3.1. Synthesis of Plasticizers
The plasticizers were prepared in three steps. In the first step, esterification of propylene glycol with oleic acid was carried out to obtain as much propylene glycol monooleate as possible. This reaction is not selective and in addition to the expected propylene glycol monoesters, propylene glycol and oleic acid diesters were also formed. The obtained esters were analyzed by GC/MS and GC/FID (
Table 3) as well as FT–IR (
Figure 2).
The first confirmation of the esterification of propylene glycol with oleic acid was the shift of the C=O stretching vibration peak at 1709 cm
−1 in oleic acid to 1739 cm
−1 in the reaction product (
Figure 2). The strong absorbance band at 1739 cm
−1 is characteristic of C=O stretching vibration in esters. The formation of an ester bond in the products of the first synthesis step was also indicated by the presence of an absorbance band at 1176 cm
−1 corresponding to a stretching C–O bond in the ester group. On the FT–IR spectra of the product of the first synthesis step, an absorbance band was also present at 3405 cm
−1, corresponding to the stretching vibration of the OH group. This was expected since the main synthesis product is propylene glycol monooleate. However, the intensity of this band was significantly lower compared to the absorbance band corresponding to the stretching vibration of the OH group at 3324 cm
−1 on the FT–IR spectra of propylene glycol. The FT–IR spectra of both oleic acid and the first step product (
Figure 2) showed absorption bands at 3005 cm
−1 and at 3004 cm
−1, respectively which correspond to C–H stretching and deformation vibrations in unsaturated –HC=CH– bonds. This result was anticipated since monounsaturated fatty acid was used for the synthesis. The chemical composition of the obtained products was then determined.
Table 3 shows the chemical compositions of the analyzed esterification products after the first step of synthesis. The identified components in
Table 3 are ordered by their retention times. When planning this research, it was taken into account that in the obtained esterification products, besides the expected monoesters, propylene glycol diesters will also be present in certain amounts. The reaction mixture after the first synthesis step contained 58.4 wt.% of propylene glycol monooleate, 25.0 wt.% of propylene glycol dioleate, and 10.4 wt.% of unreacted propylene glycol. Very small amounts (0.4 wt.%) of dipropylene glycol were also formed during the reaction. The components described as volatile medium-molecular-weight components and volatile high-molecular-weight components are most likely propylene glycol monoesters and diesters of fatty acids other than oleic acid, respectively. Their presence is due to the fact that oleic acid tech. 90.0% was used for the synthesis.
The presence of propylene glycol dioleate may be an additional advantage since these compounds have a symmetrical linear structure with a long hydrocarbon chain on each side (
Figure 3).
Figure 3 also shows the molecule of the epoxidized diester. This is due to the fact that the last step of the discussed synthesis was the epoxidation step. This means that in the final product, propylene glycol dioleate will also contain oxirane rings in its structure. Both structures (
Figure 3) can act as effective plasticizers and will be important components of the final plasticizer mixture.
For the second step of the synthesis, the reaction mixture obtained in the first step was used. The aim of this step was to obtain the highest possible yield of succinic acid, propylene glycol, and oleic acid esters. For this purpose, four samples of the plasticizer, differing in the ratio of COOH groups (derived from succinic acid) to OH groups (derived from propylene glycol and propylene glycol monoester) were synthesized. The amount of succinic acid was calculated according to the mentioned proportion and was based on the data presented in
Table 3. The esterification reaction was carried out in the same manner as in the first step. The reaction mixture was analyzed by GC/MS and GC/FID gas chromatography.
Table 4 shows the chemical compositions of the analyzed esterification products after the second synthesis step.
The data in
Table 4 indicate that almost all succinic acid reacted. Depending on the synthesis variant, its content in all products was very low: 0.0 wt.% (Sample 1), 0.1 wt.% (Sample 2), 0.2 wt.% (Sample 3), and 0.3 wt.% (Sample 4). However, the amount of succinic acid introduced into the reaction mixture clearly affected the chemical composition of the obtained products. The higher the amount of succinic acid used, the lower the content of unreacted propylene glycol in the reaction mixture. In Sample 1, where the least amount of succinic acid was used, the propylene glycol content in the product was 3.1 wt.%, while in Sample 4, where the highest amount of succinic acid was used, the propylene glycol content in the product was only 0.3 wt.%. The same correlation was observed for dipropylene glycol, which decreased to 0.1 wt.% in Sample 4. In addition, very small amounts of volatile low-molecular-weight components (0.5–0.1 wt.%) indicated that the obtained plasticizers contained small amounts of compounds that could easily migrate from the polymer matrix.
As expected, the second step reaction products contained esters of oleic acid and propylene glycol. However, it is worth nothing that the introduced amount of succinic acid affected not only the content of oleic acid monoesters but also diesters. In the variant in which the molar ratio of COOH groups to OH groups was 0.3: 1.0 (Sample 1), the content of propylene glycol monooleate was 38.2 wt.% and that of propylene glycol dioleate was 36.3 wt.%. On the other hand, in the variant in which the molar ratio of COOH groups to OH groups was 0.9:1.0 (Sample 4), the content of propylene glycol monooleate was 9.3 wt.% and that of propylene glycol dioleate was 26.2 wt.%. On this basis, it may be concluded that under the applied synthesis conditions, propylene glycol monooleate undergoes an esterification reaction with succinic acid, but it may also be assumed that propylene glycol mono- and dioleate undergoes a transesterification reaction with succinic acid. This was confirmed by the high content of free oleic acid, e.g., 8.8 wt.% in Sample 4 or 4.3 wt.% in Sample 3.
The main purpose of this synthesis step was to obtain succinic acid esters. According to the data in
Table 4, the total content of succinic acid, propylene glycol, and oleic acid mixed esters was 11.6 wt.% (Sample 1), 12.0 wt.% (Sample 2), 15.2 wt.% (Sample 3), and 14.0 wt.% (Sample 4). At first glance, these appears to be relatively low values. However, it may be assumed, that under the applied synthesis conditions, succinic acid and propylene glycol may undergo an oligomerisation reaction resulting in oligoesters of propylene glycol and succinic acid in the reaction products. This was indicated by the high content of non-volatile components in the obtained products. The higher the amount of succinic acid used in the synthesis, the higher the content of non-volatile components: 2.0 wt.% (Sample 1), 22.9 wt.% (Sample 2), 18.0 wt.% (Sample 3), and 36.0 wt.% (Sample 4). In addition to non-volatile compounds, the plasticizer mixture also contained volatile medium- and high-molecular-weight components. The content of volatile medium- and high-molecular-weight components ranged from 2.3 wt.% to 1.1 wt.%, and they occurred near the retention time of propylene glycol monooleate and are most likely monoesters of fatty acids other than oleic acid. Their content was lower in response to more succinic acid used for synthesis. In turn, the volatile high-molecular-weight components listed in
Table 4 are the components near the retention time of the propylene glycol dioleate and succinic acid, propylene glycol, and oleic acid mixed ester region. In this case, we suspect that these are analogous esters of fatty acids other than oleic acid.
These components have similar properties to the corresponding oleic acid esters. Their presence in certain amounts should not significantly influence the properties of obtained mixtures as PVC plasticizers.
The third step of synthesis was the epoxidation reaction, which was performed for each plasticizer sample in the same way, oxidizing the unsaturated bonds derived from oleic acid with performic acid. The third step was carried out to introduce oxirane groups into the plasticizer structure. The composition of the mixture after the third stage did not change; only unsaturated bonds were oxidized, which was confirmed by the determination of the oxirane value. The products obtained in the particular variants differed significantly in chemical composition, but all of their components have the potential to act as PVC plasticizers. In the next part of this work, it will be verified if their composition is an effective plasticizer.
The structures of the obtained esters (Samples 1–4) were confirmed by FT–IR spectroscopy (
Figure 4).
The FT–IR spectra for all obtained plasticizers (Sample 1, 2, 3, and 4) were very similar, which is understandable since the individual samples differed only in the amount of succinic acid added in the second step of the synthesis. On the FT–IR spectra of Samples 1–4, at 1736 cm
−1, there was a strong absorbance band corresponding to the stretching vibration of the double carbonyl bond (C=O) in the ester group, at 1158 cm
−1, there was a strong absorbance band indicating a stretching C–O bond in the ester group, and at 1081 cm
−1, a stretching C–O–CH2– bond occurred in the ester group. This confirms that the esterification reaction occurred in all reaction routes. The presence of a stretching C–O bond corresponding to the oxirane group at 835 cm
−1 and the absence of deformation vibrations in the unsaturated –HC=CH– bond indicated that the epoxidation reaction was successful for all synthesized samples. The FT–IR spectra also showed weak O–H stretching vibration bonds of the alcohol group (
Figure 4, Samples 1, 2, 3, and 4). The presence of this band corresponds to the hydroxyl groups in the unreacted diol used for the synthesis of the esters, as well as from the obtained monoesters. The presence of free propylene glycol and propylene glycol monoesters in the analyzed samples was also confirmed by GC analysis.
At 2925 cm
−1 and 2856 cm
−1, C–H stretching vibrations bands were present, and at 1460 cm
−1, 1378 cm
−1, and 723 cm
−1, C–H deformation vibrations bands appeared, indicating hydrocarbon chains. The small band at 1115 cm
−1 may be attributed to stretching vibrations in ethers because propylene glycol can undergo oligomerization reactions under esterification conditions, which was also confirmed by GC analysis (
Table 4).