Development of bio polyurethane coated-urea for controlled release fertilizer

Yao Lyu; Azizah Baharum; Lih Jiun Yu; Zibao Yan; Khairiah Haji Badri

doi:10.25259/AJC_22_2025

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Original Article

2025

:18;

222025

doi:

10.25259/AJC_22_2025

Development of bio polyurethane coated-urea for controlled release fertilizer

Yao Lyu^a, Azizah Baharum^a, Lih Jiun Yu^b, Zibao Yan^c, Khairiah Haji Badri^a,*

aDepartment of Chemical Sciences, Universiti Kebangsaan Malaysia, Faculty of Science and Technology, Bangi, 43600, Malaysia

bFaculty of Engineering, Technology and Built Environment, UCSI University, KL, 56000, Malaysia

cSichuan College of Architectural Technology, Deyang City, 618000, China

* Corresponding author: E-mail address: kaybadri@ukm.edu.my (K. Haji Badri)

Received: 2025-01-07, Accepted: 2025-07-26, Epub ahead of print: 2025-10-03, Published: 2025-10-30

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Abstract

Traditional fertilizers have been known to improve crop yield significantly, but they also have glaring disadvantages, such as excessive nutrient release, low utilization rate, waste of resources, and environmental pollution. Petroleum-based polymer-coated fertilizers are not environmentally friendly and can cause secondary pollution. In our study, palm kernel oil-based polyurethane (PU) was successfully used as a renewable coating material for controlled nutrient release for the first time. The bio-PU coating at varying NCO/OH ratios (0.8:1, 1:1, 1.2:1) was investigated by physical and chemical testing methods. The PU coating with a higher NCO/OH ratio (1.2:1) showed poor thermal stability, lower water absorption (2.02%), lower surface free energy (40.85 γSV), higher shore hardness (92.83), and higher degree of crosslinking (90.55%). The effect of the NCO/OH ratio on bio-PU-coated urea properties was then explored. The bio-PU was coated on the surface of the urea, effectively controlling urea release in water, while a higher NCO/OH ratio did not necessarily give better results. When the coating amount was 9%, the release longevity of bio-PU-coated urea was delayed to 56 (1:1 NCO/OH ratio) and 42 days (1.2:1 NCO/OH ratio), respectively. Without the modification of PU and treatment of urea surface, the controlled release performance of the bio-PU by adjusting the NCO/OH ratio alone has exceeded that of some modified bio-based PU. Palm kernel oil-based PU is a potential coating to produce controlled-release fertilizers (CRFs) to support the green and sustainable agriculture applications.

Keywords

Bio polyol

Bio-based polyurethane-coated fertilizer

Controlled-release fertilizer

Release mechanism

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1. Introduction

A large amount of inorganic fertilizer is used in the cultivation of crops to protect human food security. According to the Food and Agriculture Organization of the United Nations statistics (2023), traditional fertilizers significantly improve crop yield, but also pose disadvantages such as excessive nutrient release, low utilization rate, waste of resources, and pollution of the environment. The total global consumption of inorganic fertilizers reached 195 million tons in 2021, including nitrogen fertilizer (109 million tons), phosphate fertilizer (46 million tons), and potassium fertilizer (40 million tons) [1]. It is estimated that due to soil, water, air, or other forms of leaching, decomposition, and volatility (ammonium), about 70% of the traditional fertilizer could be lost to the environment [2]. Therefore, excessive or inappropriate use of chemical fertilizers can lead to a series of environmental and health hazards. For instance, long-term heavy use of chemical fertilizers can lead to several soil-related issues. These include soil hardness, reduced soil fertility, decreased levels of essential nutrients and minerals, weakened microbial activity, altered soil pH levels, increased pest populations, soil acidification and crusting, reduced organic matter and humus content, and a decline in the number of beneficial organisms [3]. Fertilizers also pollute water sources through soil erosion and runoff loss, leading to the eutrophication of water bodies [4]. In addition, greenhouse gases are generated during chemical fertilizer production and use, which affect air quality and contribute to climate change [5]. Through the environment and the food chain, fertilizers can also cause damage to the nerves system, brain, kidneys, liver, and other organs, causing diseases such as stomachache, rashes, heart disease, and cancer [6]. To reduce pollution caused by chemical fertilizers, enhance nutrient utilization efficiency and crop yields, and promote sustainable agriculture, the development and use of controlled-release fertilizers (CRFs) have gained increasing attention in recent years as an effective solution.

Controlled release fertilizers improve nutrient efficiency and reduce environmental pollution by controlling nutrient release rate, which could be classified according to the principle of controlled release, release type, nutrient quantity, nutrient type, preparation process, chemical composition, etc [7]. Shaviv systematically classified CRFs as organic low solubility compounds, inorganic low solubility compounds, and compounds with a physical barrier (coated and matrix fertilizers) [8]. Sulfur-coated fertilizer, polymer-coated fertilizer, urea-formaldehyde fertilizers, and isobutylidene diurea are common examples of CRF [9]. Coated CRF is a kind of novel fertilizer that delays the release rate of the traditional soluble fertilizer by coating technology [10]. The basic principle is to cover the surface of the conventional soluble fertilizer particles with one or more layers of coated material, be it organic or inorganic, usually polymer, resin, or sulfur [11]. Fu et al. [7] compared the merits and demerits of inorganic and organic coating materials. It was regarded that inorganic materials have poor nutrient release controllability and are brittle. However, petroleum-based polymer coating is disadvantageous, including high cost of raw materials, non-renewability, and secondary pollution. Polymer-coated CRFs have been widely used in agriculture because of their better-controlled release properties. Polyon, Multicote, and Plantacte are commercial CRFs that use polyurethane (PU) as the coating material [8,12]. Meister and Nutricote chose polyethylene (PE) as the coating agent and alkyd-resin for the Osmocote CRFs [13,14]. These synthetic polymers are all derived from petroleum, which is non-renewable, non-biodegradable, and causes environmental problems [15]. Therefore, bio-based polymer-coated CRFs, which are environmentally friendly, renewable, and conducive to sustainable agriculture, have become a research hotspot in recent years.

PU is a reaction product of polyols and isocyanates. Both can be derived from bioresources to replace petroleum-based materials [16]. Bio-based PUs have become widely studied as membrane materials due to their sustainability, tunable properties, and superior performance. Derived from renewable resources such as vegetable oils, lignin, and polysaccharides [17], bio-based PUs reduce reliance on petroleum-based raw materials, offering a lower carbon footprint and enhanced biodegradability. Their chemical versatility allows for the design of membranes with tailored mechanical strength, flexibility, and thermal stability, suitable for various applications. Additionally, the balance between hydrophilic and hydrophobic properties enables selective permeability, making them effective for water purification, gas separation, and biomedical uses. With growing regulatory and market demand for eco-friendly materials, bio-based PUs present a greener alternative that aligns with the need for sustainable and high-performing membranes. Bio-based polyols have the potential to be comparable with traditional petroleum-based polyols in cost and performance in some fields, while bio-based isocyanate production technology is not mature enough to rival traditional isocyanate. Therefore, most research and development of bio-based PU-coated CRFs are based on bio-based polyols [18,19]. The sources of bio-based polyol are extensive. In recent years, bio-based PUs from starch, lignin, vegetable oil, and cellulose have been investigated in CRFs [20]. Among them, vegetable oil has been favored, such as castor oil [21], soybean oil [22,23], rapeseed oil [24], palm oil [25], and so on.

Researchers generally agree that the controlled release performance of pure vegetable oil-based PU-coated fertilizers is limited; therefore, improved controlled release periods of CRFs through different modification methods. The commonly used methods are composite modification [15], co-polymerization [26], hydrophilicity modification of bio-based PU [27], density of bio-based PU [28], coating percentage [29], and fertilizer surface treatment [30]. The controlled-release period of the modified bio-based PU coated increased to dozens of days. Especially, Tian et al. [31] modified urea’s surface with polyolefin wax, then bio-based PU from starch-based polyol and castor oil was coated on the fertilizer. The release longevity of double-layered coated urea reached 218 days. Many complex methods have been adopted to improve the controlled release performance of bio-based PU-coated fertilizer. However, these methods increased the production process and materials of CRFs, which would directly increase the production cost.

In addition, these studies mainly focus on castor oil and are limited to China [32]. Therefore, they do not apply to the actual situation in other countries. As an important cash crop, palm trees are widely planted in the world, and their fruits and oils occupy a significant position in the global market. The oil that comes from the oil palm fruit pulp is palm oil, and that which comes from the kernel of the palm fruit is called palm kernel oil. A few bio-based PU-coated CRFs’ research is about palm oil [25,33]. Furthermore, the controlled release performance of palm kernel oil-based PU has not been explored. In our study, we simply used pure palm kernel oil-based polyols (PKOPs) and MDI as raw materials to explore the controlled release mechanism of pure bio-based PU coated urea and the effect of different MDI dosages on the controlled release performance.

2. Materials and Methods

2.1. Materials

Palm kernel oil-based polyol (PKOP, OHV 320-340 mg KOH/g, viscosity 420 cps, Table 1) was manufactured by Palma Tech Sdn Bhd and prepared by the one-step esterification [34]. 4,4-diphenylmethane diisocyanate (MDI) was purchased from Growchem Sdn Bhd, Kajang, Malaysia. Acetone was supplied by Sigma Aldrich Sdn Bhd. Coating machine (HBY-300) was produced by Ruian Hanbo Electromechanical Co., LTD. Soil was purchased from ECO-Shop Marketing Sdn Bhd, Kajang, Malaysia. Granular urea was produced by PETRONAS Chemicals Group Berhad, Kuala Lumpur, Malaysia.

Table 1. Physical properties of PKOP.

Property	Test mothed	Value
State at 25°C	Physical observation	Liquid
Appearance	Physical observation	Viscous light brown liquid
Viscosity at 25°C, cps	ASTM D4878-08	480-520
Molecular weight	GC-MS	442-480
Specific Gravity at 25°C	ASTM D4669 - 18	0.95-1.05
Hydroxyl value, mg KOH/g	ASTM D4274-88 (Method C)	380-420
Cloud Point, °C	ASTM D5773	12
Water, %	ASTM E203 - 16	<1
pH	ASTM D5773	12-14

2.2. Preparation of palm kernel oil-based PU-coated urea

About 300 g of urea particles were placed in a rotary drum machine (speed at 300 rpm) and preheated for 30 min at 70 ± 5°C. Then, the mixture of PKOP, MDI, and acetone (20%) was slowly and uniformly sprayed on the surface of the rotating urea particles. The particles were further rolled and heated for another 20 min to allow for complete curing of the bio-PU coating. Five coated urea samples with different NCO/OH ratios (0.8:1, 1:1, and 1.2:1) were prepared, and the formulations have been shown in Table 2. In addition, the prepolymer with varying NCO/OH ratios was spread on a Teflon board. The Teflon board was put in the oven at 70°C and dried for 30 min. After the bio-based PU film was cured for 1 day at room temperature, it was peeled off the Teflon plate. The bio-based PU film was characterized by the Fourier Transform Infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), contact angle, water absorption, and several other tests. The pristine bio-based PU coating with varying NCO/OH ratios at 0.8:1, 1:1, and 1.2:1 is labeled as 0.8PU, 1PU, and 1.2 PU, respectively. The reaction scheme has been shown in Figure 1, as investigated by the co-workers [35]. The coating amount was calculated using Eq. (1) [36]:

(1)

coating Amount (w t %) = \frac{(m_{a f t e r} - m_{b e f o r e})}{m_{b e f o r e}}

Table 2. Composition of CRFs from bio-PU coated urea.

Sample	Coating amount (wt%)	Mass ratio of NCO:OH	Bio content (wt%)
0.8CRF9	9	0.8:1	55
1CRF9	9	1:1	50
1.2CRF9	9	1.2:1	45

Schematic illustration for the pre-polymerization of palm kernel oil-based PU.

Where m_before is the mass of granular urea before coating, g; m_after is the mass of granular urea after coating.

2.3 Characterization of the bio-PU film and the palm kernel oil-based PU coated urea

2.3.1. FTIR

The chemical structure of palm kernel oil-based PU coating was confirmed using an FT-IR spectrophotometer (PerkinElmer Spectrum 400 FT-IR, USA). The spectral range was between 4000 cm^-1 and 650 cm^-1 with a scanning resolution of 2 cm^-1. Prior to FTIR data acquisition of the samples, a background scan was performed for background correction.

By analyzing the FTIR spectrum, the hydrogen bonding index of carbonyl (R), the degree of phase separation (DPS), and the degree of phase mixing (DPM) of bio-PU can be calculated by measuring the intensities and location of the hydrogen-bonded and free hydrogen-bonded carbonyl (C=O) of the urethane and urea groups. These characteristic bands appear in the 1650 to 1750 cm⁻¹ range and overlap each other [37]. The multiple carbonyl signals were deconvoluted by the Origin software (Gaussian deconvolution technique) to determine the location and absorbance of the peaks. The calculating Eqs. (2-5) are as follows:

(2)

= - \log_{10} T

(3)

= \frac{A_{b}}{A_{f}}

(4)

PS = \frac{R}{R + 1}

(5)

PM = 1 - DPS

T is the transmittance of the characteristic bands; A is the absorbance of the characteristic bands; A_b is the absorbance of the hydrogen-bonded carbonyl; A_f is the absorbance of the free carbonyl.

2.3.2. TGA

The thermal stability of bio-based PU film was investigated by TGAs (TGA-50, Shimadzu). The sample mass was about 10 mg, and an alumina crucible was used. The thermogravimetric test was performed in a high-purity nitrogen atmosphere of 50 mL/min. The test temperature was in the range of room temperature to 600°C with a heating rate of 10°C/min.

2.3.3. Contact angle test

The surface free energy of a material can be determined by measuring the contact angle between two liquids with varying polarities. This technique is frequently referred to as the contact angle method. Among the various models employed, the Owens-Wendt model, also recognized as the Owens-Wendt-Rabel-Kaelble (OWRK) model, is particularly popular. The total surface tension, along with the dispersive and polar components of water and formamide, were sourced from the Handbook of Chemistry and Physics [38] and as tabulated in Table 3. The contact angles of two different liquids (water and formamide) on the bio-based PU surface were measured.

Table 3. Surface properties of water and formamide.

Liquid	Total surface tension (γ_LV) mN/m	Dispersive component (γ^d_LV) mN/m	Polar component (γ^p_LV) mN/m
Water (H₂O)	72.8	21.8	51.0
Formamide (CH₃NO)	58.0	39.0	19.0

The measured values of the contact angle were substituted with the Owens-Wendt equation.

Eq. (6) was applied for water.

(6)

γ_{LV}^{A} (1 + \cos θ_{A}) = 2 (\sqrt{γ_{SV}^{d} γ_{LV}^{A, d}} + \sqrt{γ_{SV}^{p} γ_{LV}^{A, p}})

Eq. (7) was applied for formamide.

(7)

γ_{LV}^{B} (1 + \cos θ_{B}) = 2 (\sqrt{γ_{SV}^{d} γ_{LV}^{B, d}} + \sqrt{γ_{SV}^{p} γ_{LV}^{B, p}})

Through the two aforementioned equations, the dispersive and polar components of the surface free energy of bio-based PU can ultimately be determined. Finally, the total surface free energy of the bio-based PU coating was calculated using the Eq. (8).

(8)

Surface free energy: γ_{SV} = γ_{SV}^{d} + γ_{SV}^{p}

2.3.4. Water absorption

The water absorption test was conducted following previous research [39-41]. The bio-based PU was cast into a film form and cut into 1 cm × 1 cm squares, and weighed. Then, the squares were immersed in distilled water. After 7 days, the squares were removed from the water, and the surface moisture was wiped off with paper. The percentage of water absorption of the bio-based PU film was determined by Eq. (9). The initial mass and the final mass are indicated by m_before and m_after, respectively.

(9)

Water absorption (%) = \frac{m_{a f t e r} - m_{b e f o r e}}{m_{b e f o r e}}

2.3.5. The degree of crosslinking

The degree of crosslinking depends on the percentage of interconnected polymer chains within the polymer network, that is, the percentage of gel content. This study employed the Soxhlet extraction method to determine the crosslinking degree of hard and soft segments in bio-based PU [42,43]. Toluene was selected as the solvent due to the solubility characteristics of the hard segments. The bio-based PU film was placed in a Soxhlet extractor with toluene and extracted for 24 h. The degree of crosslinking was then calculated based on the mass difference of the PU before (m_before) and after extraction (m_after). The percentage of gel content was determined by the following formula Eq. (10):

(10)

Gel content (%) = \frac{m_{a f t e r}}{m_{b e f o r e}} \times 100 %

2.3.6. Hardness shore A

The mechanical properties of the bio-based PU coating are an important indicator for protecting urea granules from damage during transportation. Shore hardness of bio-based PU films with different NCO/OH was measured using a durometer hardness tester type A (Teclock Durometer Hardness Tester GS-719 Series). Each sample was measured three times at a rate of 15 s, following ISO 7619A.

2.3.7. Scanning electron microscopy (SEM) of coated CRFs

Bio-based PU-coated urea particles were freeze-fractured by liquid nitrogen. Randomly selected unbroken and broken coated urea particles were plated with a layer of gold by sputtering in an ion coating machine. The surface and cross-section of coated urea particles were observed by Zeiss LEO 1450VP field emission SEM (Carl Zeiss, Jena, Germany). The thickness of the PU coating was measured and determined at 10 random points, and its average was calculated [44].

2.3.8. Nutrients release in the water of coated CRF

2.3.9. Determination of urea concentration in water

UV-VIS spectroscopy was used to determine the concentrations of urea solutions. A calibration curve of urea in water needs to be established, and the procedure is as follows. A standard solution of urea in water at the corresponding concentration is configured (0.005 g/mL, 0.01 g/mL, 0.015 g/mL, 0.02 g/mL, 0.025 g/mL). Then, the absorption spectrum (shown in Figure 2a) is obtained by measuring each standard solution between 190 and 220 nm using a UV-VIS spectrophotometer. Analysis of the absorption spectra showed that the absorbance increased linearly with the concentration at a wavelength of 213 nm (shown in Figure 2b). The relationship between absorbance and concentration at 213 nm was denoted as a fitted Eq. (11). The R² value is 0.9986, indicating that the concentration of urea in water is strongly linearly related to the UV absorbance, and the fitted equation can be used to determine the concentration of urea in water when the wavelength is 213 nm.

(11)

y = 0.7692 x - 0.0678 (R^{2} = 0.9986)

(a) UV spectra of standard urea solution with different concentrations and (b) calibration curve of urea in water.

2.3.10. Nutrient release test

Nitrogen release rate in water was determined according to previous research [45,46]. Uncoated and coated urea samples were subjected to the dissolution test. About 5.0 ± 0.01 g of the sample was weighed and immersed in 200 mL (V₀) distilled water in a plastic bottle at 25°C. The fluid in the bottle was gently shaken three times, and 10 mL (V_e) of the solution was removed at specified intervals (1, 3, 5, 7, 14, 21, 28, 32, and 56 days). Another 10 mL of distilled water was added to keep the sample at a 200 mL level. The total urea content of each sample was determined by the coating amount. The absorbance of the collected solution was measured by a UV-VIS spectrophotometer (Shimadzu UV-1800 UV-Visible Scanning Spectrophotometer) to determine the nitrogen release rate of the sample. The cumulative nutrient release rate was calculated using Eq. (12).

(12)

Cumulative nutrients release (%) = \frac{V_{e} \sum_{1}^{n - 1} C_{n - 1} + V_{0} C_{n}}{M \times (1 - C o a t i n g %)} \times 100

C_n is the concentration of the n^th time test; V_e is the sample volume for UV-VIS; C_crush is the concentration of the sample at 7 days, and V₀ is the initial volume in the bottle.

2.3.11. Release kinetics of coated CRFs

To reveal the mechanism of nutrient release and predict the release behavior of CRFs, four release models were fitted to the release curves of bio-based PU-coated urea in water, following Eq. (13-16).

(13)

Zero-order model: C_{t} = K_{0} t

(14)

First-order model: C_{t} = 1 - e^{- k_{1} t}

(15)

Higuchi model: C_{t} = K_{H} t^{1 / 2}

(16)

Korsmeyer-Peppas model: C_{t} = K_{k p} t^{n}

Where C_t is the cumulative nutrients release at time t; t is the release time; K (K₀, K₁, K_H, K_kp) is the release constant corresponding to the model, and n is the release index, which represents the release rate mechanism.

3. Results and Discussion

3.1. FTIR spectroscopy

The FTIR spectra of palm kernel oil-based PU with different NCO/OH ratios have been displayed in Figure 3(a). From the Figure, it can be seen that the characteristic peaks of bio-based PU are all clearly shown in three curves. The intensity of bands at 3300 cm^-1 was related to the stretching vibrations of N-H bonds in the area of hydrogen bonds (hydrogen-bonded N-H). The peak at 2971 cm^-1 is derived from the stretching vibrations of -CH₃ [47]. The peaks at 2920 cm^-1 and 2870 cm^-1 represent the asymmetric and symmetric stretching vibration of CH₂ in the aliphatic chains, respectively [37]. The amide C=O vibration is observed at 1705 cm^-1 [48]. The bands at 1532 cm^-1 and 1511 cm^-1 are related to the coupling of N-H out-of-plane bending and C-N stretching vibration [49]. The C-O-C stretching vibration can be observed at 1070 cm^-1 and 1223 cm^-1 [50]. It could be seen that the absorption peak appeared at 2280 cm^-1 in the spectrum of PU with a 1.2 NCO/OH ratio, which is related to N=C=O of MDI. This indicates that there has been a residual of MDI, which has been incompletely reacted with PKOP [51].

FTIR spectra of bio-based PU coating: (a) Characteristic peak of bio-based PU with different NCO/HO ratio; (b) Significant different in the intensity of the transmittance of bio-based PU with different NCO/HO ratio at 1800-1100 cm-1

By comparing the intensity of the characteristic peaks (N–H, C–N, C=O, and C–O–C) in bio-based PU with different NCO/OH ratios, it was observed that the intensity of these peaks increased as the NCO/OH ratio increased (shown in Figure 3b). This result is consistent with the previous results. The absorbance of bio-based PUs from castor oil [50] and cellulose [49] increased along with the NCO/OH ratio. Material absorbance is related to the optical path, the concentration, and the absorption coefficient according to the Beer-Lambert law. Since the extinction coefficient and optical path could not be determined, a semi-quantitative comparison of concentrations could be made by absorbance. Therefore, the intensity of peaks is stronger along with the NCO/OH ratio, indicating that there are more hard segment domains in bio-based PU.

C=O from urethane [52] or urea [53] groups can form hydrogen bonds with the hydrogen (N-H). The hydrogen bonds in the hard segments lead to the microstructure of the soft and hard segments phase separation in PU, which helps to enhance the mechanical and thermal properties of bio-PU. Four peaks, hydrogen-bonded C=O of urethane and free and hydrogen-bonded C=O urea groups, formed a multipeak from 1660-1740 cm^-1 [49,54-56]. The DPS in PU was determined by the deconvolution of carbonyl peaks of this multipeak in the IR spectra. Table 4 shows the calculation results of R index, DPS, and DSM of bio-based PU with different NCO/OH ratios. The peak intensity, R-index, and DPS increased with the increase in NCO/OH ratio, which is consistent with the results of other studies [49,50]. Under the premise of using the same polyol, the DPS of PU mainly depends on the content of hard segments [37]. The reason is that the long-range connectivity of hard segments increases due to the high content of hard segments, which can form mutually connected hard phases [57].

Table 4. R, DPS, DSM of bio-PU with different NCO/OH ratios.

Sample No.		0.8PU		1PU		1.2PU
Peak No.	Group type	Wavenumber (cm^-1)	A (a.u)	Wavenumber (cm^-1)	A (a.u)	Wavenumber (cm^-1)	A (a.u)
Peak 1	Urea-free	1697	0.06842	1687	0.06644	1696	0.184
Peak 2	Urea-bonded	1703	0.05667	1687	0.03064	1705	0.162
Peak 3	Urethane-free	1715	0.09105	1702	0.10597	1715	0.035
Peak 4	Urethane-bonded	1728	0.11419	1722	0.16858	1728	0.191
R		1.072		1.16		1.615
DPS		0.517		0.536		0.618
DSM		0.483		0.464		0.382

3.2. TGA and DTG

Bio-based PU was evaluated by TGA at a heating rate of 10°C/min under a nitrogen atmosphere. Figure 4 displays the TG and DTG curves of bio-based PU film with different NCO/OH ratios. Obviously, bio-based PU decomposed in three stages, which corresponded to different decomposition processes and chemical reactions: 1. Release of trapped volatile substances; 2. Depolymerization and fracture, more weight loss, and degradation of mechanical properties; 3. Complete cleavage of the chain, yielding simple hydrocarbons [58].

Thermograms of the bio-based PU with different NCO/OH ratios exhibited (a) the TG and (b) DTG curves.

In the first stage of degradation from 30°C to 250°C, the weight loss is less than 5%. From 30°C to 100°C, the weight loss is caused by evaporation of water and solvent residues. From 100°C to 250°C, the weight loss is attributed to the degradation of labile urethane and urea linkages from hard segmented copolymer [41]. This is because the C-N bond has the lowest bond energy relative to the other bonds (C-C, C=C, C-O, C=O). The value and ordering of the bond energies in PU are as follows: C-N (290 kJ/mol) < C-C (348 kJ/mol) < C-O (358 kJ/mol) < C-H (412 kJ/mol) < C=O (743 kJ/mol).

In the second stage from 270°C to 440°C, the weight loss reaches 70% and the rate of weight loss increases to a maximum. This stage corresponded to the decomposition of free isocyanate, which could be from the first stage [51]. Qu et al. [59] explored the TGA of PU and thought that a high amount of isocyanates and polyol were released as a yellow smoke. In addition, the free isocyanates released during thermal decomposition were very active and can polymerize with themselves to form polymeric isocyanates with slightly lower O and C content. Another potential is to form carbodiimide (N=C=N) and release carbon dioxide [51,59]. Therefore, the hard segment is mainly decomposed at the first and second stages. In general, the thermal degradation reaction of the hard segment in PU could be divided into three steps: 1) Decompose into alcohols and isocyanates; 2) Decompose into primary amines, olefin, and carbon dioxide; 3) Decomposition into secondary amines and carbon dioxide [51,58,60-62]. The hard and soft segments decomposition temperatures of the three PU samples are about 360°C and 450°C. As the content of MDI increased, the decomposition peaks of the hard segments became broader and deeper, while the decomposition peaks of the soft segments occurred in the opposite direction.

In the third stage of degradation from 440°C to 570°C, this range corresponds to the soft segment decomposing into volatile organic compounds, such as CO, CO₂, benzene, methane, polycyclic aromatic hydrocarbons (PAHs), ethylene, and acetylene [62]. It is contributed by the broken of ester linkages in the soft segment. This stage involves further cracking of the residue to produce carbonization products. The reason why the decomposition temperature of soft segments is higher than hard segments is that the decomposition temperature of PKOP [63] is apparently higher than MDI [64]. The soft segment is from PKOP, and the hard segment is from MDI. In addition, the soft segments have longer molecular chains and higher molecular mass than hard segments.

3.3. Contact angle test of the bio-based PU coating

The hydrophobicity of the coating material is a key parameter to prevent moisture infiltration into the core of CRFs. The hydrophobicity and wettability of the PU coating surface can be assessed by contact angle measurements. The surface free energy is a reflection of the force between molecules on the surface of the object, which can be calculated by the contact angle. In our study, the bio-based PU film was applied as a waterproof coating for the controlled release of urea, so the high contact angle between PU and water indicates that the material has good hydrophobicity and controlled release ability. Figure 5 illustrates the contact angles of water and formamide on bio-based PU film surfaces with diverse NCO/OH ratios, respectively. With the increase of MDI content in bio-based PU, the water contact angle gradually increased from 56.45° to 80.31° and the formamide contact angle increased from 42.21° to 52.66°. Therefore, the increase of MDI weakens the hydrophilicity of bio-based PU, and the opposite is true for PKOP. This result is consistent with the results of Sonalee Das et al. [65] and El-Sayed Negim et al [66], who suggested that the reason is that the increased hard segment leads to the formation of more hydrogen bonds, which increases the physical cross-linking inside PU, thus reducing the wetness of the membrane surface. On the other hand, aromatic groups are hydrophobic, whereas the hydroxyl group is hydrophilic. The lower the NCO/OH ratio, the more hydroxyl groups are introduced in bio-based PU by PKOP, which leads to better hydrophilicity. However, the water contact angle of all PU samples was less than 90°, indicating that the synthesized bio-based PUs were hydrophilic, even though the NCO/OH ratio was higher than the stoichiometric ratio. The same situation also occurred for other bio-based PUs, such as sunflower oil-based PU [67], castor oil-based PU [50], and lignin-based PU [68]. This is because bio-based PU from natural resources, and its free-bond moisture is more difficult to remove than petroleum-based PU. Polyols containing more hydroxyl groups are hydrophilic, which makes them easy to absorb water from the air in the process of storage and use. It can also be hygroscopic. When preparing PU, the reaction between isocyanate and water will generate hydrophilic urea groups, which will affect the hydrophilicity of bio-based PU. Xu and Guo also found that the water contact angle of the polymer coating decreased when urea was added to the coating material [69].

The water and formamide contact angle of bio-based PU coating with different NCO/OH ratios. (the images on each bar are representing the water/formamide droplets).

The surface free energy is closely related to the wettability energy of the solid surface and can be divided into two main components: the dispersion component and the polar component [70]. The dispersion component is due to the dispersion of electron clouds on the surface of the material’s molecules [71]. In polymer materials, dispersed components are the main non-polar forces between molecular chains or chain segments, especially in non-polar polymer materials, such as PE, polypropylene (PP), and so on [72]. The dispersed components are the main attraction between their molecules. Although this force is relatively weak, the dispersive force can still have a significant effect in the sum due to the long polymer chain and large area of action. The polar component represents the sum of the charge attraction between polar molecules [73]. It reflects the electrostatic interaction and surface concentration of polar functional groups such as hydroxyl, carboxyl, and amine on the surface of the material [74,75].

The surface free energy of the bio-based PU film was calculated using the Owens two-liquid method, and the results have been shown in Table 5. As the MDI content increases, the dispersive component gradually increases and the polar component decreases, while the surface free energy decreases. This is because nonpolar benzene rings were introduced into PU by MDI, an aromatic diisocyanate [76]. The π-electron cloud of the benzene ring is highly polarized and can effectively generate an instantaneous dipole and an induced dipole, thereby increasing the dispersive component. At the same time, the reaction of the isocyanate with the polyols leads to the reduction of the polar group hydroxyl in the bio-based PU, so the polar component decreases. As the NCO/OH ratio increases, the polar component decreases faster, resulting in a decrease in the surface free energy of the bio-based PU film. In contrast to the contact Angle results, a higher surface free energy indicates that bio-based PU with higher polyols content is more hydrophilic and more easily wetted by water. Mihaela Lupu et al. [77] studied the effect of isocyanate content and species on the surface free energy of PU and obtained results that were consistent with this study.

Table 5. The surface free energy of bio-based PU with different NCO/OH ratios.

Sample No.	Dispersive component (γ^d_SV)	Polar component (γ^p_SV)	Surface free energy (γ_SV)
0.8PU	22.16	23.39	45.55
1PU	31.72	11.06	42.77
1.2PU	36.91	3.93	40.85

3.4. The hardness shore A, degree of crosslinking, and water absorption of bio-based PU coating

The degree of cross-linking refers to the degree of mutual connection through chemical bonds between polymer macromolecular chains [78]. Cross-linking would reduce the flexibility of polymer chains but increase stiffness and water resistance [79]. Table 6 presents the degree of crosslinking, hardness, and water absorption of bio-based PU films at various NCO/OH ratios. The degree of crosslinking and hardness of bio-based PU increased with increasing ratio of MDI, while water absorption reduced conversely. This is because the structure of the isocyanates involved in the reaction is rigid, and MDI can react with a variety of functional groups (such as hydroxyl, amine, water, etc.) to form cross-linked structures. Therefore, MDI increased the cross-linking points of the bio-based PU, thus increasing the cross-linking density and hardness of bio-based PU. From FTIR results, it could be known that MDI increased the hydrogen bond content, which improved the hardness of bio-based PU. Aiga Ivdre et al. [80] and Mudri et al. [81] studied the effects of isocyanate content and structure on PU properties, respectively. The mechanical properties of PU mainly depend on the degree of cross-linking. A higher crosslinking degree will increase the stiffness and deformation resistance of PU. The larger error in water absorption is due to the existence of tiny voids in the PU coating, which can store more water. These tiny voids were formed by carbon dioxide produced from the reaction of MDI and water. Defects in coating were regarded as the major reason for poor controlled release performance in pure bio-based PU-coated fertilizers. However, higher water content of polyol from bio-resource is unavoidable.

Table 6. The hardness and degree of crosslinking of the bio-based PU with different NCO/OH ratios.

Sample No.	hardness index shore a	Degree of crosslinking (%)	Water absorption (%)
0.8PU	83.83±0.76	87.24	4.79±0.84
1PU	87.50±1.5	88.73	2.6±0.54
1.2PU	92.83±1.25	90.55	2.02±0.87

Table 7. Kinetic parameters for urea release of bio-based PU-coated urea with different NCO/OH ratios.

Kinetic model	Parameter	0.8CRF9	1 CRF9	1.2CRF9
Zero-order	K₀	0.025	0.018	0.021
Zero-order	R²	0.96134	0.922	0.99149
First-order	K₁	0.041	0.321	0.33
First-order	R²	0.94306	0.98916	0.96012
Higuchi	K_H	0.121	0.198	0.11
Higuchi	R²	0.84192	0.93383	0.87183
Korsmeyer-Peppas	K_KP	0.031	0.53	0.24
	n	0.943	0.703	0.964
	R²	0.95721	0.9728	0.9909

In our study, the bio-PU coating film was used to control the release of urea in water, so both water absorption and desorption are key properties of the PU coating. Table 6 exhibits the water absorption of bio-based PU films with different NCO/OH ratios at 4.79% (0.8CRF9), 2.60% (1CRF9), and 2.02% (1.2CRF9), respectively. MDI enhanced the water resistance of bio-based PU, which is consistent with previous research [40,82-84]. It is generally believed that the low water absorption rate of PU is caused by the poor hydrophilicity of PU. A higher NCO/OH ratio (>1) means that the relative content of hydrophilic groups (OH) in PU decreases, while the content of non-hydrophilic groups increases [85,86]. These were discussed in the contact angle results.

In addition, some researchers believe that the increase of the NCO/OH ratio leads to the enhancement of the chain entanglement in PU, thus resulting in a denser internal structure of the material [87]. The dense structure reduces the space between molecules, and the channels and storage space for water molecules to enter and stay become smaller, thus the water absorption of the material is low. Figure 6 exhibits the mechanism of water absorption in bio-based PU with different degrees of cross-linking. In Figure 6(a), soft segments from PKOP have long flexible chains and more hydroxyl groups to attract water molecules and to provide more space for water molecules. In Figure 6(b), there are more hard segments and crosslinks in the inner structures of the PU. The space available for water access and storage is significantly reduced.

Water absorption mechanism in bio-PU with low (a) and high (b) degree of crosslinking.

3.5. Surface morphology analysis of the coated CRFs

The surface morphology of uncoated urea and bio-based PU-coated urea with different NCO/OH ratios at different scales has been shown in Figure 7. Uncoated urea particles have uneven surfaces and are formed by the binding of small urea crystals. The surface of bio-based PU-coated urea is smooth, like a very thin film covering the uneven surface of urea. All samples were coated well, and no urea surface was exposed outside. With the NCO/OH ratio increasing, the surface of bio-based PU coating becomes rough and nonuniformity. According to our coating process, the curing of the coating is done in motion. Different from the conventional static PU coating curing, the dynamic curing coating surface is formed between fertilizer particles, or between fertilizer particles and the inner wall of the coating machine, under continuous collision and fracture. Other researchers also found this phenomenon on the fractured face of PU, and they believe that this is caused by the increased phase separation between the hard phase and soft phase, and those parts with uneven protrusions are the hard phases [41,65,88,89]. The results of FTIR can also support this opinion. In addition, the mixture of raw materials with high MDI content has higher viscosity during the coating process, which may also lead to inhomogeneous coating. Figure 8 displays the surface of 1.2CRF9 buried in soil after 4 months. After long-term buried, the coating surface becomes irregular, rough, and a few micro holes appear. This is because bio-based PU was easily attacked by water and microorganisms in the natural environment. Water in soil, no matter in the form of steam or liquid, may cause PU hydrolysis [90]. Bacteria in the soil can metabolize PU into simpler compounds or elements, such as Bacillus amyloliquefaciens, Comamonas acidovorans, and Corynebacterium [91]. Tian et al. [92] studied the polymer coating from commercial CRFs and found that the biodegradation of petroleum-based polymers (PE, epoxy resin, and PU) after 807 days buried was 16.7%, 2.79%, and 4.86%. Whereas, the biodegradation of vegetable oil-based PU coatings was 5% to 30% in 1 year [25,31,93,94].

SEM of bio-based uncoated urea and bio-based PU-coated urea with different NCO/OH ratios. Note: a0 is sample of uncoated urea with yellow arrow showing b0, the surface of a0, a1 is sample of CRF with 0.8:1 NCO to OH, 9 wt% PU (0.8CRF9) and b1 showing the surface of a1, a2 is sample of CRF with 1:1 NCO to OH, 9wt% PU (1CRF9)and b2 showing the surface of a2, a3 is sample of CRF with 1.2:1 NCO to OH, 9wt% PU (1.2CRF9) and b3 showing the surface of a3.

SEM of bio-based PU coating (1.2CRF9) buried in soil for 4 months (300×).

3.6. Nutrients release of coated CRFs in water

Figure 9 shows the urea release rate of uncoated urea and coated urea in water. The release longevity of uncoated urea is very short, only 1 day, while the release longevity of all coated fertilizer samples is more than 30 days. The palm kernel oil-based PU coating has effectively controlled urea release in water. Comparing bio-based PU coated urea with different NCO/OH ratios, the urea release rate slowed down with the increase of MDI content of bio-based PU during the first 28 days. From SEM, it could be known that there are no defects on the PU coating, such as micropores [95], cracks [96], or uncovered surface [97]. In the case of the same coating amount, the main factor affecting the difference in the urea release rate of each sample is the difference in the performance of the material itself. When the NCO/OH is higher, the bio-based PU films have poor hydrophilicity, higher crosslinking degree, lower surface energy, and water absorption. Therefore, the bio-based PU containing harder segments can control urea release better in water. However, after 28 days, the urea release of the sample with a 1.2 NCO/OH ratio is apparently faster than the sample with an NCO/OH ratio of 1. Yu et al. [98] observed the surface of bio-based PU-coated fertilizer with a 2.5 NCO/OH ratio after nutrients were released. More micropores were observed on the coating surface and cross-section in SEM photographs. Yu et al. [99] and Bochao Wei et al. [100] also studied the morphology of the bio-based PU coating after nutrient releasing. The bio-based coating deteriorated, and some holes formed inside of coating. These defects would undoubtedly accelerate the release of urea. We believe that this is due to an excess of MDI remaining in the bio-based PU coating. Isocyanates (MDI) are very reactive and react with water to form carbamate and carbon dioxide, which could form defects on the coating in a very short time. FTIR spectra have also indicated the presence of unreacted MDI in bio-based PU. Coating Defects could be seen in the SEM results after urea release from the CRF. Ultimately, when the NCO/OH ratio is 1:1, the controlled release period is achieved at 56 days. The performance has exceeded that of some modified bio-based PU coating fertilizers [20].

Cumulative percentage of urea release in water for the bio-based PU-coated urea as a function of time (days)

Furthermore, the error bar in the graph becomes larger in the medium-term, then reduces in the last stage. The observed discrepancy can be attributed to the non-uniform distribution of urea solution concentrations. Although the sample vial is shaken three times prior to analysis, this protocol proves insufficient to achieve complete homogenization. Over time, the concentration gradient naturally equilibrates through diffusion processes, thereby reducing measurement error. This phenomenon has been consistently documented in multiple CRF studies [36,67,101-103] and controlled release drug studies [104].

3.7. Release the kinetic model of bio-based PU-coated CRFs

The release kinetics of bio-based PU-coated urea fertilizers in water was investigated by fitting four different kinetics models, namely: Zero order model, First order model, Higuchi model, and Korsmeyer-Peppas model [101,105,106]. Four kinds of models were introduced by Marcos Luciano Bruschi [107]. In the zero-order model, the release of the active agent (drug or nutrient) is time-dependent only, independent of the concentration of the active agent, and the release rate is constant. In the first-order dynamic model, the release rate is exponentially decaying, and the release rate is proportional to the active agent concentration in the delivery system. The Higuchi model describes the rate of the active agent release from the matrix system, which requires some assumptions: Firstly, the initial concentration of the active agent in the matrix is much higher than that in solution. Secondly, the rate of drug release is completely controlled by diffusion, and the diffusion coefficient remains constant during the release process. Thirdly, the matrix does not undergo swelling, degradation, or other changes during release, such that the active agent diffusion path remains consistent, and finally, the distribution of the active agent in the matrix is uniform. In the Korsmeyer-Pepas Model, different constants n represent different release mechanisms: When 0 <n≤0.5, the release follows Fickian diffusion. The active agent is released from the matrix by diffusion. When 0.5 < n < 1, the release follows non-Fickian diffusion. The release is controlled by both diffusion and expansion, which is a hybrid mechanism. When n=1, the release is controlled entirely by matrix swelling or relaxation rather than diffusion, and the release rate is usually constant. When n>1, the model becomes Super Case II transport. The release rate increases with time. Usually seen in the case of significant expansion or degradation of the carrier structure.

Table 7 lists the kinetic parameters of coated samples (0.8CRF9, 1CRF9, 1.2CRF9). Among them, the R² value of sample 0.8CRF9 and sample 1.2CRF9 in the zero-order model are 0.96134 and 0.99149, respectively, which are closer to 1 than other kinetic models. The results showed that when the NCO/OH ratio of bio-based PU coating was 0.8:1 or 1.2:1, the urea release rate remained unchanged throughout the release process, which was not affected by the residual amount of urea inside the bio-based PU coating. The coating of sample 0.8CRF9 had a low crosslinking degree, and the urea was rapidly released in the lag period and the constant release period. The release rate of sample 1.2CRF9 was not affected by the internal urea concentration in the late release period, because of the defects on PU coating. For sample 1CRF9, the release kinetic was more fitted with the First order model (R²=0.98916). Bio-based PU with 1.2 NCO/OH ratio formed a uniform and excellent waterproof coating on urea surface and it could control well urea release in the whole longevity of the CRF.

4. Conclusions

The utilization rate of traditional fertilizer is low, and it pollutes the environment, while petroleum-based polymer-coated fertilizer is not sustainable and would bring secondary pollution. In this study, bio-based PU derived from palm kernel oil-based polyol was successfully applied to control the release of urea for the first time. Bio-based PU films and PU-coated urea with varying NCO/OH ratios (0.8:1, 1:1, and 1.2:1) were prepared and characterized. The higher the MDI content in PU film, the higher the crosslinking degree and hardness; on the contrary, the lower the water absorption, thermal stability, and hydrophilicity of the coating. These properties could enhance water resistance and protect the fertilizer core against rapid dissolution in water. The bio-based PU formed a uniform and consistent coating on the surface of urea granules, which effectively delayed the urea release period without modification. Notably, the CRFs sample with nine coating amounts and a 1:1 NCO/OH ratio demonstrated an extended nutrient release period of up to 56 days, significantly prolonging the release time compared to uncoated urea in water. Besides, prolonged burial in soil caused degradation of bio-PU coating. These findings suggest that the palm kernel oil-based PU-coated urea developed in this study holds substantial potential as a CRF to support sustainable agriculture.

Acknowledgment

The authors would like to acknowledge Universiti Kebangsaan Malaysia (UKM) and Faculty of Science and Technology (FST) for allowing this research to be carried out. A special gratitude to i-CRIM Laboratory, Centre for Natural and Physical Laboratory Management UKM (ALAF-UKM) for material characterization facilities. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Major funding was from the Principal Researcher whom also acts as the Corresponding Authors. The project was partially funded by Universiti Kebangsaan Malaysia through the GP-2019-K012785 incentive funding.

CRediT authorship contribution statement

Lyu Yao: Conceptualization, Writing - original draft, Writing - review and editing, Visualization, Methodology, Investigation, Data curation, Formal analysis. Yu Lih Jiun: Supervision, Resources; review and editing. Azizah Baharum: Supervision, review and editing. Yan Zai Bo: Supervision, review and editing. Khairiah Haji Badri: Conceptualization, Writing - review and editing, Supervision, Validation, Methodology, Resources, Funding acquisition, Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Declaration of Generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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Development of bio polyurethane coated-urea for controlled release fertilizer

Abstract

Keywords

Bio polyol

Bio-based polyurethane-coated fertilizer

Controlled-release fertilizer

Release mechanism

1. Introduction

2. Materials and Methods

2.1. Materials

2.2. Preparation of palm kernel oil-based PU-coated urea

2.3 Characterization of the bio-PU film and the palm kernel oil-based PU coated urea

2.3.1. FTIR

2.3.2. TGA

2.3.3. Contact angle test

2.3.4. Water absorption

2.3.5. The degree of crosslinking

2.3.6. Hardness shore A

2.3.7. Scanning electron microscopy (SEM) of coated CRFs

2.3.8. Nutrients release in the water of coated CRF

2.3.9. Determination of urea concentration in water

2.3.10. Nutrient release test

2.3.11. Release kinetics of coated CRFs

3. Results and Discussion

3.1. FTIR spectroscopy

3.2. TGA and DTG

3.3. Contact angle test of the bio-based PU coating

3.4. The hardness shore A, degree of crosslinking, and water absorption of bio-based PU coating

3.5. Surface morphology analysis of the coated CRFs

3.6. Nutrients release of coated CRFs in water

3.7. Release the kinetic model of bio-based PU-coated CRFs

4. Conclusions

Acknowledgment

CRediT authorship contribution statement

Declaration of competing interest

Declaration of Generative AI and AI-assisted technologies in the writing process

References

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