Dr Dipa Roy, School of Engineering, The University of Edinburgh, UK
What is Cellulose?
Cellulose is the most abundantly available organic macromolecule on Earth. Cellulose is found in different sources like wood, agricultural biomass, sea animals, algae, and fungi. It is the main compound found in plant cell walls which helps the plant to remain stiff and strong.
The chemical structure of cellulose resembles that of starch, but unlike starch, cellulose is extremely rigid. This rigidity imparts great strength to the plant body and protection to the interior of plant cells. Cellulose is 1, 4 linkage of beta glucose monomers, as shown in Figure 1. It is, therefore, a polysaccharide (Latin for “many sugars”). Several polysaccharide chains remain arranged in parallel arrays to form cellulose microfibrils. The individual polysaccharide chains are strongly held together within the microfibrils via hydrogen bonds. Due to the presence of hydrogen bonds, they are crystalline in nature and are rigid, strong and stiff. The microfibrils are bundled together to form cellulose macrofibrils. Cellulose fibrils remain embedded in an amorphous matrix which comprises of mostly lignin and hemicellulose (Figure 2).
There are several techniques available which can separate out the crystalline cellulose from the non-crystalline, amorphous matrix and make them available for various applications. Microcrystalline or nanocrystalline cellulose, extracted from various cellulosic resources, can be used as a valuable material in developing sustainable products.
What is Nanocellulose?
With the advent of nanoscience, researchers have focused on producing nanocellulose by breaking down the structure of cellulose microfibrils using various mechanical, chemical or enzymatic techniques (Figure 3). Cellulose nanofibrils or nanoparticles are generated by mechanical or chemical treatment. Mechanical treatments include high-pressure refining, grinding or high-pressure homogenization. Acid hydrolysis is the most commonly used chemical treatment. During acid hydrolysis, hydrolytic cleavage of the glycosidic bonds take place mainly in the amorphous regions of the cellulose, releasing individual crystallites.
The increase in the nanocellulose research lies in their interesting properties such as low density, low cost and high mechanical properties. Cellulose in its nanocrystalline form has a very high tensile strength, high Young’s modulus and is a very good reinforcing filler for various composite materials. Nanocellulose can be obtained in various shapes and sizes depending on the type of treatment. They are called by different names, as given below in Table 1. However, the surface of cellulose/nanocellulose are highly hydrophilic as they have many hydroxyl groups in their chemical structure. This hydrophilicity makes them prone to moisture absorption. Various chemical modifications are carried out, if required, in order to tailor their moisture absorption behaviour to suit diverse range of applications. This also increases their compatibility with less hydrophilic materials.
Table 1: Different Forms of Nanocellulose (Angew. Chem. Int. Ed. 2011, 50(24), 5438; Composites: Part A, 2016, 830 19).
*Bacterial, or microbial, cellulose has different properties from plant cellulose and is characterized by high purity, strength and increased water holding ability.
Nanocellulose from Agrowaste
The original article can be found in Ind. Eng. Chem. Res. 2011, 50, 871–876.
Apart from various plant resources, agricultural waste can be used as a resource for the extraction of nanocellulose. The utilisation of agrowaste, such as rice husk, rice straw, sesame husk, sugarcane bagasse, potato peel, pineapple leaf fibre etc. can offer the benefit of low cost. Below is an example where sesame husks have been used as a resource to extract nanocellulose (Figure 4).
The microscopic images of the sesame husk and the extracted cellulose whiskers are shown in Figure 5.
The cellulose whiskers when subjected to further mechanical treatment produces cellulose nanoparticles, as shown in Figure 6.
Application Potential
Use of natural materials can lead to greener and low cost sustainable products that reduce environmental burden, as these materials can be safely returned to the natural carbon cycle by simple biodegradation in a composting environment. Cellulose, being strong and stiff, has the potential to be used in a diverse range of applications. Cellulose is a cheap and abundant natural resource which are key requirements for industrial application. Nanocellulose, having a smaller size and higher surface area, can impart interesting properties to products. It is used in various applications such as nanocomposites, smart coatings, paper and paper board, as thickeners, flavour carriers, and suspension stabilizers in food, medical, cosmetic and pharmaceuticals. However, a uniform and steady supply chain, and variabilities which might arise in the nanocellulose properties due to different sources, must be kept in consideration for successful application in larger scale. Further research can open up new avenues to us Feature image: Greg Rosenke on Unsplash
Title image: Casey Horner on Unsplash
