Study On Pyrolysis,Oxidation Behavior And Associated Reaction Mechanism Of Tea Polyphenol Derivatives

The analysis on pyrolysis and oxidation properties of materials as well as their thermal decomposition mechanism lays the theoretical basis for assessing utilization conditions and developing new application prospects of materials.The present work is dedicated to exploring the behaviors and nature of pyrolysis and oxidation of natural tea,tea polyphenols and their further extraction products i.e.tea catechin,for the purpose of evaluating the thermal stability in material utilization and exploring the potential of tea polyphenols as intumescent fire retardants.The chemical compositions and micromorphology of prolysed residues of samples at different temperatures were monitored,and then their expansion process as well as the reasons behind were explored.By comparing the characteristics appearing in pyrolysis and oxidation process,including thermal decomposition,gases release and residues evolution among tea leaves and their polyphenol derivatives,the pyrolysis and oxidation mechanisms were studied and the performance of different samples acting as potential fire retardants was also analyzed.The present work not only enhances the scientific understanding on thermochemical and physical properties of specific samples by revealing the intrinsic link between material properties and their chemical structure,but also lays the theoretical foundation for the development of environment-friendly fire retardants.The mass change and gaseous products of pyrolysis of tea polyphenols were tested by using simultaneous thermal analyzer(TGA/DSC),on-line Fourier transform infrared spectrometer(FT-IR)in conjunction with mass spectroscopic(MS)instrument.The chemical compositions of solid residue at different heating temperatures were also monitored.The results indicated that tea polyphenols can be classified as aromatic compounds with rich side chains,mainly consisting of C,H and O elements as well as tiny N element.The weight loss of sample pyrolysis below 200 ? was essentially the result of evaporation of free water and bound water.Pyrolysis began at about 200 ?,which could be attributed to the stability of the side chains in the chemical structure of the samples.At the temperatures between 200 and 350 ?,tea polyphenols dramatically decomposed by breaking side chains in their structure and the recombination of free radicals,leading to the generation of enormous gaseous products such as H2O and CO2,etc.At the temperatures between 350 and 800 ?,the pyrolysis reactions became sustainably slow,highlighting the generation and accumulation of more stable chemical structure in the pyrolysed residues.The established understanding provides the theoretical basis for effective utilization of tea polyphenols and the development of new application prospects in fire prevention and protection field.The expansion process and relevant mechanism of tea polyphenols by heating were explored by conducting thermal experiments in air atmosphere.The macroscopic performance of tea polyphenols during pyrolysis and oxidation at elevated temperatures were examined using a heating furnace.Mass change,heat evolution and gas products of tea polyphenols during heating in air were also monitored by using on-line TGA/DSC-FTIR-MS techniques.It was observed by raising the temperature to?550 0C at a rate of 10 ?/min,the mass of a sample reduces by nearly 70%to form a large quantity of inert gases that are mainly composed of H2O and CO2.Finally,we obtained highly-porous black chars,which are stable at high temperature.Based on the chemical compositions and micromorphology of the aerial oxidation products of tea polyphenols in the solid phase,it was found that a tea polyphenol sample first becomes semi-fluid after heating,and the volume of the melt sample rapidly expanded due to the substantial gases generated by breaking side chains.At the same time,the solid residues had accumulated a large amount of benzene structure,leading to the increase of the melting point,so that the volume of melt sample was fixed to form a black char with dense surface and porous interior.It is obvious that the abundant side chain substituents of tea polyphenols can generate large amounts of blowing agent,and the benzene contained can provide structure basis for carbon source.The characteristics of the structure showed that tea polyphenols have huge potential in fire retardant field.We conducted comparative analysis on the characteristics of pyrolysis and oxidation of tea leaves and their polyphenol derivatives.The reaction mechanism of pyrolysis and oxidation of tea leaves and their polyphenol derivatives were explored by using on-line TGA/DSC-FTIR-MS techniques.The process of sample mass change and gas release by linear heating at inert and aerial atmosphere was monitored and the chemical composition of residues at different temperature and atmosphere was also characterized.The results showed that the gas production at different temperature range is related to the breakdown sequences of different structures contained in materials.Below 250 ?,the breakdown and reorganization take place among the structures containing alcoholic C-OH bonds,ether linkage C-O-C and-CH2-;between 250 and 400 ?,the C=O and carboxylic acids C-OH structures breakdown and reorganize to form CO2 and H2O;at the temperatures higher than 400 ?,it is mainly because of the pyrolysis and oxidation of phenols C-OH as well as conjugated substituents of aromatics to generate gases.It is noted that oxygen adsorption occurs at low and moderate temperatures in polyphenols under air condition,which slows the rate of weight loss.For the samples studied,tea leaf can produce large amounts of gases at low and moderate temperatures due to its substantial unstable structures.The aromatic side chains contained in tea polyphenols and tea catechins largely decomposed at the low and moderate temperatures to generate massive gases,with some of them remained and evolved into substituents conjugated to the aromatics with increasing temperature.As a result,the stability of substituents was enhanced,and it was going to gradually breakdown and oxidize with temperature further increasing,leading to more gases released from polyphenols at high temperature.Among these materials,tea polyphenols have abundant benzene structures,making it available to form a more stable and dense expanded char,which declares a better fire retardant ability.This finding lays a theoretical basis for the development of effective and environment-friendly fire retardants.