Preparation and properties of cellulose nanofibrils from pulps with different lignin content

Nanocellulose, especially cellulose nanofibrils (CNF), have been an extensively researched topic for the last few years. The CNF possesses an incredible weight to strength ratio, is biodegradable, and the raw materials are cheap and easy to come by. The CNF have been utilized in many different ways and have in recent years especially been studied in biomedical applications and composites. Other areas where CNF has been successfully implemented are in packaging, agriculture, membranes, hygienic devices, and insulations.The most common method for CNF isolation is the use of a high-energy homogenizer with a pretreatment of TEMPO-oxidation. Pulps that are converted into CNF can contain different kinds of residues such as hemicelluloses and lignin, and it is therefore of importance to understand the fate of these residues and how these residues affect the fibrillation process, as well as the prepared CNF hydrogels.In this work, four different pulps were used in the preparation of CNF. These pulps were an industrially bleached kraft pulp, a self-bleached (bleached in the laboratory) kraft pulp, an unbleached kraft pulp, and blotters (mostly made of cotton fibers). The pulps did have different lignin amounts and the effect of lignin on  the fibrillation process was studied. It was, however, proven that lignin is oxidized in the TEMPO-oxidation step and then removed to a large extent in the following washing step. It was therefore concluded that the amount of lignin in the pulp does not significantly affect the TEMPO-oxidized CNF.The four laboratory TEMPO-oxidized CNF were accompanied by a commercially available CNF and were together with the pulps extensively analyzed. Both the pulps and the CNF were analyzed for their cellulose content, hemicellulose content, lignin content, strength properties, and content of carboxylic groups. The extractive content and fiber characteristics (fiber length, fiber width, amount of fines, cell wall thickness, and curl) were also analyzed for the pulps. The CNF were studied with a transmission electron microscope and made into films to be used in cell growth tests; cell growth studies were done outside this thesis.The results show that the kraft pulps were sufficiently fibrillated while the CNF made from blotters were of low quality. The blotter CNF did not form a hydrogel due to non-fibrillated fibers still being left in the oxidized fiber suspension, even though three times more mechanical energy was transferred into the suspension. The strength values were similar for all the CNF (approximately 90 MPa) and was comparable to the commercial CNF (93 MPa). The amount of carboxylic groups for the CNF were 1.2 mmol/g for the unbleached CNF, 1.6 for the self-bleached CNF, 1.8 for the bleached CNF, and 0.4 for the blotter CNF. The commercial CNF had a much lower value of 0.03 mmol/g, due to the difference in the preparation method.