Synthesis And Properties Of Flexible Polymer-based Electrothermal Composite Films

Electrothermal composites,a kind of functional material,can translate electric energy into heat directly,which can realize self-regulating temperature.Nowadays,the study of the flexible electric heating materials is becoming a hot spot in flexible smart devices.However,some problems still need to be solved before the large-scale application,including unsteady electrothermal behavior,poor self-regulating temperature,binder aging.Consequently,in order to solve the above issues,a series of polymer-based electrothermal composite films are fabricated to investigate the electric properties and electrothermal performance in this study.The main results are summarized as follows:(1)Thermoplastic polyurethane elastomer(TPU)is used as the binder due to its excellent mechanical properties and resistance to chemical corrosion.Conductive fillers are Super-P and multi-walled carbon nanotubes(MWCNTs),respectively.TPU and one-component conductive fillers are milled and then mingled mechanically in gradient proportions,and the obtained mixture slurry is coated on the PET substrate.The electrical conductivity,electrothermal performance and cyclic performance of various carbons/TPU composite films are investigated.The experimental results indicate that the percolation thresholds of TPU/Super-P composites and TPU/MWCNTs composites are both 15 wt.%of carbon fillers in the composites.When the conductive fillers content ranges from 10 to 20 wt.%,the electrical conductivity of TPU/Super-P composites is better than TPU/MWCNTs composites.While the conductive fillers content is above 25 wt.%,TPU/MWCNTs composites has better conductivity compared to TPU/Super-P composites.The lowest sheet resistance is only 23.91 ? sq'1 for TPU/MWCNTs composites.The electrothermal films with 10-20 wt.%conductive fillers under various input voltages can reach a certain constant temperature within 50 s.However,the electrothermal films with 25-30 wt.%conductive fillers attain a certain constant temperature within 100 s.Moreover,the stability of TPU/Super-P electrothermal composite films is better than that of TPU/MWCNTs films.TPU/Super-P and TPU/MWCNTs electrothermal films with 20 wt.%conductive fillers both exhibit the remarkable cyclic performance under 30 V and 20 V.(2)Hybrid carbon materials composed of Super-P and MWCNTs are employed as conductive fillers to synthesize TPU-based composite films.The effects of the proportion of two kinds of conductive fillers on electric properties,electrothermal performance and cyclic performance of electrothermal films are investigated.The experimental results reveal that when the mass ratio of Super-P and MWCNTs is 5:20,the sheet resistance of the sample is 30.5 ? sq-1,which is lower than TPU/Super-P and TPU/MWCNTs composite films with 25 wt.%conductive fillers.And when the mass ratio of Super-P and MWCNTs is 12.5:12.5,the sample can attain a constant temperature within 50 s under various input voltages,which is faster than TPU/Super-P and TPU/MWCNTs composite films with 25 wt.%conductive fillers.Moreover,the sample exhibits steady temperature of 47 ? under 15 V after 6 cycles.Furthermore,the current(I)-voltage(U)curves maintain approximate linearity,implying that the curves are according with the Ohm's law.(3)Polyoxyethylene(PEO)and polyurethane(PU)as mixed binders are studied in terms of electrothermal performance of electrothermal composite films.First,the influences of the content of various binders in composites on the electrical properties,electrothermal performance and cyclic performance are studied in the electrothermal films with Super-P as conductive filler.The results show that when the mass ratio of PEO and PU is 5:2,the PEO/PU/Super-P sample can reach a constant temperature within 25 s under 20 V.Based on the above results,the electrothermal performance is further investigated by combining TPU-based optimized ratios of Super-P and MWCNTs.When the mass ratio of Super-P and MWCNTs is 12.5:12.5,the sample under 20 V can reach 40 ? within 25 s and display good cycle performance under 25 V.Additionally,I-U curves strictly keep linear relationship,indicating that the curves are conforming to the Ohm's law.And the electric power(P)of the composite films increases quadratically with the applied voltage,according with the relation of P = U2/R.(4)A novel wearable application of electrothermal film is designed and fabricated by combining electrothermal composite films with reversible thermochromic paint.The results show that the color of the thermochromic paint changes gradually from red to white after the electrothermal film is electrified.Under the input voltage of 10 V,it is clearly observed that the pattern changes from red to white within 120 s when the temperature rises to 43 ?.When the power is turned off,the color of the pattern is restored within 60 s.