The effect of grinded glass and sisal fibers (25 vol%) on the cure kinetics of composites of unsaturated polyester resin (UPR) was investigated by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The DSC analysis was carried out at four different heating rates (5, 10, 20 and 40 °C/min), and the cure enthalpy and activation energy (Ea) were determined according to the Flynn-Wall-Ozawa (FWO) method. The results showed that increasing heating rates promoted reduced reaction times. The sisal fiber-containing composites exhibit higher activation energy values for the cure process in comparison with the neat polyester resin and the glass fiber composites. This can be due to the presence of polar groups in the sisal components, which physically interact with the polyester resin and retard the cure reaction. Hence, as sisal fiber retarded the cure reaction of the UPR resin, it is suggested that the use of natural fibers in polymer matrix composites can affect the cure kinetics of the polyester resin.

The use of ramie fibers as reinforcement in hybrid composites is justified considering their satisfactory mechanical properties if compared with other natural fibers. This study aims to verify changes in chemical composition and thermal stability of the ramie fibers after washing with distilled water. One additional goal is to study glass fiber and washed ramie fiber composites focusing on the effect of varying both the fiber length (25, 35, 45 and 55 mm) and the fiber composition. The overall fiber loading was maintained constant (21 vol.%). Based on the results obtained, the washed ramie fiber may be considered as an alternative for the production of these composites. The higher flexural strength presented being observed for 45 mm fiber length composite, although this difference is not significant for lower glass fiber volume fractions: (0:100) and (25:75). Also, by increasing the relative volume fraction of glass fiber until an upper limit of 75%, higher flexural and impact properties were obtained.