Research On The Preparation Technics And Sensing Properties Of Multi-walled Carbon Nanotubes/Natural Rubber Composites

With good isolation and energy dissipation,rubber isolation bearings are widely employed in seismic isolation and anti-seismic structures,such as buildings and bridges.It will be the future development direction to grasp the health status of the bearing during the installation and service stages,and realize the intelligent and real-time monitoring of the isolation bearing,which is essential to ensure the service performance of the bearing and the safety of people’s lives and property.Traditional sensors feature poor rubber compatibility,low deformation ability,and poor anti-damage performance,making it difficult to monitor bearing performance.However,conductive rubber composites have the characteristics of large deformation,anti-damage and easy processing,and are expected to become sensing materials for monitoring the service performance of rubber isolation bearings.Therefore,this dissertation focuses on multi-walled carbon nanotube(MWCNT)/natural rubber(NR)composites,and conducts systematic research on the relationships between preparation process-structure-properties,the optimization of sensing characteristics,the resistance-strain response under various loading modes and environments,and the theoretical model establishment for MWCNT/NR composites.The main contents are as follows:1.MWCNT/NR composites with good sensitivity(>27)and wide strain sensitive range(>200%)are prepared based on the laminated vulcanization process.The effects of the process on the microstructure,mechanical properties,electrical conductivity and sensing properties of MWCNT/NR composites are investigated.The resistance-strain response mechanism of the composites is explained.Digital image correlation analysis shows that the shoulder peak phenomenon is related to the deformation recovery ability of composites.2.The effects of dispersion process on the mechanical properties,conductivity,sensing capability,and filler-matrix interface interaction of MWCNT/NR composite are systematically researched from macro-and micro-scale aspects.The results reveal that the separated conductive network structure is constructed by the two-roll method,and the agglomeration and stacking of MWCNT are effectively reduced.MWCNT/NR composites with low percolation threshold(～ 1 wt%),high sensitivity(974.2)and large strain(> 100%)are prepared,and show good resistance-strain response stability,symmetry,repeatability and durability.When the MWCNT content is greater than 3 wt%,the mechanical properties of the composites prepared by two roll method increase with the increase of MWCNT content.Compared with strong interfacial interaction,good dispersion and excellent conductive network contribute more to the improvement of mechanical properties and electromechanical response for composites.Based on the tunneling theory,the resistance-strain response mechanism of the composites prepared by different dispersion processes is analyzed.3.The resistance-strain response of MWCNT/NR composites is optimized,and the linearity and hysteresis of the resistance-strain response of the composite are improved,and the uniformity,monotonicity and symmetry of the sensing response are promoted.The shoulder peak effect of the sensing response is reduced.Based on the microscopic analysis and coarse-grained method,the influence of optimization on the conductive network structure is discussed,which shows that the orientation of conductive network structure increases along the stretching direction with the increase of pre-stretch,and a large amount of pre-stretch leads to matrix slip,irreversible damage and pull-off of conductive network,resulting in a significant attenuation of sensitivity.4.The sensing response continuity of MWCNT/NR composite under interval loading in stretch and compression is explored.With the increase of interval time,the variation amplitude of resistance tends to be stable,and the established theoretical model can effectively predict the variation law of this amplitude.The effect of constant compressive strain on the compressive resistance-strain response of MWCNT/NR composites was explored.The results showed that the resistance-strain response stability,repeatability,monotonicity,symmetry and shoulder effect of MWCNT/NR composites depended on the constant strain load;MWCNT/NR composites show different piezoresistive behavior under different delamination forms.The behavioral mechanism similar to the bidirectional "negative Poisson’s ratio" structure in the compression state is verified by Digimat and Workbench simulation methods.A mathematical model of compressive cyclic resistance-strain response is established,which effectively characterizes the cyclic resistance-strain response relationship.5.The effects of temperature on the resistance-strain response of MWCNT/NR composites under compression loading are investigated,which shows that temperature has a strong effect on the resistance-strain response of the composites,and it is clarified that thermal fluctuation is the dominant factor causing the change of electrical conductivity.The effects of loading frequency on the resistance-strain response of the composites are studied,and it is shown that the electromechanical response of MWCNT/NR composites have a wide range of frequency response.With the progress of loading cycles,the electromechanical response is gradually independent of the loading frequency and a large loading frequency is more likely to form a strong shoulder peak phenomenon and low sensitivity.6.The sensing properties of MWCNT/NR composites under shear loading are systematically analyzed,and the results show that: MWCNT/NR composites exhibite good electromechanical response behavior under shear loading,and large shear amplitudes caused strong electromechanical responses.The resistance-strain hysteresis effect weaks the attenuation of the resistance amplitude of the MWCNT/NR composite,while the large shear rate causes severe resistance amplitude attenuation and conductive network loss.Combined with the theory of continuum mechanics,the cyclic resistancestrain response mathematical model is optimized,and the shear cyclic electromechanical response relationship is characterized.These research results will provide experimental support for the application of MWCNT/NR composite in different loading states and environments and have guiding significance for the design and preparation of the high-performance conductive rubber composites,which lay the research foundation for the application of MWCNT/NR composite in the health monitoring of rubber isolation bearings.