Nanocellulose
Nanocellulose is a term referring to a family of cellulosic materials that have at least one of their dimensions in the nanoscale. Examples of nanocellulosic materials are microfibrilated cellulose, cellulose nanofibers or cellulose nanocrystals. Nanocellulose may be obtained from natural cellulose fibers through a variety of production processes. This family of materials possesses interesting properties suitable for a wide range of potential applications.
Terminology
Microfibrilated cellulose
Micro cellulose is a type of nanocellulose that is more heterogeneous than cellulose nanofibers or nanocrystals as it contains a mixture of nano- and micro-scale particles. The term is sometimes misused to refer to cellulose nanofibers instead.Cellulose nanofibers
Cellulose nanofibers, also called nanofibrillated cellulose, are nanosized cellulose fibrils with a high aspect ratio. Typical fibril widths are 5–20 nanometers with a wide range of lengths, typically several micrometers.The fibrils can be isolated from natural cellulose, generally wood pulp, through high-pressure, high temperature and high velocity impact homogenization, grinding or microfluidization.
Cellulose nanocrystals
Cellulose nanocrystals, or nanocrystalline cellulose, are highly crystalline, rod-like nanoparticles. They are usually covered by negatively charged groups that render them colloidally stable in water. They are typically shorter than CNFs, with a typical length of 100 to 1000 nanometers.Bacterial nanocellulose
Some cellulose producing bacteria have also been used to produce nanocellulosic materials that are then referred to as bacterial nanocellulose. The most common examples being Medusomyces gisevii and Komagataeibacter xylinus, see bacterial cellulose for more details. This naming distinction might arise from the very peculiar morphology of these materials compared to the more traditional ones made of wood or cotton cellulose. In practice, bacterial nanocellulosic materials are often larger than their wood or cotton counterparts.History
The discovery of nanocellulosic materials can be traced back to late 1940s studies on the hydrolysis of cellulose fibers. Eventually it was noticed that cellulose hydrolysis seemed to occur preferentially at some disordered intercrystalline portions of the fibers. This led to the obtention of colloidally stable and highly crystalline nanorods particles. These particles were first referred to as micelles, before being given multiple names including cellulose nanocrystals, nanocrystalline cellulose, or cellulose whiskers, though this last term is less used today. Later studies by O. A. Battista showed that in milder hydrolysis conditions, the crystalline nanorods stay aggregated as micron size objects. This material was later referred to as microcrystalline cellulose and commercialised under the name Avicel by FMC Corporation.Microfibrillated cellulose was discovered later, in the 1980s, by Turbak, Snyder and Sandberg at the ITT Rayonier labs in Shelton, Washington. This terminology was used to describe a gel-like material prepared by passing wood pulp through a Gaulin type milk homogenizer at high temperatures and high pressures followed by ejection impact against a hard surface. In later work, F. W. Herrick at ITT Rayonier Eastern Research Division finding new uses for the MFC, including using as a thickener and binder in foods, cosmetics, paper formation, textiles, nonwovens, etc. and 2) evaluate swelling and other techniques for lowering the energy requirements for MFC production. The first MFC pilot production plant of MFC was established in 2010 by Innventia AB.
Manufacture
Cellulose sources
Nanocellulose materials can be prepared from any natural cellulose source including wood, cotton, agricultural or household wastes, algae, bacteria or tunicate. Wood, in the form of wood pulp is currently the most commonly used starting material for the industrial production of nanocellulosic materials.Nanocellulose fibrils
Nanocellulose fibrils may be isolated from the cellulose fibers using mechanical methods that expose the fibers to high shear forces, delaminating them into nano-fibers. For this purpose, high-pressure homogenizers, grinders or microfluidizers can be used. This process consumes very large amounts of energy and values over 30 MWh/tonne are not uncommon.To address this problem, sometimes enzymatic/mechanical pre-treatments and introduction of charged groups for example through carboxymethylation or TEMPO-mediated oxidation are used. These pre-treatments can decrease energy consumption below 1 MWh/tonne. "Nitro-oxidation" has been developed to prepare carboxycellulose nanofibers directly from raw plant biomass. Owing to fewer processing steps to extract nanocellulose, the nitro-oxidation method has been found to be a cost-effective, less-chemically oriented and efficient method to extract carboxycellulose nanofibers. Functionalized nanofibers obtained using nitro-oxidation have been found to be an excellent substrate to remove heavy metal ion impurities such as lead, cadmium, and uranium.
A chemo-mechanical process for production of nanocellulose from cotton linters has been demonstrated with a capacity of 10 kg per day.
Cellulose nanocrystals
Cellulose nanocrystals are formed by the acid hydrolysis of native cellulose fibers, most commonly using sulfuric or hydrochloric acid. Disordered sections of native cellulose are hydrolysed and after careful timing, the remaining crystalline sections can be retrieved from the acid solution by centrifugation and dialysis against water. Their final dimensions depend on the cellulose source, its history, the hydrolysis conditions and the purification procedures. CNCs are commercialised by various companies that use different sources and processes, leading to a range of available products.Other cellulose based nanoparticles
Spherical shaped carboxycellulose nanoparticles prepared by nitric acid-phosphoric acid treatment are stable in dispersion in its non-ionic form.Structure and properties
Dimensions and crystallinity
The ultrastructure of nanocellulose derived from various sources has been extensively studied. Techniques such as transmission electron microscopy, scanning electron microscopy, atomic force microscopy, wide angle X-ray scattering, small incidence angle X-ray diffraction and solid state 13C cross-polarization magic angle spinning, nuclear magnetic resonance and spectroscopy have been used to characterize typically dried nanocellulose morphology.A combination of microscopic techniques with image analysis can provide information on fibril widths, it is more difficult to determine fibril lengths, because of entanglements and difficulties in identifying both ends of individual nanofibrils. Also, nanocellulose suspensions may not be homogeneous and can consist of various structural components, including cellulose nanofibrils and nanofibril bundles.
In a study of enzymatically pre-treated nanocellulose fibrils in a suspension the size and size-distribution were established using cryo-TEM. The fibrils were found to be rather mono-dispersed mostly with a diameter of ca. 5 nm although occasionally thicker fibril bundles were present. By combining ultrasonication with an "oxidation pretreatment", cellulose microfibrils with a lateral dimension below 1 nm has been observed by AFM. The lower end of the thickness dimension is around 0.4 nm, which is related to the thickness of a cellulose monolayer sheet.
Aggregate widths can be determined by CP/MAS NMR developed by Innventia AB, Sweden, which also has been demonstrated to work for nanocellulose. An average width of 17 nm has been measured with the NMR-method, which corresponds well with SEM and TEM. Using TEM, values of 15 nm have been reported for nanocellulose from carboxymethylated pulp. However, thinner fibrils can also be detected. Wågberg et al. reported fibril widths of 5–15 nm for a nanocellulose with a charge density of about 0.5 meq./g. The group of Isogai reported fibril widths of 3–5 nm for TEMPO-oxidized cellulose having a charge density of 1.5 meq./g.
Pulp chemistry has a significant influence on nanocellulose microstructure. Carboxymethylation increases the numbers of charged groups on the fibril surfaces, making the fibrils easier to liberate and results in smaller and more uniform fibril widths compared to enzymatically pre-treated nanocellulose, where the fibril widths were 10–30 nm. The degree of crystallinity and crystal structure of nanocellulose. Nanocellulose exhibits cellulose crystal I organization and the degree of crystallinity is unchanged by the preparation of the nanocellulose. Typical values for the degree of crystallinity were around 63%.
Viscosity
The rheology of nanocellulose dispersions has been investigated. and revealed that the storage and loss modulus were independent of the angular frequency at all nanocellulose concentrations between 0.125% to 5.9%. The storage modulus values are particularly high compared to results for CNCs. There is also a strong concentration dependence as the storage modulus increases 5 orders of magnitude if the concentration is increased from 0.125% to 5.9%. Nanocellulose gels are also highly shear thinning. The shear-thinning behaviour is particularly useful in a range of different coating applications.It is pseudo-plastic and exhibits thixotropy, the property of certain gels or fluids that are thick under normal conditions, but become less viscous when shaken or agitated. When the shearing forces are removed the gel regains much of its original state.