Synthesis and Characterization of Renewable Polyester Coil Coatings from Biomass-Derived Isosorbide, FDCA, 1,5-Pentanediol, Succinic Acid, and 1,3-Propanediol

Biomass-derived polyester coatings for coil applications have been successfully developed and characterized. The coatings were constituted by carbohydrate-derived monomers, namely 2,5-furan dicarboxylic acid, isosorbide, succinic acid, 1,3-propanediol, and 1,5-pentanediol, the latter having previously been used as a plasticizer rather than a structural building unit. The effect of isosorbide on the coatings is widely studied. The inclusion of these monomers diversified the mechanical properties of the coatings, and showed an improved performance against common petrochemical derived coatings. This research study provides a range of fully bio-derived polyester coil coatings with tunable properties of industrial interest, highlighting the importance of renewable polymers towards a successful bioeconomy.


Introduction
The environmental and sustainability problems currently faced worldwide call for the implementation and usage of renewable materials across industries. Within polymers, the future in polyesters relies on the inclusion of monomers sourced from bioderived feedstocks [1]. In this vein, our group has previously developed the synthesis, kinetic modelling, and process optimization and intensification of bioderived polyesters based on furan 2,5-dicarboxylic acid (FDCA), succinic acid, 1,3-propanediol, and 1,5-pentanediol [2][3][4]. Common polyester coatings are normally prepared from diacids such as terephthalic acid, isophthalic acid, phthalic anhydride, and adipic acid. The polyalcohols could include difunctional monomers such as neopentyl alcohol, ethylene glycol, or polyfunctional compounds, including trimethylol propane, among other compounds exceeding two functionalities [5]. In this regard, carbohydrates are a vast source of renewable, biomass-derived diols suitable for coil coatings, such as 1,4:3,6-dianhydrohexitols [6]. Dianhydrohexitols are a by-product of the starch industry obtained by the reduction of hexose sugars followed by dehydration. 1,4:3,6-dianhydro-D-glucitol (isosorbide), 1,4:3,6-dianhydrohexitol-D-mannitol (isomannide), and 1,4:3,6-dianhydro-L-iditol (isoidide) are the three main diastereoisomers derived from D-glucose, D-mannose, and L-fructose [7], respectively, and are shown in Figure 1. The reader is encouraged to refer to additional sources for detailed information on the synthesis, chemistry, and properties of 1,4:3,6-dianhydrohexitols [8][9][10]. 1,4:3,6-dianhydrohexitols [8][9][10]. Isosorbide bears a considerable potential for the production of new tailored chemicals from renewable resources as it is conformed by two cis-connected tetrahydrofuran rings with secondary hydroxyl groups in the 2-(endo) and 5-(exo) positions, which allow for further functionalization or direct processing [9]. The use of isosorbide in polyesters can be motivated by several features: rigidity, chirality, non-toxicity [8], and recently, its use as a monomer for the preparation of UV-cured coatings has been highlighted [11]. Isosorbide has a relatively high thermostability and low segmental mobility, and can be used to improve the glass transition temperatures of polyesters [12]. However, the hydroxyl group in endo position easily forms intramolecular hydrogen bonds with the oxygen in the other ring, which leads to the poor reactivity of the secondary hydroxyl group and the low number average molecular weights of copolyesters [13]. Previous work on isosorbide-based polyesters has been reported, mainly based on a variety of biomass-derived monomers, mainly citric acid [14], lactic acid [15], succinic acid [16][17][18][19], sebacic acid [17,20], itaconic acid [18], 1,4-cyclohexanedimethanol [21], and dimethyl-2,5-furan dicarboxylate [21,22]. Goerz et al. [18] studied the synthesis of polyesters from isosorbide, itaconic acid and succinic acid. The obtained polyesters had glass transition temperatures (Tg) from 57 °C to 65 °C and molecular weights from 1200 Da up to 3500 Da, depending on the molar ratio of the monomers. In the field of coatings, Noordover et al. [14,19,23] reported the synthesis of terpolyesters for powder coatings based on isosorbide, succinic acid, citric acid, and aliphatic diols such as 1,3-propanediol, 1,4-butanediol, and neopentyl glycol, showing number average molecular weights (Mn) from 2700 up to 4600 Da, and highlighting the effect of isosorbide content on the glass transition temperature [19]. Gioia et al. [24] synthesized polyesters based on recycled PET, succinic acid, and isosorbide for powder coating applications Jacquel et al. [25] prepared bioderived copolyesters of succinic acid and isosorbide by varying the mol % isosorbide from 5 to 20%. The polyesters had Tg from −28 °C to −11 °C, which increased by increasing the mol % isosorbide, while the esterification yield decreased. Zhou et al. [12,13,20] studied the properties and crystallization kinetics of copolyesters based on isosorbide, sebacic acid, and either 1,10-decanediol [20] or 1,3-propanediol [12]. The authors varied the mol % isosorbide from 5.3 mol % to 66.2%, reporting Tg ranging from −26 °C to −5 °C, although no glass transition was observed when the mol % isosorbide was below 30%. The Mn range was broad, with the polyester of isosorbide and sebacic acid showing the lowest Mn (2800 Da), whereas the copolyester with 25.4 mol % isosorbide had the highest Mn, 17000 Da. No relationship was found between the Mn and the amount of incorporated isosorbide [20].
Besides the influence of isosorbide on the final polymer, the effect of catalysts has been studied. For instance, during the synthesis of poly(ethylene terephthalate-co-isosorbide terephthalate), the authors found that combinations of antimony oxide with lithium, magnesium or aluminum based salts successfully increased the efficiency of the transesterification step [26].
Furan 2,5-dicarboxylic acid has been subject to extensive research over the last years, with a drive to develop it as a green chemical building block for polyesters [27][28][29][30][31][32][33]. Furan 2,5-dicarboxylic acid is a versatile, bioderived carboxylic acid which was envisioned as a replacement for terephthalic acid in the synthesis of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), although many other polyfuronoates have been developed. Poly(ethylene 2,5-furandicarboxylate) (PEF) shows greatly improved barrier and mechanical properties, higher glass transition Previous work on isosorbide-based polyesters has been reported, mainly based on a variety of biomass-derived monomers, mainly citric acid [14], lactic acid [15], succinic acid [16][17][18][19], sebacic acid [17,20], itaconic acid [18], 1,4-cyclohexanedimethanol [21], and dimethyl-2,5-furan dicarboxylate [21,22]. Goerz et al. [18] studied the synthesis of polyesters from isosorbide, itaconic acid and succinic acid. The obtained polyesters had glass transition temperatures (T g ) from 57 • C to 65 • C and molecular weights from 1200 Da up to 3500 Da, depending on the molar ratio of the monomers. In the field of coatings, Noordover et al. [14,19,23] reported the synthesis of terpolyesters for powder coatings based on isosorbide, succinic acid, citric acid, and aliphatic diols such as 1,3-propanediol, 1,4-butanediol, and neopentyl glycol, showing number average molecular weights (M n ) from 2700 up to 4600 Da, and highlighting the effect of isosorbide content on the glass transition temperature [19]. Gioia et al. [24] synthesized polyesters based on recycled PET, succinic acid, and isosorbide for powder coating applications Jacquel et al. [25] prepared bioderived copolyesters of succinic acid and isosorbide by varying the mol % isosorbide from 5 to 20%. The polyesters had T g from −28 • C to −11 • C, which increased by increasing the mol % isosorbide, while the esterification yield decreased. Zhou et al. [12,13,20] studied the properties and crystallization kinetics of copolyesters based on isosorbide, sebacic acid, and either 1,10-decanediol [20] or 1,3-propanediol [12]. The authors varied the mol % isosorbide from 5.3 mol % to 66.2%, reporting T g ranging from −26 • C to −5 • C, although no glass transition was observed when the mol % isosorbide was below 30%. The M n range was broad, with the polyester of isosorbide and sebacic acid showing the lowest M n (2800 Da), whereas the copolyester with 25.4 mol % isosorbide had the highest M n , 17000 Da. No relationship was found between the M n and the amount of incorporated isosorbide [20].
Besides the influence of isosorbide on the final polymer, the effect of catalysts has been studied. For instance, during the synthesis of poly(ethylene terephthalate-co-isosorbide terephthalate), the authors found that combinations of antimony oxide with lithium, magnesium or aluminum based salts successfully increased the efficiency of the transesterification step [26].
Lately, the thermal properties of FDCA polyesters have been extensively studied, such as poly(octylene furanoate) [44], along with thermal degradation of different polyfuronoates [45,46]. The potential of polyesters based on FDCA is steadily increasing and their industrialization and commercialization will eventually become a reality. Papageorgiou et al. [47] recently reviewed the current status and latest progress of polyfuranoates.
This research study presents the development and synthesis of new polyester resins based on isosorbide, FDCA, succinic acid, and 1,5-pentanediol to be used in coil coating applications. 1,5-pentanediol has normally been used as a polymerization additive (plasticizer) [48] rather than a main diol during poleysterification; however, we have considered it since its bioderived synthesis is playing a major role within the biorefinery concept, with promising prospects [49][50][51][52][53][54]. Resins with 1,3-propanediol are included as comparison. The paper focuses on the real applicability of the bioderived coatings instead of providing a detailed description on their compositions or chemical characterization. Instead, common mechanical testing analyses for coatings are included. Our objective is therefore to compel a fully biomass-derived, coil coatings library not only of the base polyester resins, but also of the final coatings. We believe our overall work delivers a good basis for the implementation of biomass-derived polymers in large scale and aims to become a strong industrial reference to embrace renewable feedstocks.

Synthesis of Isosorbide-Based Renewable Polyesters
The structures and the synthetic procedure of the isosorbide polyesters with 1,3-propanediol (PPFIS) and 1,5-pentanediol (PPeFIS) are depicted in Scheme 1, whereas Tables 1 and 2 summarize the polyesters synthesized, along with their M n , weight-average molecular weight (M w ), and dispersity (Đ), which were calculated by gel permeation chromatography (GPC), following the method described later on in Section 2. The polyesters' nomenclature is based on the diol used, the diacid molar ratio, and the molar proportion of isosorbide present. For a typical polyester, the first P refers to the suffix poly, followed either by P or Pe, if synthesized with either 1,3-propanediol or 1,5-pentanediol, respectively.
Next, the notation F x , indicates the presence of FDCA in x mol %. Similarly, S y refers to the succinic acid present in y mol % whilst I z corresponds to the z mol % isosorbide. Polyesterification reactions were performed at two different scales: 250 mL and 500 mL. The choice of scale depended on the mol % FDCA in the feed, because the bulk viscosity of the system increased as the mol % FDCA was increased. Hence, all the polyesters bearing furanic content above 50 mol % were synthesized at the 500 mL scale. The different scales also facilitated cleaning and recovery of the product, while minimizing the use of solvents. Despite working with two different volumes, the geometry of the stirrer and the shape of the reactor (round-bottom) were the same in both configurations, as well as the nitrogen flow rate. (No flowmeter was in place but the nitrogen flow was set to 2 bubbles·s −1 ). The general polymerization reactors set-up and procedure are available in our previous publications [2][3][4].   Polyesterification reactions were performed at two different scales: 250 mL and 500 mL. The choice of scale depended on the mol % FDCA in the feed, because the bulk viscosity of the system increased as the mol % FDCA was increased. Hence, all the polyesters bearing furanic content above 50 mol % were synthesized at the 500 mL scale. The different scales also facilitated cleaning and recovery of the product, while minimizing the use of solvents. Despite working with two different volumes, the geometry of the stirrer and the shape of the reactor (round-bottom) were the same in both configurations, as well as the nitrogen flow rate. (No flowmeter was in place but the nitrogen flow was set to 2 bubbles·s −1 ). The general polymerization reactors set-up and procedure are available in our previous publications [2][3][4].
The experimental synthesis was followed for all polyesters by adjusting the ratio of the monomers accordingly. The exact quantities for each polyester are available in Supplementary Material Tables S1 and S2, along with the purification polyester method. In a typical polymerization to synthesize PPF 15  the reactor was heated up to 150 • C and 74 g (0.62 mol) of succinic acid and SnCl 2 were added. The temperature was increased to 215 • C and was continuously stirred at 350 ppm. The esterification stage was completed after 2 h when all the water had been released and the head temperature on top of the distillation column was back to ambient temperature. Then, the polycondensation reaction was carried out by azeotropic distillation by changing the packed column to a Dean Stark trap, adding 3 wt % xylene as azeotropic agent under atmospheric pressure for 5 hours. The reaction was finally cooled down and the polymer was collected and purified for characterization. The polyesters' color depended on the amount of FDCA and isosorbide in the mixture. Although SnCl 2 was the only catalyst used, tin catalysts showed less intense coloration in a comparative polycondensation study using titanium catalysts with furanoates [35]. The intensity color range of the polyesters went from a light yellow to brown with increasing FDCA, although no color space system measurements were undertaken, as our coatings did not require any color specification.

Gel Permeation Chromatography
Gel permeation chromatography was carried out on an Agilent 1260 Infinity with two Agilent ResiPore Organic 250 × 4.6 mm columns, a guard column, and a refractive index detector. The eluent was tetrahydrofuran (THF) at a flow rate of 0.3 mL/min. Molecular weights were calculated using a conventional calibration with polystyrene standards.

Differential Scanning Calorimetry (DSC)
Differential Scanning Calorimetry measurements were performed using a TA Instruments Q2000 analyzer with a RC590 cooling system using a standard heat-cool-heat method. The temperature range was −50 • C to 200 • C. Both the heating and cooling rates were 10 • C/min in N 2 . The amount of sample was approximately 6 ± 0.1 mg. The samples were deposited in Tzero aluminum pans. The thermal stability of the polyesters was determined by TGA using TA Instruments Q5000 equipment under N 2 atmosphere. The samples were placed in aluminum pans and heated from room temperature to 550 • C at a rate of 10 • C/min.

Coatings Characterization
Coatings were applied at 18-20 microns directly onto a smooth polyester pre-primed steel substrate and cured for 40 s to reach a peak metal temperature of 224-232 • C. The physical testing methods carried out on the metal panels are described in Supplementary Material, Section 2 [55][56][57][58][59][60][61].

1 H NMR
The intention of the present 1 H NMR analysis was to solely provide a general insight into the regions that identify the carbohydrate-derived polyesters. Furthermore, no quantitative analysis or precise definition of every signal was undertaken. The analysis by 13 C NMR and 2D NMR could facilitate the study. The NMR spectra of the isosorbide monomer is included in Supplementary Material, Figure S1, whilst illustrative 13 C NMR and 2D NMR spectra of a representative PPeFIS polyester are depicted in Figures S2 and S3.
The chemical shifts and assignments are summarized in Table 3. Figure 2 shows the NMR spectra for 1,5-pentanediol copolyesters PPeF 85 I 10 S 15 , PPeF 30 I 30 S 70 , and PPeF 70 I 50 S 30 , which contain 10, 30, and 50 mol % isosorbide, respectively. The identification of single peaks was a complex process since the incorporation of isosorbide could possibly facilitate the formation of cyclic structures and short chain oligomers, along with the probable presence of unreacted isosorbide (potentially between 3.5-4.5 ppm, Figure 2) at the end of the chains, leaving the secondary hydroxyl unreacted. The presence of impurities in the isosorbide monomer was a possibility as well (>98% purity), as it is one of the limitations of industrially-sourced isosorbide, along with residual monomer. The formation of the FDCA and succinic acid esters is confirmed by the shifts at 4.34 (i) and 4.09 (f ), respectively, as the protons of the FDCA esters tend to shift to higher ppm values [29][30][31]. The assignment a (7.2 ppm) corresponds to the protons of the furan ring of FDCA. The signals between 3.8 ppm and 5.4 ppm (e,d,m,n,o,p) are attributed to the protons of isosorbide, which is in good agreement with previous literature of different isosorbide-based polyesters: 3.9-5.6 ppm [24], 3.7-5.2 ppm [17], 3.8-5.4 ppm [20], and 3.8-5.15 ppm [19]. The differences in shifts between peaks p (5.40-5.46 ppm) and m (5. 21-5.25ppm) are the suggested result of the endo (p) and exo (m) stereochemistry of the two hydroxyl groups of isosorbide, as well as between peaks d (4.62-4.67 ppm) and n (4.84-4.97 ppm) [19]. Gioia et al. [24] identified the endo and exo OH groups at 5.2 ppm and 5.5 ppm, respectively. The broadening of the CH 2 -signal of succinic acid (b and b', 2.62 and 2.69 ppm) was derived from the presence of two diols, 1,5-pentanediol and isosorbide, coupled with the endo and exo stereochemistry of isosorbide. A similar behavior was reported for polyesters conformed by 1,3-propanediol and isosorbide with succinic acid, where the succinic acid shifts were observable between 2.6 and 2.8 ppm [19]. Apparently, from PPeF 85 I 10 S 15 down to PPeF 30 I 50 S 70 with 50 mol % isosorbide, the characteristic peaks of 1,5-pentanediol (1.42-1.80 ppm) decrease as the isosorbide concentration increases [20], although a quantitative analysis should be performed to confirm the assumption. The formation of the FDCA and succinic acid esters is confirmed by the shifts at 4.34 (i) and 4.09 (f), respectively, as the protons of the FDCA esters tend to shift to higher ppm values. [29][30][31] The assignment a (7.2 ppm) corresponds to the protons of the furan ring of FDCA. The signals between 3.8 ppm and 5.4 ppm (e,d,m,n,o,p) are attributed to the protons of isosorbide, which is in good agreement with previous literature of different isosorbide-based polyesters: 3.9-5.6 ppm [24], 3.7-5.2 ppm [17], 3.8-5.4 ppm [20], and 3.8-5.15 ppm [19]. The differences in shifts between peaks p (5.40-5.46 ppm) and m (5. 21-5.25ppm) are the suggested result of the endo (p) and exo (m) stereochemistry of the two hydroxyl groups of isosorbide, as well as between peaks d (4.62-4.67 ppm) and n (4.84-4.97 ppm) [19]. Gioia et al. [24] identified the endo and exo OH groups at 5.2 ppm and 5.5 ppm, respectively. The broadening of the CH2-signal of succinic acid (b and b', 2.62 and 2.69 ppm) was derived from the presence of two diols, 1,5-pentanediol and isosorbide, coupled with the endo and exo stereochemistry of isosorbide. A similar behavior was reported for polyesters conformed by 1,3propanediol and isosorbide with succinic acid, where the succinic acid shifts were observable between 2.6 and 2.8 ppm [19]. Apparently, from PPeF85I10S15 down to PPeF30I50S70 with 50 mol % isosorbide, the characteristic peaks of 1,5-pentanediol (1.42-1.80 ppm) decrease as the isosorbide concentration increases [20], although a quantitative analysis should be performed to confirm the assumption.

GPC
Mn, Mw, and Ð were measured by GPC. Tables 1 and 2 show the results obtained for the isosorbide-based polyesters, whilst Figure 3 shows the chromatographs for the PPEF15IS85 and PPeF85IS15 families, respectively.

GPC
M n , M w, and Ð were measured by GPC. Tables 1 and 2 show the results obtained for the isosorbide-based polyesters, whilst Figure 3 shows the chromatographs for the PPEF 15 IS 85 and PPeF 85 IS 15 families, respectively.
The range of M w for the polyesters was between 700 and 10200 Da, whereas M n fell within 500 and 3100 Da, indicating the great influence that the addition of isosorbide imparts, which allows a great versatility within the properties of these biomass-derived polyesters. The results obtained suggest that in general, M n and M w decreased as the isosorbide content increased. The dispersity of PPeFIS fell within 1.5/2.4 which was close to the expected value of two for polyesters [62], whilst PPFIS was between 1.3-1.6, suggesting moderate polydisperse samples. The narrowness of the distributions with isosorbide concentrations from 10% to 30% (Figures 3 and 4) were an indication of the strength and toughness of the polyesters. On the other hand, the narrow peaks at higher reaction times were an indication of the presence of short, oligomeric species, and potentially residual isosorbide, as previously seen in the NMR spectra. The highest M n and M w were achieved with the furan rich polyesters PPeF 85 IS 15 and PPeF 70 IS 30 and the trend is followed as the FDCA content decreased, although PPeF 70 IS 30 had slightly higher molecular weight when the mol % isosorbide was between 30 and 50%. The incorporation of isosorbide was limited to 50 mol % for these two polyester families, as the mixture becomes extremely viscous and highly unprocessable above that concentration. The range of Mw for the polyesters was between 700 and 10200 Da, whereas Mn fell within 500 and 3100 Da, indicating the great influence that the addition of isosorbide imparts, which allows a great versatility within the properties of these biomass-derived polyesters. The results obtained suggest that in general, Mn and Mw decreased as the isosorbide content increased. The dispersity of PPeFIS fell within 1.5/2.4 which was close to the expected value of two for polyesters [62], whilst PPFIS was between 1.3-1.6, suggesting moderate polydisperse samples. The narrowness of the distributions with isosorbide concentrations from 10% to 30% (Figures 3 and 4) were an indication of the strength and toughness of the polyesters. On the other hand, the narrow peaks at higher reaction times were an indication of the presence of short, oligomeric species, and potentially residual isosorbide, as previously seen in the NMR spectra. The highest Mn and Mw were achieved with the furan rich polyesters PPeF85IS15 and PPeF70IS30 and the trend is followed as the FDCA content decreased, although PPeF70IS30 had slightly higher molecular weight when the mol % isosorbide was between 30 and 50%. The incorporation of isosorbide was limited to 50 mol % for these two polyester families, as the mixture becomes extremely viscous and highly unprocessable above that concentration.
The molecular weight as a function of FDCA/succinic acid composition is depicted in Figure 4 for copolyesters bearing 10 mol % isosorbide. The chromatograms for 30 and 50 mol % isosorbide are available in Supplementary Material Figures S4 and S5.
The copolyesters PPeF70I10S30 and PPeF85I10S15 with 10 mol % isosorbide presented Mw of 3800 Da and 5400 Da, respectively and the dispersities of both compositions with different mol % isosorbide were above two. These values represent the highest molecular weights among the copolyesters of FDCA and succinic acid with isosorbide, whereas the lowest Mw figures correspond to PPeF15IS85 and PPeF30IS70 with either 60 or 70 mol % isosorbide (1000 Da). In the case of PPeF15IS85 and PPeF30IS70, dispersities of two and Mw above 2000 Da were obtained when the isosorbide content was limited to 30%. and 3100 Da, indicating the great influence that the addition of isosorbide imparts, which allows a great versatility within the properties of these biomass-derived polyesters. The results obtained suggest that in general, Mn and Mw decreased as the isosorbide content increased. The dispersity of PPeFIS fell within 1.5/2.4 which was close to the expected value of two for polyesters [62], whilst PPFIS was between 1.3-1.6, suggesting moderate polydisperse samples. The narrowness of the distributions with isosorbide concentrations from 10% to 30% (Figures 3 and 4) were an indication of the strength and toughness of the polyesters. On the other hand, the narrow peaks at higher reaction times were an indication of the presence of short, oligomeric species, and potentially residual isosorbide, as previously seen in the NMR spectra. The highest Mn and Mw were achieved with the furan rich polyesters PPeF85IS15 and PPeF70IS30 and the trend is followed as the FDCA content decreased, although PPeF70IS30 had slightly higher molecular weight when the mol % isosorbide was between 30 and 50%. The incorporation of isosorbide was limited to 50 mol % for these two polyester families, as the mixture becomes extremely viscous and highly unprocessable above that concentration.
The molecular weight as a function of FDCA/succinic acid composition is depicted in Figure 4 for copolyesters bearing 10 mol % isosorbide. The chromatograms for 30 and 50 mol % isosorbide are available in Supplementary Material Figures S4 and S5.
The copolyesters PPeF70I10S30 and PPeF85I10S15 with 10 mol % isosorbide presented Mw of 3800 Da and 5400 Da, respectively and the dispersities of both compositions with different mol % isosorbide were above two. These values represent the highest molecular weights among the copolyesters of FDCA and succinic acid with isosorbide, whereas the lowest Mw figures correspond to PPeF15IS85 and PPeF30IS70 with either 60 or 70 mol % isosorbide (1000 Da). In the case of PPeF15IS85 and PPeF30IS70, dispersities of two and Mw above 2000 Da were obtained when the isosorbide content was limited to 30%. The molecular weight as a function of FDCA/succinic acid composition is depicted in Figure 4 for copolyesters bearing 10 mol % isosorbide. The chromatograms for 30 and 50 mol % isosorbide are available in Supplementary Material Figures S4 and S5.
The copolyesters PPeF 70 I 10 S 30 and PPeF 85 I 10 S 15 with 10 mol % isosorbide presented M w of 3800 Da and 5400 Da, respectively and the dispersities of both compositions with different mol % isosorbide were above two. These values represent the highest molecular weights among the copolyesters of FDCA and succinic acid with isosorbide, whereas the lowest M w figures correspond to PPeF 15 Figure S6. Unfortunately, the results for PPF 70 IS 30 and PPF 85 IS 15 are not available since the samples were insoluble in THF.
The decrease in M n as the isosorbide content increases has been reported in the literature [12,15,19,20,63]. One of the possible explanations was the decreased reactivity of the secondary OH groups when compared with the primary OH groups present in aliphatic linear diols, possibly corresponding to Polymers 2018, 10, 600 9 of 19 a lower acidic character [24], and most importantly, nucleophilicity. Nucleophilicity is affected by steric hindrance, since the bulkier a given nucleophile is, the slower the rate of its reactions and therefore the lower its nucleophilicity. In the case of isosorbide, although the nucleophilicity of the endo hydroxyl group is increased, the steric hindrance caused by hydrogen bonding makes the exo hydroxyl group more reactive [64]. Consequently, the difference in reactivity of the OH groups present in isosorbide was mainly due to their stereochemical nature-endo and exo-where the steric hindrance of the endo hydroxyl group is known to decrease the whole reactivity of the system [24]. The OH in endo position is more likely to form intra-molecular hydrogen bonding with the oxygen in main chains, while the other in exo position is more reactive in polycondensation reactions [65]. In addition, the endo hydroxyl is protected by the steric bulk of the rest of the molecule [66]. It might be worthwhile to do a kinetic study to explore different catalysts and reaction times and their influence on the final molecular weight of the polyesters. Wei et al. [20] showed that increasing the isosorbide content above 30% resulted in a significant decrease in the number average molecular weights of the polyesters. In fact, the M n of the homopolymer poly(isosorbide sebacate) was lower than 3000 Da. Sadler et al. [63] synthesized unsaturated polyesters of phthalic anhydride, maleic anhydride, ethylene glycol, and isosorbide. The polyesters had isosorbide contents from 10 to 25 mol %, and the molecular weight decreased accordingly from 7000 to 3500 Da. In the same vein, Noordover et al. [19] reported M n from 2000 to 3100 Da for polyesters of succinic acid and isosorbide, and M n from 2700 to 4600 Da with the addition of 1,4-butanediol or neopentyl glycol as the second diol monomer. Our results suggest that the incorporation of more than 50 mol % isosorbide when processing via azeotropic distillation is considerably detrimental to the molecular weight of the polyesters, even though a catalyst is added to the reaction mixture. This could be due either to changes in stoichiometry, so more hydroxyl functionality will reduce the molecular weight, or due to the isosorbide reacting as a mono-functional monomer. If the isosorbide content is kept between 10 and 30 mol %, the molecular weights are in the desirable range for coatings (i.e., 2000 to 6000 Da) [5,19].

DSC
The DSC scans for PPeF 30 IS 70 and PPeF 70 IS 30 are shown in Figure 5, whilst the T g , melting temperatures (T m ), and the melting enthalpies (∆H m ) are summarized in Table 4. The T g of the parent resins with no isosorbide is included as reference. The isosorbide content and glass transition temperature kept a linear relationship as expected. In all the different compositions, the T g increased as a function of the mol % isosorbide. Sadler et al. [63] demonstrated that adding as little as 10 mol % isosorbide to the reaction mixture in place of an equivalent amount of a linear diol resulted in a significant increase in T g . Moreover, adding 60 and 80 mol % isosorbide to resins based on succinic acid and neopentyl glycol achieved final glass transition temperatures from 30.5 • C and 47.1 • C, respectively [19].
Within the range of molecular weights of the resins and taking into account the diacid composition of the base polyester, a minimum content of 50 mol % isosorbide was needed in order to achieve glass transition temperatures above 0 • C and above room temperature. The copolyesters PPeF 70 IS 30 and PPeF 85 IS 15 exhibited the highest T g among the 1,5-pentanediol polyesters, achieving values of 31 • C and 22 • C, respectively. Surprisingly, PPeF 70 I 50 S 30 had higher T g (31 • C) than PPeF 85 I 50 S 15 (22 • C), with 50 mol % isosorbide, but when only 10 mol % was added, the difference in respect to the base polyester is either negligible (−11 • C for PPeF 70 I 10 S 30 and −10 • C for PPeF 70 S 30 ) or very little as per 85 mol % FDCA (4 • C, PPeF 85 I 10 S 15 and 1 • C, PPeF 85 S 15 ). Nonetheless, the slight difference might not be significant and could come down to the acid value that was processed.
For the succinic acid rich polyesters PPeF 15 IS 85 and PPeF 30 IS 70 , there is an abrupt increase in the glass transition temperature when the isosorbide content is 60% or above, going from values of approximately −40 • C up to 30 • C. This suggests that our library of polyester resins, based in three main monomers, would have different end properties just by tuning the desired content of isosorbide, expanding the potential applications of our renewable coatings. Figure 6 shows T g and M n as a function of isosorbide content for PPeF 70 IS 30 and PPeF 85 IS 15 . The corresponding M n -T g -isosorbide relationships for PPeF 15   For the succinic acid rich polyesters PPeF15IS85 and PPeF30IS70, there is an abrupt increase in the glass transition temperature when the isosorbide content is 60% or above, going from values of approximately −40 °C up to 30 °C. This suggests that our library of polyester resins, based in three main monomers, would have different end properties just by tuning the desired content of isosorbide, expanding the potential applications of our renewable coatings. Figure 6 shows Tg and Mn as a function of isosorbide content for PPeF70IS30 and PPeF85IS15. The corresponding Mn-Tg-isosorbide relationships for PPeF15IS85 and PPeF30IS70 are depicted in Figures S10 and S11, Supplementary Material. It is observed how Mn decreases as the isosorbide content increases, as explained in the GPC section.   Nevertheless, some other factors, such as impurities, chemical conformation, degree of crystallinity, and molecular weight could be relevant for the analysis of thermal transitions. Consequently, further work could be focused on the particular effect of any of the above factors for selected polyesters, potentially those with closer molar masses or bearing the greatest industrial feasibility.
The glass transition temperatures of the PPFIS family, prepared with 1,3-propanediol, are slightly higher since they present lower chain flexibility than their 1,5-pentanediol counterparts. The results are available in Supplementary Material Table S3. The inclusion of 30 mol % isosorbide leads to an increase of T g of about 30 • C (−45 • C to −14 • C) achieving a T g at 6.2 • C with 70 mol % isosorbide. The copolyesters PPF 70  Nevertheless, some other factors, such as impurities, chemical conformation, degree of crystallinity, and molecular weight could be relevant for the analysis of thermal transitions. Consequently, further work could be focused on the particular effect of any of the above factors for selected polyesters, potentially those with closer molar masses or bearing the greatest industrial feasibility.
The glass transition temperatures of the PPFIS family, prepared with 1,3-propanediol, are slightly higher since they present lower chain flexibility than their 1,5-pentanediol counterparts. The results are available in Supplementary Material  Figures S7 and S8.
Tailoring the different ratios of diacids and diols allowed to synthesize amorphous and semicrystalline polyesters. Regarding melting temperatures (Tm) 1,5-pentanediol polyesters with FDCA contents of 30% and above showed melting endotherms from 77.3 °C to 154.7 °C, whereas for 1,3-propanediol the only observable Tm's (97-131 °C) correspond to those containing 70 and 85 mol % FDCA. In the case of PPF85IS15 and PPeF30IS70, Tm decreased with an increase of the isosorbide content, a phenomenon that was previously observed in copolyesters of sebacic acid, 1,3-propanediol, and varying isosorbide content [12], whereas the other compositions exhibit the opposite trend.
Particularly, polyesters PPF70IS30 and PPF85IS15 did not show a single melting peak in the DSC heating scan, as observed in Figure 7 for PPF85IS15 and Figure S9 for PPF70IS30. This behavior has been reported in the literature [12,[67][68][69] for different copolyesters, and it is thought to be potentially the result of the existence of different crystal types in the same polymer sample [67], meltingrecrystallization-remelting processes [68,70,71], or the presence of different molecular weight species [12]. It is necessary to confirm the assumptions regarding crystallinity features by performing an X-Ray Diffraction (XRD) analysis of the polyesters. There is a strong possibility that the polyesters are conformed by different oligomers with varying molecular weights, as the presence of isosorbide tends to form cyclic oligomeric structures [19,72]. Again, the confirmation of these structures requires further robust analysis, such as MALDI-TOF-MS, although this is outside our current scope.
Previous syntheses of isosorbide-derived polyesters have reported glass transition temperatures in the range presented herein or below. For instance, the Tg's of poly(isosorbide sebacate) and poly(isosorbide itaconate) were 6.4 °C (Mn = 19200 Da) and 34.5 °C (Mn = 8700 Da), respectively [17]. Wei et al. [20] synthesized polyesters of 1,10-decanediol, sebacic acid and isosorbide with different mol % isosorbide, namely, 43.9%, 66.2%, and 100%. The polyesters showed Tg's at −26 °C, −18 °C, and −5 °C, with Mw = 8900, 8699, and 2800 Da, respectively. The authors did not observe glass transitions for 5.3 or 16.7 mol % isosorbide, which they attributed to the fact that long chain aliphatic polyesters have a strong crystallization capacity and crystallize very fast. Terzopoulou and coworkers [22] showed that poly(isosorbide-2,5-furanoate) presented a Tg at 157 °C, whilst the Tg's for copolyesters Tailoring the different ratios of diacids and diols allowed to synthesize amorphous and semicrystalline polyesters. Regarding melting temperatures (T m ) 1,5-pentanediol polyesters with FDCA contents of 30% and above showed melting endotherms from 77.3 • C to 154.7 • C, whereas for 1,3-propanediol the only observable Tm's (97-131 • C) correspond to those containing 70 and 85 mol % FDCA. In the case of PPF 85 IS 15 and PPeF 30 IS 70 , T m decreased with an increase of the isosorbide content, a phenomenon that was previously observed in copolyesters of sebacic acid, 1,3-propanediol, and varying isosorbide content [12], whereas the other compositions exhibit the opposite trend.
Particularly, polyesters PPF 70 IS 30 and PPF 85 IS 15 did not show a single melting peak in the DSC heating scan, as observed in Figure 7 for PPF 85 IS 15 and Figure S9 for PPF 70 IS 30 . This behavior has been reported in the literature [12,[67][68][69] for different copolyesters, and it is thought to be potentially the result of the existence of different crystal types in the same polymer sample [67], melting-recrystallization-remelting processes [68,70,71], or the presence of different molecular weight species [12]. It is necessary to confirm the assumptions regarding crystallinity features by performing an X-Ray Diffraction (XRD) analysis of the polyesters. There is a strong possibility that the polyesters are conformed by different oligomers with varying molecular weights, as the presence of isosorbide tends to form cyclic oligomeric structures [19,72]. Again, the confirmation of these structures requires further robust analysis, such as MALDI-TOF-MS, although this is outside our current scope.   Figure 8 shows the thermograms for the polyesters PPeF70IS30 and PPeF85IS15. Thermal gravimetric analysis indicates that the thermal stability ranges from 308 °C to 371 °C for the 1,5pentanediol library. Expectedly, PPFIS polyesters showed a slightly higher and narrower Td range between 362 and 372 °C, suggesting that the diol is a fundamental factor for thermal stability. Previous syntheses of isosorbide-derived polyesters have reported glass transition temperatures in the range presented herein or below. For instance, the T g 's of poly(isosorbide sebacate) and poly(isosorbide itaconate) were 6.4 • C (M n = 19200 Da) and 34.5 • C (M n = 8700 Da), respectively [17]. Wei et al. [20] synthesized polyesters of 1,10-decanediol, sebacic acid and isosorbide with different mol % isosorbide, namely, 43.9%, 66.2%, and 100%. The polyesters showed T g 's at −26 • C, −18 • C, and −5 • C, with M w = 8900, 8699, and 2800 Da, respectively. The authors did not observe glass transitions for 5.3 or 16.7 mol % isosorbide, which they attributed to the fact that long chain aliphatic polyesters have a strong crystallization capacity and crystallize very fast. Terzopoulou and coworkers [22] showed that poly(isosorbide-2,5-furanoate) presented a Tg at 157 • C, whilst the Tg's for copolyesters of FDCA, isosorbide, and 1,4-cyclohexane-dimethanol (CHDM) increased from 75.3 to 103.5 • C, with decreasing CHDM/isosorbide ratios from 95/5 to 60/40 [21]. Figure 8 shows the thermograms for the polyesters PPeF 70 IS 30 and PPeF 85 IS 15 . Thermal gravimetric analysis indicates that the thermal stability ranges from 308 • C to 371 • C for the 1,5-pentanediol library. Expectedly, PPFIS polyesters showed a slightly higher and narrower T d range between 362 and 372 • C, suggesting that the diol is a fundamental factor for thermal stability. Previous studies have shown that the thermal stability decreases with increasing methylene units. For example, the Bikiaris group [43] showed that poly(1,4-butylene furanoate) (PBF) was less thermally stable than the furanoates synthesized with shorter chain diols, such as ethylene glycol and 1,3-propanediol.

Coatings
Tables 5 and 6 summarize the results obtained. For succinic acid-rich polyesters, the chosen samples were those with higher isosorbide concentrations. A reference, petro-derived coating R has been included for comparison. Specifically, in the case of 1,3-propanediol resins, it was not possible to make paints with PPF15I70S85 and PPF30I70S70 (70 mol % isosorbide) as they were immiscible with the paint solvent system, which consisted of a 50:50 mixture of butanol and dibasic ester. In the case of For both linear aliphatic diols, the incorporation of isosorbide to the main polymers resulted in a decrease of approximately 30-40 • C in the maximum T d of the resins, which were close (~385 • C) or above 400 • C. Within each diacidcomposition, the results suggest that the PPeFIS polyesters with highest mol % isosorbide present the highest decomposition temperature. However, the same trend was not observed for PPFIS, where all the polyesters had similar T d values.
Specifically, for 1,3-propanediol, the thermogravimetric data suggests the presence of diverse species within the polymer matrix, as three different decomposition temperatures (T d1 , T d2 , and T dmax , Table S5) are observed. All the diacid compositions present the first decomposition temperature between 98.7 • C and 194 • C, followed by a second transition at 180-290 • C. The lowest T d 's appear to be prompted by isosorbide concentrations of above 50 mol %. The nature of these species has not been determined; however, previous research has shown that polyesters undergo decomposition mechanisms initiated by scission of an alkyl-oxygen bond, suggesting a random-chain scission [73]. Also, the decomposition of these polyesters is suggested to be dominated by cyclic or open chain oligomers with carboxylic-end groups. The formation of these cyclic oligomers is done through an intramolecular exchange reaction which happens below 300 • C [74] which could be the same process that took place in our polyesters. Cyclization of isosorbide-based polyesters has indeed been reported by other authors [15,19]. In the case of polyesters with low FDCA and rich isosorbide contents, data suggests the formation of more volatile products, which some authors have determined to arise from the secondary breakdown of end-groups, which follows the primary step of cyclic concerted decomposition [75].

Coatings
Tables 5 and 6 summarize the results obtained. For succinic acid-rich polyesters, the chosen samples were those with higher isosorbide concentrations. A reference, petro-derived coating R has been included for comparison. Specifically, in the case of 1,3-propanediol resins, it was not possible to make paints with PPF 15 I 70 S 85 and PPF 30 I 70 S 70 (70 mol % isosorbide) as they were immiscible with the paint solvent system, which consisted of a 50:50 mixture of butanol and dibasic ester. In the case of PPF 70 I 50 S 30 , the resin was a brittle solid, which prevented solution in the same paint solvents. As similarly reported for some coating applications [14,19,24,63], the introduction of isosorbide into the synthesis of the polyester resins improved the thermomechanical properties of the resulting paints, compared to the parent polyesters. The no-isosorbide parent polyesters, PPeF1 5 S 85 and PPeF 30 S 70 , had very low microhardness (11 N·m −2 and 12 N·m −2 , respectively) and low T g (−28 • C and −17 • C, respectively), but when 70 mol % isosorbide was added (PPeF 15 I 70 S 85 and PPeF3 0 I 70 S 70 ) the microhardness abruptly increased to 237 N·m −2 and 287 N·m −2 , with T g at 66 • C and 53 • C. The resins were not very flexible as T-Bend was 5T and 6T and both presented severe cracking after the Erichsen testing, which suggested that the concentration of FDCA should be increased. We previously reported the effect of the FDCA on the final polyester resin [2]. PPeF 70 I 30 S 30 with an isosorbide concentration of 30 mol % and an increased FDCA concentration of 70% showed an improvement in flexibility (2.5T) respect to the reference resin (3T), T g on specification at 34 • C and better impact resistance as it presented a slight cracking. Moreover, PPeF 70 I 50 S 30 had above-specification Erichsen (No cracking), T g (67 • C) and microhardness (270 N·m −2 ) than the reference resin. Likewise, comparing PPeF 70 I 50 S 30 with its parent resin with no isosorbide, the performance was considerably improved, as the properties recorded for the latter were T g = 6 • C, Microhardness = 20 N·m −2 and T-Bend no cracking of 1.5T.
PPeF 85 IS 15 resins again showed better microhardness and higher T g than PPeF 85 S 15 , as the results were 21 N·m −2 and 15 • C, respectively. PPeF 85 I 50 S 15 (Table 6), despite having the highest T g (69 • C) and enhanced microhardness (299 N·m −2 ) was slightly harder (Pencil Hardness 2H) and presents poor flexibility measured by T-Bend no cracking (5.5T). PPeF 85 I 30 S 15 therefore presented the best overall properties ( Table 6).

Conclusions
We successfully diversified and enhanced the properties of novel biomass-derived polyester coatings based on FDCA, succinic acid, and either 1,3-propanediol or 1,5-pentanediol by incorporating isosorbide into the polyesters' backbone. Depending on the concentration of isosorbide, the molecular weight of the polyesters could be tuned from 700 to 10,200 Da. In general, the molecular weight decreased as the isosorbide content increased. This behavior could be attributed to the difference in reactivity of the OH groups present in isosorbide. The thermal results suggested that a minimum 50 mol % isosorbide is needed to achieve glass transition temperatures above 0 • C and above room temperature. The T g could vary approximately 40 degrees by solely adjusting the molar isosorbide concentration.
Paint testing results showed that the best isosorbide resins for coil coating applications were PPeF 85 I 30 S 15 and PPeF 70 I 30 S 30 , along with PPF 85 I 30 S 15 . The consideration of bioderived 1,5-pentanediol as a main building block opens the possibility towards the development of flexible coatings but with better hardness than currently used diols, such as 1,6-hexanediol, enhanced by the presence of FDCA and isosorbide.
The inclusion of the carbohydrate-derived diol isosorbide to our coatings did promote better performance in terms of thermomechanical properties, and allowed for the tuning of fully biomass-derived resins that could easily replace the current petrochemical-derived ones in different applications, with controlled molecular weights and glass transition temperatures. Exterior durability testing for top coats would still be needed, but our bio-derived coatings could be used as backing coatings and interior finishes. Future work however needs to be done in terms of the variables effects on the T g, the identification of the nature of the different oligomers or cyclic structures formed during the polymerization, as well as a kinetic study to optimize the reaction conditions to overcome the isosorbide's low-reactive nature.