Study On Preparation And Thermal-conductive Mechanism Of Gutta-percha Composite For Medical Use

With the improvement of people’s living standards,oral health problems are getting more and more attention.As a common treatment method for oral diseases,root canal therapy(RCT)addresses infected pulp tissue and uses a filling material to obturate the root canals to avoid the future infection.Dental gutta-percha(GP)point is the "gold standard" filling material in RCT,and its properties have crucial effects on the complete obturation of root canals.The GP point is primarily composed of GP,zinc oxide(ZnO)and barium sulfate.During the treatment,the GP point is heated by a plugger from its upper end,and then it is softened because of the low crystal melting temperature of GP.Consequently,the GP point flows into the root canals under a certain pressure.After that,the crystallization occurs again with the temperature decreases,thereby achieving the purpose of filling and maintaining a long-term stability.Based on the process above,the GP composite needs high thermal conductivity(TC),good flowing property,and volume stability,etc.However,the TC and flowing property of commercial GP point are not high or good enough.Therefore,a higher heating temperature and a longer time in clinical are required,which may bring huge pain to the patients and even damage the tissues around the root of the tooth.Therefore,the goal of this research is to prepare a new GP material with high TC and good flowing property to meet the requirements of RCT well.Through the combined use of carbon nanotubes(CNTs)and ZnO fillers,together with the orientated and modified CNTs,the GP composite with excellent comprehensive properties is prepared.In addition,the thermal-conductive mechanism is revealed and a new thermal conductive model was proposed to predict the TC of composites.Finally,the volume shrinkage of GP point is a serious problem,but the mechanism is not clear,so the research about the mechanism of volume shrinkage is also included in this paper.In the first section,by analyzing the relationship between the composition and TC of traditional commercial GP points,it was found that the TC was positively related to ZnO.Furthermore,a series of GP composites with different contents of ZnO were designed and prepared by the two-roll mill.The test results showed that the TC of the GP composites gradually increased and the growth rate became faster as the content of ZnO increased.The "three-stage"thermal-conductive mechanism was revealed according to the microstructure of"island-chain-network" of ZnO in the GP matrix.In addition,the shear viscosity of the GP/ZnO composite also rised with the increase of ZnO,especially when the ZnO content exceeded 80 wt%.The result proved that it was impossible to improve the properties of the GP composites by increasing the content of ZnO solely,and 80 wt%was the best value to obtain good comprehensive performance of TC and flowing reletively.In the second section,a series of GP/ZnO@CNTs composites were prepared by filling different contents of CNTs on the basis of GP/ZnO(80wt%)composite obtained in the first section.The TC of GP/ZnO@CNTs(50phr CNTs and 400phr ZnO)composites increased by 75.2%compared with GP/ZnO composite filled with the same content of ZnO(450phr),indicating that ZnO and CNTs had a synergistic effect on the improvement of the TC.Therefore,the thermal-conductive mechanism of the "bridge model" is proposed to explain the synergistic effect.In addition,in vitro filling experiments showed that,compared to commercial GP point,GP/ZnO@CNTs composites containing 20phr CNTs had higher TC and better flowing property.It was found that the new GP composite containing CNTs did not have cytotoxicity.Although the new GP composite still needs a series of clinical evaluations before actually used in patients,the research in this chapter shows that it has good application prospects.In the third section,the GP/ZnO@CNTs composite containing 20phr CNTs was shear-oriented by the capillary rheometer,which significantly improved the TC in the orientation direction.Compared with the composite without shearing by the capillary rheometer,the TC was up to 2.5 W/(m·K),an increase of 81.2%.In vitro filling experiments proved that the orientation of CNTs was conducive to heat transfer along the axial direction of the GP point.As a result,it is easy to soften the tip to obtain a good filling effect,which was of great significance for obtaining a better GP composites.In the fourth section,the silane coupling agent KH560 was used to modify the thermally conductive fillers.Compared to the unmodified composite,the TC of the composite containing 10phr CNTs-OH and 10phr KH560 was increased by 11.9%,and after orientation,it was increased by 34.0%.At the same time,the average shear viscosity decreased by 18.5%,indicating that the addition of KH560 not only increased the TC effectively,but also improved the flowing property obviously.Therefore,the addition of KH560 solves the contradiction between TC and flowing property in the preparation of GP comosites,and provides guides for further improving the performance of GP point.In the fifth section,a new thermal-conductive model was derived and proposed on the basis of the classic Agari thermal-conductive model,named parallel-series(P-S)model.The P-S model could be used to predict the TC of our three-phase GP/ZnO@CNTs composite well.In addition,it was also used to analyze the TC of GP/ZnO@CNTs composites after orientation.In order to verify the universality of the P-S model,other experimental data quoted from other different references were used for predictive analysis,and good fitting results were obtained.The P-S model is of great significance to enrich the theory of thermal-conductive model and guide the preparation of high TC composites in the future.In the sixth section,we explored the factors that affect the volume stability of GP point.Through the comparative analysis of the experiment and calculation of the volume shrinkage,it was found that the main reason for the volume shrinkage of GP point was the crystalline phase transition of the polymer matrix,accounting for 99%of the total volume shrinkage rate.The volume shrinkage of the filler caused by the thermal expansion and contraction effect accounted for only 1%.Furthermore,the content,crystallinity and crystal form of the GP matrix had important effects on the volume shrinkage.Generally,the higher the content and crystallinity of the GP,the greater the volume shrinkage rate.The research in this chapter helps us to fully understand the mechanism of volume shrinkage,which can provide direction for solving the problem of volume shrinkage and have practical significance.