l-Arginine as a Bio-Based Curing Agent for Epoxy Resins: Glass Transition Temperature, Rheology and Latency

Abstract

The need for sustainable practices in the processing chain of fiber-reinforced thermosets has led to the development of bio-based epoxy resins and curing agents. As a contribution to sustainable composites, this study focuses on the glass transition temperature (Tg), viscosity and latency of diglycidyl ether of bisphenol a (DGEBA) cured with l-arginine in the presence of a urea-based accelerator. These characteristics are decisive features for application as a matrix in fiber-reinforced polymer composites produced via prepreg technology in which low viscosity and sufficient latency, meaning low reactivity of the one-component system, are necessary. The homogeneous mixture of amino acid and epoxy resin was prepared via three-roll milling. Two formulations, Argopox-1 with 1wt.% accelerator and Argopox-2.5 with 2.5wt.% accelerator, were prepared and parts of each formulation were stored at 22∘C and −18∘C, respectively. Both formulations were tested via differential scanning calorimetry (DSC) and small amplitude oscillatory shear rheology (SAOS) after 0 day, 30 day, 60 day, 90 day and 180 day of storage to determine the influence of accelerator weight fraction, storage temperature and storage period on the glass transition temperature of the uncured resin system Tg0, and their viscosity. The Tg of the thermosets is about 100∘C. The DSC and SAOS measurements show that the Tg0 of Argopox-1 shifts about 5∘C in 60 day, while its viscosity is still low enough to be processed in a prepreg production line. Furthermore, Argopox-1 is storable for at least 180 day at −18∘C without significant changes in its Tg0 and viscosity. Consequently, Argopox-1 possesses a sufficiently high Tg and adequate latency, as well as a low viscosity for application as prepreg matrix material.

l-Arginine as Bio-Based Curing Agent for Epoxy Resins: Temperature-Dependence of Mechanical Properties

“…It is likely that the lower of Argopox compared to DGEBA cured with dicyandiamide stems from the aliphatic side chain of l -arginine which contains three carbon atoms between its -amine group and the next functional group. The carbon chain is highly flexible which facilitates rearrangements of network segments and thus lowers [ 17 ]. During the glass transition, the storage modulus drops down to about 28 .…”

“…As there are 22 proteinogenic amino acids, it would be interesting to see the effect of different amino acids on the mechanical properties of the resulting thermoset. Since Argopox possesses a low viscosity for prepreg production, as well as sufficient latency [ 17 ] and mechanical properties that reach performance levels similar to that of materials which are already used as matrix materials over a wide temperature range, it is suitable as matrix for sustainable fiber reinforced composites.…”

“…The preparation of the mixture of DGEBA and l -arginine follows the procedure already applied and described in [ 17 ]. One weight percentage of DYHARD®UR500 (see Table 1 ) was added before mixing in a dual asymmetric centrifuge speed mixer by Hauschild Engineering (Hamm, Germany) at 3000 min −1 for 120 .…”

“… Fracture surfaces of compact tension specimens made from Argopox after testing at T = ( a ), 22 ( b ), and 80 ( c ). Scanning electron microscopy image of cured Argopox at 50,000 × magnification showing the nanometer-sized l -arginine particles ( d ) [ 17 ]. …”

“…Previous investigations on epoxy resins cured with amino acids focused mainly on the reaction kinetics and of diglycidyl ether of bisphenol A (DGEBA) cured with l -tryptophan [ 11 , 12 , 13 , 14 , 15 , 16 ]. Rothenhäusler et al [ 17 ] investigated the and viscosity of DGEBA cured with l -arginine (see Figure 2 ) as well as its latency at T = 22 and T = . They found that the thermoset possesses a of about 100 , while its viscosity and latency are suitable for prepreg production.…”