Structure And Properties Of EVA/CR TPV

Thermoplastic vulcanizates(TPV)based on ethylene-vinyl acetate copolymer(EVA)/chloroprene rubber(CR)blends was prepared by dynamic vulcanization,the mechanical properties,microstructure,compressive stress relaxation,compressive permanent deformation and Mullins effect under tearing mode were researched syetematicallly.The influence of the weight ratio and heat treatment temperature on compression stress relaxation,compression permanent deformation and reversible recovery of Mullins effect were discussed.The mechanism and influence rule of heat-triggered shape memory polymers(HSMPs)of EVA/CR TPV were investigated.The relation between the weight ratio,viscoelastic behavior,deformation temperature,crystallization behavior and the shape memory propertiyes of the EVA/CR TPV were investigated by dynamic thermal mechanical analysis(DMTS),differential scanning calorimeter(DSC)and the theoretical mechanism was proposed.Carbon black(CB)was used to enhance EVA/CR TPV,and the influence of the existence of the incorporation of CB in TPV on the shape memory behavior of materials was researched.The main conclusions were shown as bleows:(1)The scanning electron microscope(SEM)images of the etched surface of the EVA/CR TPV samples showed the sea-island microstructure,while the cross-linked CR vulcanizate particles were uniformly dispersed in the thermoplastic matrix with the size range of 4～8μm.The tensile strength,tear strength,hardness and elongation at break were increased with the increasing EVA content,while the elongation at break was decreased obviously.(2)The stress relaxation behavior under compression mode of EVA/CR TPV showed that the ratio of weight,strain,heat treatment temperature and time had the significant influences on the stress relaxation behavior.The stress relaxation degree could be reduced effectively by increasing the content of CR phase,the stress relaxation behavior of TPV was more remarkable prominent when increasing the compression strain,and increasing the heat treatment time or the heat treatment temperature could accelerate the recovery degree of stress relaxation under compression mode.The research on the permanent deformation of EVA/CR TPV under compression mode and its reversible recovery showed that the initial permanent deformation and the residual permanent deformation were decreased gradually with the increasing of CR phase content in TPV,the mechanism of permanent deformation and its recovery behavior were discussed by using the"core-shell"model and experimental data was fitted by theoretical model.The reversible recovery was fast at the initial period;however,increasing the recovery time,the permanent deformation was decreased obviously.The reversible recovery behavor could be improved by increasing the heat treatment temperature,the residual permanent deformation was reduced,and the degree of reversible recovery was increased with the increasing of CR phase content.Mullins effect could fould clearly duing the cyclic loading-unloading process of EVA/CR TPV under the tear mode.After the first cyclic loading,the tear strength was decreased obviously in the subsequent cycles of loading-unloading,accompanied by increasing residual deformation.Mullins effect was more significant when increasing EVA content in TPV and increasing the tear strain.With the increasing of heat treatment temperature,the reversible recovery ability of Mullins effect became better,and the recovery ability of TPV was the best when the heat treatment temperature was set at 70℃.(3)A new type of HSMP material based on EVA/CR was designed.There was the obvious orientation structure in the specimen during the tensile process,while the morphology of the fracture surface was not smooth and the obvious plastic deformation could be found in the fracture surface,indicating that the strong interface interaction in the EVA/CR TPV.The SMPs switching temperature of the EVA/CR TPV was similar to that of the melting temperature of the EVA phase.The HSMPs material with the excellent properties could be obtained under the proper EVA phase content,deformation temperature and recovery temperature.DMA results showed that higher EVA content significantly increased the storage modulus of TPV,and EVA and CR had the strong interface interaction.In general,the temperature and weight ratio were the key factors to determine the shape memory properties.It was found that EVA/CR TPV have the excellent shape memory properties when T_d and T_r were set at 75℃;moreover,the shape fixation rate(～97%)and shape recovery rate(～95%)of EVA/CR(weight ratio 70/30)TPV specimen were high,and the recovery time was less than 30 s.The factors such as deformation,and the recovery conditions of hot oil and hot air were studied.It was found that large strain would lead to the decreasing shape fixing rate and shape recovery rate,the recovery time was only affected by different recovery environment.The shape memory behavior of TPV incorporated with CB reinforcing agent was researched.Compared with TPV without CB,when the CB dosage was less than 10 phr in TPV,it could promote the shape memory behavior of TPV effectively,and the shape recovery rate was improved.