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Tuesday, 6 September 2016
Review of the recent developments in cellulose nanocomposite processing
Published Date
April 2016, Vol.83:2–18, doi:10.1016/j.compositesa.2015.10.041
Special Issue on Biocomposites
Open Access, Creative Commons license
Title
Review of the recent developments in cellulose nanocomposite processing
Author
Kristiina Oksman a,b,,
Yvonne Aitomäki a
Aji P. Mathew a
Gilberto Siqueira c
Qi Zhou d
Svetlana Butylina a
Supachok Tanpichai a
Xiaojian Zhou a
Saleh Hooshmand a
aDivision of Materials Science, Luleå University of Technology, Sweden
bFiber and Particle Engineering, University of Oulu, Finland
cApplied Wood Materials Laboratory, Empa, Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland
dSchool of Biotechnology, Royal Institute of Technology (KTH), AlbaNova University Centre, Stockholm, Sweden
Available online 7 November 2015. Abstract This review addresses the recent developments of the processing of cellulose nanocomposites, focusing on the most used techniques, including solution casting, melt-processing of thermoplastic cellulose nanocomposites and resin impregnation of cellulose nanopapers using thermoset resins. Important techniques, such as partially dissolved cellulose nanocomposites, nanocomposite foams reinforced with nanocellulose, as well as long continuous fibers or filaments, are also addressed. It is shown how the research on cellulose nanocomposites has rapidly increased during the last 10 years, and manufacturing techniques have been developed from simple casting to these more sophisticated methods. To produce cellulose nanocomposites for commercial use, the processing of these materials must be developed from laboratory to industrially viable methods. Abbreviations
A-CNC, acetylated cellulose nanocrystals
AFM, atomic force microscopy
BC, bacterial cellulose
CA, cellulose acetate
CAB, cellulose acetate butyrate
CMC, carboxymethyl cellulose
CNC, cellulose nanocrystals and cellulose nanowhiskers
CNF, cellulose nanofibers and nanofibrillated cellulose and microfibrillated cellulose
B-CNC, tert-butanol cellulose nanocrystals
DMAc, dimethylacetamide
EP, epoxy
GO, graphene oxide
GMA, glycidyl methacrylate
GTA, glycerol triacetate
HPC, hydroxypropyl cellulose
IL, ionic liquid
LDPE, low density polyethylene
LiCl, lithium chloride
LPEG, linear unsaturated polycondensate (oxyethylene)
MAPP, maleated polypropylene
MCC, micro crystalline cellulose
MF, melamine formaldehyde
NR, natural rubber
PA6, polyamide
PCL, polycaprolactone, PE, polyethylene
PF, phenol formaldehyde
PAH, polyallylamine hydrochloride
PAN, polyacrylonitrile
PANI, polyaniline
PDMS, polydimethyl siloxane
PEDOT, poly(3,4-ethylenedioxythiophene)
PHBV, polyhydroxy butyrate hydroxy valerate
PHO, poly-b-hydroxyoxanoate
PLA, polylactic acid
PP, polypropylene
PS, polystyrene sulfonic acid
PTFE, polytetrafluoroethylene
PVA, polyvinyl alcohol
PVAc, polyvinyl acetate
PVOH, polyvinyl alcohol
PU, polyurethane
SEM, scanning electron microscopy
S-MA, styrene maleic anhydride copolymer
THF, tetrahydrofuran
TS, thermoplastic starch
TEC, triethylene citrate
TEM, transmission electron microscopy
TOC, tempo oxidized cellulose
WF, wood fibers
Keywords
A. Cellulose
Nanocomposites
E. Casting
Extrusion
Liquid composite molding
Spinning
1 Introduction
The research subject of cellulose nanomaterials started in the mid-90s. The pioneering group led by Chanzy and Cavaille at CERMAV in Grenoble, France [1], published the first paper on cellulose nanocomposites. This group led the research in this area until the beginning of 2000, by which time many researchers had joined the field. Cellulose nanocomposites are manufactured using different processes, and these processes affect the composite properties, such as the dispersion, distribution and alignment of the reinforcing phase. Thus, the research and development of the manufacturing process of celluloses nanocomposites is an essential part of the development of cellulose nanocomposites. The processing of nanocomposites initially involved solvent casting of water-soluble or water-dispersive polymers, which were mixed with cellulose nanomaterials [1], [2], [3], [4] and [5] because both nanofibers and nanocrystals are easily dispersed in water. In 2005, Yano and co-workers in Japan made another breakthrough, developing cellulose nanopapers and their impregnation with thermoset polymers [6], [7], [8] and [9], thus producing composites that showed much higher mechanical properties than those based on starch and latex, i.e., biopolymers used in solvent casting. During the same time period, Oksman and co-workers started the processing development of cellulose nanocomposites in which different thermoplastic polymers were mixed with cellulose nanocrystals and nanofibers using twin-screw extrusion [10], [11], [12], [13], [14] and [15]. Since that time, the research on cellulose nanocomposites has grown exponentially, and this growth is reflected in the increase in the number publications. At the time of writing, there are almost 6000 publications on nanocellulose materials. However, many of these publications address the isolation of nanocelluloses from different raw material sources and the nanomaterial structure and properties rather than nanocomposites and their processing. A Web of Science search performed in June 2015 on nanocomposites (see Fig. 1) resulted in 1700 journal publications.
Fig. 1. Number of publications on cellulose nanocomposites, showing the country, most publishing journals and popular subjects dealing with processing.
The countries with the highest number of publications on this subject are China, followed by the USA, Sweden, France, Canada and Japan, showing for example that currently China is very active on this field and has over taken that role from USA. It is also seen that solvent casting is the traditional and simplest way to make lab-scale nanocomposites and has been the most popular method to prepare nanocomposites, with 253 articles in the search using this method. The use of extrusion or melt processing has increased, especially in recent years. Impregnation of prepared paper is also a common way to prepare nanocomposites, however, it was difficult to estimate the number of impregnation studies because only a few studies have focused on processing. Therefore, these articles lack keywords associated with this method of processing. Interest in foaming and fiber spinning of cellulose nanocomposites has also increased in recent years.
The number of publications on cellulose nanocomposites in composite and polymer material journals is lower (317), but the trend is similar, with the first article published in 1996 by Helbert et al. [16], increasing to 58 publications last year (2014).
The aim of this review is to provide an overview of the processing techniques of cellulose nanocomposites, including new development areas, such as nanocomposite foams and fibers. The main focus is on casting, melt-processing and resin impregnation, which are the most important processes of cellulose nanocomposites.
2 Pre-treatment of nanocelluloses prior to composite processes
In nanocomposites, the surface properties of nanocellulose determine the fiber–fiber bonding within the cellulose network and the interfacial adhesion between the fiber and matrix, which ultimately dictates the structure and properties of the composites [17]. The critical challenge to achieve the transfer of exceptional mechanical properties of nanocellulose of single fiber level to the macroscale properties of the bulk nanocomposites is not only the ability to obtain well-dispersed hydrophilic reinforcing nanocellulose in the polymer matrices but also to optimization of the fiber–matrix interface [18]. Although nanocomposites have been successfully prepared from water suspensions of nanocellulose or from an organic medium (N,N-dimethylformamide) suspension [19], the hydrophilic nature and low thermal stability of nanocellulose limits the choice of polymer matrices and processing technologies for composites [2]. Since cellulose has a glass transition temperature in the range of 200–230 °C and thermal decomposition starts at ca. 260 °C, the compounding temperature is commonly restricted to about 200 °C in the extrusion of thermoplastic composites reinforced with cellulosic fibers. Previous study showed that the thermal stability of CNFs decreased due to the homogenization and drying process and CNCs obtained by acid hydrolysis also showed decreased thermal stability due to the charge groups on the surface. To increase the surface hydrophobicity while maintaining the thermal stability, the surface pretreatment and chemical functionalization of CNCs and CNFs is a challenging and important pre-processing step in nanocomposite preparation. There are generally two approaches: covalent coupling of hydrophobic moieties directly on the surface of cellulose nanoparticles and covalent coupling of moieties directly on the surface of cellulose nanoparticles. Table 1 shows examples of chemical functionalization methods that have been used for nanocellulose applications in nanocomposite preparation.
Table 1. Examples of surface functionalization of nanocellulose in nanocomposites preparation.
The surface functionalization by acetylation [15] and [20], esterification [21] and [22], silanization [23], silylation [22] and [24], glyoxalization [25] or grafting of PCL [26], PEG [27] or GMA [28] on cellulose nanocrystals (CNCc) has, in some cases, improved the mechanical properties of PLA nanocomposites.
Upon coating with an anionic surfactant, cellulose nanocrystals have been effectively dispersed in PLA [12] (Fig. 2), but the use of surfactant had a negative effect on the mechanical properties of the PLA, which were improved by the addition of CNC.
Fig. 2. Improved dispersion of CNC in the PLA matrix as the surfactant content increases. Fractured surfaces of (a) PLA–CNC without surfactant show agglomerated CNC, (b) PLA–CNC with 5% surfactant, (c) PLA–CNC with 10% surfactant and (d) PLA–CNC with 20% surfactant. (Copyright permission Taylor & Francis [12]).
Non-ionic surfactant has also been utilized to improve the dispersion properties of cellulose nanocrystals in polystyrene [29]. Cellulose nanocrystals [35] and nanofibrils [36] modified with quaternary ammonium salts have shown high degrees of nanodispersion in organic solvents, and nanocomposites of modified cellulose nanocrystals with PVAc have been prepared.
To obtain surface-functionalized CNF, it is more efficient to perform the corresponding chemical reaction on micrometer-scale wood pulp fibers before the final mechanical disintegration step. In this fashion, TEMPO-mediated oxidation [37], carboxymethylation [38] and [39], cationization, and pegylation reactions [27] have been performed on WF and CNF. However, the major drawback in covalent functionalization to increase the hydrophobicity of nanocellulose is the tedious solvent exchange process and the use of organic solvents in these reactions. Recently, a solvent-free, one-pot process for surface esterification of cellulose nanocrystals was developed using carboxylic acids that act not only as a grafting solvent but also as a solvent media above their melting point [40]. Such green processes for surface hydrophobization of nanocellulose have the potential for application in large volume or even online composite processing.
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