Study On Fractionation Characteristics Of Eucalyptus Lignin And Its Effect On Mechanical Properties Of Lignin Material

In this paper,eucalyptus was used as raw material to study dissolved lignn content by alkaline pulping process.At the same time,the alkali lignin was fractionated and analyzed for chemical structure characteristics of different fractions.The relationship between the molecular weight of lignin and its mechanical properties was systematically discussed.The conclusions are as follows:(1)The alkaline pulping process was optimized by single factor experiment,and the optimal conditions were obtained:the liquid ratio 1:5,the Alkali charge 20%(calculated as Na,O),temperature 160 ?,and holding time 2h.The dissolved lignin content was 24.33%,which was close to the lignin content in the raw material analysis.(2)The alkali lignin was fractionated with ethyl acetate,acetone,acetone/water(9/1,v/v).The molecular weights Mn of F1,F2 and F3 were 1856 g/mol,2525 g/mol,3976 g/mol,and PDI was 1.45?1.51?2.84.The phenolic hydroxyl group content decreased gradually with the increase of molecular weight,and the F1,F2 and F3 were 2.8844 mmol/g,1.8477 mmol/g,and 1.3694 mmol/g.FTIR analysis showed that the infrared spectras of different fractions of lignin were very similar,and the absorption peak corresponding to the functional group appeared at each position,but the shape of the peaks were different.13C-NMR analysis showed that the content of S units in the fractions were higher than that of the G units,and the low molecular weight lignin fraction F1 had a higher S/G ratio.The signal intensity of the F1 fraction in the 60-90 ppm region is weaker than F2 and F3,indicating that the linkages F1 fraction lignin units were largely destroyed during the pulping process.2D-HSQC NMR analysis indicated the ratio of ?-0-4 and ?-O-4 linkages increase with the increase of molecular weight,While the ratio of ?-? linkage not much different.Ferulic acid and stilbene were present in the F1 fraction and were completely separated by ethyl acetate during fractionation,resulting in the absence of such compounds in subsequent fractionation.TGA analysis showed that with the increase of molecular weight,the char yield of lignin increased,the maximum degradation temperature decreased,and the thermal stability increased.(3)The lignin itself had little mechanical properties.After hot pressing,much lignin was filled in the form of solid small particles.Comparison of different fractions of lignin founded that lignin with high molecular weight had higher mechanical properties than lignin with lower molecular weight.The MOR of F'1,F'2,and F'3 at the hot pressing temperature 180? and 12 min was 11.91 MPa,13.58 MPa,and 14.24 MPa,and the IB was 0.27 MPa,0.32 MPa,and 0.33 MPa.After lignin hot pressing,the molecular weight increased and the phenolic hydroxyl group content decreased.,The Mn of F'1,F'2 and F'3 were 3764 g/mol,4883 g/mol and 6514 g/mol,respectively,and the phenolic hydroxyl group content was 2.2528 mmol/g,1.5363mmol/g,and 1.2118 mmol/g.2D-HSQC NMR analysis showed that the ether bond in the side chain region was greatly affected by the hot pressing,and the carbon-carbon bond was less affected.The G-type lignin in the aromatic zone was greatly affected by the hot pressing,and the S-type lignin had less influence.TGA analysis showed that the thermal stability of different lignin fractions after hot pressing was improved.The addition of HCl(CH3OH)lignin reduced the mechanical properties of the alkali lignin sheet.When PLA added 30%,it basically reached the standard of ordinary medium density fiberboard.When the amount of PLA added exceeded 40%,the sheet is not affected by the molecular weight of lignin.