Use of Castor Oil in Polyester Resins

Castor Oil in Polyester Resins Polyester resins are known as the most economical resin systems used in engineering applications. They are widely employed particularly in the paints and coatings, maritime, automotive, and electronics sectors. In organic chemistry, the reaction of an alcohol with an organic acid produces an ester and water. The reaction of dibasic acids with special alcohols such as glycols produces polyester and water. This reaction, combined with the addition of components such as dibasic acids and cross-linkable monomers (multifunctional), forms the basic process for polyester production [1]. Polyesters with many different properties can be produced from different acids, glycols and monomers. The advantage of these materials is that they can be tailored by the end user to meet a wide range of special requirements. Therefore, it is not surprising that some of the first initiatives to convert renewable sources into polymers have been in the polyester field. In the traditional synthesis of polyester resins, the main components are polyols containing hydroxyl groups, phthalates and petrochemical products. In recent years, increasing environmental concerns have led to the gradual reduction of petrochemical-based sources and prompted industries and research institutions to replace petro-based sources with renewable oleochemical sources for these syntheses. Particularly with the EU Green Deal, issues such as improved biodegradability, cost-effectiveness and environmental sustainability have gained importance [2]. Vegetable oils and fatty acids are the most widely used renewable raw materials as reliable monomers for the development of biodegradable polyester resins with a wide variety of structural and functional variations due to their accessibility, low toxicity and relatively lower cost [3]. Vegetable oils obtained from various plants (sunflower, flax seed, soya bean, castor, rapeseed, etc.) are triglycerides produced during the esterification reaction of glycerol with fatty acids. These fatty acids can be in the form of double-bonded aliphatic unsaturated chains (oleic/linoleic/ricinoleic acid) or reactive non-saturated aliphatic chains (stearic/palmitic). In polyester resins, those containing unsaturated chains receive greater interest than those containing saturated chains. Among various unsaturated fatty acids, castor oil has gained significant commercial importance in the production of synthetic and bio-based polyester resins. Castor oil is the only naturally occurring fatty acid source that contains the natural hydroxyl group required for polyester resin synthesis and is commercially available directly from plant sources. Castor oil contains approximately 90 percent ricinoleic acid in its structure. The other main components in its structure are oleic and linoleic acid. Ricinoleic acid has a hydroxyl group on the 12th carbon and a double bond between the 9th and 10th carbons. Castor oil has an average of 2.7 hydroxyls per triglyceride and a hydroxyl (OH) number of 160-168 mg KOH/g [4]. While the double bonds serve as graft centers, the long suspended fatty acid chain of castor oil imparts flexibility, hydrophobicity and a thermosetting structure to the network [5]. The significant amount of ricinoleic acid in castor oil makes castor oil unique among other vegetable oils. Figure 1 shows the possible chemical modifications of the different reactive regions of castor oil. During the use of polyester resins in paints, castor oil is used to enhance the adhesion properties of the resin. This ensures better surface adhesion of the paint and longer durability. In addition, castor oil helps the paint to mix better and achieve a homogeneous color. Castor oil plays an important role in the use of polyester resins in adhesives. By improving the adhesion properties of adhesives, it ensures materials bond more strongly to each other, increasing the quality of the adhesive by improving durability and preventing problems such as cracking or peeling. At the same time, castor oil reduces the viscosity of the adhesive, providing ease of application. Polyester resins are used in the coatings industry to obtain durable and protective coatings. During the use of polyester resins in coatings, castor oil improves the adhesion properties of the resin, ensuring better surface adhesion of the coating. In addition, castor oil can be used to adjust the viscosity and rheology of the resin, which helps the coating to spread more homogeneously and create a smoother surface. The different uses of castor oil and its modifications in the sectors are given in Figure 2. However, there are certain limitations that prevent the commercial success of polyester resins derived from castor oil. One of these limitations is that some polyester resins derived from castor oil show low mechanical strength properties. Another limitation stems from the secondary OH group of ricinoleic acid present in the structure of castor oil creating a steric hindrance during reactions. This leads to structural irregularities created by dangling chains. Particularly in the paints sector, this affects high cure rates and the gloss/matte ratio of the paint. However, these disadvantages of castor oil can be overcome by modification through transesterification reactions with other alcohols such as glycerol, pentaerythritol and trimethylol propane [4]. Currently, castor oil is effectively used as a polyol component in the synthesis of polyester resins. With this review, the use of castor oil in polyester resins and its impact on a sectoral basis has been examined. The presence of the natural hydroxyl group, ester bond and unsaturated bonds in castor oil shows that there are many chemical reactions, transformations and modifications for effective use as a green polyol for polyester resin synthesis. Through these modifications, new properties, biodegradability, low toxicity and environmentally friendly polyester resins are produced. The importance of castor oil, one of the oleochemicals, in bringing improved performance properties for coatings, paints, adhesives and polyurethane materials with effective use of renewable raw materials has been emphasized. At the 3-S Research and Development Center, there are polyester resins made with various oleochemicals including castor oil under the TRIEST brand. With awareness of environmentally friendly production, various esters and polyesters are produced for many sectors.   References [1] M. Ionescu, D. Radojčić, X. Wan, M. Laxmi Shrestha, Z.S. Petrović, T. Upshaw, Z. Petrović, Hıghly Functıonal Polyols From Castor Oıl For Rıgıd Polyurethanes, 2016. http://www.elsevier.com/open-access/userlicense/1.0/2. [2] D. Wei, J. Zeng, Q. Yong, High-Performance Bio-Based Polyurethane Antismudge Coatings Using Castor Oil-Based Hyperbranched Polyol as Superior Cross-Linkers, ACS Appl Polym Mater. 3 (2021) 3612–3622. https://doi.org/10.1021/acsapm.1c00503. [3] J.H. Lee, S.H. Kim, K.W. Oh, Bio-based polyurethane foams with castor oil based multifunctional polyols for improved compressive properties, Polymers (Basel). 13 (2021) 1–12. https://doi.org/10.3390/polym13040576. [4] S. Das, P. Pandey, S. Mohanty, S.K. Nayak, Insight on Castor Oil Based Polyurethane and Nanocomposites: Recent Trends and Development, Polymer - Plastics Technology and Engineering. 56 (2017) 1556–1585. https://doi.org/10.1080/03602559.2017.1280685. [5] K.R. Kunduru, A. Basu, M. Haim Zada, A.J. Domb, Castor Oil-Based Biodegradable Polyesters, Biomacromolecules. 16 (2015) 2572–2587. https://doi.org/10.1021/acs.biomac.5b00923.   Ertuğrul Kaya Research and Development Manager 3-S Engineering Consulting Inc.