| Due to the concern for the environment and sustainability,research on plant fibers has increased to improve their utilization,and biocomposites containing plant fibers have become the focus of scientific research in recent years.The biodegradable plastic polylactic acid(PLA)has the advantages of a widely renewable raw materials,but its industrial applications are greatly limited due to its high cost and partial performance defects.The preparation of fully bio-based green composites using plant fibers and PLA can reduce the material cost and integrate the performance of both,which also helps to alleviate the petroleum resource constraints and achieve the sustainable development of resources.Coconut tree is a valuable food or energy resource,and fibers can be extracted from many parts of it.The aim of this paper is to extract fibers at the coconut petiole and use them as reinforcing fillers to prepare biocomposites by blending with PLA.Firstly,the extracted coconut petiole fiber(PF)were treated with alkali and the microscopic morphological structure,mechanical and thermal properties of PF,as well as the interfacial properties between PF and PLA matrix were investigated.Then,the orthogonal test method was introduced to study the effects of mixing temperature,rotational speed and initial fiber length on the size change of PF and overall mechanical properties of the composites;PLA/PF composites with fiber loadings of 10%,20%,30%,40%and 50%were prepared by optimized processing parameters to investigate the thermal or crystallization properties of the composites.The fracture form or strengthening mechanism of PF during the material break were analyzed by the composite cross-sectional morphology.The mechanical properties of PLA/PF composites were tested to investigate the strengthening effect of PF.Results showed that:1.Micron-sized holes appeared on the surface of PF after alkali treatment,which increased the roughness of the fiber surface and improved the interface shear strength between PF and PLA greatly.The microfibrous bundles in the PF all exhibited an obvious"spiral"structure.The best overall effect was obtained when the alkali treatment time was 2 h.The interfacial shear strength of PF-PLA was 14.06 MPa,which was 130.9%higher than that of untreated PF at 2 h of alkali treatment.And the tensile strength of PF was 355.77 MPa,which was 55.6%higher than that of untreated PF.The elongation at break of PF remained above 20%before and after alkali treatment,indicating that the PF had a good tensile toughness,while the effect of alkali treatment on it was not obvious.2.The effect of initial fiber length on the final fiber length in the composite was significant,but the effect of mixing temperature and rotational speed is not.The influence of each factor on the final length of fibers in the composite in descending order was:fiber incorporation length,mixing temperature,and rotational speed.As the fiber incorporation length grew up,the dispersion of the final length of the fibers in the composite and the proportion of longer size fibers increased.The critical fiber length L_c(2.28～5.69)mm for PF in PLA matrix was calculated.Increasing the temperature,reducing the speed,and longer fiber incorporation length can eventually increase the number of fiber lengths in the composite that exceed the critical fiber length to play the reinforcement effect.3.The thermal stability of PF was poorer than PLA,and the thermal stability of PLA/PF composites was reduced and lower than that of pure PLA.The addition of PF increased the thermal friction effect on the preparation of the composites,which caused some degradation of PLA and a slight shift in the glass transition temperature of the PLA/PF composites toward lower temperatures;The crystallization ability of the composites was enhanced and the crystallinity was improved.As found by the micromorphology,the two phases of PF and PLA were well bonded with no obvious interfacial defects,and the PLA/PF composites had good interfacial properties,which was mainly the improvement on interfacial adhesion of fibers and the matrix caused by the uniform distribution of micron-sized holes on the surface of the fiber surface.4.The fibers were randomly oriented in the matrix and the final length of the fibers had a certain dispersion.When the material break,the PFs were damaged by fiber fracture,fiber pulled-out and fiber tearing.The tensile modulus,flexural modulus and impact strength of PLA/PF composites kept a continuous increase with the addition of fiber loadings.The tensile modulus,flexural modulus and impact strength of PLA/50%PF composites reached 2793.90MPa,6959.70 MPa and 8.19 KJ/m~2,respectively.And the tensile strength and flexural strength of PLA/PF composites increased with the addition of fiber.The tensile strength,flexural strength of PLA/50%PF composites reached 63.54 MPa and 111.73 MPa,respectively.With higher fiber addition,PF reinforced PLA composites maintain good mechanical properties,which can greatly improve the utilization of plant fibers and reduce the overall cost of the material.High-performance PF had great potential as reinforcing fillers. |
