4.2.1 BIO-BASED EPOXY RESINS

Bio-based aliphatic epoxy resins such as glycerol polyglycidyl ether (GPE), poly­glycerol polyglycidyl ether (PGPE), sorbitol polyglycidyl ether (SPE), epoxidized soybean oil (ESO), epoxidized linseed oil (ELO), diglycidyl ester of dimer aid (DGEDA) and limonene diepoxide (LMDE), etc. are industrially available in large volumes at a reasonable cost (Fig. 4.1). Glycerol is an abundant and inexpensive bio-based aliphatic polyols, which can be derived from triglyceride vegetable oil. The biodiesel boom of the recent decade led to a significant increase in biomass — derived glycerol.27 At present, the worldwide glycerol production is around 1.2-1.4 million tons. This amount will further increase to 1.54 million tons in 2015.28 How­ever, the total global demand is less than 1 Mt. Thus, the market for petrochemical derived glycerol that is available via propene, allyl chloride and epichlorohydrin (ECH) no longer exists. The utilization of glycerol for the production of other in­termediate chemicals and final materials will become very important in near future. Indeed, some of these products are close to commercialization or already introduced into the market. One such example is the Epicerol® Process introduces by Solvay in 2007 enabling the ECH synthesis from bio-derived glycerol.29 Also, glycerol is an attractive renewable building block for synthesis of polyglycerols which have several uses in different field. Polyglycerols, especially diglycerol and triglycerol are the main products of glycerol etherification.30 Therefore, GPE and PGPE which are synthesized by the reactions of glycerol and polyglycerol with ECH should be very promising bio-based epoxy resins. Although the industrially available GPE and PGPE have been used in textile and paper processing agents and reactive diluents, etc., their epoxy resins have not yet been applied to matrix resins for fiber-reinforced plastics.

Sorbitol is also abundant and inexpensive bio-based aliphatic polyols, which can be produced by the catalytic hydrogenation of glucose derived from corn starch.3132 It is widely used in the food industry, not only as a sweetener but also as a humec — tant, texturizer, and softener. The SPE which is prepared by the reaction of sorbitol and ECH has been mainly used for tackifier, coatings, and paper and fiber-modifier, etc. Epoxidized vegetable oils such as ESO and ELO are manufactured by the epoxi — dation of the double bonds of vegetable oils with hydrogen peroxide, either in acetic acid or in formic acid,3335 and have mainly been used as plasticizer or stabilizer to modify the properties of plastic resins such as poly(vinyl chloride). Because ELO has a 30% more oxirane content than ESO does, the cured ELO has a higher cross­linking density, which results in a better performance.

Dimer acid-based DGEDA is a flexible epoxy resin produced from dimer acid and ECH. Although dimer acid can be also obtained from animal fats or vegetable oils, most of the dimer acids appearing in the market are synthesized from the crude tall oil provided as a byproduct of Kraft pulp.36 The commercially available dimer acid usually contains monomer (1-5%) and trimer or more (14-16%) in addition to dimer. Limonene-based LMDE (1-methyl-4-(2-Methyl-2-oxiranyl)-7-oxabicyc- lo[4.1.0]heptane) is commercially produced by the reaction of limonene and per­acetic acid, which is used as cationically curable resins and reactive diluents. (+)-d — Limonene is a popular monoterpene which is commercially obtained from citrus fruits.

However, their bio-based aliphatic epoxy resins had not been versatile mate­rials because of inferior mechanical and thermal properties to the bisphenol-A or novolac-based epoxy resins. Therefore, the selection of hardener and the addition of natural fibers are important for the use of the bio-based aliphatic epoxy resins in wide applications. In the following section, we used GPE, PGPE, SPE, and ESO as bio-based aliphatic epoxy resins for the preparation of green composites. Their physical properties and suppliers are summarized in Table 4.1. The average number of epoxy groups per molecule of GPE, PGPE, SPE and ESO are 2.0, 4.1, 3.6, and 4, respectively.