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Evaluation of the Thermosensitive Release Properties of Microspheres Containing an Agrochemical Compound
2017 Volume 65 Issue 1 Pages 49-55
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2017 Volume 65 Issue 1 Pages 49-55
The purpose of this study was to develop a deeper understanding of the key physicochemical parameters involved in the release profiles of microsphere-encapsulated agrochemicals at different temperatures. Microspheres consisting of different polyurethanes (PUs) were prepared using our previously reported solventless microencapsulation technique. Notably, these microspheres exhibited considerable differences in their thermodynamic characteristics, including their glass transition temperature (Tg), extrapolated onset temperature (To) and extrapolated end temperature (Te). At test temperatures below the To of the PU, only 5–10% of the agrochemical was rapidly released from the microspheres within 1 d, and none was released thereafter. However, at test temperatures above the To of the PU, the rate of agrochemical release gradually increased with increasing temperatures, and the rate of release from the microspheres was dependent on the composition of the PU. Taken together, these results show that the release profiles of the microspheres were dependent on their thermodynamic characteristics and changes in their PU composition.
Developing sustained-release formulations for agrochemicals is required to diminish the need for the frequent application of these materials, to reduce the environmental exposure, as well as to improve overall agricultural efficiency.1) Numerous studies have been reported pertaining to microencapsulation, which is a well-known technique for the controlled release of small molecules.2–6) Microencapsulation involves the use of liquid droplets or solid particles as core materials, which are covered with a shell-like material, such as an inorganic compound or polymer, to give microencapsulated particles on the micron scale. Products prepared in this way are known as microcapsules, microspheres or microparticles, depending on their morphological characteristics and internal structure.7,8) Microencapsulation has been used to mask the taste and odor characteristics of specific substances, control the release of active ingredients, protect core materials from external environmental factors and improve the ease with which toxic materials can be handled in the pharmaceutical, agricultural, printing and food industries.9–13)
Solventless microencapsulation processes have recently been studied by several groups because they are simple, cost effective and environmentally friendly.14–16) We previously reported the development of a new solventless microencapsulation process involving agitation granulation and chemical reaction steps.17) In our previous report, solid agrochemicals were agglomerated and covered with polyurethane (PU), which was synthesized in situ from polyol and isocyanate monomer units (Fig. 1(A)) during the agglomeration process. The resulting PU-covered microencapsulated samples had a median diameter of less than 75 µm and showed sustained release over several months at 25°C. From a result of inner structural analysis by synchrotron X-ray computed tomography, multiple cores of agrochemical compound and other solid excipient were dispersed in PU. In addition, after agrochemical compound was released, no change in its framework was observed, although voids emerged in the microencapsulated following the release of agrochemical compound. The experimental release data were highly consistent with the Baker–Lonsdale model derived from drug release of spherical monolithic dispersions.18) These results therefore suggest that our solventless microencapsulation techniques could be used to address some of the issues described above for the formulation of agricultural materials.
The optimal temperature for agricultural cultivation can vary considerably depending on the nature of the crop targeted. The optimum daytime cultivation temperatures of many Japanese crops are in the range of 15–30°C. To ensure that agrochemicals reach their maximum efficacy, further drug release studies are necessary to develop a deeper understanding of their release characteristics from microencapsulated samples at a variety of different temperatures consistent with the optimum cultivation temperature. It was envisaged that the release profiles of agrochemicals from microencapsulated samples would change depending on the temperature, according to the thermodynamic characteristics of the polymer used to coat the core structure. However, there have been very few reports pertaining to the relationship between the release profiles of microencapsulated materials at different temperatures. Furthermore, very little is known about the roles played by the thermodynamic characteristics of the polymeric shell during the release process in terms of its transition from a glass state to a rubber state. Further work is also required to develop a deeper understanding of the key physicochemical parameters affecting the release profiles of microencapsulated materials.
In this study, we used our previously reported microencapsulation method to prepare several microencapsulated samples with different PU compositions and a wide variety of thermodynamic characteristics. All of these samples were prepared using one isocyanate and two trifunctional and one bifunctional polyol. The physicochemical properties and release profiles of these microencapsulated samples were determined at a variety of different temperatures. The ultimate goal of this study was to not only evaluate the effects of the PU composition on the physicochemical properties of the microencapsulated samples, but to also develop a deeper understanding of the thermosensitive release properties of these materials.
Clothianidin (CTD), which is an agrochemical compound (Fig. 1(B)), and CTD pre-mix (CTD/clay=70/30, d50=10–20 µm) were supplied by Sumitomo Chemical Co., Ltd. (Tokyo, Japan). Fumed silica (AEROSIL300) was procured from Nippon Aerosil Co., Ltd. (Tokyo, Japan). Trifunctional polyether polyol (Sumiphen S429 (polyol S429), molecular weight (MW): 700; Sumiphen TM (polyol TM), MW: 450) and bifunctional polyether polyol (Sumiphen 1600U (polyol 1600U), MW: 1000) were procured from Sumika Bayer Urethane Co., Ltd. (Hyogo, Japan). Poly(phenylene methylene isocyanate) (polymeric MDI) (Sumidur 44V-10; Functionality, 2.3–2.5) was procured from Sumika Bayer Urethane Co., Ltd. 2,4,6-Tris(dimethylaminomethyl)phenol (TAP) was procured from Kayaku Akzo Corporation (Tokyo, Japan).
Preparation of PU FilmsThe PU films were prepared according to the following method to prevent the occurrence of any thermodynamic effects on the CTD in the microspheres. The compositions of these materials are shown in Table 1. The reactant ratio of isocyanate to hydroxyl moieties was set to one. Isocyanate was added to the polyols, and the resulting mixture was vigorously stirred for 1 min to give a homogenous mixture. The mixture was then cast onto a glass plate using an applicator set at 375 µm. The glass plate was then held in an oven at 80°C for over 3 h to terminate the urethane reaction.
| PU name | PU compositions (%w/w) | ||||
|---|---|---|---|---|---|
| Isocyanate | Polyol mixture | ||||
| Sumidur 44V-10 | Sumiphen TM (polyol TM) | Sumiphen 1600U (polyol 1600U) | Sumiphen S429 (polyol S429) | TAPa) | |
| TM (100) | 47.7 | 51.3 | 1.0 | ||
| TM/1600U (60/40) | 39.4 | 35.8 | 23.8 | 1.0 | |
| TM/1600U (53/47) | 37.7 | 32.5 | 28.8 | 1.0 | |
| TM/1600U (50/50) | 36.9 | 31.1 | 31.0 | 1.0 | |
| TM/1600U (40/60) | 34.3 | 25.9 | 38.8 | 1.0 | |
| S429 (100) | 36.9 | 62.1 | 1.0 | ||
a) TAP was used as a catalyst for the PU reaction and mixed with the polyol mixture.
DSC thermograms of PU were measured in an aluminum pan using a DSC Q100 system (TA Instruments, New Castle, DE, U.S.A.). The DSC curves were recorded a temperature in the range of −90 to 200°C at a heating rate of 10°C min−1 under a nitrogen atmosphere. The extrapolated onset temperature (To), extrapolated end-point temperature (Te) and midpoint temperature were determined from the DSC curves.19) The midpoint temperature is commonly used as the glass transition temperature (Tg).
Preparation of CTD Microsphere (CTD-MS) Samples Using an Agitation Granulation ProcessAll of these samples were prepared according to the agitation granulation process (Fig. 2) reported in our previous study.17) Briefly, 300 g of CTD pre-mix (CTD content: 69.1%) was loaded into a high-speed mixer (EarthTechnica Co., Ltd., Tokyo, Japan) and heated to 80±5°C with the agitator and chopper blades set at 850 and 2500 rpm, respectively, to mix the material. A predetermined amount of polyol mixture (Table 2) was added over 2 min to the pre-mixed material, and mixing was continued for 3 min. A predetermined amount of isocyanate (Table 2) was then added to the mixture over 2 min, and mixing was continued for 6 min. This sequential addition process (i.e., polyol mixture and isocyanate) was repeated totally 24 times. The encapsulated samples were then annealed for 30 min at 80±5°C to complete the PU polymerization process. Fumed silica (2.4 g) was then added to the encapsulated sample, and the resulting mixture was mixed for 3 min to prevent agglomeration. After cooling to ambient temperature, the encapsulated samples were sieved thorough a JIS standard sieve (sieve size 300 µm); the sieved samples were defined as CTD-MS. The coating ratio and microsphere yield were calculated using the following equations:
 | (1) |
 | (2) |
| Sample name | Amount of isocyanate per addition (g) | Amount of polyol mixture per addition (g) |
|---|---|---|
| TM (100)-CTD-MS | 3.58 | 3.93 |
| TM/1600U (60/40)-CTD-MS | 2.96 | 4.54 |
| TM/1600U (53/47)-CTD-MS | 2.83 | 4.67 |
| TM/1600U (50/50)-CTD-MS | 2.77 | 4.73 |
| TM/1600U (40/60)-CTD-MS | 2.57 | 4.93 |
| S429 (100)-CTD-MS | 2.77 | 4.73 |
Five grams of each CTD-MS sample was sieved for 5 min on a sieve shaker (Vibratory Sieve Shaker AS 200, Retsch) equipped with two sieves (75, 150 µm). The sieved samples were weighed. Each CTD-MS sample was evaluated in triplicate.
Scanning Electron Microscopy (SEM)The surface morphologies of the CTD-MS samples were observed by SEM (S-3000N, Hitachi, Ltd., Tokyo, Japan). Each CTD-MS sample was sputter-coated with a Pt–Pd alloy in an ion sputter before being observed by SEM.
CTD Content in CTD-MSOne hundred milligrams of each CTD-MS sample was extracted with 100 mL of tetrahydrofuran in a glass vial. The resulting suspension was filtered through a syringe filter (pore size=0.45 µm) and the filtrate was analyzed by HPLC with a reversed-phase column (Inertsil ODS-EP 5 µm, ϕ4.6×150 mm; GL Sciences, Tokyo, Japan). A wavelength of 270 nm was selected for the detection of the CTD by ultraviolet absorption. The column was eluted with a mobile phase consisting of acetonitrile–water–phosphoric acid=200 : 800 : 1 (v/v/v) at a flow rate of 1.0 mL/min. Three measurements were carried out for each of the CTD-MS samples. The encapsulation ratio was calculated using Eq. 3:
 | (3) |
A small portion (52 mg) of each CTD-MS sample was added to 450 mL of three-degree hard water, which was prepared by dissolving MgCl2 (21.7 mg) and CaCl2 (47.4 mg) in 1000 mL of distilled water under stirring at 100 rpm. The temperature of the test solution was kept at 15.0±0.3, 20.0±0.3, 25.0±0.3 or 30.0±0.3°C. Two milliliters of the test solution was taken at a predetermined time and the CTD content in the solution was quantified by HPLC, as described above. The release amount of CTD was determined using Eq. 4:
 | (4) |
Each CTD-MS sample was evaluated in triplicate. Release tests were continued up to 56 d until almost 100% of the CTD was released.
Statistical Analysis of Release ProfilesThe difference factor (f1) was used to evaluate the difference between two release profiles.20) The f1 was calculated by the following equation:
 | (5) |
CTD pre-mix and CTD-MS samples were analyzed by PXRD (Rigaku Rotaflex RU-200B, Rigaku Corp., Tokyo, Japan) under the following operating conditions: voltage, 30 kV; current, 15 mA; target, Cu; scanning speed, 4°/min; 2θ range, 3–40°.
Six different PU films (Table 1) were prepared and analyzed by DSC to determine some of their thermodynamic characteristics, including their To, Tg and Te values. The results in Table 3 show that the PU films containing larger amounts of isocyanate had higher To, Tg and Te values. As shown in Fig. 3, there was a linear relationship between the amount of isocyanate in the PU films and their Tg values. This result therefore indicated that the thermodynamic characteristics of PU films could be tailored by adjusting the amount of isocyanate in the PU composition. However, a comparison of the TM/1600U (50/50)-film with the S429 (100)-film, which both contained the same amount of isocyanate, revealed that whilst both of these films had the same Tg value, the S429 (100)-film had higher To and lower Te values. The S429 (100)-film was therefore found to change in a narrow range from the glass state to the rubber state. This change could be attributed to differences in the chemical structures of these films. For example, the PU used to prepare the S429 (100)-film was composed entirely of trifunctional polyol, making its structure much more uniform than that of the TM/1600U (50/50)-film.
| Sample name | Extrapolated onset temperature To (°C) | Glass transition temperature Tg (°C) | Extrapolated end temperature Te (°C) |
|---|---|---|---|
| TM (100)-film | 67.5 | 78.7 | 90.0 |
| TM/1600U (60/40)-film | 15.1 | 30.4 | 45.7 |
| TM/1600U (53/47)-film | 3.5 | 21.3 | 39.1 |
| TM/1600U (50/50)-film | −2.6 | 16.2 | 35.0 |
| TM/1600U (40/60)-film | −12.3 | 5.3 | 22.8 |
| S429 (100)-film | 3.1 | 15.4 | 27.6 |
Six CTD-MS samples with different PU compositions but the same coating ratio were prepared using our solventless microencapsulation process. Table 4 shows the physicochemical properties of these six CTD-MS samples, including their microsphere yield and encapsulation ratio data. With the exception of the TM/1600U (40/60)-CTD-MS, the microsphere yields of the other five samples were >95%. The encapsulation ratios of all six samples were >98%. Our previous result also showed all encapsulation ratios of 14 samples using this technique were >95%,17) indicating that this microencapsulation process would be robust even if the PU compositions changed. As shown in Fig. 4, the median diameters of all the CTD-MS samples were less than 75 µm, except for TM/1600U (40/60)-CTD-MS. Furthermore, the median diameter increased as the amount of isocyanate in the PU composition increased. The results of our previous report17) showed that the solventless microencapsulation process involved two phases, including (i) an aggregation phase, where the primary particles were aggregated by PU; and (ii) a coating phase, where the aggregated particles were covered by PU. Given that PU materials with low isocyanate contents are highly adhesive because of their low Tg values, they can form large aggregates in their aggregation phase. Figure 5 shows SEM images of the different CTD-MS samples. These images revealed that there were no discernible differences between the different samples. The PU composition was therefore found to have very little impact on the surface morphology of the CTD-MS samples.
| Sample name | Microsphere yielda) (%) | CTD content in microspheres (%) | Encapsulation ratiob) (%) |
|---|---|---|---|
| TM (100)-CTD-MS | 95.1 | 42.65±0.08 | 99.2 |
| TM/1600U (60/40)-CTD-MS | 97.5 | 42.54±0.12 | 99.0 |
| TM/1600U (53/47)-CTD-MS | 97.3 | 42.40±0.10 | 98.7 |
| TM/1600U (50/50)-CTD-MS | 96.3 | 42.52±0.10 | 99.0 |
| TM/1600U (40/60)-CTD-MS | 85.4 | 42.34±0.06 | 98.5 |
| S429 (100)-CTD-MSe) | 96.5 | 42.82±0.10 | 99.7 |
a) Microsphere yield (%)=Amount of encapsulated sample passed through a No. 50 sieve/Total amount of encapsulated sample before sieving. b) Encapsulation ratio (%)=CTD content in the CTD-MS/Theoretical CTD content in the CTD-MS.c) c) Theoretical CTD content in the CTD-MS=(CTD content in the CTD pre-mixd)×100)/((100+coating ratio)×1.005)=42.97%. d) CTD content in CTD pre-mix: 69.1%. e) The values for physicochemical properties of this sample are taken from Terada et al.17)
Figure 6 shows the release profiles of six different CTD-MS samples at four different test temperatures (15–30°C), and Table 5 shows the differences between two release profiles. As for the five CTD-MS samples with PU containing polyol TM, CTD was released at a much greater rate from CTD-MS samples containing lower amounts of polyol TM for all of the temperatures tested in the current study. The amount of CTD released from the CTD-MS samples increased with increasing temperature, except for TM (100)-CTD-MS. Notably, the differences observed in the release profiles of the different CTD-MS samples at different temperatures were much more pronounced for the CTD-MS prepared with lower amounts of polyol TM. The results of the release tests for these samples showed that their release temperatures were greater than their To values, except for TM (100)-CTD-MS. The thermodynamic characteristics of the PU materials changed from those of a glass state to a rubber state across all of the temperature ranges tested in the current study. The increase in the free volume within the PU as a result of the transition to the rubber state made it much easier for the CTD to diffuse through the PU. Furthermore, pronounced changes in the thermodynamic characteristics of the PU led to considerable differences in the release profiles of CTD-MSs at different temperatures. In contrast, the release profile of TM (100)-CTD-MS remained largely unchanged for all of the temperatures tested in the current study, with 5–10% of the CTD being released within 1 d, followed by negligible CTD releases thereafter. The glass state of the PU probably prevented the diffusion of CTD through its structure, only allowing for the CTD near to the surface of the CTD-MS to be released, as exemplified by the Te values being higher than all of the test temperatures.
(A) 15°C, (B) 20°C, (C) 25°C and (D) 30°C. The release data of S429 (100)-CTD-MS at 25°C was obtained from our previous report.17)
| Release profile 1 | Release profile 2 | f1 | Equivalence |
|---|---|---|---|
| TM (100)-CTD-MS at 15°C | TM (100)-CTD-MS at 20°C | 5.9 | Yes |
| TM (100)-CTD-MS at 15°C | TM (100)-CTD-MS at 25°C | 5.7 | Yes |
| TM (100)-CTD-MS at 15°C | TM (100)-CTD-MS at 30°C | 0.1 | Yes |
| TM (100)-CTD-MS at 20°C | TM (100)-CTD-MS at 25°C | 11.0 | Yes |
| TM (100)-CTD-MS at 20°C | TM (100)-CTD-MS at 30°C | 5.7 | Yes |
| TM (100)-CTD-MS at 25°C | TM (100)-CTD-MS at 30°C | 6.0 | Yes |
| TM/1600U (60/40)-CTD-MS at 15°C | TM/1600U (60/40)-CTD-MS at 20°C | 17.4 | No |
| TM/1600U (60/40)-CTD-MS at 15°C | TM/1600U (60/40)-CTD-MS at 25°C | 32.4 | No |
| TM/1600U (60/40)-CTD-MS at 15°C | TM/1600U (60/40)-CTD-MS at 30°C | 54.7 | No |
| TM/1600U (60/40)-CTD-MS at 20°C | TM/1600U (60/40)-CTD-MS at 25°C | 18.1 | No |
| TM/1600U (60/40)-CTD-MS at 20°C | TM/1600U (60/40)-CTD-MS at 30°C | 45.2 | No |
| TM/1600U (60/40)-CTD-MS at 25°C | TM/1600U (60/40)-CTD-MS at 30°C | 33.0 | No |
| TM/1600U (53/47)-CTD-MS at 15°C | TM/1600U (53/47)-CTD-MS at 20°C | 20.1 | No |
| TM/1600U (53/47)-CTD-MS at 15°C | TM/1600U (53/47)-CTD-MS at 25°C | 43.0 | No |
| TM/1600U (53/47)-CTD-MS at 15°C | TM/1600U (53/47)-CTD-MS at 30°C | 65.0 | No |
| TM/1600U (53/47)-CTD-MS at 20°C | TM/1600U (53/47)-CTD-MS at 25°C | 28.6 | No |
| TM/1600U (53/47)-CTD-MS at 20°C | TM/1600U (53/47)-CTD-MS at 30°C | 56.2 | No |
| TM/1600U (53/47)-CTD-MS at 25°C | TM/1600U (53/47)-CTD-MS at 30°C | 38.6 | No |
| TM/1600U (50/50)-CTD-MS at 15°C | TM/1600U (50/50)-CTD-MS at 20°C | 17.4 | No |
| TM/1600U (50/50)-CTD-MS at 15°C | TM/1600U (50/50)-CTD-MS at 25°C | 44.4 | No |
| TM/1600U (50/50)-CTD-MS at 15°C | TM/1600U (50/50)-CTD-MS at 30°C | 60.2 | No |
| TM/1600U (50/50)-CTD-MS at 20°C | TM/1600U (50/50)-CTD-MS at 25°C | 32.7 | No |
| TM/1600U (50/50)-CTD-MS at 20°C | TM/1600U (50/50)-CTD-MS at 30°C | 51.8 | No |
| TM/1600U (50/50)-CTD-MS at 25°C | TM/1600U (50/50)-CTD-MS at 30°C | 28.4 | No |
| TM/1600U (40/60)-CTD-MS at 15°C | TM/1600U (40/60)-CTD-MS at 20°C | 19.1 | No |
| TM/1600U (40/60)-CTD-MS at 15°C | TM/1600U (40/60)-CTD-MS at 25°C | 28.8 | No |
| TM/1600U (40/60)-CTD-MS at 15°C | TM/1600U (40/60)-CTD-MS at 30°C | 40.5 | No |
| TM/1600U (40/60)-CTD-MS at 20°C | TM/1600U (40/60)-CTD-MS at 25°C | 12.5 | No |
| TM/1600U (40/60)-CTD-MS at 20°C | TM/1600U (40/60)-CTD-MS at 30°C | 27.3 | No |
| TM/1600U (40/60)-CTD-MS at 25°C | TM/1600U (40/60)-CTD-MS at 30°C | 17.6 | No |
| S429 (100)-CTD-MS at 15°C | S429 (100)-CTD-MS at 20°C | 27.1 | No |
| S429 (100)-CTD-MS at 15°C | S429 (100)-CTD-MS at 25°C | 59.9 | No |
| S429 (100)-CTD-MS at 15°C | S429 (100)-CTD-MS at 30°C | 70.0 | No |
| S429 (100)-CTD-MS at 20°C | S429 (100)-CTD-MS at 25°C | 45.1 | No |
| S429 (100)-CTD-MS at 20°C | S429 (100)-CTD-MS at 30°C | 62.1 | No |
| S429 (100)-CTD-MS at 25°C | S429 (100)-CTD-MS at 30°C | 35.7 | No |
We also compared TM/1600U (50/50)-CTD-MS and S429 (100)-CTD-MS, which contained the same amount of isocyanate in their PU. Although the release profiles of these two materials were almost identical at 15°C, CTD was released at a much greater rate from S429 (100)-CTD-MS at temperatures higher than 20°C (Fig. 6, Table 6).
| Release profile 1 | Release profile 2 | f1 | Equivalence |
|---|---|---|---|
| TM/1600U (50/50)-CTD-MS at 15°C | S429 (100)-CTD-MS at 15°C | 2.1 | Yes |
| TM/1600U (50/50)-CTD-MS at 20°C | S429 (100)-CTD-MS at 20°C | 13.5 | Yes |
| TM/1600U (50/50)-CTD-MS at 25°C | S429 (100)-CTD-MS at 25°C | 29.4 | No |
| TM/1600U (50/50)-CTD-MS at 30°C | S429 (100)-CTD-MS at 30°C | 32.7 | No |
As shown in Table 3, the Tg value of the S429 (100)-film was almost identical to that of the TM/1600U (50/50)-film, but the difference between the Tg and Te values of the S429 (100)-CTD-film was narrower than that of the TM/1600U (50/50)-film. The differences in thermodynamic characteristics of these PU films may be responsible for the differences observed in the release profiles of S429 (100)-CTD-MS and TM/1600U (50/50)-CTD-MS at temperatures above 20°C (>Tg). As shown in Fig. 7, the PXRD patterns between CTD pre-mix and two CTD-MSs were almost same, indicating that the changes of crystallinity of CTD before and after microencapsulation did not occur and this was not involved in the changes of release behaviors of CTD from the CTD-MSs.
We have investigated the thermodynamic characteristics of the PU materials used during the microencapsulation of CTD by adjusting their composition. The release profiles of the CTD-MS were found to be strongly affected by the thermodynamic characteristics of the PU. At test temperatures below the To of the PU, around 5–10% of the CTD was rapidly released from the CTD-MS within 1 d, and none was released thereafter. In contrast, at test temperatures above the To of the PU, the rate of the release of CTD from the CTD-MS gradually increased with increasing temperatures, and the rate of release was dependent on the composition of the PU. This could be attributed to changes in the thermodynamic characteristics of the PU as it transitioned from a glass state to a rubber state, leading to an increase in its free volume. In addition, the particle size and surface morphology characteristics of the CTD-MS showed very little dependence on the composition of the PU, except for PUs with very low glass temperatures. In conclusion, we have developed an optimized procedure for the microencapsulation of agrochemicals using a specially designed PU.
The authors declare no conflict of interest.
