Properties and application
Cellulose fiber © Ingo Bartussek / fotolia.com
Smallest fibrils and crystals of cellulose show excellent properties for different applications. These materials, also known as nano-cellulose, are used for example as a reinforcing material in composite materials as well as for medical implants and widely in the life sciences sector. Cellulose is an indispensable part of our everyday life. It abounds in wood and plant fibers and it is used as a textile raw material, as a building material and also as an energy source. Today, almost all sustainable materials are including cellulose components. For years, cellulose structures in the nanometer range are isolated out of different raw materials (e.g. wood, annual plants, tunicin). Such cellulose is then referred to as nano-cellulose.
There are three kinds of nano-cellulose: microfibrillated cellulose (MFC), nanocrystalline cellulose (NCC) and bacterial nano cellulose (BNC).
MFCs have a diameter of approximately 5-100 nm and a length of several micrometers . They are used in different packaging materials and in nanocomposites as well as in food emulsions and dispersants. Furthermore, MFCs are used in medical, cosmetic and pharmaceutical products. When incorporated into plastics, they increase the tensile strength of these composites by many times.  Their ability to store a large amount of water at a low weight makes them interesting for applications in absorption and hygiene products.
NCC has a small diameter (5-20 nm) and a length of only 100-250 nm when it is obtained from plant cellulose. In the production from algae, bacteria or tunicates, the length may vary between 100 nm and a few microns . Nano crystalline cellulose possesses unique optical and liquid crystalline properties. So far, there doesn't seem to be any product with NCC on the market, but a variety of industries are working on a commercialization, i.e. paper industry, (food) packaging or coating companies.
The diameter of bacterial nano cellulose (BNC) is smaller and much more homogeneous than the diameter of mechanically isolated MFC. It forms nanofiber networks of different morphology . In sharp contrast to the MFC and NCC, bacterial nano cellulose, also known as biocellulose, is not produced from existing cellulose. BNC is obtained by a biotechnology process using bacteria, for example from sugars. The biocellulose has a high strength, similar to steel or Kevlar (based on the BNC single fiber) and usually has a very high crystallinity of 80-90%. Mostly rigid aerogels are formed out of these BNC networks during the manufacturing process. With a pore diameter of less than 10 microns, these gels are extremly transparent. Currently biocellulose is of interest to researchers in the fields of fuel cells, electronics, membranes, medical engineering (especially BNC-based implants) and composite materials. There exist numerous patent claims, especially in the medical sector, even without the existence of a proof-of-principle or a prototype.
Cellulose is not self-inflammable. As a mixture with air (dust) cellulose is inflammable (dust explosion) under the influence of an ignition sourc,. The behaviour in a dust explosion is similar to that of other, carbon-based materials.
MFC are obtained by isolating wood pulp by mechanical pressure or by grinding methods and previous chemical, enzymatic or mechanical (Ultra-Turrax, inline disperser, ...) treatment. Wood cellulose fibers are compressed at high pressures (up to 1500 bar) through interaction chambers of homogenizers, thereby shear forces are induced and the fibrils are released. The energy consumption of this production method was reduced significantly in recent years thanks to a chemical or enzymatic pretreatment.
The nano-crystalline cellulose is currently produced by acid hydrolysis of cellulose from wood, cotton, hemp, but also algae, bacteria, and tunicates. Currently there exits plans of a Canadian company to build an industrial-scale production facility for NCC.
BNC is produced synthetically by using acetic acid bacteria and based on low molecular weight sugars such as glucose. Such production methods are part of the so called white biotechnology. The industrial production of BNC in a large scale is currently still in development.