Nanocellulose biorefineries – A biomaterial with unparalleled perspectives – 1st part: Introduction

Publication date: 18/08/2016.

Last update: 22/04/2019.

Second part: Cellulose NanoCristals (CNC)

Cellulose

Cellulose is the most important and abundant organic biopolymer on Earth. It is the basic structural component of plant cell walls. It is a natural linear polymer (polysaccharide) with a molecular repeat unit comprised of a pair of d-anhydroglucose ring units joined by β-1→4 glycosidic oxygen linkages around which the molecular chain can bend and twist. Anhydroglucose is the monomer and cellobiose is the dimer of cellulose. The β-1,4-glycosidic bonds build an ordered crystalline structure by van der Waals forces and inter- and intramolecular hydrogen bonding. The extremely large number of hydrogen bonds results in a strong lateral association of the linear cellulose molecules. Amorphous region results from the breakage and disorder of hydrogen bonds.

Figure 1. a) 3D structure of cellulose / b) Structural formula of cellulose (taken from Reference [6])

Nanocellulose

The plant cell wall can be classified into two parts, namely, primary and secondary. The primary cell wall is the external thin layer (less than 1 μm) and the secondary cell wall chiefly contains cellulose microfibrils. With intensive defibration of this macroscopic fiber structure, smaller elements (fibrils and crystals) can be separated. When these elements are nano-scale (at least, one dimension less than 100 nanometers in size), we are talking about nanocellulose.

The hierarchical configurations from wood fibers to cellulose nanocrystals are shown in Figure 2. The plant cell wall consists of bundles of the cellulose fibrils, and their diameters are only a few micrometers. Each cellulose bundle consists of millions of microfibrils, these microfibrils are composed with elementary fibrils or nanofibrils. The diameter of the nanofibril is about 5 nm, whereas in the case of the microfibrils, the diameters will vary from 10 to 50 nm. Every nanofiber is composed of flexible amorphous and strong crystalline parts.

Figure 2. From wood fibers to cellulose molecules (taken from CelluForce website)

Categorization

The Technical Association of the Pulp and Paper Industry (TAPPI) and multiple concerned bodies have recommended that nanocellulose be categorized into two main groups:

- Cellulose NanoCrystals (CNC). Synonyms: NanoCrystalline Cellulose (NCC); Cellulose whiskers.

- Cellulose NanoFibrils (CNF). Synonyms: NanoFibrillated Cellulose (NFC).

Besides, there are some elements referred to as nanocellulose despite they are not nano-scale but micro-scale materials: MicroFibrillated Cellulose (MFC), Cellulose MicroFibrils (CMF) and Cellulose Filaments (CF). The terms CNF and MFC are being used interchangeably. There is considerable overlap in specifications and many of these materials contain a mix of nano-scale and micro-scale particles.

Sources

To date, cellulose can be obtained from a broad range of sources including algae, bacteria, plants and tunicates (sea or marine invertebrate animals). The source of the cellulose determines not only its size and properties, but also the energy consumption of the extraction process to produce nanocellulose. On an industrial level, the most typical starting materials for nanocellulose are wood pulp, cotton, agricultural by-products and bacterial cellulose.

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References

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[8] P. Phanthong, P. Reubroycharoen, X. Hao, G. Xu, A. Abudula, G. Guan: “Nanocellulose: Extraction and application”. Carbon Resources Conversion 1 (2018) 32–43.

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[10] Thomas Rosenau (Editor), Antje Potthast (Editor), Johannes Hell (Editor): “Cellulose Science and Technology: Chemistry, Analysis, and Applications”. Publisher: John Wiley and Sons, December 2018.