This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Particuology 9 (2011) Contents lists available at ScienceDirect Particuology j our nal ho me p ag e: Experimental modeling of polymer latex spray coating for producing controlled-release urea Rui Lan, Yonghui Liu, Guanda Wang, Tingjie Wang, Chengyou Kan, Yong Jin Department of Chemical Engineering, Tsinghua University, Beijing , China a r t i c l e i n f o Article history: Received 28 October 2010 Received in revised form 29 December 2010 Accepted 5 January 2011 Keywords: Urea Latex Controlled release Spray coating Experimental modeling a b s t r a c t Spray coating of polymer latex onto fertilizer particles in a fluidized bed for producing controlled-release urea is an environment friendly technology as it does not need any toxic organic solvent. Since the spray coating process in a fluidized bed occurs in the presence of particle collisions, the coating of the particles is random, intermittent and multiple, thus making it difficult to investigate the film formation process. In this paper, an experimental model apparatus was designed and used to investigate the effects of the key factors in the spray coating process. This apparatus reasonably simplified the complex process to avoid particle collisions and randomness in the coating. The intermittent coating in the fluidized bed was modeled by periodic coating and dewatering in the experimental apparatus. A large area film was obtained, and the film permeability was measured. The effects of atomizing gas flow rate, spray rate of latex, solid content of latex and gas temperature on film structure and film permeability were investigated. It was found that water transfer played a dominant role in the spray coating process Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. 1. Introduction The use of slow- and controlled-release fertilizers is an effective way to solve the problems of resource waste and environmental pollution that would be caused by indiscriminate use of huge quantities of fertilizers, especially in China. A new technology for producing slow- and controlled-release fertilizers using polymer latex as the coating material by spray coating in a fluidized bed has the advantages of simplicity, economy and not involving any organic solvent, thus making it environmentally benign (Chen, Wang, Yu, & Jin, 2002). The effective control of the performance of slow/controlledrelease fertilizers is the research focus in this field. Shaviv, Raban, and Zaidel (2003) showed that the film permeabilities of water vapor and urea were important parameters that affect the release process because they determined the release rate, release time and release pattern. Therefore, film permeability coefficients as crucial parameters in the spray coating process can be used to facilitate prediction of the release performance of the coated urea. Existing research on the preparation of slow/controlled-release fertilizers have mostly focused on spray coating conditions. Researches on polymer latex spray coating in various fluidized beds Corresponding author. Tel.: ; fax: address: wangtj@tsinghua.edu.cn (T. Wang). showed that many factors such as spray rate of latex, solid content of latex, temperature of fluidizing gas, velocity of fluidizing gas, etc. affect the spray coating process and release characteristics of the coated urea (Donida & Rocha, 2002; Liu, 2009; Tzika, Alexandridou, & Kiparissides, 2003). McGinity (1997) reported that in a fluidized bed more than 20 factors can affect this process. The spray coating process in a fluidized bed occurs in the presence of particle collisions so that the surface coating is random, intermittent and multiple, thus making it very difficult to study. Moreover, since the film surface area of the coated urea is very small and the samples and sampling are affected by randomness, it is hard to characterize and compare film permeabilities and mechanical properties. Among works on spray coating for producing slow-release medicine, Mendoza-Romero et al. (2009) modeled film coating by spraying in a rotating drum. They achieved effective control of sprayed film formation conditions such as substrate temperature, drying time, etc., and explored conditions for improving film formation. Sun, Huang, and Chang (1999) prepared films by spraying a turning table and analyzed the structural differences between sprayed films and cast films. The film permeability coefficients and the mechanical properties were measured. In this work, an experimental model apparatus was designed to simulate the process of spray coating on particles in a fluidized bed. The proposed experimental model simplified the spray coating process by eliminating the influences of random particle collisions in the fluidized bed, to better reflect the effects of key factors in the /$ see front matter 2011 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. doi: /j.partic

3 R. Lan et al. / Particuology 9 (2011) occurs with particle collisions in the fluidized bed. All these cause differences in the film on the particles, thus making it difficult to characterize the film properties Design of experimental apparatus to model spray coating Fig. 1. Schematic of spray coating process in a Wurster fluidized bed (Tzika et al., 2003). 1, partition; 2, nozzle; 3, perforated plate area A; 4, perforated plate area B. process. Various films were prepared under different conditions, and their film structures were characterized and their permeability coefficients were measured. The key factors in the spray coating process were analyzed, in order to more effectively design and control the film structure so as to better promote research and development of slow-release fertilizer production technology. 2. Experimental model 2.1. Analysis of spray coating process in a fluidized bed Fig. 1 shows a typical spray coating process in a Wurster fluidized bed (Tzika et al., 2003). Urea particles are coated in a spray region near the nozzle at the bottom, after which the particles are carried by the fluidizing gas along the partition to the top of the fluidized bed. Latex sprayed onto the particle surface was dewatered in the gas flow, and then the particles moved down in the annular region to the spray region to be coated again. The coating and dewatering processes were repeated many times in the fluidized bed, and spray coating was complete when a film of specified thickness was formed. During the multiple coating and dewatering processes the fluidizing gas was humidified while the latex was dewatered, and subsequently a coated film was formed. The spray coating process occurs with water input and output, to promote a randomintermittent-multiple film formation process on the particles. The particles pass through the spray region intermittently as they move alternately through the spray region and the annular region. The circulation times of the particles vary, even for the same particle, under different circulation actions. In addition, the process As shown in Fig. 2, an experimental apparatus was designed to simulate the characteristics of the spray coating process described above. A rotating inner cylinder was used to simulate the continuous spraying on a moving particle in the spray region, in order to overcome the limitations of a small spray area and non-uniformity of droplet distribution due to the nozzle. In this experimental apparatus, fluidized coating of the surface of urea particles was modeled by first coating a layer of urea of about 1 mm thick on the inner cylinder as substrate. The system shown in Fig. 2 includes a motor, a nozzle and two cylinders with the inner cylinder 100 mm in diameter and the outer cylinder 150 mm in diameter. There was an opening of 80 mm in width on the outer cylinder that was used for latex spraying onto the inner cylinder. Compressed gas at a set temperature was supplied to the inner cylinder and the gap between the inner and outer cylinders. The gas temperature and flow rate were controlled to model the temperature field and dewatering process in the fluidized bed. During coating, latex fed at a controlled rate by a peristaltic pump, was atomized by pressurized gas through the nozzle and sprayed onto the inner cylinder through the opening, and, as the inner cylinder rotated, this spray area was moved away from the opening and the latex on the cylinder surface was dewatered by the gas flow. In order to simplify the random, intermittent and multiple coating process, periodic spraying was used. The spray time and interval were controlled by a solenoid valve which controlled the peristaltic pump. 3. Experimental 3.1. Material The materials used in the experiments were large granular urea (China Blue Chemical Ltd.) and polyacrylic acid latex (40% solid content, Department of Chemical Engineering, Tsinghua University). For characterizing film permeability, analytical grade urea (Fine Chemicals Co., Ltd., China) was used to prepare the urea solution, the concentration of which was first measured. 4-Dimethylaminobenzaldehyde (Analytical Grade, Sinopharm Chemical Reagent Co., Ltd., China), absolute ethyl alcohol (Analytical Grade, Moderneastern Fine Chemicals Co., Ltd., China), and hydrochloric acid (Analytical Grade, Moderneastern Fine Chemicals Fig. 2. Experimental apparatus to model spray coating. 1, gas compressor; 2, flow meter; 3, gas temperature controller; 4, heater; 5, gas outlet; 6, nozzle; 7, opening; 8, outer cylinder; 9, inner cylinder; 10, gas inlet; 11, motor; 12, time controller (solenoid valve); 13, peristaltic pump; 14, latex vessel.

4 512 R. Lan et al. / Particuology 9 (2011) Co., Ltd., China) were used for preparing the colorimetric solution, which was mixed into the urea solution for urea concentration measurement using an HP 8453 ultraviolet spectrophotometer, with 4-dimethylaminobenzaldehyde as an indicator. Silica gel (Modern Eastern Fine Chemicals Co., Ltd., China) was used as desiccant. Deionized water was used in the preparation of the solutions Equipment As shown in Fig. 1, a Wurster fluidized bed with a distributor diameter of 150 mm was used for preparing the film coated urea particles. In the experimental apparatus used, as shown in Fig. 2, a constant temperature and constant humidity chamber (LRH- 250-HS, Taihong Medical Equipment Co., Ltd., Shaoguan, China) was used for the measurement of film permeability. A micrometer (0 25 mm, accuracy 10 m) was used to measure film thickness. An electronic balance (range 210 g, accuracy 0.1 mg, METTLER TOLEDO AL204, Switzerland) was used for weighing. A scanning electron microscope (JSM7401, JEOL, Japan) was used for characterizing surface morphology Film preparation Urea substrate preparation. Urea in a beaker was melted by heating in an oil bath (145 C). The inner cylinder was immersed in the molten urea and then removed to form a uniform urea substrate on the inner cylinder surface. Spray coating of the film. Latex was pumped to the nozzle by a peristaltic pump, atomized by pressurized air and sprayed onto the inner cylinder through the opening in the outer cylinder. With rotation of the inner cylinder, periodic coating and dewatering were accomplished. After multiple spraying, a film thickness of 100 m was obtained. According to exploratory experiments, satisfactory films could be obtained with the conditions shown in Table 1. In order to obtain film with about the same thickness, the latex amount used in each experiments was the same which was 20 ml with 40% solid content. Latex was sprayed onto the inner cylinder while rotating at 20 rpm. In order to simulate the intermittent-multiple spray coating process, a spray time and period were set. It was estimated that in the Wurster fluidized bed, particle circulation time was 19 s and residence time 0.5 s in the spray region. Therefore, a 16-s interval and 3-s spraying were used in the spray coating simulation experiments. The opening of outer cylinder was 1/6 of the circumference, so as to simulate particles being sprayed continuously for 0.5 s in the spray region of the fluidized bed. The distance between the nozzle and inner cylinder was 300 mm, which was considered enough to allow the latex droplets to be distributed uniformly on the spray area. Latex droplets present in the shear field of the spray flow and the environment of low humidity will experience dewatering, which is more significant for latex droplets with a high solid content. This may influence the quality of the sprayed films. Therefore, latex with Table 1 Design conditions from exploratory experiments. Gas temperature ( C) Spray rate (ml/min) Latex solid content (%) Spray distance (mm) Rotation speed (r/min) Drying gas (m 3 /h) Atomizing gas (m 3 /h) Spray interval Spray 3 s, pause 16 s Fig. 3. Measurement of the water vapor permeability coefficient of the film. 1, sealing container; 2, hygrometer; 3, support; 4, water; 5, sealing cover; 6, sealing washer; 7, film; 8, plastic vial; 9, desiccant. a low solid content (13.3%) was used except for experiments that investigated the effects of latex solid content. For the apparatus design and experimental verification of the feasibility of the simulation of the spray coating process in a fluidized bed, typical operating conditions were used. The optimal conditions for getting an ideal film yet needs more experimental study Water vapor permeability measurement An improved version of the method reported by Devassine, Henry, Guerin, and Briand (2002) was used for the measurement of the water vapor permeability of the films. The film was sealed with a rubber washer onto the mouth of a plastic vial containing desiccant. The vial was placed inside a sealed constant-temperature container, as shown in Fig. 3. At any set temperature, the water vapor pressure in the container was thus constant, and it was zero inside the vial due to the desiccant. Therefore, water would enter the vial through the film as driven by the pressure difference of water vapor. A hygrometer was placed inside the sealing container to measure its humidity. The vial was weighed after a set time. The mass change of water is described by dm(t) dt = p h w A P L, (1) where m(t) is water mass in the vial, A is the film area, P is the water vapor partial pressure difference between the inside and outside of the vial, L is the average thickness of the film, p h is the water vapor permeability coefficient of the film, and w is the water vapor density Urea permeability measurement The method reported by Sun et al. (1999) and Yao (2005) was employed for urea permeability measurement, using two cells connected by a union as shown in Fig. 4. The film was clamped between the two cells. At the beginning of measurement, one cell was filled with 70 ml urea saturated solution containing 10 g urea to ensure saturation during measurement. The other cell was filled with 70 ml deionized water. The temperature during measurement was controlled at 25 C. The urea concentration in the deionized water cell was measured after a set time. The urea permeability coefficient of the film was determined using Eqs. (2) (5). The diffusion rate of urea across the film is much lower than that of water, so that it was taken that urea dispersed quickly in the water once it diffused across the film. Since the urea solution was kept saturated during measurement, variation of urea concen-

5 R. Lan et al. / Particuology 9 (2011) Results and discussion 4.1. Experimental modeling of spray coating Fig. 4. Measurement of urea permeability coefficient of the film. 1, film; 2, cell; 3, urea saturated solution; 4, urea; 5, union; 6, deionized water. tration in the cell of urea solution could be ignored. By neglecting the variation of film volume, the diffusion-controlled process is described by J s = p s F, (2) where p s is the urea permeability coefficient of the film, F is the process driving force. At steady state, F is the urea concentration difference between the two cells, F C s = L. (3) Inasmuch as urea solution in one cell was kept saturated during measurement and the urea concentration in the deionized water cell was very low, the concentration difference could be considered constant, i.e., C s = C in C out = Csat, (4) where C sat is the saturated urea concentration. Therefore, the rate of urea diffusion is dm(t) dt = J s A = p s A C sat L, (5) where m(t) is the urea mass that diffuses into the deionized water cell. The structures of the films prepared by spray coating in the fluidized bed and the experimental model apparatus are shown in Fig. 5. Fig. 5A shows that there were many porous regions in the films prepared in the fluidized bed at low spray rates of latex and the film became smooth with an increase of the spray rate of latex. Fig. 5B shows that the films prepared in the experimental model apparatus have structures similar to those prepared in the fluidized bed. For both cases, the characteristics of how the porous structures change with latex spray rate were also similar, although the structure and flow patterns in the fluidized bed and experimental model apparatus were quite different. This shows that the experimental model apparatus is adequate for studying the spray coating process in a fluidized bed Factors affecting the spray coating process Flow rate of atomizing gas Fig. 6 shows the effect of the flow rate of the atomizing gas on film surface structures. From exploratory experiments, the atomizing gas flow rate of 1.2 m 3 /h is considered optimal, while 0.5 m 3 /h and 1.5 m 3 /h are near the marginal condition and beyond the suitable operation range. At an atomizing gas flow rate of 0.5 m 3 /h, the latex droplets formed are very large, leading to latex accumulation. When the urea substrate was dissolved, there resulted an obvious porous structure of the film. At an atomizing gas flow rate of 1.5 m 3 /h, there were also some, though not many, pores in the film. The effect of the flow rate of the atomizing gas on film permeability is also shown in Table 2: no film formation at 0.5 m 3 /h and permeability coefficients for 1.2 m 3 /h much lower than those at 1.5 m 3 /h Spray rate of latex Fig. 7 shows the effect of latex spray rate on film structure: many pores ( 10 m) for spray rate of 5 ml/min, possibly due to poor spreading of latex droplets. At a spray rate of 10 ml/min, smooth and dense films were obtained, and at 15 ml/min, the film surface was loose and porous, suggesting that a low dewatering capacity Fig. 5. Comparison of surface structures of films prepared in (A) the fluidized bed and in (B) the experimental model apparatus.

6 514 R. Lan et al. / Particuology 9 (2011) Fig. 6. Effect of atomizing gas flow rate on film surface structure. Table 2 Effect of atomizing gas flow rate on the film permeability. Atomizing gas (m 3 /h) p h ( m 2 /(s Pa)) p s ( m 2 /s) Other conditions are as listed in Table 1. resulted in urea dissolving-recrystallization that formed needlelike urea crystalline grains, which left holes in the film after the crystalline grains were dissolved. In order to study films prepared at a high spray rate, 1/3 the total amount of latex was sprayed at 10 ml/min onto the urea substrate in advance to prevent urea dissolution, and the remainder was then sprayed at 20 ml/min to complete the coating. The films prepared under this condition (denoted as ml/min ) were smooth and dense. The effect of spray rate on film permeability is shown in Table 3, and film permeability coefficients vary with surface structure, as shown in Fig. 7. Permeability coefficients of the film prepared at 15 ml/min were much higher than those of the other three samples. At this spray rate of 15 ml/min, water can accumulate on the substrate, causing urea to dissolve, thus leading to Table 3 Effect of latex spray rate on the film permeability. Spray rate (ml/min) p h ( m 2 /(s Pa)) p s ( m 2 /s) Other conditions are as listed in Table 1. some large holes left in the film when it was immersed in water. Permeability coefficients of film prepared at 5 ml/min were similar to those prepared at 10 ml/min, suggesting that the pores seen on the film surface (Fig. 7) were mainly cavities. The morphology and permeability of the films prepared at 10 ml/min and ml/min were similar, suggesting that satisfactory films can also be obtained at a higher spray rate of latex by preventing urea dissolving-recrystallization in the spray coating process. p h and p s listed in Tables 2 5 were the average values of two measurements. For the same film, the measured values showed little difference between the two indicating very good reproducibility. However, p h and p s were very sensitive to film structure, especially for p s, which was more sensitive Fig. 7. Effect of latex spray rate on film surface structure.

7 R. Lan et al. / Particuology 9 (2011) Fig. 8. Effect of latex solid content on film surface structure. Table 4 Effect of latex solid content on the film permeability. Latex solid content (%) p h ( m 2 /(s Pa)) p s ( m 2 /s) Other conditions are as listed in Table 1. Table 5 Effect of gas temperature on the film permeability. Gas temperature ( C) p h ( m 2 /(s Pa)) p s ( m 2 /s) Other conditions are as listed in Table 1. to the porous structure and increased sharply when a relatively large size hole appeared, suggesting that pore structure decisively affects the permeability coefficient. However, details call for further studies Solid content of latex Fig. 8 shows the effect of latex solid content on film structure, that is, film smoothness decreased with increasing latex solid content. The rough structure on the film prepared with 20% latex solid content became more obvious when the solid content increased to 30%. Table 4 shows the effect of latex solid content on film permeability, indicating how the permeability coefficients increased with increasing latex solid content, and this trend for urea permeability coefficient was more so than that for the water vapor permeability coefficient. The urea permeability coefficient of the film prepared at 30% solid content was so large as to suggest the presence some large holes in the film Gas temperature The glass temperature (T g ) of the latex used in this research was 23 C. Liu (2009) studied spray coating in a fluidized bed using latex and reported that C was a good temperature for the formation of a dense and uniform film. Thompson and Kelch (1991, 1992, 1993) reported that better coating and drying could be achieved at temperatures of C when T g of latex was C. So their gas temperature was set at C. The film surface morphology Fig. 9. Effect of gas temperature on film surface structure.

8 516 R. Lan et al. / Particuology 9 (2011) Fig. 10. Film formation in the spray coating process. 1, latex droplet; 2, urea substrate; 3, film. was found to be significantly affected by the gas temperature, as shown in Fig. 9. The sprayed films prepared at 35 C were smooth. When the gas temperature was higher than 40 C, there were some porous structures in the films and the area of the porous structures increased with increasing gas temperature. The film permeability coefficients also increased, as shown in Table Analysis of spray coating process Our experiments showed that film permeability strongly depended on film structure, which depended in turn on the conditions of film formation. The features of random, intermittent and multiple coating-dewatering are the characteristics of the spray coating process in a fluidized bed, which includes three steps: (1) latex droplets reaching the substrate, (2) latex droplets spreading on the substrate, and (3) latex dewatering and film formation. These steps occur repeatedly in the spray coating process until a whole film is formed, as shown in Fig. 10. Water in the latex was sprayed onto the substrate in the 3-s spraying process and removed by hot gas flow in the succeeding 16-s interval. The dewatering capacity significantly affected the spray coating process. When the dewatering capacity matched the water input, satisfactory films were obtained. When the dewatering capacity was insufficient, urea dissolving-recrystallization occurred, which resulted in formation of urea crystallites mixed in the films. However, too fast dewatering caused the droplets to be deprived of water, leading to bubbles embedded in the film because of the high viscosity of the droplets which hindered water transfer and weakened droplet spreading. The following droplet deposition on the substrate formed holes in the film thus deteriorating its smooth and dense structure. Factors such as flow rate of the atomizing gas, spray rate of latex, solid content of latex and gas temperature affect water transfer during film formation. They are the key factors in the spray coating process. The flow rate of the atomizing gas determines the diameter of the droplets and water loss from the droplets. The spray rate of latex determines the quantity of water input. The solid content of the latex affects both water input and droplet properties. The gas temperature affects the dewatering rate of the latex on the substrate. 5. Conclusions It is argued that the characteristics of the spray coating process in a fluidized bed are random-intermittent-multiple coating and dewatering. Experimental results showed that films prepared using our experimental model apparatus were similar to films coated on the urea particles in a fluidized bed. The effects of operating parameters on film properties were also similar to those in the fluidized bed. Therefore, experimental modeling can be used for studying the spray coating process. When the gas temperature, solid content of latex or flow rate of atomizing gas were relatively high, the films prepared had a porous structures due to poor spreading of the droplets on the substrate. When the latex spray rate was relatively high or atomizing gas rate was relatively low, urea dissolving-recrystallization occurred, as was caused by poor dewatering capacity, to result in urea crystallites in the films. These remained as pores in the films after the crystallites dissolved. Factors that affect water transfer, such as latex spray rate, solid content of latex, gas temperature and flow rate of atomizing gas are the key factors in the spray coating process. Acknowledgement The authors wish to express their appreciation of financial support of this study by the National Natural Science Foundation of China (NSFC No ). References Chen, D. M., Wang, T. J., Yu, S. J., & Jin, Y. (2002). Review on the research and development of control-release urea and slow-release urea. Chemical Industry and Engineering Progress, 21(7), (in Chinese). Devassine, M., Henry, F., Guerin, P., & Briand, X. (2002). Coating of fertilizers by degradable polymers. International Journal of Pharmaceutics, 242(1 2), Donida, M. W., & Rocha, S. C. S. (2002). Coating of urea with an aqueous polymeric suspension in a two-dimensional spouted bed. Drying Technology, 2(3), Liu, Y. H. (2009). Studies on preparation and release characteristics of slow/controlled release urea by spray film coating. Unpublished doctoral dissertation, Tsinghua University. McGinity, J. W. (1997). Aqueous polymeric coatings for pharmaceutical dosage forms (2nd ed.). New York: Marcel Dekker., pp Mendoza-Romero, L., Pinon-Segundo, E., Nava-Arzaluz, M. G., Ganem-Quintanar, A., Cordero-Sanchez, S., & Quintanar-Guerrero, D. (2009). Comparison of pharmaceutical films prepared from aqueous polymeric dispersions using the cast method and the spraying technique. Colloids and Surface A: Physicochemical and Engineering Aspects, 337(1 3), Shaviv, A, Raban, S., & Zaidel, E. (2003). Modeling controlled nutrient release from a population of polymer coated fertilizers: Statistically based model for diffusion release. Environmental Science & Technology, 37(10), Sun, Y. M., Huang, W. F., & Chang, C. C. (1999). Spray-coated and solution-cast ethylcellulose pseudolatex membranes. Journal of Membrane Science, 157(2), Thompson, H. E., & Kelch, R. A. (1991). Encapsulated slow release fertilizers. WO Patent No. 1991/ Thompson, H. E., & Kelch, R. A. (1992). Encapsulated slow release fertilizers. US Patent No. 5,089,041. Thompson, H. E., & Kelch, R. A. (1993). Encapsulated slow release fertilizers. US Patent No. 5,186,732. Tzika, M., Alexandridou, S., & Kiparissides, C. (2003). Evaluation of the morphological and release characteristics of coated fertilizer granules produced in a Wurster fluidized bed. Powder Technology, 132(1), Yao, J. J. (2005). The preparation of controlled-release urea by film coating and the investigation of the release mechanism. Unpublished masteral dissertation, Tsinghua University.