Study On Water Absorption Behavior And Internal Stress Of Jute Reinforced Poly(Lactic Acid)(PLA) Composites

In recent years,with energy shortages and increased environmental awareness,green or renewable materials are trying to replace traditional petroleum-based polymers and composites.Among them,plant fibre reinforced composites,especially jute fibre reinforced poly lactic acid(PLA)composites,have good mechanical properties,lower prices,and good biodegradability,making them a preferred choice for sustainable materials.At present,the most promising fields of plant fibre/PLA composites are structural parts such as automotive interior parts,which are usually used for years or even decades.However,compared with traditional fibres,plant fibres are extremely hydrophilic,and are very sensitive to environmental factors such as external temperature and h umidity.This has caused the water absorption mechanism of plant fibres/PLA composites to become extremely in hygrothermal environment.At the time,the mechanism of damage caused by water absorption is also difficult to explore,which will have a great impact on its durability.In this paper,the jute/PLA composite prepared by melt extrusion injection molding technology is used as the research object to study its water absorption mechanism in hygrothermal environment,focusing on the stage where the irreversible chemical or physical change did not occur in the early period of water absorption.Specific research contents include:1)Characterize and test the microstructure of jute/PLA composites based on X-ray tomography technology(XCT),the preliminary work has been completed by the research group.Through 3D visualization analysis of Avizo 9.0 software,quantitative analysis of tomographic data is performed.The results show that the distribution of jute fibre in PLA matrix has a certain tendency.Meanwhile,combined with 3D reconstruction technology,the separation of jute fibre and PLA matrix is realized,and then a 3D finite element(FE)model conforming to the true microstructure of the material is realized.2)Experiments on water absorption behavior of jute/PLA composite specimens at different aging temperatures were performed to obtain the water absorption curve of the composite,and its water diffusion law was described in combination with Fick's law.At the same time,the water absorption kinetic coefficients(water diffusion coefficient D and saturated water contentM_m)and water absorption expansion coefficient(?)of PLA and jute/PLA composites are calculated,and the Arrhenius law is used to predict D and?at any temperature.3)The FE software ABAQUS was used to simulate the water diffusion behavior and water swelling behavior of the jute/PLA composite.Based on the experimental data,theD_f and?_f of the jute fibre were first determined.At the same time,based on the simulation of water diffusion behavior,the law of water diffusion of jute/PLA composite is described more intuitively,and the dispersion of jute/PLA composite is non-uniform in the matrix.The stress evolution process caused by the expansion mismatch between fib re and matrix was quantitatively detected by the simulation of water swelling behavior.Result show that the more concentrated the fibre distribution is,the more obvious the stress concentration is.4)Finally,the jute/PLA composites were tested for tensile and flexural properties at different temperatures,and the change curves of tensile and flexural strength with water absorption time were obtained.The results show that the mechanical properties of jute/PLA composites are affected by the absorption rate of water,and then gradually decline after rapid decline.The decrease in the mechanical properties of the composite during this period is reversible,and the material strength is restored after removing water.The decline in the mechanical properties in the early stage of water absorption is caused only by the plasticization caused by moisture.The long-term prediction of the mechanical properties of jute/PLA composites under the influence of moisture can be achieved through the principle of time-temperature superposition(TTS)equivalence of the thermodynamic law.At the same time,the FE simulation can count the areas inside the PLA matrix that exceed the tensile strength(59MPa)and flexural strength(106MPa).The data showed that the largest area exceeded 55%(50?,>59MPa),the smallest area in about 25%(23 and 37?,>106 MPa).These areas are very prone to damage.Using the principle of TTS equivalence,the evolution of potential damage areas inside the PLA m atrix can be predicted.Finally,combined with the above research content,the mechanism of water absorption damage of jute/PLA composites was described.