*3.3. TG/DTG Analysis*

Thermo-gravimetric analysis provides essential information on the relative thermal stability of the analyzed compounds, the content of water or other volatile ingredients of synthetic or natural materials [48]. From thermo-analytical curves, recorded for the synthesized esters in the range of 25 and 600 ◦C in nitrogen atmosphere, the inflection points and the total weight loss at 600 ◦C (Table 3) indicate that the method used does not affect the physicochemical properties of the obtained esters.


**Table 3.** The inflection points in the TG analysis.

All of the obtained bioesters present low weight loss until 200 ◦C (lower than 3.5%). Therefore, the products exhibited good thermal stability below 200 ◦C so they may be used in technologies where such property is required, for example as natural adjuvant and surfactant in increasing crop yields with lowering costs. Above 300 ◦C, the weight loss is more important as the temperature increases.

For all studied bioesters the total weight loss appears around 600 ◦C, and the percentage of the residual mass remaining is insignificant. Three decomposition steps were observed. The first step corresponds to the water loss, are max. 3.5% (Table 3).

The observed mass loss values are similar and are not affected by the nature of the synthesis method. The second decomposition step (between 210 ◦C and 350 ◦C) and the third step (between 350 ◦C and 550 ◦C) were associated with the destruction of esteric group, the C-C and the C-H bonds. The decomposition process presents two inflection points at T1 and T2, associated with the highest decomposition rates.

Nitrogen atmosphere thermal stability analysis of methyl and ethyl esters of soybean oil [49] showed that they have a lower thermal stability compared to the samples synthesized by us by the three obtaining methods.

Comparable results were also obtained for erythritol tetra myristate and erythritol tetra laurate esters [50] or lubricants of the type gallate ester oils, respectively [51].

In contrast the polyester amides series [52] and the esters obtained by transesterification of palm oil-based methyl ester to trimethylolpropane esters [53] show better thermal stability compared to our samples.

Figure 2 presents an example of the degradation process of the B3 product, in nitrogen atmosphere.

**Figure 2.** TG/DTG curves of bioester B3, in nitrogen atmosphere.

#### *3.4. GS-MS Analysis*

GS-MS spectra (Figures 3–5) are also presented for the B3 bioester. The chromatogram indicates the presence of two main reaction products. The major compound was separated in the analysis conditions at the retention time of 19.15 min. Its peak had the highest intensity from the whole GC chromatogram. The second reaction product, pentyl palmitate, was eluted in the GC chromatogram at the retention time of 17.70 min (Figure 3).

**Figure 3.** The chromatogram of the reaction mixture resulted from the esterification reaction in the case of B3 bioester.

**Figure 4.** The mass spectrum of the ester, corresponding to the peak separated on the GC column at 19.15 min retention time (M = 348 g/mol), in the case of B3 bioester.

**Figure 5.** The mass spectrum of the ester, corresponding to the peak separated on the GC column at 17.70 min retention time (M = 326 g/mol), in the case of B3 bioester.

In the Figure 4, the molecular peak at *m*/*z* 348, as expected for the main product, was observed with very low intensity, mainly due to its fragmentation. From the MS spectrum it is obvious that the structure of the analyzed compound that the structure is most probably (9Z,12Z,15Z)-octadeca-9,12,15-trienoate. This is proved because the first fragmentation of this compound is expected to occur at carboxyl group, resulting in the signal observed at 262.2 *m*/*z*. Therefore, this signal belongs to the fragment remained after losing thepenthoxy group.

Furthermore, on the region of lower mass values, several differences of around 14–15 *m*/*z* units were observed. This proved that CH3 and/or CH2 fragments are present, either from penthyl or from octadeca-9,12,15-trienoate or even from both.

The MS spectrum of the compound which eluted first (see Figure 3) at 17.7 min, but with a lower intensity, is presented in Figure 5. From this MS spectrum (Figure 5) the molecular peak at 326 *m*/*z* can be easily observed, with a better intensity. Moreover, in the same mass spectrum presented in Figure 5, the signals observed at 257.2 and 239.2 *m*/*z* indicated the presence of palmitate fragments. Nevertheless, the peak observed at 257.2 *m*/*z* showed also the highest intensity from the whole MS spectrum of pentylpalmitate.

This proved that palmitate fragment had a higher ionization in the used ion source, in comparison with the other fragments formed during MS analysis. The signal obtained at 87.1 *m*/*z* proved the presence of penthoxy fragment, as for the previous analyzed ester. Moreover, in this region of the mass spectrum, were also observed more signals, at 71 *m*/*z*, 70 *m*/*z* and 69 *m*/*z* respectively. All of those peaks belong to pentyl fragments.

#### *3.5. FTIR Analysis*

The FTIR is often used in polymer analysis [54,55]. The FTIR spectrum of the bioester B2 (Figure 6) reveal peaks at 2926 cm−<sup>1</sup> and 2855 cm−1. Those peaks can be attributed to

the stretching of C-H bonds. At 1736 cm−<sup>1</sup> the peaks are attributed to the stretching of C=O, typical of esters spectra [56]. The =C–H and C=C bands appear at 3012 cm−<sup>1</sup> and 1657 cm<sup>−</sup>1. The peak at 1465 cm−<sup>1</sup> correspond to the asymmetric stretching of -CH3 present in the biodiester. This peak is absent in soy oil spectrum. The peak at 1358 cm−<sup>1</sup> was attributed to the O-CH2 group and the peak at 1173 cm−<sup>1</sup> is corresponding to the stretching of O-CH3. Nevertheless, the peak at 1049 cm−<sup>1</sup> was to attributed in plane deformation vibration of =C-H bond and the peaks at 926 cm−<sup>1</sup> and at 725 cm−<sup>1</sup> are attributed to the C–H wagging bond vibration. All of these proved also the absence of alcohol impurities.

**Figure 6.** FT-IR analysis of the bioester B3.

#### *3.6. Color Study*

For the bioester B3 color study was also performed. The ester was introduced in different concentrations (0.1–2%) in an acrylic resin—that is often used in film industry [57,58]. The film was then deposited on a cellulosic support (wood). Color properties reveal that the reflectance increased with the ester concentration (Figure 7) according to other studies [59,60].

For all dried films, color *CIEL\*a\*b\** parameters were determined: lightness *(L\*)*, redness *(a\*)*, yellowness *(b\*)*, chroma or saturation *(C\*)* and hue angle *(h\*)* respectively.

The *CIEL\*a\*b\** color properties (Figure 8) reveal that the ester acrylic composition is in the light yellow-olive domain, i.e., the *a\** parameter is in the light green domain and the *b\** parameter is in the yellow domain. As expected, the film darkness increasing with the ester concentration. The same result were presented in scientific literature [18,60,61].

The total color difference Δ*E\*ab* may be calculated with the Equation (1) [17,62,63], results being presented in Table 4.

$$
\Delta E\_{ab}^\* = \sqrt{(\Delta L^\*)^2 + (\Delta a^\*)^2 + (\Delta b^\*)^2} \,\,\,\tag{1}
$$

**Figure 7.** Reflectance spectra of the bioester B3 of the ester acrylic film at different ester concentration.

**Figure 8.** *CIEL\*a\*b\** properties of the bioester B3 of the ester acrylic film at different ester concentration.



#### *3.7. Rheology Comparative Study of the Bioesters*

An important parameter that can remarkably influence the rheological properties of all fluids is temperature. Rheology measurements for all esters obtained in this research reveal a non-Newtonian behavior at the temperatures where their viscosity was measured (25–70 ◦C) and at different share rates (1333–13,333 s−1) [64]. The characteristic equation of Ostwald de Waele model (Equation (2) may be used for interpretation. All bioesters submitted to the rheology tests present a decrease of viscosity when increasing temperature. The same behavior was reported in other papers [18,65,66]. Although the allure of the curves is similar, the viscosity is higher as the number of the carbon atoms increases, exemplified for esters B1–B3 in Figure 9.

*η<sup>a</sup>* = *Kγn*−1, (2)

where: *K*—the index of consistency, Pa·sn; *n*—the flow behavior index.

**Figure 9.** The dependence of the apparent viscosity on temperature for bioesters (B1–B3).

The pseudoplastic flow behavior is proved both by the decrease of the apparent viscosity with the increase of shear rate (shear-thinning behavior) [67], as well as by the sub-unit values of the flow index *n* (Table 5). The decreasing of the apparent viscosity with increasing temperature is also evidenced by the variation of the consistency index *k* correlated with the value of the flow index *n*.

Power law model R<sup>2</sup> values indicate that *k* and *n* values are a good fit. It was observed an increase of consistency coefficient (*k*) with the increase of carbon atoms number in alkyl group from alcohol. As expected, the increase in temperature reduces the consistency coefficient (*k*) and increase the flow behavior index (*n*) and at the same temperature, the decrease of consistency coefficient (*k*) is more evident for ester with long alkyl chain in alchool (B3). The modification of k with temperature revel some influence of Brownian movement. Regardless of the temperature and of the obtaining method, the apparent viscosities of the prepared with n-propyl alcohol (B1, M1, C1), at the same shear rate are very close to one another. For the esters prepared with *n*-butyl alcohol (B2, M2, C2) or *n*-pentanol (B3, M3, C3) the viscosities became more different, their values depending on the technology used (Figure 10).

*Polymers* **2021**, *13*, 4190


**Figure 10.** The dependence of the esters apparent viscosity on the share rate.

They can act as thickening agents. According to rheological results, addition of bioester B2 as ingredient for paints, plastics, fibers, detergents, cosmetics and lubricants, is able to modify easily the rheological data and thicken the formulations. The temperature increase leads to micro drops mobility intensification, which influences the activation energy of the system. The phenomenon may be explained by an Arrhenius type equation (Equation (3)):

$$
\eta\_a = A' \cdot \exp^{Ea/RT},
\tag{3}
$$

where: *Ea* is the activation energy of viscous flow, (J/mol); *R* is the gas general constant, (J/mol·K); *T* is absolute temperature and *A* represents the material constant, (Pa·s).

The dependence ln *η<sup>a</sup>* = f(*1/T*) was graphically represented, as obtained from the logarithmic form of Equation (3), for apparent viscosity values corresponding to the three chosen values of the shear rate. Particular expressions of Equation (3) as well as the values of the activation energy are presented in Table 6. The activation energy decreases with the increasing of the shear rate, due to the increase of the turbulence and its effect on the linearization tendency of the molecules, considering the reduction of the degree of association of the molecules as well. In both cases, the effort to move molecules is diminished. The activation energy of the bioesters varies in opposite direction with their


**Table 6.** Particular types of the Arrhenius equation for all esters.

#### **4. Conclusions**

Table 6).

The synthesis of a series bioesters, using soybean oil fatty acids as the acid component, through three different technologies (in a bubble column reactor, in a reactor heated in a microwave field, and in a classic batch reactor) was performed. Energetic evaluation of the processes pointed out that the processes in the microwave field and in the bubble column reactor are more energetic efficient then in the classic batch reactor. The physicochemical and thermal properties of all esters were determined, and they present similar properties, regardless of the used synthesis routes. Rheological comparative study shows a pseudoplastic behavior for all esters. Equations of dependence of share stress on share rate, and of apparent viscosity on 1/T are proposed. Activation energy was determined for all samples and revealed an opposite variation with the bioesters viscosity when increasing the number of the carbon atoms in the alcohols from the constitution of esters. It should be noted that the results obtained herein can contribute to the development of new applications containing esters or to the synthesis of biopolymers using the low energy consumption and environmental friendly technologies.

viscosity when increasing the number of the carbon atoms in alcohols (Figure 10 and

**Author Contributions:** Conceptualization, S.P., D.J., S.B. and G.M.; methodology, S.P., A.T., V.S., D.J., S.B. and G.M.; software, S.P., S.B. and G.M.; validation, S.P., A.T., V.S., D.J., S.B. and G.M.; formal analysis, S.P., A.T., V.S., D.J., S.B. and G.M.; investigation, S.P., S.B. and G.M.; resources, S.P., S.B. and G.M.; data curation, S.P.; writing—original draft preparation, S.P., D.J. and S.B.; writing—review and editing, S.P., V.S. and G.M.; visualization, S.P. and S.B.; supervision, S.P.; project administration, S.P and D.J.; funding acquisition, S.P and D.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was supported by the Romanian Ministry of European Funds, Managing Authority for the Increasing Economic Competitiveness Operational Program, grant POS CCE Grant No. PO102418 12/5124/22.05.2014, SMIS 50328, New energetic efficient technologies for some polyesteric copolymer synthesis.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All the experimental data obtained are presented, in the form of table and/or figure, in the article.

**Conflicts of Interest:** The authors declare no conflict of interest.
