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Technical Article!

Technical Article


PREPARATION AND APPLICATIONS OF CELLULOSE NANOMATERIALS

The properties of cellulose nanomaterials (mechanical, rheological, optical, film-forming properties) make them interesting materials for many applications and they have a high potential for an emerging industry. An overview is proposed but this list is of course not exhaustive. It is difficult to classify these applications by degree of achievement and maturity because this information is difficult to apprehend.

 

 

INTRODUCTION

The potential of nanotechnology and nanocomposites in various sectors of research and application is promising and attracting increasing investment. Unexpected and attractive properties can be observed when decreasing the size of a material down to the nanoscale. Cellulose is no exception to the rule. In addition, due to its abundance, renewability, high strength and stiffness, non-toxicity, low weight and biodegradability, the highly reactive surface of cellulose resulting from the high density of hydroxyl groups is exacerbated at the nanoscale. Cellulose nanomaterials serve therefore as promising candidates for the preparation of bionanocomposites and other nanodevices.

Despite being the most available natural polymer on earth, it is only quite recently that cellulose has gained prominence as a nanostructured material, in the form of cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs). However, the pioneering works have been initiated a long time ago, 1947 for CNCs (Nickerson, 1947), and 1983 (Herrick, 1983; Turbak, 1983) for CNFs. They have been academic curiosities for many years and have been recently described as new ageless bionanomaterials (Dufresne, 2013). Technological development depends on economic conditions. Companies primarily focus on their own interests and dig into the science where it seems financially attractive. Today there is a substantial amount of research on these cellulose nanomaterials, and commercial development is now underway with some very promising applications.

 

 

RESULTS AND CONCLUSIONS

Different forms of cellulose nanomaterials, resulting from a top-down deconstructing strategy (CNCs, CNFs) or bottom-up strategy (bacterial cellulose, BC) can be prepared and are potentially useful for a large number of industrial applications. Multiple mechanical shearing actions applied to cellulosic fibers release more or less individually the nanofibrils. A controlled strong acid hydrolysis treatment can be applied to cellulosic fibers allowing dissolution of amorphous domains. Both nanomaterials are obtained as aqueous suspensions (Dufresne, 2017) and their morphology is shown in Figure 1.

Fig.1 TEM from a dilute suspension of (a) cellulose nanocrystals from ramie fibers (Habibi, 2008) and (b) cellulose nanofibrils from Opuntia ficus-indica fibers (Malainine, 2003), and anticipated markets.

 

The mechanical modulus of crystalline cellulose is the basis of many potential applications. Moreover, the low thermal expansion coefficient caused by the high crystallinity of cellulose nanomaterials and high transparency without the presence of any existing polymer is highly advantageous for flexible display panels and electronic devices. For papermaking, in addition of improving the tensile strength, burst strength, tear, density, smoothness and also increasing the air permeability, the capacity of retaining the filler and the adsorption of a dye are also improved by the nanoparticles. Besides, the inherent high reactivity of cellulose and the pervasive surface hydroxyl groups associated with the nanoscale dimensions of cellulose nanomaterials open up opportunities to develop new functional nanomaterials (Lin, 2012).

Now after intensive research several initiatives have emerged in the perspective of producing cellulose nanomaterials at large scale. A number of organizations have announced cellulose nanomaterial demonstration plants. This development has been driven in most cases by economic reasons and public awareness. Sustainability and industrial ecology are probably additional factors. The forestry industry in traditional locations such as North America and Scandinavia are going through a major transition, and are strongly affected by the growing competition from emerging countries in Asia and South America. The economic struggle has simply become uneven. The utilization of new technologies is expected to provide a means for strengthening the competitiveness in the sector.

Cellulose nanomaterials are being developed for use over a broad range of markets and applications (some of them are shown in Figure 1), even if a high number of unknown remains at date. Tens of scientific publications and experts show their potential even if most of the studies focus on their mechanical properties as reinforcing phase and their liquid crystal self-ordering properties. The sound markets impacted by cellulose nanomaterials include composites, electronics (flexible circuits), energy (flexible batteries, such as Li-ion batteries, and solar panels), packaging, coatings (paints, and varnishes), detergents, adhesives, construction, pulp and paper, inks and printing, filtration, medicine and life science (scaffolds in tissue engineering, artificial skin and cartilage, wound healing, and vessel substitutes), optical devices (including reflective properties for security papers and UV or IR reflective barriers), rheological modifiers, cosmetics, and aerogels. Broadly, one can consider that the use of CNF is more related to the paper and coating applications, whereas CNC is more composite oriented. The interest of paper and packaging industries for CNF is quite recent. In the field of polymer nanocomposites, cellulose nanomaterials offer the opportunity to process stiff thin films. However, the serious issue of dispersion in the polymer melt and development of suitable processing technologies for large scale has to be solved to broaden the applicability of these nanoparticles. The strong interactions between cellulose nanomaterials through hydrogen-bonding are beneficial to exploit their full potential and reach the highest mechanical reinforcement effect that can be obtained from these nanoparticles. At the same time, it limits their dispersion within a continuous medium.

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