Katarina Dimic-Misic

Influence on Pore Structure of Micro/Nanofibrillar Cellulose in Pigmented Coating Formulations

Nano and microfibrillated cellulose (NFC and MFC, respectively, collectively termed MNFC) is known to interact strongly with water, related to its high polarity and surface area. The swelling behaviour acts to form a gel with high water retention properties. The observation that nanocellulose could possibly be used in paper or other coating formulations, as a co-binder, for example, raises a question about the possible effects it could have on coating pore structure. In this study, we analyse the pore structure of pigmented coatings, liquid absorption and permeability, in respect to the influence of partially substituting traditional co-binder carboxymethyl cellulose with MNFC. The contrast between polar water and non-polar liquid, such as alkane, is used to probe the water interactive and extractable in-coating (internal) gel-formation properties of the nanocellulosic materials. These contrasting liquids are important in many processes, such as offset printing, but also in respect to exposure of coatings in general to environmental factors in application. Results show that permeability to liquid water is dramatically reduced when nanocellulosic material is present, though water can permeate by diffusion through the nanocellulose gel network. Long timescale exposure to water during absorption leads to extraction of any soluble salts remaining after the chemical treatment of the fibrillar material during production. Inert alkane, on the other hand, can absorb and permeate freely without interactive hindrance from the nanocellulose, with no extractive effect. Such a construct could in principle be considered for use as an oil-water differential membrane or for slow release concepts in aqueous systems by loading soluble deliverable materials within the nanocellulosic gel.

From colloidal spheres to nanofibrils: Extensional flow properties of mineral pigment and mixtures with micro and nanofibrils under progressive double layer suppression

Suspensions of mineral pigment and cellulose fibrillar derivatives are materials regularly found in the forest products industries, particularly in paper and board production. Many manufacturing processes, including forming and coating employ flow geometries incorporating extensional flow. Traditionally, colloidal mineral pigment suspensions have been considered to show little to no non-linear behaviour in extensional viscosity. Additionally, recently, nanofibrillar materials, such as microfibrillar (MFC) and nanofibrillar cellulose (NFC), collectively termed MNFC, have been confirmed by their failure to follow the Cox-Merz rule to behave more as particulate material rather than showing polymeric rheological properties when dispersed in water. Such suspensions and their mixtures are currently intensively investigated to enable them to generate likely enhanced composite material properties. The processes frequently involve exposure to increasing levels of ionic strength, coming either from the weak solubility of pigments, such as calcium carbonate, or retained salts arising from the feed fibre source processing. By taking the simple case of polyacrylate stabilised calcium carbonate suspension and comparing the extensional viscosity as a function of post extension capillary-induced Hencky strain on a CaBER extensional rheometer over a range of increasing salt concentration, it has been shown that the regime of constriction changes as the classic DLVO double layer is progressively suppressed. This change is seen to lead to a characteristic double (bimodal) measured viscosity response for flocculated systems. With this novel characteristic established, more complex mixed suspensions of calcium carbonate, clay and MNFC have been studied, and the effects of fibrils versus flocculation identified and where possible separated. This technique is suggested to enable a better understanding of the origin of viscoelasticity in these important emerging water-based suspensions.

Micro nanofibrillated cellulose (MNFC) gel dewatering induced at ultralow-shear in presence of added colloidally-unstable particles

Aqueous nanogels are notoriously difficult to dewater. An example of such a gel is that of a suspension of micro nanofibrillated cellulose, in which water is both bound to the fibrillar surface and held within the interfibril matrix. We demonstrate a phenomenon in which dewatering of nanocellulose based gel-like suspensions can be induced by adding a colloidal particulate component, which itself can undergo autoflocculation when suspended in water. The mechanism is exemplified by the addition of undispersed precipitated calcium carbonate, which in equilibrium remains stabilised in the gel, but when the gel mix is exposed to ultralow shear, acting below the yield stress, demixing of the combination between the nanofibrils and the autoflocculating pigment leads to separation of the unbound water phase. This novel mechanism is proposed to enhance the dewatering capability in general of complex gel-like water-holding suspensions.

The investigation of rheological and strength properties of NFC hydrogels and aerogels from hardwood pulp by short catalytic bleaching (Hcat)

Alkaline washed nanofibrillated cellulose (NFC) was obtained via TEMPO-mediation of hardwood pulp which had been short catalytically bleached (Hcat) under a controlled range of conditions. With careful combination of bleaching conditions, together with the subsequent alkali pre-treatment prior to TEMPO oxidation, it is possible to reduce xylan content of Hcat hardwood birch pulps as compared to traditional elemental chlorine free (ECF) bleaching. The defined amount of xylan is seen to affect the static water-holding and agglomeration state of the respective NFC. It is shown that colloidal interactions between the nanofibrils are dependent on the amount of water present in the NFC hydrogels as water retained within xylan induced swelling of the fibrils that resulted in a weaker aerogel structure.