Effects Of Hygrothermal Treatment On The Physical And Chemical Properties Of Moso Bamboo

With decreased resources and increased price of wood in China,bamboo has become the best substitute for wood because of wide distribution,short growth cycle and excellent mechanical properties.However,bamboo is easy to absorb and desorb moisture and has low dimensional stability.It contains more nutrients such as starch,sugar and protein than wood,thus leading to deform and even crack or mildew,which affects its use in environments with greatly changed humidity.Hygrothermal treatment has been proved to improve the dimensional stability and chemical properties of wood at certain humidity and temperatures.However,few studies have applied this treatment on bamboo.It’s significant to explore and analyse the effects of hygrothermal treatment on the physical and chemical properties of bamboo.This can provide theoretical value and practical significance to obtain the best conditions of hygrothermal treatment on bamboo,improve its service life,expand its scope of application,and meet its utilization in high-humidity environments.In this work,the effects of hygrothermal treatment on the physical and chemical properties of four-year-old Moso bamboo at different temperatures（160°C、180°C、200°C、220°C）were investigated.Various methods of characterizing physical and chemical properties,such as SEM,spectrophotometer,contact angle meter,TGA,FTIR and 2D-HSQC NMR,were used.The variations of surface morphology,color,contact angles,swellings,ASE,thermal stability,main chemical components,characteristic functional groups and chemical bond were characrized.The mechanism of the effects of hygrothermal treatment on the physical properties and chemical compositions was revealed.It was pointed out that 180°C was the best temperature for hygrothermal treatment on bamboo.The following are the main conclusions of this paper:（1）After treatment at 200°C,fibers and parenchyma cells delaminated and some pores appeared within them.Both phenomena became more obvious after treatment at higher temperature（220°C）.（2）Compared with the untreated samples,the color of the treated bamboo became darker.The lightness decreased obviously,while the color difference increased with temperature.The hydrophobicity and dimensional stability of Moso bamboo improved obviously after hygrothermal treatment.The peak value of DTG curves,statistic heat-resistant index temperature and ash content increased with temperature,indicating improved thermal stability of bamboo.The temperature corresponding to the fastest degradation rate reached the maximum after treatment at 200°C.Both of the initial decomposition temperature and the change rate of statistic heat-resistant index temperature increased most obviously after hygrothermal treatment at 180°C,confirming that 180°C was the best temperature for the hygrothermal treatment on bamboo.（3）FT-IR showed that xylan in hemicelluloses gradually degraded with the increase in temperature.The acetyl in hemicelluloses degraded to acetic acid,which further catalyzed the hydrolysis of hemicelluloses;the intensity of the absorbance band at 1026cm^-1（asymmetric aromatic ether bending vibration）gradually decreased,and it decreased by 21%at 220°C,illustrating the breakage of ether bonds in lignin.The relative intensity of the absorbance band at 1597 cm^-1（corresponded to the aromatic skeletal vibrations and the C=O stretching vibrations linked to the aromatic skeleton）decreased by 18%after treatment at 220°C,indicating the decreased C=O in bamboo.The absorbance bands at899 cm^-1and 1153 cm^-1were characteristic of C-H in b-（1-4）-glycosidic linkages and C-O-C stretching vibrations in cellulose,respectively.Their intensities increased after treatment at 180°C,and the bands shifted to 903 cm^-1and 1154 cm^-1.This indicated increased crystallinity and changed structure of cellulose.The lower intensity of the band at 3340 cm^-1revealed that most O-H bonds were destroyed and stable hydrogen bonds were formed after hygrothermal treatment,resulting in improved dimensional stability.（4）2D-HSQC NMR showed that the intensities of xylan signals decreased.After hygrothermal treatment at 220°C,β-O-4 bonds decreased by 37.5%;β–βbonds disappeared and the hydroxyl in S units were oxidized;the S units in lignin condensed,thus improving the dimensional stability of bamboo.The relative content of H units increased at higher temperature,indicating that a small number of G units were transformed into H units,and S/G increased from 2.0（control sample）to13.1（the sample treated at 220°C）.The decrease of ferulic acid indicated the breakage of the connection between lignin and hemicelluloses,resulting in the looser structure of bamboo.The ester bonds between PCE and lignin were not completely broken even at 220°C.