| Nowadays, with the petroleum shortage and the serious environment issue, natural fibers reinforced biocomposites are receiving more and more attention by researchers worldwide because of their ecologically friendly behavior when considering carbon foot print. Sisal fibers have many excellent properties over synthetic fibers and can be used to reinforce the polylactide(PLA), which is a kind of biodegradable polymer. This can not only improve the properties of PLA and expand its application, but also solve the above issues thus achieving sustainable development.Because of the bad interface compatibility of untreated sisal fiber(USF) reinforced PLA composites, this paper prepared the lactide-grafted sisal fiber(SF-g-LA) basing on grafting copolymerization in order to improve the interfacial properties between sisal fiber and PLA matrix. Orthogonal test was done to investigate which process parameter affected the grafting ratio mostly. The effects of different treatments on the surface topography, thermal stability, tensile strength and tensile modulus of sisal fibers were also discussed. Then the three kinds of sisal fibers were mixed with PLA to prepare composites, respectively. The thermal properties, crystallization and mechanical properties of composites were studied. The effect of annealing treatment on thermal properties and crystallization of composites was also investigated. Gel permeation chromatography(GPC) was used to investigate changes of the relative molecular weight of PLA after mixing. At last, the Fukuda computation model was introduced and the applicability of this model was discussed. Results demonstrated that:After alkali treatment, the small substances such as hemicellulose, lignin, pectin etc. were removed from sisal fiber thus reducing the fiber diameter. Both the tensile strength and tensile modulus decreased while the thermal stability of alkali-treated sisal fiber(ASF) increased. The diameter of SF-g-LA increased because of the occurrence of PLA molecular chains onto the fibers. All of the thermal stability, tensile strength and tensile modulus of SF-g-LA decreased compared with ASF. The initiator amount was found to be the most important determinant of the grafting ratio. The grafting ratio improved rapidly with the increase of initiator amount at the very start, then the growth rate slowed down and the grafting ratio kept stable eventually. The increase of grafting ratio does not decrease the thermal stability of SF-g-LA.With the addition of sisal fibers, the thermal stability of composites decreased compared with neat PLA, together with the temperature of maximum pyrolysis rate and initial pyrolysis temperature. Both the Mw and Mn of PLA in composites reduced compared with neat PLA. The addition of sisal fibers can accelerate crystallization and improve the crystallinity of PLA, and SF-g-LA improves the most. The polarizing microscope results demonstrated that transcrystallinity appeared in PLA around the sisal fibers with the addition of sisal fibers and the transcrystalline layer of PLA/SF-g-LA composites was the thickest while that of the PLA/USF composites the thinnest. The glass transition temperature, crystallinity and crystal perfection of neat PLA and composites were improved with annealing treatment.The tensile strength and flexural strength of PLA/USF decreased while the tensile modulus, flexural modulus and impact strength increased compared with neat PLA. The tensile strength, tensile modulus, flexural strength and flexural modulus improved with the increase of ASF and SF-g-LA, while the impact strength increased first and then decreased. The tensile elongation at break of three composites all decreased compared with neat PLA. The interface compatibility of PLA/USF was the worst while the PLA/SF-g-LA best. The modified model when considering compression ratio fitted better than Fukuda model. However, the model still needs to be modified and the interface compatibility factor should be considered. |
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