The objective of this thesis was to explore the possible ways of using the intrinsic properties of cellulosic pulp fibres and inorganic pigments, by combining these elements using non-standard procedures, as a means to engineer a new composite material – a novel uncoated woodfree paper with improved physical properties. Precipitated calcium carbonate (PCC) was used as the inorganic pigment in this study. To accomplish this objective, we conducted a preliminary study on web addition followed by detailed studies on in-situ precipitation in fibrillated pulp suspensions and blending of novel furnish materials. We evaluated the different technological approaches by analysing the production process and product quality.
In the first approach, chemical precipitation and spraying of filler dispersion onto a fibrous web, and mechanically pressing to assist penetration into the network, were compared against conventional filler dosage before web forming. The results showed that web addition approach results in higher tensile strength and lower light scattering of paper. Filler agglomeration and optical crowding, in chemical precipitation and web application respectively, resulted in significantly deteriorating the light scattering of the handsheets. The experimental conditions were not sufficient to obtain an even distribution of filler along the thickness direction of paper and the filler characteristics were not optimised in this study.
In the second technique, precipitation onto fibrillated pulp suspensions was investigated by varying the pulp substrate, PCC crystal structure, and pre-refining a mixture of pulp and milk of lime. According to the research findings, PCCs formed by precipitation of calcium carbonate onto cellulosic fibrils and fibres do not necessarily have the same characteristics as reference PCCs formed by carbonizing milk of lime. Precipitation of calcium carbonate onto fibrillated fibres and microfines increases the retention of filler but impairs the dewatering of handsheets during pressing. Higher amount of fibrillated cellulosic substrates in combination with appropriate filler morphology, scalenohedral or rhombohedral, contribute to increased bond strength and light scattering of traditional fine paper. Pre-refining a mixture of pulp and calcium hydroxide results in grinding of lime, and hence, the composites have a greater surface area than the reference filler. Composite filled handsheets, from this study, exhibited high light scattering.
In the third method, the microfines-filler composite was envisioned as the backbone structure for a new composite material – uncoated fine paper. In the new composite paper, strength properties arise from the microfibrillated cellulose, bulk and pores originate from the filler surrounded by fibrils, whereas tear strength is imparted by a minimal proportion of pulp fibres in the composite. Compared to conventional fibre based fine paper, even at high filler loading the new composite material showed higher bending stiffness, tensile and tear indices, internal bond strength, light scattering and brightness properties. The new concept of fines-pigment-based furnish enables us to load pigments in uncoated wood free paper up to 50%-60%. However, dewatering time is considerably longer. This method needs to be optimised, with further research on dewatering, and printability, before scaling it to an industrial process.
This study shows the potential of different approaches, novel furnish components and addition of pigment onto a formed web, in the creation of new composite fine paper. Novel composite structure for fine paper can be achieved by employing the smallest component of pulp fibres, cellulosic microfines, in combination with pigments. The characteristics of microfines and crystal structure of pigments are important control variables in the formation and properties of the new composite paper. On the other hand, cellulosic microfines are highly swollen and retard dewatering. Therefore, further optimisation of process methodology and product quality can be expected to lead to some useful advances in commercialisation of this technology
