Defence of doctoral thesis: Magdalena Kaplan – Holding paper fibres together – the role of fibre joints in cellulose network materials

With the transition towards a circular economy and sustainable solutions happening in society today, paper-based materials are excellent replacements for fossil-based plastics in semi-structural applications. Despite being one of the oldest engineered materials still in use today, paper is not fully understood from a mechanical perspective. It is a complex system consisting of a network of cellulose fibres held together by molecular interactions also called fibre joints. To fully utilise its benefits and to tailor the material to specific applications, a further understanding of the phenomena affecting the mechanical properties of cellulose fibre-based materials is necessary.

This thesis is the result of a study investigating several phenomena and effects related to the fibre joints covered at several material scales. The drying process of the fibre joint is investigated by numerical simulations of a cellulose bead – a simplified model of a fibre joint. The effects of the drying process are also evaluated for several types of materials at the network scale by hygroexpansion measurements. The network properties are further evaluated by uniaxial tensile testing of several types of materials: different pulp types at a range of network densities (600-1000 kg/m³) and with differing joint properties. The joint properties are varied by surface modifications, yielding strong joints, and by creating networks from dry fluff pulp, a novel technique called dry forming, yielding weaker joints. A selection of the physically tested networks with enhanced joint properties is reproduced in a fibre network model, and the fibre and joint parameters are tailored to mirror the experimental stress-strain response. The dry-formed material is further characterised in three loading modes: in-plane tension, out-of-plane shear, and out-of-plane compression, for an even broader range of densities (60-1000 kg/m³), following the development of mechanical properties during the forming process.

The results of this study highlight the importance of the fibre joints, both in the making and breaking of the material. A more realistic representation of a fibre joint can be made by considering the drying process. It is also shown that increasing network density and adding surface modifications will both affect the internal stress state imposed during drying. Combining these two effects has been proven to improve network mechanical properties further, although the effect of surface modifications saturates earlier for highly densified networks than for lower densities. These effects are somewhat captured in fibre network models, where the modifications are linked to a change in the network structure, conceivably driven by an increase in the number of joints in the network. Finally, the materials with weaker joint properties challenge many of the well-established truths: a consolidated network can be made without the addition of water, but the failure mechanisms are different compared to wet-formed materials at similar density levels.

Nonetheless, many possible explorations and unanswered questions regarding fibre joints remain. The results and advancements presented here can be extended to investigate and better understand the role of the joint properties and aid in developing new products and tailoring their mechanical properties.

https://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-360025