| High density polyethylene(HDPE)is a widely used general-purpose plastic.Partial crosslinking through irradiation can improve performance especially in improving shape memory properties which can be used in heatshrinkable pipes.This study investigates the basic properties of irradiated materials and their blends,tests their shape memory properties and analyzes the deformation process of tube during thermal shrinkage,laying the foundation for their engineering applications.The main contents include:(1)The microstructure,mechanical properties and shape memory properties of electron beam irradiated HDPE are characterized.By testing through FTIR,PLM and DSC,HDPE undergoes mild degradation with more complete crystals and higher crystallinity after 5k Gy low-dose radiation treatment,and the crosslinking reaction is enhanced with the crystal integrity and crystallinity decreasing after irradiation with 5-25 k Gy.With using Avrami equation modified by Jeziorny for crystallization kinetics analysis,the Avrami index reflects the transformation of HDPE into multi-dimensional complex crystals after radiation treatment,and the crystallization rate gradually decreases.Through testing with a universal tensile testing machine,it is found that radiation treatment increases the yield strength and tensile modulus of HDPE.The relaxation and creep curves are fitted by using a three component model.The increase in relaxation strain and loading rate reduces the relaxation time while the increase in temperature accelerates the relaxation and creep processes.Stress growth will lead to increased creep.The composite curve is generated by the time-temperature equivalence principle.The relationship between the horizontal shift factor and temperature is fitted to the WLF equation and Arrhenius equation.At 5k Gy,HDPE has a smaller apparent activation energy,which is prone to relaxation with good shape memory properties of a fixation rate of 67.8% and a recovery rate of 69.6%.(2)The microstructure,mechanical properties,fluidity and shape memory properties of HDPE/EUG materials are characterized.By testing through PLM,DSC and TG,the compatibility between EUG and HDPE is poor,and when the EUG content increases,there is aggregation phenomenon and the relative crystallinity of HDPE decreases.The thermal decomposition of HDPE/EUG blends shows two stages.As the EUG content increases,the first weight loss peak temperature decreases,and the second weight loss peak temperature increases.The increase in temperature leads to an increase in the thermal expansion of HDPE/EUG blends when its thermal expansion behavior changes significantly near the melting point of EUG.The increase in EUG content increases the thermal expansion rate.The increase in temperature will reduce the tensile strength and modulus of HDPE/EUG blend materials.More EUG increases the creep of the blend materials.The increase in EUG content leads to an increase in the viscosity of the blend system,an increase in extrusion swell ratio.A decrease in shear viscosity happens due to heating.An increase in die aspect ratio leads to an increase in shear viscosity of the blend system.More EUG in content improves the shape fixation rate of HDPE/EUG blends.When the blending ratio is 5:5,a good shape fixation rate of 93.8%and a recovery rate of 76.9% are obtained.(3)The microstructure,mechanical properties,fluidity and shape memory properties of low-dose irradiated HDPE/EUG blends at 5:5 are characterized.The results of DSC,PLM and TG shows that both mixed EUG and radiation reduce the crystallinity and thermal stability of HDPE.Adding EUG to HDPE during stretching improves its elongation at break below 40℃.Through capillary rheometer research,it is found that the addition of EUG and radiation reduce the viscosity of HDPE resulting in a smaller extrusion swell ratio.An increase in aspect ratio will reduce the extrusion swell ratio.When the blending ratio of HDPE and EUG is 5:5,good bending shape memory properties are obtained with a shape fixation rate of 94.4% and a recovery rate of 79.4%.(4)Research on thermal shrinkage testing of the tube body is conducted.DSC testing shows that the crystal structure of heat-shrinkable tube forms when the annealing temperature increases,and causes double peaks or large span melting peaks.Thermogravimetric testing shows that the thermal decomposition process of heat-shrinkable tubes consists of two stages.Halogen-free flame retardants prevent the thermal weight loss curve from dropping to zero.Uneven stress releases during the shrinkage process of tube can lead to surface roughness.The morphology of the heat-shrinkable tube remains almost unchanged during annealing at lower temperatures.After reaching the thermal transition temperature,the heat-shrinkable tube undergoes significant morphological changes being manifested as a decrease in outer diameter and an increase in wall thickness as well as fluctuations in axial length.The outer diameter shrinkage rate can be up to 50%.The linear expansion rate,tensile modulus,relaxation time and viscosity coefficient of the heat-shrinkable tube are obtained through TMA,tensile,relaxation and creep experiments.Based on the three-dimensional shape memory thermodynamic constitutive equation,the compiled UMAT subroutine are embedded into ABAQUS software to simulate the expansion,cooling and thermal shrinkage processes of the tube,which can basically reflect the morphological changes and stress changes of the tube. |
