Laser and ultrasound radiation pretreatment of cellulose in dissolving wood pulp.
Ocwelwang, Atsile Rosy.
MetadataShow full item record
Dissolving wood pulp (DWP) refers to wood extracted, chemically refined bleached pulp that comprises more than 90% pure cellulose. This type of pulp is mainly utilised for the production of various cellulose derivatives such as rayon, cellophane, cellulose ethers and cellulose esters. Production of these valuable products is achieved by dissolution of DWP in chemical solvents such as sodium hydroxide (NaOH) and carbon disulphide (CS2). However, cellulose dissolution is not easily achievable due to the strong hydrogen bond interactions that give this biopolymer its highly ordered crystalline structure. High cellulose crystallinity limits the accessibility and chemical reactivity of this biopolymer. Various forms of pretreatment techniques have been developed to solve this problem. The primary aim of the pretreatment is to disrupt the rigid crystalline structure of cellulose, and this increases its structural accessibility and chemical reactivity. However, most pretreatment techniques have drawbacks such as being energy intensive, costly, corrosive, posing risks to personnel operating them, and not being readily accessible. For example, gamma ray irradiation has been shown to be an effective pretreatment technique for cellulose and other lignocellulosic materials, but it is a costly technology. For this reason, research into environmentally friendly and profitable pretreatment techniques is ongoing. This study aimed to evaluate the effect of two procedures, namely, ultrasound and laser irradiation, as possible pretreatment techniques for cellulose activation. Ultrasound irradiation is a conventional method that has been used for pretreatment of a wide range of polymeric materials, whereas the use of laser irradiation as a pretreatment technique for cellulose activation has never been reported. Therefore, using lasers as a pretreatment for activation of DWP with the aim of modifying the cellulose structure to improve its chemical reactivity is a novel aspect of this thesis. The effects of these two pretreatment techniques on the structure of cellulose were studied and compared. The effects of the two techniques were investigated separately in two parts. The first part focused on the effect of ultrasound irradiation on the structure of cellulose, and the dissolution behaviour of the ultrasonicated pulp samples in aqueous NaOH solution. The second part evaluated the effect of laser irradiation on the structure of cellulose, and the chemical reactivity of the pretreated cellulose pulp samples was measured using the Fock test method. A range of analytical techniques was employed to characterise the pretreated samples to evaluate the structural modifications that resulted from the pretreatments. Size exclusion chromatography with multi-angle light scattering (SEC-MALS) analysis was used to study the molecular structural properties of cellulose. X-ray diffraction (XRD), and Solid-state CP/MAS 13C-NMR were used to investigate the crystalline structure of cellulose and to measure its degree of crystallinity (CrI). Ultrastructural and morphological properties of cellulose were also studied by atomic force microscopy (AFM), scanning electron microscopy (SEM), and morphological fibre analyser (MorFi). Untreated DWP samples were used as control samples for all procedures and analysis conducted in the study. In the first part of this study, DWP samples were pretreated with ultrasound irradiation and subsequently dissolved in aqueous NaOH solution. SEC-MALS results showed a shift in average molar weight (Mw) from high to lower region after ultrasonication. Broadening of the molecular weight distribution (MWD) was displayed by increase in polydispersity index (PDI). A decrease in Mw was also observed with increasing pretreatment time; this confirmed the effect of the ultrasonic pretreatment on the molecular structure of cellulose. SEC-MALS analysis of the ultrasonicated and alkali treated samples displayed different dissolution behaviour compared to the control, and the changes in the MWD data did not follow any trend relative to treatment time. XRD results indicated that ultrasound irradiation had minor or no effect on the degree of cellulose crystallinity (CrI). However, treatment of the ultrasonicated pulp samples with aqueous NaOH significantly decreased the CrI. This reduction in CrI indicates that alkali treatment transformed natural cellulose to regenerated cellulose. A CrI decrease of more than 50% was also observed for the samples treated for 60 minutes (alkcell-UT60 min). AFM ultrastructural results revealed that ultrasonication did not induce visible changes on the surface of the S2 layer. An overall decrease of lateral fibril aggregate dimensions (LFAD) was observed after ultrasound irradiation, but the reduction did not show a linear relationship with increasing treatment time. Fibre distribution and dimensions results from the MorFi analyser showed that with prolonged treatment time, the number of fibres in solution increased, average fibre length decreased, while the average fibre width did not display significant changes. SEM surface morphology results showed that after ultrasonication the surface of the fibres became smooth, and there was less or no fibrillation. SEM analysis revealed that after alkali treatment the surface of the fibres displayed folds and trenches which ran parallel to the length of the fibres. This is an indication of an increase in the surface area of the fibres due to the treatment with an alkali solution. Moreover, the fibres appeared compact and agglomerated. In the second part of this study, laser radiation was used for pretreatment of DWP samples. The pretreatment caused visible morphological and molecular changes in the structure of cellulose. SEC-MALS results demonstrated that laser irradiation decreased the average Mw of the cellulose polymer and caused noticeable modifications on the molecular structure of this biopolymer. The PDI also showed increase; this rise in PDI is indicative of the size distribution of the Mw and the heterogeneity of the cellulose chain lengths as a result of the pretreatment. XRD results demonstrated that laser irradiation disrupted the crystalline structure of cellulose; an overall significant decrease in CrI was noticed for all the laser irradiated samples. NMR characterization of the irradiated samples also displayed a decline in CrI. SEM characterization results illustrated that laser pretreatment disrupted the morphology of the cellulose fibres and created porous cavities on the fibre surfaces. AFM images for laser irradiated samples also displayed similar features. The S2 layer within the cross-sections of the samples irradiated with Nd:YAG laser at 266 nm and 355 nm also had small pores and dark areas between the fibril aggregates which represent the less stiff or potential accessible regions. Calculated LFAD results also confirmed these observations in that the data displayed a noticeable decline in LFAD after laser irradiation. Finally, the Fock test results showed that the laser pretreatment caused a linear increase in cellulose reactivity with increasing irradiation time. From the results and observations presented in this study, it can be concluded that laser radiation pretreatment caused noticeable structural disruptions on the structure of cellulose compared to ultrasound irradiation. It disrupted and damaged the surface morphology of cellulose and further caused a significant degradation of the molecular structure. Moreover, increase in cellulose reactivity as measured by the Fock test method was observed in all samples pretreated with lasers. The highest reactivity increase of more than 35% was obtained in samples irradiated with the Nd:YAG laser at a wavelength of 266 nm compared to Nd:YAG and CO2 lasers at 355 nm, and 10.6 μm respectively. This observation showed that laser wavelength was influential in cellulose modification compared to laser power. Furthermore, the increase confirmed that laser radiation pretreatment reduced the cellulose Mw and disturbed its crystalline structure thus enhancing its accessibility and reactivity to chemical solvents. Therefore, the novel aspect of the thesis is that: Pretreatment of DWP with laser radiation modified the structural features of cellulose, and this led to an overall cellulose reactivity increase of about 20%. Consequently, this should result in a reduction of dosages of chemical reagents used for induction of cellulose reactivity and processing. The use of lasers for cellulose pretreatment could be a more affordable option compared to the conventional high energy radiation sources because they are less expensive and readily accessible.