Synthesis of MPAA based on polyacrylamide and gossypol resin and applications in the encapsulation of ammophos

2.1 Experimental materials

Polyacrylamide (C₃H₅NO) _n is a water-soluble polymer synthesized with a high degree of polymerization, easily soluble in water, insoluble in benzene, ether, lipids, and other common organic solvents. The appearance is a white powder that decomposes at 120°C.

Gossypol resin (C₃₀H₃₀O₈) is a black-brown mass consisting of fatty acids of uniform viscosity, with a molecular mass Mr = 518.563 g·mol⁻¹, density ρ = 1.4 g·cm⁻³, melting point t = 177°C, and boiling point t = 707°C. The mass fraction of the acid during synthesis is 70–80 mg, the degree of saponification in the calculation of NaOH ranges from 80–130 mg, humidity is 4.0%, solubility in acetone is 75%, specific gravity is 0.98–0.99 g·cm⁻³, and fatty acids (C₁₁–C₁₇–COOH) during synthesis is 55–65%.

Formalin (CH₂O) is a colorless, transparent liquid. During storage, turbidity or a white precipitate appears and melts at a temperature not exceeding 40°C. It has a molecular weight Mr = 30.03 g·mol⁻¹, density ρ = 1,110–1,120 kg·m⁻³, boiling point t = 99°C, and specific heat cm = 2.5 kJ·kg⁻¹·K⁻¹.

Sodium hydroxide (NaOH) – State Standard 4,328-77. The molar mass = 39.997 g·mol⁻¹, melting point = 323°C, boiling point = 1,403°C, and density= 2.13 g·cm⁻³.

Ammophos, also known as monoammonium phosphate (MAP), is a highly concentrated nitrogen-phosphorus fertilizer used in agriculture. Chemical formula: mixture of NH₄H₂PO₄ + (NH₄)₂HPO₄, and appears as a light-grey granule, soluble in water. Used in agriculture as a concentrated granulated NP fertilizer. Mass share of assimilable phosphates (%): 46 ± 1, mass share of nitrogen in total( %): 10 ± 1, static strength of granules (MPa (kg·cm⁻²)), min: 3.0 (30).

2.2 Instrumental methods of analysis

In conducting this work, the following analysis techniques were used: a scanning electron microscope JSM-6390LV (Jeol, Japan) integrated with an Oxford INCAEnergy energy-dispersive X-ray microanalyzer (Oxford Instrument, UK) was used to determine the elemental composition and microstructure of the sample under investigation; a capillary viscometer VPL-3 (viscosity of transparent liquids) with a caliber of 1.2 mm was used for determining the kinematic viscosity of polymers; an IR Fourier spectrometer Zhimadzu IR Prestige-21 with an attenuated total internal reflection attachment (Miracle Pike Technologies, Tokyo, Japan) was used to study the structural features of the studied sample.

2.3 Synthesis of gossypol resin

Gossypol resin contains 55–65% derivatives of fatty acids, while the fatty acids are released as a bottom residue during simple distillation. Gossypol resin is isolated as a distillate, then condensed and polymerized. Fatty acids obtained by simple distillation in a bottom residue are the main raw materials for the production of modified polymers. Gossypol resin is saponified with a 20% sodium hydroxide solution at a temperature of 90°C for 90 min. In this case, polycondensation and saponification of naphthenic hydrocarbons and gossypol resin occur simultaneously. To extract unsaponifiable fractions, white spirit is introduced through a vacuum evaporator, followed by the draining of these fractions. The resulting semi-product consists of 60–70% sodium salt, mainly unsaturated fatty acids with a predominant fraction of C₁₁–C₁₇–COOH. Esterification of fatty acids of gossypol resin is carried out with hexyl alcohol in the temperature range of 70–80°C for 30–40 min (R–COO). The resulting gossypol and its derivatives are used to produce a modified polymer.

2.4 Synthesis of the modified polymer

The synthesis of the modified polymer is carried out in two stages: heterophasic hydrolysis of PAA with a solution of sodium hydroxide in the presence of formalin and gossypol resin. Under laboratory conditions, a three-neck flask with a capacity of 0.5 L equipped with a stirrer, a reflux condenser, and a dropping funnel was used. Then, to 10 g of PAA, 90 ml of a 2% aqueous solution of sodium hydroxide was added, and the mixture was heated with stirring to 80–90°C for 90 min. With the formation of a number of carboxylate groups, the further development of the saponification reaction in the macromolecule chain created difficulties for the hydrolysis process in space due to the negative influence of negatively charged anionic groups and amide groups. The duration of the hydrolysis process was regulated by a certain ratio of carboxyl and amide groups in the polymer macromolecule. The (–COO–) and (–CO–NH₂–) groups are polar, and the total hydrophilicity of the reaction mass increases and passes from the gel state to the homogeneous state. In this case, the resulting intermediate is a viscous liquid mass that is highly soluble in water.

During the process, the polyacrylamide particles swell, and the reaction mass gradually turns from light yellow to yellow. Then, while stirring, 5 ml of 40% formalin solution and 4 g of gossypol were added dropwise. The temperature of the medium was raised to 80–90°C for 90 min with the release of ammonia. The mechanism of the process is as follows.

Polyacrylamide is easily hydrolyzed in the presence of alkaline compounds. Alkaline hydrolysis is carried out under the action of hydroxides. As a result of the partial conversion of amide groups into carboxylate groups (reaction 1), as well as an increase in the size of macromolecular compounds and the viscosity of the solution due to electrostatic repulsion of like charges of the chain, the thickening and structuring properties of polymers are enhanced [26,27].

Methylation of hydrolyzed polyacrylamide occurs in an alkaline medium using formalin (reaction 2), followed by the formation of polymethylolacrylamide. Carboxylate groups, resulting from the hydrolysis of polyacrylamide, easily react with fatty acids (reaction 3) of gossypol resin to form macromolecular compounds of polyacrylamide. When the polymethylolacrylamide formed is heated, the chains in the molecule are cross-linked to form an oxygen bridge (reaction 4), followed by doubling of the structure of the polymethylolacrylamide in the high-molecular-modified polymer. The chemistry of the process is as follows: (1) (2) (3) (4)

where R₁ is the fatty acids of gossypol resin (C₁₁–C₁₇–COOH).

2.5 Methods for the determination and encapsulation of ammophos using MPAA

The microstructure and the polymer film layer around potassium humate were determined with a JSM-6490LV scanning electron microscope (Jeol, Japan) using the energy dispersive method. The essence of the electron microscopy method is that an electron beam of different energies is fed through the sample under study. Under the influence of an electromagnetic field, it is focused on the surface in the form of a spot with a diameter not exceeding 5 nm. As a result, the elemental composition and microstructure of the sample under study are recorded. The sensitivity of the method for an individual component is 0.1 wt%, and the range of elements determined is from beryllium (B) to uranium (U). The spectra were obtained in the range of 0–20 keV. The voltage at the accelerating electrode was 15 kV.

To obtain encapsulated fertilizers, the obtained ammophos was soaked in a 0.25–0.75% solution of water-soluble organic polymers and further dried at 25–75°C. Granulation in a plate granulator was performed. Modified derivatives of polyacrylamide (MPAA) were used as water-soluble polymers.

2.6 Determination of static strength and study of the granule structure

The measurement of the static strength of sample granules was carried out on an IPG-1M device (fracture force: up to 200 N, punch speed: 0.8–1.2 mm·s⁻¹). In addition, the static strength of the granules was determined using a TAXTplus texture analyzer (Stable Microsystems, UK). This device allows one to vary the load feed rate from 0.01 to 40 mm·s⁻¹ and has a force resolution of 0.001 N. The static strength was determined for 40 granules (according to the method, 20 granules are enough) of 3.0–3.15 mm and 3.0–3.35 mm fractions. The structure of the granules was studied using an SEM JSM-6490LV (Jeol, Japan). The spectra were obtained in the range of 0–20 keV. The voltage at the accelerating electrode was 15 kV. The sensitivity of the method for an individual component is 0.1 wt%.

3.1 Elemental and IR spectrum analysis of polyacrylamide

Scanning electron microscopy was performed to determine the elemental composition and microstructure of polyacrylamide (Figure 1). The main chemical composition of polyacrylamide is 98.0%, including carbon (C) content of 73.26% and oxygen (O) of 24.74%, and the remaining 2.0% is chlorine (Cl) – 1.21%, magnesium (Mg) – 0.43%, and sulfur (S) – 0.36%.

Figure 1

Microstructure of polyacrylamide.

From Figure 1, it follows that the microstructure of polyacrylamide is composed of macromolecules of hydrocarbon compounds and amide groups, which have crystalline and insignificant amorphous forms.

To determine the structural features of polyacrylamide, IR spectral studies were carried out. The results are shown in Figure 2.

Figure 2

IR spectrum of polyacrylamide.

Figure 2 shows the IR spectrum of polyacrylamide, from which it follows that:

1. absorption spectra with a wavelength of 3,371.57 cm⁻¹ characterize the presence of RC–NH bonds of the amide group;

2. absorption spectra with wavelengths of 1,481.33–1,400.32 cm⁻¹ characterize the presence of the C–C bonds between the carbons groups in polyacrylamide;

3. absorption spectra with wavelengths of 1,249.87–1,026.13 cm⁻¹ characterize the presence of C–H bonds between the hydrocarbon groups in polyacrylamide.

3.2 Elemental and IR spectrum analysis of the hydrolyzed polyacrylamide

Scanning electron microscopy was performed to determine the elemental composition and microstructure of the hydrolyzed polyacrylamide (Figure 3). The hydrolyzed polyacrylamide has the following chemical composition: carbon (C) – 71.21%, oxygen (O) – 22.34%, sodium (Na) – 4.7%, chlorine (Cl) – 1.02%, magnesium (Mg) – 0.42%, and sulfur (S) – 0.31%.

Figure 3

Microstructure of the hydrolyzed polyacrylamide.

From Figure 3, it follows that during the hydrolysis of polyacrylamide with sodium hydroxide, the transition of the amide group to the carboxyl group occurs due to hydrophilization and dispersion of the system, and the microstructure of the polyacrylamide changes to a homogeneous amorphous system [27,28,29].

Figure 4 shows the IR spectrum of hydrolyzed polyacrylamide, from which it follows that:

1. absorption spectra with a wavelength of 3,360.0 cm⁻¹ characterize the presence of RC–NH bonds of the amide group;

2. absorption spectra with a wavelength of 2,360.87 cm⁻¹ characterize the presence of RC–N bonds of the nitrile group;

3. absorption spectra with wavelengths of 1,724.36–1,631.78 cm⁻¹ characterize the presence of RC–OH bonds for the carbonyl group of the amide;

4. absorption spectra of deformation vibrations in the region of 1,543.05–1,481.33 cm⁻¹ characterizing imide cycles as they are covered by absorption spectra of stretching vibrations of the carboxylate (C–ONa) groups;

5. absorption spectra with wavelengths of 1,384.33–1,045.42 1 cm⁻¹ characterize the presence of C–C bonds between the carbons and (C–H) hydrocarbon groups in the hydrolyzed polyacrylamide.

Figure 4

IR spectrum of the hydrolyzed polyacrylamide with sodium hydroxide.

3.3 Elemental and IR spectra of the modified polymer MPAA

Scanning electron microscopy was performed to determine the elemental composition and microstructure of the modified polymer (Figure 5).

Figure 5

Microstructure of the modified polymer.

From Figure 5, it follows that its microstructure changes during the stepwise modification of hydrolyzed polyacrylamide; the possibility of the formation of branched bonds of methylol groups via formalin has also been determined. It was observed that gossypol resin prevents the preservation of its amorphous state as a result of the penetration of fatty acids into macromolecules, resulting in the hydrophilization of the overall system.

Figure 6 shows the IR spectrum of the modified polymer MPAA, from which it follows that:

1. absorption spectra with wavelengths of 3,371.57–3,263.56 cm⁻¹ characterize the presence of RC–NH bonds of the amide group;

2. absorption spectra with wavelengths of 2,889.37–2,816.07 cm⁻¹ characterize the presence of RC–NH–CH₂O bonds of the methylamide groups in the chain of the modified polyacrylamide;

3. absorption spectra with wavelengths of 2,360.87–2,330.01 cm⁻¹ characterize the presence of RC–N bonds of the nitrile group;

4. absorption spectra with a wavelength of 1,724.36 cm⁻¹ characterize the presence of RC–OH bonds for the carbonyl group in modified polyacrylamide;

5. absorption spectra of deformation vibrations in the region of 1,543.05 cm⁻¹ characterize imide cycles since they are covered by the absorption spectra of stretching vibrations of the carboxylate (C–O–Na) groups;

6. absorption spectra with a wavelength of 1,505.06 cm⁻¹ characterize the presence of RC–OOH bonds for the carboxyl groups in modified polyacrylamide;

7. absorption spectra with wavelengths of 1,404.18 cm⁻¹ characterize the presence of the (C═O) bonds of a carbonyl group in modified polyacrylamide;

8. absorption spectra with wavelengths of 1,384.33–1,045.42 1 cm⁻¹ characterize the presence of C–C bonds between carbons and (C–H) hydrocarbon groups in modified polyacrylamide.

Figure 6

IR spectrum of the modified polymer MPAA.

3.4 Physicochemical properties of the modified polymer MPAA

Experimental work was carried out to determine the specific viscosity of solutions of modified MPAA polyacrylamides in a wide range of concentrations (0.01–1.0%). This is consistent with the fact that polymer solutions obey general laws characteristic of polymer solutions with dissociating functional groups [30,31,32].

The kinematic viscosity of the modified polymer was calculated according to GOST 33-2000 using a VPL-3 capillary viscometer with a diameter of 1.2 mm. When polyacrylamide is modified in the presence of formaldehyde with the addition of fatty acids from gossypol resin, the kinematic viscosity decreases at a temperature of 25°C from 16.22 to 15.95 mm²·s⁻¹. This is explained by the fact that the polymer composition of the modified polyacrylamide is composite. The results obtained are shown in Table 1.

The molecular weight of the modified polyacrylamide MPAA was calculated using the Mark–Houwink–Kuhn Eq. 5:

where [η] is the specific viscosity; M is the molecular weight; K is the stability of the polymer–water system, K = 0.631 × 10⁻⁴; and α (= 0.80) is the polyacrylamide indicator. The molecular weight is calculated using Eq. 5 (M = 19.0366·10⁵ Da).

To determine the surface tension of a modified polymer, the presence of high molecular weight surfactants at the polymer–water interface leads to a decrease in the surface tension of water. This decrease in surface tension can be explained by the orientation ability of the macromolecules associated with the adsorption layer. With increasing concentration of the modified polymer, the pH values of aqueous solutions increase. At a concentration of C-0.4%, the surface tension of PAA is 55.8 N·m⁻¹ and that of the modified polymer is 43.6 N·m⁻¹. This is explained by the peculiarities of the composition of the modified polyacrylamide.

3.5 Mathematical modeling of the process of the modified polymer

For the reliability [33] of the experimental data obtained, mathematical methods of processing were carried out. For processing, the methods of mathematical planning of the experiment according to the orthogonal plan of the second order for k = 4 were used. The number of experiments in the center of the plan was 4, and the stellar arm α = ±1.61 when N = 28. The experimental conditions are listed in Table 2.

The significance of the coefficients of the regression equations was verified (9) by the Student’s criterion. For the significance level, the tabulated values of Student’s measurement p = 0.05 and the number of degrees of freedom f = n ₀ – 1 = 4 − 1 = 3 have a value of t _0.05(3) = 3.18. The adequacy of the regression equation to the experiment was verified by the Fisher criterion. After excluding insignificant coefficients of regression equations, the regression equations adequate to the experiment have the form:

This equation must be reduced to canonical form. The coordinates of the center of the surface S are equal to

Eq. 13 is as follows:

After transformation, Eq. 14 takes the following form:

As a result of identifying Eqs. 12–15, the following conclusion is obtained:

Using the equation, we calculate the value of Y = 127 N·m⁻¹, which is the surface tension of the modified polymer. The ratios of the mass fractions of the initial components,

By solving Eq. 17, we find the ratio of the mass fraction of the initial components for obtaining the modified polyacrylamide.

The results of modified polymer MPAA are shown in Figure 7(a) and (b) by 3D mathematical modeling.

From Figure 7(a) and Eqs. 10 and 16, it follows that when mathematically processing the technological parameters using mathematical equations, the optimal temperature T = 90°C and time t = 2 h = 120 min for the process of obtaining the modified polyacrylamide MPAA. From Figure 7(b) and Eqs. 11 and 17, it follows that when processing the initial raw materials, the optimal ratio of the initial components was established, that is, for obtaining the modified polymer, polyacrylamide (62.5%), gossypol resin (25%), and sodium hydroxide (12.5%) = 5:2:1.

3.6 Heat treatment of the modified polymer MPAA

To determine the thermal stability of the modified PAA, studies were conducted using DTA. During dynamic heating of the polymer reagent in the temperature range from 20 to 1,000°C, thermal curves associated with the decomposition of its structure and a number of endothermic and exothermic effects were investigated. The derivatogram of modified MPAA with fatty acids in the presence of formalin is shown in Figure 9.

Figure 7

Dependence of the modified mass yield of the polymer on the ratio of the starting substances, time, and temperature.

Differential thermal analysis (DTA) and thermogravimetric (DTG) curves show the peaks at temperatures below 100°C, which are associated with the release of significant amounts of aromatic substances in this temperature range. The thermal decomposition of modified polyacrylamide in the temperature range of 185–280°C causes the decomposition of the binding compounds, while the mass of CO_org gradually decreases. The specified value does not exceed 1.25% of the initial mass of the sample. The remaining residues (Δm ₁, Δm ₂, and Δm ₃) at temperatures of 330, 385, 430, and 555°C exhibit small, but clearly visible exothermic effects on the DTA curve, which is associated with the combustion of the relatively “heavy” part of the organic matter. The thermogravimetric properties of MPAA in the range of 20–1,000°C are listed in Table 5.

In the last part of thermal decomposition (670–855°C), a weak endothermic effect is observed on the DTA curve, associated with the release of carbon dioxide (CO₂) to the atmosphere. The results show that the modified polymer is stable up to a temperature of 185°C; however, above 200°C, the polymer is subject to destruction.

3.7 MPAA application in the encapsulation of ammophos

Based on the results of the studies, it was determined that during encapsulation, an increase in the concentration of MPAA and the process temperature positively affects the strength of the granules. This is evidenced by numerous experimental works on the encapsulation of mineral fertilizers. In the encapsulation process, the concentration of the organic polymer MPAA plays the main role in the strength of granular ammophos, and the temperature of the process plays an auxiliary role. Therefore, we focus on the concentration of the organic polymer. The process of encapsulation of ammophos using the MPAA polymer agrees with the results of previous published works [24,34,35,36].

In the process of encapsulation of ammophos with a temperature change from 25 to 75°C, in the absence of MPAA, the static strength of ammophos granules reaches 1.5–1.81 kg. An increase in the temperature in the range of 25–75°C leads to the strengthening of the initial structure of the granules.

An increase in the concentration of organic polymer in the system leads to a change in the structure of the fertilizer. The influence of the concentration of MPAA and temperature on the strength of ammophos in the process of encapsulation by MPAA is presented in Table 6.

From Table 3 it follows that with an increase in the MPAA concentration from 0 to 0.25%, the static strength of encapsulated granulated ammophos increases from 3.11 to 10.7 MPa in the temperature range of 25–75°C. Next, with an increase in the concentration of MPAA by 0.5%, the static strength of encapsulated granulated ammophos increases from 4.42 to 18.4 MPa in the temperature range of 25–75°C. Due to the increase in the concentration, a protective film is formed around the granules, and a strong capsule layer appears due to the organic functional groups of the MPAA. This layer has a stable shape and provides strength to the granules (Figure 9 and Table 4).

Figure 8

Microstructure (a) and a cut of the sample (b) of the encapsulated and granulated ammophos at 75°C in the presence of 0.5% water solution of MPAA.

Next, with an increase in the concentration to 0.75%, MPAA does not have a positive effect on the strength and decreases to 12.7 MPa in the temperature range of 25–75°C. With an increase in the concentration, the viscosity of the organic polymer increases, and further use is not favorable; that is, the consumption of the polymer increases, while the strength of the granules does not increase (Figure 8).

Figure 9

Derivatogram of the modified polymer MPAA.