Materials Too Cool for
Materials Too Cool for School Nanofibrillar cellulose and their industrial promising future in combination with bioplastics. Fig. 1. Top: Cellulose kraft pulp fibres. Bottom: The surface structure of a single cellulose fibre, where the microfibrils are clearly visualized Fig. 2. Cellulose nanofibrils. Cellulose is the most frequently used biopolymer in material science, occurring in wood, cotton, hemp and other plant-based materials and serving as the dominant reinforcing phase in plant structures. Cellulose is used already for many purposes that include its use in packaging, composites, structural materials (wood is still the principal element in building constructions in many countries) and many other applications. The biorefinery concepts introduced in the last 30 years and the advancement of research in the nano area are now allowing new possible developments for cellulose: nanofibrillar cellulose (MFC) is the new trend in the industry (Fig.1). From their first discovery in the 1980’s until today MFCs have gained increasing attention due to their unique properties in improving the mechanical, optical and barrier performance of a given material. Today, their properties are becoming well-known in many areas, but it is in the field of composites where those properties can give their best in combination with bioplastics. MFCs are being produced by fibrillation of cellulose fibres. The most common ways to produce the fibrils is by using high pressure homogenization or grinding. In order to facilitate the fibrillation, various ways to pre-treat the fibres are often carried out. The pre-treatment can be mechanical, chemical, enzymatic or a combination of these. Pre-treatment lowers the energy consumption in the fibrillation step which otherwise can be very high and after substantial chemical pre-treatment it is also possible to fibrillate the fibres by just using sonication. The various pre-treatment and fibrillation methods also influence several parameters of the produced fibrils, such as degree of polymerization, fibril length, surface chemistry, average fibril diameter, rheological properties and fibril diameter size distribution [1]. Thus, it is possible to produce the material in several qualities and to adjust the product so that it is at its optimum for a specific application. At the Paper and Fiber institute today we distinguishe between tailor-made MFC dispersions and MFC is no longer used as a general term. Nanofibrils constitute the major fraction of properly produced MFC materials [2]. Nanofibrils have diameters in the nano-scale (1 µm) (Fig. 2). Such nanofibrils are expected to play a key role in improving the mechanical, optical and barrier properties of a given material. Recently, advances have been reported in the production of cellulose nanofibrils on an industrial-scale [3], which opens up new possibilities in the proper utilization of this natural and promising material. Adequate morphological characterisation of nanofibrils requires microscopy techniques with suitable resolution. Several advanced microscopy techniques exist for micro- and nano-assessments, including the commonly applied atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and their corresponding different modes of operation. FESEM is a most versatile technique for structural studies. Samples can rapidly be 20 bioplastics MAGAZINE [04/11] Vol. 6
Materials assessed at several scales, providing also high-resolution of for example 1 nm. This is a valuable property as the morphology of a given MFC material can be assessed properly and in detail [4]. Nowadays MFCs are used commercially or at research level in different areas of expertize: Packaging, paper, emulsions, membranes, as a thickener, as well as in filters and in medical applications - to list just a few of them. Among all of them, the production of composite materials is the area where they could be the right partner for bioplastics. Nanocomposites based on nanocellulosic materials such as microfibrillated cellulose or bacterial cellulose have been prepared with petroleum-derived non-biodegradable polymers such as polyethylene (PE) or polypropylene (PP) and also with biodegradable polymers such as PLA, polyvinyl alcohol (PVOH), starch, polycaprolactone (PCL) and polyhydroxybutyrate (PHB). Chemical modification of cellulose has been explored as a route for improving filler dispersion in hydrophobic polymers. Due to compatibility problems of nanocellulosic materials and hydrophobic matrices, it is clear that nanocomposites based on hydrophilic matrix polymers will be easier to produce and commercialize. The improvement of compatibility with apolar materials, on the other hand, requireschemical modification of nanocelluloses. Because of the hydrophilic nature of the material it is easy to understand why MFC and Bioplastics are perfect partners to develop new and totally renewable composite materials. www.pfi.no By: Marco Iotti, Gary Chinga Carrasco, Kristin Syverud Paper and Fiber research Institute Trondheim, Norway [1] Iotti, M; Gregersen, Ø; Møe, S; Lenes, M. (2011): Rheological Studies of Microfibrillar Cellulose Water Dispersions. Journal of Polymers and the Environment, 19(1), 137- 145. Open access. [2] Chinga-Carrasco, G. (2011): “Cellulose fibres, nanofibrils and microfibrils: The morphological sequence of MFC components from a plant physiology and fibre technology point of view”. Nanoscale Research Letters 2011, 6:417. Open access. [3] Syverud, K. (2011): “Industrial-scale production of nanofibres from wood”. http://www.pfi.no/PFI_Templates/ NewsPage____450.aspx [4] Chinga-Carrasco, G., Yu, Y, Diserud, O. (2011): “Quantitative electron microscopy of cellulose nanofibril structures from Eucalyptus and Pinus radiata pulp fibres. Microscopy and microanalysis. In press. bioplastics MAGAZINE [04/11] Vol. 6 21
