Effects Of The Structure And Properties Of Cellulose Fiber On Microwave Treatment And Its Applications

Cellulose fiber is a kind of abundantly important textile raw materials.People not only pay much attention to cellulose fiber performance,but also take interested in gaining new insight into its processing and utilization.Microwave heating treatment (MHT) is a heating technology with advantages of energy saving and high efficiency. Based on a combination of microwave technology with textile industry,textile materials treated by MHT open up new areas.Despite the early development of MHT,studies on MHT acting on fibers are mainly focused on the application fields and little on the aspects of the micro mechanism of MHT to fibers.Neither complete set of theories to describe the interaction between microwave and wet cellulose fiber nor studies on the structure and properties of cotton fiber after MHT has been reported yet.Cotton and ramie are selected as the representative of the natural cellulose fibers while rayon,loycell and cellulose acetate fibers as the representative of the regenerated cellulose fibers.Under different MHT and fiber parameters,the changes of morphology,micro structure and crystalline structure of five-kind cellulose fibers are studied systematically.The law of influence of structure and properties of cellulose fiber after MHT is set a theoretical basis for the application of MHT towel drying of cellulose industries.MHT Dry dynamics model of cellulose fibers are firstly established in this paper,which describes systematically the degree of water lose and the cellulose fibers after MHT.Characterizations and comparisons of crystalline structure of cotton fibers before and after MHT were investigated using ¹³C-CP/MAS NMR spectroscopy technology as well as trends was studied.Besides,MHT technology was applied to towel drying process in industry production.Contents and conclusions of the studies are mainly as follows.The dielectric properties of cellulose fibers were studied and the water existent form in fibers including free water and bound water were analyzed and described. Microwave loss of cellulose fibers were analyzed and the mathematical relationship between fiber fineness(N_tex) and microwave loss of unit length of cellulose fibers(P_l) were also discussed.The water loss laws of the five-kind cellulous fibers were studied.The differences between cellulose acetate fiber and the other four-kind fibers are significant.Initially, cellulose acetate fiber shows the highest drying rate.However,with the loss of water, the drying rate of rayon and loycell becomes the biggest,followed by cotton and ramie fibers.As far as the same kind of fibers with different initial moisture regains are concerned,the drying curves are basically the same,that is to say,they share the similar drying laws.Furthermore,with increasing initial moisture regain,the time of MHT drying is increased;with increasing degree of water swelling in fibers, water removal became difficult;with increasing MHT power and impurity contents in fibers,drying rate of cotton is dramatically increased.Microwave dry dynamics model was studied,and the dynamics model of cellulose fibers was expressed by PAGE equation MR=exp(-ktⁿ).The equation after regression fitting can describe moisture changes in microwave drying process of cotton.The changes of fiber tensile properties after MHT with different moisture regains were studied.Except cellulose acetate fiber,the strength of other four-kind fibers in completely dry state is decreased notably after MHT.With the initial moisture regain increased,the breaking elongations of cotton,rayon and loycell are improved remarkably after MHT.The increasing level is rayon＞loycell＞cotton,but ramie seems little improved.As the moisture regain is low,the fiber initial modulus is enhanced.However,when the moisture regain is higher,the fiber initial modulus did not change obviously.The influences of microwave power and time on fiber tensile properties were studied.For cotton,rayon and loycell fibers,with microwave power increases while MHT time keeps constant,fiber strength and breaking elongation initially increase and then decrease.Likewise,with MHT time increases while MHT power keeps constant,fiber strength of cotton,rayon and loycell increase and then decrease.Ramie, however,shows only decreased strength.With MHT time increases while MHT power keeps constant,breaking elongation of loyeell remains increasing but cotton,rayon increase and then decrease,still ramie only decreases.At the same time,all the initial modulus increased at first and subsequently decreased,but the resultant initial modulus are still higher compared with the fibers without MHT.Through SEM observation,we observed that axial morphology and cross section of fibers after MHT become slightly rough,but tiny cracks appear on the surface of ramie and loycell.For cotton and rayon,changes are very tiny,and cellulose acetate fibers have few changes.Through X-ray diffraction,we found that the basic crystalline structure of cellulose fibers can not be changed by MHT in the experiment.But after MHT,both the crystallinity and crystalline size of cotton,rayon and loycell are decreased. Meanwhile,the influence of MHT on regenerated cellulose fibers is stronger than natural cellulose fibers.Based on IR analysis,the chemical composition of cotton after MHT is not changed. At the wave number range of 3100～3600cm^-1,the two spectrums are different, which exhibits that the function of hydrogen bond between intermolecular and intramolecular is changed after MHT.Moisture index of fibers after MHT improve.Characterization of cotton fiber using solid-state ¹³C-CP/MAS NMR spectroscopy shows that the crystallinity after MHT is lower than the original sample,but the cotton after MHT has a higher lateral orientation index,which has a good definition in C2,3,5 resonance region.¹³C-CP/MAS NMR spectroscopy and X-ray diffraction reveal that cellulose I_βis the main cellulose in cotton,and microwave change the allomorphs content.In addition,the allomorphs content I_αand I_βdecreased,but the ratio keep stable.The paracrystalline of cotton fibers increased after MHT.Using towel drying as an example to study the microwave application in cellulose industries and compare to traditional way of drying(TD),a comparison has been drawn between microwave drying(MD) and TD with first-class cotton towels with 300%initial moisture regain,and the results show that MD save much more time and dry faster than TD.The strength and moisture absorption properties of towel after MD improved.The two way of towel drying show the same effect on unhairing rate and color fastness of towel.Structure and properties of cellulose fibers is a basic research subject.Under the guidance of the microwave-cellulose interaction theory,we applied microwave to cellulose fiber industries with a view to provide a theoretical basis for the application of microwave radiation treatment in cellulose industries.