Most of epoxy hardeners such as polyamines, polyphenols, and carboxylic acid an­hydrides are derived from petroleum resources. The past studies on bio-based ep­oxy hardeners are less than that on the bio-based epoxy resins. Basically, bio-based polyamines, polyphenols, and carboxylic acid anhydrides can be used as epoxy­hardener. We investigated a possibility of bio-based polyamines and polyphenols as hardeners of bio-based epoxy resins for biocomposites with lignocellulosic fibers considering an interfacial adhesion between matrix resin and fibers. Although the reaction of polyamine or polyphenol with epoxy resin generates hydroxy groups which can form hydrogen-bonding with lignocellulosic fibers, carboxylic acid an­hydrides do not produce hydroxy groups but ester groups.

As a bio-based polyamine hardener, e — poly(L-lysine) (PL) was investigated.37 The PL is produced by aerobic bacterial fermentation using Streptomyces albulus in a culture medium containing glucose, citric acid, and ammonium sulfate.3839 The PL has been used as food preservatives,40 while has not yet been applied to the industrial polymeric materials. PL differs from usual proteins in that the amide linkage is not between the a — amino and carboxylic groups in typical of peptide bonds, but is be­tween the e — amino and carboxy group. The pendant a -amino groups are expected to react with epoxy groups. When GPE or PGPE was cured with PL, a soft cured resin (GPE/PL or PGPE/PL) with a tensile modulus lower than 10 MPa and glass transition temperatures (Tg’s) measured by DSC lower than 50 °C was obtained. Although PL is interesting as a hardener for bio-based epoxy/clay nanocomposites,37 we did not use PL as a hardener for bio-based epoxy/natural fiber biocomposites because of the inferior mechanical and thermal properties.

As bio-based polyphenol hardeners, we investigated tannic acid (TA) and quer­cetin (QC) as are shown in Fig. 4.2.2124 Commercial TA is comprised of mixtures of gallotannins from sumac galls, Aleppo oak galls, or sumac leaves.41 The chemical formula for commercial TA is often given as C76H52O46 as shown in the figure. But, in fact it contains a mixture of related compounds. Its structure is based mainly on glucose ester of gallic acid. QC (3,3,’4,’5,7-pentahydroxyflavone) is one of the most abundant flavonoid found in glycosylated forms in plants such as onion, capers and tea. TA is industrially available from various Makers (for example, Fuji Chemical Industry, Co., Ltd. (Wakayama, Japan)), we used the reagent grade TA of Kanto Chemical Co., Inc. (Tokyo, Japan). QC can be obtained from plants via extraction of the quercetin glycosides followed by hydrolysis to release the aglycone and sub­sequent purification.42 Although QC is used as an ingredient in supplements, bever­ages and foods, it has not been used as an ingredient of polymer materials to the best of our knowledge. When QC is used as an epoxy hardener, it is expected that the cured resin has a high Tg and superior adhesiveness to plant fibers because of the rigid and polar polyphenol moiety. We used the QC purchased from Sigma-Aldrich Japan Co. Ltd. (Tokyo, Japan).

Also, bio-based phenols such as pyrogallol (PG) and cardanol (CD) are promis­ing raw materials for the preparation of bio-based phenolic epoxy hardeners (Fig. 4.3). PG is obtained by decarboxylation of gallic acid, which is a basic component of hydrolysable tannin. Utilization of PG as a raw material of the preparation of epoxy hardeners was described in the following section in detail. CD is a phenol meta-substituted with a flexible unsaturated hydrocarbon chain, which is derived from cashew nutshell liquid.43 There have been many studies on the utilization of CD to phenol resins44,46 and epoxy resins4649, and some of them have been already commercialized by Cardolite Corp. (Newark, NJ, USA) and Shanghai Meidong Biomaterials Co. Ltd (Shanghai, China), etc.