Insights into Hydrophobic (Meth)acrylate Polymers as Coating for Slow-Release Fertilizers PDF

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Polymer Chemistry View Article Online PAPER View Journal | View Issue Insights into hydrophobic (meth)acrylate polymers This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Cite this: Polym. Chem., 2024, 15, as coating for slow-release fertilizers to reduce Open Access Article. Published on 17 July 2024. Downloaded on 10/22/2024 9:30:03 PM. 3327 nutrient leaching† a Asma Sofyane, Salima Atlas, a,b Mohammed Lahcini,a,c Elvira Vidović,d Bruno Ameduri *e and Mustapha Raihane *a,f To solve the problem of the low utilization rate of conventional fast-release water-soluble fertilizers and to minimize their negative impact on the environment, slow-release fertilizers (SRFs) have emerged as a sustainable solution to limit their losses, reduce fertilizer dosage and improve crop production. In this study, new hydrophobic (meth)acrylate polymers ( poly(2,2,2-trifluoroethyle methacrylate) (PTFEMA) and poly(2-(perfluorohexyl)ethyl acrylate) (PPFEHEMA)) with different fluorinated side chains were synthesized by free radical polymerization and used as coatings for SFRs. These polymers were characterized by 1H and 19F NMR, FTIR, WCA, TGA and DSC. Compared to PTFEMA, PPFEHEMA with a higher content of F atoms displayed improved thermal stability and an elastomer property (Tg = −10 °C) leading to satisfactory film formation. Indeed, water contact angle (WCA) measurements were carried out on films of both materials: PPFEHEMA with WCA = 109° indicated a highly hydrophobic character with an excellent water- repellent surface, resulting in a coating layer. The use of these polymers as SFR coatings was explored using dip-coating. SEM and EDX mapping were performed to study the morphology of the coated fertilizer granules and showed the formation of a cohesive film with good adhesion between the DAP fertilizer and the coating films, limiting water diffusion. The release profiles of N and P nutrients were studied, and the corresponding release times increased with coating thickness (single layer: 1L or second layer: 2L). Compared to uncoated DAP granules which are totally solubilized after less than 2 h, DAP coated with 2L Received 24th May 2024, of PPFEHEMA shows the slowest release of N and P nutrients, and the times to reach maximum N and P Accepted 15th July 2024 releases were 30 and 38 times higher than those of uncoated DAP. The significant delay in the release of DOI: 10.1039/d4py00573b nutrients from DAP coated with PTFEMA or PPFEHEMA is consistent with nutrient demand during crop rsc.li/polymers growth and increases the efficiency of fertilizer use and therefore enhances agricultural productivity. 1. Introduction people by 2050,1 with a planned increase in food supply of 70%.2 The forecast of the Food and Agriculture Organization From the data of the population projections published by the of the United Nations (FAO) estimates that a quarter of this United Nations, the world population will reach 9.5 billion growing population could suffer from food insecurity. Around 30% of arable land will be lost due to soil degradation.3 In order to meet the growing global demand for food and to a IMED-Lab. Faculty of Sciences and Techniques, Cadi-Ayyad University, address food security challenges by promoting sustainable Av. A. Khattabi. BP 549, 40000 Marrakech, Morocco. E-mail: [email protected] agriculture, the use of inorganic nitrogen (N), phosphorus (P) b ERSIC, FPBM, Sultan Moulay Slimane University, PO. Box. 592, Mghila, 23000, and potassium (K) fertilizers is expected to increase because Beni Mellal, Morocco c they can improve crop productivity by about 60%.4 However, Mohammed VI Polytechnic University, 43150 Ben Guérir, Morocco d Faculty of Chemical Engineering and Technology, University of Zagreb, current conventional fertilizers are highly water-soluble, Marulićevtrg 19, 10000 Zagreb, Croatia meaning that only 30–60% of N, 10–20% of P and 30–50% of K e ICGM, University of Montpellier, CNRS, ENSCM, 34095 Montpellier, France. could be absorbed by plants. A large amount of these micronu- E-mail: [email protected] trients is released into the environment through leaching, f Applied Chemistry and Engineering Research Centre of Excellence (ACER CoE), runoff, volatilization, etc., which has a negative impact on eco- Mohammed VI Polytechnic University, Lot 660, Hay Moulay Rachid Ben Guérir, 43150, Morocco systems and biodiversity, such as soil disturbance and ground- † Electronic supplementary information (ESI) available. See DOI: https://doi.org/ water contamination. These losses result not only in low 10.1039/d4py00573b absorption efficiency of the nutrients by plant roots,5 but also This journal is © The Royal Society of Chemistry 2024 Polym. Chem., 2024, 15, 3327–3340 | 3327  View Article Online Paper Polymer Chemistry in financial losses due to the waste of energy associated with and continuous wax layers improve the structural stability of their production.6,7 Therefore, in order to maximize crop pro- the coating materials and enhance the slow-release perform- duction one of the major challenges is to rationalize the use of ance by preventing water penetration into the fertilizer core. fertilizers. Slow-release fertilizers (SRFs) are proposed as a With the above problems in the use of hydrophilic superab- promising technology to improve nutrient uptake by plants sorbent polymers, hence hydrophobic polymer coating films and to minimize environmental pollution.8 SRFs are designed present an answer to this challenge by acting as good barrier to release nutrients slowly to meet their needs during crop membranes to limit the diffusion of water, and thus delay growth.9 Polymers coated fertilizers are the most important nutrient release from coated fertilizers. Among these polymers, candidates for SRFs because the polymer acts as a diffusion fluorinated acrylate polymers are the most commonly pro- This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. barrier membrane. Polyolefin, alkyd-resin and polyurethane- posed materials thanks to their remarkable properties, such as coated fertilizers are important commercially available syn- UV photo-chemical stability, remarkable weatherability, semi- Open Access Article. Published on 17 July 2024. Downloaded on 10/22/2024 9:30:03 PM. thetic polymer coatings for SRFs, manufactured by JCAM AGRI permeable membranes, and self-cleaning surfaces.21–23 Homo- Co, ICL Specialty Fertilizers and Koch Agronomic Services, Inc. and copolymers of fluorinated (meth)acrylates with perfluoro- under the Nutricote®, Osmocote® and Polyon® trademarks10 alkyl side chains (CnF2n+1) are an important class of such (more details are given in ESI†). The synthetic polymer coat- materials that exhibit unexpected hydrophobicity in compari- ings can be divided into two classes: (i) hydrophobic poly- son to the corresponding n-alkyl chains (PAs). In fact, the olefins which are soluble in an organic solvent (e.g., polyethyl- fluorocarbons side chains pack less densely on the surfaces, ene11 or polyacrylonitrile12), and (ii) superabsorbent hydrogels leading to poorer van der Waals interactions with water and as three-dimensional or crosslinked matrices composed of thus to good water-repellent properties.21,24–26 Their low linear or branched polymers with abundant hydrophilic surface energies, attributed to the properties of fluorine groups,13 which in agriculture lead to increased water storage atoms, enable them to be widely used in high-performance capacity, a limited amount of irrigation, and increased crop coatings.25–29 production in semi-arid and arid areas.14 To our knowledge, there have been only two papers report- Poly(acrylate)s (PAs) have been widely used to produce SRFs ing the use of hydrophobic fluorinated polymers as SRF coat- to increase agricultural yields of corn and wheat15 and as ings. To enhance the performance of polymer-encapsulated superabsorbents.16 PA waterborne coatings using an aqueous urea fertilizers, Chen et al.30 developed a novel waterborne solution in their preparation are known for their appropriate hydrophobic polymer coating using nano-SiO2 and viscosity, good film-forming ability, and strong adhesion to 1H,1H,2H,2H-perfluorooctyltriethoxysilane to modify water- substrates through polar groups.17 Polysaccharides such as based polyvinyl alcohol. More recently waterborne copolymers starch or cellulose are used as biopolymers for the synthesis prepared by Pickering emulsion copolymerization of butyl of bio-superabsorbents in which vinyl monomers such methacrylate (BMA) with 2-( perfluorohexyl)ethyl acrylate methacrylic acid, acrylamide, or acrylic acid are grafted onto (PFEHEMA) were reported by our team. The resulting water- their backbones to increase the hydrophilicity and swelling borne latexes were tested as coating materials for granular capacity of these superabsorbents.18 To elaborate these net- water-soluble fast-release fertilizers.31 A P(BMA-co-PFEHEMA) works to give them enhanced water-retention capacity and copolymer containing 8 wt% starch nanocrystals and a low regulated slow-release of nutrients, grafting reactions have PFEHEMA percentage (6.5 mol%) showed better slow-release been performed in an aqueous solution by free radical (co) properties than those of non-fluorinated P(BMA), attributed to polymerization of these monomers using ammonium persul- the presence of fluorinated units conferring improved hydro- fate and N,N‘-methylenebisacrylate (MBA) as initiator and phobic properties on a P(BMA-co-PFEHEMA) copolymer crosslinking agent, respectively.16 Recently, Zhu et al.19 pre- coating. pared superabsorbent hydrogel composites based on okara, a The aim of this work is the preparation of hydrophobic poly byproduct derived from soybean oil milk, grafted onto poly (meth)acrylates with different fluorinated side chains, such as (acrylic acid), by in situ radical polymerization to improve vege- poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) and poly(2- table cultivation through increasing the water holding capacity ( perfluorohexyl)ethyl acrylate) (PPFEHEMA), by free radical in soils. Jumpapaeng et al.20 prepared bionanocomposite polymerization. These polymers were characterized by 1H and 19 hydrogels (BHMs) as a promising material by combining F NMR and IR spectroscopy, WCA, DSC and TGA, and cassava starch, polyacrylamide, natural rubber, and various applied as coating materials to diammonium phosphate (DAP) montmorillonite clay loadings. These low-cost biohydrogels fertilizers. The morphology and chemical composition of the exhibit high-strength properties and serve as coating mem- coated fertilizer surfaces and cross-sections were investigated branes for slow-release urea fertilizers. However, these hydro- using SEM-EDX mapping, while a UV-visible spectrophoto- gels present some defect pores when used as a coating on the meter was utilized to monitor the release rates of phosphorus surface of urea, increasing the solubility of the N nutrient and (P) and nitrogen (N) in water. Finally, the relationship between thus reducing the slow-release effect. To address this issue, a the structure of fluorinated polymers and the release profiles wax hydrophobic polymer solution was used to encapsulate of N and P nutrients was studied to evaluate the performance the BHM hydrogel surfaces as an outer layer by filling in all in terms of slowing the release rate of nutrients through these cracks and defects detected on the surface. These hydrophobic fluorinated hydrophobic polymer coatings. 3328 | Polym. Chem., 2024, 15, 3327–3340 This journal is © The Royal Society of Chemistry 2024  View Article Online Polymer Chemistry Paper 2. Experimental section cipitated from methanol. The obtained PPFEHEMA polymer was collected by filtration, washed with methanol, and dried 2.1. Materials under vacuum at 60 °C. The yield of PPFEHEMA (white Diammonium phosphate (DAP) ((NH4)2HPO4) was chosen as a powder) was close to 60%. The synthesis route of PPFEHEMA granular phosphate fertilizer to prepare SRFs. This commercial is displayed Scheme 1a. granular fertilizer, containing 46% phosphorus (P2O5) and 2.2.2. Synthesis of poly(2,2,2-trifluoroethyl methacrylate) 18% nitrogen (N), was generously provided by the OCP Group (PTFEMA). 2,2,2-Trifluoroethyl methacrylate (TFEMA) in aceto- in Morocco. 2-(Perfluorohexyl)ethyl acrylate (PFEHEMA) (CAS: nitrile (MeCN) was polymerized according to the same proto- 17527-29-6) was kindly provided by Atofina (Pierre Bénite, col described above (Scheme 1b). Briefly, 8 mL of MeCN, This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. France), while 2,2,2-trifluoroethyl methacrylate (TFEMA) was 36 mmol of TFEMA (6.02 g; 5.1 mL; 2.7 mol L−1) and kindly supplied by Tosoh Finechemical Corp, Shunan, 0.18 mmol of AIBN (0.06 g, 1 wt% of monomer) were used to Open Access Article. Published on 17 July 2024. Downloaded on 10/22/2024 9:30:03 PM. Yamaguchi (Japan). Azobisisobutyronitrile (AIBN) and all the charge a glass Schlenk flask reactor with a magnetic stirrer. solvents (hexafluorobenzene, acetonitrile, pentane, tetrahydro- After the polymerization reaction was complete, the resulting furan, and methanol) were purchased from Sigma-Aldrich solid was solubilized in a minimal amount of tetrahydrofuran (France). Before use, TFEMA and PFEHEMA were purified by and then the resulting polymer was purified by precipitation distillation under reduced pressure. in pentane and subsequently dried in an oven under vacuum at 50 °C. The yield of the obtained PTFEMA (white powder) 2.2. Synthesis of fluorinated homopolymers was 75%. 2.2.1. Synthesis of poly(2-( perfluorohexyl)ethyl acrylate) (PPFEHEMA). The bulk radical polymerization of 2-( perfluoro- 2.3. Coating technique hexyl)ethyl acrylate, PFEHEMA, was performed according to To provide a suitable viscosity for a coating process, we pre- the procedure described by Stone et al.32 Briefly, 12 mmol pared PTFEMA and PPFEHEMA polymer solutions in THF and (5.03 g) of PFEHEMA monomer were placed in a glass flask hexafluorobenzene, respectively (in 40% w/v ratio). The com- equipped with a reflux condenser, thermometer, and a mag- mercially available granular DAP fertilizers were coated by a netic stirrer. AIBN as initiator (0.3 mmol, 2.5 mol% related to dip-coating process, as described in previous studies.9,31 Dip- the monomer) was then added; the solution was purged with coating was achieved by immersing DAP granules (with dia- nitrogen gas for 15 min and heated in an oil bath at 70 °C for meters of 2–4 mm and a weight of ca. 35 mg) in the corres- 24 h to complete the polymerization. After cooling the reactor, ponding solutions for 10 min. The DAP pellets were then the final mixture was dissolved in hexafluorobenzene and pre- removed from solution and placed on a Teflon® film surface. Scheme 1 Radical polymerization of: (a) PFEHEMA and (b) TFEMA monomers. This journal is © The Royal Society of Chemistry 2024 Polym. Chem., 2024, 15, 3327–3340 | 3329  View Article Online Paper Polymer Chemistry Subsequently, the coated DAP granules were dried at room 2.4.6. Differential scanning calorimetry (DSC). DSC ana- temperature, leading to a coated fertilizer with a single layer lyses were performed on 10–15 mg samples under nitrogen (1L). To create a coated DAP with a second layer (2L), this oper- flow on a Netzsch DSC 200 F3 instrument to observe thermal ation was repeated a second time on the coated DAP (1L) using transitions using the following cycles: first heating from the same coating solution. −60 °C to 120 °C at 10 °C min−1, cooling from 120 to −60 °C at The percentage coating (CC) was calculated according to 20 °C min−1, and finally second heating from −60 °C to 120 °C the equation: at 10 °C min−1. From the DSC thermograms (second heating), mf  mi the inflection point of the step-change in heat capacity corres- CC% ¼  100 ð1Þ ponds to Tg. An indium sample (Tm = 156.6 °C) was used to This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. mi calibrate the instrument. where mf and mi are the weights of the granular fertilizer after 2.4.7. Scanning electron microscopy (SEM). SEM analysis Open Access Article. Published on 17 July 2024. Downloaded on 10/22/2024 9:30:03 PM. and before coating, respectively. was recorded to characterize the morphology of uncoated and coated fertilizers, using a VEGA-3 instrument (TESCAN-France) 2.4. Characterizations with an accelerating voltage of 10 kV. Energy-dispersive X-ray 2.4.1. Fourier-transform infrared spectroscopy (FTIR). (EDX) analysis was also used to identify the chemical compo- Powder samples were taken into KBr pellets. FTIR analyses sition of the coatings. Indeed, SEM was utilized to examine the were carried out using a PerkinElmer 1725X spectrometer in maps of the spatial distribution of elements within the samples. transmittance mode. The spectra were recorded at room temp- For this analysis, an axial rupture containing the fertilizer erature with scanning in the range 400–4000 cm−1 with 16 and the coating material was created using a razor blade. The acquisitions. coated granule and its cross-section were spread out on a 2.4.2. Nuclear magnetic resonance (NMR) spectroscopy. carbon band and fixed to the surface of a metal disc using The 1H and 19F NMR spectra were recorded at room tempera- double-sided adhesive tape. Additionally, by examining the ture using a Bruker AC 400 spectrometer at ambient tempera- cross-sectional surface of coated DAP granular fertilizer, the ture. Deuterated chloroform (CDCl3) and a 1 : 1 mixture of coating thicknesses were determined. CDCl3/CF3CO2H were used as NMR solvents for PTFEMA and 2.4.8. Release assays of nitrogen and phosphorus in water. PPFEHEMA, respectively. Chemical shifts are given in ppm. 1H The P and N release profiles for the coated and uncoated TSP and 19F NMR spectra were performed under the following fertilizers were determined according to the protocol described experimental conditions: a flip angle of 90° for 1H (30° for in our previous work.9,31 Briefly, uncoated and coated DAP 19 F), acquisition time of 4.5 s (0.7 s), pulse delay of 2 s (5 s), granules (50 mg) were placed in a 125 mL beaker filled with 16 scans (64 for 19F), and a pulse width of 5 µs for 19F NMR. distilled water and gently stirred at room temperature. 2.4.3. Size exclusion chromatography. Size exclusion Samples (100 μL) were collected at different time intervals, chromatography (SEC) analysis was carried out on a Polymer diluted 100 times, and analyzed in the spectrophotometer. Laboratories PL-GPC 50 Plus instrument using 2 PL gel mixed- Nitrogen (NH4+) and phosphorus (P2O5) release profiles were C 5 μm columns (molar masses ranging from 200 to 2 × 106 g then conducted by a colorimetric process, using AFNOR-T90- mol−1) thermostatted at 35 °C equipped with a refractive index 015 and AFNOR-T90-023 norms, respectively. An ultraviolet– detector. Tetrahydrofuran (THF) with 1% LiBr was used as visible (UV–vis) spectrophotometer (UV-2600, Shimadzu) was eluent (1.0 mL min−1). The calibration was performed using used to characterize the resulting complex-colored solutions at Varian polymethylmethacrylate (PMMA) standards. 630 nm and 880 nm for NH4+ and P2O5, respectively. The 2.4.4. Water contact angle (WCA) measurements. Water absorbance of all solutions was measured, and the standard contact angle (WCA) measurements were performed to investi- curve was drawn. Linear fitting was undertaken and yielded gate the degree of hydrophobic character of the synthesized correction equations of Y = 0.741X (R2 = 0.997) and Y = 0.615X fluorinated polymers. A KRUSS GmbH Easy Drop goniometer (R2 = 0.997) for N and P nutrients, respectively. (Germany) equipped with a charge-coupled device camera was used to measure the contact angle of a water droplet in contact with a solid surface. An image capture program using 3. Results and discussion SCAT software was utilized to record the measurements. To measure the contact angles, a circle was defined around the 3.1. Preparation and characterization of PTFEMA and drop, and the tangent angle formed at the substrate surface PPFEHEMA (coating materials) was recorded. To ensure the reproducibility of the measure- For excellent weatherability, a semi-permeable membrane ments, three experiments were conducted for each based on fluorinated acrylic polymers should be covered by as formulation. many fluorine-containing groups as possible.25,27,29,32 In con- 2.4.5. Thermogravimetric analysis (TGA). To determine the trast to some low-molar-mass per- and polyfluoroalkyl sub- thermal stability of the obtained polymers, TGA was performed stances (PFASs), well established as being water soluble, toxic, on TA-55 discovery equipment. A few milligrams of each persistent, bioaccumulative and mobile, fluoropolymers are in- sample were heated at rate of 10 °C min−1 from room tempera- soluble in water and thus not mobile, are bio-inert, safe and ture to 800 °C under nitrogen gas (60 mL min−1). have unique properties that are essential for our daily lives 3330 | Polym. Chem., 2024, 15, 3327–3340 This journal is © The Royal Society of Chemistry 2024  View Article Online Polymer Chemistry Paper (coatings, electronics, internet of things, energy, transpor- The assignments of the chemical shifts were derived by tation, etc.). Indeed, these materials are possibly irreplaceable, comparison with the values reported in the literature for since suggested alternative products such as hydrocarbon poly- TFEMA-based polymers38 and poly( perfluoro(meth)acrylate)s29 mers failed when used in similar conditions. Interestingly, and are summarized in Table 2. For example, the 1H NMR these high-performance polymers satisfy the 13 polymer of low spectrum of PTFEMA shows a signal of the methylene of ester concern (PLC) criteria in their recommended conditions of group (–O-CH̲2-CF3) centered at 4.3 ppm. The methyl group of use.33 Therefore, these specialty polymers must be separated PTFEMA (–CH̲3) was observed in the range 0.8–1.1 ppm, while from the PFAS family. Shirai et al.34 recently reported that poly the methylene protons of the backbone (CH̲2) appear between (fluoroalkyl (meth)acrylate)s containing extended perfluoro- 1.8 and 2.1 ppm. The 19F NMR spectrum of PTFEMA exhibits This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. alkyl groups (CnF2n+1) can degrade, leading to the formation of the CF̲3 peak at −73 ppm. The vinylic proton signal centred at perfluorooctanoic acid (C7F15CO2H, PFOA). These authors also 6.1 ppm for TFEMA and peaks at 6.5, 5.9 and 5.0 for Open Access Article. Published on 17 July 2024. Downloaded on 10/22/2024 9:30:03 PM. showed that polymers featuring short fluorinated side chains PFEHEMA were not present in these spectra. (n ≤ 6 fluorocarbons) present less bioaccumulative PFAS com- 3.1.3. Size exclusion chromatography. The molar masses of pared to those with n ≥ 7. Therefore, taking in account the poly(2,2,2-trifluoroethyl methacrylate), PTFEMA, were deter- hydrophobic coating performances with fluorine-containing mined using size exclusion chromatography (SEC) in tetra- groups and to address environmental concerns with less bioac- hydrofuran (THF), calibrated with polymethylmethacrylate cumulation of PFOA, we have chosen to use TFEMA (–CF3) and (PMMA) standards. The obtained M ˉ n and M ˉ w and dispersity PFEHEMA (C6F13) as fluorinated monomers to prepare ˉ ˉ (Đ = Mw/Mn) values are equal to 26 000 g mol−1, 52 000 g mol−1 PTFEMA and PPFEHEMA polymers with high molar masses and 2.0, respectively. Of course, these are relative values. compared to those of PFASs, which can be applied as coating However, it is not possible to determine the molar masses of fertilizers to achieve the slow release of nutrients. These poly- poly(2-( perfluorohexyl)ethyl acrylate) (PPFEHEMA) as it is not mers were successfully synthesized by free radical polymeriz- soluble in organic solvents, but only in fluorinated solvents ation (Scheme 1). The resulting polymers were then analyzed, such as hexafluorobenzene or 1,1,1,3,3,3-hexafluoro-2-propa- and finally used as coating materials to cover diammonium nol. Actually, our SEC apparatus is not equipped with columns phosphate (DAP) fertilizers. related to fluorinated solvents. 3.1.1. Infrared spectroscopic analysis (FTIR). Fig. S1 (ESI†) 3.1.4. Water contact angle (WCA). WCA is one of the most shows the FTIR spectra of the TFEMA and PFEHEMA mono- important parameters affecting release kinetics since the mers and the corresponding PTFEMA and PPFEHEMA homo- hydrophilic character of polymer films reduces the diffusion of polymers. Fig. S1† displays the characteristic FTIR absorption water through these films and gives them water-repellent pro- peaks assigned to different chemical bonds, as summarized in perties.40 The WCA value for the PTFEMA film was about 97°, Table 1. These assignments agree with those of fluorinated while that of PPFEHEMA reached a value of 119°, as shown in (meth)acrylate polymers described in the literature.35–39 Fig. S2 (ESI†). The difference in WCA between the two homo- Meanwhile, the characteristic stretching of the TFEMA and polymers (22°) indicates a more pronounced hydrophobic PFEHEMA double bond observed at 1649 and 1638 cm−1, character for PPFEHEMA film compared to PTFEMA film. This respectively, disappeared, indicating that the polymerization can be attributed to the difference in the surface energy value reaction and purification of the resulting polymers had been in the chemical structure at the surface of both fluorinated successfully achieved. polymers.39 Barbu et al.41 reported that the constituent groups 3.1.2 1H and 19F NMR spectroscopy. The white powders affect the surface energy in the following order: CH2 (36 mN ( purified copolymers) were characterized by NMR spec- m−1) > CF2 (23 mN m−1) > CF3 (15 mN m−1). Tsibouklis et al.42 troscopy. Fig. 1 provides the 1H and 19F NMR spectra of the also studied the surface organization phenomena and the PTFEMA polymer recorded in deuterated chloroform, while surface energy of poly( perfluoroalkyl methacrylate)s films. the NMR spectra of PPFEHEMA were recorded in a They observed the influence of the length of the pendant fluor- 1 : 1 mixture of CDCl3/CF3CO2H, since PPFEHEMA is not ocarbon moiety on the surface energy, and concluded that soluble in organic solvents. increasing the chain length induces a lower surface energy. Indeed, as the pendant chain length increases, the average surface roughness (Ra) of the corresponding film structures, Table 1 Principal FTIR characteristic bands of PTFEMA and PPFEHEMA determined by AFM, follows the same trend, and therefore polymers serves to inhibit the absorption of liquids by the bulk PTFEMA PPFEHEMA sample.42 In fact, the Ra value of PPFEHEMA is close to 3.1 nm Band (cm−1) (cm−1) (ref. 42) while that of PTFEMA is 0.41 nm.43 The surface pro- perties of comb-shaped polymers with perfluoroethyl side C–F symmetric stretching 1225 1202 C–F asymmetric stretching 1176 1145 chains (Rf ) are also strongly related to the ordered structure of CvO ester stretching 1753 1737 the side chains at the surface. Our team reported the thermal C–H: symmetric and asymmetric 2850 and 2960 2875 and 2972 behavior, liquid-crystalline structure, and functional group stretching C–H (out of plane) 973 844 orientation of comb-shaped polymers poly(2-( perfluorooctyl) C–O stretching 1176 1116 ethyl acrylate) containing perfluorooctyl side chains.24 A tilted This journal is © The Royal Society of Chemistry 2024 Polym. Chem., 2024, 15, 3327–3340 | 3331  View Article Online Paper Polymer Chemistry This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 17 July 2024. Downloaded on 10/22/2024 9:30:03 PM. Fig. 1 1H and 19F NMR spectra of PTFEMA (left) (CDCl3 as the solvent) and PPFEHEMA (right) (a mixture of CDCl3 and CF3COOH as NMR solvent in 1 H–{19F} decoupling mode NMR (chemical shifts in the inserts correspond to CF̲3 and CO2H̲ groups). Table 2 Assignments of chemical shifts/ppm for PTFEMA and to the different parameters involved above, the hydrophobic PPFEHEMA polymers characteristic of the fluorinated polymer, PPFEHEMA, was improved compared to that of PTFEMA, based on the fluori- Type of proton PTFEMA PPFEHEMA nated chain length with a lower surface energy, a high average 1 H NMR surface roughness (Ra) and the well-ordered structure of CH3 0.8–1.1 — PPFEHEMA. This result is in good agreement with previous CH2 (main chain) 1.8–2.1 1.2–2.2 CH (main chain) — 2.2–2.7 work.31,44 Comparing two fluoroalkyl methacrylate polymers, OCH2CF3 4.3 — Phillips and Dettre44 found that a polymer bearing a longer OCH2CH2C6F13 — 4.2–4.5 fluoroalkyl side chain displays the highest WCA value. OCH2CH2C6F13 — 2.2–2.7 3.1.5. Thermal properties (TGA and DSC). DSC and TGA Type of fluorine PTFEMA PPFEHEMA analyses were used to study the thermal properties of PTFEMA 19 and PPFEHEMA (Fig. 2). F NMR The degradation of PTFEMA takes place in two steps OCH2CF3 −73.0 — O(CH2)2CF2CF2CF2CF2CF2CF3 — −114.8 (Fig. 2a). The first one, in the range 200–300 °C, corresponds O(CH2)2CF2CF2CF2CF2CF2CF3 — −124.8 to the volatilization of side-chain fragments, including CO2, O(CH2)2CF2CF2CF2CF2CF2CF3 — −122.8 vinylidene fluoride and 2,2,2-trifluoroethanol, which are deter- O(CH2)2CF2CF2CF2CF2CF2CF3 — −123.9 O(CH2)2CF2CF2CF2CF2CF2CF3 — −127.5 mined to be pyrolytic decomposition products (weight loss O(CH2)2CF2CF2CF2CF2CF2CF3 — −82.6 26%). The second decomposition step, in the range 305–420 °C (weight loss 74%), is attributed to a depolymeriza- tion reaction.45,46 PPFEHEMA decomposes in a single step, in hexatic smectic-B phase was obtained, with the Rf side chains the range 280–420 °C, which is attributed to a random cleavage playing the role of mesogens to form a bilayer lamellar struc- leading to a depolymerization mechanism (Fig. 2a). ture with lateral hexagonal translational order producing a PPFEHEMA exhibits higher thermal stability than PTFEMA well-ordered structure, which exhibits better liquid repellency (Fig. 2a), which can be attributed to the better thermal stability than those analogues with short fluorinated chains. According of the C6F13pendant group in PPFEHEMA due to the strong 3332 | Polym. Chem., 2024, 15, 3327–3340 This journal is © The Royal Society of Chemistry 2024  View Article Online Polymer Chemistry Paper This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 17 July 2024. Downloaded on 10/22/2024 9:30:03 PM. Fig. 2 TGA (a) and DSC (b) thermograms of PTFEMA and PPFEHEMA (N2 gas). C–F bond (EC–F = 450 kJ mol−1) that makes it possible to increase centages of the different coating materials (calculated accord- the heat resistance performance of the polymeric materials ing to eqn (1)) are given in Table 4. by adding more fluorinated components.31 Table 3 lists the To investigate the quality of the coating between the fertili- thermal characteristics of PTFEMA and PPFEHEMA. zer and the coating, the morphology of the surface and the The DSC second heating thermograms of both fluorinated cross-section of uncoated and PTFEMA and PPFEHEMA coated polymers showed no melting temperature when the samples DAP with a single layer (1L) and a second layer (2L) was investi- were heated from −60 °C to 120 °C (Fig. 2b). Only a sharp tran- gated by SEM (Fig. 3). sition from the glassy state to the viscoelastic one was A first overview of the SEM results showed that the surface observed, as evidenced by the presence of a neat Tg, indicating of the uncoated DAP granule has an irregular and rough struc- that these fluorinated polymers exhibited amorphous behavior ture (Fig. 3a; scale bar: 1 mm). (Table 3); the Tg were close to −10 and 75 °C for PPFEHEMA The highly magnified surface (Fig. 3a; scale bar: 100 μm) and PTFEMA, respectively. showed some pinholes and an irregular morphology, due to The decrease in Tg for PPFEHEMA compared to PTFEMA is the granulation process during the production of DAP related to the structure of the PFEHEMA units. In fact, the fertilizers.40,48 When the DAP fertilizer was coated with the two long alkyl dangling chains of the acrylate moiety PTFEMA and PPFEHEMA polymers, the coating surfaces (–CO2CH2CH2C6F13) serve as internal plasticizers, resulting in exhibited a smoother and denser structure compared to low Tg and giving PPFEHEMA a more elastomeric behavior at uncoated DAP, especially when the fertilizers were covered room temperature, as shown in Fig. S3.† 47 The decrease in Tg with the second layer, as the content of the coating membrane leads to excellent film-forming properties at room temperature on the surface of the fertilizers increased (Fig. 3b–d). This is in for fertilizer coating. The PPFEHEMA coating films also help good agreement with our previous work.9,31,40,48,49 to improve the physical quality of granular fertilizer and are When analyzing the outer surface of the DAP granules expected to have a positive effect on their compressive strength coated with PTFEMA (Fig. 3b and c), we found that there are so that they do not break easily, preventing the generation of some microcracks in the surface compared to the granules excessive dust during the handling and storage process. coated with PPFEHEMA, which may be related to the structure of these polymers. PPFEHEMA has a Tg that is lower than the ambient temperature (−10 °C), so the PPFEHEMA coating has 3.2. Morphological characterization of coated DAP fertilizers high flexibility and good film-formation, resulting in improved Film forming from polymer solution coatings for DAP fertili- impact and crack resistance (Fig. S3†). In contrast, PTFEMA zers was performed using the dip-coating method.9,30 The per- with a Tg of around 70 °C (Table 3) exhibits a glassy state at Table 3 Thermal data of PTFEMA and PPFEHEMA by TGA and DSC Table 4 Percentages of coating materials, PTFEMA and PPFEHEMA, with different layers (L) TGAa DSC Weight coating percentage Average thickness Polymer Td10% (°C) Td50% (°C) Residue at 600 °C (%) Tg (°C) DAP coating (%) (μm) PTFEMA 256 345 0.0 75 PTFEMA 1L 4.5 51.0 PPFEHEMA 339 371 2.0 −10 PTFEMA 2L 10.7 90.0 PPFEHEMA 1L 7.7 27.0 a Tdx%: temperature of x% of decomposition (N2 gas, 10 °C min−1). PPFEHEMA 2L 16.0 73.0 This journal is © The Royal Society of Chemistry 2024 Polym. Chem., 2024, 15, 3327–3340 | 3333  View Article Online Paper Polymer Chemistry This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 17 July 2024. Downloaded on 10/22/2024 9:30:03 PM. Fig. 3 SEM analysis of a fertilizer granule and its cross-section containing the interface between DAP and tested PTFEMA and PPFEHEMA (single layer (1L) and second layer (2L)). room temperature, which leads to some cracking when the hydrophilic side (ester groups –CO2–) in the PTFEMA and solvent evaporates. These cracks could be reduced when the PPFEHEMA coatings could be responsible for the good second layer was applied to the surface of the coated fertilizer. adhesion by both compounds.47 Indeed, the border lines To eliminate these cracks or prevent their formation, between fertilizer and the film coatings are irregular due to the Devassine et al.50 reported that controlling the rate of solvent non-spherical irregular shape of the initial DAP granules evaporation or performing annealing could prevent the for- (Fig. 3b and d; scale bar: 100 μm). Poly(fluorinated (meth)acry- mation of cracks and pores. Yadavalli et al.51 observed some late)s are a viable option for use in agriculture as coatings for cracks in the SEM of the composite thin films and reported SRFs, as confirmed by the formation of cohesive films.31 another explanation, which is the electron-beam-induced rapid From the cross-section of core (fertilizer)–shell (coating) volatilization of the organic species, such as residual solvent (Fig. 3b–e), the thicknesses of polymer coating were assessed from the surface of these films during SEM analysis, leading by SEM at different points due to the irregular shape of DAP to a buildup of tensile stress that causes cracks in the grain fertilizers, and the average thicknesses were calculated boundaries. (Table 4). These values are a function of the type of coating The cross-sectional images of coating materials observed by (PTFEMA or PPFEHEMA) and their content (1L or 2L), as dis- SEM with different magnitudes are shown in Fig. 3b–e. The played in Fig. S4.† contact surface between PTFEMA or PPFEHEMA (1L and 2L) The thicknesses of the different PTFEMA and PPFEHEMA coatings and the DAP core fertilizers is continuous with no coatings are also shown in Fig. S4.† The average thicknesses of gaps or voids present within it. In fact, the interaction between DAP coated with PTFEMA (1L) and (2L) are close to 51 and the hydrophilic inorganic DAP granules ((NH4)2HPO4) and the 90 μm, respectively, while those achieved when PPFEHEMA is 3334 | Polym. Chem., 2024, 15, 3327–3340 This journal is © The Royal Society of Chemistry 2024  View Article Online Polymer Chemistry Paper used as the coating are around 27 and 73 μm for 1L and 2L, and PPFEHEMA coatings covered the granular fertilizers suc- respectively. The measured thicknesses of the two-layer (2L) cessively with good adhesion and without any diffusion of the coating are 1.5 and 2.7 times higher than those of the single- macronutrients N and P of the DAP fertilizer. These results layer (1L) PTFEMA and PPFEHEMA coatings, respectively. are also corroborated by the SEM analyses. In the DAP coated Energy dispersive X-ray analysis (EDX) was used to reveal with PTFEMA and PPFEHEMA membranes, the carbon the chemical compositions on the surface of the coated and content increases compared to that of the non-coated DAP fer- uncoated DAP fertilizers to evaluate the quality of the coatings. tilizer, which is attributed to the carbon atoms in the fluori- The results are shown in Fig. 4 and Table 5. nated (meth)acrylate units of the polymer coatings. As As essential macronutrients, the N and P signals of the DAP expected, the DAP coated with PPFEHEMA has a higher per- This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. fertilizer were detectable only on the uncoated DAP surface. centage of F-atoms than that coated with PTFEMA (Table 5 Their percentages were 21.55% and 10.10%, respectively and Fig. 4). Open Access Article. Published on 17 July 2024. Downloaded on 10/22/2024 9:30:03 PM. (Table 5). Other microelements with a low content (0.67%), The spatial distribution of the elements was investigated including Mg, Al and Ca, were also observed. The signal using the EDX technique. For example, Fig. 5 shows the related to carbon (19.54%) was related to the metallization of element mapping (C, N, P, O and F) in the cross-section of DAP granules because the samples needed to be conductive to DAP encapsulated with PPFEHEMA 2L. The C, N, P, O and F perform the SEM analysis. are the constituent elements of the core–shell that display a The absence of N and P macronutrients on the outer more homogeneous distribution on the cross-section of DAP surface of the coated DAP granules confirms that the PTFEMA coated with PPFEHEMA. Fig. 4 EDX analysis on the surface of uncoated DAP and DAP coated using PTFEMA and PPFEHEMA 1L and 2L. Table 5 EDX elemental weight percentages of uncoated and coated DAP with different tested polymers (1L and 2L) Detected nutrients (wt%) C F O N P Other elements Uncoated DAP 19.54 0 46.98 10.10 21.55 0.67 DAP coated with PTFEMA 1L 67.03 15.42 17.55 0 0 0 2L 73.41 14.37 12.21 0 0 0 DAP coated with PPFEHEMA 1L 44.28 50.12 05.59 0 0 0 2L 44.79 49.57 05.64 0 0 0 This journal is © The Royal Society of Chemistry 2024 Polym. Chem., 2024, 15, 3327–3340 | 3335  View Article Online Paper Polymer Chemistry This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 17 July 2024. Downloaded on 10/22/2024 9:30:03 PM. Fig. 5 Chemical mapping obtained from the cross-sections of DAP coated with PPFEHEMA (2L) (scale bar: 10 μm). 3.3. Phosphorous and nitrogen release behavior of coated 50.5 h, indicating significantly slower P release or delaying per- and non-coated DAP fertilizers formance properties of DAP fertilizers, and thus their potential applications as coating films in crop agriculture.31,40,47,48 DAP To predict slow macronutrient releases for practical appli- coated with 2L of PPFEHEMA presents the slowest macronutri- cation, the nitrogen (N) and phosphorus (P) release patterns of ent release: the times to reach the maximum N and P release uncoated and coated DAP fertilizer granules in water were are 30 and 38 times higher than those of uncoated DAP, studied according to the procedures described by Li et al.52 respectively. Indeed, compared to PTFEMA, the PPFEHEMA and Pereira et al.53 This allows an evaluation of the effects of coating shows significantly slower release of nutrients (Fig. 6 the coating on the slow release and retarding performance of and Fig. S5†). In fact, the chemical structure of the coating is the coatings. The total percentage releases of P and N in water one of the key parameters determining the release rate of P versus time for the uncoated and DAP coated with PTFEMA or nutrient from the coating. The presence of a larger number of PPFEHEMA polymers (1L or 2L) at pH 7 and ambient tempera- F atoms and C–F bonds in the PFEHEMA monomer with ture are shown in Fig. 6. hydrophobic properties, attributed to the -C6F13 side groups, Fig. 6 shows that the uncoated DAP is completely dissolved gives the PPFEHEMA coating a very hydrophobic character, in water in less than 2 h, whereas the rate of dissolution of that acts as a physical barrier and reduces water diffusion, con- nutrients in water is much slower with the encapsulated fertili- tributing to the slow release of P and N nutrients compared to zers than with uncoated DAP. For example, the times to reach PTFEMA-coated DAP.21,22,26 This hydrophobic character was the maximum percentage release of P are 3.3 and 14.5 times confirmed by water contact measurement (WCA) (Fig. S2†), higher than for uncoated DAP when the fertilizer is covered with where the value of PPFEHEMA (WCA = 109°) is higher than PTFEMA single-layer (1L) and double-layer (2L), respectively. that of PTFEMA (WCA = 79°). The soft structure of When the DAP was coated with PPFEHEMA 1L and 2L, PPFEHEMA, which was confirmed by DSC (Fig. 2b and respectively, the P release profiles of the coated granules Table 3) gives the polymer good film-forming ability and good reached the equilibrium stage at approximately 7.5 h and adhesion properties.31 Fig. 6 Release rate of P and N for uncoated DAP and coated DAP using PTFEMA and PPFEHEMA 1L and 2L in water at pH = 7 and ambient temperature. 3336 | Polym. Chem., 2024, 15, 3327–3340 This journal is © The Royal Society of Chemistry 2024