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Review

A Vegetable-Oil-Based Polyurethane Coating for Controlled Nutrient Release: A Review

by
Lyu Yao
1,*,
Azizah Baharum
1,2,
Lih Jiun Yu
3,
Zibo Yan
4 and
Khairiah Haji Badri
1,2,*
1
Department of Chemical Sciences, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia
2
Polymer Research Center, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia
3
Faculty of Engineering, Technology & Built Environment, UCSI University, Cheras 56000, Kuala Lumpur, Malaysia
4
Department of Materials Engineering, Sichuan College of Architectural Technology, Deyang 618000, China
*
Authors to whom correspondence should be addressed.
Coatings 2025, 15(6), 665; https://doi.org/10.3390/coatings15060665
Submission received: 7 May 2025 / Revised: 26 May 2025 / Accepted: 29 May 2025 / Published: 30 May 2025
(This article belongs to the Special Issue Preparation and Applications of Bio-Based Polymer Coatings)
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Abstract

:
Bio-based polyurethane (PU) is synthesized either via the prepolymerization or addition polymerization of bio-based polyols and isocyanates. PU synthesized from vegetable-oil-based polyols has excellent properties for various application needs. Bio-based PU coatings from renewable vegetable oil show good degradability in soil while controlling the nutrient release process. Castor oil, soybean oil, palm oil, olive oil, linseed oil, rapeseed oil, cottonseed oil, and recycled oil have been explored in the study of bio-based PU coatings for controlled nutrient release. Castor oil as a natural polyol has been widely studied. Generally, the epoxidation ring opening method is preferred to prepare bio-based polyols. Almost all of these studies used a drum coating machine to complete the coating process. To obtain better controlled release performance, a vegetable-oil-based PU (VPU) coating was modified by increasing the degrees of crosslinking and hydrophobicity and improving the coating uniformity. The nutrient release duration of the modified castor-oil-based PU-coated fertilizer reached 200 days. VPU-coated fertilizers, in contrast to traditional fertilizers, effectively reduce the detrimental impact on the environment. Although the preparation of VPU-coated fertilizers is still at the laboratory scale, application research has been carried out in field crops.

Graphical Abstract

1. Introduction

Polyurethane (PU) is a polymer material formed by the reaction of isocyanates (-NCO) with polyols (-OH), invented by Otto Bayer’s team in the 1930s [1]. PU has many excellent properties, such as its durability, toughness, wear resistance, water resistance, and flexibility, as well as heat insulation and sound insulation properties. It is widely used in various fields in the form of PU films, PU coatings, PU foams, and PU structural parts [2,3,4,5]. Its applications include the manufacture of shoes, cushions, medical devices, clothing, thermal insulation, anticorrosion products, adhesives, and many others [1]. The global PU market is expected to reach $101 billion by 2028, with an average annual growth rate of 4.0% [6]. PU coatings are durable, high-performance protective coatings that protect a variety of surfaces from external intrusion, such as the surfaces of metal, wood, and concrete materials [7,8,9]. Due to its excellent wear resistance, flexibility, environmental stability, and chemical resistance, it is commonly used in the industrial, automotive, and construction sectors [10].
Traditional PU is primarily synthesized from petroleum-based raw materials, which are limited and non-renewable resources [11,12,13,14]. In response to the growing need for environmental sustainability and pollution reductions, bio-based PUs derived from abundant and renewable sources have garnered increasing attention in recent years. These materials are typically synthesized from vegetable oils [15], lignin [16], starch [17], and other biomasses [18,19], offering a promising alternative to petroleum-based feedstocks and reducing the reliance on non-renewable resources.
Bio-based PU coatings have broad applicability in sectors such as the construction, automotive, and industrial manufacturing industries, providing protection against external environmental factors. Additionally, they show promise in the controlled release of drugs [20] and nutrients [21]. With global population growth and the rising demand for higher agricultural yields, coupled with a heightened awareness of environmental sustainability, the demand for controlled-release fertilizers (CRFs) continues to rise [22,23]. Recent advancements in CRF coating technologies have led to the development of numerous biodegradable materials, including modified starch, lignin, natural rubber, polymers, and beneficial microbial binding agents [24,25]. These materials not only offer effective nutrient release profiles but also degrade in soil, thereby reducing environmental pollution. Among the renewable raw materials, vegetable oil stands out as a particularly promising substitute for petrochemical-based inputs due to its many advantages [26]. Vegetable oil can be sourced from a variety of crops [26], as well as from waste streams such as used cooking oil [27] and gutter oil [28], making it cost-effective, renewable, biodegradable, and environmentally friendly. Moreover, the processing and conversion technologies for vegetable oil are well-established, and continued scientific investment has further advanced its application in polymer materials [29,30,31]. Modified vegetable-oil-based polyurethane (VPU) has recently demonstrated excellent mechanical, chemical, thermal, antiseptic, and antibacterial properties [29,32,33].
Although several reviews have examined VPU coatings in anticorrosion and antimicrobial applications within industrial fields [29,32,34,35], their potential in controlled-release fertilizers remains underexplored. While some reviews have discussed bio-polymer-coated fertilizers [24,36,37], they have not emphasized the emerging importance and value of VPU in this context. This review aims to address this gap by focusing on recent advances in the use of VPU coatings for controlled nutrient release, covering their preparation, modification, application, and degradation in CRF systems.

2. Preparation of Vegetable-Oil-Based PU-Coated Fertilizer

2.1. Bio-Based Polyol from Vegetable Oil

Polyurethane is a type of polymer material synthesized through a chemical reaction between polyols and isocyanates. Among the bio-based raw materials, the development of bio-based polyols, particularly those derived from vegetable oils, is more technologically advanced than that of bio-based isocyanates [38,39]. As a result, most studies on VPU coatings for fertilizers have predominantly utilized conventional (petroleum-derived) isocyanates, with limited application of bio-based alternatives.
Vegetable oils are primarily composed of triglycerides, which consist of three fatty acid chains esterified to a glycerol backbone. The chemical structure of triglycerides is illustrated in Figure 1. Different vegetable oils contain varying compositions and proportions of fatty acids such as oleic acid, lauric acid, palmitic acid, linoleic acid, and linolenic acid—enabling their tailored design for specific application needs [40]. Except for castor oil, which naturally contains hydroxyl groups, most vegetable oils must undergo chemical modification to introduce hydroxyl functionalities before being used as polyols in PU synthesis. In other words, unlike castor oil, other vegetable oils require functionalization to be converted into vegetable-oil-based polyols suitable for PU production.

2.1.1. Cater-Oil-Based Polyols

As a non-edible oil, castor oil has an annual global production rate of approximately 500,000 tons, offering significant economic advantages and drawing substantial research interest [41]. It is primarily composed of ricinoleic acid (approximately 80–88%), along with smaller amounts of oleic, linoleic, palmitic, and stearic acids. Notably, castor oil is the only vegetable oil that naturally contains hydroxyl groups, with a hydroxyl value exceeding 155 mg KOH/g and an average functionality of around 2.7 [41,42]. Its chemical structure is shown in Figure 2.
Due to its inherent hydroxyl functionality, castor oil can be directly used as a natural polyol in the synthesis of polyurethanes by reacting with various isocyanates, including toluene diisocyanate (TDI) [43], diphenylmethane diisocyanate (MDI) [44,45,46], polyaryl polymethylene isocyanate (PAPI) [47,48,49,50,51], and isophorone diisocyanate (IPDI) [52], particularly in the development of coatings for CRFs.
In addition to its direct use, castor oil has also been chemically modified by researchers to enhance its properties as a polyol. For instance, Tang et al. [53] performed the alcoholysis of castor oil using glycerol to obtain polyols with altered characteristics. Bortoletto-Santos et al. [54] introduced elemental sulfur directly into castor oil to improve the film-forming ability, adhesion, and flexibility of the resulting PU coating. Zhao et al. [55] prepared vulcanized castor oil using industrial sulfur powder at 160 °C. Feng et al. [56] carried out a transesterification reaction between methanol and castor oil to produce methyl castor oleate, followed by a double-bond addition reaction with molten sulfur to synthesize polysulfide castor-oil-based polyols.
Uniquely, Dong et al. [57] produced bio-based PU from a bio-based polyol (castor oil) and bio-based isocyanate (ethyl ester L-lysine triisocyanate). Therefore, their coating material was produced from the highest bio-source, called fully bio-based polyurethane coating in the current CRF research.

2.1.2. Other Vegetable-Oil-Based Polyols

In addition to castor oil, various other vegetable oils have been investigated for use in controlled-release fertilizer (CRF) coatings. Bortoletto-Santos et al. [46] synthesized soybean-oil-based polyols by first epoxidizing soybean oil with peracetic acid, followed by a ring-opening reaction using methanol to introduce hydroxyl groups. Similarly, Pang et al. [58] prepared five types of vegetable-oil-based polyols derived from rapeseed oil, olive oil, soybean oil, and linseed oil via epoxide ring-opening reactions. The epoxidation was carried out using hydrogen peroxide and acetic acid, and hydroxylation was achieved using methanol in the presence of hydrochloric acid.
Liu et al. [59] explored the use of transgenic soybean oil as a bio-based raw material for PU coatings in CRFs. They employed an epoxide ring-opening method using peroxyacetic acid as the oxidizing agent, 1,2-propylene glycol and methanol as alcohols, and fluoroboric acid as a catalyst. In contrast, Sun et al. [60] adopted a different approach to synthesize soybean-oil-based polyols. Rather than using traditional alcohols for hydroxylation, they utilized castor-oil-derived fatty acids to induce the ring-opening of epoxidized soybean oil, using a solvent-free and catalyst-free process.
Gharrak et al. [61] studied rapeseed-oil-based polyols for CRF coatings, applying hydrogen peroxide for their epoxidation and using diethylene glycol in the ring-opening step. Sair et al. [62] initially synthesized monoglycerides from rapeseed oil, which were then reacted with anhydrides to form alkyd polyols. Xie et al. [63] prepared cottonseed-oil-based polyols through transesterification with glycerol. Palm oil has also been investigated for bio-based PU coatings in CRFs [64]; it was first epoxidized using hydrogen peroxide and acetic acid, followed by ring-opening with methanol and hydrochloric acid to produce polyols.
Waste cooking oil (WCO) is generated in large quantities globally, and its disposal presents a significant environmental challenge due to the lack of appropriate treatment infrastructure. WCO is primarily composed of triglycerides, which contain ester groups and reactive double bonds that make it a suitable candidate for chemical modification and conversion into value-added products. While most research studies have focused on converting WCO into biodiesel, its application in producing polymeric materials such as polyurethanes, epoxies, polyhydroxyalkanoates (PHAs), and acrylics is gaining momentum [65].
Several studies have explored the use of WCO in PU coatings for controlled nutrient release. Waste frying palm oil [66], recovered oil [67], and waste kitchen oil [68] have been converted into bio-based polyols via epoxide ring-opening reactions. These polyols were then reacted with MDI to form PU coatings for CRFs. Among these, recovered oil often contains a high level of impurities and requires purification prior to use [67].

2.2. Coating Process of Vegetable-Oil-Based PU-Coated CRFs

Controlled-release fertilizer (CRF) coatings can be applied using various methods, including via impregnation, spray coating, pan coating, simple mixing, and fluidized bed techniques [69,70,71]. In all reported studies involving VPU coatings for CRFs, the coating was performed using a drum coating machine. The general process involves fertilizer granules first being loaded into the rotating drum and preheated. Then, a specified amount of coating liquid comprising vegetable-oil-based polyols and isocyanates is either evenly sprayed using a spray system or directly dropped onto the surface of the rotating fertilizer particles. During the coating process, the drum rotates continuously, typically at a temperature of around 70 °C, to facilitate the polymerization and solidification of the coating. The processing temperature used during this step generally ranges from 40 °C to 80 °C, with 70 °C being the most commonly selected. A coating temperature of 75 °C is considered optimal, as it provides a balanced curing time, fast enough to prevent clumping due to excessive adhesion, yet slow enough to avoid incomplete coating caused by premature solidification [44]. After each curing cycle, the coating process is repeated until the desired coating thickness is achieved. Finally, the coated fertilizer particles undergo a final curing step at the same temperature for several tens of minutes to ensure complete polymerization and film formation. This completes the fabrication of the VPU-coated fertilizer.
Notably, most researchers have employed in situ polymerization under solvent-free conditions to form the VPU coatings directly on the fertilizer particles. In some studies, surface modification of the fertilizer granules was conducted prior to coating to improve the coating adhesion or performance. Figure 3 illustrates the uniform distribution of the coating liquid across the surfaces of the fertilizer particles, forming a continuous encapsulating layer. Unlike conventional coatings used in industries such as automotive or construction, CRF coatings require a dynamic application process. This dynamic nature—from spraying to in situ curing—is essential to achieving a complete, uniform coating on each individual granule, distinguishing it from the more static methods used in other applications.

2.3. Modification of Vegetable-Oil-Based PU Coating for CRFs

Bio-based PU materials have excellent properties, such as high elasticity and wear resistance, as well as good mechanical properties, but there are still some limitations in their practical application. Therefore, three modification methods have been explored for improving the controlled nutrient release of VPU coatings, which are co-polymerization, composition modification, and surface modification.

2.3.1. Co-Polymerization

Co-polymerization refers to the synthesis of co-polymers by combining two or more different polymer components within the same molecular chain. This approach is considered a relatively straightforward and effective modification technique [72], as it introduces additional physical or chemical crosslinks within the polymer network, thereby significantly enhancing the material’s overall properties.
In controlled nutrient release applications, co-polymerization is commonly employed by blending vegetable-oil-based polyols with other types of polyols to tailor the performance of the PU coating. For instance, castor oil has been co-polymerized with another bio-based polyester polyol and subsequently reacted with methylene diphenyl diisocyanate (MDI) to produce a PU coating for urea granules [44]. In another study, a crosslinked interpenetrating polymer network was formed by combining castor oil, a starch-based polyol, and PAPI, resulting in a robust PU coating [48].
Similarly, castor oil was blended with liquefied polyhydric alcohols derived from waste cardboard, hydroxy-terminated polydimethylsiloxane (PDMS), and PAPI to formulate a coating material applied to the surfaces of urea particles [50]. In another example, a lignin-based polyol and rapeseed-oil-based polyol were combined and reacted with toluene diisocyanate (TDI) to synthesize a PU coating for controlled-release fertilizer applications [61].

2.3.2. Composite Modification

Composite modification is a physical technique that involves incorporating inorganic or organic fillers into a polymer matrix or blending two or more materials to form a composite. This method allows the resulting material to synergistically combine the advantages of each component, thereby enhancing its overall performance. In the field of CRFs, various composite modification methods have been explored to improve the mechanical, thermal, barrier, and functional properties of VPU coatings. For instance, pyrophyllite modified with a titanate coupling agent was incorporated into castor-oil-based PU to prevent surface cracking and ensure a more uniform coating distribution on fertilizer particles [43]. Beeswax was blended with 1,4-butanediol, castor oil, and ethyl ester L-lysine triisocyanate to enhance the hydrophobicity of a fully bio-based PU coating [73]. Organosilicon additives were introduced to improve the weather resistance and water repellency of PU coatings synthesized from castor oil and poplar-catkin-derived polyols [51]. Halloysite nanotubes were used to impart superhydrophobic properties to castor-oil-based PU nanocomposites [52], while soy protein isolate microcapsules were embedded into a PU matrix consisting of castor-oil-based PU and polyvinyl alcohol to develop a self-healing coating [49]. In another study, modified ethylene glycol dimethacrylate, acrylonitrile, and waste palm-oil-based PU prepolymers were combined to produce a composite coating with enhanced physical crosslinking density [66]. Additionally, multiwalled carbon nanotubes were incorporated into waste kitchen oil-based PU to achieve superhydrophobic surface characteristics [68]. Recycled-oil-based PU coatings were further modified using hydroxyl-terminated polydimethylsiloxane and γ-aminopropyl triethoxy silane as additives, resulting in improved moisture resistance and adhesion performance [67].

2.3.3. Surface Modification

Surface modification involves altering the surface properties of materials to enhance the performance of coatings, particularly in terms of their nutrient release behavior. One common technique is water polishing, which smooths the surfaces of fertilizer granules, reducing their specific surface area and thereby increasing the coating efficiency [47,68]. Hydrophobic agents such as wax [48] and paraffin [56,74] can also be applied to the surface of the fertilizer core for improved water resistance and delayed nutrient release.
Surface modification can be applied not only to the fertilizer core but also to the bio-based PU coating itself. For example, surface roughening followed by silanization was performed on cottonseed-oil-based PU coatings to create a superhydrophobic surface, minimizing water contact and enhancing the release control [63]. In another study, a secondary bio-coating composed of starch (from maize or cassava), sulfur, and Aspergillus niger was layered onto a castor-oil-based PU-coated fertilizer, forming a dual-layer coating system for improved nutrient regulation [45].

2.3.4. A Summary of the Modification Methods

Table 1 summarizes studies on vegetable-oil-based PU (VPU) coatings for fertilizers, detailing the types of vegetable oils, fertilizer core materials, and modification methods used. Castor oil appears most frequently as the bio-polyol of choice due to its natural hydroxyl content and wide availability, particularly in China, a leading global producer of castor oil.
Most modification methods focus on enhancing the water repellency, increasing the crosslinking density, and improving the uniformity of the PU coating. These improvements are crucial for optimizing the release profile of the nutrients. Among the various fertilizers studied, urea is the most commonly used core material. It contains approximately 46% nitrogen and is globally favored due to its low cost and fast nutrient release rate. In comparison, diammonium phosphate (DAP) contains 18% nitrogen and 46% phosphorus (P2O5), while NPK (15-15-15) is a balanced compound fertilizer offering 15% each of nitrogen, phosphorus (P2O5), and potassium (K2O). As of 2022, the global fertilizer consumption rate reached approximately 185 million tons, with urea accounting for the largest share due to its widespread use [75]
To further enhance the controlled nutrient release performance of PU coatings, many studies combine multiple modification strategies. The effects of these modifications on the nutrient release behaviour will be discussed in detail in Section 3.

3. Controlled Release Properties of Vegetable-Oil-Based PU Coating for CRFs

VPU are applied on fertilizers’ surfaces to control the release of nutrients; therefore, the controlled release properties of a PU coating is the main concern in its performance. There are three methods used to characterize controlled release properties, namely nutrients released in water, nutrients released in soil, and planting experiments. The controlled release properties of different VPUs for different nutrients are summarized as follows.

3.1. Nutrients Released in Water

CRFs are placed in water and their controlled release performance is evaluated by measuring the amount of nutrient dissolution at different time points. It is the simplest, quickest, and most commonly used method to evaluate the release behavior of a coated fertilizer. Testing the release time of a CRF in water is done to evaluate its effect, mainly for the following reasons. Initially, the release duration in water can reflect the controlled release characteristics of the fertilizer. Secondly, water testing has ease of operation and cost advantages. Thirdly, a CRF’s release behavior in water is correlated with soil release. Lastly, testing in water can provide standardized evaluation indicators. Uncoated traditional fertilizers could be released very quickly in water, such as urea (within an hour) [77], diammonium phosphate (less than 2 h) [78], and NPK fertilizer (a few hours) [61]. Nevertheless, the pure bio-based PU exhibited poor controlled release performance, meaning it cannot meet the needs of agricultural users. The release duration of castor-oil-based PU-coated urea with a 3% coating and 1:1 NCO/OH ratio was no more than 20 days [43]. Except for castor oil, the nutrient release durations of other unmodified pure vegetable-oil-based PU-coated fertilizers are also short, at 10 days (soybean oil) [59], 7 days (palm oil) [64], 7 days (rapeseed oil) [61], and 14 days (cottonseeds oil) [63].
After modification, the controlled release duration of the VPU-coated fertilizer was obviously increased. The bio-based PU with a 1:6 NCO/OH ratio from castor oil and PAPI was coated on the water-polished diammonium phosphate particles [47]. After water polishing the fertilizer particles, the coated fertilizer had a phosphorus release duration of more than 80 days at a 2% coating content. When the coating content was 4%, the release duration exceeded 100 days. Through building a multilayer coating composed of a wax coating and the castor-oil-based PU coating on fertilizer surfaces, the nitrogen release period was extended to 178 days [44]. A bioactive starch coating was added to the PU coating to enhance the release period of coated urea [45]. When the PU coating amount was 7% and the bioactive starch coating was composed of maize starch, sulfur, and A. niger, the nitrogen cumulative release amount of the CRF with a double coating was about 35% at 800 h (33 days). The influence of the castor oil ratio, PAPI ratio, and coating percentage of the crosslinked interpenetrating PU coating (made from castor oil, starch-based polyol, and PAPI) on the nitrogen release rate was studied [48]. The results showed that the higher content of castor oil or PAPI or the coating percentage improved the nutrient release period. Especially, when the castor oil ratio increased to 49.88%, the release period (the cumulative N release reached more than 80%) reached 218 days. A castor-oil-based PU coating filled with elemental sulfur also showed better controlled nutrient release performance [54]. When the coating amount was 8%, the cumulative nitrogen release was 70% in 45 days. The effects of the proportion of PMDI and S content on the nutrient release control of vulcanized castor-oil-based PU-coated urea was explored [55]. When the S content was 14 g, the 56 day cumulative nutrient release rate of the CRF was minimized to 50.53%. When the PMDI content was more than 28.57% (31.03%, 33.33%), the 56 day cumulative nutrient release rate of the CRF was lower than 10%. The performance of polysulfide castor-oil-based PU-coated urea as a CRF was characterized [56]. When the sulfur content of the polysulfide castor oil–based polyol was 30% and the coating content was 7%, the cumulative release times increased to 60 days. A bio-polymer coating based on recyclable poplar catkin and castor oils was applied to the urea-controlled release [51]. The nutrient release durations of the CRFs had risen from 1 h to 35 days via enhanced water repellency and improved compactness. The superhydrophobic halloysite nanotubes also improved the controlled release properties of the castor-oil-based PU-coated urea as a CRF. The nitrogen release duration of the CRF reached 78 days, when the superhydrophobic halloysite nanotube content was 3% and the coating amount was 7%. Hydroxy-terminated polydimethylsiloxane enhanced the controlled release abilities of a VPU coating via hydrophobic modification [50]. When the ratio of castor oil to liquefaction polyhydric alcohols was 90:10 and the content of hydroxy-terminated polydimethylsiloxane was 5%, the controlled release duration of the coated fertilizer with a 4.5% coating content increased to 120 days. The microcapsules loaded with soy protein isolate were filled in the castor-oil-based PU coating to reinforce the controlled release properties via self-healing [49]. The nitrogen release duration of a CRF with a 5% coating content and 5% microcapsule content (in the coating material) was 70 days. The glycerol addition amount in the castor-oil-based polyol and the proportion of castor-oil-based polyol in the PU coating both influenced the controlled release property of the CRF [53]. When the content of castor oil polyol was 50% and the coating amount was 5%, the nitrogen release duration was 70 days.
In addition to castor oil, the nutrient release control properties of other VPUs have also been studied. The cottonseed-oil-based PU coating was improved in terms of its controlled nutrient release ability via a surface roughening and subsequent silanization process [63]. The nitrogen release duration of a CRF with a 7% superhydrophobic coating was 66.3 days. The effect of the hydroxy value of a palm-oil-based polyol on the controlled release properties of palm-oil-based PU coating was studied [64]. While the hydroxy value of the palm-oil-based polyol was 123.3 and the NCO/OH ratio was 1.7:1, the nitrogen release duration of the CRF with a 7% coating content was prolonged to 73 days. The N release, P2O5 release, and K2O release rate of a bio-based PU-coated NPK fertilizer from the lignin-based polyol and rapeseed-oil-based polyol was investigated [61]. The results showed that the higher amount of rapeseed-oil-based polyol effectively increased the nutrient (N, P, K) release duration of the coated fertilizer. The maximum release period in water was 70 days for the N, P, and K’s complete release from the bio-based coating. Rapeseed-oil-based polyols made with different phthalic/maleic anhydride ratios were compared in the controlled release properties of bio-based PU-coated NPK granules [62]. The results indicated that the phthalic content was higher and the bio-based PU had better controlled release properties for N, P, and K nutrients. The maximum release duration of the coated NPK fertilizer was 90 days. Elastic waste palm oil-based PU-coated urea, whose coating was modified with acrylonitrile, displayed 77 days of controlled release [66]. Superhydrophobic modified waste kitchen oil-based PU-coated diammonium phosphate had a controlled release period of more than 60 days [68]. The nitrogen release duration of a CRF with a bio-based PU coating from a soybean-oil-based polyol and polyester polyol (from petrochemical resources) increased to 137 days [75]. The controlled release property of a bio-based PU coating from 80% castor oil and 20% epoxidized soybean oil was adjusted in terms of the NCO/OH ratio and coating amount [76]. When the coating amount was 6% and the NCO/OH ratio was 1:1, the nutrient release duration of the CRF was 168 days. A comparative study of the nitrogen-controlled release properties between bio-based PUs from a soybean oil polyol and castor oil was conducted [46]. The castor-oil-based PU showed better performance than the soybean-oil-based PU. The nitrogen release duration of the soybean-oil-based PU was half that of the castor-oil-based PU. Bio-based PUs made from five types of vegetable-oil-based polyols (palm oil, rapeseed oil, olive oil, soybean oil, linseed oil), 1,4-butanediol, and PAPI were applied in a CRF study [58]. Among them, the linseed oil-based PU (50 days) showed more sufficient controlled of nutrients than the other vegetable oils. The results showed that the type and proportion of fatty acids have an important effect on the nitrogen release control performance of coated urea, and the crosslinking density is more important than the hydrophobicity of the PU coating. The duration of nitrogen release from hydroxyl-terminated dimethyl-silicone-modified soybean-oil-based PU-coated urea reached 70 days, while the coating amount was 7% and the proportion of hydroxyl-terminated dimethyl silicone was 40% in the PU [59]. When the molar ratio of mixed vegetable oil (80.0% castor oil and 20.0% soybean oil polyol) and PPI was 5:4 and the coating amount was 6%, the release duration of the coated compound fertilizer was 164 days [60]. Figure 4 shows the controlled release performance of bio-based PU-coated urea with different modification methods and vegetable oil types in water.
In general, there are many factors affecting the controlled release performance of a VPU coating, such as the types of vegetable oil, hydroxy value of the bio-based polyol, NCO/OH ratio, crosslinking degree, hydrophobic ability, and defects. Whether modified or not, the controlled release duration of the castor-oil-based PU was the longest among the above different types of vegetable oils. Regarding the fertilizer core, the VPU-coated compound (NPK) fertilizer had a longer release duration compared with the urea or phosphate fertilizer core. The surface modification of fertilizer particles can not only improve the controlled release performance of CRFs but also reduce the coating material’s usage and coating waste, for instance using water polishing [47,68], polyolefin wax surface modification [48], and paraffin surface modification [56,74]. Although it is true that the thicker the coating, the longer the release duration of CRFs, considering the nutrient content and cost, the coating amount of all studies was controlled within 10%. The vegetable-oil-based PU coating works best in controlled nutrient release when the NCO/OH ratio is close to or slightly greater than 1:1. It is not the case that a higher NCO/OH ratio is always better. Relative to the hydrophobicity of the coating, the density, defects, and crosslinking density of the PU coating are more important for its controlled release properties. The defects in the coating obviously were reduced after the modification, as shown in Figure 5.

3.2. Nutrients Released in Soil

The release of nutrients in soil from CRFs can simulate the conditions under which CRFs will be used in real fields, including the physical, chemical, and biological properties of the soil. This test method can more accurately reflect the actual performance of CRFs with different soil types and environmental conditions. However, due to the complexity of the soil environment, it usually takes a long time to test the release performance of CRFs in the soil, and the results may not be obtained quickly. Therefore, few studies on vegetable-oil-based coated fertilizers have chosen a nutrient release test in soil.
Wang et al. [67] mixed a recycled oil-based coted fertilizer with soil, then put the mixture in a polyvinyl chloride pipe. A thin layer of gauze was placed at the bottom of the tube for filtration. Every 5 days to 30 days, 100 mL of distilled water was added to the tube and the filtrate was collected through the gauze. When the hydroxyl-terminated poly dimethyl silicone/γ-aminopropyl triethoxy silane ratio was 0.2:0.8, the cumulative release rate of the modified oil-based PU-coated fertilizer was 50% after 30 days. Liu et al. [68] tested the nutrient release performance not only in water but also in soil. The release rate of an unmodified waste kitchen oil-based PU-coated fertilizer in water was slightly higher than that in soil. They thought the reason was that the concentration difference between the inside and outside of the PU coating in the water was relatively large, prompting water to quickly enter the membrane shell to dissolve the fertilizer core. However, for a superhydrophobic modified bio-based PU-coated fertilizer, the P was released slightly faster in the soil. The possible reasons were the high hydrophobicity on the surface of the membrane shell and the large crosslinking inside the PU to offset most water effects, and the decomposition of soil microorganisms to accelerate the release of nutrients. Gharrak et al. [61] conducted soil release tests in 150 mL plastic cups filled with about 100 g of homogenized soil. Two grams of coated fertilizer was buried 4 cm deep and covered by soil with controlled temperature and humidity (30 ± 2 °C and 62% RH). The plastic cups were regularly weigh and deionized water was added if necessary to keep the soil moisture level constant during incubation. At specified intervals, the buried fertilizer particles were collected, washed to remove residual soil, then dried. Finally, the nutrients released in the soil were determined by measuring the remaining nutrients in the coating. They also compared the nutrients released in water and soil, and found that the initial release rate of the bio-based PU-coated fertilizer in soil was slower than in water. A rapeseed-oil-based PU was effective in delaying the nutrient release duration, and 80% of the cumulative nutrients (N, P, K) were released within 35 days (N), 40 days (P), and 45 days (K).

3.3. Application in Planting Experiment

Planting experiments are crucial for evaluating the effectiveness of CRFs in promoting plant growth and for translating laboratory advances into practical agricultural applications. Field and pot studies have demonstrated the promising performance of VPU-coated fertilizers in enhancing nutrient efficiency and crop productivity.
In a field experiment, maize treated with a castor-oil-based PU-coated CRF at a 5% coating rate exhibited the largest stem diameter at maturity, with a grain yield increase of 3.09% and a biological yield improvement of 1.56% compared to conventional urea treatment [53]. Similarly, the maize yields were enhanced by 8.14–23.69% following the mixed application of castor-oil-based PU-coated phosphate fertilizer with uncoated fertilizer [47]. Pot studies further confirmed the benefits of VPU coatings. The use of sulfur-modified castor-oil-based PU-coated urea led to significantly improved corn growth, resulting in a 150% increase in plant height, 180% increase in stem width, and an impressive 520% rise in dry weight after 30 days of growth [55]. However, these results also emphasize that “longer nutrient release” does not necessarily equate to better performance—the optimal release period must align with crop nutrient demand patterns.
Additional field trials showed that applying sulfur-modified castor-oil-based PU-coated urea at different coating ratios slightly increased the grain yield and nitrogen uptake, confirming that VPU coatings can meet nitrogen demands effectively across the maize growth cycle [54]. In rice cultivation, double-modified castor-oil-based PU-coated urea significantly boosted the above-ground biomass at both the tillering and harvesting stages, alongside improvements in nitrogen use efficiency, rice yield, and overall productivity [51].
For oilseed rape, pot experiments using hydrophobically modified castor-oil-based PU-coated urea revealed that CRFs with a 60-day release period were better aligned with the nutrient uptake curve of the crop, outperforming the 30-day variant in supporting both early and peak-stage nutrient requirements [50]. In Chinese cabbage trials, both single fertilization (linseed-oil-based PU-coated urea) and mixed fertilization (a blend of uncoated and coated urea) led to improvements in fresh and dry weights, plant height, and nutrient use efficiency. Notably, the nitrogen use efficiency in the mixed treatment group increased by 36.16% compared to uncoated urea alone [58].
While VPU-coated fertilizers have shown substantial success in controlled nutrient release and crop yield improvements, the cost remains a critical barrier to widespread industrial adoption. With the exception of Su et al. [44], few studies have rigorously evaluated the economic feasibility of VPU-coated CRFs. The inclusion of additional raw materials or complex modification steps—although effective in enhancing performance—inevitably increases production costs, which may hinder their commercialization and large-scale application. Thus, we propose a dual-pathway strategy for enhancing their cost competitiveness:
1.
Design-Phase Cost Integration:
Adopt design-for-cost (DfC) principles during the early development of bio-based CRFs. By integrating a cost analysis alongside technical optimization from the outset, formulations can be tailored to balance economic viability with performance.
2.
Retrospective Life-Cycle Assessment (LCA):
Conduct comprehensive life-cycle cost assessments of current high-efficiency CRFs to identify cost bottlenecks across raw material acquisition, production scalability, and regulatory compliance processes. This retrospective analysis can highlight opportunities for streamlining costs and improving process sustainability.
Through such proactive and strategic approaches, the agricultural sector can accelerate the adoption of VPU-coated CRFs, aligning environmental sustainability with practical economic incentives.

4. Environmental Impact of Vegetable-Oil-Based PU Coating for CRFs

4.1. Degradability of Vegetable-Oil-Based PU

Traditional plastics are difficult to degrade and will remain in the natural environment for a long time, causing serious “white pollution”. Tian et al. [79] tested the biodegradation of petroleum-based polymer coatings from CRFs over more than 2 years, and the final PU degradation rate was 4.86%. Bio-based materials can be broken down into harmless substances in the natural environment, such as water and carbon dioxide, thereby reducing environmental pollution. Therefore, the degradability of bio-based PU is extremely important. The degradation properties of the full bio-based PU coating of CRFs in soil were evaluated using the weight loss method [57]. The degradation rate of the PU coating reached 27% after 12 months in the soil. This was because the bio-based PU coating was rich in ester bonds and carbamate bonds, which were easily hydrolyzed in the soil, leading to the degradation of the envelope. The XPS analysis showed that the content of nitrogen on the surface of the coating increased significantly after 12 months of degradation, indicating that microorganisms were involved in the degradation process. Microorganisms promote the degradation of the envelope through adsorption and biofilm formation. The surface of the coating gradually became rough during the degradation process, and there were holes and layered structures, which were caused by the synergistic effect of microbial erosion and chemical degradation. The degradation rate of bio-based coatings from a starch-based polyol and castor oil reached 5.05% in 9 months, while that of petroleum-based coatings was only 3.74% [48]. The degradation products could better interact with soil microorganisms and promote the health of soil ecosystems. The microstructure of a kitchen-oil-based PU coating were changed, and the defects were increased by microorganisms or enzymes in the soil [68]. After 240 days of soil burial, the degradation rates of the coatings with different formulations varied from 17% to 30%. The palm-oil-based PU coating initiated degradation, with mass loss rates ranging between 5.1% and 6.3% after 180 days of soil burial [64]. Generally, the biodegradation of the VPU coating were between 5% and 30% in 1 year, standing out against the petroleum-based PU coating (shown in Figure 6).

4.2. Other Effects of Vegetable-Oil-Based PU-Coated CRFs on the Environment

To understand the harmful effects of VPU-coated fertilizers on the environment during and after use, the following studies were carried out. The effects of bio-based PU coating residues on plant growth showed that a low concentration of PU coating could promote the germination of Chinese cabbage seeds, while a high concentration of coating could slightly inhibit the germination [57]. After 12 months of coating degradation, the inhibition effect of the high-concentration coating on germination was weakened, while the low concentration still had a promoting effect. The authors also studied the effects of coating residues on soil flora. Envelope degradation significantly changed the relative abundance of Proteobacteria, Actinobacteria, and Acidobacteria, and also changed the relative abundance of Bacillus and Streptomyces. Therefore, the envelope residue has a significant effect on the soil’s bacterial community structure. Tang et al. [53] carried out an ammonia volatilization experiment that showed that the ammonia volatilization of a CRF with 7% coating decreased by 77.5% compared with uncoated urea. This indicates that the castor-oil-based PU-coated fertilizer can effectively reduce nitrogen volatilization and reduce environmental pollution. The slow release mechanism of CRFs can maintain a relatively stable level of soil nutrients and avoid the decrease in soil fertility caused by nutrient loss. In exploring the effects of CRFs on enzymes, the combined application of a bio-based polyurethane-coated fertilizer and uncoated fertilizer could significantly elevate the activity rates of phosphoribosyl pyrophosphate amino transferase and adenosine monophosphate synthase in soil, which would contribute to enhancing the capacity for soil phosphorus metabolism [47]. Soil incubation experiments on CRFs with castor oil and starch-based polymer coatings showed that the ammonia volatilization loss of the CRFs was reduced by 40–50% relative to the uncoated urea [45]. At the same time, the available NH4+ and SO42− increased significantly in the soil due to the Aspergillus Niger in the starch coating. The Aspergillus Niger metabolized organic acids to attenuate the local pH value of the soil and alkaline environment caused by urea hydrolysis, leading to improved soil fertility. N2O emissions were studied in a corn field treated with sulfur-modified castor-oil-based PU-coated urea with different coating ratios, and the results showed that the cumulative N2O emissions decreased with the increase in the coating amount [54]. Compared with uncoated urea, the N2O emissions of the corn field treated by a CRF with 8% coating amount decreased by about 70%. The toxicity of the vegetable-oil-based PU coating were evaluated via seed germination experiments. The palm-oil-based PU coating had no toxic effect on cucumber seed germination and seedling growth, even at a coating solution concentration of up to 100 mg/L [64]. The starch- and castor-oil-based PU coating had no toxic effect on the germination of rice seeds and the growth of rice seedlings, and even promoted the growth of rice seedlings at low concentrations (15 mg/L) [48]. In summary, VPU-coated fertilizers have high nutrient utilization rates and cause less losses, which consequently means they have limited influence on the soil structure and cause less pollution to water and air environment.

5. Trends of Vegetable-Oil-Based PU-Coated Fertilizers

The development trends of vegetable-oil-based polyurethane (VPU) coated fertilizers were analyzed following the method proposed by Panda et al. [80]. Due to the diverse naming conventions used for vegetable oils, the search term “oil-based PU controlled release fertilizer” was used in the scholarly works section of the Lens database. Between 1975 and 2025, a total of 361 peer-reviewed journal articles were identified related to this topic. As illustrated in Figure 7a, the number of publications remained relatively low until 2015. However, a sharp increase in interest has been observed since 2016, with the publication number peaking at 58 articles in 2024. This surge reflects the growing academic and industrial attention being given to sustainable agricultural technologies, particularly in response to environmental concerns and the global demand for improved fertilizer efficiency.
The increasing research activity in this field can be attributed not only to technological advancements but also to national policies and international collaborations aimed at promoting green agriculture [81]. According to Figure 7b, India and China together contributed 117 publications, representing a significant portion of the global output. This is likely due to their status as large agricultural nations with high population densities, where the need for efficient nutrient management and environmental protection is especially critical. The heightened environmental pressures in these regions—such as water pollution from nutrient runoff and over-fertilization—have further accelerated their research efforts.
In terms of funding support, Figure 7c shows that the top three contributors are the U.S. National Science Foundation, the National Natural Science Foundation of China, and the Science and Engineering Research Council of India. This funding landscape indicates strong governmental commitment to developing sustainable fertilizer technologies through interdisciplinary research.
As demonstrated in Figure 7d, the research on VPU-coated fertilizers spans multiple scientific disciplines, including chemistry, environmental science, materials science, and biology. This interdisciplinary nature underscores the complexity of developing controlled-release fertilizers that balance environmental performance, material innovation, and agronomical effectiveness.

6. Conclusions and Prospects

The recent advancements in bio-based polyurethane (PU) coatings derived from various vegetable oils mean they have shown promising potential for use in controlled nutrient release fertilizers (CRFs). Among these, castor oil has been the most extensively studied due to its inherent hydroxyl functionality and availability. The research has primarily focused on enhancing the nutrient release performance of vegetable-oil-based PU (VPU) coatings by employing various modification strategies such as increasing the hydrophobicity, improving the crosslinking density, and enhancing the coating uniformity. These improvements have successfully extended the nutrient release durations, with some castor-oil-based PU coatings achieving controlled release periods exceeding 200 days. Urea remains the most commonly used fertilizer core in these studies, given its high nitrogen content and widespread agricultural applications. The transition of VPU-coated CRFs from experimental research to applied studies underscores their potential for improved nutrient use efficiency, increased crop yields, and mitigating environmental pollution, contributing significantly to the goals of sustainable agriculture. However, comprehensive studies on the production costs, scalability, and economic feasibility are still lacking. These factors are critical to determining the commercial viability and broader adoption of VPU-coated CRFs.
Looking ahead, future research studies should consider region-specific agricultural conditions, such as the climate, soil type, and local industrial infrastructure, to develop tailored VPU–CRF solutions. There is also a pressing need for systematic evaluations of the long-term environmental impacts and cost–benefit analyses. VPU-coated CRFs that balance performance, sustainability, and economic viability hold great promise for addressing global food security challenges and advancing eco-friendly agricultural practices.

Author Contributions

L.Y.: Conceptualization, writing—original draft, writing—review and editing, visualization, methodology, investigation, data curation, formal analysis. L.J.Y.: Supervision, resources, review and editing. A.B.: Supervision, review and editing. Z.Y.: Supervision, review and editing. K.H.B.: Conceptualization, writing—review and editing, supervision, validation, methodology, resources, funding acquisition, project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partially funded by a grant GP-2019-K012785 by the Universiti Kebangsaan Malaysia.

Institutional Review Board Statement

Not applicable. This study did not involve humans or animals.

Informed Consent Statement

Not applicable. This study did not involve humans.

Data Availability Statement

No new data were created in this study.

Acknowledgments

The authors would like to acknowledge Universiti Kebangsaan Malaysia (UKM) and the Faculty of Science and Technology (FST) for allowing this research to be carried out.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
PUPolyurethane
VPUVegetable-oil-based polyurethane
CRFControlled release fertilizer
MDIDiphenylmethane diisocyanate
PAPIPolyaryl polymethylene isocyanate
IPDIIsophorone diisocyanate
WCOWaste cooking oil
PHAPolyhydroxyalkanoate

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Coatings 15 00665 g001
Figure 1. Chemical structure of triglycerides.
Figure 1. Chemical structure of triglycerides.
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Figure 2. Chemical structure of castor oil.
Figure 2. Chemical structure of castor oil.
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Figure 3. Coating liquid on rotating urea granule surfaces.
Figure 3. Coating liquid on rotating urea granule surfaces.
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Figure 4. Release properties of VPU-coated urea in water: (a) full bio-based PU-coated urea [57]; (b) starch- and castor-oil-based PU-coated surface-modified urea [48]; (c) castor-oil-based PU-coated water-polished urea [47]; (d) bio-based PU-coated urea from palm oil, rapeseed oil, olive oil, soybean oil, and linseed oil [58]; (e) comparison of soybean oil and castor-oil-based-PU-coated urea with different coating amounts [46].
Figure 4. Release properties of VPU-coated urea in water: (a) full bio-based PU-coated urea [57]; (b) starch- and castor-oil-based PU-coated surface-modified urea [48]; (c) castor-oil-based PU-coated water-polished urea [47]; (d) bio-based PU-coated urea from palm oil, rapeseed oil, olive oil, soybean oil, and linseed oil [58]; (e) comparison of soybean oil and castor-oil-based-PU-coated urea with different coating amounts [46].
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Figure 5. Defects in vegetable coatings under different modification methods: (a1–a4) copolymerization with different proportion [50]; (b1–b4) double-modified VPU coating [51]; (c1–c3) VPU modified by different ceresin wax additions [44]; (d1–d3) VPU from different bio-based polyol [64]; (e1,e2) before and after acrylonitrile modification [66].
Figure 5. Defects in vegetable coatings under different modification methods: (a1–a4) copolymerization with different proportion [50]; (b1–b4) double-modified VPU coating [51]; (c1–c3) VPU modified by different ceresin wax additions [44]; (d1–d3) VPU from different bio-based polyol [64]; (e1,e2) before and after acrylonitrile modification [66].
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Figure 6. Degradation rate of VPU coating from CRFs: (a) full bio-based PU coating [57]; (b) kitchen-oil-based PU coatings with different ratios [68]; (c) starch- and castor-oil-based PU coating and petroleum-based PU coating [48]; (d) bio-based PU coatings from different palm oil polyols [64].
Figure 6. Degradation rate of VPU coating from CRFs: (a) full bio-based PU coating [57]; (b) kitchen-oil-based PU coatings with different ratios [68]; (c) starch- and castor-oil-based PU coating and petroleum-based PU coating [48]; (d) bio-based PU coatings from different palm oil polyols [64].
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Figure 7. Publication metrics of VPU for controlled-release fertilizer: (a) scholarly studies over time; (b) most active countries; (c) top study funds; (d) top fields of study.
Figure 7. Publication metrics of VPU for controlled-release fertilizer: (a) scholarly studies over time; (b) most active countries; (c) top study funds; (d) top fields of study.
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Table 1. Modification methods of bio-based PU-coated fertilizers from different vegetable oils.
Table 1. Modification methods of bio-based PU-coated fertilizers from different vegetable oils.
Vegetable OilFertilizer CoreCoating TechniqueModificationReference
Castor oilUreaDrum
Coating
machine		Sulfur modification[56]
Castor oilUreaSulfur modification[55]
Castor oilUreaWater-repellent and compact double-modification[51]
Castor oilUreaHydrophobic modification[52]
Castor oilUreaHydrophobic modification and co-polymerization[50]
Castor oilUreaSelf-healing[49]
Castor oilDAPSurface modification[47]
Castor oilUreaSulfur modification[54]
Castor oilUreaAdd bio-composite layer[45]
Castor oilUreaSurface modification and co-polymerization with starch-based polyol[48]
Castor oilUreaComposite modification[43]
Castor oilUreaNone[53]
Castor oilUreaCo-polymerization and surface modification[44]
Castor oilUreaComposite modification[57]
Soybean oilUreaCo-polymerization[75]
Soybean oilUreaSilicone modification[59]
Soybean oil/castor oilUreaNone[46]
80% castor oil and 20% epoxidized soybean oilNPK (15-15-15)Co-polymerization[76]
80% castor oil and 20% hydroxylated soybean oilNPK (15-15-15)Stannous chloride[60]
Palm oil/olive oil/linseed oil/rapeseed oil/soybean oilUrea1,4-butanediol[58]
Rapeseed oilNPK (15-15-15)Co-polymerization[61]
Rapeseed oilNPK (15-15-15)Composite modification[62]
Palm oilUreaStannous octoate[64]
Cottonseed oilUreaSurface hydrophobic modification[63]
Recycled OilUreaSilicone modification[67]
Recycled OilDAPSurface modifcaiton and composite modification[68]
Recycled palm OilUreaComposite modification[66]
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Yao, L.; Baharum, A.; Yu, L.J.; Yan, Z.; Badri, K.H. A Vegetable-Oil-Based Polyurethane Coating for Controlled Nutrient Release: A Review. Coatings 2025, 15, 665. https://doi.org/10.3390/coatings15060665

AMA Style

Yao L, Baharum A, Yu LJ, Yan Z, Badri KH. A Vegetable-Oil-Based Polyurethane Coating for Controlled Nutrient Release: A Review. Coatings. 2025; 15(6):665. https://doi.org/10.3390/coatings15060665

Chicago/Turabian Style

Yao, Lyu, Azizah Baharum, Lih Jiun Yu, Zibo Yan, and Khairiah Haji Badri. 2025. "A Vegetable-Oil-Based Polyurethane Coating for Controlled Nutrient Release: A Review" Coatings 15, no. 6: 665. https://doi.org/10.3390/coatings15060665

APA Style

Yao, L., Baharum, A., Yu, L. J., Yan, Z., & Badri, K. H. (2025). A Vegetable-Oil-Based Polyurethane Coating for Controlled Nutrient Release: A Review. Coatings, 15(6), 665. https://doi.org/10.3390/coatings15060665

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