This project focused on the (i) efficient production of nanocellulose from hemp fibres, and (ii) the application of nanocellulose, including the modification of hemp fibres with nanocellulose and the reinforcement of epoxy with nanocellulose.
The oxidation/sonication method was developed successfully to fabricate nanocellulose. RSM based on a five-level-four-variable CCD was employed to optimize the preparation conditions of nanocellulose, the yield of nanocellulose up to 54.11 % under the optimal condition, i.e. hydrolysis time 5h, hydrolysis temperature 67 °C, dosage of swelling agent 4.00 % and dosage of oxidant 70 %.
The nanocellulose was then characterized systematically by employing various analytical instruments. Three main characteristics of the nanocellulose, namely, size distribution and morphologies, chemical structure and crystalline, and thermal properties, were characterized. NTA results disclosed that oxidation/sonication method was able to fabricate nano-scale cellulose. This result was evidenced by the morphologies characterization with AFM and FEG-SEM. Morphologies analysis revealed that nanocellulose fabricated by oxidation/sonication had a low aspect ratio value (only 4) far less than that of acid method or mechanical method. XPS analysis revealed that hydroxyl groups in cellulose were oxidized into carboxyl groups during the oxidation/sonication process. ATR-FTIR results further reveal that parts of hydroxyl group in C2, C3 and C6 were during the process. XRD measurement showed that CI for hemp yarn and nanocellulose were 84.66 % and 86.59 % respectively. This indicated that the oxidation/sonication did not damage the crystalline structure of cellulose. DSC results showed that nanocellulose had lower carbonization temperature and thermostability.
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The first application of nanocellulose in this project was the modification of hemp fibres with nanocellulose. Deformation of hemp fibres was the main research point in this part.
Four forms of deformation were found from hemp fibres, the MFA of S2 inner layer at fracture point which was measured by optical and their mean value was 6.16°. This indicated that deformation is the weak link point of fibres. The mechanism of deformation on the mechanical properties of hemp fibres was further revealed by employing spectroscopy technology. The crystallinity index examined by FTIR was 48.4 % for the hemp without deformations and 41.3 % for those within deformation regions, showing a significant reduction of crystallinity in the deformations. The chemical structure of deformations in hemp fibres was further revealed by FTIR. Weaker inter-and intra-molecular hydrogen bonding in the deformations of hemp
fibres were found by the deconvolution of FTIR spectra in the OH stretching region. This may be the main cause that induced the decrease of tensile strength in the hemp fibres, especially the intramolecular hydrogen bond of O(3)H---O(5). The FTIR
spectra from 1370 cm-1 to 1330 cm-1 illustrated that the band at 1368 cm-1 and 1363
cm-1 disappeared in deformation regions, indicating the removal of the hemicelluloses
in deformations and hence possible loss of lignin. The deconvolved FTIR spectra
from 1330 cm-1 to 1215 cm-1 showed the S ring stretching, CH2 rocking at C6 in
cellulose, G ring stretching, C-C plus C-O plus C=O stretch and COH bending at C6 in cellulose, indicating reduction of lignin content in the deformation regions. The
ratio of S (Syringyl, 1325cm-1)/ G (Guaiacyl, 1259 cm-1) was 1.1 for the hemp
without deformations comparing to 0.9 for the deformation regions, indicating higher cellulose content in the deformation regions. The effect of deformation on the modification which was carried out by DTAB-nanocellulose nanotechnology was revealed by EDX which showed that deformations affected the absorption of DTAB significantly. In the deformation of hemp fibres, the absorption ratio of DTAB was 0.98, while in the region without deformation this value was only 0.59. This indicated that much more nanocellulose was adsorbed on the deformation of hemp fibres
The reinforcing mechanism of the two-step nanocellulose modification on hemp fibres was also revealed by employing other two instruments, i.e. FEG-SEM and XRD. FEG-SEM results showed that nanocellulose covers the deformation of fibres with two ways, namely, (i) nanocellulose filling in the stria and (ii) bonding the inter-fibril on the gap between two fibrils. XRD results showed that the CI of un-modification, DTAB pretreatment and two-step modification were 55.17 %, 65.95 % and 76.39 % respectively. This indicated that the two-step nanocellulose modification could improve the crystallinity index of hemp fibres. We conjecture that the increase of CI is caused by the formation of hydroxyl bonds between nanocellulose with hemp fibres in the S2 layers and the non-crystalline regions of hemp fibres.
The interfacial property improvement of two-step modification on hemp fibres was investigated by employing XPS and ATR-FTIR. XPS confirmed that much more unsaturated polyester could be absorbed on the fibres with two-step modification. ATR-FTIR characterization showed that (i) the higher resin adsorption may be due to the esterification between hydroxyl groups at C-2 and C-6 of nanocellulose and carboxyl groups of unsaturated polyester, and (ii) the nanocellulose modification can benefit the adhesion of styrene on the surface of the modified fibres.
The second application of nanocellulose was the fabrication of nanocellulose/epoxy nanocomposite. DETA was used to modify nanocellulose. The preparation of nanocellulose/epoxy nanocomposite was optimized by single factor method. The optimized results showed that the maximal mechanical properties of nanocomposite can be obtained under the conditions: curing temperature 130 ºC, dosage of nanocellulose 0.035 %. Compared with epoxy, the modulus, tensile stress and tensile
strain of the modified nanocellulose/epoxy nanocomposite increased 1.42 %, 15.44 % and 27.47 % respectively. The DETA modification facilitates the dispersion of nanocellulose into epoxy. FEG-SEM showed that the size of nanocellulose particle in the matrix can decrease from 785 nm to 350 nm. Compared with the unmodified nanocellulose/epoxy nanocomposite, the modulus, tensile stress and tensile strain of the modified nanocellulose/epoxy nanocomposite were increased by 4.93 %, 29.36 % and 57.49 % respectively.
DSC was used to investigate the effect of nanocellulose on the cure kinetics of epoxy. Three methods were employed for the analysis of cure kinetics, i.e. KAS, Friedman and Málek method. Substantial difference in activation energy values can be found between Friedman method and KAS. The cure kinetics results which were calculated by Málek method showed that the DETA modified nanocellulose/epoxy system still have autocatalytic feature of epoxy and Šesták-Berggren kinetic model can describe the curing process of DETA modified nanocellulose/epoxy resin systems very well.