Biobased Polyurethane, Epoxy Resin, and Polyolefin Wax Composite

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Applications of Polymer, Composite, and Coating Materials

Bio-based Polyurethane, Epoxy Resin and Polyolefin Wax Composite Coating for Controlled-release Fertilizer Hongyu Tian, Zhiguang Liu, Min Zhang, Yanle Guo, Lei Zheng, and Yuncong C. Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16030 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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ACS Applied Materials & Interfaces

Bio-based Polyurethane, Epoxy Resin and Polyolefin Wax Composite Coating for Controlled-release Fertilizer Hongyu Tian†, Zhiguang Liu*, †, Min Zhang*, †, Yanle Guo†, Lei Zheng‡, Yuncong C. Li§ †

National Engineering Laboratory for Efficient Utilization

of Soil and Fertilizer Resources, College

of Recourses and Environment, Shandong Agricultural University, Taian, 271018, China ‡

State Key Laboratory of Nutrition Resources Integrated Utilization, Kingenta Ecological Engineering

Group Co., Ltd., Linshu, 276700, China. §Department

of Soil and Water Science, Tropical Research and Education Center, IFAS, University of

Florida, Homestead, FL 33031, USA

Corresponding Author * (Z. L.) E-mail: [email protected]. Phone: 86-538-824 1531. Fax: 86-538-824 2500. (M.Z.) E-mail: [email protected]. Phone: 86-538-824 1531. Fax: 86-538-824 2500. Notes The authors declare no competing financial interest. Synopsis This study provides a novel combined application of starch-based polyurethanes, epoxy resins and polyolefin waxes for controlled release fertilizers.

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ABSTRACT: Reducing the use of petrochemical products in coated controlled-release fertilizers while regulating the release rate is a popular research topic in the field of controlledrelease fertilizers. In this study, a novel bio-based polyurethane (BPU), epoxy resin (ER) and polyolefin wax (PW) composite coating method for the controlled release of urea was successfully established. The method involved:1) the use of PW as a modified inner coating, which improved fertilizer surface performance and reduced urea surface roughness; 2) the degradable BPU film was synthesized with liquefied starch (LS) as the outer coating material; and 3) epoxy resin is a protective layer, which improved the hydrophobicity of the coated urea for controlled release. The chemical structure, thermostability and microscopic morphology of composite-coated urea (CCU) were examined by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM), respectively. A central composite design of response surface methodology was used to examine the effects of different film percentage, PW contents, and BPU/ER ratios on nutrient release behavior. The results showed that PW optimized the fluidity, thermal insulation properties and microscopic surface of the particles and improved the uniformity of the heating of urea. When the same amount of ER was used, the CCU has a three-fold increase in the release period compared to that of the cross-linked interpenetrating coated urea. Polynomial mathematical models were established for CCU preparation and could be an effective tool for manufacturing CCUs with specific nutrient release characteristics which could meet the nutrient requirements of crops in different cropping systems. The new coating method introduced in this study could guide the development of a new generation of bio-based controlled-release fertilizers. KEYWORDS: Composite-coated urea; Surface response model; Nutrient release; Coated controlled-release fertilizers; Coating material

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INTRODUCTION Urea is the most used fertilizer in the world and has contributed greatly to crop production since it was synthesized commercially and used for agriculture.1-4 However, fertilization with more urea than a crop needs has resulted in low nitrogen (N) use efficiencies, which may negatively affect groundwater, surface water, and the atmosphere through leaching, runoff, and volatilization of N.5-6 Additionally, low N use efficiencies lead to decreased economic returns to growers from their fertilizer investments.3,5 Polymer-coated controlled-release urea (CRU) has been proven as a technology to solve the problem of low N use efficiencies and to relieve environmental pressures.7-8 Compared with those obtained by using treatments with normal urea, the yields of wheat and maize obtained by using CRU increased by 8.2–11.9% and 6.8-9.8%, and the N use efficiencies were improved by 35.7–37.6% and 13.2–14.3%, respectively.9-11 Researchers also found that wheat and maize yields were increased by using CRU compared with using normal urea.12-14 In addition, CRU not only improved the N use efficiencies but also decreased NO3 −-N leaching.15-16 Applying only once at planning with CRUs saved labor costs and time compared to the multiple applications during crop growing season with conventional urea.6,8 Because of the mechanism of nutrient transmembrane diffusion, the coating materials are the key factor restricting the further development of CRU. Although considerable research has focused on synthesizing novel polymers for CRU, previous studies have paid much attention to petroleum-based synthetic materials such as polyenes, acrylic resins, glycerol ester and polysulfones for CRU and other controlled-release fertilizers.17 However, due to the costly 3



nature of the above materials and the expensive and complicated production processes, which cause serious pollution, the industrial production and large-scale application in agriculture of CRUs from petroleum-based polymers are difficult to realize.18 Moreover, these degradationresistant synthetic materials are made from nonrenewable resources and could be harmful to the environment.5,19 Recent studies have focused on bio-based coating materials including vegetable oil, lignin, chitin, keratin, cellulose, and starch, starch is used as coating material due to its positive characteristics of easy modification, low cost, nontoxic and degradable.20,21 However, due to the abundant hydrophilic groups and micropores of bio-based coating materials, the initial nutrient release rates and longevity of those fertilizers are often below the standard of general controlled-release fertilizer requirements.4,18,22 Not only the above features but also regional crop requirements, which regulate the nutrient release characteristics of such systems, greatly limit their large-scale commercial application.4 Thus, the development of a better controllable management strategy for CRU (seeking a superior performance coating) to decrease the initial nutrient release rate and to improve longevity.23-24 Al-Zahrani used wax to coat the fertilizer and found that the wax has the function of controlling nutrient release, and the wax is low in cost, has hydrophobic property. However, wax coated fertilizers release nutrients too quick and polyolefin wax is better than paraffin wax regarding controlling nutrient release. 3 Epoxy resin is a kind of widely used thermosetting polymer, with high adhesion strength, good water resistance and other excellent properties, has a wide range of applications in adhesives, coatings

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and other fields.18 Therefore, the release period of nutrients was prolonged by the combination of biomaterials and epoxy resins. In this research, a bio-based polyurethane (BPU) polymer was derived from liquefied starch and used as the coating material for CRU. Polyolefin wax (PW) was coated in the innermost layer to modify the core of the fertilizer surface and prevent the fertilizer from contacting water. Epoxy resin (ER) was further used as a hard shell to protect the bio-based coating material from water infiltration and external extrusion, thus optimizing the nutrient slow-release characteristics of CRU. Among all bio-based materials, starch is the most attractive candidate that is also suitable as a membrane material because of its low impurity, low cost, high yield, ready availability, renewability, and excellent biodegradability. Glucose in an aggregated state is the primary constituent of starch, which could be liquefied to provide hydroxyl groups for the synthesis of a BPU polymer for use as the coating material of CCUs. The organic matter and nutrients (mainly N and P) released from the starch-based coating material improved soil fertility after the coating was degraded by microbes. PW could eliminate defects and fill holes and cracks. Hence, it plays an important role in preventing water from penetrating into the urea surface. As the hydrophobic and hard protective shell, ER could prevent the film from being crushed and delay the process of water molecules entering the inner part of the non-water-tight BPU, thereby strengthening the control of N release and reducing the particle damage rate in the material transportation process. When PW, BPU and ER are coated in layers, the composite coating could be formed, and a response surface model is established by producing fertilizers with different parameters, including film percentage, PW content, and BPU/ER ratios. It is 5



hypothesized that the composite structure will make the new coatings more hydrophobic and wearable than most BPU polymers and that the response surface model will be an effective tool for manufacturing CCUs with specified nutrient release characteristics to meet regional crop requirements. To the best of the authors' knowledge, none of the studies attempted to use PW, BPU, and ER as a composite coating for CRU. The objective of this study was to develop a new method using bio-based polyurethane (BPU), epoxy resin (ER) and polyolefin wax (PW) composite coating materials for producing the controlled release urea fertilizer. First, starch was liquefied to produce bio-based polyols. Second, the fertilizer surface was modified with PW. Then, the BPU was synthesized using the bio-based polyols and polyaryl polymethylene isocyanate (PAPI) to coat urea fertilizer prills. After that, ER was applied to the last layer of the fertilizer. The relationships among the N release characteristics, film percentage, PW content, and BPU/ER ratios were investigated. Finally, a response surface model was established to provide a reference for different growing crops. This kind of coating method is novel, cheap and environmentally friendly and could provide a variety of possibilities for controlled-release fertilizers. EXPERIMENTAL SECTION Materials. Starch was obtained from the experimental farm of the National Engineering Laboratory for Efficient Utilization of Soil and Fertilizer Resources in Tai’an, Shandong, China. Polyolefin wax (PW), sulfuric acid (97%, v/v), diethylene glycol, and triethylenetetramine were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Polyaryl polymethylene isocyanate (PAPI) with 31.1% wt NCO groups was obtained from Yantai 6


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Wanhua Polyurethane Co., Ltd. (Shandong, China). E-44 was purchased from Shanghai Yugao Co., Ltd. (Shanghai, China). Urea prills (3−5 mm in diameter and 46.4% N) were purchased from Shandong Hualu Hengsheng Chemical Industry Co., Ltd. (Shandong, China). Preparation of Liquefied Starch. The liquefied starch was prepared by mixing diethylene glycol (127.5 g) and sulfuric acid (7.5 g) in a double-layer glass reaction kettle (3000 mL) equipped with a flux condenser, thermometer, and motor-driven stirrer. After preheating to 120°C, the reflux condenser was opened, and the motor-driven stirrer was set to rotate at a speed of 800 rpm. The starch (15 g) was then poured into the reaction kettle, mixed with the solution, refluxed, and continuously stirred. The mixture was reacted for 90 min. Finally, the liquefied starch (the bio-based polyols, Scheme 1a) was removed from the kettle for analysis and used as one of the coating materials. The starch could be completely liquefied, and the resulting liquid was used to synthesize the bio-based polyurethane (BPU) without separation or purification. As a result, the technology is easier and cheaper than other methods. Optimizing Experimental Design Using the Surface Response Model (SRM). The traditional polynomial modeling method is generally limited to second-order polynomials, while the response surface method, which is a statistical method to solve multivariate problems, uses the multiple quadratic regression equation to fit the functional relationship between the factor and the response value. To design the surface response model (SRM), center composite design in the Design-Expert V8.0.6 software was implemented. The PW content and ER ratio in the coating materials and the film percentage were selected as the design variables. Based on preliminary tests, the ratio of the amount of wax to the amount of urea was set at 0.25-0.8%, 7



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the ratio of epoxy resin (ER) to the total amount of coating material was set at 25-80%, and the percentage of the composite coating material was set at 3-7%. The initial nitrogen release rate, 28-day cumulative nitrogen release rate, 56-day cumulative nitrogen release rate , and 15-28 days cumulative nitrogen release rate were set as response values. The test design table (Table 1) is as follows: Table 1. Factors and Levels in Three-factor and Three-level Response Surface Analysis. polyolefin wax (%) epoxy resin ratio（%） film percentage (%) No. A

B

C

1

0.06

53

5.00

2

0.25

25

3.00

3

0.25

25

7.00

4

0.25

80

3.00

5

0.25

80

7.00

6

0.53

6

5.00

7

0.53

53

8.36

8

0.53

53

1.64

9

0.80

25

3.00

10

0.80

25

7.00

11

0.53

53

5.00

12

0.53

99

5.00

13

0.80

80

3.00

8


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14

0.80

80

7.00

15

0.99

53

5.00

16

0.53

53

5.00

17

0.53

53

5.00

18

0.53

53

5.00

19

0.53

53

5.00

20

0.53

53

5.00

Preparation of Composite-coated Urea (CCU). Fertilizer coated with the BPU polymer and ER composite was prepared on the laboratory scale from 1 kg of urea prills (3−5 mm in diameter and 46.4% N) for each coating sample. The prills were loaded into a rotating drum and preheated at 80 ± 2°C for 10 min. After the preheating stage, the PW particles was added based on Table 1 and the reaction was continued for approximately 5~10 min. Polyolefin wax-modified urea (PWU) was produced. Then, mixed BPU coating materials (6.5 g of PAPI and 3.5 g of liquefied starch) were dropped onto the surfaces of the rotating urea prills. The heat-curing reaction of the mixed coating materials was finished in the rotating drum in 8-10 min, and the BPU coating (Scheme 1b) was then synthesized and attached to the surface of the urea prills. The mechanism of BPU synthesis is illustrated in Scheme 1b. Each BPU coating material weighs approximately 1% of the weight of the urea. Bio-based polyurethane-coated urea (BPCU) particles of different percentage were produced with different coating rates by repeating the coating process for different times. Finally, the epoxy resin and triethylenetetramine are mixed and added in a ratio 9



of 8.5:1.5. Each ER coating material weighs approximately 1% of the weight of the urea, reaction about 5min. The final CCUs granules have a three-layer structure with urea as a core, polyolefin wax as the innermost layer, starch and polyurethane as the middle layer, and epoxy resin as the outermost layer. Scheme 1. Liquidation Mechanism of Starch (a) and Reaction Mechanism of the Biobased Polyurethane (b) and Epoxy Resin (c).

Next, the BPU membrane materials were coated. The same coating method as the one described above was used to coat the prills with ER (Scheme 1c). E-44 was first heated to 80°C in a water bath to soften it. A total of 10 g of coating material consisting of 8.5 g of E-44 and 1.5 g of triethylenetetramine was then used to coat urea prills in the drum. It took approximately 10


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6 min to finish the reaction. The mechanism of ER synthesis is illustrated in Scheme 1c. The weight of the ER coating was approximately 1% wt relative to urea. The process was repeated until the film was complete. Finally, 20 samples with different PW coating contents, ER ratios and coating percentage were obtained. Characterization. Fourier transform infrared (FTIR) spectra of starch, liquefied starch, E44, PAPI, ER, BPU and their composites were recorded using 380 FTIR spectrometer (Thermo Nicolet Corporation; Maine, USA) at a scanning range from 4000 to 500 cm−1. The thermal stability of the coating shells was evaluated by thermogravimetric analysis (DTG60A; Shimadzu Corporation; Tokyo, Japan). The morphologies of the coatings were examined using scanning electron microscopy (QUANTA250; FEI Company; Oregon, USA). The particle pressure strengths of urea, PWU, ERU and BPU were analyzed by a particle pressure strength meter (FT-803; Ruike Weiye Instrument Co., Ltd.; Zhejiang, China). The specific surface area of 0% and 0.53% PW-modified fertilizer cores of different percentage was analyzed with an ultra-precise three-dimensional laser scanning system (LDI Surveyor-ZS; Minnesota, USA). The water contact angles (WCAs) were measured using a contact angle meter (JC2000A; Jianduan Photoelectricity Technology Co., LTD; Shanghai, China) . Effect of Polyolefin Wax Dosage on Heating Rate. Six glass petri dishes with 15.00 g each of fertilizers containing 0, 0.06, 0.25, 0.53, 0.8 or 0.99% wax and another petri dish with 15 g pure wax as a control, were heated in an oven at 103°C. Every 5 mins, pictures were taken with an infrared camera (TESTO-869, TESTO, Germany) while recording the temperature. 11



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Measurement of Flow Performance. First, 200 mL of urea granules and 5.3% PWU were added to an angle of repose tester (FBS-104, FURBS, China), and the angle of repose (ϕ) was determined by the particle accumulation height (h). The fertilizer accumulation angle was calculated according to Equation 1. This procedure was repeated 5 times, and the results were averaged. ϕ=(arctan (h/5))*180/π

(1)

Nitrogen Release Characteristics of Controlled Release Urea with Various Coatings. The percentages of N release from different treatments of urea in the first 24 h were measured in water at 25°C. With three replicates, 10 g of the coated fertilizer was placed in a glass bottle containing 200 mL of deionized water and kept in an electroheating, standing-temperature incubator at 25 ± 0.5°C. The N released from each of the coated fertilizers at 1, 3, 5, 7, 15, 28, and 56 days or the time until the cumulative N release of fertilizers reached over 80% was measured. The N concentration was determined using the Kjeldahl method. 25 The N release longevity of the coated fertilizers is defined as the time when the cumulative N release reached 80% of the total N. Statistical Analysis. All statistical analyses were conducted using Excel 2010 and the Statistical Analysis System (SAS) package, version 9.2 (SAS Institute, Cary, NC). Comparisons of various treatments were evaluated using analysis of variance (ANOVA). Image processing was conducted with Sigmaplot version 12.5. Mean separation tests (Duncan’s multiple range test) were used to determine significant differences among treatments. Regression equations and coefficients were calculated between the N release longevity and 12


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coating rate for BPCU and ERU. The differences among means and correlation coefficients were considered significant when p < 0.05. RESULTS AND DISCUSSION Determination of Urea Particle Flowability and Particle Pressure Strength. The particle mobility and the comparison for repose angle of urea, polyolefin wax-modified urea at 80°C and polyolefin wax-modified urea at 25°C were measured, the results are illustrated in Figure 1. It was also found that the angle of repose of urea (A in Figure 1) is 28.9°, which is higher than that waxed urea at room temperature (angle of repose of 26.6°) (C in Figure 1); therefore, after modification, the friction decreases, and the flowability improves. For the waxed urea at a higher temperature (80°C), the flowability of the waxed urea is better than that of the waxed urea at room temperature (25°C). The angle of repose of the waxed fertilizer particles at 80°C (B in Figure 1) was 25.3°, which increased the fluidity and thus enabled more even wrapping of the film material.

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Figure 1. Repose angles of urea (A), polyolefin wax-modified urea measured at 80°C (B) and polyolefin wax-modified urea measured at 25°C (C).

Measurement of Surface Area. Fertilizer granules of urea and polyolefin wax-modified urea (PWU) have the same weight (0.0373 g), while modification with PW increased the surface area. The common urea particles (Figure 2a) had a surface area of 44.91 mm2, while the surface area of the PWU (Figure 2b) was 46.31 mm2, this is because the density of the wax (0.9g/cm3) is much smaller than the density of urea (1.335g/cm3), when urea and waxed urea have same weight, the waxed urea has a larger volume and therefore a larger surface area, thereby reducing the amount of coating and the cost of CRU.25 Still, but wax played a lubrication role to prevent the defects, the roundness of the PWU surface increased, and the pits present on the surface of 14


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the ordinary urea particles were covered, thereby making the particles smoother. The smooth surface is more easily covered, and the degree of binding of the membrane material to the fertilizer becomes better, thus making the fertilizer less vulnerable to the external environment.

Figure 2. Custom views of the laser scanning tests of (a) urea and (b) urea modified with 5% wt polyolefin wax. The tests were conducted using an ultra-precise three-dimensional laser scanning system. Morphologies of Urea, PWU, BPCU and ERU. SEM micrographs showed that the surface of urea (panel a1 of Figure 3) is rough with a layered superposition phenomenon (panel a2 of Figure 3) and that there is an irregular cascading bulge (surface). Many recessed holes and an irregular structure were observed in the micrographs of the urea surface (panel a3 of Figure3), which might be easily permeated by water, causing quick release of nutrients from the coated fertilizer. These features increase the contact area between the water molecules and fertilizer and accelerates the dissolution of the urea particles by water, causing rapid nutrient release. The surface of the PW-modified fertilizer particles (panel b1 of Figure 3) is smoother than that of the unmodified urea particles (panel a1 of Figure 3) because the wax filled the gap and 15



penetrated into the surface of the fertilizer, facilitating the adhesion of the outer polyurethane film. However, the PW coating has also produced concave extrusion (panel b1 of Figure 3) because the PW coating increases the surface brittleness and reduces the grain pressure intensity, especially during the cooling process. Nonetheless, as a natural hydrophobic polymer, PW is effective in separating soluble urea from water and thus reduces the initial nutrient release rate. Excess PW makes adhesion between particles easier, which could lead to destruction of the wax coating film integrity. Therefore, controlling the amount of PW and adding a layer with good pressure resistance, abrasion resistance, and strong rigidity are necessary to protect the internal coating.

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Figure 3. Scanning electron microscopy (SEM) images of urea and polyolefin wax-modified urea (PWU). Panels a1, a2 and a3 show the urea surface under 50×, 1000× and 5000× image magnification. Panels b1, b2 and b3 show the surface of the PWU under 50×, 1000× and 5000× image magnification. The BPU membrane material (panel b1 of Figure 4) has a very rough surface and could be seen from the plane. Due to the biological origin of the polyurethane membrane material, the structure is relatively loose and heterogeneous but not dense, and the boundaries between the membrane and fertilizer are not obvious because of the formation of gas during the preparation of the polyurethane, which causes foaming and leads to the membrane material surface having a certain porosity. Previous studies have also shown that in the preparation of bio-based membranes, the high porosity of coating materials is associated with rapid degradation. Adding starch increased porosity of coating materials. 27,28

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Figure 4. Scanning electron microscope (SEM) images of epoxy resin-coated urea (ERU), biobased polyurethane-coated urea (BPCU) and composite-coated urea (CCU). Panels a1, b1 and c1 show the surface of the ERU, BPCU and CCU under 30× image magnification. Panels a2, b2 and c2 show sections of the ERU, BPCU and CCU under 1000× image magnification. Water resistance is very important in the application of controlled release membrane shells. 26

The WCA of the BPU (Figure 5a) was nearly 43°, which is because the surface of the BPU

is porous and could be rapidly penetrated by water molecules. Compared to those of the BPU, the surface (Panel a1 of Figure 4) and section (Panel a2 of Figure 4) of ERU are smooth, dense and uniform. ER has good hydrophobicity and anti-extrusion and wear resistance abilities, so it 18


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can protect the inner coating from microorganisms and the external environment. The surface of the ERU coating shell was much smoother than that of BPCU, with a WCA of approximately 48° (Figure 5b). This result indicates that the epoxy resin is more hydrophobic than the BPU. 18

Figure 5. Water contact angle of (a) bio-based polyurethane and (b) epoxy resin measured using a contact angle meter. Finally, the cross-section of the CCU (panel c2 of Figure 4) shows a three-layer structure. The surface of the CCU (panel c1 of Figure 4) has the characteristics of ERU, and the interior has PW and BPU coatings, indicating that the CCU structure has been successfully prepared. Fourier Transform Infrared Analysis of Coating Shells. The FTIR spectra of starch, liquefied starch, PAPI, E-44, ER, and BPU revealed that chemical changes occurred during biomass liquefaction and the syntheses of BPU and ER (Figure 6). For example, in starch (the red line in Figure 6), the presence of −OH, C−C and C−O bonds corresponded to three characteristic absorption peaks for corn starch at 3423, 1157 and 1018 cm−1, respectively. 19



In LS (the blue line in Figure 6), however, a rightward shift was observed for the −OH peaks at 3384 cm−1 and 1354 cm−1, and the two characteristic absorption peaks at 1238 cm−1 and 1130 cm−1 represented the stretching vibration peak of the C−O bond in the ether group. This result indicates that after the liquefaction of starch, abundant alcoholic hydroxyl groups and partial ether bonds are formed, and these functionalities provide a soft segment for the formation of BPU. For PAPI (the black line in Figure 6), there are no peaks corresponding to NCO bonds at 2277 cm−1. A benzene ring skeleton vibration peak was observed in 1577 cm−1. The absorption peaks of the BPU shell (the green line in Figure 6) observed at 1135, 1065 and 3428 cm−1 were assigned to vibration of the O−H bond, and the peak observed at 1413 cm−1 was assigned to the stretching vibration of the C−O−C bond. The peak of 1614 cm-1 was assigned to the C=C stretching vibration of the double bond or benzene ring. The peaks at 1699 cm-1 correspond to the stretching vibration peak of the C=O bond in the carbonyl groups. The peak at 3032 cm-1 was ascribed to the telescopic vibration peak of the C−H bonds on the carbon atoms of the epoxy ring and the contraction vibration peak of the C−H bonds on the benzene ring. The characteristic peak of the benzene ring observed in BPU was also observed in PAPI, confirming that BPU was prepared from liquefied starch and PAPI. 29,30 The presence of −OH bonds was indicated by two absorption peaks characteristic of E-44 (the orange line in Figure 6) at 3486 and 1036 cm−1. The peaks at 3056, 1607 and 1510 cm-1 correspond to the C=C stretching vibration of the benzene ring, and the stretching vibration peaks of the C−O bond in the ether group are at 1248 and 1184 cm-1. 20


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The three characteristic absorption peaks of the ER shell (the purple line in Figure 6) at 3428, 1610, and 828 cm−1 corresponded to the stretching vibrations of the −OH groups, the C=C bonds in the aromatic ring, and the C−O−C bonds, respectively. 30

Figure 6. Fourier transform infrared spectra of liquefied starch (LS), starch, polyaryl polymethylene isocyanate (PAPI), E-44, epoxy resin (ER) and bio-based polyurethane (BPU). Effect of Wax Dosage on Heating Rate. Wax is a good heat storage material, and its specific heat capacity is 2.14–2.9 KJ (kg·K). In contrast, the specific heat capacity of urea (solid) is 1.55 KJ (kg·K). In Figure 7, which shows the temperature distribution of fertilizer particles in petri dishes, the color represents the temperature of the fertilizer in the petri dish. The more uniform the color is, the more uniform the temperature of the fertilizer particles. Black 21



represents a higher temperature, sea green represents a lower temperature, and the ordinate axis represents the number of fertilizer granules at a certain temperature. Among the samples tested, the U temperature distribution is the most uneven (Figure 7a), and the temperature rise, which is mostly in the range of 83.4-85.9°C, is rapid. In contrast, the temperature of the urea containing 0.53% PW (Figure 7c) is mostly 82.1-85.6°C, which is colder than urea. Additionally, 0.99% PWU (Figure 7d) is generally lighter in color than 0.53% PWU, and the particle temperature is mostly in the range of 81.1-84.7°C, indicating that the rate of change of the PWU temperature decreases as the amount of wax increases, thus indicating that the wax coated on the fertilizer particles has efficiency strong effect on temperature control. With simultaneous heating, the temperature of the PW particles (Figure 7b) is obviously lower than that of the urea particles (mostly at 67.6-71.0°C), which also proves the above postulation.

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Figure 7. The surface temperature distribution of fertilizer particles of (a) urea, (b) polyolefin wax, (c) urea modified with 0.53% wt polyolefin wax and (d) urea modified with 0.99% wt polyolefin wax. Distributions were measured using an infrared camera. Decomposition of Membrane Materials at High Temperatures. The thermal properties of the BPU and ER composites (Figure 8) were evaluated by TGA and differential thermal analysis (DTA). At a certain heating rate, the pyrolysis of BPU and ER proceeds through several different stages as the temperature increases. The results show that the ER has better thermal stability than the BPU. In contrast, the mass change of the BPU is more complicated than that of the ER. The process is divided into three weight loss stages, the first of which is in the 100210°C stage. The initial stage of pyrolysis is generally the loss of moisture, this process mainly 23



involves dehydration of the BPU, and the weight loss rate was approximately 15%.31The main weight loss peak of the BPU appears at 300-400°C, with a weight loss rate of 30%. This process is mainly the decomposition of the BPU. The final weight loss peak at 400-580°C is mainly the decomposition of ash formed by the coating materials in BPU or the cleavage of some residual covalent bonds. The residual weight for the BPU is 23%. 32 The ER has only weight loss one peak at 330-470°C, which is the main stage of the thermal degradation process of the ER. Most of the weight loss occurs in this region, and the differential value rapidly changes during this stage, which is mainly due to the decomposition of the ER polymer. The weight loss rate is as high as 90% and the ER residual mass is 7%. At 25-330°C, the ER weight loss rate was only 4.5%, and the BPU weight loss was 33%; thus, the thermal stability of the ER was better than that of BPU. The DTA experiments agree well with the TGA thermograms.

Figure 8. Thermogravimetric analysis (a) and differential thermal analysis curves (b) of the ER and BPU composite. 24


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Relationships among the Coating Ratio, Proportion of ER, Amount of PW, and Nutrient Release Characteristics. Under the same conditions, the nutrient release period for 0.53% PW and 53% ER gradually increases with increasing film percentage. The percentage of the coating was relatively large in a certain range, especially the prophase nutrient release rate. When the membrane percentage was 1.64% (Figure 9), the initial release rate of nutrients was 30.86%. When the membrane percentage increased to 5.00% and 8.36%, the initial nutrient release rate became 5.49% and 1.77%, respectively, because when the percentage of the coating is too low, the membrane material could not completely cover the surface of the fertilizer. Therefore, the nutrients are easily eluted to the portion not covered by membrane material, and the overly thin membrane material could not stop the infiltration by water molecules, thereby increasing the nutrient release rate of the fertilizer. The total nutrient release rate of urea with 1.64% coating percentage reached 80% on the 7th day, while the fertilizer with 5% mulching amount released 51.69% on the 28th day. The 28-day cumulative release rate fertilizer with 8.36% percentage coating was only 17.93%, demonstrating that the greater the percentage of the coating is, the lower the nutrient release rate, and the longer the release period. However, when a large amount of membrane material is needed to suitably ensure that the nutrient release rate is in line with crop growth, the production costs of the membrane increase. Similarly, the results for the fertilizer with 53% ER of and a film percentage of 5% under the same conditions indicate that increasing the amount of wax could reduce the nutrient release rate. When the amount of PW used is 0.99%, the initial nutrient release rate is 0.21%. The initial nutrient release rate was reduced to 5.49% and 12.96% when the PW content was reduced to 25



0.53% and 0.06%, respectively. In the late period of nutrient release (after 28 days), the effect of wax on nutrient release was reduced. In summary, the cumulative nitrogen release rate of fertilizers with low wax content was always less than that of fertilizers with high wax content. Because the PW has a good hydrophobic effect and could flatten the surface of the fertilizer, it could slow the rapid release of nutrients. However, an overly high wax content could make the particles stick together and increase the cost, so the amount of wax should be appropriately controlled.

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Figure 9. Nitrogen release characteristics of composite coated urea with different amount of polyolefin wax (PW), epoxy resin content (ER) and film percentage. 27



During the prophase, the quantity of ER coating had little effect on the nutrient release when the coating film percentage was 5.00% with 0.53% polyolefin wax, the nutrient release rates of CCUs were 0.8, 5.49 and 0% for 6, 53 and 99% ER coatings, respectively. The corresponding 7-day total nutrient release rates were 15.64, 17.86, and 0.17%, but from the seventh day to the 28th day, the difference in nutrient release rate between the fertilizers became apparent. During this period, the nutrient release rate of the CCU with 6% ER increases suddenly, that of the CCU with 53% ER slowly rises, and that of the CCU with 99% ER remains at a low level. The 28-day cumulative nitrogen release rates of the CCU with 6, 53 and 99% ER reached 85.16, 51.69 and 1.54%, respectively. These results showed that increasing the film percentage, the amount of PW and the coating ratio of ER could reduce the release of nutrients, although the mechanism of and effect on reducing the nutrient release were different. The results indicated that increasing the film percentage, increasing the amount of PW and the coating ratio of ER significantly improved the release longevity of coated fertilizers. Because the materials (triethylenetetramine and E-44) that are required for synthesizing 1 g of ERU are more expensive than that of 1 g of BPCU, BPCU could be a cheaper replacement of ERU. The ER is sticky, so that the prills were easily coherent. In previous studies, bio-based materials such as locust sawdust modified with ER were formed as BPCU with a cross-linked interpenetrating structure.30 However, in this work, the starch-based synthesis of the BPU combined with composite coating structure provides better results. For example, the release period of the cross-linked interpenetrating-type BPCU with a 28


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coating percentage of 5% using 50% ER is approximately 14 days, whereas the release period for the composite coating is approximately 56 days. At the same time, because the ER membrane is in the outermost layer of the fertilizer, it also causes the fertilizer to have superior physical and chemical properties in terms of compressive and wear-resistance, making it easy to transport and store. The release mechanism of nitrogen is that water molecules as vapor were diffused through the coating membranes and dissolved core materials such as urea. The dissolved urea molecules then were leaked out through micropores on the membranes which were expand depending on temperature. 24,27-29 Epoxy resin and polyolefin wax delay the entry of vapor and subsequently delayed nutrient release. 25,29 The effects of the coating ratio, proportion of ER and amount of PW on the design array for variable Y for nutrient release characteristics were evaluated. The model was selected according to the significance of the response value. The initial nitrogen release rate (Y1) response value was fitted with a quartic model, the process order of the 18-day and 56-day cumulative nitrogen release rate was quadratic. The resulting models relating the coating ratio, the proportion of ER and the amount of PW were determined as follows (Equations 1-3). Y1=2.11-3.79A-0.24B-8.65C+1.36AB+0.77AC+1.1BC+1.58A2-0.61B2+5.02C2-0.82ABC1.34A2B+7.46A2C+2.41AB2-6.20A2B2

(Equation 1)

Y2=62.47-2.99A-23.13B-20.86C+6.02AB-6.41AC+2.34BC-8.76A2-7.78B2-2.26C2 (Equation 2) Y3=86.75-5.1A-23.53B-15.02C+2.33AB-2.38AC-3.15BC-6.40A2-11.40B2-3.18C2 (Equation 3) 29



Here, Y1 was the initial nitrogen release rate , Y2 was the 28-day cumulative nitrogen release rate, Y3 was the 56-day cumulative nitrogen release rate, and A (the amount of PW), B (the proportion of ER), and C (the coating ratio) were the independent variables. The correlation coefficients R2 and adjusted R2 were evaluated. The latter coefficient reflects an adjustment for the number of model parameters relative to the number of points in the study. The variance analysis of the above model equations shows that the polynomial model used in this experiment is extremely significant, as the p-value for Y1, Y2, and Y3 are the proportion of ER, while the sequence of significance for the 28-day cumulative nitrogen release rate and 56-day cumulative nitrogen release rate was the proportion of ER > the coating ratio; the impact of the amount of PW minimal. The results of ANOVAs and other statistical analyses showed that in terms of interactions, A, B and C interact with each other on the initial release rate, indicating that the initial release rate is susceptible, while the long-term cumulative release rate and release period are less affected. The 3D images Figures 10 and 11 represent the fitting relationship between the variables and response values. Contour lines represent interactions between factors. The denser the contour lines are, the more that the response values are affected by the factors. The close relationship indicated that the N initial release rate of coated fertilizers could be well predicted with the three-dimensional fitting equation. When the initial nutrient release rate and total release longevity were given, the coating content, the proportion of ER, and the amount of PW could be easily calculated, which is useful to guide practical production. Based on factors such as the industry standard for controlled release fertilizer and control of the cost of the membrane material, the optimum process is a 5.4% coating ratio, a 22% proportion of ER and 0.5% PW.

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Figure 11. Simulated 3D modeling of Y1 (a1), Y2 (b1), Y3 (c1) by A and B factors, and the contour map of the interaction of AB factors with Y1 (a2), Y2 (b2), and Y3 (c2). 33



CONCLUSIONS The findings from this work showed that the composite coating is an effective and controllable approach for modifying the core surface of fertilizer, improving the release performance for controlled-release fertilizer, and reducing the use of petroleum-based products. The appropriate coating process could be selected according to the length of crop growth and nutrient absorption characteristics. The surface properties and physical properties of urea can be effectively improved by using PW, as observed from the results of SEM, SGKS, the determination of the angle of repose, and the measurements of the particle pressure strength and variation in fertilizer temperature after waxing. The CCUs has excellent nutrient release properties, which enable long-term nutrient control release. This study established a multi-item model that could regulate the release of nutrients by adjusting the amount and proportion of different layers. Therefore, it can provide nutrients to plants at different growth stages. ACKNOWLEDGMENTS The present study was supported by the National Key Research and Development Program of China (Grant no. 2017YFD0200706, 2018YFD0200604), the Natural Science Foundation of China (Grant no. 41571236), and Shandong Key Research and Development Plan Project (Grant no. 2017CXGC0306). We also thank Cong Jia for technical assistance. ABBREVIATIONS ANOVA, analysis of variance; BPU, bio-based polyurethane; BPCU, bio-based polyurethanecoated urea; CCU, composite coated urea; CRU, controlled-release urea; ER, epoxy resin; ERU, epoxy resin-coated urea; FTIR, Fourier transform infrared; LS, liquefied starch; PAPI, polyaryl 34


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polymethylene isocyanate; PW, polyolefin wax;

PWU, urea modified with polyolefin wax;

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 40


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Biobased Polyurethane, Epoxy Resin, and Polyolefin Wax Composite

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