According to some market reports, the worldwide production of alkyd resins will grow at a compound annual rate of 6.78% during the period 2019–2023. This growth is attributed to the requirements of the automotive industry, which have increased the demand of binders to be applied in the manufacture of automotive paints and coatings [1,2]. About 70% of the conventional binders used in surface coatings are constituted by alkyd resins [3].
Alkyd resins are polyesters obtained from the polycondensation between polyols, polyacids, and fatty acids derived from vegetable oils that contribute to the flexibility of the films, improve the solubility properties, and increase the degree of crosslinking of the resin through oxidative polymerization [[4], [5], [6]]. These polymers are widely applied in the field of coatings due to their low cost, excellent adhesion, gloss properties, and weathering capabilities [7,8]. In comparison with the conventional petroleum-based polymers, the presence of vegetables oil and glycerol in alkyd resins promotes materials environmental friendly [9,10].
Vegetable oils are one of the sustainable materials widely used in polymer synthesis due to the versatile chemical functionalization, low toxicity, and availability [11]. The compositions of vegetable oils allow a series of chemical reactions for obtaining new monomers [12,13]. The main chemical transformations of vegetable oils are the synthesis of polyacids, polyalcohols and epoxy derivatives.
Several types of vegetable oils have been used in the synthesis of alkyd resins. The most commonly studied include linseed, sunflower, soybean, among others [4,14]. Depending on the amount of unsaturations in the fatty acids, oils can be classified as drying, semi-drying, and non-drying. Drying oils have the highest degree of unsaturation, which favors the radical crosslinking of the fatty acid chains with the atmospheric oxygen, forming hard and solid films. For this reason, drying oils have been used to synthesize oil-based coatings like alkyds [15].
Castor oil is an oleaginous material obtained from Ricinus communis through a process that include extraction, purification, and a combination of mechanical processes. The castor oil chemistry is based mainly on the ricinoleic acid structure, that bears a carboxyl group, hydroxyl group, and a single point of unsaturation [16,17]. The carboxylic group in the molecule allows production of a wide range of esterification products. The hydroxyl group on the 12th carbon can be acetylated or eliminated through a dehydration process that transforms to castor oil in semi-drying oil [18]. Due to the composition of the fatty acids, castor oil is considered a non-drying oil. However, it has been reported the increase of unsaturation grade in the oil through catalytic dehydration or by blending it with highly unsaturated oils. Among the catalyst used in the dehydration process, sulfuric acid, sodium bisulfate, and acid-activated clays have been have been extensively investigated. In our previous work [19], the dehydration process of castor oil using KHSO4 as catalyst, 230 °C and 190 min was evaluated. The dehydrated castor oil obtained presented an iodine value of 127.46 g I2/100g.
During an interesterification reaction, the ester bonds link to fatty acids in the glycerol backbone are split. Then, newly released fatty acids are randomly shuffled within a fatty acid group and re-esterified in a new position in the same glycerol (intraesterification) or in another glycerol backbone (interesterification) [20]. Hence, it is achieved intermediate properties with respect to the starting oils due to the random distribution of fatty acids that form a new triacylglycerol [21]. Therefore, the new product obtained from the interesterification of castor and sacha inchi oils will have very interesting properties, such as a higher unsaturation content, required for its drying, and secondary 
OH groups (presented in the original castor oil) that will also participate in the esterification reactions to obtain the alkyd resin.
There is relatively little research respect to the used of interesterification reaction for the production of drying oil to be applied in alkyd resins, especially using castor oil. The reaction has been mainly studied for the food fats with plasticity curves in the range of commercial soft-tub margarine oils. In particular, food fast obtained from wholly hydrogenated soybean, palm stearin, or cottonseed hard stocks; and liquid vegetable oils have been evaluated. To this end, the reaction has been conducted using different compositions of the initial mixture, temperature range of 70–90 °C, variable reaction times, and using sodium methoxide (0.2–0.5%) as catalyst [21,22]. On the other hand, De Melo et al., evaluated the effect of blends of soybean/coconut oil, and soybean/castor oil in the biodiesel production through interesterification reaction [23]. Saravari and Praditvatanakit, synthesized alkyd urethane resins obtained from vegetable oils by the interesterification reaction between jatropha and castor oil [5]. The resins exhibited lower molecular weight and viscosity, slightly hardness, and longer drying time than the conventional and commercial urethane alkyds.
Plukenetia volubilis L. also known as sacha inchi is an oleaginous seed of the Euphorbiaceae family that grows in the Peruvian Amazonian forest. Sacha inchi seeds are widely used for the production of oils, cakes and protein meals, cosmetics, food, and medicine [24,25]. The seeds have a high oil content (35–60%) rich in linoleic and linolenic acids. The sacha inchi oil contains α-linolenic (50.8%) and linoleic (33.4%) acids, with low levels of oleic (9.1%), palmitic (4.4%), and stearic (2.4%) acids. Due to the high unsaturation content (iodine value: 189.16 g I2/100 g oil), the sacha inchi oil has been considered as a drying employed in the synthesis of alkyd resins. Flores et al., studied the synthesis and characterization of alkyd resins based on sacha inchi oil. The authors concluded that sacha inchi oil can be used as a raw material alternative to linseed oil in the synthesis of alkyd resins for industrial applications [26]. On the other hand, Obregón et al., evaluated the use of sacha inchi oil in the synthesis of alkyd resins. The results showed that alkyd resins prepared with trimethylolpropane and sacha inchi oil exhibited low viscosities, which are suitable for design high solids protective coatings [27].
In this context, the aim of this work is to study the potential interesterification reaction of vegetable oils as an alternative for the production of a drying oil used in the synthesis of alkyd resins. Considering that the oxidation stability, lubricity, and availability of castor oil, as well as the average unsaturation of sacha inchi oil, can be improved the drying properties of alkyd resins, sacha inchi and castor oil with different grades of purity were selected. Particularly, the effect of sacha inchi/castor oil blends before and after interesterification on the physical-chemical properties of medium alkyd resins was evaluated. In general, the physical-chemical properties of starting oils and the interesterification products such as iodine value, acid value, saponification value, moisture content, and Gardner color were determined according to ASTM standards. Alkyd resins were synthetized and physio-chemically characterized following the ASTM international standards (viscosity and color Gardner, acid value, volatile compounds, to-touch time, dry-to-touch time, dry-hard time, and dry-to handle time, adhesion, pencil hardness, water resistance, and chemical resistance).
