Demand for water dispersible granules (WDGs) in the pesticide market has been increasing since the 1980s, mainly due to their good flowability and low risk of dust inhalation when used ascompared to the conventional wettable powder (WP) formulation.WDGs are manufactured in many different ways, such as extrusion granulation, fluidized bed granulation, high shear granulation, tumbling granulation, dry roller compaction granulation and spray dry granulation. In particular, WDGs prepared by a fluidized bed granulator have an irregular shape with a large surface area, resulting in good disintegration ability in water. The typical fluidized bed granulation of WDGs has been conducted by spraying a binder solution onto a fluidizing powder mixture containing a solid active ingredient prepared by a dry milling process in advance. In order to obtain better WDG performance, a direct granulation method is proposed, in which an agrochemical suspension containing particles of a solid active ingredient prepared by a wet milling process is sprayed and granulated in a fluidized bed granulator.This production method has the following advantages as compared to the conventional
fluidized bed granulation of WDGs:
(i) the size of active ingredient particles in the granules becomes smaller than those prepared by conventional fluidized bed granulation, leading to good dispersion stability in a spraying tank and efficient coverage of the plant surface in use;
(ii) dust explosion risks are minimized due to the wet milling process; (iii) risks of crosscontamination
are low, as this operation is conducted in a closed system from milling to granulation. Despite these advantages,only a few studies have been reported, and no systematic work has been done on this granulation method.
In our previous study, the granule growth mechanism of this granulation method was elucidated, and the effect of operating conditions on the physicochemical properties of WDGs was investigated experimentally. In this paper, the effect of the binder composition on the physicochemical properties of WDGs was investigated, and the main factors that affect the granule mass median diameter and disintegration time in water were determined. The composition of the agrochemical suspension used in this study is given in Table 1a. Procymidone (Sumitomo Chemical Co., Ltd., Tokyo., Japan) was chosen as a model active ingredient due to its low water solubility and high thermal stability. Sodium
lignosulphonate (REAX85A: Ingevity Corporation, South Carolina,USA) and polyvinyl alcohol (PVA) (Gohsenol GL-05: The Nippon Synthetic Chemical Industry Co., Ltd., Osaka, Japan)were used as a binderand a dispersant. The mass ratio of sodium lignosulphonate and PVA was varied while maintaining the total weight at 23.8%, and the effect of the binder composition on the physicochemical properties of the WDG was investigated. In this paper, to represent the composition of binders, only
the PVA content in the composition of each WDG (Table 1b) is used for simplicity. Procymidone was mixed into an aqueous solution of sodium lignosulphonate and PVA containing a defoamer (Dow Corning® Antifoam C emulsion, food grade: Dow Corning Silicones Malaysia Sdn Bhd., Selangor Darul Eshan, Malaysia), and the obtained mixture was pre-milled using a homogenizer (Polytron PT6100, Kinematica AG.) to decrease the size of coarse particles. The obtained agrochemical suspension was milled again using a bead mill (Dyno-mill, Shinmaru Enterprises Co., beads diameter: 1mm, beads filling rate: 80% of chamber volume, speed of agitator disc: 10m/s) by adjusting the feed rate and pass times to decrease the procymidone particle size to 1–2 µm. The viscosity of the agrochemical suspension was measured using a rheometer (AR-G2, TA Instrument Japan Inc.). The spray mist size was measured as previously reported.14) In order to evaluate the effect of the binder composition on the water evaporation efficiency, the relationship between drying time and water content of each agrochemical suspension was measured using a halogen moisture analyzer (Halogen Moisture Analyzer HR, Mettler-Toledo International Inc.) at a sample quantity of 5 g and a drying temperature of 80°C. In this study, the same operating conditions were applied for the granulation of all agrochemical suspensions. A schematic diagram of the fluidized bed granulator (FS-100, Dalton Co., Ltd.) is shown in Supplemental Fig. S1. In the beginning of the granulation, around 25% of the agrochemical suspension was spraydried under the drying condition to prevent spray mists from adhering to the wall of granulator (Inlet air temperature: 80°C, Air flow rate: 1.1m3 /min, Spray air pressure: 0.30MPa, Liquid flow rate: 5.9 g/min). Then the fluidized air flow rate and liquid flow rate were gradually increased to enlarge the granules (Inlet air temperature: 80°C, Air flow rate: 0.16→0.84 m3 /min, Spray air pressure: 0.10MPa, Liquid flow rate: 2.9→15.8 g/min) . The particle-size distribution, scanning electron microscope (SEM) images, and disintegration time were measured as previously reported. The WDG hardness was measured using a method similar to that previously described14) (size of glass beads: 4mm, weight of glass beads: 10 g, weight of granules: 5 g, mesh size of a coarse sieve to remove the beads: 1400µm). In this study, the hardness results were described as the percent of fine powder quantity under 125 µm. In order to evaluate the water solubility of the binder mixture of sodium lignosulphonate and PVA, which might impact the disintegration properties of WDGs in water, the following experiments were carried out. Aqueous solutions containing sodium lignosulphonate and PVA, as shown in Table 1a, were spread on a stainless bat and dried in an oven at 80°C. The obtained solid was ground and sieved to obtain the granules (250–300µm) of the binder mixture. The time to complete dissolution was measured using the same method as for the disintegration time of WDGs. Fig. 1a shows the effect of PVA content on the mass median diameter of the WDG. The mass median diameter almost linearly increased with an increase in PVA content. The main factors to affect the mass median diameter of the WDG were considered to be
(i) spray mist size,
(ii) retention of water in a spray mist when dried and
(iii) viscosity of the agrochemical suspension.
Fig. 1b shows the relationship between volume median diameter of spray mist and PVA content under two types of spraying conditions. The spray mist size under the granulation condition was larger than the one under the spray-drying condition and no significant impact of the binder composition on spray mist size was seen. Fig. 1c illustrates the relationship between drying time and the mass ratio of remaining water to initial water quantity in the agrochemical suspension. As compared at the same drying time, the remaining water ratio decreased with an increase in PVA content between 0.0% and 2.0%. The results of the suspension containing 2.0% and 5.0% PVA, however, showed almost the same water content while the mass median
Fig. 1. Effect of PVA content on various physicochemical properties of agrochemical suspension, WDG and binder mixture.
(a) Mass median diameter of WDG,
(b) Spray mist size,
(c) Mass ratio of remaining water to initial water quantity in agrochemical suspension after 30min, 40min and 50min drying,
(d) Shear rate on viscosity,
(e) Disintegration time of WDG,
(f) Hardness(percent of fine powder quantity) of WDG,
(g) dissolution time of each binder mixture.
diameter of the WDG with 5.0% PVA was much larger than that of the WDG with 2.0% PVA. The retention of water in the spray mist when dried could not explain the relationship between PVA content and mass median diameter. The reason for the change in the evaporation property at a high PVA content might be that PVA formed a film on the water surface during drying; however, after water surface was completely covered with the PVA film, the excess PVA dissolved in water did not affect the film formation, resulting in no additional suppression effect of water evaporation. Fig. 1d demonstrates the viscosity of each agrochemical suspension against the PVA content. Although all of the agrochemical suspensions showed a Newtonian flow property, regardless of PVA content, the viscosity increased with an increase in PVA content. It was thus found that the mass median diameter of the WDG increased with an increase in viscosity of the agrochemical suspension. Based on the above results, the main factor to affect granule growth was considered to be the viscosity of the agrochemical suspension, which increased as the PVA content increased. Thus, it could be possible to estimate granulation efficiency by evaluating the viscosity of agrochemical suspensions without preparing WDGs. Fig. 1e shows the effect of PVA content on disintegration time. The disintegration time increased with an increase in PVA content. Factors that may explain this tendency are
(i) hardness of the WDG,
(ii) solubility of the binder, and
(iii) surface morphology of the WDG.
Fig. 1f demonstrates the effect of PVA content on WDG hardness (percent of fine powder quantity). It was found that the hardness increased with an increase in PVA content. However, as seen in Fig. 2a, the correlation between hardness and disintegration time was not very clear. Fig. 1g illustrates the effect of the binder composition on the water solubility of the binder mixture. The water solubility of the binder mixture tended to decrease with an increase in PVA content. Figure 2b shows the relationship between the water solubility of the binder mixture and the disintegration time. As water solubility decreased, disintegration time increased. The correlation was much clearer than the relationship between hardness and disintegration time, which is shown in Fig. 2b. As seen in Supplemental
Fig. S2, no significant difference was observed in the surface morphology of each granule, indicating that surface morphology of the WDG is not a main factor to explain difference in disintegration properties observed in this study. Considering the above results, it was concluded that the main factor to affect disintegration time was the water solubility of the binder mixture. Thus it could be possible to estimate the disintegration time by the evaluating water solubility of the binder mixture without preparing a WDG. In conclusion, a WDG was prepared through a direct granulation of an agrochemical suspension containing a mixture of sodium lignosulphonate and PVA as a binder using a fluidized bed granulator. The effect of the binder composition on the physicochemical properties of a WDG was investigated experimentally. The volume median diameter increased with an increase in viscosity of the agrochemical suspension, which increased with an increase in PVA content. The disintegration time increased with a decrease in the water solubility of the binder mixture, which decreased with an increase in PVA content. It was thus concluded that the direct granulation of an agrochemical suspension using a fluidized bed could be controlled by appropriate binders considering the viscosity of the agrochemical suspension and the water solubility of the binder mixture, although further studies using other binders are desired to confirm the generality of the findings obtained in this study.
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