Environmentally Friendly Slow-Release Urea Fertilizers Based on

Research Article pubs.acs.org/journal/ascecg

Environmentally Friendly Slow-Release Urea Fertilizers Based on Waste Frying Oil for Sustained Nutrient Release Xiaoqi Liu,† Yuechao Yang,*,†,§ Bin Gao,‡ Yuncong Li,§ and Yongshan Wan§ †

National Engineering Laboratory for Efficient Utilization of Soil and Fertilizer Resources; National Engineering & Technology Research Center for Slow and Controlled-Release Fertilizers, College of Resources and Environment, Shandong Agricultural University, Daizong Street No. 61, Taishan District, Taian, Shandong 271018, China ‡ Agricultural and Biological Engineering, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, Florida 32611-0570, United States § Department of Soil and Water Science, Tropical Research and Education Center, University of Florida, Homestead, Florida 33031, United States ABSTRACT: Novel biobased polyurethane (PU) was synthesized by using waste frying oil-based polyol (WFOP) and isocyanate for slow-release fertilizers (SRFs). Epoxy resin (EP) was used to modify PU to synthesize the interpenetrating network (IPN) for improving the properties of the coating film. Nine polymer-coated nitrogen (N) fertilizers were prepared from these composite coating materials. The N release characteristics of the PU-coated urea (PCU) in water and soil were determined. Degradation behavior of coatings in the soil was evaluated. Results showed that 20% EP in the coating increased the cross-linking density by 77.9%, reduced the coating porosity by 28%, and thus slowed the nutrient release from the PCU significantly. Because of its compact structure, the water absorption rate of the EP-modified PCU shell was slower than that of the unmodified ones. All the PCUs, especially the EP-modified ones, showed excellent slow N release behaviors. In addition, the biopolymers derived from waste frying oil also displayed good biodegradability in the soil environment. Findings from this work indicated that the slow release and environmentally friendly PCUs have great potential for applications in horticulture and agriculture. KEYWORDS: Polymer-coated urea, Waste frying oil, Polyurethane, Nitrogen release

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and atmospheric environments.12 These drawbacks restrict the large-scale use of PCFs in the field. Thus, development of renewable PCFs is necessary and urgent.13 A potentially effective way to solve this problem is to search for environmentally friendly and renewable sources as coating materials.14 Biomass sources, especially vegetable oils as a potential renewable energy source, have attracted much attention.15 Vegetable oil is renewable, available everywhere, and cleaner than fossil energy sources.16 It has been used as the raw material for resin preparation including vegetable oil-based polyurethane.17 Waste frying oil (WFO) is the main waste resource of the catering industry. In the literature, there are increasing reports about using WFO as a resource such for biodiesel, soap, washing powder, and chemical products.18 However, there is no report on using WFO-based polymer coating materials in slow-release fertilizers.

INTRODUCTION With a rapidly growing world population, demand for fertilizers has greatly increased to meet the agricultural production levels needed to satisfy world food demand.1 About half of the world’s grain production is attributed to fertilizer.2 On a global scale, nitrogen (N) fertilizer consumption is expected to increase to 130−150 million tons/year by 2050.3 However, low use efficiency of N fertilizer (about 30−50%) due to losses via runoff, leaching, and volatilization poses significant environmental concerns.4 Development of new fertilizers with low cost and high efficiency is of paramount significance for a sustainable agricultural system.5−7 Slow-release fertilizer (SRF), especially polymer-coated fertilizer (PCF) as a new type of fertilizer, can significantly improve nutrient use efficiency and crop yield.8 In fact, some PCFs have been used in agriculture because of their positive role in increasing production of cereal.9 However, the prices are about 2 times higher than conventional fertilizer. Besides, the coating polymer is usually derived from fossil energy sources.10,11 The petroleum-based coating is usually nondegradable and may release toxic gases, so it is harmful to soil © 2017 American Chemical Society

Received: March 23, 2017 Revised: May 24, 2017 Published: May 26, 2017 6036

DOI: 10.1021/acssuschemeng.7b00882 ACS Sustainable Chem. Eng. 2017, 5, 6036−6045

Research Article

ACS Sustainable Chemistry & Engineering

stirred for 15 h in a dark place to produce PAA. With agitation, PAA was slowly added into the flask through the isobaric funnel. The molar ratio of reagents was DB:H2O2:AA = 1:2:1.5. The reaction was carried out for 6 h with the water bath temperature held near 60 °C. The mixture was then put inside a refrigerator to stop the epoxidation reaction. After stratification, the aqueous layer was drawn off, and the oil layer was washed with dilute sodium carbonate until the pH value was neutral. The samples were washed with saturated salt water followed by distilled water. Finally, EWFO was obtained with vacuum distillation. The reaction mechanism is illustrated in Scheme 1a. The epoxy value content of the sample was 0.265 mol/100g, according to the methods of AOCS (Cd9-57).

WFO-based polymer coating materials have poor water resistance, partly due to WFO’s high acid value, low iodine value, and high water content.19 Meanwhile, WFO contains many free fatty acids, which is adverse for film forming. The prepared film has large pores and is easy to break.20 Recently, research efforts have been spent on modification of WFO to overcome these shortcomings. Some researchers enhanced the relative unsaturation of WFO through gradient freeze centrifugation. Others improved the functionality of oil-based coating materials by grafting functional groups, especially the hydrophobic group.21,22 Previous studies also proved epoxy resin had a positive effect on improving the physicochemical properties of oil-based polymers and was used for a series of high performance coatings in the manufacture of leather and rubber products.23−26 However, nobody applied it for preparation of coated fertilizers. The objective of the present work was to explore the potential application of WFO for development of PCFs. Herein, we increased the relative unsaturation of WFO by gradient freeze centrifugation. We then used it to prepare WFO-based polyurethane (WFOPU) through a series of chemical reactions. In addition, epoxy resin diglycidyl ether of bisphenol A was used to modify the coating to improve the properties of the film. We synthesized polyurethane/epoxy resin interpenetrating polymer networks (PU/EP IPNs) for preparing polymer-coated urea (PCU). The release behavior was determined in water and soil, respectively. Finally, the degradation behavior of the coating materials in soil was investigated. The ultimate goal was to develop a low cost, high efficiency, and environmentally friendly fertilizer.

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Scheme 1. Preparation Process of WFOP

Preparation of Waste Frying Oil-Based Polyol (WFOP). The EWFO and 1,2-propylene glycol with a molar ratio of epoxy groups to methanol of 1:11 were added to a 1-L flask with 1% fluorin boric acid added as a catalyst. The reaction was carried out at a temperature of 98 °C for 1.5 h to prepare WFOP. After cooling to room temperature, the reaction system was neutralized by adding ammonia. The unreacted 1,2-propylene glycol was rinsed out with distilled water. Finally, WFOP was obtained by distillation through vacuum pumping for 2 h. The reaction is illustrated in Scheme 1b. The hydroxyl value content of the sample was 198 mg KOH/g according to the methods of ASTM D 1957-86. Preparation of EP/PU IPNs Composites. A mixture of 28.3 g of WFOP and 16.8 g of MDI (with molar ratio of −NCO and −OH of 1.2) was prepared in a flask and kept at 80 °C. The reaction was completed after 10 min, and then, PU was obtained. The reaction forms a PU prepolymer at 3 min. Then, amounts of quantitative E44 (2.26, 4.51, 6.77, 9.02, 22.55, 36.08 g) liquated at 85 °C were added into the reaction system, respectively. After 10 min, 10 wt % triethylene tetramine as the epoxy hardener of E44 was added in the system. Six composites (labeled as PU/EP 5, 10, 15, 20, 50, 80) were obtained. Preparation of PCUs. PCU was prepared at the laboratory scale with 1 kg of urea prills (about 4 mm in diameter and 44% of N). A rotating heating drum machine (WKY-300, China) was used to coat the urea prills. After the urea prills were preheated to 80 °C for 10 min, 10 g of coating materials containing 3.73 g of MDI and 6.27 g of WFOP were sprayed slowly onto the surface of urea prills rotated at 35 rpm. The reaction was complete in 10−15 min, and WFO−PCU was produced. The weight of the PU coating was approximately 1% of the total weight of PCU. The above coating process was repeated until PCU containing 3%, 5%, and 7% of coating was prepared. The obtained PCUs were labeled as PCU1, PCU2, and PCU3, respectively. Preparation of EP-Modified PCUs. EP-modified PCUs were prepared following the method of preparation of PCUs. Urea prills were put into the rotating drum machine with 45 rpm rotation speed at 85 °C. Then, 10 g of each coating material, which included 3.39 g of MDI and 5.71g of WFOP, were dropped onto the rotating urea prills. After 2 min, 0.9 g of E44 preliquated at 85 °C was added into the machine. When the reaction was carried out for 5 min, 0.09 g of triethylene tetramine was added. The reaction was complete after 10 min. The reaction mechanism is shown in Scheme 2. The coating process was repeated with addition of 0.5 wt % of coating materials each time until the coating reached 3 wt %. Additional two types of modified PCUs that contained 10 and 20 wt % of E44 in the coating materials were prepared. The three EP-modified PCUs and were labeled as PCU1-1, PCU1-2, and PCU1-3, respectively.

MATERIALS AND METHODS MATERIALS

Waste frying oil was provided by the Shandong Agricultural University student cafeteria. Activated clay, as a special crystallization agent, was purchased from Nanjing Yadong Aotu Mining Co., Ltd. (Nanjing, China). Activated carbon, as a pigment adsorbent, was purchased from Chengde Xing Yuan Activated Carbon Co., Ltd. (Hebei, China). The E44 ([epoxy value] 0.41−0.47 equiv/100 g) was purchased from Guangzhou Kaiwei Green Chemical Co., Ltd. (Guangzhou, China). 1,2-Propylene glycol, sulfuric acid (98%), sodium hydroxide, hydrogen peroxide(30%), ethyl alcohol absolute, ether, and triethylene tetramine were provided by Kaitong Chemical Reagent Co., Ltd. (Tianjin, China). Fluorin boric acid was purchased from Aikeda Chemical Reagents Co., Ltd. (Chengdu, China). Diphenylmethane diisocyanate (MDI) with 30.03 wt % NCO group was bought from Wanhua Polyurethane Co. (Yantai, China). Urea (about 4 mm in diameter with 44% N) was obtained from Shandong Hualu Hengsheng Chemical Industry Co., Ltd. (Shandong, China). Preparation of High Unsaturated WFO. Preparation of high unsaturated WFO started with removing the residue impurity from WFO by filtering. Then, 8% activated carbon was added into WFO to adsorb pigment. The activated carbon was separated out by filtering after 20 min. The moisture was evaporated under vacuum by rotary evaporation. Activated clay (0.6%) was used as a crystallization agent, stirred into the WFO, and aged at 2 °C for 10 h, 1 °C for 8 h, and 0 °C for 6 h. The sample was then centrifuged under 10,000 rpm for 10 min, and impurities were removed by filtering to obtain high unsaturated WFO. Preparation of Epoxy Waste Frying Oil (EWFO). Synthesis of EWFO was performed in a 1-L, four-necked flask equipped with a mixer, a thermometer, a condenser, and an isobaric funnel. Here, 250 g of WFO was added to the flask. Peroxy-acetic acid (PAA) was preprepared by mixing 92.3 g of glacial acetic acid with 69.7 g of hydrogen peroxide and 3% concentrated sulfuric acid. The mixture was 6037


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ACS Sustainable Chemistry & Engineering Scheme 2. Synthesis Route of EP-Modified PCU by WFOP, MDI, E44, and Triethylene Tetramine

Figure 1. GC-MS chromatogram of original (A) and high (B) unsaturated WFO. between the wet shell (Ww) and dry shell (Wd) and using ρw and ρp as the densities of water and polymer, respectively, using eq 1

Characterization. The fatty acid contents of the original WFO and high unsaturated WFO obtained through gradient freeze centrifugation were examined using gas chromatography−mass spectrometry (GCMS, MSQ8100 GC/MS, China). Fourier transform infrared (FTIR) spectra of WFO, EWFO, WFOP, MDI, EP, PU, and EP/PU IPNs composites were analyzed by a Nicolet 380 FTIR spectrometer (USA) at a scanning range from 4000 to 500 cm−1. The 1H NMR and 13C NMR spectra of PU and EP/PU were recorded on a Bruker AVANCE III 500 MHz in CDCl3 as solvent. The thermal stability of the coating shells was evaluated by thermogravimetric analysis (TGA, DTG60A, Japan). Morphologies of the coatings were examined using scanning electron microscopy (SEM, QUANTA250, USA). The cross-linking degree of the coating materials was tested by a nuclear magnetic resonance cross-link density analyzer (VTNMR20-010 V_T, China). Coating porosity (ε) was determined based on the weight difference

ε=

Ww − Wd ρw Ww − Wd ρw

+

Wd ρp

× 100% (1)

Water Absorption Rate of Coating Materials. Water absorbency of coating materials was determined according to ref 27. Briefly, specimens (2 cm × 2 cm × 0.5 mm) of seven coatings were immersed in 300 mL of distilled water at ambient temperature (25 ± 0.5 °C). The samples were removed at specified time intervals and gently blotted with tissue paper to remove excess water on the surface. The weight of each swollen sample was recorded. Then, the samples were dried in an oven at 40 °C to constant weight. The water swelling and mass loss ratios were calculated according to eq 2 6038


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ACS Sustainable Chemistry & Engineering Water absorption (%) =

W 2 − W1 × 100% W1

Table 2. Relative Contents of Fatty Acids in High Unsaturated WFO

(2)

where W1 is the weight of the dried sample and W2 is the weight of the swollen sample. Nitrogen Release Characteristics of PCUs. To determine the release characteristics of the PCUs, 10 g of sample was placed into a glass bottle containing 200 mL distilled water and then cultivated at 25 ± 0.5 °C. The N release rates were measured at 1, 3, 5, 7, 14, 21, 28, and 77 days until total N released reached 80%.28 N concentrations were determined using the Kjeldahl method.29 The buried bag method was employed to determine the N release rate of the coated fertilizers in soil.30 Soil used in the release study is a calcaric ochriaquic Cambosol, which was sampled from Taian, China. The N release rate from PCUs was determined by burying 10 g of PCUs placed in 5 cm × 5 cm polypropylene bags with a mesh size of 1.0 mm2 approximately 6 cm below the soil surface at ambient temperature. After 1, 3, 5, 7, 14, 21, 28, and 91 days, the bags were retrieved and rinsed in the laboratory with distilled water to remove soil attached to the coating materials. Fertilizer granules were removed from the bags to determining residual N by the Kjeldahl method for calculating the N release rate of PCU. Determination of Degradation of Coatings in Soil. Degradation of coating materials was conducted in garden soil (the experimental field at the National Engineering Research Center for Slow/Controlled Release Fertilizers, Shandong Agricultural University, Taian, China). To examine the degradation behavior of the coatings, three coatings were chosen to incubate in soil at 25 °C. At 10, 30, 60, 90, 120, 150, 180, and 210 days after incubation, the sample was picked up, cleaned with deionized water, dried, and weighed (Mi) to compare with the initial weight (M0) before incubation. The degree of degradation (De) was calculated using eq 3 De (%) =

M 0 − Mi × 100% M0

fatty acid

shorthand

residence time

mass fraction (%)

myristic acid palmitic acid 12-methyl-palmitic acid heptadecanoic acid 9-octadecenoic acid (Z) 11-octadecenoic acid (Z) 7,10-octadecadienoic acid (Z,Z) octadecanoic acid N-nonadecanoic acid

C14:0 C16:0 C16:0 C17:0 C18:1 C18:1 C18:2

13.16 18.88 19.88 22.58 28.17 28.58 29.21

0.05 0.05 18.6 1.3 56.1 2.2 8.5

C18:0 C19:0

30.83 31.22

1.9 1.1

(3)

Statistical Analysis. All experiments were conducted in triplicate, and average values were reported. Analysis of variance among treatments and mean separation tests (Duncan’s multiple range test and least significant difference test) were performed using the Statistical Analysis System (SAS) package version 9.2 (2010, SAS Institute, Cary, NC). Regression equations and coefficients were determined using SAS. The differences among means and correlation coefficients were considered significant when p < 0.05.

Figure 2. FTIR spectra of WFO (a), EWFO (b), and WFOP (c).

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RESULTS AND DISCUSSION General Property of High Unsaturated WFO. The high unsaturated WFO made in the laboratory is more suitable for Table 1. Relative Contents of Fatty Acids in Original WFO fatty acid

shorthand

residence time

mass fraction (%)

myristic acid palmitic acid 12-methyl-palmitic acid hexadecanoic acid 9,12-octadecadienoic acid (Z,Z) 11-octadecenoic acid 9-octadecenoic acid octadecanoic acid

C14:0 C16:0 C16:0 C17:0 C18:2

13.16 18.71 19.72 22.43 27.89

0.05 0.04 18.8 6.5 58.7

C18:1 C18:1 C18:0

28.22 28.99 30.38

1.5 8.5 0.02

Figure 3. FTIR spectra of WFOP (a), isocyanate (b), PU (c), E44 (d), and EP/PU IPN composite (e).

polyol. Figure 1 shows the GC-MS of the original and high unsaturated WFO. The relative degree of saturation (U/R) was 2.9. The relative content of heptadecanoic acid decreased from 6.5 to 1.3 (Tables 1 and 2). FTIR Analyses. Different FTIR spectra (Figure 2) of WFO, EWFO, and TWFOP suggest they had different chemical structures. For WFO, the characteristic peak at 3008 cm−1 was attributed to the C−H stretching of the C=CH bond (Figure 2a). This peak disappeared in the spectrum of EWFO (Figure

the preparation of biobased polyurethane than the original WFO. The acid value was reduced from 16 to 8.4 mg/g, while pH was raised from 5.8 to 6.4. The conductivity at 25 °C was reduced from 35.2 to 23.8 μs/cm. The iodine value was greatly improved from 58 to 95 g/100 g, which is an important reference index for the preparation of the vegetable oil-based 6039


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Figure 4. 1HNMR spectrum of pure PU (A) and EP/PU (B).

Figure 5. 13CNMR spectrum of pure PU (A) and EP/PU (B).

2.90 ppm (Figure 4A and B). The 1HNMR spectrum of pure PU and EP/PU were in accordance with proposed structures, cross-linking reaction between epoxy resin and polyurethane through hydroxyl group in epoxy resin, and the epoxy terminated was cured by triethylene tetramine. The 13CNMR spectrum (Figure 5A and B) of pure PU and EP/PU were also in accordance with 1HNMR. The 13C NMR spectrum showed methylene carbon of the epoxide ring at 45.6 ppm, methane carbon of the epoxide ring at 50.9 ppm, and methylene carbon attached to urethane linkage at 69.9 ppm. The epoxy resin group and a new carbon chain were introduced into the WFO-based polyurethane; thus, the physical and chemical properties of the films were enhanced. Morphologies of PCU and EP-Modified PCU. SEM images (Figure 6) illustrate the morphologies of unmodified and EP-modified PCU shells. The surfaces of the EP-modified PCU shells were relatively smoother than those of the unmodified ones (Figure 6A1 and A2). When the SEM images were enlarged 2000 and 5000 times, numerous gaps or pin holes were observed in the surface of unmodified PCU (Figure 6B1 and C1) but were relatively compact in the surface of EPmodified PCU (Figure 6B2 and C2). This phenomenon is also illustrated in the cross section (Figure 6D1, D2, E1, and E2). Many obvious pores in the interior of the unmodified PCU shells were observed in the cross section at 5000× magnification (Figure 6E1). After adding 10% or 20% E44, the structure of the coating film was more compact (Figure 6E2), suggesting EP played a positive role in improving the membrane structure; the membrane structure had significant improvement in preparing PCUs.

2b), suggesting that the epoxidation reaction (Scheme 1A) successfully opened the C=C bond. New peaks appeared at 835 cm−1 (Figure 2b), attributed to the epoxy group. After EWFO was transformed into WFOP, the characteristic peaks at 835 cm−1 disappeared, and new peaks were observed at 3400 cm−1 (attributed to −OH) (Figure 2c), indicating the reaction had opened the epoxy group to form hydroxyl groups (Scheme 1B). The other peaks observed from the FTIR spectra were 722 cm−1 (methylene in-phase rocking) and 1380 cm−1 (methyl symmetric deformation). The C=O double bonds at 1730 cm−1 remain unaltered throughout the whole reaction (Figure 2c). Figure 3 displays the FTIR spectra of WFOP, MDI, PU, EP, and EP/PU composites. In the spectrum of WFO-based PU, the characteristic carbonyl stretching can be observed at 1645 cm−1 (Figure 3c), indicating the presence of a urethane linkage and esters from WFO-based polyols. The peak at 2270 cm−1 indicates the presence of unreacted −NCO in WSO-based PU (Figure 3e). Furthermore, the characteristic peak at 912 cm−1 for the epoxy group disappeared (Figure 3d), indicating the epoxy curing agent reacted with epoxy groups. The FTIR spectra explained that EP is not only physically blended with PU but also facilitated chemical reactions in the process of forming coating materials. 1 HNMR and 13CHNMR spectra. The 1HNMR spectrum of pure PU and EP/PU are shown in Figure 4. Peaks due to epoxy groups were observed at about 2.75−2.90 ppm (Figure 4A). Urethane NH groups were presented at about 7.11−7.27 ppm (Figure 4A and B). CH2 groups attached to a urethane oxygen atom were presented at about 3.89−4.19 ppm, and CH2 groups attached to a urethane nitrogen atom appeared at about 2.75− 6040


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Figure 7. TGA and DTA curves of pure PU and EP/PU composites.

Table 3. Influence of Mass Ratio of E44 on Thermal Stability of Polymer Film

Figure 6. SEM images of unmodified and EP-modified PCU coatings. (A1) Surface of unmodified PCU coating shell at 100× magnification. (A2) Surface of EP-modified PCU coating shell at 100× magnification. (B1) Surface of unmodified PCU coating shell at 2000× magnification. (B2) Surface of EP-modified PCU coating shell at 2000× magnification. (C1) Surface of unmodified PCU coating shell at 5000× magnification. (C2) Surface of EP-modified PCU coating shell at 5000× magnification. (D1) Cross section of unmodified coating at 200× magnification. (D2) Cross section of HTMS-modified coating at 200× magnification. (E1) Cross section of unmodified coating at 5000× magnification. (E2) Cross section of HTMS-modified coating at 5000× magnification.

E44 content (%)

T5% (°C)

T50% (°C)

T80% (°C)

Tmax (°C)

0 5 10 20 50 80

94.0 216.4 261.3 204.6 172.6 144.4

376.6 409.3 445.3 409.3 406.5 405.6

470.6 500.7 693.4 484.0 484.0 486.1

415 438 472 428 426 422

Figure 8. Degree of cross-linking and porosity of different E44 content in coatings.

through covalent bonds.31 Temperatures of 5% and 50% weight loss (i.e., T5% and T50%) significantly increased when EP was added (Table 3). Three degradation stages were also obvious in the TG profiles. The first degradation stage occurring from about 50 to 113 °C is associated with evaporation of water molecules in polymers. The second degradation stage from 113 to 235 °C is indicative of some unreacted components in the polymer. The onset temperature of decomposition of E44 was at 113 °C.32 The last stage (235−510 °C) was the decomposition of some covalent bonds in polymers. DTA experiments agree well with the TGA thermograms. There were two exothermic peaks at about 400 and 480 °C, likely associated with the decomposition process of hard

Thermal Properties. The thermal properties of pure PU and EP/PU composites were evaluated by TGA and DTA (Figure 7). TG profiles of six coatings show obvious differences in the process of thermal degradation. EP/PU composites show higher thermal resistance than pure PU, likely due to the presence of cross-linking structures between PU and EP chains 6041


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segments (i.e., isocyanate + chain extender) and soft segments (i.e., polyol).33 Porosity and Degree of Cross-Linking. Coating porosity measurements showed that E44 had a great influence on porosity (Figure 8). For PCUs with 3% coating material, the addition of 0%, 5%, 10%, 20%, 50% and 80% E44 resulted in coating porosities of 10.0%, 8.5%, 7.2%, 7.9%, 10.4%, and 15.3%, respectively. The coating porosity was affected by both physical mixing and chemical cross-linking of polyol, isocyanate, and epoxy resin. The coating porosity was reduced from 10.0% to 7.2% when E44 increased from 0% to 10%. However, the phenomenon of coating porosity growth has emerged as E44 continues to increase, due to the fact that excess E44 caused a physical barrier to the reaction of polyol and isocyanate. The cross-linking density, proportion of cross-linked chain, suspension chain specific gravity, and qMrl of coatings under different E44 are shown in Table 4. The coating cross-linking density, proportion of cross-linked chain, suspension chain specific gravity, and qMrl all were affected by the E44 content. The proportion of cross-linked chain was improved, while the proportion of suspension chain specific gravity was reduced when the E44 content increased from 0% to 10% (Table 4). However, the opposite pattern appeared when E44 continued to increase since the branched chain of excess E44 did not participate in the reaction. The degree of cross-linking and proportion of cross-linked chain are at maximum when the E44 content is 10%, while the suspension chain specific gravity is lowest. There was a close negative correlation between coating porosity and coating cross-linking density (Figure 8). Water Absorption Rate of Six Coating Materials. The water absorption of the material is greatly influenced by the EP content (Figure 9). After 3 days of incubation, the water absorption of EP/PU 10 was 6.5%, while that of pure PU was 12.3%. When the content of E44 increased to 80%, the water absorption was 20.43%, indicating complex interactions

Table 4. Cross-Linking Density, Proportion of Cross-Linked Chain, Suspension Chain Specific Gravity, and qMrl of Coatings under Different E44 EP content (%) 0 5 10 20 50 80

cross-linking density, E-04 mol/mL a

4.62e 5.46d 8.43a 8.22b 5.8c 2.91f

proportion of cross-linked chain

suspension chain specific gravity

qMrl

52.05e 58.25d 80.43a 65.34c 69.98b 47.47f

47.95b 41.75c 19.57f 34.66d 30.02e 52.53a

9.92e 13.83d 32.93a 31.34b 15.59c 3.94f

a

Different letters within the same column indicate differences at the 0.05 significance level.

Figure 9. Water absorption of material modified by different (0%, 5%, 10%, 20%, 50% and 80%) EP contents.

Figure 10. Cumulative nitrogen release rate of different coated fertilizers at 25 °C in water and soil. Panels A, B, and C show cumulative nitrogen release curves of PCUs in water at 25 °C. Panel D shows the cumulative nitrogen release rate of coated fertilizers at 25 °C in soil. PCU1, PCU2, and PCU3 were unmodified ones with 3%, 5%, and 7% of coating materials, respectively. The corresponding E44-modified ones are labeled with -1 (10% E44) and -2 (20% E44). 6042


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Figure 11. Degradation rate and the surface morphologies of the film containing 20% E44 with different weight losses. (A) Degradation rate of the film containing 20% E44 in soil. (B−D) Surface morphologies of the film containing 20% E44 after 0, 120, and 210 days in soil, respectively.

release rates is consistent with Mayer.34 Liu et al. modified transgenic soybean oil-based polyurethane with organic silicone and prepared polymer-coated urea with good controlled release properties.35 Similarly, Xie et al. successfully prepared an innovative cottonseed oil-based polymer-coated urea with a superhydrophobic surface by diatomite and SiO2.36 These modifications improved the hydrophobic performance of the coating film through interface chemical modification. Compared with their PCUs, the waste frying oil-based polymercoated urea developed in this work had several advantages including lower cost and simpler preparation process under the premise of a good controlled release effect. Biodegradability of Coatings. The biodegradability of coating materials was established through a soil burial test. The surface morphologies of the film containing 20% EP with different weight losses were examined. The degradation rates of films containing 0%, 10%, and 20% EP buried after 210 days were 31.2, 29.1, and 20.3 wt %, respectively (Figure 11A). The surface morphology images exhibited increasing gaps emerging on the surface of the film in the process of degradation (Figure 11B−D), suggesting degradation of the vegetable oil by microorganisms and enzymes in the soil. Morphologies of other membranes also show this phenomenon (data not shown). The coating materials in this study are biodegradable in the soil environment.

between E44 and PU. It was probably because the cross-linking degree of EP/PU 80 is lower, and the porosity is higher than for pure PU. There is a close relationship between water absorption of coatings and nutrient release of CRFs. N Release Characteristics of PCUs. The N release characteristics of PCUs were significantly affected by the different coating rates of the materials (Figure 10). The amounts of N released in 24 h (i.e., initial release rate) were 9.25%, 1.25%, and 1.25% for PCU1, PCU2, and PCU3, respectively. The N release longevity of the PCUs increased from 23 days to 36 and 37 days when the coating material was increased from 3% to 5% and 7%, respectively. Then, the longevity of N releases no longer increased with the increase in coating materials because of the material properties. A modifying film is necessary for increasing the longevity of N releases. The release curves of PCUs showed that EP strongly affected the N release rate (Figure 10). EP-modified PCUs showed significantly slower N release than unmodified PCUs. The amounts of N release in 24 h (i.e., initial release rate) were 9.25%, 3.94%, and 0.44% for PCU1, PCU1-1, and PCU1-2, respectively (Figure 10A). The N release longevity of 3% PCU increased from 23 days to 58 and 47 days when the E44 was increased from 0% to 10% and 20%, respectively (Figure 10A). The N release longevity increased from 36 days to 37, 49, 57, 57, and 58 days for PCU2, PCU3, PCU 2-2, PCU2-1, PCU3-2, and PCU3-1, respectively (Figure 10B and C). The cumulative N release curves for EP-modified PCUs showed an “S” shape with a low release rate in the early stage, followed by an accelerating release rate caused by membrane expansion, and finally, another slow release with micropore or pore increases in the late stage. The same trend was observed in soil. The initial N release rate was greater in water than in soil at the same temperature (Figure 10A and D). This similarity among N

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CONCLUSIONS Findings from this work showed that waste frying oil was successfully converted into biobased PU through chemical reactions. The PU was modified with E44 to synthesize EP/PU IPN with improved properties as the coating film. These PUs were used to coat urea prills to control the release of N. The PCUs showed excellent slow N release behaviors, particularly 6043


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the one with E44 modification. Furthermore, the biopolymers were biodegradable in the soil environment, indicating that the as-prepared PCUs have great potential for applications in horticulture and agriculture.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 305-246-7001, ext: 355 (USA) or +86-538-824-2900 (China). Fax:+86-538-824-2250 (China). E-mail: [email protected]. ORCID

Yuechao Yang: 0000-0003-4045-0252 Bin Gao: 0000-0003-3769-0191 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the National Natural Science Foundation of China (Grant No. 31572201), Shandong Agricultural Innovation Team (SDAIT-17-04), Natural Science Foundation of Shandong Province (Grant No. ZR2015CM035), projects of commercialization of research findings of Shandong Province (Grant No. 2014 183), and Great Innovation Projects in Agriculture of Shandong Province (Grant No. 2013 136).

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ABBREVIATIONS PU, polyurethane; N, nitrogen; SOPU, soybean oil-based polyurethane; PCFs, polymer-coated fertilizers; WFO, waste frying oil; EWFO, epoxy waste frying oil; WFOP, waste frying oil-based polyol; PAA, peroxyacetic acid; MDI, diphenylmethane diisocyanate; PCU, polymer-coated urea

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REFERENCES

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Environmentally Friendly Slow-Release Urea Fertilizers Based on

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