The relationship between parameter of coating material and nutrient release characteristics of the LDCUs
The effects of lignin content, -CNO/-OH molar ratio, amount of EP, and coating ratio on the design array of variable Y for nutrient release characteristics of the LDCUs were evaluated (Fig. 1). The model was selected according to the significance of the response value. The process order of the nitrogen cumulative release rate for 35 days was quadratic. The resulting models relating lignin content (A), -CNO/-OH molar ratio (B), amount of EP (C), and coating ratio (D) were determined as follows.
Here, Y was the nitrogen cumulative release rate for 35 days. A (lignin content), B (-CNO/-OH molar ratio), C (amount of EP), and D (coating ratio) were the independent variables. The correlation coefficients R2 and adjusted R2 were calculated. The latter coefficient reflected an adjustment for the number of model parameters relative to the number of points. The variance analysis of the model equations showed that the polynomial model used in this experiment was extremely significant, as the p-value for Y was < 0.0001 (P < 0.05). The correction coefficient R2 of the model for Y was 94.09%, the lack of fit Y was 0.0974 (P > 0.05). The data indicated that the N release rate (35 days) of the LDCUs was accurately predicted by using the three-dimensional (3D) fitting equation. Central composite design of response surface methodology experiments revealed the following sequence of the nitrogen cumulative release rate for 35 days: coating ratio > content of lignin > amount of EP > -CNO/-OH molar ratio. All of the statistical analyses results showed that AC and AD items interacted with each other on the long-term cumulative release rate and release period.
The fitting relationship between the variable and the response value (a1-f1) as well as contour lines (a2-f2) were shown in Fig. 1. The denser the contour, the more the response value was affected by the factor 3D. By response surface analysis, the combined with industry standard for controlled release fertilizer and the cost of the film material, the optimal cumulative nutrient release rate (79.4%) of LDCU was obtained (lignin of 50%, -CNO/-OH molar ratios of 1.15, EP of 35%, and coating ratio of 5%). This result may be related to less material gaps or holes in the coating material. The coated fertilizer with few holes showed a longest longevity . Based on the optimal conditions, the physical and chemical properties of the coating material and the LDCUs were systematically evaluated.
The nitrogen release characteristics of the LDCUs were significantly affected by the different parameters (a: lignin content; b: -CNO/-OH molar ratio; c: amount of EP; d: coating ratio) (Fig. 2). The lignin content was directly negative correlated to the longevity of the LDCUs based on nitrogen cumulative release rate (Fig. 2a). The LDCUs prepared with 30% lignin content showed that the nitrogen cumulative release rate of the LDCUs reached more than 80% during the 45 days of incubation. The reason for this was mainly that the change of the type and content of hydroxyl due to the different lignin content. Meanwhile, the longevity of the LDCUs was also closely related to the -CNO/-OH molar ratio (Fig. 2b). The longevity of the LDCUs first increased from 24 days (-CNO/-OH molar ratios of 1.00) to 36 days (-CNO/-OH molar ratios of 1.15), but then decreased to 32 days in the experiment performed at -CNO/-OH molar ratios of 1.30, suggesting that the weakening of mechanical properties of coating material occurred when the -CNO/-OH molar ratios was higher than 1.15. It was noted that a strong positive correlation was obtained between the longevity of the LDCUs (20-42 days) and amount of EP (20-50%) (Fig. 2c). A similar phenomenon was also found between the longevity of the LDCUs and coating ratio (Fig. 2d).
Physicochemical properties of coating materials
Fig. 3 exhibits the chemical structural changes of the AL, PCLD, HDI, E-44, EP, and LPUs by FT-IR spectra during the syntheses of LPUs and EP (a1-3). Fig. 3a1 displays the FT-IR spectra of the AL, PCLD, HDI, E44, and EP. The intense bands at 1607, 1512, and 1425 cm−1 correspond to aromatic skeletal vibrations of AL . The strong intensity of the carbonyl absorption band at 1731 cm−1 typified the carbonyl absorption of PCLD polyester. The carbonyl groups of lignin usually have a broad absorption band at 1700–1680 cm−1. However, the hydroxyl-stretching mode of lignin was weaker than that of PCLD due to the low concentration of lignin hydroxyl chain-end groups. The characteristic band of HDI (-CNO) was observed at 2273 cm−1. The bands at 3056 and 1607 cm−1 originate from the C-C stretching vibration of E44 (the benzene ring). The two characteristic absorption bands of the EP at 2883 and 1243 cm−1 corresponded to the stretching vibrations of the C-H groups and C-O bonds, respectively. The intensity of bands at 3346 cm−1 (-OH stretching vibration) and 2859-2933 cm−1 (C-H vibration) reduced with increasing lignin content in LPU (Fig. 3a2). The fact was possibly related to the vibration intensity of -OH in AL lower than that of PCLD (Fig. 3a1). It was found that the bands at 2260–2280 cm−1 (-CNO groups in HDI) were disappeared in the all spectra of LPU, implying that HDI was totally consumed after the reaction with polyols . In addition, the intensity of band at 3400 cm−1 (-OH stretching vibration) reduced with increasing -CNO/−OH molar ratios, indicating that the −OH of AL and PCL reacted strongly with -CNO (Fig. 3a3). A small band appeared at 2265 cm−1 under a high -CNO/-OH molar ratio condition, indicating that the incomplete reaction of -CNO (HDI) and -OH (AL and PCLD). This may be related to the complex hydroxyl type of lignin. It is well known that not all -OH groups from lignin molecules are easily accessible for the reaction with other groups. Also, phenolic, guaiacyl and syringyl -OH were less reactive towards -CNO group compared to the aliphatic -OH [34,35]. In short, the LPUs were successfully prepared by reactions of the HDI and polyols (AL and PCLD) under the given conditions.
To investigate the potential relationship between structural features and thermal behaviors, the TGA of the LPUs were comparatively investigated (Fig. 3b1-2). The thermal degradation processes of the LPUs were mainly divided into four temperature stages, which were 137–285 ℃, 285–351 ℃, 351–450 ℃ and higher than 450 ℃ (Fig. 3b1-2). As can be seen, EP had worse thermal stability than the LPUs. During the first stage of thermal degradation, the weight of LPUs was reduced by 15%, which was mainly caused by the hydroxyl dehydration reaction . The second stage of thermal degradation was the fastest weight loss (about 36%) process due to the degradation of the unstable carbamate groups in LPUs . At this stage, the content of lignin had no obvious effect on the weight loss of LPUs (Fig. 3b1). During the third stage, the weight of the LPUs reduced (33.56-19.37%) with the increasing lignin content (30-70%). The weight of all LPUs was mainly closely related to the decomposition of PCLD ester bonds in LPU and the rupture of carbon-carbon bonds between lignin structural units . This reduced trend was attributed to an increased benzene ring and carbon of lignin in the LPUs . For the last stage, the weight loss of the LPUs was mainly related to the further thermal oxidation of the LPUs. This stage and the third stage showed the same change trend, that was, the weight loss of the LPUs reduced with the increasing lignin content. This similar behavior was founded in previous literature. Llovera et al.,  synthesized lignin-based polyurethane film using 4, 4-diphenylmethane diisocyanate and polyethylene glycol as raw materials, and analyzed the thermal stability of lignin-based polyurethane film by thermogravimetric analysis. It was found that the weight loss of the LPUs was reduced with the increasing lignin content (0-70%). It was worth noting that the effects of -CNO/-OH molar ratios and lignin content on the weight loss of the LPUs appeared the same trend in the first and second stages during thermal degradation processes (Fig. 3b2). In the third and fourth stages, the weight loss of the LPUs decreased with reducing -CNO/-OH molar ratios. This was similar to the pervious results . In other words, decreasing -CNO/-OH molar ratios caused continued improvement of the thermal stability of the LPUs.
The WCAs were performed to determine the wettability properties of surfaces of the LPUs (Fig. 3c1-2). The wettability of the LPUs was determined by the hydroxyl groups on the surface under the same -CNO/-OH molar ratio condition. The WCAs of the LPUs gradually decreased from 75.6° to 63.6° due to the presence of a large number of carboxyl and hydroxyl groups with the increase of the lignin content (30-70%) (Fig. 3c1). In addition, the value of -CNO/-OH also significantly affected the WCAs of the LPUs (Fig. 3c2). As can be seen, the WCAs of the LPUs (64.7–75.1°) increased with the raise of -CNO/-OH molar ratio, indicating that the wettability of the LPUs was weakened . Zhang et al reported that the -CNO/-OH molar ratio was positively correlated to the WCAs . The increment of the WCAs was mainly attributed to the efficient reaction of the -OH with –CNO during the preparation of LPUs. Meanwhile, the reaction increased the content of hard segments in the LPUs and then limited the wettability of LPUs . It was noted that the EP-LPU had a higher WCA (84°) than the LPUs (63.6-75.6°). A possible reason was that the EP resulted in reduced the penetration of water molecules due to block the pores of LPU , which could prevent water from entering the coating shells and thus slow down N release.
To assess the biodegradability of the coating layer of LDCU, the degradation ratio of the coating layer in soil was presented in Fig. 3d1. The degradation ratio of the coating layer increased with the soil burial time prolonged. As expected, the degradation ratio of the coating layer reached a maximum value (36.5%) after 180 days, suggesting that the prepared coating layer was an environmentally friendly and degradable material. Furthermore, this was similar to the results of the biodegradability of lignin-poly(ε-caprolactone)-based PU . The SEM images of the 0 (d2) and degraded after 180 days (d3) coating layer of the LDCU in the soil are also presented in Fig. 3. Compared with the initial dense structure, the dense structure of the coating layer was destroyed and exhibited fragmented structures of different sizes after 180 days of degradation. Generally, the soil environment contains abundant microorganisms (bacteria and fungi), which promote the hydrolysis of the coating layer in the LDCU . In addition, this may also be related to the decomposition of lignin macromolecular structure, biodegradable LDCU and epoxy resin in the soil by microorganisms and enzymes . In summary, the coating layer of the LDCU appeared excellent biodegradability in the soil, suggesting that lignin could be widely applied in the layer construction of coated fertilizer.
Surface properties, physical properties and swelling performance of the LDCUs
Different technologies significantly alter the morphology of coating material, which also reflects the property of the coated fertilizer. To investigate the morphology changes caused by the various technologies, the surface (a1, b1, and c1) and section (a2, b2, and c2) images of the LPUCU, EPCU and LDCU were obverted by SEM images (Fig. 4). Most of the surface of the coating of the LPUCU was relatively smooth, and some areas were rough (a1). A few holes were observed in the section micrographs of the coating of the LPUCU (a2). The holes were easily permeated by water, causing quick release of nutrients from the coated fertilizer. The roughness of the surface of the coating of the LPUCU was due to the fact that the coating materials were inhomogeneously dispersed and agglomerated because the strong hydrogen bonding in lignin structure. The surface (b1) and section (b2) of the coating of the EP appeared more smooth, compact, and uniform morphologies than that of the EPCU. When EP was added to the LPU coating material, the surface (c1) and section (c2) of the coating of the obtained LDCU were more compact and less holes than that of LDCU. In brief, the EP was an effective amendment for the LPU during the preparation of the LDCUs.
The particle hardness is critical to the quality of the fertilizer . Compared to the average particle hardness of conventional urea (39.3 N), the average particle hardness of the LDCUs significantly enhanced (55.6-67.0 N) (Fig. 5d1-2). The increasing phenomenon was possibly attributed to the formation of dense coating layer. When the LDCUs were extruded, the particles were not easily broken due to the high hardness . The average particle hardness of the LDCUs first increased from 58.1 N (lignin of 30%) to 67.0 N (lignin of 60%), but then decreased to 62.3 N in the experiment performed at lignin of 70% (Fig. 5d1). There was no significant difference in the average particle hardness of the LDCUs produced by using different lignin content. As compared to other LDCUs, the higher average particle hardness (67.0 N) was obtained under lignin of 60% in this study. The results showed that the addition of lignin in the coating layer could improve the compression resistance of particles due to enhanced proportion of hard segments with high-quality aromatic structure . In addition, the -CNO/-OH molar ratio also greatly affected the average particle hardness of the LDCUs (Fig. 5d2). The average particle hardness of the LDCUs gradually increased as the -CNO/-OH molar ratio rose and reached a maximum value (64.1 N). The increased trend suggested that the crosslink density and hard segments were continuously promoted. It was noted that the there was no significant difference in the average particle hardness of the LDCUs obtained under the -CNO/-OH molar ratio exceeded 1.00 conditions. This could be related to hard links that no longer increased due to insufficient -OH with -CNO reaction.
The photo (e1) and swelling volume (e2) of the LDCUs under different incubation time are shown in Fig. 5. The image change and swelling volume of the LDCUs within 7 days did not change significantly due to the only a small amount of water penetrated into the LDCUs . However, after 7 days, the LDCU granules began to absorb water to dissolve nutrients, which expanded and increased their swelling volumes due to the elevated osmotic pressure difference inside and outside of the coating. The swelling volume of the LDCUs increased by 1.5 times after 35 days compared to the initial fertilizer. Interestingly, the surface of the LDCUs became smoother as the swelling volumes increased, which was similar to the findings of Liu et al . However, as the incubation time further prolonged to 49 days, the swelling volume of the LDCUs showed a decreasing trend because the nitrogen concentration in the coating layer reduced after the osmotic pressure difference decreased [51,52]. In addition, the dissolved urea molecules were released to the outside through the coating layer, which was another reason for the decreasing the swelling volume of the LDCUs [53,54].
Release mechanism of nitrogen in the LDCUs
Through the above analysis, the release longevity of coated urea was closely related to the preparation parameters of the coating material (lignin content, -CNO/-OH molar ratio, amount of EP, coating rate). For instance, the WCA of the coating material affected nutrient release longevity of the LDCUs . Specifically, the higher WCA of the coating material could result in relatively higher nutrient release longevity of the LDCUs. Likewise, the release longevity of the LDCUs was also positively related to the coating rate . The pores on the surface of the coating increased the water permeability of the coating, which speeded up the dissolution of urea . The nutrient of the LDCUs was released by the penetration and diffusion of hydrone into the inside of the urea particles. The aggregates of hydrone on the fertilizer particles caused the nutrients to dissolve and create an osmotic pressure. The release processes of the LDCUs were mainly divided into three stages (Fig. 6) [57,58]. During the initial stage, the hydrone squeezed into the urea core through permeation and diffusion (lag period Fig. 6a2). During the urea dissolution stage, the dissolution rate of urea was significantly faster than the diffusion rate of the urea solution through the coating. The dissolution of urea led to an increase in osmotic pressure and swelling of the coating layer (stable period, Fig. 6b). During the third stage, the nutrients of urea were released in large quantities through the dissolution process, resulting in a decrease in osmotic pressure (recession, Fig. 6c). In addition, the swelling phenomenon of the LDCUs could reflect the release process of nitrogen in the LDCUs to a certain extent. In the early stage of swelling, the volume of the LDCUs did not change. During the middle stage of swelling, the nutrients were dissolved and swelled in the coated urea. As the swelling progresses, the nutrients were expelled from the coating layer, eventually resulting in a continuous decrease in volume of the LDCUs. The holes of the coating layer were easily permeated by water, causing quick release of nutrients from the coated fertilizer. The average particle hardness significantly affected the release performance of the LDCUs. Specifically, when the fertilizer particles were impacted and squeezed, the integrity of the coating layer directly affected the release longevity of nitrogen in the LDCUs . In short, although the nutrient release of the LDCUs was affected by many factors, the successful development of the LDCUs will help improve the rapid development of the coated fertilizer industry.