# Development of Bio-Sourced Epoxies for Bio-Composites
## 2 Rosin-Sourced Epoxy as a Matrix Resin  
Rosin is an abundantly available natural product, it is nontoxic and odorless, and contains various isomerized acids (>90%) and some neutral substances [5]. Reactive double bonds and carboxyl groups of rosin acids render them suitable for the Diels-Alder reaction, esterification and condensation reaction usage. Therefore, rosin acid has received increasing attention as a bio-sourced form of renewable feedstock in polymer science. Anhydride is one of the curing agents for epoxies (EPs). Owing to their characteristic bulky hydrogenated phenanthrene ring structure, rosin acids are analogous to many aromatic compounds in rigidity. However, their curing temperature is generally high, about 200-250 \({}^{\circ}\)C, and the curing time is long, which limits the use of the anhydride in some products and apparatus. The complex reactionsof epoxy/anhydride curing include the carboxyl group reaction with an epoxy ring in the catalyzed action of accelerator, alternate ring-opening copolymerization of epoxy, and anhydride catalyzed by amine and polyetherification at high temperatures, initiated by amine and catalyzed by hydroxyl groups [6]. These processes occur when an amine is added as the catalyst. An excellent reference [6] on this issue can be found in the literature. In this study [7; 8], an anhydride-type epoxy curing agent, maleopimaric (Scheme 1), was synthesized as a hardener from rosin acid (Scheme 1). This was supplied by the Ningbo Institute of Materials and Technology Engineering. A resin mixed with an E51-type epoxy and a solid phenolic epoxy was then prepared as the main component, together with an accelerator mixed of two amino imidazole salts as the thermally latent curing agent and also as the catalyst for the anhydride. The experimental details can also be find in reference [7]. Figure 1 exhibits the glass transition behavior of the formulated resin system. As shown, the curing degree increased with the curing temperature for a constant curing time of 3 h, as well as the glass transition temperature. The formulated matrix resin was finally designated as AGMP3600, with a bio-content of about 30%. Figure 2 shows the typical viscosity behavior of the trial product AGMP3600. It behaved well in the film manufacturing and subsequent prepreg production. The process condition for the prepreg using AGMP3600 as the matrix in the autoclave is shown in Figure 3.  
The mechanical properties of AGMP 3600 laminates reinforced with different kinds of fibers and weaves were determined and are listed in Tables 1-4 in some cases compared with the state-of-the-art counterparts as reference. Figure 1: Differential Scanning Calorimetry (DSC) curves of the rosin-sourced epoxy resin system designated as AGMP3600 at different temperature conditions. In parallel, a 180 \({}^{\circ}\)C/2 h cured-rosin epoxy was also developed, designated as AGMP5600, with a higher bio-content of about 40%. It is particularly interesting to note that the glass transition temperature of AGMP5600 matrix composite reinforced with EW250F glass weave was about 220 \({}^{\circ}\)C. Table 4 lists the mechanical properties of the laminated composite. The temperature-dependent mechanical properties were also studied, as well as those after the hot/wet exposure (1000 h/70 \({}^{\circ}\)C/85% r.H.) \begin{table}
\begin{tabular}{c c c c} \hline
**Properties** & **UNIT** & **3233B/EW250F** & **AGMP3600/EW250F** \\ \hline Ply thickness & mm & 0.241 & 0.237 \\ Bending strength & MPa & 696 & 674 \\ Bending modulus & GPa & 19.9 & 21.1 \\ Short beam shear strength & MPa & 45 & 60.9 \\ Tensile strength & MPa & 470 & 540 \\ Tensile modulus & GPa & 22.5 & 25.3 \\ Compression strength & MPa & 474 & 483 \\ Compression modulus & GPa & 23.9 & 27 \\ \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties of AGMP3600/EW250F (glass fabric, 8 satin, 47 vol %), a rosin-sourced epoxy laminates, and of 3233B/EW250F, a standard intermediate-temperature epoxy laminate for reference and comparison.