A. Ketola: Surface and inter-fibre interactions in aqueous cellulose-based systems for open fibrous structures

Abstract: 

In the thesis, the morphology of cellulose-based fibre surfaces immersed in water and their interactions with air bubbles in the presence of surfactants were investigated. The cellulose-based fibres included those from wood pulps obtained by chemical and chemi-thermomechanical processing as well as regenerated cellulose. The revealed surface characteristics and interaction mechanisms were connected to the fibre behaviour in water and in foam forming and to the resulting dry material properties.

Fibre surface morphology and cellulose microfibrils (CMF) were explored using helium ion microscopy. Freeze drying and critical point drying were used to preserve the open cellulose fibril structure. Well preserved fibre surface fibrils had a significant resemblance with CMF. As wet CMF was known to be gel-like, it was suggested that fibrillated fibre surfaces bore similar gel-like behaviour. The gel-likeness of fibre surfaces was connected to the increased force transmittance capacity of fibre webs, seen as higher shrinkage and elongation of water-formed fibre webs.

Bubble-cellulose interactions were investigated with a stepwise approach: 1) Hydrophobised and hydrophilic cellulose model surface studies were conducted with a captive bubble method in anionic sodium dodecyl sulphate (SDS) and non-ionic Tween 20 solutions with varying surface tensions. 2) CMF model surface was used to study the effect of surface roughness. 3) The bubble-fibre interaction was elaborated with actual cellulose fibres by using a fibre bed method. 4) Low-density foam-formed materials were prepared to test the findings from the model surface studies in the context of fibre-foam behaviour and dry structure properties.

In conclusion, the bubble interaction with hydrophilic cellulose fibres was generally very weak. The hydrophobic content of cellulose (amphiphilicity and lignin) together with the entrapped air on a fibre surface can drive the bubble adhesion to the fibre. This weak interaction can be cancelled out by higher surfactant concentration. Lightweight materials prepared with hydrophilic and hydrophobized cellulose fibres (with SDS and Tween 20) showed differences in structure and mechanical behaviour, as well as formation of microscale fibre networks inside the structure.

These findings increase the general understanding of fibre surface properties and interaction mechanisms between bubbles and cellulose fibres and can be applied in material design in both water-and foam-forming processes.

Follow the remote defence: TBA

Opponent: Professor Chunlin Xu, Åbo Akademi University, Finland

Supervisor: Professor Orlando Rojas,  Aalto University, Department of Bioproducts and Biosystems

Link to electronic thesis: Surface and inter-fibre interactions in aqueous cellulose-based systems for open fibrous structures