Preparation And Interfacial Characterization Of Modified-Hemp-Fibers Reinforced Unsaturated Polyester Composites

In this paper, the morphology and chemical composition of hemp fibers were investigated. The fibers were modified with3-isopropenyl-dimethylbenzyl isocyanate (TMI), using dibutyltin dilaurate as a catalyst, to improve the interfacial adhesion between hemp fibers and unsaturated polyester (UPE) resin. The usage of TMI was based on the dry weight of the fibers, and was adopted by3%,5%,7%of the dry weight of the fibers respectively. The usage of catalyst was based on the TMI weight, and was adopted by5%,10%,15%,20%of the TMI weight. The hemp fibers reinforced UPE composites were prepared by compression molding. The effect of surface modification of hemp fibers with TMI on the mechanical properties and water resistance of the resulting hemp-UPE composites was investigated. The surface chemical composition and functional groups of hemp fibers were characterized by Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) analyses. The interfacial adhesion of hemp-UPE composites was analyzed by scanning electron microscope (SEM) using the fractured-surface of composites as samples. The main conclusions obtained from above research are as follows:(1) The length, width and length-width ratio of hemp fibers were distributed majorly from20mm to60mm,20μm to100μm, and300to900, respectively. The contents of cellulose, pentosan and lignin of hemp fibers were37.34%,20.10%and24.50%, respectively.(2) The FT-IR spectra of hemp fibers showed that TMI-treated hemp fibers had a strong peak at1731cm-1resulting from the stretch vibration of C=O and a peak at1281cm-1resulting from the stretch vibration of C-N. The C=O and C-N groups might result from the structure of carbamate of the reaction products between the isocyanate groups of TMI and the hydroxyl groups of hemp fibers only due to the fibers were extracted by ethyl acetate before FI-IR testing. This indicated that TMI was bonded covalently onto the fibers surface. The stretch vibration of hydrogen bonds in hemp fibers were analyzed from the deconvolved FT-IR spectra between3650cm-1and3000cm-1, which demonstrated that the primary hydroxyl groups in hemp fibers had a higher reaction possibility than that of the secondary hydroxyl groups, and that TMI was reacted majorly with the primary hydroxyl groups.(3) The XPS analysis showed that TMI was bonded covalently onto the surface of hemp fibers because the TMI-treated hemp fibers had a lower O/C atomic ratio, and other two more C1s peaks (C-N and N-C=O) than that of the untreated hemp fibers.(4) The treatment of hemp fibers with TMI significantly increased the tensile strength, flexural strength, and impact strength of the resulting hemp-UPE composites, and yet did not change flexural modulus of the composites significantly. When the TMI usage was7wt%based on the weight of hemp fibers and the corresponding catalyst usage was20wt%based on the TMI weight, the resulting composites had37.74%,22.75%and81.70%higher tensile strength, flexural strength, and impact strength than that of the untreated-hemp-UPE composites.(5) The treated-hemp-UPE composites had a much better water resistance than the untreated-hemp-UPE composites. The water absorption in hemp-UPE composites follows Fickian behavior since the experimental data of water absorption of composites was fit reasonably well with the Fickian mode of diffusion.(6) SEM graphs of the tensile-fractured surface of hemp-UPE composites revealed that the modification of hemp fibers with TMI greatly improved the interfacial adhesion between hemp fibers and UPE resins.