Introduction

Alkyd resins are modified oil polyesters, generally synthesized by polycondensation of polyols and polyacids with fatty acids [1]. Kienle introduced in 1927 the use of unsaturated vegetable oils with phthalic anhydride (glyptal polyester) for resins synthesis [2]. These unsaturated polyesters are soluble in aliphatic solvents that quickly crosslink with the oxygen from the air, forming flexible and durable films [3]. Moreover, alkyd resins are used for the commercial production of paints, enamels, lacquers and varnishes, covering all market segments, being the most prominent the automotive, decorative, marine and aerospace industries [4].

Linseed oil (LO), followed by soybean oil, rapeseed oil, castor oil and sunflower oil are mainly used in large scale production of alkyd resins [4]. However, non-traditional sources, like Sacha inchi oil (SIO) [5, 6], have recently been employed for the production of high performance alkyd resins for coating applications. Both SIO and LO have high degrees of unsaturation [7, 8], which improve the crosslinking process and/or polymerization per unit weight of material, resulting in a stronger and more resistant product. Moreover, previous studies revealed that SIO has higher unsaturated fatty acid content than soybean, corn, peanut, sunflower, cotton, palm and olive seeds [9]. Likewise, it has been reported that SIO has similar content of alpha linoleic acid, lower oleic acid content and higher linolenic acid content than LO [10].

Among the polyols most used for obtaining alkyd resins are glycerol and pentaerythritol (PE). PE is one of the most important polyol in the production of alkyd resins; it has four primary hydroxyl groups of the same reactivity arranged in a neopentyl structure that provides stability against heat, light and moisture [4]. Therefore, compared to glycerol resins, it has been indicated that PE-based resins may have better physicochemical properties, such as viscosity, cured film hardness, gloss retention, color stability, thermal stability and external durability [6, 10,11,12]. On the other hand, it has been reported that the use of a trifunctional polyols, like trimethylolpropane (TMP), might modify the properties of alkyd resins, by lowering their viscosity, and improving their resistance to light, heat and moisture, as well as resistance to alkaline media [4, 13]. This study compares the use of multifunctional polyols for the synthesis of SIO-based alkyds. The main objective is to find good performance alkyd resins with highly branched structures and low viscosities.

Materials and experimental methods

Materials

The alkyd resins synthesized had as their basic components LO or SIO, PE, TMP and phthalic anhydride (PA) ((C6H4(CO)2), ACS grade, Merck brand). SIO from the Starseed brand (Amazon Health Products S.A.C.), and LO from a commercially available brand were used. In addition, lithium carbonate (Li2CO3, ACS grade, Merck Company) was used as catalyst, whereas xylol ((CH3)2C6H3OH, ACS grade, Mallonckrodt Chemicals), as a solvent to extract the water produced in the synthesis reaction.

Synthesis of alkyd resins

Medium-chain alkyd resins were synthesized with LO and SIO with different TMP and PE contents, as shown in Table 1. The resins synthesized prepared with LO and SIO were identified by the initials LM and SM, respectively. The polyol acronyms, TMP and PE, were used to differentiate resin names and identify the resin’s polyol content. The weight ratio in percentage between the polyols used is also specified in parentheses, next to the sample’s name. For example, a Sacha inchi resin with 50% TMP and 50% PE was identified as SM-TMP/PE (50/50). Resins prepared directly with fatty acids extracted from SIO and LO were identified with the letter “A”; the fatty acid extraction process has been recently published by our research group [12].

Table 1 Composition of alkyd resins
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Alkyd resins were synthesized by the alcoholysis-esterification method. Initially, the oil/fatty acid fraction was heated to approximately 200 °C, and the lithium carbonate catalyst was added. Then, PE was added, and the mixture was heated to approximately 230 °C. Subsequently, the mixture was allowed to cool to 200 °C, before adding the TMP. The first stage concluded when the mixture showed solubility in ethanol (mixture: ethanol = 1:3). In the second stage, the obtained monoglyceride was heated to approximately 230 °C, and then PA and xylol were added. The reaction concluded after reaching an acid value lower than 20. The reaction was carried out under constant stirring and inert atmosphere.

Preparation of alkyd resins mixtures

A mixture of resin with 10% by weight of solvent (white spirit:xylol = 3:1) was prepared to evaluate physicochemical properties, such as hardness, drying time, viscosity, colour and density of resin solutions (uncured) and resin film (cured).

Viscosity, colour and density

Glass tubes filled with resin solutions were placed on a support to measure viscosity and colour. The relative viscosity was determined following the procedure based on ASTM D1545-13, while color was determined by comparison with Gardner liquid color standards on a scale from 1 (lighter yellow) to 18 (darker brown), according to ASTM 1544. Density of resin solutions was determined using Gardco mini wpg cup model WG-SS-8.32/T/C; weight was recorded, and density was calculated as specified in ASTM D1475-13.

Curing test

For the curing tests, the resins mixed with a cobalt octoate base catalyst were applied to 10 × 4.5 cm glass specimens with a 30 μm applicator (Erichsen Model 360, Germany). The dry-to-touch and dry-hard times were determined according to the procedure described in ASTM D1640.

Hardness test

Samples with cured resins were used for hardness. The surface hardness measurement of the resin was performed using charcoal pencils with hardness between 6B (soft) and 6H (hard), according to ASTM D3363.

Chemical resistance

Samples with cured resins were used for chemical resistance tests. The chemical resistance of the cured resins was evaluated by immersing the samples for three days at room temperature, in 10% aqueous hydrochloric acid (HCl) solution, 10% sodium chloride (NaCl) solution, ethanol and distilled water.

FTIR analysis

The FTIR spectra of the alkyd resins were obtained with FTIR Spectrum Two (Perkin Elmer, USA) using KBr disks.

NMR analysis

The 1H-NMR and 13C-NMR spectra of the resins were obtained at 500 MHz with a NMR Ascend equipment (Bruker, USA), using deuterated chloroform as the solvent.

Thermal analysis

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed with STA PT 1600 equipment (Linseis, Germany) with a heating rate of 10 °C/min.

Scanning electron microscopy (SEM)

Scanning electron microscope (SEM) images were obtained using a Quanta 650 (by FEI). The used acceleration voltage was 10 kV with a spot size of 3.5. To obtain a suitable result for the weakly conductive samples, the low vacuum mode and a low vacuum secondary electron detector (LFD) was used taking advantage of the water vapor on the sample surface as conductive medium. The samples were measured with magnifications between 50 and 5000x.

Results and discussion

Acid value (A.V.) from synthesized alkyd resin varied from 4.7 to 19.2 mg KOH/g (Table 2) as expected according to Wick et al., who established that the adequate value should be between 5 and 10 mg KOH/g [14]. The acid values obtained in this study were lower than those reported for alkyd resins synthesized with Tung oil [15] (A.V. = 30 mg KOH/g), Jatropha curcas oil [16] (A.V. = 23–44 mg KOH/g), yellow oleander seed oil [17] (A.V. = 24.63–35.25 mg KOH/g), and Ricinodendron heudelotii [18] (A.V. = 21–40 mg KOH/g). It is worth noting that the sample obtained from SIO fatty acids, A-SM-TMP/PE (50/50), reached the lowest acid value without gelling (Table 2).

Table 2 Physico-chemical properties of LO and SIO alkyd resins
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Viscosity, colour and density

The viscosity of the SIO-based alkyd resins ranged from Z5 to Z8, increasing with PE content. Previous researches [6, 11] agreed on their findings by indicating that branched structures, like PE-based resins, have high viscosities because of the high functionality of the polyol. The highest viscosity was found in LM-TMP/PE (50/50) and SM-TMP/PE (25/75) samples, those containing more amount of PE. Table 2 showed that the SIO-based resins have lighter Gardner colours than those synthesized with LO; tendency that is observed in both groups, prepared with oil, and with fatty acids. As a general trend, also TMP content increased the colour darkening of the synthesized resin. For all samples, density was around 1 g/ml.

Curing test

Drying times and hardness of the alkyd resins are presented in Table 3. In the alkyd resins, the drying process occurs due to the oxidation of the unsaturated fatty acids, leading to the formation of hydro peroxides and covalent bonds between the fatty acid chains (cross-linking) [19]. The oxidation begins with a hydrogen extraction of the methylene group activated by the unsaturation of the fatty acids, through a process of radical polymerization caused by the use of drying agents [19, 20].

Table 3 Drying times and pencil hardness of alkyd resins
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As shown in Table 3, drying times of SIO-based resins diminished with TMP content. This probably be caused by the voluminous methyl group of TMP, which slightly reduces intermolecular interactions in comparison with PE [4], allowing the oxygen to enter more easily throughout the resin film. The drying times of the resins produced with fatty acids were shorter than those obtained from their respective oils. Sample A-SM-TMP/PE (50/50) had the shortest drying time compared to the other samples of the same TMP/PE content.

Hardness test

Hardness of the alkyd resins reached a value of 4H after a curing time of 14 days, with the exception of the samples SM-TMP/PE (50/50) and SM-TMP/PE (75/25).

Chemical resistance tests

Chemical resistance of alkyd films was evaluated by immersion tests in five different mediums (Table 4). Four physical characteristics were compared after immersion: opacity, wrinkling and/or loss of adhesion, blistering and dissolution of the film, and assigned to a numerical value as follows: 0: No defects; 1: Opacity; 2: Wrinkle and/or loss of adhesion; 3: Blisters; and 4: Dissolved film.

Table 4 Chemical resistance of the cured resins
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The chemical resistance data indicated that alkyd resins have a high resistance to water, HCl(ac), and NaCl(ac) solutions; similar results are reported by other authors [16, 17, 21, 22]. The increase in PE content did not promote a greater resistance of the alkyd resin. On the other hand, resins exhibited a poor resistance to alkaline solution due to the presence of hydrosable ester groups in alkyd structure [16,17,18, 21, 23,24,25]. Sample A-SM-TMP/PE (50/50) was the only resin that presented resistance in alkaline medium. This behavior could be attributed to its lower acid value (4.74 mg KOH/g) in comparison with entire group of synthesized resins.

FTIR analysis

The FTIR spectra obtained for the synthesized alkyd resins are shown in Fig. 1 and Fig. 2. A characteristic broad band of the hydroxyl group was observed at approximately 3500 cm−1, and asymmetric and symmetrical stretching bands and bending of (C-H)CH2 were observed at 2935, 2848 and 1466 cm−1, respectively [26,27,28]. Bands due to strong C = O stretching appeared at approximately 1726 cm−1, while bands from C-O stretching, at 1262 and 1120 cm−1 [29]. The peaks generated by the aromatic rings, present in the main polymer structure, were observed at 1600 and 1580 cm−1; an unsaturated ring with strong in-plane deformation at 1071 cm−1, and out-of-plane deformation between 741 and 750 cm−1. All these peaks, typical of an alkyd resin, confirmed the polyesterification process [30].

Fig. 1
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FTIR spectra of SIO-based alkyd resins

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Fig. 2
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FTIR spectra of oil and fatty acid based alkyd resins

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NMR analysis

The 1H-NMR spectra of SI TMP/PE (25/75) and SI TMP/PE (100/0) samples are presented in Fig. 3. Peaks (a) to (f), and (h), are signals from protons located on fatty acid chains. Peak (a) at δ 0.81–1.05 ppm corresponds to protons from terminal methyl groups. Peak (b) at δ 1.22–1.42 ppm (b) comprises internal protons of the -CH2- groups, while methylene protons next to -CH2COO groups are identified in peak (c) at δ 1.47–1.55 ppm. Peak (d) at δ 1.94–2.14 ppm corresponds to allylic protons, whereas the protons of the carbon located next to the ester linkage on the fatty acid chain appeared at 2.23–2.39 ppm (peak (e)). Peak (f) at δ 2.73–2.83 ppm represents methylene groups of double allylic protons. Protons of unsaturated carbons are assigned to peak (h) at δ 5.31–5.46 ppm. Peaks (i) are aromatic protons of the phthalic anhydride moiety (main component of the resin); they are observed at δ 7.45–7.79 ppm. Results of 1H-NMR are similar to those reported in previous studies [23, 31, 32].

Fig. 3
figure 3

1H-NMR spectra of synthesized alkyd resins

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Figure 4 shows 1H-NMR spectra from methylene protons located in the polyalcohol moiety, whose signals appear between δ 3.55 and 4.49 ppm. Peaks from PE-based resins at δ 3.2–3.75 ppm presented a higher intensity than those containing more amount of TMP. Rämamen and Maunu [32] studied PE/TMP based alkyd resins and attributed larger peaks intensity at δ 3.45 and 3.82 ppm to protons from methylene group attached to unreacted -OH groups. In addition, they suggested that a lower intensity of polyol proton signal at δ 4.11–4.65 ppm in PE-based alkyd resins may be related to the greater functionality of PE (branched structures).

Fig. 4
figure 4

1H-NMR spectra of the polyol area of synthesized alkyd resins

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Figure 5 showed the 13C-NMR spectrum of SM-TMP/PE (25/75) and SM-TMP/PE (100/00) alkyd resins. In the high field region between δ 0–35 ppm, peaks (A) and (B) in Fig. 5 contains mainly the methylene and methyl carbon peaks of the fatty acid chains. The δ 40 to 70 ppm region has been recognized as the polyol region, as it contains signals of carbons bonded directly with oxygen atoms located in the polyalcohol moiety (60–70 ppm), and quaternary carbons of PE and TMP (40–60 ppm) [31]. The aromatic carbon region identified as peaks (C) at 125–135 ppm contains signals corresponding to aromatic carbons of PA and vinyl carbons of the unsaturated fatty acid chains. The carbonyl groups of PA and fatty acid esters appeared in the δ 160–180 ppm region (peaks (D) and (E)). Results from 13C-NMR analysis were similar to those reported in previous studies [23, 31, 32].

Fig. 5
figure 5

13C-NMR spectra of synthesized alkyd resins

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Thermogravimetric analysis

Thermogravimetric (TGA) and differential thermal analysis (DTA) curves of the alkyd resins prepared with SIO are observed in Fig. 6 and Fig. 7. Weight loss (%) of each alkyd resins is presented in Table 5. Alkyd resins prepared with more amount of TMP had less weight loss during heating between 244–360 °C. However, at temperatures above 500 °C, only the sample SM-TMP/PE (75/25) showed greater thermal stability by having the least weig.

Fig. 6
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TGA curves of SIO-based alkyd resins

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Fig. 7
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DTA curves of SIO-based alkyds

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Table 5 Weight loss (%) of SIO-based alkyd resins
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Regarding the DTA analysis (Fig. 7), it is observed that, in general, thermal degradation of alkyd resins takes place near 470 °C. It can also be corroborated that a greater amount of TMP improves the thermal behavior of alkyd resins.

Figure 8 and Fig. 9 show the TGA and DTA curves of the alkyd resins prepared from SIO and LO, as well as, those of fatty acid-based resins. Weight loss (%) of alkyd resins is presented in Table 6. At 360 °C weight loss was higher in oil-based alkyd resins in comparison with fatty acid-based alkyd resins (Table 6). LO oil-based alkyd resin LM-TMP/PE (50/50) exhibited a loss weight below 90% at 546 °C. Exothermic peaks of DTA curves (Fig. 9) of fatty acid-based resins were displaced at temperatures higher than 470 °C, suggesting a higher thermal resistance.

Fig. 8
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Comparison of TGA curves of oil and fatty acid based alkyd resins

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Fig. 9
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Comparison of DTA curves of oil and fatty acid based alkyd resins

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Fig. 10
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Magnification of 500 × for SEM of (a) LM-TMP/PE (50/50), (b) SM-TMP/PE (50/50), (c) SM-TMP/PE (25/75), and (d) SM-TMP/PE (100/0)

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Table 6 Weight loss (%) of oil and fatty acid based alkyd alkyd resins
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Scanning electron microscopy (SEM)

The SEM micrographs of alkyd resins revealed a homogenous morphology (show smooth surface) attributed to the efficient crosslinking (Fig. 10).

Conclusion

It was possible to synthesize alkyd resins obtained from sacha inchi oil, TMP, PE, and PA in different proportions in a satisfactory way. The structures of the synthesized alkyd resins were confirmed by FTIR, 1H-NMR, and 13C-NMR analyses.

Alkyd resins with the highest TMP content exhibited the lowest viscosities and the best thermal stabilities. Sacha inchi oil-based alkyd resins presented clearer Gardner colours and lower viscosities than their LO-based counterparts. The drying times were shorter in the resins produced with fatty acids compared to those produced with oil. Drying-touch times increased with PE content.

The alkyd resins obtained with multifunctional polyols have adequate colour properties, drying times, hardness, chemical resistance and thermal stability to be used as surface coating.