Newly epoxy resin synthesis from citric acid and the effects of modified almond shell waste with different natural acids on the creation of bio-based composites

Abstract

Synthesis of sustainable and bio-based epoxy resins that can replace petro-based and potentially harmful existing epoxy resins is important. In this sense, a novel biobased epoxy resin (ECiA) was synthesized through the esterification reaction of biobased citric acid (CiA) with epichlorohydrin (ECH). The chemical structure of ECiA was verified using FT-IR, 1H NMR, MALDI-TOF mass spectrometry, and epoxy group determination. Moreover, two biobased carboxylic acids-oxalic (OA) and linoleic (LA) acids were used in the chemical treatment process. Raw (ASh) and acid-modified AShs (LA-ASh and OA-ASh) were applied as inexpensive natural reinforcement materials in the bisphenol A-type epoxy resin (ER)-ECiA blend matrix system. The composites were produced with 10-20-30-40-50 % of ASh particles inserted in the epoxy blend matrix, and the mechanical properties, thermal behavior, dynamic-machanical properties, water absorption, and surface wettability were determined. The investigation highlighted the impact of modifying ER and reinforcing elements on the quality of the composite materials. Chemical treatments show an enhancement in adhesion between the ASh fillers and ER-ECiA matrix, as evidenced by the morphological results. The chemically modified AShs-based composites exhibit higher tensile strengths compared to both the neat ER-ECiA and untreated ASh composites. The study revealed that the composite material consisting of 15 wt% LA-ASh (70:30 wt%) demonstrated elevated values for tensile strength (88.9 MPa) and elasticity modulus (E-modulus) (7.7 GPa). TGA analysis showed that AShs improved the thermal stability of pure ER-ECiA to a certain extent. Composites prepared with modified AShs exhibited hydrophobic surfaces. According to DSC and DMA results, the glass transition temperatures (Tg) of the composites were found to be higher than the pure ER-ECiA mixture. The same results are valid for the degrees of cure calculated from FT-IR spectra.