Study On The Thermal Properties Of Shellac Resin

Lac resin is the main constituent of lac.Its application stemed from the years B.C.In recent years,lac resin has been used widely in the food and pharmaceutical industries,etc,such as the coating materials and host materials in these fields.However,there is no comprehensive studies concerning lac resin's physico-chemical properties changing with the rising temperature.This research will not only stop the gap in these aspects,but also some guidance could be provided in keeping and processing the lac resin.Research about these is necessary.For this purpose,methods in researching the amorphous polymers' transition along with the temperature were used in this study,such as the glass trasition,viscous flow transition?the curing reactions may take place when the polymers possess reaction activity?and pyrolysis.Based on lac resin's crystallinity,the differential scanning calorimetry?DSC?and dynamic thermomechanical analysis?DMA?were used to confirm its glass transition temperature?Tg?.Then the optimum conditions in measuring lac resin's Tg using DSC method was studied through the single factor experiment.Lac resin's viscous flow transition was defined the combination of DSC method and conventional methods in lac-research field.According to the experimental data,the fitting curve of lac resin's curing time and curing temperature would be obtained.The significance of this fitting curve was tested.Finally,lac resin's thermal decomposition process and kinetics were studied based on the principle and methods of pyrolysis using thermogravimetric analysis?TG?and DSC.There were three conclutions from this whole study:?1?Glass transition and viscous flow transition was firstly introduced in lac-research field.As a result lac resin will experience glassy state,glass transition,rubbery state,viscous flow transition and viscous state with increased temperature.In the viscous state,the active groups of lac resin's molecular chain could react with each other rapidly until to the formation of insolubles and unmelts.This period was called thermal curing process.After that,lac resin will enter thermal decomposition state when it is continuely heated.?2?The Tg of lac resin was 27.3? under the DSC measurement and was 36.0?under the DMA measurement.DSC measyrement is the priority choice in measuring lac resin's Tg and defined the peak temperature of the DSC curve's first-order differential as the Tg.The optimum conditions of DSC measurement were:the velocity of heating the sample was 7±0.5?/min,the weight of the sample was 7±0.1 mg,the amount of the gas flowed into the apparatus was 15±1mL/min,the size of the sample was 125?250?m,the repetitive temperature of heating the sample was 35±0.5?.Based on these optimum conditions,the Tg of Kerria yunnanesis lac,Kerria pusana lac,Kerria rangoonensis lac,Kerria lacca lac,alkaline extracted lac,seed lac,flake shellac,NaCIO bleached shellac,H₂O₂ bleached shellac were 22.7?29.8?30.8?32.9?34.3?33.9?28.6?23.3?16.4?,respectively.In the different heating velocities,the began and end temperature and enthalpy change of lac resin's viscous flow transition are different,while the entropy change is of little difference.And the impact on the physico-chemical property of lac resin's viscous flow transition could be ignored.The fitting equation of lac resin's curing time and temperature was significative after the significance testing.It is of reference value to other lac types.The anaerobic thermal decomposition process occurred in one step with lower heat release and higher heat absorption.While the aerobic thermal decomposition process occurred in two steps with higher heat release and lower heat absorption.The activation energy in the anaerobic thermal decomposition process was?254.52±31?kJ·mol-1.In the aerobic condition,the caculated results were of low reliability because of the complexity in the whole process.The results of Kissinger and Starink methods were 287.192 kJ/mol and 287.294kJ/mol.The results of FWO method could still be used to analyze lac resin's aerobic thermal decomposition mechanism.The most probable thermal decomposition mechanism function under the two conditions was J-M-A equation.The mechanism is random nucleation and subsequent growth according to this equation.?3?The interpretation of the instability of lac resin with the glass transition and Tg is:lac resin could be stored for a long time at the temperature below Tg.If temperature is above Tg,the storage time will be shortened because of the chemical reactions among these reactive and removable end-groups.Therefore lac resin's Tg is its stagnation temperature in storage.The advices to lac resin's processing conditions are:? in lac resin's heating process to its of viscous state,the lower heating velocity could save more energy;? the characteristic temperatures of viscous flow transition rises along with the heating velocity.Therefore,the measured characteristic viscous flow transition temperatures only apply to the production practice in which the conditions are homologous to the testing conditiongs.