The exceedingly high tensile strength of clear timber is explained by the cellulose fibrils in the cell wall. A Top-Nano-21 feasibility study performed by Empa sets out to isolate these fibrils or fibril agglomerations, with diameters below 100 nanometres, from mass-produced wood pulp. This would allow the extraction of high-strength, highly reactive, biologically produced starting materials for technical applications such as (bio-)polymer reinforcement.

Cellulose fibrils may be extracted from mass-produced sulphite and sulphate pulp. The fineness of the fibrils varies according to the isolation process (mechanical or chemical/mechanical). Purely mechanical processing produces fibril structures several micrometres long and between 20 and 100 nanometres in diameter. Combined chemical and mechanical processes achieve even finer fibril structures, only a few nanometres in diameter.

Morphological, biological and chemical characterization of the fibrils

The dimensions and morphology of the isolated fibrils are investigated at Empa using electron microscopy. Individual fibrils form networks that can become clearly visible after preparation on mica sheets. Biological characteristics are also being examined in collaboration with the Anatomical Institute at the University of Berne: in an in vitro test to determine the biological impact of cellulose fibrils on the lung, fibrils are incubated in different concentrations for varying periods in macrophages of pigs. The chemical characterization entails a determination of the chain length of the polysaccharide cellulose forming the fibrils. It appears that particularly chemical disintegration achieves a significant reduction in chain length. Nonetheless, even these fibrils still exhibit lengths in the micrometre range. Knowledge of these properties will permit systematic modification of the reactivity and nature of the fibrils.

Tremendous storage capacity

Intensive dispersion of disintegrated cellulose fibrils in water produces a stable gel – even for solid contents as low as approx. 2%. Cellulose fibril networks consequently exhibit a high water storage capacity. This property makes the transparent fibrils ideal for use in paints, for example, where the dispersion behaviour and strength characteristics can be substantially enhanced.

Cellulose-fibril-reinforced polymers

Initial embedding tests have shown the tensile strength achieved by the polymer composite to be many times that of polymers without fibrils. As a natural product with promising strength properties and miniature size, cellulose fibrils lend themselves particularly for use as reinforcement in conjunction with (bio-)polymers. Cellulose-fibril-reinforced (bio-)polymers could be used for applications where a combination of biodegradability, high mechanical strength and, possibly, visual transparency is required (e.g. in the medical and food sectors or in garden architecture). Empa is seeking to collaborate with industry partners in raising the production of cellulose fibrils to the level of kilograms. Further, research will be performed at Empa into compounding methods for cellulose fibrils and polymers and investigations will be conducted into the characteristics and potential applications of the resulting composite.

Your contact:

Tanja Zimmermann, Wood Laboratory, Phone +41 1 823 43 89, E-mail: tanja.zimmermann@empa.ch (weitere Expertenartikel von T. Zimmermann)

Evelyn Pöhler, Wood Laboratory, Phone +41 1 823 40 74, E-mail: evelyn.poehler@empa.ch

Dr. Thomas Geiger, Laboratory Functional Polymers, Phone +41 1 823 47 23, E-mail: thomas.geiger@empa.ch

T. Zimmermann, E. Pöhler, P. Schwaller: Mechanical and Morphological Properties of Cellulose Fibril Reinforced Nanocomposites
Advanced Engineering materials, 2005, Vol. 7, N°. 12, pp 1156-1161

T. Zimmermann, E. Pöhler, T. Geiger: Cellulose Fibrils for Polymer Reinforcement
Advanced Engineering materials, 2004, Vol. 6, N°. 9, pp 754-761