Eco-friendly cellulose nanofibre membranes

Abstract

Due to the rapid population growth, mounting energy concerns and increasing environmental crisis, the development of new and more energy efficient membranes for water and air purification is becoming an important task. Currently, membranes made from petro-based polymers are used. Although they have been widely recognized because of their excellent properties, they pose environmental problems during production and disposal stages of the life cycle. The essential characteristics of next generation membranes should be high efficiency, high durability, low operating pressure, low cost, and be able to be fabricated using environmentally friendly processes. Cellulose nanofibres are of technological interests as renewable, sustainable and eco-and bio-friendly raw materials to produce such nanomembranes. They possess excellent properties and can be used in environmental, biomedical and other functional filtration applications. The research in this thesis focused on developing the ‘green’ nano-membrane having high mechanical strength and pore size control. Cellulose nanofibre dimensions, diameter and length, are critical for establishing the structure-property relationships in the later part of this research. Cellulose nanofibre diameter controls the pore size of the nanofibre membranes and aspect ratio controls the preparation of nanofibre membrane via filtration. There is currently no easy means of rapidly quantifying aspect ratio of nanofibres because the two ends of nanofibres cannot be seen in microscopic images due to entanglement between fibres, although diameter distributions can be measured from microscopic images at high magnification. A simple method was developed to estimate aspect ratio of nanofibres from sedimentation and yield stress measurements. The gel-point was measured both from the height of a layer of cellulose nanofibres sedimented from a dilute suspension or from the lowest solids concentration at which a yield stress could be measured using a vane rheometer. The two methods were closely in agreement for all samples. Aspect ratio was then calculated using either the Effective Medium (EMT) or Crowding Number (CN) theories. Fabrication of nanofibre sheet is time consuming for the existing methods because they use fine filter media to retain the cellulose nanofibres on filter medium. A rapid and commercially feasible method for preparing cellulose nanofibre sheet was developed to produce high quality nanofibre sheets using large pore size filter medium. The retention of nanofibres is improved by using the concentration of the forming suspension higher than connectivity threshold to allow the connected fibre suspension to bridge over the large pore openings. Nanofibre sheet preparation required 13 minutes in total. The mechanical strength of cellulose nanofibre paper is a key property but tensile strength measurement requires a substantial amount of test material. To minimise the time for testing mechanical properties and the loss of sample, it has been established that zero/short span test can be used instead of standard tensile test to measure the tensile strength for nanofibre sheets. This is because there are no fibres bridging between and directly gripped by both jaws, just as the case in tensile tests. The zero-span test was shown to be a sensitive measure of the sheet strength at a short scale. Cellulose nanofibre sheets possess good dry and wet strength compared to standard cellulose fibre sheets. However, wet strength of nanofibre sheet is low for use in certain applications like filtration. The effect of the addition of two cationic polymers, CPAM and PAE, on cellulose nanofibre sheet forming characteristics such as water drainage and nanofibre retention and strength of cellulose nanofibre sheet is investigated. It is found that the retention of nanofibre increased with the addition of either polymer; in addition wet strength of nanofibre sheet greatly increased with the addition of PAE. The method developed to prepared nanofibre sheet with high wet strength, was adapted to prepare nanofibre membranes and nanofibre composite membranes. Their performance in filtration applications was then evaluated. Cellulose nanofibre composite membranes were prepared using suspensions of cellulose nanofibres, silica nanoparticles (22nm) and polyamide-amine-epichlorohydrin (PAE) via filtration. It was demonstrated that silica nanoparticles act as spacers to control pore size of nanofibre network. PAE was added to adhere the negatively-charged nanoparticles to the nanofibres and also to improve the wet strength of the membrane. Membranes prepared with nanofibres alone showed high flux but low rejection due to large pore size. In contrast, nanofibre composite membranes showed water flux of 80 LMH (litres per square meter per hour) and Molecular Weight Cut Off of 200 kDa. The addition of silica nano particles controlled pore size. These results demonstrate the potential of cellulose nanofibre composite membranes in ultrafiltration. The produced membranes are readily recyclable as a feed stock to a conventional paper making process. By conducting these studies, a novel strategy to rapidly produce eco-friendly cellulose nanofibre composite membranes for ultrafiltration applications was developed.