Process Intensification in Particle Technology: Production of Powder

A novel intensive granulation process for the production of powder coatings is described. The method is based on nonisothermal flow-induced phase inve...
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Process Intensification in Particle Technology: Production of Powder Coatings by Nonisothermal Flow-Induced Phase Inversion Galip Akay,*,† Bandara Dissanayake,† and Andy Morgan‡ † ‡

School of Chemical Engineering and Advanced Materials, Newcastle University, Newcastle upon Tyne, NE1 7RU, U.K. AkzoNobel Powder Coatings Ltd., Stoneygate Lane, Felling, Gateshead, NE10 0JY, U.K. ABSTRACT: A novel intensive granulation process for the production of powder coatings is described. The method is based on nonisothermal flow-induced phase inversion (FIPI) phenomenon. A powder coating composition in melt state is transformed to coating particles in a purpose-built granulator that operates in a rotor-stator configuration in which rotor-stator elements have cavities to achieve mixing, pumping, particulate conveying, heat transfer, and crumbling (phase inversion) which takes place at a certain distance (radius) from the center of the disk. The rotor disk is sandwiched between two stator disks. Heat transfer is further improved and melt quenching under deformation is achieved by injecting water or carbon dioxide into the melt while a temperature gradient is maintained across the upper rotor and stator. Injection of fluids reduced the average particle size of the product considerably and also intensified the granulation process through crumbling in a small volume when the coating melt is transformed to particles. The product was characterized by particle size analysis. Methods to control the crumbling radius and average particle size well-below the rotor-stator gap are discussed. The influence of fluid (water or carbon dioxide) injection on the granulation mechanism and the extension of the method to cylindrical rotor-stator system are also discussed.

1. INTRODUCTION One of the main problems of solvent-based paints is the emission of volatile organic compounds (VOC).1,2 It has been reported that the coating industry accounts for ca. 6% of manmade VOC emission in Europe.3 Powder coatings are a green alternative for solvent-based paints. High-quality films, extremely low wastage during application, and broad application range coupled with environmental advantages made powder coating a growing business with an annual global production of 1.785 billion tonnes and market value of $4.5 billion in 2006.4,5 Powder coatings are commercially manufactured by a wellestablished process that comprises several unit operations: (1) mixing, (2) melt extrusion, (3) cooling, (4) flake granulation, (5) fine grinding, and (6) classification. Despite being widely adopted in the industry, the conventional process suffers from several major drawbacks. Powder coating particles obtained from the conventional process have irregular morphologies and wide particle size distributions due to mechanical grinding. Irregular morphologies and wide particle size distributions are undesired since they cause poor powder flow, nonuniform feed rate, deposition in spray guns, and high wastage during the application of powder coatings.6 Furthermore, the thickness of the coating layer may be nonuniform, and hence, the powder coating film may not be smooth.7 The product of the hot melt compounding may be a nonhomogeneous mixture that results in uneven composition of powder coatings particles.8 It may cause shade variations in powder coating films. Since the hot melt compounding is carried out at elevated temperatures, premature reactions may occur to a considerable extent.9 These may result in gelated materials that reduce the quality of the powder coatings film. Mechanical size reduction is generally energy inefficient and causes material and energy wastages. In addition, the conventional process contains a number of dissimilar, noncontinuous r 2011 American Chemical Society

operations that complicate the process. Intermediate products need to be stored between steps. The production of small batches is uneconomical. Moreover, the process needs large floor area and the capital is relatively high.10 It is becoming difficult to ignore the limitations of the conventional process; thus, some radical alternatives to the conventional process have been introduced, including emulsion aggregation,7 spray drying,8 melt atomization,11 lyophilization,12 microfine milling,13 and particle bonding.14 The aim of the present study is to develop an alternative powder coating manufacturing process based on intensive granulation, which is a fundamentally different size enlargement process based on nonisothermal flow-induced phase inversion (FIPI), which was discovered by Akay15-18 in solid-liquid15-17 and liquid-liquid17-19 as well as in multiphase systems in general.19 FIPI formed the bases of a generic process intensification19 (PI) technique with industrial applications.20-22 The use of FIPI in process intensification was further advanced in particle technology through nonisothermal, thermomechanical techniques.22-26 In intensive isothermal or nonisothermal FIPI granulation,16,19,22-26 a solid phase suspended in a liquid phase is inverted to a liquid phase encapsulated by particulate matter which themselves are dispersed in air. The liquid phase is a molten polymer binder that solidifies during cooling, resulting in the formation of discrete particles. The phase inversion (crumbling) is achieved by subjecting a mixture of molten binder and filler particles, such as pigment particles, to a prescribed flow field17-19 while being cooled. The fundamental difference of the intensive granulation Received: July 20, 2010 Accepted: January 7, 2011 Revised: December 16, 2010 Published: February 02, 2011 3239

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Industrial & Engineering Chemistry Research process as compared to the conventional granulation process is the granulation mechanism. The main steps of the conventional granulation processes27,28 are (1) wetting and nucleation, (2) coalescence or growth, (3) consolidation, and (4) attrition or breakage, while the main steps of the intensive granulation17,19,25 are (1) nucleation and growth, (2) crumbling, and (3) attrition and breakage. Akay and co-workers carried out a substantial amount of work on intensive granulation using a purpose-built granulator.24-26 The primary aims of their studies were to evaluate the granulator and establish the granulation mechanism. Very recently, a population balance model has been developed29 to model the process. In this process, a melt of coating binder (containing fine filler particles) is extruded into a rotating disk granulator that contains cavities. It is possible to superimpose a temperature gradient in the radial direction as well as across the rotor and stator (vertical direction) in order to achieve fractional solidification as the material is conveyed radially by the cavities present in the rotor and stator. It has been reported that the average particle size of the product is approximately equal to the rotor-stator gap.24 The reason for the observation is, however, not established. One question that needs to be asked, however, is whether it is possible to reduce the average particle size well below 500 μm, which is the rotor-stator gap, since very fine particles are needed for powder coatings. Furthermore, the dynamics of the granulation process are such that polymer melt/pigment dispersion should ideally phase invert within a reasonable distance (crumbling radius24-26) after entering into the granulator space between the rotor and stator. In previous studies, it was observed24-26 that the crumbling zone occupied most of the rotor due to insufficient rate of heat transfer, implying that a larger rotor may be needed if nucleation is comparatively slow. This happens when viscous heat generation is high as a result of small gap size and when binder has low molecular weight and high concentration (which is the case in powder coating production). However, far too little attention has been paid to control the crumbling radius, although it dictates the size of the granulator. This paper introduces an alternative powder coating manufacturing process based on nonisothermal FIPI in which the crumbling radius is controlled, effectively making the process industrially possible while reducing the average particle size below the rotor-stator clearance, which can be varied between 500 and 3000 μm in the current granulator.

2. EXPERIMENTAL SECTION 2.1. Materials. The powder coating composition comprises a reactive binder and filler. The filler was titanium dioxide (TiO2) particles (coded Kronos 2310) with mean particle size (D50) of 0.2 μm supplied by Kronos Inc. The binder was a carboxylic acid functional polyester (coded Polyester IP4125) with an average molecular weight of 104 supplied by Nuplex Resins Ltd. The glass transition temperature range and softening point of the polymer were 55-65 and 120 °C, respectively (as specified by the supplier). All materials were used without any modification. 2.2. Equipment. The experiments were carried out in the system shown in Figure 1. It consists of an extruder and a granulator and their controller units. (a). Extruder. A Haake Rheomix 252 single screw extruder controlled by a Haake Rheocord 9000 was used in this study to melt and mix raw materials and then feed them to the granulator.

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Figure 1. Diagrammatic illustration of the granulator system containing an extruder and granulator. Water or compressed carbon dioxide can be injected into the granulator.

Figure 2. Diagrammatic representation of an intensive granulator. The melt is introduced from the melt port and water is injected through injection points.

The torque of the extruder drive motor can be recorded online. The rotational speed of the screw and temperatures of the barrel, which comprises three heating zones, can be set and monitored online. Temperatures of heating zones are controlled using compressed air. (b). Granulator. The granulator and extruder are connected by a thermally insulated metal transfer pipe. A schematic diagram of the granulator is shown in Figure 2. The granulator consists of a rotor block that is sandwiched between two stators. The upper stator comprises a heating zone and nonheating zone as illustrated in Figure 3. A thermal barrier, in the form of a circular groove on the upper stator, separates the heated and nonheated zones of the stator. Heat transfer from the heating zone (necessary to prevent immediate solidification of the melt) to the nonheating zone can effectively be reduced by running compressed air in the groove. After flowing around the groove channel, heated air is vented. Small cavities, which can be broadly classified as circular and elongated based on their shape, are present on the surface of the rotor and stators. The function of the circular cavities is to mix the melt while that of the elongated cavities is to pump (when melt is present) or convey (when particles are present). The melt is introduced to the granulator through the opening of the upper stator to the center of the upper rotor. It is subject to an angular motion due to the rotation of disks and a radial motion due to the pressure generated by the extruder screw as well as by the elongated cavities once the melt enters into granulator. The cavities on rotors and stators are arranged in such a way that the rotor cavities and stator cavities are offset by half a cavity length so that the melt follows a vertical path to transfer from the rotor to the stator and back again to the rotor cavity while being displaced in radial and axial directions. As a result of rotation, cavity arrangement, and pressure gradient, the melt flows in a 3240

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Table 1. Temperature Profile of the Granulator Upper Stator When Experiments Started

a

Figure 3. Diagrammatic illustration of the upper stator comprising a heating zone and nonheating zone (not scaled). The internal diameter of injection points is 2.5 mm. Radii of various zones are indicated. All dimensions are given in millimeters.

three-dimensional flow path while being mixed and pumped. Another important function of cavities is to increase the heat transfer area to intensify heat removal from the melt. Eight fluid injection points at two different radii are available to inject fluids, as illustrated in Figure 3. The function of fluid injection is to remove heat, transport particles, and reduce viscous heating once crumbling occurred to prevent size enlargement. 2.3. Experimental Procedure. A blend of carboxyl polyester (65% w/w) and TiO2 (35% w/w) was prepared by dry mixing. The extrusion was carried out at 130 °C temperature and 10 rpm screw speed. Akay et al.24-26 used the crystallization and melt temperature ranges of the blend to determine the temperature profile of the granulator. However, that procedure can only be applied for semicrystalline polymers when a relatively sharp melt temperature distribution is present. Since the polymer is amorphous, the determination of the optimum temperature profile of the granulator can only be done by a trial and error approach using the glass transition and softening temperatures as a guide. The experimental investigation may be divided into three parts: (1) dry granulation (no gas or water injection into the granulator), (2) wet granulation (with water injection), and (3) gas-phase granulation (with gas injection). In wet granulation experiments, water at different temperatures was injected using a pump through tubes connected to the injection points of the upper stator. A Haake refrigerated chiller (model K15) was used to chill water. In gas-phase granulation, CO2 gas from gas cylinder was injected through tubes connected to the injection points. The temperature of CO2 gas at the injection points was measured and the pressure of the gas was regulated to control CO2 flow rate. Initially, dry granulation experiments were conducted with different temperature settings and angular speed of the rotor to determine the optimum temperature profile. Following several trials, the following operating conditions were established and used in all the experiments throughout this study. Unless stated otherwise, the following conditions were applied: upper stator

location of temperature gaugea (mm)

33

65

150

set steady state temperature (°C)

70

40

30

Location was measured from the centre of upper stator.

center temperature (heating zone) was 70 °C (just above glass transition point of the binder), rotor and lower stator were kept at room temperature, rotor speed was 30 rpm, rotor-stator gap was 0.5 mm, and melt (polymeric binder and filler) flow rate was 10 g/min. The product was collected from the collection tray and from cavities immediately after the crumbling zone. The extruder torque graph was constantly monitored during the run. The upper stator was removed immediately after the experiment after allowing for the melt in the center of the granulator disk to solidify. In dry granulation, polymer melt from the extruder was fed into the granulator and the experiment was started when a steady temperature profile of the upper stator was achieved, as shown in Table 1. In the case of wet or gas-phase granulation, the experiments were started when the temperature in the heating zone reached 70 °C. 2.4. Analytical Techniques. Particle size distribution of the product was determined by a Beckman Coulter LS-230 laser diffraction particle size analyzer. Scanning electron microscopy (SEM) studies were performed with a JEOL JSM5300LV microscope.

3. RESULTS AND DISCUSSION 3.1. Particle Size Distribution. The effect of rotor speed, upper stator center temperature, and rotor temperature on the average particle size (D50) and particle size span (S), defined below, was studied in dry granulation.

S¼

D90 -D10 D50

where D10, D50, and D90 are the diameters below which 10, 50, and 90% of particles lie, respectively. Table 2 shows the particle size characteristics obtained from dry granulation experiments. It is apparent from Table 2 that the average particle size was high when the center temperature of the upper stator was high. It can be reduced by nearly a factor of 2 by reducing the center temperature of the upper stator from 100 to 70 °C. However, the center temperature cannot be reduced below 70 °C due to rapid solidification of the melt in the heating zone. Increasing rotor speed increased the average particle size and span. The influence of the rotor temperature was also profound, since the finest particles were produced when the rotor temperature was 30 °C. Table 2 also indicates the dependence of the crumbling radius on the processing conditions. In wet granulation, the effect of the water temperature and water/powder coating flow rate ratio on the average particle size and particle size span was studied, and the results are plotted in Figure 4. This figure shows that, in the temperature range studied, there is a clear trend of increasing the average particle size linearly with the increase of the water temperature, indicating the possibility of producing fine particles by injecting cold water (or fluids). Figure 4 further shows that the average particle size depended on the water/powder coating weight ratio, although at lower water temperatures it was less significant. The corresponding variation of the particle size span is presented in Figure 5. It can be seen that the particle size span 3241

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Table 2. Variation of the Average Particle Size (D50), Particle Size Span, and Crumbling Radius with Granulator Parameters in Dry Granulation final product upper stator center set temperature (°C)

rotor speed (rpm)

rotor temperature (°C)

D50 (μm)

span

crumbling radius (mm)

100

30

20

856

1.40

76

70

30

20

459

0.75

65

70 70

60 30

20 30

640 408

1.58 0.95

76 89

Figure 4. Variation of average particle size (D50) with water temperature and water:powder coating ratio (w/w) in wet granulation.

Figure 5. Variation of particle size span with water temperature and water:powder coating ratio (w/w) in wet granulation.

decreased linearly as the temperature of water increased when the water to powder coating weight ratio was 1. However, as the water amount was increased, the particle size span remained nearly constant. Gas-phase granulation was studied by injecting compressed CO2 gas, and the results are presented in Figure 6. The most interesting finding of the gas-phase granulation was that the

average particle size exponentially decreased with increasing CO2/powder coating weight ratio. The corresponding change of the particle size span is presented in Figure 7. The span remained approximately constant, except when the CO2/powder coating weight ratio was increased to 1.34. The lowest average particle size obtained from three different processing methods along with the corresponding crumbling radia are summarized in Table 3. As can be seen from the data of Table 3, the average particle size can considerably be reduced by injecting water or CO2. Water injection is preferred, since water is more economical at industrial scale. However, a limitation of wet granulation is that the particle size distribution was wide as compared to dry granulation or gas-phase granulation. Another interesting finding was that the gas-phase granulation and wet granulation reduced the average particle size below the rotorstator gap (which was set at 500 μm), which was initially thought to be a limitation of intensive granulation. Furthermore, the crumbling radius was considerably reduced in wet granulation and gas-phase granulation. The crumbling radius in dry granulation was a function of the processing parameters such as melt entry temperature, rotor speed, and rotor temperature and therefore could not be controlled as an independent variable. In contrast, the crumbling radius was constant, irrespective of the processing conditions in wet and gas-phase granulation. The crumbling radius in wet granulation was smaller compared to gasphase granulation. In both cases, crumbling radii were smaller than the radius at which fluid was first injected (first stage fluid injection radius was 60 mm). This indicates that due to enhanced heat transfer, nucleation was more effective before crumbling took place. The second-stage fluid injection helped to transport the granules without size enlargement. The importance of this finding is that the injection of fluids may be considered as a method of controlling the crumbling radius. 3.2. Influence of the Water Injection on the Granulation Mechanism. Figure 8a shows the typical appearance of the rotor after wet granulation experiments. It has been reported that, in the typical dry granulation process, four distinct zones exist: (1) a melt zone, in which the melt flows radially outward; (2) a nucleation zone, in which the phase volume of the dispersed phase increases due to fractional solidification of polymer; (3) a crumbling zone, in which the melt crumbles to form particles; and (4) a granule transport zone, in which newly formed granules are transported radially outward. The presence of these zone can be observed on the rotor.24,25 In Figure 8a, although the melt zone and granule zone can be distinguished, the nucleation zone and crumbling zone are unclear. This is because these zones are now very small due to rapid cooling due to fluid injection. Nucleation and crumbling take place nearer to the center and more importantly within one cavity ring, suggesting that the phase inversion is almost instantaneous. However, as the steady 3242

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Industrial & Engineering Chemistry Research state is established, due to heat transfer, the crumbling radius shrinks toward the rotor center. We note that crumbling in dry granulation process is is also instantaneous, except that the crumbling radius is large compared with the granulation with fluid injection. As seen in Figure 8a, the granulator zone is unnecessarily very large. Figure 8b shows an SEM of the powder coating material recovered just before crumbling occurred. It shows the presence of particles loosely held together shortly before crumbling. This is an important observation, since previous investigations on intensive granulation showed that the nucleation zone occupies most of the rotor, resulting in higher crumbling radius.24,25 The above findings may be explained with the granulation mechanism, which is diagrammatically illustrated in Figure 9. This illustration is based on SEM examination of samples recovered from various locations in the nucleation zone. A filled polymer

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melt can be considered as a dispersion where the dispersed (discrete) phase consists of filler particles and the continuous phase consists of polymer melt. When the melt enters the granulator, it initially flows through the cavities, where the melt is subject to high shear and extensional forces. Flow-induced phenomena (such as flow-induced orientation and crystallization15-19,30 and bound polymer formation15,16) and fractional solidification increase the volume fraction of the dispersed phase. This occurs at the nucleation zone. The increase of the dispersed phase volume eventually results in phase inversion, which occurs at the crumbling zone.24,25 In the absence of any heat removal, the increase of the discrete phase volume is primarily due to flow field-microstructure interactions.16,17 As a result, the increase of the discrete phase volume is slow, resulting in large crumbling radius. However, heat removal from the melt results in an increased

Figure 6. Variation of average particle size with CO2:powder coating ratio (w/w) in wet granulation. The temperature of the gas is also given.

Figure 7. Variation of particle size span with CO2:powder coating ratio (w/w) in wet granulation. The temperature of the gas is also given.

Figure 8. Evaluation of the wet granulation process by visualization: (a) rotor and (b) SEM image of the crumbling zone in which the frozen material is recovered from the cavity for visualization. Experimental conditions were water temperature = 20 °C, water:material ratio = 3:1 (w/w).

Table 3. Comparison of Average Particle Size (D50), Particle Size Distribution Span (S), and Crumbling Radius in Dry, Wet, and Gas-Phase Granulation, and the Two-Stage Fluid (Water or Carbon Dioxide) Injection Radii a fluid injection radius (mm) granulation method

D50 (μm)

span

crumbling radius (mm) 76

stage 1

stage 2

dry

408

0.95

wet

299

2.76

36

60

100

gas-phase

277

0.85

50

60

100

Experimental conditions for dry granulation: rotor speed = 30 rpm, rotor temperature = 30 °C, upper stator center temperature = 70 °C. Experimental conditions for wet granulation: water temperature = 5 °C, water:powder coating ratio = 1:1. Experimental conditions for gas-phase granulation: CO2: powder coating ratio (w/w) = 1.34. Crumbling radius was measured from the rotor center. a

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Figure 10. Variation of extruder torque in dry granulation and wet granulation. Experimental conditions for dry granulation: rotor speed = 30 rpm, rotor temperature = 20 °C, upper stator center temperature = 70 °C. Experimental conditions for wet granulation: water temperature = 20 °C, water:powder coating ratio = 3:1 (w/w).

Figure 9. Granulation mechanism: (a) dry granulation and (b) wet or gas-phase granulation.

fractional solidification rate, which also contributes to the increase of the discrete phase volume, thus reducing the size of the crumbling zone. The injection of quenching fluid (water or CO2) can be considered a method of removing heat directly from the melt. As a result, the processes of nucleation and crumbling are intensified with fluid injection. An SEM image of frozen material in a cavity at crumbling zone is presented in Figure 8b. Large particles can be identified in the center of the cavity, while a small particle can be observed at the exit and entrance of the cavity. It is because melt is subjected to high extensional and shear deformation rates at the entrance and exit to a cavity.16 High deformation rates and stresses can reduce the dispersed phase particle size16,17 and cause melt fracture.31 Another way of achieving high deformation rates is to increase the rotor speed or decrease the gap between the rotor and upper stator. However, an increase in the deformation rate also increases viscous heat dissipation, which gradually increases the temperature of the melt, thus reducing the stresses generated in the fluid. An alternative model for the present granulation process is thermomechanical melt fracture. Melt fracture is a term used for the flow instability associated with the flow of polymer melts or filled polymer melts (see, for example, ref 31). However this melt fracture does not create particles even under superimposed cooling. Unlike flow-induced phase inversion, melt fracture in polymers is a function of deformation rate, rather than stress power (i.e., energy expanded per unit time).15,17 Flow-induced phase inversion in filled polymer melts can be scaled with respect to stress power.17 In FIPI emulsification,18 phase inversion takes place from water-in-oil to oil-in-water through an intermediate stage of [water-in-oil]-in-water multiple emulsion. Phase inversion is not spontaneous but gradual, requiring the maintenance of the deformation.18 This is also true for the isothermal FIPI process in

the inversion of filler-in-polymer melt. In this case also, the phase inversion is through [filler-in-polymer melt]-in-particle, where the new particle phase is generated as a result of several flow-induced phenomena such as flow-induced crystallization and bound polymer formation. Due to the formation of the particle phase as the continuous phase, air is also part of the continuous phase, and this phase inversion is physically marked by the increased volume of this dispersed system and sudden decrease in the mixer torque in a batch mixer. At this stage, no further stresses can be generated and hence the phase inversion from the filler-in-polymer to [filler-in-polymer melt]-in-particle/air type of dispersion remains stable and frozen due to further cooling. In nonisothermal FIPI, solidification in the filler-in-polymer melt dispersion is very rapid. As the continuous polymer melt phase undergoes solidification, the dispersed phase volume fraction increases above ca. 0.7, and the phase inversion takes place in the form of [filler-in-polymer melt]-in-particles, which upon further cooling is frozen in structure. Another important characteristic that is linked with the granulation mechanism is the variation of the extruder torque with time,16,23 which is shown in Figure 10. The torque of the extruder increased slowly until the melt entered the granulator. It then suddenly increased due to the increased flow path and cooling of melt. When the melt crumbled, the extruder torque reduced significantly and did not recover subsequently. The location where the extruder torque reduced (crumbling occurred) is shown in Figure 10. As can be seen, crumbling delayed significantly in dry granulation as compared with wet granulation. It suggests that the melt flowed comparatively longer distance in dry granulation before crumbling. Another interesting finding to emerge from Figure 10 is that the extruder torque was lower in wet granulation. As the crumbling occurred nearer to the center of the rotor in wet granulation, the melt flow path is reduced as compared to dry granulation. Thus, a smaller extruder torque can be expected in wet granulation, since the extruder torque depends on the length of the melt path. Figure 11 shows the variation of the average particle size and particle size span as a function of the location in wet granulation experiments. As can be seen, the average particle size of the 3244

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Figure 11. Variation of particle size data as a function of the location on the rotor in the wet granulation. Experimental conditions were as follows: water temperature = 20 °C, water:powder coating ratio = 1:1. The x-axis represents the radial distance of the cavity ring where samples were recovered from at the rotor surface.

product initially decreased but then increased as particles traveled radially outward in the rotor. Interestingly, the particle size span followed exactly the opposite trend. A possible explanation for the decrease of the particle size and increase of the span just after the crumbling might be the comminution of the particles, which is the final step of the established granulation mechanism.25 Nevertheless, it is certain from Figure 11 that when the overall process is taken into consideration, comminution was not significant, as the average particle size started to increase and the particle size span started to decrease after the fifth cavity ring (65 mm). The increase of particle size may be due to particle reagglomeration32 and/or particle segregation.33,34 This finding is in agreement with previous similar observations made by Akay et al.24 in dry granulation. These results show that the residence time of the granulator is crucial in controlling the properties of the final product. Wet granulation initially produces a highly concentrated dispersion with high viscosity. Although the cavities of the rotating disk agglomerator are capable of conveying highly viscous melts and particles, they are not efficient for pumping concentrated slurry due to particle segregation. As a result, compaction of particles can occur, leading to particle enlargement. Within the cavities, water under pressure appears to channel through the particle bed in the cavities. However, when carbon dioxide is used as the cooling/quenching fluid, particles generated in the crumbling zone are fluidized and rapidly removed; thus, there is no particle size enlargement after crumbling. It is clear that the injection of the coolant/quenching fluid essentially starts crumbling, but the cheaper fluid (water) is not effective in conveying particles in the present granulator. This drawback of the rotary disk agglomerator can be eliminated by redesigning of the cavities after the crumbling zone. In this new design, it is possible to use the cavity types developed for the pumping and emulsification of highly viscous fluids using cylindrical rotor-stator configuration. These equipment are called controlled deformation dynamic mixers (CDDMs).35 In these mixers/reactors, the flow geometry of different mixing/conveying zones can be arranged for a given product and then further refined during processing in order to achieve optimum conditions by shifting the rotor and stator cavities relative to each other. The facilities of CDDMs are most readily achieved by using cylindrical flow geometry in which the inner cylindrical

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rotor and the outer cylindrical stator have cavities for mixing, conveying, and heat transfer enhancement. Tailoring of the flow field during processing is achieved by sliding rotor cavities along its axis with respect to stator cavities to obtain either predominantly extensional (elongational) or shear flows. Pumping and mixing of the fluid is achieved by cavity shape and rotor speed. Heat transfer facilities of the rotor (inner cylinder) and stator (outer cylinder) are also effectively achieved while the injection of fluids followed by rapid mixing are facilitated. Therefore, the use of such devices at the end the extruder will overcome the drawbacks of the rotating disk granulator with fluid injection. Nevertheless, the current agglomerator configuration is very useful in the evaluation of the mechanism of the nonisothermal FIPI agglomeration technique. The CDDMs were originally designed to achieve FIPI emulsification22,35 and subsequently adopted to cover particle generation using nonisothermal FIPI in the presence of phase transformation. As seen in Table 2 and Figures 4 and 5, the granule characteristics are highly dependent on temperature. Particle generation methods (agglomeration, encapsulation, emulsification) based on the flow-induced phase inversion process utilizes the interchangeability of the thermodynamic state variables (temperature, concentration, pressure, surface activity) with the objective measure of deformation state variables such stress power, G (power per unit volume), which in Cartesian tensor notation is given by36 G ¼ Eij τij where Eij is the rate of deformation tensor and τij is the total stress tensor, which is a function of viscosity. The effect of temperature is therefore 2-fold; it can directly influence the thermodynamic conditions but also the deformation state variables by altering viscosity of the fluid. The interchangeability of thermodynamic and deformation state variables makes nonisothermal FIPI a powerful process involving multiphase, high-viscosity systems.

4. CONCLUSIONS The aim of this study was to develop a more efficient form of intensive granulation process for powder coatings production. The study has found that the average particle size can be reduced by injecting fluids into the material being processed. It was also found that the crumbling radius can be controlled and the average particle size can be reduced well below the rotor-stator gap. The fluid injection intensified the granulation process by reducing the crumbling radius and making nucleation almost instantaneous. This is due to increased heat removal rate through the injection of a cooling/quenching fluid (water or CO2). Although the particle size just after the phase inversion is low in wet granulation, it increases due to particle segregation and/or reagglomeration. Hence, the quality (particle size and distribution) of the final product is also a function of the residence time and heat transfer and coolant injection conditions, thus making the process highly versatile for product quality control. Particle characteristics and their comparison with those obtained from conventional process will be presented separately.37 ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: þ44 191 222 7269. Fax: þ44 191 222 5292. 3245

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Industrial & Engineering Chemistry Research

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