Strain Sensor Based On Thermosetting Resin Pyrolytic Carbon Material And Its Performance Research

With a rapid development of the plastics industry,thermosetting resins are increasingly used in buildings,mechanical equipment,electronic appliances and aerospace industries due to their excellent mechanical properties and good heat resistance.According to statistics,in 2013 the total plastic production around the world exceeded 299 million tons,of which thermosetting resins reached 42 million tons.Although the widespread use of plastics has greatly facilitated our lives,the large amount of waste plastics produced after use also poses serious environmental problems.In particular,once the thermosetting resin is cured,it cannot be melted or dissolved,which poses a great challenge to recycle and reuse them.In this paper,carbon materials with good electrical conductivity were firstly prepared from thermosetting plastics through a high-temperature carbonization process.Then,we fabricated some flexible strain sensors based on those carbon materials and studied their sensing behavior.The main research work and results are as follows:(1)The polybenzoxazine(PBa)thermosetting resin was prepared by heat curing of bisphenol A type benzoxazine monomer.The chemical structure and thermal stability were characterized by FTIR and TGA.The PBa resin is subjected to ball milling and carbonization to obtain carbonized PBa powder(F-cPBa).It is then blended with the PDMS elastomer by a solution-assisted mixing method and cured to obtain an F-cPBa/PDMS flexible tensile strain sensor.This sensor's response current is positively correlated with the tensile strain.Under cyclic loadings at the same strain,the relative current variation(RCV)of strain sensors have good stability and repeatability.In addition,this sensor shows a small response hysteresis,and its corresponding response time is only 25.5 ms.(2)PBa plastic sheets were subjected to high-temperature carbonization treatment in a N2 atmosphere to obtain a kind of carbonized porous material(P-cPBa).Through SEM observation,the pore size distribution of P-cPBa is wide,which is related to the formation and growth rate of the "carbonization point" of PBa resin during the carbonization.The electrical conductivity of P-cPBa is measured by using the two-probe method and it is found that a higher carbonization temperature uses,a higher volume conductivity of P-cPBa reaches.This is because that some amorphous carbon in P-cPBa have transformed into graphite type carbon when the carbonization temperature increases.After P-cPBa is encapsulated by PDMS,a PDMS@P-cPBa flexible compression strain sensor is obtained.During the entire compression deformation,the sensor shows a tendency to decrease from the response current to an increase,which is due to the evolution of the so-called "fragmented conductive network" formed by the strain sensor after pre-compression.When under a large range of compressive strain,such as 7%<?<30%,the sensor has high sensitivity and its GF reaches 1954.3.(3)Carbonized porous materials(cPFF)with different carbonization temperatures were prepared by high-temperature carbonization of phenolic resin foam(PFF)under N2 atmosphere.The cross-section microscopic morphology of the cPFF was observed by SEM,where the amount of open-cell structure of cPFF is increased compared with P-cPBa.Some cPFF were then carefully sliced and encapsulated by PDMS to obtain PDMS@cPFF multifunctional strain sensors,which are able to monitor tensile,compressive and curved strains.In contrast to the F-cPBa/PDMS tensile strain sensor,the RCV of the PDMS@cPFF strain sensor is negatively correlated with the tensile strain;Similar to the PDMS@P-cPBa compressive strain sensor,the response current of PDMS@cPFF strain sensor also decreases first and then increases.The versatile strain sensor has good durability and it can accurately feedback different forms of deformation even after more than 1000 times loadings.In addition,we have successfully applied the PDMS@cPFF multifunctional strain sensor on monitoring of various human movements,including human joints activities(finger,knee,neck),facial micro-expressions(smile,blinking)as well as human speech recognition.