Study On The Improvement Of Fiber-Matrix Interphase Strength Of Ramie Fibers Reinforced Poly (Lactic Acid) Bio Composites Via Fiber Surface Modification And Composite Thermal Annealing

Polylactic acid(PLA)is a known biodegradable thermoplastic polymer with appreciated mechanical properties.It is one of a naturally synthesized thermoplastic resin into usable format in laboratory from agricultural crops.When it is molded to composite,it is proven to degrade at different rates and profiles based on the kind of reinforcement,applied modification and composition of its lactide monomer.Due to this promising property,it has been widely used as disposable packaging material,scaffolds in tissue engineering and cell support materials and other applications in the garment and automobile industries.However,the use of PLA is still limited by its inherent lack of toughness,low heat of softening and low thermal stability.To overcome these limitations,in this study,PLA-based composites were reinforced with natural ramie fibers so as to improve the toughness and thermal resistance properties.The defined orientation of reinforcing fibers in a composite can increase the strength and resistance to deformation of the PLA polymer.Because,ramie fiber is cheaper and lighter than conventional fibers and at the same time environmentally friendly.Ramie fiber provides the good heat resistance and superior mechanical strength than the other lignocellulosic fibers owing to its high percentage of cellulose.On top of that,the mechanical and chemical properties of ramie fibers/PLA composites are mainly determined by the type of surface modification introduced to fibers,orientation of fibers in the matrix and the nature of PLA matrix used.The ramie fibers/PLA reinforced composite system has two distinct phases,the polar fibers(hydrophilic)and the nonpolar(hydrophobic)polymer that require defined a strategy to improve compatibility and interphase strength.Aimed to improve the surface compatibility between these two materials(ramie fibers and PLA)during composite formation,the following surface modification approaches were studied and,in the meantime,applied on ramie fibers as a means of solutions to the problems:(1)Sodium hydroxide(alkali),(2)silane coupling agent,(3)combination of alkali and silane,(4)heat and(5)pectinase enzymes treatments.Furthermore,di-ammonium phosphate(DAP)treatment was also introduced on the ramie fibers following the above steps.It was assumed that modification of fiber surface could engineer the adhesion between fiber/matrix interface strength to the optimal level.Thus,the bond strength at the ramie fiber/PLA matrix interface could be obtained from the surface energies of the reinforcement,which have an indicative nature for the adhesive properties of the fiber.In particular,the properties of natural fibers reinforced composites are strongly determined by bond strength forces at fibers/matrix interfaces.After all,the treated ramie fibers were prepared in some defined ratios(5%,10%,20%,30% and 40%)and opened to ramie/PLA fibers mat first,and then by taking six layers of these ramie/PLA fiber mat stacks were used to obtain each composite sample.The prepared layers of ramie/PLA mats stacks were molded together between two metal plates by heat compression molding with the molding pressure of 6-8 MPa,at molding temperature of 170 ? and molding time of 6 min.To study the thermal stability and melt crystallization properties of ramie/PLA bio composite,the molded samples were produced either by direct cooling(without heat annealing)or melt crystallization(constant heat annealing).Then,the interfacial adhesion properties of untreated and treated ramie fibers in PLA composites were characterized by contact angle measurements and mechanical test analysis.The interfacial adhesion of composites was estimated from the wetting analysis of ramie fibers/PLA composites.The polar liquids,water and disperse diiodomethane were taken to measure the wetting properties of composites.For the mechanical interface strength characterization,transverse three points bending test,tensile strength,and Izod impact tests were performed.FTIR characterization was also carried out to obtain the surface chemistry,and functional groups constituted on the fiber/matrix interface before and after modifications.The resultant bio composites obtained from the study showed tunable mechanical and thermal resistance properties.These improved properties are ascribed to the improved interfacial bonding properties of modified ramie fibers and PLA polymer courtesy of newly introduced reactive groups onto fibers which were capable of building strong interactions between the polymer-fiber interfaces.Additionally,the thermal stability of the resultant composite is attributed to the thermal annealing condition which provided crystallized PLA molecules upon exposure to high temperatures and the steady cooling responsible for the increased degree of crystallinity in the composites as confirmed from X-ray Diffraction(XRD)and Differential Scanning Calorimetric(DSC)data results.The thermal study results indicated that DAP modified ramie fibers/PLA composites exhibited outstanding performance towards lowering thermal decomposition,providing unique decomposition patterns and increasing residual char at the decomposition temperature.The tensile,flexural and impact properties of DAP modified ramie fibers were superior to untreated ramie fibers composite.Moreover,the morphological structures were also observed on SEM that wellmodified ramie fibers enhanced fracture mechanism of failure under tensile,flexural and impact tests.Besides,this study was also highlighted the molding process optimization and degradation properties of ramie fibers reinforced PLA composites by underground burial experimental procedures.