One-Pot Synthesis of Bio-Based Waterborne Polyester as UV

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Article Cite This: ACS Omega 2018, 3, 16812−16822

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One-Pot Synthesis of Bio-Based Waterborne Polyester as UVResistant Biodegradable Sustainable Material with Controlled Release Attributes Geeti Kaberi Dutta and Niranjan Karak*

ACS Omega 2018.3:16812-16822. Downloaded from pubs.acs.org by 95.181.177.157 on 12/19/18. For personal use only.

Advanced Polymer and Nanomaterial Laboratory, Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784028, Assam, India ABSTRACT: Research in the field of biodegradable formulation has seen a huge upsurge owing to environmental concerns. Hence, fundamental shifting to fully bio-based material productions represents an essential task. In this context, the authors directed their efforts to synthesize fully bio-based waterborne polyesters by one-pot synthesis using vegetable oil-based dimer acid, glycerol, and citric acid via a solvent-free environmentally benign route. Three different compositions of the polyester were synthesized by varying the amount of citric acid and dimer acid by melt polycondensation without using any organic solvent. The physicochemical structure of the synthesized polyesters was characterized by Fourier transform infrared and NMR spectroscopies. Glycerol-based epoxy and fatty acid-based poly(amido amine) cured thermosets of the synthesized polyesters showed excellent performances including tensile strength (8.55−13.72 MPa), elongation at break (128.43−182.13%), toughness (12.95−17.31 MJ/m3), impact resistance (15.99 to >19.5 kJ/m), scratch hardness (8−10 kg), gloss value at 60° (56.2−62.3), and good thermal stability (above 203−230 °C). Moreover, the thermosets also showed good resistance against chemical and UV aging with high microbial biodegradability and controlled release ability toward urea as fertilizer. Thus, the synthesized fully bio-based polyester has potential applications as UV-resistant biodegradable sustainable material with controlled release attributes for fertilizers.

1. INTRODUCTION With the growing modernization of today’s material world, synthetic polymers have started substituting natural materials in almost every area. They become an indispensable part in our daily life ranging from clothes, food packaging, paints, etc. to essential components in every emerging advanced technology. However, these synthetic polymers are dependent on a wide variety of petroleum-based raw materials due to their easy handling and good balance of properties.1 But the issues related to the gradual depletion of petroleum reserves and growing awareness toward environmental impact on petrobased waste disposal and emission of dangerous greenhouse gases stimulate investigators to develop bio-based and biodegradable polymers.2−7 Polyesters are one of the significant interests to the research enthusiasts among different synthetic polymers due to their inborn versatile and tunable properties along with their wide variety of applications.8 Biodegradability is one of such important properties of polyesters, which is due to the presence of hydrolytically labile ester linkages in them. Under biological conditions, these ester linkages undergo scission via hydrolysis and generate CO2, carboxylic acid, small aliphatic chains, etc., thereby degrading the polymeric chains.9,10 Nowadays, such bio-based and biodegradable polyesters have found applications in different fields including © 2018 American Chemical Society

agriculture, transportations, etc. and are of great interest. It is necessary for such bio-based polyesters to be durable and have adequate mechanical properties under different environmental conditions such as light, water, ultraviolet radiations, chemical media, etc.11 Although many studies have already gained wings on partially bio-based polyester with varied applications, they are not fully bio-based and hence not completely eco-friendly. In addition, most of them involve the use of hazardous organic solvent during their synthesis and process, which led to the release of volatile organic compounds (VOCs).12,13 Therefore, driven by environmental pressures, researchers are trying to make efforts in fundamental shifting from petroleum-based to complete bio-based polyester productions and focus on the use of sustainable nonhazardous solvent during their processing. This has led to the invention of several low-solvent or green solvent alternatives such as waterborne systems, powder coatings, high solid dispersions, etc. Among these sustainable and eco-friendly substitutes of traditional organic solvent systems, waterborne systems have the widest potential as they have many favorable attributes like ease of cleaning, no or very low emission of VOC, nonflammability, nontoxicity, and low Received: October 13, 2018 Accepted: November 27, 2018 Published: December 6, 2018 16812

DOI: 10.1021/acsomega.8b02790 ACS Omega 2018, 3, 16812−16822

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reactants such as citric acid, glycerol, and dimer acid. The polyester resin was synthesized with a yield of 99.5%, and the reaction scheme of the synthesized polyester is shown in Scheme 1. In the first step, as the reactivities of the primary hydroxyl and carboxylic acid groups are high leading to the formation of linear aliphatic ester linkages at first, some of the secondary hydroxyl groups of the glycerol unit start reacting as the reaction time passes leading to a branched structure of the polyester with increased molecular weight. Here, it is pertinent to mention that in the first step, although the probable major product is shown in the scheme, other oligomeric polyols may also form, which are subsequently converted to the desired polyester in the second step of polycondensation reaction. Further, water molecules formed during the condensation process were removed by purging the nitrogen gas. The acid value and hydroxyl values of the synthesized polyester were determined and are given in Table 2. It is noted from the table that the acid value and hydroxyl values of the polyester resins increase gradually with increase in citric acid content, which is due to the presence of free tertiary carboxylic acid group and free tertiary hydroxyl groups in the citric acid moiety. The synthesized polyester becomes waterborne on addition of triethyl amine toward the end of the reaction. This is because of the presence of free −COOH groups in the polyester resin, which get converted to their ammonium salts, and its solubility in water increases. The ease of water solubility of all of the three compositions of the polyesters was checked and found to be increasing with increase in the concentration of citric acid content. Later, the water solubility behavior of WBPE 3 was evaluated for different weight percentages and found to be easily soluble up to 5 wt % of the polyester and becomes easily dispersed in water up to 30 wt % of the polyester (Figure 1). 2.2. Structural Analysis. 2.2.1. Fourier Transform Infrared (FT-IR) Spectral Study. The Fourier transform infrared (FT-IR) spectra of the synthesized polyester resins are displayed in Figure 2, which support the chemical linkages present in them. The characteristic absorbance at 1743 cm−1 for all WBPEs is due to the stretching vibration of CO of aliphatic ester group in the resin, which confirms the formation of ester bond during the polycondensation reaction. The stretching vibration of O−H in all three WBPEs appeared at 3456 cm−1, and a small peak of low intensity at 721 cm−1 determines the O−H bending out of plane. Again, the absorption peaks at 2926 and 2854 cm−1 indicate the asymmetric and symmetric stretching vibrations of aliphatic C−H present in the resins. 2.2.2. 1H NMR Spectral Study. Different chemical environments of the protons in the structure of the synthesized polyesters were confirmed by 1H NMR spectral analysis. A representative of the 1H NMR spectrum of WBPE 1 is shown in Figure 3. The chemical shift value at δ = 0.83 ppm (j) is due to the terminal methyl protons. The remaining methylene protons of the dimer acid unit show peaks with a chemical shift value at δ = 1.22 ppm (f) and 1.37 ppm (g). Again, the peaks at δ = 2.90 ppm (k) and 2.97 ppm (l) are due to the methylene protons of the citric acid unit, whereas the peaks at δ = 3.68 ppm (a) and 4.16 ppm (b) are for the protons of the methylene groups of glycerol units. The signals observed at δ = 5.04 ppm (c) and 5.20 ppm (m) are assigned for the methine protons of glycerol unit and hydroxyl protons of the citric acid unit, respectively. The signals at δ = 2.28 ppm (d) and 2.04 ppm (i) are designated to the protons attached to the carbons at α- and β-positions with respect to the carbonyl groups,

odor, which in combination with the low or no cost of water gradually expand the use of waterborne polyester systems.14 It is necessary for the polyester systems to have ionic or hydrophilic polar functional groups to become waterborne, which further interact with water molecules to solubilize or disperse the polyester.15 Due to hydrophilicity and biodegradability of the waterborne polyesters, they can be further used for controlled release applications. A colossal number of reports have been found in the literature on partially bio-based polyesters with different properties and applications. Noordover et al. synthesized coand ter-polyesters based on renewable resources like isosorbide, succinic acid, and aliphatic diols via bulk condensation. The synthesized polyesters showed good impact resistance, high hardness, and solvent resistance. Later, the same research group had used citric acid to enhance the functionality of the synthesized polyester and enhanced the properties of the material.16,17 Gioia et al. synthesized polyesters for coating purposes by combining chemically recycled poly(ethylene terephthalate) (PET) with renewable resources like isosorbide and succinic acid. These coatings showed good properties in terms of solvent and water resistance and long-term stability.18 Kim et al. synthesized biodegradable and elastomeric polyesters with electrospray coating application using polycondensation between glycerol and sebacic acid. Although the synthesized polyesters possessed elongation as high as 237.8%, the tensile strength was of 0.28 MPa only.19 Dai et al. synthesized bio-based waterborne polyesters from itaconic acid and different diols with UV-curable nature and exhibited good water resistance, solvent resistance, and hardness.20 Again, Ma et al. synthesized water-soluble hyperbranched polyester with phthalic anhydride, adipic acid, pentaerythritol, trimethylol propane, and neopentyl glycol by a one-step method. The polyesters showed excellent resistance to acid, saltwater, and alkali with distinction at hardness test, adhesion test, and gloss.21 However, the reported polyesters were not synthesized from fully bio-based reactants through eco-friendly route. Thus, to our knowledge, synthesis of environmentally benign bio-based waterborne polyester with desirable properties was not reported so far. Therefore, in the present work, the authors attempted to synthesize bio-based waterborne polyester via an environmentally benign route. The mechanical, thermal, biodegradation, controlled release of fertilizers, UV aging, and chemical resistance properties of the synthesized polyesters were further evaluated to judge their suitability for advanced potential applications. The authors also tried to generate the structure− property relationship of the waterborne polyester (Table 1).

2. RESULTS AND DISCUSSION 2.1. Synthesis of Waterborne Polyester (WBPE). Biobased polyester was synthesized by a simple one-pot synthesis via solvent-free polycondensation technique using bio-based Table 1. Composition (mol) of the Synthesized Polyester Resin reactants (mol)

glycerol

dimer acid

citric acid

WBPE 1 WBPE 2 WBPE 3

2 2 2

1 0.75 0.5

1 1.25 1.5 16813


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Scheme 1. Synthetic Route of the Polyester

Table 2. Physical Properties of the Polyester Resins and Their Thermosets property

WBPE 1

WBPE 2

WBPE 3

acid value (mg/g) hydroxyl value (mg/g) density at 25 °C (g/cm3) viscosity (Pa s) gloss at 60° (thermosets) number-average molecular weight (g/mol) weight-average molecular weight (g/mol) polydispersity index (PDI)

70.8 ± 9 124.6 ± 4 0.91 385.17 56.2 ± 3 11 336

101.4 ± 4 179.9 ± 5 0.95 632.207 60.3 ± 0.6 8234

140.1 ± 8 193.8 ± 4 0.97 731.04 62.3 ± 3 5674

14 045

10 333

7013

1.238

1.255

1.235

Figure 2. FT-IR spectra of WBPE 1, WBPE 2, and WBPE 3.

respectively, and the signals at δ = 1.57 ppm (h) and 1.81 ppm (e) are designated to the ring protons on the dimer acid unit in the polyester resin. A very small peak at δ = 2.51 ppm (n) is designated to the −OH proton of the remaining unbranched glycerol unit, and another small peak at δ = 11.2 ppm (o) is due to the proton of free −COOH group of the citric acid unit. The chemical shift values for the other two compositions of WBPEs are almost the same as those of WBPE 1. 2.2.3. 13C NMR Spectral Study. Similarly, 13C NMR spectral study confirms the different chemical environments of the carbon atoms in the structure of the synthesized polyesters (Figure 4). The terminal methyl carbons of the dimer acid unit

Figure 1. Water solubility behavior of WBPE 3 at different weight percentages.

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calculated by the following equation by determining the integration of the peaks (D, L, and T) and found to be 0.73 degree of branching = (D + T)/(D + T + L)

(1)

The chemical shift values for the other two compositions of the WBPEs are almost the same as those of WBPE 1, and the degrees of branching were found to be 0.75 and 0.72, respectively, for WBPE 2 and WBPE 3. 2.3. Physical Properties. The physical attributes (Table 2) of the synthesized polyesters were further investigated to develop a proper understanding of the polyesters. The WBPE resins come out as an odorless brown viscous mass. The acid and hydroxyl values of WBPEs increase with increasing amount of citric acid content, which can be attributed to the increase of free tertiary carboxylic and hydroxyl groups. The density of the resins with respect to hexane was also found to be increasing, which can be attributed to increase in inter- and intramolecular interactions with increase in free polar functional groups of the citric acid unit. The high dimensional stability leading to a rigid structure of the WBPE thermosets can be determined by the gloss value, which increases with the increase in citric acid content. The molecular weights of the synthesized resins vary from 7013 to 14 045 g/mol with a polydispersity index of ∼1.2, indicating easily processable freeflowing resins with well-defined properties. 2.4. Curing of the Polyesters. The synthesized polyesters were cross-linked with glycerol-based epoxy and fatty acidderived poly(amido amine) as the hardener to obtain the thermosets through the transformation of a low viscous twodimensional liquid to a relatively rigid three-dimensional solid. Different chemical reactions are possible among different functional groups present in the polyester, epoxy, and hardener during the cross-linking process. The amine group of the hardener reacts with the carboxyl group of the polyester to get amide linkage and also reacts with the epoxy ring to get another secondary amine linkage. Again, the reaction between the hydroxyl groups of the polyester with the epoxy rings forms ether linkage. One more kind of reaction that is possible during cross-linking is the transesterification reaction, which takes place between hydroxyl group and ester at high temperature in the presence of basic catalyst. Further, the remaining free −OH and CO of the polyester form hydrogen bond between them. The curing time and swelling value of all of the polyesters are given in Table 3. 2.5. Mechanical Properties. Mechanical performances of thermosetting polyester films are strongly influenced by their molar mass, presence of polar linkages, chain entanglement, flexibility and rigidity, physical and chemical cross-linking, different inter- and intramolecular interactions, etc. Tensile strength, elongation at break, and toughness of the synthesized

Figure 3. 1H NMR spectrum of WBPE 1.

Figure 4. 13C NMR spectrum of WBPE 1.

in the polyester resin show a signal at δ = 14.20 ppm (k), and the methylene carbon adjacent to the terminal methyl groups of the dimer acid unit shows a chemical shift value at δ = 22.74 ppm (j). The peaks at δ = 34.20 ppm (d) and 24.93 ppm (e) are due to the methylene carbon at α- and β-positions of the carbonyl group of the dimer acid unit, respectively. The remaining methylene protons of the dimer acid unit show peaks at δ = 29.1−30.2 ppm (f) and 32.02 ppm (g). The signals for carbonyl carbons of acid and ester groups were observed at δ = 174.69 ppm (c). Again, the multiplet signals at δ = 40.07 ppm (l, m, n) are for methylene protons of the citric acid unit and ring carbons in the dimer acid unit. On the other hand, the tertiary carbon atom of the citric acid unit shows a peak at δ = 72.16 ppm (i). The chemical shifts at δ = 25.11 ppm (a) and 70.28 ppm (b) are due to the methylene and methine carbons of the glycerol unit, respectively. Some extra peaks at 62.30 ppm (D), 63.58 ppm (L), and 67.90 ppm (T) were observed in the 13C NMR spectrum of WBPE 1 representing the carbon atoms of tri-, di-, and monosubstituted glycerol moieties in the polyester, respectively, which further confirms a branching structure of the synthesized polyester resin. The degree of branching of WBPE 1 was further

Table 3. Performance Characteristics of the Polyester Thermosets

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WBPET 1

WBPET 2

WBPET 3

tensile strength (MPa) elongation at break (%) toughness (MJ/m3) scratch hardness (kg) impact resistance (kJ/m) curing time (min) swelling value (%)

8.55 ± 0.35 182.13 ± 5 12.95 ± 0.4 8 ± 0.5 15.99 ± 0.6 300 24.83

11.71 ± 0.14 165.32 ± 2 17.31 ± 0.6 10 ± 0.3 >19.5 ± 0.5 270 20.97

13.71 ± 0.19 128.43 ± 6 15.65 ± 0.3 10 ± 0.2 19.11 ± 0.3 180 20.72


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WBPE thermosets (WBPETs) are calculated from the stress− strain curves as presented in Figure 5, and the values of the

Figure 6. TGA thermograms with inset differential thermogravimetry curves of WBPET 1, WBPET 2, and WBPET 3.

pattern. The initial degradation temperature, peak temperature of first and second stages of degradation and weight residue are provided in Table 4. The peak temperature of first-step

Figure 5. Stress−strain profiles of WBPETs.

different mechanical properties are tabulated in Table 3. It is seen from the table that tensile strength of the WBPETs followed an increasing trend with increase in the weight percentage of citric acid content. This can be attributed to the increasing number of surface functionalities such as −COOH and −OH groups, which form more number of ester and amide linkages and intramolecular hydrogen bonding during cross-linking, thereby leading to a rigid three-dimensional geometry of WBPEs. Further, the thermosets followed a decreasing pattern for the elongation at break (flexibility) with decrease in dimer acid units. This view can be attributed to the decreasing long-chain aliphatic hydrocarbons present in dimer acid, followed by less number of secondary bonds, which helps in extension of the molecular chains and thereby decreases the elongation at break value for WBPETs. The toughness, which is the combination of both tensile strength and flexibility of the WBPET films, was calculated from the area under stress−strain curves and was found to be the highest for WBPET 2. The WBPETs showed the highest limit of impact resistance (limit of the instrument is 1 m), which demonstrates their high toughness character. In addition, the scratch hardness of the thermosets was also high, indicating their overall toughness. Further, it is relevant to mention that the synthesized polyester thermosets showed better tensile strength, elongation at break, and toughness compared to the previously reported bio-based polyesters. Chongcharoenchaikul et al. synthesized bio-based poly(glycerol azelate) polyester from azelaic acid and glycerol with tensile strength and elongation at break varying from 0.26 to 0.55 MPa and 38 to 11%, respectively.22 The tensile strength and elongation at break of the polyesters synthesized by Fan et al. vary from 1.3 to 2.8 MPa and 83 to 182%, respectively.23 Later, Hazarika et al. synthesized waterborne hyperbranched polyester whose tensile strength varies from 4 to 7.8 MPa and elongation at break varies from 175 to 245%.24 2.6. Thermal Properties. The thermal stability of polyester generally depends on various factors like chemical composition, structure, ester linkages, molecular weight, and intra-/intermolecular forces. From the thermograms and their derivative curves of thermogravimetric analysis (TGA) as shown in Figure 6, similar thermal stability can be seen for all of the three thermosets following a two-step degradation

Table 4. Thermal Degradation Parameters of the Polyester Thermosets parameter Ton (°C) first-stage degradation peak temperature (°C) second-stage degradation peak temperature (°C) weight residue (%) at 700 °C

WBPET 1

WBPE 2 WBPE 3

230 331

207 327

203 341

436

435

432

6.49

6.93

9.87

degradation varies from 331 to 341 °C, which is attributed to the thermolabile ester linkages, amide linkages, ether linkages, and aliphatic moieties, whereas the peak temperature of second-step degradation varies from 431 to 436 °C, which is attributed to the aromatic and cycloaliphatic moieties present in the thermosets. The aromatic moieties were generated during cross-linking of the polyesters with the glycerol-based epoxy and poly(amido amine) hardener, whereas the cycloaliphatic moieties were present in the dimer acid unit in the polyesters. The weight residues left after degradation up to 700 °C were found to be 6.49, 6.93, and 9.87% for WBPET 1, WBPET 2, and WBPET 3, respectively, due to the formation of carbonaceous products. The weight residue was found to increase with increase in citric acid content in the polyester. 2.7. Chemical and UV Resistance. The polymeric materials should be weather-resistant and durable as they are exposed to different weathering environments during their service life. Therefore, to determine the weather resistance and durability of the synthesized WBPETs, a series of chemical resistance and UV aging tests were performed as artificial accelerated weathering conditions to mimic the natural weathering process. 2.7.1. Chemical Aging Resistance. The chemical resistance of the synthesized polyester thermosets was studied under different chemical environments at room temperature for 20 days, and stress−strain profiles of the thermosets were evaluated as shown in Figure 7. Weight losses of the thermosets upon chemical aging are also tabulated in Table 5. It is seen from both the table and Figure 7 that all of the thermosets showed good resistance toward 10% NaCl solution 16816


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Figure 7. Stress−strain profiles showing UV aging effect of (a) WBPET 1, (b) WBPET 2, and (c) WBPET 3, and stress−strain curves showing chemical aging effect of (d) WBPET 1, (e) WBPET 2, and (f) WBPET 3.

it contains more free −OH and −COOH groups due to the presence of high amount of citric acid, which leads to a high cross-linked density by forming more number of ester and amide linkages during cross-linking. Additionally, the presence of more number of secondary interactions in WBPET 3 enhanced their chemical resistance. 2.7.2. UV Aging Resistance. The synthesized WBPEs also exhibited excellent resistance against UV radiation, which is supported by the retention of their mechanical properties. The variations in percentage retention of mechanical properties and stress−strain curves of the WBPET films before and after UV aging are shown in Table 6 and Figure 7, respectively. It is

Table 5. Weight Loss (%) of the Polyester Thermosets in Different Chemical Media weight loss (%) chemical medium

WBPET 1

WBPET 2

WBPET 3

acidic medium (pH = 4−5) basic medium (pH = 8−9) 10% NaCl tap water

0.219 1.551 0.0041 0.0024

0.343 1.752 0.0054 0.0026

0.422 2.011 0.0059 0.0019

and tap water, moderate resistance toward acidic medium of pH 4−5, but poor resistance toward basic medium of pH 8−9. In general, Fick’s law is followed for the diffusion of any solvent through the polymer matrix.25−27 As far as a solution with pH 4−5 and 8−9, salt solution, and tap water are concerned, the absorbed solution molecules occupy the voids of the WBPETs and disrupt the interfacial bonds via hydrolysis.28,29 There are also possibilities of secondary (virtual) cross-linking between the polyester films and water molecules via hydrogen bond, leading to stiffening and embrittlement of the polyester matrix. As a result, the elongation at break of all of the WBPETs decreases, whereas the tensile strength of the films either decreases or remains almost constant. The tensile strength of the films kept in tap water and salt solution remains almost same and decreases for the films kept in acidic and basic media due to the hydrolysis of the ether and ester linkages, respectively. However, the decrease is quite less in acidic medium compared to basic medium due to the presence of less number of ether linkages formed during cross-linking with the glycerol-based epoxy than the ester linkages. Hence, the overall mechanical performance of the films decreases in basic solution. Among the different compositions of WBPETs, WBPET 3 shows better chemical resistance than other two thermosets as

Table 6. Retention (%) of Mechanical Properties of WBPEs after UV Aging retention (%) polyester compositions

tensile strength

elongation at break

toughness

WBPET 1 WBPET 2 WBPET 3

97.13 99.17 97.30

88.32 90.40 89.11

91.51 87.29 70.67

observed that after 2 weeks of UV irradiation, the tensile strength of the films remains almost constant, but the flexibility of the films decreases as measured from elongation at break. This can be endorsed to the scission of the chemical bonds leading to photooxidation which further decreases the strength and flexibility, while UV-exposure decreases the flexibility but chain recombination leading to additional cross-linking which increases the tensile strength. These two effects are contradictory to mechanical strength, and hence the effects are almost canceled from each other, and the resulting strength decreases marginally, while both the effects decrease the overall 16817


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Figure 8. Variation of optical density value against exposure time by the bacterial growth of (a) B. subtilis and (b) P. aeruginosa on WBPET 1, WBPET 2, and WBPET 3, and (c) weight loss % of WBPET 1, WBPET 2, and WBPET 3 due to the bacterial growth of B. subtilis and P. aeruginosa.

flexibility of the films. Again, decrease in both flexibility and strength resulted in decrease of toughness of the films. 2.8. Biodegradation. The biodegradation of the synthesized polyester thermosets against Pseudomonas aeruginosa and Bacillus subtilis bacterial strains was studied for a period of 5 weeks by determining the optical density (OD) value and the weight loss % of the degraded films against exposure time (Figure 8). It was observed that after 5 weeks of incubation, the WBPETs were degraded to a considerable amount, and the rate of degradation increases with the increase of citric acid content. This can be attributed to the presence of large numbers of additional hydrolyzable ester and amide linkages formed during cross-linking. It was also observed that the rate of degradation of WBPETs was more by the P. aeruginosa bacterial strain compared to that by the B. subtilis strain, which can be seen from high weight loss percentage of the thermosets on exposure to the respective bacterial strain. This is further due to the high biosurfactant activity and cell surface hydrophobicity of the P. aeruginosa bacterial strain, leading to a faster bacterial colonization. In the general mechanism of the polyester degradation (Figure 9), microorganisms are not able to pick up the polyesters directly into their cells for biochemical processes due to their large size. Hence, they excrete extracellular enzymes, which depolymerize the polyesters outside the cells via hydrolysis of the ester linkages causing surface erosion of the films. Once the depolymerization generates small watersoluble intermediates, they are transported into the microorganisms where the metabolic pathways produce products

like water, CO2, and humus.1,30,31 The scanning electron microscopy (SEM) images of the controlled and degraded WBPET 3 taken after 5 weeks of exposure to the bacterial strains are shown in Figure 9 to reveal the extent of bacterial growth. The surface erosion of the degraded films confirmed the biodegradation of the thermoset. 2.9. Controlled Release of Urea. The urea-loaded polyester resins were first characterized by FT-IR spectroscopy, and a representative FT-IR spectrum of WBPE 3 resin, urealoaded WBPE 3 resin, and urea is given in Figure 10a. The characteristic peaks at 1628 and 1669 cm−1 are for the carbonyl group of amide linkage, and another characteristic peak at 1456 cm−1 is for the C−N stretching present in urea. However, these peaks can be observed only in urea and urealoaded WBPE 3 resin, which supports the presence of urea in the WBPE 3 resin. The urea release profile by the synthesized polyester thermosets was determined for 30 days in acidic medium (pH 6−7), basic medium (pH 8−9), and tap water. The thermosets showed good encapsulation efficiency for urea, 87.6, 89.9, and 90.5% for WBPET 1, WBPET 2, and WBPET 3, respectively, which are due to hydrogen bonding between the urea molecules and the functional groups present in the polyester thermosets. It was observed that WBPET 3 showed maximum encapsulation efficiency compared to the other two compositions, which can be attributed to the large number of hydrogen bonding due to the presence of a large number of free −COOH and −OH groups. The urea release performance by the synthesized polyester thermosets depends on the presence of hydrolyzable hydrophilic bonds in them. The 16818


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Figure 9. General mechanism of polyester biodegradation with SEM images of WBPET 3 of (a) control, (b) degraded by B. subtilis, and (c) degraded by P. aeruginosa.

by varying the amount of citric acid. Thus, the studied polyester thermosets can find decent position as potential sustainable and eco-friendly polymeric materials.

irrigation water penetrates through the polymer coating and reaches the fertilizer core, which is followed by partial dissolution. Afterward, the dissolved fertilizer is thought to be released slowly via diffusion to the soil. As the number of hydrolyzable hydrophilic bonds increases in the thermosets, more water can penetrate through the coating, thereby increasing the release rate of the fertilizer.32 Thus, from all of the three compositions of the thermosets, WBPET 3 showed good release profile for the fertilizer, as can be seen from Figure 10. Also, the release study was also checked in different media comprising acidic medium (pH 6−7), basic medium (pH 8−9), and tap water. It was observed that in basic medium, the release rate was quite higher compared to acidic medium and tap water. This can be attributed to more disruption of the hydrolyzable bonds present in the thermosets in basic medium due to which the dissolved fertilizer can easily pass through the thermosets. Since WBPET 3 has the highest number of hydrolyzable bonds, it shows the highest release rate of urea in basic medium, which is about 74.4%. The unreleased urea will slowly release to the soil as soon as the thermosets undergo biodegradation naturally.

4. METHODS 4.1. Materials. Dimer acid (Mn ∼ 570, Sigma-Aldrich), glycerol (Merck, India), and citric acid (Merck, India) were dried in a vacuum oven for about 6 h at 70 °C prior to use. Bisphenol-A (Sisco Research Laboratories Pvt. Ltd., India) was recrystallized from toluene before use. Oxalic acid (Rankem, India), potassium hydroxide (Rankem, India), para-toluene sulfonic acid (SRL, Mumbai, India), sodium sulfate anhydrous (Merck, Mumbai, India), sodium hydroxide (Rankem, India), poly(amido amine) (Asian Paints, India, gift sample), epichlorohydrin (Merck, India), and urea (Merck, India) were used as received. Tetrahydrofuran (THF, SD Fine Chem., India) was distilled before use. An epoxy based on glycerol was prepared using glycerol, bisphenol-A, and epichlorohydrin following an earlier reported method by our group.33 The bacterial strains of P. aeruginosa and B. subtilis were obtained from Department of Molecular Biology and Biotechnology, Tezpur University. All other chemicals used in this work were of reagent grade and used without any further purification. 4.2. Synthesis of Bio-Based Waterborne Polyester (WBPE). The synthesis of the bio-based polyester via solventfree polycondensation reaction requires a three-neck roundbottom flask equipped with a nitrogen gas inlet, an oil bath, a mechanical stirrer, and a thermometer. In the first step, a hydroxyl-terminated polyol was synthesized using dimer acid and glycerol in the presence of 0.05 wt % p-toluene sulfonic acid as catalyst. The reaction mixture was then allowed to react

3. CONCLUSIONS From the study, it can be concluded that bio-based waterborne polyesters can be synthesized through one-pot synthesis through an environmentally benign route. The glycerol-based epoxy and poly(amido amine) cured thermosets of the polyester showed excellent mechanical, thermal and chemical resistance and UV aging retention property. The notable features of the thermosets are high biodegradability and good controlled release profile of urea, which can be further tuned 16819


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Figure 10. (a) FT-IR spectra of WBPE 3 resin, urea-loaded WBPE 3 resin, and urea; release profiles of urea by WBPET 1, WBPET 2, and WBPET 3 in (b) tap water, (c) acidic medium (pH 6−7), and (d) basic medium (pH 8−9).

for 3 h at 160 °C until the acid value drops down to 10 mg of KOH/g. Once this reaction is completed, the reaction mixture was allowed to cool down up to 80 °C with constant stirring, and then, a calculated amount of citric acid was added. The reaction mixture was then allowed to react under constant mechanical agitation at 140 °C for another 2 h to get the desired polyester. During the course of reaction, it was monitored by the change of acid value and stopped before any gel formation. After completion of the reaction, a calculated amount of triethyl amine was added slowly to neutralize the free −COOH groups present and stirred continuously for about 40 min. This was followed by slow addition of warm water with continuous stirring for another 30 min to get the desired bio-based waterborne amination-modified polyester. By ensuing to the same procedure, three different compositions of the waterborne amination-modified polyesters were prepared by varying the weight percentages of dimer acid and citric acid, which were further coded as WBPE 1, WBPE 2, and WBPE 3 (Table 1). 4.3. Characterization. 4.3.1. Structural Analysis. Fourier transform infrared spectra of WBPEs were recorded over the range of wavenumber from 4000 to 400 cm−1 by a Nicolet (Madison) FT-IR impact 410 spectrophotometer using KBr pellets. The proton and the carbon environments of WBPEs were determined by 1H and 13C NMR spectrometers of 400 MHz (JEOL, Japan) using deuterated chloroform as the solvent and trimethyl silane as the internal standard.

4.3.2. Determination of Physical Properties. The standard liquid displacement method with ASTM D7932 was used to measure the specific gravity of the WBPEs. The gloss of the polymer films was measured by a mini gloss meter (Sheen Instruments Ltd., U.K.) at an incidence angle of 60°. The weight-average molecular weight (Mw), number-average molecular weight (Mn), and polydispersity index (PDI) of all of the WBPEs were evaluated by gel permeation chromatography (Waters, U.K.), where linear polystyrene was used as the standard and THF was used as the eluent with flow rate maintained at 0.75 mL/min. 4.3.3. Curing of Polyesters. WBPEs were cured by mixing the required amount of glycerol-based epoxy and poly(amido amine). A ratio of 1:1 was maintained for the polyester and the epoxy resin, while 50 wt % of the poly(amido amine) compared to that of the epoxy resin was used for this purpose. To facilitate homogeneous mixing of all of these three components in a small glass beaker, a minimum amount of THF was used at room temperature with continuous hand stirring. The reaction mixture was then evenly coated on glass plates (75 mm × 25 mm × 1.3 mm) for the measurement of tensile strength, gloss, and scratch hardness and on steel plates (150 mm × 50 mm × 1mm) for the measurement of impact resistance. The plates were then left undisturbed in an open environment for 2 days followed by curing at 150 °C for a required period of time. The optimized curing time was calculated by checking the swelling value of the cured films. The synthesized polyester thermosets were further named 16820


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2H2O, 7H2O, 100 mg of CuSO4·7H2O, 10 mg of H3BO3· 5H2O, 70 mg of ZnSO4, 10 mg of MoO3, and 1 mg of FeSO4· 7H2O. This medium was then autoclaved for 15 min under a pressure of 103.5 kPa at 120 °C for sterilization and then used to culture the bacterial strains inside an incubator for 48 h at 37 °C. An amount of 100 μL (108 microbes/mL) of the cultured medium was then taken in a conical flask containing 10 mL of the above sterilized light of wavelength 254 nm and then added to the above prepared medium, which was later kept incubated at 37 °C. The negative control was the above medium without the WBPE films. The bacterial growth was monitored by measuring the increase in turbidity with exposure time. The bacterial growth on the polyester films with respect to the control was measured by the absorbance at 600 nm as optical density (OD) of the microorganism. The reported values were the average of three experimental results, and the experiment was performed for a period of 4 weeks. The surface images of biodegraded samples were taken by a JEOL JSM-6390LV scanning electron microscope. 4.3.9. Study of Controlled Release of Urea. The release profile of urea by the polyester thermosets was determined artificially in three different media by using an optimal spectroscopic methodology. In the release study, p-dimethylaminobenzaldehyde (DMAB, 20 mmol/L) was used as a colorimetric reagent, which forms a yellow-green complex with urea in acidic condition based on Ehrlich reaction and further absorbs visible light at 425 nm. A standard curve, concentration versus absorbance, was first plotted by detecting known concentration of urea, where DMAB- (1800 μL) and urea-containing solutions (1000 μL) were taken in glass vials in the presence of 68 μL of concentrated HCl, and then their absorbance was determined using a UV−vis spectrophotometer. The polyester resins were loaded with urea dissolved in water after making them waterborne and characterized by FTIR spectroscopy. These urea-loaded polyester resins were further cross-linked with glycerol-based epoxy and poly(amido amine) as hardener to get the thermosets and cured at 60 °C until they become touch-free. The encapsulation efficiency of the urea-loaded polyester thermosets was then determined by washing the known dried mass of the thermosets into 25 mL of distilled water at room temperature and detecting the washedout urea content from the standard curve. Encapsulation efficiency was then calculated using the following equation

WBPET 1, WBPET 2, and WBPET 3, respectively, for the polyester resins WBPE 1, WBPE 2, and WBPE 3. 4.3.4. Swelling Test. The swelling test of the cured polyester films was performed by immersing small weighted pieces of the dried films in THF. After every 24 h, the swelled films were taken out from the solvent and weighed after removing the excess solvent by using blotting paper. This process was continued until a constant weight was obtained (equilibrium swelling reached). The swelling % for the films was calculated by the difference in weight between the dried and swollen films as follows swelling% = [(Ws − Wd)/Wd ] × 100

(2)

where Ws is the weight of the swelled film and Wd is the weight of the dried film. 4.3.5. Mechanical Properties Measurement. The tensile strength and elongation at break of all WBPEs were measured according to the ASTM D 638 by using Universal Testing Machine, model WDW-10 (Jinan, China), with a 1.0 kN load cell and a crosshead speed of 10 mm/min. The scratch hardness of the films was measured by using a scratch hardness tester, model number 705 (Sheen Instruments Ltd., U.K.), with stylus accessory and a travel speed of 50 mm/s. An impact tester (S.C. Dey & Co., Kolkata, with 1 m as the maximum height) using the standard ASTM D4272 falling weight method was used to measure the impact strength of WBPETs. A weight of 850 g was allowed to fall on the WBPETs coated on steel plates from minimum to maximum falling heights, and the energy per unit thickness corresponding to the maximum height was taken as the impact strength. 4.3.6. Thermal Properties Measurement. The decomposition profiles of the WBPETs were thermogravimetrically scrutinized by a TGA-4000 (PerkinElmer) instrument under inert atmosphere of pure nitrogen gas with a flow rate of 30 mL/min and a heating rate of 5 °C/min in a temperature range of 30−700 °C. 4.3.7. Chemical and UV Resistance Test. 4.3.7.1. Chemical Resistance. The chemical resistance of the WBPETs was determined according to the standard ASTM D 543-95 method by measuring the weight loss and mechanical properties of the films upon exposure to the chemical media such as acidic media (pH 4−5), basic media (pH 9−10), 15% NaCl solution, and tap water for 20 days. 4.3.7.2. UV Aging Test. The artificial UV aging test of the WBPETs was carried out in an accelerated UV aging chamber (Labtech, India) with UV light of wavelength and power 256 nm and 8 W, respectively. The sample was placed at 25 cm from the UV lamp. The tensile test of UV aged films was performed after a total duration of 2 weeks to study the influence of UV radiations on the mechanical properties of WBPETs, and the retention (%) of tensile strength was calculated as follows retention (%) = 100 − [{(P0 − P1)/P0} × 100]

encapsulation efficiency (%) = {1 − (Δw/Cwo)} × 100 (4)

where Δw = washed-out urea content wo = dry weight of the thermosets C = total urea content initially added during loading After washing, the thermosets were then dried and exposed to different chemical media such as acidic media (pH = 6−7), basic media (pH = 8−9), and tap water.

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(3)

where P0 and P1 represent the values of tensile strength before and after UV aging, respectively. 4.3.8. Biodegradation Test. The biodegradation of the WBPETs was studied by using P. aeruginosa (Gram-negative) and B. subtilis (Gram-positive) bacterial strains by following the McFarland method. First, a medium was prepared in 1 L of demineralized water by adding different mineral salts such as 4.75 g of KH2PO4, 2 g of Na2HPO4, 2 g of (NH4)2SO4, 1.2 g of MgSO4·7H2O, 100 mg of MnSO4·5H2O, 0.5 mg of CaCl2·

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-3712-267009. Fax: +91-3712-267006. ORCID

Niranjan Karak: 0000-0002-3402-9536 Notes

The authors declare no competing financial interest. 16821


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resources and recycled polymers. J. Coat. Technol. Res. 2015, 12, 555− 562. (19) Kim, M. J.; Hwang, M. Y.; Kim, J.; Chung, D. J. Biodegradable and elastomeric poly(glycerol sebacate) as a coating material for nitinol bare stent. BioMed Res. Int. 2014, No. 956952. (20) Dai, J.; Ma, S.; Liu, X.; Han, L.; Wu, Y.; Dai, X.; Zhu, J. Synthesis of bio-based unsaturated polyester resins and their application in waterborne UV-curable coatings. Prog. Org. Coat. 2015, 78, 49−54. (21) Ma, S.; Qian, J.; Zhuang, Q.; Li, X.; Kou, W.; Peng, S. Synthesis and application of water-soluble hyperbranched polyester modified by trimellitic anhydride. J. Macromol. Sci., Part A: Pure Appl. Chem. 2018, 55, 414−421. (22) Chongcharoenchaikul, T.; Thamyongkit, P.; Poompradub, S. Synthesis, characterization and properties of a bio-based poly(glycerol azelate) polyester. Mater. Chem. Phys. 2016, 177, 485−495. (23) Fan, C.; Pang, C.; Liu, X.; Ma, J.; Gao, H. Self-curing furanbased elastic thermosets derived from citric acid. Green Chem. 2016, 18, 6320−6328. (24) Hazarika, D.; Karak, N. Waterborne sustainable tough hyperbranched aliphatic polyester thermosets. ACS Sustainable Chem. Eng. 2015, 3, 2458−2568. (25) Pavan, R. M. V.; Saravanan, V.; Dinesh, A. R.; Rao, Y. J.; Srihari, S.; Revathi, A. Hygrothermal effects on painted and unpainted glass/ epoxy compositesPart A: moisture absorption characteristics. J. Reinf. Plast. Compos. 2001, 20, 1036−1047. (26) Ferreira, J. M.; Pires, J. T. B.; Costa, J. D.; Errajhi, O. A.; Richardson, M. Fatigue damage and environment interaction of polyester aluminized glass fiber composites. Compos. Struct. 2007, 78, 397−401. (27) Zhang, S.; Karbhari, V. M.; Mai, L.-Y.; Mai, Y.-W. Evaluation of property retention in E-glass/vinylester composites after exposure to salt solution and natural weathering. J. Reinf. Plast. Compos. 2000, 19, 704−731. (28) Smith, W. F. Foundations of Materials Science and Engineering; McGraw-Hill Science/Engineering/Math, 1993. (29) Papanicolaou, G. C.; Kosmidou, T. V.; Vatalis, A. S.; Delides, C. G. Water absorption mechanism and some anomalous effects on the mechanical and viscoelastic behavior of an epoxy system. J. Appl. Polym. Sci. 2006, 99, 1328−1339. (30) Rücker, C.; Kümmerer, K. Modeling and Predicting Aquatic Aerobic Biodegradation − a Review from a User’s Perspective. Green Chem. 2012, 14, 875. (31) Hazarika, D.; Karak, N. Biodegradable tough waterborne hyperbranched polyester/carbon dot nanocomposite: approach towards an eco-friendly material. Green Chem. 2016, 18, 5200−5211. (32) Azeem, B.; KuShaari, K.; Man, Z. B.; Basit, A.; Thanh, T. H. Review on materials & methods to produce controlled release coated urea fertilizer. J. Controlled Release 2014, 181, 11−21. (33) Barua, S.; Dutta, G.; Karak, N. Glycerol based tough hyperbranched epoxy: synthesis, statistical optimization and property evaluation. Chem. Eng. Sci. 2013, 95, 138−147.

ACKNOWLEDGMENTS The authors express their gratitude to Council of Scientific and Industry Research (CSIR), New Delhi, for financial assistance through granting the project No. 22(0759)/17/EMR-II and Sophisticated Analytical Instrumentation Centre (SAIC), Tezpur University, for helping in instrumental analyses.

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REFERENCES

(1) Mueller, R. J. Biological degradation of synthetic polyesters enzymes as potential catalysts for polyester recycling. Process Biochem. 2006, 41, 2124−2128. (2) Miyagawa, H.; Mohanty, A. K.; Burgueño, R.; Drzal, L. T.; Misra, M. Development of biobased unsaturated polyester containing functionalized linseed oil. Ind. Eng. Chem. Res. 2006, 45, 1014−1018. (3) Gustini, L.; Finzel, L.; Lavilla, C.; Noordover, B. A.; Hendrix, M. M.; Gehrels, C.; Koning, C. E. Understanding the limitations of the solvent-free enzymatic synthesis of sorbitol-containing polyesters. ACS Sustainable Chem. Eng. 2016, 4, 2259−2268. (4) Jin, L.; Geng, K.; Arshad, M.; Ahmadi, R.; Ullah, A. Synthesis of Fully Biobased Polyesters from Plant Oil. ACS Sustainable Chem. Eng. 2017, 5, 9793−9801. (5) Dutta, N.; Karak, N.; Dolui, S. K. Synthesis and characterization of polyester resins based on Nahar seed oil. Prog. Org. Coat. 2004, 49, 146−152. (6) Zia, K. M.; Noreen, A.; Zuber, M.; Tabasum, S.; Mujahid, M. Recent developments and future prospects on bio-based polyesters derived from renewable resources: A review. Int. J. Biol. Macromol. 2016, 82, 1028−1040. (7) Fan, C.; Pang, C.; Liu, X.; Ma, J.; Gao, H. Self-curing furan-based elastic thermosets derived from citric acid. Green Chem. 2016, 18, 6320−6328. (8) Bakare, I. O.; Pavithran, C.; Okieimen, F. E.; Pillai, C. K. S. Polyesters from renewable resources: preparation and characterization. J. Appl. Polym. Sci. 2006, 100, 3748−3755. (9) Djonlagic, J.; Nikolic, M. S. Biodegradable Polyesters: Synthesis and Physical Properties. In A Handbook of Applied Biopolymer Technology; Sharma, S. K.; Mudhoo, A. K., Eds.; RSC Green Chemistry, 2011; pp 149−196. (10) Edlund, U.; Albertsson, A. C. Polyesters based on diacid monomers. Adv. Drug Delivery Rev. 2003, 55, 585−609. (11) Park, H. S.; Kim, S. R.; Park, H. J.; Kwak, Y. C.; Hahm, H. S.; Kim, S. K. Preparation and characterization of weather resistant silicone/acrylic resin coatings. J. Coat. Technol. 2003, 75, 55−64. (12) Pathan, S.; Ahmad, S. s-Triazine ring-modified waterborne alkyd: Synthesis, characterization, antibacterial, and electrochemical corrosion studies. ACS Sustainable Chem. Eng. 2013, 1, 1246−1257. (13) Yu, Q.; Liang, Y.; Cheng, J.; Chen, S.; Zhang, A.; Miao, M.; Zhang, D. Synthesis of a Degradable High-Performance Epoxy-Ended Hyperbranched Polyester. ACS Omega 2017, 2, 1350−1359. (14) Islam, S.; Inglefield, D. L.; Velev, O. D. Revisiting the colloidal fundamentals of water-dispersible polyesters: interactions and selfassembly of polymer nanoaggregates in water. Soft Matter 2018, 14, 2118−2130. (15) Asif, A.; Shi, W. Synthesis and properties of UV curable waterborne hyperbranched aliphatic polyester. Eur. Polym. J. 2003, 39, 933−938. (16) Noordover, B. A. J.; van Staalduinen, V. G.; Duchateau, R.; Koning, C. E.; van Benthem; Mak, M.; Heise, A.; Frissen, A. E.; van Haveren, J. Co- and terpolyesters based on isosorbide and succinic acid for coating applications: synthesis and characterization. Biomacromolecules 2006, 7, 3406−3416. (17) Noordover, B. A. J.; Duchateau, R.; van Benthem, R. A. T. M.; Ming, W.; Koning, C. E. Enhancing the functionality of biobased polyester coating resins through modification with citric acid. Biomacromolecules 2007, 8, 3860−3870. (18) Gioia, C.; Minesso, A.; Cavalieri, R.; Marchese, P.; Celli, A.; Colonna, M. Powder coatings for indoor applications from renewable 16822


One-Pot Synthesis of Bio-Based Waterborne Polyester as UV

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