Use of Castor Oil in Polyester Resins
Polyester resins are widely recognized as the most costeffective resin systems utilized in various engineering applications.
Polyester resin is a commonly used material in various industries, including paint, coating, marine, automotive, and electronics.
In organic chemistry, the reaction of an alcohol with an organic acid result in the formation of an ester and water. The reaction of dibasic acids with specific alcohols, such as glycol, results in the formation of polyester and water.
The basic process for polyester production involves a reaction that includes the addition of components such as dibasic acids and polyfunctional crosslinkable monomers [1]. Polyesters with diverse properties can be produced by using various acids, glycols, and monomers.
The benefit of these materials is that they can be customized by end-users to meet specific requirements. Consequently, it is unsurprising that the initial endeavors to transform
renewable resources into polymers were focused on the polyester industry.
Hydroxyl-containing polyols, phthalates, and petrochemical products are the primary components in the conventional synthesis of polyester resins.
However, growing environmental concerns have prompted industries and research institutions to gradually replace petrochemical-based sources with renewable oleochemical
sources for these syntheses.
The European Union’s Green Deal, in particular, has placed significant emphasis
on enhanced biodegradability, cost-effectiveness, and environmental sustainability [2].
Vegetable oil and fatty acids are widely used as renewable raw materials for the development of biodegradable polyester resins due to their availability, low toxicity,
and relatively low cost [3].
Various plants (sunflower, linseed, soybean, castor, and rapeseed), produce triglycerides
during the esterification reaction of glycerol with fatty acids.
These fatty acids can exist as doublelinked aliphatic unsaturated chains, such as oleic, linoleic, and ricinoleic acid, or non-reactive saturated aliphatic chains, such as stearic and palmitic. Polyester resins that contain unsaturated chains are of greater interest
than those containing saturated chains.
Among the various unsaturated fatty acids, castor oil has gained great commercial importance in the production of synthetic and bio-based polyester resins.
Castor oil is the only commercially available natural source of fatty acids that is produced directly from the plant source, with the natural hydroxyl group required for the synthesis
of polyester resins.
Castor oil contains about 90% ricinoleic acid content, with the other main components
in its structure being oleic and linoleic acid. Ricinoleic acid has a hydroxyl group at carbon 12 and a double bond between carbons 9 and 10. Castor oil has an average hydroxyl (OH) number of 2.7 per triglyceride and 160-168mg KOH/g [4].
The double bonds serve as grafting centers, while castor oil’s long suspended fatty acid chain imparts flexibility, hydrophobicity, and a thermosetting structure to the web [5].
The significant amount of ricinoleic acid in castor oil makes it unique among other vegetable oils. Figure 1 shows the possible chemical modifications of the different reactive sites of castor oil.
Castor oil is used during the application of polyester resins in paints to increase the adhesion properties of the resin.
This ensures that the paint adheres better to the surface and lasts longer. Additionally, castor oil helps to achieve a homogeneous color by facilitating the mixing of the paint.
Castor oil also plays a vital role in the use of polyester resins in adhesives. By improving
the adhesion properties of the adhesive, it ensures that the materials stick together more strongly, increasing the durability of the adhesive and preventing issues such as cracking or peeling.
Consequently, the quality of the adhesive is enhanced. Furthermore, castor oil reduces the viscosity of the adhesive, making it easier to apply. Polyester resins are utilized in the coating industry to create durable and protective coatings.
During the use of polyester resins in coatings, castor oil enhances the adhesion properties of the resin, allowing for better adhesion of the coating to the surface.
Additionally, castor oil can be used to adjust the viscosity and rheology of the resin, which helps the coating to spread more evenly, leading to a smoother surface. Figure 2 provides an overview of the different uses of castor oil and its modifications in various industries.
However, certain limitations hinder the commercial success of castor oil-derived polyester resins. One of these limitations is that some polyester resins derived from castor oil exhibit low mechanical strength.
Additionally, the secondary OH group of ricinoleic acid present in castor oil creates steric hindrance during reactions, resulting in structural irregularities caused by fringing chains.
This, in turn, impacts the curing rates and gloss/matte ratio of paints, particularly in the paint industry. Nevertheless, these drawbacks of castor oil can be overcome through its modification by transesterification reaction with other alcohols, such as glycerol,
pentaerythritol, and trimethylol propane. Currently, castor oil is effectively employed as a polyol component in synthesizing polyester resins.
This review investigates the use of castor oil in polyester resins and its sectoral effects. The natural hydroxyl group, ester bond, and unsaturated bonds present in castor oil offer many chemical reactions, transformations, and modifications that enable its effective use as a green polyol for synthesizing polyester resins.
Such modifications result in the production of polyester resins with new properties, including biodegradability, low toxicity, and environmental friendliness.
The significance of castor oil, one of the oleochemicals, was emphasized for improving performance properties, such as those of coatings, paints, adhesives, and polyurethane materials, by effectively using renewable raw materials.
At the 3-S Engineering R&D Center, various oleochemicals, including castor oil, are used to produce polyester resins under the brand name TRIEST, with an emphasis on environmentally friendly production. Different esters and polyesters are manufactured for numerous sectors.
Ertuğrul Kaya
R&D Executive
3-S Mühendislik Müşavirlik San. ve Tic. A.Ş.
References
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[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.
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