Nano-Mechanical Properties And Potential Applications Of Straw Fiber

This study investigated the nano-mechanical properties of fiber cell walls of ten hardwoods and five crop stalks by nanoindentation and atomic force microscopy(AFM).And also the rice straw cellulose fiber was used to produce rice straw micro/nano fibrils(RSF) by high intensity ultrasonication(HIUS) and high pressure homogenizer(HPH).The fibrils were used to reinforce polypropylene(PP) and poly(vinyl alcohol) by compression molding and film casting. The effects of the compounding methods,technical conditions,compatilizer addition and fibril loadings on tensile properties,dynamic mechanical and thermal properties of resultant nanocomposites were investigated.The conclusions were listed as following:The SilviScan elastic modulus of Keranji appeared to exhibit the highest value of 35.4 GPa, whereas poplar exhibited the lowest elastic modulus of 14.2 GPa.Moreover the scanned wood density is the predominant factor that determines the SilviScan elastic modulus.The higher the scanned wood density,the higher is the SilviScan elastic modulus.The SilviScan elastic modulus increased with wood density and decreased with microfibril angle(MFA).The highest cell-wall elastic modulus of 24.6 GPa was observed in Keranji,which was lower than its SilviScan modulus(35.4 GPa).The lowest elastic modulus of 16.9 GPa was observed in Poplar. The percentage of the difference in elastic modulus between Keranji and Poplar was 31.3%. The wood cell-wall elastic modulus increased with wood density and decreased with MFA.The values of hardness of cell wall varied from 0.44 to 0.56 g/cm³.It appeared that there was no obvious relationship between cell-wall hardness and MFA.Wheat straw has the highest elastic modulus of 20.8 GPa within five crops.The soybean stalk and cotton stalk both exhibit the lowest values of 16.3 GPa among the five crop stalks.The mean value of five crop stalks was 18.4 GPa.The values of the elastic moduli of cell walls of crop stalks(16.3-20.8 GPa) were comparable to that of most of hardwood species(16.9-24.6 GPa) and was higher than that of some softwoods(15.0-18.0 GPa) and lyocell fiber(11.5-13.2 GPa).The cotton stalk cell walls showed the highest hardness of 0.85 GPa.The lowest hardness of 0.48 GPa was observed in soybean stalks.The percentage of difference in hardness was up to 43.5%among five crops.The mean value of hardness in crop stalks was the highest one(0.60 GPa) compared with the lower value of 0.39 GPa in lyocell fiber and 0.49 GPa in hardwoods and 0.41 GPa in softwoods.This indicates that crops stalk fibers have relatively better nano-mechanical properties than wood and lyocell fiber. The distribution of diameters of RSF ranged from 0.1μm to 80μm by HIUS treatment.The percentage was 6.3%of RSF which diameters were less than 500 nm;almost 90 percents of RSF distributed between 7.0μm and 80μm;the average diameter was 41μm.The distribution of diameters of RSF ranged from 0.1μm to 25μm by HPH treatment.The percentage was 16% of RSF which diameters were less than 500 nm;almost 90 percents of RSF distributed between 0.7μm and 25μm;the average diameter was 8.8μm.The relative crystallinities of untreated rice straw cellulose fibers and rice straw cellulose fibers treated by HIUS and HPH were 71.3%, 72.9%and 68.0%,respectively.The FTIR spectra showed that the -OH peaks moved to higher wavenumbers in rice straw cellulose fibers treated by HIUS and HPH than untreated fibers.The SEM images showed:the fiber cell walls were crashed by HIUS and HPH treatments,and the fibers appeared the phenomena of layered and fibrillation.The fibrils were isolated from the fiber aggregations.The elastic modulus increased of RSF/PP nanocomposite comparing with PP/MAPP polymer. However,the tensile strength and elongation at break of 5%rice straw fibril reinforcing PP nanocomposite was lower than PP/MAPP polymer at different extruder compounding conditions.The tensile strength at 5%RSF loading was up to the maximum value,31.7 MPa, which was a little higher than the value of PP/MAPP polymer,30.83 MPa.The maximum was 1621 MPa at the 8%RSF,which was 17%higher than the value of PP/MAPP polymer.The elongation at break showed a significant decreasing trend with increasing the fibril loading. There was no distinct influence of tensile strength and elongation of nanocomposite and PP/MAPP polymer with increasing MAPP content.The maximum of elastic modulus was 1509 MPa at the 4%of MAPP.The tensile properties increased distinctly with increasing ultrasonication treat time.The tensile strength of 0.5%rice straw cellulose fiber content was lower than 1%rice straw cellulose fiber content at different ultrasonication treat time.After adding MAPP and RSF into PP matrix,the FTIR spectra had big changes.The absorption peak of ester bonds(C-O-C) appeared at 1224 CM^-1,1074 CM^-1 and 1028 CM^-1,which proved that there had been a good compatibility between PP matrix and RSF.The SEM images showed:the fibrils were embedded into the PP/MAPP matrix,during the tensile test,which were not pulled out from the matrix.The tensile strength of the nanocomposites was higher than PVA matrix.Moreover,the tensile strength of the nanocomposites casting at -20℃freezing and thawing 3 cycles condition were higher than the nanocomposites casting at room temperature.When casting at room temperature,the storage flexural modulus(E') of the nanocomposites higher than PVA by adding 2%～11%RSF into the PVA matrix.When casing at -20℃freezing and thawing three 3 cycles condition,the E' of nanocomposites were higher than PVA matrix at up 60℃condition;and there was no distinct discrepancy at under 60℃condition.When comparing of the damping factor(tanδ) of RSF/PVA nanocomposites and PVA matrix,which had no clear difference at whole temperature range.However,the glass transition temperatures (T_g) were higher than the PVA matrix.There were no big changes of melt temperature of nanocomposites casting at room temperature and -20℃freezing and thawing three 3 cycles conditions.The crystallinity(X_C) of nanocomposites casting at -20℃freezing and thawing three 3 cycles condition was higher than the nanocomposites casting at room temperature.The adding of fibrils improved the thermo stability of PVA matrix.The maximum ratio of weightlessness(T_p) of nanocomposites casting at -20℃freezing and thawing three 3 cycles condition was higher than the nanocomposites casting at room temperature,and T_p increased with fibrils loadings.The FTIR spectra of RSF/PVA nanocomposites contained the characters of PVA matrix,however,because of the chemical reactions during blending,the peak of-OH changed,which proved a good compatibility between RSF and PVA matrix.