1. Introduction
Every year, around 300 million tons of plastics are produced in the world [
1], most of it made from petrochemical raw materials. Plastics, however, due to their petrochemical nature, may take many centuries to degrade, which has become a well-known problem for the environment and health of both animals and plants, pressing industries, people, and governments to search for less harmful alternatives [
2,
3]. Among the kinds of plastics produced and consumed yearly, polyurethanes (PU) are one of the most versatile, being possible to make into soft foams, rigid foams, films, adhesives, and many other applications. Despite PUs being usually mostly made from petrochemical raw material, they have the advantage of being easily produced from bio-renewable compounds, such as vegetable oils [
4].
Vegetable oils are usually seen as promising replacements for petrochemical materials as they are readily available from many different sources, and with variable structural features. Most oils are easy and cheap to obtain and process [
5,
6]. They are particularly useful for the synthesis of PUs and constitute one of the two main reagents for the synthesis of polyols. There are many different synthetic routes for vegetable oil-based polyols, leading to different PUs [
7]. Macauba kernel oil (MKO) is one of the two main oils that can be extracted from the macauba (
Acrocomia aculeata) fruit. Macauba is the fruit of the macauba tree, found throughout South America, especially in Brazil. Macauba cultivation is still not as well-studied and optimized as other more common crops, such as soybeans, and is usually done by family farming with the fruits being picked up after they fall from the trees (palms that grow up to 30 m tall) [
8]. Macauba oil is rich in short- to medium-chain fatty acids and is mostly used in the biofuel industry, with health benefits being studied due to its content of carotenoids and tocopherols.
Epoxidation is a well-studied chemical modification for vegetable oils, and the epoxidation of MKO was studied in a previous work of the authors where the oil was epoxidized to study the effect it would have on its physical chemical properties [
9]. The work has shown an increase of approximately 1.8% in density, 12 °C in
Tonset and 74.92% in oxidative stability caused by the epoxidation of the MKO. This paper can be seen as a continuation of that work.
In contrast to MKO, macauba pulp oil (MPU) is more commonly used and studied, probably due to the kernel of the fruit being confined inside a hard endocarp, making it hard to access. While MPU is rich in oleic acid (~69%), MKO is largely made from saturated fatty acids, with lauric acid usually making up from 32.58% to 44.14% of its composition, and oleic acid representing 18.7–36.27% of the fatty acid chains [
10,
11]. The lower unsaturation index of MKO makes it harder to modify and functionalize than MPU. Despite being usually neglected for oil production, the macauba kernel is actually richer in oil than the pulp (40–50% oil in the kernel versus 28–30% in the pulp) [
12]. Additionally, the leftover cake after oil extraction from the kernel is nutritious, containing about 17.73% in proteins and 41.48% in fibers [
13], which makes it a higher-quality feed for animals and humans in comparison to pulp cake (4.11% protein and 36.73% fibers) [
14]. A more in-depth study of macauba can add value to the fruits, generating more income for the families that make a living from this crop [
15].
Glycerol (or propane-1,2,3-triol) is a tri-functional alcohol that is present in all vegetable oils as the backbone of their triglyceride molecules and can be easily obtained as a byproduct of the transesterification of vegetable oils during the production of biodiesel. The rapid increase in demand for biofuels, caused by the rise in environmental awareness, created such a big surge in glycerol production that its usual applications, such as cosmetics, animal feed, fermentation, and food packaging cannot keep up with it. In 2021, approximately 2.8 billion liters of glycerol were produced by only five of the biggest biodiesel producers in the world [
16]. Global glycerol production will most likely keep on increasing for the foreseeable future, partly due to governmental efforts to popularize biodiesel (e.g., the PROBIODIESEL program in Brazil, which mandates that ever-increasing proportions of biodiesel should be mixed with regular diesel, starting in 2008 with 3% and aiming at 20% by 2030). Despite the already mentioned supply of glycerol having already reached the order of 2.8 billion liters in 2021, it is estimated that the demand for glycerol in 2025 will be only about 4 million tons. This drastic difference can lead to massive price drops [
17]. Other than the environmental interest in using glycerol for this reaction, using glycerol to open the epoxide ring makes it possible to add three -OH for each unsaturation, in contrast to only two obtained when using other mono-functional nucleophiles. Therefore, the use of glycerol as a ring-opening agent promotes the conversion of an oil with a low degree of unsaturation, such as macauba oil, into a suitable polyol for polyurethane synthesis.
The excessive glycerol supplies sparked research interest in its valorization. Ben et al. [
16] reviewed the pros and cons of using glycerol as a plasticizer in starch films for food packaging, noting how the hydrophilic nature of the molecules interrupts hydrogen bonds between starch molecules, working as a lubricant between them. Calderon et al. [
18] used glycerol acetate and coconut oil to synthesize thermoplastic PUs. While the integration of glycerol in polymers by itself is not new, using glycerol as an epoxide ring opener, the simple and straightforward way to produce polyols used in this work, is still novel. Polyol can be produced from a vegetable oil by the epoxidation of its oil, followed by the opening of the epoxide ring by water in a one-step reaction. The work has for objective the use of polyol synthesized by ring-opening reaction of epoxidized macauba kernel oil (MKO), employing glycerol as the ring-opening agent for the production of a novel polyurethane to be used as the polymeric matrix of thermally treated wood composites. The composites will be presented and discussed in a separate article. The PUs produced in this work, despite having different NCO/OH ratios, presented similar results in relation to the analysis by Fourier-transform infrared spectroscopy (FTIR), thermogravimetric (TG) and dynamic-mechanical analysis (DMA), especially the PU 1.0 and 1.2 samples. Their derivative thermogravimetric (DTG) curves, however, were quite different, with PU 1.0 showing a DTG profile with the usual three degradation peaks of PUs (urethane bond, rigid segments and soft segments) [
19], while PU 0.8 and PU 1.2 presented five degradation peaks, which were likely caused by the uneven quantities of NCO and OH creating a higher density of crosslinking in these samples, which results in segments of intermediate hardness and softness degrading at different temperatures.
2. Materials and Methods
2.1. Materials
MKO was purchased from Central do CerradoTM, Brasília, Brazil. Formic acid 85%, hydrogen peroxide, sodium carbonate and sodium hydroxide were procured from DinânimcaTM, all from São Paulo, Brazil. Glycerol was obtained from SynthTM, while boron trifluoride etherate (BF3·OEt2) was acquired from Fluka, Germany. Chloroform-D 99.8% and sodium carbonate were purchased from Sigma-Aldrich, St. Louis, MO, USA. 4,4′-diphenylmethane diisocyanate (MDI) was procured from Dow Chemical Brazil. Sodium chloride was obtained from Brazil, as a commercial salt from the Cisne brand.
2.2. Epoxide Synthesis
The first step for the epoxide synthesis was the determination of molar mass and unsaturation index of the vegetable oil that is going to be used. The reagent quantities used for the epoxidation of MKO were calculated based on unsaturation per mole of vegetable oil, as determined by the integration of the peaks in
1H nuclear magnetic resonance spectroscopy (
1H NMR) spectrum for olefinic protons (5.4 ppm), methylene protons in the glyceryl group (4.2 ppm), α-methylene protons adjacent to the carbonyl carbon (2.3 ppm), allyl methylene protons (2.1 ppm), β-methylene protons from the carbonyl carbon (2.6 ppm), methylene protons on saturated carbons (2.25 ppm), terminal methyl protons (0.85 ppm) and bisallylic hydrogens (2.79 ppm). It is worth noting that the chemical shifts listed are approximate in quantitative terms [
20,
21]. The reagent quantities listed on
Table 1 for epoxide synthesis are empirical values derived from previous work [
9].
For the synthesis of epoxidized MKO, formic acid and MKO were added to a 500 mL round-bottom flask. Hydrogen peroxide was subsequently added dropwise into the reaction mixture under mechanical agitation. The temperature was kept at 60–65 °C for 2 h. The reaction mixture was then transferred to a separatory funnel, where the aqueous phase is removed, followed by two extractions with a saturated NaCl solution approximately the same volume of the original aqueous phase of the mixture. The reaction mixture was then neutralized with a 5% (m/v) Na2CO3 solution, usually talking about 9 mL, and after removing the aqueous phase, the product was filtered to remove impurities and dried in a rotary evaporator. Finally, the product was washed with deionized water to remove residuals salts. Each extraction step was allowed to decant overnight to improve separation results.
2.3. Polyol Synthesis
Epoxidized MKO and glycerin, at 1:1 proportion (40 mL of each) were refluxed in a 2-neck round bottom flask under slow magnetic agitation. BF3·OEt2 (10% v/v of the epoxide) was then slowly added dropwise into the reaction mixture over the course of 20 min through the second neck of the flask in the proportion of 10% the volume of epoxide. The dropwise, slow addition ensures the reaction proceeds in a controlled fashion. Since each drop of BF3·OEt2 reacts readily with the other reaction contents, there is no build-up in its concentration, therefore allowing for a smooth synthesis. After BF3·OEt2 is added, the reaction mixture’s viscosity increases significantly. At this point, diethyl ether is added to reduce the viscosity and ensure proper mixing of all components for the duration of the reaction. The reaction mixture was agitated at room temperature for 5 h and 40 min. The agitation speed was slowly adjusted until the reaction mixture homogenizes. The reagent proportion used in this reaction was 1:1:2 (epoxidized MKO–glycerin–diethyl ether). After a homogeneous mixture was obtained, it was transferred to a separatory funnel and decanted overnight, following neutralization with a 10% (m/v) NaOH solution. Subsequently, 2 extractions were carried out with a saturated NaCl solution approximately the same volume as the glycerol added for the reaction, followed by filtration and water evaporation.
2.4. Polyol Hydroxyl Index
The hydroxyl index (OH index) of the polyols was determined according to the Standard Test Method for Hydroxyl Value of Fatty Oils and Acids, ASTM Standard D1193 [
22] Specification for Reagent Water, an acetylation of the polyol with acetic anhydride followed by a titration with potassium hydroxide. The method was based on the acetylation of the polyol 9 g to 11 g by pyridine-acetic anhydride solution, and 25 mL of neutralized n-butyl alcohol, which were mixed for 30 min under water reflux. The mixture then was titrated with 0.5 M alcoholic KOH using phenolphthalein indicator. The measurements were made in triplicate, resulting in an OH index of 85.57 mgKOH·g
−1 for the GlyMKO.
2.5. Polyurethane Synthesis
For the synthesis of polyurethanes (PUs), polyol and MDI were mixed in 0.8, 1.0, and 1.2 NCO/OH ratios. The reagent quantities for each PU are shown in
Table 2. This ratio was calculated using the previously determined free NCO index for aromatic isocyanates ASTM D 5155-96 [
23] on the available MDI and the OH index in the polyol. The polyol was firstly added in a Teflon beaker, followed by the NCO. The reagents were then mixed by mechanical stirring for one min. The mixture was poured into Teflon molds and then left to cure in an oven pre-heated at 65 °C for 24 h. The final products were identified and characterized.
2.6. Fourier-Transform Infrared Spectroscopy
Fourier-transform infrared (FTIR) spectroscopy analyses were performed on a Varian 640 spectrometer equipped with an attenuated total reflectance (ATR) accessory. The spectra reported correspond to an average of 16 scans with a spectral window from 400 cm−1 to 4000 cm−1 with a normal resolution of 4 cm−1. The presented spectra have been normalized by the 2900 cm−1 peak and all spectra were analyzed with Origin 9.0 software.
2.7. Thermogravimetric Analysis
The PU samples from 5 mg to 10 mg (aiming near 7 mg) were analyzed by thermogravimetric analysis (TGA) and its derivative (DTG) under synthetic air atmosphere on a NETZSCH STA 449F3 analyzer. The TGA analysis temperature profile is as follows: raising the temperature from room temperature to 103 °C, 20 °C·min−1, followed by a 30 min isothermal, after which the temperature is raised to 1000 °C at the same temperature rate. The temperature is then decreased to room temperature at −50 °C·min−1. The whole process happens under 100 mL·min−1 synthetic air atmosphere.
2.8. Dynamic Mechanical Analysis
Dynamic mechanical analysis (DMA) was performed on a Q800 (TA Instruments, New Castle, DE, USA) using a three-point bend fixture. Specimens of size 5.0 mm × 9.0 mm × 5.0 mm (length × width × thickness) were cut and initially cooled to 0 °C (in preliminary tests, some DMA specimens broke before the start of the experiments when temperatures below 0 °C were employed, limiting the temperature range used), followed by heating to 150 °C at a heating rate of 3 °C·min
−1 under an iso-strain mode with an amplitude of 14 µm and a frequency of 1 Hz. Two replicates of each sample were tested individually, and the results disclosed herein represent the average of the two trials. The crosslink density (ε) was calculated based on the rubber elasticity theory according to Equation (1) [
24,
25], where
E’ is the storage modulus at temperature T and R is the gas constant.
4. Conclusions
The unique structure obtained from the ring-opening of epoxidized MKO with glycerol is unprecedented and resulted in PUs with four distinct thermal degradation stages, as shown in
Figure 10, instead of the typical three thermal degradation stages observed for other vegetable oil-based polyurethane foams. While the use of glycerol to produce polymers is not new, as demonstrated by the production of polyols, plasticizers, and chain extenders, the use of an epoxide ring-opening reaction to insert glycerol into a triglyceride molecule results in a novel polyol (GlyMKO). This polyol provides new and excellent types of PUs, confirmed by FTIR and
1H NMR spectroscopies and with unique thermal properties, shown by TG and DTG. The FTIR data showed that both the epoxide formation and the epoxide ring-opening reaction were successful and evidenced by the disappearance of signals associated with the presence of carbon-carbon double bonds in MKO accompanied by the appearance of epoxide peaks in EPMKO, and their subsequent disappearance in GlyMKO along with the appearance of a large OH band in the FTIR spectrum. Those results agree with other reports in the literature. The FTIR spectra also revealed that, although different NCO/OH ratios were used to obtain the PUs, there were no significant differences in the final products, especially between PU 1.0 and PU 1.2. Despite this, PU 1.2 exhibits lumps not present in considerable quantities in the other samples, which correspond to denser regions within the sample. The three samples prepared presented similar thermal behavior under synthetic air atmosphere, showing a difference of a few degrees in
Tonset,
T5, and
T10, with PU 0.8 having higher temperatures and PU 1.2 lower temperatures, except for
Tonset. Notwithstanding that the PUs have similar thermal stability, the samples presented different behaviors in relation to DTG curves. PU 1.0 followed the known degradation profile seen in the literature for PUs, with three well-defined degradation stages. The first stage corresponds to degradation of urethane bonds, followed by rigid segments, and lastly by soft segments. The other two samples (PU 0.8 and PU 1.2) present four degradation stages, likely caused by different levels of rigidity and softness associated with changes in crosslink density due to the uneven quantities of NCO and OH, resulting in complex thermal degradation profiles. The lower temperatures for the rigid segments’ thermal degradation might also explain why PU 1.2 exhibits the lowest
Tonset,
T5, and
T10, while PU 0.8 showed the highest values. DMA analysis was used to calculate the crosslink density of the samples and help explain their thermo-mechanical behavior. The PUs obtained in this research are excellent candidates as a matrix material for bio-based composites. The reinforcement of these materials with thermally treated wood biomass will soon be investigated by the authors. It is hypothesized that excess NCO in the PUs can react with hydroxyl groups on the surface of wood to enhance matrix-reinforcement interactions and maximize mechanical properties.