Study On Pyrolysis Kinetics Of Polyvinyl Alcohol And Mechanical Properties Of Cement-based Materials After High Temperature

In recent years,fiber-reinforced cement-based materials(FRCC)have been widely used in different building structures.Among different types of reinforced fibers,PVA fiber is a commonly used fiber in FRCC due to its advantages of large specific modulus,high strength,and low cost.Generally,PVA fiber-reinforced cement-based composite materials(PVA-FRCC)exhibit the excellent tensile property,crack resistance,and deformation capability at room temperature,but the poor thermal stability of PVA fibers would have a bad influence on the overall strength of FRCC and even cause damage at high temperature environments.Currently,there are scarce studies on the mechanical property and micro-mechanism of PVA-FRCC under high temperature.Therefore,this paper kept a foothold on thermal stability analysis of PVA material,and thus revealed the mechanical property and internal degradation mechanism of PVA-FRCC after high temperatures by conducing pyrolysis and machanics experiments,and reaction dynamic analysis.Consequently,the thermal pyrolysis mechanism of PVA,the reaction mechanism and thermal stability of PVA fiber enhanced by coating flame retardant(DMMP),and the influence of flame retardant on the mechanical properties of PVA-FRCC after high temperature were deeply analyzed and discussed.This research provides scientific basis and support for the design and development of fiber-reinforced cement-based materials with excellent mechanical properties,which mainly includes the following parts:(1)Based on the previous researches on thermal analysis of PVA,this paper used differential scanning calorimetry(DSC)analysis and non-isothermal thermogravimetric analysis(TGA)experiments to study thermal decomposition process of PVA in the temperature range of 20 ～ 600℃,and analyzed its pyrolysis mechanism.The results show that the melting point and initial pyrolysis temperature of PVA are about 218℃ and 230℃,respectively.And its whole decomposition process briefly includes two main steps.In order to accurately describe such process,three kinetic triplets,i.e.reaction-activation energy(Ea),pre-exponential factor(A),and reaction mechanism model(f(α)),of each step were precisely calculated and the mechanism models were reconstructed.Finally,the reconstructed model shows good independence to the heating conditions,upon which the simulated results can fit the experimental data well.These provide a reference to study and simulate the reaction kinetic mechanism of PVA.(2)In this paper,the PVA particles impregnated with dimethyl methyl phosphonate(DMMP)were studied by thermogravimetric-infrared spectroscopy(TG-FTIR)experiments.The results indicate that the existence of DMMP could suppress the dehydration reaction of PVA at high temperature,of which the whole process primainly contains three decomposition stages.Accoring to the experimental data,three kinetic triplets involved in the main pyrolysis stages were obtained by kinetic analysis,and thus the mechanism models were optimized and reconstructed.The results show that the reconstructed mechanism model can describe the real thermal decomposition process well.(3)To further gain insight into the impact of DMMP on decomposition behavior of PVA,DMMP was used to coat the surface of PVA fiber to carry out the pyrolysis experiments.The thermogravimetric analysis(TGA)results show that DMMP can inhibite the thermal decomposition process of PVA fiber.Moreover,it could reduce mass loss rate of PVA fiber during main thermal decomposition zone,and thereby improve its thermal stability.(4)Two sets of PVA-FRCC specimens were made by using PVA fibers with and without DMMP coating.Herein,three-point bending test and cubic compression test were carried out to explore the flexural and compressive strengths of FRCC after different temperatures.With respect to the quality changes and scanning electron microscopy results,the strength degradation law and microstructures of FRCC after high temperature were analyzed.The results illstrate that DMMP could reduce the compressive and flexural strengthes of FRCC at room temperature.However,DMMP has a certain inhibitory effect on the thermal decomposition of PVA fiber at high temperatures,particularly for itsstrengthening effect on the matrix at 400℃.