| Rosin is a significant nontimber forest material.It contains a variety of resin acids.Rosin esters were the product of rosin esterificated with monohydric or polyhydric alcohols as widely used modified-resin.However,the conjugated double bonds in rosin have high reactivity and are prone to oxidation reaction;it reduces the performance of rosin products and limits many applications.Therefore,the oxidation reaction of rosin and colophony is a popular research field at home and abroad.In order to improve the quality of rosin and its modified-resin products,and to reduce and avoid the oxidation reaction during production,storage and transportation,which leads to product quality decline,it is necessary to study the oxidation process of rosin acids and modified rosin esters,to understand the oxidation reaction,and to provide theoretical basis for thermal stability of rosin products and the development of antioxidant technology.The research content and main research results are as follows:?1?.Thermal stability and characteristics during abietic acid oxidation processThermogravimetric analyzer?TG?was used to study the thermal decomposition characteristics and decomposition kinetics of abietic acid under oxygen atmosphere.The self-designed mini closed pressure vessel test?MCPVT?device was used to study thecharacteristics of abietic acid oxidation.The iodimetry method was used to track the formation of the peroxide in abietic acid oxidation reaction.The structure of the primary peroxide was separated and characterized.The oxidation products were analyzed by gas chromatography-mass spectrometry?GC-MS?and the oxidation pathway was explored.The TG experiment results showed that at 308 K,the combination of abietic acid and oxygen appeared to increase weight.The weight loss of abietic acid under oxygen atmosphere is one step.The initial temperature of thermal decomposition increases with the increasing heating rate.The peak temperature increased with the increasing heating rate.The thermal decomposition kinetics of abietic acid in oxygen was calculated by Kissinger method and FWO method.The activation energy Ea obtained by Kissinger method was 96.59 k J·mol-1,and by the FWO method contained the value calculated by the Kissinger method.The activation energy obtained by the FWO method increased with the progress of the reaction??0.1 to 0.9?,and the overall trend was rising.Iodimetry results indicated that abietic acid was very sensitive to generate peroxides.In the oxidation process below 343 K,a higher concentration of peroxide can be formed;when the reaction time is 7 h,the maximum peroxide value can reach 21.77 mmol·kg-1.The primary oxidation product was characterized by MS,NMR and IR.It was 7-hydroperoxy-13-cedar-8?14?-enoic acid.The DSC parameters of the thermal decomposition of peroxide were as follows:exothermic onset temperature?T0?was 353.94 K,and the heat of decomposition(QDSC)was 545.37 J·g-1MCPVT experiment and oxidation product analysis results showed that after peroxide decomposition above 348 K,an exothermic peak would appear in the abietic acid oxidation process,the release of free radical pool would accelerate the rapid oxidation of free radicals,and consumed a large amount of oxygen by accelerating the hydrogen extraction reaction.When the temperature reached the melting point of abietic acid?448 K?,a secondary oxidation process was first discovered in abietic acid:complex isomerization,decarboxylation,and oxidation products were produced during the abietic acid oxidation process,including dehydroabietic acid,neoabietic acid,7-oxodehydroabietic acid,7-methoxydehydroabietic acid,2-deoxyketoabietic acid,12-methoxyabietic acid and desmethylabietic-8,11,13-tri ene.As a result,the abietic acid oxidation reaction has three stages:1.the primary oxidation stage is mainly the combination of abietic acid and oxygen to generate abietic acid peroxide;2.the abietic acid peroxide undergoes a rapid thermal decomposition reaction,generating a large amount of active free radicals 3.the termination period of abietic acid oxidation reaction,accompanied by the formation of a large number of secondary oxidation products.?2?.Thermal stability and oxidation process characteristics of levopimaric acidThe thermogravimetric analyzer?TG?was used to study the thermal decomposition characteristics and decomposition kinetics of levopimaric acid in an oxygen atmosphere.The oxidation reaction characteristics of levopimaric acid were studied by a self-designed mini closed pressure vessel experimental device?MCPVT?.The peroxide value of the levopimaric acid oxidation reaction was measured by iodometry.The peroxide formation of levopimaric acid was studied,and its thermal decomposition parameters were determined.The oxidation products were analyzed by gas chromatography-mass spectrometry?GC-MS?and explored the oxidation pathway.The TG experiment results showed that at 303 K,the weight increased with the combination of levopimaric acid and oxygen.The weight loss of levopimaric acid under oxygen atmosphere is one step.The peak temperature moved toward the direction of higher temperature as the heating rate increased.The thermal decomposition kinetics of levopimaric acid in oxygen was calculated by Kissinger method and FWO method.The activation energy Ea obtained by Kissinger method was 104.20 energy calculated by the FWO method contained the value calculated by the Kissinger method.The activation energy obtained by the FWO method increased with the increasing reaction progress??0.1 to 0.9?,and the overall trend rised.The results of iodometric analysis indicated that levopimaric acid is very sensitive to oxidation to generate peroxides.In the oxidation process below323 K,a higher concentration of peroxide can be formed;when the reaction time is 2 h,the maximum peroxide value can reach 30.44 mmol·kg-1.Thin layer chromatography?TLC?analysis of levopimaric acid peroxide showed that the oxidation of levopimaric acid produces multiple peroxides.The exothermic onset temperature?T0?of levopimaric acid peroxide is 375.37 K,and the decomposition heat(QDSC)is 338.75 J·g-1.MCPVT experiment and oxidation product analysis results showed that after the decomposition of peroxides above 325 K,an exothermic process will occur in the oxidation process of levopimaric acid;the release of free radical pool will accelerate the rapid oxidation of free radicals,and accelerate the consumption of oxygen through hydrogen extraction reaction.When the temperature reached the melting point of levopimaric acid?423.15 K?,the second oxidation process was first found in levopimaric acid.Complex products were produced during the oxidation of levopimaric acid,mainly including dehydroabietic acid,abietic acid,neoabietic acid,palustric acid,7-oxodehydroabietic acid,7-vinyl-1,2 3,4,4a,4b,5,6,7,9,10,10a-dodecane 1,4a,7-trimethyl-7,15-spimadiene-18-phenanthrene 1-carboxylic acid and 1,2,3,4,4a,9,10,10a-octahydro-6-methoxy-1,4a-dimethyl-7-?1-ethyl?-phenanthrene-1carboxylic acid.The oxidation reaction of levopimaric acid undergoes three stages:1.the primary oxidation stage is mainly the combination of levopimaric acid and oxygen to formlevopimaric acid peroxide;2.the rapid thermal decomposition reaction of levopimaric acid peroxide produces a lot of active radicals.3.the termination period of the oxidation reaction of levopimaric acid accompanied by the formation of a large number of secondary oxidation products.?3?.The effect of antioxidants on the oxidation reaction of abietic acid and levopimaric acidMCPVT was used to determine the reaction of abietic acid and levopimaric acid with oxygen in an oxygen atmosphere.The effects of four antioxidants on the oxidation reaction were investigated to evaluate the antioxidant effect.The results showed that the exothermic peak of the oxidation reaction of abietic acid or levopimaric acid gradually increased with the increasing mass of abietic acid or levopimaric acid.When the mass of abietic acid and levopimaric acid is 4.8 g,their maximum exothermic temperature?T reached 25.1 K and 16.8 K,respectively.When antioxidants were added to abietic acid and levopimaric acid,their initial oxidation reaction temperature was significantly increased,and their?T were significantly reduced.It showed that antioxidants can stabilize abietic acid and levopimaric acid,and slow down the oxidation reaction rate.The order of the antioxidant effects of 4 kinds of antioxidants on abietic acid is as follows:vitamin E>antioxidant 1010>BHT>tea polyphenols;the order of the antioxidant effects of 4 kinds of antioxidants on lwvopimaric acid is as follows:BHT>antioxidant 1010>vitamin E>tea polyphenols.?4?.Study on photooxidation of glycerol ester of rosin?GER?and rosin pentaerythritol ester?RPE?A self-designed gas-solid photo-oxidation reaction device was used to measure the oxidation reaction of GER and RPE in the air,and explore the weather resistance of GER and RPE.The photo-oxidation kinetic parameters of the two rosin esters were obtained by studying the oxidation reaction of GER and RPE in the air under no UV radiation,365 nm UV,and 254 nm UV irradiation.The results showed that in the air at room temperature,GER and RPE can be oxidized under UV 254 nm irradiation,but no oxidation reaction was observed at room temperature under no UV irradiation or 365 nm UV irradiation.The photo-oxidation process of GER is pseudo-first-order kinetics,and the activation energy?Ea?has a linear relationship with the logarithm of I under experimental conditions:Ea=-3.575ln I+34.943;the photo-oxidation process of RPE is also a pseudo-first-order kinetics.Under the experimental conditions,the logarithm of Ea and I has a linear relationship:Ea=-4.937ln I+45.565.GER and RPE will form high levels of peroxides during their photooxidation.The increase of temperature?T?and light intensity?I?will make GER and RPE unstable by accelerating the formation of peroxide. |
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