The effects of the crystallinity index of cellulose on the flexural properties of hybrid-cellulose epoxy composites

This work investigated the effects of the crystallinity index (CI) of cellulose on the flexural properties of hybrid-cellulose epoxy composites. The CI was varied by combining cellulose microfibrils (CMF) and microcrystalline cellulose (MCC), extracted from coir and eucalyptus, respectively, in various mixing ratios. From the XRD analysis, it was shown that the CI values and the CMF/MCC content follow a parabolic-fitting pattern, reaching a maximum value of 82.06% for the CMF/MCC of 30/70. The composites with cellulose of different CI values were fabricated and the flexural properties were measured. The results showed that the flexural strength increased as the CI increased, reaching a maximum of 105.45 MPa for a CI value of 81.87%. However, the flexural modulus displayed a parabolic relationship with respect to the CI value, reaching a maximum of 3,015.53 MPa for a CI value of 81.45%. FTIR spectroscopy was used to study the correlation between the interfacial bonding via hydrogen bonding of hydroxyl groups in cellulose and epoxy, the CI value, and the flexural properties. It was suggested that the strength of the cellulose was an important factor for the flexural strength of the composite, whereas both cellulose strength and interfacial bonding were crucial for the enhancement of the flexural modulus.