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Synthesis of Bio-Based Epoxy Resins

Piotr Czuband Anna Sienkiewicz

Cracow University of Technology, Department of Chemistry and Technology of Polymers, ul. Warszawska 24, 31-155 Cracow, Poland

1.1 Introduction

The term "epoxy resin" is understood to mean compounds containing at least one active epoxy group in their structure and which are capable of forming a cross-linked three-dimensional structure in the curing process involving these groups. Naturally, epoxy rings are found only in vernonia oil. However, epoxy functionality can be easily introduced into the compound structure, even by the oxidation of unsaturated bonds to oxirane rings. This is a typical method of obtaining cycloaliphatic resins, applied on a large scale in electronics to encapsulate electronic systems. The second method is the use of epichlorohydrin, which is commonly applied to prepare epoxy compounds via the reaction with polyalcohols or polyphenols. Epichlorohydrin together with bisphenols (mainly bisphenol A or F, and S) are the main raw materials used in industrial methods for the synthesis of epoxy resins most often produced and used on a large scale. All these compounds are of petrochemical origin. There are three main reasons for the search of new raw materials of natural origin for the synthesis of epoxy resins. The first is the need to replace petrochemical raw materials. The volatility of oil and gas prices and their strong connections with the changing political situation in various regions of the world, as well as the inevitable prospect of imminent exhaustion of their sources, and ecological considerations are the main reasons for the search of alternative sources of raw materials. Moreover, potential toxicological and endocrine disrupting properties of bisphenol A are discussed and emphasized, especially in recent years. The second reason is the need to solve the problem of annually increasing amount of postconsumer plastic waste. Epoxy resins belong to the category of polymeric materials practically not biodegradable. The application of bio-based raw materials can enable and facilitate their decomposition under the influence of biological factors. Epoxy resins are widely used as coating materials in products intended for contact with food or even storage of food (e.g. can-coating or paints for securing ship hold walls). Therefore, the third reason is the need to limit the penetration of harmful substances such as bisphenol A into food from the coating material, preferably by eliminating them already at the stage of synthesis.

While searching for new bio-based resources for the synthesis of epoxy resins, particularly bisphenol substitutes, the crucial issue must be remembered. One of the most important challenges is to provide new bio-based resins with comparable performance properties to the currently manufactured and applied petrochemical-based commercial products, i.e. primarily high mechanical strength, thermal stability, and chemical resistance. The mentioned properties are characteristic of the resins based on bisphenol A (or other bisphenols), thanks to which these materials are produced on a large scale for many applications. Therefore, this chapter presents the most promising raw materials whose structure can provide the desired final properties of the epoxy system after cross-linking. At the same time, they must be raw materials easily available in large quantities from renewable sources, nontoxic and cheap to obtain and in preparation.

1.2 Plant Oil Bio-Based Epoxy Resins

Vegetable oils, as a material of natural origin and from renewable sources, are the subject of numerous studies aimed at their application for the synthesis or modification of various polymers [1]. Soybean, castor, linseed, rapeseed, sunflower, cotton, peanut, and palm oils are primarily used on a larger scale depending on the type of oil produced in a given region [2]. From the chemical point of view, plant-based oils are a mixture of esters derived from glycerol and free fatty acids, mainly unsaturated acids (primarily oleic, linoleic, linolenic, ricinoleic, and erucic acids) and in a small amount of saturated acids (stearic and palmitic acids) (Figure 1.1), depending on the type of oil.

When choosing vegetable oil for use in the synthesis of polymers, first of all, its structure should be taken into account: the presence of unsaturated bonds and possibly other functional groups (e.g. hydroxyl in castor oil or epoxy in vernonia oil), the amount of unsaturated bonds present in the molecule (referred as the oil functionality), and chain length alkyl derived from fatty acids (Table 1.1).

Figure 1.1 Schematic structure of triglycerides.

Table 1.1 The content of various fatty acids in selected vegetable oils.

The functionality of oils (understood as the content of unsaturated bonds) primarily determines the cross-linking density of oil-based chemosetting polymers or polymers obtained by free radical polymerization as well as oil-modified polymeric materials. In turn, the final polymer properties such as mechanical strength, thermal stability, and chemical resistance strongly depend on the cross-linking density. The elasticity of the polymers with the addition of vegetable oil or based on them depends on the length of the alkyl chains in the oil molecule derived from fatty acids.

Vegetable oils can be easily and efficiently converted into epoxy derivatives by oxidizing unsaturated bonds present in fatty acid residues. Several methods of double bond oxidation in triglyceride molecules are known and commonly used [3]: the method based on the Prilezhaev reaction, the radical oxidation, the Wacker-type oxidation, dihydroxylation of oils and fats, and enzyme-catalyst oxidation. The Prilezhaev reaction is the most often used method for natural oil epoxidation, commonly applied in the industry. In this method, the process of epoxidation of natural fatty acids and triglycerols is carried out in the system consisting of hydrogen peroxide, an aliphatic carboxylic acid, and an acidic catalyst. The organic peracid formed in situ by the reaction of acid with hydrogen peroxide is the real oxidizing agent in this method (Figure 1.2).

Carboxylic acids with one to seven carbon atoms are the most commonly used (in practice, mainly acetic acid). Inorganic or organic acids and their salts, as well as acidic esters, can be used as catalysts; however, sulfuric and phosphoric acid are the most often used in industrial practice. A promising method is oxidation in the presence of enzymes [4], heteropolyacids [5], and even ion exchange resins [6] as catalysts. The most commonly used oxidizing agent is hydrogen peroxide in the form of solution with a concentration of 35-90% (usually 50%). Epoxidation of plant oils in ionic liquids, as well as in supercritical carbon dioxide, is also described [7].

The earliest epoxidized esters of higher fatty acids have found wide applications as both plasticizers and stabilizers for thermoplastics, mostly poly(vinyl...