| Natural fiber reinforced composites are very popular with the advantages of low cost and easy processibility, which can meet the global sustainable development. Ramie fiber reinforced Poly lactic acid (PLA) composites have good mechanical performance, excellent dimensional stability and degradation. However, it is known that the hydrophilic nature of plant fiber make it less attractive for reinforcement of polymeric composite materials because of the poor adhesion between plant fiber and polymeric materials, leading to the limited applications of these kinds of composites. In this dissertation, the aim was to discuss effect of fiber chemical surface modification on ramie/PLA composites.First of all, two different surface treatment methods, alkalization and coupling silane agent treatment, were applied on ramie fibers. Effect of fiber surface treatments on the interfacial properties was studied at the microstructure level. The concentrations of alkalization and silane coupling agent were set as3%,5%and7%. For the optimized results,5%alkali and5%silane coupling agent treatment was also applied to ramie fibers. The conclusions are listed as followed:(1) The results of Fourier Transform Infrared Spectroscopy (FT-IR) showed that siloxane groups were grafted on silane coupling agent treated ramie fibers with two new bands at780cm-1and1203cm-1, which were corresponding to-Si-C and-Si-O-C-bond, respectively.(2) The results of fiber dynamic contact angle test showed alkali treated ramie fiber become more hydrophilic. And the contact angle of fibers treated by the solution concentration of5%reached the highest one. However, silane coupling agent treated fibers reduced their wettability, especially for the solution concentration of5%.(3) The results of micro-bonding test showed that when the solution concentration was5%, the interfacial shear strength (IFSS) of alkali and silane coupling agent treated fiber was respectively increased by34.7%and19.9%, compared to that of the untreated one. The fibers treated by the combined method reached the value of55.27MPa with the increase of39.9%.(4) The results of sScanning Electron Microscope (SEM) showed that after the alkali treatment, the ramie fiber surface became clean. However, there was no appreciable morphological difference found for the silane coupling agent treatment.(5) The results of X-Ray Diffraction (XRD) spectroscopy showed that the crystalinity index of the alkali treated fibers increased, except that of fibers treated by the solution with the concentration of7%. But the crystalinity of silane coupling agent treated fiber decreased compared with that of the untreated one. The lowest crystalinity index was36.27%for the silane coupling agent solution with the concentration of5%.(6) The results of fiber tensile strength test showed that the alkali treated fiber with two different concentrations of3%and5%performed higher mean values of tensile strength, except that with the concentration of7%. All silane coupling agent treated fibers enhanced the tensile strength, compared with the untreated one.Then, the ramie fabric reinforced PLA composites were hot-pressed. The fabric were pre-treated with5%concentration of alkali and silane coupling agent solution, and treated by the combined treatment in order to investigate effect of fiber surface treatments on the composite properties. The conclusions are as follows:(1) The results of mechanical test exhibited the improvement of tensile, flexural and impact properties. Tensile stress of the composites pre-treatcd with5%alkali solution,5%silane coupling agent solution and combined method increased by30.3%,44.8%, and51.2%, respectively. The combined method make the composites perform the highest flexural and impact strength, which increased by37.8%and47.1%, respectively.(2) The results of Thermal Degradation Analysis (TGA) indicated that the thermal stability of composite pre-treated by5%alkali solution respectively increased by41.75℃,9.98℃and10.06℃, when the composite weight degraded to20%,50%, and80%. But the flame resistant ability was enhanced when treated with5%silane coupling agent.(3) The results of water absorption ability showed that the mass water absorption increased by5.42%,5.07%and5.34%, respectively, when the composites were treated with5%alkali solution, silane coupling agent and combined treatment method.(4) The results of Scanning Electron Microscope (SEM) showed that, most of fibers in the composites without any treatment pulled out from the PLA matrix while those in the composites pretreated with chemical treatments failed.Lastly, the degradation ability of ramie/PLA composites was observed. The conclusions are as follows:(1) Treatment with alkali solution increased the weight loss and tensile strength loss of ramie/PLA composites. Pre-treatment with alkali solution accelerate ramie/PLA composites’ degradation speed, because of residual alkali left in the ramie fabric.(2) Treatment with APS solution reduced the composites’ weight loss and tensile strength loss, the results indicated the ramie surface modification with APS solution could slow down the degradation rate. |
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