam irradiated to the surface of a wet coated film of clay particle dispersion is scattered and a scattering pattern is projected on a white paper. Since the intensity distribution of the irradiated laser beam is expressed by Gaussian function, the scattering pattern becomes a bright spot with a sharp decay of brightness when the surface of interest is sufficiently flat (reflection). However, the laser beam scattered on a rough surface generates broad distribution of brightness (scattering). As a result, a superposed brightness distribution of reflection and scattering is observed as a scattering pattern, which changes with the progression of drying, as shown in Fig. 7a. Note that the brightness distribution is affected not only by the increase in packing density of particles but also by the change in the balance between reflection and scattering. In the early stage of drying, a bright spot speckle pattern is interpreted as reflection dominant because the surface is occupied largely by liquid phase and also because clay particles lie over the surface. However, as drying proceeds, the bright spot disappears and then dim light is observed, because a laser beam is scattered at the surface where clay particles are aligned at random. Then, we have calculated span-wise distribution of brightness by averaging brightness distribution in each pixel row. If the maximum brightness in a pixel row was more than 250 in 8-bit grayscale (saturation), the row was omitted in calculating the average.

As can be seen in Fig. 7b, the brightness distributions at any time during drying were reasonably correlated by a superposition of two Gaussian functions having different intensity, I and standard deviation, d expressed by equation (1). The Gaussian function with smaller standard deviation corresponds to reflection, whereas that with larger standard deviation relates to scattering. These figures indicate that the standard deviation of the reflection component was roughly constant after 20 min and that the scattering component was more widely distributed as drying proceeded.

I = [I.sub.refl]exp (-[x.sup.2]/[[sigma].sup.2.sub.refl]) + [I.sub.scat]exp (-[x.sup.2]/[[sigma].sup.2.sub.scat]) (1)

Although surface roughness can be calculated from these fitting parameters based on the scattering theory, the adequacy of the theory to the wet surface has not been sufficiently discussed. Therefore, in the present study, we will use these fitting parameters in order to discuss the surface roughness change.

Figure 8 shows the effect of shear strain applied in the coating process on the time variations of characteristic parameters of reflection and scattering components in the drying process. The time at the end of a constant drying rate period was determined based on the film weight change, and was expressed as a dashed line in the figure. Since the number of saturated pixel rows omitted in calculating average brightness distribution is not constant, the intensity changed irregularly around the end of the constant drying rate period. Additionally, since the brightness of the image is also affected by the configuration of the camera and light source, the time variation of the intensity of each component was not constant even though thickness variation has a good reproducibility. However, it was found that the intensity ratio of scattering to reflection was stable against saturated row omission and exposure conditions, and plays a sufficient role on discussing the dominant component of the scattering pattern. On the contrary, the scattering of the irradiated laser beam is caused by the packing status and alignment of clay particles at the surface. In other words, the standard deviation of a scattering component is kept small when plate-like clay particles are flattened, and a widely distributed scattering component with large standard deviation is obtainable when the particles are packed randomly. The standard deviation of the reflection component also increased gradually in the latter part of the drying process. However, since the reflection component became significantly small in the falling drying rate period, the standard deviation contains a relatively large fitting error and was not used in the characterization of the particle packing process.

In this section, we discuss the change of surface structure during drying based on the variation of intensity ratio and standard deviation of the scattering component. We carried out thickness decrease measurement at the same time as scattering pattern analysis, and confirmed that the film thickness change showed good agreement with those shown in Fig. 6. Therefore, the end of the constant drying rate period at each shear strain condition determined in Fig. 6 is expressed by a dashed line in Fig. 8. The coated film of particle dispersion in which particle networks are sufficiently destroyed with a larger shear strain showed gradual increases in both intensity ratio and standard deviation during the constant drying rate period. Although both parameters continued increasing in the falling drying rate period, they each attained steady values after 50 min even when the film weight is still decreasing. These facts indicate that the alignment of dispersed clay particles has already started in the constant drying rate period and the packing of the particles is finished even though silicone oil remains in the gap spaces between particles. Since the intensity ratio becomes larger than unity at the end of the constant drying rate period, the scattering pattern in the falling drying rate period is dominated by scattering. On the contrary, when particle dispersion was coated with a smaller shear strain and then particle network structure remained, both intensity ratio and standard deviation of the scattering component were not changed in the constant drying rate period. However, they both increased rapidly in the following falling drying rate period even though the film thickness was not decreased anymore. Therefore, in the constant drying rate period, no significant structural change took place at the surface because loosely connected particle networks were just shrunk without any remarkable structural change. In the constant drying rate period, the intensity ratio smaller than unity indicates that the reflection is still dominant in the scattering pattern. In the falling drying rate period, structural change lasted long, similar to the change in void-less film thickness. Since actual film thickness was not decreased anymore in this period, clay particles started to align randomly without changing void fraction of the particle-packed layer. Therefore, loose-connected particles are first formed as a loosely packed particle layer, which is filled with sufficient amount of dispersing medium. After a while, a relatively large void near the surface not filled with the dispersing medium anymore collapses and then the surface roughness increases, whereas the actual film thickness scarcely decreases. The drastic and rapid increase in the scattering component without changing the film thickness is a somewhat a curious behavior. It is considered that this is caused by the shape of plate-like clay particles or by the strong interaction between the clay particles. Further investigation is required in the future for detailed discussion. However, the difference in the particle packing process between the dispersed and connected particle dispersion has been successfully characterized by the measurements performed in the present study, as illustrated in Fig. 9.

Conclusion

In the present study, we investigated the effect of shear strain applied in the coating process on the particle packing of the drying process. For this purpose, we chose a gelled-clay dispersion as a coating material, because a loosely connected network structure of clay particles can be destroyed depending on the shear strain applied when it is coated on a substrate. The destruction or yielding behavior of the clay particle dispersion was investigated using a rheometer, and the shear strain to completely destroy particle networks was determined. After the status of particle aggregation in the coated film of the clay particle dispersion was controlled by the shear strain applied in the coating process, the packing behavior of the dispersed particles in the film was investigated from the viewpoints of void formation and surface roughness. In the case when particle networks are destroyed in the coating process with a larger shear strain, particles are aligned randomly and packed tightly in the constant drying rate period. Since particles are not packed further in the falling drying rate period, the evaporation rate is gradually decreased, accompanied by the formation of void in the film. In contrast, when the particle dispersion is coated with a smaller shear strain, no significant structural change was observed at the surface in the constant drying rate period, because the network structure of loosely connected particles remains and is shrunk. Terminated packing density of a particle layer is small compared to that of a larger strain condition. Although the film thickness does not change in the falling drying rate period, the roughness of the film surface is increased rapidly and drastically. The findings in the present study largely depend either on the particle shapes or the rheological properties of the particle dispersion. The universality of the findings need to be confirmed by expanding this experiment to more simple systems like the dispersion of monodispersed spherical particles containing well-known additives.

DOI 10.1007/s11998-015-9719-7

Y. Komoda ([mail]), S. Kobayashi, H. Suzuki

Chemical Science and Engineering, Kobe University, 1-1, Rokkodai-cho, Nada, Kobe, Hyogo 657-8501, Japan

e-mail: komoda@kobe-u.ac.jp

S. Kobayashi

e-mail: 125t435t@stu.kobe-u.ac.jp

H. Suzuki

e-mail: hero@kobe-u.ac.jp

R. Hidema

Organization of Advanced Science and Technology, Kobe

University, 1-1, Rokkodai-cho, Nada, Kobe. Hyogo 657-8501.

Japan

e-mail: hidema@port.kobe-u.ac.jp

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Author:Komoda, Yoshiyuki; Kobayashi, Shigeyuki; Suzuki, Hiroshi; Hidema, Ruri
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:Sep 1, 2015
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